Molecular simulation of hydroxyl activated by methanol adsorption on Zn-modified Sr/γ-Al2O3 surface

Biodiesel has been one of the most promising types of renewable and clean energy. Replacing fossil fuels with biodiesel can effectively reduce environmental pollution and the consumption of non-renewable energy. It is often produced by a transesterification reaction between methanol and triglyceride under the action of a catalyst. The catalyst is indispensable during reactions. Homogeneous catalyst has been widely used in industrial applications. But the homogeneous catalyst is difficult to separate and easy to saponify with oil. Fortunately, solid catalysts behave the high activity and are easy separation from the solution. In this study, a solid catalyst was fabricated with γ-Al2O3 as a carrier, while Sr and Zn as active atoms. γ-Al2O3 has been widely used as the carrier of active substances, due to the high thermal stability and developed micro pore structure. The strontium doped γ-Al2O3 presented a high catalytic activity for transesterification reaction, due to the high basic strength of Sr atom. Zinc atom was added to form the diatomic catalyst. Zn atom served as the dual properties of acid and base, which adsorbed the acid grease for esterification reaction. Doping both Sr and Zn atoms into γ-Al2O3 can effectively improve the yield of biodiesel. A γ-Al2O3 (110) surface model, strontium doped γ-Al2O3 (110) surface model, and Sr-Zn doped γ-Al2O3 (110) surface model were constructed. The surface doping energy of Sr and Sr-Zn doped catalysts was calculated to obtain the most stable surface. A system evaluation was made to clarify the influence of atomic doping on the stability of the catalysts. After that, the catalyst adsorption model of methanol was established to take the most stable surface as the adsorption carrier. The adsorption energy of methanol on different catalysts was calculated to obtain the best adsorption site of methanol on the catalyst surface. An investigation was made to explore the influence of zinc doping on the catalyst adsorption effect of methanol. Mulliken charge layout was selected to analyze the charge density of each atom in methanol molecule under various adsorption states. The state density and differential charge density of methanol adsorption were also calculated on different catalysts, and the atomic bonding trend, charge transfer amount and charge transfer direction before and after methanol adsorption on the catalyst surface were obtained to explore the mechanism of the catalyst in the transesterification reaction. There was the minimum doping energy on the catalyst surface when the Sr atom was added to the different sites on γ-Al2O3 (110) surface, where the Sr atom was tended to replace Al3c-1 and Al4c-2 coordination aluminum. Zn atom tended to coordinate with Al4C-4 when doping, indicating the minimal doping energy on the catalyst surface. Specifically, the O atom of the methanol molecule gained charge, but the H atom lost charge, where the H atom on the methanol hydroxyl group was bonded with the O atom on the catalyst surface when Sr/γ-Al2O3 and Sr-Zn/γ-Al2O3 (110) surfaces adsorbed methanol. The absolute value of adsorption energy on the surface of the catalyst adsorption methanol increased, when Zn atom doped Sr/γ-Al2O3 (110) surface, indicating the adsorption energy of ?305.60 kJ/mol. The number of electrons also increased to gain by oxygen atom on methanol. Hydrogen atoms on the hydroxyl group were bonded better to the surface of the catalyst. Both Sr/γ-Al2O3 and Sr-Zn/γ-Al2O3 can activate methanol effectively, particularly with the stability of Sr/γ-Al2O3 catalyst that is enhanced by Zn atomic doping. The solid catalyst can also greatly contribute to the subsequent biodiesel synthesis.