Hierarchical SAPO-5 catalysts active in acid-catalyzed reactions

Crystalline silicoaluminophosphate (SAPO) of the AFI framework type with a bimodal pore system and high silicon content was hydrothermally synthesized using a soft template with a silicon-containing head group and cyclohexylamine to maintain microporosity. The structural and catalytic properties of the hierarchical structure were extensively characterized and compared to those of microporous SAPO-5 and H-ZSM-5. Temperature-programmed desorption of n-propylamine, isomerization of 2-methyl-2-pentene, and monomolecular cracking of propane indicated a large number of Brønsted acid sites and their high reactivity, similar to those of microporous H-SAPO-5. The catalytic activity of the hierarchical H-SAPO-5 was much higher in space-demanding alkylation of benzene with benzyl alcohol. At 353 K and under autogenous pressure, the conversion of benzyl alcohol over the hierarchical sample reached 98%, whereas the conversion over the microporous H-SAPO-5 was around 66% and over H-ZSM-5 below 10%. In contrast to H-ZSM-5, the external surfaces of H-SAPO-5 and H-SAPO-5M contribute to the alkylation reaction. Other procedures to yield mesoporous SAPO, such as hard-templating with carbon, soft-templating with amphiphilic surfactants, and phosphorus- or aluminum-containing molecules, resulted in impure, amorphous, non-mesoporous, or much less acidic materials.