Stabilization strategies in biomass depolymerization using chemical functionalization

Combined conversion of lignocellulosic biomass into high-value products with ultrasonic cavitation and photocatalytic produced reactive oxygen species - A review

The production of high-value products from lignocellulosic biomass is carried out through the selective scission of crosslinked CC/CO bonds. Nowadays, several techniques are applied to optimize biomass conversion into desired products with high yields. Photocatalytic technology has been proven to be a valuable tool for valorizing biomass at mild conditions. The photoproduced reactive oxygen species (ROSs) can initiate the scission of crosslinked bonds and form radical intermediates. However, the low mass transfer of the photocatalytic process could limit the production of a high yield of products. The incorporation of ultrasonic cavitation in the photocatalytic system provides an exceptional condition to boost the fragmentation and transformation of biomass into the desired products within a lesser reaction time. This review critically discusses the main factors governing the application of photocatalysis for biomass valorization and tricks to boost the selectivity for enhancing the yield of desired products. Synergistic effects obtained through the combination of sonolysis and photocatalysis were discussed in depth. Under ultrasonic vibration, hot spots could be produced on the surface of the photocatalysts, improving the mass transfer through the jet phenomenon. In addition, shock waves can assist the dissolution and mixing of biomass particles.

Creative biological lignin conversion routes toward lignin valorization

Lignin, the largest renewable aromatic resource, is a promising alternative feedstock for the sustainable production of various chemicals, fuels, and materials. Despite this potential, lignin is characterized by heterogeneous and macromolecular structures that must be addressed. In this review, we present biological lignin conversion routes (BLCRs) that offer opportunities for overcoming these challenges, making lignin valorization feasible. Funneling heterogeneous aromatics via a 'biological funnel' offers a high-specificity bioconversion route for aromatic platform chemicals. The inherent aromaticity of lignin drives atom-economic functionalization routes toward aromatic natural product generation. By harnessing the ligninolytic capacities of specific microbial systems, powerful aromatic ring-opening routes can be developed to generate various value-added products. Thus, BLCRs hold the promise to make lignin valorization feasible and enable a lignocellulose-based bioeconomy.

A review on lignin pyrolysis: pyrolytic behavior, mechanism, and relevant upgrading for improving process efficiency

Lignin is a promising alternative to traditional fossil resources for producing biofuels due to its aromaticity and renewability. Pyrolysis is an efficient technology to convert lignin to valuable chemicals, which is beneficial for improving lignin valorization. In this review, pyrolytic behaviors of various lignin were included, as well as the pyrolytic mechanism consisting of initial, primary, and charring stages were also introduced. Several parallel reactions, such as demethoxylation, demethylation, decarboxylation, and decarbonylation of lignin side chains to form light gases, major lignin structure decomposition to generate phenolic compounds, and polymerization of active lignin intermediates to yield char, can be observed through the whole pyrolysis process. Several parameters, such as pyrolytic temperature, time, lignin type, and functional groups (hydroxyl, methoxy), were also investigated to figure out their effects on lignin pyrolysis. On the other hand, zeolite-driven lignin catalytic pyrolysis and lignin co-pyrolysis with other hydrogen-rich co-feedings were also introduced for improving process efficiency to produce more aromatic hydrocarbons (AHs). During the pyrolysis process, phenolic compounds and/or AHs can be produced, showing promising applications in biochemical intermediates and biofuel additives. Finally, some challenges and future perspectives for lignin pyrolysis have been discussed.

Hydrogen Borrowing: towards Aliphatic Tertiary Amines from Lignin Model Compounds Using a Supported Copper Catalyst

Upcoming biorefineries, such as lignin-first provide renewable aromatics containing unique aliphatic alcohols. In this context, a Cu-ZrO₂ catalyzed hydrogen borrowing approach was established to yield tertiary amine from the lignin model monomer 3-(3,4-dimethoxyphenyl)-1-propanol and the actual lignin-derived monomers, (3-(4-hydroxyphenyl)-1-propanol and dihydroconiferyl alcohol), with dimethylamine. Various industrial metal catalysts were evaluated, resulting in nearly quantitative mass balances for most catalysts. Identified intermediates, side and reaction products were placed into a corresponding reaction network, supported by kinetic evolution experiments. Cu-ZrO₂ was selected as most suitable catalyst combining high alcohol conversion with respectable aliphatic tertiary amine selectivity. Low pressure H₂ was key for high catalyst activity and tertiary amine selectivity, mainly by hindering undesired reactant dimethylamine disproportionation and alcohol amidation. Besides dimethylamine model, diverse secondary amine reactants were tested with moderate to high tertiary amine yields. As most active catalytic site, highly dispersed Cu species in strong contact with ZrO₂ is suggested. ToF-SIMS, N₂ O chemisorption, TGA and XPS of spent Cu-ZrO₂ revealed that imperfect amine product desorption and declining surface Cu lowered the catalytic activity upon catalyst reuse, while thermal reduction readily restored the initial activity and selectivity demonstrating catalyst reuse.

Simultaneous Generation of Methyl Esters and CO in Lignin Transformation

Lignin is an abundant renewable carbon source. Due to its complex structure, utilization of lignin is very challenging. Herein, we describe an efficient strategy for the simultaneous utilization of lignin, in which the methoxy groups in lignin react with carboxylic acids to generate methyl carboxylates and the other alkyl and phenyl carbons react with oxygen to predominantly form CO that can be used directly in carbonylation reactions. The method was applied to the methylation of various functionalized aryl and alkyl carboxylic acids, including natural compounds, to produce valuable chemicals, including pharmaceuticals. No solid or liquid residues remain after the reaction. Mechanistic studies demonstrate that a well-ordered C-C and C-O bond activation sequence takes place to realize total transformation of lignin. This work opens a way for transformation of the entire lignin polymer into valuable products, exemplified by the synthesis of the pharmaceutical, Ramipril, on a gram scale.

Primary amines from lignocellulose by direct amination of alcohol intermediates, catalyzed by RANEY® Ni

Primary amines are crucially important building blocks for the synthesis of a wide range of industrially relevant products. Our comprehensive catalytic strategy presented here allows diverse primary amines from lignocellulosic biomass to be sourced in a straightforward manner and with minimal purification effort. The core of the methodology is the efficient RANEY® Ni-catalyzed hydrogen-borrowing amination (with ammonia) of the alcohol intermediates, namely alkyl-phenol derivatives as well as aliphatic alcohols, obtained through the two-stage LignoFlex process. Hereby the first stage entails the copper-doped porous metal oxide (Cu20PMO) catalyzed reductive catalytic fractionation (RCF) of pine lignocellulose into a crude bio-oil, rich in dihydroconiferyl alcohol (1G), which could be converted into dihydroconiferyl amine (1G amine) in high selectivity using ammonia gas, by applying our selective amination protocol. Notably also, the crude RCF-oil directly afforded 1G amine in a high 4.6 wt% isolated yield (based on lignin content). Finally it was also shown that the here developed Ni-catalysed heterogeneous catalytic procedure was equally capable of transforming a range of aliphatic linear/cyclic primary/secondary alcohols - available from the second stage of the LignoFlex procedure - into their respective primary amines.

One-Pot Catalytic Conversion of Lignin-Derivable Guaiacols and Syringols to Cyclohexylamines

Cyclic primary amines are elementary building blocks to many fine chemicals, pharmaceuticals, and polymers. Here, a powerful one-pot Raney Ni-based catalytic strategy was developed to transform guaiacol into cyclohexylamine using NH3 (7 bar) and H2 (10 bar) in up to 94 % yield. The methodology was extendable to the conversion of a wider range of guaiacols and syringols into their corresponding cyclohexylamines. Notably, a crude bio-oil originating from the reductive catalytic fractionation of birch lignocellulose was transformed into a product mixture rich in 4-propylcyclohexylamine, constituting an interesting case of catalytic funneling. The isolated yield of the desired 4-propylcyclohexylamine reached as high as 7 wt % (on lignin basis). Preliminary mechanistic studies pointed at the consecutive occurrence of three key catalytic transformations, namely, demethoxylation, hydrogenation, and amination.

Thermochemical depolymerization of lignin: Process analysis with state-of-the-art soft ionization mass spectrometry

Lignin valorization via thermochemical approaches has the potential to produce renewable fuels and value-added chemicals, which are of great significance to the sustainable development of human beings. During the thermochemical depolymerization which involves acid-catalyzed, alkali-catalyzed, oxidative, reductive, pyrolytic, and other reactions, the lignin structure will undergo a series of bond cleavage, condensation, and functional group changes, while the mechanism is still unclear. To improve the efficiency, the analysis of the evolution of intermediates during depolymerization is very important, among which soft ionization mass spectrometry plays a vital role. This review aims to summarize the research progress of process analysis of lignin depolymerization in both gas-phase, typically thermal and catalytic pyrolysis, and liquid-phase via online mass spectrometry. The challenges and our insights into the future development of the lignin valorization as well as soft ionization mass spectrometry methods are also discussed.

Rational highly dispersed ruthenium for reductive catalytic fractionation of lignocellulose

Producing monomeric phenols from lignin biopolymer depolymerization in a detachable and efficient manner comes under the spotlight on the fullest utilization of sustainable lignocellulosic biomass. Here, we report a low-loaded and highly dispersed Ru anchored on a chitosan-derived N-doped carbon catalyst (RuN/ZnO/C), which exhibits outstanding performance in the reductive catalytic fractionation of lignocellulose. Nearly theoretical maximum yields of phenolic monomers from lignin are achieved, corresponding to TON as 431 molphenols molRu-1, 20 times higher than that from commercial Ru/C catalyst; high selectivity toward propyl end-chained guaiacol and syringol allow them to be readily purified. The RCF leave high retention of (hemi)cellulose amenable to enzymatic hydrolysis due to the successful breakdown of biomass recalcitrance. The RuN/ZnO/C catalyst shows good stability in recycling experiments as well as after a harsh hydrothermal treatment, benefiting from the coordination of Ru species with N atoms. Characterizations of the RuN/ZnO/C imply a transformation from Ru single atoms to nanoclusters under current reaction conditions. Time-course experiment, as well as reactivity screening of a series of lignin model compounds, offer insight into the mechanism of current RCF over RuN/ZnO/C. This work opens a new opportunity for achieving the valuable aromatic products from lignin and promoting the industrial economic feasibility of lignocellulosic biomass.

Hydrogenation of Xylose to Xylitol in the Presence of Bimetallic Nanoparticles Ni3Fe Catalyst in the Presence of Choline Chloride

Hydrogenation of sugars to sugars alcohols is of prime interest for food applications for instance. Xylose obtained from the hemicellulose fraction of lignocellulosic biomass can be hydrogenated to xylitol. Herein, we conducted catalytic hydrogenation reactions in a non-conventional media approach by using choline chloride, a non-toxic naturally occurring organic compound that can form a deep eutectic solvent with xylose. Acknowledging the benefits of cost-effective transition metal-based alloys, Ni3Fe1 bimetallic nanoparticles were utilized as a hetero-catalyst. Under optimized reaction conditions (110 °C, 3 h and 30 bar H2), a highly concentrated feed of xylose (76 wt.%) was converted to 80% of xylitol, showing the benefit of using choline chloride. Overall, the catalytic conversion activity and the product selectivity in the substrate-assisted DES media are relatively high but, the recyclability of the catalyst should be improved in the presence of such media.

Technology Overview of Fast Pyrolysis of Lignin: Current State and Potential for Scale-Up

Lignin is an abundant natural polymer obtained from lignocellulosic biomass and rich in aromatic substructures. When efficiently depolymerized, it has great potential in the production of value-added chemicals. Fast pyrolysis is a promising depolymerization method, but current studies focus mainly on small quantities of lignin. In this Review, to determine the potential for upscaling, systems used in the most relevant unit operations of fast pyrolysis of lignin are evaluated. Fluidized-bed reactors have the most potential. It would be beneficial to combine them with the following: slug injectors for feeding, hot particle filters, cyclones, and fractional condensation for product separation and recovery. Moreover, upgrading lignin pyrolysis oil would allow the necessary quality parameters for particular applications to be reached.

Biocatalytic valorization of lignin subunit: Screening a carboxylic acid reductase with high substrate preference to syringyl functional group

Lignin is inexpensive and the most abundant source of biological aromatics. It can be decomposed to three types of subunits, 4-hydroxybenzoic, vanillic and syringic acids, each of which can be valorized to value added compounds. Syringaldehyde is a versatile phenolic aldehyde implicated with multiple bioactive properties as well as intermediates for biofuels. Herein, fourteen microbial carboxylic acid reductases (CARs) were screened for the biocatalysis of the energetically unfavorable reduction of syringic acid to syringaldehyde. Nine CARs were positive to syringic acid reduction, among which Mycobacterium abscessus CAR exhibited the highest analytical yield of the product. By the optimization of the reaction condition, the whole-cell biocatalyst (i.e., recombinant Escherichia coli expressing the gene) successfully converted syringic acid to syringaldehyde with a yield of 90%. Furthermore, structural features of the screened CAR responsible for the specificity toward the syringyl subunit were analyzed that helps to further engineer the biocatalyst for improved performances.

Decoupled temperature and pressure hydrothermal synthesis of carbon sub-micron spheres from cellulose

The temperature and pressure of the hydrothermal process occurring in a batch reactor are typically coupled. Herein, we develop a decoupled temperature and pressure hydrothermal system that can heat the cellulose at a constant pressure, thus lowering the degradation temperature of cellulose significantly and enabling the fast production of carbon sub-micron spheres. Carbon sub-micron spheres can be produced without any isothermal time, much faster compared to the conventional hydrothermal process. High-pressure water can help to cleave the hydrogen bonds in cellulose and facilitate dehydration reactions, thus promoting cellulose carbonization at low temperatures. A life cycle assessment based on a conceptual biorefinery design reveals that this technology leads to a substantial reduction in carbon emissions when hydrochar replacing fuel or used for soil amendment. Overall, the decoupled temperature and pressure hydrothermal treatment in this study provides a promising method to produce sustainable carbon materials from cellulose with a carbon-negative effect.

Lignin-First Depolymerization of Lignocellulose into Monophenols over Carbon Nanotube-Supported Ruthenium: Impact of Lignin Sources

Lignin-first depolymerization of lignocellulosic biomass into aromatics is of great significance to sustainable biorefinery. However, it remains a challenge, owing to the variance between lignin sources and structures. In this study, ruthenium supported on carbon nanotubes (Ru/CNT) exhibits efficient catalytic activity toward lignin hydrogenolysis to exclusively afford monophenols in high yields. Catalytic tests indicate that the yields of aromatic monomers are related to lignin sources and decrease in the order: hardwoods > herbaceous plants > softwoods. Experimental results demonstrate that the scission of C-O bonds and the high selectivity to monomeric aromatic compounds over the Ru/CNT catalyst are enhanced by avoiding side condensation. Furthermore, the fabricated Ru/CNT shows good reusability and recyclability, applicability, and biomass feedstock compatibility, rendering it a promising candidate for lignin valorization. These findings pave the way for rational design of highly active and stable catalysts to potentially address challenges in lignin chemistry.

Fully lignocellulose-based PET analogues for the circular economy

Polyethylene terephthalate is one of the most abundantly used polymers, but also a significant pollutant in oceans. Due to growing environmental concerns, polyethylene terephthalate alternatives are highly sought after. Here we present readily recyclable polyethylene terephthalate analogues, made entirely from woody biomass. Central to the concept is a two-step noble metal free catalytic sequence (Cu20-PMO catalyzed reductive catalytic fractionation and Raney Ni mediated catalytic funneling) that allows for obtaining a single aliphatic diol 4-(3-hydroxypropyl) cyclohexan-1-ol in high isolated yield (11.7 wt% on lignin basis), as well as other product streams that are converted to fuels, achieving a total carbon yield of 29.5%. The diol 4-(3-hydroxypropyl) cyclohexan-1-ol is co-polymerized with methyl esters of terephthalic acid and furan dicarboxylic acid, both of which can be derived from the cellulose residues, to obtain polyesters with competitive Mw and thermal properties (Tg of 70-90 °C). The polymers show excellent chemical recyclability in methanol and are thus promising candidates for the circular economy.

Transition-metal-free synthesis of pyrimidines from lignin β-O-4 segments via a one-pot multi-component reaction

Heteroatom-participated lignin depolymerization for heterocyclic aromatic compounds production is of great importance to expanding the product portfolio and meeting value-added biorefinery demand, but it is also particularly challenging. In this work, the synthesis of pyrimidines from lignin β-O-4 model compounds, the most abundant segment in lignin, mediated by NaOH through a one-pot multi-component cascade reaction is reported. Mechanism study suggests that the transformation starts by NaOH-induced deprotonation of Cα-H bond in β-O-4 model compounds, and involves highly coupled sequential cleavage of C-O bonds, alcohol dehydrogenation, aldol condensation, and dehydrogenative aromatization. This strategy features transition-metal free catalysis, a sustainable universal approach, no need of external oxidant/reductant, and an efficient one-pot process, thus providing an unprecedented opportunity for N-containing aromatic heterocyclic compounds synthesis from biorenewable feedstock. With this protocol, an important marine alkaloid meridianin derivative can be synthesized, emphasizing the application feasibility in pharmaceutical synthesis.

Unraveling the mechanism for paired electrocatalysis of organics with water as a feedstock

Paired electroreduction and electrooxidation of organics with water as a feedstock to produce value-added chemicals is meaningful. A comprehensive understanding of reaction mechanism is critical for the catalyst design and relative area development. Here, we have systematically studied the mechanism of the paired electroreduction and electrooxidation of organics on Fe-Mo-based phosphide heterojunctions. It is shown that active H* species for organic electroreduction originate from water. As for organic electrooxidation, among various oxygen species (OH*, OOH*, and O*), OH* free radicals derived from the first step of water dissociation are identified as active species. Furthermore, explicit reaction pathways and their paired advantages are proposed based on theoretical calculations. The paired electrolyzer powered by a solar cell shows a low voltage of 1.594 V at 100 mA cm-2, faradaic efficiency of ≥99%, and remarkable cycle stability. This work provides a guide for sustainable synthesis of various value-added chemicals via paired electrocatalysis.

A molecular motor from lignocellulose

Lignin is the largest natural source of functionalized aromatics on the planet, therefore exploiting its inherent structural features for the synthesis of aromatic products is a timely and ambitious goal. While the recently developed lignin depolymerization strategies gave rise to well-defined aromatic platform chemicals, the diversification of these structures, especially toward high-end applications is still poorly addressed. Molecular motors and switches have found widespread application in many important areas such as targeted drug delivery systems, responsive coatings for self-healing surfaces, paints and resins or muscles for soft robotics. They typically comprise a functionalized aromatic backbone, yet their synthesis from lignin has not been considered before. In this contribution, we showcase the synthesis of a novel light-driven unidirectional molecular motor from the specific aromatic platform chemical 4-(3-hydroxypropyl)-2,6-dimethoxyphenol (dihydrosynapyl alcohol) that can be directly obtained from lignocellulose via a reductive catalytic fractionation strategy. The synthetic path takes into account the principles of green chemistry and aims to maintain the intrinsic functionality of the lignin-derived platform molecule.

Coproduction xylo-oligosaccharides with low degree of polymerization and glucose from sugarcane bagasse by non-isothermal subcritical carbon dioxide assisted seawater autohydrolysis

High pretreatment temperature is necessary to obtain xylo-oligosaccharides (XOS) with low degree of polymerization (DP). However, traditional isothermal pretreatment for XOS production may increase the generation of xylose and furfural with the reaction time extending (10-100 min). In this study, non-isothermal subcritical CO₂-assisted seawater autohydrolysis (NSCSA) firstly used seawater and CO₂ for the coproduction of XOS with low DP and glucose. 51.44% XOS was obtained at 205 °C/5 MPa, and low-DP (2-4) XOS accounted for 79.13% of the total XOS. Furthermore, the specific surface area and total pore volume of the pretreated sugarcane bagasse (SCB) were 1.96 m²/g and 0.011 cm³/g, respectively, increased by 148% and 83% than that of nature SCB. Compared with subcritical CO₂ pretreatment, NSCSA is an efficient method for the coproduction of XOS with low DP and glucose through inorganic salts in seawater and H₂CO₃ formed from CO₂.

High-Value Chemicals from Electrocatalytic Depolymerization of Lignin: Challenges and Opportunities

Lignocellulosic biomass is renewable and one of the most abundant sources for the production of high-value chemicals, materials, and fuels. It is of immense importance to develop new efficient technologies for the industrial production of chemicals by utilizing renewable resources. Lignocellulosic biomass can potentially replace fossil-based chemistries. The production of fuel and chemicals from lignin powered by renewable electricity under ambient temperatures and pressures enables a more sustainable way to obtain high-value chemicals. More specifically, in a sustainable biorefinery, it is essential to valorize lignin to enhance biomass transformation technology and increase the overall economy of the process. Strategies regarding electrocatalytic approaches as a way to valorize or depolymerize lignin have attracted significant interest from growing scientific communities over the recent decades. This review presents a comprehensive overview of the electrocatalytic methods for depolymerization of lignocellulosic biomass with an emphasis on untargeted depolymerization as well as the selective and targeted mild synthesis of high-value chemicals. Electrocatalytic cleavage of model compounds and further electrochemical upgrading of bio-oils are discussed. Finally, some insights into current challenges and limitations associated with this approach are also summarized.

Catalytic self-transfer hydrogenolysis of lignin with endogenous hydrogen: road to the carbon-neutral future

Due to the depletion of fossil sources, it is imperative to develop a sustainable and carbon-neutral biorefinery for supporting the fuel and chemical supply in modern society. Lignin, the only renewable aromatic source, is still an underutilized component in lignocellulose. Very recently, it has been found that hydrogenolysis is a promising technology for lignin valorization. However, high-pressure H₂ is necessary during lignin hydrogenolysis, resulting in safety problems. Furthermore, H₂ is mainly produced from steam reforming of fossil sources in industry, which makes the conversion of renewable lignin unsustainable and costly. Plentiful aliphatic hydroxyl and methoxy groups exist in native lignin and offer a renewable alternative to H₂, and can be hydrogen sources for the depolymerization and upgradation of lignin via the intramolecular catalytic transfer hydrogenation. The hydrogen source in situ generated from lignin is a type of green hydrogen, decreasing the carbon footprint. The purpose of this review is to provide a summary and perspective of lignin valorization via self-transfer hydrogenolysis, mainly focusing on a comprehensive understanding of the mechanism of catalytic self-transfer hydrogenolysis at the molecular level and developing highly effective catalytic systems. Moreover, some opportunities and challenges within this attractive field are given to discuss future research directions.

A hexasaccharide from capsular polysaccharide of carbapenem-resistant Klebsiella pneumoniae KN2 is a ligand of Toll-like receptor 4

Klebsiella pneumoniae serotype KN2 is a carbapenem-resistant strain and leads to the health care-associated infections, such as bloodstream infections. Its capsular polysaccharide (CPS) was isolated and cleaved by a specific enzyme from a bacteriophage into a hexasaccharide-repeating unit. With GC-MS, NMR, and Mass analyses, the structure of KN2 CPS was determined to be {→3)-β-D-Glcp-(1→3)-[α-D-GlcpA-(1→4)-β-D-Glcp-(1→6)]-α-D-Galp-(1→6)-β-D-Galp-(1→3)-β-D-Galp-(1→}_n. We demonstrated that 1 μg/mL CPS could stimulate J774A.1 murine macrophages to release tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in vitro. Also, we proved that KN2 CPS induced the immune response through Toll-like receptor 4 (TLR4) in the human embryonic kidney (HEK)-293 cells. Strikingly, the hexasaccharide alone shows the same immune response as the CPS, suggesting that the hexasaccharide can shape the adaptive immunity to be a potential vaccine adjuvant. The glucuronic acid (GlcA) on other polysaccharides can affect the immune response, but the GlcA-reduced KN2 CPS and hexasaccharide still maintain their immunomodulatory activities.

Bioplastics for a circular economy

Bioplastics - typically plastics manufactured from bio-based polymers - stand to contribute to more sustainable commercial plastic life cycles as part of a circular economy, in which virgin polymers are made from renewable or recycled raw materials. Carbon-neutral energy is used for production and products are reused or recycled at their end of life (EOL). In this Review, we assess the advantages and challenges of bioplastics in transitioning towards a circular economy. Compared with fossil-based plastics, bio-based plastics can have a lower carbon footprint and exhibit advantageous materials properties; moreover, they can be compatible with existing recycling streams and some offer biodegradation as an EOL scenario if performed in controlled or predictable environments. However, these benefits can have trade-offs, including negative agricultural impacts, competition with food production, unclear EOL management and higher costs. Emerging chemical and biological methods can enable the 'upcycling' of increasing volumes of heterogeneous plastic and bioplastic waste into higher-quality materials. To guide converters and consumers in their purchasing choices, existing (bio)plastic identification standards and life cycle assessment guidelines need revision and homogenization. Furthermore, clear regulation and financial incentives remain essential to scale from niche polymers to large-scale bioplastic market applications with truly sustainable impact.

Size effect of Ru particles on the self-reforming-driven hydrogenolysis of lignin model compound

Particle size always has a great influence on catalytic performance. In this work, we investigated the size effect of Ru colloids on the self-reforming-driven hydrogenolysis of lignin model compound by...

Saccharification of acid-alkali pretreated sugarcane bagasse using immobilized enzymes from Phomopsis stipata

In this study, a mild-temperature two-step dilute acid and alkaline pretreatment (DA-AL) process was developed to generate highly digestible cellulose pulp from sugarcane bagasse for producing fermentable sugars by novel thermophilic cellulases derived from Phomopsis stipata SC 04. First, DA pretreatment of sugarcane bagasse at 2% (w/v) H2SO4 and 121 °C for 71 min, followed by AL pretreatment at 2.2% (w/v) NaOH and 110 °C for 100 min led to the pulp containing 86% cellulose. The cellulose pulp was hydrolyzed by the immobilized P. stipata cellulase on Ca-alginate beads, following optimization of immobilization conditions. The results showed that mixing the cellulase extract and sodium alginate solutions at a volume ratio of 1:4 led to the highest immobilization efficiencies of 99.83% for β-glucosidase and 97.52% for endoglucanase while the enzyme leakage was the lowest. The use of the immobilized cellulases led to a cellulose digestibility of 30% in the initial batch and recycling of the immobilized cellulases reduced cellulose digestibility to 18% after s recycling for two times (a total of third rounds). Overall, this study provides useful information in the use of a mild pretreatment process to produce highly digestible cellulose pulp and in the immobilization of thermophilic cellulases to produce fermentable sugars from pretreated biomass.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03101-2.

Simulation and optimization of organosolv based lignocellulosic biomass refinery: A review

Organosolv pretreatment can be considered as the core of the lignocellulosic biomass fractionation within the biorefinery concept. Organosolv facilitates the separation of the major fractions (cellulose, hemicelluloses, lignin), and their use as renewable feedstocks to produce bioenergy, biofuels, and added-value biomass derived chemicals. The efficient separation of these fractions affects the economic feasibility of the biorefinery complex. This review focuses on the simulation of the organosolv pretreatment and the optimization of (i) feedstock delignification, (ii) sugars production (mainly from hemicelluloses), (iii) enzymatic digestibility of the cellulose fraction and (iv) quality of lignin. Simulation is used for the technoeconomic optimization of the biorefinery complex. Simulation and optimization implement a holistic approach considering the efficient technological, economic, and environmental performance of the biorefinery operational units. Consequently, an optimized organosolv stage is the first step for a sustainable, economically viable biorefinery complex in the concept of industrial ecology and zero waste circular economy.

Oxidase enzymes as sustainable oxidation catalysts

Oxidation is one of the most important processes used by the chemical industry. However, many of the methods that are used pose significant sustainability and environmental issues. Biocatalytic oxidation offers an alternative to these methods, with a now significant enzymatic oxidation toolbox on offer to chemists. Oxidases are one of these options, and as they only depend on molecular oxygen as a terminal oxidant offer perfect atom economy alongside the selectivity benefits afforded by enzymes. This review will focus on examples of oxidase biocatalysts that have been used for the sustainable production of important molecules and highlight some important processes that have been significantly improved through the use of oxidases. It will also consider emerging classes of oxidases, and how they might fit in a future biorefinery approach for the sustainable production of important chemicals.

From Lignin to Valuable Aromatic Chemicals: Lignin Depolymerization and Monomer Separation via Centrifugal Partition Chromatography

Lignin has long been recognized as a potential feedstock for aromatic molecules; however, most lignin depolymerization methods create a complex mixture of products. The present study describes an alkaline aerobic oxidation method that converts lignin extracted from poplar into a collection of oxygenated aromatics, including valuable commercial compounds such as vanillin and p-hydroxybenzoic acid. Centrifugal partition chromatography (CPC) is shown to be an effective method to isolate the individual compounds from the complex product mixture. The liquid-liquid extraction method proceeds in two stages. The crude depolymerization mixture is first subjected to ascending-mode extraction with the Arizona solvent system L (pentane/ethyl acetate/methanol/water 2:3:2:3), enabling isolation of vanillin, syringic acid, and oligomers. The remaining components, syringaldehyde, vanillic acid, and p-hydroxybenzoic acid (pHBA), were resolved by using ascending-mode extraction with solvent mixture comprising dichloromethane/methanol/water (10:6:4) separation. These results showcase CPC as an effective technology that could provide scalable access to valuable chemicals from lignin and other biomass-derived feedstocks.

Processive Enzymes Kept on a Leash: How Cellulase Activity in Multienzyme Complexes Directs Nanoscale Deconstruction of Cellulose

Biological deconstruction of polymer materials gains efficiency from the spatiotemporally coordinated action of enzymes with synergetic function in polymer chain depolymerization. To perpetuate enzyme synergy on a solid substrate undergoing deconstruction, the overall attack must alternate between focusing the individual enzymes locally and dissipating them again to other surface sites. Natural cellulases working as multienzyme complexes assembled on a scaffold protein (the cellulosome) maximize the effect of local concentration yet restrain the dispersion of individual enzymes. Here, with evidence from real-time atomic force microscopy to track nanoscale deconstruction of single cellulose fibers, we show that the cellulosome forces the fiber degradation into the transversal direction, to produce smaller fragments from multiple local attacks ("cuts"). Noncomplexed enzymes, as in fungal cellulases or obtained by dissociating the cellulosome, release the confining force so that fiber degradation proceeds laterally, observed as directed ablation of surface fibrils and leading to whole fiber "thinning". Processive cellulases that are enabled to freely disperse evoke the lateral degradation and determine its efficiency. Our results suggest that among natural cellulases, the dispersed enzymes are more generally and globally effective in depolymerization, while the cellulosome represents a specialized, fiber-fragmenting machinery.

Preparation and properties of novel bio-based epoxy resin thermosets from lignin oligomers and cardanol

A series of lignin-based epoxy resins (LEPs) were prepared by the reaction of epichlorohydrin with lignin oligomers derived from partial reductive depolymerization of lignin. To overcome the high viscosity and brittleness defects in practical applications, the LEPs were blended with renewable epoxied cardanol glycidyl ether (ECGE) and then cured with methyltetrahydrophthalic anhydride (MeTHPA) to form high-performance epoxy thermosets. The effects of degree of lignin depolymerization, chemical composition of lignin oligomers and dosage of ECGE on thermal and mechanical properties of the cured products were investigated. The LEP/MeTHPA thermosets exhibited good thermal and mechanical properties. Especially, by separating monomer-rich fractions from lignin oligomers, the thermal and mechanical properties of the cured product were improved obviously. Notably, the incorporation of ECGE also possessed a positive effect on reinforcing and toughening the cured products. With 20 wt% ECGE loadings, the tensile, flexural and impact strength of the cured product reached the maximum value of 77 MPa, 115 MPa and 14 kJ/m², respectively, which were equivalent to the commercial bisphenol A epoxy resins thermosets. These findings indicated that the novel bio-based epoxy resins from lignin oligomers and cardanol could be utilized as renewable alternatives for BPA epoxy resins.

Oxidative Catalytic Fractionation of Lignocellulosic Biomass under Non-alkaline Conditions

Biomass pretreatment methods are commonly used to isolate carbohydrates from biomass, but they often lead to modification, degradation, and/or low yields of lignin. Catalytic fractionation approaches provide a possible solution to these challenges by separating the polymeric sugar and lignin fractions in the presence of a catalyst that promotes cleavage of the lignin into aromatic monomers. Here, we demonstrate an oxidative fractionation method conducted in the presence of a heterogeneous non-precious-metal Co-N-C catalyst and O2 in acetone as the solvent. The process affords a 15 wt% yield of phenolic products bearing aldehydes (vanillin, syringaldehyde) and carboxylic acids (p-hydroxybenzoic acid, vanillic acid, syringic acid), complementing the alkylated phenols obtained from existing reductive catalytic fractionation methods. The oxygenated aromatics derived from this process have appealing features for use in polymer synthesis and/or biological funneling to value-added products, and the non-alkaline conditions associated with this process support preservation of the cellulose, which remains insoluble at reaction conditions and is recovered as a solid.

Extraction of Noncondensed Lignin from Poplar Sawdusts with p-Toluenesulfonic Acid and Ethanol

The traditional pretreatment leads to the recalcitration of C-C bonds during lignin fractionation, thus hindering their depolymerization into aromatic monomers. It is essential to develop an applicable approach to extract noncondensed lignin for its high-value applications. In this work, noncondensed lignins were extracted from poplar sawdust using recyclable p-toluenesulfonic acid for cleaving lignin-carbohydrate complex bonds effectively and ethanol as a stabilization reagent to inhibit lignin condensation. Lignin yield of 83.74% was recovered by 3 mol/L acid in ethanol at 85 °C for 5 h, and carbohydrates were well preserved (retaining 98.97% cellulose and 50.01% hemicelluloses). During lignin fractionation, the acid concentration and extraction time were the major drivers of condensation. Ethanol reacted with lignin at the α-position to prevent the formation of the condensed structure. The extracted lignin depolymerized over the Pd/C catalysts gave a yield of 50.35% of aromatic monomers, suggesting that the novel extraction process provided a promising way for noncondensed lignin production.

Tunable and functional deep eutectic solvents for lignocellulose valorization

Stabilization of reactive intermediates is an enabling concept in biomass fractionation and depolymerization. Deep eutectic solvents (DES) are intriguing green reaction media for biomass processing; however undesired lignin condensation is a typical drawback for most acid-based DES fractionation processes. Here we describe ternary DES systems composed of choline chloride and oxalic acid, additionally incorporating ethylene glycol (or other diols) that provide the desired 'stabilization' function for efficient lignocellulose fractionation, preserving the quality of all lignocellulose constituents. The obtained ethylene-glycol protected lignin displays high β-O-4 content (up to 53 per 100 aromatic units) and can be readily depolymerized to distinct monophenolic products. The cellulose residues, free from condensed lignin particles, deliver up to 95.9 ± 2.12% glucose yield upon enzymatic digestion. The DES can be recovered with high yield and purity and re-used with good efficiency. Notably, we have shown that the reactivity of the β-O-4 linkage in model compounds can be steered towards either cleavage or stabilization, depending on DES composition, demonstrating the advantage of the modular DES composition.

Sustainable Production of Benzylamines from Lignin

Catalytic conversion of lignin into heteroatom functionalized chemicals is of great importance to bring the biorefinery concept into reality. Herein, a new strategy was designed for direct transformation of lignin β-O-4 model compounds into benzylamines and phenols in moderate to excellent yields in the presence of organic amines. The transformation involves dehydrogenation of C_α -OH, hydrogenolysis of the C_β -O bond and reductive amination in the presence of Pd/C catalyst. Experimental data suggest that the dehydrogenation reaction proceeds over the other two reactions and secondary amines serve as both reducing agents and amine sources in the transformation. Moreover, the concept of "lignin to benzylamines" was demonstrated by a two-step process. This work represents a first example of synthesis of benzylamines from lignin, thus providing a new opportunity for the sustainable synthesis of benzylamines from renewable biomass, and expanding the products pool of biomass conversion to meet future biorefinery demands.

Techno-economic analysis and life cycle assessment of a biorefinery utilizing reductive catalytic fractionation

Reductive catalytic fractionation (RCF) is a promising approach to fractionate lignocellulose and convert lignin to a narrow product slate. To guide research towards commercialization, cost and sustainability must be considered. Here we report a techno-economic analysis (TEA), life cycle assessment (LCA), and air emission analysis of the RCF process, wherein biomass carbohydrates are converted to ethanol and the RCF oil is the lignin-derived product. The base-case process, using a feedstock supply of 2000 dry metric tons per day, methanol as a solvent, and H2 gas as a hydrogen source, predicts a minimum selling price (MSP) of crude RCF oil of $1.13 per kg when ethanol is sold at $2.50 per gallon of gasoline-equivalent ($0.66 per liter of gasoline-equivalent). We estimate that the RCF process accounts for 57% of biorefinery installed capital costs, 77% of positive life cycle global warming potential (GWP) (excluding carbon uptake), and 43% of positive cumulative energy demand (CED). Of $563.7 MM total installed capital costs, the RCF area accounts for $323.5 MM, driven by high-pressure reactors. Solvent recycle and water removal via distillation incur a process heat demand equivalent to 73% of the biomass energy content, and accounts for 35% of total operating costs. In contrast, H2 cost and catalyst recycle are relatively minor contributors to operating costs and environmental impacts. In the carbohydrate-rich pulps, polysaccharide retention is predicted not to substantially affect the RCF oil MSP. Analysis of cases using different solvents and hemicellulose as an in situ hydrogen donor reveals that reducing reactor pressure and the use of low vapor pressure solvents could reduce both capital costs and environmental impacts. Processes that reduce the energy demand for solvent separation also improve GWP, CED, and air emissions. Additionally, despite requiring natural gas imports, converting lignin as a biorefinery co-product could significantly reduce non-greenhouse gas air emissions compared to burning lignin. Overall, this study suggests that research should prioritize ways to lower RCF operating pressure to reduce capital expenses associated with high-pressure reactors, minimize solvent loading to reduce reactor size and energy required for solvent recovery, implement condensed-phase separations for solvent recovery, and utilize the entirety of RCF oil to maximize value-added product revenues.

Acid Hydrotropic Fractionation of Lignocelluloses for Sustainable Biorefinery: Advantages, Opportunities, and Research Needs

This Minireview provides a comprehensive discussion on the potential of using acid hydrotropes for sustainably fractionating lignocelluloses for biorefinery applications. Acid hydrotropes are a class of acids that have hydrotrope properties toward lignin, which helps to solubilize lignin in aqueous systems. With the capability of cleaving ether and ester bonds and even lignin-carbohydrate complex (LCC) linkages, these acid hydrotropes can therefore isolate lignin embedded in the plant biomass cell wall and subsequently solubilize the isolated lignin in aqueous systems. Performances of two acid hydrotropes, that is, an aromatic sulfonic acid [p-toluenesulfonic acid (p-TsOH)] and a dicarboxylic acid [maleic acid (MA)], in terms of delignification and dissolution of hemicelluloses, and reducing lignin condensation, were evaluated and compared. The advantages of lignin esterification by MA for producing cellulosic sugars through enzymatic hydrolysis and lignin-containing cellulose nanofibrils (LCNFs) through mechanical fibrillation from the fractionated water insoluble solids (WIS), and for obtaining less condensed lignin with light color, were demonstrated. The excellent enzymatic digestibility of maleic acid hydrotropic fractionation WISs was also demonstrated by comparing with WISs from other fractionation processes. The recyclability and reusability of acid hydrotropes were also reviewed. Finally, perspectives on future research needs to address key technical issues for commercialization were also provided.