High-Silica Faujasite by Direct Synthesis - ACS Symposium Series

Chapter 30

High-Silica Faujasite by Direct Synthesis

Downloaded by UNIV ILLINOIS URBANA on April 16, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch030

Harry Robson Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803

The preparation of high-silica faujasite (LTY) by D. W. Breck certainly is one of the outstanding successes in zeolite synthesis because it established synthetic faujasite as a catalytic material. In the 30 years since, our efforts have only raised the product from Breck's 4.9 SiO /Al O to about 6.0. But there are good indications that a still more silica-rich synthetic product would be a superior catalyst. For catalytic cracking, the optimum base-faujasite is about 10 SiO /Al O . In order to produce such a material, the synthesis batch should have a pH=11; but under these conditions, the rate becomes impossibly slow. Our experience in other zeolite synthesis systems indicates four approaches to increase the crystallization rate in these high-silica gels: 1) increased temperature, 2) amine additives, 3) non-aqueous solvents, and 4) improved seeding. Of these, the first three would impose a major cost increase on the faujasite product. Improved seeding could be compatible with existing equipment and processes. 2

2

3

2

2

3

We shouldrememberthat low temperature zeolite synthesis started with Milton's gel synthesis of A and X in the early 1950's(l). Large pore zeolites were substantially unavailable at that time. Natural faujasite was rare as it still is. Large port mordenite was still ten years in the future as were zeolites L and Omega. Zeolites were regarded as ion exchangers or as selective sorbents, not as catalysts. Breck's preparation of type Y faujasite in the late 1950's still stands as the outstanding success in zeolite synthesis (2). Type X might have had some catalytic applications but I doubt the International Zeolite Association would exist without the interest and support generated by the catalytic applications of the Type Y materials. It didn't seem that critical at the time; after all Breck hadreproduceda material which exists naturally. Synthetic counterparts of natural zeolites have been prepared dozens of times since (3). But the extra silica content, or perhaps the diminished alumina content, was enough to give high temperature stability in the acid form and to get zeolites into catalysts for petroleum processes (4). In the thirty years since, we have increased the silica content of Breck's product by only about 4% in the composition of the total product (S1O2/AI2O3:4.9 -> 6.0) as shown in Table I. This increase is not trivial, but it can't be said to have created new products or new processes. Perhaps the better way to consider it is the 0097-6156/89/0398-0436$06.00/0 ο 1989 American Chemical Society

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

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number of tetrahedral positions in the faujasite unit cell which are occupied by A l . For type X (2.7 S1O2/AI2O3), 82 of the total 192 T-positions are A l ; for type Y , it is 56 A l per unit cell. At 6.0 S1O2/AI2O3, we have reduced this number only to 48. 3 +

3 +

Optimum Synthetic Faujasite. As a goal for synthesis research, we might seek a procedure which would yield any sihca/alumina ratio up to pure S1O2 in faujasite form. Although such a sample would be interesting from a research perspective, it would almost certainly be a disaster as a catalyst (No alumina = no acidity). For catalytic cracking applications, the optimum zeolite component in the finished catalyst has been reported to be a SiO2/Al20^ of about 14 (24 A 1 / U . C ) (5). Such a material when dispersed in a silica-alumina matrix and finished by current manufacturing procedures should have a unit cell size of about 24.29Â which is near optimum for catalytic cracking. The optimum zeolite component for a hydrocracking catalyst is not so narrowly defined but the evidence is such a material would be an improvement on current US Y technology (6).


3+

Table I. Synthetic Faujasites S1O2/AI2O3

Type X Y Ζ

2.7 4.9 10

Al/UC 82 56 32

Cell Size 24.92A 24.72Â 24.45À

Ordered A l Distribution. But I believe we can be more explicit about our goal for the next generation of synthetic faujasite than simply more, i.e.higher S1O2/AI2O3. There can be little doubt that the acid sites which are most effective in strong acid catalysis are the alumina tetrahedra isolated in the zeolite structure by at least two silica tetrahedra This was defined by Wachter (7) as zero next-nearest neighbors (0-NNN). We still do not have the technology to observe these alumina sites directly, but we can infer them from the S i M A S - N M R spectra as the absence of Si(OAl)2 and higher resonances. To achieve a faujasite-type product with the maximum 0-NNN sites requires a 10 S1O2/AI2O3 product with ordered distribution of A l in the T-sites. This is the material I choose to designate type Ζ faujasite. In terms of the unit cell, this is 32A1 /UC (32/(192-32)=0.2). It seems unlikely that stabilization or extraction treatments will yield such a product. The optimum zeolite reported by Pine, et al. as 24A1 /UC is at least 25% off the maximum possibly due to a significant concentration of Si(OAl)2 in the parent zeolite. 2 9

3 +

3+

3+

SiUca/alumina by Unit Cell Size. Unit cell size has proven to be a reliable indicator of the S1O2/AI2O3 ratio of faujasite-type materials. Figure 1 plots unit cell size (Â) vs the number of A l ions per unit cell. Conventional type Y materials are at the upper right (A1 /UC greater than 56). The anchor point at the lower left (A1 /UC=0) at 24.19Â is the consensus of many observations of the end product of repeated dealuminations. It is difficult to get reliable data in between. It is tempting to extrapolate the Dempsey, Kuhl, and Olson (8) curve to zero alumina with just enough curvature to terminate at 24.19 Â. Skeels and Breck data on USY-type materials (LZ-Y82 ^ LZ-Y20) probably show non-frame work dumina(9) The SiRi-treated materials (LZ-210) reported by Skeels and Breck fall on the silica-rich side of the extrapolated curve. They may indicate non-framework silica; however, there could be a break in the curve at 56 A 1 / U C . The dashed segment is linear over the range 24.64 to 24.19Â. 3 +

3+

3+

3+


ZEOLITE SYNTHESIS

438


25.0

Figure 1. Change in faujasite unit cell(Â) with increasing A l (192 total A13+ + S i per unit cell).

3 +

in T-positions

4 +


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Admittedly the case for type Ζ faujasite (10 S1O2/AI2O3,0-NNN) being a superior catalytic material is somewhat speculative at this point Dealuminated type Y has given us materials with a higher S1O2/AI2O3, but has not necessarily produced optimum alumina distribution (0-NNN). Until such materials are prepared and tested in catalytic applications we can only conjecture on their advantages. Type Y got us over the hump and into catalysis, but it was never the material we wanted. US Y and LZ-210 are compromises to give us, at a considerable price, the materials we cannot prepare by crystallization. High-Silica Faujasite bv Means Other Than Synthesis U S Y (ultra-stable type Y) is a good material which has served us well but which has probably been pushed to its limit (10). In simplified terms, as A l is eliminated from the T-positions in the structure by thermal treatment in the presence of H2O, they are replaced by S i * from some other portion of the crystal. Table Π compares a typical U S Y (LZ-Y82) to the parent material, NaY. The S1O2/AI2O3 ratio (5.77) probably understates the transformation because of non-framework alumina retained in the structure. Reduced crystallinity is evidence of structural damage; this same effect would be expected to reduce the zeolite character of its sorption properties. The reduction in cation content (0.38 Na/Al) renders it unsuitable for an alkaline application such as the ELF-Aquitaine aromatization catalyst


3 +

4

Table Π. Composition of High-Silica Faujasites Sample

NaY 4.92 0 0.98 100 24.69

S1O2/AI2O3

Fluoride(%) M+/A X-ray Cryst U-CelliÂÏ

LZ-210 9.31 0 0.99 106 24.49

USY 5.77 0.05 0.38 73 24.52

Exp 5.99 0 0.99 87 24.57

LZ-210 (10) is a new material which doesn't seem to have attracted the attention its properties warrant Unlike USY, S i * is supplied from external (NH4)2SiF6 as A l is eliminated. There is no loss in crystallinity at least for a moderate treatment (In this case, an apparent gain, 100-»106). There is some retained flouride which might be a problem in a commercial cracking unit. Skeels and Breck indicate this residual flouride increases with more severe (NH^SiFo treatments. It also does not bode well for an alkaline catalyst application. The unit cell size (24.49Â) compares favorably with USY. But even with the proper S1O2/AI2O3 ratio, there is a second requirement for type Z: 0-NNN. It would be fortuitous if the ammonium fluorosilicate extraction selectively removed A l (1-NNN) leaving A l (0-NNN). Since 0-NNN sites are the most acidic, they would be expected to react most rapidly with (NH4) SiF . The final column in Table Π is a product of direct synthesis in my laboratory: I have labelled it "Exp" because it falls considerably short of the 10 Si02/Al2C>3 which I believe is optimum for catalytic applications. At 5.99 S1O2/AI2O3, it should have 48A1 /UC compared to 34 for LZ210 and 32 for the optimum type Z. Crystallinity is respectable (87%) but should be improved; additional work and study is needed in type Ζ synthesis. Na/Al = 0.99 indicates a good cation balance; the material has tested favorably in an alkaline application. 4

3 +

2

6

3+


440

ZEOLITE SYNTHESIS

Is High-Silica Possible bv Direct Synthesis? For many years, the attitude has prevailed that we had reached the limit when the synthetic product duplicated natural faujasite (5.1 S1O2/AI2O3). What nature has accomplished in 10,000 years we should not expect to exceed in hours or days in the laboratory. Fortunately our vision has not been so severely limited in the case of other zeolite phases. Table ΠΙ compares natural materials to their high-silca synthetic counterparts for mordenite, ferrierite, and sodalite.


Table m . High-Silica Forms of Natural Zeolites Phase

Means

Si02/Al 03 ll-»28

Amine Additive

Ferrierite/ZSM-35

14->60

Amine Additive

Sodalite/Silica-sodalite

2->oo

Faujasite/Type Ζ

5->10

Glycol Solvent ?

2

Mordenite/HSM

High-silica mordenite (HSM) is the product of the Tonen group in Japan (11). It can be prepared with tetrapropylammonium hydroxide addition or by control of the timetemperature cycle in synthesis. At 28.5 Si02/Al203, it is comparable to the product of strong acid extraction. ZSM-35 is the counterpart of natural ferrierite; it was prepared by the Mobil group with pyrrolidyne, ethylenediarnine, or butanediamine additives (12). Silica-sodalite was recendy announced by Bibby and Dale (13). Natural sodalite has S1O2/AI2O3 =2; silica-sodalite exists in silica-rich forms up to pure SiC>2- This is the first example of a 4-6 ring structure which can be prepared in very high silica form. The principal change in their synthesis is substitution of glycol for water. At this stage we would be happy to make type Ζ faujasite by any of these methods in order to have the material to prove its catalytic properties. But if it is to escape the laboratory and see service as a commercial catalyst it will have to be produced at a cost which can be justified by its advantages over other materials. The organic templates are expensive and sometimes toxic. No templated synthesis is yet a commercial pocess, with the possible exception of ZSM-5. Non-aqueous synthesis is not difficult in the laboratory; on commercial scale, it is a completely new undertaking. We will continue to test these techniques, but always with the hope that a better way will be found. It is unlikely that a single change in current synthesis procedure will open up the high-silica type Ζ region. Commercial Synthetic Faujasite. At present commercially supplied type Y materials have increasedfrom4.6 to about 5.1 S1O2/AI2O3 on the average. Laboratory syntheses now approach 6 S1O2/AI2O3. Infigure2, a plot of synthesis experiments from the literature, I have chosen to plot composition of the synthesis batch as two ratios: Al/Si as abscissa and OHVSi as ordinate. The point code shows a number of series where the investigator pushed toward a more silica-rich product (lower left). These plots show a sequence of experiments leading to the point where the batch failed to crystallize and the series ended, only to begin again at some other point or by another investigator. My own experiments in this area are somewhat wider ranging but also are plagued by less than fully-crystalline products. Consistendy I have observed that the initial faujasite crystals which form are more silicarichthan thefinalproduct by a



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0.6\-

0.1

0.2

0.3

0.4

0.5

AL/SI—•

Figure 2. Overall batch composition of faujasite syntheses reported by several investigators (H2O content not shown).


ZEOLITE SYNTHESIS

442

crystallization which continues to where faujasite is the major product. In some cases, the reaction ceased with less than 10% faujasite crystallinity but enough x-ray diffraction lines visible to calculate a cell size as low as 24.48 Â.


pH Controls Si/Al in the Zeolite Product. For zeolite A and other durnina-rich phases, the synthesis is done in strongly alkaline systems which are not ordinarily described in pH terms. Further, measuring pH in alkaline systems at 100°C or above is not a trivial problem. But for silica rich systems, pH becomes a useful indicator of synthesis conditions. For most synthetic zeolites, pH is the primary control of Si/Al in the product. This was nicely demonstrated by Donahoe and Liou (14) on phillipsite synthesis from (K, Na) alumino-silicate solutions. They tested a number of synthesis variables but concluded only pH was significant in determining product S1O2/AI2O3. A plot of their data (Figure 3) shows a linear relationship, admittedly over a limited range. Our experience on other phases is similar although less precisely defined. It is instructive to project this relationship to the silica/alumina range for type Ζ faujasite. Granting the uncertainty in extrapolation, the predicted value of pH = 11 for Si/Al = 5 (S1O2/AI2O3 = 10) is entirely reasonable. Current laboratory typs Y synthesis is near pH = 12.3; certainly reduction of the pH of the synthesis batch should produce a more silica-rich product. The size of the pH reduction required to produce 10 S1O2/AI2O3 is in doubt, but die direction is unmistakable. How Can We Promote Fauiasite Crystallization at pH = 11? The problem is the rate of crystallization which becomes impossibly slow using current technique if operating in this pH range. Table IV is a list of possible remedies and is definitely open-ended; new ideas are welcome. Table IV. How Can We Promote Faujasite Crystallization at pH=l 1? -^Increase crystallization time or temperature -»Use amine additives (templating agents) ->Switeh to non-aqueous solvents ~»Improve nucleation Longer crystallization treatments are not very promising experimentally. Figure 4 is from Kacerek & Lechert (15); it shows the rate of faujasite crystallization to be an exponential function of S1O2/AI2O3 of the zeolite product They indicate a break in the function to a markedly slower rate above 5.5 S1O2/AI2O3. If we accept the lower rate which they predict, the synthesis of a 10 S1O2/AI2O3 product requires nearly 10,000 years. Even if we extrapolate the low-silica data without the break (dashed line), our synthesis requires 200 days. In either case, the conclusion is the same: synthesis of type Ζ at 88 C is not a viable process. Higher temperature treatments usually promote competing phases (gismondine /phillipsite or analcine) more than they promote faujasite. The 100 - 150°C region has not been adequately investigated in faujasite synthesis probably because autoclave experiments are more difficult than atmospheric pressure experiments and the more interesting results came at 100°C or less.Furthermore, faujasite is a metastable product and can easily degrade upon prolonged contact with the mother liquor, particularly at e


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-

\P-6


NP-9 1.90

-

|P-7

\ p - 4 \ P - 8

1.80 13.40

13.50

13.70

13.60

M E A S U R E D pH

Figure 3. Framework Si/Al of phillipsite products vs. pH of the synthesis batch; ref (14). 1000

β

Figure 4. Rate of faujasite crystallization at 88 C vs. SiO^/A^O. of the framework; ref (15).


443

444

ZEOLITE SYNTHESIS

higher temperature. Table V shows loss of crystallinity and appearance of other phases in a preformed NaY faujasite after exposure to sodium silicate solutions. The degradation is rapid at 100°C in 10% Na2Si03, but delayed for at least seven days in the less-alkaline water glass (Na20 · 3.3 S1O2). For 120°C or a more concentrated solution (20%), three days exposure is too much.


Table V. Recrystallization of NaY Treating Solution

(10g solution/g zeolite) pH Days/°C

Product

Ipfe

133

3 7 î œ

FAU(S)+ÛIS

7/100

10% Na 03.3Si02

11.4

GIS(m)+ANA 7/100 12/100

FAU(100) FAU(95)

10% Na2O3.3Si02

11.6

3/120

FAU(96)

20% Na Q-3.3SiQ2

UA

3/100

FAU(82)

2

2

Amine Additives. It is precisely in this area, suppressing an undesired product in favor of the desired one, that additives can be most useful. High silica zeolites have been formed in the presence of amine additives. Vaughan (16) has prepared faujasite with 7.0 S1O2/AI2O3 (24.52 A ) by addition of bis-(2-hydroxyethyl) dimethyl ammonium chloride in a slurry composition whose cation composition is 69% Na and 31% organic template (T). To scale this product up to commercial synthesis would require almost total recovery of the organic template. But its silica content makes it an interesting candidate for catalytic testing. Perhaps we have concentrated too much on the quaternary amines as templating agents. Another approach is suggested by Verdijn in the synthesis of zeolite L(17). As showing in Table VI, by addition of only trace amounts of alkaline earth cations, especially strontium, the gel composition range which produced the desired zeolite L product was widened, the formation rate increased, smaller crystallites were obtained, and stirred synthesis became possible. The alkaline earths were very probably present when natural faujasite was formed. But their solubility in alkaline silicate liquors is low; 200 ppm S r is near saturation in the synthesis slurry. 0

2+

Table VI. Synthesis of Zeolite L The use of trace amounts of divalent cations significantly affects the synthesis aspects of zeolite L : -> Gel composition range widened -» Formation rate increased -» Smaller crystallites obtained —» Stirred synthesis possible These observations suggest that the nucleation mechanism is greatly influenced.


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The largest area of ignorance and therefore the most promising field for investigation lies in nucleation. Breck's synthesis required S1O2 in the form of colloidal silica sol to make silica-rich type Y . We have since learned to seed the batch with a freshly prepared slurry (18), and use cheaper silica sources. But we are far from a chemical definition of the nucleating agent There is considerable data showing the synthesis can stall at low levels of crystallinity even though all the nutrients are present for continued growth. It has not been established that fresh addition of the "seed" slurry will reinitiate crystal growth, but growth of faujasite cryustals at 24.48À, initiated but not sustained as noted in the previous section, indicates reinitiation should be possible. This "growth hormone" seems to be self generated in X and Y synthesis but lacking at conditions which would yield type Z. Centrifugation of the Nucleation Slurry Conventional wisdom ascribes nucleation to rrticrocrystalline type X which in the presence of the proper nutrients grows into a full yield of type Y with no trace of its type X initiator. The "seed" composition is essentially sodium metasilicate solution with the addition of a small amount of sodium aluminate: Na2Si03 -0.16 NaA102 · 21H2O. Comparatively little "seed" is necessary for a good Y synthesis; as little as 1% of the total AI2O3 may come from the seed. But the actual nucleaant is probably a much smaller entity than the incipient faufasite crystal, even smaller than a sodalite cage which has been proposed as a secondary building unit It seems to; be a metastable unit which exists only temporarily in the depolymerization of the silica network and integration of A l into a more stable structure. The nucleation slurries we have now are limited in shelf life and sensitive to the materials used in their preparation. In my laboratory, I have attempted to concentrate the active component by centrifuging the nucleation slurry. As shown in Table V u , the solid cake removed by centrifrigation was almost inactive as a nucleating agent while the clear supernatant liquid was more active than the original slurry. Table V u indicates two synthesis batches of the same overall compostion (0.25 Ai/Si, 0.51OH7SÎ, I6H2O/S1) but prepared one with the total nucleation slurry, the other with the clear liquor. Whereas whole nucleation slurry gave a mixture of nearly equal amounts of faujasite and gismondine at this composition, the clear decantate gave full faujasite crystallinity with only a small amount of gismondine. By analysis, the clear decantate is richer in NaOH and leaner in NaA102 than the whole slurry. Note that the initial faujasite crystals in poorly crystallized product appear to be more silia-rich than a fully crystalline product 3 +

Table VII. Centrifugation Improves Nucleation Slurry Seeding ll%N.S.-whole 10%N.S.-clear

Days/°C 4/100 5/100 4/100 4/90

Product FAU(w)+LTA FAU(24)+GIS FAU(96)+GIS FAU(100)+GIS

U-Cell 24.58 24.57 24.61 24.59

The ionic composition of dilute sodium silicate is a very complex problem involving Na20/SiU2 ratio, water content, and even trace impurities. Equilibration seems to be very slow at ordinary temperatures. As shown in Figure 5, Harris, et.al, were able to identify a wide variety of structures in potassium silicate solutions which bear a striking resemblence to the secondary building units proposed for zeolite



446

Z E O L I T E

SYNTHESIS

4-e- s Figure 5. Structures of Silicate Ions in K-silicate Solution. (Reproduced from r e f . 19. Copyright 1981 American Chemical Society.)


30. ROBSON

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structures. For either sodium or potassium silicate, addition of alumina to the system should change the relative stability of these structures and create new ones. In order to make type Ζ a reality, we need to continue to feed the nucleation component to the crystallizing batch. This could mean circulating the liquor from the crystallizer through an environment which recreates the essential ingredient. Figure 6 is a block diagram of what such a process might look like. There are many problems still to be solved beyond the compositon of the nucleation liquor, for example pH control in the crystallizer, but the product could be of great technological interest.


Na-Aluminate «—Nucleation Liquor « -

[NUCLEATION] liquor

iMIXERl—• gel -^CRYSTALLIZER]

Τ

No-Silicate

slurry

•!CENTRIFUGE]

Τ

slurry i [FILTER"]—•Zeolite type Y Mother Liquor

Figure 6. Flow diagram for a proposed modified batch synthesis process promoted by a nucleation agent

Literature Cited 1. Milton, R. M. U.S. Patents 2 882 243 & 2 882 244, 1959. 2. Breck, D.W. U.S. Patent 3 130 007, 1961 3. Robson, H. E. Chemtech1978,8,176. 4. Rabo, J.Α.; Poutsma, M. L. Molecular SievesZeolites-II.Advances in Chem. Series No. 102; American Chemical Society: Washington DC, 1971p284 5. Pine, L. Α.; Maher, P. J.; Wachter, W.A. J. Catal. 1984, 85, 466 6. Ward, J. W.; Carlson, T. L. U.S. Patents 4 517 073, 1985 and 4 576 711, 1986. 7. Wachter, W. A. Proc. 6th Int. Zeolite Conf. 1983,p141. 8. Dempsey, E.; Kuhl, G. H.; Olson, D. H. J. Phys. Chem. 1969, 73, 387 9. Skeels, G. W.; Breck, D. W. Proc. 6th Int. Zeolite Conf. 1983,p87. 10. Lynch, J. et.al.,Zeolites 1987,7,333. 11. Sakurada, S. et.al., European Patent 40 104, 1981. 12. Rollman, L.D. U.S. Patent 4 107 195, 1978. 13. Bibby, D. M.; Dale, M. P. Nature 1985, 317, 157 14. Donahoe, R. J.; Liou, J. G. Geochemica et Cosmochemica Acta 1985,p34. 15. Kacirek, H.; Lechert, H. J. Phys. Chem. 1976, 80, 1291 16. Vaughan, D. E. W. U.S. Patent 4 714 601, 1987. 17. Verdijn,J.P.European Patent 142 355, 1984 18. McDaniel, C. V.; Duecker, H. C. U.S. Patent 3 574 538, 1971. 19. Harris, R. K.; Knight, C. T. G.; Hull, W. E. J. Am. Chem. Soc. 1981, 1577

RECEIVED February 22, 1989


High-Silica Faujasite by Direct Synthesis - ACS Symposium Series

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