Glycolic acid-based deep eutectic solvents boosting co-production of xylo-oligomers and fermentable sugars from corncob and the related kinetic mechanism.

BACKGROUND: Xylo-oligomers are a kind of high value-added products in biomass fractionation. Although there are several chemical methods to obtain xylo-oligomers from biomass, the reports about the deep eutectic solvents (DESs)-mediated co-production of xylo-oligomers and fermentable sugars and the related kinetic mechanism are limited. RESULTS: In this work, glycolic acid-based DESs were used to obtain xylo-oligomers from corncob. The highest xylo-oligomers yield of 65.9% was achieved at 120 °C for 20 min, of which the functional xylo-oligosaccharides (XOSs, DP 2-5) accounted for up to 31.8%. Meanwhile, the enzymatic digestion of cellulose and xylan in residues reached 81.0% and 95.5%, respectively. Moreover, the addition of metal inorganic salts significantly accelerated the hydrolysis of xylan and even the degradation of xylo-oligomers in DES, thus resulting in higher selectivity of xylan removal. AlCl3 showed the strongest synergistic effect with DES on accelerating the processes, while FeCl2 is best one for xylo-oligomers accumulation, affording the highest xylo-oligomers yield of 66.1% for only 10 min. Furthermore, the kinetic study indicates that the 'potential hydrolysis degree' model could well describe the xylan hydrolysis processes and glycolic acid/lactic acid (3:1) is a promising solvent for xylo-oligomers production, in particular, it worked well with FeCl2 for the excellent accumulation of xylo-oligomers. CONCLUSIONS: Glycolic acid-based deep eutectic solvents can be successfully applied in corncob fractionation with excellent xylo-oligomers and fermentable sugars yields on mild conditions, and the large amount of xylo-oligosaccharides accumulation could be achieved by specific process controlling. The strategies established here can be useful for developing high-valued products from biomass.

Enhancing the Thermostability of Papain by Immobilizing on Deep Eutectic Solvents-Treated Chitosan With Optimal Microporous Structure and Catalytic Microenvironment.

Deep eutectic solvents (DESs) have attracted an increasing attention in the fields of biocatalysis and biopolymer processing. In this study, papain immobilized on choline chloride- lactic acid (ChCl-Lac) DES-treated chitosan exhibited excellent thermostability as compared to the free enzyme. The properties of native or DES-treated chitosan and immobilized enzyme were characterized by FT-IR, SEM, surface area and pore property analysis. Like the common enzyme immobilization, papain immobilized on DES-treated chitosan resulted in a lower catalytic efficiency and a higher thermostability than the free enzyme due to the restricted diffusion. The results also revealed that DES could control the active group content, thus achieving the appropriate microporous structure of immobilized enzyme. Meanwhile, it could also help to construct the optimal microenvironment by hydrogen-bonding interaction between enzyme, chitosan, and residual DES, which are benefit for maintaining an active conformation and subsequently a high thermostability of papain. Moreover, it was found that trace DES (10 mM) significantly promoted the activity of free papain (145%). Deactivation thermodynamics study showed that the DES could enhance the thermostability of papain especially at high temperature (half-life of 7.4 vs. 3.5 h) because of the increased Gibbs free energy of denaturation. Secondary structure analysis by circular dichroism spectroscopy (CD) agreed well with the activity and thermostability data, further confirming the formation of rigid conformation induced by a specific amount of DES. This work provides a new way of enzyme immobilization synergistically intensified by solvents and supporting materials to achieve better microporous structure and catalytic microenvironment.

Preparation, characterization and application of rod-like chitin nanocrystal by using p-toluenesulfonic acid/choline chloride deep eutectic solvent as a hydrolytic media.

Chitin nanocrystal (ChiNC) was fabricated based on p-toluenesulfonic acid -choline chloride deep eutectic solvent treatment. The obtained ChiNC was about 12-44 nm in width and 206-399 nm in length. The crystalline structure and the functional groups of ChiNC were maintained during the preparation process. Moreover, porcine pancreas lipase (PPL) was successfully immobilized onto the ChiNC to form the immobilized PPL (PPL@ChiNC). The resulting PPL@ChiNC has enzyme loading and activity recovery of 35.6 mg/g and 82.5%, respectively. The thermal stability, pH and temperature adaptabilities of PPL@ChiNC was improved, comparing with free PPL. The demonstrated DES treatment process was efficient for ChiNC preparation and the as-prepared ChiNC exhibited great potentials in biocatalysis and biomedical field.

Rice straw pretreatment using deep eutectic solvents with different constituents molar ratios: Biomass fractionation, polysaccharides enzymatic digestion and solvent reuse.

Lignocellulosic biomass pretreatment with deep eutectic solvents (DESs) is a promising and challenging process for production of biofuels and valuable platform chemicals. In this work, rice straw was mainly fractionated into carbohydrate-rich materials (CRMs) and lignin-rich materials (LRMs) by 90% lactic acid/choline chloride (LC)-water solution with different molar ratio of hydrogen bond donor (HBD, lactic acid) and hydrogen bond acceptor (HBA, choline chloride). It was found that high HBD/HBA molar ratio of DESs was favorable for achieving CRMs and LRMs with high purity, and both HBD and HBA were responsible for effective biomass fractionation possibly due to their synergistic effect on highly efficient breakage of the linkage between hemicellulose and lignin and thus lignin extraction. About 30%-35% of lignin in native rice straw was fractionated as LRMs, and exceeding 70% of xylan were removed and fractionated into the liquid stream as forms of xylose, furfural and humins after pretreatment using aqueous LC (3:1, 5:1) solution. Consequently, polysaccharides enzymatic hydrolysis of the CRMs were significantly enhanced. Moreover, all the DESs could be recovered with high yields of around 90%, and 69% of the LC (3:1) was recovered after 5 cycles reuse at 90 °C. Besides, the recycled DES maintained a good pretreatment ability, and glucose yields of 60-70% were achieved in the enzymatic hydrolysis of CRMs obtained in each cycle. The facile process established in present work is promising for large scale production of fermentable sugars and other chemicals.

Insight into the structure-function relationships of deep eutectic solvents during rice straw pretreatment.

Rice straw pretreatment mediated by choline chloride (ChCl) or lactic acid (Lac) sequences deep eutectic solvents (DESs) was investigated in this work. Hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) proved to be both important for DESs pretreatment efficiency. DESs containing lots of hydroxyl or amino groups with a high intermolecular hydrogen-bond (H-bond) strength exhibited weak biomass deconstruction abilities. The presence of strong electron-withdrawing groups in DESs was benefit for xylan removal, thus furnishing higher cellulose digestibility. The relationships between the properties of DESs, xylan removal and cellulose digestibility of pretreated biomass were established. It was found that xylan removal was negatively correlated with the pKa values of HBDs, and the enzymatic cellulose digestibility of the residues was linearly and positively related to xylan removal instead of delignification. These results provide a preliminary reference for rational design of novel DESs for biomass pretreatment.

Significantly enhanced enzymatic hydrolysis of rice straw via a high-performance two-stage deep eutectic solvents synergistic pretreatment.

A two-stage deep eutectic solvents (DESs) treatment was shown to be an effective method for improving the utilization of certain DESs, and the specific order of pretreatment, such as malic acid/proline (MP) or choline chloride/oxalic acid (CO) during the first stage and choline chloride/urea (CU) during the second stage, resulted in better performance for enhancing the sugar yield due to the synergistic effect of the two DESs on biomass fractionation. Moreover, the presence of water during these processes could balance the loss of components by tuning the pretreatment severity, thus ensuring higher sugar yields. By eliminating the washing step after the first stage treatment, enhanced cellulose recovery and glucose yield were achieved for the CO-CU pretreatment in the presence of 5% water, and a simpler process was established with a glucose yield of 90.2% after a 3-h treatment at 100°C.

Effect of anion structures on cholinium ionic liquids pretreatment of rice straw and the subsequent enzymatic hydrolysis.

In this work, 28 cholinium ionic liquids (ILs), most of which are good solvents for dissolving lignin, were used for rice straw pretreatment to improve subsequent enzymatic hydrolysis. The anion exerted a significant effect on the pretreatment effectiveness of the IL as well as the subsequent enzymatic hydrolysis efficiency of rice straw residues. The presence of the basic group(s) in the anion significantly enhanced the IL pretreatment effectiveness, while the carboxyl, hydroxyl and aromatic groups had a negative impact on IL delignification. Except for amino acid-based ILs, the delignification abilities of the ILs are linearly and positively correlated with the pKa values of the conjugate acids of the anions. Of the ILs tested, amino acid-based ILs, especially basic amino acid-based ILs, are the most effective pretreatment solvents. Satisfactory reducing sugar yields (81% for glucose and 26% for xylose) were obtained in the enzymatic hydrolysis of rice straw pretreated by cholinium argininate ([Ch][Arg]) under a pretty mild pretreatment severity (60°C, 6 h). The results presented in this work may be useful for rational design of novel and green ILs for delignification of lignocellulose.

Significantly enhancing enzymatic hydrolysis of rice straw after pretreatment using renewable ionic liquid-water mixtures.

Pretreatment of rice straw by using renewable cholinium lysine ionic liquid ([Ch][Lys] IL)-water mixtures and subsequent enzymatic hydrolysis of the residues were conducted in this work. There is a clear correlation between the delignification capacity of the pretreatment solvent and its basicity. After pretreatment, surface area and pore volume of rice straw increased significantly, which substantially improved polysaccharides accessibility to enzymes and thus enhanced polysaccharides digestion. By carefully controlling the pretreatment severity (IL content, temperature and duration), loss of readily extractable xylan could be minimized. The sugar yields of 81% for glucose and 48% for xylose were achieved in the enzymatic hydrolysis of rice straw after pretreatment with 20% [Ch][Lys]-water mixture at 90 °C for 1 h. This pretreatment process is highly promising for industrial application because of high sugar yields, low energy input, short pretreatment time, and being environmentally benign and highly tolerant to moisture.

Evaluation of toxicity and biodegradability of cholinium amino acids ionic liquids.

Cholinium amino acid ionic liquids ([Ch][AA] ILs), which are wholly composed of renewable biomaterials, have recently been demonstrated to have very promising properties for applications in organic synthesis and biomass pretreatment. In this work, the toxicity of these ILs toward enzymes and bacteria was assessed, and the effect of the anion on these properties is discussed. The inhibitory potentials of this type of ILs to acetylcholinesterase were weaker approximately an order of magnitude than the traditional IL 1-butyl-3-methylimidazolium tetrafluoroborate. Additionally, the [Ch][AA] ILs displayed low toxicity toward the bacteria tested. Furthermore, the biodegradability of the [Ch][AA] ILs was evaluated via the closed bottle and CO(2) headspace tests using wastewater microorganisms. All the ILs were classified as 'readily biodegradable' based on their high levels of mineralization (62-87%). The presence of extra carboxyl or amide groups on the amino acid side chain rendered the ILs significantly more susceptible to microbial breakdown. In addition, for most of the [Ch][AA] ILs, low toxicity correlated with good biodegradability. The low toxicity and high biodegradability of these novel [Ch][AA] make them promising candidates for use as environmentally friendly solvents in large-scale applications.

Renewable bio ionic liquids-water mixtures-mediated selective removal of lignin from rice straw: visualization of changes in composition and cell wall structure.

Pretreatment of rice straw by using renewable cholinium amino acids ionic liquids ([Ch][AA] ILs)-water mixtures and the subsequent enzymatic hydrolysis of the residues were conducted in the present work. Of the eight mixtures composed of ILs and water, most were found to be effective for rice straw pretreatment. After pretreatment with 50% ILs-water mixtures, the enzymatic digestion of the lignocellulosic biomass was enhanced significantly, thus leading to satisfactory sugar yields of >80% for glucose and approximately 50% for xylose. To better understand the ILs pretreatment mechanism, confocal laser scanning microscopy combined with immunolabeling and transmission electron microscopy were used to visualize changes in the contents and distribution of two major components--lignin and xylan. The results coupled with changes in chemical structures (infrared spectra) of the substrates indicated occurrence of extensive delignification, especially in cell corner and compound middle lumen of cell walls, which made polysaccharides more accessible to enzymes. This pretreatment process is promising for large-scale application because of the high sugar yields, easy handling, being environmentally benign and highly tolerant to moisture, and significantly reduced cost and energy consumption.

Novel renewable ionic liquids as highly effective solvents for pretreatment of rice straw biomass by selective removal of lignin.

Cholinium amino acids ionic liquids ([Ch][AA] ILs), a novel type of bio-ILs that can easily be prepared from renewable biomaterials, were investigated for pretreatment of rice straw by selective extraction of lignin from this abundant lignocellulosic biomass material. Of the eight ILs examined, most were demonstrated to be excellent pretreatment solvents. Upon pretreatment using these ILs, the initial saccharification rates of rice straw residues were substantially improved as well as the extent to which polysaccharides could be digested (>90% for cellulose and >60% for xylan). Enzymatic hydrolysis of pretreated rice straw by Trichoderma reesei cellulase/xylanase furnished glucose and xylose with the yields in excess of 80% and 30%, respectively. Detailed spectroscopic characterization showed that the enhancement of polysaccharides degestibility derived mainly from delignification rather than changes in cellulose crystallinity. The yields of fermentable reducing sugars were significantly improved after individual optimization of pretreatment temperature and duration. With [Ch][Lys] as the solvent, the sugar yields of 84.0% for glucose and 42.1% for xylose were achieved after pretreatment at 90°C for 5 h. The IL [Ch][Lys] showed excellent reusability across five successive batches in pretreatment of rice straw. These bio-ILs performed as well as or better than previously investigated non-renewable ILs, and thus present a new and environmentally friendly way to pretreat lignocellulose for production of fermentable sugars and total utilization of the biomass.