Role of the Tetramethylammonium Cation in the Synthesis of Zeolites

Downloaded by STANFORD UNIV GREEN LIBR on April 16, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch011

Chapter 11

Role of the Tetramethylammonium Cation in the Synthesis of Zeolites ZK-4, Y, and HS P. D. Hopkins Amoco Oil Company, Naperville, IL 60566

The synthesis of three z e o l i t e s whose frameworks include the sodalite 14-hedron were investigated. In two reaction series the product changed from z e o l i t e Y to ZK-4 as the TMA/Na r a t i o i n the reactant mixture was increased; i n a third series the product changed from gmelinite to omega, and f i n a l l y to HS as the TMA/Na r a t i o increased. In agreement with published work, e s s e n t i a l l y all sodalite cages i n ZK-4 occluded a TMA ion. Sodalite cages i n Y z e o l i t e s were occupied s t a t i s t i c a l l y by one TMA or approximately two sodium ions. Mechanisms for the synthesis of the z e o l i t e s , that are consistent with these observations, are proposed.

E s s e n t i a l l y a l l of the z e o l i t e s that can be synthesized i n the presence of the tetramethylammonium cation (TMA) have frameworks that contain one of two types of 14-hedra U). The most common z e o l i t e s synthesized with TMA are ZK-4 (LTA)(2), omega (MAZ)(3), Ε (EAB)(3), and o f f r e t i t e (OFF)(4). (The three l e t t e r code i n parentheses following each z e o l i t e i s the IUPAC structure code ( 5 ) . These codes i d e n t i f y each z e o l i t e when f i r s t mentioned and are used elsewhere i n this paper when structure types, rather than s p e c i f i c products are discussed). ZK-4 consists of sodalite or beta cages (14-hedra type I) joined through double four rings (D4R). Omega consists of columns of gmelinite cages (14-hedra type I I ) . O f f r e t i t e and Ε also contain columns of gmelinite cages but have substantial numbers of double s i x rings (D6R). Molecular modelling

0097-6156/89/0398-0152$06.00/0 ο 1989 American Chemical Society In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


11. HOPKINS

Tetramethylammonium Cation and Zeolite Synthesis

153

shows that the spherical TMA ion f i l l s the spherical sodalite cage well; TMA also f i t s i n the almost spherical gmelinite cage but somewhat more loosely. These f i t s suggest that TMA may function as a template during the synthesis of these z e o l i t e s as well as acting as the counterion to the negatively charged framework. TMA cannot pass through single 6-rings or 8-rings (S6R, S8R) so any TMA occupying space i n either 14-hedron must be incorporated during synthesis. O f f r e t i t e , Ε and omega syntheses are strongly aided by the presence of TMA. Gmelinite cage occupancy by TMA i n o f f r e t i t e and omega i s near unity(6). TMA i s e s s e n t i a l to the synthesis of ZK-4. With TMA products having S i / A l atomic ratios up to about three have been produced(7). E s s e n t i a l l y a l l sodalite units i n ZK-4 contain a TMA ion(8). In the absence of TMA the i s o s t r u c t u r a l z e o l i t e A, with S i / A l invariably equal to one, i s produced. Zeolites Y (FAU) and HS (hydroxysodalite, SOD) both contain sodalite units i n their framework. Both can be synthesized e a s i l y without TMA but can also be synthesized i n the presence of TMA(9,10). A l l sodalite cages i n HS were found to contain a TMA(8) but quantitative results were not reported for Y(10). The present work describes synthesis of the three z e o l i t e s , ZK-4, HS, and Y, that contain the sodalite u n i t . These syntheses were carried out to prepare samples for NMR and neutron scattering studies, results of which w i l l be reported elsewhere. Some results of these syntheses are presented here; these results may help to elucidate the role that TMA plays i n the synthesis of the three z e o l i t e s . Experimental Syntheses were carried out according to published procedures(7,9) or procedures developed by us. Reactants employed were Ludox HS-40 c o l l o i d a l s i l i c a (DuPont, 40% S i l i c a ) , sodium aluminate trihydrate (Nalco or EM), sodium hydroxide, tetramethylammonium hydroxide pentahydrate (Aldrich), and d i s t i l l e d water. A l l reactant mixtures, except those intended to produce HS, were aged f o r one or more days at room temperature before heating to ca. 100°C f o r c r y s t a l l i z a t i o n . HS synthesis mixtures were not aged. C r y s t a l l i z a t i o n s were carried out i n three-neck flasks at r e f l u x with s t i r r i n g or i n Teflon bottles placed i n a 100°C oven without stirring. The z e o l i t e s were synthesized i n one of s i x reaction series. Reactant ratios f o r the s i x series are l i s t e d i n Table I. Series 1 i s one that we have used, i n the absence of TMA, to synthesize z e o l i t e Y (FAU). Series 2 i s based on a published recipe(7) f o r synthesis of ZK-4 (LTA). Series 3 i s one which we use to synthesize z e o l i t e A (LTA) at the low s i l i c a end of the range used here. Series 4 and 5 are Series 3 with added TMAOH or NaOH; each increases the pH approximately equally. Series 6 i s based on a published recipe(9) f o r synthesis of z e o l i t e HS. In Series 1, 2, and 6, substitution of TMAOH for NaOH causes only minor increases i n pH.

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

154

ZEOLITE SYNTHESIS

Table I. Reactant Ratios

Mole Ratios SiO Al 6 Na^0 (TMA) 0 Na 0 + (TMA) 0 Downloaded by STANFORD UNIV GREEN LIBR on April 16, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch011

J

1

2

17.3 1.0 (b) (b) 13.6 445

9.5 1.0 (b) (b) 4.6 325

Series 4 3 (a) 1.0 1.1 0 1.1 85

(a) 1.0 2.0 0 2.1 85

5

6

(a) 1.0 1.1 1.0 2.1 85

20.3 1.0 (b) (b) 7.4 280

(a) Varied from 1 to 8. (b) Varied with sum indicated i n f i f t h l i n e kept constant.

A l l products were washed thoroughly and dried at ca. 105°C overnight. Product i d e n t i t i e s and phase p u r i t i e s were determined by powder XRD. S i l i c o n and aluminum were determined by wet chemical methods, carbon and hydrogen were determined by a combustion process, and sodium was determined by atomic absorption spectroscopy. Framework S i / A l ratios were determined by established XRD correlations f o r Y ( l l ) and ZK-4(12); some ratios were also determined by S i NMR. TMA, i n t o t a l and i n s p e c i f i c locations, was determined by C NMR(8). Results Effect of the TMA/Na Ratio on the Product. Changing the TMA/Na r a t i o i n reaction series 1 and 2 had a pronounced e f f e c t on the nature of the z e o l i t e product as shown i n Table I I .

Table I I .

Effect of Tetramethylammonium on the Nature of the Zeolite Product

Series 1 TMA/Na 0.0 0.2

8.8

Product Y Y Y Y,ZK-4 ZK-4 ZK-4

TMA/Na 0.0 0.6 0.8 1.0 1.2 1.5 2.5

Series 2 Product Y Y Y,ZK-4,E Y,ZK-4,E ZK-4,E ZK-4 ZK-4


11. HOPKINS

155


Both reaction series, employing substantially d i f f e r e n t reactant r a t i o s , followed the same pattern. When sodium was the predominant cation the synthesis product was Y but when there was more TMA than sodium the product was ZK-4. (A trace of z e o l i t e Ε was found i n Series 2 at TMA/Na ratios near 1.0). Clearly, TMA s t a b i l i z e s the LTA structure over a wider range of S1O2/AI2O2 reactant ratios than has been reported previously. The S i / A l atomic ratios of the products from series 1, both Y and ZK-4, were nearly invariant as shown i n Table I I I .


Table I I I . S i / A l Atomic Ratios i n Series 1 Products Reactant TMA/Na 0.0 0.2 1.0 8.8

Product

XRD

NMR

Y Y Y ZK-4

1.59 1.64 1.59 1.57

1.71 1.86

„

—

The S i / A l r a t i o f o r the ZK-4 product from reaction series 2 was 1.92 by XRD, 2.20 by NMR and 2.26 by chemical analysis; the l a s t i s i n good agreement with a r a t i o of 2.39 found by chemical analysis of a product from a similar synthesis(6). The TMA/Na r a t i o also had an e f f e c t on the products synthesized i n Series 6 as shown i n Table IV.

Table IV. Effect of TMA/Na on Series 6 Products

Reactant TMA/Na

Zeolite Product

0.00 0.24 0.40 0.62 1.39

gmelinite omega omega and HS HS HS

Synthesis of HS at a TMA/Na r a t i o of 0.25 had been reported previously(6) but the same reactant mixture has also been reported to produce omega(_1). As shown i n Table IV we synthesized omega at this TMA/Na r a t i o , but by increasing the TMA/Na r a t i o we were able to synthesize HS. Sodalite Cage Occupancy i n Y. The sodalite cage occupancy by J^A of three Y z e o l i t e s synthesized i n Series 1 was determined by C


156

ZEOLITE SYNTHESIS

NMR using the i n t e n s i t y for sodalite cages i n ZK-4 (which are completely f i l l e d with TMA) as a standard. The r e s u l t s are compared i n Table V with values calculated for random f i l l i n g by one TMA ion or by either two or three sodium ions based on the reactant compositions. Table V.

Sodalite Cage Occupancy by TMA i n Y Zeolites


Calculated for Random Occupancy by 1 TMA or η Na/Cage Reactant TMA/Na 0.17 0.41 0.99

J

C NMR 26% 45 75

n=2 25% 45 66

n=3 33% 54 74

The r e s u l t s suggest that the sodalite cages of Y synthesized i n the presence of TMA are f i l l e d by a random process; the f i t for two sodium ions/cage i s s l i g h t l y better than for three sodium ions/cage. Sodalite cage occupancy by approximately two sodium ions i s consistent with many XRD studies(13)· TMA i n Z e o l i t e A Reactant Mixtures. Series 3, 4, and 5 d i f f e r e d only i n that the cation and hydroxyl concentrations of Series 3 were increased by adding NaOH (Series 4) or TMAOH (Series 5) to the reactant mixture. Series 5 products maintained higher c r y s t a l l i n i t y than series 3 products as s i l i c a i n the reactant mixtures was increased as shown i n Figure 1. Series 4 products l o s t c r y s t a l l i n i t y faster than those of Series 3 as the s i l i c a content increased, proving that the e f f e c t observed i n Series 5 was due to TMA and not to pH (since the difference i n pH between TMAOH and NaOH systems was small). The c r y s t a l l i n i t y i n Figure 1 includes only material having the LTA structure; other c r y s t a l l i n e phases appeared i n some products but were not included i n the c r y s t a l l i n i t y assessments. Series 3 products became amorphous as the s i l i c o n content of the reactant mixture increased. Series 4 changed from A to Ρ (GIS) then to Y and gmelinite (GME) and f i n a l l y became amorphous. Series 5 changed from ZK-4 to HS. Obviously TMA s t a b i l i z e s the LTA structure i n high s i l i c a reaction environments. Discussion The facts uncovered here that any synthesis mechanisms must account for are: 1. In two reaction s e r i e s , with s u b s t a n t i a l l y d i f f e r e n t s i l i c a to alumina r a t i o s , the z e o l i t e synthesized changed from Y to ZK-4 as the TMA/Na r a t i o increased.


11.

HOPKINS


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Figure 1. C r y s t a l l i n i t y (of the LTA structure) as a function the S i / A l r a t i o i n the reactant mixture. Series 3, 4, and 5 d i f f e r i n their Na and TMA contents; see Table I.


ZEOLITE SYNTHESIS

158 2.

3.

4.


5.

As the TMA/Na r a t i o increased i n another reaction series the z e o l i t e product changed f i r s t from gmelinite to omega and f i n a l l y to HS. Sodalite units of z e o l i t e Y were f i l l e d s t a t i s t i c a l l y by one TMA or about two Na ions based on the r e l a t i v e concentrations of the two ions i n the reaction medium. The Si/Al atomic r a t i o of z e o l i t e Y was independent of the amount of TMA incorporated and was comparable to that of a ZK-4 synthesized from a reactant mixture having s i m i l a r s i l i c a and alumina contents. As the s i l i c a content of a t y p i c a l z e o l i t e A reaction mixture was increased, the presence of TMA maintained the product (with the LTA structure) i n a higher state of c r y s t a l l i n i t y than that obtained i n i t s absence.

The f i r s t two observations have an important s i m i l a r i t y . At low TMA contents i n a l l three series z e o l i t e s composed predominantly of double s i x rings (D6R) are synthesized, z e o l i t e Y in two series and gmelinite i n the other. Gmelinite does include the gmelinite 14-hedron i n i t s framework but the framework can be considered as being made up of p a r a l l e l layers of D6R, the gmelinite cage being the consequence of D6R stacking rather than an important building block. As the TMA content of reaction mixtures increases structures that do not include D6R appear. In two series ZK-4, which may be thought of as b u i l t from D4R rather than D6R, appears. In the HS series f i r s t omega, i n which gmelinite cages are an important part of the framework, appears followed at higher TMA contents by HS, which contains only sodalite cages. The s t a t i s t i c a l occupancy of sodalite units i n Y implies that j o i n i n g of D6R, i f that i s the synthesis mechanism, i s not sensitive to the nature of the cation occluded. A reasonable assumption i s that sodalite units i n Y form without templating but cations are required for charge balancing during some step i n the synthesis procedure. The constant alumina content observed i n the Y synthesis supports this i n t e r p r e t a t i o n . S t a b i l i z a t i o n of the LTA structure at high reactant Si/Al ratios by TMA indicates some role for TMA i n the synthesis. Published(8) and our own findings that sodalite cages i n ZK-4 a l l contain one TMA suggest that the role i s as a template for sodalite cage formation. Templating of sodalite cages apparently i s not required for synthesis of z e o l i t e A (Si/Al r a t i o of one) because the reaction i s f a c i l e i n the absence of TMA. The observations made here are not s u f f i c i e n t to prove any p a r t i c u l a r synthesis mechanism. However we may speculate as to mechanisms that are consistent with the observations. The synthesis of LTA probably proceeds by formation of sodalite units from D4R. This mechanism has been postulated before(14,15). D4R are present i n s i l i c a t e solutions containing sodium(16), potassium(17), and TMA(18) and i n aluminosilicate solutions containing TMA(19). In TMA s i l i c a t e solutions the f r a c t i o n of S i in D4R decreases with dilution(20) and i n TMA aluminosilicate solutions D4R s decrease with decreasing Si/Al(21). D4R with s t r i c t a l t e r n a t i o n of S i and A l , as required for z e o l i t e A, can j o i n i n only one way and this i s apparently f a c i l e as no template f



11. HOPKINS


159

i s required. However, i n situations where D4R contain more S i than A l , a TMA template i s required; the template could act to bring the D4R together i n the absence of a strong tendency to do so or the TMA could act to s t a b i l i z e high s i l i c a sodalite units which have recently been shown to be less stable than low s i l i c a sodalite units(22). Synthesis of A by a mechanism involving three S4R joined to form a central S6R has been proposed recently(23). The arguments above would also f i t this mechanism. However the mechanism using D4R i s more s a t i s f a c t o r y because i t provides the sodalite unit with six D4R to d i r e c t further reaction to the LTA structure, but not to FAU or SOD. Analogously z e o l i t e Y i s probably formed by j o i n i n g of D6R. The formation of Y or gmelinite, i n both of which every S i and A l i s i n a D6R, suggests that D6R are p l e n t i f u l i n reactant mixtures. The existence of D6R i n s i l i c a t e solutions has been inferred(16,24). Joining of D6R to form sodalite units appears to be f a c i l e and not affected by the presence of TMA, possibly because the S i / A l ratios of the D6R change i n a n o n - c r i t i c a l range; that i s the S i / A l r a t i o does not approach one or become very large. A similar mechanism, using S6R, has been proposed (25). Again, the D6R mechanism i s more s a t i s f y i n g because a sodalite unit with 4 D6R attached i s a nucleus only for the FAU structure. The mechanism of HS formation i s less obvious. The s t a b i l i z i n g effect of TMA on high s i l i c a sodalite units(22) may have some bearing. The d e s t a b i l i z a t i o n of D4R and D6R at high Na/Si ratios i n solution (high pH?) has been observed(16). HS forms as higher pH than the other z e o l i t e s studied here. This may lead to a mechanism involving SnR. Acknowledgments I wish to thank S-C. J . Lee and J . L. Yedinak for t h e i r assistance in the z e o l i t e syntheses, G. J . Ray for the NMR analyses, and R. H. Jarman for helpful discussions. Literature Cited 1.

Breck, D. W. Zeolite Molecular Sieves; John Wiley: New York, 1974; Table 4.17. 2. Kerr, G. T. Inorg. Chem. 1966, 5, 1537-1541. 3. A i e l l o , R.; Barrer, R. M. J . Chem. Soc. (A) 1970, 1470-1475. 4. Whyte J r . , T.E.; Wu, E. L.; Kerr, G. T.; Venuto P. B. J . Catal. 1971, 20, 88-96. 5. Meier, W. M.; Olson, D. H. Atlas of Zeolite Structure Types; Butterworths: London, 1987. 6. Barrer, R. M. Hydrothermal Chemistry of Z e o l i t e s , Academic Press: London, 1982; 166. 7. Jarman, R. H.; Melchior, M. T.; Vaughan, D. E. W. Intrazeolite Chemistry, Stucky, G. D.; F. G. Dwyer, Eds.; American Chemical Society: Washington, DC, 1983; 267-281. 8. Jarman, R. H.; Melchior, M. T. J . Chem. Soc. Chem. Commun. 1984, 414-416. 9. Jarman, R. H. J . Chem. Soc. Chem. Commun. 1983, 512-513.


ZEOLITE SYNTHESIS

160


10.

Hayashi, S.; Suzuki, K.; Shin, S.; Hayamizu, K.; Yamamoto, O. Chem. Phys. Lett. 1985, 113, 368-371. 11. Reference 1, p. 94. 12. Jarman, R. H. Zeolites 1985, 5, 213-216. 13. Mortier, W. J . Compilation of Extra Framework Sites i n Z e o l i t e s , Butterworth: Guildford, Surrey, UK, 1982; 19-31. 14. Dutta, P. K.; Shieh, D. C. J . Phys. Chem. 1986, 90, 2331-2334. 15. Melchior, M. T. In Intrazeolite Chemistry; Stucky, G. D.; Dwyer, F. G., Eds.; American Chemical Society: Washington, 1983; 243-265. 16. McCormick, Α. V.; B e l l , A. T.; Radke, C. J . Zeolites 1987, 7, 183-190. 17. Harris, R. K.; Knight, C. T. G. J . Chem. Soc. Faraday Trans. 2 1983, 79, 1539-1561. 18. Hoebbel, D.; Weiker, W. Z. Anorg. A l l g . Chem. 1971, 384, 43-52. 19. Groenen, E. J . J . ; Kortbeek, A. G. T. G.; Mackay, M.; Sudmeijer, O. Zeolites 1986, 6, 403-411. 20. Hoebbel, D.; Garzo, G.; Engelhardt, G.; Vargha, A. Z. Anorg. A l l g . Chem. 1982, 494, 31-42. 21. Hoebbel, D.; Garzo, G.; Ujszaszi, K.; Engelhardt, G.; Fahlke, B.; Vargha, A. Z. Anorg. A l l g . Chem. 1982, 484, 7-21. 22. Mabilia, M.; Pearlstein, R. Α.; Hopfinger, A. J . J . Am. Chem. Soc. 1987, 109, 7960-7968. 23. Dutta, P. K.; Puri, M.; Shieh, D. C. Microstructure and Properties of Catalysts; M. M. J . Treacy; J . M. Thomas; J . M. White, Eds.; Materials Research Society: Pittsburgh, PA 1988; 101-106. 24. Hoebbel, D.; Garzo, G.; Engelhardt, G.; Ebert, R.; Lippma, E.; A l l a , M. Z. anorg. a l l g . Chem. 1980, 465, 15-33. 25. Dutta, P. K; Shieh, D. C.; Puri, M. J . Phys. Chem. 1987, 91, 2332-2336. RECEIVED December 22, 1988


Role of the Tetramethylammonium Cation in the Synthesis of Zeolites

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