Radicals from the Gas-Phase Pyrolysis of Hydroquinone: 2. Identification of Alkyl Peroxy Radicals

Abstract

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 CO 2 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 O 2 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.

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

“…49 These radicals may arise in the presence of traces of oxygen in the vacuum system. 50 However, they cannot be the dominant species in the radical mixture detected here at high temperatures, i.e. , 700 to 1000 °C.…”

Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds. Cinnamyl alcohol

“…Another candidate to explain the anisotropy of the observed EPR spectra could serve the alkylperoxy radical (RO 2 ) readily formed in the presence of trace amounts of oxygen [39] and described by a high g-value between 2.008 and 2.010 [39,40]. However, the formation of RO 2 traces was ruled out based on the power dependence experiments, Fig.…”

Stable Organic Radicals in Lignin: A Review

“…A representative spectrum of trapped radicals is depicted in Figure (spectrum 1). The spectrum was a singlet with g =2.0072 and spectral line width (Δ H p–p) of 14.0 G. Minor signals on both sides of the main spectrum (marked with an asterisk in Figure A) originated from the presence of trace quantities of oxygen as E lines ( K =1, J =2, M =1→2); it was believed that these could be easily removed by annealing …”

“…An EPR spectrum from a Burley tobacco sample was compared with a lignin sample because the pyrolysis of tobacco was similar to that of the pyrolysis of lignin (Figure A, spectrum 2) . These similarities originated from radicals formed, such as catechol, hydroquinone, and other organics, in the presence of trace quantities of oxygen . If the spectrum of tobacco (Figure , spectrum 2, black line) was subtracted from the spectrum of EPR radicals from the lignin pyrolysis (Figure , spectrum 1, red line), the resulting subtraction spectrum was obtained with an elevated g value of 2.0064 and Δ H p–p=18 G (Figure , spectrum 3, blue line).…”

“…The radicals from phenol and hydroquinone/catechol pyrolysis (and photolysis) were generated from lignin degradation, previously identified as phenoxy and semiquinone radicals, respectively . The phenoxy radical spectrum showed a broad spectral line width (Δ H p–p=16 G), whereas the semiquinone radical showed a narrower line width (Δ H p–p=12 G), possibly due to phenoxy linkages, which were present at higher concentration than those of semiquinones in lignin pyrolysis . Consequently, from these results, it was concluded that the free radical species were mostly created from phenolic linkages in lignin and were possible precursors to the formation of phenolic compounds, such as 2,6‐dimethoxyphenoxy (syringyl groups), 2‐methoxyphenoxy (guaiacyl groups), and phenols for (phenoxy groups)c. GC‐MS analyses also supported these conclusions.…”