Shape-selective alkylation of biphenyl catalyzed by H-Mordenites

Abstract

Liquid phase alkylation of biphenyl was studied over large pore zeolites. Selective formation of the narrowest products, 4,4′’-diisopropylbiphenyl (4,4′’-DIPB), occurred only over HM among the zeolites with twelvemembered pore openings. These shape-selective catalyses are ascribed to steric restriction of transition state and to entrance of bulky substrates into the pores. The dealumination of HM enhanced catalytic activity and the selectivity of 4,4′’-DIPB because of the decrease of coke-deposition, while the activity and the selectivity were low over HM with the low SiO2/Al2O3 ratio. Non-regioselective catalysis occurs on external acid sites because severe cokedeposition 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. 4,4′’-DIPB yielded selectively under high propylene pressure (<0.3 MPa) at 250 °C, while the selectivity of 4,4′’-DIPB decreased under such low propylene pressure as 0.2MPa. Selective formation of 4,4′’-DIPB was observed at moderate temperature such as 250 °C, whereas the decrease of the selectivity of 4,4′’-DIPB occurred at higher temperature as 300 °C. However, 4,4′’-DIPB was almost exclusively isomer in the encapsulated DIPB isomers inside the pores under every pressure and temperature. These decreases of the selectivity of 4,4′’-DIPB are due to the isomerization of 4,4′’-DIPB on the external acid sites. The deactivation of external acid sites of HM was examined to reduce non-regioselective alkylation and isomerization. External acid sites were deactivated by calcination after impregnation of cerium on HM without the decrease in pore radii. Selectivities of 4,4′’-DIPB were improved even at high temperatures in the isopropylation of biphenyl because of the suppression of non-regioselective alkylation and isomerization at the external acid sites. The selectivity of 4,4′’-diethylbiphenyl (4,4′’-DEBP) in the ethylation of biphenyl was much lower than that in the isopropylation. Among the DEBP isomers, 4,4′’-DEBP has the highest reactivity for the ethylation to polyethylbiphenyls inside the pores, whereas the isopropylation of 4,4′’-DIPB was negligibly low inside the pores. These differences are ascribed to the difference in steric restriction at the transition state composed of substrate, alkylating agent, and acid sites inside the pores.