In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon–carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.

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Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading

Algae-bacteria interactions: Evolution, ecology and emerging applications.

Bioethanol is one of the most promising and eco-friendly alternatives to fossil fuels, which is produced from renewable sources. Although almost all the current fuel ethanol is generated from edible sources (sugars and starch), lignocellulosic biomass (LCB) has drawn much attention in recent times. However, the conversion efficiency as well as ethanol yield of the biomass differs greatly with respect to the source and nature of LCB, primarily due to the variation in lignocellulosic content. Two major polysaccharides in LCB, namely, cellulose and hemicellulose firmly link to lignin and form a complex lignocellulosic network, which is highly robust and recalcitrant to depolymerization. For this reason, generation of ethanol from LCB requires a complicated conversion process that has made it commercially non-competitive. As attempts to exploit LCBs into commercial ethanol production, recent research efforts have been devoted to the techno-economic improvements of the overall conversion process, in addition to screen out promising feedstocks. This review paper presents an overview on the diversity of biomass, technological approaches and microbial contribution to the conversion of LCB into ethanol.

Fuel ethanol production from lignocellulosic biomass: an overview on feedstocks and technological approaches.

More than 70 years ago, the filamentous ascomycete Trichoderma reesei was isolated on the Solomon Islands due to its ability to degrade and thrive on cellulose containing fabrics. This trait that relies on its secreted cellulases is nowadays exploited by several industries. Most prominently in biorefineries which use T. reesei enzymes to saccharify lignocellulose from renewable plant biomass in order to produce biobased fuels and chemicals. In this review we summarize important milestones of the development of T. reesei as the leading production host for biorefinery enzymes, and discuss emerging trends in strain engineering. Trichoderma reesei has very recently also been proposed as a consolidated bioprocessing organism capable of direct conversion of biopolymeric substrates to desired products. We therefore cover this topic by reviewing novel approaches in metabolic engineering of T. reesei.

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Cellulases and beyond: the first 70 years of the enzyme producer Trichoderma reesei.

There is an immediate need to drastically reduce the emissions associated with global fossil fuel consumption in order to limit climate change. However, carbon-based materials, chemicals, and transportation fuels are predominantly made from fossil sources and currently there is no alternative source available to adequately displace them. Gas-fermenting microorganisms that fix carbon dioxide (CO2) and carbon monoxide (CO) can break this dependence as they are capable of converting gaseous carbon to fuels and chemicals. As such, the technology can utilize a wide range of feedstocks including gasified organic matter of any sort (e.g., municipal solid waste, industrial waste, biomass, and agricultural waste residues) or industrial off-gases (e.g., from steel mills or processing plants). Gas fermentation has matured to the point that large-scale production of ethanol from gas has been demonstrated by two companies. This review gives an overview of the gas fermentation process, focusing specifically on anaerobic acetogens. Applications of synthetic biology and coupling gas fermentation to additional processes are discussed in detail. Both of these strategies, demonstrated at bench-scale, have abundant potential to rapidly expand the commercial product spectrum of gas fermentation and further improve efficiencies and yields.

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Gas Fermentation—A Flexible Platform for Commercial Scale Production of Low-Carbon-Fuels and Chemicals from Waste and Renewable Feedstocks

The fermentation inhibition of yeast or bacteria by lignocellulose-derived degradation products, during hexose/pentose co-fermentation, is a major bottleneck for cost-effective lignocellulosic biorefineries. To engineer microbial strains for improved performance, it is critical to understand the mechanisms of inhibition that affect fermentative organisms in the presence of major components of a lignocellulosic hydrolysate. The development of a synthetic lignocellulosic hydrolysate (SH) media with a composition similar to the actual biomass hydrolysate will be an important advancement to facilitate these studies. In this work, we characterized the nutrients and plant-derived decomposition products present in AFEX™ pretreated corn stover hydrolysate (ACH). The SH was formulated based on the ACH composition and was further used to evaluate the inhibitory effects of various families of decomposition products during Saccharomyces cerevisiae 424A (LNH-ST) fermentation. The ACH contained high levels of nitrogenous compounds, notably amides, pyrazines, and imidazoles. In contrast, a relatively low content of furans and aromatic and aliphatic acids were found in the ACH. Though most of the families of decomposition products were inhibitory to xylose fermentation, due to their abundance, the nitrogenous compounds showed the most inhibition. From these compounds, amides (products of the ammonolysis reaction) contributed the most to the reduction of the fermentation performance. However, this result is associated to a concentration effect, as the corresponding carboxylic acids (products of hydrolysis) promoted greater inhibition when present at the same molar concentration as the amides. Due to its complexity, the formulated SH did not perfectly match the fermentation profile of the actual hydrolysate, especially the growth curve. However, the SH formulation was effective for studying the inhibitory effect of various compounds on yeast fermentation. The formulation of SHs is an important advancement for future multi-omics studies and for better understanding the mechanisms of fermentation inhibition in lignocellulosic hydrolysates. The SH formulated in this work was instrumental for defining the most important inhibitors in the ACH. Major AFEX decomposition products are less inhibitory to yeast fermentation than the products of dilute acid or steam explosion pretreatments; thus, ACH is readily fermentable by yeast without any detoxification.

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Designer synthetic media for studying microbial-catalyzed biofuel production

A catalytic membrane nanoreactor (CMNR) with Cu-Agx (where x is the millimolar concentration of AgNO3) bimetallic catalysts immobilized in membrane pores has been fabricated via coupling flowing synthesis and replacement reaction. Surface characterization by transmission electron microscopy (TEM) gives obvious evidence of the formation of Cu-Ag bimetallic core-shell nanostructures with Ag islands deposited on the Cu core metal. An apparent high shift phenomenon for the Cu element and a low shift phenomenon for the Ag element was determined by X-ray photoelectron spectroscopy (XPS), indicating a close interaction with the transfer of electron density from the Cu atom to the Ag atom. The hydrogenation catalysis of p-nitrophenol (p-NP) was tested to evaluate the catalytic performance. During the catalytic process, the Cu core acts as an electron-deficient site to adsorb and activate the -NO2 group for p-NP, and the Ag shell is beneficial for enhancing active H spilling to the Cu surface and then performing hydrogenation. A volcano-shaped apparent reaction rate constant can be achieved, which rises initially with the increasing Ag content and subsequently drops with a further increase in the Ag content. The highest value of 1071 min-1 can be achieved for CMNR immobilized with Cu-Ag2 owing to the suitable adsorption activation behavior and the best hydrogen spillover behavior.

Zeyi Xiao

Papers

Designer synthetic media for studying microbial-catalyzed biofuel production

Catalytic Membrane Nanoreactor with Cu-Agx Bimetallic Nanoparticles Immobilized in Membrane Pores for Enhanced Catalytic Performance.

Catalytic Membrane Nano Reactor with Two-Dimensional Channels Assembly of Graphene Oxide Nanosheets with Zif-67 Derived Co3s4 Catalyst Immobilized on

A flow-through electrode for hydrogen production from water splitting by mitigating bubble induced overpotential

Enhanced Benzyl Alcohol Oxidation Coupled with Hydrogen Evolution by Co3O4@SS Electrocatalytic Membrane Structured Reactor via Flow-Through Operation