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

摘要

Experimental and theoretical studies on the oxidation of saturated hydrocarbons (n-hexane, cyclohexane, n-heptane, n-octane and isooctane) and ethanol in 28 Torr O-2 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 - (+) 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(+), H2O, olefins (and/or cycloalkanes) and oxygen-containing compounds were eliminated from these ions. Ozone as one of the terminal products in the O-2 plasma was postulated as the oxidizing reagent. As an example, the reactions of C6H14+ with O-2 and of C6H13+ (CH3CH2CH+CH2CH2CH3) with ozone were examined by density functional theory calculations. Nucleophilic interaction of ozone with C6H13+ leads to the formation of protonated ketone, CH3CH2C(=OH+)CH2CH2CH3. In air plasma, M - H + O(+) became predominant over carbocations, M - H(+) and M - 3H(+). For ethanol, the protonated acetic acid CH3C(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 (C) 2016 John Wiley Sons, Ltd.