Formation of Cyclopentadienyl Radical from the Gas-Phase Pyrolysis of Hydroquinone, Catechol, and Phenol

Abstract

The formation of radicals from the gas-phase pyrolysis of hydroquinone, catechol, and phenol over a temperature range of 400-750 degrees C was studied using the technique of low-temperature matrix isolation electron paramagnetic resonance (LTMI EPR). Cooling the reactor effluent from pyrolysis in a nitrogen carrier gas to 77 K produces a cryogenic matrix that exhibits poorly resolved EPR spectra. However, using carbon dioxide as a carrier gas formed a matrix that, upon annealing by slowly raising the matrix temperature followed by rapid recooling to 77 K, yielded more resolved, identifiable spectra. Annealed spectra of all three samples resulted in the generation of EPR spectra above 700 degrees C with 6 lines, hyperfine splitting constant approximately 6.0 G, and peak to peak width approximately 3 G that was readily assignable, based on comparison to the literature and theoretical calculations, as that of cyclopentadienyl radical. Pyrolysis at temperatures below 700 degrees C generated a carbon dioxide matrix isolation spectrum with a high g-value (>2.0040) that is attributed to oxygen-containing radicals such as semiquinone or phenoxyl. Conclusive identification of anticipated semiquinone, phenoxyl, and hydroxycyclopentadienyl radicals was complicated by the ability of these radicals to exist in carbon-centered and oxygen-centered resonance structures that can give different EPR spectra.

Formation of dibenzofuran, dibenzo-p-dioxin and their hydroxylated derivatives from catechol

“…3 Dellinger's group has thoroughly investigated thermal decomposition of CT reporting that both pyrolytic and oxidative decomposition reactions of CT result in the formation of dibenzo-p-dioxin (DD) and dibenzofuran (DF). 2,[5][6][7][8] In one of the contributions of the group, Khachatryan et al, 5 using the technique of low-temperature matrix isolation electron paramagnetic resonance spectroscopy (LTMI-EPR), detected the formation of an o-semiquinone (o-SQ) radical as the most prominent initial intermediate from decomposition of the CT molecule, via fission of one of its phenolic O-H bonds (Scheme 1).…”

Gas Chromatography Mass Spectrometry Identification of Labile Radicals Formed during Pyrolysis of Catechool, Hydroquinone, and Phenol through Neutral Pyrolysis Product Mass Analysis

“…Therefore OHCPD is formed but was not persistent enough to acquire its EPR spectrum with the various techniques we used in the previous studies [39,40,42].…”

“…Phenol thermal degradation leads to phenoxy radical by elimination of the hydrogen atom of the hydroxyl group of phenol. Further reaction of phenoxy proceeds by CO elimination to form CPD [39]. In our gasphase phenol thermal degradation study, CPD and phenoxy radicals are, respectively, identified as pyrolysis products and their EPR gas-phase spectra are acquired [39][40][41][42].…”

“…p-semiquinone (p-SQ), CPD, phenoxy, and hydroxycyclopentadienyl (HO-CPD) radicals are more likely the most Persistent Free Radicals (PFRs) to form from the pyrolysis/photolysis of HQ. We have positively identified and acquired p-SQ and CPD radicals EPR spectra [39,40].…”

The Modified Electrode by PEDOP with MWCNTs-Palladium Nanoparticles for the Determination of hydroquinone and Catechol

“…[3][4][5][6] Nevertheless, HQ and its isomers, catechol (CA; 1,2-dihydroxybenzene) and resorcinol (RE; 1,3-dihydroxybenzene), are harmful to both humans and animals, even at low concentrations, and they are difficult to degrade due to their coexistence and interference during determination. [7][8][9][10] Moreover, they are considered to be environmental pollutants by the United States Environmental Protection Agency (EPA) and the European Union (EU) and banned its use in cosmetics and such formulations.…”