Alkylation of Biphenyl over Zeolites: Shape-Selective Catalysis in Zeolite Channels

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Steric interaction of reactants, products, and transition state intermediates with zeolites with 12- and 14-membered ring pore entrances is discussed in the alkylation of biphenyl (BP) by propene, 1-butene, and 2-methylpropene as alkylating agents. The selectivities for the least bulky 4,4′-dialkylbiphenyl (4,4′-DAB) depended on the types of zeolite and alkylating agent. The zeolites are classified as two types: Category I: mordenite (MOR), SSZ-24 (AFI), SSZ-31 (STO), CIT-5 (CFI), MCM-68 (MSE), and ZSM-12 (MTW) with straight (or slightly corrugated) channels with 12-MR or 14-MR pore entrances. Category II: SSZ-42 (IFR), SSZ-55 (ATS), SSZ-60 (SSY), Y (FAU), β (BEA), CIT-1 (CON), UTD-1 (DON), and SSZ-53 (SFH) with large channels. Category I zeolites have shape-selective natures in their channels in the isopropylation. Among them, MOR supported the highest selectivities around 85 % for 4,4′-diisopropylbipneyl (4,4′-DIPB) at moderate temperatures as 250 oC. The selectivities of 4,4′-DIPB are: MOR > AFI, MSE > MTW, CFI, STO: the differences are due to the recognition of 4,4′- and 3,4′-DIPB by the channels. Category II zeolites allow the accommodation and the formation of bulky DIPB isomers in their channels, resulting in non-shape selective catalysis under kinetic and/or thermodynamic controls. Bulky alkylating agents, 1-butene and 2-methylpropene, enhance selective formation of the least bulky 4,4′-DAB: 4,4′-di-sec-butylbiphenyl (4,4′-DSSB) and 4,4′-di-tert-butylbiphenyl (4,4′-DTBB), respectively, even over Category II zeolites. The bulky moieties enhance the exclusion of bulky isomers by their interaction with zeolite channels. The selectivities for 4,4′-DSSB were increased remarkably to higher than 80 % for Category I zeolites and to 70 % for ATS in Category II zeolites in the sec-butylation. However, kinetic and thermodynamic controls still worked with the formation of bulky isomers over Category II zeolites: IFR, FAU, BEA, CON, DON, and SFH in the sec-butylation. The high selectivities for 4,4′-DTTB higher than 80 % were observed for the zeolites, except FAU in the tert-butylation. These results indicate that the selectivities for 4,4′-DAB are controlled primarily by space and shape of zeolite channels for active sites, and not always by the size of pore entrances.