Optimization of bio-chemical conversion of rice straw to 5-(hydroxymethyl) furfural

Abstract

Agricultural waste-based biorefinery merges waste and production sectors to develop a circular economy. One such biorefinery product, 5-(hydroxymethyl) furfural (5-HMF) has gained increasing interest as a versatile platform chemical to produce chemicals and fuels, and rice straw will be an ideal feedstock to produce 5-HMF. Limited studies on the direct conversion of rice straw to 5-HMF reveal the requirement of extensive research. This MPhil research study focuses to identify the most feasible conversion process to convert rice straw into 5-HMF, and optimize the required process parameters in laboratory-scale to be used in unit processes in scaled-up implementation. As the first objective, this thesis has proposed a roadmap, elucidating the existing methods for rice straw pretreatment to convert cellulose and cellulose conversion to 5-HMF processes. Then cellulose conversion to 5-HMF was evaluated, considering catalyst used, solvent system, process temperature, and process time. Eventually, an evaluation method, based on a generalized objective function with penalty scores, was developed and used its minimum value to find the optimal process configuration at the lowest cost for large-scale 5-HMF production. However, considering the feasibility of large-scale application of these processes in Sri Lankan context, combinatory acid/ alkaline pretreatment method and mineral acid-catalyzed cellulose to 5-HMF conversion process were selected for the optimization. The final objective was experimentally optimization of the process parameters of each, using an advanced optimization technique, response surface methodology. Central composite design-based experiments were used to develop statistical models for each process. Quantitative analyses were performed, using regression techniques, analysis of variance, and residual analysis, whereas qualitative analyses were carried out via Fourier Transform Infrared Spectroscopy. The pretreatment process was carried out to maximize the outcome of rice straw biorefinery. It included two steps: (1) dilute sulphuric acid treatment at reduced temperatures to optimize hemicellulose removal, and (11) dilute sodium hydroxide treatment at reduced temperatures to optimize lignin removal. The maximum hemicellulose removal (15.78%) was observed at optimal conditions of 0.26 moldm−3 acid concentration, 98.1 ∘C reaction temperature, and 30.48 min reaction time. The maximum removal of lignin (20.98%) was obtained at 2.55 moldm−3 sodium hydroxide concentration, 80.5 ∘C reaction temperature, 106.48 min reaction time. An optimized acid- catalyzed hydrothermal process for the in-situ production of 5-HMF from rice straw extracted cellulose was obtained, using a biphasic reaction system. The maximum yield of 5- HMF was 23.51% at optimal conditions of 0.046 moldm−3 dilute hydrochloric acid concentration, 180 ∘C process temperature, and 107 min process time. Finally, validation experiments were performed, and the observed optimum results showed close agreement with the predicted, confirming rice straw biorefinery process optimization. Keywords: Rice straw, Biorefinery, 5-(hydroxymethyl) furfural, Pretreatment, Cellulose, Catalytic conversion, Optimization, Respo