Computational analysis for shape-selective alkylation of naphthalene over zeolite catalysts

This study is a computational analysis for shape-selective alkylation of naphthalene (including isopropylation, ethylation and methylation) by MINDO-PM3 calculations with MOPAC. There are 10 possible dialkylnaphthalene (DAN) isomers, with up to 3 from the so-called β, β-selective dialkylation including 2,6-, 2,7- and 2,3-DAN. However, only 2,6-DAN is the desired product. We calculated the frontier electron density f_r(E) and molecular dimensions for alkylnaphthalene (AN) isomers and dialkylnaphthalene (DAN) isomers. The calculation shows that the 6-position in 2-AN has higher f_r(E) value than 7-position. The more reactive position for electrophilic substitution has higher f_r(E) value which represents the density of electrons in the highest occupied molecular orbital (HOMO). The formation of the cationic intermediate that leads to 2,6-DAN is favored energetically as compared to that for 2,7-DAN or 2,3-DAN. This suggests that during 2-AN alkylation inside a shape-selective channel, the formation of 2,6-DAN is favored electronically more than that of 2,7-DAN or 2,3-DAN, and thus the catalyst with lower acidity may favor 2,6-DAN. The results are consistent with experimental data in literature, and suggest that there is a restricted electronic transition-state selectivity to 2,6-DAN on some molecular-sieve catalysts with lower acidity, since both 2,6- and 2,7-DAN isomers are sterically permitted. In general, 2,6-DAN has a more linear structure with a similar or smaller critical diameter compared to 2,7-DAN, while 2,3-DAN always has a larger diameter than both 2,6- and 2,7-DAN.