Valorization of lignocellulosic waste through sustainable transformation and selective separation processes
- Autores: Nora Vidal Duarte
- Directores de la Tesis: Juan Antonio Melero Hernández (dir. tes.), Fernando Martínez Castillejo (codir. tes.)
- Lectura: En la Universidad Rey Juan Carlos ( España ) en 2025
- Idioma: español
- Tribunal Calificador de la Tesis: Brenda Cecilia Soledad Ledesma (presid.), Gema Gómez Pozuelo (secret.), Marcos Larriba Martínez (voc.)
- Programa de doctorado: Programa de Doctorado en Tecnologías Industriales: Química, Ambiental, Energética, Electrónica, Mecánica y de los Materiales por la Universidad Rey Juan Carlos
- Materias:
- Enlaces
- Tesis en acceso abierto en: TESEO
- Resumen
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.
However, catalyst stability emerged as a critical challenge due to the formation of Mg(OH)2 under hydrothermal conditions of the reaction which promotes the loss of selective delignification and decrease of carboxylic acids production. Hence, further research was subsequently addressed to improve catalyst robustness.
Hence, in the second stage of the research, transition metals (Fe, Cu, and V) were supported on the MgO support to enhance oxidative capacity with the purpose of improvement of the carboxylic acids production. Among all the tested formulations, 10 wt. % V/MgO emerged as the optimal catalyst, combining high selective lignin removal (SLR) of 0.8 with minimal conversion of cellulose and hemicellulose. This catalytic performance arises from its moderate oxidative capacity as well as high macroporosity in comparison with Fe and Cu containing materials Moreover, the catalyst maintained its catalytic performance over three reaction cycles with negligible metal leaching, thereby demonstrating both catalytic efficiency and operational stability. Notably, the catalytic modifications also led to a higher overall yield of valuable monomers, including volatile and non-volatile fatty acids in comparison of non-metal supported MgO (I) (achieving a carbon selectivity toward carboxylic acids of 37%) The solid fraction obtained after catalytic treatment, primarily holocellulose, was subjected to dark acidogenic fermentation assays. Initial fermentation attempts using untreated holocellulose resulted in poor yields due to the lignin content which hinders microbial accessibility. However, the hydrothermal catalytic pretreatment of the waste significantly improved bioconversion rates. Subsequent fermentation tests yielded a total carboxylic acid concentration of 1.10 g/L, dominated by acetic and butyric acids, and a cumulative gas production of 55 mL CO2/gVS and 17 mL H2/gVS. Interestingly, rare fermentation byproducts such as isobutyric and hexanoic acids were also detected, suggesting potential metabolic pathway shifts likely due to the specific composition of the substrate and microbial community dynamics.
A final experimental step of the current research was addressed to the selective separation of carboxylic acids from the fermentative broth, particularly those with longer carbon chains (C greater than or equal to 5). Hydrophobic eutectic solvents (HES) based on trioctylphosphine oxide (TOPO) combined with menthol or thymol in different molar ratios were thoroughly characterized and tested in extraction assays. Three stable eutectic mixtures were identified with molar ratios of: 1TOPO:2Thymol, 1TOPO:1Thymol, and 1TOPO:2Menthol, respectively. These mixtures kept the liquid state at room temperature for long storing times. The molecular interactions of the molecules were determined by ¹H and ³¹P nuclear magnetic resonance (NMR) spectroscopy indicating that the modification of 1H and 31P environments are strong evidence of the formation of stable liquid mixtures. The 1TOPO:2Thymol mixture demonstrated the highest selectivity achieving over 70% recovery of long-chain carboxylic acids from model aqueous mixtures (C greater than or equal to 5) with negligible co-extraction of short-chain carboxylic acids (C< 5) and monomeric sugars. Moreover, with a negligible loss of P in the aqueous medium indicating the good stability of the HES. Moreover, this HES was used in a second extraction cycle after washing with NaOH showing an extraction performance like the fresh one which confirm its good reusability which is a critical point for prospective industrial applications.
This selected eutectic solvent was further tested in a continuous glass-packed counter-current extraction systems with continuous recovering of the solvent in a second column via alkaline back-extraction using 0.1 M NaOH. The results showed that the excellent performance of the solvent is kept in continuous operation in terms of selective extraction of long-chain carboxylic acids and reusability. Finally, the optimized HES were tested in the selective extraction of carboxylic acids from real fermentation broths coming from the acidogenic fermentation of holocellulose. Again, the HES solvent showed an excellent performance in terms of selectivity, stability and reusability for this real effluent.
In the final stage, a conceptual preliminary design and techno-economic assessment of the potential integrated valorization process was developed. The proposed biorefinery was designed to treat approximately 1,370 kg/h of pruning waste, based on municipal waste generation data from Móstoles city (Madrid). The process includes three sections: (i) hydrothermal depolymerization of the lignocellulosic waste to yield an aqueous effluent rich in carboxylic acids, (ii) acidogenic fermentation of the remaining solid holocellulose phase after hydrothermal catalytic treatment and (iii) extraction of long-chain acids from fermentative broth with recovery of the HES. Mass and energy balances were performed, and the capital (CAPEX) and operational expenditures (OPEX) were estimated. The unitary treatment cost was estimated at approximately 250 €/ton, which is much higher than current landfill disposal rates (~40 €/ton). However, with projected increases in landfill taxes and regulatory restrictions across the European Union, this technology could become economically competitive in the near future.
In terms of material efficiency, the integrated system achieved a 36% reduction in solid waste, a carboxylic acid yield of 12.7%, and biohydrogen production of 1.4% by mass. These outputs highlight the potential of lignocellulosic biomass as a renewable feedstock to produce bio-based chemicals and energy carriers and thereby contributing to decarbonization and waste minimization goals in the framework of a circular economy.
Concluding, the current research shows that the integration of catalytic hydrothermal treatment, biological fermentation, and selective extraction via HES might be a viable pathway for the valorization of lignocellulosic waste. Key findings include the identification of an optimal depolymerization catalyst (10 wt. % V/MgO), effective mechanical pretreatment of the holocellulose to enhance fermentation, and the successful design of selective and reusable extraction solvent. Although proposed treatment cost exceeds currently landfill disposal rates, the added value of the obtained products combined with the anticipated tightening of environmental regulations, might make this approach promising for future implementation. Nevertheless, future work must be addressed focus on: (i) exploring mixed oxide as depolymerization catalysts, (ii) elucidation of the reaction mechanisms involved in lignin depolymerization, (iii) optimizing fermentation for continuous operation and higher butyric acid yields, (iv) studying post-fermentation chain elongation processes, and (v) conducting comprehensive environmental and social impact assessments. Finally, scaling up of the proposed technologies to higher technology readiness levels (TRL > 4) will also be essential for an industrial future deployment.
