Matthew T. Clough

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis

Abstract Lignin is an abundant biopolymer with a high carbon content and high aromaticity. Despite its potential as a raw material for the fuel and chemical industries, lignin remains the most poorly utilised of the lignocellulosic biopolymers. Effective valorisation of lignin requires careful fine‐tuning of multiple “upstream” (i.e., lignin bioengineering, lignin isolation and “early‐stage catalytic conversion of lignin”) and “downstream” (i.e., lignin depolymerisation and upgrading) process stages, demanding input and understanding from a broad array of scientific disciplines. This review provides a “beginning‐to‐end” analysis of the recent advances reported in lignin valorisation. Particular emphasis is placed on the improved understanding of lignins biosynthesis and structure, differences in structure and chemical bonding between native and technical lignins, emerging catalytic valorisation strategies, and the relationships between lignin structure and catalyst performance.

The Influence of Hemicellulose Sugars on Product Distribution of Early‐Stage Conversion of Lignin Oligomers Catalysed by Raney Nickel

We recently introduced catalytic upstream biorefining, a fractionation process performed on whole lignocellulosic materials, based on the early‐stage conversion of lignin by hydrogen‐transfer reactions (using Raney Ni as the catalyst and 2‐PrOH as a hydrogen‐donor). The process fractionates lignocellulose, isolating lignin as an extensively depolymerised oil, opening up new avenues in the catalytic upgrading of bio‐derived phenolic streams to chemicals and fuels. In addition, highly delignified holocellulose pulps are obtained, holding potential as a feedstock for the production of paper, chemicals, and biofuels. Herein, we report our first results on the chemistry underlying this process under nearly neutral to slightly alkaline conditions achieved by the addition of inorganic bases. This report sheds light on the influence of hemicellulose sugars on the product distribution obtained from the early‐stage catalytic conversion of lignin oligomers released from lignocellulose. The increase in the pH value of the medium suppressed the hydrolysis of xylans. As a result, a dramatic increase in the xylans retention from 10 % (at pH 4.5) up to 60 % (at pH>7.5) was achieved. Interestingly, the pH value of the liquor did not affect the delignification extent of lignocellulose or the absolute content of glucans retained in the holocellulose. By enhancing xylans retention, we provide evidence that hemicellulose sugars decrease the activity of Raney Ni towards full hydrogenation of the aromatic species composing the lignin stream. In fact, the yield of selected cyclohexanols increases from 0.8 % (no added bases) to 4.4 % (added NaOH), whereas the yield of selected phenols decreases from 12.9 % (no added bases) to 7.2 % (added NaOH).

Catalysis meets nonthermal separation for the production of (alkyl)phenols and hydrocarbons from pyrolysis oil.

A simple and efficient hydrodeoxygenation strategy is described to selectively generate and separate high-value alkylphenols from pyrolysis bio-oil, produced directly from lignocellulosic biomass. The overall process is efficient and only requires low pressures of hydrogen gas (5 bar). Initially, an investigation using model compounds indicates that MoCx /C is a promising catalyst for targeted hydrodeoxygenation, enabling selective retention of the desired Ar-OH substituents. By applying this procedure to pyrolysis bio-oil, the primary products (phenol/4-alkylphenols and hydrocarbons) are easily separable from each other by short-path column chromatography, serving as potential valuable feedstocks for industry. The strategy requires no prior fractionation of the lignocellulosic biomass, no further synthetic steps, and no input of additional (e.g., petrochemical) platform molecules.

Biphasic extraction of mechanocatalytically-depolymerized lignin from water-soluble wood and its catalytic downstream processing

Mechanocatalysis is an efficient method for deep depolymerization of lignocellulosic materials. It renders full conversion of lignocellulose into water-soluble products, a mixture of oligosaccharides and lignin oligomers. In an aqueous monophasic system, it is known that further saccharification of these products affords high yields of fermentable sugars in addition to a lignin precipitate. Although much attention has been devoted to upgrading of the carbohydrate fraction, the reactivity of mechanocatalytically-isolated lignin has hitherto been overlooked. In this report, we demonstrate the saccharification of water-soluble lignocellulose in a biphasic mixture of water and 2-methyltetrahydrofuran, to prevent lignin fragments from recondensation, not only with other lignin fragments but also with oligosaccharides and their degradation products. The lignin stream extracted in the biphasic system exhibits a molecular weight much lower than that of the lignin precipitate obtained from monophasic aqueous saccharification, and that of lignin isolated by an Organosolv process with no added acid catalyst. Surprisingly, β-O-4 and phenylcoumaran (β-5 + α-O-4) linkages are quantitatively hydrolyzed during saccharification in the biphasic system. However, the extracted lignin still incorporates resinol bonding motifs (β-β + γ-O-α). Because of the lower molecular weight, low carbohydrate content, and high solubility in organic solvents, the catalytic downstream processing of the extracted lignin leads to higher yields of liquid products (e.g. 2-methoxyphenols and 2,6-dimethoxyphenols) by RANEY® Ni-catalyzed hydrogenolysis (200 °C, 7 MPa H2), compared to reactions performed on the lignin precipitate or Organosolv lignin.

A Convergent Approach for a Deep Converting Lignin-First Biorefinery Rendering High-Energy-Density Drop-in Fuels

Summary Herein, a lignin-centered convergent approach to produce either aliphatic or aromatic bio-hydrocarbons is introduced. First, poplar or spruce wood was deconstructed by a lignin-first biorefining process, a technique based on the early-stage catalytic conversion of lignin, yielding lignin oils along with cellulosic pulps. Next, the lignin oils were catalytically upgraded in the presence of a phosphidated Ni/SiO2 catalyst under H2 pressure. Notably, selectivity toward aliphatics or aromatics can simply be adjusted by changes in H2 pressure and temperature. The process renders two distinct main cuts of branched hydrocarbons (gasoline: C6-C10, and kerosene/diesel: C14-C20). As the approach is H2-intensive, we examined the utilization of pulp as an H2 source via gasification. For several biomass sources, the H2 obtainable by gasification stoichiometrically meets the H2 demand of the deep converting lignin-first biorefinery, making this concept plausible for the production of high-energy-density drop-in biofuels.

On the Meaning and Origins of Lignin Recalcitrance: A Critical Analysis of the Catalytic Upgrading of Lignins Obtained from Mechanocatalytic Biorefining and Organosolv Pulping

In the broad context of catalysis for lignin valorization, the term “recalcitrance” is often used to describe the resistance of lignin to undergo chemical transformations (generally, reductive processes) rendering small molecules soluble in the reaction medium. Unfortunately, the current usage of the term “recalcitrance” often remains vague in meaning, hindering the search for better catalysts for lignin valorization. In the quest to address the research question—What is lignin recalcitrance?—we present our search for the factors responsible for the resistance of lignin to reductive catalytic processes, from various perspectives. In this study, lignins isolated as a precipitate obtained from the saccharification of water‐soluble lignocelluloses (produced by solvent‐free mechanocatalytic depolymerization of beechwood, pinewood, or sugarcane bagasse) and their counterparts isolated by solvent extraction (organosolv pulping) are investigated. The critical analysis of structure and bonding, in addition to the in‐depth understanding of results from the catalytic upgrading of lignin streams, in the presence of Raney Ni and H2 pressure under mild and extreme conditions, reveals that the simple evaluation of the total yield of liquid products provides no quantitative measure of the lignin recalcitrance. Our results shed light on the real meaning, origins and implications of “lignin recalcitrance” for catalysis research. The results demonstrate that lignin recalcitrance is associated not only with its intrinsic properties (i.e. molecular weight, the occurrence of native linkages, and their bond dissociation enthalpies) but also with its extrinsic properties (e.g. residual polysaccharides and solubility). Overall, this study presents a detailed evaluation of recalcitrance of lignin through the critical analysis of the product mixture properties (e.g. H/C and O/C ratios, molecular weight distribution, yield of key individual products, and several others).

Thermally Triggered Phase Separation of Organic Electrolyte–Cellulose Solutions

Organic electrolyte solutions (OES)-binary mixtures of an ionic liquid (IL) with a neutral polar aprotic co-solvent-are being recognized as excellent candidate solvents for the dissolution, derivatization, and sustainable processing of cellulose. These solutions exhibit the beneficially combined properties of rapid-to-instantaneous cellulose dissolution, raised thermal stability, and reduced viscosity, compared to cellulose solutions in the parent ILs. Herein, we report the reversible, thermally triggered phase separation of cellulose solutions in 1-ethyl-3-methylimidazolium acetate with 1,3-dimethyl-2-imidazolidinone. In these solutions, cellulose drives the process of phase separation, resulting in a lower, IL-rich layer in which the biopolymer is segregated. In turn, the upper phase is enriched in the neutral co-solvent. We show that the temperature of phase separation can be fine-tuned by modification of mole fractions of IL, co-solvent, and cellulose. This finding holds promise for the design of strategies for separation and solvent recycling in cellulose chemistry.

Organic electrolyte solutions as versatile media for the dissolution and regeneration of cellulose

Organic electrolyte solutions – mixtures of a (room-temperature) ionic liquid with a neutral, organic, polar co-solvent – are attracting increasing attention as solvents for the regeneration and derivatisation of cellulose. Despite advantages (in comparison to simple ionic liquid analogues) associated with rapid or instantaneous dissolution, reduced viscosity, enhanced thermal stability and fine-tunable physicochemical properties, a firm understanding of the precise solvent–solute interactions and the relative kinetic versus thermodynamic contributions to dissolution remains elusive. The incorporation of a co-solvent introduces an additional layer of complexity, therefore an informed choice of both ionic liquid and co-solvent is necessary in order to achieve the desired properties. This article first provides an overview of the structure and bonding properties of native and non-native cellulose allomorphs, and a brief history of strategies for cellulose dissolution. Subsequently, organic electrolyte solutions are introduced as versatile solvents for cellulose, and the underpinning thermodynamic, kinetic and mechanistic phenomena behind cellulose solubility are critically discussed. The final sections summarize recent advances in the development of organic electrolyte technologies for derivatisation and regeneration of cellulose and critically discuss whether organic electrolytes can, or could be, rightly regarded as ‘green solvents’.

On the Reactivity of Dihydro-p-coumaryl Alcohol towards Reductive Processes Catalyzed by Raney Nickel

There are several established approaches for the reductive fractionation of lignocellulose (e.g., “catalytic upstream biorefining” and “lignin‐first” approaches) that lead to a lignin oil product that is composed primarily of dihydro‐p‐monolignols [e.g., 4‐(3‐hydroxypropyl)‐2‐methoxyphenol and 4‐(3‐hydroxypropyl)‐2,6‐dimethoxyphenol]. Although effective catalytic methods have been developed to perform reductive or deoxygenative processes on the lignin oil, the influence of the 3‐hydroxypropyl substituent on catalyst activity has previously been overlooked. Herein, to better understand the reactivity of the depolymerized lignin oil obtained from catalytic upstream biorefining processes, dihydro‐p‐coumaryl alcohol was selected as a model compound. Hydrogenation of this species in the presence of Raney Ni with molecular hydrogen led to ring saturation (100 % selectivity) in the absence of hydrodeoxygenation, whereas under hydrogen‐transfer conditions with 2‐propanol, hydrogenation occurred (≈55 % selectivity) simultaneously with hydrodeoxygenation (≈40 % selectivity). In a broader context, this study sheds light not only on the reactivity of dihydro‐p‐monolignols but also on the intricacies of the catalytic upstream biorefining reaction network in which these species are revealed to be key intermediates in the formation of less‐functionalized p‐alkylphenols.

1D and 2D NMR Spectroscopy of Bonding Interactions within Stable and Phase-Separating Organic Electrolyte–Cellulose Solutions

Organic electrolyte solutions (i.e. mixtures containing an ionic liquid and a polar, molecular co-solvent) are highly versatile solvents for cellulose. However, the underlying solvent-solvent and solvent-solute interactions are not yet fully understood. Herein, mixtures of the ionic liquid 1-ethyl-3-methylimidazolium acetate, the co-solvent 1,3-dimethyl-2-imidazolidinone, and cellulose are investigated using 1D and 2D NMR spectroscopy. The use of a triply-13 C-labelled ionic liquid enhances the signal-to-noise ratio for 13 C NMR spectroscopy, enabling changes in bonding interactions to be accurately pinpointed. Current observations reveal an additional degree of complexity regarding the distinct roles of cation, anion, and co-solvent toward maintaining cellulose solubility and phase stability. Unexpectedly, the interactions between the dialkylimidazolium ring C2 -H substituent and cellulose become more pronounced at high temperatures, counteracted by a net weakening of acetate-cellulose interactions. Moreover, for mixtures that exhibit critical solution behavior, phase separation is accompanied by the apparent recombination of cation-anion pairs.