Annual Reports on NMR Spectroscopy, Volume 28

ANNUAL REPORTS ON

NMR SPECTROSCOPY Edited by I. ANDO" and G. A. WEBBY *Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan WDepartment of Chemistry, University of Surrey, Guildford, Surrey, England

VOLUME 28

ACADEMIC PRESS Harcoutt Brace & Company, Publishers London 0 San Diego New York Boston 0 Sydney 0 Tokyo Toronto

ACADEMIC PRESS LIMITED 24-28 Oval Road, LONDON NW17DX

U.S. Edition Published by ACADEMIC PRESS INC. San Diego, CA 92101 This book is printed on acid free paper

Copyright 0 1994 ACADEMIC PRESS LIMITED

AN Rights Reserved

N o part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical including photocopying, recording, or any information storage and retrieval system without permission in writing from the publisher A catalogue record for this book is available from the British Library

ISBN 0-12-505328-2 ISSN 0066-4103

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LIST OF CONTRIBUTORS I . Ando, Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan. S . Ando, NTT Interdisciplinary Research Laboratories, Midori-cho, Musashino-shi, Tokyo, Japan.

F. D . Blum, Department of Chemistry and Materials Research Center, University of Missouri-Rolla, Rolla, MO 65401, USA. H . Ernst, Universitat Leipzig, Fachbreich Physik, Linnkstr. 5 , 04103 Leiprig, Germany. S . Hayashi, National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305,Japan.

H . Kawazoe, Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Natatsuta, Midori-ku, Yokohama 227, Japan.

H . Kurosu, Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan. H . Pfeifer, Universitat Leipzig, Fachbreich Physik, Linnkstr. 5 , 04103 Leipzig, Germany. H . Yoshimizu, Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Japan.

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PREFACE This is the first of a series of special issues of Annual Reports on NMR Spectroscopy, and it is devoted to Applications of NMR to New Materials. It is a great pleasure for me to welcome as co-editor of this issue Professor I. Ando of the Tokyo Institute of Technology. Professor Ando and I have cooperated in many areas of NMR over the past 15 years and I am especially happy to have been able to draw upon his considerable knowledge of new materials in the planning and execution of this volume. The topics covered are high-temperature superconductors, ceramics, zeolites, high-order polymer structure, and organic thin films. While not being exhaustive this coverage represents timely reports on many of the most active areas of new materials science. I wish to express my sincere thanks to all of the contributors, and my co-editor, for their very considerable help and cooperation in the production of this volume. University of Surrey Guildford, Surrey England

G. A. WEBB May 1993

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CONTENTS List of contributors Preface .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

...

111

V

Application of NMR Spectroscopy to the Science and Technology of Glasses and Ceramics H. KAWAZOE 1. Introduction . . . . . . . . . . . . . . . . . 2. Identification and quantization of silicate anions in solid silicates by 29Si MAS-NMR and effects of bonding characteristics on the chemical shift. . . . . . . . . . . . . . . . . . 3. Distribution of silicate structures in silicate glasses by using 29Si MAS-NMR . . . . . . . . . . . . . . . . . . 4. Structure determination of “defects” in amorphous silica . . . 5. Structure determination of phosphate glasses by using 31P NMR . 6. Application of high field “B NMR to structure determination of borate and borosilicateminerals and glasses . . . . . . . 7. Application of NMR imaging to detect flaws in composite ceramics in green state . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

1 2

5 11 16

20 24 26

High-resolution Solid-state NMR Studies on Ceramics S. HAYASHI 1. Introduction . . . . . . . . . . 2. Experimental aspects of NMR techniques 3. Interpretation of NMR parameters. . . 4. Conventional ceramics . . . . . . . 5 . High-performance ceramics . . . . . 6. Bioceramics . . . . . . . . . . 7. Concluding remarks. . . . . . . . References. . . . . . . . . . .

, . . . . . . . . . , . . . . . . . . . , . . . . . . . . . , . . . . , . . . .

. . . . . . . . . .

. . . . .

29 30 33 36 66 83 83 84

. . . . . . . . . . . . . . . .

91 93

. . . . . .

.

. .

NMR Studies of Zeolites H. PFEIFER and H. ERNST 1. Introduction . . . . . 2. Frameworkof zeolites . . .

CONTENTS

3. Bronsted acid sites . . . . . . . . . . . . . . . . 4 . Lewis acid sitedextra-framework aluminium . . . . . . . 5. Structure of adsorbed molecules . . . . . . . . . . . 6. Molecular diffusion . . . . . . . . . . . . . . . . 7. Chemical reactions . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

106 126 132 138 159 176

NMR Studies of Higher-order Structures of Solid Polymers H . KUROSU. S. ANDO. H . YOSHIMIZU and I . ANDO 1. Introduction . . . . . . . . . . . . . 2. Engineering plastics and high-performance polymers 3. Polymer alloys . . . . . . . . . . . . 4 . Natural polymers . . . . . . . . . . . . 5. Conclusion. . . . . . . . . . . . . . References . . . . . . . . . . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

189 190 239 251 269 269

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

277' 278 280 300 315 315 315

. . . . . . . . . . . . . . . . . . . . .

323

NMR Studies of Organic Thin Films F . D . BLUM 1. Introduction . . . 2. Background . . . 3. Polymers at interfaces 4 . Surface-active agents 5 . Conclusions . . . Acknowledgement . References . . . .

Index

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

Application of NMR Spectroscopy to the Science and Technology of Glasses and Ceramics H. KAWAZOE Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Natatsuta, Midori-ku, Yokohama 227, Japan

1. Introduction 2. Identification and quantization of silicate anions in solid silicates by 29Si MAS-NMR and effects of bonding characteristics on the chemical shift 3. Distribution of silicate structures in silicate glasses by using 29SiMASNMR 4. Structure determination of “defects” in amorphous silica 5. Structure determination of phosphate glasses by using 31PNMR 6. Application of high field “B NMR to structure determination of borate and borosilicate minerals and glasses 7. Application of NMR imaging to detect flaws in composite ceramics in green state References

1

2

5 11 16 20

24 26

1. INTRODUCTION Nuclear magnetic resonance (NMR) spectroscopy has a characteristic among spectroscopic methods in that it is appropriate for identifying and quantifying structural units involved in molecules or solids rather than determining their detailed geometrical structure. This is especially useful in structure determination of amorphous materials and glasses for which the diffraction methods are less effective. Information on structure is essential to understanding the properties of solid materials. The first systematic study of glass structure by using NMR spectroscopy’ appeared shortly after the first observation of nuclear magnetic resonance phenomena in 1945.2.3In the 1960s broad-line NMR instruments were only available for solids. Therefore, the application was limited to elucidation of local structures around “B in borate and borosilicate glasses. Boron in these glasses usually has two different local structures, tetrahedral and trigonal, ANNUAL REPORTS ON NMR SPECl’ROSCOPY VOLUME zs ISBN n-12-~05328-2

Copyright 0 1994 Academic Prws Limited A / / righu of reproduction in any form reserved

2

H.KAWAZOE

depending upon local chemical fields. In the case of "B NMR, line shape and line width are markedly affected by the interaction between the nuclear quadrupole moment of "B and an electric field gradient at the position of the nucleus. The line width broadening due to dipolar interactions between the nuclei and the anisotropy of chemical shifts does not exceed the broadening due to the nuclear quadrupole effect. This enables one to analyse local structures around boron atoms in the oxide glasses without using the recently developed techniques such as magic angle spinning (MAS) .4 The structure determination contributed considerably to solving the problem of the "boric oxide anomaly", which has been important from the viewpoints of glass science and glass technology. The magic angle spinning (MAS) technique4 for solids opened new applications of high resolution NMR in the sciences and technologies of glasses and ceramics. Decrease in line width by MAS is not satisfactory in some cases for the detailed analysis of chemical structures of the materials, but is still effective in some applications. The distinction of Si04 structural units in silicates, in solids and solution^,^ which are usually abbreviated as Qo, Q1, Q2, Q3 and Q4,where Q stands for the tetrahedrally coordinated Si by four oxygens and the superscripts for the number of bridging oxygens (nearest neighbouring Si04 units), has been established. The chemical shifts in 29Si MAS NMR of the respective Q' species were given empirically by collecting the spectra of the minerals with known crystal structures. After the pioneering work on 29Si MAS-NMR, MAS or MAS-CP methods on 27A16and 31P7 were applied to a wide variety of problems in ceramics and For instance, the structure of zeolite gel was determined by a combination of 27Aland 29SiNMR." The purpose of the present article is to overview the recent advances in applications of NMR to the sciences and technologies of glasses and ceramics from the viewpoint of the materials scientist. There have been some published and excellent reviews from this standpoint' '-12 and this chapter will deal with the topics which might be epoch making in the applications. 2. IDENTIFICATION AND QUANTIZATION OF SILICATE ANIONS IN SOLID SILICATES BY 29SiMAS-NMR AND EFFECTS OF BONDING CHARACTERISTICSON THE CHEMICAL SHIFT

Identification and quantization of various types of silicate anions by reliable experiments have long been an important problem in glass science and mineralogy. 29Si MAS-NMR enables one to evaluate the distribution of silicate anions in the materials. Lippmaa ef ~ 1 . applied ~ ~ ' ~the technique to establish one by one correspondence between the isotropic chemical shift value and a particular Q' unit in solid silicates with known crystal structures.

APPLICATION OF NMR SPECTROSCOPY TO GLASSES AND CERAMICS

/"i Q"

Silicate anions in solution

Q'

Q2

3

d

Q3

Solid silicates

-110 ppmlTMS)

t

-70

-80

-90

-100

Fig. 1. Ranges of 29Si chemical shifts of different structural units of silicate anions in solid silicates and silicate solutions.

Figure 1 shows ranges in the observed chemical shift values of 29Si in Q' species dissolved in solutions and solid silicates with respect to tetramethyl silane (TMS). The chemical shift is affected by the type of charge compensating cations. The empirical correlation can be applied to Na, K, Ca, Mg, Zn and alkylammonium salts in the case of solid silicates. The ranges in the chemical shift of the respective Q' units were well separated from each other for a particular counter cation, and this relation is extremely useful in identifying and quantizing Q' units contained in amorphous silicates. A considerable effort has been concentrated to elucidate the relation between the chemical shift values of 29Si in Q4 units and their structural characteristics. Smith and Bla~kwell'~ found that a linear relation holds between the chemical shift values and mean secant of Si-0-Si bond angles involved in several types of silica polymorphs. The relation was used in estimating a distribution in Si-0-Si bond angle in silica g1a~ses.l~ Comparing with the results of X-ray analysis, the bond angles were found to distribute from 90" to 180" consistent with X-ray analysis, but a shape of the distribution function was different. Ramdas and Klinowski16examined extensively the effect of A1 neighbours on the chemical shift of 29Si, which is important in zeolite chemistry. The number of second neighbouring A1 to a central Q4 unit ranges from 0 to 4, and the structures are differentiated by the expression Si(0-Al)n(O-Si)4 or in short Si(nA1). A Iinear correlation was found to hold between the isotropic chemical shift values of 29Siand the sum of bond lengths between a central silicon and respective second neighbouring silicon or aluminium atoms. However, a different straight line was necessary in reproducing the chemical shifts for the structures with the same n value, the number of A1 second neighbours. The above-mentioned relations were included in the more unified and extended relation found for silicates involving Q4, Q3 and Q2 species with known structures. l7 Agreement of the calculated chemical shifts with the

4

H.KAWAZOE

Experimental chemical shift ( p p rn) Fig. 2. Experimental chemical shift plotted against chemical shift calculated from sorosilicates; A, inosilicates; 0 , equations (1) and (2). X , orthosilicates;

+,

phyllosilicates;0, tectosilicates.

observed shifts was examined in Fig. 2, and was found to be satisfactory. The correlation coefficient was 0.991 over 76 reliable data. Here the chemical shifts were calculated by using a relation:

S = 650.08f - 56.06 (ppm with respect to TMS)

(1)

where ,f was obtained by , I ! =

2 Sj[(1- 3 cos2e i ) / 3 ~ :[cos ] cril(C0S

Cyi

- l)]

i

In the calculation, summation runs over four bonds and Si is the bond valence18 of the ith bond, given by Si = exp[(ro - rj)/0.37].

(3)

APPLICATION OF NMR SPECTROSCOPY TO GLASSES AND CERAMICS

5

Si Fig. 3. Diagram showing the definitions of the angles 8 and a,and the lengths r and R. X is any second neighbour cation.

Oi, Ri, and cyi are defined as presented in Fig. 3. The success of the estimation is thought to be derived from the fact that this approach appropriately takes into consideration the geometrical characteristic of the surroundings of the central silicon and the effect of types of the second nearest cations.

3. DISTRIBUTION OF SILICATE STRUCTURES IN SILICATE GLASSES BY USING 29SiMAS-NMR This is one of the topics frequently discussed in the literature.'' In this section it will be discussed from a slightly different viewpoint, that is from the eyes of the glass scientist. The isotropic component of the chemical shift tensors of silicate anions, which are dissolved in solids, with different polymerization degree to Q4) measured from the resonance frequency of TMS, was given in Fig. l.5,'3 Each ionic species is characterized by the corresponding chemical shift and the overlapping between those for neighbouring Q's is not large. The above-mentioned empirical relation between the structures of silicate anions and the corresponding chemical shifts were successfully used in the structure determination of R20-Si02 glasses,193mwhere R means alkali cations. Distribution of different types of silicate anions in the glasses has been thought to be similar with those in the crystalline phases having the

(ao

6

H.KAWAZOE

same chemical composition. One mole of R 2 0 incorporated into Si02 is assumed to generate two non-bridging oxygens in the following reaction:

In this reaction two Q4 building units are converted to two Q3s. The reaction model is extended to the composition regions with higher alkali concentrations. Total concentration of the non-bridging oxygens in the glasses was first and directly determined by using XPS spectra of 0 ls.21 A close agreement was observed between the experimental estimations and the predictions from the chemical reaction model. However, XPS results can not identify different Q' species having different numbers of non-bridging oxygens. Figure 4 shows 29Si MAS-NMR spectra of NazO-SiOZ glasses with NazO concentration of &SO mol%.19 It is noted that in the spectrum for silica glass (NAO; Omol% Na20) only a single peak with the chemical shift value of -109 ppm with respect to TMS was observed. From a comparison with the data in Fig. 1, the resonance absorption was assigned to Q4 species. A shoulder with chemical shift of -92 ppm appeared at the tail of the Q4 peak in the spectrum of NA6 (Na20 concentration of 10mol%). The additional peak was assigned similarly to Q3. Growth of the Q3 peak and decay of Q4 resonance with increasing Na20 concentration up to 33.3 mol% were clearly seen in the spectra for NAO, NA6, NAS, NA8 and NA7. Upon the further increase in NazO content to SO mol%, the Q3 peak was similarly replaced by a new peak with chemical shift of -78ppm, which was assigned to Q2 species. Small and additional peaks were observed in both upper and lower fields of the central absorption for the most of the spectra. These are spinning sidebands (SSB), which originate from an anisotropy of chemical shift tensor. In the case of Q4 units, four oxygens coordinating to a central Si are all bridging oxygens and chemically equivalent to each other. Thus Si has an almost isotropic chemical shift tensor in Q4, which results in a weak SSB in the spectra for NAO and NA6. In the case of Q3 and Q2 units one or two oxygens among the four ligands are non-bridging oxygens and an isotropic chemical shift tensor can no longer be expected. This gives rise to much stronger SSB in the spectra of the glasses with higher concentration of Na20. The gradual replacement of Q4 with Q3 on the addition of NazO into Si02 up to 33.3 mol% Na20 and a similar change from Q3 to Q2 roughly agreed with the prediction of equation 4. However, as seen in Fig. 5,19 which exhibits a comparison of 29Si MAS-NMR spectra of sodium di- and metasilicate crystals and glasses, line widths of the crystals were far narrower than those of the corresponding glasses. The extra broadening derives from site to site structural distribution in a particular Q' species. The

APPLICATION OF NMR SPECTROSCOPY TO GLASSES AND CERAMICS

7

NA12

NAll

-50 400 -150 CHEMICAL YllFT (PPM)

Fig. 4. 29Si spectra obtained by MAS-NMR €or a series of sodium silicate glasses: (left) 0-33.3 mol% Na20; (right) 33.3-50 mol% Na20.

absorption peak of the glass with di- or metasilicate composition, for which only Q3 or Q2 species is expected, is too broad to be assigned to a single kind of Q' species. We notice here that there is a hierarchy in the structural distribution: one is the chemical distribution, that is the coexistence of different types of Q' species or polysilicate anionic species. The chemical distribution is not limited to anionic structures. Since the polyanions constitute ligand fields for the cations, coexistence of different kind of silicate anions gives rise to coexistence of Na ions having different counter anions or chemical fields. Alkali cations in silicate glasses are well known to be responsible for a wide variety of transport phenomena such as ionic conduction, alkali diffusion, ionic exchange and internal friction. The differently coordinated alkali ions are expected to contribute differently to the properties.22

8

H.KAWAZOE

Na20.2Si02 -CRYSTAL

Na20.SiOz CRYSTAL

o

-50 -100 -150 CHEMICAL SHIFT (PPM)

Fig. 5. 29Si MAS-NMR spectra for glass and crystalline modifications of Na20-2Si02 and Na204i02. Spinning sidebands are visible in the spectra.

The second kind of structural distribution is physical distribution or topological distribution, which is the site to site structural distribution within an ensemble of a single kind of chemical species. We think about Q3 units as an example. A tremendous number of Q3 species are contained even in a small piece of the glass. Principally all Q3 units have slightly different geometrical structures reflecting variations in second nearest neighbouring structures. Magnetic resonance spectroscopy is a very appropriate method to evaluate quantitatively the structural distributions in glasses.23 This is because the line broadening by the structural distribution is large enough to evaluate quantitatively and at the same time small enough not to disturb a reliable assignment.

APPLICATION OF NMR SPECTROSCOPY TO GLASSES A N D CERAMICS

PPM

9

PPM

7-

-120.00 -100.00 -80.00 PPM

.120.00 -lOo'.OO

-80.00 -6O:OO PPM

Fig. 6. Static (upper) and MAS (lower) NMR spectra of potassium tetrasilicate (left) and potassium disilicate (right).

The coexistence of two or more types of silicate anions in the stoichiometric alkali silicate glasses with di- and metasilicate compositions was confirmed clearly by a combination of high field MAS and static measurements. Figure 6 displays NMR spectra of 29Si in potassium disilicate glass.24 The upper trace is a static spectrum obtained without spinning of the sample and the lower is the MAS spectrum obtained at 4.5 kHz rotation. Although

10

H.KAWAZOE

Table 1. Chemical shift tensor values of "Si in each of Q" units present in alkali silicate glasses and distribution of each Q" in the glasses.

Glass composition Li20.4sio2 Li20.2Si02 Li20.SiOz

Q" Q4 Q3 Q2 Q4

Q3 Q2 Q3 Q2

Q' Na20 ' 4sio2 Na20 * 2Si02

-

Na20 Si02

K 2 0 .4sio2

Q4 Q3 Q2 Q4 Q3 Q2 Q3 Q2

Q' Q4

Q3 Q2

K 2 0 . 2Si02

Q4

K 2 0 . Si02

Q3 Q2 Q3 Q2

Q' R b 20 * 2Si02

Q4 Q3 Q2

Per cent contribution 63 24 13 14.5 71 14.5 8.5 83 8.5 54

40 6 7.5 85 7.5 7.5 85 7.5 57 35 8 5.5 89 5.5

2 96 2 3 94 3

miso

*I

u2

a3

-111 - 104 - 102 -107 -93 -85 - 105 -73 -102 -109 -100 -85 - 105 -92 -85 - 105 -78 -90 - 103 -92 -85 - 103 -95 -86 -98 -88 -85 - 108 -98 -89

-111

-111

-64

-64

-111 -184 - 158 - 107 - 153 - 155 -205 -163 -186 -109 -196 - 171 -105 - 176 - 171 - 175 -148 - 170 -103 - 192 - 171 - 103 - 179 - 172 -182 - 174 - 169 -108 -184 -159

(ppm)

-46

- 107 -63 -25 -55 -8 -60 -109 -52 -26 - 105 -50 -26 -70 - 18 -50 -103 -42 -26 - 103 -53 -27 -56 -33 -43 - 108 -55 -54

- 102 - 107

-63 -75 -55 -48 -60 -109 -52 -58 - 105 -50 -58 -70 -68 -50 - 103 -42 -58 - 103 -53 -59 -56 -57 -43 - 108 -55 -54

o,chemical shift values.

line width was relatively broad, we see only a single absorption, which was assigned to Q3, in the MAS spectrum. On the other hand, the static spectrum, whose line shape was highly affected by anisotropies of the chemical shift tensors, cannot be reproduced without assuming the coexistence of Q4, Q3, and Q2. The concentrations and principal values of chemical shift tensors of the respective Q' species in several alkali silicate glasses estimated by computer simulation of the spectra are given in Table l.24 In the case of the potassium disilicate glass the simple MAS spectrum indicates that almost all of Q' species involved in the glass are Q3, whereas analysis for both the MAS and static spectra suggests that 5.5% of Q4,89% of Q3 and 5.5% of Q2 are coexisting in the stoichiometric glass. Use of the

APPLICATION OF NMR SPECTROSCOPY TO GLASSES AND CERAMICS

11

static measurement is important in quantitative evaluation of the coexisting Q’ species in silicate glasses. It must be added that even the isotropic chemical shift value itself cannot be determined for the broad resonance absorption. 4. STRUCTURE DETERMINATION OF “DEFECTS” IN AMORPHOUS SILICA Neutron irradiation with the dose of 1 x lo2’ neutrons/cm2 to quartz crystal and silica glass induces a dramatic change in the densities of both materials.25 In the case of quartz crystal it decreased from 2.64 to 2.26. Inversely the density of silica glass increased from 2.21 to the same value with the irradiated quartz. The observation suggested a possibility at least from the macroscopic viewpoint that the irradiated materials have a similar structure. Measurements of Raman spectra on the materials before and after the irradiation confirmed that the assumption from the microscopic viewpoint is correct.26 As noted in Fig. 7 quartz crystal changes into amorphous silica upon heavy irradiation. In the case of silica glass the material remained in the amorphous state during the irradiation and the closely identical Raman spectra were obtained for both the irradiated quartz and silica g l a ~ s . ~ The ~ . ~ ’most pronounced change in the spectra of the glass before and after the irradiation is an enhancement of intensities of the peaks at 500cm-’ and 609cm-’. Since the peaks grew on irradiation, these features were ascribed to “defects”,27 and numerous authors have subsequently proposed specific structural models for their origin.2L35Most of the models assume the presence of dangling bonds or wrong bonds, because the continuous random network model frequently assumed as the structure of Si02 failed to explain the appearance of “defect” lines. However, there was no unique model which can satisfy the related data. Assignment of the Raman lines had been an open question. The “defect” lines in the Raman spectra were found to be enhanced without the irradiation under the polycondensation process of silica gels prepared by the sol-gel method.36737Drying of the gel at under 200°C resulted in enhancement of the D1 (500cm-’) line, while heating at 600°C induced the D2 line (609cm-I). The finding suggests that the structures responsible for the “defect” lines formed upon the dehydration and condensation reaction of hydrated silica gels. A planar ring consisting of three Si04 structure units was proposed to be the intermediate structure responsible for D2 line.35 This model has been experimentally illustrated by using solid state NMR.37 Figure 8 shows the changes in Raman spectra of the initially heated silica gel at 600”C, in which a strong D2 line was discernible, upon subjecting the sample to 100% relative humidity (RH) air at 25°C for the times indicated in the figure. It is

12

H.KAWAZOE l

~

~

'

f

'

l

'

i

'

l

~

Q-OUARTZ

L

I 10"'n/cm'

UNIRRAOIATEO a-

1

1200

.

4

1

1000

,

1

, Mx)

1

,

800 400 FREQUENCY Icm")

~

.

~

.

200

noticed that the intensity of the D2 line decreased during the exposure. This indicates that the structure responsible for the D2 line is attacked by moist air even at room temperature. The NMR spectra of the differently processed silica samples are shown in Fig. 9.37MAS and CP MAS spectra were obtained for the samples heated at the temperatures given in the figure. Hydrated MAS spectra were collected for the respective and heated samples after exposing to 100% RH for 24 h at 25°C. The NMR spectra were obtained at an Ho field strength of 8.45 T ("Si frequency of 71.49MHz). 29Si-'H CP MAS spectra were obtained using

APPLICATION OF NMR SPECTROSCOPY TO GLASSES AND CERAMICS

13

b"""'/

lXxJt200

Irnmo

#x)

800 700 6(30 500 FREQUENCY tun")

400 300

200

!Oo

0

(b)

Fig. 7. Raman spectra of unirradiated and neutron irradiated a-quartz single crystals (a) and glasses (b).

contact times of 7.5 ms. The three distinct peaks at chemical shifts of about -91, -101, and -110ppm in the CP MAS spectra of 50°C and 200°C gels and in the rehydrated spectrum of the 600°C sample correspond to Q2, Q3 and Q4 structural units, respectively. The non-bridging oxygens in Q2 and Q4 species are bonded to hydrogen as shown by a pronounced enhancement of the Q2 and Q3 peaks in the CP-MAS spectra of the corresponding samples. The MAS spectrum of the 600°C sample had no resolved peaks and centred at about - 107 ppm. Statistical deconvolution of the broad band using the chemical shift values of -91 and -101 ppm for Q2 and Q3 units require the presence of a third peak at about -105 ppm. The additional peak must be due to Q4 sites with a small value of Si-0-Si bonding angle, because the chemical shift of the Q3 structure containing the OH group is

14

H.KAWAZOE

1

0

1

1

1

, 500

1000

1560

RAMAN SHIFT (CM-’) Fig. 8. Raman spectra of SiOz gel initially heated to 600°C and subsequently exposed to 100% RH water vapour at 25°C for the times indicated.

not expected to appear in this chemical shift range. This assignment is further supported by the preferential enhancement of the hydrated Q3 and Q2 resonances in the CP-MAS spectrum of the 600°C sample. A similar deconvolution of the hydrated MAS spectrum of the 600°C sample does not require the presence of the -105 ppm peak; the spectrum was reproduced from the weighted sum of the peaks with -91, -101 and -110 ppm. The NMR data thus unambiguously associate the formation of the species responsible for D2 with the presence of Q4 species with reduced value of the bonding angle. These observations uniquely identify the D2 species as a cyclic trisiloxane (three-membered ring as originally proposed by Galeener).35 Three-membered rings are absent in the gels heated at room temperature. They form in intermediate temperature regions, predomi-

APPLJCATION OF NMR SPECTROSCOPY TO GLASSES AND CERAMICS 1H CPMASS

MASS

15

HYDRATED MASS

-100

-150

-50

-100

-150

-50

.roo

-150

-50

-100

-150

-50

200°C

.so

-100

-150

ppm FROM T M S Fig. 9. 29Si MAS and CP-MAS spectra of silica gels after heat treatments between 50 and 1100°C. Hydrated MAS spectra were collected after exposure of the original samples to 100% RH for 24 h at 25°C. The 'H CP-MAS spectrum of the 1100°C sample is greatly scale expanded in order to reveal the Q2 and Q3 resonances.

nantly at the surface by the condensation of silanol groups via the following react ion :

16

H.KAWAZOE

Changes in the average Si-0-Si bond angle upon the compaction of silica glass by neutron irradiations was recently demonstrated by 29Si MASNMR.38 The resonance peak with chemical shift of -112.0 & 0.5 ppm relative to TMS and of full width at half maximum height of 14.3ppm was obtained for the sample before the irradiation. In comparison, the irradiated silica had a chemical shift of -106.5 f 0.5 ppm and a line width of 17.6 ppm. Thus the change in chemical shifts from -112.0 to -106.5ppm can be unambiguously interpreted in terms of a change in the Si-0-Si bond angle distribution. The macroscopic compaction can be related to the decrease in the average bond angle of Si-0-Si.

5. STRUCTURE DETERMINATION OF PHOSPHATE GLASSES BY USING "P NMR

As in the case of silicates, PO4 building units in polyphosphates are termed as Q', where i ranges from 0 to 3. Q3 stands for the PO4 unit having three nearest neighbouring PO4 groups around the central PO4 unit, which are involved in crystalline ultraphosphates. Similarly, i denotes the number of neighbouring PO4 units; Qo, Q' and Q2 are respectively contained in crystalline ortho-, pyro-, and metaphosphates. Distribution in polyphosphate anions in a particular phosphate glass was estimated by applying chromatographic methods39 in which dissolution of the glass into aqueous solution is unavoidable. A long-standing question has been whether a distribution in polyphosphate anions with different polymerization degree in the glass is changed or not on the dissolution process. NMR has an advantage over the chromatographic methods because it is a non-destructive analysis. 31P MAS-NMR has applied to the structure determination of phosphate minerals and In the case of 31P NMR it was demonstrated that the estimation of anisotropies in the chemical shift tensor in addition to the isotropic part is important in the identification of Q' species in phosphate glasses.44 Glass samples with the composition xCaO.(l- x)P2O5, xSrO(1 - x)P205, andxBaOm(1- x)P205were obtained by melting the appropriate mixtures of starting reagents. 31P CW NMR spectra were measured at a field of 6.4T and a frequency of 109.31MHz. The spectra were accumulated up to 80 times to achieve a signal to noise ratio larger than 150. MAS spectra were recorded on a JEOL GX 270 at a frequency of 109.35MHz and spinning frequencies of 4.7-4.9 kHz. All chemical shifts have been measured relative to 85% aqueous phosphoric acid. Negative shifts correspond to higher field strength. Figure 10 denotes 31P MAS-NMR spectra of strontium phosphate glasses. The three spectra had a shoulder in the higher field side, whose intensity increased with decreasing SrO/P205ratio. Results of a deconvolution of the

APPLICATION OF N M R SPECTROSCOPY TO GLASSES AND CERAMICS

0

10

20

- experiment

30

40

50

60

17

70

- cs [PPml theory

-single-components

Fig. 10. Three typical central lines of 31PMAS spectra for xSrO-$l.- x)P205glasses. The intensity of the Q3 line decreases and the intensity of the Q hne increases with growing x .

observed spectra into two absorptions are given as dotted lines. The growth of the shoulder on going from the glass with high SrO content to that of low concentration corresponds to the expected increase in the content of Q3 species. The other resonance is assigned to Q2 groups and consequently a decrease in the intensity with respect to the shoulder. The chemical shift values and half widths were evaluated and the results

Table 2. Isotropic chemical shifts, anisotropy and asymmetry of the chemical shift tensors of 31Pin each of Q" units present in xCaO. (1 - x ) P205 glasses, and distribution of each Q" in the glasses. X

0.30 0.375 0.40 0.45

Q" content

4 ,

(PP~)

Half line width (ppm)

A6 ( P P 4

11

Q2

Q3

Q2

Q3

Q2

Q3

Q2

Q3

Q2

Q3

0.52 0.61 0.72 0.89

0.48 0.39 0.28 0.11

-36.8 -35.5 -31.3 -27.3

-48.2 -47.8 -43.5 -36.9

11.1 12.1 11.2 10.6

15.1 10.4 13.5 16.2

-260 -247 -234 -209

-193 -180 - 142

-201

0.37 0.39 0.41 0.44

0.23 0.22 0.21 0.20

The error for the Q" content is 20.02, for isotropic cs ) , 6 ( f0.05.

and half line width k0.5 ppm, for the anisotropy (As) 25 ppm, for the asymmetry (7)

APPLICATION OF NMR SPECTROSCOPY TO GLASSES AND CERAMICS

19

31P- BL - NMR 6.4T

x(SrO)(l - x)(P206)

. -200

-100

-experiment

0

100

200

:300'

- cs [PPml theory

- single-components

Fig. 11. Three ty ical 31P CW NMR spectra for xSrO.(l -x)P205 glasses. The intensity of the Q line decreases and the intensity of the Q2 line increases with growing x .

P

are given in Table 2. Gaussian line shape was assumed in the deconvolution. A distribution in isotropic chemical shift resulted in a remarkably broad absorption, and spinning sidebands could not be observed. This disturbs a quantitative evaluation of content of the coexisting Q' groups. Figure 11 shows the CW-NMR spectra of 31P in the same glasses. Two distinct and anisotropic absorptions were seen in the spectra. The anisotropy and asymmetry of the chemical shift tensor were evaluated by the computer simulation, and given in Table 2. The calculated spectra were denoted as

20

H.KAWAZOE

1.0 -

0.2

0.4

0.3

0.5

X

-theory

B C ~ Q ~ oSr tlCaQ3 OSr

@ Q3

rBaQ2 aBaQ3

Fig. 12. Q" content versus chemical composition for xRO.(l(R = Ca, Sr, Ba).

x)P205

glasses

dotted lines in the figure. The less anisotropic and less asymmetric signal was ascribed to Q3 units. Figure 12 denotes the relative concentration of the respective Q' species in the phosphate glasses. Calculated values from chemical compositions of the glasses are shown in the solid lines. Agreement between the observed and calculated is satisfactory for high RO content, but the discrepancy is pronounced for the lower content of RO, which might be due to high hygroscopicity of the glasses. 6. APPLICATION OF HIGH FIELD IlB NMR TO STRUCTURE DETERMINATION OF BORATE AND BOROSILICATE MINERALS AND GLASSES

As stated in the introduction, "B NMR was the first example successfully applied to structure analysis in inorganic materials. The width of the central transition caused by the second order quadrupole interaction is far greater than the width induced by dipolar broadening and chemical shift anisotropy. This enables one to determine the ratio trigonal B03/tetrahedral B04. Application of "B MAS-NMR to a determination of the ratio in borate glasses and minerals is naturally expected to result in a more reliable value than those evaluated by the CW broad line technique, but this is not so simple4-' because of the overlapping of the two resonances and the broadening due to 'H-l'B dipolar interactions, the hydrogen being included accidentally or from water of hydration, Optimum conditions for obtaining well resolved "B MAS-NMR was found for borate glasses and minerals.48 It was shown that rapid sample

APPLICATION OF NMR SPECTROSCOPY TO GLASSES AND CERAMICS

21

spinning larger than 6 kHz and high power proton decoupling are desirable for rapid acquisition of IIB NMR spectra of the borates, from which accurate trigonavtetrahedral ratios may be determined. NMR spectra were recorded on a 11.7 T spectrometer, the magnetic field corresponding to IIB NMR frequency of 160.4 MHz. All spectra were recorded using 2 ps pulse excitation. The solution 90" pulse width for boron trifluoride etherate was 9ps. Chemical shifts are reported in ppm with respect to an external standard of BF3.Etz0 and positive values correspond to low field and high frequency. Figure 13 shows static and MAS-NMR spectra of "B in three minerals; kernite (NazB406(OH)z/3HzO), inderite (MgB303(OH)5/(Hz0)4/HzO)and borax (NazB407/10HzO).48TrigonaYtetrahedral ratios of the minerals are 2/2, 1/2 and 2/2, respectively. The spectra in A and B were obtained under a static condition (without sample spinning), and those in A and B were recorded without and with proton decoupling, respectively. It is noted from a comparison of the spectra in A and B that proton decoupling contributes to reduce the width of the absorptions, this being marked in borax in which WB ratio is high, and that even the use of a high field of 11.7T is insufficient for obtaining well resolved spectra. 3.8 kHz sample spinning at the magic angle was applied in measuring the spectra shown in C and D. Again in this case, those in C are proton coupled spectra and in D are the decoupled. Although the spectra were more resolved compared with the static ones, MAS at 3.8 kHz alone was not sufficient especially for inderite and borax with high WB ratio. The width caused by 'H-"B dipole interaction was reduced in D by proton decoupling and well resolved spectra were obtained. Distinction of quadrupolar broadened absorption due to a trigonal boron which appears in large chemical shift regions from the narrow resonance of a tetrahedral boron having a chemical shift value close to 0, is possible. However, the spectra in D are still insufficient because of the presence of spinning sidebands which disturbs the accurate estimation of the trigonavtetrahedral ratios. Much higher sample spinning (6.4 kHz) and proton decoupling and the use of high frequency are the optimum conditions for the reliable estimation of the ratios. By using computer simulation of the spectra in F the theoretical ratios were well reproduced for the three samples. Quadrupole coupling constants, e2qQ/h in MHz, and isotropic chemical shift with respect to BF3.Etz0 were estimated for the trigonal and tetrahedral borons and given in Table 3. The observed quadrupole constants were in the range 2.3-2.6MHz for trigonal borons, and 0-0.5 MHz for tetrahedral ones. These are in close agreement with those previously reported. '1,42 A remark must be added here. The above-mentioned conditions are undoubtedly appropriate for accurate estimation of the ratios, but inadequate for determining asymmetry parameter q for the quadrupole coupling tensor of tetrahedral borons. In this case low field measurements are more reliable.

22

H.KAWAZOE

Kernile WB.2

H/8=5

-50

20

I

0

Borax WB.5

lnderiie

-20

50

0

-50

20 0 -2b ppm from BF3.Et20

50

20

0

-so

0

-20

Fig. 13. 11.7T "B static and MAS-NMR spectra of kernite, inderite and borax showing the effects of specimen rotation rate, and of 'H decoupling, on spectrum appearance. WB refers to the hydrogen to boron ratio. A , Static, coupled spectra; B, static, decoupled spectra; C, 3.8kHz MAS, coupled; D, 3.8kHz MAS, decoupled; E, 6.4 kHz MAS, coupled; F, 6.4 kHz MAS, decoupled. Spectra were recorded using 2 ps pulses, 219 of the 90" pulse for the solution, the recycle time was 1s, and a 10 Hz line broadening due to exponential multiplication was applied.

Table 3. "B nuclear quadrupole coupling constant and chemical shift data for a series of borates and borosilicates.

IIB NMR parameters Trigonal Sample Boracite Colemanite Danburite Datolite Inderite Inyoite Kernite Tourmaline

Formula and source

QCC

Mg3B7OI3C1,Stramfurt, Germany CaB3O4(OH),-H20, Turkey CaBzSizOs, Mexico CaB(SiO4)(OH),W. Patterson, NJ MgB303(0H)5-(H20)4*H~0, Kern co., calif. CaB303(0H)5- 4Hz0, Death Valley, Calif. NaZB406(OH)2-3H20, Kern Co., Calif. (Na, Ca)(Li, Mg, AI)(Al, Fe,

2.6 2.4

8:

Theory

Experiment

Tetrahedral QCC

sb 1.o

No. of No. of No. of No. of B O ~ B O ~ BO; BO$

d 2.4

16.0 17.0 d d 18.1

-0.3 -0.3 -0 -0 -0.2

1.4 -0.7 1.0 1.o

-

2.3

17.4

-0.2

2.4

18.5

e

-

1

6 2 1 1 2

1

6 2 1 1 2

1.5

1

2

1

2

-0.2

0.9

2

2

2

2

12.7f

d

d

1

-

1

1

2.45 d 2.5 2.5 2.4 2.5 2.3

18.2 d 17.9 18.9 19.0 12.6 16.0

1.2 -3.3 1.7 1.4 2.0 1.8

2

3 1 2

2

3 1 2 1 2 2 1

d d

d d

-0.3 0 -0.2 -0.3 -0.3 -0.5 -0.5 -0

d

1 1

1 1

0

TI

z

Lo

Mn)6(B03)3(Si6018)(OH)41

Ulexite Boron phosphate Lithium borate Potassium pentaborate Borax Pyrex (1) (2) Reedmergnerite Alkali feldspar

Newry, Mass. NaCaB506(OH),-5Hzo, Unknown BP04, Alfa Products Li2B4O7,Noah Chemical KZB10016-8Hz0, Alfa Products NazB407-10HzO,MCB Reagents NaBSi30s,Duchesne Co., Utah NaA1Si308 (B)

-0

0.2

-

2 4 1 2 2 Unknown Unknown

2 4 2 6 1

-1.9 -1.1, -2.5 ~

"Quadrupolecoupling constant, in MHz. Error is fO.1 MHz. "In ppm from BF,-Et,O. Error is f0.2ppm. 'Error is ? 10%. dNo boron of this coordination present. 'Not determined. fCentre of peak.

-

~

~~~

h)

w

24

H.KAWAZOE

7. APPLICATION OF N M R IMAGING TO DETECT FLAWS IN

COMPOSITE CERAMICS IN GREEN STATE

Composite ceramic materials, such as S i c fibre reinforced alumina, are believed to be the most promising ceramics of high fracture toughness and high reliability. There are several factors such as agglomerates of the fibres, metallic inclusions, and pores formed during the formation of the green compact, all of which can either cause fracture of the composite or contribute to the formation of even larger flaws.49-50A considerable improvement in the reliability can be achieved by a complete examination in the green state, where most of the conditions that contribute to the fracture rigi in ate.^' In this section the application of NMR imaging as a nondestructive technique to detect physical flaws in green state fibre reinforced ceramic matrix composites is disc~ssed.~' NMR imaging experiments were first reported in 197353 and several different approaches to data acquisition and processing developed The two-dimensional spin warp method57 was used. This technique uses a standard 90" pulse-delay-180" pulse-delay echo sequence in which the 90" pulse, and occasionally the 180" pulse, are selective. A field gradient is applied along the direction of the magnetic field (2) during the selective pulse (s) in order to control the width of the cross-section of the sample in which the NMR signal is excited. A second gradient, applied orthogonally to the t gradient immediately following the 90" pulse and during the acquisition of the echo signal, provides frequency encoding, while a third gradient, orthogonal to the first two, is incremented stepwise for phase encoding. These latter two gradients yield spatial information in the plane of the cross-sectional slice. The samples were fabricated by slipcasting slurries of alumina containing 20 vol% S i c fibres (diameter 8 p m and length 1mm). Total solid content was 60 wt% of the slip. Slipcast samples of cylindrical shape, 8 mm diameter and 20 mm high, were sealed in 10 mm outside diameter NMR tubes as soon as their surface was dry enough to handle. In the imaging experiment 'H is chosen as an NMR nucleus due to its high sensitivity, and a high field instrument of 9.2 T (lH frequency is 400 MHz) was used. Representative images obtained from the sample are shown in Fig. 14. The images were obtained using a 10 mm Helmholtz coil. A soft 90" pulse of 1.8ms duration and a hard 180" pulse of 54ps duration were employed in the imaging sequence. TE, defined as the time from the centre of the soft 90" pulse to mid-echo, was 8ms, and TR, the delay between repetitions, was set to 500ms. Sixteen scans were acquired at each of 256 phase encoding steps, for a total acquisition time of approximately 34 min per image. Resolution of 70 X 70 pmZ was achieved with X,Y gradients of 9.6 X 10d4T/cm and acquisition of a 256 x 256 data point matrix. Slice thickness in each image is approximately 0.8 mm.

APPLICATION OF NMR SPECTROSCOPY TO GLASSES AND CERAMICS

25

Fig. 14. Proton NMR images of three 0.8mm-thick slices of the sample, spaced 4 mrn apart. The pixel resorution in each case is 70 x 70 pm'. The diameters of the flaws marked with arrows are (a) 300 pm, (b) 350 p m (flaw near edge) and 375 p m (flaw near centre), (c) 425 pm. Diameter of the tube is 9 mm.

The images reflect the relative amount of 'H spins across a transverse cross-section through the sample. Bright areas indicate high water content and dark areas represent pockets of air between particles or large physical flaws such as inclusions and agglomerates of fibres. Fibre reinforced composites are very porous in the green state, due to the loose packing of the ceramic grains around the fibres. The fine distribution of small spots across the images indicated the presence of water in these open porosities. The dark spots or areas indicate flaws in the composites, caused either by foreign particles or open pores and fibre bundles formed during the casting process. Dimensions of the larger flaws detected in the images are noted in the figure caption.

26

H.KAWAZOE

REFERENCES A. H. Silver and P. J. Bray, J . Chem. Phys., 1958, 29, 984. E. M. Purcell, H. C. Torrey and R. V. Pound, Phys. Rev., 1946, 69, 37. F. Bloch, W. W. Hansen and M. E. Packard, Phys. Rev., 1946, 69, 127. E. R. Andrew, Progr. Nucl. Mag. Res. Spectrosc., 1971, 8 , 1. E. Lippmaa, M. Magi, A. Samoson, G. Engelhardt and A.-R. Grimmer, J. A m . Chem. SOC.,1980, 102, 4889. 6. D. Miiller, W. Gessner, H.-J. Behrens and G. Scheler, Chern. Phys. Lett., 1981, 79, 59. 7. A.-R. Grimmer and U. Haubenreisser, Chem. Phys. Lett., 1983,99,487. 8. R. Dupree, D. Holland and D. S. Williams, Phys. Chem. Glasses, 1985, 26, 50. 9. U. Haubenreisser, Glastechn. Ber., 1986, 59, 174. 10. C. A. Fyfe, G. C. Gobbi, J. S. Hartman, J. Klinowski and J. M. Thomas, 1. Phys. Chem., 1982, 86, 1247. 11. P. E. Stallworth and P. J. Bray, in Nuclear Magnetic Resonance in Glass In Glass: Science and Technology, Vol. 4B (eds D. R. Uhlman and N. J. Kreidl), pp. 77. Academic Press, London, 1990, pp. 77-149. 12. J. F. Stebbins, MRS Bull., 1992,45. 13. M. Magi, E. Lippmaa, A. Samoson, G. Engelhardt and A.-R. Grimmer, J . Chem. Phys., 1984, 88, 1518. 14. J. V. Smith and C. S. Blackwell, Nature, 1983, 303, 223. 15. R. Dupree and R. F. Pettifer, Nature, 1984, 308,523. 16. S. Ramdas and J. Klinowski, Nature, 1984, 308,521. 17. B. L. Scherriff and H. D. Grundy, Nature, 1988,332,819. 18. D. Altermatt and I. D. Brown, Acra Crystullogr., 1985, B41, 240; I. D. Brown and D. Altermatt, ibid., 1985, B41, 244. 19. R. Dupree, D. Holland, P. W.McMillan and R. F. Pettifer, J. Non-Cryst. Solids, 1984,68, 399. 20. A.-R. Grimmer, M. Magi, M. H b e r t , H. Stade, A. Samoson, W. Wieker and E. Lippmaa, Phys. Chem. Glasses, 1984, 25, 105. 21. R. Briickner, H.-U. Chun, H. Goretzki and M. Sammet, J. Non-Cryst. Solids, 1980, 42, 49. 22. H. Kawazoe, M. Takagi, T. Kanazawa and I. Yasui, Glasrechn. Ber., 1983, 56K, 1035. 23. P. C. Taylor, J. F. Baugher and H. M. Kriz, Chem. Rev., 1975,75,203. 24. J . F. Emerson, P. E. Stallworth and P. J . Bray, J . Non-Cryst. Solids, 1989, 113, 253. 25. E. LeII, N. J. Kreidl and J. R. Hensler, in Progress in Ceramic Science, Vol. 4 (ed. J. E. Burke), p. 6. Pergamon, Oxford, 1966. 26. J. B. Bates, R. W. Hendricks and L. B. Shaffer, J . Chern. Phys., 1974, 61, 4163. 27. R. H. Stolen, J. T. Krause and C. R. Kurkjian, Disc. Faruday Soc., 1970, 50, 103. 28. F. L. Galeener, J. C. Mikkelsen, Jr and N. M. Johnson, in The Physics o f S i O z and i&s Interfaces (ed. S . T. Pantelides), p. 284. Pergamon, New York, 1978. 29. R. B. Laughlin, J. D. Joannopoulous, C. A. Murray, K. J. Hartnett and T. J. Greytak, Phys. Rev. Lett., 1978, 40,461. 30. C. A. Murray and T. J. Greytak, J . Chem. Phys., 1979, 71, 3355. 31. G. N. Greaves, J . Non-Cryst. Solids, 1979, 32, 295. 32. G. Lucovsky, Phil. Mag., 1979, 39, 513. 33. E. J. Friebele, D. L. Griscom, M. Stapelbroeck and R. A. Weeks, Phys. Rev. Lett., 1979, 42, 1346. 34. A. R. Silin and P. J. Bray, Bull. A m . Phys. SOC., 1981, 26, 218. 35. F. L. Galeener, 1. Non-Cryst. Solids, 1982,49, 53. 36. V. Gottardi, M. Guglielmi, A. Bertoluzza, C. Fagnano and M. A. Morelli, J. Non-Crysr. Solids, 1984, 63, 71. 1. 2. 3. 4. 5.

APPLICATION OF NMR SPECTROSCOPY TO GLASSES AND CERAMICS

27

37. C. J. Brinker, R. J. Kirkpatrick, D. R. Tallant, B. C. Bunker and B. Montez, J. Non-Cryst. Solids, 1988, 99,418. 38. A. C. Wright, B. Bachra, T. M. Brunier, R. N. Sinclair, L. F. Gladden and R. L. Portsmouth, J. Non-Cryst. Solids, 1992, 150, 69. 39. J . R. van Wazer, Phosphorus and its Compounds, Vol. 1, Interscience, New York, 1958. 40. U. Haubenreisser, G. Scheler and A.-R. Grimmer, Z. unorg. allg. Chem., 1986, 532, 157. 41. T. M. Duncan and D. C. Douglas, Chem. Phys., 1984, 87,339. 42. G. L. Turner, K. A. Smith, R. J. Kirkpatrick and E. Oldfield, J. Mugn. Res., 1986, 70, 408. 43. I . L. Mudrakovskii, V. P. Shmachkova, N. S. Kotsarenko and V. M. Mastikhin, J. Phys. Chem. Solids, 1986,47,335. 44. P. Losso, B. Schnabel, C. Jager, U. Sternberg, D. Stachel and D. 0. Smith, J. Non-Cryst. Solids,1992, 143, 265. 45. S. Schramm and E. Oldfield, J . Chem. SOC. Chem. Commun., 1982, 980. 46. C. A. Fyfe, G. C. Gobbi, J. S. Hartman, R. E. Lenkinski, J. H. O’Brien, E. R. Beange and M. A. R. Smith, J. Magn. Res., 1982,47, 168. 47. S. Ganapathy, S. Schramm and E. Oldfield, J. Chem. Phys., 1982,77,4360. 48. G. L. Turner, K. A. Smith, R. J. Kirkpatrick and E. Oldfield, J. Magn. Res., 1986, 67, 544. 49. P. S. Nicholson, Can. Cerum. Q., 1987, 60,26. 50. D. B. Marshall and J. E. Ritter, A m . Ceram. SOC. Bull., 1987, 66, 309. 51. R. W. McClung and D. R. Johnson, MRS Bull., 1988, 13, 34. 52. S. Karunanithy and S. Mooibroek, J. Mater. Sci., 1989, 24, 3686. 53. P. C. Lauterbur, Nature, 1973, 242, 190. 54. P. A. Bottomley, Rev. Sci. Instrum., 1982, 53, 1319. 55. S. L. Smith, Anal. Chem., l985,57,595A. 56. M. A. Foster and J. M. S. Hutchinson, 1. Biomed. Engng, 1985, 7, 171. 57. W. A. Edelstein, J. M. S. Hutchinson, G. Johnson and T. W. Redpath, Phys. Med. Biol., 1980, 25, 751.

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High-resolutionSolid-state NMR Studies on Ceramics S. HAYASHI National Institute of Materials and Chemical Research, Tsukuba, Zbaraki 305, Japan and Department of Chemistry, University of Tsukuba, Tsukuba, lbaraki 305, Japan 29 30 30 31 32 32 33 33 34 35 35 36 36 43 49

1. Introduction 2. Experimental aspects of NMR techniques 2.1. High-resolution techniques 2.2. Signal enhancement techniques 2.3. Long spin-lattice relaxation time 2.4. Multinuclear approach 3. Interpretation of NMR parameters 3.1. Chemical shift interaction 3.2. Dipolar interaction 3.3. Quadrupole interaction 3.4. Mechanism of spin-lattice relaxation 4. Conventional ceramics 4.1. Clay minerals 4.2. Cements 4.3. Glasses 5. High-performance ceramics 5.1. Silicon carbide 5.2. Silicon nitride 5.3. Silica and silicates 5.4. Alumina, aluminates and aluminium nitride 5.5. Sialon and its analogues 5.6. Newly accessible nuclei 6. Bioceramics 7. Concluding remarks References

66

67 74 74 77 79 83 83 83 84

1. INTRODUCTION

It is difficult to define “ceramics” strictly, since new ceramics are developed day by day. Conventional ceramics include pottery, porcelain, cement, glass, and so on. Recently, an enormous number of “new ceramics” have ANNUAL REPORTS ON Nh4R SPECTROSCOPY VOLUME 28 ISBN 0-12-505328-2

Copyrighf @ 1994 Academic Press Limited A / / rights of reproduction in any form resewed

30

S.HAYASHI

been developed, which include high-performance ceramics such as silicon carbide, silicon nitride, sialon, etc. Nowadays, ceramics can include all the inorganic materials except for metals. Consequently, “ceramics” include too many materials to be covered here. Limiting the range of the materials, we describe the conventional ceramics, typical high-performance ceramics, and bioceramics. The following materials are not reviewed, irrespective of their importance; zeolites, zeolite-like aluminophosphates, hydrogenated amorphous silicon, and high-T, superconductors. They should be reviewed in separate chapters. Since the pioneering works of Pines et al.’ and Schaefer and Stejskal? high-resolution solid-state 13C NMR has been used widely in the field of organic polymer^.^ The high-resolution techniques have been applied also to silicates and aluminosilicates since the works of Lippmaa et and Klinowski et uL6 NMR is a powerful tool to investigate local structures in those solid materials. Although 29Si is the most popular nucleus in NMR studies on ceramics, other nuclei are important as well; for example, ‘H, llB, 13C, 23Na, 27Al, 31P, and so on. Multinuclear approach is important in NMR studies on inorganic solids including ceramics. Several review papers and books have been published for NMR on inorganic solids.”” In the present chapter, high-resolution solid-state NMR studies on ceramics are reviewed. Although a large number of works using the conventional NMR techniques have been published up to now, they are out of scope in this chapter. Since chemical shift is the most important parameter in high-resolution solid-state NMR, almost all the works reviewed here are concerned with local structures. 2. EXPERIMENTAL ASPECTS OF NMR TECHNIQUES 2.1. High-resolution techniques

Chemical shift parameters are the most important to identify structural units. To extract accurate values of isotropic chemical shift, several techniques are used to narrow the resonance line. In this section, highresolution techniques are summarized briefly. The techniques used depend on the properties of the observed spins. From the viewpoint of the high-resolution techniques, the spins can be classified into three groups; rare spins with a spin quantum number (S) of 1/2 (for example, I3C and 29Si), abundant spins with S = 1/2 (lH and 19F), and quadrupolar spins with S 2 1 (“B, 23Na, and 27Al). The words “rare” and “abundant” mean low and high natural abundances, respectively. In practical experiments, however, the abundant spins can become “rare” when the spin concentration is low in the samples studied. For example, although 31P has a natural abundance of loo%, techniques for the rare

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

31

S = 1/2 spins can be applied to the 31P measurements of phosphoruscontaining materials such as inorganic phosphates. The line width in the spectra of the rare S = 112 spins is dominated by chemical shift anisotropy and dipole-dipole interaction with other abundant spins such as 'H and 19F. Homonuclear dipole-dipole interaction is negligibly small. Magic angle spinning (MAS) of the sample is commonly used to reduce the line width due to dipole-dipole interaction and chemical shift anisotropy. l2?l3 The spinning rate is ordinarily lower than 10 kHz, which is insufficient to suppress the dipolar interaction completely. For the case of 'H, 'H dipolar decoupling (DD) is carried out only during the signal acquisition. Other spins such as 19F have rarely been decoupled, since the second channel in commercial spectrometers is usually tuned to the 'H frequency. When the observed nucleus belongs to the abundant S = 1/2 spins, homonuclear dipolar interaction is dominant, which is suppressed by multiple pulse sequences such as WHH-4,14 MREV-8,'5,'6 and BR-24. Combining with MAS, both the homonuclear dipolar interaction and chemical shift anisotropy are reduced, which method is called "combined rotation and multiple pulse spectroscopy'' (abbreviated as CRAMPS). Heteronuclear dipolar interaction can also be suppressed by MAS to some extent, even if it exists. For quadrupolar nuclei, quadrupole interaction should be considered additionally. The first-order quadrupole interaction spreads out the signals of satellite transitions to an extent of megahertz order. This interaction can be averaged out by MAS, and an enormous number of spinning sidebands are observed frequently. The central transition (rn = 1/2 -1/2) for half-integer spins is not affected by the first-order interaction but by the second-order interaction. The second-order quadrupole interaction is partially reduced by MAS, and the line width is reduced only by a factor of 3 or 4.1'),20The second-order interaction can be averaged out by either dynamic angle spinning (DAS)21or double rotation (DOR) techniques.22

''

f,

2.2. Signal enhancement techniques Cross-polarization (CP) from 'H to rare spins is used to enhance the signal intensity and to improve the accumulation e f f i ~ i e n c ywhen , ~ ~ the observed nucleus is in the neighbourhood of '€3. Another use of the CP is selective observation of those spins dipolarly interacting with 'H spins. The matching condition for CP from 'H is described as y I H l l = ysHls, where yI and ys are nuclear gyromagnetic ratios of 'H and the observed spin, respectively, and and Hls are magnetic field strengths of the RF pulses. For quadrupolar nuclei with a large coupling constant, the condition is yIYrHlr = (S + 1/2) ysHls. Recently, Vega has studied the spin dynamics of cross-polarization from Z = 1/2 to quadrupolar S = 3/2 nuclei.24

32

S.HAYASHI

Another enhancement method is enrichment of NMR active nuclei. I7O has a natural abundance of only 0.037%, and Oldfield's group has prepared 1 7 0 enriched samples to trace 170 spectra.25

2.3. Long spin-lattice relaxation time Ceramics have often very long spin-lattice relaxation times ( T I ) . For example, the 29Si relaxation time in kaolinite with high crystallinity is ca. 2000 seconds.26The 13C Tl of synthetic diamond powder is 3600 s, while the spectra of a natural diamond have been traced 3.5 days after the sample was set in the magnet.27 The waiting time between RF pulses should be long enough to equilibrate the spin state. If 'H spins are in the neighbourhood of the observed nucleus, crosspolarization from 'H can be used, as is the case of kaolinite.28 For synthetic samples, paramagnetic impurities can be added artificially to reduce the TI values. Grimmer et u L . ~have ~ added 0.1 mol% MnO to Na20-Si02 glasses. Maekawa et uL3' have added 0.05 0.1 wt% Fez03 to Na20-Al2O3-SiO2 glasses.

-

2.4. Multinuclear approach A multinuclear approach is necessary, since ceramics consist of a variety of elements. The chemical shift value is expressed with respect to the signal position of a standard compound for each nucleus. Experimentally, second reference compounds are used for instrumental set-up. Solid compounds with the following properties are d e ~ i r a b l e : ~ ~

(1) The line width is very sharp under MAS. (2) The compound contains the observed nuclei with high concentration. (3) The TI value is short. (4) The compound is available easily. (5) The compound is stable in air for a long time. (6) For quadrupolar nuclei, the crystal symmetry is very high, resulting in no quadrupolar broadening. Table 1 summarizes reference compounds and their chemical shifts for nuclei frequently included in ceramics. For alkali and halogen nuclei, alkali halides are good reference corn pound^.^^ Reference compounds containing hydrogens are adequate to set up cross-polarization experiments. For quadrupolar nuclei, the pulse width should be shorter than 1/2(2S+ 1) of the T pulse width calibrated for solution, if one wishes to quantitate the spectra.36

33

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

Table 1. Reference compounds. ~

Compounds

Shifdppm

Tetramethylsilane Silicone rubber (&H5)20*BF3 BPO, NaBH4 Tetramethylsilane Adamantane Glycine Hexamethylbenzene Silicone rubber Nitromethane "NH4CI 1M NaCl aqueous soh. NaCl 1 M A1 (N03)3aqueous soh. AlK(S04)z * 12H.20 AIN&(SO4)2 12H20 Sodium 3-(trimethylsilyl)-propionate-d4 Sodium 3-(trimethylsilyl)-propionate Sodium 3-(trimethylsilyl)-propane-l-sulphonate Silicone rubber 85% &Po4 (NH4)2HP04 NH4HzP04

0 0.119 0 -3.60 -42.06 0 38.520,29.472 176.46,43.67 132.07, 17.17 1.412 0 -341.168 0 7.21 0 -0.21"

Nucleus

'H I'B 13c

I5N

23Na 27A1

*'si 3'P

-

-0.54"

1.445 1.459 1.534 -22.333 0 1.33 1.00

~

Ref. 32 31 31 32 32 32 32 33 31 34 34 32 32 32 32 31 31

~

"Measured at 104.26 MHz. No correction was made for the second-order quadrupole shift.

3. INTERPRETATION OF NMR SPECTRA 3.1. Chemical shift interaction

In high-resolution solid-state NMR, the isotropic chemical shift value is the most important parameter to identify the structural units. For organic polymers, the I3C spectral assignments can be made based on solution data, since an enormous amount of spectral data have been accumulated up to now. However, for other nuclei lacking in data accumulation, various attempts have been made to correlate the chemical shift with crystal structure, especially for 29Si, 31P, and "Al nuclei. In silicates, 29Si chemical shifts correlate with the degree of condensation of the Si04 tetrahedra.4,37 Figure 1 shows 29Si chemical shift ranges of Q" units. Q" means Si04 tetrahedron linked with n Si04 tetrahedra. The range moves toward lower frequency with increase in n . Incorporation of aluminium into the network shifts the 29Si signal of the neighbouring silicon towards higher frequency.- Various attempts have been made to establish

34

S. HAYASHI

t

-60

I

-70

-80

-90

-100

-110

-120 ppm(TMS1

Fig. 1. "Si chemical shift ranges of silicates with a different degree of condensation of Si04 tetrahedra. Q" units have n bridging oxygen^.^' (Reprinted with permission from J. Phys. Chem., 1984, 88, 1518. 0 1984 American Chemical Society.)

empirical correlations between the chemical shifts and structural parameters, especially Si-0-T angles (T = Si, Al),3w3 Si-0 bond length^,^^,^^ Si-T distances,39cation-xygen bond strengths,43 and group ele~tronegativity.~~ Several works have been reported on 31P chemical shifts and their anisotropies in inorganic phosphates.4s56 PO4 tetrahedra are interconnected by sharing an oxygen atom. Lower frequency shift in the isotropic value is observed as the condensation proceeds, similarly to the Si04 units. In contrast to 29Si, 31P has a considerable magnitude of chemical shift anisotropy. The magnitudes of the anisotropy have a trend Qo < Q' < Q2. The coordination of aluminium in solids can be distinguished by 27Al chemical shift.57 Aluminium coordinated by four oxygens (A104) shows a 27Al signal at ca. 60 ppm with respect to A1(H20)z+, while A106 units at ca. Oppm. A105 units are much rarer than A104 and A106. Alemany et uf.58,59 have traced good 27Al spectra of aluminophosphate AlP04-21 using high-field and very fast MAS, in which two inequivalent A105 units have chemical shifts of 14 and 16ppm and quadrupole coupling constants of 5.1 and 7.4 MHz, respectively. Consequently, the isotropic chemical shift moves toward lower frequency with increase in the coordination number. 3.2. Dipolar interaction

MAS can average out the dipole-dipole interaction in principle, if the spinning rate is fast enough. Although the dipolar interaction with 'H is too

HIGH-RESOLUTION SOLID-STATE NMR STUDIES O N CERAMICS

35

large to be suppressed by only MAS practically, it is erased by the 'H dipolar decoupling. However, if the observed nucleus interacts with quadrupolar nuclei through the dipole-dipole interaction, MAS cannot average out the dipolar interaction completely. This phenomenon has been analysed theoretically, especially for nitrogen-containing organic compounds.60-62The same phenomenon should be encountered in ceramics containing nitrogen and aluminium. For example, 29Si spectra of silicon nitride have fine structure caused by 29Si-14N dipole-dipole i n t e r a ~ t i o n . ~ ~ 29Si-27A1 dipole-dipole interaction contributes the line broadening in 29Si spectra of aluminosilicates.28The residual broadening due to the dipolar interaction is inversely proportional to the magnetic field (if expressed in hertz). Therefore, high-field experiments are preferable to suppress this effect. 3.3. Quadrupole interaction

For 1 3 1 spins, quadrupole interaction broadens the resonance line much more than the other interactions, when the observed nucleus locates in a site with low symmetry, Signals of the satellite transitions are spread out, and only the central transition (rn = 1/2 c, -1/2) is observed for half-integer spins. The central transition is broadened by the second-order quadrupole interaction, which is reduced only by a factor of 3-4 under the MAS conditions. 19*20 The second-order quadrupole interaction gives characteristic line shapes in both the static and the MAS spectra,19920364 and thus quadrupole coupling parameters, quadrupole coupling constant and asymmetry factor, can be estimated from simulation of the line shape. It must be noted that the second-order quadrupole interaction shifts the resonance position toward lower frequency, and the intrinsic chemical shift can be extracted only after the correction for the second-order quadrupole shift ." Sites with different symmetries can be distinguished by the difference in the quadrupole coupling parameters. A good example is "B NMR study of boron coordination in b o r a t e ~ .Three-coordinated ~~ boron, B 0 3 , has a quadrupole coupling constant of ca. 2.5MHz, while the constant of B 0 4 units is less than 1MHz. 3.4. Mechanism of spin-lattice relaxation

Mechanisms of spin-lattice relaxation associated with ceramics are discussed in this section. When the material contains no mobile species in it, relaxations by paramagnetic impurities are dominant .269M9 The relaxation rate depends on the sample even if the crystal structure is the same. This

36

S.HAYASHI

relaxation mechanism has been studied t h e o r e t i ~ a l l y , 2 ~and , ~ ~can ~ ~ be classified into two mechanisms roughly: spin-diffusion limiting case and direct relaxation due to the dipole-dipole interaction with electron spins on paramagnetic impurities. The former is applied to abundant spins such as 'H and 27Al,28and the latter to rare spins such as 29Siand 13C.27,28,69 Although nuclei in the neighbourhood of paramagnetic impurity relax very fast due to the direct relaxation, their signals are too broad and too shifted to be detected. For materials including mobile species like H 2 0 and cations, motion of those species relaxes the surrounding nuclei through fluctuation of the dipole-dipole interaction and of the electric field gradient for quadrupolar O2 molecules in micropores also contribute to the relaxation .73,76 Replacement of O2 by organic molecules causes an increase in the relaxation time.77 4. CONVENTIONAL CERAMICS 4.1. Clay minerals

Pottery and porcelain are made of clays and clay minerals through a sintering process. Clay minerals are classified into layered aluminosilicates. Since there are an enormous number of NMR studies on silicates and aluminosilicates," the present chapter cannot cover the whole literature. Kaolins have been used as the starting materials widely in the world, and they have been the most intensively studied among the clay minerals. Consequently, attention is focused on kaolins. 4.1.1. Structure of kaolins Kaolins, A12Si205(OH)4, are layered aluminosilicates with a dioctahedral 1:1 layer structure consisting of an octahedral aluminium hydroxide sheet and a tetrahedral silica sheet. Kaolinite, dickite, and nacrite belong to the kaolins, and the layer stacking manner is different from each other. Figure 2 shows the structure of kaolinite. The 29Si chemical shift of kaolinite is about -91ppm from pure tetramethylsilane, which is attributed to Si04 tetrahedra linked with three S O 4 tetrahedra and having a non-bridging oxygen, denoted as Q3(OAl). Barron et ~ 1 first. detected ~ ~ two 29Sisignals with different chemical shifts in kaolinite, nacrite, and dickite, suggesting the presence of two silicon environments. Magnetic field dependence experiments have recently confirmed the presence of two crystallographically inequivalent sites experimentally.28 Figure 3 shows 29Si spectra of kaolinite, measured with two different fields. The splitting of the two peaks is unchanged (in ppm) by

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

37

Fig. 2. Projection of the structure of kaolinite from (100) direction." (Reprinted with permission from J. Phys. Chem., 01992 American Chemical Society.)

the magnetic field, indicating that the splitting is caused by the difference in the isotropic chemical shift. The presence of two silicon environments agrees with recent results of the crystal structure obtained by diffraction studi e ~ . The ~ ~origin ' of the splitting has been discussed by Thompsong2 and Thompson and B a r r ~ n . *The ~ latter work attributed the origin to hydrogen bonding at the surface of the silicate sheet. As shown in Fig. 3, the resolution of the "Si spectra becomes worse as the magnetic field decreases. This can be explained by the contribution of the dipole-dipole interaction between 29Si and 27Al spins.28 Disorder and paramagnetic impurities also play an important role in broadening the resonance Effect of disorder is easily masked by that of paramagnetic i r n p u r i t i e ~ . ~ ~ . ~ ~ Aluminium is coordinated by six oxygen atoms, and the 27Al NMR signal is observed at about 0 ppm from 1M A1(N03)3 aqueous solution.28Figure 4 shows 27AlMAS-NMR spectra at two different fields. Lower-frequency shift and broadening of the line width at the lower field clearly demonstrate that the line is broadened by the second-order quadrupole interaction. Shulepov

38

S.HAYASH1

-85

-90

-95

PPm Fig. 3. 29Si CPMAS-NMR spectra of Kanpaku kaolinite, measured at (A) 79.496MHz and (B) 39.683MH~.~~ (Reprinted with permission from J . Phys. Chem., 0 1992 American Chemical Society.)

et aLB6have estimated quadrupole coupling parameters from a low field measurement (6.0 MHz); 2 Q q = 3.12 MHz and 77 = 0.9. Simulation of the spectra at the high fields suggests the presence of two aluminium sites.28The diffraction studies also indicate the presence of two aluminium The CRAMPS technique has been successfully applied to kaolinite ,28 as shown in Fig. 5 . The static line width, 30 kHz, is narrowed to 0.6 kHz by use of the CRAMPS. The chemical shift of hydroxyl groups in kaolinite is 2.8 ppm from pure tetramethylsilane. The line width is considerably narrowed by only MAS, provided that the concentration of hydrogen is relatively OW.^*,*^ 29Si spin-lattice relaxation times in clay minerals may vary widely and be extremely long.26*84*88 Care should be taken when one wishes to discuss the . ~ signal intensity quantitatively. Barron et aLS8 and Watanabe et ~ 1 have suggested that the relaxation is caused by paramagnetic impurities. Hayashi et aL2‘ have presented relaxation curves showing exp(-(t/l;)”*) behaviour,

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

39

I

A

100

O

50

-50

-100

PPm Fig. 4. 27Al DD/MAS-NMR spectra of Kanpaku kaolinite, measured at (A) 104.263MHz and (B) 52.051 MHz. Marks ssb indicate spinning sidebands.28 (Reprinted with permission from J . Phys. Chem., @ 1992 American Chemical Society.)

as shown in Fig. 6, and they analysed it quantitatively. The 29Si spins relax by the dipole-dipole interaction with electron spins on paramagnetic impurities directly, and contribution of spin diffusion is negligible. 27Al and 'H spins in kaolinite also relax by paramagnetic Spin diffusion plays an important role in the relaxations of those spins.26 4.1.2. Thermal transformation of kaolins When kaolins are heated, transformation takes place in a stepwise manner. They are transformed to metakaolin at about 500-7OO0C, and then to mullite and crystobalite at about 1000°C. The first transformation from kaolins to metakaolin is accompanied by dehydroxylation.

-

-

A1203.2Si02 2H20 + A1203 2Si02 + 2 H 2 0

40

%HAYASHI

I

40

8

1

I

20

II

0 k Hz

4

-20

-40

0 PPm

Fig. 5. 'H NMR spectra of Kanpaku kaolinite, measured at 400.136MHz. (A) The ordinary single-pulse sequence is used for a static sample. (B) The CRAMPS spectrum measured with the BR24 pulse sequence in the quadrature detection mode.** (Reprinted with permission from J. Phys. Chem., 0 1992 American Chemical Society.)

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

41

100 80

60 Y

E 40

I

I

I

10

0

1

I

I

I

20 30 t v 2 (S”* )

I

Fig. 6. 29Si magnetization recovery curves for Kanpaku kaolinite, plotted as a function of square root of time, measured at 79.496MHz and at room temperature with the magic angle spinning at 3.0 kHz (0)and without spinning (A).26(Reprinted with permission from J . Phys. Chem., 0 1992 American Chemical Society.)

The second transformation has no loss of weight, being expressed by

-

3(AI2O3 2Si02) + 3AI2o3 2Si02 + 4sio2 The dehydroxylation process has been monitored by ‘H NMR spectra by Gastuche et and Otero-Arean et aL91 Dehydroxylation occurs layer by layer when the lost fraction of the hydroxyl groups is from 0% to 70%. For dehydroxylation more than 70%, hydrogens are left rather inhomogeneously ; patches of undehydroxylated regions are distributed in regions containing isolated hydroxyl groups. Environments of silicon and aluminium atoms during the thermal transformation have been monitored by high-resolution solid-state NMR of 29Si and *’A1 n ~ c l e i . ~Short-range ~-~~ structures can be discussed, although clear interpretation of the spectra is difficult because of the broad line width and the mixed composition. Rocha and Klinowski9’ have traced the spectra, carefully setting the pulse width and the recycle delay. Figure 7 shows 29Si MAS-NMR spectra measured at room temperature after thermal treatment. The parent sample has a -91.5 ppm peak without any splitting, being ascribed to Q3(OAl). At

42

S.HAYASHI

1OOoaC

85ooC

650%

. -20

-40

0100

-140

- .

-180

Ppm trom TMS Fig. 7. 29Si MAS-NMR spectra of kaolinite after thermal treatment at different temperatures given in the figure. They were measured at 79.5MHz and at room temperature. (Reproduced by permission of Springer-Verlag from ref. 97.)

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

43

500°C just below the onset of the dehydroxylation, two new signals are observed at -97 and -101ppm. Above 500°C the signal is broadened, whose centre of gravity is between -99 and -101 ppm. The broad line width reflects an amorphous nature with variations of Si-0-T (T = Si and Al) angles and bond lengths. Although Lambert et ~ 1 . ' ~and Rocha and K l i n ~ w s k attempted i~~ to deconvolute the spectra, it was difficult to assign each peak to a certain conformational unit. At 1000°C the signal is at ca. -110ppm, and a new peak at about -87ppm begins to emerge. The -1lOppm peak is assigned to crystobalite, Si02, and the -87ppm peak to mullite. Figure 8 shows 27A1MAS-NMR spectra after the thermal treatment. The untreated sample has only one peak at ca. Oppm, being assigned to six-coordinated Al. At about 500"C, two other signals are observed at ca. 28ppm, assigned to five-coordinated Al, and at 57ppm due to fourcoordinated Al. In the 550-900°C range (metakaolinite range) the three A1 coordinations coexist. The amounts of the four- and five-coordinated A1 increase at the expense of the six-coordinated A1 until ca. 80O0C, and then begin to decrease. The six-coordinated A1 reaches a minimum at 750 800"C, above which it rises again. The five-coordinated A1 disappears at ca. 900°C, and simultaneously the four-coordinated A1 begins to increase again. At ca. 900°C the chemical shifts of the six- and four-coordinated A1 are changed, suggesting creation of new phases, Rocha and K l i n ~ w s k i ~ ~ suggested phases of y-alumina and the precursor of mullite. On the other hand, the presence of y-alumina was excluded by Sanz et ~ 1 . ~ ~ A number of compounds, known as mineralizers, have been used to promote selectively the formation of certain species during the thermal transformation. Rocha et ~ 1 have . studied ~ ~ the effect of lithium nitrate mineralizer on the thermal transformations of kaolinite by means of 'Li, 27A1, and 29Si high-resolution solid-state NMR. The temperatures for the transformation are changed by the mineralizer, but no new phases are found. The dehydroxylation of kaolinite to metakaolinite can be completely reversed.'OOy'O1Metakaolinite is reacted with water at 150 250°C for several days, producing kaolinite, which has been monitored by 29Si and 27Al MAS-NMR.'007'o' The reaction is initiated at the edges of metakaolinite particles and is followed by diffusion of water into the bulk. When metakaolinite is treated with NaOH solution, Na-A type zeolite is formed.lo2 The solution phase might play a role in the formation of the zeolite.

-

-

4.2. Cements

CaO-Si02, Ca0-SiO2-Al2O3, and Ca0-SiOZAl2O3-Fe2O3 systems are solidified by reaction with water, and they are the main components in

44

S. HAYASHI

cements. Ordinary Portland cement typically contains 50% tricalcium silicate (3Ca0 SiO,; denoted as C3S) and 25% dicalcium silicate (2Ca0 .SiO,; p-GS). The remaining 25% consists largely of tricalcium aluminate and calcium aluminoferrite phases.lo3 C3S and p-C2S play an important role in the solidification process.

-

4.2.1. Calcium silicates and cements

The CaO-SiO, system has four compounds; Ca0.Si02 (denoted as CS), 3Ca0-2Si02(C3S,), 2Ca0.SiOz (C&, and 3Ca0-Si02 (C3S). C3S and the p phase of &S are main components in the ordinary Portland cement clinker, as described above. Figure 9 shows 29Si MAS-NMR spectra of p-&S, C3S, and two cements. The crystalline calcium silicates have very narrow signals. Table 2 summarizes 29Si chemical shift values of GS, C3S, and their hydrates. C2S and C3S have only Qo environment. C2S has only one equivalent site, showing one 29Sipeak. C3S has nine crystallographically inequivalent sites, among which eight sites are distinguished from the spectra. lociThe ordinary Portland cement is amorphous, and thus it shows a broad signal. The spectra of the two Portland cements are interpreted as the sum of the spectra of C3S and

p-(2,s. The hydration process has been monitored by high-resolution solid-state 29Si NMR.""'16 As a model reaction in the setting of the ordinary Portland cement, the hydration process of C3S and C2S has also been studied. Figure 10 shows 29Si spectra of the materials corresponding to Fig. 9, but after hydration for one week. In the initial stage of hydration of the cements, there is an induction period of several hours. After the induction period, a resonance appears at -79 ppm, being ascribed to end units in silicate chains (Q'). After longer times, a further resonance is clearly resolved at -84 ppm, which is middle units in polymer chains (0'). As shown in the figure, the hydration rate of p-C;S is slower than that of C3S. The hydration of C3S has been studied in detai1.'07-112 The crosspolarization experiments have demonstrated that hydrated monomeric silicate units (ao-H) are formed during the induction period.'" Figure 11 shows the formation of hydrated materials versus time in conjunction with a calorimetric data. After the end of the induction period, the concentration of Qo-H levels off to ca. 2%, and the Q' species (end units in silicate chains) begin to increase. 29Sispectra have also been used to monitor the hydration process of C3S Fig. 8. 27Al MAS-NMR spectra of kaolinite after thermal treatment at different temperatures given in the figure. They were measured at 104.2MHz and at room temperature. (Reproduced by permission of Springer-Verlag from ref. 97.)

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

1ooooc

9000c

800°C

550%

5000c

20% 200

100

0

-100 -200

45

46

S. HAYASHI

ORDINARY PORTLAND CEMENT

AALBORG WHITE PORTLAND

-3r-40

-

-

-

-

-

- 00

-110

PPm Fig. 9. 29Si MAS-NMR spectra of P-GS, C$, ordinary Portland cement, and Aalborg white Portland cement, measured at 39.76 MHz. (Reproduced by permission of Chapman and Hall Ltd. from ref. 107.)

in the presence of admixtures such as CaC12, sucrose,11o and The induction period becomes shorter for CaC12, and it becomes longer for sucrose. On the other hand, silica accelerates greatly the polymerization process without affecting the induction period. Hydration of actual cement samples has been studied by 29Si NMR. 101,11?-116 Figure 12 shows 29Sispectra of cement pastes. Three peaks are observed at -71, -79, and -84 ppm, being assigned to Qo, Q1, and Q2,

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

47

Table 2. 29Si chemical shifts in calcium silicates. ~~

Formula

Mineral

Shift/ppm

C3S-H C3S-H Hillebrandite

-70.8 -70.3 -71.5 -71.34 -71.4 -73.4 -73.66 -73.5 -72.5 -68.93 -69.04 -71.67 -72.67 -73.44 -73.66 -73.84 -74.46 -82.6 -84 -86.3

2Ca0. SiO2 2Ca0 SiOz 2Ca0. Si02 2Ca0 SiO2 2Ca0. SiO2 2Ca0. SiO2 2Ca0 Si02 2Ca0 SiOz 2Ca0. SiOz.H 2 0 3Ca0. SiOz

~~~

Type

Ref.

Qo

105 37 105 106 37 105 106 37 4 106

Q0

Qo Qo

Q0

Qo Qo Q0 Q0

Qo Q0 Q0

Q0

Q0

Q0 Q0 Q0

Q' Q'

Q2

4 104 4

respectively. The fraction of the sum of Q 1 and Q2 increases with curing time and temperature. The compressive strength increases with curing time and temperature (21-55°C). At 80°C the strength is lower because of the non-uniform distribution of the hydration products, although the degree of hydration is higher.

4.2.2. Calcium aluminates "Al NMR is useful to study A1 coordination^.^^^"^ Six-coordinated A1 (A106) gives a signal at ca. Oppm, while four-coordinated A1 (A104) is at 55-80 ppm. The ordinary Portland cement contains calcium aluminate phase. Table 3 summarizes 27Al chemical shifts of various calcium aluminates. Tricalcium aluminate, 3Ca0 AI2O3(denoted as C,A), is an important component in Portland cements. Skibsted et a1."' have analysed the spectra of C3A in detail, estimating isotropic chemical shifts and quadrupole coupling parameters for the two crystallographically inequivalent sites. Mueller et al."' have monitored the hydration process of monocalcium aluminate by means of 27Al MAS-NMR, as shown in Fig. 13. The hydrated species having six-coordinated A1 increase at the expense of the unhydrated compounds with four-coordinated Al. The conversion of the A1 coordination is divided into three periods; an induction period, a conversion period,

-

48

S.HAYASH1

A A LB0RG WHITE PORTLAND

V'

. , - - J w w

-33

-40

-5,

-60

-?O

PPm

-80

-90

-100

- 1 10

Fig. 10. 29Si MAS-NMR spectra of p-C2S, C3S, ordinary Portland cement, and

Aalborg white Portland cement, after hydration for 1 week. They were measured at 39.76 MHz. (Reproduced by permission of Chapman and Hall Ltd, from ref. 107.)

and an after-conversion period. Hjorth ec ~ 1 . "have ~ observed the conversion of the A1 coordination in a commercial cement upon hydration. Four-coordinated A1 is converted to six-coordinated Al. ' studied A1 coordination in calcium silicate hydrate Stade et ~ 1 . ' ~have containing a small amount of A1203by means of 27AlMAS-NMR. Specimens with CaO/Si02 = 1 contain almost entirely four-coordinated Al, and those with CaO/Si02 = 1.5 only six-coordinated Al. With increasing CaO/SiOz

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

49

I '

/.

/' /"'

End units in silicate chains

/'

.i /*

/

/

Hydrated monomeric silicate units

Time (mins)

Fig. 11. Graph showing the formation of hydrate material as a percentage of the total silicate material present versus time, together with calorimetric data. (Reproduced by permission of the American Ceramic Society from ref. 110.)

ratio the amount of four-coordinated A1 decreases, while that of sixcoordinated A1 increases. 4.3. Glasses

Conventional inorganic glasses consist of network-forming units and network modifiers. Networks of oxide glasses are ordinarily formed by silicate, phosphate, and borate anions, and network modifiers are cations. Many new glasses have recently been developed, most of which were designed for specific applications such as optical fibre, ionic conductor, photochromism, laser, light filter, and so on. NMR is a suitable method to study the short-range order in glasses. In this section, we discuss the structure of glasses studied by means of NMR. 4.3.1. Silicate glasses

Silicate glass with the simplest composition is vitreous silica. Si04 tetrahedra are interconnected without symmetry. The 29Si MAS-NMR spectrum has a relatively broad peak at ca. -110ppm with a full width at half-maximum (FWHM) of about 13ppm,lz2 which is assigned to Q4. The line shape is asymmetric, reflecting distributions of Si-0-Si bond angles, Si-0 distances,

50

S. HAYASHI

Table 3. "A1 chemical shifts in calcium aluminates.

Compounds CaO * 6A1203 CaO .2A1203

-

4Ca0 * 3A1203 3Ca0 2A1203 12Ca0.7Al2o3

-

3Ca0 A1203 6H20 2Ca0 * AIzO3. 8H20 CaO .A1203* 10HzO

Chemical shift/ppm

Ref.

6 6 4 4

9 f0.3 16 k 0.3 65 0.3 78 f 1 60 f 20 76" 80.5 f0.5 83.3 k 0.5 80.3 f 1 71 85 f 8 79f 1 85f3 79.50 f0.50 78.25 k 0.50 12" 9" 3"

119

4 4

CaO ~ 1 ~ 0 ~ CaO A1203

3Ca0. A1203 3Ca0. A1203

A14 coordination

Ib rIc

4 4 4 4 4 4 4 4 4 6 6 6

*

119

118 119 119 117 119 119 120 118 57 57

"A correction of second-orderquadruple shift is not included. bQuadrupole coupling constant (QCC) is 8.69 k 0.05 MHz, and asymmetry factor ( 7 ) is 0.32 +_ 0.02. 'QCC = 9.30 k 0.05 MHz, and q = 0.54 k 0.02.

and Si-Si distances. Dupree and Pettifer'22 have estimated the distribution of the Si-0-Si bond angles by analysing the line shape. The addition of alkali and alkaline earth metals to silica glass introduces bond breakings between SiO4 tetrahedra. Consequently, Q" units with n less than 4 are formed. 29Si MAS-NMR has been used to study the Q" distribution in the binary g l a s ~ e s . ~ ~ * 'Figure ~ " ' ~ ~14 shows 29Si spectra of Li20-Si02 glasses. The spectra were deconvoluted into three and four Q" components, each of which was assumed to have a Gaussian line shape. Since the spectral resolution is not good, the deconvolution procedures might produce considerable errors. Grimmer et al.29 have attempted to reduce the deconvolution errors in the study of Na20-Si02 glasses by use of a high magnetic field and a high spinning rate. In the M20-Si02 (M = alkali

Fig. 12. 29Si MAS-NMR spectra of cement paste specimens of waterlcement ratio 0.45 at 21,35,45,55, and 80°C (A, B, C, D, and E, respectively) for 3,7, 14, and 31 days (a, b, c, and d, respectively). They were measured at 53.54MHz and at room temperature. (Reproduced by permission of the American Ceramic Society from ref. 116.)

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

5

PPM

51

52

S. HAYASHI

I

100

I

0

I

-100 ppm

Fig. 13. 27Al MAS-NMR spectra of hydrated monocalcium aluminate samples obtained after selected reaction times at 70°C curing temperature. They were measured at 70.4MHz and at room temperature. (Reproduced by permission of

Academic Press, Inc. from ref. 118.)

metal) glasses, the observed Q" contents are not explained by the random statistical distribution of the five Q" units. The distribution is rather constrained, but with a small randomness. In the case of alkaline-earth silicate glasses, the overlap between the Q" peaks is too severe to deconvolute the spectra.13' Other binary glasses have also been studied by 29Si NMR. Lippmaa et ~ 1 . ' ~have ' studied PbO-Si02 glasses. Only one broad signal was observed at -85ppm for a glass with a composition Pb0.Si02, at -80ppm for 2Pb0.Si02, and at -76 ppm for 4Pb0.Si02. No clear interpretation was presented for the network structure. Coordination of aluminium is an important problem in the structure of

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

26Li20-74Si02

53

n

& calculated

Fig. 14. 29Si MAS-NMR spectra of three lithium silicate glasses, calculated spectra, and Gaussians used to construct calculated spectra. The resonance frequency was 39.7 MHz.Iz3 (Reprinted with permission from J . Am. Chem. Soc., 1984, 106, 4396. 0 1984 American Chemical Society.)

54

S.HAYASHI

1

ZOO

.

I

100

-

I

0

PPY

'

-

I

100

.

I

-200

Fig. 15. *'A1 MAS-NMR spectra. of SiOrAIOz glasses. SQ, super quench; NQ, normal quench. Sample compositions in mol% SiOz (S) and A1203 (A) are given for each spectrum. Spectra were obtained at 8.45T (93.83MHz) at spin speeds near 15 ~ H z . (Reprinted '~~ with permission from J . Phys. Chem., 1991, 95, 4483. 01991 American Chemical Society.)

aluminosilicate glasses. Risbud et ~ 1 . and ' ~ ~Sat0 et ~ 1 . lhave ~ ~ studied the structure of Si02-A1203 glasses by means of 27Al and 29Si MAS-NMR. Aluminium is present in four-, five-, and six-coordinations in rapidly quenched glasses, while samples with slower quench rate show only fourand six-coordinations, as shown in Fig. 15. Different local structures are formed by the quench rate. In contrast, only four-coordinated silicon is detected in "Si NMR spectra.

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

55

In aluminosilicate glasses, ternary glass systems of M20-AI2O3-SiO2 and MO-AI2O3-SiO2, where M is an alkali or alkaline-earth metal, have been studied e x t e n ~ i v e l y . ~Hallas ~ ~ ' ~ et~ 'al. ~ 134 ~ have studied A1 coordination in Na20-A1203-Si02 glasses, and have observed only four-coordinated A1 with moderately high symmetry. Engelhardt et ~ 1 . lhave ~ ~ studied network structure in Ca0-AI2O3-SiO2 glasses with widely varying compositions. 29Si spectra show only one peak, and 27Alspectra show the presence of four- and six-coordinated Al. They have reached several important conclusions: (1) Up to three Q"(mA1) units are coexisting, which differ by only one Si-0-T (T = Si, Al) bond. (2) A1 is preferentially bound to the most polymerized Q"(mA1) units. (3) With CaO = A1203S 0.5Si02, the network is fully polymerized. (4) With CaO 2 A1203S 0.5Si02, A1 is four-coordinated, being completely incorporated in the network. ( 5 ) With CaO0.5Si02 (and 2 C a 0 S S i 0 2 ) , A1 is present as neutral "extra-network'' species and does not act as a network modifier. (6) If CaO is present in great excess over A1203 and S O 2 , calcium aluminate is formed. Oestrike et ~ 1 . have ' ~ ~ deduced several correlations between chemical shifts of 2ySi, 27Al, and 23Na and atomic compositions of the ternary systems. have studied the network structure in Na20-AI2O3-SiO2 Maekawa et glasses with N a a Al. With the addition of A1203, the resolution between the Q2 and Q3 peaks in 29Sispectra becomes worse, as shown in Fig. 16. The network is fully polymerized in their compositions, and all A1 atoms are four-coordinated. The amount of non-bridging oxygen atoms is determined by an excess amount of Na over Al. The sol-gel process has increasingly been used in recent years as a method of preparing g 1 a ~ s e s . lThe ~ ~ structure of sol-gel derived glasses and the sol-gel process have been studied by high-resolution solid-state NMR.13s146 Dupree et ~ 1 . and ' ~ ~Wies et ~ 1 . have ' ~ ~ studied Li20-SiOz sol-gel glasses. 29Si MAS-NMR spectra are shown in Fig. 17. The -1lOppm peak is assigned to Q4, and the peak at -101ppm to Q3-H. The Q3-H peak is enhanced in the CP spectra compared with the Q4peak. The glasses contain ca. 20mol% H 2 0 before the heat treatment. The heat treatment removes H 2 0 and allows the formation of Si-O-Li+ units. Crystallization begins above 575"C, which is revealed by narrow crystalline lines at -74.8 ppm, -92.4ppm, -102ppm, and -114ppm. The structures of AI2O3-SiO2 gel and glass have been studied by 27Aland 29Si NMR.'*'" 27Alspectra are shown in Fig. 18. Three A1 coordinations are observed; 1,29, and 52 ppm for six-, five-, and four-coordinated Al. The amount of the five-coordinated A1 increases with the thermal treatment. The amounts of each species depend on the preparation method of the gel.

56

S.HAYASH1

PPm

+

Fig. 16. 29Si MAS-NMR spectra for glasses with 3Na20-4SiO2 NaA102, measured at 3 9 . 7 6 M H ~ ~ (Reprinted ' with permission from J . Phys. Chem., 1991, 95, 6822. @ 1991 American Chemical Society.)

When the gels were prepared from separate alkoxides, the amount of five-coordinated A1 is much smaller. The six-coordinated A1 decreases with addition of Na20. In contrast, the 29Si signal consists of only one broad line, having limited information. Wies et a/.144have studied SiOrTi02-Zr02 sol-gel glasses by means of 29Si and 'H MAS-NMR. Q4 units increase at the expense of Q3 and Q2 by thermal treatment. Selvaraj et have studied the sol-gel process in the synthesis of cordierite. Six-coordinated A1 is converted to four-coordinated A1 by heat treatment, and at the same time five-coordinated A1 is formed. The structural role of H 2 0 in sodium silicate glasses has been studied by Kuemmerlen et ~ 2 1 Q2 . ~ and ~ ~ Q3 silicon environments increases at the expense of Q4 with H 2 0 addition. H 2 0 depolymerizes the silica network. 'H CRAMPS spectra shown in Fig. 19 show two signals at 0 ppm and 7.5 ppm

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

I

I

-50

. . I

-1 50

PPm

I

I .

. . I . . . . I ,

-50

57

. . I

-1 50

PPm

Fig. 17. 29Si MAS-NMR spectra of Li@-SiOz sol-gel glasses after heat treatment. (a) single-pulse and (b) CP spectra. They were obtained at 71.535 MHz and at room temperature.13’ (Reprinted by permission from “Multinuclear magnetic resonance study of Li20-SiOz sol-gel glasses”, R. Dupree ef al. @ 1990 John Wiley & Sons, Ltd.)

with respect to H20. They concluded that both Si-OH (7.5ppm) and molecular water (0 ppm) are present in hydrous sodium silicate glasses. 4.3.2. Phosphate glasses

Tetrahedral PO4 units are condensed in inorganic phosphate glasses by sharing an oxygen atom. The PO4 condensation can be identified by 31P NMR, although the line width in glasses is broader than the corresponding crystalline compounds.56~’47-151 Isolated PO4 units end units (Q’), and middle units in the chain (Q’) are observed separately in the 31P spectra.

(a’),

58

S.HAYASH1

0

100

PPm

-1 00

Fig. 18. "Al MAS-NMR spectra of A1203--Si02 gels derived from di-s-butoxyaluminoxytriethoxysilaneat various stages of thermal treatment. (a) 40T, 6 h; (b) 150°C, 6 h; (c) 450°C, 2 h; (d) 800°C, 2 h. They were traced at 78.2 MHz and at room temperature. (Reproduced by permission of Chapman and Hall Ltd, from ref. 141.)

Figure 20 shows 31P MAS-NMR spectra of AgI-Ag2O-P2O5 glasses, which are known as superionic conducting glasses. In this system, 31P signals are observed at 23 21, 4 2, and -15 -22 ppm for the Qo, Q', and Q2 units, respectively. Average lengths of the chain can be estimated from the integrated peak areas including spinning sidebands. The NMR results confirmed that the chain length is determined by the ratio of AgZO to PZO5. AgI does not modify the network. Hayashi and Hayamkd6 and Brow et ~ 2 1 . 'have ~ ~ presented 31P spectra of metaphosphate glasses of alkali and alkaline-earth metals. Many spinning sidebands are observed, from which the magnitudes of chemical shift anisotropy have been estimated as well as isotropic chemical shifts. Prabhakar ef al. lS1 have studied several phosphate glasses; LiP03, AgPO3, Zn2P207,PbO-P205, M003-P205, W03-P205, V20rP205, K20-

-

-

-

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

' t i CRAMPS

59

MREV-8

hydrous Na Si40g

30

20

10

0 ppm

Fig. 19. 'H CRAMPS spectra of sample I1 (Na2Si40+l.8%H20) and sample I11 (Na2Si4Or9.1%H20), measured at 300.1 MHz. The chemical shift is with respect to H20.'& (Reprinted with permission from J. Phys. Chem., 1992, 96, 6405. @ 1992 American Chemical Society.)

Mo03-P205, Pb0-Mo03-P205, and NaP03-NaV03. They suggested that the network consists of metaphosphate units. Mueller et ~ 1 . have l ~ studied ~ A1 coordination in CaO-A1203-P205 glasses. Three resolved lines are observed; at -21 ppm (isolated A106 octahedra linked with phosphorus atoms), 4 ppm (A106 partially connected via Al-O-A1 bridging bonds), and 37 ppm (A104 tetrahedra incorporated in the phosphate network). The number of A104 groups increases for decreasing P205/A1203ratios. 4.3.3. Borate glasses Boron atoms are three- and four-coordinated in borate glasses. Bray's group has studied boron coordination in various borate glasses by means of "B continuous-wave (CW) NMR of static ~ a r n p l e s . ' ~ ~B' ~0*3 units have a

60

S.HAYASHI

No.4

I

No. 13

* I

* +

-

A

*

+

~ ~ ~ ' " ' " ~ ' " " ' ' ' " " ' ~ ' ' ' '

100

0

-100

ppm

Fig. 20. 31P MAS-NMR spectra of AgI-Ag20-P205 glasses, measured at 80.76 MHz. Compositions (in mol%) are (No. 4) 54.9%AgI-30.1%Ag2015.O%P2O5 and (No. 13) 60.0%AgI-25.O%Ag20-15.O%P~O~. (Reproduced by permission of Academic Press, Inc. from ref. 149.)

quadrupole coupling constant of ca. 2.5MHz, while B 0 4 units have a constant much less than 1MHz. Consequently, the line width of the B 0 3 units is much broader than that of the B 0 4 units, which is mainly broadened by the second-order quadrupole interaction. Typical spectra for static samples are shown in Figs 21A and 21B. The central peak in Fig. 21A is ascribed to B 0 4 units, whereas the broad line with the outer two peaks is

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

61

-

5kHz

+ J

+

+

+

I

I

I

1

200

100

0

-100

I

1

I

100

50

0

+

+ 1

I

I

-50

L

-200 ppm

-100 ppm

A

d

I

1000

I

0

I

-1000 ppm

Fig. 21. "B NMR spectra of a glass sample whose composition is 0.2Ag14.3Ag200.5BzO3.The spectra were measured (A) at 28.88MHz in a static state, (B) at 128.38MHz in a static state, and (C) at 64.03MHz with MAS (spinning rate: 4.5 kHz). (Reproduced by permission of Elsevier Science Publishers B.V. from ref. 159.)

62

S.HAYASH1

ascribed to B 0 3 units. Since the line width due to the second-order quadrupole interaction is inversely proportional to the magnetic field (in hertz), the broad B 0 3 line is collapsed into the B 0 4 line at high magnetic fields, as shown in Fig. 21B. The use of low magnetic fields is important to divide the two species for static samples. B2O3 consists of B 0 3 units. In alkali borate glasses, both B 0 4 and B 0 3 units coexist. The fraction of the B 0 4 units increases with addition of alkali oxides, until it takes the maximum value of about 0.5 at M20/B203= 0.5 (M = alkali metal). In the range M20/B203>0.5, B03 units with nonbridging oxygens are formed, and the fraction of B 0 4 decreases as the amount of alkali oxide increases, The magic angle spinning can reduce the second-order quadrupolar broadening by a factor of three or four. Figure 21C shows a ''B MAS-NMR spectrum, in which the B 0 4 line at ca. 0 ppm overlaps partially with the B 0 3 line spreading over 20 to -50ppm. Although the B03 units have isotropic chemical shifts of ca. 20ppm, the centre of gravity shifts towards lower frequency due to the second-order quadrupole shift. The use of high magnetic fields can reduce the overlap of the signals between the B 0 4 and B 0 3 units. Figure 22 shows "B MAS-NMR spectra of borate minerals and a borosilicate glass. By the use of the high magnetic field (11.7T), the signal of the B 0 3 units spreads over 20 to 0 ppm, and the overlap between the two species is considerably suppressed. Bunker et aZ.lm have studied MO-B203-A1203 glasses (M = alkaline earth). The glasses contain B03, B04, A104, A105, and A106. The presence of three-coordinated oxygen and non-bridging oxygens is inferred. The structure is complex because of the large numbers of local structures. 4.3.4. Mixed and other oxide glasses Networks of silicate, phosphate, and borate can be mixed, resulting in complexity of the structure. Weeding et ~ 1 . ' ~have ' studied silicon coordination in SO2-P205 glasses, and found that the silicon is four-coordinated in the glasses. The fourcoordinated Si is transformed to six-coordinated Si by devitrification. Yang and Kirkpatrick'62 have studied P205-doped alkaline-earth metasilicate With glasses. Phosphorus is present as monomeric structural units increasing P2O5 content the silicon network becomes more polymerized, as indicated by the lower frequency shift of the 29Si signal. Bunker et ~ 1 .have l ~ studied ~ Na2O-B2O34iO2 glasses by means of 29Si, "B, 1 7 0 , and 23Na NMR. For boron-rich glasses, phase separation into sodium borate and silicate-rich phases takes place. The alkali-rich glasses consist of a borosilicate network containing B 0 4 units and non-bridging oxygens. They have also studied structural changes of the glasses during ' ~ ~ traced 29Si leaching in water at pH1, 9, and 12.1M Martin et ~ 1 . have

(a').

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

20

0

63

-20

ppm from BF3*Ef20 Fig. 22. "B MAS-NMR spectra of (A) kernite, (C) Pyrex glass, and (E) ulexite, together with their sirnilations (B, D, F). The spectra were measured at 160.4MHz. (Reproduced by permission of Academic Press, Inc. from ref. 65.)

MAS-NMR spectra of Li20-B203-Si02 glasses. The spectra consist of only one line, whose position and line width depend on the composition. From the analysis of the 29Si NMR results, they suggested that a proportional sharing of the alkali between the borate and silicate systems begins at the initial stage of the alkali addition. On the other hand, a "B NMR study'% has concluded that at the initial stage of the alkali addition the borate groups consume all the added oxygen to form B 0 4 units. The discrepancy of the above two results has not been solved yet. ' studied PO4 condensation in M2O-B2O3-P2O5 glasses Villa er ~ f . ' ~ have (M = Li, Ag) by means of 31P NMR. In addition to the units observed in the Ag20-P2OS systems, three units are newly observed in the Ag20-B203Pz05 systems; P-[O-P*02-01-B at - 18 to -16 ppm, (B-AgP04-B)-[OP*02-01-B at -2 ppm, and (B-Ag2P04) at +12 ppm, The deconvolution of the 31P spectra of the Li20-B203-P20s glass is difficult because of their broad and featureless nature. Calcium aluminate glass contains none of the traditional glass formers such as Si, P, and B. Almost all of A1 in the glasses with compositions 63Ca0 - 37Al2O3and 48Ca0 - 15CaF2.37A1203 are four-coordinated.'68 It is

64

S.HAYASH1

speculated that fluorine enters as a non-bridging species in the form of an A103F tetrahedron. 4.3.5. Non-oxide glasses

Instead of the oxide network, sulphide, selenide, and telluride can form a glass network. Eckert's group has studied various non-oxide glasses such as P-Se,16s172 P-S,173 LiI-Li2S-SiS2,174 Ag2S-P2SS,175 Li2S-P2S5, Li2S-B2S3, Li2S-P2SrB2S3,176 and CaGeP2177by means of NMR. In the P-Se glasses, various phosphorus conformations have been observed by 31P MAS-NMR,'69 as are partly shown in Fig. 23. The 31P resonances at 63 and -75 -85ppm are ascribed to molecular P4Se3, a broad background signal not narrowed by MAS to a more phosphorus-rich environment, the ca. 135 ppm peak to PSe3,2 units, the ca. 125 ppm peak to Se2/2P-PSe2n units, and the 10ppm peak to Se=PSe3/2units. The peak at 148.5 ppm cannot be assigned. Concentrations of the above units depend on the composition. The microstructure in the glasses has been studied quantitatively also by spin echo double and spin echo experiments. 172 It is concluded that the formation of P-Se bond is preferred to those of P-P and Se-Se bonds. In the P-S system, S=PS3,* units and molecular P4S9and P4S10 are identified by 31PMAS-NMR.173 The local structure in LiI-Li2SSiS2 glasses has been investigated by 29Si, 6Li, and 7Li MAS-NMR.174 In analogy to crystalline silicates, the 29Si resonance in crystalline silicon sulphides is shifted towards higher frequency with decreasing the SiS4 condensation , with an overall chemical shift range of ca. 30ppm. In contrast, the 29Si resonance of the glasses appears invariant over the entire region of glass formation. This can be interpreted by assuming that the sulphide introduced with LiZS is shared by more than two SiS4 tetrahedra, and that the coordination of the sulphide undergoes systematic changes. Lithium iodide does not change the Q" distribution of the SiS4tetrahedra, and the formation of LiI microdomains is not suggested. Ag2S-P2S5 glasses show two 31P resonance lines centred at 97 98ppm (assigned to P2S$-: Q') and at 86 87ppm (Q2),17' as shown in Fig. 24. The fraction of Q' increases with increasing Ag2S content, which is similar to the oxide analogue. The Li2S-P2S5system shows similar b e h a ~ i 0 u r . In l~~ the Li2S-B2S3 system,176the fraction of BS4 increases with addition of Li2S at the initial stage. After it takes the maximum at a certain composition, the BS4 fraction decreases with addition of Li2S. This behaviour is similar to the oxide analogue again. Ternary chalcogenide glasses with compositions (Li2S)0.67(B2S3)1 - ,,(P2S5),, have also been studied.'76 However, no final structure has yet been deduced. The structure of CdGeP2 has been studied by '13Cd MAS and spin echo and 31P-113Cd spin echo double resonance NMR.'77 In contrast to the

-

-

-

HIGH-RESOLUTIONSOLID-STATE NMR STUDIES ON CERAMICS

65

1 " " 1 " " 1 " " l " " 1

300

200

100

-100

0

-200

-300

PPM

Fig. 23. 31P MAS-NMR of P-Se glasses. Top: 121.65MHz spectrum of a glass containing 66.6 atom% P. Bottom: 121.46MHz spectrum of a lass containing 50 atom% P (top trace) and of crystalline P4Se4 (bottom trace).lg9 (Reprinted with permission from J . Phys. Chern., 1989, 93, 7895. 0 1989 American Chemical Society.)

66

S.HAYASH1

4

x = 0.40 1

T

,

200

I

T

I

130

I " " 0

-100

PPM

Fig. 24. 31PMAS-NMR s ectra of glasses (Ag2S),(P2S5)1-,. The spectra were measured at 121.65MHz.'' (Reprinted with permission from J . Am. Chem. Soc., 1992, 114, 5775. @ 1992 American Chemical Society.)

crystalline analogue, glassy CdGePz contains a substantial fraction of P-P bonds, and the number of Cd-P bonds is significantly reduced.

5. HIGH-PERFORMANCE CERAMICS Several new ceramics have been developed in recent years. High-resolution solid-state NMR is a powerful tool to investigate the local environment of the observed nuclei. In this section, we discuss microstructures of highly refractory materials, most of which contain silicon (silicon carbide, silicon nitride, silica, etc.) and aluminium (alumina and related materials).

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

67

Table 4. Population of sites in silicon carbide. Polytype

Stacking sequence

Population of site ~

A 2H 3c 4H 6H 15R

ab abc abcb abcacb abcbacabacbcacb

B

C

D 2

3

2 3

~~

2 2 6

2 2 6

Although many works in this section are concerned with 29Siand "Al NMR, we describe works of newly accessible nuclei as well. 5.1. Silicon carbide

5.1.I . Structure of polymorphs Silicon carbide is a highly refractory material. Roughly speaking, two groups of structure are present; a and p phases. The a phase has many polytypes; 2H, 4H, 6H, and 15R are typical ones. H is hexagonal, and R is rhombohedral. The p phase has the ZnS-type structure, being designated as 3C, where C means a cubic structure. In general, there are two manners for the closest packing; face-centred cubic and hexagonal closest packed structures. The packing sequences for the above two structures can be described as abc and ab, respectively. Many polytypes are present in Sic, which differ in their stacking sequence, as are listed in Table 4. Crystallographically inequivalent sites are also listed in Table 4, together with their populations. The four types of sites are schematically expressed in Fig. 25. The environments of silicon and carbon have been studied by highresolution solid-state 29Si and 13C NMR.l7&la Typical spectra are shown in Fig. 26. The number of peaks corresponds to the number of inequivalent sites. Spectral assignment has been carried out on the basis of peak intensity, comparison with other polytypes, and contribution of the second and higher neighbour atoms. 29Si and 13C chemical shifts are summarized in Tables 5 and 6, respectively. The 13C signal of the 3C polytype has not been detected in some work^.'^^+'^ The spin-lattice relaxation time might be too long for pure samples. The estimated TI values are 35 +- 4 min for 6H("Si), 120 k 20 s for 4H(29Si), and 130k 30 s for 4H('3C).178 The recycle delay should be long enough to analyse the spectra quantitatively. Elemental boron is added as a sintering aid to silicon carbide, and the

68

S. HAYASHI

\

Si2

c3 I

6H:A

4H:B

6H:B -

-

6H:C

4H: C

2H:D -

Fig. 25. Local environments for silicon in SIC polytype in a plane containing the crystallographic c axis. The atoms Sil, C1 (the subscripts referring to nearestneighbour silicon and carbon atoms), and C2 (second-neighbour atoms) define four different silicon environments A, B, C, and D. The more remote Siz and C3 atoms allow further distinctions to be made. (Reproduced by permission of the American Ceramic Society from ref. 178.)

I

~

-10

I

-10

PPm

I

- 30

L

PPm

- 30

+

I

30

I

ppm

I

- 10

I

PPm

*10

I

-30

Fig. 26. 29Si and 13C MAS-NMR s ectra of silicon carbide. (a) 6H polytype, ?%; (b) 6H polytype, I3C; (c) cubic P-SiC, 29Si; (d) mixture of polytype, SI. (Reproduced by permission of the Royal Society of Chemistry from ref. 179.)

F.

$

70

S.HAYASH1

Table 5. 29Si chemical shifts in silicon carbide."

PoIytype

29Si chemical shift from Th4S (ppm) ~

2H 3c

4H 6H

15R

Ref. ~

~~

-20.0 (D) -18.4(A) -18.3(A) - 16.1(A) -18.4(A) -17.2(A) -20 (A) -16 (A) -22.5(B) -19.7(C) -14.7(A) -25.4(B) -20.9(C) -13.9(A) -20.2; -24.5(B;C) -14.3(A) -24.9(B) -20.4(C) -14.4; -20.5; -25(A; B; C) -14.9(A) -20.8; -24.4(B; C) -14.6(A) -24.1(B) -20.5(C) -15.2; -20.2; -24(A; B; C)

"Each signal is assigned to one of the sites A

178 178 180 181 181 182 182 183 178 178 180 181 183 180 181 183

- D.

Table 6. 13C chemical shifts in silicon carbide." ~

~

Polytype 2H 3c

4H 6H 15R

13Cchemical shift from T M S (ppm) 15.O(D) No signal No signal 23.7( A) 18.4(A) 24.7(A)

13.5(B)b 20.9(C)b 23.O(A) 15.2(B)b 20. 3(C)b 15.2(A)b 20.2; 23.2(B; C)b 15.6; 20.6; 23.8 16.0(A)b 20.7; 22.7(B; C)b

"Each signal is assigned to one of the sites A Ventative assignment.

Ref. 178 178 180 182 182 183 178 178 180 183 180

- D.

coordination of boron has been investigated by "B NMR.lWWhen sintered in argon, boron penetrates the grain boundaries and is incorporated into the bulk in a tetrahedral form. In contrast, when sintered in nitrogen atmosphere, boron nitride is formed on the intergranular surface, where boron is in a trigonal form.

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

71

5.1.2. Curing pracess of polycurbosilune Polycarbosilane is a precursor for ceramic materials such as silicon carbide fibres, The curing process of polycarbosilane has been studied by 29Si and I3C NMR.18s189 Figure 27 shows 29Si MAS-NMR spectra of polycarbosilanes. Polycarbosilanes were synthesized by thermal decomposition and condensation of tetramethylsilane and of polydimethylsilane.190 Uncured polycarbosilanes show two intense 29Si resonance lines at -0.5 and -17.5ppm (see Fig. 27a), which are assigned to Sic4 and SiC3-H, respectively. The presence of the Si-H bond for the -17.5ppm peak is confirmed by the dipolar dephasing experiment, as shown in Fig. 27b. The third weak peak is present at -38.5ppm, corresponding to Si*C3Si. Only one 13Cline is observed at 5 ppm, being assigned to CH, units (n = 1, 2, 3). Thermal conversion in argon atmosphere begins at 600°C and finishes at 700°C. 187 At 600°C the Sic3-H peak diminishes, suggesting the breaking of the SCH bond. At 700°C both 29Si and 13C lines become broad, reflecting the amorphous nature of the network. Further heating produces amorphous Sic. When the curing is carried out in oxygen at 145 195"C, three new lines appear in 29Si spectra at 9.5, -17, and -53 ppm.'85,189The 9.5 pprn peak is ascribed to Si*C3-OSi, the -17ppm peak to Si*C3-OCH2-Si, and the -53 ppm peak to Si*&(0CH2)2. Dipolar dephasing experiments can enhance the lines at -17 and -53 ppm compared with the Sic3-H peak. Nitridation of polycarbosilane is carried out by thermal treatment in NH3 gas flow, and the process has been monitored by 29Siand 13CNMR.188 The nitridation begins at 500"C, and is completed at 700°C. After breaking Si-H and Si-C bonds, Si-N bonds are created. Finally amorphous Si3N4 is formed, which gives a 29Siline at -45 ppm. No new peaks have been observed in 29Siand I3C spectra, when polycarbosilanes are cured by electron irradiation.186Si-H and C-H bonds are broken, and Si-C and S i S i bonds are newly created. Si-Ti-C fibre can be prepared by the pyrolysis of polytitanocarbosilane, which process has been monitored by 29Si NMR. 19' Polytitanocarbosilane has a 29Sisignal at 10 ppm in addition to the -0.8 and -18.1 ppm lines. The latter two lines are similar to those in polycarbosilane, and are ascribed to Sic4 and Sic3-H, respectively. The 10ppm peak is assigned to Sic3-0 units. Pyrolysis process up to 1500°C in argon atmosphere has been followed by 29Si NMR measurements at room temperature after heat treatment,'" as shown in Fig. 28. In the first stage up to lOOO"C, Si-C bonds are broken, and SiC4-xOx units are formed. Three new lines are observed at -35 pprn (%GO2), -60 -78 ppm (SiCO3), and -95 -111 ppm -15 (Si04). Above 10o0"C the number of Si-0 bonds decreases. At 1500°C the product is a mixture of crystalline SIC and Tic. Polycarbosilane modified with aluminium alkoxide is based on AI(OH)6

-

-

-

-

72

S. HAYASHI

Sic,

(

b

SiC,H

Dipolar P151-F D ephasing

(

CP/MAS

(C)

CP/MAS

(dl

I

I

I

+50 0 -50 29Si Chemical Shift (relative toTMS) ppm

Fig. 27. 29Si MAS-NMR spectra of polycarbosilane, measured at 11.9 MHz. (a,b) Uncured polycarbosilane fibres, (c) polycarbosilane fibres cured by electron irradiation of 7.5 MGy and (d) of 15 MGy, and (e) polycarbosilane fibres cured by thermal oxidation. (Reproduced by permission of Chapman and Hall Ltd, from ref. 186.)

HIGH-RESOLUTION SOLID-STATE N M R STUDIES ON CERAMICS

73

1500°C

1400°C

1m0c

PTC I

'

'

'

'

' 0

* ' a

1

'

'

'

- 100

1

' ' 1

(PPm 1 Fig. 28. Evolution of 29Si MAS-NMR spectra of polytitanocarbosilane during the pyrolysis process. The spectra were obtained at 59.6 MHz and at room temperature. (Reproduced by permission of Chapman and Hall Ltd, from ref. 191.)

74

S.HAYASHI

particles dispersed in polycarbosilane chains, which has been suggested by 29Si, 27Al,and 13C NMR. 192 The polyaluminocarbosilane is pyrolysed to produce Sic 2H polytype at 1500°C. 5.2. Silicon nitride

There are two crystalline forms of silicon nitride, a and p. In both structures silicon is tetrahedrally coordinated by four nitrogens, and nitrogen is trigonally bound to three silicons. The a phase contains two crystallographically inequivalent Si sites, while the p phase contains only one Si site. Figures 29 and 30 show 29Si MAS-NMR spectra of a-Si3N4 and p-Si3N4, respectively. Roughly speaking, the a phase has two 29Si lines at -46.8 and -48.9ppm, while the p phase has one line at -48.7ppm.’9s196 This corresponds to the number of inequivalent sites. The chemical shift assignment has been based on the idea of “sphere of i n f l ~ e n c e ” . ’The ~ ~ Sicrl and the Sip have eight Si neighbours within a 3.8 A sphere, while the Sia2 has ten Si neighbours. The Sial and Sip are expected to have the same chemical shift, since they both have eight Si neighbours. Consequently, the -48.9ppm peak is ascribed to the Sial, and the -46.8ppm peak to the Sia2. The 29Si spectra shown in Figs 29 and 30 show fine structure, which is dependent on the magnetic field strength. The MAS cannot average out the dipole-dipole interaction when the observed nuclei interact with quadrupolar nuclei. In Si3N4 a 29Si spin interacts with four 14N spins which nuclear spin is 1. Olivieri and Hatfield63 have analysed the spectra theoretically, estimating that isotropic scalar coupling constant Ji,(29Si-’4N) = 15 Hz,and 14N quadrupole coupling constant = -2.1 MHz. The dipolar coupling constant between 29Si and I4N is estimated from the interatomic distance rsrN (1.73 A), which is 335 Hz. Carduner et have recommended the use of high magnetic fields, if crystallographically inequivalent sites are to be observed. Various impurity phases in Si3N4powders have been distinguished by 29Si NMR. 194~195The chemical shifts of those phases are -46.4 ppm (amorphous Si3N4), -57 and -60 ppm (non-stoichiometric phases of silicon oxynitride), -63 ppm (Si2N20), elemental silicon (-81.1 ppm) , and - 110 ppm (Si02). Although it is difficult to trace I5N spectra of samples with natural abundance 15N, spectra with good S/N ratio can be obtained for 15N enriched silicon nitride.7 The chemical shift values are -306 and -317 ppm with respect to CH3N02for p-Si3N4 and Si2N20, respectively. 5.3. Silica and silicates

It is well known that silica is both in amorphous state and in numerous crystalline polymorphs. 197 Table 7 summarizes 29Si chemical shifts in various

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

75

Fig. 29. 29Si MAS-NMR spectra of a-Si3N4 obtained at (A) 39.5, (B) 59.5, and (C) 79.5 M H z . ' ~(Reprinted ~ with permission from J . Am. Chem. Soc., 1990, 112, 4676. 0 1990 American Chemical Society.)

forms of silica. Ordinarily silicon is coordinated by four oxygens in silica, giving 29Si signals in the range from -107 to -120 ppm. On the other hand, silicon is six-coordinated in stishovite, showing a 29Si chemical shift of -191 ppm. The chemical shifts are correlated with Si-Si distances and with Si-0-Si Cordierite is a material with a low expansion coefficient. The ideal composition is Mg2Al3(Si5A1)OI8.Two polymorphs are present; the hightemperature form with hexagonal r tructure and the low-temperature form

76

S. HAYASHI

p - Si3N,

l

~

-44

l

'

-45

l

-46

'

l

-47

'

-40

l

'

-49

l

-50

~

-51

l

~

l

~

-52 ppm

Fig. 30. *'Si MAS-NMR spectra of /3-Si3N4 obtained at (A) 59.5 and (B) 79.5 M H z . ' ~(Reprinted ~ with permission from J . Am. Chem. SOC., 1990, 112, 4676. 0 1990 American Chemical Society.)

with orthorhombic structure. Fyfe et dZo1 have studied the Si,Al ordering process by 29Si NMR. Figure 31 shows 29Si MAS spectra, On heating the glass at 1185"C, crystallization is complete within less than 1min. The amorphous structure reflected in Fig. 31A changes to the hexagonal structure, where the arrangement of Si and Al are disordered, as shown in Fig. 31B. After prolonged annealing, the spectra consist of two peaks at -79.2 and -100.2ppm with intensities of 0.144 and 0.698 (see Fig. 31J), and the crystal structure is orthorhombic. The two main peaks are assigned to Si(4Al) (SO4 tetrahedron.coordinated by four A1 atoms) for the chains and Si(3Al) for the rings. The number of Al-O-A1 bonds decreases during the ordering process.

l

~

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

77

Table 7. 29Sichemical shifts in silica.

Polymorph

29Si chemical shifts/ppm

Ref.

a-Quartz Quartz Quartz Coesite Low crystobalite Cristobalite Tridymite Silicalite Holdstite Stishovite Stishovite

-107.4 -107.1 - 108 -108.1, -113.9 -109.9 -108.5 -109.3 -114.0 -109.9- -117.0 -108.9, -115.0, -119.4 -191.1 -191.3

4 39

-

44 39 4 39 39 198,199 39 39 200

Senegas et aL202 have studied the Si,Al distribution in K-substituted cordierite, KxMg2A14+.Si5-,01s ( O C x S l), by means of 29Si and 27Al NMR. Aluminium excess compensating for the potassium insertion is accommodated in the ring part.

5.4. Alumina, aluminates, and aluminium nitride Alumina has several polymorphs, a, p, y, and so on. A1 coordination in alumina can be identified by 27Alchemical shift, as listed in Table 8. Table 8 also lists chemical shift values of alkali and alkaline-earth aluminates except for calcium aluminates (for calcium aluminates, see Table 3). Magnesium aluminate has a spinel structure. Ideally, Mg and A1 occupy tetrahedral and octahedral sites, respectively. Figure 32 shows "Al MASNMR spectra of synthetic and natural MgA1204 spinels. Natural samples show almost complete A1 ordering, since they are cooled through geometric times. However, the aluminium ordering is incomplete in synthetic samples, and both six- and four-coordinated A1 are observed in the 27Alspectra.204In yttrium aluminium garnet Y3Al5OI2,aluminium is known to occupy both octahedral and tetrahedral sites. Massiot et aL2& have analysed the spectra theoretically, and have obtained accurate populations; three octahedral for two tetrahedral sites. Sodium aluminate NaA102 is formed by dehydration of Na20 A1203 3H20. The dehydration process has been studied by 23Na and 27AlMAS-NMR.208For untreated Na20 -A1203.3H20, two major types of sodium sites are present, which have pseudo-octahedral structure. Aluminium is four-coordinated in the form A1*02(0A1)2. For the dehydrated sample, the sodium site has pseudotrigonal-bipyrimidal structure, and aluminium is four-coordinated, A104.

-

-

78

S.HAYASHI

J

-do

-do

-100

w m irom TYS

-110

-60

-90

-160

-iio

pom irom TMS

Fig. 31. 29Si MAS-NMR spectra of synthetic cordierite, Mg2AI4Si5Ol8.The crystalline cordierites were prepared from glass of the same composition (shown in (A)), by annealing at 1185°C for (B) 2 min, (C) 6.5 min, (D) 20 min, (E) 6 h, (F) 23.5 h, (G) 48.5h, (H) 96h, (I) 408h, and (J) ca. 2000h. The spectra were obtained at 79.5MHz and at room temperature.”l (Reprinted with permission from J . Am. Chem. Soc., 1 9 8 6 , 1 0 8 , 3 2 1 8 . ~1986 American Chemical Society.)

HIGH-RESOLUTION SOLID-STATE NMR STUDIES ON CERAMICS

79

Table 8. "Al chemical shifts." Compounds

Al-0 coordination

Chemical shift/ppm

Ref.

6 6 6

5 3 11.5 6479 64,12 68, 0 6692 66, 8 77 76 73 77 22 0 69.7, 9.8 69 74, 0.8 76, 0.8' 9.4, 0.8 113 109

57 117 205 57 117 203 117 205 57 117 57 57 57 117 204 57 205 206 205

4,6 4, 6 4, 6 4,6 4,6 4

4 4 4 6 6

4,6

4 4, 6 4, 6 6 4 4

205

207

A ' correction for the second-orderquadrupole shift is not included, unless otherwise stated. 'A correction for the second-orderquadruple shift is included:

Aluminium nitride has a high heat-resisting property and large thermal conductivity. Ultrafine powder of aluminium nitride is easily reacted with water, producing AI(OH)3, as shown in Fig. 33.207,209AIN shows a 27Al signal at 103ppm, which corresponds to aluminium coordinated by four nitrogens. The hydrated sample has a new "Al signal at -5ppm, which is assigned to A1 coordinated by six oxygens in A1(OH)3. 5.5. Sialon and its analogues

Sialons are phases in Si-Al-0-N and M-Si-A1-0-N systems. They are constructed of mainly (Si, Al)(O,N)4 tetrahedra, and sometimes contain six-coordinated Al. The local coordination of A1 in the Si-AI-0-N system has been Sialon X-phase, of approximate composition Si3Al6OI2N2,contains both A106 and A104 units. A mixture of sialon polytypoids shows A106 and AIN4 units. Local environments of Si have been studied by 29Si NMR for Y-Si-0N,196,212 La-Si-0-N, and La-Si-AI-0-N systems.213The 29Sichemical shift

00 0

octahedral

I

c

A

B i=Q.36fo.03

tetrahedral

i=0.12fo.06

I

100

$0

0

-50

100

50

0

-50

ppm from AI(H20)63+ Fig. 32. "Al MAS-NMR spectra of (A) MgA1204spinel synthesized at 1400°C and (B) natural spinel from Thailand. The spectra were obtained at 104.2 MHz. (Reproduced by permission of the Chemical Society of Japan from ref. 204.)

HIGH-RESOLUTION SOLID-STATE! NMR STUDIES ON CERAMICS

81

3

I

I

200

I

I

I

100

8

I

1

I

0 -100 /PPm

1

I

-200

Fig. 33. 27Al MAS-NMR spectra of (a) ultrafine powder of AlN and (b) its hydrated sample. The spectra were obtained at 52.00 MHz. (Reproduced by permission of the Chemical Society of Japan from ref. 207.)

ranges for the tetrahedral units SiOxN4-x (x = 0 to 4) are summarized in Fig, 34. p'-Sialon is produced from a nanocomposite between dodecylammoniumexchanged montmorillonite and polyacryonitrile through carbothermal reduction in N2 atmosphere. The process has been monitored by 27Aland 29Si MAS-NMR.214A106 units change to AlN4 units via Al(N,0)4. In contrast, S i 0 4 units are converted to SiN4 by 1200°C without passing intermediate Si(N,O), environments. By 1600°C the Sic4 environment becomes dominant.

w Tclrahcdron

L" SiO,

SiOJN

-

SIOZNz

SON,

SiN4 -20

-30

-40

.-SO

-60

-70

-80

-90

-100

-110

-120

Chemical shil1 in ppm from TMS

Fig.

34. 29Si isotropic chemical shift ranges for SiOxN4-x (0 S x S 4) tetrahedra from the (Y, LaMi-Al-0-N systems.213 (Reprinted with permission from J. Am. Chem. Soc., 1989,111, 5125. @ 1989 American Chemical Society.)

HIGH-RESOLUTION SOLID-STATE NMR STUDIES O N CERAMICS

83

5.6. Newly accessible nuclei In this section, we discuss other nuclei newly accessible, such as 170, 25Mg, 89Y, 91Zr, and '39La. 1 7 0 has a low natural abundance (0.037%), resulting in very low sensitivity. Oldfield's group has traced 170NMR spectra for 1 7 0 enriched ~ a m p l e s . ~In~ contrast, . ~ ~ ~ ~Bastow '~ and Stuart have traced the spectra for natural-abundance samples with relatively small quadrupole interactions.218 Mueller et ul. have applied DAS and DOR techniques to 170-enriched silicate 25Mg spectra of simple inorganic solids have been presented by Dupree and Smith.220No enrichment was carried out, since the natural abundance is 10%. MgO has a chemical shift of 26 ppm with respect to 3 M MgS04 solution. "Y spectra have been traced for Y2O3, various yttrium aluminates, various yttrium silicates, Y2Sn207,221 Y2Ti207, and Y2 -,Ln,M2O7 (Ln = Ce, Pr, Nd, Sm, Eu, Yb; M = Sn, Ti).222 91Zr spectra of oxide materials have been published by Hartman et 13'La spectra have been measured for LaA103.213 6. BIOCERAMICS

Bioceramics are classified into bioinert and bioactive materials. Alumina and zirconia belong to the former group. On the other hand, calcium phosphates have a good affinity to a living body, being classified in the latter group. Calcium phosphates have been studied by means of 31P NMR.224-226 Table 9 summarizes 31P chemical shifts of various calcium phosphates. The isotropic chemical shifts tend to move to lower frequency upon protonation of the phosphate. The spectra of non-stoichiometric calcium hydroxyapatite resemble those of stoichiometric hydroxyapatite, indicating that no additional compounds are formed by the Ca deficits. Human dental enamel shows spectra similar to hydroxyapatite. Amorphous calcium phosphate is an important compound, since it is a precursor of hydroxyapatite. 31P NMR r e s ~ l t s support ~ ~ ~ * a~ model ~ ~ in which amorphous calcium phosphate consists of Ca9(P04)6 clusters. Water resides in the interstices between clusters. 7. CONCLUDING REMARKS Novel ceramics are being created day by day, while new techniques are being developed in the field of solid-state NMR. At the same time, the number of nuclei accessible by solid-state NMR is increasing. Correlation between performance of materials and atomic-level chemistry will become

84

S. HAYASHI

Table 9. "P chemical shifts in calcium phosphates.

Compounds

Form

Monocalcium phosphate monohydrate Monocalcium phosphate Dicalcium phosphate dihydrate (brushite) Dicalcium phosphate (monetite) Octacalcium phosphate P-Tricalcium phosphate Hydroxyapatite Fluoroapatite Amorphous calcium phosphate

ShiWppm

Ref.

-0.1, -4.6 -0.6, -5.0 0.5, -0.5 1.7 1.0 0.0, -1.5 -0.7, -1.8 3.4, -0.1 2.6, 1.2, -0.8 9.2, 5.2, 4.2 2.8 2.8 1.7 2.6

224 226 224 224 226 224 226 224 226 226 224 224 225 226

much more important to improve the performance and to create novel materials with high performance. NMR is a powerful method to investigate atomic-level local structures in principle, and is an indispensable method for material researches. Although both materials and NMR techniques are limited in the present chapter, application of solid-state NMR to material researches will be enlarged greatly.

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NMR Studies of Zeolites H. PFEIFER and H. ERNST Universitat Leipzig, Fachbreich Physik, Linnkstr. 5, 04103 Leipzig, Germany 1. Introduction 2. Framework of zeolites 2.1. High-resolution solid-state NMR of nuclei in zeolite lattices-general aspects 2.2. 29Si MAS-NMR studies 2.3. 27AIMAS-NMR studies 3. Bronsted acid sites 3.1. Unloaded (evacuated) zeolites 3.2. Accessibility of Bronsted acid sites, hydrated zeolites 4. Lewis acid sitedextra-framework aluminium 4.1. ”Al NMR spectroscopy 4.2. Use of probe molecules 5. Structure of adsorbed molecules 5.1. High-resolution NMR of adsorbed molecules-general aspects 5.2. 13C and ‘H MAS-NMR studies on sealed samples 6. Molecular diffusion 6.1. Basic principles 6.2. Intracrystalline diffusion of hydrocarbons 6.3. Anisotropic molecular diffusion 6.4. NMR tracer desorption technique 7. Chemical reactions 7.1. Experimental methods for in situ MAS-NMR reaction studies on sealed samples 7.2. Conversion of methanol 7.3. Synthesis of methylamines References

91 93 93 97 101 106 106 123 126 127 129 132 132 133 138 138 147 152 154 159 159 164 166 176

1. INTRODUCTION

Zeolites’4 are porous inorganic crystallites built from TO4 tetrahedra. In the case of the original zeolites T represents silicon or aluminium, for the ALPOs (aluminophosphates) aluminium or phosphorus, for the SAPOs (silicoaluminophosphates) silicon, aluminium or phosphorus while in the so-called MeAPOs and MeAPSOs additional metal atoms Me such as Mg, Mn, Fe, Co, or Zn are incorporated. The word ZEOLITE, which means “boiling stone” in Greek, was coined by the Swedish scientist Cronstedt’ in ANNUAL REPORTS ON NMR SPECTROSCOPY VOLUME 28 ISBN 0-12-505328-2

Copyright 0 1994 Academic Press Limited AN rights of reproduction in any form reserved

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1756 to describe the behaviour of the newly discovered mineral stilbite which, when heated, rapidly loses water and thus seems to boil. The original zeolites can be represented by the general formula (Si02),,(AlOz)-M+ where n 3 1 denotes the silicon-to-aluminium ratio of the zeolite framework (Si02),,(A102)-. This framework is built from corner-sharing SiOj- and A102- tetrahedra and contains regular systems of intracrystalline cavities and channels of molecular dimensions. The inner surface is formed nearly entirely by the oxygen atoms due to their relatively large ionic radius (the ionic radii of 02-,A13+, and Si4+ are 0.132 nm, 0.051 nm, and 0.042 nm, respectively). The negative charge of the framework and hence the concentration of the exchangeable cations M+ which are located on extra-framework sites6 can be made to zero by enhancing the silicon-toaluminium ratio so that in this limiting case zeolites exhibit a perfect homogeneous solid surface formed by oxygen atoms. The cations M+ neutralizing the electric charge of the framework can be exchanged for other cations of corresponding net charge (e.g. 1/2 M2+). An exchange with ammonium ions followed by a heat treatment leads to the formation of bridging OH groups (SiOHAl), which are known to be able to protonate adsorbed molecules (Bronsted acid sites). In the last 40 years more than 30 natural zeolites and more than 100 synthetic zeolites were identified. Several classifications of zeolite structures, based on the framework topology, have been proposed. For practical reasons it is useful to distinguish between large-pore zeolites with 12membered oxygen rings (e.g. zeolites of the faujasite type: X,Y, ZSM-20, see Table l), medium-pore zeolites with 10-membered rings (e.g. zeolites of the pentasil group: ZSMJ or silicalite, see Table 1) or small-pore zeolites with eight-membered rings (e.g. A-zeolites or ZK-4, see Table 1). The spatial arrangements of cavities and channels of the zeolites characterized in Table 1 are shown in Fig. 1 where each corner is occupied by a T-atom which is connected to four others by oxygen bridges (indicated by the straight lines). Zeolites are largely used in industry for a wide range of applications including adsorption and separation of gases and of hydrocarbons (“molecular sieves”), drying, ion exchange and catalysis. Catalysts based on zeolite Y containing bridging OH groups have a cracking activity which is orders of magnitude greater than that of conventional silica-alumina catalysts and have displaced them now almost completely. The synthetic zeolite ZSM-5 is very useful for many applications since its high silica content (n is typically not less than about 15) gives it high thermal stability, while the channel diameter leads to quite striking effects of shape selectivity including the ability to synthesize gasoline from methanol in a single step.

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93

Table 1. Characteristic data of typical large-pore, small-pore and medium-pore zeolites. Faujasite group: zeolites X,Y, ZSM-20 Unit cell

Large cavities/ channels

Zeolite A, ZK-4

Cubic a = b = c = 2.47nm

Cubic a = b = c = 2.46nm

8 cubooctahedra

8 cubooctahedra +24 oxygen bridges 384 0; 192 T Inner diam.: 1.14nm window (8 O-ring): 0.41 nm (NaA) 0.50 nm (CaA)

+16 oxygen bridges 384 0; 192 T Inner diam.: 1.16nm window (12 O-ring): 0.74 nm

Cubooctahedra (small cavities)

Inner diam.: 0.66 nm; window (6 O-rings): 0.25 nm

Number of T-atoms

24 per large cavity

Number of large cavitiedintersections per g zeolite

4.18 X

Id"( n

= m)

3.53 X I d n ( n = 1)

Pentasil group: ZSM-5, silicalite Orthorhombic a = 2.01 nm b = 1.99nm c = 1.34nm 4 intersections 129 0;96 T Straight channels: 0.53 x 0.56 nm2 zig-zag channels: 0.51 x 0.55 nm2 (10 O-rings)

24 per intersection 4.18 X 10" (n = 0 0 )

Summarized, zeolites share the following five properties: (1) well-defined crystalline s t r ~ c t u r e , ~ (2) large internal surface areas (a 600 m2/g), (3) uniform pores, (4) good thermal stability, and ( 5 ) Bronsted acidity of adjustable concentration and strength, that make them attractive not only for practical application (see above) but also for basic research studies and especially to NMR spectroscopists since virtually every chemical element they contain can be studied by N M R with a sufficiently large signal-to-noise ratio due to the large internal surface areas. 2. FRAMEWORK OF ZEOLITES 2.1. High-resolution solid-state NMR of nuclei in zeolite lattices-general aspects

The field of N M R has developed very rapidly during the last 30 years especially due to the invention of techniques that allow a measurement of

94

H. PFEIFER AND H. ERNST

12 RING

ZEOLITE

PENTASIL ZEOLITE

@ 8 RING

A ZEOLITE

Fig. 1. Framework structures of typical large-pore, medium-pore, and small-pore zeolites (cf. Table 1).

highly resolved NMR spectra in solids. As is well known, the Hamiltonian which determines the properties of a spin system can be written as

HO = H z + H D + HCsA+ H j + H o + HRF

(1)

where Hz is the Zeeman interaction including the isotropic chemical shift between the nucleus and the applied static magnetic field Bo. HD denotes the magnetic dipolar interaction, HCsAthe interaction due to chemical shift

NMR STUDIES OF ZEOLITES

95

anisotropy, HJthe indirect electron-coupled nuclear spin interaction, HQ the electric quadrupolar interaction which is only present for nuclei with spin Z> 1 and HRF the interaction with alternating magnetic fields, mostly applied in the form of short rf pulses. In non-viscous liquids HD and Ho average to zero, which permits the much weaker isotropic value of the chemical shift and the J coupling to be studied. In the solid state, however, the homonuclear dipolar, the heteronuclear dipolar and the quadrupolar interactions as well as the chemical shift anisotropy are dominant, which leads to such a broadening of the NMR lines that in general the valuable information contained in the chemical shift and J-coupling (fingerprint system of liquid state NMR7) is hidden. To obtain highly resolved NMR spectra in solids, a selective removal or alteration of each of the broadening interactions is necessary. Generally, manipulations of spatial or spin variables of the Hamiltonian given in equation (1) can be performed. The new techniques of solid-state NMR, namely multiple pulse sequences,g2o high-power dipolar decoupling,21 cross-polarization (CP) in combination with high-power d e c ~ u p l i n g , ~ ~ ~ ~ magic angle spinning (MAS) of the sample,25726combined rotation and multiple pulse sequence (CRAMPS) ,27-30 variable angle spinning (VAS),31"4 dynamic angle spinning (DAS)35-37and double rotation technique (DOR)38-41eliminate or at least reduce to a certain degree the influence of HD,HQ and the chemical shift anisotropy HCSA, thus making possible the observation of one- and two-dimensional highly resolved NMR spectra in solids. The benefits of strong magnetic fields lie at least in three factors: (1) the sensitivity is roughly proportional to Bo to the power 3/2; (2) for quadrupolar nuclei with half-integer spin, lines tend to become narrower at higher fields, since the second-order quadrupolar broadening is inversely proportional to Bo, and (3) since the chemical-shift dispersion is proportional to Bo, potentially more chemical-shift information becomes available. More details and reviews of the present state of the high resolution NMR in solid^^^,^^ and applications to zeolites and related molecular sieves and catalysts4357 can be found in the literature. In ref. 55 an instructive and concise survey on the applicability of multinuclear solid-state NMR in structural studies of silicates, aluminosilicates and zeolites is given. From the point of view of NMR, each of the four basic atomic constituents of the framework of zeolites including aluminophosphatesoxygen, silicon, aluminium and phosphorus-are amenable to measurements by their naturally occurring isotopes 1 7 0 , "Al, 29Siand 31P (see Table 2). However, the 1 7 0 isotope has a very low natural abundance and a nuclear quadrupole moment giving rise to a large line broadening. Moreover, the isotopic "0enrichment is difficult. Therefore, 1 7 0 NMR on zeolites has been little used till now. The 27Al isotope has a natural abundance of 100% but a quadrupole moment ( I = 5/2), which may also cause a

96

H. PFFJFER AND H. ERNST

Table 2. N M R properties of some selected nuclei which may be used in NMR studies of zeolites. The relative sensitivity is defined as the product of natural abundance and NMR relative sensitivity to protons. The resonance frequency is given for a magnetic field of ca. 11.74T.

Isotope

Spin

Natural abundance (%)

Relative sensitivity

NMR frequency (MW

~~

'H 7 ~ i I*B 170

23Na 2 7 ~ 1

29~i 3*P 69Ga 'lGa

73Ge

112 312 312 512 312 512 112 112 312 312 912

99.985 92.58 80.42 0.037 100.00 100.00 4.70 100.00 60.4 39.6 7.76

1.om 0.27 0.13 1.08x 10-5 9.25 x lo-' 0.21 3.69 x 10-4 6.63 x lo-' 4.17 x 5.62 X lo-' 1.08 x 10-4

500 194.3 160.4 67.8 132.3 130.3 99.3 202.4 120.0 152.5 17.4

strong broadening of the NMR signals. The 29Si isotope is 4.7% abundant and isotopic enrichment is also difficult. But in contrast to 1 7 0 and 27Al, the 29Sinucleus has a spin 1/2, so that Ho = 0 in equation (1) and one may get narrow resonance lines. Compared with the 27Al isotope the longitudinal relaxation time of 29Si is often very long. In these cases the NMR experiments are time-consuming. Due to its natural abundance of 100% and the absence of an electric quadrupole moment, 31P solid-state NMR is the most widely used analytical method for zeolites of type A L P 0 and SAPO. Several other elements that replace silicon or aluminium in the zeolitic framework (e.g. B, Ga, Ge) or are present as charge-compensating cations (e.g. Li+, Na+) can be studied by solid-state NMR of their respective NMR-active isotopes ("B, 69Ga, 71Ga, 73Ge, 'Li, 23Na). With respect to the 'H MAS-NMR see the sections on Bronsted acid sites and Lewis acid sites/extraframework aluminium. Since the first application of solid-state NMR to zeolites, important information about the zeolitic framework has been retrieved5* concerning especially (1) the coordination and local atomic environment of silicon, aluminium and phosphorus; (2) the number of distinct tetrahedral sites and of the framework composition (silicon-to-aluminium ratio) from an analysis of the 27Si MAS-NMR spectra; (3) the de- and re-alumination, and de- and re-gallination, and last but not least (4) the characterization of Bronsted acid and Lewis acid sites. In the following, typical examples for the information of type (1)-(3) derived from 29Si, "Al and 1 7 0 MAS-NMR studies shall be given.

NMR STUDIES OF ZEOLITES

.................................................. ................................................... J - C

S1(4AI)-SOD

Si(4AI)

f SI I 2A I)

I

- 80 I

97

I

- 90 I

I

- 100 I

1

I

-110

1

-120

dsi/ P P ~ Fig. 2. Ranges of %i chemical shifts of Si(nA1) units in zeolites. Si(4Al)-SOD denotes Si(4Al) units in ~odalites.~'

2.2. 29Si MAS-NMR studies

The 29Si chemical shifts of silicates and ahminosilicates depend sensitively on the number and type of T-atoms (T = Si, A1 or other tetrahedral framework atoms) connected with a given Si04 tetrahedron. That means, distinct signals appear in the "Si MAS-NMR spectra for the five different Si(nA1) environments (n = 04).The typical shift ranges established from a large number of data measured in various types of zeolites and other framework silicates, are shown in Fig. 2.47The largest range observed for a Si(nA1) unit is that of Si(4A1) in aluminosilicate sodalites. As usual, all values of the chemical shift of the 29Sisignals are with respect to TMS. For an interpretation of the spectra obtained it is important to note that the relative signal intensities of the 29Sisignals are directly related to the relative concentrations of these units in the zeolite framework. Figure 3 shows the 29SiMAS-NMR spectra of a series of zeolites X and Y with different Si/Al ratios.59 With increasing SYA1 ratio, the decrease of the signal intensities of aluminium-rich units and a corresponding increase of the aluminium-poor Si(nA1) signals can be seen. Provided that no AlOAl linkages are present in the zeolite (i.e. Loewenstein's rule appliesa), the

98

H.PFEIFER AND H.ERNST

L

1.02

1.60

2.82 3 ,

2 1

3

4 l d . 4 h I

.

.

-80 -100 -120

-80 -100 -120 1

2.02

1.17

2

4

I

-80 -100 -120

-80 -100 -120

4

I

I

I

,

I

I

I

-80 -100 -120

2

,

-80 -100 -120

:.k

,

- 80

,

I

-160 -1'O ;

.

,

-80 -100 -120

Fig. 3. "Si MAS-NMR spectra of zeolites X and Y. The SilAl ratio is indicated for each spectrum, the peak assignments are given b the number n of the corresponding Si(nA1) unit. J

N M R STUDIES OF ZEOLITES

99

framework Si/AI ratio can be directly calculated from the 29Si MAS-NMR spectra according to the equation (Si/AI)F = 4.1I,,/Cn.I,

(2)

where n = 0-4 and I, are the intensities of the Si(nA1) peaks.59 Equation (2) is independent of the specific structure of the zeolite, but excludes the existence of silanol groups. Figure 4 shows the 29Si MAS-NMR spectra of some typical zeolites with relatively low SUAI ratio together with the corresponding highly siliceous forms obtained after a hydrothermal dealumination.61 In all highly siliceous materials, the resonances observed are only due to Si(0AI) units and are extremely narrow. Because of their high resolution, these spectra can be used in various ways to get subtle information regarding zeolite structures which are not easily obtainable by other techniques. The residual line broadening in the 29Si MAS-NMR spectra of highly siliceous zeolites is due to the chemical shift distribution arising from the distribution of the residual aluminium atoms and imperfections of the structure.61 The effect of a careful optimization of all factors affecting the spectral resolution is shown in Fig. 5 for a zeolite ZSM-5.62 From the optimized spectrum, 21 or 22 of the 24 postulated signals can be clearly observed depending also on the temperature of the measurement. In NMR studies on liquids, the application of two-dimensional (2D) techniques has provided a wealth of information on the connectivities between atoms within molecular structures. In principle, 2D MAS-NMR experiments can also be used to establish connectivities in the solid state. As typical examples, in Fig. 6 a 2D COSY and in Fig. 7 a 2D INADEQUATE 29Si MAS-NMR spectrum of a zeolite ZSM-12 is shown.49 The lattice framework of the zeolite ZSM-12 is represented schematically in Fig. 8.63In this figure, the seven crystallographically inequivalent tetrahedral lattice sites are indicated, and it can be seen that their relative concentrations are equal. Accordingly the 29Si MAS-NMR spectrum shows seven clearly resolved resonances with equal intensities for six of them. The fact that one of the signals exhibits a lower intensity was used by the authors to assign it to the T5 site which is the only site not accessible by adsorbed oxygen. Therefore the longitudinal relaxation rate and hence also the signal intensity of the 29Si nuclei occupying sites T5 must be reduced with regard to the others. Starting from this assignment of T5 the authors could proceed to assign the connectivities yielding the labelling of the cross-peaks given in Fig. 6. All of the connectivities found and assigned in the 2D COSY experiment are confirmed in the 2D INADEQUATE experiment shown in Fig. 7. This result includes the connectivity between the sites T4 and T6, which is clearly resolved in the 2D INADEQUATE experiment but was ambiguous in the 2D COSY experiment due to the close proximity of the cross-peaks to the diagonal.

100

H. PFEIFER AND H. ERNST

Si(1AI) Si(0AI)

Si(2AI) Si(2AI)

1M Si(0AI)

Si(1AI)

-90

-100

Si(0AI)

-110

-90

-1201

-110

-100

-120

Si(1AI) Si(2Al) Si(0Al)

-90 -100 -110 -120

-90 -100 -110 -120 SsllPPm

Fig. 4. 29SiMAS-NMR spectra of some typical zeolites with a relatively small Si/A ratio together with the corresponding highly siliceous forms: (A) zeolite Y, (B) mordenite, (C) offretite, (D) zeolite omega.6'

N M R STUDIES OF ZEOLITES

n

A

101

5 Hr

B

C

-

I

I

I

I

108

I

I

1

d5;/ PPm

I

I

I

-

I

118

Fig. 5. 29Si MAS-NMR spectra of a highly siliceous zeolite ZSM-5: (A) without special pretreatment, (B) after optimization of all experimental variables, (C) the simulated spectrum.62

2.3. "Al MAS-NMR studies

Compared with 29Si the nuclei of the other most abundant atoms of the zeolite framework, "Al and 1 7 0 , show a more complex NMR behaviour, since even the 112 to - 112 transition which yields the smallest line widths is distorted and shifted by the second-order quadrupolar interaction. The shift of the centre of gravity of this signal is given by:52

+

w c c - 0 ~=

-(1/30).0&/%.[I(I+

1)- (3/4)](1 +v2/3)

(3)

102

H. PFEIFER AND H.ERNST

T3

T4

T6

7-2

~ ~ " ' " " " " " " ' ~ ' " ~ ' ' . ' - . ' ~ . . -108.0

-109.0

-110.0 -111.0 -112.0 -113.0 &l/PPm

Fig. 6. Contour plot of the 2D COSY "Si MAS-NMR spectrum of a zeolite ZSM-12.

The projection in the F2 dimension (1D "Si MAS-NMR spectrum) is shown above.49

103

N M R STUDIES OF ZEOLITES

r

- S6

'TqT6

s5 s4

s3

s2 SI

L

-108.0

-109.0

-110.0

-111.0

WPPm

-112.0

-113.0

Fig. 7. Contour of the 2D INADEQUATE *'Si MAS-NMR spectrum of the same zeolite ZSM-12 as in Fig. 6. The projection in the F dimension (1D "Si MAS-NMR spectrum) is shown above.492

with oQ = 32qQ/(21(21- 1)fi). Here eq denotes the z-component of the electric field-gradient tensor, eQ the quadrupole moment of the nucleus, 7 the asymmetry parameter, and y the resonance frequency for Ho = 0. Therefore it is of advantage to measure at high fields Bo. This is illustrated in Fig. 9 which shows the "A1 MAS-NMR spectra of a zeolite Y at 23.5 and 1 0 4 . 2 M H ~ As . ~ ~usual, all 27Al NMR shifts are given with respect to the signal of an aqueous solution of aluminium chloride. Since the framework aluminium nuclei of hydrated zeolites give rise to a relatively narrow line in the 27Al MAS-NMR spectra, the higher NMR sensitivity of "Al compared with 29Si (cf. Table 2) allows a quantitative

104

H. PFEIFER AND H. ERNST

Fig. 8. Schematic representation of the lattice framework of a zeolite ZSM-12. The seven crystallographically inequivalent tetrahedral lattice sites are indicated by TI (=Sil) to T, (=Si7).63Ts is the only site which is not located on the surface of a channel wall.

determination of small concentrations of framework aluminium also in those cases where it cannot be seen in the 29SiMAS-NMR spectra. In contrast, for the non-framework aluminium species the 27Al MAS-NMR signals may be very broad which reduces dramatically the sensitivity of this method. Moreover, complications may arise if bmad signals of aluminium sites subject to different quadrupolar interactions overlap in the spectra. The separation of these lines can be achieved by application of the twodimensional quadrupole nutation MAS-NMR technique As an example, Fig. 10 shows "Al MAS-NMR spectra of framework dealuminated Y zeolites.65 The different lines could be resolved by 27Al nutation MASNMR. It has been concluded that the 27Al MAS-NMR spectra consist of three superimposed lines which correspond to framework ( A F ) , non-

N M R STUDIES OF ZEOLITES

200

100

0

105

-100

101.22 MHz

Fig. 9. 27A1MAS-NMR spectra of a zeolite Y obtained at (a) 23.45MHz and (b)

104.22M H z . ~ ~

framework octahedral (AINFo) and non-framework tetrahedral (AINFT) aluminium. Details of the dealumination process are described in a recent paper from a theoretical point of view on the basis of ab initio SCF MO calculations.% The results given in this study may contribute to a better understanding of the nature of non-framework aluminium species in hydrothermally treated zeolites. In Table 3 a survey is given of solid-state NMR studies performed on zeolites including aluminophosphates (zeolites of type ALPO, SAP0 etc.) since 1987. With regard to references before 1987 the reader is referred to ref. 43.

106

H. PFEIFER AND H. ERNST AIF

AINm

60

40

AtNFo

1

SiIAI

2

SilAl

3

io

0

6dPm

Fig. 10. "A1 MAS-NMR spectra of framework dealuminated zeolites Y. Integral intensities (in %) of the lines have been determined by the use of line sha es measured in a two-dimensional 27AI nutation experiment. A f , AINm, and AlgF0 denote tetrahedral framework Al, tetrahedral non-framework Al, and octahedral non-framework Al, re~pectively.~~

3. BRONSTED ACID SITES

3.1. Unloaded (evacuated) zeolites The elementary step of a catalytic reaction effected by a so-called Bronsted acid site is the proton transfer from an acidic surface hydroxyl group ZOH to the adsorbed molecule M:

ZOH + M 2 ZO-

+ MH+

(4)

N M R STUDIES OF ZEOLITES

107

Therefore, apart from a possible rate-determining influence of diffusion, the efficiency of a catalyst will be determined by three independent parameters for each sort of acidic surface hydroxyl groups (Bronsted acid sites): (1) the strength of acidity as defined qualitatively by the ability to protonate an adsorbed molecule; (2) the concentration; and (3) the accessibility of the respective OH gtoups. Apparently the ability to protonate an adsorbed molecule will depend both on the properties of the acidic site ZOH and of the molecule M. In order to define a quantity which characterizes the protonation ability of the ZOH group but does not depend on the particular molecule, the reaction described by equation (4)is decomposed into two processes:

ZOH T)c ZO-

+ H+

H+ + M jc MH+

(5) (6)

Denoting the standard Gibbs free energy change of the first process (equation 5) by AGgp we define the strength of acidity S, of the ZOH group in vacuum (the so-called gas phase acidity261)by S, = 1iAGgp

(7)

so that decreasing values for the standard Gibbs free energy of the deprotonation of ZOH correspond to increasing values of the strength of acidity as it should be. To compare the strength of acidity with the deprotonation energy of a ZOH group, AEDp,a quantity which follows from quantum chemical calculations, one must take into consideration that the standard Gibbs free energy change AGgp is the sum of the deprotonation energy AEDP,of the zero-point energy change AEgp and of the Gibbs free energy change which results from the conversion of the three vibrational degrees of freedom of the proton as part of the ZOH group into its three translational degrees of freedom after leaving this group. Assuming that the zero-point energy change AEgp is a constant and that the the reciprocal value of the deprocontribution of A G & F is tonation energy AEDpcan be taken approximately as a measure for the strength of acidity. With respect to nuclear magnetic resonance spectroscopy there are at least four arguments for the statement that the position of the 'H MAS-NMR signal of an OH group is related to its strength of acidity: (1) A qualitative argument. An increase of the so-called chemical shift S, of an OH group which is defined by &= -I UTMS-UH

(8)

108

H.PFEIFER AND H. ERNST

Table 3. Survey of solid-state NMR measurements which were performed on zeolites (including alurninophosphates)since 1987.

Zeolite Nucleus

Faujasitetype

ZSM-type, mordenite

Aluminophosphates

Others

”A1

65,’ 87: 88,’ 90, 56,75,’ 76,390, 110,’ 126,’ 130, 104,3117, 124, 131; 153,’ 165,’ 125, 126; 130, 183,4200,’ 131; 185,226: 231, 233,’ 234,’ 227,4*5229,4 237,3 238, 241,’ 243; 245 247

67,68, 78,3 80, 83,389,90, 98,3 81: 93, 107,6 104,3153: 217, 155, 156,3157, 245 160,161, 162,6 173,6 176,3180,6 198,205, 222, 223,3 228, 240 253, 256: 2575

29Si

82,’ 84,’ 101, 102,112,152, 154; 170, 171, 179,’ 189; 197, 211,’225,’249’

70,77,‘79,105; 106,3108, 118, 121,135, 136, 138,5139, 140,5 142-149,’ 150: 152,163: 172, 187,192; 239; 246, 258,’ 259’

127; 181, 194, 203

70, 97,3 100,119, 120, 122, 136, 137, 138; 140,5 141,’ 184, 194, 195, 215, 216, 230, 244

27A1

73,74,’86,91,’ 92,109: 115,3 116,3133,’ 158,’ 159; 164,’ 166,3 168, 169, 174, 188,’ 190, 193, 199,’ 204,* 212, 220,224; 232, 235,’ 242,2 252’

92,94,’95,’96, 98; 99,’ 114, 115,3123,133,’ 134,151: 186; 190,193,209, 210,213, 214,3 236,251’

69,85,92, 111,3 113; 132,167, 177,’ 193,201,’ 202; 218: 219: 232,250,’ 254,’ 2553

71,372, 103,’ 128,129: 151; 167,175, 178, 182,191,196; 206-208, 220, 221, 248

232

98, 99,238, 2604 67,78,80, 81,6 194 85,92, 111, 113, 132, 155-157, 160,161,167, 177,194, 198, 201,202,218, 219,222,223, 232,240,250, 254,255, 256,4 2575

+

29Si

31P

NMR STUDIES OF ZEOLITES

109

Table 3-contd.

"B

152

Other

86,116,130,131, 75,92,94-96,98, 92, 132, 181,254 83, 122, 128, 129, 151, 184, 208, 217, 133, 183, 18899,118, 123, 124, 190,245 130, 131, 133, 230,245, 248 151, 172, 185, 190,213,214, 226,231

152, 186

92,93

83,97

'insertion of nuclei in the framework of the zeolites; 'dealurnination of the zeolite framework; 3synthesisof zeolites; 4CP (cross-polarization) MAS-NMR experiments; '2D (two-dimensional) MAS-NMR experiments; 6DOR (double-rotation) MAS-NMR experiments.

with uTMS and uH as the chemical shielding of the 'H nucleus in tetramethysilane and the OH group, respectively, corresponds to an increase of the net atomic charge of the hydrogen atom. On the other hand, an enhancement of the net atomic charge will be accompanied by a reduction of the deprotonation energy. Hence, the chemical shift of OH groups should increase with increasing strength of acidity. (2) Experimental results for molecules in the gaseous state. In Fig. 11 values of S, which were measured by Chauvel and for various molecules in the gaseous state are plotted as a function of experimental values of AGL, taken from ref. 264. As can be seen, in agreement with the qualitative argument, the chemical shift increases with increasing strength of acidity (cf. equation 7). (3) Experimental results for zeolites of varying Si/AI ratio. In Fig. 12 values for Sanderson's intermediate electronegativity S, which were computed for zeolites of composition HA102(Si02), according to the formula265

with the following values of the atomic electronegativitie~:~~~ SH = 3.55, SAl = 2.22, So = 5.21, Ssi = 2.84 are plotted together with experimental , in dependence on the silicon-to-aluminium ratio n. The nearly values for 6 identical functional dependence of both quantities provides ample evidence for the usefulness and sensitivity of 8, as a measure for the strength of acidity. In this connection it should be mentioned that due to the use of methane as an inner standard2& the absolute values of 8~ plotted in Fig. 12 are smaller by about 0.3 ppm with respect to a former paper.267

110

H. P F E m R AND H. ERNST

.4t E

4-

Q P

*=

3-

2-

't

i-PrOH

0

1550

-AG&

1500

1450

1400

/ kJ rno1-l

Fig. 11. Experimental values for the chemical shift S, (ref. 263) and for the standard Gibbs free energy of deprotonation AGODp of the OH group of various molecules in the gaseous state.

4.4

'4.3

H-Y

OH-2

OH-Z

0HwM

H-M

-4.2 E

v, -4.1

I

1

4.0

nFig. 12. Values for the intermediate electronegativity S,, and the chemical shift S, of the accessible bridging OH groups (i.e. those OH groups which give rise to line b, corresponding to the HF-band in IR spectroscopy) in dependence on the silicon-toaluminium ratio ~tfor various zeolites.268

111

NMR STUDIES OF ZEOLITES

t3 5 30 -

E

Q Q \

bx

25 /

/

20 r

I

Hi0

/

+

I

I

I

I

800

1000

1200

1400

I

1600

A E , , ~ / kJ rnol‘’

I

1800 c

Fig. 13. Quantum chemically calculated values for the chemical shielding aH and for the deprotonation energy AEDp of various OH groups.262

(4) Quantum chemical calculations. In Fig. 13 the results of non-empirical quantum chemical calculations for the shielding constant oH and the deprotonation energy AEDPare plotted for OH groups of various species.262 In agreement with the qualitative argument presented above, the chemical shielding increases with increasing values of the deprotonation energy. Summarized one can state that all four arguments are in favour of a direct correlation between the chemical shift and the strength of acidity of OH groups. However, it is necessary to add two comments: (1) The OH groups must be isolated which requires the absence of

adsorbed molecules (all measurements have to be performed in vacuum: gas phase acidity) and of additional electrostatic interactions with the framework as caused, for example, by formation of hydrogen bonds to neighbouring oxygen atoms. The latter interaction leads to different values of the chemical shift for the OH groups pointing into large and small cavities of zeolites (lines b and c, see below). (2) The OH groups must be of similar type as e.g. those compared in Figs 11-13. In a recent paper however, Sauer et ~ 1 were. able~to show ~ that the direct correlation between 6~ and S, is valid for all sorts of surface OH groups of zeolites and related catalysts.

~

112

H.PFELFER AND H. ERNST

- 1

6,l PPm

I

I

I

7

4.5

2

I

Fig. 14. Enhancement of resolution for a typical zeolite catalyst (90 H-Y 300 SB) by spinning of the evacuated powder samples which are contained in sealed glass ampoules about the magic angle. Top: usual 'H NMR spectrum. Bottom: 'H MAS-NMR spectrum with a spinning rate of 2.5 kHz. 270 MHz resonance frequency, room temperature."'

Hence, however, the chemical shift of the 'H MAS-NMR signal can be used as a reliable and sensitive measure for the strength of acidity S, of isolated Bronsted acid. The high sensitivity follows from an inspection of Figs 11 and 13, which yield a slope of ca. 35-100Id/mol ppm, and from the fact that the position of the 'H MAS-NMR signals can be determined with an accuracy of better than 0.1ppm. Hence, it should be possible to measure a change of the deprotonation energy of as low as ca. 5 kJ/mol. The first highly resolved 'H NMR spectrum of a catalyst which could be achieved by a fast rotation of the sample about the so-called magic angle ('H MAS-NMR technique) is shown in Fig. 14. The problem in these experiments is the fast rotation of evacuated powder samples which requires a high degree of axial symmetry of the sealed glass ampoules. In the 'H MAS-NMR spectra of evacuated zeolites containing only oxygen, silicon and aluminium in the framework, five lines can be separated in general which have been denoted266,269.272 as lines a, b, c, d, and e.

NMR STUDIES OF ZEOLITES

113

Line a which appears in the interval between ca. 1.8 and 2.3ppm is caused by non-acidic (silanol) groups. In contrast to an older ~ a p e ? ~ it' seems necessary to state that a distinction between single (SiOH) and geminal (Si(OH)2) hydroxyl groups is not possible by this technique since the difference between the corresponding values of S, is less than 0.1 ppm.268In 29SiMAS-NMR spectra, however, well-separated signals due to single and geminal silanols appear at about -100ppm and about -90 ppm, re~pectively.~~ Unfortunately, the possibility of confirming these two species is limited to adsorbents built up only by silicon and oxygen since the insertion of other metal atoms, as e.g. aluminium, leads to overlapping signals in the 29SiMAS-NMR spectra. Line b at 3 . 8 4 . 4 ppm is ascribed to acidic OH groups which are known to be of bridging type (SiOHAl). The value of S, increases with increasing silicon-to-aluminium ratio of the zeolite (cf. Fig. 12). Line c at 4.8-5.6 ppm is also ascribed to acidic OH groups of the bridging type but under the influence of an additional electrostatic interaction, as e.g. in the case of formation of hydrogen bonds to neighbouring oxygen atoms ("non-isolated" OH groups). The same effect has been found for the stretching vibration of OH groups giving rise to the so-called LF band (3540 cm-I) in addition to the H F band (3650 cm-1).271 In a former ~ a p e ? ' ~line c was ascribed tentatively to OH groups located in the large cavities which are known to be responsible for the HF band in the IR spectrum. In a later paper,267 however, it could be shown unambiguously by the use of deuterated pyridine as a probe molecule that the correlation line b line c

f,

HF band

LF band

is the correct one. The fact that for the bridging OH groups pointing into the large and into the small cavities separate lines appear in the 'H MAS-NMR spectra (lines b and c, respectively) excludes the possibility of a fast proton exchange among the four oxygens around an aluminium atom of the zeolite framework. Line d at 6.5-7.0 ppm is due to residual ammonium ions. Line e at 2.5-3.6 ppm represents hydroxyl groups associated with extraframework aluminium species. Due to the limited space available for these OH groups their S, value will be affected by additional electrostatic interactions. Accordingly, for isolated AlOH groups the chemical shift is much smaller with values in the interval between -0.5 and +l ppm. In Fig. 15 measured values for the 'H NMR chemical shift of isolated (black) and interacting (hatched area) OH groups appearing in zeolite catalysts of various type and composition are presented.274

114 AlOH

Y

CaOH

e

I

MgOH

I

BOH

- a

SiOH POH

SiOHAl

m c

-

&,I

-

H. PFEIFER AND H.ERNST

I

PPm 5.0

m

b

I

I

I

I

I

I

4.0

3.0

2.0

1.0

0

-1 .o

Fig. 15. Measured values for the 'H NMR chemical shift of isolated (black) and interacting (hatched area) OH groups observed in zeolite catalysts.274

As has been shown by a systematic study of the influence of the susceptibility of the zeolite crystallites upon the measured value of the 'H NMR signal, errors of the order of 0.5-1.0ppm must be taken into account.275Hence, the use of an inner standard is strongly recommended if one wants to determine absolute values for the 'H NMR shift of OH groups in zeolites. In this respect, methane has been proved to be suitable and convenient to handle.'% The ultimate resolution of the 'H MAS-NMR spectra of zeolites is a problem of basic interest since the line widths of the signals apparently determine the lower limit of the strength of acidity which can be measured by this method. Line-broadening mechanisms which must be considered are:276 magnetic field inhomogeneity, misadjustment of the magic angle, anisotropy of the magnetic susceptibility, influence of the homo- and heteronuclear magnetic dipole interaction and of the anisotropy of the chemical shift, heteronuclear magnetic dipole interaction with quadrupolar nuclei (27Al)and thermal motion. The dominating mechanisms are the magnetic dipole interaction with protons of neighbouring O H groups (homonuclear magnetic intera~tion)'~~ and with aluminium nuclei. The former line broadening mechanism can be reduced by partial deuteration of the hydroxyl see e.g. Fig. 16, or by application of combined rotation and multipulse sequences (CRAMPS te~hnique)~'or by an enhancement of the spinning rate. An example for the latter possibility is shown in Fig. 17. The line broadening effect of the magnetic dipole interaction of the protons with quadrupolar nuclei (neighbouring 27Al)

NMR STUDIES OF ZEOLITES

10

115

1

5

0

6,l PPm

Fig. 16. 'H MAS-NMR spectra of a zeolite H-Y with a relatively high concentration of OH groups silicon-to-aluminium ratio of 2.6): (A) non-deuterated, (B) deuterated, measure&77 at room temperature and at a resonance frequency of 300MHz with a spinning rate of 3 kHz. * denotes spinning sidebands.

decreases with increasing resonance frequency.278 The enhancement of resolution which can be achieved by a transition of the resonance frequency from 300 MHz to 500 MHz is also demonstrated in Fig. 17, and one can see that the line width for the signal of the bridging OH groups is reduced to less than 0.5 ppm or 250 Hz. However, it is not clear at present whether this value can be reduced further by an increase of the spinning rate above 11 kHz. In contrast, for highly siliceous zeolites like H-ZSM-5, due to the small concentration of bridging O H groups the ultimate resolution has been achieved at relatively small spinning rates (3 kHz). The observed line width of ca. 0.8ppm for these zeolites cannot be further reduced by the NMR technique, it is the natural line width which is given by the distribution width of the bridging O H groups in zeolites H-ZSMJ. of the strength of With respect to a measurement of the concentration of hydroxyl groups (Bronsted acid sites and non-acidic OH groups) nuclear magnetic resonance

116

H. PFEIFER AND H. ERNST

90H-Y 400 SB

b

I

I

500 MHz, 11.O kHz

500 MHz, 3.4 kHz C

4

10

0

-10

10

0

-10

J,lPPm Fig. 17. 'H MAS-NMR spectra of a zeolite H-Y with a relatively high concentration of OH groups (silicon-to-aluminium ratio of 2.6) measured at resonance frequencies of 300 MHz and 500 MHz and with a spinning rate of ca. 3.0 kHz and 11.0 kHz. * denotes spinning sidebands.

spectroscopy has an extremely important advantage compared with infrared spectroscopy since the area of a 'H MAS-NMR signal is directly proportional to the concentration of the hydrogen nuclei contributing to this signal irrespective of their bonding state, so that any compound with a

NMR STUDIES OF ZEOLITES

117

known concentration of hydrogen atoms can be used as a reference (mostly water). In Fig. 18 'H MAS-NMR (Bruker MSL 300) and infrared stretching vibration spectra (Digilab FTS-20) are shown268for two differently synthesized specimens of SAPO-5. While the positions of the various signals in the NMR and IR spectra correspond to each other quite well and are in agreement with IR results published by other authors,279there are dramatic differences in the relative intensities. From the IR spectrum of the first specimen one would erroneously conclude that the concentrations of bridging OH groups of type b and c are approximately equal, and from the IR spectrum of the second specimen that the concentration of POH groups is about three times larger than that of the bridging OH groups of type b. Therefore, even the relative intensity of an OH stretching vibration band cannot be taken as a measure for the concentration of the respective hydroxyl groups in contrast to the intensity of an NMR signal. On the other hand, there is a surprisingly good correlation between the positions of the various signals in the IR and 'H MAS-NMR spectra. Experimental results which were available to uszxoare collected in Table 4 and plotted in Fig. 19. With the exception of the cationic OH groups there is a nearly linear interdependence between the wave number vOH of the IR band and the chemical shift S, of the 'H MAS-NMR signal. The straight line approximating the experimental results is given by:280

-

vOH cm-' = 3870 - 67.8 SH ppm

(10)

Hence, the above-mentioned minimum line width of less than ca. 0.5 ppm for the 'H MAS-NMR signal of bridging OH groups which is tantamount to a resolution of less than ca. 0.1-0.05ppm, corresponds to an equivalent resolution of less than ca. 6.8-3.4cm-' for the OH stretching vibration band. This value has to be compared with the ultimate resolution of 2 cm-' achieved in ref. 281 which leads to the remarkable statement that for both methods the ultimate resolution is of the same order of magnitude. As has been shown above, the resonance position of the 'H MAS-NMR lines, which is given by the isotropic value of the chemical shift, can be used as a reliable and sensitive measure for the strength of acidity is isolated Bronsted acid sites are compared. On the other hand, for not too high spinning rates of the sample, rotational sidebands of the various lines appear which contain information about the anistropy of the chemical shift and the magnetic dipole interaction between the 'H and the neighbouring 27Al nuclei, or in other words, about their distance. In order to analyse these sideband patterns for each of the various signals it is necessary to separate them carefully. This can be accomplished by the following experiment which is a typical example of two-dimensional NMR spectroscopy.282At first a 7r/2

118

H.PFEIFER AND H.ERNST SAPO-5/1

SAPO-5/2 POH

IR

c

b

1

3630 vOH

'H MAS NMR

J cm-'

I C

b

I I POH

H,o

3.8

4.8

1.8

A SiOH

SiOH

6.8

3630

4

6, J P P ~

I

1

1

1

3.7 1.1

1.9 0.2

Fig. 18. IR stretching vibration (Digilab FTS-20) and 'H MAS-NMR s ectra (Bruker MSL 300) of two differently synthesized specimens of SAPOJ.2 8

N M R STUDIES OF ZEOLITES

I 3500

I

I

3600

3700

uo

119

I 3800

/cm-

Fig. 19. Experimental values (cf. Table 4) for the wave number VOH of the IR band the 'H MAS-NMR signal of the stretching vibration and of the chemical shift of OH groups in zeolites.

pulse and then after a time interval tl a T pulse is applied to the sample. Under the condition that the following equation is fulfilled

where vrot denotes the spinning rate, the nuclear magnetic resonance signal is measured at time t2 after the T pulse. Hence, this signal which we denote by F(t,,t,) depends both on tl and 1,. A twofold Fourier transformation of F(t,,t,) then gives the two-dimensional 'H MAS-NMR spectrum S(ol, 0,). A typical example is shown in Fig. 20. The important fact is that S(wl = const., 02) yields the ordinary 'H MAS-NMR spectrum, i.e. the central lines together with all sideband patterns, while S ( q , ~2 = constant) gives the 'H MAS-NMR spectrum without the sidebands. Therefore the sidebands for each line can be analysed separately. In ref. 283 a procedure is described which allows a determination of both the distance rHAl between the 'H and the neighbouring 27Al nuclei and of the chemical-shift anisotropy ACTH for the bridging OH groups (lines b and c in the 'H MAS-NMR spectra) of zeolites. In Table 5 results283for the distance rHAl of bridging OH groups which give rise to lines b and c of zeolites H-Y are collected and compared with theoretical data found by a computer

120

H. PFEIFER AND H. ERNST

Table 4. Experimental values for the chemical shift SH and for the wave number vOH of the various OH groups of zeolites.280 ~

Zeolite H-Y H-ZSM-5 H-ZSMJ (dealuminated) H-SABO SAPO-5

SAPO-11

Mg-Y* * Ca-Y**

OH group SiOH SiOHAl (HF) SiOHAl (LF) SiOH SiOHAl SiOH AlOH (extra framework) SiOHAl SiOH SiOHB SiOHAl SiOH* POH* SiOHAI(1) SiOHAI(I1) SOH* POH* SiOHAI(1) SiOHAl(I1) M~:OHCa OH-

~

~~~~~~~

SH ppm

vOHcm-'

2.0 4.1 5.0 2.1 4.3 2.1 3.0 4.3 2.1 2.5 4.3 1.7'

3742 3636 3543 3734 3604 3738 3654

3.8 4.8 1.9' 3.9 4.9 0.0 0.0

Data point in Fig. 19

3740 3720 3604 3739 3674 3623 3523 3741 3671 3615 3538 3620 3612

*assignment uncertain; **SOH and SiOHAl as in H-Y;' these NMR signals are the sum of two or more components.

~ i m u l a t i o n ~and ~ with results of neutron powder diffraction mea~urements.**~ It can be seen that the values for the distances between the hydrogen and aluminium nuclei derived from the neutron powder diffraction measurements are much smaller and moreover that the ratio of the distances for the two sorts of bridging OH groups is at variance with the result of the two other papers. Presumably this discrepancy is due to the fact that in ref. 285 the AI-0 and Si-0 distances have been assumed to be equal. Table 6 contains experimental results for rHA1 and the chemical-shift anisotropy AuH which have been found by the 'H MAS-NMR spinning sideband analysis283for zeolites of type H-Yand SAPO-5. For the bridging OH groups which point into the small cages (line c) the H-A1 distance has been found to be 2 3 7 k 4 p m for the zeolite H-Y and 2 3 4 k 4 p m for the zeolite SAPO-5. In contrast, for the bridging OH groups in the large cavities the corresponding distances are equal and distinctly larger, viz. 248 f.4 pm. Within the limits of error, the values for the anisotropy of the chemical shift are equal (1922ppm) except for line b of the zeolite SAPO-5 which exhibits a smaller value (14.5 k 2 ppm).

NMR STUDIES OF ZEOLITES

121

Fig. 20. Two-dimensional 'H MAS-NMR spectrum of a zeolite S A P O J . The pulse

sequence applied is shown in the inset. The measurements were performed at room temperature, at a resonance frequency of 300 MHz with a spinning rate of 2 kHz, with 64 consecutive values of tl and 100 accumulations per free induction decay F(tl,t2). The resonance frequencies w1 and are related to that of the reference tetramethylsilane (TMS) by (w1mS - wlb)/wlmS = 3.8 ppm and (wIms - w& alms = 4.8ppm.

Summarizing these results one can state that neither the isotropic chemical shift of the 'H NMR signals nor their anisotropy seems to be simply related to the 1H-27Al distance of the bridging OH groups. It should be the goal of forthcoming quantum chemical calculations to throw some light upon the interdependence between the geometrical (rHAI) and electronic (chemical shift) parameters of the bridging OH groups in zeolites and related catalysts. Sometimes it is claimed that a crucial disadvantage of NMR spectroscopy is its relatively small signal-to-noise ratio. However, with the present state of the art allowing measurements at high resonance frequencies, a linenarrowing by magic angle spinning of the sample and accumulation of

122

H.PFEIFER AND H. ERNST

Table 5. Results for the distance rHAl of bridging OH groups which give rise to lines b and c of zeolites H-Y derived from an analysis of the 'H MAS-NMR spinning sideband pattern283 compared with theoretical data found by a corn uter simulation284and with results of neutron powder diffraction measurements.$5

Line b HF band 01H

Line c LF band 03H

Ref. 283 Ref. 284 Ref. 285

248 k 4 pm 238.6 pm 213.2 pm

237 f4 pm 233.2 pm 219 pm

Ref. 283 Ref. 284 Ref. 285

-

-

169.4 pm 167.7 pm

169.7 pm 165.4 pm ~~

rAlO

-

Ref. 283 Ref. 284 Ref. 285

191.O pm 167.7 pm

~

-

~~

193.0 pm 165.4 pm

Table 6. Experimental resultsB3 for the isotropic value S, of the chemical shift and for the anisotropy AUH, as well as for the distance r ~ A between l the 'H and 27Al nuclei of bridging OH groups in zeolites H-Y and SAPOJ.

'H MAS-NMR signal

Line b

Line c

4.0 f 0.1 ppm 18.3 +_ 1.5ppm 248 k 4 pm

5.0 k 0.1 ppm 20.2 +_ 1.5 ppm 237 +_ 4 pm

3.8 f 0.1 ppm 14.5 -+ 1.5 ppm 248 f4 pm

4.8 +_ 0.1 ppm 19.5 2 1.5 ppm 234 f4 pm

Zeolite H-Y 6 ,

AUH

~HAI

Zeolite SAPOJ 6H

AUH

rHA1

signals, the signal-to-noise ratio seems to be quite sufficient for a study of OH groups in zeolites. For the minimum number of hydrogen nuclei Nmin detectable by 'H MAS-NMR one may write:286

Nmin (

*

(TI * Av/ T,)ln

where T denotes the temperature, vo the resonance frequency, Av the line width, TI the longitudinal relaxation time and T, the time of measurement.

NMR STUDIES OF ZEOLITES

123

The factor of proportionality depends on the quality and the filling factor of the rf coil, the noise of the electronic system and similar other parameters. For a signal-to-noise ratio of 10, a measuring time of 10 minutes, a resonance frequency of 300 MHz, room temperature, a longitudinal relaxation time of ca. 1 second and a line width of 500 Hz which are typical values for 'H MAS-NMR measurements of OH groups in zeolites, Nminis of the order of 10" hydrogen nuclei per sample. Hence, for a sample of 0.2 g zeolite with typically 4 X lo2' cavities per g this value corresponds to 0.01 O H groups per cavity which can be measured by 'H MAS-NMR spectroscopy. In a recent paper287solid-state deuterium NMR has been used to study hydroxyl groups in deuterium-exchanged H-Y zeolites. Through a line-shape analysis it was possible to separate the signals from Bronsted acid sites and non-acidic silanols, and to determine the quadrupolar coupling constant of the former sites as 234 k 2 kHz. However, the resolution of this method is not sufficient to separate the lines from the two sorts of Bronsted acid sites (lines b and c in the 'H MAS-NMR spectra), of residual ammonium ions (line d), and of OH groups at extra-framework aluminium species (line e), so that at present *H NMR may not compete with 'H MAS-NMR in characterizing Bronsted acidity of zeolites and related catalysts. 3.2. Accessibility of Bronsted acid sites, hydrated zeolites

The accessibility of hydroxyl groups can be easily determined through a study of the 'H MAS-NMR spectra after loading the adsorbent with a suitable molecule which, however, must be fully deuterated in order to avoid an unwanted additional 'H NMR signal. Using deuterated pyridine, the concentrations of accessible and non-accessible silanol groups of silica since the formation of a hydrogen bond between could be pyridine and the silanol group shifts the 'H MAS-NMR signal of the latter by ca. 8 ppm to higher values. This effect is demonstrated in Fig. 21A where a zeolite H-ZSMJ with a high concentration of non-acidic silanols (3.7 x lo2' SiOWg) has been loaded with about the same concentration (4 x lo2') of deuterated pyridine molecules. In the case of acidic OH groups (Bronsted acid sites), however, the adsorption of pyridine leads to a protonation, i.e. to a formation of pyridinium ions with a larger shift of ca. 12ppm to higher values. The spectra of a zeolite H-ZSMJ with 4.9 x lo2' Bronsted acid sites per g zeolite and only a negligible concentration of non-acidic silanols (this can be achieved e.g. by a synthesis of the zeolite ZSM-5 without template2") unloaded and after loading with a concentration of 6 X lo2' deuterated pyridine molecules are shown in Fig. 21B. With the same probe molecule it was also possible to show unambiguously that line b in the 'H MAS-NMR spectra of zeolites H-Y is due to bridging OH groups which are easily

124

H. PFEIFER AND H.ERNST

B

A

_sL a

4.1OZ0 Pyr

d

A* SiOHAL I g

Q

6.1Oto Pyr I g

1

I

I

1

10

2

16

4.3 2

6,

I PPm

1

Fig. 21. 'H MAS-NMR spectra of zeolites H-ZSMJ unloaded and after loading with deuterated pyridine: (A) Prevailing concentration of non-acidic OH groups (zeolite synthesized with template289):formation of a hydrogen bond between the OH group and the adsorbed pyridine molecule. (B) Prevailing concentration of Bronsted acid sites (zeolite synthesized without template): formation of pyridinium ions.

accessible by pyridineZ6' and that line c is caused by bridging O H groups pointing into the small cavities (LF band in infrared spectroscopy). In Fig. 22 resultsza are shown for a shallow-bed (400°C) pretreated zeolite SAPO-5 unloaded (A), and after keeping it loaded with deuterated n-hexane for 1hour at 50°C (B). There is no doubt that the 3.9 ppm signal is caused by bridging OH groups which are easily accessible to n-hexane in contrast to the 4.9ppm signal. 'H MAS-NMR studies of hydrated zeolites may be complicated by a superposition of three effects: (1) Hirschler-Plank mechanism, i.e. the adsorption and dissociation of water molecules on extra-framework multivalent cations like Ca2' with a formation of cationic and bridging OH groups.2w The 'H MAS-NMR signal of the former hydroxyls appear at S, values of ca. Oppm and 2.8ppm for calcium ions in the large and small cavities of zeolites Y,re~pectively,'~~ and an inspection of Fig. 19 shows that equation (10) is not fulfilled for those OH groups. Up till now a simple interpretation of this experimental result cannot be given. (2) Formation of hydroxonium ions (chemical shift ah+) at Bronsted acid sites which, however, take part in a fast proton exchange with physically adsorbed water molecules (chemical shift 6,) and with accessible bridging OH groups (chemical shift SOH). Physically adsorbed water molecules may

NMR STUDIES OF ZEOLITES

125

B

4.9 3.9

1.8

4.9

1.8

Fig. 22. ‘H MAS-NMR spectram of a shallow-bed (400°C) pretreated zeolite SAPOJ: (A) unloaded, (B) after keeping the sample loaded with deuterated n-hexane for 1 hour at 50°C.

include water molecules adsorbed on extra-framework cations (adsorption energy 791 kJ/mol, 335 kJ/mol, and 117 kJ/mol for A13+, Mg2+, and Na+, respectively292i293)and water molecules hydrogen bonded to bridging O H groups, to other water molecules and to silanol groups (58.4kJ/mol, 20.1 kJ/mol, and 16.4 kJ/mol, r e s p e c t i ~ e l ~ ~ ~Hence, * ~ ” ) . the resulting ‘H MAS-NMR shift 6 can be described quantitatively by

where cOH,ch+/3, and c, denote the concentration of the ‘H nuclei in the accessible bridging OH groups, in the hydroxonium ions, and in the physically adsorbed water molecules, respectively. With aOH= 4.3 ppm, 6, = 3.2-4.8ppm (the upper limit is for hydrogen bonded water molecules) and &+ = 13ppm2w it is possible to determine quantitatively the concentration of hydroxonium ions from the position 6 of the ‘H MASNMR signal. In shallow-bed (400°C)treated zeolites H-Y the probability of finding a water molecule in the state of a hydroxonium ion is ca. 0.2-0.3 for a rehydration corresponding to one water molecule per accessible bridging OH group (line b).2” (3) Adsorption of water molecules on Lewis acid sites giving rise to a narrow tine at S,= 6.5ppm. In the case of hydrothermally pretreated zeolites H-Y (540°C; 20 h; 4 kPa water vapour pressure) a concentration of 2k0.5 Lewis acid sites of this type per unit cell could be found. Surprisingly, the MAS sideband pattern of the signal at 6.Sppm could be

126

H. PFEIFER AND H. ERNST

explained quantitatively only if these Lewis acid sites are not connected with aluminium. Therefore it was concluded that the signal at 6.5 ppm is caused by water molecules adsorbed on threefold coordinated and positively charged silicon atoms of the zeolite framework,294i.e. on sites which were proposed in order to explain an infrared band at 4035cm-’ for hydrogen adsorbed on a zeolite H-Y activated at 400°C under deep-bed conditions, see refs 295, 296. Further experimental and theoretical work seems necessary to prove whether threefold coordinated and positively charged silicon atoms do exist in the framework of zeolites. Another method to study the formation of hydroxonium ions in hydrated zeolites has been introduced by J. Fraissard et a1.297Through a measurement at very low temperatures (4 K) the fast proton exchange among the various species can be excluded and a line-shape analysis should yield quantitative information about the concentration of the various species. Unfortunately, however, the line widths of the respective ‘H NMR signals are so large that they cannot be distinguished by their chemical shift. Instead it is necessary to make use of the different line shape for the ‘H NMR signal of rigid one-, two-, and three-spin systems. Assuming that there are only isolated hydroxyl groups (one-spin systems), isolated water molecules (two-spin systems), water molecules hydrogen bonded to OH groups (three-spin systems with the configuration of an isosceles triangle) and hydroxonium ions (three-spin systems with the configuration of an equilateral triangle) the shape of the observed ‘H NMR broad line is decomposed into contributions of these species by a fitting procedure where, however, the magnetic dipole interaction among these species must be considered by introducing a further (“line-broadening”) parameter. Since the concentration of hydroxonium ions ch+ thus determined will depend both on the concentration of the acidic OH groups c,, and on their strength of acidity S, the ratio ch+/c, should be a reasonable measure for S,. Apparently this method fails if the zeolite catalyst contains various sorts of OH groups differing in their concentration and strength of acidity. With respect to the ‘H MAS-NMR technique it should be of interest whether the sensitivity of the broad line method is sufficient to confirm, for example, the dependence of the strength of acidity on the electronegativity of zeolite catalysts (Fig. 12).

4. LEWIS ACID SITESEXTRA-FRAMEWORK ALUMINIUM Apart from relatively weak signals in the IR spectra of adsorbed hydrogen295and carbon monoxide296which are ascribed to an interaction of these molecules with threefold aluminium and silicon atoms of the zeolite framework affected by, for example, a hydrothermal treatment, it is generally accepted that the Lewis acidity of zeolites298and related catalysts (silica-alumina, y-alumina) is connected with the presence of aluminium

NMR STUDIES OF ZEOLITES

127

species on the surface. Therefore two possibilities should exist in principle to study the Lewis acidity of zeolites: (1) an analysis of highly resolved 27Al NMR spectra similar to the ‘H

MAS-NMR method described in Section 3, and (2) the use of probe molecules as e.g. ~ y r i d i n or e ~the ~ ~above-mentioned molecules in IR spectroscopy. 4.1.

27AIN M R spectroscopy

In well-crystallized zeolites exhibiting no Lewis acidity, aluminium is tetrahedrally coordinated with an isotropical chemical shift between 55 and 6 5 p ~ m with ~ ~respect to an aqueous dilute solution of A13+. After a hydrothermal treatment and/or dealumination, however, extra-framework aluminium species are formed (Lewis acid sites) and the 27Al MAS-NMR spectrum becomes more complicated. The initial signal at ca. 60ppm changes its intensity due to ejection of aluminium from the framework into the intracrystalline space and due to 27Al signals from tetrahedrally coordinated extra framework aluminium, which must exist at least partly as Al(OH), species as could be shown by ‘H-27Al cross-polarization (CP MAS-NMR) experiment^."^ The other extra framework aluminium species give rise to distinct resonance lines in the interval between -15 and +4 ppm which could be assigned to polymeric aluminium species,3o1and to a line near 30 ppm which is interpreted as being due to either penta~oordinated”~or tetrahedrally c o ~ r d i n a t e d ’ extra ~ ~ ~framework ~ ~ ~ . ~ ~aluminium. ~ In addition, a very broad hump extending from ca. -180ppm to ca. +230ppm (for an external magnetic field strength of 7.1 T) appears below these more distinct resonances. As has been shown recently,lg3this broad hump merges into the lines near 0, 30, and 60 ppm for ultrahigh (518 kHz) MAS speeds. In hydrated samples a narrow line at Oppm is often observed, suggesting the presence of A13+ cations balancing lattice charges. A survey on these experimental results is given in Table 7. 27Alnutation experirnend5 performed on hydrothermally treated zeolites Y suggest that the quadrupolar interaction of the extra framework aluminium species is relatively large corresponding to a value of at least ca. 1MHz for the quadrupole frequency vQ defined by equation (15) (see below). Large values of vQ give rise to a large second-order quadrupolar shift:52

where vcG and vL denote the centre of gravity of the signal and the Larmor frequency (signal for vQ = 0) in Hz, respectively, Z is the spin quantum

128

H. PFEIFER AND H. ERNST

Table 7. Experimental results for the chemical shift 8, observed on zeolites.

of 27Al NMR signals

Species 0 4..

Octah. coord. extra framework aluminium (A13+)53

. -15

Extra framework polymeric aluminium speciesM’

~

~

~~

30

Penta-~oordinated”~ extra framework aluminium

30 (50?,ref. 159)

Tetrahedrally coordinated extra framework aluminium: AlOOH associated with two framework oxygenslW aluminium in amorphous sili~a-alumina”~

-~~~~~~

-180. . . +230 (Bo = 7.1 T)

Extra framework aluminium of low symmetry (“NMR invisible”). The very broad hump merges into the lines at 0, 30, and 60 ppm for ultrahigh spinning speeds ( a 1 8 kHz)ls3

60

Framework aluminium183and tetrah. coord. extra framework aluminium (CP MAS-NMR183)

number ( I = 512 for 27Al), r) the asymmetry parameter ( 0 s 7 s 1) and uQ the quadrupole frequency VQ

= 3e2qQ/(21(2Z- 1)h)

(15)

with eq, eQ and h denoting the electric field gradient, the electric quadrupole moment and Planck’s constant, respectively. In addition to this effect which must be taken into consideration if one wants to determine the true chemical shift of a ”A1 NMR signal, a strong quadrupole interaction leads to a dramatic broadening of the lines. The line widths in ppm for a static sample (A), for a sample under conditions of magic angle spinning (AMAS) and under conditions of double rotation (ADoR) are given by:40,303

A = (u ~ u* (25 ~ +) 229 ~ + q2)* 106/18 AMAS = h13.64 if ur 3 ps ADOR

= Ac

if ui = 5 . ~ *OR ~ 5

where u, is the spinning rate of the rotor in the case of MAS, and q and vo denote the corresponding rates for the inner and outer rotor of DOR, respectively. The critical values for the rates are given by

NMR STUDIES OF ZEOLITES

129

Table 8. Values for the nuclear magnetic resonance frequency % of *'Al, for the line and width AMAs under magic angle conditions and for the critical spinning rates &'OR of MAS and DOR experiments, respectively, if the signals of extra-framework condensed (Al-A1 distance ca. 300pm) aluminium species of low symmetry (YQ = 3 MHz) should be resolved.

eAS

7.05

11.7

78.2 130.3

562 202

25 25

44 26 ~~

~

Bo denotes the strength of the external magnetic field.

Ac denotes the line width in ppm due to a distribution of the chemical and/or quadrupole shift and AD the line width in ppm due to homonuclear magnetic dipole interaction. It is the influence of these two latter quantities which determines the ultimate resolution of 27Al MAS and of 27Al DOR-NMR spectra. In Table 8 numerical data are given for vQ = 3 MHz (aluminium species of low symmetry), 11 = 0 and for an assumed mean distance of ca. 300pm between neighbouring 27Al nuclei in condensed extra framework species. The latter value corresponds to AD = 5 kHz. An inspection of this table shows that the conditions of equations (17) and (18) cannot be fulfilled experimentally so that we must state that neither MAS nor DOR NMR spectroscopy of 27Al will be able to detect extra framework condensed aluminium species of low symmetry. 4.2. Use of probe molecules

In analogy to infrared spectroscopy, probe molecules may be used to study Lewis acid sites. 15NCP MAS-NMR spectra of adsorbed pyridine have been studied by Maciel et aL3" (pyridine on silica-alumina), Ripmeester305 (pyridine on y-alumina, mordenite), and by Majors and Ellis3& (pyridine on y-alumina). Values for the resonance shifts relative to solid pyridine are collected in Table 9. The major drawbacks of these experiments are that (1) an absolute determination of concentrations is connected with large errors due to the strong dependence of the line intensities on quantities controlling the efficiency of cross-polarization, see e.g. ref. 307, and (2) often, caused by molecular exchange, some or all of the lines merge into a single averaged line so that an unambiguous separation into its components, namely the concentrations of physisorbed pyridine, pyridinium ions and pyridine adsorbed on Lewis acid sites is not possible. Compared with pyridine, phosphines are roughly three orders of magnitude stronger bases. 31P CP MAS-NMR spectra of trimethylphosphine

130

H.PFEIFER AND H.ERNST

Table 9. Values for the "N NMR shift of pyridine-* in the liquid state, physisorbed, and adsorbed on Bronsted and Lewis acid sites relative to the resonance of solid pyridine. ~~

~

s,PPm

Species Solid pyridine (- 105OC) Liquid pyridine Physisorbed pyridine Pyridinium ion (Bronsted acid sites) Pyridine on tetrah. A13' (Lewis) Pyridine on octah. A13+(Lewis)

0 -26 -10 f 10 88f2 22.5 f 3.5 46.5 k 7.7

Table 10. Values for the 31P NMR shift of trimethylphosphine physisorbed and adsorbed on Bronsted and Lewis acid sites, relative to the liquid state.3087309

4 PPm

Species ~~

Liquid trimethylphosphine Physisorbed trimethylphosphine Protonated trimethylphosphine (Bronsted acid sites) Trimethylphosphine on Lewis acid sites

0 0.7 f 6

59f2 12 f 10

adsorbed on zeolite H-Y and on y-alumina were investigated by Lunsford et al. ,308 and of various trialkylphospines adsorbed on silica-alumina and on y-alumina by Maciel et al.309 Values for the resonance shift relative to liquid trimethylphosphine are shown in Table 10. For this probe molecule the same drawbacks hold as mentioned above for pyridine, although due to the larger signal-to-noise ratio of the 31PNMR signal compared with I5N, cross-polarization must not be applied. The sensitivity of trimethylphosphine to distinguish between different sorts of Lewis acid sites seems to be less than that of pyridine since for y-alumina only one sort of Lewis acid sites could be found (see Table 10). In principle also water can be used as a probe molecule since the chemical shift of water molecules adsorbed on Lewis acid sites (6.5 ppm, see ref. 294) is different from that of physisorbed water (less than 4.8 ppm) and of water molecules adsorbed on Bronsted acid sites (hydroxonium ions: 13 ppm). As has been shown in section 3.2, however, the 'H MAS-NMR spectrum of hydrated zeolites is complicated by a superposition of various effects so that water is not very suitable to probe Lewis acidity of zeolites and related catalysts. Another way to study quantitatively Lewis acidity results from the fact that probe molecules could be found for which the resonance shift caused by Lewis acid sites is much larger than that caused by Bronsted acid sites or physisorption. Typical examples are carbon monoxide31c312 and dinitrogen

NMR STUDIES OF ZEOLITES

131

Table 11. Values for the 13C NMR shift of carbon monoxide physisorbed and

adsorbed on Bronsted and Lewis acid sites, relative to the gaseous state.31s312 Sc PPm

Species ~~~

~

Carbon monoxide gas Physisorbed carbon monoxide Carbon monoxide on Bronsted acid sites Carbon monoxide on Lewis acid sites

~

~~~

0 -3+2 -3f2

300 . . . 400 (ref. 310, 311) 590 f 60 (ref. 312)

Table 12. Values for the "N NMR shift of dinitrogen oxide physisorbed and

adsorbed on Bronsted and Lewis acid sites relative to the gaseous state.313

Species Dinitrogen oxide gas Physisorbed dinitrogen oxide Dinitrogen oxide on Bronsted acid sites Dinitrogen oxide on Lewis acid sites

s,PPm 0 4.5 k 3 4.5 k 3 50 k 20

oxide.313 Since at room temperature and above these molecules exchange rapidly between the various adsorption sites, only a single averaged line for the I3C NMR of carbon monoxide and for the 15N NMR of the terminal nitrogen of dinitrogen oxide appears, and it becomes necessary to measure the position of this line as a function of loading to determine the resonance shift of the molecules adsorbed on the Lewis acid sites and their concentration. The procedure developed by Borovkov and described in detail in ref. 275 makes use of an extrapolation of the resonance shift of the single averaged line to zero loading which is connected with relatively large errors due to the decreasing signal-to-noise ratio. Results for the resonance shift of the probe molecules carbon monoxide and dinitrogen oxide physisorbed and adsorbed on Bronsted and Lewis acid sites are collected in Tables 11 and 12, respectively. At present the accuracy is far from the goal to take the values of the resonance shift for the molecules adsorbed on Lewis acid sites as a measure for their strength of acidity. Assuming an average value for this resonance shift, however, the method seems suitable to determine the concentration of Lewis acid sites. Summarizing the results obtained up till now with probe molecules, the following statements can be made: (1) At room temperature and above no separate lines for water, carbon monoxide and dinitrogen oxide adsorbed on Lewis acid sites can be observed due to fast molecular (or proton) exchange. Hence, a measurement of coverage dependence becomes necessary, yielding

132

H. PFEIFER AND H. ERNST

large errors for the chemical shift (strength of acidity) and for the concentration of the Lewis acid sites. (2) In those cases where due to a poor signal-to-noise ratio crosspolarization has been applied (pyridine, trimethylphosphine) only a semiquantitative determination of the concentration of Lewis acid sites is possible. (3) Steric effects connected with larger probe molecules may lead to large errors. As an example the result presented in a paper by Lunsford et d 3 1 4 shall be mentioned: with trimethylphosphine as a probe molecule, 37 protonated species per unit cell have been found for a zeolite H-Y in contrast to the real value of 54 Bronsted acid sites (framework aluminium atoms) per unit cell.

5. STRUCTURE OF ADSORBED MOLECULES 5.1. High-resolution NMR of adsorbed molecules-general aspects

The surface properties of zeolites have received much attention since the pioneering work of B a t ~ e r . ~The ’ ~ application of high-resolution NMR to study adsorbed molecules is of interest from a standpoint both of basic and applied re~earch.~’,~’~ The systems under study are closely related to such important processes in chemical industry as selective adsorption or heterogeneous catalysis. In general, the mobility of adsorbed molecules is intermediate between the solid and the liquid state, which gives rise to a more or less strong broadening of the NMR signals. In those cases where a sufficient resolution can be achieved, an analysis of the spectra may yield valuable information about the existence of adsorption sites and their interaction with adsorbed molecules. In this connection the influence of other interactions, as, for example, the van der Waals interaction between the molecules and the surface or intermolecular interactions, susceptibility effects and exchange processes of molecules between different adsorption sites must be taken into con~ideration.~~*~~~ In 1972 the first highly resolved ‘’C N M R spectra of adsorbed hydrocarbon molecules could be m e a ~ u r e d . ~ ~As ~ ~ a’ ”typical example, the study18 of adsorption complexes formed between but-1-ene and exchangeable cations in zeolites X and Y shall be mentioned. The carbon atoms in this molecule are denoted as follows:

133

NMR STUDIES OF ZEOLITES

Table 13. Experimental values for the difference ($9- ~ 9 ) o~ f the ' 13C chemical shielding in ppm of but-1-ene adsorbed on zeolites TI-X and NaAg-X (60% Ag+) and dissolved in AgN03 H20.

+

c1

c2

c3

c4

0 (113.5)

0 (140.5)

0 (27.4)

0 (13.4)

NaAg-X

-1.1 11.5

-1.6 -1.5

0 -0.6

0 -0.6

Dissolved in AgN03 + H2O

12.2

Neat liquid ~~

~

TI-X

0.9

0.5

0.2

Values in parenthesis are referred to Th4S. The loading of the zeolites corresponds to ca. 4 molecules per large cavity."'

and in Table 13 experimental values for the difference of the chemical shielding in the neat liquid and for the adsorption complexes are given. For comparison also values are included for the complex which is formed in an aqueous solution of silver nitrate. From these results it must be concluded that in NaAg-X zeolites silverlbut-1-ene complexes are formed of the same structure as in solution while in TI-X zeolites the but-1-ene is differently bonded to the thallium ion. By use of the MAS (magic angle spinning) technique a significant enhancement of the resolution and hence of the sensitivity can be achieved. In the first proton MAS-NMR experiments, performed on ethanol adsorbed on diatomaceous earth, the static line width could be reduced by more than two orders of r n ag n i t ~ d e . ~This ' ~ reduction can be ascribed to the fact that the broadening caused by the magnetic inhomogeneity of the adsorbateadsorbent system and by the anisotropy of the motion of the adsorbed molecules is controlled by the same angular factor as in the case of magnetic dipole interaction in solids. Furthermore, in a recent paper320 it could be shown both experimentally and theoretically that the line width of the 'H MAS-NMR signals approximates zero if (1) the molecule performs rotational motion only around axes fixed in space and (2) the intermolecular homonuclear dipole interaction can be neglected.

5.2. I3C and 'H MAS-NMR studies on sealed samples MAS-NMR experiments (cf. for example refs 321-325) performed on sealed samples containing zeolites loaded with different hydrocarbons have shown that the enhancement in spectral resolution brought about by the use of MAS allows a detailed assignment of NMR spectral lines to individual molecular species including those which are strongly bound to the surface.

134

H. PFEIFER AND H. ERNST

- CH,

‘c = o

0

I

230

I

.

I

.

I

I

220

.

PPH

I

.

I

21 0

I

. ” {

’

l . . . . I . . , . I . , . ,

40

30

PPH

20

Fig. 23. I3C NMR spectra (resonance frequency 74.5 MHz) of acetone molecules adsorbed on a Na-X zeolite pretreated at 400°C with a loading of 4 molecules per large cavity: (a) static NMR spectrum; (b) MAS-NMR spectrum with proton decoupling (spinning rate 2 W ) ; (c) MAS-NMR spectrum without proton decoupling.330

N M R STUDIES OF ZEOLITES

135

I

0 Na'

Fig. 24. Geometry of possible arrangements between an acetone molecule and a sodium ion.32g

As an example in Fig. 23 the I3C static NMR spectrum (a) as well as the 13C MAS-NMR spectra (b, c) with (b) and without (c) proton decoupling of acetone adsorbed on a Na-X zeolite is shown for a resonance frequency of 74.5 MHz. The values of the chemical shifts of the 13C MAS-NMR signals could be determined with an error of less than 0.1 to be 29.0ppm for the CH3 groups and 213.5ppm for the CO group compared with 30.6 and 206.0 ppm in the liquid state, respectively. In order to understand the formation of adsorption complexes between an acetone molecule and an adsorption site in the large cavities of the zeolites semiempirical and ab initio quantum chemical calculations have been carried o ~ t . In~ Fig. ~ ~24 , two ~ possible ~ ~ arrangements between an acetone molecule and a sodium ion are shown. The result of ab initio calculations of the energy as well as of the chemical shifts lead to the conclusion that position I has to be preferred where the sodium ion interacts with the lone pair electrons of the oxygen atoms.328 Anderson et aZ.'90,329have used 'H MAS-NMR spectroscopy to study the adsorption of methanol on zeolites H-ZSM-5, Na-ZSM-5 and several Si,AIand Si,AI,P-based zeolites. Due to the relatively high resolution of the spectra (cf. Fig. 25) and a comparison of the 'H MAS-NMR results after loading the zeolites with CH30H, CH30D, CD30H, and CD30D under identical conditions, the authors were able to conclude that in the case of acidic zeolites at low coverages each methanol molecule is bonded to a Briinsted acid site and forms a methoxonium ion (CH30H;, cf. Fig. 26a). At high coverages, however, charged clusters of methanol molecules associated with each Bronsted acid site are formed (cf. Fig. 26b). In the case of non-acidic zeolites (Na-ZSM-5) methanol is coordinatively adsorbed on the cation.

136

H. PFEIFER AND H. ERNST

4.1 ppm

(b) H.Y

5.1 ppm

\

9.1 ppm

~

15

-~

10

5

-0

-5

Fig. 25. 'H MAS-NhfR spectra (resonance frequency 400 MHz, spinning rate up to 3 kHz) of methanol molecules adsorbed on zeolites: (a) Na-ZSM-5, (b) H-Y, (c) H-ZSM-5. The loading corresponds to six molecules of methanol per Bronsted acid

site .329

N M R STUDIES OF ZEOLITES

SI

At

137

Si

Fig. 26. Structures for methanol molecules hydrogen bonded to a Bronsted acid site:329(a) one methanol molecule forms a methoxonium ion (CH,OH,+); (b) at high coverages protonated clusters are formed.

The advantage of magic angle spinning is clearly demonstrated by Figs 27 and 28, where static 13C and 'H NMR43 as well as MAS-NMR330spectra are compared which were taken on the same sample of but-1-ene molecules (in the case of Fig. 28 I3C enriched in the =CH2 position) adsorbed on Na-Y zeolites. Besides the much higher accuracy of the 13C MAS-NMR measurements, the liquid-like high-resolution 'H MAS-NMR spectrum confirms the 13C: NMR results for b ~ t - l - e n eand ~ ~ other olefinic hydrocarbons331 adsorbed on Na-Y zeolites: with respect to the liquid, the change of the 13C chemical shift (4.1 ppm) as well as of the 'H chemical shift (0.38 ppm) of the =CH- group (to lower magnetic field) is much higher than the corresponding values for the other C and H atoms in the but-1-ene molecule.

138

H. PFEIFER AND H. ERNST

ZCH-

CD ,,

=CH, 1

Fig. 27. Static NMR spectra of but-1-ene molecules adsorbed on a Na-Y zeolite43 with a loading of 3.3 molecules per large cavity: (a) 13C NMR spectrum (resonance frequency 22.6 MHz); (b) 'H NMR spectrum (resonance frequency 90.0 MHz).

In Table 14 recent results of 13C MAS-NMR studies on adsorbateadsorbent systems are given. With regard to results of 13C NMR measurements the reader is referred to ref. 43. 6. MOLECULAR DIFFUSION

6.1. Basic principles Since in general zeolites are only available as crystallites with a size of the order of a few micrometers, measurements of molecular diffusion have to be carried out with assemblages of zeolite crystallites (powder samples). The conventional way of measuring molecular diffusion in such systems is to follow the rate of mass change of the sample after changing the pressure of the surrounding a t m o ~ p h e r eHowever, .~~ the interpretation of such sorption or uptake experiments is not always unambiguous (cf. below) and allows only an indirect determination of the intracrystalline self-diffusion coefficients. A completely different way to study molecular migration is provided by the pulsed field gradient NMR ~ p e c t r o s c o (PFG-NMR). p~~~~

N M R STUDIES OF ZEOLITES

139

Although this method is generally combined with the formation of a so-called spin echo in order to eliminate the influence of an inhomogeneity of the constant magnetic field Bo, its essential features can be explained by a consideration of the free induction decay, i.e. the NMR signal following a short intense rf pulse. During the decay time of this signal ( t > O ) , two succeeding magnetic field gradient pulses of different sign are applied so that the intensity of the external magnetic field is given by

where g denotes the intensity and S the duration of the magnetic field gradient pulses. Assuming that S is much smaller than the time interval A between the two magnetic field gradient pulses, the ratio J, of the amplitudes of the free induction decay at time t = 28 + A and t = 0 can be shown to be :348

Here T2 denotes the transverse nuclear magnetic relaxation time of the adsorbed molecules and y the magnetogyric ratio of the resonating nuclei. p(z’ - z , A) is the mean propagator given by

p(z’-z,A) =

I

p(z)*p(z,z’,A)*dz

with the a priori probability density p ( z ) to find a molecule at z , and the conditional probability density, the ‘propagator’, p ( z , z ’ ,A) to find a molecule at time A at z f if it has been at z at time 0. Assuming T2

>> A

(24)

the first factor in equation (22) can be neglected and the thus simplified equation offers two different possibilities to evaluate the experimentally determined data, namely the ratio J, as a function of g or of S: (1) By a Fourier transformation of J, the mean propagator can be directly determined:

I

p ( ~-‘ 2 , A ) = ( 2 ~ ) ~I,/.J*cos[(z~ ’ -~)~gS]*d(ygS)

(25)

H. PFEIFER AND H. ERNST

140

=CH2

115

O

.

I

140

.

i

.

120

*

.

100

I

.

80

I

110

.

60

’

.

40

1

.

1

20

Sc/PPm

Fig. 28. MAS-NMR spectra of but-1-ene molecules adsorbed on a Na-Y zeolite with a loading of 3 molecules per large cavity (unpublished): (a) 13CMAS-NMR spectrum with proton decoupling (resonance frequency 74.5 M H z , spinning rate 3 kHz); \b) ”C MAS-NMR spectrum of the =CH* group without proton decoupling; (c) H MAS-NMR spectrum (resonance frequency 300 M H Z ) .

(2) The initial slope of a plot of In+ as a function of (yg8)’ yields the mean square displacement ((2’ - 2)’) of the adsorbed molecules: (dln $/d(ygS)2),=o = ((2’ - 2)’)/2

(26)

Since for an unrestricted molecular diffusion the following equation holds:

where D denotes the self-diffusion coefficient, we introduce for the general case an apparent self-diffusion coefficient Dappby the equation

Dapp= ( ( 2 ‘ - z ) ~ ) / ~ = A (dln +/d(ygti)’),,,/A

(28)

141

NMR STUDIES OF ZEOLITES

-CY

-CH2I

=CH, 1

=CH4

L 1

.

1

7.0

.

l

6.0

.

~

5.0

.

4.0

~

.

3.0

l

.

8.0

l

.

l

1.0

.

,

0.0

.

l

.

,

-1.0

6, I ppm

Fig. S o n t d .

The limits of the PFG-NMR method are determined by the magnetogyric ratio y of the resonating nuclei and the maximum achievable values of g, A, and S for which we assume

D,,,

= T2; ,,g

20 T/m; S,,

=

s

(29)

where in principle the value of a,,, can be enhanced further as long as the condition S,,