Insight into the mechanism of methanol assistance with syngas conversion over partially hydroxylated γ-Al2O3(110D) surface in slurry bed

Abstract

Despite numerous studies devoted to the various properties of γ-Al₂O₃, the explorations of its catalytic activity remain scarce. In this study, density functional theory calculations are performed to study the elementary adsorption and reaction mechanisms for syngas conversion on partially hydroxylated γ-Al₂O₃(110D) surface in liquid paraffin. It is found that the partially hydroxylated γ-Al₂O₃(110D) surface with the hydroxyl coverage of 8.9 OH nm⁻² is formed by two dissociative adsorptions of H₂O on the dry γ-Al₂O₃(110D) surface. The hydroxyl coverage conditions play a key role in determining the dominant reaction mechanism on account of the existence of strong hydrogen bonds. The preferential pathway for syngas conversion with assistance of methanol over the partially hydroxylated γ-Al₂O₃(110D) surface in liquid paraffin has been proven to be CH₃OH → CH₃O + H → CH₃ + OH, CH₃ + CO → CH₃CO. C₂H₅OH is then formed by successive hydrogenation via the pathway CH₃CO + 3H → CH₃CHO + 2H → CH₃CH₂O + H → C₂H₅OH. Here, CH₃CHO formation by CH₃CO hydrogenation is not inhibited. Actually, with the assistance of partially hydroxylated γ-Al₂O₃, CH₃CHO has been synthesized with high selectivity in our previous experiment by the reaction of methanol and syngas, which provides favorable evidence for our results. The rate-limiting step is the formation of CH₃O from CH₃OH dehydrogenation with an activation barrier of 122.2 kJ mol⁻¹. Moreover, the reaction barrier of CO insertion into the adsorbed CH₃ group is at least 89.4 kJ mol⁻¹, lower than those of CH₄, C₂H₆, and CH₃OCH₃ formations. ADCH charge and ESP analyses indicate that the typical (Al, O) Lewis acid–base pair may have a significant effect upon the initial C–C chain formation. Thus, the present study provides a new approach for the rational tailoring and designing of new catalysts with superior reactivity involved in syngas conversion.