Skeletal Isomerization of n-Butenes to Isolbutylene on Ferrierite and H-ZSM-5 Catalysts

摘要

Four ferrierites and two H-ZSM-5 zeolites were tested in the skeletal isomerization of n-butenes to isobutylene at temperatures from 573 to 773 K. At the same reaction condition the H-ZSM-5 zelites showed a higher conversion of n-butenes and a lower selectivity to isobutylene thant he ferrierites. Even at comparable conversion the selectivity was lower for the H-ZSM-5 zeolites. Wioth increasing weight hourly space velocity (decreasing residence time) increasing selectivity to isobutylene and in some cases also increasing yield of isobutylene was observed both on ferrieites and H-ZSM-5 zeolites. Therefore the monomolecular mechanism is proposed for the siomerization of n-butenes to isobutylene on both zeolite types. By-product formation takes place in consecutive reactions by cracking of dimers formed from isobutylene. Using a H-ZSM-5 zeolite with a high Si/Al ratio, decreasing conversion of n-butenes and increasing yield of isobutylene was observed with increasing temperature. Furthermore, a selectivity to isobutylene of 87 percent could be reached with this zeolite. This catalytic behaviour is seen as a further evidence for the proposed scheme of consecutive reactions. Since TPAD emasurements indicated a higher Broensted acidity for the ferrierites than for the H-ZSM-5 zeolites, it was concluded that the pore size and pore structure affect the catalytic behaviour more strongly than the acidity.