Catalytic Mechanisms of Methanol Oxidation to Methyl Formate on Vanadia–Titania and Vanadia–Titania–Sulfate Catalysts

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• 作者：Na Li ; Shibin Wang ; Qinghua RenShenggang Li ; Yuhan Sun
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：December 29, 2016
• 年：2016
• 卷：120
• 期：51
• 页码：29290-29301
• 全文大小：708K
• ISSN：1932-7455

文摘

Density functional theory calculations were carried out using both cluster and slab models to investigate the catalytic reaction network and the effect of sulfate promoter on methanol selective oxidation using the V₂O₅/TiO₂ catalyst. The hemiacetal mechanism was reaffirmed to be the most favorable reaction pathway for the formation of methyl formate (MF) by the prediction of another reaction pathway involving formic acid. Molecular oxygen was found to assist the desorption of the reaction products from the reduced catalyst active site, both formaldehyde and methyl formate (MF). The mechanism of catalyst regeneration was elucidated based solely on first-principles calculations, which involve the conversion of another methanol molecule to formaldehyde over a peroxo species. This leads to our formulation of a complete catalytic reaction network for methanol selective oxidation to formaldehyde and MF on the V₂O₅/TiO₂ catalyst. In addition, the detailed mechanism for the formation of formaldehyde and MF was also predicted on the sulfate-promoted V₂O₅/TiO₂ catalyst. Based on the calculated reaction networks, the preferred formation of MF on the V₂O₅/TiO₂-based catalysts was attributed to the lower energy barrier of the oxidative dehydrogenation (ODH) of the CH₃OCH₂O* intermediate than that of CH₃O*. Furthermore, our calculated energy barriers also suggest that the sulfate-promoted V₂O₅/TiO₂ catalyst has not only higher catalytic activity for methanol conversion but also higher selectivity of MF over CH₂O, consistent with previous experimental observations. The sulfate promoter was found to increase the positive charge at the V site, leading to a lower energy barrier for the ODH of CH₃O* than the unpromoted catalyst.