Perspective on Overcoming Scale-Up Hurdles for the Reductive Catalytic Fractionation of Lignocellulose Biomass

Advancements and Perspectives toward Lignin Valorization via O-Demethylation

Lignin represents the largest aromatic carbon resource in plants, holding significant promise as a renewable feedstock for bioaromatics and other cyclic hydrocarbons in the context of the circular bioeconomy. However, the methoxy groups of aryl methyl ethers, abundantly found in technical lignins and lignin-derived chemicals, limit their pertinent chemical reactivity and broader applicability. Unlocking the phenolic hydroxyl functionality through O-demethylation (ODM) has emerged as a valuable approach to mitigate this need and enables further applications. In this review, we provide a comprehensive summary of the progress in the valorization of technical lignin and lignin-derived chemicals via ODM, both catalytic and non-catalytic reactions. Furthermore, a detailed analysis of the properties and potential applications of the O-demethylated products is presented, accompanied by a systematic overview of available ODM reactions. This review primarily focuses on enhancing the phenolic hydroxyl content in lignin-derived species through ODM, showcasing its potential in the catalytic funneling of lignin and value-added applications. A comprehensive synopsis and future outlook are included in the concluding section of this review.

Valorisation of lignocellulose and low concentration CO2 using a fractionation-photocatalysis-electrolysis process

The simultaneous upcycling of all components in lignocellulosic biomass and the greenhouse gas CO2 presents an attractive opportunity to synthesise sustainable and valuable chemicals. However, this approach is challenging to realise due to the difficulty of implementing a solution process to convert a robust and complex solid (lignocellulose) together with a barely soluble and stable gas (CO2). Herein, we present the complete oxidative valorisation of lignocellulose coupled to the reduction of low concentration CO2 through a three-stage fractionation-photocatalysis-electrolysis process. Lignocellulose from white birch wood was first pre-treated using an acidic solution to generate predominantly cellulosic- and lignin-based fractions. The solid cellulosic-based fraction was solubilised using cellulase (a cellulose depolymerising enzyme), followed by photocatalytic oxidation to formate with concomitant reduction of CO2 to syngas (a gas mixture of CO and H2) using a phosphonate-containing cobalt(ii) bis(terpyridine) catalyst immobilised onto TiO2 nanoparticles. Photocatalysis generated 27.9 ± 2.0 μmolCO gTiO2-1 (TONCO = 2.8 ± 0.2; 16% CO selectivity) and 147.7 ± 12.0 μmolformate gTiO2-1 after 24 h solar light irradiation under 20 vol% CO2 in N2. The soluble lignin-based fraction was oxidised in an electrolyser to the value-added chemicals vanillin (0.62 g kglignin-1) and syringaldehyde (1.65 g kglignin-1) at the anode, while diluted CO2 (20 vol%) was converted to CO (20.5 ± 0.2 μmolCO cm-2 in 4 h) at a Co(ii) porphyrin catalyst modified cathode (TONCO = 707 ± 7; 78% CO selectivity) at an applied voltage of -3 V. We thus demonstrate the complete valorisation of solid and a gaseous waste stream in a liquid phase process by combining fractioning, photo- and electrocatalysis using molecular hybrid nanomaterials assembled from earth abundant elements.

Efficient mapping of lignin depolymerization product pools and quantification of specific monomers through rapid GPC-HPLC-UV/VIS analysis

BACKGROUND

Growing research on lignin depolymerization to functionalized bio-aromatics has necessitated dedicated analysis techniques. However, immense variability in molecular weight and functional groups of the depolymerization products impedes fast analysis of a large number of samples while remaining in-depth enough for catalyst screening or reaction condition optimization. While GPC-HPLC-UV/VIS has been a promising technique, up until now, the information it provides is largely qualitative. By enabling quantification of key monomeric products and through further reduction of overall analysis time, this study aims to increase the potential of GPC-HPLC-UV/VIS for fast and in-depth characterization of lignin depolymerization product pools.

RESULTS

Analysis of selected samples, isolated from GPC-HPLC-UV/VIS analyses of lignin depolymerization product pools, with gas chromatography (GC) equipped with an Orbitrap high-resolution accurate mass spectrometer (Orbitrap-HR/AM-MS) is successful in identifying the main low monomeric products. Moreover, these identifications are further substantiated through GPC-HPLC-UV/VIS analysis of standards. Furthermore, straight forward quantification of these products directly within GPC-HPLC-UV/VIS is successfully developed with limits of detection ≤0.05 mmol/L, which is at least on par with more complex analysis techniques. Additionally, several different reversed phase columns are assessed to reduce 2nd dimension (2D) time and, hence, overall analysis time while maintaining the possibility for quantification. A reduction in overall analysis time of about 30% as compared to the state-of-the-art is achieved by using a YMC Triart BIO C4 column as 2D.

SIGNIFICANCE

Through the enhancements introduced in this study, GPC-HPLC-UV/VIS emerges as a unique technique for the analysis of lignin depolymerization product pools, which is capable of fast yet sufficiently in-depth analysis of a large volume of samples. This capability is indispensable for catalyst screening and fine-tuning reaction conditions.

Lignin Hydrogenolysis: Phenolic Monomers from Lignin and Associated Phenolates across Plant Clades

The chemical complexity of lignin remains a major challenge for lignin valorization into commodity and fine chemicals. A knowledge of the lignin features that favor its valorization and which plants produce such lignins can be used in plant selection or to engineer them to produce lignins that are more ideally suited for conversion. Sixteen biomass samples were compositionally surveyed by NMR and analytical degradative methods, and the yields of phenolic monomers following hydrogenolytic depolymerization were assessed to elucidate the key determinants controlling the depolymerization. Hardwoods, including those incorporating monolignol p-hydroxybenzoates into their syringyl/guaiacyl copolymeric lignins, produced high monomer yields by hydrogenolysis, whereas grasses incorporating monolignol p-coumarates and ferulates gave lower yields, on a lignin basis. Softwoods, with their more condensed guaiacyl lignins, gave the lowest yields. Lignins with a high syringyl unit content released elevated monomer levels, with a high-syringyl polar transgenic being particularly striking. Herein, we distinguish phenolic monomers resulting from the core lignin vs those from pendent phenolate esters associated with the biomass cell wall, acylating either polysaccharides or lignins. The basis for these observations is rationalized as a means to select or engineer biomass for optimal conversion to worthy phenolic monomers.

Engineering Innovations, Challenges, and Opportunities for Lignocellulosic Biorefineries: Leveraging Biobased Polymer Production

Alternative polymer feedstocks are highly desirable to address environmental, social, and security concerns associated with petrochemical-based materials. Lignocellulosic biomass (LCB) has emerged as one critical feedstock in this regard because it is an abundant and ubiquitous renewable resource. LCB can be deconstructed to generate valuable fuels, chemicals, and small molecules/oligomers that are amenable to modification and polymerization. However, the diversity of LCB complicates the evaluation of biorefinery concepts in areas including process scale-up, production outputs, plant economics, and life-cycle management. We discuss aspects of current LCB biorefinery research with a focus on the major process stages, including feedstock selection, fractionation/deconstruction, and characterization, along with product purification, functionalization, and polymerization to manufacture valuable macromolecular materials. We highlight opportunities to valorize underutilized and complex feedstocks, leverage advanced characterization techniques to predict and manage biorefinery outputs, and increase the fraction of biomass converted into valuable products.

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.

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.

Sustainable Sorbitol Dehydration to Isosorbide using Solid Acid Catalysts: Transition from Batch Reactor to Continuous-Flow System

Isosorbide is one of the most interesting cellulosic-derived molecules with great potential to be implemented in wide range of products that shaping our daily life. This Review describes the recent developments in the production of isosorbide from sorbitol in batch and continuous-flow systems under hydrothermal conditions using solid acid catalysts. Moreover, the current hurdles and challenges regarding the synthesis of isosorbide from cellulosic biomass in continuous-flow process using solid acid catalysts are summarized, as well as the scaling-up of this process into pilot level, which will lead to an established industrial process with high sustainability metrics.

Multi-fractal structure features of corn stalks and their correlation with pretreatment homogeneity and efficacy

Lignocellulose biomass is a natural porous medium with fractal characteristics, which tend to be distinct after certain pretreatment and relational with processing effects. In this work, multi-scale fractal features of corn stalks after steam explosion pretreatment were systematically characterized employing digital image processing and mercury intrusion porosimetry methodologies. The macroscopic surface fractal features (D_s: 2.8278 ∼ 2.8523) and microscopic pore fractal features (D_p: 2.3003 ∼ 2.8867) presented same variation tendency with pretreatment severity, revealing the self-similarity of processing results of corn stalks with the scale. In association with pretreatment homogeneity and efficacy, the decrease in fractal dimensions corresponded to morphologically homogeneous and structurally complex samples with preferable auto-hydrolysis degree of structural components, which led to high reactivity with enzymes. Quantitatively, there were strongly linear correlation between fractal dimensions and enzymatic digestibility with r² > 0.95. Fractal dimension was expected to theoretically guide the rational evaluation, prediction and promotion of the key pretreatment technique in biorefinery.

Ambient-pressure lignin valorization to high-performance polymers by intensified reductive catalytic deconstruction

Chemocatalytic lignin valorization strategies are critical for a sustainable bioeconomy, as lignin, especially technical lignin, is one of the most available and underutilized aromatic feedstocks. Here, we provide the first report of an intensified reactive distillation–reductive catalytic deconstruction (RD-RCD) process to concurrently deconstruct technical lignins from diverse sources and purify the aromatic products at ambient pressure. We demonstrate the utility of RD-RCD bio-oils in high-performance additive manufacturing via stereolithography 3D printing and highlight its economic advantages over a conventional reductive catalytic fractionation/RCD process. As an example, our RD-RCD reduces the cost of producing a biobased pressure-sensitive adhesive from softwood Kraft lignin by up to 60% in comparison to the high-pressure RCD approach. Last, a facile screening method was developed to predict deconstruction yields using easy-to-obtain thermal decomposition data. This work presents an integrated lignin valorization approach for upgrading existing lignin streams toward the realization of economically viable biorefineries.

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.

A highly efficient RANEY® Ni-based amination methodology using ammonia that is capable of converting lignocellulose-derived alcohol intermediates into their corresponding primary amines in high selectivity, following hydrogen borrowing methodology.

Catalytic Hydrogenolysis of Lignin: The Influence of Minor Units and Saccharides

The precise elucidation of native lignin structures plays a vital role for the development of "lignin first" strategies such as reductive catalytic fractionation. The structure of lignin and composition of the starting material has a major impact on the product yield and distribution. Here, the differences in structure of lignin from birch, pine, reed, and walnut shell were investigated by combining detailed analysis of the whole cell wall material, residual enzyme lignin, and milled wood lignin. The results of the 2D heteronuclear single quantum coherence NMR analysis could be correlated to the product from Ru/C-catalyzed hydrogenolysis if monomeric products from ferulate and p-coumaryl and its analogous units were also appropriately considered. Notably, residual polysaccharide constituents seemed to influence the selectivity towards hydroxy-containing monomers. The results reinforced the importance of adequate structural characterization and compositional analysis of the starting materials as well as distinct (dis)advantages of specific types of structural characterization and isolation methods for guiding valorization potential of different biomass feedstocks.

Deconstruction of biomass into lignin oil and platform chemicals over heteropoly acids with carbon-supported palladium as a hybrid catalyst under mild conditions

In this work, near-complete conversion of lignocellulosic biomass and high products yields were achieved through catalytic transfer hydrogenolysis (CTH) in isopropanol using a heteropoly acid SiW₁₂ synergistic with Pd/C catalyst at a relatively mild condition. The performances of various heteropoly acids and catalytic conditions were extensively examined. The results confirmed that SiW₁₂ exhibited the highest activity on disrupting C-C linkages and C-O linkages than H₂WO₄, PW₁₂, and PMo₁₂. 34.91 wt% and 43.55 wt% monophenols were achieved for hydrogenolysis of bagasse and eucalyptus, respectively, at their optimal temperature for 5 h. Characterization studies on the lignin oil revealed that the notable structural changes were observed including the breaking of the side chain alkyl-aryl ether bonds and glycosidic bonds, while methoxyl groups were retained. Additionally, particle size of feedstock also has significant impact on the distribution and yields of the monophenols.

Lignin-to-chemicals: Application of catalytic hydrogenolysis of lignin to produce phenols and terephthalic acid via metal-based catalysts

Lignin is the only renewable aromatic material in nature and contains a large number of oxygen-containing functional groups. High-value and green utilization of "lignin-to-chemicals" can be realized via using lignin to produce fine chemicals such as phenols and carboxylic acids, which can not only reduce the waste of lignin in the process of lignocellulosic biomass treatment, but gradually make the substitution of traditional fossil fuels come true. The hydrogenolysis process under catalysis of metal catalyst has high product selectivity and less impurity, which is suitable for the production of same type or single fine chemicals. Hydrogenolysis of lignin via metal catalysts to produce lignin oil, and further modification of functional groups (e.g. methoxyl, alkyl and hydroxyl group) of depolymerized monomers in the bio-oil to yeild phenols and terephthalic acid are reviewed, and catalytic mechanisms are briefly summarized in this paper. Finally, the problems of lignin catalytic conversion existing currently are investigated, and the future development of this field is also prospected.

β-Zeolite-Assisted Lignin-First Fractionation in a Flow-Through Reactor*

In the present work, a hydrogen-free one-step catalytic fractionation of woody biomass using commercial β-zeolite as catalyst in a flow-through reactor was carried out. Birch, spruce, and walnut shells were compared as lignocellulosic feedstocks. β-Zeolite acted as a bifunctional catalyst, preventing lignin repolymerization due to its size-selective properties and also cleaving β-O-4 lignin intralinkages while stabilizing reactive intermediates. A rate-limiting step analysis using different reactor configurations revealed a mixed regime where the rates of both solvolytic delignification and zeolite-catalyzed depolymerization and dehydration affected the net rate of aromatic monomer production. Oxalic acid co-feeding was found to enhance monomer production at moderate concentrations by improving solvolysis, while it caused structural changes to the zeolite and led to lower monomer yields at higher concentrations. Zeolite stability was assessed through catalyst recycling and characterization. Main catalyst deactivation mechanisms were found to be coking and leaching, leading to widening of the pores and decrease of zeolite acidity, respectively.

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.

Fast screening of Depolymerized Lignin Samples Through 2D-Liquid Chromatography Mapping

Lignin valorization and particularly its depolymerization into bio-aromatics, has emerged as an important research topic within green chemistry. However, screening of catalysts and reaction conditions within this field is strongly constrained by the lack of analytical techniques that allow for fast and detailed mapping of the product pools. This analytical gap results from the inherent product pool complexity and the focus of the state-of-the-art on monomers and dimers, overlooking the larger oligomers. In this work, this gap is bridged through the development of a quasi-orthogonal GPC-HPLC-UV/VIS method that is able to separate the bio-aromatics according to molecular weight (hydrodynamic volume) and polarity. The method is evaluated using model compounds and real lignin depolymerization samples. The resulting color plots provide a powerful graphical tool to rapidly assess differences in reaction selectivity towards monomers and dimers as well as to identify differences in the oligomers.

Low molecular weight and highly functional RCF lignin products as a full bisphenol a replacer in bio-based epoxy resins

Herein, we present a full lignocellulose-to-chemicals valorization chain, wherein low molecular weight and highly functional lignin oligomers, obtained from reductive catalytic fractionation (RCF) of pine wood, were used to fully replace bisphenol A (BPA) for synthesizing bio-based epoxy resins.