Specific catalytic activity at the HZSM-5 surface for the polymerization of tetrahydrofuran

Abstract

The polymerization of tetrahydrofuran has been investigated by using medium- and large-pore zeolites as catalysts. HZSM-5 (SiO 2/Al 2O 3 = 70; small crystals) shows distinctly the highest activity. Under the same conditions, medium pore zeolite HZSM-23 (SiO 2/Al 2O 3 = 110; small crystals) and large-pore zeolites Hbeta, (SiO 2/Al 2O 3 = 37; small crystls), HMCM-22 (SiO 2/Al 2O 3 = 23; small crystals) and HECR-1 (SiO 2/Al 2O 3 = 8; small crystals) are far less active while Ultrastable HY (SiO 2/Al 2O 3 = 13; medium size crystals) is inactive. In addition, the activities of HZSM-5 (SiO 2/Al 2O 3 = 70; small crystals) and zeolite Hbeta (SiO 2/Al 2O 3 = 37; small crystals) are greatly reduced when they are surface deactivated by tetraphenylphosphonium bromide. Large crystal HZSM-5 (SiO 2/Al 2O 3 = 70; about 100 times larger than small crystals) appears inactive. 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-ClC 6H 5NN +PF 6 − is a superior catalyst for the polymerization of tetrahydrofuran in homogeneous systems, resulting in a high degree of polymerization. We have shown that under the same experimental conditions, the degree of polymerization attained by using HZSM-5 (small crystals) as the catalyst is higher than that achieved using p-ClC 6H 5NN +PF 6 −. We infer that the reason for this higher degree of polymerization is that the anion in the zeolite framework offers even higher protection to the propagating cation than the already superior anion PF 6 − in homogeneous systems. For the same reason, far fewer occurrences of chain transfer take place during the polymerization with the HZSM-5 system than with the p-ClC 6H 5NN +PF 6 − system when trimethyl orthoformate is used as a chain transfer agent. Based on the observations, we have proposed that zeolite-catalyzed polymerization of tetrahydrofuran takes place at the external surfaces and HZSM-5 has a surface geometry which is particularly favorable for the propagation of tetrahydrofuran polymerization.