Oxidation of Methanol by FeO2+ in Water: DFT Calculations in the Gas Phase and Ab Initio MD Simulations in Water Solution

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• 作者：Manuel J. Louwerse ; Peter Vassilev ; Evert Jan Baerends
• 刊名：Journal of Physical Chemistry A
• 出版年：2008
• 出版时间：February 7, 2008
• 年：2008
• 卷：112
• 期：5
• 页码：1000 - 1012
• 全文大小：378K
• 年卷期：v.112,no.5(February 7, 2008)
• ISSN：1520-5215

文摘

We investigate the mechanism of methanol oxidation to formaldehyde by ironoxido ([Fe^IVO]²⁺), the allegedactive intermediate in the Fenton reaction. The most likely reaction mechanisms are explored with densityfunctional theory (DFT) calculations on microsolvated clusters in the gas phase and, for a selected set ofmechanisms, with constrained Car-Parrinello molecular dynamics (CPMD) simulations in water solution.Helmholtz free energy differences are calculated using thermodynamic integration in a simulation box with31 water molecules at 300 K. The mechanism of the reaction is investigated with an emphasis on whetherFeO²⁺ attacks methanol at a C-H bond or at the O-H bond. We conclude that the most likely mechanismis attack by the oxido oxygen at the C-H bond ("direct CH mechanism"). We calculate an upper bound forthe reaction Helmholtz free energy barrier in solution of 50 kJ/mol for the C-H hydrogen transfer, afterwhich transfer of the O-H hydrogen proceeds spontaneously. An alternative mechanism, starting withcoordination of methanol directly to Fe ("coordination OH mechanism"), cannot be ruled out, as it involvesa reaction Helmholtz free energy barrier in solution of 44 ± 10 kJ/mol. However, this coordination mechanismhas the disadvantage of requiring a prior ligand substitution reaction, to replace a water ligand by methanol.Because of the strong acidity of [FeO(H₂O)₅]²⁺, we also investigate the effect of deprotonation of a first-shellwater molecule. However, this is found to increase the barriers for all mechanisms.