A Mechanistic Investigation on 1,5- to 2,6-Dimethylnaphthalene Isomerization Catalyzed by Acidic Beta Zeolite: An ONIOM Study with a Newly Developed Density Functional Theory

2,6-Dimethylnaphthalene (DMN) is the key intermediate in the synthesis of poly(ethylene naphthalate) (PEN), a high-performance polymer. The detailed reaction mechanism of the catalyzed 1,5to 2,6DMN isomerization via 1,6-DMN by acidic beta zeolite is investigated at the ONIOM(M06-L/6-31G(d,p):UFF) level of theory. The M06-L method, a newly developed density functional theory, is applied for the first time to investigate such reaction within the nano-reactor of zeolite. The stepwise mechanism is proposed to proceed with three steps: protonation, methyl shift and proton backdonation. The methyl shift is the rate-determining step with the activation energies of 24.0 and 20.8 kcal/mol for 1,5to 1,6-DMN and 1,6to 2,6-DMN steps, respectively. The calculation confirmed the experiment that the 1,6-DMN formation is kinetically controlled. The calculated adsorption and activation energies are in good agreement with experimental data. Our findings demonstrate that the influence of the pore size of the beta zeolite perfectly fit for the enhancement of the isomerization of 1,5into 2,6-DMN.