Zeolite Synthesis (ACS Symposium Series 398)

ACS SYMPOSIUM SERIES 398

Zeolite Synthesis Mario L. Occelli, EDITOR Unocal Corporation

Harry E. Robson, EDITOR Louisiana State University

Developed from a symposium sponsored by the Division of Colloid and Surface Chemistry at the 196th National Meeting of the American Chemical Society, Los Angeles, California, September 25-30, 1988

American Chemical Society, Washington, DC 1989

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Library of Congress Cataloging-in-Publication Data Zeolite synthesis/ Mario L. Occelli, editor; Harry E. Robson, editor. p. cm.—(ACS Symposium Series, ISSN 0097-6156; 398). "Developed from a symposium sponsored by the Division or Colloid and Surface Chemistry at the 196th National Meeting of the American Chemical Society, Los Angeles, California, September 25-30, 1988." Includes index. ISBN 0-8412-1632-0 1. Zeolites—Congresses. I. Occelli. Mario L., 1942- . II. Robson, Harry E., 1927- . III. American Chemical Society. Division of Colloid and Surface Chemistry. IV. American Chemical Society. Meeting (196th: 1988: Los Angeles, Calif.). V. Series. TP245.S5Z387 1989 549'.68—dc20 89-6884 CIP

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In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

ACS Symposium Series M. Joan Comstock, Series Editor 1989 ACS Books Advisory Board Paul S. Anderson Merck Sharp & Dohme Research Laboratories

Mary A. Kaiser Ε. I. du Pont de Nemours and Company

Alexis T. Bell University of California—Berkeley

Michael R. Ladisch Purdue University

Harvey W. Blanch University of California—Berkeley Malcolm H. Chisholm Indiana University

John L. Massingill Dow Chemical Company Daniel M. Quinn University of Io wa

Alan Elzerman Clemson University

James C. Randall Exx Inc.

John W. Finley Nabisco Brands, Inc.

Wendy A. Warr Imperial Chemical Industries

Natalie Foster Lehigh University

Robert A. Weiss University of Connecticut

Marye Anne Fox The University of Texas—Austin G. Wayne Ivie U.S. Department of Agriculture, Agricultural Research Service

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Foreword The ACS S Y M P O S I U M S E R I E S was founded in 1974 to provide a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing A D V A N C E S IN C H E M I S T R Y S E R I E S except that, in order to save time, the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously pub­ lished papers are not accepted. Both reviews and reports of research are acceptable, because symposia may embrace both types of presentation.

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Preface THE

FIRST ZEOLITE, STILBITE, W A S D I S C O V E R E D I N 1756 by Baron Cronstedt, a Swedish mineralogist. He named these types of minerals zeolites from the Greek words zeo (boil) and lithos (stone), because when gently heated, the stones evolved water vapor. Because of the multiplicity of properties possessed by natural zeolites, it is not surprising that extensive attempts at their synthesis began so long ago. In fact, efforts to achieve the hydrothermal synthesis of analogs of natural zeolites date back to 1845, although the elevated temperatures and pressures employed and the lack of proper identification techniques precluded a high degree of success for more than a century. The bulk of successful work began in the 1940s when X-ray diffraction provided easy product identification and R. M. Barrer developed the gel synthesis. This approach was based on starting with very reactive components in closed systems and employing temperature and crystallization conditions that were more typical of the synthesis of organic compounds than of mineral formation. Zeolite Synthesis, and the symposium on which it is based, is a review of the progress that, to date, has been made toward under­ standing the various aspects of thisfieldon a molecular level. By 1959, under the leadership of R. M. Milton, the Linde Division of Union Carbide had successfully synthesized nearly all the commercially important zeolites. Thefirstchapter of this volume is a personal account by Milton of how Union Carbide pioneered the synthetic molecular sieve zeolite business. In the synthesis area, results have indeed been impressive. Of the 35 now-recognized naturally occurring zeolites, 24 have been duplicated in the laboratory. In the process, more than 200 new synthetic phases have been discovered, including VPI-5, ZSM-5, and ALPO, a new family of molecular sieves. Today, ZSM-5 is considered to be one of the most important catalytic materials to be found since the cracking properties of faujasite were established in the early 1960s by Plank and Rosinsky. Aluminophosphates, such as ALPO, SAPO, and MEAPO, equip the chemist with an almost endless supply of crystalline molecular sieves with unique composition and structural characteristics. Synthesis of VPI-5, thefirst18-membered ring molecular sieve, suggests that many more new and technologically important molecular sieves (some of which exist already as models) await to be synthesized.

xi In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Until fairly recently, zeolite synthesis has been mainly an empirical science in which a large number of experiments were used to systematically change synthesis parameters in the hope of obtaining new phases and crystal composition. Good luck and the execution of the right "mistakes" were thought by many to be essential to the synthesis and discovery of new zeolites. This Edisonian approach to zeolite synthesis is now being gradually replaced by methods based on the use of new characterization techniques that will provide a better understanding of gel chemistry, zeolite nucleation, crystal growth, crystallization kinetics, and structure-directing phenomena.

The two founders of zeolite synthesis technology, R. M. Barrer (right) and R. M. Milton (left), September 22,1988, Los Angeles, CA (photo by Mario L. Occelli).

A great part of the success of the symposium on zeolite synthesis can be attributed to the generous contributions from several industrial sponsors and to the support of the Division of Colloid and Surface Chemistry of the American Chemical Society and of the International Zeolite Association. A special acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support provided during the early stages of this project. xii In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

We would also like to thank D. E . W. Vaughan, Ε. M. Flanigen, T. Inui, D. Bibby, and G. Volyocsik for helping to chair the symposium, and to express our gratitude to the many colleagues who acted as technical referees. Mario L. Occelli is particularly grateful to Unocal for permission to participate in and complete this project, and to G. Smith for her invaluable secretarial help. Finally, we would like to thank the authors of Zeolite Synthesis for the time and effort they gave to presenting their research at the symposium and preparing the manuscripts for this book. MARIO L. OCCELLI

Unocal Corporation Science and Technology Division Brea, CA 92621 HARRY E. ROBSON

Department of Chemistry Louisiana State University Baton Rouge, LA 70803 February 16, 1989

xiii In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Chapter 1

Molecular Sieve Science and Technology A Historical Perspective 1

Robert M. Milton

5991 Set-N-Sun Place, Jupiter, FL 33458

Union Carbide pioneered the synthetic molecular sieve zeolite busi­ ness, initiating research in 1948, entering the market in 1954, and turning a profit on an annual basis beginning in 1960 with an up­ front cost of a little over $7 million. Based on this effort, Union Carbide dominated the field of synthetic molecular sieves for many years. This state-of-the-art volume on zeolite synthesis represents a giant step from 1949, when I first began hydrothermal synthesis studies on zeolites at Union Carbide. Although this chapter is not intended to provide a thorough, historical perspective, it should give readers a view of the atmosphere and the people contributing to the initial development of the field of zeolite synthesis.

The work leading to Union Carbide's success began in 1948 after I had been at Linde's Tonawanda Research Laboratory for nearly two years. Management asked me to investigate physical adsorption as a potentially useful methodology in the purification and separation of air. At that time the only group working actively in zeolites was R. M . Barrels group abroad. This introduction represents a condensed history of Union Carbide's work on molecular sieve zeol­ ites from discovery to commercial success. New Zeolite Synthesis Methodology In the fall of 1948, I was measuring the adsorption characteristics of numerous commercial adsorbents and of the natural zeolite, chabazite. Several uses for silica gel in air separation plants were identified. But the more we learned about chabazite, the more intrigued I became by its potential as a commercial adsorbent as well as its possible use in air purification and separation. I envisioned, as others had before me [1-5], major new separation processes based on a series of different pore size zeolites. The stumbling blocks were that (1) chabazite was the only known zeolite with seemingly practical adsorption ^ O T E : Retired from Union Carbide Corporation, 39 Old Ridgebury Road, Danbury, CT 06817 0097-6156/89/0398-0001$06.00/0 ο 1989 American Chemical Society In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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ZEOLITE SYNTHESIS

characteristics, (2) large deposits of the mineral had never been found, and (3) although many had tried, no one had succeeded in synthesizing chabazite [6]. I requested and received permission for a limited exploratory zeolite synthesis pro­ gram in mid 1949. We started hydrothermal syntheses in September 1949, using relatively inso­ luble forms of silica and alumina in mildly alkaline solutions (pH 8-11) contain­ ing calcium, magnesium, and sodium cations at temperatures of 200°-300 C . Reaction periods varied from a few days to a week. In some experiments there was no reaction; in others we made analcime or small pore mordenite. These had been made by others under similar conditions and neither was considered a commercially useful adsorbent [7, 8]. Our first change in synthesis procedures was to repeat some of the earlier experiments while applying a hydrostatic pressure of about 2,000 psig. This addi­ tional pressure had no apparent effect on zeolite crystallization. By October 1949, I started experimenting with crystallization at 100 C , rea­ soning that the higher water content zeolites with larger pore volumes and, presumably, larger pore sizes, would be more likely to crystallize at temperatures lower than 200°-300 °C. In nature the anhydrous aluminosilicates were formed at relatively high temperatures, and the hydrous ones were believed to have been formed later as the earth's surface cooled. Not surprisingly, when we first tried low temperature synthesis with relatively insoluble silica and alumina in mildly alkaline solutions, there was no reaction in reasonable time periods. This was solved by using soluble forms of silica and alumina under highly alkaline conditions. We dissolved sodium aluminate in water, or dissolved alumina trihydrate in hot sodium hydroxide solutions and then mixed the aluminate solution with sodium silicate solutions. On mixing a gel usually formed. The gel was stirred thoroughly and placed in metal or glass containers, which were sealed and immersed in a 100 C water bath. The results were dramatic. Within a few hours, hydrated solid species usually settled out of the mother liquor and in most cases these were crystalline zeolites. The relative amounts of sodium oxide, alumina, silica, and water in the initial gel were key variables in determining what materials were formed. Temperature, gel stirring, and gel aging were also important variables. Following crystallization, the solid was separated from the mother liquor by filtration, washed with distilled water, and air dried in an oven at 100 °C to remove loosely bound water. Samples of the dried powder were sent routinely to the x-ray laboratory. The fact that we could obtain a strip chart recording of the x-ray powder pattern within 30 minutes was an important factor in the pace of our work. Adsorption evaluations were facilitated by use of multiple, quartz spring, McBain-Bakr balances connected in parallel. As many as 16 adsorbent samples could be evaluated simultaneously. By year end 1949 we had developed not only a new and widely applicable method for synthesizing zeolites but had discovered the A zeolite, the Β zeolite later shown to be gismondite, the C zeolite later identified as basic sodalite, and a crystalline impurity named X [9-10]. I first identified the X zeolite by its x-ray peaks as an impurity in the Β zeolite in 1949 at about a 20% concentration. We next saw it in February 1950 at about a 50% concentration with the Β zeolite when N . R. Mumbach brought in the x-ray pattern from his first attempt to scale up the synthesis of B. By mid 1950, I had discovered how to routinely make pure zeolite X [11]. Chabazite was synthesized in late 1950, and by mid 1951 I had made three new e

e

e

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Molecular Sieve Science and Technology

zeolites in the potassium-aluminosilicate system [12-14]. Adsorption properties of A and X had been determined including the discovery that the pore size could be reduced or enlarged by appropriate ion exchange with potassium, cal­ cium, or magnesium ions. Both A and X had proven stable at 500 C . [15,16]. By mid 1953 we had made a total of 20 crystalline zeolites, including erionite, gmelinite, and 14 with no know natural counterparts [15]. e

Crystal Structure In mid 1951 two young scientists joined the laboratory staff, Drs. T. B. Reed, a crystallographer, and D. W. Breck, an inorganic chemist. Reed had hoped to work with me on molecular sieves. In the applicant interview, I had chal­ lenged him to determine the detailed structures of the A and X zeolites from powder x-ray data, pore size and volume information, and ion exchange proper­ ties. He was assigned, however, to a different project in another group. Dr. Breck was assigned to my molecular sieve group. He quickly appreci­ ated the value of knowing the crystal structures of A and X if we were to fully understand their unique properties. Early in 1952 Reed and Breck decided to work together on the A and X structures. Progress was slow. Their studies were conducted frequently during lunch hour and in the evenings, since Reed had another full time assignment and Breck was busy with continuing zeolite synthesis and characterization studies. By March 1954 they achieved their final structure for the A zeolite, verifying it shortly thereafter with single crystal x-ray data on a 30 micron A crystal that Breck and Nancy A . Acara had recently grown [17]. By mid 1954, they also completed the structures of X and faujasite [18], with help from single crystal xray data on faujasite. This was completed two to six years before these struc­ tures were described by others [19-21]. Interestingly, a manuscript by Breck et al. on the synthesis, structure, and properties of X was rejected by the Journal of Physical Chemistry in 1958 on the basis of insufficient reader interest. These were remarkable accomplishments considering the size of the unit cells and the complexity of the structures. I remember posing the problem to Dr. Linus Pauling when he visited our laboraory in the mid fifties. Some 25 years earlier he had published on the structure of the zeolite natrolite and several feldspathoids [22]. He assured me that it was a waste of time to even try to determine the detailed structure of zeolites from powder x-ray data. X Yy Faujasite 9

In August 1952 Breck located the powder x-ray data for mineral faujasite and realized that it was very similar to that of the X zeolite. We obtained about 50 mg. of faujasite and studied it carefully. The x-ray pattern was indeed very similar to that of X . The adsorption capacity was somewhat lower but similar. The silica/alumina ratio was 4.7 compared to 2.5 for X . The cations in faujasite were calcium, magnesium, and barium, not sodium as in X . It was clear that X and faujasite were isostructural but with different compositions. Further similari­ ties and differences could not be studied at that time due to the limited supply of faujasite. In mid 1954 Breck proposed that it should be possible to synthesize the X structure with silica/alumina ratios as high as 4.7, found in faujasite, and possibly higher. He further hypothesized that the higher ratio materials, with lower aluminum and exchangeable cation content, would be more stable to acid attack

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and to heat in the presence of water vapor than the 2.5 ratio X . If true, this could be a valuable property as X had limited stability to the high temperature burn-off of carbon deposits necessary in regeneration of petroleum cracking catalysts and to acid containing gases such as cracker gases. N . A . Acara, Brack's synthesis assistant, collected samples of X made over the past few years and sent some of them in for chemical analysis. Heretofore most lots of X had never been analyzed. Those that had, always gave silica/alumina ratios very close to 2.5 with none higher than about 2.7. This analysis of old lots of X showed some lots with ratios as high as 2.83 and 2.92. With this background, Breck and Acara soon learned how to routinely synthesize X with ratios between 3.0 and 4.0. Later analyses of additional old lots of X showed ratios of 3.05, 3.47, and 3.48. We had made high silica X before but had not recognized it. Beginning in 1956, Ε. M . Flanigen learned how to make X with silica/alumina ratios between 4.0 and 5.7. Now with a full range of ratios from 2.5 to 5.7, we were able to study systematically the variation in properties with alumina content. Breck's original hypothesis proved to be correct. The high sil­ ica forms were more stable to acid attack and to high temperatures in the pres­ ence of water vapor than the low silica forms [23]. Since X had been defined in our patent applications as having silica/alumina ratios between 2.0 and 3.0, and because there was a significant change in proper­ ties at a ratio of 3.0, the isostructural zeolites with ratios above 3.0 and up to 6.0 were named and patented as zeolite Y . The Y zeolite was not introduced into the market place until we had time to file appropriate patents and evaluate it as a catalyst [24].

Patents Scientists do not usually get deeply involved in the intricacies of patent coverage, but it was essential in the case of the A and X zeolites because they involved concepts and science totally new to Union Carbide patent lawyers. During 1952 and 1953 I spent probably 20% of my time on patent matters. The first lawyer assigned to molecular sieves planned to base protection on the process of manufacture. Since we could not possibly cover all practical methods of making A and X , we asked for composition of matter coverage, with process claims only to protect our actual manufacturing methods, and broad use claims covering as many applications as possible, so as to minimize the possibil­ ity of restrictive use claims by others. Carbide's patent department had reservations because in their experience, composition of matter claims had to be drawn very narrowly to be valid and as such were frequently easy to circumvent. We convinced them that the unique properties that make A and X useful are singular results of their specific chemi­ cal composition and the arrangement of atoms in the crystal lattice of the zeol­ ites. To our knowledge, the use of powder x-ray data as a finger print to uniquely identify a specific crystal structure was a new concept in patent protec­ tion. Wfaen the first drafts of the patent applications came back from our patent department for checking, it was clear that major changes were needed. After several unsuccessful attempts to revise the original drafts, I had to rewrite both applications. At about that time a young patent lawyer, J. B. Browning, replaced the original attorney. He was quick to comprehend the significance of

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the project, approved the basic strategy, made several important suggestions to clarify the disclosure and simplify the claims, and the applications were filed on December 24, 1953. Both applications were rejected a number of times primarily based on the examiner's inability to understand the differences between our new A and X zeolites and prior art in non-crystalline zeolites used in water softeners and their unfamiliarity with compositions of matter identified by x-ray patterns. Finally Dr. Breck and I, armed with crystal models, models of variously sized adsorbate molecules, x-ray patterns, and comparative data on water softening zeolites, visited the examiner and his assistants in Washington. With these props we were able to explain molecular sieve adsorption, relate it to the crystal structures of A and X , and relate those to the x-ray patterns. The examiners understood, and after they removed the use claims for separate continuation-in-part filing, they issued the patents on April 14, 1959 [9,11].

Catalysis In September 1951, I authored an Idea Memorandum arguing that both the A and X zeolites should make good catalysts or catalyst supports, specifically men­ tioning hydrocarbon cracking, abnormally strong adsorption forces, molecular size selectivity, and the possibility of atomically dispersed metals on the internal sur­ faces. In March 1952, I discussed these ideas in a paper at a Union Carbide catalyst conference and later at our Bakélite Division laboratory. First results came from the Bakélite Division in 1953. They were catalytically cracking ditolyethane to make methyl styrene. They found that magnesium/hydrogen exchanged A and calcium/hydrogen exchanged A were both more active and more selective catalysts than the best commercial silica-alumina cracking catalysts. The pure sodium A form was inactive and the pure hydrogen exchanged form was unstable. Ditolyethane is too large a molecule to enter the A zeolite pore structure. In February and March of 1954, Breck and I prepared hydrogen exchanged X with greater than 60% of the sodium cations replaced, both by direct exchange with acid and by exchange with ammonium ions followed by heating to drive off ammonia. Both forms of hydrogen X were tested in cracking of ntetradecane and isopropylbenzene. Both forms were extremely active catalysts producing primarily C - C hydrocarbons from n-tetradecane and benzene and propylene from isopropylbenzene. This was the first discovery of the unique hydrocarbon cracking activity of the X structure, providing at least 60% of the sodium cations are removed from the lattice. A patent application [25] was filed on March 13, 1956 on the catalytic cracking of hydrocarbons with hydrogen X in the names of Milton and Breck. This application, too, was rejected by the examiner because he could not under­ stand wherein our crystalline zeolite X was different from the amorphous, water softening zeolites of the prior art. This objection should have been easily over­ come as it was in the basic A and X cases. For some unknown and still unex­ plained reason, however, this application was abandoned by our patent depart­ ment on June 30, 1959. By mid 1954 Dr. Breck and I had developed methods for dispersing metals on the inner surfaces of the A, X , and Y zeolites and had initiated catalytic stu­ dies with them [26-27]. Dr. C. R. Castor and S. W. Bukata were added to this effort; during 1954-1955 they carried out exploratory studies with platinum, other 6

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metals, and select metal oxides dispersed on the X and Y zeolites. Very active hydrocarbon hydrogénation and dehydrogenation catalysts were made, tested, and patented [28-31]. These catalytic studies were not continued at Linde as Research management decided to emphasize other zeolite programs. With this hiatus in catalytic research on zeolites at Carbide, we began encouraging outside groups to evaluate A and X as catalysts. In an invited paper on molecular sieves at the Gordon Research Conference on Separations in 1955, I concluded my presentation with an explanation of why I thought the A and X zeolites would show unusual catalytic effects- highly polar surfaces, unusually high heats of adsorption, molecular size selective availability of the internal surfaces, and the ease of dispersal of metal atoms on the internal sur­ faces. Dr. P.B. Weisz and another Mobil Oil scientist attending the confer­ ence questioned me at length after the paper. I made similar presentations in 1955 and 1956 at Esso Research and Engineering, Mobil Oil, Union Oil, Stan­ dard of California, and Shell Development laboratories. In 1956 I organized a new in-house catalytic research group in Linde's Development Department, reporting to Dr. T. L Thomas. Dr. J. E . Boyle joined in June 1956 and P. E. Pickert, in early 1957. Dr. J. A . Rabo was brought in as group leader in June 1957. The charter we gave Dr. Rabo was to develop zeolite based catalysts for use in major petroleum refining operations such as catalytic cracking, isomerization, reforming, and hydrocracking. Our con­ fidence was based on the earlier results with exchanged A and with hydrogen and decationized X , the availability of the more stable Y zeolite, and the promising initial studies with platinum on X and Y . When Dr. J. Rabo arrived, the group was studying reforming with plati­ num impregnated X zeolite. He broadened the program to include cracking and isomerization, shifted emphasis from X to Y , replaced sodium ions in the lattice with ammonium and polyvalent cations, and worked with unimpregnated as well as impregnated Y zeolite. By mid 1958 we had developed a unique zeolite catalyst for the dealkylation of alkyl aromatics to produce benzene. By early 1959 we had developed and completed laboratory testing of a superior pentane and hexane isomerization catalyst based on a decationized, 0.5 wt.% platinum loaded Y zeolite. By December 30, 1959 we had filed a patent application on hydrocarbon conversion processes and catalyst using polyvalent exchanged Y with greater than 40% removal of sodium cations from the lattice. This patent issued on February 22, 1966 was the first one to cover the Y based catalysts now used world wide in the cracking of gas oils [32]. Simultaneously scientists at Esso Research and Engineering and Mobil Oil were working with X based catalysts [33-35]. Mobil Oil introduced the first zeolite based catalysts for cracking gas oils in 1962 using rare earth exchanged X in a silica-alumina matrix. This replaced the older silica-alumina catalysts. When we made Y available, the Y based catalysts largely replaced the X based catalysts in this application. In mid 1959 a significant decision was made, with the sales manager and me dissenting, not to enter the finished catalyst business at that time with our new isomerization catalyst, and to concentrate instead on the adsorbent portion of the molecular sieve business. Accordingly, a major reduction was made in the level of our catalytic studies with zeolites shortly thereafter.

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Commercialization Early in 1950, only a few months after the discovery of A and B, an application group and a process group were set up to develop uses, scale up synthesis, and agglomerate the powder into useful forms. By mid 1951 all signs were positive [Milton, R.M., Linde Technical, B-148, 2/7/51, unpublished], and the level of activity was raised. In June 1953 we issued a project status report recommend­ ing test marketing [Burdick, J.N., Milton, R . M . , Peters, P.E., Linde Technical Memorandum, B-173, 6/9/53, unpublished]. In the spring of 1954, test marketing was initiated with molecular sieve types 4A, 5A, and 13X. Research manage­ ment, R. AJones, and I made most of the customer visits, usually accompanied by a member of the regular Linde sales department. Interest was very high and by fall of that year, Linde announced it was entering the business of manufactur­ ing and selling molecular sieve zeolites. At this time prime responsibility for the project was shifted from the Research Department to the Development Department under W.B. Nicholson. E.R. Behnke was given responsibility for molecular sieve sales, and I was made Manager of the Tonawanda Development Laboratory, responsible for molecular sieve applications, and in time, process development and manufacturing. Major expansions in both development and sales groups occurred in the period 1955-1958. A sales force was staffed with specially trained engineers working full time on molecular sieves. Laboratory engineers and chemists stu­ died specific applications and developed the technology to optimize the design of adsorption systems. Moving bed systems were evaluated jointly with Union Oil. Catalysis research was reactivated. Latent catalyst systems were developed for curing rubber and plastics with active accelerators adsorbed on the molecular sieves. Synthesis, filtering, drying, ion exchange, and activation processes were suc­ cessfully scaled up. Pellet forming procedures were perfected and bead forming techniques developed. A pilot plant and modest scale production facility were built and operated at Tonawanda. In a cooperative effort, Linde Research and Union Carbide Nuclear Co. prospected for and located deposits of natural zeolites in Western United States. No deposits of A, X , Y , or faujasite were found. Numerous and extensive depo­ sits of other useful zeolites were located (chabazite, erionite, mordenite, clinoptilolite), claimed and at a later date some were mined and sold for special uses. We learned how to dealuminate zeolites while maintaining crystal structure, opening the pore and increasing the silica/alumina from 10 to about 20 in mor­ denite. Procedures for synthesizing A, X , and Y from clays were discovered. In early 1958 responsibility for the molecular sieve project was shifted to Linde's New Product Department where it was to be treated as an independent business, no longer a development project. I was made Assistant Manager of New Products and was responsible for the molecular sieve business. E.R. Behnke became Sales Manager, and R.W. Pressing became Manager of Develop­ ment and Production; both reported to me. In February 1959 I was transferred back to research as Director of Linde's Tonawanda Research Laboratory, including its molecular sieve research group. Mr. Behnke took over as General Manager of the Molecular Sieve Department with Mr. R.A. Jones as Sales Manager and Dr. T.L. Thomas as Director of Technology. Mr. Langerhans became Manager of the Mobile Plant when it was opened.

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ZEOLITE SYNTHESIS

Major Contributors Many people have contributed to the success of the molecular sieve project at Union Carbide over the years. I would like to mention the following whose contributions were very important during this initial period, through approxi­ mately 1959. • Dr L I . Dana, Linde Vice President, hired me and put me to work on air separation. He and Dr. J.M. Gaines, Director of Research, asked me to work on physical adsorption and strongly supported the molecular sieve pro­ ject at all times. Dr. Dana made the decision to test market and set the ini­ tial prices. • Dr W.G. Eversole was my supervisor from 1946 to 1953. Besides being a fine scientist, he was inspiring, imaginative, fully supportive, and a valuable consultant. • W.B. Nicholson was responsible for the decision to enter the molecular sieve business, for selling top management on the heavy expenditures needed to launch the new business, and for guiding the project through its most diffi­ cult period. • E.R. Behnke developed the initial sales strategy, built the sales force, was put in charge of the molecular sieve business when I returned to research, and shepherded the business until it was secure and profitable. • Dr. T . L Thomas participated in the earliest research on adsorption/desorption kinetics, air separation, pressure swing adsorption systems, liquid phase separa­ tions, and ion exchange applications. He directed many of the application studies between 1955 and 1959. • R . L Mays was the technical expert on drying of gases and liquids, recovery of olefins, and regeneration of molecular sieve beds, and a key supervisor in application technology. • R.A. Jones initiated application studies in 1950, was coinventor of the first major application of molecular sieves [36], led our entire application engineering group prior to going into field sales, and later became Sales Manager. • R . L Langerhans was my assistant during the initial discovery period (19481950), the first person to work with the clay bonding of zeolite powders, and assisted in siting and design of the Mobile plant and was its first plant manager. • Dr. D.W. Breck was the discoverer of the Y zeolite, a co-discoverer of the unique catalytic cracking activity of the X zeolite, and with Dr. T.B. Reed worked out the crystal structures of A and X . He was certainly one of the world's outstanding zeolite scientists. • Dr. E . M . Flanigen is a world expert on zeolite synthesis. In the early years she was first to synthesize high silica Y with silica/alumina rations above 4.0, first to remove aluminum from zeolite lattices without loss of structure, and was responsible for identification and evaluation of the myriad of samples from Union Carbide's investigation of sedimentary zeolite deposits in Western United States. Additionally, I would like to acknowledge her assistance in

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

1.

MILTON

Molecular Sieve Science and Technology

9

preparation of this paper, particularly for reviewing old data books to check the accuracy of my memory. • Dr. J.A. Rabo led our catalyst research group from 1957 to 1961 and played a key role in the discovery of the catalyticly active ingredient used world wide in the catalytic cracking of gas oils to produce gasoline. • When we started selling molecular sieves, little was known about heat and mass transfer in fixed bed adsorption/desorption. Design was an art. G.J. Griesmer, J.J. Collins, F.W.Leavitt, W.F. Avery, and K. Kiyonaga made it a science, a unit process we understand and can optimize. Griesmer led much of this activity and was primarily responsible for Union Carbide's Iso Sieve process for separating normal from iso paraffins [37]. • E . H . Westerland and G.L. Ribaud conducted and/or supervised most of our very successful process and manufacturing development studies including development of our pellet forming technology. • W. Drost was my synthesis assistant from 1950 to 1952 and later developed our methods for making hard, attrition-resistant molecular sieve beads. • R. E. Cuddeback led the design team for the Union Carbide Mobile Plant for manufacture of molecular sieve products.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Lamb, A. B. U.S. Patent 1 813 174, 1931. Lamb, A. B.; Woodhouse, J. C. J. Am. Chem. Soc. 1936, 58. 2637. Barrer, R. M. U.S. Patent 2 306 610, 1943. Barrer, R. M. J. Soc. Chem. Ind. 1945, 64, 130. Barrer, R. M. British Patent 574 911, 1946. Barrer, R. M. Discuss. Faraday Soc. 1949, 5, 326. Straub, F. G. Ind Eng. Chem. 1936, 28, 113. Barrer, R. M. J. Chem. Soc. 1948, 2158. Milton, R. M. U.S. Patent 2 882 243, 1959. Milton, R. M. U.S. Patent 3 008 803, 1961. Milton, R. M. U.S. Patent 2 882 244, 1959. Milton, R. M. U.S. Patent 2 996 358, 1961. Milton, R. M. U.S. Patent 3 010 789, 1961. Milton, R. M. U.S. Patent 3 012 853, 1961. Breck, D. W.; Eversole, W. G.; Milton, R. M. J. Am. Chem. Soc. 1956, 78, 2338. Breck, D. W.; Eversole, W. G.; Milton, R. M.; Reed, T. B.; Thomas, T.L. J. Am. Chem. Soc. 1956, 78, 5963. Reed, T. B.; Breck, D. W. J. Am. Chem. Soc. 1956, 78, 5972. Breck, D. W.; Flanigen, Ε. M.; Milton, R. M.; Reed, T. B. Abstracts 134th Natl. Meeting, Am. Chem. Soc. Chicago, Sept. 1958. Bergerhoff, G.; Koyama, H.; Nowacki, W. Experienta 1956, 12, 418. Bergerhoff, G.; Baur, W. H.; Nowacki, W. Neues Jahrb. Mineral, Monatsh 1958, 9, 193. Broussand, L.; Shoemaker. D. P. J. Am. Chem. Soc. 1960, 82, 1041. Pauling, L. Proc. Nat. Acad. Sci. 1930, 16, 453. Breck, D. W.; Flanigen, Ε. M. Molecular Sieves, Soc. Chem. Ind., London, 1968, 47.

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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ZEOLITE SYNTHESIS

24. Breck, D. W. U.S. Patent 3 130 007, 1964. 25. Milton, R. M.; Breck, D. W. U. S. Patent Application, Ser. No. 571,129, 1956. 26. Breck, D. W.; Milton, R. M. U. S. Patents 3 013 982, 3 013 983, 3 013 985, 1961. 27. Milton, R. M. U.S. Patents 3 200 083, 1965, 3 236 903, 1966. 28. Milton, R. M.; Castor, C. R. U.S. Patent 3 013 987, 1961. 29. Breck, D. W.; Castor, C. R.; Milton, R. M. U.S. Patent 3 013 990, 1961. 30. Bukata, S. W.; Castor, C. R.; Milton, R. M. U.S. Patents 3 013 988, 1961, 3 236 910, 1966. 31. Breck, D. W.; Bukata, S. W. U.S. Patent 3 200 082, 1966. 32. Rabo, J. Α.; Pickert, P. E.; Boyle, J. E. U.S. Patent 3 236 762, 1966. 33. Kimberlin, C. N. Jr.; Gladrow, Ε. M. U.S. Patent 2 971 903, 1961. 34. Plank, C. J.; Rosinski, E. J.; Hawthorne, W. P. Ind. Enq Chem., Prod. Res. Dev. 1964, 3, 165. 35. Weisz, P. B.; Frilette, V. J. J. Phys. Chem. 1960, 64, 382. 36. Jones, R. Α.; Milton, R. M. U.S. Patent 2 810 454, 1957. 37. Griesmer, G. J.; Avery, W. F.; Lee, Μ. Ν. Y. Hydc. Proc. Petr. Ref. 1965, 44 [6], 147. RECEIVED December 22, 1988

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Chapter 2

Zeolites: Their Nucleation and Growth R. M. Barrer Chemistry Department, Imperial College, London SW7 2AZ, England

Zeolite synthesis proceeds via nucleation, which is a consequence of local fluctuations, small in extent but considerable in degree of departure from the mean for the solution, followed by spon­ taneous growth of nuclei exceeding a critical size. A physico-chemical basis for the critical size requirement has been described. There is evidence that chemical events rather than diffusion can govern subsequent linear growth of zeolite crystals. To succeed in synthesis it is essential during growth to stabilise the open structure by inclusion of quest molecules. This requirement has a thermo­ dynamic origin which has been developed and applied to formation of zeolites, porosils and AlPO's. The explanation of some experimentally observed features of zeolite synthesis follows from the treatment. A distinction is made between zeolitic stabilisers and nucleation templates. As t h e p o t e n t i a l i t i e s o f m i c r o p o r e s i n c r y s t a l s r a t h e r s l o w l y became r e a l i s e d t h e r e d e v e l o p e d an a r e a o f s y n t h e t i c c h e m i s t r y which has y i e l d e d a remarkable v a r i e t y o f microporous s t r u c t u r e s . Most a r e 3 - d i m e n s i o n a l 4-connected n e t s , o f which t h e numbers e n v i s a g e d v a s t l y exceed t h e numbers p r e p a r e d e x p e r i m e n t a l l y . A c c o r d i n g l y t h e s e a r c h f o r c h e m i c a l pathways t o new porous c r y s t a l s p r o c e e d s apace, both f o r i t s s c i e n t i f i c i n t e r e s t and i t s p o s s i b l e i n d u s t r i a l rewards. T h i s account w i l l emphasise s e v e r a l p h y s i c o - c h e m i c a l a s p e c t s o f s y n t h e s i s which h e l p i n u n d e r s t a n d i n g t h e "green f i n g e r s " a p p r o a c h o f much c u r r e n t r e s e a r c h on t h e f o r m a t i o n o f porous c r y s t a l s . Aluminate

and S i l i c a t e S o l u t i o n s

In t h e h i g h pH range needed f o r z e o l i t e s y n t h e s i s a l u m i n a t e s o l u t i o n s a r e r e l a t i v e l y s i m p l e i n t h a t t h e a n i o n s p r e s e n t a r e almost e x c l u s ­ i v e l y A1(0H) On t h e o t h e r hand i n s i l i c a t e s o l u t i o n s around room

0097-6156/89A)398-0011$06.00/0 ο 1989 American Chemical Society In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

12

ZEOLITE SYNTHESIS

t e m p e r a t u r e v a r i o u s a n i o n s c o - e x i s t a c c o r d i n g t o t h e r a t i o o f base t o s i l i c a , the n a t u r e o f t h e base, and t h e c o n c e n t r a t i o n . There has, on t h e o t h e r hand, been l i t t l e study o f s i l i c a t e s o l u t i o n s i n t h e t e m p e r a t u r e range most i m p o r t a n t f o r z e o l i t e s y n ­ t h e s i s (up t o ~ 2 5 0 ° C ) . However K n i g h t e t a l (J_) made an i n t e r e s t i n g o b s e r v a t i o n on tetramethylammonium s i l i c a t e s o l u t i o n s 1 M i n S i and w i t h S i / N = 1/2. The room t e m p e r a t u r e e q u i l i b r i u m was p e r t u r b e d by ~30 s h e a t i n g t o 100°C, q u e n c h i n g i n l i q u i d n i t r o g e n and warming t o room temperature. F o r t h e f i r s t few minutes the NMR spectrum t h e n showed p r e d o m i n a n t l y monomeric s i l i c a t e a n i o n s w i t h s m a l l amounts o f dimer and t r i m e r . T r i g o n a l p r i s m a n i o n s next began t o appear f o l l o w e d a f t e r s e v e r a l hours by c u b i c a n i o n s . The l a t t e r i n c r e a s e d s t e a d i l y i n r e l a t i v e amount u n t i l a f t e r 16 days e q u i l i b r i u m was e s t ­ ablished. The e x p e r i m e n t shows slow e q u i l i b r a t i o n a t room tempera­ t u r e , r a p i d p e r t u r b a t i o n o f t h i s e q u i l i b r i u m a t 100°C, and a p r i m ­ a r i l y monomeric a n i o n i c c o n s t i t u t i o n a t 100°C. T h i s experiment s h o u l d be extended t o o t h e r systems s i n c e i t may have g e n e r a l i m p l i ­ c a t i o n s f o r m o l e c u l a r mechanisms o f n u c l e a t i o n . T h i s would be e s p e c i a l l y true of the c l e a r a l u m i n o s i l i c a t e s o l u t i o n s r e f e r r e d t o below. Reaction

Mixtures

The u s u a l r e s u l t o f m i x i n g a l u m i n a t e and s i l i c a t e s o l u t i o n s i s t h e f o r m a t i o n o f a g e l , which may l a t e r s e p a r a t e i n t o a c l e a r s u p e r ­ n a t a n t l i q u i d and a g e l . One may ask whether n u c l e a t i o n i s homoge­ neous ( i n s o l u t i o n ) o r h e t e r o g e n e o u s ( i n g e l ) . A p a r t i a l answer may be p r o v i d e d i f i t can be d e m o n s t r a t e d t h a t z e o l i t e s can grow from c l e a r s o l u t i o n s . Guth e t a l (2^) and Ueda e t a l have shown how such s o l u t i o n s can be p r e p a r e d . From c l e a r Na- a l u m i n o s i l i c a t e s o l u t i o n s Ueda e t a l have c r y s t a l l i s e d a n a l c i m e , s o d a l i t e h y d r a t e , m o r d e n i t e , f a u j a s i t e (Na-Y), z e o l i t e s ( g m e l i n i t e t y p e ) and z e o l i t e Ρ (gismondine t y p e ) , so t h a t homogeneous n u c l e a t i o n i s a t l e a s t p o s s i b l e . Where g e l i s p r e s e n t c r y s t a l s n u c l e a t e d homogeneously would, as growing c r y s t a l s , become enmeshed w i t h g e l so t h a t homogeneous and heterogeneous n u c l e a t i o n a r e d i f f i c u l t t o d i f f e r e n t i a t e i n g e l - c o n t a i n i n g media. Whether n u c l e a t i o n i s homogeneous, heterogeneous or both t h e r e i s s t r o n g e v i d e n c e t h a t i n subsequent c r y s t a l growth t h e g e l , i f p r e s e n t , p r o g r e s s i v e l y d i s s o l v e s and t h a t t h e d i s s o l v e d m a t e r i a l t h e n feeds t h e growing c r y s t a l s . This also applies i n successive trans­ f o r m a t i o n s such as (4^) : s o l u t i o n -+ Na-Y Na-S -> Na-P, because each c r o p o f c r y s t a l s i n the s u c c e s s i o n has i t s own s i z e range and morphology r a t h e r t h a n b e i n g a pseudomorph o f i t s p a r e n t , as would be t h e c a s e i n an i n t e r n a l s o l i d s t a t e t r a n s f o r m a t i o n . Z e o l i t e C r y s t a l l i s a t i o n from c l e a r S o l u t i o n s The c l e a r a l u m i n o s i l i c a t e s o l u t i o n s from which Ueda e t a l (j4) s t u d i e d c r y s t a l l i s a t i o n o f z e o l i t e s Y, S and Ρ were based on t h e composition range 10Na O.(0.35-0.55)A1 0 . ( 2 2 - 2 8 ) S i 0 . ( 2 5 0 - 3 0 0 ) H O . Figure 1 shows t h e r e g i o n i n which g e l and s o l u t i o n c o - e x i s t e a (cross-hatched), t h e r e g i o n of c l e a r s o l u t i o n , and the f o r m a t i o n f i e l d s o f t h e t h r e e z e o l i t e s from t h e c l e a r s o l u t i o n . 2

2

2

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

2.

BARRER

Zeolites: Their Nucleation and Growth

L

I

0.40

Figure

1.

ι

I

045

ι

I

13

;

050

C r y s t a l l i s a t i o n f i e l d s o f z e o l i t e s Y, S and Ρ a t 100°C from c l e a r a l u m i n o s i l i c a t e s o l u t i o n s . In t h e c r o s s h a t c h e d a r e a g e l and s o l u t i o n c o - e x i s t . (Reproduced w i t h p e r m i s s i o n from Ref. 4. C o p y r i g h t 1984 B u t t e r w o r t h s . )

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

14

ZEOLITE SYNTHESIS

For c r y s t a l l i s a t i o n o f Y t h e optimum c o m p o s i t i o n o f s o l u t i o n was 10Na O.O.45A1 0 26SiO .270H O. F a u j a s i t e ( z e o l i t e Y) appeared a f t e r ~12 h o u r s , and t h e c r y s t a l s were o f r a t h e r c o n s t a n t c o m p o s i t i o n , h a v i n g , o v e r a l l t h e e x p e r i m e n t s , r a t i o s SiO^/Al^O between 5.1 and 5.6. Because t h e s e r a t i o s a r e so much g r e a t e r i n t h e p a r e n t s o l u ­ t i o n s t h e A l has been s e l e c t i v e l y removed from s o l u t i o n and i t s c o n c e n t r a t i o n t h e r e i n drops much f a s t e r t h a n t h o s e o f Na and S i . When S i O /Al^O^ i n s o l u t i o n had i n t h i s way r i s e n from 58 t o 73 z e o l i t e δ began t o appear and t h e y i e l d o f Y t o d e c r e a s e . When S i O / A l 0 had r e a c h e d about 102 z e o l i t e Ρ s t a r t e d t o form and t h e y i e l d of S Began t o d e c l i n e . 2

2

A l s o , i f t h e s o l u t i o n s had an i n i t i a l S i 0 / A l 0 r a t i o o f 73,S formed b u t no Y, w h i l e i f t h i s i n i t i a l r a t i o was 102,Ρ formed b u t no S. T h i s b e h a v i o u r s u g g e s t s c a u t i o n i n i n t e r p r e t i n g a l l c r y s t a l ­ l i s a t i o n sequences as examples o f Ostwald's r u l e o f s u c c e s s i v e transformations. The r u l e s t a t e s t h a t i n a c r y s t a l l i s a t i o n sequence the new phases r e p l a c e each o t h e r i n t h e o r d e r o f a s t e p by s t e p d e s c e n t o f a l a d d e r o f i n c r e a s i n g thermodynamic s t a b i l i t y . An example i n a h y d r o t h e r m a l system i s {5): Amorphous S i O ^ cristobalite keatite quartz. The optimum check o f Ostwald's r u l e would, as i n t h e above sequence, i n v o l v e p a r e n t g e l and s u c c e s s i v e phases a l l o f t h e same c o m p o s i t i o n . T h i s c o n d i t i o n i s n o t met i n many c r y s t a l l i s a t i o n sequences. 2

2

N u c l e a t i o n and C r y s t a l Growth S t u d i e s o f Raman (2) and NMR (6) s p e c t r a o f s o l u t i o n s o f a l u m i n a t e s and s i l i c a t e s , and o f t h e i r m i x t u r e s under c o n d i t i o n s y i e l d i n g c l e a r a l u m i n o s i l i c a t e s o l u t i o n s , agree i n showing a t l e a s t p a r t i a l s u p p r e s ­ s i o n o f t h e A l ( O H ) ^ i o n when s i l i c a t e a n i o n s a r e p r e s e n t , s u p p o r t i n g the view t h a t a l u m i n o s i l i c a t e a n i o n s form around room t e m p e r a t u r e . I t i s t h e n p o s s i b l e t o v i s u a l i s e how such a n i o n s c o u l d form more com­ p l e x u n i t s and germ n u c l e i . An example i s shown i n F i g u r e 2 i n which 4 - r i n g a n i o n s y i e l d t h e c u b i c u n i t found i n t h e z e o l i t e A framework, o r t h e d o u b l e c r a n k s h a f t c h a i n found i n f e l s p a r s o r i n p h i l l i p s i t e - h a r m o t o m e z e o l i t e s {!_). W h i l e a c t u a l c h e m i c a l e v e n t s i n v o l v e d i n n u c l e a t i o n and c r y s t a l growth a r e n o t known a p h e n o m e n o l o g i c a l t r e a t m e n t (8_) g i v e s some insight. W i l l a r d G i b b s (9_) c o n s i d e r e d p r o c e s s e s o f phase s e p a r a t i o n of two extreme k i n d s . In t h e f i r s t , f l u c t u a t i o n s i n c o n c e n t r a t i o n o c c u r which a r e minute i n volume b u t l a r g e i n e x t e n t o f d e p a r t u r e from t h e mean ( t h e c a s e o f b i n o d a l phase s e p a r a t i o n ) . In t h e second t h e volume o f t h e f l u c t u a t i o n i s l a r g e but t h e d e v i a t i o n from the mean f o r t h e s o l u t i o n i s minute ( r e s p o n s i b l e f o r s p i n o d a l phase separation). In n u c l e a t i o n o f z e o l i t e s one i s conerned o n l y w i t h fluctuations of the f i r s t kind. One may e n q u i r e what f a c t o r s oppose t h e immediate appearance o f v i a b l e n u c l e i growing s p o n t a n e o u s l y . Whether i n g e l o r s o l u t i o n a p o s i t i v e i n t e r f a c i a l f r e e energy term Δς^. a r i s e s which i s i n c r e a s ­ i n g l y i m p o r t a n t r e l a t i v e t o o t h e r f r e e energy terms t h e l a r g e r t h e s u r f a c e t o volume r a t i o . In a r e s t r a i n i n g m a t r i x t h e germ, t h r o u g h m i s f i t , may a l s o produce a p o s i t i v e s t r a i n f r e e energy, Ag . Both t h e s e terms g r e a t l y r e d u c e t h e p r o b a b i l i t y o f a germ n u c l e u s becoming v i a b l e . The n e t f r e e energy o f f o r m a t i o n o f a germ

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

2.

BARRER

F i g u r e 2.

Zeolites: Their Nucleation and Growth

F o r m a t i o n from 4 - r i n g a n i o n s o f c u b i c u n i t s found i n z e o l i t e A and d o u b l e c r a n k s h a f t c h a i n s found i n f e l s p a r s and p h i l l i p s i t e - h a r m o t o m e z e o l i t e s . (Reproduced w i t h p e r m i s s i o n from Ref. 7. C o p y r i g h t 1982 Academic P r e s s . )

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

16

ZEOLITE SYNTHESIS

c o n s i s t i n g of j s t r u c t u r a l u n i t s of a p a r t i c u l a r k i n d i s t h e r e f o r e

AG Ag. = - — - j D N

+

J

Ag *

A

+

σ

Ag ^s

( 1 )

where AG i s t h e f r e e energy o f f o r m a t i o n o f a mole o f b u l k c r y s t a l , i . e . t h e amount o f c r y s t a l c o n t a i n i n g an Avogadro number, N^, o f t h e structural units. AG i s n e g a t i v e ^ i n s i g n . Ag i s proportional to t h e i n t e r f a c i a l a r e a , and so t o j , w h i l e Ag i s proportional to j. Thus, w i t h A = -AG/N , one has A

Ag.. = - A j + B j

2

/

3

+ C j

(2)

where A, Β and C a r e p o s i t i v e c o e f f i c i e n t s . F o r s m a l l j , wj^eçe t h e s u r f a c e t o volume r a t i o i s v e r y l a r g e , t h e p o s i t i v e term B j can be dominant. In s o l u t i o n o r g e l we e x p e c t C t o be z e r o o r s m a l l , i . e . A > C. A s J i n c r e a s e s t h e n e g a t i v e term - ( A - C ) j w i l l grow more r a p i d l y than B j . A c c o r d i n g l y , when Ag. i s p l o t t e d a g a i n s t j t h e c u r v e i n i t i a l l y has a p o s i t i v e s l o p e and ^Ag. i s p o s i t i v e , but a t some v a l u e o f j t h i s c u r v e p a s s e s t h r o u g h a maximum and t h e r e a f t e r d e c r e a s e s , as shown i n F i g u r e 3 (JO. A t t h e maximum dAg ./dj = 0 and so from E q u a t i o n 2 t h e v a l u e s o f j and o f Ag_. a t t h e maximum a r e 2

D_ = m

8B

3

3

. 27(A-C) 3

;

A

Ag

= ^

. (A-C) j , m

/ o l

(3)

2

Any n u c l e u s i n which j exceeds j w i l l add more l a t t i c e - f o r m i n g u n i t s w i t h a d e c r e a s e i n f r e e energy and t h e r e f o r e t e n d s t o grow s p o n t a n e o u s l y ; but any germ n u c l e u s i n which j i s l e s s t h a n j will l o s e l a t t i c e - f o r m i n g u n i t s w i t h a d e c r e a s e i n f r e e energy, anS w i l l t h e r e f o r e tend t o disappear. Even so, f l u c t u a t i o n s e n s u r e t h a t some germs e v e n t u a l l y r e a c h and c r o s s t h e s a d d l e p o i n t i n F i g u r e 2 and then grow s p o n t a n e o u s l y . As t h e t o t a l s u r f a c e a r e a o f growing c r y s t a l s i n c r e a s e s , and v a s t l y exceeds t h e t o t a l a r e a o f germ n u c l e i , c r y s t a l s w i l l dominate more and more s t r o n g l y o v e r f r e s h n u c l e i i n consuming t h e l a t t i c e forming u n i t s a v a i l a b l e . A c c o r d i n g l y the n u c l e a t i o n r a t e should b u i l d t o a maximum e a r l y i n t h e c u r v e o f y i e l d a g a i n s t t i m e , but w i l l t h e r e a f t e r , through the c o m p e t i t i o n w i t h c r y s t a l s f o r chemical n u t r i e n t s , t e n d t o decay towards z e r o . T h i s b e h a v i o u r was found by Zdhanov and Samuelevich (_1_0) from an a n a l y s i s o f t h e l i n e a r growth r a t e s o f i n d i v i d u a l c r y s t a l s o f Na-X and t h e s i z e d i s t r i b u t i o n o f the f i n a l crop of c r y s t a l s . A l s o , as t h e t o t a l a r e a o f growing c r y s t a l s i n c r e a s e s so does t h e r a t e a t which c h e m i c a l n u t r i e n t s a r e consumed. The s l o p e o f t h e c u r v e o f y i e l d a g a i n s t time t h e r e f o r e i n c r e a s e s . L a t e r , as t h e s u p p l y o f n u t r i e n t s becomes more and more exhausted, t h e s l o p e o f t h e c u r v e d e c r e a s e s t o z e r o . As a r e s u l t c u r v e s o f y i e l d v s . time a r e s i g m o i d i n c o n t o u r , as i l l u s t r a t e d i n F i g u r e 4 ( 1 1 ) . Two r e s u l t s o f p r a c t i c a l i n t e r e s t f o l l o w from t h e above reasoning. F i r s t l y , t h e s m a l l e r t h e number o f v i a b l e n u c l e i t h e l a r g e r w i l l be t h e average c r y s t a l l i t e s i z e i n t h e f i n a l c r o p o f c r y s t a l s . Secondly,

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

2.

BARRER

Zeolites: Their Nucleation and Growth

17

β;
' NaA10 ..3., o ( % ; .400H 0 .92 3. 0 8 ° 4 > NaA10 .,5. ,400H 0 o . ,78 3. 2 2 ° 4 ' NaA10 ..7.,5(Na ,400H 0 S i ,73 3. 2 7 V 2

3ι

2

2

0 ( N a

2

0

f o r Growth R a t e s on

Ε

H

S i

2

1 .53 .

49.4

H

S i

2

1 .78 .

51.5

H

S i

2

2,.20

59.0

2

2..54

65.3

H

Mineralisers Water and h y d r o x y l i o n a r e t h e c l a s s i c m i n e r a l i s e r s i n h y d r o t h e r m a l s y n t h e s i s , f i r s t l y because aqueous a l k a l i d i s s o l v e s amphoteric o x i d e s and so promotes m o b i l i t y and m i x i n g o f m o l e c u l a r and i o n i c s p e c i e s as a p r e - r e q u i s i t e f o r r e a c t i o n . A second v i t a l r o l e i s t h a t of m o l e c u l a r water which (see below) s t a b i l i s e s aluminous z e o l i t e s by f i l l i n g c h a n n e l s and c a v i t i e s . T h i s r o l e can be s h a r e d o r t a k e n o v e r by o r g a n i c m o l e c u l e s ( e . g . i n p o r o s i l s , s i l i c a - r i c h z e o l i t e s o r A l P O ' s ) , and by s a l t s (e.g. i n s c a p o l i t e s , s o d a l i t e and c a n c r i n i t e ) . S t a b i l i s i n g Porous C r y s t a l s ;

Host-Guest S o l u t i o n s

The z e o l i t e , p o r o s i l o r A1PO i s t h e " h o s t " and t h e z e o l i t i c component t h e "guest". The h o s t - g u e s t complex i s a s o l u t i o n , amenable a t e q u i l i b r i u m t o s o l u t i o n thermodynamics (J_4 ,J_5 ), and t h e h o s t - g u e s t r e l a t i o n s h i p t h e r e b y d e s c r i b e d i s one o f t h e most i m p o r t a n t i n t h e c h e m i s t r y o f porous c r y s t a l s because, w i t h o u t t h e z e o l i t i c g u e s t ,

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

2.

19

Zeolites: Their Nucleation and Growth

BARRER

m i c r o p o r o u s c r y s t a l s c o u l d not be s y n t h e s i s e d . The g u e s t i s r e q u i r e d o n l y d u r i n g s y n t h e s i s and i s t h e r e a f t e r removed b e f o r e t h e porous c r y s t a l s a r e put t o use. F o r p o r o s i l s t h e n a t u r a l c h o i c e o f t h e gram m o l e c u l e (or mole) i n h o s t - g u e s t s o l u t i o n s i s S i O ^ , and f o r c o m p a r a b i l i t y among a l l p o r o s i l s and z e o l i t e s t h e mole w i l l t h e r e f o r e be t a k e n as M ^ A l ^ S i ^ ^ _ w h e r e 0 < χ < 0.5 and M i s an e q u i v a l e n t o f c a t i o n s . χ

For AlPO's t h e n a t u r a l c h o i c e o f mole i s AlPO^. F o r a two component s o l u t i o n a t c o n s t a n t temperature t h e Gibbs-Duhem e q u a t i o n i s

n

H

d y

+

H

n

G

d y

=

G

V

d

P

( 4 )

where n and n denote numbers o f moles o f h o s t and q u e s t i n t h e s o l i d s o l u t i o n o f volume V a t t o t a l p r e s s u r e P. y and y are the r e s p e c t i v e c h e m i c a l p o t e n t i a l s o f h o s t and g u e s t i n t h e s o l u t i o n . E q u a t i o n 4 can be r e - a r r a n g e d , w i t h dy_ = RTdlna, and i n t e g r a t e d t o G give fl

Q

fl

Q

The a c t i v i t y , a, o f t h e g u e s t i s z e r o when t h e h o s t i s g u e s t - f r e e and has c h e m i c a l p o t e n t i a l i s t h u s t h e change i n c h e m i c a l p o t e n t i a l o f t h e h o s t due t o t h e i n c l u s i o n o f t h e 9^||^· ® i f r a c t i o n a l s a t u r a t i o n o f t h e h o s t by t h e g u e s t , V = n /n and η i | £ h e number o f moles o f g u e s t a t s a t u r a t i o n o f t h e h o s t , so that n Θ = n . / ^ volume o f t h e h o s t because t h e r i g i d h o s t framework r e n d e r s V v i r t u a l l y independent o f n . The t o t a l p r e s s u r e , P, i s o f t e n t h e vapour p r e s s u r e , p, o f t h e g u e s t , and t h e a c t i v i t y , a, can sometimes be r e p l a c e d by t h e r e l a t i v e vapour p r e s s u r e , x, o f t h e g u e s t , so t h a t s

t

n

e

Q

a

V

Q

Q

=

v

n

H

s

t

h

e

m

o

l

a

r

H

Q

Δ μ

Η

=

V

H

P

"

R T V

0

X

d

( 6 )

£*< / ) x

T h i s i n t e g r a l can be e v a l u a t e d g r a p h i c a l l y from t h e s o r p t i o n i s o t h e r m o f t h e g u e s t p l o t t e d as Θ/χ a g a i n s t x. From E q u a t i o n 6, Δ μ ^ ί ε seen t o c o n s i s t o f two p a r t s : a p o s i t i v e term Δμ = V P ; and a n e g a t i v e term Δ μ

2

= -RTV

Ί

jf^ (0/x)dx.

These two

parts w i l l

π

be e v a l u a t e d i n t u r n .

Table II g i v e s values of V and o f V P a t 100°C where Ρ = 1 atm. V i s based upon t h e number o f S i + A l atoms per 1000 A of z e o l i t e (16). A t 100°C f o r water Δμ = V P i s insignificant for a l l zeolites. However, i n a c l o s e d system above 100°C,P-p f o r water r i s e s rapidly. The s m a l l e s t and l a r g e s t m o l e c u l a r volumes a r e t h o s e o f b i k i t a i t e and f a u j a s i t e r e s p e c t i v e l y , and i n T a b l e I I I v a l u e s o f V p a r e t h e r e f o r e g i v e n f o r t h e s e two z e o l i t e s up t o 365°C n e g l e c t i n g any s m a l l change i n V w i t h t e m p e r a t u r e . V^p f o r a l l o t h e r z e o l i t e s l i e s between t h e v a l u e s f o r t h e s e two. A c c o r d i n g l y i n the temperature range most r e l e v a n t f o r z e o l i t e s y n t h e s i s , Δμ^ , i f water i s t h e g u e s t , w i l l not be l a r g e . However, i f , b y u s i n g an i n e r t p i s t o n f l u i d , one moves i n t o t h e k i l o b a r p r e s s u r e range t h e term V P w i l l have a major e f f e c t on Δμ . H

3

)

H

fl

fl

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

20

ZEOLITE SYNTHESIS

TABLE I I .

M o l a r Volumes, V , o f Z e o l i t e s and V P (100°C f o r Water a s G u e s t )

v

Zeolite cm Bikitaite Li-ABW Analcime Ferrierite Dachiardite Mordenite Sodalite Hydrate Heulandite LTL Mazzite

V P

mol

J mol

29.8 31.7 32.4 34.0 34.8 35.0

3.03 3.21 3.28 3.45 3.53 3.55

35.0 35.4 36.7 37.4

3.55 3.59 3.72 3.79

v

Zeolite

H

H

a t one

cm

1

Merlinoite Phillipsite Erionite Offretite Paulingite Chabazite Gmelinite RHO LTA Faujasite

Atm

V P H

H

mol

37.6 38.1 38.6 38.9 38.9 41.3 41.3 42.1 46.7 47.4

J mol

1

3.82 3.88 3.92 3.94 3.94 4.18 4.18 4.29 4.73 4.83

We c o n s i d e r now t h e term Δ]^ . T a b l e IV g i v e s v a l u e s o f V i n Equation 6 f o r a v a r i e t y of h y d r o p h i l i c z e o l i t e s , according t o the u n i t c e l l c o m p o s i t i o n s g i v e n by M e i e r and O l s o n (JM6 ). To p r o c e e d f u r t h e r a s i m p l e model o f t h e h o s t - g u e s t s o l u t i o n w i l l be employed. A Model f o r H o s t - G u e s t S o l u t i o n s I n t r a c r y s t a l l i n e s o r p t i o n i s n o r m a l l y o f Type 1 i n B r u n a u e r ' s c l a s s ­ i f i c a t i o n (_V7) and i s o t h e r m c o n t o u r s t h e r e f o r e resemble t h o s e a c c o r d i n g t o Langmuir's i s o t h e r m e q u a t i o n . T h i s can d e s c r i b e a c t u a l i s o t h e r m s w e l l enough (_1_8) t o be o f v a l u e i n p r e d i c t i n g , t h r o u g h E q u a t i o n s 5 o r 6, some f e a t u r e s o f z e o l i t e c h e m i s t r y . The maximum v a l u e o f t h e r e l a t i v e p r e s s u r e , x, i s u n i t y and t h i s v a l u e w i l l be c l o s e l y a p p r o a c h e d f o r t h e aqueous phase where the guest i s water. Then f o r Langmuir's i s o t h e r m Θ

=

Kx/(1

+ Kx)

(7)

Isotherm c o n t o u r s i n F i g u r e 5 show how l a r g e Κ must be t o g i v e r e c t ­ a n g u l a r i s o t h e r m s l i k e t h o s e u s u a l l y o b s e r v e d f o r water i n z e o l i t e s and a l s o show some v a l u e s o f Θ a t χ = 1. W i t h E q u a t i o n 7, E q u a t i o n 6 integrates to Δμ

Η

= Δμ

ι

+ Δρ^

= V p + RTVln(1-0) fl

= V„p π

- RTVln(1+TCx)

(8)

The n e g a t i v e term, Δ μ ^ , becomes i n c r e a s i n g l y so t h e l a r g e r t h e v a l u e o f K, and hence t h e n e a r e r t h e v a l u e o f Θ a t χ = 1 i s t o u n i t y and t h e more r e c t a n g u l a r t h e i s o t h e r m .

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

2.

BARRER

TABLE I I I .

Zeolites: Their Nucleation and Growth V p f o r F a u j a s i t e and B i k i t a i t e Temperatures

i n Water* a t D i f f e r e n t

fl

T°C

p/atm

V p / J mol"

1.413 1.959 2.665 3.565 4.695 6.100 7.811 9.888 12.378 15.334 18.81 22.88 27.59 33.01 39.22 46.28 54.28 63.29 73.41 84.72 97.32 111.32 195.50 1 ,000 5,000

—

1

H

Bikitaite

Faujasite

110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 365

21

4.29 5.95 8.08 10.82 14.25 18.50 23.71 29.98 35.57 46.54 57.1 64.4 83.8 99.5 119.0 140.4 164.7 192.1 222.8 257.1 295.4 337.9 456.7 3,033 15,165

6.82 9.46 12.87 17.22 22.67 29.94 37.71 47.69 59.76 74.0 90.8 110.5 133.2 159.8 189.4 223.5 262.0 305.5 352.5 409.1 469.9 537.5 726.5 4,828 24,140

* Vapour p r e s s u r e s o f water from NBS/NRC Steam T a b l e s . Hemisphere P u b l i s h i n g Co., 1984. C o n v e r t e d from b a r s t o atm.

TABLE IV.

M o l e s o f Water p e r Mole o f Z e o l i t e (V)

Examples

Examples 0.33 0.40

Analcime, b i k i t a i t e Natrolite

0. 2 0, 75

0.50

Li-ABW, d a c h i a r d i t e f e r r i e r i t e , mordenite, yugawaralite

0, 77 0, 80 0, 91

5 8

°- 3 0.60 0.62 0.66

5

L

T

7 2

L

Thomsonite Brewsterite E p i s t i l b i t e , heulandite laumontite, s o d a l i t e hydrate

1

0.92 1.00 1.04 1.11 c 1.25 1

2

EAB Erionite, merlinoite, phillipsite Mazzite, o f f r e t i t e Edingtonite RHO Levynite Gismondine, g m e l i n i t e Paulingite Chabazite LTA Faujasite

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

22

ZEOLITE SYNTHESIS

X F i g u r e 5.

Isotherms o f Θ = Kx/(1 + K x ) , showing Θ a t χ = 1 and t h e changes i n c u r v a t u r e w i t h i n c r e a s i n g v a l u e s o f K.

T a b l e V g i v e s v a l u e s o f -à\x^/RTV f o r d i f f e r e n t values of Κ or of Θ a t χ = 1, a c c o r d i n g t o E q u a t i o n 8. F o r l a r g e Κ, Δμ dominates t h i s e q u a t i o n and t h e z e o l i t e i s t h e r e f o r e much s t a b i l i s e d r e l a t i v e to i t s guest-free s t a t e . Thus f o r f a u j a s i t e a t 100°C, Δμ = V ρ = 0.005 k J mol ( T a b l e I I ) and V = 1.25 ( T a b l e I V ) . The f i n a l two columns i n T a b l e V g i v e Δμ a t 100°C f o r d i f f e r e n t Κ when V = 1.25 TABLE V.

Κ

Δ μ i n k J mol a t χ = 1 f o r d i f f e r e n t Values o f Κ using t h e Langmuir Model 2

θ at χ = 1

-Δμ

-Δμ /κτν 2

V = 1 2 3 5 10 15 25 50 100 500 1000

0.500 0.6é 0.750 0.833 0.90909 0.93750 0.96154 0.98039 0.99010 0.99800 0.99900

0.6932 1.0986 1.3863 1.7918 2.3979 2.7726 3.2581 3.9318 4.6151 6.2166 6.9088

1.A

2.,687 4..259 5.,374 6..946 9..295 10..75 12,.63 15..24 17..89 24..10 26..78

a t 100°C V = 0.33 0.717 1 . 136 1 .433 1 .853 2.479 2.867 3.368 4.064 4.771 6.427 7.141

and 0.33, and so f o r t h e extremes among t h e v a l u e s o f V f o r z e o l i t e s (Table I V ) . One may c o n c l u d e from F i g u r e 5 and T a b l e s IV and V t h a t : (i) Because as z e o l i t e s become r i c h e r and r i c h e r i n s i l i c a t h e y a r e known t o become l e s s h y d r o p h i l i c and e v e n t u a l l y h y d r o p h o b i c , water w i l l i n c r e a s i n g l y l o s e i t s e f f e c t i v e n e s s as a s t a b i l i s e r because as Κ d e c l i n e s so does - Δ μ .

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

2.

Zeolites: Their Nucleation and Growth

BARRER

23

(ii) F o r s i l i c a - r i c h z e o l i t e s , p o r o s i l s and AlPO's water must t h e r e f o r e i n p a r t o r w h o l l y be r e p l a c e d by o t h e r g u e s t m o l e c u l e s which a r e more s t r o n g l y s o r b e d g i v i n g more r e c t a n g u l a r i s o t h e r m s t h a n water. These a r e u s u a l l y m u l t i a t o m i c o r g a n i c m o l e c u l e s . They o f t e n c o n t a i n a b a s i c group t o improve t h e i r s o l u b i l i t y i n water and h e l p m a i n t a i n a h i g h pH. (iii) The s t a b i l i s i n g r o l e i s a g e n e r a l one e x e r c i s e d by any g u e s t m o l e c u l e s a b l e t o e n t e r t h e porous c r y s t a l o r be i n c o r p o r a t e d d u r i n g i t s growth. T h i s a p p l i e s e q u a l l y t o permanent gases a t p r e s s u r e s h i g h enough t o g i v e s i g n i f i c a n t u p t a k e s . Thus c r y s t a l l i s a t i o n o f t h e p o r o s i l m e l a n o p h l o g i t e was e f f e c t e d f o r t h e f i r s t time a t 170°C under a p r e s s u r e o f 150 b a r o f C H (J_9 ). T a b l e V I g i v e s some o f t h e g u e s t m o l e c u l e s used t o a i d f o r m a t i o n o f ZSM-5 {20) and s e r v e s t o i l l u s t r a t e the generality of the e f f e c t . 4

TABLE V I .

Some G u e s t M o l e c u l e s

NPr OH NEt OH NPr NH à H NH OHC Β OHC,H*NH^ 3 6 2 4

NH +C H OH Glycerine n-CHJH (n-Cl)NH NH ( & C ) ÎNH 3

fa

0

The

2

5

Triethylenetetramine Diethylenetriamine / \ θ4 )* \ / Hexanediol Propylamine

HH

C(CH OH) Dipropylenetriamine

C H OH 2

u s e d t o a i d S y n t h e s i s o f ZSM-5 (20)

5

E f f e c t o f Temperature upon H o s t

Stabilisation

Temperature i n f l u e n c e s t h e vapour p r e s s u r e , p, and hence changes Δμ = V p as a l r e a d y c o n s i d e r e d ( T a b l e I I I ) . F o r t h e Langmuir moael Δ μ = - R T V i n M + Kx) i s a l s o dependent upon t e m p e r a t u r e . The e f f e c t i s r e a d i l y e v a l u a t e d assuming t h a t V i s independent o f Τ and that fl

2

Κ

=

K

q

exp(-AH/RT)

(9)

where Κ i s a c o n s t a n t and ΔΗ i s t h e h e a t o f w e t t i n g o f t h e w a t e r f r e e z e o l i t e by l i q u i d water (x = 1 ) . Relevant heats o f wetting f o r s e v e r a l z e o l i t e s i n d i f f e r e n t c a t i o n i c forms f e l l i n t h e range -6.8 t o -3.2 k c a l mol of l i q u i d water (2J_). C a l c u l a t i o n s o f Δη + Δμ a r e here made f o r ΔΗ =.,-6.0, -4.0 and -2.0 k c a l mol ( i . e . -Ί25.1, -16.7 and 8.37 k J mol ) t a k i n g Κ a t 100°C t o be 100, and a l s o 10. The r e s u l t s i n T a b l e V I I show t h a t Δμ^ becomes l e s s n e g a t i v e as t e m p e r a t u r e increases. F o r a g i v e n Κ t h e change i n Δμ i s l a r g e r t h e g r e a t e r t h e heat o f w e t t i n g . The c a l c u l a t i o n s l e a d one t o e x p e c t t h a t :

In Zeolite Synthesis; Occelli, M., el al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

24

ZEOLITE SYNTHESIS

(i) As Τ i n c r e a s e s z e o l i t e s w i l l t e n d t o be r e p l a c e d a t e q u i l i b r i u m by denser phases, because, i n t h e r e l a t i o n Δμ = (μ - μ ), μ of the z e o l i t e s t a b i l i s e d by water may no l o n g e r be l e s s a t h i g h Τ t h a n t h e μ f o r dense phases ( f e l s p a r s , non-porous f e l s p a t h o i d s , s i l i c a t e s o r oxides). C o n v e r s e l y , as Τ i s lowered so t h a t Δμ^ becomes i n c r e a s ­ i n g l y n e g a t i v e , z e o l i t e s w i l l i n c r e a s i n g l y r e p l a c e dense phases Η (ii) The more porous t h e h o s t t h e g r e a t e r one e x p e c t s i t s c h e m i c a l potential, μ when g u e s t - f r e e , t o become. The c r y s t a l l i s a t i o n o f t h e most porous z e o l i t e s w i l l t h e r e f o r e r e q u i r e u n u s u a l l y b i g n e g a t i v e v a l u e s o f Δ μ t o compensate f o r t h e l a r g e μ · Δμ i s seen i n T a b l e V I I t o become more n e g a t i v e t h e lower the t e m p e r a t u r e so t h a t t h e most porous c r y s t a l s w h i c h need t h e g r e a t e s t s t a b i l i s a t i o n s h o u l d form b e s t towards t h e low end o f t h e t e m p e r a t u r e range f o r z e o l i t e formation. ( μ