Microwave-Assisted Urea-Based-Hydrotropic Pretreatment of Rice Straw: Experimental Data and Mechanistic Kinetic Models

The three major lignocellulose components can be transformed into various biomass-derived platform fuels, chemicals, and materials upon pretreatment and chemical upgrading. Lignocellulose pretreatment is an important step to obtain an eco-friendly, economical, and effective biomass utilization process. The combination of microwave heating and hydrotropic pretreatment is considered as a green method of lignocellulose pretreatment. Experimental data and two mechanistic kinetic models of microwave-assisted pretreatment of rice straw are presented. Here, the use of urea solution as the hydrotropic agent was examined to facilitate the degradation of three major lignocellulose components. The first kinetic model assumes that the soluble lignin does not undergo condensation, while the second one assumes that part of the soluble lignin condenses to a solid product. The mechanistic models were validated with a series of experimental data obtained from microwave-assisted hydrotropic pretreatment of rice straw. The results show that both models could generally describe the experimental data well. However, based on the evaluation of the results of the kinetic models, it turned out that the rate of lignin condensation was relatively slow compared to the rate of lignin degradation to soluble lignin (the value of k _c is relatively small compared to the value of k _l1). Hence, the kinetic model with exclusion of lignin condensation is suggested more since it is mathematically simpler. The proposed mechanistic model can also predict the cellulose and hemicellulose dissolution and thereby can be used as a process optimization tool. The microwave-assisted urea-based hydrotropic pretreatment conducted at a solid-liquid ratio of 1:35, a urea concentration of 36.8%, a reaction temperature of 90 °C, and a pretreatment duration of 73.6 min is predicted to give a solid residue with low lignin content and high cellulose content which resulted in a cellulose to lignin ratio of 5.53. Cellulosic biomass characterization revealed that microwave-assisted hydrotropic pretreatment was able to produce higher crystallinity and thermally stable cellulosic biomass.