Catalyst-free toluene hydrogenation to methyl-cyclohexane by pulsed DBD plasma under ambient conditions

[Display omitted] •H2 DBD plasma enables the toluene hydrogenation under ambient conditions without catalysts.•Minimum energy consumption is 0.12 kW∙h/mmol with methyl-cyclohexane of 80.1 mol%.•It is the H radicals from the plasma that step-by-step hydrogenates the aromatic ring. Hydrogenation of aromatic compounds is an important reaction in the petrochemical, pharmaceutical, and organic industries. In this paper, we report a novel catalyst-free hydrogenation process that converted gaseous toluene into methyl-cyclohexane at ambient pressure and room temperature using microsecond pulsed dielectric barrier discharge (DBD). The hydrogenation performance of the toluene was investigated in terms of the pulse repetition frequency, reactor temperature, and toluene concentration. The experiments show that the toluene was easily hydrogenated to methyl-cyclohexane by the H2 DBD plasma, in which higher PRF, lower reactor temperature and higher H2 concentration were favorable for the production of methyl-cyclohexane. The highest molar fraction of the methyl-cyclohexane was 80.1% with energy consumption of 0.12 kW∙h/mmol. Controlled experiments with Ar atmosphere or different feedstocks (d-toluene, methyl-cyclohexadiene and methyl-cyclohexene) indicate that the H radicals from the H2 DBD plasma realize the toluene hydrogenation. The possible reaction mechanism of the plasma-enabled hydrogenation was discussed via the optical emission spectroscopy (OES), density functional theory (DFT) and molecular dynamic simulations, which confirms that the H radicals produced by the electron-impact reactions induce the step-by-step hydrogenation reaction of the toluene. Overall, this work provides not only new insights into the plasma-enabled hydrogenation reaction, but also a potentially effective catalyst-free method for hydrogenation applications.