The Nature of Stable Char Radicals: An ESR and DFT Study of Structural and Hydrogen Bonding Requirements

Pyrolysis is a promising way to convert biomass into fuels and chemicals. This reaction is complex and inevitably involves a cascade of radical reactions that lead to char formation, in which some radicals become trapped and stabilized. Their nature is difficult to characterize, and in this respect computational chemistry can be a strong supplementary tool to electron spin resonance spectroscopy and other experimental methods. Here biomass char radicals and oxidation reactivity are studied experimentally, and density functional theory is used to predict the thermodynamic stability and g-values of carbon- and oxygen-centered radicals of polyaromatic char models including defect structures. Hydroxylated and especially certain dihydroxylated structures provide exceptional stabilization of oxygen-centered radicals. Hydrogen bonding plays a crucial role, and it is proposed that hydrogen atom transfer couples radical localizations. This is a new proposal on the structural requirements for stabilization of char radicals, which impacts our understanding of pyrolysis mechanisms and char reactivity.