Shape-selective alkylation of biphenyl over metalloaluminophosphates with AFI topology

Abstract

Metalloaluminophosphates with AFI topology (MAPO-5; M: Mg, Ca, Sr, Ba, and Zn) synthesized by the dry gel conversion (DGC) method were used as catalysts for the alkylation of biphenyl (BP). MgAPO-5 and ZnAPO-5 had high catalytic activities of the isopropylation; however, activities of CaAPO-5, SrAPO-5, and BaAPO-5 were much lower than those of MgAPO-5 and ZnAPO-5. These results suggest that the catalytic activity of MAPO-5 molecular sieves was originated by the acidity due to the isomorphous substitution of M 2+ for Al 3+. The selectivities for 4,4′-diisopropylbiphenyl (4,4′-DIPB) for all MAPO-5 molecular sieves are at the level of 70%. On the basis of various reaction behaviors, the predominant formation of 4,4′-DIBP was considered to be mainly due to the steric restriction on the transition state of the isopropylation to the least bulky isomer inside the channel of MAPO-5 molecular sieves. The isopropylation of BP over MgAPO-5 and ZnAPO-5 was accompanied by the isomerization of 4,4′-DIPB to thermodynamically more stable isomers, 3,4′-DIPB at the higher temperatures; the decrease in the selectivity for 4,4′-DIPB was observed in both bulk and encapsulated products. These results show that 4,4′-DIPB formed inside channels was isomerized inside the channels in addition to external acid sites. The channels of MAPO-5 molecular sieves are large enough to allow the isomerization of 4,4′-DIPB especially at higher temperatures: this is quite different from those of H-mordenite. The sec- and tert-butylations of BP over MgAPO-5 and ZnAPO-5 gave selectively the least bulky isomers, 4,4′-di- sec-butylbiphenyl (4,4′-DSBB) and 4,4′-di- tert-butylbiphenyl (4,4′-DTBB), respectively. The selectivity for the least bulky isomer increased with increasing the bulkiness of alkylating agents in the order: isopropylation < sec-butylation < tert-butylation. This order reflects the spatial constraints of the transition sates inside the AFI structure.