Parameters Affecting the Growth of Large Silicalite Crystals

Parameters Affecting the Growth of Large Silicalite Crystals

and J. C. Lee of Chemical Engineering, Cleveland State University, Cleveland, Ohio, 44115, U.S.A. A statistical method was employed to optimize the synthesis of large silicalite crystals. Using a starting reactant batch composition of 10NaZO o100SiZO.Z490HZO o11TPABr, a factorial method was used to adjust the contents of NaOH and TPABr in the synthesis mixture in order to maximize the crystal size. It was found that lesser amounts of NaOH and TPABr favored the growth of large crystals. Crystals up to a size of 400x80x80PID were identified, with the optimum reaction batch mixture being 2.25NaZOo100SiZOo2832HZOo5.22TPA. Additional experiments were performed using KOH, CsOH and sodium salts to replace NaOH in the batch formulation. Products from these runs were found to be well-crystallized, uniform material with crystal lengths of up to 200)Jm. INTRODUCTION Synthesis of the pentasil zeolite ZSM-5 and its aluminum deficient end-member silicalite has been the topic of a large number of investigations. A few citations are provided [1-5]. Typical reactants used to produce these pentasil zeolite are silica, often a colloidal silica and an alumina such as sodium aluminate. Sodium hydroxide, tetrapropylammonium bromide and water are typically required. The base, sodium hydroxide, has the function of dissolving silica to form a reactive liquid phase. This is considered the first step in zeolite crystallization. The tetrapropylammonium bromide dissociates to form the large organic cation, TPA+. These ions act as structure directing templates around which the zeolite cage structure forms. The crystallization reactions for zeolites are autocatalytic, requiring a long nucleation period followed by rapid crystal growth. This long nucleation followed by rapid crystallization is considered to be responsible for the very fine crystal size often observed for zeolites. In this scheme, a large number of nuclei forms initially. When a critical number of nuclei is achieved, growth proceeds rapidly, depleting the solution of nutrients and limiting the size of the resulting crystals. Reports of the growth of large zeolite crystals are quite limited. For the pentasil family of zeolites, there are reports of modifying the chemical reactant system by replacing the sodium hydroxide and TPA ions with other species. One purpose for this modification is to grow larger crystals. Chao et al.[6] found that ZSM-5 crystallized without the addition of TPA ion. Nakamoto and Takahashi reported the synthesis of ZSM-5 using tetrapropylammonium hydroxide as the only source of base in the ZSM-5 synthesis [7]. Neither of these modifications were found to enhance crystal size. Mostowicz and Sand reported on the use of sodium salts as the only source of alkali metal in ZSM-5 crystallization [8]. In their experiments, they found that the ZSM-5 formed as crystal aggregates of up to 80 )Jmin diameter. It was proposed that this increase in crystal size was due to the low water content of the reaction mixture. This reduction in water increased the "dryness" of the gel thereby increasing the concentration of reactants. The growth of large single crystals generally was not observed in their system. Other syntheses of large ZSM-5 crystals include reports by von Ballmoos and Meier [9, 10]. They indicated that ZSM-5 crystals ranging from 60 - 200)Jmin size were produced by using very high concentrations of TPA ions in the reaction mixtures. Crystals were found to form with a 113

114 (SY-7-3)

distribution of sizes. No mechanistic explanation was provided for this observation. The primary objective of this study was to grow large zeolite crystals by modifying the chemical composition of a reacting system and to determine which parameter was most critical to the growth of larger crystals. A second objective was to apply an optimization scheme to maximize the resulting size of the zeolite crystals using the fewest number of test runs. The gradient method was chosen to achieve this second objective, that is to identify the best starting chemical composition of the reacting batch mixture for maximizing crystal size. In this experimental design, concentrations of TPABr and NaOH were chosen as variables in a reaction mixture. Additional batch compositions were tested where NaOH was rep;aced with cat!ons other than sodium in order to delineate the role of both the Na and the OH in the crystallization process. EXPERIMENTAL 1. Material -- Experiments conducted in this laboratory have shown in general that for ZSM-5 synthesis, a reduction of the total aluminum content of the starting mixture will result in an enhancement in the size of the crystals produced. As it was the purpose of this project to synthesize large crystals, aluminum was de1ibrate1y omitted from the reaction mixtures tested (although no attempt was made to reduce the trace aluminum content of the starting reactants). In addition, as it is well recognized that reactant source will have a strong influence both on the type and morphology of zeolite derived by hydrothermal synthesis, the types and sources of the starting materials used in this study were maintained as constants. The following is a list of the reagents used. Ludox AS-40 (du Pont) was chosen as the silica source. Other reagents included a 50 wt% sodium hydroxide solution, reagent grade sodium citrate and potassium hydroxide pellets (Fisher Chemicals), tetrapropy1ammonium bromide (TPABr) and 99% cesium hydroxide solution (Aldrich Company). Design of Synthesis Program In general, zeolite synthesis has a large number of experimental variables such as time, temperature, reaction mixture composition, source of reactants, order of reactant mixing etc •• Each of these variables may affect the structure-type, purity and crystal size of the zeolite formed from the reaction. In order to gain insight into the key parameters which affect zeolite crystal size, it is necessary to minimize the number of these variables. In this study, reaction composition was chosen as the key variable. Time, temperature, zeolite structure-type, reactant source and order of reactant mixing were held as constants. As stated previously, reactant system tested was that in which aluminum delibrately was excluded from the reacting mixture. The only zeolite phase produced in our research program was silica1ite. For this experimental design, the number of composition variables was limited to three; namely, the tetrapropylammonium bromide, the sodium hydroxide and water. In the initial experiments, the optimum crystal size was found by varying TPABr and NaOH concentrations while maintaining water addition as a constant. When the optimal TPABr and NaOH concentrations were determined, the water content of the system was varied to further optimize crystal size. A 2x2 factorial with center point optimization design was adopted for this study [111. Concentrations of alkaline (Na20) and the temp1ating agent (TPABr) were chosen as independent variables. The amounts of water and silicate were kept constant for all the experimental runs. Typical batch compositions for a set of experimental run are shown in Figure 1. Upon making the runs indicated in Figure 1, the following mathematical model was used to relate product crystal size to the control or key variables of the reaction batch [11]: (1)

In this equation, y is the observed crystal size, and x2 represent the normalized values of the key variables and £ is the random error. The calculated values of and b2 indicated the direction of component change that will result an improvement in the product size. Based on these calculated coefficients, a new set

D.T. Hayhurst and J.Co Lee of experimental test compositions was established. Testing was stopped improvement of crystal size was found with compositional changes or crystalline product was observed for the set crystallization time.

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(11,10)

115

when no when no

(14,12)

Xl

(TPA Br)

o

(8,8)

o

Fig. 1. Experimental design of 2 x 2 experiments using lONaz O' 100Si2 O' 2490Hz O' 11TPABr as the starting batch compos!tion. Experimental Protocol For each of the batch compositions tested, the following experimental procedure was used. First, a calculated amount of sodium hydroxide was weighed into a tarred plastic beaker. To this, tetrapropylammonium bromide was added, followed by the rapid addition of approximately 20 grams of water. The reactants were mixed to produce a solution of the NaOH and TPABr. The remaining water was then used to wash the reactants from the stirring rod into the mixing beaker. The final step was the addition of the colloidal silica, Ludox. This was done with mixing. The mixture usually formed a uniform clear solution, although for some compositions with a high concentration of sodium hydroxide, a viscous opaque mixture was found to form. The starting solution was poured into five 15-ml teflon-lined Morey-type reaction vessels. Each of the vessels was sealed and immediately placed into a forced convection oven preset to 18Soc. Upon completion of the reaction (one week for all runs), vessels were removed and immediately quenched under cold tap water. The product crystals were washed five times with SOml of distilled water. The products were then characterized for size by both optical and scanning electron microscopy. RESULTS AND DISCUSSION As a starting point for applying the 2x2 optimization scheme, a batch composition known to yield only silicalite was chosen. The mole oxide formula of the starting reacting batch is shown below. 10Naz 0 '100Si 20 '2490Hz 0 '11TPABr The design of the first 2x2 factorial experiment is shown in Figure 1. Five experimental batch compositions were prepared. The crystal dimensions of the final products was used to determine values of b l and b 2for Equation 1. From the equation, a second set of experimental batch compos1tions were extrapolated. This procedure was followed for several iterations, with each iteration producing successively

116 (SY-7-3) larger silicalite crystals. Experiments were terminated when either the product crystals were no longer larger than those produced in previous experiments or no silicalite crystals were produced within the pre-set one-week synthesis time period. Table 1 is a summary of data taken for this first series of experiments. Table 1. Series # 100 1Z0 130 150 160

Compositions of experimental batches (oxide ratio)

SiZO

HZO

NaZO

TPABr

100 100 100 100 100

Z491 2491 2490 2491 2491

1.07 1.19 1.18 0.97 0.97

4.04 3.58 4.47 4.49 3.58

Crystal length

()l

m)

85-90 85 65-70 85-100 110-120

To insure that the size obtained from the first set of experiments was truly optimal, a second starting batch composition was chosen for a new sequence of 2 x 2 factorial runs. For this second set of experiments, the starting composition of the reactant mixture is listed below. 11. 5Naz 0 '100Siz 0 '2490Hz 0 'I. 5TPABr The optimum batch composition determined for this second series of tests was identical to that arrived at using the first batch composition. That is, the amounts of TPABr and NaOH found to be optimum from the first set of experiments indicated a true maximum for the test system. A plot of the range of crystal size obtained from all of the experiments performed in this study is shown in Figure 2. As is shown in this figure, the maximum crystal sizes occurred in a relatively narrow compositional range. In general, it was observed that lesser amounts of both NaOH and TPABr resulted in the formation of large crystals. As TPA ions are considered as structure directing agent for the ordering the tetrahedra units during zeolite formation [12], a lower concentration of TPA appears to lower the total number of possible nuclei that may form. This lower nuclei population favors the formation of fewer crystals, resembling the A type mechanism as proposed by Derouane et al. [12]. Fewer nuclei presented in the system leads to the formation of larger crystals because of the limited growth opportunities for the silica nutrient. There appears to exist a minimum TPA concentration required for silicalite crystallization. In addition to the effects of lower TPABr, a lower NaOH concentration is considered to control the dissolution of the colloidal silica source; thereby controlling the availability of the silica required for the crystal growth. The limited nutrient will be consumed preferentially in the growth of the existing crystals rather than in the production of new nuclei. A second manifestation of this limited secondary nucleation is the observation of narrow crystal size distribution. Borade et al. [13] has indicated that larger ZSM-5 crystals form when both the rates of nucleation and crystallization decrease. In this research, the nucleation was considered to occur separately from crystallization; therefore, nucleation may be limited by the limited concentration of TPA ions. The growth of larger crystals is further enhanced by limiting the solubility of silica as a result of limiting the addition of the alkaline component, NaOR. With the 2x2 factorial approach, the amounts of TPABr and NaOR were optimized at values of TPABr equals to 5.22 and Na20 equals to 2.25. The effect of water addition was also studied. Using optimal amounts of TPA and NaOR, the quantity of water in the reactant mixtures was varied. In Figure 3 results are shown for crystal size variations due to changes in the water addition. These changes were found to be quite large. A small composition difference was shown to dramatically alter crystal size of the product. Consequently, crystals ranging in length from 100)lm to 400)lm were observed in batch mixtures having small changes in water composition. In all the experimental runs, crystals were observed to form preferentially on the teflon-lined wall of the autoclave. Some larger crystals were found to adhere to the vessel wall. These crystals showed a high degree of tWinning. Crystal aggregates

D.T. Hayhurst and J.C. Lee

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Crystal size for each experimental composition tested.

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Fig. 3. Variation of crystal size with water content for the 2.25Na20·lOOSi20·5.22TPABr starting batch composition.

117

118 (SY-7-3) up to 300 in diameter frequently were observed. This type of growth implied that the teflon liners were not inert in these synthesis reactions but rather acted as active sites for heterogeneous nucleation. In these same experimental runs, the largest crystals were found to be suspended in a gel-like bubble in the reaction solution. These crystals were considered to be formed by a homogeneous nucleation mechanism. Although the size of these suspended crystals was larger and crystals were more uniform than those which formed at the wall, the product yield of these suspended crystals was found to be low. Studies were also conducted using KOR and CsOR to replace the NaOR in the reaction mixture. Gabelica et al.[14] have reported cations larger than Na+ will have a structure-breaking effect during the growth of a zeolite crystal. This would suggest that larger crystal may result in a system which contains larger cations as fewer nuclei would be formed in the system. To test the validity of this hypothesis, KOR and CsOR were substituted for NaOR in several reaction batch mixtures. Results of these substitutions are listed in Table 2. For several experimental runs, crystals of approximately 200~m were produced, indicating that it is possible to produce silicalite in systems containing the large cesium or potassium cations. Although experiments conducted with sodium hydroxide produced crystals of up to 400~m in length, the size of the silicalite crystals was substantially reduced in these non-sodium systems. In addition, the product yields were found to increase substantially when KOR and CsOR were used to replace NaOR. No gel phase was found in the final product of the KOR and CsOR runs; while for the sodium systems a gel phase often existed in the final product. Further experiments are underway to optimize the crystal size in the CsOR and KOR systems. Table 2. Starting batch compositions for runs where NaOR was replaced by either CsOR or KOR Si 20 2529 2470 3008 2786 2845 2801

100 100 100 100 100 100

Alkaline oxide

TPABr

2. 04K2 0 2.16K2O 3. 37K2 0 2.28Cs 2O 2.40Cs20 2.62Cs 2O

4.44 4.47 5.24 5.42 5.67 5.43

Crystal length 220 160 110 250 80 90

x 30 x 60 x 40 x 60 x 20 x 20

The final phase of this research program focused on producing large silicalite crystals by eliminating base from the reaction mixture. For these experiments, the sodium hydroxide in the starting batch composition was replaced by a variety of sodium salts, including sodium citrate, ethylenedinitrilo tetraacetic acid tetrasodium salt and sodium chloride. Listed in Table 3 are the starting batch formulation for these hydroxide-free runs. Table 3. Run 1

Salts Na citrate

2

3 4 5

6

7

8

Na4(E?,TA)

9

10 11 12 13

NaCI

Reactant compositions for runs with salt addition Si 20

R20

Na+

TPABr

100 100 100 100 100 100 100 100 100 100 100 100 100

2787 2797 2794 2705 2798 2766 2808 2850 2789 2844 2773 2785 2672

21 25 27 5 21 10 39 3 5 19 44 19 2

5.4 5.8 5.7 5.7 5.5 5.5 5.6 5.5 5.4 5.7 5.5 5.4 5.4

Crystal length 150 150 180 35 150 70 100 40 50 60 10-150 10-100 10-50

D.T. Hayhurst and

J.e.

Lee

119

For these sodium salts runs, the pH of the initial reaction batch mixture was found to range from 8.0 to 8.5. The pH measured for the final product solution was approximately the same. For this study, the concentration of each of these salts was varied. The effect of this variation on crystal size is shown in Figure 4. As the concentration of the salt was increased, the crystal size of the product was found to increase proportionally. A maximum crystal length of 180 ~ was observed for the Na citrate system. Results observed for salt addition are in contrast to those found for the sodium hydroxide system. For experiments where sodium hydroxide was used as a reactant, lower concentrations of NaOH was found to favor the formation of larger crystals. For the salt experiments, higher sodium concentrations favored the formation of larger crystals. As it is impossible to increase sodium addition for the sodium hydroxide system without increasing the pH of the reaction mixture, the low sodium hydroxide value arrived at by using the 2x2 factorial technique indicates a need to maintain a relatively neutral pH in the reaction mixture in order to favor large crystal growth. In the NaOH system, an optimal sodium concentration may not have achieved. The role of the anion during crystal growth may be equally important. Data measured to date, however, is too limited to draw any conclusions about a possible role played by the anion during the growth process.

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50

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20

30

40

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produced in the systems with and sodium chloride ~.

CONCLUSION In this research, the 2x2 factorial method is introduced for optimizing the growth of large silicalite crystals by varying a set of synthesis parameters. Crystals grown to date have length up to 400~ with little or no twinning being observed by scanning electron-microscopy (see Figure 5). It was determined that these large crystals form from reaction mixture where both NaOH and TPABr additions were limited, although yields were also found to be limited. Product yields were increased substantially by replacing NaOH by either KOH or CsOH, however, the final crystal length observed in these systems was reduced by more than a factor of two. Large silicalite crystals also were observed to form in systems where hydroxide was replaced by neutral salt anions. For these runs, crystal size was found to increase with increasing sodium concentration. The mechanism for zeolite crystallization and the role of the anion in these salt systems remains a question.

120 (SY-7-3) The results of this research indicate that zeolite crystallization is a complex process. We have demonstrated an approach to the design of a synthesis experiment which can be used to optimize a single parameter such as crystal size. This technique is quite powerful and should be equally effective in maximizing (or minimizing) other experimental parameter such as reaction time, temperature or reactant composition in a wide variety of synthesis systems.

Fig. 5. Scanning electron microgasph of a crystal grown in the 2.25Na20·100Si20·2832~0·5.22TPABr system. REFERENCE 1. R. J. Arganer and G. R. Landolt, U.S. 3702886. 2. D. M. Bibby, N. B. Milestone, and L. P. Aldrige, Nature,285, 30(1980). 3. R. Mostowicz and L. B. Sand, ZEOLITES, 2, 143(1982). 4. A Erdem and L. B. Sand, J. Catal., 60, 241(1979). 5. E. M Flanigan, J. M. Bennett, R. W. Grose, J. P. Cohen, R. L. Patton, and R. M. Kircner, Nature, 271, 512(1978). Chen, and I. Wang, J. Chem. Soc., Faraday 6. K. Y. Chao, T. C. Tsai, ~S. Trans. 1., 77, 547(1981). 7. H. Nakamotoland H. Takahasi, Chem. Lett., 1739(1981). 8. R. Mostowicz and L. B. Sand, ZEOLITES, 3, 219(1983). 289, 782(1981). 9. R. von Ballmoos and W. M. Meier, Nature~ 10. R. von Ballmoos, R. Busber, and W. M. Meier;- Proceedings of the Sixth International Zeolite Conference, 1983, p. 803. 11. J. S. Hunter, Chemical Engineering Progress Symposium Series, 56, 10(1960). -12. E. G. Derouane, S. Detremmerie, Z. Gabelica, and N. Blom, Applied Catalysis, 1, 201(1981). 13. R. B. Borade, A. J.-Chandradkar, S. B. Kulkarni, and P. Ratnasamy, Indian Journal of Technology, 21, 358(1983). 14. S. Gabelica, N. Blom, and E. G:-Derouane, Applied Catalysis, 1, 227(1983). 15. D. T. Hayhurst and J. C. Lee, Chem. Lett., January, 1986.