SILICOALUMINOPHOSPHATE-5 MOLECULAR SIEVE: CHARACTERIZATION AND CATALYTIC ACTIVITY FOR ALKYLAROMATIC REACTIONS

Abstract

Highly crystalline silicoaluminophosphate-5 (SAPO-5) was prepared according to a Union Carbide patent. This solid material was characterized by a number of physical techniques and used as a catalyst for several alkylaromatic interconversion reactions. Information about the reaction mechanisms and the active sites was provided by kinetic studies involving tracer molecules labeled with \sp{13}C and D isotopes. The bulk composition of SAPO-5 was shown by neutron activation and atomic adsorption to be (Si\sb{0.10}Al\sb{0.56}P\sb{0.34})O\sb2. A fresh calcined sample had a BET surface area of 235 {\rm m}\sp2\over{\rm g}. X-ray analysis indicated that the structure of the catalyst was not affected by coking/regeneration cycles. The catalyst contained 1.3 \sp{\*}\ 10\sp{14} {\rm H\ atoms}\over{\rm cm\sp2} that could be readily exchanged. Xylene isomerization was the primary reaction used to test the activity of SAPO-5 in a CSTR. While the three xylene isomers were the principle products observed in the temperature range 300-400\sp\circC, some disproportionation products were also observed. Xylene isomerization followed first order kinetics with observed activation energies for xylene interconversion in the range of 21-24 kcalmole{\rm kcal}\over{\rm mole}. The disproportionation reactions follow reversible second order kinetics with an observed activation energy of 8.5 kcalmole{\rm kcal}\over{\rm mole}. The xylene isomerization reactions are primarily intramolecular. More than 90% of the m-xylene formed initially from mixtures of o-xylene and o-xylene labeled with \sp{13}C in the methyl groups was either completely unlabeled or labeled. However, at higher conversion where disproportionation becomes significant, intermolecular scrambling increases. While aryl hydrogens are extremely labile under these conditions, the hydrogens in the methyl groups are relatively inert. Even at short contact times, the ring hydrogens of mixtures of perdeuterated and unlabeled o-xylene exchanged to statistical equilibrium, whereas the methyl groups remained either fully labeled or unlabeled. It was shown, however, that during isomerization one H (or D) atom was exchanged on the average in the migrating methyl group. The activities of other acidic oxide catalyst (amorphous silica-alumina, ZSM-5, and AlPO-5) were compared with SAPO-5. It was concluded that all exhibit similar types of catalytic behavior with the rates varying over several orders of magnitude and ZSM-5 being the most active. (Abstract shortened with permission of author.

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