A hierarchical surrogate approach to biomass ethanolysis reaction kinetic modelling†

The reaction mechanism and kinetics of the sulfuric acid catalysed ethanolysis of glucose, cellulose, xylan, and corncob were investigated using a combination of experiments and empirical reaction mechanism modelling. The experimental study was carried out in ethanol at various temperatures between 150 °C and 200 °C. Ethanol mediates the depolymerisation and formation of ethyl levulinate from the carbohydrates in the substrates. Ethanol itself is converted to the corresponding ether in a parallel acid-catalysed condensation reaction. The complementary synergistic thermal and combustion properties of the main components in the resulting mixture, ethyl levulinate, diethyl ether, and ethanol, create the potential for the use of the product mixture as a tailored drop-in biofuel. The concentrations of the main species in the product mixtures from the reaction experiments were used to build a hierarchical surrogate kinetic model based on feedstock composition. The reaction mechanism provided to the surrogate kinetic model is informed by a comparative experimental mechanistic study of the ethanolysis of glucose and fructose. The study shows that the major reaction species formed from glucose ethanolysis are ethyl glucoside and ethyl levulinate, whereas fructose ethanolysis primarily forms 5-hydroxymethylfurfural, 5-ethoxymethylfurfural, ethyl fructoside and ethyl levulinate. The study shows that fructose produces a higher yield of ethyl levulinate than glucose and that fructose does so at a rate approximately ten times faster than glucose. The rate of formation of both ethyl levulinate and diethyl ether increases with increasing temperature. The maximum yields (mass%) of ethyl levulinate achieved from the ethanolysis of glucose, cellulose, xylan, and corncob are 39.3, 39.1, 7.9, and 18.6%, respectively. Ethyl levulinate yields reach a maximum steady state value for each feedstock that is independent of temperature. The conversion of the model compounds, glucose, cellulose, and xylan, to ethyl levulinate in the presence of ethanol and sulfuric acid is a catalytic process. However, for corncob, the yield of ethyl levulinate is dependent on the concentration of sulfuric acid in the reaction. This effect is also observed in the mass fraction of diethyl ether formed, indicating that the hydrogen cation supplied by sulfuric acid is not being fully replenished in the corncob ethanolysis process. A corncob : acid mass ratio of 10 : 1 is identified as a sufficient sulfuric acid concentration to achieve a maximum steady state yield of ethyl levulinate. An empirical analysis of the experimental data show that the apparent activation energies of the global reaction of glucose to ethyl levulinate and ethanol to diethyl ether are 21.5 and 23.0 kcal mol⁻¹, respectively. The hierarchical surrogate kinetic model for the ethanolysis of corncob based on its composition of cellulose, hemicellulose, and lignin was developed and has an overall R² value of 0.88. The model was exercised to predict the major trends of the reaction system at various hypothetical conditions, demonstrating its utility as tool for process development.