Selectivity of large pore zeolites in the alkylation of naphthalene with tert-butyl alcohol:: analysis of experimental results by computational modelling

Liquid phase alkylation of naphthalene with tert-butanol has been studied using HY and H-beta zeolites with varying silicon to aluminium ratios. Both series of zeolites underwent efficient activities and high selectivities for the mono- and di-(tert-butyl) derivatives. In all cases, 2-(tert-butyl)naphthalene (2-TBN) was the only monoalkylated product (100%,beta-selectivity). Over H-beta zeolites, 2-TBN was obtained as the main product with relatively small amounts of dialkylated compounds. Over HY samples, a 2,6-di(tert-butyl)naphthalene (2,6-DTBN) selectivity up to 84% was obtained, with a 2,6-DTBN/2,7-DTBN ratio of 5.6-5.9 and beta,beta'-selectivity (2,6-+2,7-) of 98-99%. This constitutes the first published observation of such high beta,beta' selectivity and 2,6-/2,7- ratio in the liquid phase alkylation of naphthalene. In order to understand such a differentiation in the formation of the two di-(tert-butyl)naphthalenes over the HY zeolites, computational analysis of both energies and molecular dimensions of these derivatives and of the monoalkylated counterparts has been performed, using quantum mechanics (AM1 and PM3) and molecular mechanics (MM+) methods. The energy calculations are in agreement with the experimental results regarding the selective formation of the beta and beta,beta' isomers. The determination of the kinetic diameters shows that, in their most stable conformation, 2,6-DTBN has a smaller kinetic diameter (7.1 Angstrom) than 2,7-DTBN (7.5 Angstrom), which may explain the selective formation of the 2,6-isomer against the 2,7- within the faujasite framework. (C) 2000 Elsevier Science B.V. All rights reserved.