Mechanism of dibenzofuran hydrodeoxygenation on the Ni (1 1 1) surface

Abstract The low-temperature coal tar contains a considerable number of oxygen-containing compounds, which results in poor quality. The catalytic hydrodeoxygenation of oxygen-containing compound to an added-value chemical compound is one of the most efficient methods to upgrade coal tar. In this study, density functional theory calculations are employed to assess and analyze in detail the hydrodeoxygenation of dibenzofuran, as a model compound of coal tar, on the Ni (1 1 1) surface. The obtained results indicate that dibenzofuran can be firstly hydrogenated to tetrahydrodibenzofuran and hexahydrodibenzofuran. The five-membered-ring opening reaction of tetrahydrodibenzofuran is more straightforward than that of hexahydrodibenzofuran (Ea = 0.71 eV vs. 1.66 eV). Then, both pathways generate an intermediate 2-cyclohexylphenoxy compound. One part of 2-cyclohexylphenoxy is hydrogenated to 2-cyclohexylphenol and consecutively hydrogenated to cyclohexylcyclohexanol, and another part is directly hydrogenated to cyclohexylcyclohexanone. The hydrogenated intermediates of 2-cyclohexylphenol have higher deoxygenation barriers than 2-cyclohexylphenol and cyclohexylcyclohexanol. During the hydrogenation process of cyclohexylcyclohexanone to cyclohexylcyclohexanol, the intermediate 26, formed by adding H to O atom of cyclohexylcyclohexanone, exhibits the lowest deoxygenation barrier of 1.08 eV. High hydrogen coverage may promote the hydrogenation of tetrahydrodibenzofuran, hexahydrodibenzofuran, and intermediate 26 to generate dodecahydrodibenzofuran and cyclohexylcyclohexanol. This dibenzofuran hydrodeoxygenation reaction mechanism corroborates well with previous experimental results and provides a theoretical basis for further optimization of the design of nickel-based catalysts.