3810
Energy & Fuels 2008, 22, 3810–3813
Radicals from the Gas-Phase Pyrolysis of Hydroquinone: 2. Identification of Alkyl Peroxy Radicals Lavrent Khachatryan, Julien Adounkpe, and Barry Dellinger* Department of Chemistry, 413 Choppin Hall, Louisiana State UniVersity, Baton Rouge, Louisiana 70803 ReceiVed June 9, 2008. ReVised Manuscript ReceiVed September 11, 2008
The formation of radicals from the gas-phase pyrolysis of hydroquinone (HQ) from 400 to 825 °C was studied using the technique of low-temperature matrix isolation-electron paramagnetic resonance (LTMI-EPR). Cooling the reactor effluent in a CO2 carrier gas to 77 K produced a cryogenic matrix that exhibited complex EPR spectra. The observed EPR spectra were very sensitive to the delivery rate of HQ, presence/absence of carrier gas, and traces of O2 in the reaction gas that required careful manipulation of the experimental conditions to separate labile and persistent radicals. Conclusive identification of peroxyl radical formed during the pyrolysis of HQ in the presence of oxygen was based on the anisotropy of EPR spectra, high g value, hyperfine splitting constant, and spectral width that matched very well with literature data. The presence of peroxyl radicals during gas-phase, oxidative pyrolysis of HQ suggests the formation of alkyl radicals as precursors of light hydrocarbons: methane, ethane, and olefins previously observed from the pyrolysis of HQ.
Introduction We have previously reported studies of the thermal degradation of hydroquinone (HQ) under controlled conditions that allowed trapping of the resulting radical(s) using the technique of low-temperature matrix isolation-electron paramagnetic resonance (LTMI-EPR) spectroscopy.1,2 Two different types of radicals were positively identified from the gas-phase pyrolysis of HQ over the temperature range of 350-950 °C. Neutral para-semiquinone radical (p-SQ) dominated from 350 to 725 °C, while cyclopentadienyl (CPD) radicals were dominant from 850 to 975 °C. At intermediate temperatures, 725-975 °C, a mixture of p-SQ and CPD radicals were identified on the basis of annihilation and microwave power saturation experiments for the frozen radicals and gas chromatography-mass spectrometry (GC-MS) analysis of pyrolysis products.2 Hydroxycyclopentadiene radicals were observed in trace quantities. It was concluded that the steady-state concentration of the phenoxy radical, one of the anticipated radicals, was below the sensitivity of EPR because of its high reactivity. In this paper, the effects of trace oxygen, reaction pressure, and initial reactant concentration on the formation of radical products and EPR spectra of p-SQ are presented. Experimental Procedure Pyrolysis Reactor and Conditions. A thermoelectrically heated reactor was used for the pyrolysis of HQ. It was interfaced to a liquid nitrogen-cooled Dewar located in the cavity of an EPR spectrometer, which allows for the detection and identification of radicals from gas-phase thermal reactions.1,2 Reactants are delivered using rotary pumps at a pressure of
