Reductive Catalytic Fractionation Research Articles

Efficient Cu-based catalysts for the selective demethoxylation of guaiacols

Lignin is an attractive feedstock for low molecular weight biobased phenols using depolymerization strategies such as reductive catalytic fractionation or fast pyrolysis. Such strategies often yield a product mixture enriched in lignin-derived monomers with methoxy substituents. Selective catalytic hydrodeoxygenation (HDO) is an effective methodology to demethoxylate these monomers into valuable alkylated phenols. Here, we report the use of non-precious Cu based catalysts supported on SiO2, ZrO2, TiO2 (various forms), MoO3-ZrO2, and MoO3-TiO2 in a continuous fixed-bed reactor at elevated temperature and pressure (300–360 °C, 10 bar) for the selective demethoxylation of guaiacol. Among the various catalysts, Cu/TiO2-P25 was found to be an effective and highly stable catalyst (100 h on stream) with a selectivity of 87% to demethoxylated compounds like phenol and cresols at a guaiacol conversion of 98%. A correlation was found between the oxygen storage capacity of the support (TiO2-P25, TiO2-A HSA, TiO2-R, and MoO3-TiO2) and guaiacol conversion, indicating that this property plays a role in the catalytic cycle. Besides, the demethoxylation of 4-n-propylguaiacol and a realistic guaiacol-rich feed isolated from a representative pyrolysis oil was successfully demonstrated. 87% of the guaiacols present in the feed were converted to demethoxylated phenols with a selectivity of 81%.

Fabrication of ultraviolet resistant and anti-bacterial non-isocyanate polyurethanes using the oligomers from the reductive catalytic fractionated lignin oil

Under the ‘green chemistry’ concept, lignin oligomers obtained from the reductive catalytic fractionation process was utilized as the renewable polyol substitute for non-isocyanate polyurethane (NIPU) films. After fractionation and characterization, the lignin oligomers underwent oxypropylation and were subsequently prepared as o-chlorohydrin resins. Then, the obtained cyclic carbonate (CC) was reacted with amines-based molecules for the fabrication of NIPUs. Owing to the low molecular weight, high hydroxyl group content, and good solubility, the obtained NIPUs substituted with the RCF lignin oligomers derived CC exhibited satisfactory mechanical properties and thermal stability. More attractively, the NIPUs possessed excellent UV resistant property and antibacterial activity, revealing that the RCF lignin-based NIPUs can be potentially applied as the antibacterial coating, protective coating, and anti-ultraviolet materials. These findings opening a new approach for valorization of the low valuable oligomer fractions in the RCF oil, which improving the overall economic feasibility of the lignin-first biorefineries.

Stepping away from purified solvents in reductive catalytic fractionation: a step forward towards a disruptive wood biorefinery process

By using unpurified solvents in a reductive catalytic wood fractionation process, its economics, sustainability and efficiency can be drastically improved.

High yield production of 1,4-cyclohexanediol and 1,4-cyclohexanediamine from high molecular-weight lignin oil.

The complete utilization of all lignin depolymerization streams obtained from the reductive catalytic fractionation (RCF) of woody biomass into high-value-added compounds is a timely and challenging objective. Here, we present a catalytic methodology to transform beech lignin-derived dimers and oligomers (DO) into well-defined 1,4-cyclohexanediol and 1,4-cyclohexanediamine. The latter two compounds have vast industrial relevance as monomers for polymer synthesis as well as pharmaceutical building blocks. The proposed two-step catalytic sequence involves the use of the commercially available RANEY® Ni catalyst. Therefore, the first step involves the efficient defunctionalization of lignin-derived 2,6-dimethoxybenzoquinone (DMBQ) into 1,4-cyclohexanediol (14CHDO) in 86.5% molar yield, representing a 10.7 wt% yield calculated on a DO weight basis. The second step concerns the highly selective amination of 1,4-cyclohexanediol with ammonia to give 1,4-cyclohexanediamine (14CHDA) in near quantitative yield. The ability to use RANEY® Ni and ammonia in this process holds great potential for future industrial synthesis of 1,4-cyclohexanediamine from renewable resources.

Ultrasound-assisted low-temperature catalytic lignin-first depolymerization of pine wood biomass to selectively produce propyl guaiacol

Lignin-first depolymerization of biomass using reductive catalytic fractionation is a promising option for the biorefinery as it selectively converts lignin to value-added phenolic monomers while preserving the carbohydrate fraction.

Feedstock-agnostic reductive catalytic fractionation in alcohol and alcohol–water mixtures

This work demonstrates that reductive catalytic fractionation can be a feedstock-agnostic process on hardwoods, softwoods, agricultural residues, and grasses, especially with water-alcohol solvent mixtures.

The substrate specificity in the O-demethylation of 4-alkylguaiacols by cytochrome P450 AgcAP450

Alkylguaiacols are lignin-derived products obtained by reductive catalytic fractionation (RCF) of lignocellulosic biomass.

Lignin-first biorefinery approach for the valorization of cotton stalks to phenolic monomers

Cotton stalks were fractionated using a lignin-first biorefinery approach by converting lignin to phenolic monomers and carbohydrates to functional carbon(s).

Flow-through reductive catalytic fractionation of beech wood sawdust

The use of flow-through (FT) systems in the lignin-first approach can improve the proficiency of the process. Herein, the reductive catalytic fractionation of beech wood sawdust was conducted in an FT system using 35Ni/NDC pellets.

Single-Standard Quantification Strategy for Lignin Dimers by Supercritical Fluid Chromatography with Charged Aerosol Detection.

The increased interest in utilizing lignin as a feedstock to produce various aromatic compounds requires advanced chemical analysis methods to provide qualitative and quantitative characterization of lignin samples along different technology streamlines. However, due to the lack of commercially available chemical standards, routine quantification of industrially relevant lignin oligomers in complex lignin samples remains a challenge. This study presents a novel method for universal quantification of lignin dimers based on supercritical fluid chromatography with charged aerosol detection (CAD). A series of lignin-derived dimeric compounds that have been reported from reductive catalytic fractionation (RCF) were synthesized and used as standards. The applicability of using linear regression instead of quadratic calibration curves was evaluated over a concentration range of 15-125 mg/L, demonstrating that the former calibration method is as appropriate as the latter. The response factors of lignin dimeric compounds were compared to assess the uniformity of the CAD signal, revealing that the CAD response for the tested lignin dimers did not differ substantially. It was also found that the response factors were not dependent on the number of methoxy groups or linkage motifs, ultimately enabling the use of only one calibrant for these compounds. The importance of chromatographic peak resolution in CAD was stressed, and the use of a digital peak sharpening technique was adopted and applied to address this challenge. The developed method was verified and used for the quantification of lignin dimers in an oil obtained by a RCF of birch sawdust.

Reductive Catalytic Fractionation of Lignocellulosic Biomass: Unveiling of the Reaction Mechanism

This work studied the reaction pathways involved in the depolymerization of cellulose and hemicellulose during the fractionation of lignocellulosic biomass using an isopropyl alcohol/water mixture as a solvent with the Raney-Ni catalyst. Model molecules such as cellulose and hemicellulose derived-products (glucose, sorbitol, erythrose, xylose, xylitol, glycerol) were used to understand the role of the catalyst on the depolymerization of these lignocellulosic fractions. The fractionation of sugar cane bagasse with the Raney-Ni catalyst was performed, a correlation between the different products formed, and the possible reaction pathways were proposed. The results obtained in this work show that the appropriate balance between hydrogenation and hydrogenolysis capacity might tune the selectivity to the desired product. It is important to notice that Raney-Ni is an unsupported catalyst. The support may add acidity, basicity, or redox properties that may play an important role in the determination of product distribution.

Lignin Stabilization and Carbohydrate Nature in H-transfer Reductive Catalytic Fractionation: The Role of Solvent Fractionation of Lignin Oil in Structural Profiling.

Reductive Catalytic Fractionation (RCF) of lignocellulosic materials produces lignin oil rich in monomer products and high-quality cellulosic pulps. RCF lignin oil also contains lignin oligomers/polymers and hemicellulose-derived carbohydrates. The variety of components makes lignin oil a complex matrix for analytical methods. As a result, the signals are often convoluted and overlapped, making detecting and quantifying key intermediates challenging. Therefore, to investigate the mechanisms underlining lignin stabilization and elucidate the structural features of carbohydrates occurring in the RCF lignin oil, fractionation methods reducing the RCF lignin oil complexity are required. This report examines the solvent fractionation of RCF lignin oil as a facile method for producing lignin oil fractions for advanced characterization. Solvent fractionation uses small volumes of environmentally benign solvents (methanol, acetone, and ethyl acetate) to produce multigram lignin fractions comprising products in different molecular weight ranges. This feature allows the determination of structural heterogeneity across the entire molecular weight distribution of the RCF lignin oil by high-resolution HSQC NMR spectroscopy. This study provides detailed insight into the role of the hydrogenation catalyst (Raney Ni) in stabilizing lignin fragments and defining the structural features of hemicellulose-derived carbohydrates in lignin oil obtained by the H-transfer RCF process.

A sustainable process to 100% bio-based nylons integrated chemical and biological conversion of lignocellulose

Considerable progress has been made in recent years to the development of sustainable polymers from bio-based feedstocks. In this study, 100% bio-based nylons were prepared via an integrated chemical and biological process from lignocellulose. These novel nylons were obtained by the melt polymerization of 3-propyladipic acid derived from lignin and 1,5-pentenediamine/1,4-butanediamine derived from carbohydrate sugar. Central to the concept is a three-step noble metal free catalytic chemical funnelling sequence (Raney Ni mediated reductive catalytic fractionation - reductive funnelling - oxidative funnelling), which allowed for obtaining a single component 3-propyladipic acid from lignin with high efficiency. The structural and thermodynamic properties of the obtained nylons have been systematically investigated, and thus obtained transparent bio-based nylons exhibited higher Mw (>32,000) and excellent thermal stability (Td5% > 265 °C). Considering their moderate Tg and good melt strength, these transparent bio-based nylons could serve as promising functional additives or temperature-responsive materials.

Structure- and computational-aided engineering of an oxidase to produce isoeugenol from a lignin-derived compound

Various 4-alkylphenols can be easily obtained through reductive catalytic fractionation of lignocellulosic biomass. Selective dehydrogenation of 4-n-propylguaiacol results in the formation of isoeugenol, a valuable flavor and fragrance molecule and versatile precursor compound. Here we present the engineering of a bacterial eugenol oxidase to catalyze this reaction. Five mutations, identified from computational predictions, are first introduced to render the enzyme more thermostable. Other mutations are then added and analyzed to enhance chemoselectivity and activity. Structural insight demonstrates that the slow catalytic activity of an otherwise promising enzyme variant is due the formation of a slowly-decaying covalent substrate-flavin cofactor adduct that can be remedied by targeted residue changes. The final engineered variant comprises eight mutations, is thermostable, displays good activity and acts as a highly chemoselective 4-n-propylguaiacol oxidase. We lastly use our engineered biocatalyst in an illustrative preparative reaction at gram-scale. Our findings show that a natural enzyme can be redesigned into a tailored biocatalyst capable of valorizing lignin-based monophenols.

Techno-economic Analysis of Sustainable Biofuels for Marine Transportation.

Renewable, low-carbon biofuels offer the potential opportunity to decarbonize marine transportation. This paper presents a comparative techno-economic analysis and process sustainability assessment of four conversion pathways: (1) hydrothermal liquefaction (HTL) of wet wastes such as sewage sludge and manure; (2) fast pyrolysis of woody biomass; (3) landfill gas Fischer-Tropsch synthesis; and (4) lignin-ethanol oil from the lignocellulosic ethanol biorefinery utilizing reductive catalytic fractionation. These alternative marine biofuels have a modeled minimum fuel selling price between $1.68 and $3.98 per heavy fuel oil gallon equivalent in 2016 U.S. dollars based on a mature plant assessment. The selected pathways also exhibit good process sustainability performance in terms of water intensity compared to the petroleum refineries. Further, the O and S contents of the biofuels vary widely. While the non-HTL biofuels exhibit negligible S content, the raw biocrudes via HTL pathways from sludge and manure show relatively high S contents (>0.5 wt %). Partial or full hydrotreatment can effectively lower the biocrude S content. Additionally, co-feeding with other low-sulfur wet wastes such as food waste can provide another option to produce raw biocrude with lower S content to meet the target with further hydrotreatment. This study indicates that biofuels could be a cost-effective fuel option for the marine sector. Marine biofuels derived from various feedstocks and conversion technologies could mitigate marine biofuel adoption risk in terms of feedstock availability and biorefinery economics.

Reductive Catalytic Fractionation of Spruce Wood over Ru/C Bifunctional Catalyst in the Medium of Ethanol and Molecular Hydrogen

Reductive catalytic fractionation (RCF) has emerged as an effective lignin-first biorefinery strategy to depolymerize lignin into tractable fragments in high yields. Herein, we propose the RCF of spruce wood over a Ru/C bifunctional catalyst in the medium of ethanol and molecular hydrogen to produce monomeric phenolic compounds from lignin, polyols from hemicelluloses, and microcrystalline cellulose. This contribution attempts to elucidate the role of the Ru/C bifunctional catalysts characteristics. The results clarify the particular effect of the carbon support acidity, catalyst grain size, content and dispersion of Ru on the effectiveness of lignin and hemicelluloses extraction and the yields of liquid and gaseous products. The most efficient catalysts for RCF of spruce wood, providing high yields of the monomeric phenols, glycols, and solid product with content of cellulose up to 90 wt%, bear 3 wt% of Ru with a dispersion of 0.94 based on an acidic oxidized graphite-like carbon support Sibunit®, and having a grain size of 56–94 μm. The Ru/C catalysts intensify the reactions of hydrodeoxygenation of liquid products from lignin. The main phenolic monomers are 4-propyl guaiacol, 4-propenyl guaiacol, and 4-propanol guaiacol. We explored the effect of the process temperature and time on the yield and composition of the liquid, solid, and gaseous products of spruce wood RCF. The optimal trade-off between the yields of phenolic monomers (30.0 wt%). polyols (18.6 wt%) and the solid product containing 84.4 wt% of cellulose is reached at 225 °C and 3 h over the most acidic Ru/C catalyst.

Co@CoO: An efficient catalyst for the depolymerization and upgrading of lignocellulose to alkylcyclohexanols with cellulose intact

The depolymerization and upgrading of lignin from raw biomass, while keeping cellulose intact is important in biorefinery and various metal-based catalysts have been used in reductive catalytic fractionation, a key method in “lignin-first” strategy. Recently, we found that a core–shell structured Co@CoO catalyst with CoO shell as the real active site had excellent performance in the hydrogenolysis of 5-hydromethylfurfural to 2,5-dimethylfuran due to its unique ability to dissociate H2 and yield active Hδ− species (Xiang et al., 2022). In this work, we report a one-pot depolymerization and upgrading of lignocellulose to alkylcyclohexanols, a flavour precursor, with intact cellulose over this unique core–shell structured catalyst, Co@CoO. Lignin model compounds (β-O-4, 4-O-5, α-O-4) were first used to clarify the activity of Co@CoO catalyst. Then, the one-pot conversion of various organosolv lignin (birch, pine and poplar) to alkylcyclohexanols was realized with the mass yield of alkylcyclohexanols up to 25.8 wt% from birch lignin under the reaction condition of 210 °C, 1 MPa H2, 16 h. Finally, the corresponding woody sawdusts were used as feedstocks and found that the Co@CoO catalyst indeed preferentially depolymerized and upgraded the lignin part and obtained the same alkylcyclohexanols products with the retention of cellulose-rich pulp. The collected alkylcyclohexanols were further esterified to obtain value-added esters, which can be used as flavors. This work will inspire the design of new efficient metal oxide catalysts in lignin fractionation and depolymerization to high-value-added chemicals with intact cellulose.

From Lignin to Chemicals: An Expedition from Classical to Modern Catalytic Valorization Technologies

AbstractFor the transition towards greener biorefineries with reduced waste, valorization of lignin to drop‐in chemicals instead of their combustion for energy purposes is a key issue for future processes. In this context, lignin should be extracted from lignocellulosic biomass (LCB) and fragmented into smaller units, followed by catalytic funneling to fine chemicals. In this review, we report on the classical approaches for lignin valorization from LCB: thermal treatments, solvolytic valorization, acid‐catalyzed process, and base‐catalyzed process. We also provide the reader with the modern approach of lignin valorization that led to an integrated LCB biorefinery. The performance of different solid catalysts in lignin‐first approach via reductive or oxidative catalytic fractionation at different conditions is discussed in detail.

Inhibitions of microbial fermentation by residual reductive lignin oil: Concerns on the bioconversion of reductive catalytic fractionated carbohydrate pulp

Reductive catalytic fractionation (RCF) is an emerging biomass valorization strategy with high atom economy, which primarily outputting extensively depolymerized reductive lignin oil and highly delignified pulp. In downstream processes, the polysaccharide rich pulp can be potentially transferred into various value-added biochemicals through fermentation. In this study, the negative influences of the residual lignin oil that remaining on the surface of the RCF pulp towards different industrial microbes (total 9 species, categorized into eukaryotes, Gram-negative, and Gram-positive strains) was investigated. Results indicated the reduced lignin fractions were the key inhibitors in the RCF lignin oil, whilst the polarities, molecular weight, and the substitution groups of lignin possessed significant impacts on the growth of strains. Meanwhile, the resistance to RCF lignin oil by different category of industrial strains was analyzed. Generally, eukaryotes exhibited better resistance than the Gram-negative and Gram-positive strains. Took bioethanol and acetone-butanol-ethanol fermentation as examples, the negative influence of the lignin oil on biochemicals production using the RCF pulp as substrate was explored. Batch fermentation results revealed that the RCF lignin oil limited both the sugars (glucose and xylose) metabolism and the pathways toward bio-alcohols production of the microorganism. Therefore, it is vital to take continuous technical improvement in order to minimize the negative influences of the residual lignin oil on downstream biotransformation of the RCF pulps.

Metal–Organic-Framework-Derived Copper Catalysts for the Hydrogenolysis of Lignin into Monomeric Phenols

Reductive catalytic fractionation (RCF) of lignocellulosic biomass has emerged as a leading biorefinery strategy. Herein, we present a copper-based catalyst (CuO/C) derived from a metal–organic backbone (HKUST-1) with enhanced catalytic performance for RCF of woody sawdust, which affords high yields of propyl and propanol end-chain monomeric phenols (15.5 wt % in softwoods and 46.7 wt % in hardwoods) via the C–O bond scission. A series of conifer β-O-4 models and their deuterated analogues revealed that the synergistic action of CuO/C and hydrogen could effectively cleave aryl ether linkages. It was deduced that lignin alcoholysis led to partial α-OH etherification of the β-O-4′ units, which promoted the C–O bond breakage of Cα-OMe and Cβ-O, thus giving propanol phenolic compounds through the hydrogenation of coniferyl alcohol over the CuO/C catalyst. When both α- and γ-OH of β-O-4′ motifs were meoxylated, the para-propyl phenolics were obtained through the scission of C–O linkages (Cα-OMe and Cβ-O), followed by the Cγ-OMe cleavage of propenyl ethers and hydrogenation of propenyl phenols. We envision that this work may pave the way for the development of non-noble catalysts with high reactivity and selectively for lignin valorization.