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
C. Kumsapaya, K. Bobuatong, S. Choomwattana, P. Khongpracha, Y. Tantirungrotechai, J. Limtrakul
Kasetsart University, TH
Keywords: Dimethylnaphthalene, Isomerization, Density functional theory, Zeolite, Beta
Abstract:2,6-Dimethylnaphthalene (DMN) is the key intermediate in the synthesis of poly (ethylene naphthalate) (PEN), a high-performance polymeric material. The processes to produce 2,6-DMN have been the focus of numerous experimental investigations, yet there is no accord on such reaction mechanism of the isomerization reaction to 2,6-DMN in detail. The detailed reaction mechanism of the catalyzed 1,5- to 2,6-dimethylnatphalene (DMN) isomerization by acidic beta zeolite is investigated at the ONIOM(M06L/6- 31G(d,p):UFF) level of theory. The M06L method is also applied with the ONIOM scheme for the first time to investigate such reaction within the nano-reactor of zeolite. The isomerization reaction occurs stepwise from 1,5-DMN to 2,6-DMN by 1,6-DMN. The catalyzed reaction proceeds with three steps: protonation, methyl shift and proton back-donation. The methyl shift step is the rate-determining step with the activation energies of 23.97 and 20.79 kcal/mol for 1,5- to 1,6-DMN and 1,6- to 2,6-DMN steps respectively. The calculation confirmed the experiment that for the 2,6-DMN production, the 1,5- to 1,6-DMN isomerization is kinetically controlled. The ONIOM (M06L/6-31G(d,p):UFF) energy profile yields the activation energy and the adsorption energy in good agreement with the available experimental data. Our findings demonstrate that the influence of the pore size of the beta zeolite which perfectly fit for enhancement of the isomerization reaction of 1,5- into 2,6-DMN.