Departament de Química Física i Analítica, Universitat Jaume I, Box 224, Castelló 12071, Spain.
J Comput Chem. 2010 Oct;31(13):2493-501. doi: 10.1002/jcc.21543.
Density functional theory was used to study the mechanism for the oxidation of methanol to formaldehyde. A vanadium oxide cluster O=V(OH)(3) has been utilized to represent the catalytic system under hydrated conditions, i.e., in the presence of V-OH hydroxyl groups. Two types of methoxy-intermediates have been considered: a penta-coordinate methoxy-intermediate (OH)(4)V(OCH(3)) and a tetrahedral methoxy-intermediate (OH)(2)VO(OCH(3))(H(2)O). The most plausible reaction pathway corresponds to the process involving first the formation of the tetrahedral methoxide, and a subsequent rate-limiting step where hydrogen is transferred from the methoxy groups toward the oxygen atom of the vanadyl V=O site. The reaction mechanism is a typical two-state reactivity process due to a change of the multiplicity (reactive singlet --> product triplet) along the reaction coordinate accompanied by a reduction of the vanadium center from V(V) (d(0)) to V(III) (d(2)). Minimum energy crossing points were localized and possible spin inversion processes are discussed by means of the intrinsic reaction coordinate approach to find the most favorable reaction pathways. The hydration effect is found to be mainly the destabilization of the methoxy intermediates. An alternative reaction pathway with a lower apparent barrier is presented.
密度泛函理论被用于研究甲醇氧化为甲醛的反应机制。选用氧化钒簇 O=V(OH)(3) 来代表水合条件下的催化体系，即存在 V-OH 羟基的情况下。考虑了两种甲氧基中间体：五配位甲氧基中间体 (OH)(4)V(OCH(3)) 和四面体形甲氧基中间体 (OH)(2)VO(OCH(3))(H(2)O)。最合理的反应途径涉及首先形成四面体形甲氧基，然后是一个速率限制步骤，其中氢从甲氧基向钒酰基 V=O 位点的氧原子转移。反应机制是一个典型的两态反应性过程，由于沿着反应坐标发生多重性的变化（反应性单重态-产物三重态），同时钒中心从 V(V)（d(0)）还原为 V(III)（d(2)）。通过内禀反应坐标方法定位最小能量交叉点，并讨论可能的自旋反转过程，以找到最有利的反应途径。发现水合作用的主要影响是使甲氧基中间体不稳定。提出了一个具有较低表观势垒的替代反应途径。
