Characterization of crystalline zirconium phosphates and their isomerization activities

Abstract

The catalytic activities for ring-opening isomerization of cyclopropane and isomerization of butenes have been examined on crystalline zirconium phosphates. ε-Zr(HPO 4) 2 catalyst was highly crystallized during the dehydration of zirconium phosphate gel with concentrated phosphoric acid solution under reduced pressure. This catalyst, which was evacuated at higher temperatures (above 770 K), exhibited higher catalytic activities (based on unit surface area) than α-Zr(HPO 4) 2 · H 2O catalyst or other conventional solid acid catalysts. The coisomerization of d 0− and d 8−-1-butene suggests that isomerization would proceed on protonic acid sites even after heat treatment at 1100 K. After evacuation at 773 K, most of phosphate groups were removed, with consequent loss of water, due to the condensation of phosphate groups between each zirconium atom layer. However, a trace amount of residual phosphate groups still remained on the surface. After heat treatment at higher temperatures, the stretching and bending vibration of POP appeared in infrared studies; their intensities increased with increasing temperatures of evacuation. Even though the protonic concentrations decreased, the reaction rates for isomerization were drastically enhanced, because of the presence of POP bonds which could withdraw the electrons from the residual phosphate groups on the surface. Thus some enhancement of acid strength of protons of phosphate groups may occur.