Study on the catalytic properties of aluminophosphate type molecular sieves

Abstract

Aluminophosphate type molecular sieves including ALPO-5, SAPO-5 and VAPO-5 were synthesized by hydrothermal crystallization technique and characterization of SAPO-5 and VAPO-5 catalysts was carried out. The crystallization rate of ALPO-5 showed sigmoid form and the length/width ratio of ALPO-5 crystals increased as the water content of initial gel increased. SAPO-5 showed much higher cation exchange capacity and catalytic activity for the acid catalyzed reactions such as isomerization, alkylation and disproportionation than the ALPO-5. Although the cation exchange capacity of SAPO-5 increased with the silicon content of SAPO-5(up to about 1 si/U.C.), the SAPO-5 having about 1 to 3 si/U.C. showed nearly constant cation exchange capacity. The EPR spectrum of cupric ion in Cu-SAPO-5 indicated that all cation sites could be equivalent. The EPR spectrum of sulfur dioxide anion radical formed on H-SAPO-5 suggested that the adsorption sites for the radical were also equivalent. These equivalences of cation or adsorption sites are probably due to the simple framework structure of SAPO-5. Most of vanadium in as-synthesized VAPO-5 was V(IV) state and was oxidized to V(V) by calcination in air. The vanadium in VAPO-5 seemed to be distributed in highly dispersed state and showed reversible redox properties changing it``s oxidation state easily between V(IV) and V(V). As the vanadium content of VAPO-5 increased, the weight loss in TGA due to the template molecules (tri-n-propylamine) shifted to higher temperature, suggesting some interaction between VAPO-5 and template molecules. This interaction, probably due to the acidity of VAPO-5 favors the interpretation that V(IV) was incorporated into the hypothetical P(V) site rather than Al (III) site in VAPO-5. TGA results and the facts that the unit cell dimensions were changed by incorporating vanadium into ALPO-5 and the vanadium in VAPO-5 had tetrahedral coordination suggested that the vanadium was probably sited in The tetra...