Structures and Energetics of the Methylation of 2-Methylnaphthalene with Methanol over H-BEA Zeolite

The methylation of 2-methylnaphthalene (2-MN) with methanol to the 2,6 (2,6-DMN) and 2,7 (2,7-DMN) dimethylnaphthalenes catalyzed over nanoporous BEA zeolite has been investigated quantum chemically using the M06-2X density functional. The catalytic cycle consists of three elementary steps: (1) formation of a methoxy species from methanol that is bound to a zeolite oxygen atom, (2) methylation of 2-MN to DMN with methoxy leading to naphthalynic carbocations, and (3) formation of DMN by proton back-donation from naphthalynic carbocations. The reaction profiles are similar for both the 2,6 and the 2,7 isomer and are in agreement with the experimental observation that they are produced in equal amounts on acidic BEA zeolite. A possible side reaction, the formation of dimethyl ether via the self-activation of methanol, is also discussed. The stability of the intermediates inside the pores is, to a large extent, governed by the steric constraints and the van der Waals dispersion interactions induced by the pore structure of BEA zeolite. These are the key parameters for understanding the relationship between zeolite topology and catalytic activity.