Synthesis and characterization of gallosilicate MFI-type zeolites: effect of the gel composition on the crystallization rate

Synthesis and characterization of gallosilicate MFI-type zeolites: effect of the gel composition on the crystallization rate G. Giannetto, J. Papa, J. Perez, and L. Garcia Escuela de Ingenieria Quimica, Facultad de Ingenieria, Universidad Central de Venezuela, Caracas, Venezuela R. Monque INTEVEP S.A., Caracas, Venezuela Z. Gabelica Namur University, Dept. Chemistry, Lab. Catalysis, Namur, Belgium Gallosilicate MFI-type zeolites ([Ga]-ZSM-5) were synthesized at 170°C, with autogeneous pressure and agitation. Samples were characterized by 71Ga MAS n.m.r., XRD, i.r., X.p.s., and chemical analysis. The influence of synthesis parameters such as the SiO2/Ga203, (TPA)20/SiO2, and Na20/SiO2 gel molar ratios on the crystallization kinetics were evaluated. Pure and highly crystalline [Ga]-ZSM-5 are formed after 20-24 h heating, independently of the gel composition, provided that traces of the template agent (tetrapropylammonium ions, TPA +) are present. Such particular behavior is related to the presence of very stable monomer gallate ions in the alkaline synthesis media that are readily incorporated in the MFI framework structure, irrespective of low oligomeric hydroxy-AI species present in similar conditions. Keywords: Zeolite; MFI-type zeolite; gallosilicate; crystallization rate; synthesis gel composition

INTRODUCTION [Ga]-ZSM-5 zeolites have proved to be very active and selective catalysts in the transformation of lower alkanes toward aromatics.~-5 Besides the zeolite acid sites that catalyze various reaction steps such as cracking, oligomerization, and cyclization, highly dispersed gallium oxidic species were shown to catalyze the dehydrogenation of alkanes to alkenes and of olefins and cycloolefins intermediates, reactions that are, respectively, responsible for the overall increase of the reaction rate and of the selectivity toward aromatics. 5-8 The gallium oxidic species, of the GaxOy or GaO(OH) type, can be prepared by appropriate activation of Ga-exchanged, Ga-impregnated, or Ga203-10aded H-[AI]-ZSM-5 zeolites 1,2.5-8 or by thermal treatment of MFI-type gallosilicates under various conditions. ~-5'9-~ This latter procedure causes partial degalliation of the zeolite and results in the formation of very dispersed GaxOy oxidic species admixed with the parent zeolite, 5,9-~3 so that the use Address reprint requests to Prof. Giannetto at the Escuela de Ingenieria Quimica, Facultad de Ingenieria, Universidad Central de Venezuela, Apartado 47100, Caracas 1041A, Venezuela. Received 19 May 1993; revised 23 March 1994; accepted 31 March 1994

© 1994 Butterworth-Heinemann

of [Ga]-ZSM-5 as precursors appears very promising for the preparation of the final bifunctional catalyst. In this paper, we report the influence of different parameters such as SiO2/Ga203, (TPA)20/SiO2, and Na20/SiO2 gel molar ratios on the crystallization rate of [Ga]-ZSM-5 zeolites. Very crystalline and pure pentasihype gallosilicates were obtained from each of the starting gel molar compositions after 20-24 h of heating.

EXPERIMENTAL The various MFI zeolites were prepared following the procedure described by Argauer and Landol04 but changing the aluminum source by a gallium salt. Synthesis gels were prepared at room temperature, by mixing two solutions vigorously. Solution A of sodium silicate (density = 1.38 g/cc, 28.9% SiO2, SiO2/Na20 = 3.22, from Glasven) was poured into solution B containing a mixture of gallium nitrate, sodium chloride, sulfuric acid, tetrapropylammonium bromide (template agent), and water. The effect of the SiO2/Ga20~ gel molar ratio on the crystallization rate was studied by preparing three hydrogels with molar ratios of 41, 150, and 300. To study the influence of the (TPA)20/SiO2 molar ratio

ZEOLITES, 1994, Vol 14, September~October

549

Synthesis of gallosilicate MFI-type zeolites: G. Giannetto et al. •

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Figure 1 Crystallization curves for [Ga]-ZSM-5 zeolites from hydrogels with different SiO2/Ga203 molar ratios.

Figure 2 71Ga MAS n.m.r, spectra of [Ga]-ZSM-5 zeolites with different Si02/Ga203 molar ratios.

on the reaction rate, three additional gels involving a SiO2/Ga20:~ molar ratio of 41 were prepared. In these gels, the original (TPA)20 concentration was reduced to 25, 12.5, and 0%, respectively, keeping the Na20/ SiO2 ratio constant. The Na20/SiOz molar ratio was varied by reducing the NaCI concentration to 50 and 25% of the original value. In both cases, the SiOz/ GazOs molar ratio was 41. All the syntheses were carried out at 170°C, under autogeneous pressure (6-7 bar), using Teflon-lined stainless-steel autoclaves filled to 75% of their full capacity and placed inside a temperature-controlled oven with rocking agitation. The percentage of crystallization with time was followed by removing autoclaves from the oven at different times and quenching them to room temperature by immersion into a water bath. Once cooled, the content of each reactor was filtered and the solids were abundantly washed with demineralized water, then dried for 24 h, and, finally, left for another 24 h at room temperature inside a humidifier with 35% relative humidity. Solids were characterized by XRD, 7]Ga MAS n.m.r., FTi.r., SEM, X.p.s., and atomic absorption spectroscopy. The degree o f crystallinity was calculated using 15 (a) the intensity of the XRD (%CRx) peaks arising between 22-25 20(CuKoQ and (b) the intensity of the i.r. absorption band (%Ci.~.) at 550 cm -I, which characterizes the vibration of the distorted double ring of the MFI framework. In both cases, the relative crystallinity was evaluated by comparison with the most crystalline sample obtained in these syntheses. Detailed experimental conditions for XRD, FTi.r., X.p.s., and VIGa MAS n.m.r, have been described previously. ]6-19

However, the effect is not as strong as that observed for [AI]-ZSM-5. ir,.~(, In all cases, the maximum of crystallinity is reached after approximately 20-24 h of heating as supported by XRD and i.r. data (Table 1). Moreover, solid-state 71Ga MAS n.m.r. (Figure 2) shows that after 24 h heating only the 150-160 ppm n.m.r, line is detected, corresponding to Ga :~+ ions incorporated in the MFI framework structure. ]°'12"17 This confirms that the solids are pure gallosilicate ([Ga]-ZSM-5) zeolites and not silicalites admixed with highly dispersed gallium oxidic species. Additional support was obtained from XPS studies of crystalline [Ga]-ZSM-5 zeolites that show that the superficial Si/Ga ratio is almost the same as in the bulk (obtained by chemical analysis), l:~'l(~''9 Currently, the extraframework gallium species are deposited mainly on the outer surface of the zeolite crystals 16 or inside

RESULTS AND DISCUSSION Effect of the SiO2/Ga203 gel molar ratio Figure 1 shows the crystallinity changes with time for hydrogels with different SiO2/Ga203 molar ratios. One can notice that an increase in the SiO2/Ga203 gel molar ratio slightly reduces the induction period.

550

ZEOLITE& 1994, Vol 14, September~October

Table 1 Physicochemical properties of [Ga]-ZSM-5 zeolites obtained from gels with different SiO2/Ga203 ratios Zeolite 8 Crystallization time (h)

SiO2/Ga203

%CxR % Ci,r.

Gel

(170°C, agitated) SiO2/Ga203

1

2

3

4

30.5

14 25 43 100 95

100 94

2 6 12 24 72

41 41 41 41 41

36.9 35.3 27.8 32.0

0 0

2 6 24

72

150 150 150 150

120.4 119.9 105.3 110.2

108 112

11 36 93 100

0 35 100 -

2 6 24 72

300 300 300 300

200 195.0 179.0 -

190 183

18 59 89 100

15 85 95 100

a(1) By atomic absorption spectroscopy) 6 (2) By 71Ga MAS n.m.r. (see Ref. 17 for detailed experimental setup for quantitative measurements). (3) Crystallinity by XRD (Ref. 15). (4) Crystallinity by i.r. (Ref. 15).

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the zeolite channels, X:~d e p e n d i n g on the preparation method (ion exchange, impregnation, thermal treatment, etc.), which led to Si/Ga ratios lower or bigher, respectively, to the bulk ratio of the as-synthesized material. Finally, as would be expected,' l.X7 a variation of the SiO,>/Ga,)O3 molar ratio in the gel yields gallosilicate crystals involving a parallel variation of this ratio in the framework (synthesis efficiency close to 100% with respect to Ga :~+ incorporation) (Table 1).17.18 Little or no change in crystal size and morp h o l o g y with the initial SiO,_,/Ga,,O:~ ratio was observed (Figure 3), as expected from previous experiments. 15,. ~5,1

Effect of (TPA)20/SiO2 and Na20/SiO~ gel molar ratios Similar kinetic results were obtained by varying the (TPA)20/SiO2 (Figure 4) and Na,_,O/SiOL, (Figure 5) gel molar ratios. In each case, highly crystalline and pure [Ga]-ZSM-5 zeolites were obtained after 20-24 h of heating. Only gels free fi-om T P A B r do not yield any crystalline ZSM-5 phase. T h e fact that the crystallization rate does not depend on the composition of the gel could be explained by considering that gallium forms very stable monomeric gallate ions, even in weak alkaline media, '-'l which is not the case for aluminum. '-''-'''-':~These gallate ions will not transform readily into other types of Ga-hydroxy-oxy species, so that their incorporation I00 -

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into the crystallizing MFI structure is expected to be more straightforward than that of aluminum.'~"-'l T h e crystallization rate obtained in this work is almost six times slower thau that observed when s o d i u m trisilicate (Merck) is used as the silica source, l'j This difference is obviously related to the difference in tbe reactivity of the silica source, as ah'eady observed for [AI]-ZSM-5. 2°''-'5 CONCLUSIONS Highly pure and crystalline [Ga]-ZSM-5 zeolites were obtained after 20-24 h of heating hydrogels with variable molar ratios. Their crystallization was found to be quasi-independent on the gel chemical composiSiO,, tion and, more specifically, on tbe initial - molar Ga20:~ ratio in contrast to [AI]-ZSM-5 zeolites synthesized u n d e r similar conditions. This is explained by assuming that monomeric gallate ions, which readily appear in alkaline solution, are easily incorporated into MFI frameworks, irrespective of low nligomeric hydroxyA1 species that less readily react with silicate species as to yield the adequate (Si, AI) ready to condense and build up the first MFI nuclei (crystallites). Finally, u n d e r our conditions (170°C, autogeneous pressure), MFI does not crystallize in the absence of the TPA + templeting agent.

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ACKNOWLEDGEMENTS i

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T h e authors wish to thank INTEVEP S.A. for their support which made this research possible and for the permission to publish.

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REFERENCES 8

Figure 4 Crystallization curves for [Ga]-ZSM-5 zeolites from hydrogels with different (TPA)20/SiO2 m o l a r ratios (SiO2/Ga203 = 41).

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