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Pretreatment with solid base catalysts addressed the challenge of recalcitrance of the lignocellulosic cell wall and provided insight into an economical and environmentally friendly biorefinery technology. Several solid base catalysts were prepared, to establish a reliable assessment of the factors governing their catalytic behaviors. The activity of solid base catalysts, allowing the more accessible lignocellulosic molecules to be depolymerized by enzymes, were evaluated by investigating the properties of the catalysts, the pretreated feedstock, and the residues, after 72 h of enzymatic hydrolysis. A statistical design (CCD) approach was applied to define the factors for the maximum saccharification and an efficient activity recovery. In the case of hybrid pennisetum, an optimized yield of 92% and a retention of 98% activity after 4 cycles were obtained. One of the mechanisms of the solid base catalysts during recalcitrance removal reactions, considering their structures and chemical compositions, was to enhance the accessibility of the cellulosic molecules.

Catalytic conversion of lignocellulosic biomass into furfural (FF) and 5-hydroxymethylfurfural (5-HMF) requires a high reaction temperature (150–230 °C) in aqueous/organic systems. In this study, a novel choline chloride–formic acid-SnCl4·5H2O (ChCl–Fa–SnCl4) catalytic system was designed to transform herbal residues (HRs) carbohydrates into furanic derivatives at temperatures below 140 °C. The total yield of FF and xylose from HR of Heteropogon contortus (L.) Beauv (HRH) was 67.5% at 140 °C for 10 min, while the total yield of 5-HMF and glucose from HR of Anemarrhena asphodeloides Bunge was 93.8% at 120 °C. Characteristics of substrates such as molecular weight had a great influence on formation of furanic derivatives and sugar release. Surprisingly, the dissolution and recrystallisation characteristics of ChCl make this catalytic system sustainable and economically attractive. Recovery of ChCl and SnCl4 was 73.3% following simple natural recrystallisation, and recovered material retained a high chemical reactivity for FF and 5-HMF production. Moreover, 100% lignin dissolution was achieved in this system at 140 °C, and ~ 77% could be precipitated and recovered by adding water.

A novel lignin-derived deep eutectic solvent (DES) was synthesized from choline chloride (ChCl) and p-coumaric acid (PCA). The results showed that after pretreatment with lignin-derived DES, the dense cellulose–hemicellulose–lignin structure of herb residues of Cortex albiziae (HRCA) was effectively broken down and this facilitated subsequent enzymatic hydrolysis. The optimum pretreatment conditions were ChCl/PCA molar ratio of 1:1, 160 °C-5 h, and a solid–liquid ratio of 1:10. Compared with untreated HRCA, 72-h enzymatic digestibility increased from 48.08 to 84.62%. When a ChCl–PCA–water pretreatment was used, the 72-h digestibility level of the residue reached 99.36%. Moreover, the recycling and reusability of this novel DES were evaluated. The results showed that synthesis and utilization of green, environment-friendly and recyclable lignin-derived DES can offer a new way of utilizing lignin which has considerable potential in the refining of biomass. A novel lignin-derived deep eutectic solvent was synthesized, this DES pretreatment can improve the enzymatic digestibility of HCRA cellulose efficiently.

Explicitly understanding biomass recalcitrance through the characterization of biomass physicochemical properties may help to develop efficient pretreatment and enzymatic hydrolysis strategies. The lignin of corncob and eucalyptus contain the same main linkage bonds: β–O–4 aryl ether bonds, β–β and β–5 structures, but the lignin of eucalyptus was of the syringyl (S)-guaiacy (G) type, while that of corncob was SG-p-hydroxyphenyl (H) type, corresponding to lignin S/G ratios of 1.6 and 1.1 respectively. Under the optimum microwave–hydrothermal pretreatment condition of 180 °C for 30 min at a 12.5% substrate concentration, the maximum total xylose yield of corncob (64.7%) was lower than that of eucalyptus (79.2%). In contrast, corncob resulted in a greater increase in enzymatic digestibility, from 59.6 to 82.4%, after pretreatments, compared with 16.7 to 74.9% for eucalyptus. There was a positive correlation between the xylose yield and lignin S/G ratio, but the lignin content was negatively correlated with enzymatic digestibility. Furthermore, based on the non-destructive characterization of three-dimensional X-ray microscopy, not only was the increase in the number and size of surface pores beneficial to the accessibility of cellulose to cellulosic enzymes, but the swelling of cell wall could reduce the recalcitrance of sugar release.

Three pairs of solid substrates from dilute acid pretreatment of two poplar wood samples were enzymatically hydrolyzed by cellulase preparations supplemented with xylanase. Supplementation of xylanase improved cellulose saccharification perhaps due to improved cellulose accessibility by xylan hydrolysis. Total xylan removal directly affected enzymatic cellulose saccharification. Furthermore, xylan removal by pretreatment and xylanase are indifferent to enzymatic cellulose saccharification. However, more enzymatic xylose and glucose yields were obtained for a substrate with lower xylan content after a severer pretreatment at the same xylanase dosage. The effectiveness of xylanase at increased dosages depended on the substrates structure or accessibility. High xylanase dosages were more effective on well pretreated substrates than on under-pretreated substrates with high xylan content. The application sequence of xylanase and cellulase affected cellulose saccharification. This effect varied with substrate accessibility, perhaps due to competition between xylanase and cellulase binding to the substrate.