Molecular Structure and Formation Mechanism of Hydrochar from Hydrothermal Carbonization of Carbohydrates

Hydrochars are solid byproducts formed during the liquid-phase biorefinery process and could be used to generate functional carbonaceous materials, but the detailed molecular structure and the formation mechanism are still unclear. Herein, the formation of hydrochars from liquid-phase carbonization of biomass-derived compounds including glucose, fructose, xylose, ribose, dihydroxyacetone (DHA), 5-hydroxymethylfurfural (HMF), furfural (FF), and pyruvaldehyde (PRV) in water and inert polar organic solvents ethyl acetate (EAC) and tetrahydrofuran (THF) was studied. The carbohydrates were found to generate hydrochars in both water and the organic solvents, while the HMF and FF could generate hydrochars only in water. The α-carbonyl aldehydes, including PRV, 3-deoxyglucosone, and 2,5-dioxo-6-hydroxyhexanal (DHH), formed during the decomposition of carbohydrates were proposed to be the key primary precursors for hydrochar formation. The molecular structures of the hydrochars were characterized by elemental analysis, Fourier transform infrared analysis, and solid-state 13C NMR analysis to confirm that the molecular formula of the hydrochars all could be approximately expressed as (C3H2O)n, and the molecular structures of the hydrochars all consisted of polyaromatic hydrocarbon, phenolic, furanic, and aliphatic framents and a small amount of carbonyl/carboxyl groups. The presence of the polyaromatic hydrocarbon and phenolic fragments in the hydrochars suggested that aldol condensation played a critical role for hydrochar formation. By regarding the aldol condensation of α-carbonyl aldehydes as the initial step for hydrochar formation, we deduced the polymerization routes of these α-carbonyl aldehydes and found that the α-carbonyl aldehydes all could undergo aldol condensation followed by acetal cyclization and etherification to form polymers (C3H2O)n rich in furanic framework or undergo aldol condensation followed by a 1,2-hydride shift, intramolecular aldol condensation, and dehydration to generate polymers (C3H2O)n rich in the phenolic framework. One molecular structure containing polyaromatic hydrocarbon, phenolic, furanic, and aliphatic fragments is proposed for the hydrochars.