A Theoretical Investigation of the Selective Oxidation of Methanol to Formaldehyde on Isolated Vanadate Species Supported on Silica

A theoretical analysis has been conducted on the selective oxidation of methanol to formaldehyde catalyzed
by isolated vanadate species supported on silica. The active site was represented by a VO group substituted
for a Si−H group in the corner of silsesquioxane, Si8O12H8. Calculations of ground and transition states were
carried out using density functional theory, whereas statistical mechanics and absolute rate theory were used
to determine equilibrium constants and rate coefficients for each elementary step. The formation of
formaldehyde was found to involve two key steps. The first is the reversible adsorption of methanol, which
occurs by addition across one of the three V−O−Si bonds of the active site. The rate-limiting step is the
transfer of a hydrogen atom from the resulting V−OCH3 species to the VO bond of the active center. The
release of formaldehyde and water from the active center leads to a two electron reduction of the vanadium
atom in the center. Rapid reoxidation of the reduced vanadium can occur via adsorption of O2 to form a
peroxide species and subsequent migration of one of the O atoms associated with the peroxide across the
surface of the support. The predicted heat of adsorption and equilibrium constant for methanol adsorption are
in good agreement with those found experimentally, as is the infrared spectrum of the adsorbed methanol.
The apparent first-order rate coefficient and the apparent activation energy are also in very good agreement
with the values determined experimentally.

对负载于二氧化硅上的独立钒酸盐物种催化的甲醇选择性氧化为甲醛的理论分析已经完成。活性位点以硅氧烷八面体，Si8O12H8，角落的Si−H基团被VO基团取代的形式表示。利用密度泛函理论进行了基态和过渡态的计算，而统计力学和绝对速率理论被用于确定每个基元步骤的平衡常数和速率系数。甲醛的形成被发现涉及两个关键步骤。首先是甲醇的可逆吸附，它通过在活性位点的三个V−O−Si键之一上添加而实现。速率决定步骤是从产生的V−OCH3物种向活性中心的VO键转移一个氢原子。甲醛和水从活性中心释放导致中心钒原子的二电子还原。还原钒的快速再氧化可以通过吸附O2形成过氧化物物种，随后与过氧化物相关的一个氧原子在载体表面迁移来实现。甲醇吸附的预测吸附热和平衡常数与实验结果相符，吸附甲醇的红外光谱也是如此。表观一级速率系数和表观活化能也与实验确定的值非常吻合。