Radicals from the Gas-Phase Pyrolysis of Hydroquinone: 1. Temperature Dependence of the Total Radical Yield

Abstract

The formation and temperature dependence of the total yields of radicals from the gas-phase pyrolysis of hydroquinone (HQ) over a temperature range of 350−950 °C was studied using the technique of low-temperature matrix isolation electron paramagnetic resonance (LTMI−EPR) spectroscopy. Annihilation procedures along with microwave power saturation studies of the frozen radicals as well as GC−MS analysis of products helped to clearly identify several types of radicals over the temperature range studied. On the basis of these results, three formation temperature regimes could be identified: low-temperature region from 350 to 725 °C, where neutral p-semiquinone (p-SQ) radical is dominant, high-temperature region from 850 to 975 °C, where cyclopentadienyl (CPD) radical is dominant, and an intermediate-temperature region from 725 to 850 °C, where a mixture of p-SQ and CPD radicals exists.

Radicals from the gas-phase pyrolysis of a lignin model compound: p-coumaryl alcohol

“…The same trend was observed earlier in the gas-phase pyrolysis of hydro-quinone (HQ) at 550 to 750 °C and that of catechol (CT) at 600 to 750 °C, indicating the compositional changes of radicals at different temperatures. 45,46 …”

“…Variation of the g -value and Δ H p–p is indicative of either the existence of different types of radicals or of a significant disturbance in the matrix environment. 32 The signal intensity of a trapped radical is also an important characteristic parameter; it is calculated as the double integrated normalized intensity (DI/N) in WINEPR. This accounts for a number of variables, such as the conversion time, receiver gain and sweep width.…”

“…, gradual warming of the Dewar flask by removing liquid nitrogen, was applied in some experiments to produce cleaner and sharper spectra by selective annihilation of more reactive radicals. 17,32–38 …”

Thermal reactivities of catechols/pyrogallols and cresols/xylenols as lignin pyrolysis intermediates

“…Lomnicki et al [9] reported that p-hydroquinone gave p-benzoquinone in a 32% yield at 550 • C, while o-benzoquinone was not observed as a product from ohydroquinone (pyrocatechol). Such difference has been discussed with the influence of the position of hydroxyl group (p-or o-) on formation of semiquinone radicals as intermediates of benzoquinones [38,40,41].…”

Ab initio study of the formation and degradation reactions of chlorinated phenols

“…Semiquinone radicals are known to be quite stable [12] and capable of reducing molecular oxygen to superoxide radical anion in a cyclic process [13]. Since monochlorinated phenoxyl radicals are valence isolectronic with semiquinone radicals and probably even more resistant to oxidation due to chlorine substitution, it is possible that chlorinated phenoxyl radicals may be even more toxic than the better studied semiquinone family of radicals.…”