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Computational mechanistic study of the gas phase oxidation of methanol with ozone

Posted on 31. July, 2014.

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One important aim of modern combustion chemistry is to design and analyse possible new forms of non-petroleum-based fuel or fuel components for use in combustion engines.Methanol can be considered as one of the simplest oxygenated hydrocarbon compounds in this regard.

Methanol’s characteristics such as low pollution, fuel supply options (manufacturing from natural gas, coal, and biomass), fire safety, high performance, and being economically attractive are a few of its advantages that make it as a material of choice as replacement for gasoline or at least for being considered as an additive to fossil fuels. Probably biofuels will soon become commercially available for use as a gasoline-blending component in combustion engines.

Methanol is toxic by nature via two mechanisms. First, it can be fatal because of its properties leading to depression of the central nervous system in the same manner as ethanol. Second, in a process of toxication, it is metabolised to yield formic acid via formaldehyde in a process initiated by the enzyme alcohol in the liver.

Methanol is produced naturally in the anaerobic metabolism of many varieties of bacteria, and is ubiquitous in the environment. As a result, there is a small fraction of methanol vapour in the atmosphere. Over a period of several days, atmospheric methanol is oxidised by OH and other tropospheric radicals to carbon dioxide and water. Also, methanol is used as a solvent in ozonolysis of alkenes in liquid phase, it is being used as a medium so unreactive as not interfere with reaction.

To the authors' knowledge, the atmospheric oxidation of methanol with ozone has not been studied theoretically or experimentally. Moreover, the kinetics of the reaction CH3OH+O3 are not very well-known. Only limited information is available in the literature on methanol pyrolysis, hydrogen abstraction from methanol, hydroperoxyl, and methyl radicals via direct experiments.

The reaction of methanol and ozone on the singlet potential energy surface has been carried out using the RMP2 theoretical approach in connection with the 6‑311++G(d, p) basis set. To obtain a more reliable energy, single point calculations were performed for all species at the CCSD(T) level. One prereactive complex C1 is formed between the reactants. From a variety of the complexes, five types of product are obtained, of which four types have enough thermodynamic stability at the RMP2 level. In the thermodynamic approach, the most favoured route begins with the formation of a pre-reactive complex C1 and gives H2CO+H2O+3O2 as the final adducts. This process is exothermic by –73.243 kcalmol– 1 in standard enthalpy and exergonic by – 83.382 kcalmol–1.

Keywords: methanol, RMP2 level, theoretical approach, oxidation reaction, ozone, combustion chemistry

Authors: Kolsoom Shayan and Morteza Vahedpour
Department of Chemistry, University of Zanjan, PO Box 45371-38791, Zanjan, Iran

Read the full article in Progress in Reaction Kinetics and Mechanism, Volume 39, Number 2, 2014, pp. 171-185.

DOI: 10.3184/146867814X13981545064973

Image: Geometries of reactants, products, intermediates and transition states optimised at the RMP2 level (bond distances are in angstrom and angles are in degrees). The values in square parentheses refer to experimental data.