CO₂ Sequestration by Humic Substances and the Contribution of Quinones and Quinone Imines: Consideration on the Molecular Scale

Abstract

Recent results support the assumption that hydroquinones (e.g., methoxyhydroquinone, MHQ), benzoquinones (e.g., methoxy-[1,4]benzoquinone, MBQ; 2,5-dihydroxy-[1,4]benzoquinone, DHBQ), and quinone imines (e.g., 2,5-dihydroxy-[1,4]benzoquinone imine, DHBQI) are pivotal compounds in the oxidative degradation of ligneous materials and in the formation of nitrogenous oligo- and polymeric materials. Furthermore, DHBQ is able to bind CO₂ under neutral aqueous conditions, forming quinone carboxylic acids and subsequently stable products. In addition, DHBQIs have a considerable CO₂-binding potential as (1) the transition state of the electrophilic addition of CO₂ to the respective dianion has a similarly low activation energy as in the case of DHBQ and (2) the tautomeric 2,5-diamino-[1,4]benzoquinone, DABQ) has the tendency to form ion-pair complexes.