Alkylation of Biphenyl and Naphthalene over Zeolites: Characteristics of Shape-selective Catalysis in Zeolite Channels

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To understand the characteristics of shape-selective catalysis over zeolites, the steric interactions of reactants, products, and transition state intermediates with zeolite channels are discussed in the alkylation of biphenyl (BP) and naphthalene (NP). The selectivities for 4,4'-dialkylbiphenyl (4,4'-DABP) and 2,6-dialkylnaphthalene (2,6-DAN) were varied by using various zeolites and alkylating agents, and determined by the exclusion of bulky isomers by the channels. The results show predominance of the least bulky isomers, 4,4'-DABP and 2,6-DAN, where the structure of zeolite channel and the bulkiness of alkylating agent are the key factors in shape-selective catalysis. Mordenite (MOR) and SSZ-24 (AFI), which have straight channels with 12-membered ring (12-MR) pore entrances, gave high selectivities for 4,4'-diisopropylbiphenyl (4,4'-DIPB) in the isopropylation of BP using propene. However, SSZ-55 (ATS) and SSZ-42 (IFR), which have caged channels with 12-MR pore entrances, gave low the selectivities in the range of 30 to 35%. These results indicate that the channels of MOR and AFI are narrow enough to exclude the transition states of bulky isomers. The isopropylation of BP over ATS and IFR occurs under kinetic and/or thermodynamic controls because their channels are too large for shape-selective catalysis. The bulkiness of the alkylating agent enhanced the selectivities for 4,4'-di-s-butylbiphenyl (4,4'-DSBB) in the s-butylation using 1-butene and 4,4'-di-t-butylbiphenyl (4,4'-DTBB) in the t-butylation using 2-methylpropene. CIT-5 (CFI), UTD-1 (DON), and SSZ-53 (SFH), which have one-dimensional channels with 14-MR pore entrances, and Y (FAU), β (BEA), and CIT-1 (CON), which are three-dimensional channels with 12-MR pore entrances, increased the selectivities for 4,4'-DSBB and 4,4'-DTBB using bulky alkylating agent due to the restrictions on the transition states by their channels.The alkylation of NP was also discussed on the basis of the selectivities for β,β- and 2,6-DAN. Shape-selective formation of β,β- and 2,6-diisopropylnaphthalene (DIPN) occurred in the isopropylation only over MOR, whereas the other zeolites gave lower selectivities for β,β- and 2,6-DIPN because their channels are too large for the exclusion of the transition states of bulky isomers. The increase in bulkiness of alkylating agent enhanced the formation of β,β-, and 2,6-DAN. However, the selectivities for 2,6-di-t-butylnaphthalene (2,6-DTBN) were around 50 to 60% over FAU, BEA, and CON in the t-butylation, although the selectivities for β,β-DTBN were almost 100%. These results indicate that these zeolites cannot recognize the differences in bulkiness between 2,6- and 2,7-DTBN.These findings in this paper indicate that accommodation of only the least bulky isomers fit with the zeolite channels and resulting exclusion of other bulky isomers, is a key factor for highly shape-selective catalysis, and that steric restriction on the transition states of product isomers by zeolite channels is essential to evaluate for the design of appropriate structures for target reactions.