Reactivity and free radical chemistry of lilac (Syringa vulgaris) charcoal

Abstract

The reactivity, and surface chemistry of charcoal determine its combustion behaviour, and these properties depend on the source of the original wood, production conditions and treatment. Here we studied the properties of charcoal derived from lilac (Syringa vulgaris). Its reactivity was tested by isothermal and non-isothermal thermogravimetric analysis and differential scanning calorimetry in air and nitrogen. The free radical concentration was determined by measuring the electron spin resonance of fresh charcoal, after washing with HCl, and after degassing in air with or without nitrogen. We found that lilac is highly reactive, igniting at 250–300 oC with peak combustion at 320–520 oC. The quantity of oxygen consumed and heat released during oxidation increased with temperature. The free radical concentration in the untreated charcoal was 5.29 x 1018 spins/g, compared to 3.49 x 1019 spins/g after acid washing, 7.06 x 1019 spins/g after exposure to air, and 3.75 x 1017 spins/g after degassing with nitrogen before exposure to air. The line width of all the charcoal samples was 11.6–11.9 G. However, degassing the charcoal in nitrogen followed by exposure to air at low temperatures resulted in a four-fold increase in the line width to 41.8 G. The exposure of lilac charcoal to air alone at low temperatures resulted in the formation of persistent peroxyl radicals superimposed on the main peak. The g-values of charcoal samples that were untreated, acid washed, degassed in N2 + air, and degassed in air alone (main peak) were 2.00481, 2.00477, 2.00260 and 2.00483, respectively. The g-values of the peroxyl radicals superimposed on the main peak were 2.0155, 2.0138, 2.0020 and 2.0007, respectively. The reactivity, and free radical content suggest that lilac charcoal is particularly suitable for applications involving energetic materials, catalysis and co-firing.