Montejo Valencia, Brian D.

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    DFT analysis on structure-property relationships of metal-substituted zeolites
    (2018-05) Montejo Valencia, Brian D.; Curet Arana, María C.; College of Engineering; Cardona Martínez, Nelson; Santana Vargas, Alberto; Martínez Iñesta, María M.; Acevedo Rullán, Aldo; Department of Chemical Engineering; Vega Olivencia, Carmen A.
    Metal-modified zeolites have proven to be effective catalysts for various important reactions involving the transformation of biomass-derived molecules and the conversion of greenhouse gases, such as CO2 and CH4. In this work, we analyzed metal-substituted zeolites and metal-exchanged zeolites in various zeolite frameworks in order to quantify the catalytic activity of these materials. Density functional theory (DFT) calculations, ONIOM calculations, which is an integrated quantum mechanical molecular mechanical method, and periodic DFT calculations were used to analyze these systems. The substitution of Ti, Sn, Ge, Zr, and Hf in various zeolite frameworks were analyzed. The preferential substitution sites of these metals were reported. The Lewis acidity was measured through the NH3 binding energies and through the charge transfer of NH3 upon adsorption on the zeolites. The deprotonation energies of the open sites, which are proportional to the Brønsted acidities, and the hydrolysis energies were also reported. We also present the properties of zeolite beta (BEA) with a single and a double Sn-substitution to compare the active sites obtained with two methods commonly employed for the synthesis of Sn-BEA. The opening of glucose and fructose rings catalyzed by M-BEA (M = Sn, Ti, Zr, Hf) zeolites were analyzed with periodic DFT calculations. We proposed a novel mechanism for the ring opening of these molecules in one elementary step, which can be achieved in the closed sites of the zeolites. The adsorption energies of glucose and fructose through their different oxygens in M-BEA were also reported. Among the zeolites studied, Sn-BEA exhibits the lowest energy barrier for the opening of the glucose ring, whereas Hf-BEA yields the lowest energy barrier for the opening of the fructose ring. For the conversion of greenhouse gases, such as CO2 and CH4 into acetic acid, we analyzed the reaction catalyzed by MFI zeolite exchanged with Be2+, Co2+, Cu2+, Mg2+, Mn2+, and Zn2+ cations. Our results demonstrated that the highest reaction barrier on the reaction mechanism is CH4 dissociation. We also demonstrated that the CO2 insertion has a low energy barrier, and the protonation of the acetate species is spontaneous. Furthermore, desorption of acetic acid can be promoted with the co-adsorption of water.