Abstract
Abstract The results of a detailed mechanistic study of aqueous-phase OH-oxidation of methanol are presented. Analysis of reaction products by specific chromatographic methods revealed that hydrated formaldehyde is not the only stable primary reaction product. Formic acid and/or formate ion are also stable primary molecular reaction products of methanol OH-oxidation. The branching ratios for their formation are highly pH dependent. At pH=7, hydrated formaldehyde is the dominant molecular reaction product (ratio 4.5 : 1 for hydrated formaldehyde : formate ion), whereas at pH=2, formic acid is the dominant product (ratio 3.7 : 1 for formic acid : hydrated formaldehyde). At all pH studied, the sum of the primary stable products represents 49 (±11)% of methanol removal, in agreement with the amount of OOCH 2 OH radicals formed relative to methanol removal 48(±2)%. The formation of primary formic acid at pH=2 is attributed to OOCH 2 OH self-reaction, and the strong pH effect is attributed to the base-catalyzed decomposition of OOCH 2 OH leading to the formation of hydrated formaldehyde. Evaporation and/or an addition reaction between CH 2 OH and HO 2 radicals leading to the formation of hydroxymethyl hydroperoxide is proposed to explain the missing yields. The implications of this mechanism to atmospheric chemistry are discussed.
Published Version
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