SPECIFIC CATALYTIC ACTIVITY AT THE HZSM-5 SURFACE FOR THE POLYMERIZATION OF TETRAHYDROFURAN

The polymerization of tetrahydrofuran has been investigated by using m edium- and large-pore zeolites as catalysts. HZSM-5 (SiO2/Al2O3 = 70; small crystals) shows distinctly the highest activity. Under the same conditions, medium pore zeolite HZSM-23 (SiO2/Al2O3 = 110; small cryst als) and large-pore zeolites Hbeta, (SiO2/Al2O3 = 37; small crystals), HMCM-22 (SiO2/Al2O3 = 23; small crystals) and HECR-1 (SiO2/Al2O3 = 8; small crystals) are far less active while Ultrastable HY (SiO2/Al2O3 = 13; medium size crystals) is inactive. In addition, the activities o f HZSM-5 (SiO2/Al2O3 = 70; small crystals) and zeolite Hbeta (SiO2/Al2 O3 = 37; small crystals) are greatly reduced when they are surface dea ctivated by tetraphenylphosphonium bromide. Large crystal HZSM-5 (SiO2 /Al2O3 = 70; about 100 times larger than small crystals) appears inact ive. In cationic polymerization, the protecting power of the companion anion contributes to the long life of the propagating cation, and in turn the high degree of polymerization. It is known that p-ClC6H5N=N+P F6- is a superior catalyst for the polymerization of tetrahydrofuran i n homogeneous systems, resulting in a high degree of polymerization. W e have shown that under the same experimental conditions, the degree o f polymerization attained by using HZSM-5 (small crystals) as the cata lyst is higher than that achieved using p-ClC6H5N=N+PF6-. We infer tha t the reason for this higher degree of polymerization is that the anio n in the zeolite framework offers even higher protection to the propag ating cation than the already superior anion PF6- in homogeneous syste ms. For the same reason, far fewer occurrences of chain transfer take place during the polymerization with the HZSM-5 system than with the p -ClC6H5N=N+PF6- system when trimethyl orthoformate is used as a chain transfer agent. Based on the observations, we have proposed that zeoli te-catalyzed polymerization of tetrahydrofuran takes place at the exte rnal surfaces and HZSM-5 has a surface geometry which is particularly favorable for the propagation of tetrahydrofuran polymerization.