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 fr(E) and molecular dimensions for alkylnaphthalene (AN) isomers and dialkylnaphthalene (DAN) isomers. The calculation shows that the 6-position in 2-AN has higher fr(E) value than 7-position. The more reactive position for electrophilic substitution has higher fr(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.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)