Article pubs.acs.org/EF
Mechanistic Insights and Kinetic Modeling of Cellobiose Decomposition in Hot Compressed Water Xiao Liang, Alejandro Montoya,* and Brian S. Haynes School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia S Supporting Information *
ABSTRACT: The decomposition of cellobiose (CB) under moderate hydrothermal conditions is studied experimentally using microtubular and pilot-scale flow reactors. The conversion of CB and formation of products, including cellobiulose, glucose, fructose, 5-hydroxymethylfurfural, and 2-furfural, are quantified, accounting for more than 70% of the initial carbon. Organic acids, such as formic, succinic, glycolic, and acetic acids, are identified. The measured proton concentration increases with CB conversion up to 90% in a manner consistent with the formation of a notional acid with a yield of 0.49 ± 0.11 mol of acid/mol of CB decomposed and pKa = 3.33 ± 0.11. Slow ongoing formation of acids occurs after CB is substantially depleted (X > 90%). The rate of CB decomposition is increased only slightly in the presence of added formic acid, but the selectivity of glucose is enhanced significantly. A thermodynamically consistent kinetic model (40 reversible steps) is developed using reaction pathways, thermodynamics, and kinetics from the literature and our own molecular modeling computations. The model is validated by comparing predictions to our current experiments and other published results. Excellent agreement is obtained without parameter adjustment over the temperature range of 124−320 °C. Sensitivity analysis reveals the competition between protoncatalyzed and uncatalyzed hydration as well as isomerization and reverse aldol condensation processes.
1. INTRODUCTION Hydrothermal liquefaction (HTL) of the carbohydrate component of biomass is a promising technique for the production of fuels and chemicals, such as furans, alcohols, ketones, and low-molecular-weight acids.1,2 Detailed analyses of the yields of products as a function of reaction parameters and the chemical nature of the starting material have been obtained mainly for monomeric carbohydrates, such as glucose (GLU),3−6 fructose (FRU),7,8 and xylose,9,10 and dimeric carbohydrates, such as cellobiose (CB),11−16 maltose,17 and sucrose.18 CB, a disaccharide containing two GLU units connected via one glycosidic bond, is a primary product of the depolymerization of cellulose.1 The HTL decomposition of CB has been analyzed using different reactor configurations, in the absence and presence of catalysts. Early analysis of CB hydrothermolysis was carried out in batch reactors between 180 and 249 °C, focusing on the extent of conversion and the yield of GLU.16 Only ∼5% conversion of CB was obtained in 14 min at 180 °C, while nearly complete decomposition occurred in less than 2 min at 249 °C.16 The reaction pathway of CB decomposition is difficult to analyze under HTL conditions as a result of the intricacy of the reaction network, the short lifetime of primary reaction products, and the complexity of reaction environments arising from the formation of numerous organic compounds. Recent studies have then focused on the reaction pathways and kinetics using continuous plug flow microreactor systems at conditions between 200 and 420 °C and between 100 and 400 bar for residence times of
