[acs symposium series] zeolite synthesis volume 398 || role of the tetramethylammonium cation in the...

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 zeolites whose frameworks include the sodalite 14-hedron were investigated. In two reaction series the product changed from zeolite Y to ZK-4 as the TMA/Na ratio in the reactant mixture was increased; in a third series the product changed from gmelinite to omega, and fi n a l l y to HS as the TMA/Na ratio increased. In agreement with published work, essentially all sodalite cages in ZK-4 occluded a TMA ion. Sodalite cages in Y zeolites 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 zeolites, that are consistent with these observations, are proposed.

Essentially a l l of the zeolites that can be synthesized in the presence of the tetramethylammonium cation (TMA) have frameworks that contain one of two types of 14-hedra U). The most common zeolites synthesized with TMA are ZK-4 (LTA)(2), omega (MAZ)(3), Ε (EAB)(3), and offretite (OFF)(4). (The three letter code in parentheses following each zeolite i s the IUPAC structure code (5). These codes identify each zeolite when f i r s t mentioned and are used elsewhere in this paper when structure types, rather than specific 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 II). Offretite and Ε also contain columns of gmelinite cages but have substantial numbers of double six rings (D6R). Molecular modelling

0097-6156/89/0398-0152$06.00/0 ο 1989 American Chemical Society

Dow

nloa

ded

by S

TA

NFO

RD

UN

IV G

RE

EN

LIB

R o

n A

pril

16, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: J

uly

31, 1

989

| doi

: 10.

1021

/bk-

1989

-039

8.ch

011

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 in 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 zeolites 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 in either 14-hedron must be incorporated during synthesis.

Offretite, Ε and omega syntheses are strongly aided by the presence of TMA. Gmelinite cage occupancy by TMA in offretite and omega is near unity(6).

TMA is essential to the synthesis of ZK-4. With TMA products having Si/Al atomic ratios up to about three have been produced(7). Essentially a l l sodalite units in ZK-4 contain a TMA ion(8). In the absence of TMA the isostructural zeolite A, with Si/Al invariably equal to one, is produced.

Zeolites Y (FAU) and HS (hydroxysodalite, SOD) both contain sodalite units in their framework. Both can be synthesized easily without TMA but can also be synthesized in the presence of TMA(9,10). A l l sodalite cages in 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 zeolites, ZK-4, HS, and Y, that contain the sodalite unit. 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 in the synthesis of the three zeolites.

Experimental

Syntheses were carried out according to published procedures(7,9) or procedures developed by us. Reactants employed were Ludox HS-40 colloidal 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 for one or more days at room temperature before heating to ca. 100°C for crystallization. HS synthesis mixtures were not aged. Crystallizations were carried out in three-neck flasks at reflux with st i r r i n g or in Teflon bottles placed in a 100°C oven without st i r r i n g .

The zeolites were synthesized in one of six reaction series. Reactant ratios for the six series are listed in Table I. Series 1 is one that we have used, in the absence of TMA, to synthesize zeolite Y (FAU). Series 2 is based on a published recipe(7) for synthesis of ZK-4 (LTA). Series 3 i s one which we use to synthesize zeolite 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 is based on a published recipe(9) for synthesis of zeolite HS. In Series 1, 2, and 6, substitution of TMAOH for NaOH causes only minor increases in pH.

Dow

nloa

ded

by S

TA

NFO

RD

UN

IV G

RE

EN

LIB

R o

n A

pril

16, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: J

uly

31, 1

989

| doi

: 10.

1021

/bk-

1989

-039

8.ch

011


154 ZEOLITE SYNTHESIS

Table I. Reactant Ratios

Mole Ratios Series

1 2 3 4 5 6

17.3 9.5 (a) (a) (a) 20.3 1.0 1.0 1.0 1.0 1.0 1.0 (b) (b) 1.1 2.0 1.1 (b) (b) (b) 0 0 1.0 (b)

13.6 4.6 1.1 2.1 2.1 7.4 445 325 85 85 85 280

SiO Al 6 Na^0J

(TMA) 0 Na 0 + (TMA) 0

(a) Varied from 1 to 8. (b) Varied with sum indicated in f i f t h line kept constant.

A l l products were washed thoroughly and dried at ca. 105°C overnight. Product identities and phase purities were determined by powder XRD. Silicon 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 Si/Al ratios were determined by established XRD correlations for Y( l l ) and ZK-4(12); some ratios were also determined by Si NMR. TMA, in total and in specific locations, was determined by C NMR(8).

Results

Effect of the TMA/Na Ratio on the Product. Changing the TMA/Na ratio in reaction series 1 and 2 had a pronounced effect on the nature of the zeolite product as shown in Table II.

Table II. Effect of Tetramethylammonium on the Nature of the Zeolite Product

Series 1 TMA/Na Product TMA/Na

Series 2 Product

0.0 0.2

8.8

Y Y Y

Y,ZK-4 ZK-4 ZK-4

0.0 0.6 0.8 1.0 1.2 1.5 2.5

Y Y

Y,ZK-4,E Y,ZK-4,E

ZK-4,E ZK-4 ZK-4

Dow

nloa

ded

by S

TA

NFO

RD

UN

IV G

RE

EN

LIB

R o

n A

pril

16, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: J

uly

31, 1

989

| doi

: 10.

1021

/bk-

1989

-039

8.ch

011



Both reaction series, employing substantially different reactant ratios, 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 zeolite Ε was found in Series 2 at TMA/Na ratios near 1.0). Clearly, TMA stabilizes the LTA structure over a wider range of S1O2/AI2O2 reactant ratios than has been reported previously.

The Si/Al atomic ratios of the products from series 1, both Y and ZK-4, were nearly invariant as shown in Table III.

Table III. Si/Al Atomic Ratios in Series 1 Products

Reactant TMA/Na Product XRD NMR

0.0 Y 1.59 „

0.2 Y 1.64 1.71 1.0 Y 1.59 1.86 8.8 ZK-4 1.57 —

The Si/Al ratio for the ZK-4 product from reaction series 2 was 1.92 by XRD, 2.20 by NMR and 2.26 by chemical analysis; the last i s in good agreement with a ratio of 2.39 found by chemical analysis of a product from a similar synthesis(6).

The TMA/Na ratio also had an effect on the products synthesized in Series 6 as shown in Table IV.

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

Reactant TMA/Na Zeolite Product

0.00 gmelinite 0.24 omega 0.40 omega and HS 0.62 HS 1.39 HS

Synthesis of HS at a TMA/Na ratio of 0.25 had been reported previously(6) but the same reactant mixture has also been reported to produce omega(_1). As shown in Table IV we synthesized omega at this TMA/Na ratio, but by increasing the TMA/Na ratio we were able to synthesize HS.

Sodalite Cage Occupancy in Y. The sodalite cage occupancy by J^A of three Y zeolites synthesized in Series 1 was determined by C

Dow

nloa

ded

by S

TA

NFO

RD

UN

IV G

RE

EN

LIB

R o

n A

pril

16, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: J

uly

31, 1

989

| doi

: 10.

1021

/bk-

1989

-039

8.ch

011



NMR using the intensity for sodalite cages in ZK-4 (which are completely f i l l e d with TMA) as a standard. The results are compared in 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 in Y Zeolites

Calculated for Random Occupancy by 1 TMA

or η Na/Cage Reactant TMA/Na JC NMR n=2 n=3

0.17 26% 25% 33% 0.41 45 45 54 0.99 75 66 74

The results suggest that the sodalite cages of Y synthesized in 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 slightly better than for three sodium ions/cage. Sodalite cage occupancy by approximately two sodium ions i s consistent with many XRD studies(13)·

TMA in Zeolite A Reactant Mixtures. Series 3, 4, and 5 differed only in 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 in the reactant mixtures was increased as shown in Figure 1. Series 4 products lost 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 effect observed in Series 5 was due to TMA and not to pH (since the difference in pH between TMAOH and NaOH systems was small). The cry s t a l l i n i t y in Figure 1 includes only material having the LTA structure; other crystalline phases appeared in some products but were not included in the crys 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 fin a l l y became amorphous. Series 5 changed from ZK-4 to HS. Obviously TMA stabilizes the LTA structure in 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 series, with substantially different s i l i c a to

alumina ratios, the zeolite synthesized changed from Y to ZK-4 as the TMA/Na ratio increased.

Dow

nloa

ded

by S

TA

NFO

RD

UN

IV G

RE

EN

LIB

R o

n A

pril

16, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: J

uly

31, 1

989

| doi

: 10.

1021

/bk-

1989

-039

8.ch

011


11. HOPKINS Tetramethylammonium Cation and Zeolite Synthesis

High- **r9 \ · * \ / K

\: \ /V' ·' L I 1

\ *

- ο \

Med. o-

\ \ I I I 1 t 1 1 1 1 1

\ Series 5 1

c \ \ \

1 1

Cry

sti

\ \ \ \ \ \ \ \ \ \

1

i 1

Low \ \ \ \ \ r"| Series 3 \ \ \ \ \ \ \ \ > \

\ Series 4 \ \ \

»

t » 1

t

X \

None

\ \ \ \

& Ί ' 1 ' 1 ' 1 ' = Ύ

0 1 2 3 4 Si/Al in reactants

Figure 1. Crystallinity (of the LTA structure) as a function the Si/Al ratio in the reactant mixture. Series 3, 4, and 5 differ in their Na and TMA contents; see Table I.

Dow

nloa

ded

by S

TA

NFO

RD

UN

IV G

RE

EN

LIB

R o

n A

pril

16, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: J

uly

31, 1

989

| doi

: 10.

1021

/bk-

1989

-039

8.ch

011



2. As the TMA/Na ratio increased in another reaction series the zeolite product changed f i r s t from gmelinite to omega and fi n a l l y to HS.

3. Sodalite units of zeolite 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 relative concentrations of the two ions in the reaction medium.

4. The Si/Al atomic ratio of zeolite Y was independent of the amount of TMA incorporated and was comparable to that of a ZK-4 synthesized from a reactant mixture having similar s i l i c a and alumina contents.

5. As the s i l i c a content of a typical zeolite A reaction mixture was increased, the presence of TMA maintained the product (with the LTA structure) in a higher state of cry s t a l l i n i t y than that obtained in i t s absence.

The f i r s t two observations have an important similarity. At low TMA contents in a l l three series zeolites composed predominantly of double six rings (D6R) are synthesized, zeolite Y in two series and gmelinite in the other. Gmelinite does include the gmelinite 14-hedron in i t s framework but the framework can be considered as being made up of parallel 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 built from D4R rather than D6R, appears. In the HS series f i r s t omega, in 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 in Y implies that joining of D6R, i f that is the synthesis mechanism, i s not sensitive to the nature of the cation occluded. A reasonable assumption is that sodalite units in Y form without templating but cations are required for charge balancing during some step in the synthesis procedure. The constant alumina content observed in the Y synthesis supports this interpretation.

Stabilization of the LTA structure at high reactant Si/Al ratios by TMA indicates some role for TMA in the synthesis. Published(8) and our own findings that sodalite cages in 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 is not required for synthesis of zeolite A (Si/Al ratio of one) because the reaction i s facile in the absence of TMA.

The observations made here are not sufficient to prove any particular 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 in s i l i c a t e solutions containing sodium(16), potassium(17), and TMA(18) and in aluminosilicate solutions containing TMA(19). In TMA s i l i c a t e solutions the fraction of Si in D4R decreases with dilution(20) and in TMA aluminosilicate solutions D4Rfs decrease with decreasing Si/Al(21). D4R with s t r i c t alternation of Si and Al, as required for zeolite A, can join in only one way and this i s apparently facile as no template

Dow

nloa

ded

by S

TA

NFO

RD

UN

IV G

RE

EN

LIB

R o

n A

pril

16, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: J

uly

31, 1

989

| doi

: 10.

1021

/bk-

1989

-039

8.ch

011



is required. However, in situations where D4R contain more Si than Al, a TMA template is required; the template could act to bring the D4R together in the absence of a strong tendency to do so or the TMA could act to stabilize 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 satisfactory because i t provides the sodalite unit with six D4R to direct further reaction to the LTA structure, but not to FAU or SOD.

Analogously zeolite Y is probably formed by joining of D6R. The formation of Y or gmelinite, in both of which every Si and Al is in a D6R, suggests that D6R are plentiful in reactant mixtures. The existence of D6R in s i l i c a t e solutions has been inferred(16,24). Joining of D6R to form sodalite units appears to be facile and not affected by the presence of TMA, possibly because the Si/Al ratios of the D6R change in a non-critical range; that i s the Si/Al ratio does not approach one or become very large. A similar mechanism, using S6R, has been proposed (25). Again, the D6R mechanism i s more satisfying because a sodalite unit with 4 D6R attached i s a nucleus only for the FAU structure.

The mechanism of HS formation is less obvious. The stabilizing effect of TMA on high s i l i c a sodalite units(22) may have some bearing. The destabilization of D4R and D6R at high Na/Si ratios in solution (high pH?) has been observed(16). HS forms as higher pH than the other zeolites studied here. This may lead to a mechanism involving SnR.

Acknowledgments

I wish to thank S-C. J. Lee and J. L. Yedinak for their assistance in the zeolite 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. Aiello, R.; Barrer, R. M. J. Chem. Soc. (A) 1970, 1470-1475. 4. Whyte Jr., 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 Zeolites, 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.

Dow

nloa

ded

by S

TA

NFO

RD

UN

IV G

RE

EN

LIB

R o

n A

pril

16, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: J

uly

31, 1

989

| doi

: 10.

1021

/bk-

1989

-039

8.ch

011



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 in

Zeolites, 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.; Bell, 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. Allg. 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. Allg. Chem. 1982, 494, 31-42.

21. Hoebbel, D.; Garzo, G.; Ujszaszi, K.; Engelhardt, G.; Fahlke, B.; Vargha, A. Z. Anorg. Allg. 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.; Alla, 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

Dow

nloa

ded

by S

TA

NFO

RD

UN

IV G

RE

EN

LIB

R o

n A

pril

16, 2

013

| http

://pu

bs.a

cs.o

rg

Pub

licat

ion

Dat

e: J

uly

31, 1

989

| doi

: 10.

1021

/bk-

1989

-039

8.ch

011


[acs symposium series] zeolite synthesis volume 398 || role of the tetramethylammonium cation in the...

Documents