Physical Chemistry Chemical Physics 2018-04-12

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

Bing Bai, Hui Bai, Haojie Cao, Zhihua Gao, Zhi-Jun Zuo, Wei Huang

文献索引：10.1039/C8CP02000K

摘要

Despite numerous studies devoted to the various properties of γ-Al2O3, the explorations of its catalytic activity remains scarce. In this work, density functional theory calculations are performed to study the elementary adsorption and reaction mechanisms for syngas conversion on the 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 adsorption 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 preference pathway for syngas conversion with methanol assisted over the partially hydroxylated γ-Al2O3 (110D) surface in liquid paraffin is proved as CH3OH→CH3O+H→CH3+OH, CH3+CO→CH3CO, and followed by successive hydrogenation C2H5OH is formed via CH3CO+3H→ CH3CHO+2H→CH3CH2O+H→C2H5OH. Here, CH3CHO formation by CH3CO hydrogenation is no barrier. Actually, under the role of partially hydroxylated γ-Al2O3, CH3CHO with high selectivity has been synthesized 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 that of CH4, C2H6, and CH3OCH3 formation. ADCH charge and ESP analyses indicate that the typical (Al, O) Lewis acid–base pair may has a significant effect upon the initial C-C chain formation. Thus the present work provides new idea for rational tailoring and designing of new catalysts with superior reactivity involved in syngas conversion.