Techno-economic optimization of a process superstructure for lignin valorization

Lignin, the most abundant aromatic biopolymer on Earth, is often considered a biorefinery by-product, despite its potential to be valorized into high-added-value chemicals and fuels. In this work, an integrated superstructure-based optimization model was set up and optimized using mixed-integer non-linear programming for the conversion of technical lignin to three main biobased products: aromatic monomers, phenol-formaldehyde resins, and aromatic aldehydes/acids. Several alternative conversion pathways were simultaneously compared to assess the profitability of lignins-based processes by predicting the performance of technologies with different TRL. Upon employing key technologies such as hydrothermal liquefaction, dissolution in solvent, or high-temperature electrolysis, the technical lignins could have a market value of 200 €/t when the market price for aromatic monomers, resins, and vanillin is at least 2.0, 0.8, and 15.0 €/kg, respectively. When lower product selling prices were considered, the aromatic monomers and the resins were not profitable as target products.

On the Oxidative Valorization of Lignin to High-Value Chemicals: A Critical Review of Opportunities and Challenges

The efficient valorization of lignin is crucial if we are to replace current petroleum-based feedstock and establish more sustainable and competitive lignocellulosic biorefineries. Pulp and paper mills and second-generation biorefineries produce large quantities of low-value technical lignin as a by-product, which is often combusted on-site for energy recovery. This Review focuses on the conversion of technical lignins by oxidative depolymerization employing heterogeneous catalysts. It scrutinizes the current literature describing the use of various heterogeneous catalysts in the oxidative depolymerization of lignin and includes a comparison of the methods, catalyst loadings, reaction media, and types of catalyst applied, as well as the reaction products and yields. Furthermore, current techniques for the determination of product yields and product recovery are discussed. Finally, challenges and suggestions for future approaches are outlined.

Iridium-Catalyzed Dehydrogenation in a Continuous Flow Reactor for Practical On-Board Hydrogen Generation From Liquid Organic Hydrogen Carriers

To enable the large-scale use of hydrogen fuel cells for mobility applications, convenient methods for on-board hydrogen storage and release are required. A promising approach is liquid organic hydrogen carriers (LOHCs), since these are safe, available on a large scale, and compatible with existing refueling infrastructure. Usually, LOHC dehydrogenation is carried out in batch-type reactors by transition metals and their complexes and suffers from slow H₂ release kinetics and/or inability to reach high energy density by weight, owing to low conversion or the need to dilute the reaction mixture. In this study, a continuous flow reactor is used in combination with a heterogenized iridium pincer complex, which enables a tremendous increase in LOHC dehydrogenation rates. Thus, dehydrogenation of isopropanol is performed in a regime that, in terms of gravimetric energy density, hydrogen generation rate, and precious metal content, is potentially compatible with applications in a fuel-cell powered car.

Oxidative depolymerization of Kraft lignin to high-value aromatics using a homogeneous vanadium–copper catalyst

Kraft lignin is efficiently depolymerized under benign conditions into value-added aromatics and high-quality bio-oil using a facile vanadium–copper catalyst system.

Lignin Depolymerization under Continuous-Flow Conditions: Highlights of Recent Developments

Lignin is a polyaromatic polymer contained in plant cell walls, and it is considered the most abundant noncarbohydrate polymer on Earth. The aromaticity and richness of its functional groups render lignin an attractive starting biomacromolecule for conversion into a variety of value-added products. The development of successful strategies for lignin valorization infers the design of effective depolymerization protocols. Most research on lignin depolymerization has focused on batch-mode processing, whereas only a few studies have investigated such lignin transformations in continuous reactor systems. In this Concept, emerging developments within the concept of continuous lignin processing and the challenges remaining in realizing the efficient valorization of lignin by using this technology are highlighted. A special focus is set on the hydrothermal conversion of technical lignin under continuous-flow conditions, together with suggestions for future research and technology development.

Green solvents-based fractionation process for kraft lignin with controlled dispersity and molecular weight

The aim of this work is to develop a novel green solvent based sustainable process to refine lignin into low molecular weight (LMW) and high molecular weight (HMW) fractions. Lignin dispersity reduction were experimentally determined using four solvent mixtures, and benchmarked against eight pure solvents. Data outputs were used for modelling the integrated fractionation process. Dispersity reduction of up to 73% was achieved for the high value LMW fraction. Also, a 90% reduction of energy requirement was achieved with an optimized process incorporating a mechanical vapor compression system. This study showed that solvent mixtures involving water can significantly reduce the cost, environment, health and safety impacts of lignin fractionation. Techno-economic evaluation confirmed the economic viability of a large-scale process processing 50 tonne/day of lignin.

Bacterial conversion of depolymerized Kraft lignin

BACKGROUND

Lignin is a potential feedstock for microbial conversion into various chemicals. However, the microbial degradation rate of native or technical lignin is low, and chemical depolymerization is needed to obtain reasonable conversion rates. In the current study, nine bacterial strains belonging to the Pseudomonas and Rhodococcus genera were evaluated for their ability to grow on alkaline-treated softwood lignin as a sole carbon source.

RESULTS

Pseudomonas fluorescens DSM 50090 and Rhodococcus opacus DSM1069 showed the best growth of the tested species on plates with lignin. Further evaluation of P. fluorescens and R. opacus was made in liquid cultivations with depolymerized softwood Kraft lignin (DL) at a concentration of 1 g/L. Size-exclusion chromatography (SEC) showed that R. opacus consumed most of the available lower-molecular weight compounds (approximately 0.1-0.4 kDa) in the DL, but the weight distribution of larger fractions was almost unaffected. Importantly, the consumed compounds included guaiacol-one of the main monomers in the DL. SEC analysis of P. fluorescens culture broth, in contrast, did not show a large conversion of low-molecular weight compounds, and guaiacol remained unconsumed. However, a significant shift in molecular weight distribution towards lower average weights was seen after cultivation with P. fluorescens.

CONCLUSIONS

Rhodococcus opacus and P. fluorescens were identified as two potential microbial candidates for the conversion/consumption of base-catalyzed depolymerized lignin, acting on low- and high-molecular weight lignin fragments, respectively. These findings will be of relevance for designing bioconversion of softwood Kraft lignin.

Retraction Note to: Bacterial conversion of depolymerized Kraft lignin

[This retracts the article DOI: 10.1186/s13068-018-1240-7.].

Bacterial conversion of depolymerized Kraft lignin

BACKGROUND

Lignin is a potential feedstock for microbial conversion into various chemicals. However, the degradation rate of native or technical lignin is low, and depolymerization is needed to obtain reasonable conversion rates. In the current study, base-catalyzed depolymerization-using NaOH (5 wt%)-of softwood Kraft lignin was conducted in a continuous-flow reactor system at temperatures in the range 190-240 °C and residence times of 1 or 2 min. The ability of growth of nine bacterial strains belonging to the genera Pseudomonas and Rhodococcus was tested using the alkaline-treated lignin as a sole carbon source.

RESULTS

Pseudomonas fluorescens and Rhodococcus opacus showed the best growth of the tested species on plates with lignin. Further evaluation of P. fluorescens and R. opacus was made in liquid cultivations with depolymerized lignin (DL) at a concentration of 1 g/L. Size exclusion chromatography (SEC) showed that R. opacus consumed most of the available lower molecular weight compounds (approximately 0.1-0.4 kDa) in the DL, but the weight distribution of larger fractions was almost unaffected. Importantly, the consumed compounds included guaiacol-one of the main monomers in the DL. SEC analysis of P. fluorescens culture broth, in contrast, did not show a large conversion of low molecular weight compounds, and guaiacol remained unconsumed. However, a significant shift in molecular weight distribution towards lower average weights was seen.

CONCLUSIONS

Rhodococcus opacus and P. fluorescens were identified as two potential microbial candidates for the conversion/consumption of base-catalyzed depolymerized lignin, acting on low and high molecular weight lignin fragments, respectively. These findings will be of relevance for designing bioconversion of softwood Kraft lignin.