Solid Lewis acids materials are effective catalysts for various important reactions involving biomass. A notorious example is the Sn substituted Beta zeolite (Sn-BEA), which has recently been used for the glucose isomerization to fructose. BEC zeolite is the polymorphism C of Beta zeolite that has three dimensional pore topology, all three with twelve-membered ring channels. Previous works by Corma and coworkers have demonstrated that Ti-BEC is a better catalyst than Ti-BEA for epoxidation reactions involving aromatic molecules.1Thus, we hypothesize that metal substituted BEC catalysts could outperform BEA zeolites in other Lewis-acid catalyzed reactions involving biomass derived molecules.
This study aims at obtaining a fundamental understanding on the relationship between structure and catalytic properties of BEC and BEA zeolites substituted with different metal atoms (Ge, Sn, Ti, V, Nb, Ta). A combination of quantum mechanical calculations and integrated quantum mechanics−molecular mechanics along with a polarizable continuum model were used to determine the preferred substitution site of the metals in zeolites and to quantify how the electronic properties of the metal atoms change upon hydrolysis.
Our results demonstrate: (1) the most favorable substitution of Ge, Sn, Ti, V, Nb and Ta in BEC is in the T1 site; (2) the most favorable substitution of Ti in BEA is in the T2 site; (3) Ti-BEC has a Lewis acidity similar to that of Sn-BEA;2 (4) the hydrolysis of Ge-BEC is energetically favorable only when the Ge/Si ratio is higher than 1/32; and (5) among the elements of the VB group analyzed in this study (V, Nb and Ta), V-BEC exhibits a higher Lewis Acidity than the well-known Sn-BEA.
(1) Moliner, M.; Corma, A. Advances in the Synthesis of Titanosilicates: From the Medium Pore TS-1 Zeolite to Highly-Accessible Ordered Materials. Microporous Mesoporous Mater. 2014, 189, 31–40.
(2) Montejo-Valencia, B. D.; Curet-Arana, M. C. DFT Study of the Lewis Acidities and Relative Hydrothermal Stabilities of BEC and BEA Zeolites Substituted with Ti, Sn, and Ge. J. Phys. Chem. C 2015, 119, 4148–4157.