Mass spectrometric monitoring of oxidation of aliphatic C6-C8 hydrocarbons and ethanol in low pressure oxygen and air plasmas.

Abstract

Experimental and theoretical studies on the oxidation of saturated hydrocarbons (n-hexane, cyclohexane, n-heptane, n-octane and isooctane) and ethanol in 28 Torr O2 or air plasma generated by a hollow cathode discharge ion source were made. Ions corresponding to [M + 15]+ and [M + 13]+ in addition to [M - H]+ and [M - 3H]+ were detected as major ions where M is the sample molecule. The ions [M + 15]+ and [M + 13]+ were assigned as oxidation products, [M - H + O]+ and [M - 3H + O]+ , respectively. By the tandem mass spectrometry analysis of [M - H + O]+ and [M - 3H + O]+ , H2 O, olefins (and/or cycloalkanes) and oxygen-containing compounds were eliminated from these ions. Ozone as one of the terminal products in the O2 plasma was postulated as the oxidizing reagent. As an example, the reactions of C6 H14+• with O2 and of C6 H13+ (CH3 CH2 CH+ CH2 CH2 CH3 ) with ozone were examined by density functional theory calculations. Nucleophilic interaction of ozone with C6 H13+ leads to the formation of protonated ketone, CH3 CH2 C(=OH+ )CH2 CH2 CH3 . In air plasma, [M - H + O]+ became predominant over carbocations, [M - H]+ and [M - 3H]+ . For ethanol, the protonated acetic acid CH3 C(OH)2+ (m/z 61.03) was formed as the oxidation product. The peaks at m/z 75.04 and 75.08 are assigned as protonated ethyl formate and protonated diethyl ether, respectively, and that at m/z 89.06 as protonated ethyl acetate. Copyright © 2016 John Wiley & Sons, Ltd.