Reactive carbons from Kraft lignin pyrolysis: Stabilization of peroxyl radicals at carbon/silica interface

Lignin, the most abundant aromatic natural polymer and the second richest source of organic raw material, is considered as a potential source of energy, chemicals and carbon-based materials. In this work we have investigated the nature and reactivity of sp2 carbons obtained by the pyrolysis of soluble Kraft lignin, a byproduct of paper making process. The formation of carbons from Kraft lignin deposited onto high surface area silica was studied by vacuum pyrolysis at different temperatures, using pristine and acid-washed lignin. Among the various techniques adopted for the material characterization, the Raman measurements showed that the procedure successfully yields carbon atoms organized in sp2 domains, on the surface of silica, whose dimensions are sub-micrometric. The identification of a very stable peroxyl radical onto carbon-modified silica represents a major result, because these species are known to be usually transient. The observed peroxyl radicals are detectable only when both pyrolytic carbon and silica are present. The reactions of pyrolytic carbon onto silica with oxygen and nitrogen monoxide were studied by electron paramagnetic resonance (EPR). The kinetics and reversibility of the reaction with oxygen, as well as the reaction with nitrogen monoxide, were investigated.