Valorization of lignocellulosic waste through sustainable transformation and selective separation processes

Abstract

This doctoral thesis has been, carried out at the Research Institute for Sustainable Technologies (ITPS) at Universidad Rey Juan Carlos and it is aligned within the strategic objective of advancing in the implementation of sustainable biorefineries through the adequate valorization of lignocellulosic biomass waste. The current study focuses specifically on urban pruning and gardening waste, which constitutes a non-edible and abundantly available lignocellulosic waste. Unfortunately, this waste is still largely disposed via landfilling or inefficiently valorised by energy recovery methods. This research addresses a novel concept by means of the integrating of catalytic hydrothermal treatment, dark acidogenic fermentation, and selective separation techniques with the purpose of creating a closed-loop waste valorization strategy and aligned with the EU circular economy paradigm. The lignocellulosic biomass used in this study was characterized through various techniques, including thermogravimetric analysis (TGA), elemental analysis (HCNS-O), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). This comprehensive characterization enabled the identification of structural and compositional properties critical to the performance of subsequent catalytic and fermentation processes. The first phase of the research explored hydrothermal catalytic treatment for the selective depolymerization of lignin in lignocellulosic biomass and coproduction of carboxylic acids. Four MgO-based catalysts were synthesized using different synthesis methods. The catalyst synthesized via a sol-gel process (MgO I) exhibited superior properties, including high macroporosity and high medium basicity, which contributed to an efficient lignin breakdown and high selectivity toward carboxylic acids. Under mild operating conditions (120°C, 30 minutes), over 76% of converted polymers were attributed to lignin degradation, and more than 30% of carbon in the raw waste was recovered as commercially valuable carboxylic acids, particularly dicarboxylic acids.