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Ionic liquids (ILs) can be used to sustainably convert lignocellulosic feedstocks into renewable bio-based materials and chemicals. To improve the prospects of commercialization, it is essential to investigate the fate of lignocellulosic biomass during IL-based processing and develop tools for designing and optimizing this “green” technology. In situ characterization during pretreatment and dissolution processes have shown that ILs reduced the inherent recalcitrance of lignocellulosic biomass via swelling of cellulose bundles and formation of fissures in the secondary cell wall layers. It subsequently enhanced the penetration of ILs into the plant cell wall leading to depolymerization and solubilization of matrix polysaccharides, mainly hemicellulose via deacetylation. Lignin also underwent dehydration or reduction reactions, depending on the IL type, with different mechanisms leading to the cleavage of inter-unit linkages. Following this process, the accessibility to cellulose microfibrils increased and induced delamination. Complementary X-ray diffraction analyses have elucidated that ILs also reduced cellulose crystallinity and altered cellulose polymorphs. High throughput in situ analyses, namely bright-field optical microscopy, nuclear magnetic resonance and Fourier transform infrared spectroscopies, have aided in monitoring the degree of swelling and chemical structural changes in lignocellulosic biomass during IL-based processing. Development of novel in situ analytical tools like IL-based gel permeation chromatography and rheometry will further shed light on molecular level changes in lignocellulose. Thus, an overall understanding of physico-chemical changes underwent by lignocellulosic biomass will help develop tools for monitoring and improving IL-based engineering of renewable materials and chemicals.

Materials generated from renewable resources are promising and attractive substitutes for petroleum-based materials. Recently, regeneration of cellulose fibers using ionic liquids (ILs) as green solvents has been a topic of interest to both industrial and academic sectors. However, extraction of cellulose from lignocellulosic biomass requires numerous energy intensive processing steps. Additionally, the deconstruction and removal of lignin and hemicellulose components from lignocellulosic biomass usually involve corrosive pretreatment and the solvation of specific biomass components. Instead, utilization of the whole biomass—particularly woody residues—to manufacture high-performance materials offers an attractive value-proposition. In this study, we demonstrated fiber regeneration of whole hybrid poplar (HP) biomass by a sustainable method. We developed an environmentally friendly approach by partially auto-hydrolyzing the biomass with water before its dissolution in 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) ionic liquid, for large-scale, roll-to-roll production of fibers by solution spinning. We report, for the first time, a fundamental understanding of the inter- and intramolecular interactions in HP biomass–IL solutions, as well as their corresponding spinnability, structural reformation, and mechanical performance of the regenerated fibers. Particularly, the molecular alignment, recrystallization, and crystallinity of the spun fibers were correlated to the chain entanglement, molecular relaxation, and rheological properties of the HP biomass–IL solutions. A window of entangled concentration (4–6.5 wt%) of biomass in the IL was determined to be favorable for fiber spinning.

The water–gas shift (WGS) reaction is an attractive process for producing hydrogen gas from lignocellulosic biomass conversion applications. The goal of this study was to investigate hydrogen production via the WGS reaction using carbon monoxide (CO), one of the significant non-condensable gases formed during biomass fast pyrolysis, as reactant over the range of the intermediate-temperature shift (ITS). WGS reaction is typically carried out as a low-temperature shift (LTS;150–300 °C) or a high-temperature shift (HTS; 300–500 °C) with each shift using a different catalyst. In this study, the WGS was conducted at an intermediate temperature range (200–400 °C) relevant to lignocellulosic biomass fast pyrolysis hydrodeoxygenation over a copper (Cu) based catalyst in a CO-lean environment (70 vol% steam, 20 vol% He, and 10 vol% CO). The experimental temperatures were tested over three different weight hourly space velocities (WHSV = 1220, 2040, and 6110 cm3 g−1 min−1). CO conversion increased with increasing temperature and catalyst weight, with a maximum CO conversion of 94% achieved for temperatures greater than 300 °C. We evaluated four models including two mechanistic Langmuir–Hinshelwood (LH) models, one redox mechanistic model, and one reduced order model (ROM). The first (LH1) and second (LH2) Langmuir–Hinshelwood models differ by the intermediate formed on the catalyst surface. LH1 forms product complexes while LH2 produces a formate complex intermediate. LH2 best described our experimental kinetic data, based on statistical and regression analysis, and provided apparent activation energies between 60 and 80 kJ mol−1 at different space velocities. Furthermore, the ROM fit the experimental data well and, due to its simplicity, has potential for incorporation into computationally expensive simulations for similar experimental conditions.

The ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate ([EMIM]Acetate) has been widely used for biomass processing, i.e., to pretreat, activate, or fractionate lignocellulosic biomass to produce soluble sugars and lignin. However, this IL does not achieve high biomass solubility, therefore minimizing the efficiency of biomass processing. In this study, [EMIM]Acetate and three other ILs composed of different 3-methylimidazolium cations and carboxylate anions ([EMIM]Formate, 1-allyl-3-methylimidazolium ([AMIM]) formate, and [AMIM]Acetate) were analyzed to relate their physicochemical properties to their biomass solubility performance. While all four ILs are able to dissolve hybrid poplar under fairly mild process conditions (80 °C and 100 RPM stirring), [AMIM]Formate and [AMIM]Acetate have particularly increased biomass solubility of 40 and 32%, respectively, relative to [EMIM]Acetate. Molecular dynamics simulations suggest that strong interactions between IL and specific plant biopolymers may contribute to this enhanced solubilization, as the calculated second virial coefficients between ILs and hemicellullose are most favorable for [AMIM]Formate, matching the trend of the experimental solubility measurements. The simulations also reveal that the interactions between the ILs and hemicellulose are an important factor in determining the overall biomass solubility, whereas lignin-IL interactions were not found to vary significantly, consistent with literature. The combined experimental and simulation studies identify [AMIM]Formate as an efficient biomass solvent and explain its efficacy, suggesting a new approach to rationally select ionic liquid solvents for lignocellulosic deconstruction.

While previous studies have shown that white-rot fungal pretreatment reduces the severity of chemical pretreatments and improves enzymatic saccharification yields, very few have investigated the synergistic effects of fungal pretreatment combined with organosolv pretreatment on lignin recovery and quality. In this study, loblolly pine chips were incubated with Trametes versicolor for 15, 30 and 45 days, prior to organosolv pretreatment and enzymatic saccharification. Fungal pretreatment for 15 days improved the saccharification yield by 23%, and higher amounts (56%) of lignin-enriched fractions were obtained. Fungal pretreatment for 45 days led to extensive depolymerization, structural modification and enrichment of lignin in the organosolv precipitates (OP). Characterization of the OP fraction showed that samples pretreated for 30 and 45 days contained higher amounts of hydroxyl groups and p-hydroxyphenyl (H) subunits, and showed increases in depolymerization of carbohydrates as well as decreases in G/H lignin ratio. It became apparent that further investigations are needed to explore the benefits of combining fungal and organosolv pretreatments for improving lignin yields from softwoods.

Correction for ‘Understanding the in situ state of lignocellulosic biomass during ionic liquids-based engineering of renewable materials and chemicals’ by Kalavathy Rajan et al., Green Chem., 2020, 22, 6748–6766, DOI: 10.1039/D0GC02582H.

Organosolv fractionation is a promising pathway to separate cellulosic biomass into high purity cellulose, hemicelluloses, and lignin. This work specifically investigates the properties of lignins isolated at specific time points as fractionation progressed, with the intent of correlating fractionation time with lignin purity, yield, thermal and chemical properties. Yellow poplar (Liriodendron tulipifera) was fractionated using a mixture of methyl isobutyl ketone, ethanol, and water with sulfuric acid as catalyst at 140 °C over a two-hour period. Aliquots of the liquor were collected by sampling every 15 min during the fractionation to generate a series of lignins. The results showed that with increased fractionation time, lignin purity improved from 90.3 to 94.6% and the glass transition temperature increased from 117 to 137 °C. The loss of aliphatic OH and increase of phenolic OH with fractionation time led to an increase in condensed structures and increased polydispersity at times greater than 90 min. Principal component analysis of Fourier transform infrared spectroscopic data confirmed the shift to higher purity and more condensed chemical structures with increasing fractionation time. Overall, this study demonstrates that thermal and chemical properties of lignin change with the organosolv fractionation time.

Recently, there has been a growing interest in developing value added uses for lignin, including the utilization of lignins as a precursor for carbon materials. Proper understanding of the association behavior of lignins during solution processing provides important structural information that is needed to rationally optimize the use of lignins in industry in a range of value added applications. In these experiments, we follow the assembly of lignin molecules from a variety of sources in dimethyl sulfoxide, a good solvent for lignins, using small angle neutron scattering. In order to mimic industrial processing conditions, concentrations of lignins were kept above the overlap concentration. At small length scales, short lignin segments with ∼4–10 monolignol units associate to form rigid rod-like/cylindrical building blocks, where the number of repeat units in a cylindrical segment decreases with increasing lignin concentration. These cylindrical building blocks associate to form aggregates with low cross-linking densities and a random coil or network like structures from highly branched lignin structures. The degree of branching of the base lignin molecule, which varies with source, plays a crucial role in determining their association behavior. The overall sizes of the aggregates decrease with increasing concentration at low cross-linking densities, whereas the opposite trend is observed for highly branched lignins.