Economic barriers preventing commercialization of lignocellulose-to-ethanol bioconversion processes include the high cost of hydrolytic enzymes. One strategy for cost reduction is to improve the specific activities of cellulases by genetic engineering. However, screening for improved activity typically uses "ideal" cellulosic substrates, and results are not necessarily applicable to more realistic substrates such as pretreated hardwoods and softwoods. For lignocellulosic substrates, nonproductive binding and inactivation of enzymes by the lignin component appear to be important factors limiting catalytic efficiency. A better understanding of these factors could allow engineering of cellulases with improved activity based on reduced enzyme-lignin interaction ("weak lignin-binding cellulases"). To prove this concept, we have shown that naturally occurring cellulases with similar catalytic activity on a model cellulosic substrate can differ significantly in their affinities for lignin. Moreover, although cellulose-binding domains (CBDs) are hydrophobic and probably participate in lignin binding, we show that cellulases lacking CBDs also have a high affinity for lignin, indicating the presence of lignin-binding sites on the catalytic domain.

Drop‐in biofuels have been defined as functionally equivalent to petroleum‐based transportation fuels and are fully compatible with the existing petroleum infrastructure. They will be essential in sectors such as aviation if we are to achieve emission reduction and climate mitigation goals. Currently, ‘conventional’ drop‐in biofuels, which are primarily based on upgrading of lipids / oleochemicals, are the only significant source of commercial volumes of drop‐in biofuels. However, the necessary increased, future volumes will likely come from ‘advanced’ drop‐in biofuels based on biomass feedstocks such as forest and agriculture residues. Biocrudes / bio‐oils produced from lignocellulosic feedstocks using thermochemical technologies such as gasification, pyrolysis, and hydrothermal liquefaction need to be further upgraded to drop‐in biofuels. However, advanced drop‐in biofuels have been slow to reach commercial maturity due to significant technical challenges, high capital costs, and the challenge of generally lower oil prices. It is likely that the co‐processing of drop‐in biofuels with conventional petroleum refining could considerably reduce capital costs. Initially, co‐processing is likely to be established through the upgrading of conventional / oleochemical feedstocks (lipids). Lipids are readily available in large volumes (global production in 2017 was ~185 million metric tonnes) and can be more easily integrated into oil‐refinery processes. In contrast, lignocellulose‐derived biocrudes / bio‐oils are not yet available in significant volumes and are more complex to co‐process in a refinery. The likely strategies for co‐processing of oleochemicals (lipids) and bio‐oil and biocrude feedstocks based on different insertion points within the refinery infrastructure are discussed. © 2019 The Authors. Biofuels, Bioproducts and Biorefining published by Society of Chemical Industry and John Wiley & Sons, Ltd.

Two fluorescence-tagged carbohydrate-binding modules (CBMs), which specifically bind to crystalline (CBM2a-RRedX) and paracrystalline (CBM17-FITC) cellulose, were used to differentiate the supramolecular cellulose structures in bleached softwood Kraft fibers during enzyme-mediated hydrolysis. Differences in CBM adsorption were elucidated using confocal laser scanning microscopy (CLSM), and the structural changes occurring during enzyme-mediated deconstruction were quantified via the relative fluorescence intensities of the respective probes. It was apparent that a high degree of order (i.e., crystalline cellulose) occurred at the cellulose fiber surface, which was interspersed by zones of lower structural organization and increased cellulose accessibility. Quantitative image analysis, supported by 13C NMR, scanning electron microscopy (SEM) imaging, and fiber length distribution analysis, showed that enzymatic degradation predominates at these zones during the initial phase of the reaction, resulting in rapid fiber fragmentation and an increase in cellulose surface crystallinity. By applying this method to elucidate the differences in the enzyme-mediated deconstruction mechanisms, this work further demonstrated that drying decreased the accessibility of enzymes to these disorganized zones, resulting in a delayed onset of degradation and fragmentation. The use of fluorescence-tagged CBMs with specific recognition sites provided a quantitative way to elucidate supramolecular substructures of cellulose and their impact on enzyme accessibility. By designing a quantitative method to analyze the cellulose ultrastructure and accessibility, this study gives insights into the degradation mechanism of cellulosic substrates.

From the aspects of green chemistry and sustainability, the use of green and sustainable materials and reagents for nanocellulose production is highly desirable. In this study, an acidic deep eutectic solvent (DES) pretreatment process was developed to fabricate lignin-containing cellulose nanocrystals (LCNCs) from undervalued thermomechanical pulp (TMP). LCNCs were successfully obtained using both binary DES (choline chloride:oxalic acid, 1:1 molar ratio) and ternary DES (choline chloride:oxalic acid:p-toluenesulfonic acid, 2:1:1 molar ratio) followed by a mild mechanical disintegration process. The LCNCs with a width of around 6 nm, thickness of 3.3 nm, retained cellulose I crystallinity of 57.4%, high lignin content of 47.8%, and high yield of 66% were obtained under the optimum conditions using ternary DES at 80 °C for 3 h pretreatment. Meanwhile, the LCNCs obtained from this process showed a high thermal stability (T max of 358 °C), which exhibited promising potential for further applications. The results demonstrate that the environmentally friendly DES is a promising solvent, which can provide a prospective future for both lignocellulosic material utilization and LCNCs isolation.

To be effective, steam pretreatment is typically carried out at temperatures/pressures above the glass transition point (Tg) of biomass lignin so that it can partly fluidize and relocate. The relocation of Douglas‐fir and corn stover derived lignin was compared with the expectation that, with the corn stover lignin's lower hydrophobicity and molecular weight, it would be more readily fluidized. It was apparent that the Tg of lignin decreased as the moisture increased, with the easier access of steam to the corn stover lignin promoting its plasticization. Although the softwood lignin was more recalcitrant, when it was incorporated onto filter paper, it too could be plasticized, with its relocation enhancing enzymatic hydrolysis. When lignin recondensation was minimized, the increased hydrophobicity suppressed lignin relocation. It was apparent that differences in the accessibility of the lignin present in Douglas‐fir and corn stover to steam significantly impacted lignin fluidization, relocation, and subsequent cellulose hydrolysis.

No abstract

BackgroundThe hydrotreatment of oleochemical/lipid feedstocks is currently the only technology that provides significant volumes (millions of litres per year) of “conventional” biojet/sustainable aviation fuels (SAF). However, if biojet fuels are to be produced in sustainably sourced volumes (billions of litres per year) at a price comparable with fossil jet fuel, biomass-derived “advanced” biojet fuels will be needed. Three direct thermochemical liquefaction technologies, fast pyrolysis, catalytic fast pyrolysis and hydrothermal liquefaction were assessed for their potential to produce “biocrudes” which were subsequently upgraded to drop-in biofuels by either dedicated hydrotreatment or co-processed hydrotreatment.ResultsA significant biojet fraction (between 20.8 and 36.6% of total upgraded fuel volume) was produced by all of the processes. When the fractions were assessed against general ASTM D7566 specifications they showed significant compliance, despite a lack of optimization in any of the process steps. When the life cycle analysis GHGenius model was used to assess the carbon intensity of the various products, significant emission reductions (up to 74%) could be achieved.ConclusionsIt was apparent that the production of biojet fuels based on direct thermochemical liquefaction of biocrudes, followed by hydrotreating, has considerable potential.

It is widely acknowledged that the rate limiting step in the enzyme-mediated deconstruction of the biomass process is the restricted ability of the enzymes to access the cellulosic substrate. An ongoing challenge has been to find reproducible and quantifiable methods for measuring enzyme accessibility to cellulose. Type A (crystalline cellulose) and type B (paracrystalline) cellulose binding modules (CBMs) were used in parallel with microscopy, fiber analysis (aspect ratio), and water retention values (WRV) to determine if the observed and anticipated changes in differentially prepared microfibrillated cellulose (MFC) substrates were similar. It was apparent that with increasing refining there was a corresponding increase in fibrillation (SEM and WRV), as well as a decrease in aspect ratio. Although the initial degree and rate of enzymatic hydrolysis increased with prolonged refining, above 1000 kWh ton–1 little improvement in either was observed. However, when cellulose accessibility was assessed by the CBM method, the observed trend followed the hydrolysis profile. Although the other methods (WRV, SEM, and aspect ratio) suggested increased refining should result in greater accessibility and a corresponding improvement in hydrolysis, the CBM method more accurately predicted enzyme accessibility, implying that refining did not significantly improve enzyme accessibility at the microfibril level of the cellulosic substrate.