Kinetic and thermodynamic evidences of the Diels-Alder cycloaddition and Pechmann condensation as key mechanisms of hydrochar formation during hydrothermal conversion of Lignin-Cellulose

Abstract

We investigated the kinetics and thermodynamics of the Diels-Alder cycloaddition and Pechmann condensation to evaluate the significance of both mechanisms toward hydrochar formation during hydrothermal carbonization and liquefaction of biomass. We performed well-designed experiments of semi-continuous hydrothermal conversion of pure compounds representative of lignin-cellulose fractions (i.e., D-(+)-cellobiose and 1-(3,4-dimethoxyphenyl)-2-(4-hydroxy-2-methoxyphenoxy)propane-1,3-diol) and corresponding hydrochar precursors at 220–370 °C for 60 min. We developed thermodynamically-feasible multi-phase reaction pathways and specified the kinetic and thermodynamic parameters of each pathway by fitting the reversible power-law kinetic model, Arrhenius equation, and correlation between conventional definition of Gibbs free energy and the Gibbs free energy isotherm equation into experimental data of time evolution of chemical species concentration measured using GC–MS, XRD, and μGC. Reaction pathways of mixtures of pure compounds were a combination of those for individual pure compounds. We discovered that endothermic Diels-Alder cycloaddition involving dienes and dienophile was prominent in D-(+)-cellobiose conversion pathways and functioned as an intermediate reaction producing precursors for various endothermic and exothermic precipitation mechanisms. The temperature dependence of solid organics formation via this indirect route depended on the net ΔH of a set of reaction pathways after Diels-Alder cycloaddition. In contrast, we found that the Pechmann condensation involving phenols and β-ketoester/β-ketoacid was significant in the conversion pathway of 1-(3,4-dimethoxyphenyl)-2-(4-hydroxy-2-methoxyphenoxy)propane-1,3-diol and facilitated direct precipitation of organics including 4,10-dimethyl-2,7-dioxo-2,7-dihydropyrano[2,3-g]chromene-5-carbaldehyde and 4,5-dimethoxy-2,7-dioxo-6,7-dihydro-2H-benzo[g]chromene-10-carboxylic acid. The endothermic nature of Pechmann condensation preferred higher temperatures for a higher equilibrium conversion toward solid product formation. Results in this study contributed to the improved understanding of fundamentals of hydrochar formation.