Biphenyl, alkylation over zeolites

Shape-selective alkylation of biphenyl over H- [Al]-SSZ-24 zeolites with AFl topology. Bull. Chem. Soc. 

Electron densities of ortho- and para-positions of biphenyl are higher than that of meta-posltions. Sterically non-controlled isopropylatlon of biphenyl at low temperature occurred predominantly at ortho and para-posltlons to give 2-and 4-IPBP because of the electrophilic nature of the alkylation. However, selective formation of 4-IPBP over HM is controlled by the sterlc restrictions depending on the elliptical pore of the zeolite and on the conformation of the transition state for the formation of products. The molecule 2-IPBP has approximately 0.75 nm of diameter In a twisted bulky conformation with an angle of 64°[8]. The formation of 2-IPBP is prevented over HM because the corresponding transition state with bulky conformation requires more space than is available at the acidic sites of HM. On the other hand, the formation of 4-IPBP proceeds unhindered because of its smaller transition state. The formation of 3-IPBP Is also less hindered because of flexible conformations at transition states In HM pore. 

Isopropylation over HY, HL and Other Zeolites - The catalysis over HY and HL zeolites is quite different from that over HM. The product distribution over HY and HL resembled that of SA the formation of 4,4 -DIPB was non-regioselective (Table l).18-1 These catalyses are not controlled by the environments of the pores because both zeolites have sufficiently large pores for the transition states form all the IPBP and DIPB isomers. Instead, the product distribution markedly changes by the reaction temperature. At low temperatures, the products are determined by the reactivity at 2- and 4-positions of biphenyl for electrophilic alkylation to yield principally 2- and 4-IPBP for IPBP isomers and... 

Selective dkylation of benzene and alkylbenzenes over various zeolites has been studied extensively since the 1960 s, as summarized in many reports [Venuto and Landis, 1968 Yashima et al., 1970 Kaeding et al., 1981 Haag, 1984 Chen and Garwood, 1986 Yashima, 1988 Dwyer, 1991 Bhat et al., 1992 Chen et al., 1996 Xu et al., 1996]. However, until recently, little attention has been paid to selective alkylation of two-ring aromatics such as naphthalene and biphenyl. Naphthalene and its derivatives are rich in liquids derived from coals via carbonization, pyrolysis and liquefaction. Due to the need for monomers for making the advanced polymer materials shown in Scheme I, (3-selective naphthalene alkylation has become an important subject. In particular, 2,6-dialkyl substituted naphthalene (2,6-DAN) is needed now for making the monomers for PEN, PBN and LCPs shown in Scheme 1. 

The alkylation of biphenyl with /ert-butanol has been carried out over different zeolites under liquid phase conditions. HM (17.5) and HY (15) zeolites have been found to be the most active, with a maximum biphenyl conversion near 60 %. Dealuminated mordenite HM (17.5) leads to very high selectivities to 4-(ter/-butyl)biphenyl (99%) and 4,4 -di(/er/-butyl)biphenyl (96%). Selectivity to linear 4-TBB and 4,4 -DTBB depends on diffusional possibilities of relatively voluminous mono tert-butyl- and di (rert-butyl)biphenyl isomers from the zeolite pores. The most suitable temperature has been found to be 160 "C. An increase of the temperature leads to a significant decrease of selectivities to the desired products, as a result of secondary reactions. 

The selective alkylation of biphenyl and naphthalene is also possible, and is of commercial interest because 4,4 -diisopropylbiphenyl and 2,6-diisopropyl-naphthalene are used in the synthesis of speciality chemicals, for example for application as liquid crystal polymer materials for displays. For these larger molecules, large-pore zeolites such as mordenite give the best performance for isopropylation of naphthalene, introducing shape selectivity to linear (or more linear) dialkylated products over dialkylated products. 

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