Effect of Si/Al ratio of high-silica HZSM-5 catalysts on the prins condensation of isobutylene and formaldehyde to isoprene.

A series of HZSM-5 catalysts with different Si/Al ratio were prepared and characterized with various characteristic methods XRD, BET, NH3-TPD, FT-IR spectra of adsorbed pyridine, etc. The effect of Si/Al ratio of HZSM-5 catalysts on the prins condensation of isobutylene and formaldehyde to isoprene was investigated in detail. It is resulted that the catalytic activities are dependent on the acid strength, decreasing with the ratio of Si/Al increases. The optimized catalyst was HZSM-5 (600) with 8.8% of isobutene conversion and 90.2% of selectivity to isoprene, indicating that the higher Si/Al ratio was termed as appropriate for prins condensation of isobutylene and formaldehyde to isoprene. On the contrary, the low Si/Al ratio may lead to the formation of by-products and coke deposits, which leads the catalysts deactivation.