Zeolite Catalyzed Alkylation of Biphenyl. Where Does Shape-Selective Catalysis Occur?

Abstract

Liquid-phase alkylation of biphenyl (BP) was studied over large-pore zeolites. Selective formation of the least bulky products, 4,4′-diisopropylbiphenyl (4,4′-DIPB) occurred only in the isopropylation of BP over one- or two-dimensional zeolites, H-mordenite (HM), ZSM-12, SSZ-24, SAPO-5, SSZ-31, and CIT-5. These shape-selective catalyses are ascribed to steric restriction of transition state and to easiness of the substrates to enter into the pores. HM gave the highest selectivity among them. The dealumination of HM enhanced catalytic activity and the selectivity for 4,4′-DIPB because of the decrease of coke deposition. Non-regioselective catalysis occurs on external acid sites over HM with the low SiO2/Al2O3 ratio because severe coke deposition deactivates the acid sites inside the pores by blocking pore openings. The selectivity of DIPB isomers was changed with propylene pressure and/or with reaction temperature. Selective formation of 4,4′-DIPB was observed at moderate temperatures such as 250 °C, whereas the decrease of the selectivity of 4,4′-DIPB occurred at higher temperatures as 300 °C. 4,4′-DIPB yielded selectively under high propylene pressure (<0.3 MPa) at 250 °C, while the selectivity of 4,4′-DIPB decreased under low propylene pressure as 0.2 MPa. However, 4,4′-DIPB was almost exclusive isomer in the encapsulated DIPB isomers inside the pores under every temperature and pressure. The decrease of the selectivity of 4,4′-DIPB is due to the isomerization of 4,4′-DIPB on the external acid sites. The deactivation of external acid sites of HM was examined by the modification with cerium and other rare earth metal oxide on HM. Selectivities of 4,4′-DIPB were improved over modified HM even at high temperatures because of the suppression of non-regioselective alkylation and isomerization at the external acid sites. The ethylation of BP to ethylbiphenyls (EBPs) and diethylbiphenyls (DEBPs) was non-regioselective. The ethylation of BP to EBPs was controlled kinetically. However, there was difference in reactivities of EBPs and DEBPs for their further ethylation. 4-EBP was ethylated preferentially among the isomers, although the formation of 4,4′-DEBP was less selective. 4-EBP and 4,4′-DEBP have the highest reactivities among EBPs and DEBPs for the ethylation to polyethylbiphenyls (PEBPs). These results show that the environments of HM pores are too loose for shape-selective formation of the least bulky isomers, 4-EBP and 4,4′-DEBP, in the ethylation of BP, and that HM pores have enough space for the further ethylation of 4,4′-DEBP.