Jointly published by Elsevier Science B.V., Amsterdam and Akaddmiai Kiado, Budapest

React.Kinet.Catal.Lett. Vol. 59, No. 2, 247-251

(1996)

RKCL3006

SYNTHESIS AND CHARACTERIZATION OF SnOx/AizO3 DERIVED GEL CATALYSTS

Ricardo Gomez*, Jaime Sanchez, Rebeca Silva and Tessy Lopez Uinversidad Autonoma Metropolitana-Iztapalapa

Department of Chemistry, POB 55-534, Mexico 09340 D.F. Mexico

Received April 30, 1996 Accepted September 3, 1996

Abstract SnOx/Al203 catalysts were prepared by the sol-gel method using as starting reactants alumim'um-tri-s-butoxide and (i) tetrabutylfin, (ii) fin tetractdoride and (iii) fin tetra-t-amyloxide. The gel derived catalysts show acidifies of 0.72, 1.08 and 1.05 pmol NH3/m 2 and BET areas of 91, 112 and 189 m2/g. The activity and selectivity pattern for isopropanol decomposition were found to depend strongly on the fin precursor used.

Keywords: Tin-alumina support, sol-gel derived alumina, isopropanol dehydration, fin-alumina acidity

INTRODUCTION

Aluminium oxide is the industrial support used in naphtha reforming catalysts since 50 years. Naphtha reforming catalysts require simultaneously metal and acid functions and a fine equilibrium between them is maintained. Concerning the alumina support, few modifications are reported. One of them is the SnOx/A1203 modified-support obtained by impregnation of alumina with tfi-~ tetrachloride. By subsequent impregnation with hexachloroplatimc acid Pt- Sn/AI203 reforming catalysts are obtained [1-3]. The effect of SnOx on alumina properties is not well established, however, the formation of tin aluminates is postulated [4]. In order to magnify the SnO• alumina interaction, in recent papers

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248 GOMEZ et al.: DERIVED GEL CATALYSTS

the synthesis of SnOx/AlaO3 catalysts by the sol-gel method has been reported [5- 7]. Important modifications in the selectivity for n-hexane and n-heptane reactions were observed. Low yield in benzene and high resistivity to deactivation was shown [5]. The goal of the present study is to determine the extent of alumina modifications occurring on SnOx/Al203 sol-gel preparations. The samples were prepared by co-gelation of aluminium alkoxide with various tin precursors and then compared with impregnated alumina.

EXPERIMENTAL

Derived gel catalysts were prepared as follows: 49.5 mL of aluminium-tri-s- butoxide (ATB) in 100 mL of ethanol were refluxed for 10 rain under constant stirring. To the ATB solution was added a solution containing 2.35 mL water, 20 mL of n-butanol and the precursors: 1) tetrabutyltin (0.33 mL), 2) tin tetrachloride (0.118 mL) or 3) tin tetra-t-amyloxide (0,43 mL). The refluxing solution was maintained under stirring until the gel was formed. The impregnated alumina was prepared by impregnating alumina prepared by ATB hydrolysis (blank) with aqueous n-butanol solutions containing the tin precursors used for the gel catalysts. The gel or impregnated catalysts were dried and calcined at 500~ in air for 2 h.

For identification the catalysts were labeled as a function of the tin precursor a n d preparation method as SnOx/AlzO3DG for the gels derived and as SnOx/Al203I for impregnated alumina. The BET specific area of the catalysts was determined in an automated Micromeritics apparatus. The acidity was determined by NH3 adsorption at 200~ in a dynamic system coupled to a conductivity detector. Activity tests were done in a flow microreactor coupled to a gas chromatograph. The reactants were fed through a saturator system using nitrogen as carrier. For isopropanol decomposition the reaction temperature was 200~

RESULTS AND DISCUSSION

In Table 1 the specific BET area as well as the pore size of the various catalysts are reported. Comparing with the alumina blank, strong modification of the gel catalysts derived is observed. In general the BET area and the pore size diameter are lower. The lowest BET area of the SnO• derived gels must have originated from the modification of the hydrolysis/condensation rate of ATB in presence of tin compounds. Aluminium alkoxide hydrolysis is very sensitive to water and solvent effects and of course to tin precursors and ATB cogelation.

GOIvIEZ et al.: DERIVED GEL CATALYSTS 249

The effect of the preparation method on the textural properties of the SnOx/Al203 catalysts can also be seen in the strong modifications of total acidity (Table 1). When the catalysts were prepared by impregnation, an increase of acidity is observed. Impregnation of alumina with tin precursors leads to the formation of great amounts of tin oxide which can act as acid centers [9]. In derived gel catalysts, the acidity is drastically diminished. During the co-gelation of aluminium alkoxide and tin precursors the interaction between alumina and tin oxides must be magnified. The nature of the tin-aluminate formed is not possible to determine by any type of spectroscopy, since the tin content (close to industrial preparation 0.3 wt.%) is so low that the sensitivity of physical methods like XRD, MOssbauer, XPS, EDS [10], etc., is not sufficient.

When tin tetra-t-amyloxide is used for co-gelation, the fast tin alkoxide hydrolysis leads to the formation of catalysts similar to those prepared by impregnation in which small modifications of the acidity are observed, 0.99 and 1.05 lamol NH3/m 2 for the alumina blank and SnOx/Al203 derived gel preparation, respectively. However, when tin tetraehloride is used in tbe co- gelation a slight modification of acidity is observed and such diminution occurs in spite of the chlorine effect. The role of chlorine is to increase the acidity of supports; in the present preparation such an effect is not observed. On the contrary, the acidity is diminished from 0.99 for the blank to 0.72 I,tmol NHB/m 2 for the tetrabutyltin preparation

Table 1

Specific BET area, pore size diameter and total acidity of derived gels and impregnated SnOx/Al203 catalysts

Catalysts" BET area Pore diameter Acidity Acidity (m2/g) (A) (I.tmol NH3/g) (wnNHdm 2)

SnOx/A1203DG1 91 70 66 0.72 SnOx/A1203DG2 112 56 122 1.08 SnOx/AI203DG3 189 50 195 1.05 SnOx/A1203I 1 229 64 314 1.37 SnOx/AI20312 235 54 262 1.11 SnOx/A120313 217 72 355 1.63 A1203 blank 256 108 254 0.99

~tin precursor: 1) tetrabutyltin, 2) tin tetrachloride, 3) fin tetra-t-amyloxide

250 GOMEZ et al.: DERIVED GEL CATALYSTS

Table 2

Isopropanol decomposition on gel derived and impregnated SnOx/A1203 catalysts

Catalysts a Specific rate Selectivity (mol%) (10 6 mol/g s) Propene Acetone Di-i-propyl ether

SnO• 0.09 55 45 - SnOx/A1203DG2 0.65 95 5 SnO• 0.30 91 9 - SnOx/Al20311 2.33 83 0.2 16.8 SnOx/A120312 1.80 83 0.3 16.7 SnOx/AlaO313 4.02 85 0.2 14.8 A1203 blank 4.24 72 1.0 27.0

atin precursor: I) tetrabutyltin, 2) tin tetrachloride, 3) tin tetra-t-amyloxide

The strong variations of acidity observed for gel derived catalysts are confirmed by the activity determined in isopropanol decomposition (Table 2). The results show that higher reaction rate corresponds to the catalysts showing higher acidity (impregnated ones). As expected, low activity is obtained for catalysts showing low acidity (tetrabutyltin derived gel preparation). When the activity of catalysts is determined in isopropanol decomposition, important effects in the selectivity pattern are observed. In Table 2, it can be seen that for impregnated alumina the activity for isopropanol decomposition is only slightly modified with respect to the alumina blank. Moreover, the selectivity pattern is close to the reference support. However, when the catalysts are prepared by co- gelation, the rate is smaller and the selectivity pattern is strongly modified. Using tin tetra-t-amyloxide, tin tetrachloride or tetrabutyltin the formation of isopropyl ether is inhibited. Such modifications confirm that in gel derived preparations the acid. site distribution is strongly affected. The most important variation in acidity is observed using tetrabutyltin. Table 2 shows that in such preparation the acid site distribution (no isopropyl ether formation) and additionally their nature is strongly affected. The high selectivity to acetone (44 %) suggests the formation of basic sites [11-13]. The extent of tin effect in the textural and structural alumina properties depends strongly on the tin precursor used and on the method to incorporate it into the alumina. The greater modifications are observed in the SnOx/Al203 derived gel using tetrabutyltin as precursor and we can assume than in such preparation the SnOx-alumina interaction is stronger. We will focus our attention on this catalyst. Such interaction can occur by two ways; i) a high incorporation of SnOx into the alumina network, ii) the SnO• is highly dispersed on the alumina support. Recently a detailed STEM-EDX (energy-dispersive X- ray microanalysis) study of Pt-Sn/AI203 sol-gel catalysts has been reported [10].

GOMEZ et al.: DERIVED GEL CATALYSTS 251

It is shown that the incorporation of tin during alumina gelation is so extensive that it was not possible to detect it. To discriminate between the two possibilities of SnOx_ alumina interaction proposed before, the selectivity pattern of isopropanol dehydration is helpful. In pure SnOx oxide the main product during the dehydration of 4-methyl-2-pentanol is the dehydrogenated molecule [4]. On the SnO• catalyst the main product formed by dehydrogenation of isopropanol is similar to that on pure SnO2. This prompted us to propose that the SnOx oxides are found as very highly dispersed conglomerates on the alumina surface. The dispersity SnOx_ alumina interaction leads to a strong effect on Pt- Sn/A1203 sol-gel catalysts [8]. The present results show that it is possible to prepare highly dispersed SnOx/AlzO3 catalysts, which can be useful for the preparation of highly selective Pt-Sn/AIzO3 reforming catalysts.

Almowledgements. We are indebted to CONACYT for the support given.

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