Copper(II) oxide mediated formation and stabilization of combustion generated persistent free radicals

Abstract

Biologically damaging semiquinone-type radicals have been reported in cigarette smoke, their likely origin being hydroquinone (HQ) and catechol (CT) molecular precursors contained by the tobacco. Since other biomass contains HQ and CT-type species, it is likely that combustion of other biomass will also form semiquinone-type radicals. All hydrocarbon fuels will form phenol and substituted phenol that can form substituted phenoxyl radicals. Because each of these radicals has the potential to be environmentally persistent and biologically active, their formation and stabilization from various molecular precursors was studied with the focus on surface-bound radicals formed in association with combustion-generated fly-ash particles. Comprehensive product yield determinations from the high-temperature, gas-phase degradation of HQ and CT revealed the formation of dibenzofuran, dibenzo-p-dioxin, benzoquinone, phenol, benzene, phenylethyne, styrene, indene, naphthalene and biphenylene. The formation of semiquinone, phenoxyl, and cyclopentadienyl radical is strongly implied because of the observation of benzoquinone, phenol, and naphthalene, respectively during the thermal decomposition of HQ and CT process. Radicals were generated and stabilized on the surface of 5% copper (II) oxide/silica dioxide (CuO/SiO2) particles, which was used as a surrogate for combustion-generated fly-ashes. These radicals were generated from the following precursors through chemisorption and electrons transfer between the matrix and the adsorbed molecules: monochlorobenzene (MCBz), 1,2-dichlorobenzene (1,2-DCBz), phenol (P), 2-monochlorophenol (2-MCP), HQ and CT. Electron paramagnetic resonance (EPR) was used to investigate the characteristic of radicals. Gas chromatography-mass spectroscopy (GC-MS) was used to identify molecular species formed during solvent extraction using polar solvents; methyl alcohol, isopropyl alcohol, and dichloromethane; and non-polar solvents; toluene, and tert-butylbenzene. All of precursors generated surface-associated radicals with maximum yields between 200oC - 230oC and were very persistent in the air. Polar solvents extracted more free radicals than non-polar solvents. GC/MS analysis identified many molecular dimer species in solution indicating radical-radical reaction in the extract solution. These studies indicate that CuO/SiO2 surface mediates the formation and stabilization of substituted phenol-type surface-associated radicals that can be environmentally persistent. Solvent extraction converts these radicals to molecular species that may results in their misidentification in the literature as molecules rather than radicals.