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Traditional pretreatment of lignocellulose is usually conducted under higher acidic and high temperature conditions, which leads to both the degradation of sugar and the condensation of lignin, hindering the subsequent conversion. An effective approach to fractionate lignocellulose into 93.9% of noncondensed lignin, 99.4% of cellulose, 17.8% of xylose, and 66.7% of xylooligosaccharides under mild conditions was developed using the formic acid solution at 80 °C for 100 min. The β-O-4 bond content of lignin fractionated with formic acid (54.6 per 100 C9 units) was higher than dioxasolv lignin (48.4 per 100 C9 units), indicating that formic acid pretreatment well protected the ether bonds in lignin. Therefore, the hydrogenolysis of fractionated lignin contributed to 28.0% of aromatic monomer yield, which was comparable to dioxasolv lignin. As cellulose possesses a large amount of porosity because lignin was separated from lignocellulose, the hydrolysis of fractionated cellulose by molten salt hydrates gave a 96.4% of glucose yield.

The conventional fractionation of lignocellulose into cellulose, hemicellulose, and lignin was generally carried out at high temperatures and under highly acidic conditions owing to the rigid matrix of lignocellulose and crystallinity of cellulose, causing the degradation of carbohydrates and condensation of lignin, thus hindering their value-added utilization. In this study, a stepwise fractionation method was proposed in which microwave-assisted fractionation of hemicellulose under mild conditions contributed to 81.9% of xylose yield; then, the cellulose part solid was rapidly dissolved in LiBr molten salt hydrate (LiBr-MSH) solution, followed by a filtration process to isolate lignin solid. It was found that the removal of hemicellulose provided more pores for the next dissolution of cellulose, and LiBr-MSH caused the cleavage of the hydrogen bonds of cellulose. Consequently, the cellulose part solid was rapidly dissolved in LiBr-MSH. The results showed that under the optimal fractionation conditions, 78.0% yield of glucan oligomers and 89.7% purity of noncondensed lignin were obtained from cellulose fractionation and filtration, respectively. A high β-O-4 bond content close to 87.3% of the theoretical maximum yield was detected in the obtained lignin, indicating that most β-O-4 bond structure of lignin was well preserved. Additionally, the isolated lignin can be selectively depolymerized into aromatic monomers with a high yield of 20.4%, which is 74.2% of the theoretical maximum yield. [ABSTRACT FROM AUTHOR]

Molten salt hydrate (MSH) is unique in cellulose dissolution and in one-pot conversion of crystalline cellulose into 5-hydroxymethylfurfural (5-HMF). However, low yield of 5-HMF was obtained using raw biomass substrate in MSHs. Herein, we investigated conversion of raw biomass towards HMF using MSHs. We showed that the release of acetic acid from hemicellulose degradation in MSH is a key to influence the yield of 5-HMF. Thus, an autohydrolysis pretreatment method was proposed to remove hemicellulose before conversion of cellulose into 5-HMF. After being pretreated at 180 °C for 40 min, hemicellulose was removed from biomass with 91.8% yield of cellulose remained in solid part. By eliminating the negative effect of acetic acid, 5-HMF yield was significantly improved from 40.9% to 64.6%, together with 9.3% yield of glucose for cellulose conversion in MSH/acetone at 180 °C for 20 min. [ABSTRACT FROM AUTHOR]

[Display omitted] • Cellulose can be selectively converted into 5-HMF in monophasic MSH solvent. • 44.5 % yield of 5-HMF together with 75.0 % of carbon balance were obtained. • The total B/L acid sites of AlSiO-20 is the key issue in affecting 5-HMF yield. • AlSiO-20 can be recycled for more than 5 times with stable performance. • 5-HMF can be separated from MSH via HCP adsorption. Production of 5-HMF from cellulose is a promising approach for advanced bio-fuels preparation, but the degradation of active 5-hydroxymethylfurfural (5-HMF) is a critical challenge affecting 5-HMF selectivity. To address this issue, biphasic reaction system consisting of water and organic solvent is generally used to in-situ extract 5-HMF into organic phase to inhibit product degradation. Despite the high 5-HMF selectivity achieved in biphasic system, low product concentration and difficulty in separation restrict its industrial applications. Molten salt hydrate (MSH) is unique in cellulose conversion and 5-HMF preservation. In this work, MSH of LiBr was used as monophasic solvent for cellulose conversion and an acidity tunable solid acid AlSiO-x was used as the catalyst. It was found that a relatively high 5-HMF yield of 44.5% together with carbon balance of 75.0% were obtained at 170 °C for 20 min, which is higher than the results ever reported. Importantly, this work proved that the ratio of total Brønsted to Lewis acid sites on solid acid is the key issue affected 5-HMF yield. After reaction, solid acid can be recycled for more than 5 times without catalytic performance decline, and the dissolved 5-HMF can be sufficiently separated from MSH via hyper cross-linked polymer (HCP) adsorption. [ABSTRACT FROM AUTHOR]

Selective hydrolysis of crystalline cellulose to cello-oligosaccharides in molten salt hydrates (MSHs) is a promising method for valorization of lignocellulose. However, the by-products derived from cellulose hydrolysis can facilitate cello-oligosaccharide degradation and decrease its selectivity. To address this issue, lignin-based hyper cross-linked polymers (LHCPs) were prepared to in situ adsorb by-products during hydrolysis and thus prevent cello-oligosaccharides from degradation. The synthesized LHCP‐33%, with a high lignin content of 33%, showed a remarkable surface area of 708.7 m2 g−1 and good hydrophilicity, exhibiting a promising adsorption capacity of 340 mg g−1 towards 5-hydroxymethylfurfural (5-HMF), which is supposed to be the major cellulose-derived by-product. We showed that 5-HMF adsorption was driven by the formation of CH-π bonds between 5-HMF and aromatic rings on LHCP. When LHCP‐33% was employed in cellulose hydrolysis, the yield of cello-oligosaccharide can be efficiently enhanced from 39.5% to 78.2%. This work offered a new application of lignin in enhanced cello-oligosaccharide production from cellulose. [Display omitted] • 78.2 % yield of cello-oligosaccharide can be selectively produced from crystalline cellulose saccharification via in situ adsorption of by-products on lignin-based hyper cross-linked polymer (LHCP). • The synthesized LHCP shows a high surface area and good hydrophilcity which is beneficial for by-products adsorption. • The adsorption of hydrolysis-derived 5-hydroxymethylfurfural (5-HMF) on LHCP is driven by the formation of CH-π bonds rather than hydrogen bonds. • This approach provides a new alternative in utilization of lignin for high-valued cello-oligosaccharide production from cellulose. [ABSTRACT FROM AUTHOR]