Advances in process design, techno-economic assessment and environmental aspects for hydrothermal pretreatment in the fractionation of biomass under biorefinery concept

Grass lignin: biosynthesis, biological roles, and industrial applications

Lignin is a phenolic heteropolymer found in most terrestrial plants that contributes an essential role in plant growth, abiotic stress tolerance, and biotic stress resistance. Recent research in grass lignin biosynthesis has found differences compared to dicots such as Arabidopsis thaliana. For example, the prolific incorporation of hydroxycinnamic acids into grass secondary cell walls improve the structural integrity of vascular and structural elements via covalent crosslinking. Conversely, fundamental monolignol chemistry conserves the mechanisms of monolignol translocation and polymerization across the plant phylum. Emerging evidence suggests grass lignin compositions contribute to abiotic stress tolerance, and periods of biotic stress often alter cereal lignin compositions to hinder pathogenesis. This same recalcitrance also inhibits industrial valorization of plant biomass, making lignin alterations and reductions a prolific field of research. This review presents an update of grass lignin biosynthesis, translocation, and polymerization, highlights how lignified grass cell walls contribute to plant development and stress responses, and briefly addresses genetic engineering strategies that may benefit industrial applications.

Green Process for Producing Xylooligosaccharides by Using Sequential Auto-hydrolysis and Xylanase Hydrolysis

Xylooligosaccharides (XOS), as prebiotic oligomers, are increasingly receiving attention as high value-added products produced from lignocellulosic biomass. Although the XOS contains a series of different degrees of polymerization (DP) of xylose units, DP 2 and 3 (xylobiose (X2) and xylotriose (X3)) are regarded as the main active components in food and pharmaceutical fields. Therefore, in the study, in order to achieve the maximum production of XOS with the desired DP, a combination strategy of sequential auto-hydrolysis and xylanase hydrolysis was developed with corncob as raw material. The evidences showed that the hemicellulosic xylan could be effectively decomposed into various higher DP saccharides (> 4), which were dissolved into the auto-hydrolysate; sequentially, the soluble saccharides could be rapidly hydrolyzed into XOS with desired DP by xylanase hydrolysis. Finally, a maximum XOS yield of 56.3% was achieved and the ratio of (X2 + X3)/XOS was over 80%; meanwhile, the by-products could be controlled at lower levels. Overall, this study provides solid data that support the selective and precise preparation of XOS from corncob, vigorously promoting the application of XOS as functional sugar products.

Efficient co-production of reducing sugars and xylooligosaccharides via clean hydrothermal pretreatment of rape straw

Hydrothermal treatment was applied to pretreat rape straw for the efficient co-production of reducing sugars and xylooligosaccharides. It was observed that hydrothermal treatment using water as solvent and catalyst destructed the compact structure of rape straw and increased its enzymatic digestion efficiency from 24.6% to 92.0%. Xylooligosaccharide (3.3 g/L) was acquired after the treatment under 200 °C for 60 min (severity factor Log Ro = 4.7). With increasing pretreatment intensity from 3.1 to 5.4, the hemicellulose removal increased from 14.4% to 100%, and the delignification was raised from 12% to 44%. Various characterization proved that the surface morphology of treated material showed a porous shape, while the cellulose accessibility, lignin surface area and lignin hydrophobicity were greatly improved. Consequently, hydrothermal pretreatment played a vital role in the sustainable transformation of biomass to valuable biobased compounds, and had a wide range of application prospects in lignocellulosic biorefining.

Emerging trends and advances in valorization of lignocellulosic biomass to biofuels

Sustainable technologies pave the way to address future energy demand by converting lignocellulosic biomass into fuels, carbon-neutral materials, and chemicals which might replace fossil fuels. Thermochemical and biochemical technologies are conventional methods that convert biomass into value-added products. To enhance biofuel production, the existing technologies should be upgraded using advanced processes. In this regard, the present review explores the advanced technologies of thermochemical processes such as plasma technology, hydrothermal treatment, microwave-based processing, microbial-catalyzed electrochemical systems, etc. Advanced biochemical technologies such as synthetic metabolic engineering and genomic engineering have led to the development of an effective strategy to produce biofuels. The microwave-plasma-based technique increases the biofuel conversion efficiency by 97% and the genetic engineering strains increase the sugar production by 40%, inferring that the advanced technologies enhances the efficiency. So understanding these processes leads to low-carbon technologies which can solve the global issues on energy security, the greenhouse gases emission, and global warming.

Sustainable valorization of pitaya (Hylocereus spp.) peel in a semi-continuous high-pressure hydrothermal process to recover value-added products

This study evaluated the use of a semi-continuous high-pressure hydrothermal process for the recovery of value-added products from pitaya peel. The process was carried out at 15 MPa, a water flow rate of 2 mL/min, a solvent-to-feed ratio of 60 g water/g pitaya peel, and temperatures ranging from 40 to 210 °C. The results show that extraction temperatures (between 40 and 80 °C) promoted the recovery of betacyanin (1.52 mg/g), malic acid (25.6 mg/g), and citric acid (25.98 mg/g). The major phenolic compounds obtained were p-coumaric acid (144.63 ± 0.42 µg/g), protocatechuic acid (91.43 ± 0.32 µg/g), and piperonylic acid (74.2 ± 0.31 µg/g). The hydrolysis temperatures (between 150 and 210 °C) could produce sugars (18.09 mg/g). However, the hydrolysis process at temperatures above 180 °C generated Maillard reaction products, which increased the total phenolic compounds and antioxidant activity of the hydrolysates. Finally, the use of semi-continuous high-pressure hydrothermal process can be a sustainable and promising approach for the recovery of value-added compounds from pitaya peel, advocating a circular economy approach in the agri-food industry.

The key role of pretreatment for the one-step and multi-step conversions of European lignocellulosic materials into furan compounds

Nowadays, an increased interest from the chemical industry towards the furanic compounds production, renewable molecules alternatives to fossil molecules, which can be transformed into a wide range of chemicals and biopolymers. These molecules are produced following hexose and pentose dehydration. In this context, lignocellulosic biomass, owing to its richness in carbohydrates, notably cellulose and hemicellulose, can be the starting material for monosaccharide supply to be converted into bio-based products. Nevertheless, processing biomass is essential to overcome the recalcitrance of biomass, cellulose crystallinity, and lignin crosslinked structure. The previous reports describe only the furanic compound production from monosaccharides, without considering the starting raw material from which they would be extracted, and without paying attention to raw material pretreatment for the furan production pathway, nor the mass balance of the whole process. Taking account of these shortcomings, this review focuses, firstly, on the conversion potential of different European abundant lignocellulosic matrices into 5-hydroxymethyl furfural and 2-furfural based on their chemical composition. The second line of discussion is focused on the many technological approaches reported so far for the conversion of feedstocks into furan intermediates for polymer technology but highlighting those adopting the minimum possible steps and with the lowest possible environmental impact. The focus of this review is to providing an updated discussion of the important issues relevant to bringing chemically furan derivatives into a market context within a green European context.

Ionic liquid recovery and recycling via electrodialysis in biomass processing: An economical assessment

Extravagant price and lack of high-efficiency recovery technology limited scale-up utilization of ionic liquids. Ionic liquids recovery with electrodialysis-based techniques has caught wide concern due to membrane-based characteristic. Economical assessment for electrodialysis-based ionic liquid recovery and recycling in biomass processing was performed by determining influence of equipment-related and financial-related factors with sensitivity analysis for each factor. Overall recovery cost of 1-ethyl-3-methylimidazolium acetate, choline acetate, 1-butyl-3-methylimidazolium hydrogen sulphate and 1-ethyl-3-methylimidazolium hydrogen sulfate varied within 0.75-1.96 $/Kg, 0.99-3.00 $/Kg, 1.37-2.74 $/Kg and 1.15-2.89 $/Kg when factors changed within investigated range. Fold of membrane cost, factor of membrane stack cost, factor of auxiliary equipment cost, factor of annual maintenance cost and annual interest rate of loan were positively related with recovery cost. While percentage of annual elapsed time and loan period were negatively correlated with recovery cost. Economical assessment confirmed economic efficiency of electrodialysis for ionic liquids recovery and recycling in biomass processing.