Acid-catalyzed fractionation of almond shells in γ-valerolactone/water

The fractionation of almond shells, an agro-industry residue available in some Mediterranean climate regions, was investigated using acid-catalyzed hydrolysis in gamma-valerolactone (GVL)/water. A set of non-isothermal experiments at nominal temperatures of 120, 140, and 160 degrees C and sulfuric acid concentrations from 25 to 75 mM were developed using a constant 80% w/w GVL in water concentration and a reaction time of up to 120 min. GVL was an efficient medium and promoted solubilization of both lignin and hemicellulose, even at low temperature during the initial period of reactor heating, while cellulose conversion was limited. A temperature of 160 degrees C gave the highest extraction of lignin and hemicellulose, but recovery of hemicellulose carbohydrates was better below 140 degrees C. Sulfuric acid concentrations above 45 mM promoted excessive dehydration of xylose and glucose to furans and humins, which were recovered with lignin. A model was developed to describe the kinetics of lignin and hemicellulose solubilization. It distinguished three fractions of different reactivity in each polymer (lignin or hemicellulose): fast-reacting, slow-reacting, and unreactive. The amount of each fraction was correlated with acid concentration and reaction temperature. Activation energies and the other parameters in the model were obtained numerically by least-squares optimization using the data from the non-isothermal experiments. Activation energies for the fast-reacting and slow-reacting fractions of hemicellulose were 142 and 39.7 kJ mol(-1), and for those of lignin 134 and 71.7 kJ mol(-1), respectively. Acid concentration had a larger influence than temperature on establishing the amounts of slow-reacting hemicellulose and lignin, whereas temperature was the dominant variable concerning the fractions of non-reacting polymers.