Preparation of metallo-silicate solid catalysts by sol-gel method with regulation of activity and selectivity

PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 1995 Elsevier Science B.V.

421

P r e p a r a t i o n of metallo-silicate solid catalysts by sol-gel method with regulation of activity and selectivity I.M. Kolesnikov, A.V. Yuablonsky, S.I. Kolesnikov, A. Busenna, M.Y. Kiljanov State Academy of Oil and Gas named after Goubkin I.M., 117296 Moscow, Leninsky Prospekt 65, Russia The petroleum industry uses metallo-silicates as catalysts for cracking processes, alkylation of benzene with propylene and ethylene, and as adsorbents. Activity and selectivity of those catalysts increase with the help of zeolites with different structures and compositions. We worked out a method of synthesis based on the application of a thermal blow upon heating of sol or gel metallo-silicates (MS). The physical and catalytical properties of the solid MS depend on the pH of the sol and gel, time of syneresis, nature of the metal in the composition of the MS, acidity or basicity of the sol and gel, and time of initial coagulation of the sol into the gel.

I n f l u e n c e of pH of the sol on the t i m e of b e g i n n i n g o b v i o u s c o a g u l a t i o n of the sol We studied the properties of the sol under the influence of different pH values at T = 298 IL The experimental data are presented in figure 1.

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q

=,

120

qo

8

40

i

~a pH

Figure 1. Influence of pH on time of beginning obvious coagulation of sol into gel; a: alumino-silicate; b: zircono-silicate. It can be seen from Fig. I that the sol has a maximum rate of coagulation at pH values between 6 and 10. Similar results were obtained for the following

422 metallo-silicates: Be-, Mg-, Zn-, Sr-, Al-, Zr-, Fe-, Mn-, Co- and Ni-Si. For those MS, the pH regions with maximum rate of coagulations are:

MS

.pH

MS

pH

MS

pH

Be-Si Sr-Si Fe-Si Ni-Si

4.6-10.0 8.0-10.0 4.0-10.4 4.0-9.2

Mg-Si A1-Si Mn-Si

5.1-10.0 6.0-10.0 5.8-9.8

Zn-Si Zr-Si Co-Si

6.1-10.0 6.0-10.0 5.0-9.2

.....

The sol stability increases in the following sequence: Be-Si > Mg-Si > ZnSi, namely in the sequence of increasing ion radius: rBe2+ < rMg2+ > rZn2+ of decreasing ~ potential: ~Be-Si < ~Mg-Si < ~Zn-Si

The kinetic of changing hydrosol properties One of the steps in the catalysts preparation is the syneresis of sol and gel. During this step, several processes occur: creation and decomposition of the micelles, formation of polyhedral structure from micelle nuclei, dissolution of the small micelles and enlargement of the large ones, solvatation and change of the thickness of the solvate shell, and so on. The process passes at time and is characterized by the kinetics of syneresis. The rate of syneresis depends on the concentration of the initial salt solution, time of syneresis, temperature and nature of Me-Si. To investigate the role of the parameters mentioned above, we used colorimetry and the formula of Lambert-Beer: J = Jo exp (- e.c.1) where

J, Jo - intensity of passing a n d initial b e a m

of light

e - m o l a r coefficient of light absorption c - concentration I - thickness of the solution - sol or gel.

To study the law ruling the change of the properties, the sol and gel were prepared using a middle pH = 3.3 or pH = 10.7. We synthesized sols at pH = 3.3 and pH = 10.7. Those two types of sols were aged at T = 298 K for 18 h. The experimental data concerning the light transmission by sol or gel aluminosilicates and zircono-silicates are shown in Fig. 2 and Fig. 3.

423

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Figure 2. Dependence of light transmitted by alumino-silicate hydrosol-1 and hydrogel-2 vs. ln(l+~), z-time, rain.; a) pU 3.3; b) pH 10.7. 10U ~

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Figure 3. Dependence of light transmitted by zircono-silieate hydrosol-1 and hydrogen-2 vs. In (1+~), ~-time, rain.; a) pH = 3.3; b) pH = 10.7. CA12(S04)3 g/l: 1/20; 2/60; 3/80; 4/100; 5/120; 6/140; 7/150.

424 It can be seen t h a t the time at which obvious coagulation begins changes with the pH value. The dotted curve indicates the time of beginning of obvious coagulation of hydrosol into hydrogel. The type of curve depends on the pH value, but z-coag - varies according to the concentration of the A12(804)3 solution. Influence of the preparation conditions on the surface area

Figure 2 presents the results of changing properties of sol and gel upon action of syneresis time, and concentration of the salt into water. Using the method of thermic blow, we prepared a series of catalysts at determined times of sol or gel aging. The surface area of this series of samples was m e a s u r e d by adsorption methods. The data are presented in Table 1. Table 1. Specific surface areas. Catalyst Aging time (rain) alumino120 silicate 240 410 600 100 200 245 600 42

pH of hydrosol

CA12(SO4)3 g/] 20 20 20 20 140 140 140 140 10

3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 10.7

S m2/g 122 128 419 337 162 110 161 115 142

84

10.7

10

101

128 170

10.7 10.7

10 10

107 107

From this table, the following conclusions can be drawn: - increasing the pH of the sol from 3.3 to 10.7 decreases S; - the value of S passes through a m a x i m u m with increasing ~ aging (for pH = 3.3); - increasing the concentration of A12(804)3 solutions decreases S. Infrared spectra

The alumino-silicate catalysts with different compositions were examined by IR spectroscopy in the following spectral domains: valence vibration" 1000 - 1200 cm -1 for ~ Si-O-Si 800,877, 750 cm -1 for ~ Si-O-AI~ 960 cm -1 for ~ AI-O-H 700, 668 cm -1 for ~ Si-O-AI~ isolated type I

560, 550, 535 cm -1 for ~ Si-O-AI~ condensed type I

425 473, 450, 430 cm -1 for -~ Si-O- condensed type. The q u a n t i t y of (SiO4.AlO4)-tetrahedra of "isolated" type increases for one of the series of A1-Si and decreases for the other one. The activity of the catalysts of the first type are higher t h a n for the second one.

Activity of the catalysts synthesized from sol and gel The activity of the catalysts synthesized from sol or gel w i t h different times of aging was studied. The d a t a are presented in Fig. 4 for the solids obtained from hydrosols or hydrogels at pH = 3.3, by the method of t h e r m a l blow. It can be seen t h a t the activity of the alumino-silicate catalysts passes t h r o u g h a m a x i m u m . The catalyst with the m a x i m u m activity was synthesized at a syneresis time of 300 rain, namely at the border of the sol into gel transformation.

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Figure 4. Influence of aging time of hydrosol (1') and hydrogen (2') at pH = 3.3 for A1-Si catalysts on the conversion of cumene at 673 K (1), 723 K (2) and 773 K (3); CA12(S04)3 - 40 gfl. The conversion data obtained on the series of A1-Si synthesized from sols and gels at pH = 10.7 are presented in Fig. 5. The same results were obtained with zirconium silicates synthesized from sol a n d gel a n d for other metallo-silicate catalysts. This new m e t h o d of preparation allows to increase the catalytic activity by a factor of 1.8 - 2.0 in the cracking reaction of individual hydrocarbons. We studied the cracking of kerosene-gasoil fraction in the presence of synthetic alumino-silicates at T = 723 K and space velocity v=0.7 h. We used the catalysts points a, b, c in Fig. 4. The yields of benzene were 41.5, 44.6, 47.8 % mass., respectively and the coke yields were 2.6, 2.4 a n d 2.25 % mass., respectively.

426

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Figure 5. Dependence of cumene conversion in A1-Si catalysts synthesized a t pH = 10.7 from sols (1') and gels (2') at 723 K (1) and 773 K (2).