Handbook of Green Chemistry, Volume 2: Heterogeneous Catalysis

Contents 1 Zeolites in Catalysis (Stephen H. Brown) 1 2 Sol–Gel Sulfonic Acid Silicas as Catalysts (Adam F. Lee and...

Author: Robert H. Crabtree

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Contents 1

Zeolites in Catalysis (Stephen H. Brown)

1

2

Sol–Gel Sulfonic Acid Silicas as Catalysts (Adam F. Lee and Karen Wilson)

3

Applications of Environmentally Friendly TiO2 Photocatalysts in Green Chemistry: Environmental Purification and Clean Energy Production Under Solar Light Irradiation 59 (Masaya Matsuoka and Masakazu Anpo)

4

Nanoparticles in Green Catalysis (Mazaahir Kidwai)

5

‘Heterogreeneous Chemistry‘ (Heiko Jacobsen)

6

Single-site Heterogeneous Catalysts via Surface-bound Organometallic and Inorganic Complexes 117 (Christophe Copéret)

7

Sustainable Heterogeneous Acid Catalysis by Heteropoly Acids (Ivan Kozhevnikov)

8

The Kinetics of TiO2-based Solar Cells Sensitized by Metal Complexes (Anthony G. Fitch, Don Walker, and Nathan S. Lewis)

9

Automotive Emission Control: Past, Present and Future (Robert J. Farrauto and Jeffrey Hoke)

10

Heterogeneous Catalysis for Hydrogen Production (Morgan S. Scott and Hicham Idriss)

11

High-Throughput Screening of Catalyst Libraries for Emissions Control (Stephen Cypes, Joel Cizeron, Alfred Hagemeyer, and Anthony Volpe)

12

Catalytic Conversion of High-Moisture Biomass to Synthetic Natural Gas in Supercritical Water 281 (Frédéric Vogel)

37

81

93

153

175

197

223

247

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1 Zeolites in Catalysis Stephen H. Brown

1.1 Introduction

Acid catalysis as a modern science is less than 150 years old. From its inception, acid catalysis has been explored as a means of producing fuels, lubes and petrochemicals. Ordinary homogeneous acids, both inorganic and organic, never proved industrially useful at temperatures much above 150 C. The first reports of aluminosilicate solid acid catalysts involved the use of clays after the turn of the century. The inspiration for the first commercial synthetic aluminosilicate catalysts came from work done co precipitating silicon and aluminum salts during WWI by a Sun Oil chemist [1]. The Brønsted acid site in these materials is most often represented as in Scheme 1.1. Useful features of this novel type of acid versus homogeneous liquid acids were their high temperature stability, moderate acidity (roughly equivalent to a 50% sulfuric acid solution), solid and non-corrosive character and regenerability by air oxidation. These features enabled acid catalyzed reactions of chemicals to be contemplated at a greatly extended range of temperatures (up to 600 C) and metallurgies. Scheme 1.1 The Brønsted acid site of an aluminosilicate.

The first embodiments of many modern refining processes including heavy oil cracking, naphtha reforming and light gas oligomerization did not use catalysts [2]. As soon as these thermal processes commercialized, exploration of the use of solid acid catalysts ensued naturally. Because of the key role played in the development of the automotive industry, heavy oil cracking to gasoline provided a focal point for the early development of heterogeneous acid catalysis. Temperatures above 400 C and pressures below 3 atmospheres are thermodynamically favorable for the conversion of heavy oils to light hydrocarbons rich in olefins. Acceptable heavy oil cracking rates are achieved without a

Handbook of Green Chemistry, Volume 2: Heterogeneous Catalysis. Edited by Robert H. Crabtree Copyright 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32497-2

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catalyst at temperatures above 600 C. This was the basis of the thermal cracking process. Thermal cracking produces high yields of methane and aromatic hydrocarbons. The goal of researchers was to find a catalyst that could crack heavy hydrocarbons selectively to gasoline with only minimal formation of gases with molecular weights of less than 30. Due to thermodynamic constraints, the catalyst had to be effective at a temperature above 400 C. In order to avoid unselective thermal cracking, the catalyst had to be effective below 550 C. The discovery in the early 1920s by Houdry that acid activated clays were active and selective in this temperature window was a breakthrough [2]. In the 1930s and 1940s methods were developed and commercialized to produce high surface area manmade aluminosilicates that were significantly improved catalysts. Examination of the aluminosilicate catalysts led to the understanding that the active site was a Brønsted acid [3]. At the time of the discovery of synthetic zeolites in the early 1950s, only two classes of solid Brønsted acids (solid phosphoric acid and aluminosilicates) were being used commercially to produce commodity fuels or petrochemicals [4]. The commercialization of silica-rich synthetic zeolites in their hydrogen form represented a breakthrough for scientists and organizations interested in the production of fuels, lubes and petrochemicals at temperatures above 200 C. Like amorphous aluminosilicates, zeolite Brønsted acid sites are active and stable up to 600 C. Shortly after Union Carbides discovery of synthetic zeolites in the late 1940s, Mobil Oil researchers in catalytic cracking of heavy oil investigated zeolites as potential catalysts [5]. The zeolite known as faujasite (FAU) was found to be three to five orders of magnitude more active than amorphous aluminosilicates. Unmodified, FAU was too active to be useful. When the activity of FAU was tuned by ion exchange with rare earth cations and/or by reducing aluminum content, it was found to have a dramatically different selectivity to cracked products. Optimized samples of FAU zeolites produced almost 5% less C2-gases and coke and increased gasoline yields by more than 10 wt%. Over the course of the past 50 years, evolving heavy oil cracking catalysts and hardware have been continuously decreasing coke and C2-gas yields while increasing the yield of gasoline. The commercialization of zeolite catalysts for heavy oil cracking unleashed the creative abilities of every organization interested in producing fuels and petrochemicals using acid catalysts between 250 and 600 C. Close to 23 processes have been commercialized (Table 1.1). About two-thirds of the processes had no real precedence using homogeneous acids. The other third involved displacement of homogeneous and amorphous acid catalysts. Introduction of zeolite catalysts for the production of commodities has proceeded at a steady pace. Each commercialization has provided an opportunity for zeolite scientists to find improved catalysts. 1.1.1 The Environmental Benefits of Zeolite-enabled Processes

The petroleum industry has been subject to environmental drivers for many decades [6]. Innovations in technology, some driven by more restrictive regulations,

1.1 Introduction Table 1.1 List of zeolite processes.

Process

Reactor type

Temperature range, C

Toluene þ C9 þ aromatics MSTDP Cumene via transalkylation Ethylbenzene via transalkylation Ethylbenzene Cumene Fluid catalytic cracking (FCC) ZSM-5 in FCC Gasoil hydrocracking Distillate hydrocracking Distillate dewaxing Wax hydrocracking Wax hydroisomerization Gasoline octane enhancement Reformate upgrading Light paraffin isomerization Butene isomerization Xylene isomerization Light paraffin aromatization Methanol to gasoline Methanol to olefins Aromatics feed treating Caprolactam

Fixed Fixed Fixed Fixed Fixed Fixed Fluid Fluid Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Moving Fixed Fluid Fixed Fluid

350–450 350–450 150–200 230–260 180–250 100–150 500–550 500–550 350–475 280–350 350–450 290–370 300–350 350–450 450–550 240–300 350–450 400–470 450–550 300–400 400–500 150–250 350–450

have continuously increased the efficiency of refining processes. The trend is to produce fuels having lower concentrations of heteroatoms and polynuclear aromatics (often referred to as clean fuels) that can be burned to carbon dioxide and water with increasingly lower emissions of NOx, SOx and particulate byproducts. For decades, nearly the entire hydrocarbon content of a barrel of oil feeding a refinery or petrochemical complex has been converted to salable products or used for fuel at the manufacturing site. Distillation of crude oil largely splits it into streams with the boiling ranges of the fuels sold to consumers and businesses (gasoline, diesel, fuel oil, etc.). The quantities of the streams produced by distillation rarely match market demand. Processes using zeolite catalysts have reduced the effort required to convert streams that are oversupplied by simple crude oil distillation into undersupplied products. Optimized zeolite catalyzed processes are often high technology operations. Performance can be sensitive to the performance of neighboring units. Operating multiple zeolite-catalyzed processes can provide refiners with an incentive to continuously work to bring the refinery closer to steady state operation. Adoption of these high technology processes and work practices has helped refiners to steadily increase the amount of clean fuel products produced from each barrel of oil, thereby reducing emissions of CO2, NOx, SOx and particulates and increasing energy efficiency.

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Zeolite catalysts are remarkably efficient. Each weight unit of zeolite produces between 3000 and 500 000 weight units of fuel or petrochemical product before its lifetime ends and it is removed from catalyst service. As a result, relatively small volumes of spent zeolite catalysts are produced. There are often other uses for spent catalysts, such as an ingredient for cement. In the many cases where reuse is an option, there are little/no catalyst waste disposal costs. Catalytic cracking (also known as fluid catalytic cracking or FCC) is by far the most economically important process in the refining and petrochemicals industry and will be described in some detail to allow the green aspects to be highlighted. World wide, FCC units process almost 20 million bbl/day of feedstock (almost 30% of the crude oil produced) and FCC catalysts generate $1 billion in sales [7]. The remarkable performance of the FCC process is achieved by both optimizing the zeolite catalyst and the reactor design. A schematic of an FCC unit is provided in Figure 1.1. The FCC catalyst spends most of its time in a large, cylindrical regeneration vessel typically 15 meters in diameter and 40 meters tall holding 300 tons of a coarse powder catalyst comprised of a bell-shaped distribution of spheres between 15 and 120 microns in diameter. The vessel is typically held at 15–25 psig and 620 to 700 C. Air is continuously blown up from the bottom of the vessel and is carefully distributed to provide uniform contacting with the solids. When properly engineered, up flowing gases mix with the coarse catalyst powder to form a mixture which behaves like a fluid. The reaction carried out in the regeneration vessel is the combustion of the solid carbonaceous reaction byproducts that accumulate on the catalyst during the cracking reaction. The FCC catalyst enters the isothermal, back-mixed regenerator at the reaction temperature (about 550 C) and is heated to the regenerator temperature by the heat of combustion of the coke. Because of its fluid-like properties in the presence of a flowing gas stream, the catalyst will flow smoothly out of the bottom of the regenerator, up a 2 meter diameter

Figure 1.1 FCC reactor process flow diagram.

1.2 General Process Considerations

pipe (called a riser) where it contacts the heavy oil feedstock and then back into the top of the regeneration vessel. A typical catalyst circulation rate for a unit filled with 300 tons of catalyst would be 3000 tons/hour. An average catalyst particle travels through the riser once every 5 or 6 minutes. Heavy oil feedstock is heated to about 300 C and sprayed into the circulating catalyst (620 to 700 C) at the bottom of the riser. Feed vaporization is accomplished by direct contact with the hot zeolite catalyst. The gaseous product is removed utilizing cyclones at the top of the riser. Feedstock is typically fed into the riser at twice the total catalyst inventory and one fifth the catalyst circulation rate (e.g. catalyst circulation of 3000 tons/h and a feed throughput of 600 tons/h). The total time of feedstock and catalyst contact is several seconds. About 5 wt% of the feedstock (no more, no less) must be converted to the carbonaceous solids (coke) that are required to provide the energy input needed to drive the feedstock vaporization and the endothermic reaction. A typical catalyst particle contains about 1 wt% coke on catalyst upon entering the regenerator. Thirty to fifty percent of a barrel of crude oil boils above the endpoint of gasoline and automotive diesel fuels. The FCC unit converts much of this material into gasoline and diesel fuels with roughly 80 wt% selectivity. Another 5 to 10% of the C4products are easily converted into high quality gasoline in a second step, resulting in an overall selectivity to gasoline and diesel fuels of 85 to 90%. Five wt% of the feed is converted to coke which is used to supply most of the fuel for the unit (regeneration and separations). The remaining ca. 5–10% of the byproducts are mostly low molecular weight gases ( 5 and 5 and 12–20 angstroms, acid sites inside the pores can be conceptualized in the same fashion as acid sites on amorphous aluminosilicates or on zeolite surfaces. Pores so large place few steric constraints on the polymerization of large molecules into larger deactivating oligomeric structures. Structures of the zeolite frameworks listed in Table 1.2 are provided at the IZA website. Although all the structures contain 10 or 12 ring pores, each structure has many unique aspects. Each ring system has its own unique size and shape. Some zeolites, e.g. FAU, have large internal cavities, while others contain only one dimensional cylindrical pores (e.g. LTL). Because there are only a handful of unique structures, it should not be surprising that there is often a large difference in performance when these structures are applied. 1.3.1 Other Properties

At temperatures >200 C zeolite Brønsted acid protons delocalize [11]. At temperatures >550 C dehydroxylation is initiated and Brønsted acid activity is lost [12]. The presence of steam can retard dehydroxylation. Activity loss by dehydroxylation is commonly reversible as the dehydroxylated zeolite can rehydrate at low temperature and resume its original structure. Zeolites exchanged with polyvalent metal ions (typically nitrate salts) become acidic upon thermal dehydration and nitrate decomposition [13–16]. Weak Brønsted acid sites can form by hydroxylation of the metal cation. The mechanism is believed to proceed by association of the cation with a specific framework aluminum accompanied by dissociation of water to form a hydroxyl group attached to the cation (Scheme 1.2). For this reason, the addition of polyvalent cations to zeolites directly impacts the number of Brønsted acid sites and total zeolite pore volume but has only a minor impact on the strength of the remaining Brønsted acid sites. Furthermore, zeolites containing polyvalent cations are considerably more complex because both the metal cations and the protons are mobile and because many metal ions are more active for redox reactions than silicon and aluminum. At reaction temperatures between 250 and 500 C these features generally lead to increased rates of hydrogen transfer reactions and more rapid deactivation explaining the limited use of zeolites exchanged with polyvalent cations. Scheme 1.2 Example of a weak Brønsted acid site in a metalexchanged zeolite.

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1.3.2 Number of Acid Sites

Loewensteins rule forbids the formation of AlOAl bonds in zeolite structures [17]. Therefore, the potential number of acid sites equals the number of aluminum atoms in any reference unit of a zeolite crystal. High silica zeolites (Si : Al > 20) can generally be synthesized and converted to the hydrogen form with minimal deviation from the idealized structure. For these materials the number of acid sites determined by analytical techniques agrees well with the number of acid sites derived from a simple analysis of bulk aluminum content. 1.3.3 Acid Strength

All of the catalysts used in the reviewed processes are aluminosilicates. The overall acid strength of a hydrogen form aluminosilicate zeolite depends upon aluminum distribution. Acidity associated with an aluminum tetrahedra is stronger with a smaller number of near aluminum atoms [18–20]. For this reason, zeolites with Si:Al ratios between 1 and 10 can have a variety of acid site strengths. However, careful studies with ZSM-5 demonstrated that acid sites with 0 and 1 next nearest neighbor aluminums are very close in acid site strength [21]. Most of the acid sites in zeolites with Si : Al ratios >10 have only a small number of their sites with more than one aluminum next nearest neighbors leading to uniform acid site strength. The strength of this site has been well characterized by NMR and IR probes of simple sorbates allowing the conclusion to be reached that the acid site strength is similar to that of 70% sulfuric acid [22–24]. Careful studies of model compound reactions uncomplicated by mass transfer limitations or fast secondary reaction provide further support for uniform acid site strength [25–27]. At the present time, aluminosilicate zeolites remain the only class of crystalline solid Brønsted acids to find broad use in the production of commodity chemicals. Although a wide range of materials with alternative framework compositions are known, few commercial uses have been found for these materials. Zeolite frameworks and novel frameworks based on aluminophosphate building blocks were discovered at Union Carbide in the early 1980s [28–30]. When phosphorus sites are substituted with silicon, a Brønsted acid is formed. The acid site in these materials is weaker than an aluminosilicate acid site.

1.4 Reaction Mechanisms 1.4.1 Hydrocarbon Cracking

Academic work in the 1930s and 40s elucidated how AlCl3 – a strong Lewis acid – is converted when dissolved in hydrocarbon fractions to a working catalytic species with

1.4 Reaction Mechanisms

strong Brønsted acidity (Scheme 1.3) [31–33]. The basic mechanistic features of hydrocarbon cracking were well understood by the end of the 1950s and are well explained in many subsequent reviews [34–39]. Any unsaturated molecules (i.e. aromatics, olefins, dienes) in hydrocarbon streams undergo protonation in the presence of a Brønsted acid catalyst. Once protonated, isomerization reactions can proceed. In general, hydride shifts proceed considerably faster than alkyl shifts (Scheme 1.4). Exact relative rates are dependent upon the structure of the hydrocarbon, the catalyst and the conditions and need to be computed or measured on a case by case basis.

Scheme 1.3 Example AlCl3 activation reactions.

Scheme 1.4 Hydride and methyl shifts.

Once protonated, a hydrocarbon molecule is destabilized, existing almost simultaneously as many different carbocations. The energy of most hydrocarbon carbocations are now well understood and can be calculated using algorithms derived from first principles [40]. In most cases it is safe to assume that a representative sample of a specific protonated hydrocarbon exists at any instant with the full pool of its possible cation isomers populated at a distribution at least approaching thermodynamic equilibrium. Because carbocations are stabilized by delocalization and electron donating groups, isomers containing these attributes dominate the instantaneous distribution (Scheme 1.5, for example). The most stable cations, however, can be less reactive and therefore may not be the most important intermediates of the reaction pathway. Scheme 1.5 Sample of a cation stabilized by conjugation and branching.

Acid cracking of cations proceeds most readily via beta scission. Aromatics dealkylation is the least complex as it is dominated by a single class of beta scission reaction (Scheme 1.6). There are many viable cracking pathways for paraffin, olefin and naphthene-derived hydrocarbon cations. Cracking of these species is dominated

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by beta scission pathways A, B1, B2 and C (Scheme 1.7) [41]. Relative rates of these reactions along with cracking reactions involving primary cations (pathways D and E, Scheme 1.7) have been determined using model compounds and ZSM-5 catalysts at cracking conditions typical of the industrial processes covered in this review (Tables 1.3 and 1.4) [42]. The combined data from the tables demonstrate that large, branched olefins are readily cracked. Through successive cracking and oligomerization reactions, zeolite catalysts can convert such molecules to a broad distribution of olefins directed by thermodynamic considerations at temperatures below 200 C.

Scheme 1.6 Aromatics cracking reaction.

Scheme 1.7 Hydrocarbon cation cracking types.

Table 1.3 Cracking rate constants of hydrocarbons over HZSM-5 at 510 C.

Carbon #

Rate constant k sec1 olefin

Rate constant k sec1 paraffin

4 5 6 7 8

10 230 1800 5700

0.1 0.3 0.8 1.5 2.2

1.4 Reaction Mechanisms Table 1.4 Relative rates of olefin cracking over HZSM-5 at 510 C.

Olefin feed

Cracking type

Relative rate constant

C6

C D E B C D E B C

40 2 1 120 40 2 1 120 40

C7

C8

Cracking of pure paraffins has been the subject of a great deal of academic interest due to the possibility that paraffin cracking might be initiated by protonation (Scheme 1.8). In fact, under forcing conditions this reaction has been observed [43– 45]. It has a higher activation energy than paraffin cracking involving hydride abstraction (Scheme 1.9). Chain cracking involving hydride abstraction [46] dominates because this pathway is autocatalytic. Furthermore, typical hydrocarbon mixtures almost always contain aromatic and/or olefinic initiators so cracking via paraffin protonation need never be invoked.

Scheme 1.8 Example of paraffin cracking via protonation.

Scheme 1.9 Example of paraffin cracking via hydride transfer.

An unusual reaction that cracks polymethylaromatics to ethylene and propylene, and mono, di and trimethylbenzenes, is known as the Paring reaction [47]. The Paring reaction is unusually important in zeolite catalysis because it provides a mechanism for removing polyalkylbenzenes trapped inside zeolite pores before they can undergo further reactions to form condensed ring polynuclear aromatics that cause permanent deactivation. The mechanistic steps that form ethyl and propyl aromatics from polymethyl aromatics are shown (Scheme 1.10). The reaction can occur via a concerted electrocyclic reaction of migrating double bonds. The reaction is facilitated by the known non-classical structure of protonated polymethylbenzenes [48–51]. Dilute solutions of pentamethylbenzene in FSO3H result in stable solutions of completely protonated aromatic. Irradiation of this cation at 78 C causes nearly quantitative conversion

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via ring contraction to a stable pentacoordinate cyclopentyl cation with a non-classical structure (Scheme 1.11). Expansion of this ion back to a 6 ring protonated aromatic occurs upon warming to 30 C.

Scheme 1.10 Formation of iso-propyl-aromatics from polymethyl-aromatics.

Scheme 1.11 Non-classical cation structure from pentamethylbenzene protonation.

1.4.2 Oligomerization and Alkylation

Like cracking, the mechanism was first well described in the 1940s using homogeneous acids [52–56]. Protonation of unsaturated hydrocarbons is governed by the same rules as just discussed for cracking. The adding cation is usually secondary or


tertiary creating branches in the product molecules. Addition is governed by Markovnikovs rule. These features are exemplified by a favored butene trimerization pathway (Scheme 1.12) [57]. Secondary alkyl and hydride shift reactions occurring at competitive rates often result in a very complex product mixture especially at the temperatures between 100 and 200 C which are typically utilized when oligomerizing light olefins over zeolites [58, 59]. Steric constraints impact oligomerization using zeolites. The branchiness and average carbon number of oligomerization products tends to decrease with decreasing dimensionality and pore size. Careful attention to choice of zeolite and conditions allows the production of oligomers with near linear structures (more linear than thermodynamic equilibrium) [60].

Scheme 1.12 Isobutylene trimerization at 0 C with 65–70% H2SO4.

Alkylations of olefins with aromatics or paraffins are reverse cracking reactions which proceed by a similar mechanism to olefin oligomerization. It was first described using homogeneous acid catalysts at near room temperature [61–63]. Aromatics alkylation proceeds by protonation of the olefin followed by addition to the aromatic ring (Scheme 1.13). When an alkylbenzene undergoes further alkylation (i.e. toluene methylation to form xylenes), electron donation by the alkyl substituent activates the ring and causes the addition to be ortho/para selective [64]. Steric constraints play a strong role. While methyl and ethyl groups readily form ortho isomers, ortho di-isopropylbenzene is a minor reaction product and dit-butylbenzene is not formed with acid catalysts [65].

Scheme 1.13 Proposed aromatics alkylation mechanism.

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Isoparaffin alkylation is a chain reaction that has a hydride transfer as the rate determining step (Scheme 1.14). Olefin oligomerization has a lower activation energy and proceeds much faster than isoparaffin alkylation at ordinary conditions. Good selectivity to C8 isoparaffins from a mixture of isobutane and n-butenes is achieved by running the reaction in a large excess of isobutane.

Scheme 1.14 Proposed isoparaffin alkylation Mechanism.

1.4.3 Isomerization

Olefins are easily isomerized upon contact with mineral acids. The olefin doublebond shift reaction is one of the fastest acid catalyzed reactions of hydrocarbons (Scheme 1.15). Shifting alkyl groups along the backbone of an olefin also occurs readily but requires more severe conditions since more than simple protonation/ deprotonation reactions are involved. This type of reaction is exemplified by a methyl shift reaction (Scheme 1.16). A third type of olefin isomerization adds or removes a branch [66, 67]. This type of isomerization involves a cyclopropyl alkylcarbonium ion often referred to as a protonated cyclopropane (Scheme 1.17).

Scheme 1.15 Proposed mechanism of olefin double-bond shift reactions.

Scheme 1.16 Proposed mechanism of a methyl shift olefin isomerization reaction.

Scheme 1.17 Proposed mechanism of olefin branching/unbranching isomerization.


At low temperatures and high pressures, oligomerization/polymerization is favored by thermodynamics. Therefore, care must be taken to achieve selective isomerization. Selectively achieving the more demanding skeletal isomerization of olefins usually requires operation at higher temperatures and lower pressures where olefin monomers dominate equilibrium. As olefin molecular weight rises, selective skeletal isomerization becomes increasingly challenging since increasing temperature and decreasing pressure leads to undesired cracking while decreasing temperature and increasing pressure leads to oligomerization. Paraffins isomerize by nearly the same mechanisms as olefins. Paraffin isomerization is of greater industrial interest than the isomerization of unsaturated hydrocarbons because it enabled conversion of C5–C10 n-alkanes of low octane values into branched alkanes with high octane and the conversion of C20 to C100 waxes into lubricant base stocks. It also enabled the production of isobutane from n-butane which was needed to make alkylate. The extensive work aimed at commercializing paraffin isomerization contributed greatly to the fundamental understanding of all acid catalyzed hydrocarbon isomerizations. Paraffin isomerization is more difficult than olefin or aromatic isomerization because it requires hydride abstraction (Scheme 1.19) or dehydrogenation using a noble metal. In the most common method, a noble metal is used to bring the paraffin to the paraffin/olefin equilibrium [68]. At temperatures below 300 C, equilibrium favors the paraffin so only trace olefin is present in the reaction medium. An acid function isomerizes the trace olefin by the mechanisms in Schemes 1.15, 1.16, and 1.17. Polyalkyl aromatics also isomerize by an alkyl shift mechanism. The reaction mechanism for the interconversion of meta and para xylene is shown in Scheme 1.18. Note that in each of Schemes 1.15, 1.16 and 1.18 the intermediate cation structures shown are all secondary. Although these secondary cations are higher energy than the available tertiary cations, they are necessary intermediates in the reaction pathway. The possible tertiary cations are not shown because formation of these favored ions does not lead to the desired transformation.

Scheme 1.18 Proposed mechanism of xylene isomerization.

Scheme 1.19 Proposed chain transfer hydride abstraction step in paraffin isomerization.

1.4.4 Transalkylation of Aromatics

The acid catalyzed reaction, disproportionation of alkylbenzenes to benzene and polyalkylbenzenes, has been investigated thoroughly. Using Friedel–Crafts catalysts

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and conditions, tert-alkyl benzenes transalkylate by cracking to olefin and benzene and realkylating [69, 70]. N-Alkylbenzenes transalkylate by a different mechanism. Mechanistic studies of the n-alkyl benzene reaction have been carried out at low temperatures in liquid media containing Friedel-Crafts or superacid catalysts [71–73], showed that the reaction proceeds via a chain mechanism initiated by the formation of benzyl cations and propagated by hydride abstraction. Sec-alkylbenzenes apparently undergo transalkylation by one or a combination of these two mechanisms depending upon the catalyst and conditions. The mechanism of n-alkylbenzene transalkylation depends on the steady state concentration of benzyl cations, which when formed abstract a hydride from a neighboring aromatic or alkylate another aromatic to form a 1,1-diphenylalkane. The mechanism for moving the simplest n-alkyl substituent, a methyl group, from one aromatic ring to another is provided (Scheme 1.21). In the presence of an acid, 1,1-diphenylethane easily cracks back to benzene and styrene (Scheme 1.20). Protonation of styrene forms a benzyl cation, which either abstracts a hydride from another ethylbenzene or alkylates. Although alkylation to 1,1-diphenylethane is much faster than hydride abstraction, repeated cracking back to styrene and benzene (Scheme 1.20) followed by protonation to benzyl cation eventually results in chain transfer of the hydride, completing the transalkylation.

Scheme 1.20 Cracking of diphenylethane to benzene and styrene.

Scheme 1.21 Proposed mechanism of toluene transmethylation.


At temperatures between 0 and 50 C, toluene disproportionation catalyzed by AlCl3 and AlBr3 is inefficient and was estimated to proceed about 107 times more slowly than transethylation [74]. Diphenylmethane lacks a b-CH bond and is more difficult to crack explaining the observed dramatic rate deceleration. Although inefficient with Friedel–Crafts catalysts, the disproportionation of toluene to xylenes is practiced industrially to convert on the order of 10 billion pounds of aromatics per year with high activity zeolite catalysts. In addition to the mechanism already discussed (Scheme 1.21), another acid catalyzed pathway involving dimers has been reported [75]. This mechanism (Scheme 1.22) is consistent with previously reported mechanistic studies on aromatics transmethylation at high temperatures using solid acid catalysts [76–78]. It proceeds via aromatics protonation and is likely to be of increasing importance as the basicity of the aromatic increases. Polyalkylbenzenes, and particularly 1,3,5-trisubstituted polyalkylbenzenes, are orders of magnitude more basic than benzene, toluene and xylenes [79], and are more likely to proceed by this mechanism.

Scheme 1.22 Proposed toluene disproportionation mechanism with a diphenylmethane intermediate.

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The Scheme 1.22 mechanism involves the formation of cyclohexadienyl toluenes as intermediates. At low temperatures in the liquid phase, these species would be expected to undergo further alkylations to form oligomers. Consistent with this prediction, benzene solutions of AlBr3–HBr have been reported to be unstable [80]. The use of zeolite catalysts can lead to mixed mechanisms. At high temperatures and moderate pressures unimolecular cracking reactions are favored relative to bimolecular condensations. For example, the vapor phase transalkylation of polyethylbenzenes with ZSM-5 at 150 psig aromatics and 300 C is believed to proceed via cracking/realkylation [81]. On the other hand, the use of zeolites beta, mordenite and faujasite to transalkylate polyisopropylbenzenes in the liquid phase at 300 psig and 200 C is believed to proceed mostly by bimolecular mechanisms [82]. 1.4.5 Hydrogen Transfer or Conjunct Polymerization

In the presence of acid catalysts, olefins can be reacted with isobutane in the liquid phase at 250 C substituted cyclopentadienes are unstable and convert by subsequent isomerization and hydrogen transfer reactions to predominantly 1-ring aromatic compounds. This reaction has recently been directly observed in ZSM5 [87]. Stoichiometric restrictions apply to the hydrogen transfer reaction. For each unit of unsaturation produced, one mole of paraffin is produced. Four moles of paraffins are produced to offset each mole of aromatic formed.


Scheme 1.23 Simplified mechanistic proposal to produce cyclopentadienes from olefins.

Like olefins, alkyl-aromatics undergo hydrogen transfer reactions to form multiring aromatics and light paraffins. Olefin co-feeds or olefins formed in-situ from aromatic cracking reactions are necessary participants in the reaction path. These reactions involve the formation of benzyl cations. When olefins are present, they compete with aromatics to scavenge the reactive benzyl cations. When a benzyl cation alkylates an olefin, an indane can be generally formed. This is illustrated for ethylbenzene and cumene (Schemes 1.24 and 1.25). Compared to isomerization, oligomerization, cracking and alkylation, hydrogen transfer is usually a slow reaction. Nonetheless, it is of critical importance in many zeolite catalyzed processes because catalyst deactivation by coking typically involves hydrogen transfer reactions. In order to achieve acceptable cycle lengths, fixed bed processes must reduce coking reactions to near negligible rates. Hydrogen transfer is favored at high pressures and is nearly inevitable in reactions carried out above 400 C. The bimolecular nature of the reaction explains why the reaction rate can be strongly influenced by the steric constraints imposed by the structure of the pores of a zeolite catalyst. For example, hydrogen transfer has been shown to proceed at higher rates in faujasite than in ZSM-5 [88, 89].

Scheme 1.24 Indanes, isobutylene and EB from cumene.

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Scheme 1.25 Indanes, propylene and toluene from ethylbenzene.

In addition to participating in alkylation reactions, the olefin intermediates will also abstract hydrides to form new benzyl or related cations and light paraffin. Because the alkylation products of isobutylene are often difficult to form due to steric hindrance, are easily cracked and monomers are relatively favored by thermodynamics, isobutylene is particularly prone to hydride abstraction to form isobutane. An example reaction path producing isobutane and naphthalene from isobutylene and an indane is shown in Scheme 1.26. Analogous reactions lead to the formation of other light paraffins and multiring aromatics. Indanes are ordinary observed only as minor products because these molecules are much better hydride donors than the feedstock aromatic and tend to undergo the secondary reactions to naphthalenes at faster rates than they are formed from benzyl cations and olefins.

Scheme 1.26 Isobutylene and indane to isobutane and naphthalene.

As just shown, hydrogen transfer reactions involving aromatics lead to the formation of large, polynuclear aromatics. These types of molecules are prone to reacting to form coke faster than they can diffuse out of the pores of zeolites. Control of zeolite structure, process conditions and feedstock composition to minimize or eliminate aromatics hydrogen transfer reactions (at least those leading to deactivating coke) is an area of constant study since increased process cycle lengths generally result in substantial process cost savings.

1.5 Mass Transport and Diffusion

1.5 Mass Transport and Diffusion

Diffusion in pore sizes in excess of 0.5 microns is not significantly different than gas phase diffusion in larger spaces. In pores between 10 and 1000 angstroms, wall effects are important and increase in importance as the pore diameter decreases. For most of this range, however, molecules typically strike other molecules much more frequently than they strike pore walls. Diffusivity in pores of this size is usually well described by the Knudsen flow equations. Below 10 angstroms, molecules strike the pore walls with similar or greater frequency than they strike other molecules. The diffusion of small molecules such as hydrogen, nitrogen, oxygen, CO and methane in large pore (12 ring channel systems) zeolites at temperatures above 200 C can usually be adequately described using the Knudsen flow equations. The reactants and products of interest in this review are molecules with sizes similar to the pore dimensions of the zeolite catalysts. Because of this, minor changes in size, shape and polarity can have a dramatic effect on the diffusion constant of the molecule. For this reason molecular diffusion in zeolites is a very complex topic which has been the subject of a large amount of research. This field has been the subject of many reviews [90–98]. The relative rates of diffusion of different possible reactants and products is usually at least partially responsible when large differences in selectivities are observed for different aluminosilicate catalysts. The relative diffusivities of para xylene, meta xylene and 1,3,5-trimethylbenzene have been measured at 1 atmosphere and 315 C and found to be about 107, 109 and 1012 cm2 per second respectively [99]. Dropping the diffusion constant from 109 to 1012 cm2 per second at these conditions leads to a situation where the intrinsic rate of reaction of 1,3,5-trimethylbenzene is much faster than its rate of diffusion. This circumstance is often described by saying that the reactant is too large to enter the catalyst pores. The ability of zeolites to discriminate between feed and product molecules in this fashion is at the heart of their utility. However, restricted diffusion of desired feed and product molecules is also an inherent problem. It is highly desirable to build a catalytic process where the diffusivity of the target feeds and products is fast enough compared to the intrinsic reaction rate to place the desired reaction under kinetic control (or near to it) rather than mass transport control. It is also highly desirable to design a commodity process to produce at least 0.5 liquid volumes of product per volume of catalyst per hour. Achieving these targets generally requires diffusion coefficients of greater than 1010 cm2 per second. As feed and product diffusion coefficients approach the minimum, increasingly smaller zeolite crystal size is required to achieve the catalyst productivity target. There is a large and growing body of literature reporting the reactions of molecules with highly polar or polarizable functional groups [100]. Increasing molecular weight, increasing polarity and decreasing temperature all decrease molecular diffusion rates. Poor diffusion rates of large, polar molecules constitute a high hurdle for commodity process development. An example of a zeolite-catalyzed process using a large, polar feedstock is the Beckmann rearrangement (Scheme 1.27).

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The process utilizes temperatures between 300 and 400 C, pressures below 4 atmospheres and zeolite catalyst with low aluminum content and small crystal size [101, 102]. These conditions minimize the impact of poor inherent diffusivity.

Scheme 1.27 The Beckmann rearrangement.

1.6 Zeolite Shape Selectivity

The interaction of acid catalyzed mechanisms, the unique pore geometry of zeolite catalysts and feed and product diffusion results in zeolite shape selectivity. Shape selectivity enables products to be produced with remarkable kinetic selectivities despite the high temperature conditions and competing acid catalyzed mechanisms to thermodynamically favored products. 1.6.1 Mass Transport Discrimination of Product Molecules

A well-studied example of zeolite shape selectivity is the production of xylenes containing >90% p-xylene from toluene by disproportionation. Toluene disproportionation is the classic example zeolite shape selectivity arising from mass-transport discrimination. This type of shape selectivity arises when there are significant differences in diffusivities among various classes of molecules with respect to the pore channel size/configuration for a given zeolite. Shape selective toluene disproportionation was discovered and commercialized by Mobil and converts low cost distilled toluene or extracted toluene directly into chemical grade benzene and high purity p-xylene [103, 104]. The benzene produced is so pure that no benzene extraction is required. Per pass p-xylene yield is limited by equilibrium to less than 20%. As product p-xylene approaches equilibrium, it slowly isomerizes to meta and ortho xylene. The much more demanding isomerization of xylenes to ethylbenzene proceeds at a faster apparent rate. The ethylbenzene then hydrocracks to ethane and benzene. Briefly, p-xylene is produced selectively by designing the ZSM-5 catalyst to serve as a reactive membrane for internally produced C8 aromatics [105–108]. Toluene diffuses in and out of the ZSM-5 pore system faster than it disproportionates. The rate of disproportionation is independent of crystal size out to significantly larger than 10 microns. Work with small crystal ZSM-5 catalysts at low toluene conversion proved that p-xylene dominates the kinetic distribution of xylenes. Like toluene, p-xylene diffuses in and out of the catalyst faster than it reacts. The ortho and meta

1.6 Zeolite Shape Selectivity

xylenes diffuse 1000–10 000 times more slowly and as a result concentrate inside the catalyst pores. Fast, secondary isomerization reactions convert the ortho and meta byproducts into p-xylene which then rapidly diffuses out. A much slower secondary isomerization reaction converts xylenes to ethylbenzene which also rapidly diffuses out. Excellent quantitative treatments of this remarkable interaction of diffusion and chemical kinetics are available [109]. 1.6.2 Molecular Sieving

Molecular sieving is an extreme boundary case of mass-transport discrimination. This type of shape selectivity arises when classes of molecules in the feedstock are completely excluded from the intracrystalline volume of the zeolite catalyst. Lubricant oil dewaxing is a good example of the industrial application of the molecular sieving properties of acidic zeolite catalysts. The most common lubricants are viscous hydrocarbon liquids with molecular weights ranging from 250 to 650 (C20 to C60). Lubricants can be produced from crude oil fractions in the lube molecular weight range by removing wax (linear and near linear paraffins) and high density aromatics (aromatic molecules with low hydrogen content). Acidic zeolites are highly active for the depolymerization (acid catalyzed cracking) of lubricant molecular weight hydrocarbons at 300–400 C. Commercial manufacturing of lubricants uses special zeolite catalysts that selectively crack linear and near linear paraffins. The structure, acidity and activity of the active sites of wax-selective zeolite cracking catalysts is similar to the active sites of non-selective zeolite catalysts. Molecular sieving is the source of selectivity. Linear and near linear paraffins (waxes) diffuse rapidly in and out of the pores of these catalysts. The diameter of the pores is just the right size to stabilize the adsorption and transport of linear and near-linear paraffins. Paraffins with three or more branches, cycloparaffins and aromatic molecules are all too large to fit into the zeolite catalyst pores. In optimized catalysts transport rates are discontinuous. The target molecules, linear and near linear paraffins, diffuse rapidly (diffusion coefficients near 104 cm2/sec), while the rest of the molecules in the feed are excluded (have diffusion coefficients of propyl.

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Scheme 2.11 Acylation of anisole with acetic anhydride.

The Fries rearrangement of phenyl acetate to o- and p-hydroxyacetophenones is an important reaction in the production of pharmaceuticals, dyes and agrochemicals and is also the first step of the Hoechst Celanese process for paracetamol synthesis [58]. This is a particularly challenging reaction as it involves an intramolecular rearrangement to form the ortho-substituent, whereas an intermolecular reaction with phenol forms the para-substituent. Cleavage of the acyl group and formation of phenol and ketene intermediates can also occur in competition, lowering product selectivity due to ketene oligomerization. Fries rearrangement has been compared over phenyl- and propylsulfonic acids and commercial Amberlyst [27, 59] (Scheme 2.12). Although conversions using the Amberlyst and phenylsulfonic acids were comparable, the sulfonic acid exhibited higher selectivity towards ortho substitution, consistent with the observed slow isomerization of alkylated phenols [27]. Interestingly, the catalyst lifetime could be extended if dichloromethane was used as a reaction solvent [46].

Scheme 2.12 Fries rearrangement of phenyl acetate.

2.3.3 Miscellaneous Reactions

The synthesis of more complex pharmaceuticals such as Biginelli-type compounds [60], which involve the sulfonic acid-mediated reaction of ethyl acetoacetate with aromatic aldehydes and urea (or thiourea) to produce 3,4-dihydropyrimidinones/

2.4 Conclusions and Future Prospects

thiones, have also been reported (Scheme 2.13). These products are of interest for their antiviral, antibacterial and antihypertensive activity, and also efficacy as calcium channel modulators and 1a-antagonists [61]. Silica-grafted sulfonic acid was found to give >90% isolated yields and could be recycled eight times without a significant loss of yield.

Scheme 2.13 The Biginelli reaction of ethyl acetoacetate with aldehyde and urea (X ¼ S or O) to form 3,4-dihydropyrimidones.

Dehydration reactions such as the pinacolol rearrangement of meso-hydrobenzoin have been studied over MCM-41 sulfonic acid, with high selectivity to diphenylacetaldehyde (DPAA) being observed [46]. The authors suggest that DPPA is favored over 1,2-diphenylethanone (DPE) since the hydride shift is less sterically demanding and pore size constrains the possible transition state conformation (Scheme 2.14).

Scheme 2.14 Pinacolol rearrangement of meso-hydrobenzoin.

2.4 Conclusions and Future Prospects

Mesporous sulfonic acid catalysts offer clean alternatives to H2SO4 in numerous liquid-phase organic transformations. Methods to tailor the electronic properties of the sulfonic acid group through the use of phenyl rather than propyl tethering groups have proven beneficial for increasing the acid strength. Likewise, fine tuning of catalyst surface polarity and hydrophobicity can be achieved through the use of spectator groups or more advanced synthesis of framework-substituted organic silicas. Such modifications can significantly increase catalyst activity by aiding adsorption of non-polar reactants at the acid site or help with the removal of water from the catalyst surface in dehydration reactions. In spite of progress in our understanding and advances in materials synthesis, there are still some fundamental questions about the nature of the catalyst surface which remain unanswered. In particular, a better understanding of how the acid strength of phenyl- and propylsulfonic acid groups varies with solvent and temperature is required. While access to calorimetric techniques to measure acid strength has

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increased the accuracy with which acid site distributions are determined, it is clear that simple NH3 adsorption measurements may not provide a true reflection of the acid strength under reaction conditions. Furthermore, it would be desirable for the effect of sulfonic acid loading, acid strength and spectator groups to be compared on materials where the morphology of the carrier remains constant across the series of samples, as in the case for post-grafted materials. There have been many advances in this area, but with porosity and surface area often changing with the loading of functional groups, a unified model for the surface chemistry of sulfonic acid silicas cannot yet be derived from the current literature. If these designer materials are to obtain more widespread application in industrial chemistry, it is critical that more detailed kinetic data are measured on well-defined catalytic systems. In contrast to gas-phase reactions, where partial pressures of reactants can readily be explored, there are no studies of liquid-phase reactions over silica-based sulfonic acid catalysts where the effect of competitive adsorption of reactants at the surface are examined. This is particularly important in the liquid phase since solvent effects may play a key role in aiding diffusion of reactants. The use of molecular modeling should also help with our understanding of reactant activation, with transition states recently identified in vapor-phase etherification processes over sulfonic acid silicas [52]. Molecular simulation studies of adsorption processes at the surface of sulfonic acid catalysts could also provide similar information for liquid-phase processes and assist our understanding of the role of different spectator groups, accessibility of the acid site or cooperative effects. Overall, sulfonic acid silicas are very promising replacements for H2SO4; however, their full potential can only be realized if they are incorporated into continuous reactors, which offer inherently more efficient processing than with conventional batch reactors. Future studies should address issues at the chemistry–chemical engineering interface such as particle forming, stability and optimization of pore networks for efficient reactant/product diffusion and mixing, to aid assimilation of these catalyst systems into modern reactor technology.

Abbreviations

BTEB BTME CSPTMS CTAB DDA DRIFTS EtOH FFA HMS MAS-NMR MCM MPTS

bis(triethoxysilyl)benzene 1,2-bis(trimethoxysilyl)ethane 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane cetyltrimethylammonium bromide dodecylamine diffuse reflectance Fourier transform infrared spectroscopy ethanol free fatty acid hexagonal mesoporous silica magic angle spinning nuclear magnetic resonance Mobil corporate material mercaptopropyltrimethoxy silane

References

PEO TCEP TEOS TEPO TGA XPS

poly(ethylene oxide) tris(2-carboxyethyl)phosphine tetraethyl orthosilicate [Si(OEt)4] triethylphosphine oxide thermogravimetric analysis X-ray photoelectron spectroscopy

Acknowledgments

Financial support from the EPSRC under grants GR/R39436/01 and EP/E013090/1 and from BP Chemicals is gratefully acknowledged.

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11 Harmer, M.A., Farmeth, W.E. and Sun, Q. (1998) Advanced Materials, 10, 1255. 12 Ying, J.Y., Mehnert, C.P. and Wong, M.S. (1999) Angewandte Chemie-International Edition, 38, 56. 13 Linssen, T., Cassiers, K., Cool, P. and Vansant, E.F. (2003) Advances in Colloid and Interface Science, 103, 121. 14 Davidson, A. (2002) Current Opinion in Colloid and Interface Science, 92. 15 Galarneau, A., Iapichella, J., Bonhomme, K., Di Renzo, F., Kooyman, P., Terasaki, O. and Fajula, F. (2006) Advanced Functional Materials, 16, 1657. 16 Gr€ un, M., Unger, K., Matsumoto, A. and Tsutsumi, K. (1999) Microporous Mesoporous Mater, 27, 207. 17 Tanev, P.T. and Pinnavaia, T.J. (1995) Science, 267, 865. 18 Attard, G.S., Glyde, J.C. and Goltner, C.G. (1995) Nature, 378, 366. 19 Margolese, D., Melero, J.A., Christiansen, S.C., Chmelka, B.F. and Stucky, G.D. (2000) Chemistry of Materials, 12, 2448. 20 Melero, J.A., Stucky, G.D., van Griekena, R. and Morales, G. (2002) Journal of Materials Chemistry, 12, 1664. 21 Wilson, K., Lee, A.F., Macquarrie, D.J. and Clark, J.H. (2002) Applied Catalysis A-General, 228, 127. 22 Wilson, K. unpublished results.

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23 Zhao, X.S., Lu, G.Q., Whittaker, A.K., Millar, G.J. and Zhu, H.Y. (1997) The Journal of Physical Chemistry. B, 101, 6525. 24 Boveri, M., Aguilar-Pliego, J., PerezPariente, J. and Sastre, E. (2005) Catal Today, 107–108, 868. 25 Mbaraka, I.K. and Shanks, B.H. (2005) Journal of Catalysis, 229, 365. 26 Dıaz, I., Marquez-Alvarez, C., Mohino, F., Perez-Pariente, J. and Sastre, E. (2001) Microporous Mesoporous Mater, 44–45, 295. 27 Lim, M.H., Blanford, C.F. and Stein, A. (1998) Chemistry of Materials, 10, 467. 28 Dıaz, I., Marquez-Alvarez, C., Mohino, F., Perez-Pariente, J. and Sastre, E. (2001) Applied Catalysis A-General, 205, 19. 29 Feng, Y.-F., Yang, X.-Y., Di, Y., Du, Y.-C., Zhang, Y.-L. and Xiao, F.-S. (2006) The Journal of Physical Chemistry B, 110, 14142. 30 Ecormier, M.A., Wilson, K. and Lee, A.F. (2003) Journal of Catalysis, 215, 57. 31 Siril, P.F., Davison, A.D., Randhawa, J.K. and Brown, D.R. (2007) Journal of Molecular Catalysis A-Chemical, 267, 72. 32 Serjeant, E.P. and Dempsey, B. (eds) (1979) Ionization Constants of Organic Acids in Solution, IUPAC Chemical Data Series No. 23, Pergamon Press, Oxford. 33 Osegovic, J.P. and Drago, R.S. (2000) The Journal of Physical Chemistry B, 104, 147. 34 van Grieken, R., Melero, J.A. and Morales, G. (2006) Journal of Molecular Catalysis A-Chemical, 256, 29. 35 Mbaraka, I.K., Radu, D.R., Lin, V.S.-Y. and Shanks, B.H. (2003) Journal of Catalysis, 219, 329. 36 Koujout, S. and Brown, D.R. (2005) Thermochimica Acta, 434, 158. 37 Koujout, S. and Brown, D.R. (2004) Catalysis Letters, 98, 195. 38 Melero, J.A., van Grieken, R. and Morales, G. (2006) Chemical Reviews, 106, 3790. 39 Dufaud, V. and Davis, M.E. (2003) Journal of the American Chemical Society, 125, 9403.

40 Zeidan, R.K., Dufaud, V. and Davis, M.E. (2006) Journal of Catalysis, 239, 299. 41 Mbaraka, I.K. and Shanks, B.H. (2006) Journal of Catalysis, 244, 78. 42 Wilson, K., Renson, A. and Clark, J.H. (1999) Catalysis Letters, 61, 51. 43 Dìaz, I., Marquez-Alvarez, C., Mohino, F., Prez-Pariente, K. and Sastre, E. (2000) Journal of Catalysis, 193, 295. 44 Yang, J., Yang, Q., Wang, G., Feng, Z. and Liu, J. (2006) Journal of Molecular Catalysis A-Chemical, 256, 122. 45 Yang, Q., Liu, J., Yang, J., Kapoor, M.P., Inagaki, S. and Li, C. (2004) Journal of Catalysis, 228, 265. 46 Rac, B., Hegyes, P., Forgo, P. and Molnar, A. (2006) Applied Catalysis A-General, 299, 193. 47 Yang, Q.H., Kapoor, M.P., Shirokura, N., Onashi, M., Inagaki, S., Kondo, J.N. and Domen, K. (2005) Journal of Materials Chemistry, 15, 666. 48 Climent, M.J., Corma, A., Iborra, S. and Primo, J. (1995) Journal of Catalysis, 151, 60. 49 Dias, A.S., Pillinger, M. and Valente, A.A. (2005) Journal of Catalysis, 229, 414. 50 Das, D., Lee, J.-F. and Cheng, S. (2001) Chemical Communications, 2178. 51 Das, D., Lee, J.-F. and Cheng, S. (2004) Journal of Catalysis, 223, 152. 52 Herman, R.G., Khouri, F.H., Klier, K., Higgins, J.B., Galler, M.R. and Terenna, C.R. (2004) Journal of Catalysis, 228, 347. 53 Sow, B., Hamoudi, S., Zahedi-Niaki, M.H. and Kaliaguine, S. (2005) Microporous and Mesoporous Mater, 79, 129. 54 Yang, Q.H., Kapoor, M.P., Inagaki, S., Shirokura, N., Konodo, J.N. and Domen, K. (2005) Journal of Molecular Catalysis A-Chemical, 230, 85. 55 Rac, B., Molnar, A., Forgo, P., Mohai, M. and Bertóti, I. (2006) Journal of Molecular Catalysis A-Chemical, 244, 46. 56 Yadav, G.D. and Kumar, P. (2005) Applied Catalysis A-General, 286, 61. 57 Melero, J.A., van Grieken, R., Morales, G. and Nuno, V. (2004) Catalysis Communications, 5, 131.

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60 Gupta, R., Paul, S. and Gupta, R. (2007) Journal of Molecular Catalysis A-Chemical, 266, 50. 61 Oliver Kappe, C. (2000) European Journal of Medicinal Chemistry, 35, 1043.

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3 Applications of Environmentally Friendly TiO2 Photocatalysts in Green Chemistry: Environmental Purification and Clean Energy Production Under Solar Light Irradiation Masaya Matsuoka and Masakazu Anpo

3.1 Introduction

In recent years, there has been great concern over many serious environmental problems and the lack of natural energy resources, all of which we are facing on a global scale. An increase in world population and industrial development have led to accelerated energy consumption and the unabated release of toxic agents into the air and waterways, leading to such adverse effects as pollution-related diseases and climatic changes such as global warming. It is therefore of the utmost urgency to develop ecologically clean and safe chemical technologies, materials and processes to sustain our present level of population and economic expansion. The development of new catalytic processes which can contribute to environmental protection and new energy production are, therefore, strongly desired. One of the most ideal catalytic processes is the so-called artificial photosynthesis which has the potential to realize safe and clean chemical processes and systems with the use of limitless solar energy. As shown in Figure 3.1a, photosynthesis in plants allows one of the most significant uphill reactions, where the photon energy is converted into chemical energy and stored in the bonds of glucose, accompanied by a large positive change in the Gibbs free energy (DG > 0): hn

H2 O þ CO2 !

1 ðC H O Þ þ O2 6 6 12 6

DG ¼ 502 kJ mol1

ð3:1Þ

Since the first energy crisis in the early 1970s, much research has been devoted to the development of efficient systems that would permit the absorption and conversion of solar light into useful chemical energy resources. One of the most promising of such artificial photosynthetic reactions is the photocatalytic splitting of water to produce H2 and O2 under solar light accompanied by a large positive change in the Gibbs free energy: hn

H2 O ! H2 þ

1 O2 2

DG ¼ 237 kJ mol1


ð3:2Þ

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Photocatalysts in Green Chemistry

Figure 3.1 (a) Mechanism of the photosynthetic reaction of green plants; (b) artificial photosynthetic reaction on a visible lightresponsive titanium oxide photocatalyst incorporated within a zeolite (Ti/zeolite).

The photosensitization effect of a TiO2 electrode on the electrolysis of water into H2 and O2 was discovered in 1972 by Fujishima and Honda [1, 2]. The refinement of this uphill reaction is significant not only for the storage of solar energy but also for the clean production of hydrogen, since its consumption is expected to increase

3.2 Principles of Photocatalysis

dramatically in future generations, especially in fuel cell applications [3–6]. In addition to the water decomposition reaction, it has been reported that an artificial photosynthetic reaction system, which produces hydrocarbons and oxygen from carbon dioxide and water under solar light irradiation, can be constructed by utilizing titanium oxide photocatalysts highly dispersed within the framework structure of zeolites, as shown in Figure 3.1b [7–15]. In addition to the uphill reaction, TiO2 photocatalysts also permit a downhill reaction. In a downhill reaction, the photocatalyst induces thermodynamically favored reactions under light irradiation such as the complete oxidation of organic compounds into CO2 and H2O, accompanied by a large negative change in the Gibbs free energy (DG < 0). Downhill reactions induced by solid semiconducting photocatalysts such as TiO2 have been applied in practice for the degradation of toxic organic compounds in air and water [2, 16]. Recently, another notable achievement in inducing artificial photosynthesis has been the development of visible light-responsive TiO2 photocatalysts which can operate not only under UV but also visible light irradiation [3–10, 17]. This chapter introduces the fundamental principles of photocatalysis, the development of such visible light-responsive TiO2 photocatalysts and their applications in environmentally friendly green chemistry and new energy production.

3.2 Principles of Photocatalysis

Semiconducting metal oxides such as TiO2, ZnO and Fe2O3 are known to act as sensitizers for light-induced redox processes due to their unique electronic structure characterized by a filled valence band and an empty conduction band [1–6]. As shown in Figure 3.2, when semiconducting metal oxide absorbs a photon having an energy larger than its bandgap, an electron is promoted from the valence band to the conduction band, leaving a hole. The electrons and holes formed are dissipated within a few nanoseconds by their recombination in the absence of suitable electron and hole scavengers [1–6]. However, if a suitable scavenger or surface defect state is available, recombination is prevented and subsequent redox reactions may occur. The holes in the valence band act as powerful oxidants, while the electrons in the conduction band are good reductants [1–6]. When the TiO2 is irradiated by UV light (l < 380 nm) in water, H þ is q reduced to H2 by the photo-formed electrons, while OH is oxidized to OH radicals by the photo-formed holes to produce O2 through several reaction steps. In this way, TiO2 can decompose water into H2 and O2, allowing the efficient conversion of light energy into chemical energy accompanied by a large positive change in the Gibbs free energy (DG ¼ 237 kJ mol1). It should be noted that the irradiation of vacuum UV light (l < 165 nm) is necessary for the direct photolysis of water molecules into H2 and O2 [1–6]. On the other hand, when TiO2 is irradiated by UV light in the presence of air and reactant molecules such as organic compounds in water, the photo-formed q electrons react with O2 to form O2, while OH is oxidized into OH radicals. The oxygen radicals formed can easily react with the organic compounds, decomposing

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2


Figure 3.2 Various reaction systems using a TiO2 photocatalyst under different reaction conditions: (a) production of electrons and holes in TiO2 by absorption of UV light in the presence of oxygen and water (reaction in atmosphere); (b) production of electrons and holes by the absorption of UV light in the presence of water alone without any oxygen.

them into CO2 and H2O accompanied by a large negative change in the Gibbs free energy. In fact, the application of illuminated semiconductors in the remediation of contaminants has been successful for a wide variety of compounds [2–6]. In many cases, complete mineralization of the organic compounds could proceed on the TiO2 photocatalysts. Among the various photocatalysts, TiO2 is the most attractive due to its low cost, availability, high photocatalytic reactivity and chemical stability. However, TiO2 has a large bandgap with an absorption edge in UV regions shorter than 380 nm so that TiO2 semiconductors are able to absorb only 3–4% of the solar light that reaches the Earth. Extensive investigations have been carried out on the synthesis of lightresponsive TiO2 photocatalysts that can extend their absorption into the visible region and the development of such visible light-responsive photocatalysts will be introduced in this chapter.

3.3 Application of Photocatalysts in Green Chemistry: Solar Energy Conversion and Environmental Protection 3.3.1 Water Splitting to Produce Pure Hydrogen as Clean Fuel

Water splitting reactions have been intensively investigated in recent years using various powdered photocatalysts for the production of H2. It has been reported, for

3.3 Application of Photocatalysts in Green Chemistry: Solar Energy Conversion

example, that Pt-loaded TiO2 acts as a photocatalyst for water splitting reactions under UV irradiation. However, a product separation process is also required for the utilization of H2 as fuel since only a gas mixture of H2 and O2 evolves from powdered TiO2 photocatalysts. The reaction yields are also low since the recombination reaction between H2 and O2 to produce H2O occurs on the surface Pt particles which are the additives that promote the water splitting reaction [1, 2]. The development of photocatalytic systems allowing the separate evolution of H2 and O2 from water is, therefore, strongly desired. Recently, an H-type reactor has been constructed and applied to the separate evolution of H2 and O2 from water, where the liquid phases are separated by a TiO2 thin-film device and proton-exchange membrane [3–6]. In order to utilize the abundance of solar light reaching the Earth, visible light-responsive TiO2 thin films were prepared by a one-step magnetron sputtering deposition method in which the substrate temperature was controlled during the sputtering process [8–10]. As shown in Figure 3.3, a visible light-responsive TiO2 thin-film device (Vis-TiO2/Ti/Pt) consisting of a Ti metal foil substrate deposited with the visible light-responsive TiO2 thin film on one side and Pt metal on the other side was constructed by magnetron sputtering deposition. The prepared Vis-TiO2/Ti/Pt device was applied in the separate evolution of H2 and O2 from water. Figure 3.4 shows the time profile of this separate evolution reaction under solar light irradiation from a sunlight gathering system.

Figure 3.3 H-type reactor for the separate evolution of H2 and O2 using a visible light-responsive TiO2 thin-film device (Vis-TiO2/Ti/Pt) under solar light irradiation. Electrolyte: 1.0 M NaOH (right side); 0.5 M H2SO4 (left side).

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Figure 3.4 Separate evolution of H2 and O2 on a visible light-responsive TiO2 thin-film device (Vis-TiO2/Ti/Pt) irradiated by light beams from a sunlight-gathering system using an H-type reactor.

Sunlight irradiation of Vis-TiO2/Ti/Pt within the H-type reactor successfully led to the stoichiometric evolution of H2 from the Pt side and O2 from the TiO2 side [3–6]. In fact, the solar energy conversion efficiency was found to reach 0.3% for this reaction. Thus, recent advances in the method of preparation of unique photocatalysts have allowed efficient solar energy conversion into useful chemical energy (hydrogen energy), which can be applied in practice for new energy production processes. Photocatalytic H2 evolution reactions from water containing biomass have also attracted much attention. A TiO2 thin-film device immersed in water containing biomass was observed to efficiently produce H2 under visible light irradiation. In this reaction, biomass, i.e. the organic compounds, efficiently scavenge the photo-formed holes while, simultaneously, the photo-formed electrons remain on the visible lightresponsive TiO2 thin film. As a result, the presence of the biomass in water leads to an enhancement of the reduction efficiency of H þ into H2 by photo-formed electrons [3–6]. This reaction system produces CO2 as a result of the oxidation of the biomass. However, this system can be applied for clean H2 production by combining it with an efficient CO2 utilization system such as in a unique agricultural system which can supply concentrated CO2 to green plants to promote their photosynthesis. 3.3.2 Photocatalytic Reduction of CO2 with H2O (Artificial Photosynthesis)

The development of efficient photocatalytic systems which are able to reduce CO2 with H2O into chemically valuable compounds such as CH3OH or CH4 under solar light irradiation is a challenging goal in research on environmentally friendly catalysts. The gas-phase reaction of CO2 and H2O on powdered TiO2 photocatalysts has been found to yield CH4 as the major product, accompanied by the formation of


CO, C2H6 and C2H4 as minor products [11–15]. The product distributions are affected by the kind of co-catalysts used such as Pt or Cu, e.g. PtTiO2 and CuTiO2 yield CH4 and CH3OH as the major product, respectively. Recently, a highly dispersed tetrahedrally coordinated titanium oxide species incorporated within zeolite frameworks has been found to exhibit high and unique photocatalytic reactivity for the reduction of CO2 with H2O [11–15]. Electrons and holes produced in the conduction and valence bands, respectively, of the semiconducting TiO2 powdered catalysts under UV irradiation were found to play a major role in various photocatalytic reactions. However, as the size of the TiO2 particles is reduced to less than 100 A, the gap between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) starts to increase, leading to an enhancement of the reduction ability of the photo-formed electrons in LUMO and also the oxidation ability of the photo-formed holes in HOMO (Figure 3.5) [11]. This size quantization effect led to high and selective photocatalytic reactions completely different from photoelectrochemical reactions occurring on bulk TiO2 powder [11]. This unique photocatalytic activity of small nanoscale TiO2 particles can be ascribed not only to an electronic modification of the TiO2 catalysts but also to the close existence of the photo-formed electron and hole pairs and their balanced contribution to the reactions. As shown in Figure 3.1b, recent advances in the preparation of catalysts have made it possible to disperse the TiO2 species within the framework structure of the zeolite to a

Figure 3.5 Advances in titanium oxide photocatalysts from extended semiconducting TiO2 particles and nanoscale molecular clusters to the isolated titanium oxide species and the changes observed in their electronic states.

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Figure 3.6 Ordinary photoluminescence spectrum observed for (a) Ti/Y-zeolite catalyst prepared by ion-exchange method; (EX) Its corresponding excitation spectrum. (b), (c) Effect of the addition of CO2 and H2O, respectively, at 77 K. Excitation at 290 nm; emission monitored at 490 nm. Amount of CO2 added: (b) 8.5 and amount of H2 added: (c) 2.9 mmol g1.

molecular level by hydrothermal synthesis. The highly dispersed titanium oxide species formed exhibits unique photocatalytic reactions different from those on bulk TiO2 powders. Various spectroscopic measurements have revealed that the titanium oxide species incorporated within various zeolite frameworks (Ti/zeolites) exist in an isolated four-fold coordination sphere having a TiO bond distance of about 1.83 A [11–15]. These titanium oxide species-containing zeolite catalysts (Ti/zeolites) exhibited a photoluminescence spectrum at around 480–490 nm by excitation at around 220–260 nm (Figure 3.6). The photoluminescence spectrum is attributed to the radiative decay process from the charge-transfer excited to ground state of the highly dispersed titanium oxide species in tetrahedral coordination, as follows [11–15]: hn

ðTi4 þ O2 Þ s

hn0

ðTi3 þ O Þ

ð3:3Þ

The addition of H2O or CO2 molecules to the Ti/zeolite catalysts led to efficient quenching of the photoluminescence and also shortening of the lifetime for the charge-transfer excited state. Such an efficient quenching of the photoluminescence suggests not only that a four-fold coordinated titanium oxide species locates at positions accessible to these small molecules but also that they interact with the titanium oxide species in both their ground and excited states. In fact, UV irradiation of the Ti/zeolite catalysts in the presence of CO2 and H2O led to the photocatalytic reduction of CO2 to form CH3OH and CH4 as major products in addition to CO, O2, C2H4 and C2H6 as minor products at 323 K, while the yields of these photoformed products increased with good linearity with respect to the UV irradiation time [11– 15]. Figure 3.7 shows the relationship between the coordination number of the


Figure 3.7 Relationship between the coordination numbers and photocatalytic reactivities of the titanium oxides.

titanium oxide species of the Ti/zeolite catalysts as obtained from XAFS analysis and the selectivity for the formation of CH3OH in the photocatalytic reduction of CO2 with H2O on various Ti/zeolite catalysts. A clear dependence of the selectivity for the formation of CH3OH on the coordination numbers of the titanium oxide species can be observed, i.e. the lower the coordination number of the titanium oxide species, the higher is the selectivity for CH3OH formation. Bulk TiO2 semiconducting photocatalysts did not show any reactivity for the formation of CH3OH from CO2 and H2O. From these results, it was proposed that a highly efficient and selective photocatalytic reduction of CO2 to CH3OH by H2O could be achieved using Ti/zeolite catalysts involving a highly dispersed four-fold coordinated titanium oxide species in their framework as the active species [11–15]. 3.3.3 Direct Photocatalytic Decomposition of NO into N2 and O2

The development of efficient photocatalytic systems which can decompose NOx directly into N2 and O2 is strongly desired in order to establish clean and environmentally-friendly deNOx systems for atmospheric purification. It has been reported that the decomposition reaction of NO can proceed photocatalytically on powdered

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Figure 3.8 View of soundproof highway walls coated with TiO2 photocatalysts for the elimination of NO (Osaka, April 1999).

TiO2 at room temperature [11–14, 18]. In fact, TiO2 photocatalysts have been practically applied for soundproof highway walls to eliminate the NO emitted from automobiles, as shown in Figure 3.8. When the TiO2 photocatalyst is irradiated by UV light in the presence of NO in atmospheric conditions, NO is oxidized into NO2 and then further oxidized into NO3. This NO3 species on the TiO2 surface can be removed as HNO3 by water in the form of, for example, raindrops. In addition to powdered photocatalysts, it has been reported that highly dispersed titanium oxide photocatalysts can decompose NO directly into N2 and O2. UV light irradiation of powdered TiO2 and Ti/Y-zeolite catalysts prepared by ion-exchange (ex-Ti/Y-zeolite) or impregnation (imp-Ti/Y-zeolite) methods in the presence of NO led to the evolution of N2, O2 and N2O in the gas phase at 275 K with different yields and product selectivity [11–14, 18]. The yields of the photo-formed products increased linearly against the UV irradiation time and the reaction ceased immediately when irradiation was discontinued. A comparison of the photocatalytic activity for the Ti/ Y-zeolite catalysts and the widely used bulk TiO2 powdered catalyst was of special interest. Specific photocatalytic reactivities for the Ti/Y-zeolite catalysts, normalized for the unit amount of TiO2 included, were much higher than that for the bulk TiO2 [11–14, 18]. Moreover, the selectivity for the formation of N2 depended strongly on the type of catalyst. The relationship between the coordination number of the titanium oxide species and the selectivity for N2 formation in the photocatalytic decomposition of NO on various types of titanium oxide-based photocatalysts are shown in Figure 3.7. There is a clear dependence of the N2 selectivity on the coordination number of the titanium oxide species [11–14, 18]. From these results,


it was shown that a highly efficient and selective photocatalytic reduction of NO into N2 and O2 could be achieved with ex-Ti/Y-zeolite incorporating a highly dispersed isolated tetrahedral titanium oxide as the active species. The formation of N2O as the major product was also observed for the bulk TiO2 and imp-Ti/Y-zeolite catalysts, which included the aggregated octahedrally coordinated titanium oxide species. Based on these results, a reaction mechanism for the photocatalytic decomposition of NO on the isolated tetrahedral titanium oxide species could be proposed, as shown in Scheme 3.1. The NO molecule could adsorb on the oxide species as weak ligands to form the reaction precursors. Under UV irradiation, the charge-transfer excited complexes of the oxides, (Ti3 þ O) , were formed. Within their lifetimes, an electron transfer from the trapped electron center, Ti3 þ , into the p-antibonding orbital of NO takes place and, simultaneously, an electron transfer from the p-bonding orbital of another NO into the trapped hole center, O, occurs. These electron transfers led to the direct decomposition of two sets of NO on (Ti3 þ O) into N2 and O2 under UV irradiation in the presence of NO even at 275 K. With the aggregated or bulk TiO2 catalysts, the photo-formed holes and electrons rapidly separate spatially from each other (with large distances between the holes and electrons), thus preventing the simultaneous activation of two NO on the same active sites and resulting in the formation of N2O and NO2 in place of N2 and O2 [11– 14, 18]. Moreover, the decomposed N and O species reacted with NO on different sites to form N2O and NO2, respectively. These results clearly demonstrate that the use of zeolites as supports enabled the anchoring of a titanium oxide species in a highly dispersed state within the zeolite cavities and such tetrahedrally coordinated titanium oxide photocatalysts are promising for applications in removal systems of toxic NOx compounds in the atmosphere.

Scheme 3.1 Reaction scheme of the photocatalytic decomposition of NO into N2 and O2 on the ex-Ti/Y-zeolite catalyst at 275 K.

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3.3.4 Application to the Purification of Air Polluted with Various Organic Compounds

In recent years, various air-purifying systems using TiO2 photocatalysts have been commercialized. One such system is equipped with a highly active rectangular column-structured TiO2 photocatalyst constructed directly on silica sheets [19]. These rectangular column-structured TiO2 photocatalysts were prepared by a wet or dry process, as shown in Figure 3.9. SEM images of the synthesized TiO2 photocatalysts are shown in Figure 3.9a and b. The rectangular column-structured crystals, with a width of 100–500 nm and a length of 1000–5000 nm, were observed to be anchored perpendicularly to the substrate in a very dense state with stable mechanical strength. TEM images revealed that the TiO2 crystal has a hollow structure which consists of an outer TiO2 shell of high density and an inner region of low density. Moreover, XRD analysis revealed that the TiO2 crystals have an anatase polycrystalline structure. The air-purifying system incorporating the rectangular column-structured titanium oxide photocatalyst is shown in Figure 3.10a. The photocatalytic performance of these purifiers for the complete oxidation of contaminants such as formaldehyde into CO2 and H2O as compared with other TiO2 photocatalytic systems is shown in Figure 3.10b. The efficiency of the air purifiers using activated carbon or absorbents, systems A and B, respectively, were seen to decrease gradually and reach zero as the

Figure 3.9 Synthesis method for rectangular column titanium oxide photocatalysts anchored on a silica sheet and SEM images of rectangular column-structured titanium oxide photocatalysts anchored on a silica sheet: (a) 6000; (b) 15 000.


Figure 3.10 (a) Air purification systems applying the rectangular column-structured TiO2 photocatalysts; (b) comparison of the capacity for formaldehyde decomposition with other purifiers under different systems.

absorbents and activated carbons became saturated with the various contaminants. In contrast, the air purifier applying the rectangular column-structured TiO2 showed high efficiency in decomposing formaldehyde, the concentration decreasing rapidly to below the guideline limits issued by the Ministry of Health, Labor and Welfare of Japan. These results clearly show that the air-purifying system using these TiO2 photocatalysts can be applied in practice to decompose harmful organic compounds which exist in our living space. 3.3.5 Application to the Purification of Water Polluted with Toxic Compounds Such as Dioxins

In a reaction system in the presence of organic compounds together with water and q air, both the produced O2 and OH formed from the photo-formed electrons and holes, respectively, easily react with the organic compounds, resulting in their complete oxidation into CO2 and H2O [2, 16]. In fact, this strong oxidation ability for the TiO2 photocatalysts has been applied to the purification of water polluted with toxic compounds such as dioxins, which were then completely mineralized into CO2, H2O and HCl under UV irradiation [2, 16]: O2

Cn Hm Oz Cly ! nCO2 þ yHCl þ wH2 O hn

ð3:4Þ

The effective photocatalytic reactivity of these TiO2 photocatalysts has been applied in practice for the purification of underground water polluted with volatile organic compounds (VOCs) such as tetrachloroethylene [2, 16].

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It has also been reported that visible light-responsive TiO2 photocatalysts can be prepared by applying a metal ion implantation method. By metal ion implantation, metal ions, such as Fe þ , Mn þ and V þ, are accelerated to have a high kinetic energy (150 keV) and are implanted into the bulk of the TiO2. TiO2 catalysts subjected to metal ion implantation were able to absorb and work efficiently as photocatalysts even under visible light irradiation. Significantly, they were found to exhibit reactivity for the liquid-phase degradation of 2-propanol diluted in water at 295 K under visible light (l > 450 nm) irradiation [20]. This advanced preparation method should open the way towards the widespread use of photocatalysts, significantly, for environmental remediation including water purification. 3.3.6 Superhydrophilic Properties of TiO2 Thin Films and Their Application in Self-cleaning Materials

TiO2 thin films have been known to exhibit superhydrophilicity under UV irradiation since the mid-1990s [2]. As shown in Figure 3.11, the contact angle of water droplets on a TiO2 thin film reaches almost zero under UV light and reaches a so-called superhydrophilic state. Under such a superhydrophilic state, water has a tendency to spread perfectly across the TiO2 surface. Moreover, the superhydrophilicity of TiO2 thin films has been applied for practical purposes such as anti-fogging mirrors, since water cannot form droplets on a TiO2 thin film under such a superhydrophilic state, as shown in Figure 3.11. The superhydrophilicity of TiO2 thin films has also been applied in self-cleaning materials such as window glass and architectural tiles since

Figure 3.11 (a) Changes in the contact angle of water droplets under UV irradiation on TiB binary oxide thin films; (b) application of superhydrophilic properties for anti-fogging mirrors in automobiles.

3.4 Development of Visible Light-responsive TiO2 Photocatalysts

the high wettability of the TiO2 thin-film surface can prevent adhesion of oil and dust on the TiO2 surface. TiB binary oxide thin films including ultrafine TiO2 nanoparticles of octahedral coordination and dispersed in the host B2O3 have also been successfully prepared by an ionized cluster beam (ICB) deposition method using multi-ion sources [10, 21]. As shown in Figure 3.11, it was found that Ti–B binary oxide thin films prepared by ICB deposition exhibit higher photoinduced hydrophilic properties than untreated pure TiO2 thin films [10, 21]. These advanced binary oxide thin films can be effectively applied in self-cleaning materials to realize a cleaner living environment.

3.4 Development of Visible Light-responsive TiO2 Photocatalysts 3.4.1 Modification of the Electronic State of TiO2 by Applying an Advanced Metal Ion Implantation Method

TiO2 semiconductors have a relatively large bandgap of 3.2 eV, corresponding to wavelengths shorter than 388 nm. Therefore, TiO2 can make use of only 3–4% of the solar energy that reaches the Earth, as mentioned previously. In order to develop TiO2 photocatalysts that can operate under visible light, a metal ion implantation method was applied to modify the electronic properties of bulk TiO2 photocatalysts by bombarding them with high-energy metal ions [8–10]. Metal ion implantation of the TiO2 with various transition metal ions such as V, Cr, Mn, Fe and Ni was found to lead to a large shift in the absorption band of the catalysts towards the visible region. As can be seen in Figure 3.12, the absorption band of the Cr ion-implanted TiO2 shifts

Figure 3.12 UV–visible absorption spectra of (a) TiO2 and (b)–(d) Cr ion-implanted TiO2 photocatalysts. Amount of implanted Cr ions (mmol g1): (a) 0; (b) 0.22; (c) 0.66; (d) 1.3.

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smoothly towards visible regions, the extent depending on the amount of metal ions implanted [8–10]. On the other hand, TiO2 catalysts impregnated or chemically doped with Cr ions exhibit a new absorption band at around 420 nm as a shoulder peak due to the formation of an impurity energy level within the bandgap. These results indicate that the method of doping causes the electronic properties of the TiO2 catalyst to be modified in completely different ways, thus confirming that only metal ion-implanted TiO2 catalysts show such shifts in the absorption band toward the visible region. The local environment of the implanted metal ion was investigated by XAFS (XANES and EXAFS) measurements [8–10]. The results show that in the TiO2 catalysts chemically doped with Cr ions by an impregnation method, the ions were present as aggregated Cr oxides having an octahedral coordination similar to that of Cr2O3 and a tetrahedral coordination similar to that of CrO3. On the other hand, in the catalysts physically implanted with Cr ions, the ions were present in a highly dispersed and isolated state in an octahedral coordination, clearly indicating that the Cr ions are incorporated into the lattice positions of the TiO2 catalyst in place of the Ti ions. These findings clearly show that modification of the electronic state of TiO2 catalysts by metal ion implantation is closely associated with the strong and long-distance interaction which arises between the TiO2 and metal ions implanted and not with the formation of impurity energy levels within the bandgap of the catalysts. The photocatalytic activity of Cr ion-implanted TiO2 was examined for the direct decomposition reaction of NO. As shown in Figure 3.13, the unimplanted original or

Figure 3.13 Time profiles of the direct photocatalytic decomposition of NO into N2 and N2O on unimplanted pure TiO2 catalyst and Cr ion-implanted TiO2 catalyst with Cr ions of 6.6 107 mol g1 TiO2 under visible light irradiation (l > 450 nm).


chemically doped TiO2 catalysts show no activity for the decomposition reaction of NO under visible light (l > 450 nm) [8–10]. However, visible light irradiation (l > 450 nm) of the Cr ion-implanted TiO2 catalysts led to the decomposition of NO into N2, O2 and N2O with good linearity with respect to the irradiation time. The metal ionimplanted TiO2 catalysts were therefore found to allow the absorption of visible light up to a wavelength of 400–600 nm and to operate effectively as photocatalysts. It is important to emphasize that the photocatalytic reactivity of the metal ionimplanted TiO2 catalysts under UV light (l < 380 nm) retained the same photocatalytic efficiency as the unimplanted original TiO2 catalyst. When metal ions were chemically doped into the TiO2 catalyst, the photocatalytic efficiency decreased dramatically under UV irradiation due to the rapid recombination of the photoformed electrons and holes through the impurity energy levels formed by the doped metal ions within the bandgap of the catalyst. These results clearly suggest that physically implanted metal ions do not work as electron–hole recombination centers but only to modify the electronic properties of the TiO2 catalyst. 3.4.2 Design of Visible Light-responsive Ti/Zeolite Catalysts by Applying an Advanced Metal Ion Implantation Method

Titanium oxide photocatalysts anchored within various zeolites exhibited unique and high photocatalytic activity for various reactions such as the direct decomposition of NO into N2 and O2 and the reduction of CO2 with H2O. However, the isolated tetrahedral titanium oxide species absorbs UV light of wavelengths below 300 nm since the HOMO–LUMO energy gap of this isolated tetrahedral titanium oxide species becomes significantly larger than that of bulk TiO2 due to the size quantization effect. In other words, titanium oxide photocatalysts cannot utilize the abundant solar energy that reaches the Earth, necessitating a UV light source for its use as a photocatalyst. From this viewpoint, photocatalysts which can operate efficiently under both UV and visible light are urgently required for practical and widespread use. A modification of the electronic properties of Ti/zeolite photocatalysts by bombarding them with high-energy metal ions led to the discovery that metal ion implantation with various transition metal ions such as V and Cr, accelerated by high electric fields, can produce a large shift in the absorption band toward visible light regions [8–12]. Figure 3.14 shows the effect of V ion implantation on the diffuse reflectance UV–visible absorption spectra of Ti-containing mesoporous materials, Ti-HMS and Ti-MCM-41. Their absorption spectra at around 200–260 nm can be attributed to the charge-transfer absorption process, involving an electron transfer from the O2 to the Ti4 þ ion of the highly dispersed tetrahedrally coordinated titanium oxide species of these catalysts [8–12]. The spectra shifted smoothly towards the visible region, the extent depending strongly on the number of V ions implanted. These results indicate that the interaction of the implanted V ions with the tetrahedrally coordinated titanium oxide species led to the modification of the electronic properties of the titanium oxide species within the zeolite framework, enabling them to absorb visible light.

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Figure 3.14 UV–visible absorption spectra of V ion-implanted (a) Ti-HMS and (b) Ti-MCM-41 catalysts. Implanted V ions (from left to right): 0, 0.66, 1.3, 2.0 mmol g1 catalyst).

The photocatalytic activity of the V ion-implanted Ti-HMS and Ti-MCM-41 was also investigated for the decomposition of NO into N2 and O2 under visible light irradiation (l > 420 nm). Visible light irradiation of the V ion-implanted Ti-HMS led to the efficient decomposition of NO into N2 and O2, whereas the unimplanted original Ti-HMS exhibited no activity for the reaction under the same reaction conditions [8–12]. Moreover, no NO decomposition could be confirmed under UV (l < 300 nm) or visible light (l > 420 nm) on the V ion-implanted HMS. These results show that ion implantation is an effective technique for the modification of the electronic properties of titanium oxide photocatalysts, enabling them to absorb and operate under visible light (l > 420 nm) with high efficiency. The local environment of the implanted metal ion was investigated by XANES and EXAFS (XAFS) analyses [8–10]. The V K-edge FT-EXAFS spectra of the Ti-HMS catalyst implanted with V ions show that the nearest neighbors of the V environment are not the same as in vanadium oxide-based catalysts (e.g., V2O5) and suggest the formation of tetrahedral titanium oxides having a VOTi instead of a VOV linkage, as shown in Figure 3.1b [8–12]. These findings show that the formation of the VOTi bridge structures between the isolated tetrahedrally coordinated titanium oxide species and implanted V ions affect the electronic structure of the isolated titanium oxide species, leading to a red shift in the absorption spectra of these catalysts. 3.4.3 Preparation of Visible Light-responsive TiO2 Thin-film Photocatalysts by an RF Magnetron Sputtering Deposition Method

The simple one-step preparation of visible light-responsive TiO2 thin films has been successfully achieved by applying an RF magnetron sputtering (RF-MS) deposition method, as shown in Figure 3.15 [3–6, 8–10]. The system is equipped with a substrate (quartz or Ti foil) center positioned in parallel just above the source material, the calcined TiO2 plate. The calcined TiO2 plate is sputtered by an Ar plasma by inducing


Figure 3.15 Schematic diagram of the RF magnetron sputtering (RF-MS) deposition method.

an RF power of 300 W in an Ar atmosphere and the TiO2 thin film is prepared on the quartz or Ti metal foil substrate mounted on a heater. The substrate temperature (TS) was held fixed in the range 473–873 K. Figure 3.16 shows the effect of the TS on the UV–visible transmission spectra of the TiO2 thin films prepared on a quartz substrate with a TiO2 thickness of 1.2 mm. TiO2 thin films prepared at 473 K (UV-TiO2) exhibited no absorption at wavelengths longer than 380 nm, whereas the TiO2 thin films prepared at a TS higher than 673 K (VisTiO2) were yellow and exhibited considerable absorption at wavelengths longer than 380 nm, permitting the absorption of visible light [3–6, 8–10]. Moreover, the onset of the absorption band of Vis-TiO2 prepared at 873 K (Vis-TiO2-873) shifted toward longer wavelength regions of around 600 nm, compared with around 400 nm for UVTiO2 prepared at 473 K. Hence it can be seen that the precise control of the substrate

Figure 3.16 UV–visible absorption (transmittance) spectra of TiO2 thin films prepared on a quartz substrate by the RF-MS deposition method. Preparation temperature: (a) 373; (b) 473; (c) 673; (d) 873; (e) 973 K.

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temperature allows the development of unique visible light-responsive TiO2 thin-film photocatalysts, while the efficiency of visible light absorption increased with increase in the substrate temperature. It should be noted that the coexistence of O2 with Ar in the sputtering chamber led to the formation of UV light-responsive TiO2 thin films regardless of the substrate temperature (TS > 473 K) during the sputtering deposition process. This suggests that TiO2 deposition under pure Ar gas without any trace of O2 is one of the major factors in the successful preparation of Vis-TiO2 thin-film photocatalysts. To clarify the origin of visible light absorption for the Vis-TiO2 thin film, SIMS and TEM analyses were carried out. The secondary ion intensity due to 18 O for UV-TiO2 exhibited a stoichiometric O : Ti value of 2.00, independent of the depth from the TiO2 surface, whereas in the case of Vis-TiO2, the O : Ti value gradually decreased from the top surface (O : Ti ¼ 2.00 0.01) to the inside bulk (1.93 0.01), showing a distinct contrast to UV-TiO2 [3–6, 8–10]. It could therefore be proposed that the unique declined composition of Vis-TiO2 thin films of an anisotropic structure causes a significant perturbation in the electronic structure of the TiO2, permitting the absorption of visible light. As shown in Figure 3.17, cross-sectional TEM observations revealed that Vis-TiO2 consists of large columnar crystals growing perpendicular to the substrate and the surface of the columnar crystals is covered with a stoichiometric TiO2 phase. Such a stable surface phase of the columnar crystals worked as a passive phase and could protect the slightly reduced TiO2 phases inside the bulk from complete oxidation [3–6, 8–10]. The prepared Vis-TiO2 thin film was found to act as an efficient photocatalyst for the separate evolution of H2 and O2 under solar light irradiation (Figure 3.4) and also for the complete oxidation of organic compounds into CO2 and H2O even under visible light irradiation [8–10].

Figure 3.17 Cross-sectional TEM image of the Vis-TiO2 thin-film photocatalyst prepared on a quartz substrate.

References

3.5 Conclusion

In this chapter, recent advances in research on the photocatalytic reactivity and photoinduced superhydrophilic properties of titanium oxide-based catalysts and their applications in green chemistry, e.g. solar energy conversion and environmental protection, were summarized. Special attention was focused on the application of an advanced metal ion implantation method for the preparation of second-generation TiO2 photocatalysts and highly dispersed titanium oxide photocatalysts, both of which can operate under visible light irradiation. Moreover, a new and cost-efficient RF magnetron sputtering deposition method to produce visible light-responsive TiO2 thin-film photocatalysts was also presented. Detailed characterizations of such unique visible light-responsive TiO2 photocatalysts were carried out along with investigations into the various photocatalytic reactions that could be initiated, significantly for processes related to environmental remediation. It has been demonstrated that advanced physical ion-engineering techniques can provide new approaches to the design of unique titanium oxide photocatalysts which can utilize solar energy on a global scale as the most abundant and safe energy source.

References 1 Fujishima, A. and Honda, K. (1972) Nature, 238, 37–38. 2 Fujishima, A., Rao, T.N. and Tryk, D.A. (2000) Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 1, 1–21. 3 Matsuoka, M., Kitano, M., Takeuchi, M., Anpo, M. and Thomas, J.M. (2005) Topics in Catalysis, 35, 305–310. 4 Kitano, M., Tsujimaru, K. and Anpo, M. (2006) Applied Catalysis A-General, 314, 179–183. 5 Matsuoka, M., Kitano, M., Takeuchi, M., Tsujimaru, K., Anpo, M. and Thomas, J.M. (2007) Catalysis Today, 112, 51–61. 6 Kitano, M., Matsuoka, M., Ueshima, M. and Anpo, M. (2007) Applied Catalysis AGeneral, 325, 1–14. 7 Anpo, M. and Che, M. (2000) Advances in Catalysis, 44, 119–257. 8 Anpo, M. and Takeuchi, M. (2003) Journal of Catalysis, 216, 505–516. 9 Anpo, M. (2004) Bulletin of the Chemical Society of Japan, 77, 1427–1442.

10 Anpo, M., Dohshi, S., Kitano, M., Hu, Y., Takeuchi, M. and Matsuoka, M. (2005) Annual Review of Materials Science, 35, 1–27. 11 Anpo, M. and Thomas, J.M. (2006) Chemical Communications, 31, 3273– 3278. 12 Anpo, M. and Matsuoka, M. (2008) Turning Points in Solid-state, Materials and Surface Science, Royal Society of Chemistry, Cambridge, pp. 492–506. 13 Anpo, M. (ed.) (2000) Photofunctional Zeolites, NOVA Science, New York. 14 Matsuoka, M. and Anpo, M. (2003) Photochemistry and Photobiology C, 3, 225–252. 15 Yamashita, H., Ikeue, K., Takewaki, T. and Anpo, M. (2002) Topics in Catalysis, 18, 95–100. 16 Anpo, M. (2000) Pure and Applied Chemistry, 72, 1265–1270. 17 Takeuchi, M., Anpo, M., Hirao, T., Itoh, N. and Iwamoto, N. (2001) Journal of the Surface Science Society of Japan, 22, 561–567.

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18 Yamashita, H., Ichihashi, Y., Anpo, M., Hashimoto, M., Louis, C. and Che, M. (1996) The Journal of Physical Chemistry, 100, 16041–16044. 19 Kudo, T., Kudo, Y., Ruike, A., Hasegawa, A., Kitano, M. and Anpo, M. (2007) Catalysis Today, 122, 14–19.

Photocatalysts in Green Chemistry 20 Yamashita, H., Harada, M., Misaka, J., Takeuchi, M., Neppolian, B. and Anpo, M. (2003) Catalysis Today, 84, 191–196. 21 Dohshi, S., Takeuchi, M. and Anpo, M. (2003) Catalysis Today, 85, 199–206.

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4 Nanoparticles in Green Catalysis Mazaahir Kidwai

4.1 Introduction

A revolution is occurring in science and technology, based on the recently developed ability to measure, manipulate and organize matter on the nanoscale from 1 to hundreds of billionths of a meter (Figure 4.1). Nanoscience has taken scientists around the world by storm. It hopes to revolutionize the world we live in with striking breakthroughs in areas such as materials and manufacturing, electronics, medicine and healthcare, environment and energy, chemicals and pharmaceuticals, biotechnology and agriculture, computation and information technology [1]. In recent years, there has been growing interest in the catalytic properties of transition metal nanoparticles. The high surface area-to-volume ratio of solid-supported metal nanoparticles (1–10 nm in size) is mainly responsible for their catalytic properties, and this can be exploited in many industrially important reactions [2]. This chapter highlights the use of different metal nanoparticles or their compounds in the organic transformations and syntheses of biologically important compounds. It is does not intended to provide a comprehensive review but to serve as a critical overview of the scope and applications of different metal structures such as spherical nanoparticles, nanorods, nanoplates and nanocubes in catalytic phenomena.

4.2 Advanced Catalysis by Gold Nanoparticles

Gold has long been regarded as a poorly active catalyst. A recent theoretical calculation has explained why the smooth surface of Au is noble in the dissociative adsorption of hydrogen [3]. However, when Au deposited as nanoparticles on metal oxides by mean of coprecipitation and deposition–precipitation techniques, it exhibits high catalytic activity for CO oxidation [4, 5]. The extraordinarily high catalytic activity of supported Au catalysts for CO oxidation at room temperature arises from the reaction of CO adsorbed on the steep edge and the corner sites of the


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Figure 4.1 Overview of nanoscience.

metallic Au particles with oxygen molecules adsorbed at the perimeter sites on the support surface. Hayashi et al. found that Au supported on TiO2 (Degussa, p-25) could catalyze the epoxidation of propylene in the gas phase containing O2 and H2 (Scheme 4.1) [6, 7]. Propylene oxide is the one of the important bulk chemicals, used for producing polyurethane and polyols.

Scheme 4.1

4.2 Advanced Catalysis by Gold Nanoparticles

In general, the hydrogenation of hydrocarbons is a structure-insensitive reaction over most metal catalysts (Scheme 4.2). A characteristic feature of gold as catalyst is that partial hydrogenation takes place very selectively [8]. In the hydrogenation of a,b-unsaturated aldehydes, selectivity for the hydrogenation of C¼O against that of C¼C has recently been reported to reach 40–50% when Au particles are larger than 2 nm in diameter [9, 10].

Scheme 4.2

Zhu and Angelici recently reported carbon monoxide oxidative amination using gold powder of size 103 nm [11]. The reaction of CO with primary amines in the presence of gold results in the formation of isocyanate, which further reacts with another molecule of amine to form urea (Scheme 4.3). The proposed mechanism suggests that CO absorbed on Au should be susceptible to attack by amines.

Scheme 4.3

Lazar and Angelici reported the oxidative amination of isocyanide to give carbodiimides using gold powder with an approximate size of less than 103 nm and with water as a by-product (Scheme 4.4) [12].

Scheme 4.4

Polymer-stabilized Au clusters dispersed in water act as efficient quasi-homogenous catalysts for the aerobic oxidation of alcohols. A monodisperse Au–poly(N-vinyl2-pyrrolidone) cluster is used for the aerobic oxidation of p-hydroxybenzyl alcohol (Scheme 4.5) [13]. The catalytic activity per unit cluster surface area increases rapidly with decreasing size, which is associated with non-metallic electronic structures.

Scheme 4.5

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Polymer microsphere-stabilized gold metallic colloids have been used for the reduction of 4-nitrophenol to 4-aminophenol with sodium borohydride as reductant at an ambient temperature of 25 C (Scheme 4.6) [14]. Gold nanoparticles are stabilized by active carboxylic acid on the surface of poly(divinylbenzene-co-acrylic acid) microspheres with carboxylic acid-to-gold molar ratios from 20 : 1 to 75 : 1. The microsphere-stabilized gold colloids were recycled by filtration over a G-6 sinteredglass filter after the catalytic reaction and the recovery of the catalyst was proven further by the activity of recycling four times. Preliminary results indicated that the resultant polymer microsphere-stabilized gold nanocolloids were recyclable with high catalytic activity in water medium, which may find further wide application in the field of green chemistry.

Scheme 4.6

The reusable nanosized gold particle-based chiral bisoxazoline catalyst acts like a homogeneous catalyst in the ene reaction between 2-phenylpropene and ethyl glyoxylate in dichloromethane (Scheme 4.7) [15]. A variety of hybrid chiral ligands having spacers of different chain lengths were converted into copper(II) triflate complexes and used in the ene reaction. The catalyst could be reused five times; however this results in a decrease in the yield but the catalyst remains highly enantioselective. The enantioselectivity is up to 86% with fresh catalyst and decreases only to 84% in the fourth run.

Scheme 4.7

Green synthesis of propargylamine via a three-component coupling reaction of aldehyde, alkyne and amine was achieved using recyclable gold nanoparticles in tetrahydrofuran (Scheme 4.8) [16]. The reaction was carried out in an inert atmosphere at 75–80 C. The maximum reaction rate was observed for an average gold particle of diameter of about 20 nm. It is important to stress that the catalyst was

4.3 Nickel Nanoparticles: a Versatile Green Catalyst

recycled and reused for five or seven runs with only a slight drop in the catalytic activity. This drop was induced by agglomeration of Au nanoparticles, which was size dependent. A significant feature of Au nanoparticles in this reaction is that they can be reused without further purification and without using any additives or cofactors.

Scheme 4.8

4.3 Nickel Nanoparticles: a Versatile Green Catalyst

In the field of heterogeneous catalysis, Group VIII metal-based catalysts such as palladium, platinum and nickel are among the most active transition metal catalysts. As the most widely available element of the triad, nickel is an important transition metal catalyst that can be used in many heterogeneous reactions. In recent years, numerous publications have reported the reduction of metal cation, especially Ni2 þ in solution, by hydrazine to produce metallic nanoparticles [17, 18]. Nickel oxide nanoparticles were found to be an effective catalyst in the catalytic activation of CCl bonds in chloroalkanes. The reaction of dichloromethane with amine in the presence of nickel oxide nanoparticles having a small size of 10–20 nm gave a quantitative yield of quaternary ammonium salts, depending on the amine (Scheme 4.9) [19]. Steric and electronic factors of the substituents on the amines played an important role in the catalytic activity. A primary alkyl substituent on the amine gave a quantitative yield whereas no product was obtained with N,N-diisopropylethylamine. It was found that nickel oxide nanoparticles were easily recovered and could be reused at least five times without loss of catalytic activity.

Scheme 4.9

In situ-generated Ni(0) nanoparticles and molecular hydrogen from the system NiCl2–Li–DTBB–ROH is a new mild and simple methodology for the efficient stereoselectivecispartialhydrogenationofalkynes(Scheme4.10).Thismethodologyhasbeen used for the synthesis of cis alkenes, partial hydrogenation of terminal alkynes and reduction of dienes to alkenes. Other conditions allow complete reduction of alkynes and alkenes to the corresponding alkane [20]. The versatility of this reducing system is that it results in complete hydrogenation of both internal and terminal alkynes. This simple protocol is used for the hydrogenation of multiple carbon–carbon bonds, including dienes, to corresponding alkane.

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Scheme 4.10

a,b-Unsaturated carbonyl compounds can be selectively reduced by in situ generated Ni(0) nanoparticles and molecular hydrogen (Scheme 4.11). One of the main advantages of this methodology is that handling of external molecular hydrogen is avoided since it is generated in situ in the reaction flask.

Scheme 4.11

The exocyclic conjugated carbon–carbon double bond of (R)-( þ )-pulegone, even though tetrasubstituted, was reduced with a high yield to give a cis–trans (75 : 25) diastereomeric mixture of ( þ )-isomenthone and ()-menthone. Most of the reduction using this protocol was regioselective with diastereomeric mixture and furnished cis isomer in major amount [21]. Even a- and b-ionone were both reduced to the expected product in good yields without any double bond isomerization. Kidwai and co-workers recently reported Ni(0) nanoparticles as a green catalyst for chemoselective reduction of carbonyl compounds using ammonium formate as a hydrogen donor (Scheme 4.12) [22, 23]; 10 mol% Ni nanoparticles in tetrahydrofuran selectively reduced the carbonyl group in the presence of other functional groups, viz. NO2, CN and alkene, to give the corresponding alcohols in excellent yields. This protocol reduces both aromatic and heteroaromatic aldehydes chemoselectively.

Scheme 4.12

NiFe bimetallic nanoparticles [24] were used for the catalytic dechlorination of the highly toxic pentachlorophenol in aqueous solution. The dechlorination is believed to take place on the surface sites of these particles. The dechlorination efficiency was 46%

4.4 Copper Nanoparticles: an Efficient Catalyst

within 30 min under optimal conditions and the use of ultrasonic irradiation enhanced the dechlorination efficiency to 96% for the same period of time.

4.4 Copper Nanoparticles: an Efficient Catalyst

Benzo-fused azoles are an important class of compounds and provide a common heterocyclic scaffold in biologically active and medicinally significant compounds. Benzoxazoles are found in a variety of natural products and are important targets in drug discovery. Benzoxazoles can be prepared by oxidative cyclization of Schiff bases using Cu nanoparticles (Scheme 4.13) [25].

Scheme 4.13

Without any catalytic effect of the Cu nanoparticles, 2-aminophenol condenses with the aldehyde to form the Schiff base, which then undergoes cyclization to form the benzoxazilidine in the presence of K2CO3. The resulting benzoxazilidine then undergoes aromatization via Cu nanoparticle catalysis to give the benzoxazole. Supported metallic Cu nanoparticles [26] were active for the selective dehydrogenation of methanol to produce formaldehyde and hydrogen with 100% H2 selectivity. Cu nanoparticles were impregnated on MoO2, MoO3, ZnO and SiO2 and used for selective dehydrogenation. A combination of Cu2O nanoparticles with P(o-tol)3 shows high catalytic activity for Stille cross-coupling reactions (Scheme 4.14). Li et al. evaluated a series of Cu catalysts and ligands and found Cu2O nanoparticles with P(o-tol)3 to be an effective catalyst for Stille cross-coupling reactions [27]. This catalytic protocol with Cu2O–P(o-tol)3–TBAB (tetrabutylammonium bromide) can be recovered and reused at least three times without any loss of catalytic activity for reactions of aryl iodides and activated aryl bromides. This methodology has good tolerance of aromatic rings.

Scheme 4.14

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In situ-generated Cu nanoparticles can be efficiently used for the Heck reaction in ionic liquids. The Heck reaction of aryl iodides and activated aryl bromides catalyzed by copper bronze in tetrabutylammonium bromide as solvent and tetrabutylammonium acetate was developed by Calo et al. [28]. The copper nanocolloids were derived from reaction of iodobenzene with copper bronze (Scheme 14.15). This catalytic system was recycled 20 times with an average total turnover number of approximately 40. These nanocolloids can be stored for months without loss of activity thanks to the stabilization effect of the TBAB.

Scheme 4.15

Freshly prepared copper nanocolloids were used for the formation of methanol. In a typical procedure, a high-pressure autoclave was charged with copper nanocolloid solution followed by sequential pressurization with synthesis gas (H2CO) and CO2 at roomtemperature [29].Theformationof the methanol was observed at130 Cwith a gas chromatographic detector attached to the high-pressure autoclave. Cu nanoparticles of size 14–17 A were efficiently used for aza-Michael reactions [30] in various N-alkyl- and N-arylpiperazines. Cyanoethylation of aryl- or alkypiperazines was carried out using acrylonitrile at room temperature with 10 mol% of Cu nanoparticles (Scheme 14.16). The reaction is highly chemoselective and only cyanoethylates secondary amines. The reaction studies showed that in the presence of anilines such as p-anisidine, p-toluidine and o-aminophenol, only secondary amines undergo the aza-Michael reaction.

Scheme 4.16

CHsp bond activation of terminal alkynes is of fundamental interest in organic synthesis. Several systems have been developed for CH activation, which mainly include transition metal complexes. Recently, Kidwai et al. reported Cu nanoparticlecatalyzed CH bond activation [31]. Cu nanoparticle-catalyzed A3 coupling via CH bond activation results in the formation of propargylamines (Scheme 4.17). This protocol can be widely used with a variety of secondary amines and aldehyde. Moreover, the nanoparticles can be recycled and reused and avoids the use of co-catalyst and gives the product in quantitative yield.

Scheme 4.17

4.5 Bimetallic Nanoparticles in a Variety of Reactions

4.5 Bimetallic Nanoparticles in a Variety of Reactions

Bimetallic salts or metals have wide application in organic synthesis. The electrode potential of these bimetallic salts can be used to carry out several transformations. Especially palladium catalysts are widely used in organic synthesis. In recent decades, their utilization in a variety of transformations has shown continuous impressive growth, achieving an important place in the arsenal of the practicing organic chemist. Highly dispersed nickel or palladium nanoparticles and silica aerogels were used as catalysts in the Mizoroki–Heck reaction [32]. Different nanocomposite silica aerogels were synthesized using Ni(OAc)2 and Pd(OAc)2 as a metal source. In situ-generated Pd nanoparticles in MCM-41 were used in the catalytic hydrogenation of alkynes in the liquid phase [33]. It was found that Pd particles incorporated in MCM-41 were significantly less active in liquid-phase alkyne hydrogenation than those on the external surface of MCM-41. Despite the difference in catalytic activities, the selectivities of Pd–MCM-41 is very similar and the Z-stereoisomer is selectively formed in the hydrogenation of 3-hexyne. The selectivity is found to be greater than 94%. 1-Pentyne, 1-hexyne and 3-hexyne were also reduced efficiently. In this methodology, hydrogen at 105 Pa was used in the hydrogenation reactor. Ultrasonically generated Pd(0) nanoparticles were found to catalyze the Sonogashira coupling reaction at ambient temperature. Reaction proceeds in acetone or ionic liquid as solvent at room temperature. Ionic liquid solvents such as 1,3-din-butylimidazolium tetrafluoroborate provide excellent chemoselectivity with considerably enhanced reaction rates through the formation of stable and crystalline clusters of zerovalent Pd nanoparticles [34]. A Pd–biscarbene complex (1) is formed by the reaction of an ionic liquid and PdCl2 and this complex can be isolated and characterized; it then undergoes sonochemical conversion to give polydisperse Pd(0) nanoparticles as catalyst for the reaction (Scheme 4.18). The reaction proceeds without copper as co-catalyst and ligand.

Scheme 4.18

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A similar reaction was also reported by Li et al. [35]. using palladium colloids in polyvinylpyrrolidone (PVP) (Scheme 4.19). The reaction takes place in 1% Pd(0) colloids with average diameter less than 7 nm. The catalyst can be recycled eight times without loss of catalytic activity.

Scheme 4.19

Palladium catalysts, however, are expensive and this may limit their utilization in some cases. To overcome this limitation, a number of new catalyst systems have been developed and Cacchi et al. reported carbon aerogel doped with Pd nanoparticles as an efficient catalyst for the hydroxycarbonylation of an aryl iodide [36] to form the corresponding carboxylic acid. High content Pd–carbon aerogels were prepared by sol–gel polymerization [37] of formaldehyde with the potassium salt of 2,4-dihydroxybenzoic acid, followed by K þ exchange with Pd2 þ ions from 0.1 M Pd(OAc)2 solution in acetone and subsequent supercritical drying with CO2. Transmission electron microscopy of the Pd–carbon aerogel showed nanoparticles with a mean particle size 19 4 nm. Hydroxycarbonylation of aryl iodides was carried out in the presence of lithium formate and acetic anhydride as an internal source of carbon monoxide with Pd–carbon aerogel (Scheme 4.20).

Scheme 4.20

Zhou et al. studied bimetallic PtCu nanoparticles as catalysts for the heterogeneous reduction of NO in the gas phase with H2 as the reducing agent [38]. Palladium nanoparticles deposited on polydimethylphosphazene (PDMP) were used for a Hecktype reaction [39]. Pd nanoparticles, obtained by the metal vapor synthesis technique, were deposited on PDMP and showed high catalytic activity in the Heck CC coupling of iodobenzene with methyl acrylate (Scheme 4.21). In the reaction, triethylamine is used as base to shift the equilibrium towards the product as it reacts with the hydrochloric acid formed during the course of reaction.

Scheme 4.21

References

The reaction results in the formation of trans-methyl cinnamate. Pd–PDMP is also used for the alkylative cyclization of 3-ethyl-3-methyl-7-octen-1-yne with iodobenzene (Scheme 4.22). NOE 1 H NMR experiments showed that the resulting product, 1,2-bis (alkylidene)cyclohexanes, possess Z-stereochemistry.

Scheme 4.22

A similar reaction was also carried out with in situ-generated Pd nanoparticles in ionic liquids [40]. Hence the nanoparticle approach has wide application in organic synthesis. Most of the nanoparticle catalysts can be recycled and reused to give quantitative yield even after the fifth or sixth cycle. Their catalytic use, recyclability and atom efficiency put nanoparticles squarely into the field of green chemistry.

References 1 Bell, A.T. Science (2003), 299, 1688. 2 R.A. Van Santen, P.W.N.M. Van Leeuwen, J.A. Moulijn and B.A. Averill (eds), (1999) Catalysis: an Integrated Approach, 2nd edn, Elsevier, Amsterdam, Stud. Surf. Sci. Catal. p. 123. 3 Hammer, B. and Norskov, J.K. (1995) Nature, 376, 238. 4 Haruta, M. (1997) Catalysis Today, 36, 153. 5 Haruta, M., Yamada, N., Kobayashi, T. and Iijima, S. (1989) Journal of Catalysis, 115, 301. 6 Nijhuis, T.A. and Weckhuysen, B.M. (2006) Catalysis Today, 117, 84. 7 Hayashi, T., Tanaka, K. and Haruta, M. (1998) Journal of Catalysis, 178, 566. 8 Jia, J., Haraki, K., Konodo, J.N., Domen, K. and Tamaru, K. (2000) The Journal of Physical Chemistry. B, 104, 11153. 9 Claus, P., Bruckner, A., Mohr, C. and Hofmeister, H. (2000) Journal of the American Chemical Society, 122, 11430. 10 Mohr, C., Hofmeister, H., Lucas, M. and Claus, P. (2000) Chemical Engineering & Technology, 23, 324.

11 Zhu, B. and Angelici, R.J. (2006) Journal of the American Chemical Society, 128, 14460. 12 Lazar, M. and Angelici, R.J. (2006) Journal of the American Chemical Society, 128, 10613. 13 Tsunoyama, H., Sakuari, H. and Tsukuda, T. (2006) Chemical Physics Letters, 429, 528. 14 Liu, W., Yang, X. and Huang, W. (2006) Journal of Colloid and Interface Science, 304, 160. 15 Ono, F., Kanemasa, S. and Tanaka, J. (2005) Tetrahedron Letters, 46, 7623. 16 Kidwai, M., Bansal, V., Kumar, A. and Mozumdar, S. (2007) Greem Chemistry, 9, 742. 17 Ni, X., Su, X., Yang, Z. and Zheng, H. (2003) Journal of Crystal Growth, 252, 612. 18 Shen, J., Hu, Z., Zhang, L., Li, Z. and Chen, Y. (1996) Applied Physics Letters, 15, 715. 19 Park, K.H., Jung, I.G., Chung, Y.K. and Han, J.W. (2007) Advanced Synthesis and Catalysis, 349, 411. 20 Alonso, F., Osante, I. and Yus, M. (2007) Tetrahedron, 63, 93.

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21 Alonso, F., Osante, I. and Yus, M. (2006) Synlett, 3017. 22 Kidwai, M., Bansal, V., Saxena, A., Shankar, R. and Mozumdar, S. (2006) Tetrahedron Letters, 47, 4161. 23 Kidwai, M., Mishra, N.K., Bansal, V., Kumar, A. and Mozumdar, S. (2008) Catal. Commun., 9, 612. 24 Zhang, W., Quan, X., Wang, J., Zhang, Z. and Chen, S. (2006) Chemosphere, 65, 58. 25 Kidwai, M., Bansal, V., Saxena, A., Aerry, S. and Mozumdar, S. (2006) Tetrahedron Letters, 47, 8049. 26 Tada, M., Bal, R., Namba, S. and Iwasawa, Y. (2006) Applied Catalysis A-General, 307, 78. 27 Li, J.H., Tang, B.X., Tao, L.M., Xie, Y.X., Liang, Y. and Zhang, M.B. (2006) The Journal of Organic Chemistry, 71, 7488. 28 Calo, V., Nacci, A., Monopoli, A., Leva, E. and Cioffi, N. (2005) Organic Letters, 7, 617. 29 Vukojevic, S., Trapp, O., Grunwaldt, J.D., Kiener, C. and Schuth, F. (2005) Angewandte Chemie-International Edition, 44, 7978. 30 Verma, A.K., Kumar, R., Chaudhary, P., Saxena, A., Shankar, R., Mozumdar, S. and Chandra, R. (2005) Tetrahedron Letters, 46, 5229. 31 Kidwai, M., Bansal, V., Mishra, N.K. and Kumar, A. (2007) Synlett, 1581. 32 Martinez, S., Manas, M.M., Vallribera, A., Schubert, U., Roig, A. and Molins, E. (1093) New Journal of Chemistry, 2006, 30.

33 Mastalir, A., Rae, B., Kiraly, Z. and Molnar, A. (2007) Journal of Molecular Catalysis AChemical, 264, 170. 34 Gholap, A.R., Venkatesan, K., Pasricha, R., Daniel, T., Lahoti, R.J. and Srinivasan, V.K. (2005) The Journal of Organic Chemistry, 70, 4869. 35 Li, P., Wang, L. and Li, H. (2005) Tetrahedron, 61, 8633. 36 Cacchi, S., Cotet, C.L., Farizi, G., Forte, G., Goggiamani, A., Martin, L., Martinez, S., Molins, E., Moreno-Manas, M., Petrucci, F., Roig, A. and Vallribera, A. (2007) Tetrahedron, 63, 2519. 37 Martinez, S., Vallribera, A., Cotet, C.L., Popovici, M., Martin, L., Roig, A., Moreno-Manas, M. and Molins, E. (2005) New Journal of Chemistry, 29, 1342. 38 Zhou, S., Varughese, B., Eichhorn, B., Jackson, G. and Mcllwrath, K. (2005) Angewandte Chemie-International Edition, 44, 4539. 39 Panziera, N., Pertici, P., Barazzone, L., Caporusso, A.M., Vitulli, G., Salvadori, P., Borsacchi, S., Geppi, M., Veracini, C.A., Marta, G. and Bertinetti, L. (2007) Journal of Catalysis, 246, 351. 40 Fei, Z., Zhao, D., Pieraccini, D., Ang, W.H., Geldbach, T.J., Scopelliti, R., Chiappe, C. and Dyson, P.J. (2007) Organometallics, 26, 1588.

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5 Heterogreeneous Chemistry Heiko Jacobsen

5.1 Introduction

The chemical philosophy of Green Chemistry originated in the early nineties and the Pollution Prevention Act that was passed in 1990 in the USA probably played a substantial role in increasing awareness of environmental and sustainability issues. With the advent of the new millennium, the importance of green chemistry has found widespread recognition. The World Summit on Sustainable Development, held in 2002 in Johannesburg, South Africa, provided ample evidence of a growing consensus that the world faces serious challenges to its sustainability. The list of major issues includes concerns regarding energy, resource depletion and the generation and dispersion of toxic substances [1]. This forum was one of the first major events in the 21st century that highlighted the need for global awareness of issues and problems relating to the worlds future. The choice of the Nobel Peace Prize awarded in 2007 [2], 5 years after the Johannesburg Summit, demonstrates that the concerns recognized at the beginning of the new millennium are far from resolved, are ongoing, are of major concern for our future, will clearly influence developments in politics, economics and science and need to become a component of our way of thinking. The prize was awarded to the Intergovernmental Panel on Climate Change and to Albert Arnold Gore Jr for their efforts to build up and disseminate greater knowledge about man-made climate change and to lay the foundations for the measures that are needed to counteract such change. The related issue of Global Warming is perhaps the most challenging, but certainly not the only problem that the world has to come to grips with and solve over the next decade to guarantee its sustainability for long-term survival. In order to achieve the goal of sustainability, Green Chemistry [3–6] is clearly evolving as a quintessential part of the foundation from which efficient and sensible solutions to the challenges at hand are derived. Green Chemistry is characterized by a move away from the command and control approach to environmental protection to a more scientifically based and economically minded approach [7].


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Since Green Chemistry is better viewed as a philosophy rather than a science, it naturally gives rise to a wide spectrum of different interpretations and therefore it behoves us to establish a good understanding of Green Chemistry before we go into more detail. Green Chemistry is an approach to the synthesis, processing and use of chemicals that reduces risks to humans and the environment. Existing chemical procedures are modified and new chemistries are developed that are effective, efficient and more environmentally benign. The benefits to industry and also the environment are all a part of the positive impact that Green Chemistry is having in the chemistry community and in society in general. As we noted above, Green Chemistry reflects a shift away from the historic command and control approach to environmental problems that dealt with issues of waste treatment, waste control and waste clean-up through regulation and towards preventing pollution at its source. Rather than accepting waste generation and disposal as unavoidable, Green Chemistry seeks new technologies that are cleaner and economically competitive. In 2002, Anastas and Kirchhoff summarized the first decade of Green Chemistry and analyzed its origins, its current status and future challenges [8]. They clearly illustrated the fact that Green Chemistry has demonstrated how fundamental scientific methodologies can protect human health and the environment in an economically beneficial manner. They further noted that significant progress is being made in several key research areas, such as catalysis, the design of safer chemicals and environmentally benign solvents and the development of renewable feedstocks. These trends and activities continue to receive major attention within the Green Chemistry community and highlight the potential of the science of chemistry to solve many of the global environmental challenges that the world faces at the beginning of the 21st century. The origins and basis of Green Chemistry chart a course for achieving environmental and economic prosperity inherent in a sustainable world and Anastas and Kirchhoff illustrate how the 12 Principles of Green Chemistry [3, 8] provide helpful guidelines in the efficient realization of the abstract concept and philosophy of Green Chemistry. The 12 Principles of Green Chemistry constitute the foundation of a new way of conducting chemistry and in principle address three major aspects of chemistry, the selection of reactants and products, synthetic methodologies and issues relating to chemical risks and energy requirements, summarized in Figure 5.1. The term Heterogreeneous Chemistry essentially relates to the fact that Green Chemistry has to be recognized as a philosophy, rather than a branch of well-defined science. From an inspection of the 12 Principles of Green Chemistry, it becomes clear that Green Chemistry cannot easily be reduced to just a few scientific principles, but incorporates a wide variety of strategies all devoted to the greater goal of achieving world sustainability. Thus, Green Chemistry is a heterogeneous subject in the purest form of the word: it consists of elements that are not of the same kind or nature, it is not uniform in structure or composition, it is composed of parts of different kinds that have widely dissimilar elements or constituents. The term Heterogreeneous Chemistry was coined with reference to a significant development which utilizes heterogeneous catalysis and follows many of the 12 Principles of Green Chemistry [9]. Heterogreeneous Chemistry also includes

5.1 Introduction

Figure 5.1 The 12 Principles of Green Chemistry. Adapted from [8].

heterogeneous aspects in a pure chemical definition, as in composed of different substances or the same substance in different phases. The most prominent heterogeneous area in chemistry is indeed heterogeneous catalysis, and it has been noted that catalysis is one of the foundational pillars of Green Chemistry [10]. The design and application of new catalysts and catalytic systems are simultaneously achieving the dual goals of environmental protection and economic benefit. Within the framework of the 12 Principles of Green Chemistry, catalysis offers numerous Green Chemistry benefits, including lower energy requirements, catalytic versus stoichiometric amounts of materials, increased selectivity and decreased use of processing and separation agents. Furthermore, catalysis allows the use of less toxic materials. Heterogeneous catalysis, in particular, addresses the goals of Green Chemistry by providing the ease of separation of product and catalyst, thereby eliminating the need for separation through distillation or extraction. In addition, environmentally benign catalysts such as clays and zeolites, may be used to replace the more hazardous catalysts currently in use. Significant progress is being made in several key research areas including environmental catalysis, which is undergoing a transformation from pollution abatement to pollution prevention [11]. The benefits to human health, the environment and the economic goals realized through the use of catalysis in manufacturing and processing are illustrated by focusing on the catalyst design and catalyst applications.

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This chapter is not intended to present an ultimate and authoritative review and reference work to the many facets of Heterogreeneous Chemistry. Instead, we will elucidate the idea of Heterogreeneous Chemistry and especially catalysis in a few recent exemplarily developments, hoping to disseminate the philosophy of Green Chemistry that continues to be a main incentive for ongoing research activities.

5.2 Heterogreeneous Catalysis

Heterogreeneous Catalysis incorporates many of the 12 Principles of Green Chemistry. To reiterate, it is not so much a breakthrough in one particular scientific approach that is at the heart of Heterogreeneous Chemistry, but rather a change in the conceptual framework dictating strategies how to conduct chemistry. This change and the accompanying increasing awareness manifest themselves in their increasing presence and recognition in the chemical literature of the last decade. The results of a simple topic search in the Web of Science database of ISI Web of Knowledge for publications that contain the topic entries Green Chemistry, Green Chemistry and Catalysis, and also Green Chemistry and Heterogeneous Catalysis, illustrate the growing awareness of the philosophy of Green Chemistry and of research on heterogeneous catalysis that addresses the topic of Green Chemistry (Figure 5.2).

Figure 5.2 Number of publications during the 10 years from 1997 to 2006 that contain topic entries Green Chemistry, Green Chemistry and Catalysis and Green Chemistry and Heterogeneous Catalysis, according to a search in the Web of Science database of ISI Web of Knowledge.


The afore mentioned simplified search is by no means exhaustive, as it does not cover all research activities of the past 10 years that are of significant importance to the field of Green Chemistry. In fact, the contribution by Dumesic and co-workers on catalysts for hydrogen production from biomass-derived hydrocarbons [12], which gave rise to the neologism Heterogreeneous Chemistry, is not captured in this basic search. What the graph and statistics in Figure 5.2 indicate, however, is more than growing awareness of a green philosophy in chemistry – they intimate a link between Green Chemistry and heterogeneous catalysis. We will explore the role of Heterogreeneous Chemistry in a few selective examples that illustrate how and why catalysis is one of the fundamental cornerstones of Green Chemistry. Before we continue, it is helpful to provide a brief definition of one of the key terms that we will encounter in our discussion: biomass. Living and recently dead biological material that can be used as fuel or for industrial production, be it specifically grown for a particular purpose or be it any type of biodegradable waste, is commonly referred to as biomass. Although the definition of biomass is broad and heterogeneous, it excludes organic material which has been transformed by geological processes into substances such as coal or petroleum. What differentiates biomass from other organic material that can be used for the same purposes is the timeline of its production. The time scale of production of biomass is on an equal footing with that of its consumption. This aspect renders biomass a renewable, rather than depletable, feedstock. 5.2.1 An Exemplarily Reaction – Catalysts for Hydrogen Production from Biomass-Derived Hydrocarbons

Concerns about the depletion of fossil fuel reserves and pollution caused by continuously increasing energy demands make hydrogen an attractive alternative energy source. Hydrogen is currently derived form non-renewable natural gas and petroleum [13], but could in principle be generated from renewable sources such as biomass and water. Dumesic and co-workers demonstrated that hydrogen can be produced from sugars and alcohols at temperatures near 500 K in a single-reactor aqueous phase reforming process using a platinum-based catalyst (Pt/Al2O3) [12]. The selectivity for hydrogen production increases greatly when oxygenated hydrocarbons are employed that have a C : O stoichiometry of 1 : 1. This study suggests that catalytic aqueous phase reforming might prove useful for the generation of hydrogen-rich fuel gas from carbohydrates extracted from renewable biomass and biomass waste streams. The transformation of oxygenated hydrocarbons into H2 and CO2 occurs according to the following stoichiometric reaction: Cn On H2n þ 2 þ nH2 O ! ð2n þ 1ÞH2 þ nCO2

ð5:1Þ

The selective generation of hydrogen by this route, however, proves to be difficult, since the products readily react at low temperatures to form alkanes and water: nCO2 þ ð3n þ 1ÞH2 ! Cn H2n þ 2 þ 2nH2 O

ð5:2Þ

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The steps shown in Scheme 5.1 are proposed to be involved in the formation of hydrogen and alkanes.

Scheme 5.1 Reaction pathways for hydrogen production by reactions of oxygenated hydrocarbons with water. The symbol represents a surface metal site; [M] indicates presence of a metal surface; denotes bond cleavage. Adapted from [12].

The reactant undergoes dehydrogenation steps on the metal surface to yield adsorbed intermediates before the cleavage of CC or CO bonds occurs. For the catalyst employed, it is noted that PtC bonds are more stable than PtO bonds and adsorbed species are probably bonded preferentially to the catalyst surface through PtC bonds. Subsequent cleavage of CC bonds leads to the formation of CO and H2, and the CO reacts with water to form CO2 and H2 by the water gas shift (WGS) reaction (Equation 5.3), a reaction which has been shown to be catalyzed by ceriabased active non-metallic Au and Pt species [14]. CO þ H2 O ! CO2 þ H2

ð5:3Þ

Davda and Dumesic have also shown how this equilibrium can be tuned in such a manner as to produce CO-poor hydrogen [15]. The expansion of gas bubbles formed in the process by the vaporization of water leads to decreasing partial pressures of H2 and CO2, thereby favoring the water gas shift and lowering the CO concentration. This process leads to the production of fuel cell-grade H2 at high pressures. Two reaction channels that influence the selectivity of hydrogen production from hydrocarbons have been identified [12]. A series-selectivity challenge is represented by the hydrogen-consuming reaction, in which either one or both of CO and CO2 react with H2, leading to alkanes and water by methanation and Fischer–Tropsch reactions. In a parallel-selectivity challenge, undesirable alkanes can form on the catalyst surface by cleavage of CO bonds, followed by hydrogenation of the resulting adsorbed species. The above-described scenario is extended by proposing further competing reactions, such as the formation of organic acids by dehydrogenation reactions, which in turn lead to the formation of alkanes from carbon atoms not bonded to oxygen atoms.

5.2 Heterogreeneous Catalysis Table 5.1 Experimental data for reforming of oxygenated hydrocarbons with Pt- and Ni-based catalysts.

SnNi b

Pt/Al2O3a Parameter

Sorbitol

Glycerol

Ethylene glycol

Sorbitol

Glycerol

Ethylene glycol

T (K) p (bar) %H2c %CnH2nþ2c

498 29 66 15

498 29 75 19

498 29 96 4

498 25.8 65 19

498 25.8 81 13

498 25.8 95 4

a

Data compiled from [12]. Data compiled from [19]. c %H2, hydrogen selectivity; %CnH2n þ 2, alkane selectivity; see [12] for a definition of selectivities. b

In addition, the proposed reaction scheme has been investigated in a theoretical study based on self-consistent periodic density functional calculations [16]. In a model reaction, the relative stabilities and reactivities of surface species on Pt(111) derived by subsequent removal of hydrogen atoms from ethanol have been considered. Transition states for CC and CO bond cleavage reactions have been located and the results from these calculations, combined with transition state theory, predict that the rate constant for CC bond cleavage in ethanol will be faster than for CO bond cleavage on Pt(111) at temperatures higher than about 550 K. Further, the calculated value of the rate constant for CC bond cleavage in ethanol is predicted to be much higher than that for CC bond cleavage in ethane on Pt(111). Similarly, the rate of CO bond cleavage in ethanol is predicted to be much higher than for CO bond cleavage in carbon monoxide on Pt(111). These calculations highlight the effectiveness of the platinum catalyst employed, which disfavors elemental reaction steps occurring in reactions competing with hydrogen formation. The experimental results for aqueous phase reforming of sorbitol (C6O6H14), glycerol (C3O3H8) and glycol (C2O2H6) are presented in Table 5.1. Sorbitol can be produced by hydrogenation of glucose [17], a compound that is directly relevant to biomass utilization. Glycerol and glycol, in turn, can be obtained from hydrogenolysis of sorbitol [18]. A variety of other biomass related sources readily provide additional access to glycerol and glycol. The reactions were performed over a 3 wt% Pt catalyst supported on nanofibers of g-alumina. Reactions were carried out under pressure in a tubular reactor at 498 K by continuously feeding an aqueous solution having a 1 wt% feed concentration of organic compound. The data in Table 5.1 indicate that a H2 selectivity as high as 96% was achieved. The corresponding alkane selectivities range from 4 to 19%. The selectivity for production of H2 improves in the order C6O6H14 < C3O3H8 < C2O2H6. The fractions of feed carbon detected in effluent gaseous and liquid streams yield a complete carbon balance, indicating that only negligible amounts of carbon have been deposited on the catalyst. Of further importance is the catalyst performance, which was stable for times on stream of at least 1 week. While the data in Table 5.1 establish that Pt-based catalysts show high activities and good selectivity for the production of hydrogen from biomass-derived alcohols,

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improvements are necessary to render the process useful. Highly active catalytic materials that satisfy the series and parallel selectivity challenges (Scheme 5.1) at lower materials costs are particularly desirable. In a follow-up study, Dumesic and co-workers introduced a new active system for hydrogen production by aqueous phase reforming of biomass-derived oxygenated hydrocarbons, namely a tin-promoted Raney nickel catalyst (SnNi ) [19]. They found that the addition of tin to nickel decreases the rate of methane formation from CO bond cleavage while maintaining the high rates of CC bond cleavage required for hydrogen formation. Referring back to Table 5.1, results for the same experiments as described above under the same experimental conditions but using an SnNi rather than a Pt/Al2O3 catalyst indicate that the cheap non-precious metal catalyst compares favorably with the expensive platinum-based catalyst. The above-described reactions obey a number of the 12 principles [3, 8] that guide Green Chemistry, such as use of feedstock derived from renewable raw materials, use of efficient and cheap catalysts, avoidance of extensive use of auxiliary materials and prevention of waste. This work also outlines how new catalysts can be expected to provide impetus and lower potential barriers for the implementation of greener industrial processes and technologies. Known chemistry and technology are tailored in a green fashion. This process is accompanied by certain by-products, which, however, do not need to be avoided, but can be maximized and open up another valuable aspect of biomass utilization. This is in accord with the second of the 12 Principles of Green Chemistry, that synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product. We will explore this aspect in more detail in the next section. 5.2.2 Transportation Fuels from Biomass – Catalytic Processing of Biomass-derived Reactants

Biomass serves as the prototypical green feedstock not only for hydrogen production, but also for transportation fuels. In fact, it has been stated that biomass is the only practical source of renewable liquid fuel [20]. In addition, biofuels generate significantly less greenhouse gas emissions than do fossil fuels and can even be greenhouse gas neutral if efficient methods for biofuels production are developed [21]. A biomass growth and manufacturing scheme relies only on CO2, H2O, light, air and nutrients to produce energy, some of which is fed back into the biomass processing scheme. The three main aspects necessary for a carbohydrate economy are growth of the biomass feedstock, biomass conversion into a fuel and fuel utilization. The format of an integrated biomass production-conversion scheme is illustrated in Figure 5.3. Of the above-mentioned three technologies, the steps of biomass conversion and fuel utilization are processes that can be addressed in the spirit of Green Chemistry and heterogeneous catalysis plays an essential role in designing and employing environmentally friendly and sustainable procedures. The biorefinery concept [21] and the chemical catalytic transformations of biomass-derived feedstocks have been extensively reviewed [22, 23] and hydrolysis, dehydration, isomerization, aldol condensation, reforming, hydrogenation and oxidation have been identified as key reactions involved in the processing of biomass. Here, we shall discuss in detail the


Figure 5.3 Integrated biomass production–conversion system for the sustainable production of transportation fuels. Adapted from [21].

production of renewable alkanes by aqueous phase reforming of biomass-derived oxygenates [24], and identify the green aspects of the technology involved. We have seen in the previous section how it is possible to produce hydrogen from biomass-derived oxygenates, such as glycerol and sorbitol, using a process of aqueous phase reforming (APR). We also noted that the production of hydrogen is accompanied by the formation of light alkanes, primarily methane. Through tuning of the reaction conditions and catalysts, it is possible to tailor the aqueous phase reforming process selectively to produce a clean stream of heavier alkanes consisting primarily of butane. The APR process offers a simple route for the production of renewable fuels from biomass and we will discuss in more detail, and as an illustrative example, how aqueous phase reforming of sorbitol, the sugar alcohol obtained by hydrogenation of glucose, can be adjusted for conversion of sorbitol to heavier alkanes consisting primarily of butane, pentane and hexane [24]. This process incorporates concepts of Green Chemistry, as it utilizes known chemical reactions, such as the WGS reaction and Fischer–Tropsch synthesis in combination with metal catalysts as well as acid catalysts, to conduct chemical reactions according to the 12 Principles of Green Chemistry. Production of alkanes by aqueous phase reforming of sorbitol takes place by a bifunctional reaction pathway that involves first the formation of hydrogen and CO2 on an appropriate metal catalyst [M] and then the dehydration of sorbitol on a solid acid catalyst These initial steps are followed by hydrogenation of the dehydrated reaction intermediates on the metal catalyst. When these steps are balanced properly, the hydrogen produced in the first step is fully consumed by hydrogenation of the dehydrated reaction intermediates, which leads to the overall conversion of sorbitol to alkanes plus CO2 and water. The essential features of the bifunctional reaction pathway for the production of alkanes from sorbitol are depicted in Scheme 5.2.

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Scheme 5.2 Reaction pathways for the production of alkanes from sorbitol over catalysts with metal [M] and solid acid components. F–T, Fischer–Tropsch; denotes bond cleavage.

Hydrogen is produced on the metal by cleavage of CC bonds followed by the WGS reaction. Dehydrated species are first formed on acid sites, typically solid acids, and then migrate to metal sites [M] where they undergo hydrogenation reactions. Repeated cycling of dehydration and hydrogenation reactions in the presence of hydrogen then leads to formation of heavier alkanes. Formation of lighter alkanes takes place by more rapid cleavage of CC bonds compared with hydrogenation of dehydrated reaction intermediates and the selectivities for production of various alkanes by aqueous phase reforming depend on the relative rates of CC bond cleavage, dehydration and hydrogenation reactions. The selectivities for the production of alkanes can be varied by changing the catalyst composition and the reaction conditions and by modifying the reactor design. In addition, the selectivities can be modified by co-feeding hydrogen with the aqueous sorbitol feed, which then leads to a process in which sorbitol can be converted to alkanes and water without the formation of CO2. Catalysts typically employed are the transition metals Pd or Pt as heterogeneous metal catalysts [M] and silica–alumina composites (SiO2Al2O3) as solid acid catalysts. Solid acids are conventional materials that have wide applications in chemical production and the arsenal of solid acids is constantly expanding, including, for example, carbon materials as strong protonic acids [25]. In Table 5.2 are presented the experimental results for aqueous phase reforming of sorbitol obtained from different catalytic systems. The data demonstrate how aqueous phase reduction of oxygenated hydrocarbons can be adjusted such that it is not hydrogen that evolves as major product of the reformation process [12], but heavier alkanes instead. The H2 selectivity is 4% or lower, the carbon selectivities are largest for heavier alkanes, such as hexane (C6H14), and alkane to total gas-phase carbon selectivities are as high as 98%. Hydrogen, which is needed for the hydrogenation reaction, can be produced in situ by aqueous phase reforming or it can be co-fed into the reactor with the aqueous sorbitol reactant. The alkanes formed are straight-chain compounds with only minor amounts of branched isomers. The selectivities for production of heavier alkanes can be controlled by the choice of the reaction conditions and by co-feeding hydrogen to the reactor.

5.2 Heterogreeneous Catalysis Table 5.2 Experimental data for aqueous phase reforming of sorbitol.a

Catalysts and additives to feed Parameter

Pt–SiAl

Pt–SiAl

Pt–SiAl H2

Pt–SiAl H2

Pd–SiAl H2

T (K) p (bar) % Carbon selectivityb CH4 C2H6 C3H8 C4H10 C5H12 C6H14 % H2 selectivityb % Alkane to total gas phase carbonc

498 52.7

538 60.7

498 29.3

498 34.8

538 58.2

10 10 9 15 19 37 1 53

7 10 11 16 21 35 4 60

0 3 6 11 25 55 — 91

0 4 7 11 24 54 — 91

2 2 4 8 28 56 — 98

a

Data compiled from [23]. See [23] for a definition of selectivities. c The gas-phase carbon consists of alkanes and carbon dioxide. b

This green protocol draws from the wealth of established chemistry and combines well-known reactions such as Fischer–Tropsch processes and the WGS reaction with new applications of old catalyst systems. This reaction qualifies as heterogreen by way of a variety of different aspects, which are combined to achieve the shared goal of creating reactions with environmentally friendly sustainability. Furthermore, this reaction employs the quintessential green solvent water. We will further elaborate on solvents in Green Chemistry, in particular in connection with heterogeneous aspects, at a later point in our discussion. 5.2.3 Diesel Fuels from Biomass – Heterogreeneous Processes for Biodiesel Production

Diesel fuel produced from petroleum is a hydrocarbon mixture, composed of about 75% saturated hydrocarbons and 25% aromatic hydrocarbons. The average chemical formula for common diesel fuel is C12H23, ranging from approximately C10H20 to C15H28. Diesel-powered engines generally have a better fuel economy than equivalent gasoline engines and produce less greenhouse gas pollution. The term biodiesel refers to a diesel-equivalent processed fuel consisting of short-chain alkyl esters of fatty acids, such as methyl or ethyl esters, made by transesterification of vegetable oils or animal fats. Biodiesel fuels can be used alone or blended in with conventional diesel fuel, in unmodified diesel engines. Biodiesel is biodegradable and non-toxic and typically produces about 60% less net-lifecycle carbon dioxide emissions. Having the characteristics of common diesel fuels, sparing the limited resources of fossil fuels and addressing current environmental concerns render biodiesel a key element in the production of energy from biomass according to the 12 Principles of Green Chemistry and a promising alternative fuel to petrodiesel.

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The advantages of biodiesel as an alternative fuel and the problems involved in its manufacturing have been reviewed [26], with one of the main problems in the production of biodiesel being the identification of a suitable catalyst that is active, selective and stable under the process conditions needed for heterogeneous transesterification. Furthermore, technology development for processing more abundant lignocellulosic biomass for fuels and materials will be critical [23]. Several commercial processes to produce biodiesel as fatty acid methyl esters from vegetable oils have been developed and are available today in recognition of its green characteristics and increasing demand. These processes use homogeneous basic catalysts such as caustic soda or sodium methylate, which lead to waste products after neutralization with mineral acids. A further undesirable effect is that the byproduct glycerol is obtained as contaminated raw product requiring further purification steps. It is highly desirable to have at hand a process for biodiesel production that requires neither catalyst recovery nor aqueous treatment steps and that produces the byproduct glycerol with high purity levels and exempt from any salt contaminants. Heterogeneous catalysis holds the promise of an efficient, green and continuous biodiesel production process and we will describe an exemplarily process developed by Casanave and co-workers in more detail [27]. The transesterfication process that leads to the production of biodiesel from fatty acids is depicted in Scheme 5.3. Groups R1, R2 and R3 typically contain hydrocarbon chains of 15–20 C-atoms. It is important to note that the methanolysis is an equilibrium process. The reaction is promoted by a completely heterogeneous catalyst. This catalyst consists of a mixed oxide of zinc and aluminum, which promotes the transesterification reaction without catalyst loss. The reaction is performed with an excess of methanol and this excess is removed by vaporization and recycled to the process with fresh methanol.

Scheme 5.3 Methanolysis of fatty acids.

A reaction scheme for biodiesel production based on heterogeneous catalysis is presented in Figure 5.4. The catalyst section includes two fixed-bed reactors R1 and R2, fed with vegetable oil and methanol at a given ratio. Excess of methanol is removed after each reactor by partial evaporation. Then, esters and glycerol are separated in a settler or separator. Glycerol outputs are gathered and the residual methanol is removed by evaporation. The purification section of methyl ester output coming from the last separator consists of a finishing methanol vaporization under vacuum followed by a final purification step. The biodiesel produced by this heterogeneous process meets industrial standards and therefore is not only of environmental, but also of economic value. Of the greatest importance, however, is the fact that the quality and the value of the crude glycerol produced by biodiesel plants are significantly improved; glycerol with purity levels of at least 98% can be obtained.


Figure 5.4 Reaction scheme for biodiesel production based on heterogreeneous catalysis.

According to the second of the 12 Principles of Green Chemistry, glycerol from waste glycerol streams that are currently generated as by-products from the production of biodiesel can be converted over platinum-based catalysts into gas mixtures of H2 and CO at temperatures from 498 to 620 K [28]. This gas mixture, known as synthesis gas, serves as a feedstock for a variety of chemical production processes. The temperatures required for the above-mentioned catalysis are lower than those for conventional gasification of biomass, which typically are around 800–1000 K; hence the catalytic conversion of glycerol at low temperatures may allow for economical operation of a small-scale Fischer–Tropsch reactor by producing an undiluted H2CO synthesis gas mixture. The capital cost of a Fischer–Tropsch plant would be greatly reduced by eliminating the need for a biomass gasifier and further scaled down if the need for gas cleaning steps due to the use of pure glycerol were eliminated. More notable incentives for the implementation of Heterogreeneous Chemistry are that the same feedstock can be used in a variety of processes and that its by-products are efficiently incorporated in green synthesis projects. Furthermore, the green concept of recycling is already incorporated in the production process. We have seen a similar strategy in the reforming of biomass-derived oxygenates, where the desired product was obtained in repeated cycles of reforming, dehydration and hydrogenation [24]. Even the principle product obtained from green processing of biomass, biodiesel, can further be fed into other green production lines and might serve as a feedstock for chemical intermediates produced in the chemical industry, such as olefins. Subramanian and Schmidt, for example, have demonstrated how renewable olefins can be obtained from biodiesel by a process of autothermal reforming [29]. Thus, the heterogeneous advantage in the use and production of biodiesel [30] truly is a heterogreeneous advantage in all respects.

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5.2.4 Other Heterogreeneous Aspects of Catalysis

Although catalysis has long been utilized in order to increase efficiency, yield and selectivity in chemical processes, it has also of late gained recognition for achieving a wide range of green chemistry goals. At the dawn of the 21st century, increasingly demanding environmental legislation, public and corporate pressure and the resulting drive towards clean technology provide an unquestionable impetus for the development of new catalysis and catalytic processes. In compliance with the 12 Principles of Green Chemistry, some salient goals in the search for new catalysis are to increase process selectivity, maximize the use of starting materials, replace stoichiometric reagents with catalysts and facilitate easy separation of the final reaction mixture, including the efficient recovery of the catalyst. The examples presented so far illustrate how catalysis has evolved into one of the foundational pillars of green chemistry [10]. To complete our discussion of heterogreeneous catalysis, we consider various other aspects of catalysis all related to Green Chemistry. Again, note that the following selections are not intended to represent an authoritative and comprehensive review of the subject, but rather to present, since Green Chemistry is still in its infancy, a snapshot of green activities and developments at the end of the first decade of the new millennium. 5.2.4.1 Solid and Solid Acid Catalysts The use of efficient solid catalysts holds great promise in the developments of Green Chemistry [31]. Polymer-supported catalysts, for example, have been widely used and their popularity comes mainly from the fact that product isolation is simplified and that less harsh conditions and higher degrees of selectivity can be attained. As an additional benefit, catalysts based on high surface area inorganic support materials show good thermal stability and have therefore attracted considerable interest as solid catalysts and reagents in liquid-phase organic reactions. Chemically modified mesoporous materials can be prepared to serve as robust catalysts suitable for application in liquidphase processes such as Friedel–Crafts reactions, selective oxidations, nucleophilic substitutions and aromatic brominations, and might form the basis of some new industrial catalysts which will then replace toxic and corrosive traditional reagents [32]. The development and use of mesoporous inorganic support materials as catalysts with chemically bound active centers is emerging as an area of research which seeks to retain the green benefits of heterogenization, enhanced activity and enhanced product selectivity while avoiding the drawbacks of catalyst instability and limited reusability. A recent example of this technology is observable in the heterogeneous aluminasupported ruthenium catalyst designed by Yamaguchi and Mizuno, which is easy to prepare, inexpensive to use, capable of being recycled and efficient in the aerobic oxidation of amines [33]. Further expanding on this technology, the same group have also synthesized an organic–inorganic hybrid support, which allows catalytically active polyoxometalate anions to be immobilized [34]. This truly heterogreeneous catalytic system is effective for liquid-phase oxidation with hydrogen peroxide, such as epoxidation and sulfonation, and is reusable without any loss of catalytic performance.


Solid acids are the most widely studied and commonly used heterogeneous catalysts, often used in large-scale continuous vapor-phase processes such as catalytic cracking and alkane isomerizations. The examples in the previous subsections further illustrate the use of solid acids in heterogreeneous catalysis. Current activities in green research are geared towards the development of solid acid catalysts which are effective in liquid-phase organic reactions such as those employed in many batch-type reactors by fine, specialty and pharmaceutical intermediate chemical manufacturers. A recently reported example of work in this direction is silica sulfuric acid, reported by Dabiri et al., which permits the rapid and green synthesis of 2,5-disubstituted 1,3,4-oxadiazoles under solvent-free conditions [35]. The development of carbon materials as strong protonic acids [25], which illustrates the diverse research activities that all are driven by or related to Green Chemistry, exemplifies the heterogreeneous nature of this subject, as mentioned previously. 5.2.4.2 Recycling Catalysts An important aspect of heterogreeneous catalysis is the recyclability of the catalyst employed. Ideally, the catalyst is not consumed in a green process, nor does it need to be recycled, but this seldom happens in reality. Researchers are therefore looking for new ways to allow easy catalyst recycling. Mizuno and co-workers have designed an easily prepared ruthenium hydroxide catalyst on magnetite, Ru(OH)x/Fe3O4, which allows for simple and straightforward product–catalyst separation [36]. After a particular reaction, the desired separation can be easily achieved with a permanent magnet, allowing more than 99% of the Ru(OH)x/Fe3O4 catalyst to be usually recovered for each reaction. Three kinds of reactions, namely aerobic oxidation of alcohols, aerobic oxidation of amines and reduction of carbonyl compounds to alcohols using 2-propanol as a hydrogen donor, can efficiently be promoted by this magnetic catalyst. A wide variety of substrates, including aromatic, aliphatic and heterocyclic compounds, can be converted to the desired products in high to excellent yields without any additives such as bases and electron transfer mediators. Ru(OH)x/ Fe3O4 is thus an intrinsically heterogeneous catalyst and the recovered catalyst can be reused without appreciable loss of the catalytic performance. In the context of the aforementioned solid acid catalyst and with emphasis on recyclability, Paul and co-workers prepared a covalently anchored sulfonic acid on silica as an efficient and recoverable interphase catalyst [37]. The catalyst is highly stable, completely heterogeneous and recyclable several times. The Biginelli reaction was investigated as a test reaction for the new catalyst and the products were obtained in good to excellent yields, requiring only very simple work-up procedures. The ease of recyclability represents one of the major advantages of heterogeneous versus homogeneous catalysis, but homogeneous catalysis also carries advantages. Homogeneous catalysts that exist in the same phase as reactants and products are usually more selective than heterogeneous catalysts and far less affected by limitations due to slow transport of reactants and products, which has stimulated the development of strategies that facilitate the recycling of homogeneous catalysts [38]. With the concept of ease of catalyst separation in mind, Dioumaev and Bullock designed a tungsten catalyst for the solvent-free hydrosilylation of ketones that retains its activity

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until essentially all of the liquid substrate is converted to liquid products [39]. The product can then simply be decanted to separate the catalyst that precipitates from the products of the reaction. The incorporation of homogeneous catalysis into Green Chemistry is not a straightforward procedure, but the work of Dioumaev and Bullock illustrates how the limitations of homogeneous catalysis might be circumvented and how homogeneous catalysis might become part of Heterogreeneous Chemistry. 5.2.4.3 One-pot Catalysis Creating one-pot synthetic routes is a challenge already spawning new chemistry, enzymes, materials and mechanistic insight [40]. Through one-pot reactions, chemical products can be produced with less waste and greater economic benefits and still one-pot reactions adhere to the philosophy of Green Chemistry. Synthetic strategies combining the framework of a one-pot synthesis with green catalytic transformations are beginning to evolve, thanks to their inherent green nature. We mention as one example the one-pot synthesis of primary amides from aldoximes or aldehydes in water in the presence of a supported rhodium catalyst, reported by Mizuno and co-workers [41]. 5.2.4.4 Photocatalysis Photocatalysis numbers among the new and emerging green technologies. Since different aspects of the 12 Principles of Green Chemistry can be addressed by the use of photocatalysis, it truly stands as an example of heterogreeneous technology. One of its aspects involves the removal of organic contaminants by way of solar energy, which mainly involves oxidative decomposition of volatile organic compounds (VOCs). Photocatalysis has many advantages over other treatment methods, such as the use of the environmentally friendly oxidant O2 and low reaction temperatures. To date, TiO2 has undoubtedly proven to be the best photocatalyst for the oxidative decomposition of many organic compounds under UV irradiation. Ongoing research activities are aimed towards expanding the electromagnetic spectrum of photocatalysts and Tang et al. have reported a novel photocatalyst, CaBi2O4, which is active in the photocatalytic oxidative decomposition of organic contaminants under visible light irradiation [42]. Another facet of photocatalysis as it gains increasing importance has to do with the development of new energy sources. In view of our limited fossil fuel reserves and the pollution caused by constantly increasing energy demands, hydrogen emerges as an attractive alternative energy source. Thus, visible light hydrogen generation from water [43, 44] holds the promise and potential to become one of the main energy sources of future generations.

5.3 Solvents for Green Catalysis

The examples given under previous headings mostly address green issues dealing with sustainability, renewable feedstocks and renewable energy resources. Setting


aside targeted goals of chemical processes, Green Chemistry, in its heterogeneous nature, also tackles the issue of how to conduct chemical reactions in a benign, sustainable and environmentally friendly manner. Solvents are key components of most chemical transformations and have gained center stage in the design of green processes. The current status of green solvents for sustainable synthesis has been reviewed by Sheldon, who addresses many of the problems, challenges and solutions in the field of solvents in chemical synthesis, and in particular organic synthesis [45]. In the context of Green Chemistry, there are several issues which influence the choice of solvent. Ideally, the solvent should be non-toxic and non-hazardous, that is, not flammable or corrosive. Further, the solvent should also be contained, that is, it should not be released to the environment. Commonly used solvents are typically separated from products by evaporation or distillation and most popular solvents are, therefore, highly volatile. Spillage and evaporation inevitably lead to atmospheric pollution, a major environmental issue of global proportions. Moreover, worker exposure to VOCs represents a serious health issue. The ideal green solvent should be modeled to avoid the above-mentioned risks and shortcomings of conventional solvents, while at the same time offering some of the same or similar properties such as polarity or boiling points. Issues surrounding a wide range of volatile and non-volatile solvents have already stimulated the fine chemical and pharmaceutical industries to seek more benign alternatives. Not all issues can be properly addressed by the nature of the solvent alone, but might be solved by the proper conduction of a chemical process. Biphasic reactions hold many green advantages such as ease of separation of products and possible catalysts, and are beginning to take their place in production processes in the chemical industry. We have repeatedly pointed out that catalysis is one of the cornerstones of Green Chemistry implementation. However, depending on the goal at hand, catalysis by itself is not always sufficient to put into practice a green synthetic process. Given that the use of solvents accounts for 50% of the post-treatment greenhouse gas emissions and 60% of the energy used in pharmaceutical processes, consideration must be given to the proper selection of solvents when designing and developing a reaction scheme [46]. Choice of catalyst and choice of solvent are ultimately intrinsically linked when problems in chemistry are solved in the green frame of mind. The term Heterogreeneous Chemistry thus reflects the complexity and multilayered nature of the many problems originating when serious consideration is given to the 12 Principles of Green Chemistry. 5.3.1 Heterogreeneous Solvent Systems

The best solvent is no solvent at all, and, if a solvent is needed, green or potentially green alternatives should be considered. Preferably, such alternative solvents should also allow easy catalyst separation and recycling. Although a number of reactions have been shown to proceed under solvent-free conditions, at present most synthetic reactions still call for the use of solvents. Therefore, alternatives to conventional organic solvents are being actively sought. Currently, one can categorize green and

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Figure 5.5 Heterogreeneous solvents and solvent systems.

heterogreeneous solvents into four classes, namely water, supercritical carbon dioxide (scCO2), ionic liquids (ILs) and fluorous solvents, as depicted in Figure 5.5. With respect to catalysis, the four solvent systems are further subdivided into those used preferentially for monophasic and those used preferentially for biphasic catalysis. Water and ILs straddle the border between mono- and biphasic applications. Three out of the four solvent classes, scCO2, ILs and fluorous solvents, are currently referred to as non-conventional solvents, which find extensive use in the process of greening organic chemistry. Water represents the most conventional solvent out of the classes of green solvents and, if a prototypical green solvent had to be named, water would represent the best choice. It is cheap, readily available, nontoxic, non-flammable and safe to the environment. Of additional importance to catalysis is the fact that water allows for facile catalyst separation and recycles through a biphasic catalysis mode due to its low miscibility with most organic compounds. We have chosen the example of a success story to illustrate the above-mentioned concept of biphasic catalysis and to stress the important role of water as a quintessential green solvent. The first profitable large-scale application of aqueous catalysts is the oxo process of Ruhrchemie/Rhône-Poulenc, which uses a rhodium catalyst dissolved in water [47]. This reaction is outlined in Scheme 5.4. In this biphasic catalytic reaction, the catalyst resides in the water phase, whereas the product dissolves in the organic phase. This hydroformylation reaction truly represents an environmentally benign technique: highly economic, environmentally sound and pollution reducing. Although water is a conventional solvent, heterogreeneous activities expand its use to non-conventional applications. Aqueous Barbier–Grignard-type reactions described by Li abandon fundamental ideas and principles of organic chemistry and open up new and greener avenues for organic synthesis [48]. This work spawned new developments in the execution of fundamental and conventional reaction steps in organic chemistry, such as CC bond formation, in aqueous media [49]. A recent


Scheme 5.4 Ruhrchemie/Rhône-Poulenc process for aqueous biphasic hydroformylation.

application of aqueous organic chemistry, and a step toward a bio-based industry, is the benign catalytic esterification of succinic acid in the presence of water, reported by Clark and co-workers [50]. Supercritical carbon dioxide (scCO2) has been attracting increasing attention as an alternative and green reaction medium in recent years [51]. It is non-toxic, nonflammable, inexpensive, relatively inert towards reactive compounds and readily separable from products upon depressurization. These characteristics of scCO2 are in line with many of the 12 Principles of Green Chemistry. Furthermore, its low viscosity and high diffusivity properties confer advantages on reactions with mass transfer problems. However, scCO2 is apolar and generally only suitable in catalytic processes for compounds of low polarity. The eleventh point of the 12 Principles of Green Chemistry refers to the need for further development of analytical methods. Supercritical fluids such as scCO2 offer opportunities of in situ spectroscopic studies, which provide valuable information useful for rendering a particular chemical process more ecological or more economical [52]. The direct synthesis of propylene oxide from propylene, hydrogen and oxygen provides a recent example of the green use of carbon dioxide as solvent for organic reactions [53]. Ionic liquids (ILs), composed entirely of organic cations and organic or inorganic anions, display physicochemical properties such as low melting point, negligible vapor pressure, low flammability, tunable polarity and miscibility with other organic or inorganic compounds, that are appealing for use in catalysis and separation processes. ILs have low solubility towards low-polarity compounds, such as ethers and alkanes, which makes the application of biphasic catalysis with ILs possible. Even for monophasic catalysis, catalyst-product separation can be readily achieved by extraction of the product, leaving the catalyst in ILs for reuse.

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In view of the toxic or hazardous properties of many solvents, notably chlorinated hydrocarbons, and in view of serious environmental issues, such as atmospheric emissions and contamination of aqueous effluents, the chemical industry is currently exploring alternative and greener reaction media. The current emphasis on the search for novel reaction media is also fueled by the need for efficient methods of recycling homogeneous catalysts. The use of ionic liquids as novel reaction media may offer a convenient solution to both the solvent emission and the catalyst recycling problems. Applications and advantages of ILs have been reviewed by Sheldon, who emphasizes the importance of ILs in Green Chemistry [54]. Biphasic methodologies heterogenize homogeneous catalysis and represent prime examples of the concept of Heterogreeneous Chemistry. Fluorous solvents are non-conventional solvents primarily used for biphasic catalysis, and are highly fluorinated alkanes, ethers and tertiary amines, with perfluorinated alkanes being the most representative. The term fluorous is analogous to aqueous and the concept of fluorous biphasic catalysis is illustrated by Horvath [55]. Fluorous solvents possess unusual physicochemical properties, such as low dielectric constants, high chemical and thermal stability and low toxicity. They commonly exhibit temperature-dependent miscibility with organic solvents and are immiscible with many common organic solvents at ambient temperature, although they can become miscible at elevated temperatures. This property is of particular relevance in biphasic catalysis since it provides the basis for performing biphasic catalysis or, alternatively, monophasic catalysis at elevated temperatures with biphasic product–catalyst separation at lower temperatures. The four classes of green solvents, as discussed above, span almost the entire range of the solvent spectrum in terms of their chemical and physical properties, which can be further tuned to fulfill the specific demands of a given synthetic task. Although their use is unlikely to solve all the solvent problems faced by industries, these solvents will contribute to the development of green and sustainable synthetic processes by allowing enhanced catalyst activity, selectivity and productivity, new selectivity patterns, reduced or eliminated waste and solvent emissions and ease of operation. The use of green solvents in green catalytic synthesis has been extensively reviewed by Liu and Xiao, who clearly illustrate the critical role that solvents play in greening synthetic chemistry and in particular catalytic organic synthesis [56]. 5.3.2 Solvent-free Heterogreeneous Chemistry

In 2000, Lippard provided a wish list identifying the grand challenges for chemistry that would affect a quiet revolution in chemical practice, on both industrial and laboratory scales [57]. This list includes the art of conducting chemical reactions without solvents. Although much attention has been given to the importance of green solvents, there is an equally desirable need to design reactions that proceed under totally solvent-free conditions. The best solvent is no solvent at all and, in the arena of Green Chemistry, solvent-free reactions give rise to major reduction and

5.4 Conclusion and Outlook

simplification. Green Chemistry as applied to chemical processes can be viewed as a series of reductions, such as reductions in energy, in auxiliaries and in waste. It should always lead to the simplification of the process in terms of the number of chemicals and steps involved. Simply put, removing the solvent factor from a chemical process will likely be the greatest reduction and simplification achievable, in many cases. The quest for solvent-free reactions is gaining momentum: Thomas et al. have developed more environmentally friendly and highly selective solvent-free alternatives for carrying out a number of important chemical conversions, such as selective oxidation of hydrocarbons and aromatics and industrial hydrogenations [58]. The procedures are based on porous heterogeneous catalysts in which the active sites have been atomically engineered. Such solid catalysts operate under solvent-free conditions and usually entail one-step processes. The concept of solvent-free heterogreeneous catalysis can be implemented in different ways. Bose et al., for example, described a general and practical green chemistry route to the Biginelli cyclocondensation reaction under solvent-free conditions [59]. Kotsuki et al. reported that the reaction of epoxides with lithium halides is efficiently promoted on the surface of silica gel in the absence of any solvent to give the corresponding b-halohydrins [60]. Mariani and co-workers used solid– liquid solvent-free phase transfer catalysis and acidic catalysis in dry media to design a solvent-free protocol for the synthesis of cosmetic fatty esters [61]. The necessity to minimize the amount of toxic waste and by-products from chemical processes has led to the development of new, more environmentally friendly synthetic methods in which fewer toxic substances are used, and has become one of key aspects of greening organic synthesis. Since solvents are generally used in large quantities and since many organic solvents are ecologically harmful, their use should be minimized to the greatest degree possible. Solventfree approaches to organic chemistry therefore provide an ideal green solution to the above referenced problems. Exemplary reactions have shown that solvent-free reactions proceed with at least the same yield and selectivity as their solution counterparts [62], catalysis being at the heart of these reactions. Solvent-free chemistry is truly an example of the concept of Heterogreeneous Chemistry.

5.4 Conclusion and Outlook

This brief review has illustrated how catalysis, and especially heterogeneous catalysis, constitutes one of the cornerstones of Green Chemistry. The heterogeneous nature of Green Chemistry is well underlined by the wide range of topics encircled in its domain, concepts ranging from employment of renewable energies and renewable feedstocks to implementation of chemical processes which maintain an ecologically friendly outlook. A broader perspective of the 12 Principles of Green Chemistry also indicates to us that Green Chemistry is more of a chemical philosophy than a chemical science.

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The chemical industry plays a key role in sustaining the world economy and in so doing underpins future technologies. Poliakoff et al. have explored some of the issues raised by the development of Green Chemistry techniques and have identified potential barriers to their implementation by industry [63]. They ask the question of what might be needed for the industry to embrace efforts to become greener. Economical barriers to the implementation of Green Chemistry necessitate a paradigm shift from the traditional concept of process efficiency, which focuses largely on chemical yield, to one that assigns economic value to eliminating waste at its source and avoiding the use of toxic and hazardous substances [45]. In this context, efforts are being made to make the philosophic concept of Green Chemistry more scientific and to introduce new concepts, such as atom efficiency [64], and measurable parameters, such as the as the E-factor [65], in order to provide a metric for Green Chemistry [66]. New strategies have been developed that allow chemists to assess clearly whether or not and to what extent chemistries and chemical processes can be considered as being green [67]. Aside from economic barriers, certain behavioral barriers exist to the implementation of Green Chemistry. These hurdles relate to the prevalent way of thinking about chemistry, which assesses value and confers weight to ongoing developments in chemistry. The concept of Heterogreeneous Chemistry elevates the 12 Principles of Green Chemistry to center field in Green Chemistry developments. It is holistic in nature and embraces all different green aspects of chemistry. Heterogreeneous Chemistry draws from the wealth of chemical knowledge and approaches old problems with well-established solutions from a different angle. It is an approach that focuses on the essential ideas and motivations behind the 12 Principle of Green Chemistry and it recognizes, for example, that technologies, for which questions of atom economy and E-factor cannot be answered, can still adhere to the 12 Principles of Green Chemistry and be truly heterogreeneous. Heterogreeneous Chemistry also necessitates a paradigm shift and recognizes that the question of the greenness of a chemical process cannot always be answered in terms of measurable parameters, just as the impact and value of a chemical discovery or new chemical procedure cannot necessarily be measured by its novelty appeal (such a paradigm shift is beginning to receive general recognition; the Journal of the American Chemical Society, for example, instructs its authors that titles of manuscripts may not contain the word First or Novel [68]). Re-evaluating chemical beliefs and chemical conceptions will be fundamental if we wish to conduct chemistry in a way that recognizes current challenges and expands views to incorporate inevitable consequences for future generations.

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3 Anastas, P.T. and Warner, J.C. (1998) Green Chemistry: Theory and Practice, Oxford University Press, Oxford.

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23 Chheda, J.N., Huber, G.W. and Dumesic, J.A. (2007) Angewandte ChemieInternational Edition, 46, 7164–7183. 24 Huber, G.W., Cortright, R.D. and Dumesic, J.A. (2004) Angewandte ChemieInternational Edition, 43, 1549–1551. 25 Hara, M., Yoshida, T., Takagaki, A., Takata, T., Kondo, J.N., Haashi, S. and Domen, K. (2004) Angewandte Chemie-International Edition, 43, 2955–2958. 26 Kiss, A.A., Dimian, A.C. and Rothenberg, G. (2006) Advanced Synthesis and Catalysis, 348, 75–81. 27 Bournay, L., Casanave, D., Delfort, B., Hillion, G. and Chodorge, J.A. (2005) Catalysis Today, 106, 190–192. 28 Soares, R.R., Simonetti, D.A. and Dumesic, J.A. (2006) Angewandte ChemieInternational Edition, 45, 3982–3985. 29 Subramanian, R. and Schmidt, L.D. (2005) Angewandte Chemie-International Edition, 44, 302–305. 30 Kiss, A.A., Omota, F., Dimian, A.C. and Rothenberg, G. (2006) Topics in Catalysis, 40, 141–150. 31 Clark, J.H. and Macquarrie, D.J. (1998) Chemical Communications, 853–860. 32 Clark, J.H., Butterworth, A.J., Tavener, S.J., Teasdale, A.J., Barlow, S.J., Bastock, T.W. and Martin, K. (1997) Journal of Chemical Technology and Biotechnology (Oxford, Oxfordshire: 1986), 68, 367–376. 33 Yamaguchi, K. and Mizuno, N. (2003) Angewandte Chemie-International Edition, 42, 1480–1483. 34 Kasai, J., Nakagawa, Y., Uchida, S., Yamaguchi, K. and Mizuno, N. (2006) Chemistry – A European Journal, 12, 4176–4184. 35 Dabiri, M., Salehi, P., Baghbanzadeh, M., Zolfigol, M.A. and Bahramnejad, M. (2007) Synthetic Communications, 37, 1201–1209. 36 Kotani, M., Koike, T., Yamaguchi, K. and Mizuno, N. (2006) Green Chemistry, 8, 735–741. 37 Gupta, R., Paul, S. and Gupta, R. (2007) Journal of Molecular Catalysis A-Chemical, 266, 50–54.

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38 Gladysz, J.A. (2001) Pure and Applied Chemistry, 73, 1319–1324. 39 Dioumaev, V.K. and Bullock, R.M. (2003) Nature, 424, 530–532. 40 Broadwater, S.J., Roth, S.L., Price, K.E., Kobaslija, M. and McQuade, D.T. (2005) Organic and Biomolecular Chemistry, 3, 2899–2906. 41 Fujiwara, H., Ogasawara, Y., Yamaguchi, K. and Mizuno, N. (2007) Angewandte Chemie-International Edition, 46, 5202–5205. 42 Tang, J.W., Zou, Z.G. and Ye, J.H. (2004) Angewandte Chemie-International Edition, 43, 4463–4466. 43 Tsuji, I., Kato, H. and Kudo, A. (2005) Angewandte Chemie-International Edition, 44, 3565–3568. 44 Zheng, N., Bu, X.H., Vu, H. and Feng, P.Y. (2005) Angewandte Chemie-International Edition, 44, 5299–5303. 45 Sheldon, R.A. (2005) Green Chemistry, 7, 267–278. 46 Butters, M., Catterick, D., Craig, A., Curzons, A., Dale, D., Gillmore, A., Green, S.P., Marziano, I., Sherlock, J.-P. and White, W. (2006) Chemical Reviews, 106, 3002–3027. 47 Cornils, B. and Wiebus, E. (1996) Recueil des Travaux Chimiques des Pays-Bas, 115, 211–215. 48 Li, C.J. (1996) Tetrahedron, 52, 5643–5668. 49 Li, C.J. (2005) Chemical Reviews, 105, 3095–3165. 50 Budarin, V., Luque, R., Macquarrie, D.J. and Clark, J.H. (2007) Chemistry – A European Journal, 13, 6914–6919. 51 Leitner, W. (2002) Accounts of Chemical Research, 35, 746–756. 52 Grunwaldt, J.-D., Wandeler, R. and Baiker, A. (2003) Catalysis Reviews-Science and Engineering, 45, 1–96.

53 Danciu, T., Beckman, E.J., Hancu, D., Cochran, R.N., Grey, R., Hajnik, G.M. and Jewson, J. (2003) Angewandte ChemieInternational Edition, 42, 1140–1142. 54 Sheldon, R. (2001) Chemical Communications, 2399–2407. 55 Horvath, I.T. (1998) Accounts of Chemical Research, 31, 641–650. 56 Liu, S.F. and Xiao, J.L. (2007) Journal of Molecular Catalysis A-Chemical, 270, 1–43. 57 Lippard, S.J. (2000) Chemical & Engineering News, 78, 64–65. 58 Thomas, J.M., Raja, R., Sankar, G., Johnson, B.F.G. and Lewis, D.W. (2001) Chemistry – A European Journal, 7, 2973–2978. 59 Bose, D.S., Fatima, L. and Mereyala, H.B. (2003) The Journal of Organic Chemistry, 68, 587–590. 60 Kotsuki, H., Shimanouchi, T., Ohshima, R. and Fujiwara, S. (1998) Tetrahedron, 54, 2709–2722. 61 Villa, C., Mariani, E., Loupy, A., Grippo, C., Grossi, G.C. and Bargagna, A. (2003) Green Chemistry, 5, 623–626. 62 Metzger, J.O. (1998) Angewandte ChemieInternational Edition, 37, 2975–2978. 63 Poliakoff, M., Fitzpatrick, J.M., Farren, T.R. and Anastas, P.T. (2002) Science, 297, 807–810. 64 Trost, B.M. (2002) Accounts of Chemical Research, 35, 695–705. 65 Sheldon, R.A. (2007) Green Chemistry, 9, 1273–1283. 66 Curzons, A.D., Constable, D.J.C., Mortimer, D.N. and Cunningham, V.L. (2002) Green Chemistry, 4, 521–527. 67 Constable, D.J.C., Curzons, A.D. and Cunningham, V.L. (2001) Green Chemistry, 3, 1–6. 68 http://pubs.acs.org/paragonplus/ submission/jacsat/jacsat_authguide.pdf.

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6 Single-site Heterogeneous Catalysts via Surface-bound Organometallic and Inorganic Complexes Christophe Coperet

6.1 Introduction

Heterogeneous catalysis fully complies with the aims and the rules of Green Chemistry because it associates catalysis – a way to shorten steps and to improve the selectivity and the energy efficiency of reactions (lower temperatures) – and heterogeneous phases, which allow an easier separation of products from the catalyst, greatly simplifying chemical processes (number of separation steps) and avoiding the contamination of products with residual metals. In fact, industry typically prefers to implement these types of processes. However, the rational optimization of these systems from a chemical point of view is more difficult, because of their complexity and the associated lack of molecular understanding of the nature and the environment of their active sites. In contrast, homogeneous catalysts can be developed more rationally through structure–reactivity relationships, which is due to the large body of data obtained in the past 40 years in molecular chemistry. In more recent years, the same molecular approach has been undertaken in heterogeneous catalysis [1–6], and in this chapter the focus will be specifically on the development of single-site heterogeneous catalysts (for generalities, see the next section) and their applications to selective chemical reactions such as hydrogenation and hydrosilylation (Section 6.3), alkene metathesis and other alkene homologation processes (Section 6.4), alkyne metathesis and other alkyne homologation processes (Section 6.5), Lewis acid-catalyzed reactions (Section 6.6), oxidation (Section 6.7) and alkane homologation processes (Section 6.8).

6.2 Generalities

Numerous heterogeneous catalysts are supported on oxides and the active sites are directly bound to their surfaces. A molecular approach to the preparation and development ofthesecatalysts dependslargelyonobtaininga molecular understanding of the structure of the surface species and relies on characterizing them at a molecular


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level using a combination of tools: chemical reactivity, spectroscopy (IR, Raman, EXAFS–XANES, solid-state NMR, UV–visible) and computational chemistry [5]. The first approach to the preparation of these systems consists in the controlled reaction (grafting) of organometallic or metallo-organic complexes with the functionalities present at the surface of an oxide support, typically hydroxyls (Scheme 6.1a), but also reactive oxygen atoms (Scheme 6.1b). It is therefore critical to have a good understanding of the surface properties and the stability of the oxide supports. For instance, the density of active sites and their environment can be controlled by a thermal treatment of the support under vacuum or a flow of dry inert gas prior to the grafting step. This treatment induces a condensation of adjacent hydroxyl groups, which tunes the density of OH groups (Scheme 6.1c). Prior to this thermal treatment, it is often preferable to perform a calcination of the support to remove adsorbed carbon-containing species. After chemical grafting on to the support, these systems can also be further treated with chemical, thermal and/or photochemical processes (Scheme 6.1d). This typically increases the number of covalent bonds between the metals and the support and can yield isolated surface species, having structures unprecedented in molecular chemistry. This also allows an increase stability and reactivity of active sites, which is translated into Greener chemistry, i.e. enhanced catalytic activities and stabilities as well as the discovery of new reactivities (see below).

Scheme 6.1

In the case of amorphous silica, the OH groups are attached to tetracoordinated Si atoms and the OH density upon thermal treatment varies as follows: ca 2.6, 1.2 and 0.7 OH nm2 at 200, 500 and 700 C, respectively. Above 800 C, the OH density still decreases, but there is also a dramatic decrease in surface area (sintering). The OH groups are mostly isolated and statistically distributed at the surface of silica treated at 700 C [SiO2-(700)] and at lower temperatures the amount of vicinal silanols increases; geminal silanols are proposed to exist only below 200 C (Scheme 6.2a) [7, 8]. The use of

6.3 Hydrogenation and Hydrosilylation

mesoporous silica supports does not dramatically influence the overall trends presented above: the OH density – expressed in OH nm–2 differ only slightly, but the OH concentration in mmol g–1 increases nearly proportionallywith increase in surface area, which allows the metal loading to be increased. Note that for these supports care should be taken because of their lower thermal stability (the mesoporous structure may collapse). In the case of alumina, the situation is more complex: the density is ca 4 OH nm2 fora g-aluminapretreatedat500 C [g-Al2O3-(500)],their distributionis not even [9] and the hydroxyls have very different natures depending on whether they are attached to a tetra-, penta- or hexacoordinated Al atoms and also if they are bound to one (h1), two (h2) or three (h3) Al (Scheme 6.2b) [10–12]. Moreover, the surface of g-Al2O3 also contains different types of Lewis acid sites, which can also react with the molecular precursors during the grafting step (see below) [13]. Each support has its own characteristic and should be studied in detail – typically by the combined use of titration studies, spectroscopy and modeling – prior to investigating grafted species [14].

Scheme 6.2 Surface functionalities on (a) silica and (b, c) alumina [(b) hydroxyls; (c) Lewis acid sites].

6.3 Hydrogenation and Hydrosilylation 6.3.1 Hydrogenation

In contrast to their homogeneous catalyst homologues, single-site heterogeneous hydrogenation catalysts are based on early transition metals, mainly Groups 4–5,

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because of their stronger MO bonds, which prevents sintering and formation of metal particles. These systems are prepared by first reaction of perhydrocarbyl organometallic complexes with silica followed by treatment under H2, at 150–200 C. In the case of Group 4 and 5 metals supported on silica, this method yields surface metal hydrides: Ti [TiH/SiO2] [15], Zr [ZrH/SiO2] [16, 17], Hf[HfH/ SiO2] [18] and Ta [TaH/SiO2] [19] (Scheme 6.3). These hydrides can be prepared on a variety of oxides supports and extended to Group 6 metals: [ZrH/Al2O3] [20], [ZrH/SiO2–Al2O3] [21], [TaH/Al2O3] [22], [WH/Al2O3] [22, 23], [WH/SiO2– Al2O3] [24], but their structures have not yet been fully elucidated.

Scheme 6.3 (a) Silica-supported group 4 hydrides; (b) silica-supported tantalum hydrides.

Noteworthily, the silica-supported Ti and Zr hydrides efficiently catalyze the hydrogenation of alkenes and even aromatics (TOF ¼ 3600–360 000 h1 for the hydrogenation of cyclohexene or benzene) [25–28]. Such types of catalysts have also been prepared in situ by directly reacting the molecular precursor with silica in an autoclave prior to adding the reagent and H2, and they display good catalytic performances on various aromatic substrates (Tables 6.1 and 6.2) [29]. Cyclopentadienyl Zr derivatives supported on alumina are also highly active alkene hydrogenation catalysts [30, 31], and the activity of these hydrogenation catalysts depends on both the molecular precursor and the temperature of thermal treatment of alumina (Table 6.3, Entries 1–6) [32]. The best systems are obtained for highly dehydroxylated alumina [Al2O3-(1000)] and this has been associated with the formation of cationic surface species. Therefore, other supports have been investigated in order to generate more electrophilic Zr systems, such as sulfated zirconia [33], sulfated alumina[34] or other sulfated oxide supports (Table 6.3, Entries 7–11) [35]. In these

6.3 Hydrogenation and Hydrosilylation Table 6.1 Hydrogenation of benzene catalyzed by silica-supported

Group 4–6 transition metal complexes. Catalytic system

Conditionsa

Time (min)

Conversion (%)

Ti(CH2SiMe3)4/SiO2 Zr(CH2Ph)4/SiO2 Hf(CH2Ph)4/SiO2 Ta(CH2Ar)5/SiO2 (Ar ¼ 4-MeC6H4) [Nb(m-CSiMe3)(CH2SiMe3)2]2/SiO2

A A A A A B A B B

360 240 600 24h 300 25 720 55 720

39 100 100 100 100 100 100 100 100

[Ta(m-CSiMe3)(CH2SiMe3)2]2/SiO2 [Mo2(CH2SiMe3)6]/SiO2

Conditions: A, 0.056 mmol of M loaded on 5 g of silica treated at 200 C under vacuum, 17.5 g of C6H6 (4000 equiv.), 80–100 bar of H2 at 120 C; B, 0.056 mmol of M loaded on 5 g of silica treated at 200 C under vacuum, 26 mmol of substrate (280 equiv.), 80–100 bar of H2 at 120 C.

a

Table 6.2 Hydrogenation of other aromatics catalyzed by silica supported Group 4–6 transition metal complexes.

Catalytic system

Substrate

Time (min) Conversion (%) Selectivity (%)

Ti(CH2SiMe3)4/SiO2 Zr(CH2Ph)4/SiO2 Hf(CH2Ph)4/SiO2 Ta(CH2Ar)5/SiO2 (Ar ¼ 4-MeC6H4) [Nb(m-CSiMe3)(CH2SiMe3)2]2/SiO2

Naphthalenea Naphthalenea Naphthalenea Naphthalenea Naphthalenea Tolueneb o-Xyleneb m-Xyleneb p-Xyleneb Naphthalenea Tolueneb o-Xyleneb m-Xyleneb p-Xyleneb Naphthalenea

150 30 60 160 20 33 30 20 25 70 64 75 70 65 70

[Ta(m-CSiMe3)(CH2SiMe3)2]2/SiO2

[Mo2(CH2SiMe3)6]/SiO2

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

55 : 45c 25 : 75c 19 : 81c 20 : 80c 15 : 85c 100 57 : 43c 78 : 22c 57 : 43c 20 : 80c 100 91 : 9c 92 : 8c 82 : 18c 20 : 80c

0.056 mmol of M loaded on 5 g of silica treated at 200 C under vacuum, 2.0 g of naphthalene (280 equiv.), 25 mL of hexane, 100 bar of H2 at 120 C. b 0.056 mmol of M loaded on 5 g of silica treated at 200 C under vacuum, 26 mmol of substrate (280 equiv.), 80–100 bar of H2 at 120 C. c cis : trans ratio. a

cases, the activities have been correlated with the surface Brønsted acidity of the support [35], but the structure of the active sites is not yet understood. This approach can also be applied to actinide complexes, which, when supported on alumina, display much higher activity than the original molecular complexes in

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Table 6.3 Activity of cyclopentadienylzirconium derivatives in the hydrogenation of alkenes.

Entry

Molecular precursor

Support

Substrate

Activity (h1)

1 2 3 4 5 6 7 8 9 10 11

Cp ZrMe3 Cp ZrMe3 Cp2ZrMe2 Cp CpZrMe2 Cp2ZrMe2 Cp CpZrMe2 Cp ZrMe3 Cp ZrMe3 Cp ZrMe3 Cp ZrMe3 Cp ZrMe3

Al2O3-(1000) Al2O3-(500) Al2O3-(1000) Al2O3-(1000) Al2O3-(500) Al2O3-(500) ZrO2-sulfated Al2O3-sulfated SnO2-sulfated SnO2-sulfated TiO2-sulfated

Propene Propene Propene Propene Propene Propene 1-Hexene 1-Hexene 1-Hexene 1-Hexene 1-Hexene

3960a 1080a 1080a 720a 360a 216a 970b 360b 10b 5b trans2-butene (2900 h1) propene (470 h1) isobutene (13 h1). Finally, aromatics can also be efficiently hydrogenated at 90 C and 13 bar of H2 with the following order of reactivity: [Th(allyl)4]/Al2O3-(1000) (1970 h1) [Th (CH2Ar)4]/Al2O3-(1000) (825 h1) > [Cp Th(CH2Ar)3] (765 h1) [39, 40]. The rate of hydrogenation also depends on the aromatic compounds: benzene (6850k) > toluene (4100k) > xylene (1450k) naphthalene (k). Note that the activities observed with [Th(allyl)4]/Al2O3-(1000) are comparable to those obtained for classical heterogeneous catalysts based on supported Rh or Pt particles.

Table 6.4 Activity of cyclopentadienyl actinide derivatives supported or not in the hydrogenation of propene at 25 Ca.

Entry

Molecular precursor

Support

Activity (h1)

1 2 3 4 5 6 7

Cp 2UMe2 Cp 2UMe2 Cp 2ThMe2 Cp 2ThMe2 Cp 2ThMe2 Cp 2ThMe2 Cp 2ThMe2

Al2O3-(1000) None Al2O3-(1000) None SiO2–Al2O3 MgCl2 SiO2–MgO

1080 68 580 0.54 160–230 25–43 0

6.3 Hydrogenation and Hydrosilylation

6.3.2 Hydrosilylation

The silica-supported lanthanide and Group 3 silylamide complexes [(SiO)Ln(N {SiMe3}2)2] (Ln ¼ Y, La, Nd and Sm), prepared by the reaction of the corresponding trisilylamide complexes [Ln(N{SiMe3}2)3] with SiO2-(700), catalyze the hydrosilylation of alkenes (Scheme 6.4). Whereas 1-hexene is transformed mainly into the linear isomer with 90–94% selectivity, styrene gives the branched product in 99% selectively [41]. Overall, the silica-supported systems are only slightly less active than the parent molecular complexes, but the trends of reactivity over a series of lanthanide metals are different: La > Nd > Sm Y for the supported systems vs La Sm > Nd Y for the molecular precursors. This has been associated with the impossibility to generate dimeric species for the supported systems.

Scheme 6.4

Hydrosilylation has also been catalyzed by a well-defined Rh supported complex [(SiO)Rh(COD)L], which is prepared by the reaction of [{(Me3SiO)Rh(COD)}2] with SiO2-(200) (Scheme 6.5) [42]. In this specific case, this catalytic system is noteworthy: the catalyst loading is fairly low (0.01%) and recycling is fairly efficient with possibility of 10–20 cycles without major loss of activity and metal leaching. Moreover, spectroscopic evidence has been obtained for the intermediate silyl hydride complex, expected from the classical Chalk–Harrod mechanism of hydrosilylation. This is again consistent with the greater stability of the supported system.

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Scheme 6.5 (a) Preparation of well-defined RhI species; (b) hydrosilylation with [(SiO)RhI(COD)].

6.4 Metathesis and Homologation Processes of Alkenes 6.4.1 Alkene Metathesis 6.4.1.1 Silica-supported Catalysts The preparation of heterogeneous alkene metathesis catalysts by grafting molecular complexes on oxide supports started in the 1970s [43–47], but it has only been recently that the first well-defined silica supported metallocarbene complexes have appeared: they are all prepared by the grafting of well-defined Mo [48–50], W [51] and Re [52–57] alkylidene complexes on silica via cleavage of one MX bond (X ¼ alkyl or amido) by the isolated silanols of a surface of SiO2-(700) yielding the surface complex and HX (Scheme 6.6). By combining reactivity studies, spectroscopy (IR and solid-state NMR and also EXAFS in some cases) and computational studies, it has been possible to determine their structure at a molecular level: they are all isoelectronic d0 tetrahedral syn-complexes, displaying an agostic interaction between the alkylidene proton and the metal center. Their catalytic properties in alkene metathesis are noteworthy (Table 6.5, Entries 1–6), because of: Their greater activities and stabilities than their homogeneous precursors. Their compatibility with functionalized alkenes such as esters without the use of co-catalysts, which is different from the classical heterogeneous alkene metathesis catalysts [58].

6.4 Metathesis and Homologation Processes of Alkenes

Scheme 6.6

The nearly quantitative formation of the cross-metathesis products (initiation step), which is consistent with single-site catalysts. Their improved stability because deactivation pathways via dimerization have been shut down (site isolation) [59–62]. Their high metathesis selectivity for amido systems; the alkyl ones leading to the formation of some amount of 1-butene, especially for the Re systems [63]. The origin of the high reactivity of these silica-supported systems (X)(Y)M(ER) (¼CHR) is due to the asymmetry at the metal center (X6¼Y) [64, 65]. Indeed, alkene metathesis is a four-step reaction pathway involving the coordination of the alkene,

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Table 6.5 Activity of well-defined silica supported alkylidene complexes in alkene metathesis.

Propeneb

Ethyl oleate (EO)c

Entry

Catalysta

Initial activityd

Selectivitye

TON

Initial activityd

Time (h)

1 2 3 4 5 6 7 8

Re–R Mo–R W–R Mo–NPh2 Mo–Pyr–Ar Mo–2,5-DiMePyr–Ar Mo–2,5-DiMePyr–CF3 Mo–2,5-DiMePyr–Ad

120 120 8.4 374 362 320 560 780

96.0 99.4 99.6 >99.9 >99.9 >99.9 >99.9 >99.9

6 · 103 22 · 103 6 · 103 138 · 103 62 · 103 101 · 103 135 · 103 275 · 103

1.2 1.2 — 6.4 2.4 3.0 72.0 30.0

24 (30%) 24 — 4 24 (10%) 3 0.5 1

a

For structures, see Scheme 6.6. Experimental conditions: in a flow reactor with a propene flow rate of ca 5000 mol mol1 M min1. c EO/M ¼ 2000, 1.16 M solution of EO in toluene. d The initial activity is defined as the moles of substrate transformed per mole of catalyst per minute. e Selectivity ¼ (Z þ E)-2-butenes/(all butenes). b

the [2 þ 2]-cycloaddition and the corresponding reverse steps (cycloreversion and decoordination), with metallacyclobutane intermediates having trigonal bipyramid (TBP) and square-based pyramid (SP) geometries, the former being on the reaction pathway and the latter being more stable (Scheme 6.7). The first step corresponds to a distortion of the complex from a tetrahedral into a trigonal pyramid structure, in order to generate an empty coordination site and to accommodate the incoming alkenes, and it is favored when the ligand trans to the incoming alkene is a strong s-donor ligand (X ¼ CH2R or NR2), whereas that entering the basal plane of the trigonal pyramid is a weak s-donor ligand (Y ¼ OSi) to avoid the strong competition of the E- and the alkylidene ligands. The second step, the [2 þ 2]-cycloaddition, has a very low activation energy and leads to very stable metallacyclobutane intermediates, which are destabilized by strong s-donor ligands. Therefore, the asymmetric systems (X ¼ CH2R or NR2 and Y ¼ OSi) associates low activation energies and not too stable intermediates, which is optimal for a catalyst.

Scheme 6.7

6.4 Metathesis and Homologation Processes of Alkenes

Moreover, the Mo systems can be further improved by tuning the imido ligands and, for instance, in the case of the 2,5-dimethylpyrrolyl systems there is a clear increase in activity and overall turnover by replacing the 2,6-diisopropylphenylimido by either a 2-CF3-phenyl or an adamantyl imido ligand (Table 6.5, Entries 6–8). The adamantylimido ligand allows high activity (ca 8 mol of propene converted per Mo per second) and overall TON to be reached (230 000) in the metathesis of propene [50], whereas the 2-CF3-phenyl group gives better stability in the metathesis of ethyl oleate. 6.4.1.2 Alumina-supported Catalysts In contrast to the silica-supported systems, obtaining a molecular understanding of the alumina-supported systems is more difficult, due to the complexity of the alumina support (see Section 6.2). In fact, despite extensive studies, Re2O7/Al2O3, a promising catalyst because of its unusual activity at room temperatures [66] and its compatibility with functional groups when activated with organotin reagents [67–69], there is still little information on the nature of its active sites, even though it is clear that the Lewis acidity of alumina is key [70]. This is also true for the oxide-supported methyltrioxorhenium, CH3ReO3 [71–75]. Combining mass balance analysis, in situ IR, EXAFS, solid-state NMR and periodic calculations shows that the Re–C bond is not cleaved upon grafting of CH3ReO3 on g-Al2O3 partially dehydroxylated at 500 C, but that several surface species are formed: (1) the major surface species (1.0 nm2) correspond to CH3ReO3 chemisorbed on aluminum Lewis acid sites through its oxo ligands and (2) the minor surface species (0.15 Re nm2) are formed via the heterolytic cleavage of the CH bond of CH3ReO3 on reactive surface sites, AlSOS [13], yielding AlSCH2ReO3 surface species along with new hydroxyls (Scheme 6.8) [76]. These reactive sites are in fact generated upon thermal treatment of alumina (calcination and partial dehydroxylation typically performed at 500 C) prior to grafting of CH3ReO3 and correspond to Al sites having an empty coordination site (Lewis acid), the reactivity of which depends on the coordination number of Al: (OS)3AlS (AlIII) > (OS)4AlS (AlIV) > (OS)5AlS(AlV) [13]. Of the various possible AlSCH2ReO3 surface species, titration studies in combination with solid-state NMR data and calculations are consistent with the formation of mainly AlVCH2ReO3 or AlVICH2ReO3 surface species.

Scheme 6.8

Noteworthily, in the metathesis of propene, this catalyst deactivates first order in products (very likely ethene) [77], and it has been possible to improve the catalytic stability of the system by changing the adsorption properties of alumina by passivating

j127


128

the surface with trimethylsilyl group (Scheme 6.9) [78]. Moreover, this change in surface adsorption properties also allows the more selective formation of (Z)-2-butene (Z/E ¼ 3), the kinetic product, even at 10% conversion, whereas the thermodynamic ratio is typically observed (E/Z ¼ 3) in the case of the parent alumina-supported systems.

Scheme 6.9

Finally, the presence of masked carbenic active sites in CH3ReO3/Al2O3 probably explains why this catalyst is compatible with functionalized alkenes, whereas the parent system based on Re2O7/Al2O3 necessitates activators. In fact, as the active sites of Re2O7/Al2O3 require the presence of Lewis acid sites, they are probably poisoned in the presence of functionalized alkenes and it is likely that an activator, typically alkylating agents such as R4Sn, for Re2O7/Al2O3 is necessary to generate active sites, closely related to AlSCH2ReO3. 6.4.2 Other Alkene Homologation Processes

The field of single-site heterogeneous catalyst has probably emerged from the polymer industry, where chemists had been trying to develop well-defined equivalents of the Ziegler–Natta catalysts [1, 2]. In fact, the silica-supported hydrides were discovered by Yermakov et al. [2] within this context and they display good polymerization activities [79–83]. This area of research has already been extensively reviewed and is not directly related to green chemistry and will therefore not been discussed here. This section will be solely devoted to homologation or cyclization processes. 6.4.2.1 Direct Conversion of Ethene into Propene Following the discovery of the alumina-supported tungsten hydrides, an alkane metathesis catalyst (see below) [23, 83], it has been shown that this system transforms ethene directly into propene at 150 C with 95% selectivity (Scheme 6.10) [84]. Kinetic studies have shown that in fact this single-site catalyst performs three reactions successively: step 1, dimerization of ethene into butenes (mainly 1-butene); step 2, isomerization of 1-butene into 2-butene; and step 3, cross-metathesis of 2-butene with ethene yielding propene.

6.5 Metathesis, Dimerization, Trimerization and Other Reactions Involving Alkynes

Scheme 6.10

6.4.2.2 Cyclization of Dienes The silica-supported zirconium hydrides (see Scheme 6.2) also catalyzes carbon–carbon bond-forming reactions, converting dienes into cyclic products (Scheme 6.11) [26]. For instance, 1,5-hexadiene is converted into 2-methylcyclopentene via the successive insertion of the diene into the hydride followed by cyclization and b-H elimination releasing methylenecyclopentane, which is then isomerized into the final product.

Scheme 6.11

6.5 Metathesis, Dimerization, Trimerization and Other Reactions Involving Alkynes 6.5.1 Alkyne Metathesis

Alkyne metathesis is analogous to alkene metathesis, but requires alkylidyne propagating species (Scheme 6.12a) [85]. In fact, the Re-based silica-supported

j129


130

catalyst presented in Section 6.3, which contains an alkylidyne ligand, also catalyzes the metathesis of 2-pentyne into 2-butyne and 3-hexyne, albeit with low turnovers (Scheme 6.12b) [52]. Recently, Weissman et al. reported that [(SiO)Mo(CEt) (NtBuPh)2], prepared by grafting [Mo(CEt)(NtBuPh)3] on silica, is a highly active catalyst in alkyne metathesis (Scheme 6.12c) [86]. Grafting the dinuclear amido W complex [(Me2N)3WW(NMe2)3] on SiO2-(700) also generates a well-defined system [(SiO)(Me2N)2WW(NMe2)3], but it is only poorly active in alkyne metathesis (Scheme 6.12d). However, upon addition of 5 equiv. of tBuOH, this material catalyzes this reaction very efficiently, converting 50 equiv. of 4-nonyne in 30 min, in comparison with the 20 min required for [(tBuO)3WCtBu] [87].

Scheme 6.12

6.5.2 Dimerization and Trimerization of Alkynes

Dimerization of alkynes yielding dienes can be catalyzed by the silica-supported silylamide lanthanide complexes [(SiO)Ln{N(SiMe3)2}2]. These systems are slightly less active than their homogeneous precursors (Scheme 6.13a) [41, 88]. Noteworthily, the lanthanum and neodymium surface species are more selective for dimerization than oligomerization and, in contrast to the homogeneous system, head-to-head dimerization is favored. Finally, the catalytic trimerization of alkynes into aromatics has been reported using silica-supported zirconium hydrides (Scheme 6.13a) [26].

6.6 Lewis Acid-catalyzed Reactions

Scheme 6.13 (a) Dimerization of phenylacetylene on silicasupported lanthanide and Group 3 amido complexes (selectivity in dimer); (b) trimerization of butylacetylene on silica-supported zirconium hydrides.

6.5.3 Hydroamination of Alkynes

Well-defined PdII surface species, cis- and trans-[(SiO)Pd(X)L2], are prepared by reaction of the corresponding methyl molecular complexes [L2Pd(X)(Me)] {X ¼ Cl, OTf and NO3; L ¼ PMe3, PPh3, dmpe and dppe} with SiO2-(500) (Scheme 6.14). These systems catalyze the cyclic hydroamination of g-aminoalkynes and the best catalyst is trans-[(SiO)Pd(NO3)L2], which can be recycled up to three times using ca 1 mol% of Pd [89, 90].

6.6 Lewis Acid-catalyzed Reactions 6.6.1 Silica-supported Group 4 Metals

Well-defined supported Group 4 alkoxides based on Ti or Zr are readily available by several routes. First, the monosiloxy derivatives are typically prepared with silica partially dehydroxylated above 500 C (Scheme 6.15a). Well-defined mononuclear systems have been obtained by grafting tetrakisalkyl [91–93], amido [94] or siloxide [95] complexes on silica followed by an alcoholysis step for the amido or alkyl surface

j131


132

Scheme 6.14

complexes. Note that grafting directly alkoxide derivatives generates polynuclear surface species [94]. The preparation of bis-siloxy systems is carried out using the same strategy (Scheme 6.15b), but using a silica dehydroxylated at lower temperatures, i.e. 200–300 C [94, 96], or a mesoporous silica, which generates the bis-siloxy system even when the pre-treatment of silica was 500 C [97, 98]. For tris-siloxy systems, this requires a thermal treatment and can be performed via two strategies (Scheme 6.15c): (1) formation of the hydrides followed by treatment with various reagents such as N2O [17], H2O [93], ROH [91–93], CO2 [17] or acetylacetone [99], and (2) calcination, generating isolated sites (Scheme 6.15d). Their Lewis acid properties have been exploited to catalyze a variety of reactions: the transfer hydrogenation of ketones (Equation 6.1), the transesterification of esters (Equation 6.2) and the selective epoxidation of alkenes by alkyl peroxide or hydrogen peroxide (see Sections 6.7.1 and 6.7.2). ð6:1Þ

ð6:2Þ

6.6 Lewis Acid-catalyzed Reactions

Scheme 6.15

6.6.1.1 Reduction of Ketones Through Hydrogen Transfer Cyclohexanone is reduced selectively to cyclohexanol by 2-propanol in the presence of a catalytic amount of [(SiO)M(OiPr)3] (Meerwein–Ponndorf–Verley reaction) with the order of reactivity Hf (73%) > Zr (50%) Ti (0%), whereas 4-methyl-2-pentanone is not converted with any of the catalysts [91, 93]. Similarly, benzaldehyde is reduced to benzyl alcohol with the same order of reactivity: Hf (90%) > Zr (65%) Ti (0%). Noteworthily, whereas Ti leaches from the silica

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134

support, no leaching has been observed with Zr and Hf. In fact, both catalysts can be recycled even if slight deactivation has been observed. In the case of Zr, it was clearly shown that the Zr(OiPr)4 does not catalyze this reaction and that preparation of the catalyst [(SiO)Zr(OiPr)3] via [(SiO)Zr(CH2tBu)3] followed by treatment with iPrOH provides a more stable catalyst than by direct grafting of Zr(OiPr)4, and this has been associated with the formation of polynuclear species in the latter case. In the case of Hf, it was shown that water had no detrimental effect on the catalytic behavior of [(SiO)Hf(OiPr)3] and in fact [(SiO)Hf(OH)3], prepared by the controlled hydrolysis of [(SiO)Hf(CH2tBu)3], shows similar catalytic performance. 6.6.1.2 Transesterification of Esters The silica-supported complexes [(SiO)Zr(acac)3] and [(SiO)3Zr(acac)] also display high activities in the transesterification of methyl methacrylate, albeit lower than that of the parent molecular system [Zr(acac)4] [99, 100]. Moreover, while leaching is observed for the mono-grafted species, the tris-grafted species is stable under the reaction conditions. In the latter case, it has been shown that the loss of activity is due to the build-up of the corresponding butanolate derivatives [(SiO)3Zr(OBu)], which can be reactivated by the addition of acetylacetone. 6.6.2 Silica-supported Group 3 Metals and Lanthanides

Group 3 and lanthanide metals are well known for their Lewis acid properties and their associated catalytic properties. Therefore, several materials have been prepared by grafting various sources of lanthanide derivatives on mesoporous silica (MCM-41) as the silicic support [101, 102]. First, the reaction of the silyl amide derivatives [Ln{N(SiR2H)2}3(thf)2] with MCM-41 yields [(SiO)xLn{N (SiR2H)2}3–x(thf)y] (Ln ¼ Sc, Y, Nd, La; x ¼ 1 or 2) [103–106], which, upon treatment with alcohol or fodh, gives [(SiO)xLn(OR)3–x(thf)y] or [(SiO)xLn(fod)3–x(thf)y], respectively (Scheme 6.16a). In these cases, note that the surface is also covered by (Me2SiHO) groups, because of the competitive subsequent reaction with surface

Scheme 6.16

6.7 Oxidation

silanols of HN(SiR2H)2, liberated during grafting. In contrast, the reaction of (fod)3Y with MCM-41 gives [(SiO)Y(fod)2] as the sole surface species (Scheme 6.16b) [107]. The surface alkoxide species display good catalytic activities for the transfer hydrogenation reaction (Equation 6.1) [108]. In the case of the fod derivatives, they have been used to catalyze selectively the hetero Diels–Alder reactions of benzaldehyde with trans-1-methoxy-3-trimethylsiloxy-1,3-butadiene yielding the silylenol ether with no sign of formation of the a,b-conjugated ketone resulting from acidcatalyzed side reactions when the catalysts were prepared by the silylamide routes (Scheme 6.17). In contrast, using a catalyst prepared directly from (fod)3Y and MCM-41 gave only the a,b-conjugated ketones, which shows the importance of the partial passivation of the silica surface [107]. Asymmetric Diels–Alder catalysts were prepared by reacting the MCM-41-supported silylamide lanthanide complexes with chiral ligands such binol, menthol and ephedrine. The reactivity of the supported complexes was enhanced compared with their homogeneous precursors, but so far only low enantiomeric and diastereomeric excesses have been obtained [109].

Scheme 6.17

6.7 Oxidation 6.7.1 Single-site Titanium Species

In industry, one of the key epoxidation processes, the production of propene oxide from propene, uses a catalyst based on TiX4 supported on SiO2 and ROOH as primary oxidants (Equation 6.3) [110]. ð6:3Þ After more than 30 years of research, the nature of the active site is still not known, but several studies on the basis of various models (well-defined heterogeneous catalysts prepared via various methods or soluble analogues) agree on the fact that the active sites are probably isolated Ti centers, probably triply bonded to the surface of silica (tripodal system, Scheme 6.18a) [92, 95, 96, 98, 111–115]. This type of active sites probably corresponds to a compromise between an accessible titanium center

j135


136

and an increase in the electrophilicity of the metal center through the presence of the right number of siloxy substituents. Moreover, the fourth ligand, not coming from the surface, called the capping ligand (Scheme 6.18b), is also critical for combining good catalytic activity, selectivity and stability for these catalysts. Stabilization of the catalyst can also be achieved by using a calixarene scaffold (Scheme 6.18c) [116, 117]. Finally, in studies devoted to the replacement of alkyl peroxides (ROOH) by hydrogen peroxide, it has been shown that the capping ligand and the hydrophobicity of the silica are critical for obtaining both high activity and selectivity (Scheme 6.18d) [118–124]. The ultimate case corresponds to TS-1, where the Ti is surrounded by four siloxy ligands of the crystalline lattice of a zeolite [125], and in fact a system related to TS-1 has been implemented recently in an industrial process for the epoxidation of propene by H2O2 (Equation 6.4) [126].

Scheme 6.18

ð6:4Þ

6.7 Oxidation

6.7.2 Single-site Zirconium Species

Similarly to the case of Ti, it was shown that tripodal isolated zirconium sites are also the best choice for catalyzing the epoxidation of alkenes. For instance, the oxidation of cyclohexene can be carried with H2O2 to give mainly the epoxide (60–70%) along with the diols (5–8%) and the compounds resulting from allylic oxidation (10–35%) [127]. 6.7.3 Single-site Vanadium Species

In several selective alkane oxidation reactions, e.g. the oxidative dehydrogenation of alkanes to alkenes and the selective oxidation of methane, VO4 species have been involved, but it is not clear yet – despite extensive studies – whether or not these species are necessarily isolated under the reaction conditions (small vanadium oxide clusters), even though high dispersion seems to be critical [128–132]. Here, only an example will be discussed, because it relies on the grafting of well-defined mononuclear precursors on silica [133]. The reaction of [{(tBuO)3SiO}3VO] and [(tBuO)3VO] with a mesoporous silica (SBA) generates the corresponding mononuclear species [(SiO)VO{OSi(OtBu)3}2] and [(SiO)VO(OtBu)2], respectively (Scheme 6.19). Calcination of these species yielded oxidation catalysts having isolated VO4 units for loading as high as 0.47 V nm2. These catalysts perform the oxidation of methane to formaldehyde by O2 with higher selectivity (30–40%) and activity (up to 0.48 mol CH4 mol V1 s1) than those prepared by standard techniques based on polyvanadates.

Scheme 6.19

6.7.4 Single-site Tantalum Species

Using the same strategy as for Ti, a well-defined Ta siloxide [(iPrO)2Ta{OSi(OtBuO)3}3] complex was grafted on a mesoporous silica, namely SBA, yielding isolated Ta centers (Scheme 6.20a) [134]. This material displays good activity in the oxidation of cyclohexene by H2O2 (6.7 mol oxidation products mol1 Ta min1), other oxidants such as TBHP being less efficient. However, the selectivity of this catalyst for

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cyclohexene oxide is low (36.0%), cyclohexenol (32.7%) and cyclohexenone (31.2%) q being concomitantly formed, probably via a radical-type mechanism (HO ). Calcination of the material as a way to stabilize the active sites (Scheme 6.20b) does not improve the catalyst performances (lower activities and selectivities), but further treatment of this calcined material with various silylylating agents, e.g. RMe2SiNMe2, to obtain capped Ta active sites improved the activity and, more importantly, increased the selectivity in epoxide to 98–99% (Scheme 6.20c) [135]. This study also strongly suggests that the capping agent stays during the catalytic test and that it is essential to obtain high activities; in fact, removing the cap by calcination induces a loss of stability of the system (Scheme 6.20d).

Scheme 6.20

6.7.5 Single-site Group 6 Species

Epoxidation with homogeneous Mo-based catalysts has also been industrially important and strategies to obtain the corresponding heterogeneous catalysts have been investigated. For instance, isolated Mo centers can prepared by reacting [Mo(N) (OtBu)3] on SiO2-(700) or MCM41-(500). During grafting, the tBuO ligand is replaced by a surface siloxy ligand to give [(SiO)Mo(N)(OtBu)2] (Scheme 6.21a) [136]. These systems are highly active and selective in the epoxidation of cyclohexene by tertbutyl hydroperoxide and are much more stable than the parent molecular precursors. However, these systems deactivate through leaching of Mo. Similarly, the molecular complexes [M(¼O)(OSiOR3)4] (M ¼ Mo and W) react with the silanol of a mesoporous silica (SBA-15) to generate the corresponding isolated oxo species [(SiO)M(¼O)(OSiOR3)3] (Scheme 6.21b), which are both highly active epoxidation catalysts [137].

6.7 Oxidation

Scheme 6.21

6.7.6 Single-site Iron Species

Fe-based heterogeneous catalysts have also been identified as interesting oxidation catalysts with the discovery of the direct and selective oxidation of benzene to phenol by N2O in the presence of Fe silicalite (Equation 6.5) [138, 139].

ð6:5Þ

Several studies have therefore investigated ways to obtain single-site Fe species at the surface of silica supports. First, well-defined FeIII surface complex [(SiO)Fe(OSi (OtBu)3)2thf ] have been prepared by grafting [Fe(OSi(OtBu)3)3thf ] on SBA-15 material (Scheme 6.22a) [140]. Using the same approach, a well-defined FeII surface complex [(SiO)Fe(OSi(OtBu)3)] has been prepared by grafting [{Fe(OSi(OtBu)3)2}2] on SBA-15 material (Scheme 6.22b). In both cases, further calcination of this material at 300 C leads to the formation of isolated FeIII species in a tetrahedral environment (Scheme 6.22c). These isolated Fe species catalyze the oxidation of aliphatic and aromatic C–H bonds by H2O2, transforming benzene into phenol (Equation 6.6); toluene into mixtures of cresols (Equation 6.7), benzyl alcohol and benzaldehyde (Equation 6.8); adamantane into the corresponding alcohols and ketones (Equation 6.9); and cyclohexene into cyclohexenol and cyclohexenone with only traces of epoxide (0–1%) (Equation 6.10) [140, 141]. A more selective cyclohexene epoxidation catalyst with H2O2 is obtained when using a well-defined FeII species having N ligands (Scheme 6.22d); however, the epoxide selectivity is still low (24%), the selectivity in

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140

Scheme 6.22

hexenol (32%) and hexenone (34%) still being very high as a result of Fenton-type chemistry [142]. ð6:6Þ

ð6:7Þ

6.8 Alkane Homologation

ð6:8Þ

ð6:9Þ

ð6:10Þ

It is possible, however, to catalyze the selective epoxidation of propene with N2O with silica-supported Fe isolated species (Equation 6.11) [143–145]. The best system is prepared by impregnating silica with FeIICl2 (0.15 wt%) and Rb2SO4 (10/Fe), followed by a calcination step. The combined use of ESR, UV–visible and Raman spectroscopy is consistent with the presence of isolated Fe species, Rb2SO4 being essential to avoid clustering of Fe into Fe2O3 and to guarantee site isolation, which is necessary to obtain good activities and selectivities [145]. ð6:11Þ

6.7.7 Single-site Cobalt Species

The selective oxidation of alkanes and aromatics is still a challenge today and necessitates the development of better catalysts. Using a silica-supported single-site Co catalyst, prepared by grafting [Co(OSi(OtBu)3)2Bipy2] (Bipy ¼ 4,40 -di-tBu-bipyridine) on SBA-15, the air oxidation of ethylbenzenes into the corresponding acetophenones has been achieved with good selectivities (82–100%) and no leaching (Scheme 6.23) [146].

6.8 Alkane Homologation 6.8.1 Alkane Hydrogenolysis

The hydrogenolysis of alkanes typically requires high reaction temperatures, but when silica-supported early transition metal hydrides are used, this catalytic reaction takes place at relatively low temperatures (50–150 C) [15, 18, 147, 148] The product selectivity depends on the metal: hydrogenolysis of propane yields a 1 : 1 mixture of ethane and methane the silica-supported zirconium hydrides, a Group 4 metal, while

j141


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Scheme 6.23

methane is the final product with the silica-supported tantalum hydrides, a Group 5 metal (Table 6.6). The difference in selectivity observed for various alkanes is consistent with a different carbon–carbon cleavage mechanism for both metal:balkyl transfer for Group 4 metals [20, 147, 149–151] and a-alkyl transfer for Ta (Scheme 6.24) [152].

Scheme 6.24

The silica-supported zirconium hydride catalyst has also been used to carry out the hydrogenolysis of a wide range of paraffinic materials, including polyalkenes, which can transformed into lower alkanes under mild conditions [149, 153]. Note that the same catalyst can polymerize alkenes and this illustrates nicely the principle of microreversibility. While depolymerization is indeed highly endothermic and necessitates elevated temperatures, the use of hydrogen allows the equilibrium to be displaced via the hydrogenation of the resulting alkenes.

6.8 Alkane Homologation Table 6.6 Hydrogenolysis of alkanes catalyzed by silica supported metal hydrides.

Catalyst

Alkane

Activitya

[ZrH/SiO2] [ZrH/SiO2]

Ethane Propane

0 70

[ZrH/SiO2]

Isobutane

70

[ZrH/SiO2]

Butane

66

[ZrH/SiO2]

Neopentane

66

[TaH/SiO2] [TaH/SiO2]

Ethane Propane

30 12

[TaH/SiO2]

Isobutane

24

[TaH/SiO2]

Butane

18

[TaH/SiO2]

Neopentane

6

Extrapolated selectivity at 0% conversion

Final product selectivity

— CH4 (50%) C2H6 (50%) CH4 (52%) C3H8 (47%)b CH4 (23%) C3H8 (20%)c CH4 (54%) C4H10 (40%)d CH4 (100%) CH4 (55%) C2H6 (45%) CH4 (55%) C2H6 (10%) C3H8 (35%) CH4 (42%) C2H6 (38%) C3H8 (20%) —e

— CH4 (50%) C2H6 (50%) CH4 (67%) C2H6 (33%) CH4 (60%) C2H6 (40%) CH4 (75%) C2H6 (25%) CH4 (100%) CH4 (100%) CH4 (100%)

CH4 (100%)

CH4 (100%)

a

Activity expressed in moles of propane transformed per mole of metal per hour. Ethane (1.5%). c Ethane (54%). d Propane (5%) and ethane (1%). e Since isobutane is hydrogenolyzed faster than neopentane, selectivity at 0% conversion is difficult to measure in a batch reactor. b

6.8.2 Alkane Metathesis

Alkane metathesis transforms a given alkane into its lower and higher homologues (Scheme 6.25a) and it could therefore become an important process in the petrochemical industry. In fact, finding ways to produce higher alkane homologues has been a major focus of research in this area [154–156]. Alkane metathesis was first investigated via a two-step process combining classical heterogeneous dehydrogenation–hydrogenation catalysts and alkene metathesis catalysts, which allows overall for a given alkane to be converted into its lower and higher homologues [157–159]. This process, however, requires higher temperatures (300–400 C) and higher pressures (>10 bar). More recently, in 1997, alkane metathesis was reported using a single-site catalyst at low temperatures and low pressures, typically 150 C and 0.8–1 bar. The original catalyst was a silica-supported tantalum hydrides (Scheme 6.25b and Table 6.7) [160]. Based on structure–reactivity relationships and kinetic studies, it was shown that, in this case also, the reaction takes place via alkene metathesis, but probably involves

j143


144

Scheme 6.25

alkylidene hydride intermediates [161–163]. This has lead to an important research effort to find new catalyst based on Group 5 and Group 6 hydrides and alkylidene systems. Note that the silica-supported tantalum hydrides is like all hydrides prepared in two steps (see Section 6.3), involving first grafting of [Ta(¼CHtBu) (CH2tBu)3], generating a well-defined alkylidene system [(SiO)Ta(¼CHtBu) (CH2tBu)2] [164, 165], followed by treatment under H2 at 150 C yielding the hydride [TaH/SiO2] (Scheme 6.25b) [19]. In the case of W, whereas [(SiO)W(CtBu) (CH2tBu)2] is a highly active alkene metathesis catalyst [166], it is nearly inactive in alkane metathesis and the corresponding W hydrides ([WH/SiO2] prepared by

6.8 Alkane Homologation Table 6.7 Performances of single-site catalysts in the metathesis of propanea.

Catalystb

Initial activityc at 24 h CH4

[Ta(¼ CHtBu)(CH2tBu)3]/SiO2 [TaH/SiO2] [TaH/Al2O3] [W(CtBu)(CH2tBu)3]/SiO2 [WH/SiO2] [W(CtBu)(CH2tBu)3]/Al2O3 [WH/Al2O3] [W(CtBu)(CH2tBu)3]/SiO2–Al2O3 [WH/SiO2–Al2O3] [Mo(NAr)(¼CHtBu)(CH2tBu)2]/SiO2 [ZrH/SiO2] [ZrH/SiO2–Al2O3]

3.0 3.5 2.5 — — 1.8 8.5 0.7 8.5 1 —e —e

13 10 10 — 6 3 2 2 2 H4SiW12O40 (445 C) > H3PMo12O40 (375 C) > H4SiMo12O40 (350 C), the strongest acid H3PW12O40 being the most stable [4]. Figure 7.2 shows the TGA profile for H3PW12O40 hydrate [4]. Three main peaks can be observed: (1) a peak at a temperature below 100 C, corresponding to the loss of physisorbed water (a variable amount depending on the number of hydration waters in the sample); (2) a peak in the temperature range 100–280 C centered at about 200 C, accounted for by the loss of ca 6 H2O molecules per Keggin unit, corresponding to the dehydration of a relatively stable hexahydrate H3PW12O406H2O, in which the waters are hydrogen bonded to the acidic protons to form the [H2OH þ OH2] ions; and (3) a peak in the range 370–600 C centered at 450–470 C, which is due to the loss of 1.5 H2O molecules, corresponding to the loss of all acidic protons and the beginning of decomposition of the Keggin structure. For tungsten HPAs, the last loss is practically irreversible, which causes irreversible loss of catalytic activity. The decomposition is complete at about 610 C to form P2O5 and WO3, which exhibits an exotherm in DTA and DSC [3, 4]. The course of thermal decomposition of H3PW12O40 is shown in Scheme 7.1 [4].

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Scheme 7.1 Thermal decomposition of H3PW12O40 hydrate.

Various aspects of catalyst deactivation and regeneration are well covered in the literature [12, 13]. Coke formation is the most frequent cause of catalyst deactivation in heterogeneous acid-catalyzed organic reactions [13–16]. Much research has been carried out into coke formation on the catalysts for petrochemical processes such as catalytic cracking, reforming and hydrotreatment. The most studied catalysts include amorphous silica–alumina, zeolites and acidic alumina, and also those doped with metals such as palladium, platinum and nickel [13–16]. Catalyst regeneration (decoking) is usually carried our by coke combustion at 450–550 C [13–16]. For the oxide and zeolite catalysts possessing sufficient thermal stability, combustion of coke is an effective method to recover catalyst activity. Solid HPA catalysts in organic reactions suffer from deactivation by coking, similar to the conventional solid acid catalysts. However, little information is available about coke formation on HPA catalysts. The problem is that the standard catalyst regeneration by coke combustion is not applicable to HPA catalysts due to their low thermal stability. This makes coking the most serious problem for heterogeneous acid catalysis by HPAs [4, 8]. Other possible causes of HPA deactivation, such as poisoning, aggregation, dehydration and decomposition of HPA, could also play a role but not as crucial as coking, at least at the moderate reaction temperatures that are typical of acid catalysis by HPAs (100–300 C). The question is how to overcome the problem of coking and make heterogeneous acid catalysis by HPA sustainable. Several directions that may be instrumental to achieve this goal will be discussed here, namely developing HPA catalysts possessing high thermal stability, modification of HPA catalysts to enhance coke combustion, inhibition of coke formation on HPA catalysts during operation, reactions in supercritical fluids and cascade reactions using multifunctional HPA catalysis [8].

7.2 Development of HPA Catalysts Possessing High Thermal Stability

In recent years, there has been considerable activity in this direction, focusing mainly on oxide composites comprising tungsten(VI) polyoxometalates and niobium(V), zirconium(IV) and titanium(IV) oxides as an oxide matrix [17–24]. These composites are usually prepared by wet chemical synthesis, followed by calcination at 500–750 C, i.e. at temperatures considerably higher than the temperature of HPA decomposition. The materials thus made contain HPA precursors or HPA decomposition products, possessing Brønsted and Lewis acid sites of moderate strength. These materials have been found active in a range of Friedel–Crafts reactions, often

7.3 Modification of HPA Catalysts to Enhance Coke Combustion

with good catalyst recycling. However, their activity is considerably lower than that of the standard Keggin HPA catalysts. For example, the H3PO4–WO3–Nb2O5 (9 : 55 : 36 wt%) composite with a surface area of 58 m2 g1 has been prepared by interaction of (NH4)10W12O41, Nb(V) oxalate and H3PO4 in aqueous solution, followed by evaporation and calcination at 500 C [17]. It has been tested in the alkylation of anisole by benzyl alcohol (Scheme 7.2) to yield 94% of the alkylation product. The catalyst is reported to be recyclable many times without loss of its activity. However, it is less active than the H3PW12O40/Nb2O5 catalyst prepared by the usual impregnation of niobium(V) oxide with H3PW12O40, but the latter is not recyclable.

Scheme 7.2

Another composite has been obtained by impregnation of 15% H4SiW12O40 on zirconium(IV) oxide and calcination at 700 C [19]. From Raman spectra, it contains ZrO2-anchored monooxotungstate and possesses Brønsted and Lewis acid sites. This solid acid material is active in the acylation of veratrole by benzoic anhydride (Scheme 7.3), with no leaching and good catalyst recycling after regeneration by coke combustion at 500 C. However, this catalyst is less active than HY zeolite per gram of catalyst, whereas the standard HPA catalysts are usually much more active than the HY. Therefore, the composite materials based on W(VI) polyoxometalates and Nb(V), Zr(IV) and Ti(IV) oxides possess relatively weak acid sites and as solid acid catalysts have practically no advantage over acidic zeolites. Work should be continued to obtain HPA materials possessing stronger acid sites.

Scheme 7.3


Doping of solid acid catalysts with platinum group metals (PGM) such as palladium and platinum to enhance catalyst regeneration by coke combustion is well docu-

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mented [13, 14]. Typical examples are zeolite and alumina doped with Pd and Pt employed as the catalysts for alkane isomerization and cracking [13, 14]. It is suggested that the role of metal sites is twofold: on the one hand, they catalyze coke combustion, and on the other, they change the nature of coke depositing on the catalyst to make it more aliphatic and hence more easily combustible [14]. 7.3.1 Propene Oligomerization

PGM doping has been found to be effective for enhancing the regeneration of solid HPA catalysts for propene oligomerization [25, 26]. The effect of Pd doping on coke combustion can be seen from the TGA/TPO for the coked catalyst 20% H3PW12O40/ SiO2 (Figure 7.3) [25, 26]. The catalyst has been coked by propene in a fixed-bed flow reactor at 200 C. In the absence of Pd, coke burns at about 500 C. In the case of Pd-doped catalyst, this temperature decreases; the higher the Pd loading, the lower is the temperature of coke combustion. With 2% Pd doping, coke burns at 350 C, which is well below the decomposition temperature for H3PW12O40. Using XPS and 31 P MAS NMR, it has been shown that coking does not affect the structure of H3PW12O40 [26]. From XPS, the oxidation state of tungsten is 6þ in both the fresh and coked catalyst. The 31 P chemical shift is the same for the as-made, Pddoped and coked catalysts (about 15 ppm versus 85% H3PO4, as expected for H3PW12O40), which indicates that the Keggin structure remains intact. As shown by 13 C CP/MAS NMR spectroscopy of the coked H3PW12O40 catalysts, Pd doping does affect the nature of coke depositing on the catalyst [25]. On the undoped catalyst, both aliphatic (soft) and polyaromatic (hard) coke are formed. In contrast, the Pd-doped catalyst builds only aliphatic coke, which will burn off more easily. From these results, the effect of palladium in HPA catalysts appears to be the same as in alumina or

Figure 7.3 TGA/TPO in air for Pd-doped 20% H3PW12O40/SiO2 coked by propene in a fixed-bed flow reactor at 200 C: (a) no Pd doping; (b) 1.6% Pd; (c) 2.0% Pd; (d) 2.5% Pd [25, 26].


Figure 7.4 Performance of fresh and regenerated 2.5% Pd/20% H3PW12O40/SiO2 catalysts in gas-phase propene oligomerization (fixed-bed flow reactor, 200 C, 2% C3H6 in N2, GHSV ¼ 6000 h1; in situ regeneration by air calcination at 350 C/2 h followed by reduction with H2 at 225 C/2 h) [26].

zeolite, i.e. Pd catalyzes the combustion of coke and inhibits the formation of hard polyaromatic coke, which is more difficult to burn off [25, 26]. Palladium doping has been found to be effective in enhancing in situ regeneration of silica-supported H3PW12O40 catalyst for the gas-phase oligomerization of propene [26]. The reaction has been carried out in a fixed-bed flow reactor, yielding C12–C18 oligomers as major products. The undoped and Pd-doped H3PW12O40 catalysts both have high initial activity, but suffer from fast deactivation due to coking. The Pd-doped catalyst can be regenerated by combustion of coke at 350 C to regain its activity fully (Figure 7.4), as expected from the TPO studies. In contrast, the undoped catalyst fails to recover its activity under such conditions. 7.3.2 Friedel–Crafts Acylation

Doping with palladium and platinum has also proved effective for regeneration and recycling of HPA catalysts for Friedel–Crafts acylation in liquid-phase batch processes [27–30]. This is illustrated by studies of Fries rearrangement of phenyl acetate, yielding acylated phenols (Scheme 7.4) [29, 30]. HPAs are very efficient solid acid catalysts for this reaction, much more active than H2SO4 and acidic zeolites. The bulk acidic salt Cs2.5H0.5PW12O40 (CsPW), which is insoluble hence easily recyclable, is an especially good catalyst for this reaction. However, CsPW is deactivated by carbonaceous deposit and requires regeneration. The TGA/TPO analysis of the coked CsPW after its use for the Fries rearrangement of PhOAc shows that coke combustion is complete at about 550 C (Figure 7.5). This temperature, however, is too high for the catalyst to retain its integrity. In the case of Pd-doped CsPW (2 wt% Pd), coke is already gone at 350 C, indicating that catalyst regeneration may be possible at this temperature [27].

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Scheme 7.4

Figure 7.5 TGA/TPO in air for coked catalysts CsPW and 2.1%Pd/ CsPW after use for Fries reaction of PhOAc (in nitrobenzene, 130 C, 2 h) [27].

Platinum doping has been found to be even more effective, enhancing coke combustion at a Pt loading as low as 0.3 wt% [28]. The TGA/TPO (Figure 7.6) shows two combustion peaks at a lower and higher temperature, which can be attributed to soft aliphatic coke and hard polyaromatic coke, respectively. PGM doping has been demonstrated to allow sustainable regeneration of solid HPA catalysts for Fries reaction [27, 28]. Figure 7.7 shows excellent recycling of the 0.3% Pt/CsPW catalyst in the Fries rearrangement of PhOAc. After each run, the catalyst has been separated and regenerated by air calcination at 350 C, followed by steaming at 200 C to restore the acid sites. As evidenced by FTIR spectroscopy, the Keggin structure of the CsPW remains unchanged after multiple catalyst regeneration and reuse [28]. It should be noted, however, that PGM doping could initiate side reactions, thus impairing the selectivity. Although no such effect has been observed in Friedel–Crafts acylation [27, 28], it may be the case in other reactions; therefore, care must be taken regarding the possible effect of PGM on reaction selectivity.

7.4 Inhibition of Coke Formation on HPA Catalysts

Figure 7.6 TGA/TPO in air for coked catalysts after use for Fries reaction of PhOAc (in nitrobenzene, 130 C, 2 h): (a) CsPW; (b) 0.3% Pt/CsPW; (c) 1% Pt/CsPW [28].

Figure 7.7 Catalyst reuse in Fries rearrangement of PhOAc: conversion and total acylation selectivity in successive runs [0.3% Pt/CsPW (2.3 wt%), in nitrobenzene, 130 C, 2 h] [28].

7.4 Inhibition of Coke Formation on HPA Catalysts

Catalyst regeneration is an expensive procedure. Obviously, it would be preferable to prevent the catalyst from coking in the first place to avoid its regeneration. Coke inhibition on HPA catalysts has been studied using propene oligomerization as a model reaction [25, 26]. The reaction occurs via the carbenium ion mechanism yielding propene oligomers and coke (Scheme 7.5). The oligomers may be considered as coke precursors. Addition of nucleophilic molecules, such as water, methanol

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Table 7.3 Effect of additives (7 vol.%) to propene flow on coke formation on 40% H3PW12O40/SiO2 at 150 C [26].

Additive

Time on-stream (h)

Amount of coke (%)

None H2O Methanol Acetic acid

3.0 3.0 3.0 3.0

3.6 0.5 1.7 2.6

and acetic acid, has been found to affect greatly the reaction selectivity by reacting with carbenium ion intermediates to yield oxygenates at the expense of the oligomers and coke (Scheme 7.5). Water has been found to be the most effective coke inhibitor, with the amount of coke decreased sevenfold compared with the background (Table 7.3) [26].

Scheme 7.5 Acid-catalyzed oligomerization of propene.

On an industrial scale, addition of water to the reactor feed has been proved effective to prolong catalyst lifetime in BPs Avada process for the synthesis of ethyl acetate [11, 31]. In this process, ethyl acetate is produced by interaction of acetic acid with ethene in the gas phase in the presence of tungstosilicic acid supported on silica as the catalyst (Scheme 7.6).

Scheme 7.6

In 2001, this process was commercialized on a scale of 220 000 t yr1 at Hull in the UK. This is the largest ethyl acetate plant in the world. Figure 7.8 shows a schematic flowchart for the Avada process. Ethene and acetic acid are fed through evaporator to the adiabatic fixed-bed reactor containing the catalyst H4SiW12O40/SiO2. In order to achieve sustained catalyst performance, steam (3–8 mol%) is added to the reactor. The addition of water leads to reversible formation of ethanol and diethyl ether as byproducts, which are recycled back to the reactor. The product ethyl acetate is obtained in several separation and purification steps. Table 7.4 compares the performance of H4SiW12O40/SiO2 with that of other solid acid catalysts, illustrating that the HPA is a highly active catalyst for the synthesis of ethyl acetate [31]. Addition of water is essential for the stable performance of HPA catalyst. Without water, the catalyst deactivates

7.5 Reactions in Supercritical Fluids

Figure 7.8 Flowchart for BPs Avada process [11].

Table 7.4 Solid acid catalysts for the synthesis of ethyl acetate from ethene and acetic acid [31].

Catalyst

C2H4:AcOH (mol:mol)

Temperature ( C)

Pressure (bar)

Contact time (s)

H2O in feed (mol%)

STY (g L1 h1)

H-montmorillonite XE386 resin Nafion-H H-Zeolite Y H4SiW12O40/SiO2

5:1 5:1 5:1 5:1 12 : 1

200 155 170 200 180

50 50 50 50 10

4 4 4 4 2

0 0 0 0 6

144 120 102 2 380

quickly due to extensive formation of coke. The effect of water on the catalyst in this process is probably manifold. In addition to coke inhibition, water stabilizes HPA by preventing dehydration. This simple remedy has allowed BP to achieve an economically viable lifetime of their HPA catalyst [11, 31]. It should be pointed out, however, that the addition of water is not a universal cure. Although effective in the watertolerant ethyl acetate process, it is unlikely to work in other reactions that are incompatible with water such as Friedel–Crafts acylation and alkane isomerization.

7.5 Reactions in Supercritical Fluids

Heterogeneous catalysis in supercritical fluids offers considerable benefits (for a review, see [32]). The use of supercritical fluids can greatly intensify mass and heat transfer, thus enhancing the reaction rate and selectivity and also product separation.

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On top of that, supercritical methodology can facilitate catalyst regeneration and increase catalyst lifetime. Supercritical fluids possess unique solvent properties which have long been utilized in separation technologies such as extraction and chromatography and are now gaining increasing interest for application in catalytic synthesis. Supercritical fluids are miscible with gases and can dissolve solids and liquids. Usually, the supercritical methodology is applied in the region near the critical point, (1.0–1.2) Tc and (1–2) Pc, where Tc and Pc are the critical temperature and pressure of the fluid, respectively. In this region, densities are close to or above the critical density of the fluid and the dissolution power of the fluid is at its maximum. Supercritical fluids exhibit considerably higher solubilities than the corresponding gases for heavy organic compounds which may deactivate catalysts and promote coking. Changing the process conditions from gas phase to dense supercritical medium can suppress this deactivation. Furthermore, enhanced diffusivity in a supercritical system can accelerate the transfer of coke precursors from the catalyst surface hence reduce the amount of coke formed. It has been reported that the lifetime of solid HPA catalysts can be significantly longer in supercritical systems than in conventional gas or liquid systems and regeneration of HPA catalysts deactivated by coking can be accomplished in supercritical systems by extracting the carbonaceous deposits from the catalyst surface [33–36]. The isomerization of n-butane has been studied in supercritical n-butane in a fixed-bed flow reactor using 20% H3PW12O40/TiO2, 20% H4SiW12O40/ TiO2 (260 C, 110 bar), sulfated zirconia (215 C, 61 bar) and H-mordenite (300 C, 138 bar) as the catalysts [34, 35]. Gas-phase isomerization on these catalysts suffers from rapid deactivation due to catalyst coking. In contrast, the supercritical system shows stable activity without catalyst deactivation, for more than 5 h on-stream in the case of HPA/TiO2. The catalysts coked in the gas-phase isomerization can be regenerated in the supercritical system at an n-butane density close to its critical value to regain almost fully their initial activity. The isomerization on HPA/TiO2 and sulfated zirconia in supercritical n-butane provides 80% selectivity to isobutane at 20–25% conversion. H-mordenite gives

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