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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]

Selective production of glucose from cellulose in a molten salt hydrate (MSH) shows high efficiency but the separation of glucose is still challenging. To address this issue, we proposed a stepwise method, where cellulose was first hydrolyzed into glucan oligomers and separated via anti-solvent precipitation, followed by being hydrolyzed into glucose under mild conditions. For potential industrial applications, using raw biomass as a substrate is more attractive due to its commercial advantages. However, directly hydrolyzing raw corn stover in an MSH resulted in a low glucan oligomer yield and sugar concentration, together with insufficient separation efficiency. This work demonstrated that the generation of acetic acid during hemicellulose hydrolysis is the critical factor affecting glucan oligomer selectivity. To address this issue, a pre-extraction method using auto-hydrolysis was proposed to remove hemicellulose before MSH hydrolysis. After pre-extraction, the obtained glucan oligomer yield could be enhanced from 59.9% to 79.5%, with the separation efficiency increased from 53.7% to 80.6% by methanol precipitation. Besides, increasing the raw corn stover concentration led to an increase in acetic acid and resulted in glucan oligomer degradation. However, after removing hemicellulose, the concentration of glucan oligomers could be significantly enhanced from less than 72.1 to 121.5 mg mL−1. Thus, pre-extraction of hemicellulose followed by MSH hydrolysis provides a potential application for directly producing glucan oligomers from raw biomass with improved yield and separation efficiency. [ABSTRACT FROM AUTHOR]

Selective production of glucose from the hydrolysis of cellulose is the key step for efficient utilization of lignocellulose biomass. Crystalline cellulose can be dissolved and hydrolyzed into glucose with a high selectivity in molten salt hydrates (MSHs). However, the separation of the formed glucose is challenging due to its high solubility in the MSHs. To address this issue, a stepwise method is introduced, where cellulose is hydrolyzed into short-chain glucan oligomers in the MSH of LiBr. We demonstrate that compared to glucose, the formed glucan oligomers with a degree of polymerization of 4–11 can be efficiently separated from the MSH hydrolysate using an anti-solvent precipitation method. The separated oligomers can be readily converted into glucose under mild conditions and used for other applications. Under optimized conditions, 90.3% of glucan oligomer can be produced from crystalline cellulose and separated from the MSH with the addition of methanol, and the precipitated glucan oligomer can be hydrolyzed into glucose with a yield of 99.7% using dilute sulfuric acid. We show that the precipitation efficiency is influenced by the glucan oligomer chain length, glycoside bond type and concentration. Moreover, the separation of a glucan oligomer from a cotton straw hydrolysate was also investigated in the MSH. 79.2% yield of glucan oligomer was obtained from the hydrolysis of cotton straw at 130 °C for 2 h. With the addition of methanol, the glucan oligomer was precipitated with a selectivity of 60.8%. [ABSTRACT FROM AUTHOR]

Molten salt hydrates (MSH) are unique in overcoming the characteristic recalcitrance of crystalline cellulose. We show that cellulose can be efficiently hydrolyzed into short-chain oligomers and glucose under mild conditions in concentrated LiBr solution without using any additional acid catalyst. Under optimized reaction conditions, short-chain oligomers with a yield of 90.4% can be obtained. Selective hydrolysis in the MSH is effective even at a high initial cellulose concentration (higher than 10 wt%). The obtained oligomers are solvated and soluble in the MSH. We show that amorphous carbon of a large surface area is effective for adsorption of the short-chain oligomers, cellobiose, and glucose from the MSH. The adsorption capacity and adsorption affinity of the carbon increases by ∼4 and ∼15 times in MSH with increasing the oligomer chain length compared to glucose and is negatively influenced by the ion concentration of the MSH. A stepwise hydrolysis process for converting crystalline cellulose into glucose is developed, whereby cellulose is firstly hydrolyzed into soluble short-chain oligomers in the MSH. The oligomers are then separated from the MSH using amorphous carbon and finally hydrolyzed into glucose under mild reaction conditions. [ABSTRACT FROM AUTHOR]

The conventional fractionation of lignocellulose is generally carried out at high temperatures, high acid or alkali, or using organic solvents due to the rigid matrix of lignocellulose and crystallinity of cellulose, leading to the condensation of lignin and environmental pollution, and hindering the further value-added utilization of lignin. In this work, a rapid fractionation method was proposed, in which hemicellulose and cellulose (holocellulose) were first dissolved and partially hydrolyzed in acidified LiCl molten salt hydrates, followed by a filtration process to isolate the lignin solid with a purity of 88.3%. It was found that the dissolution of holocellulose required the addition of a small amount of acid to break the cellulose chain into short chains. Due to the small amount of acid added and the rapid dissolution, the obtained lignin has as high as 89.5% of β-O-4 bond content close to the theoretical value, suggesting that a promising native lignin structure was preserved. The isolated lignin can be selectively depolymerized into monophenol over Ru/C catalyst with a high yield of 24.2%. In addition, the dissolved holocellulose without containing lignin was selectively hydrolyzed into sugars in acidified LiCl molten salt hydrates, giving 63.1% of xylose, 43.9% of glucose, 29.5% of xylan oligomers, and 48.2% of glucan oligomers, respectively. [Display omitted] • LiCl molten salt hydrates required little acid to dissolve holocellulose. • LiCl-AMSH could quickly dissolve holocellulose and separate noncondensed lignin. • 88.3% purity of noncondensed lignin was fractionated. • 24.2% of monophenol (88.0% to theoretical) was obtained from noncondensed lignin. • 63.1% of xylose, 43.9% of glucose, 29.5% of xylan oligomers, and 48.2% of glucan oligomers were obtained. [ABSTRACT FROM AUTHOR]