Novel betaine-amino acid based natural deep eutectic solvents for enhancing the enzymatic hydrolysis of corncob

Selective adsorption of Cr(VI) by nitrogen-doped hydrothermal carbon in binary system

Selective adsorption of heavy metal ions from industrial effluent is important for healthy ecosystem development. However, the selective adsorption of heavy metal pollutants by biochar using lignin as raw material is still a challenge. In this paper, the lignin carbon material (N-BLC) was synthesized by a one-step hydrothermal carbonization method using paper black liquor (BL) as raw material and triethylene diamine (TEDA) as nitrogen source. N-BLC (2:1) showed excellent selectivity for Cr(VI) in the binary system, and the adsorption amounts of Cr(VI) in the binary system were all greater than 150 mg/g, but the adsorption amounts of Ca(II), Mg(II), and Zn(II) were only 19.3, 25.5, and 6.3 mg/g, respectively. The separation factor (SF) for Cr(VI) adsorption was as high as 120.0. Meanwhile, FTIR, elemental analysis and XPS proved that the surface of N-BLC (2:1) contained many N- and O- containing groups which were favorable for the removal of Cr(VI). The adsorption of N-BLC (2:1) followed the Langmuir model and its maximum theoretical adsorption amount was 618.4 mg/g. After 5th recycling, the adsorption amount of Cr(VI) by N-BLC (2:1) decreased about 15%, showing a good regeneration ability. Therefore, N-BLC (2:1) is a highly efficient, selective and reusable Cr(VI) adsorbent with wide application prospects.

Natural arginine-based deep eutectic solvents for rapid microwave-assisted pretreatment on crystalline bamboo cellulose with boosting enzymatic conversion performance

Development of amino acid-based natural deep eutectic solvents (DESs) pretreatment technology on lignocellulosic biomass is a promising approach to biorefinery. In this study, for arginine-based DESs with different molar ratios, the viscosity and Kamlet-Taft solvation parameters were quantified to evaluate the pretreatment performance on bamboo biomass. Further, microwave assisted DES pretreatment was eminent, evidenced by 84.8% lignin removal and increased saccharification yield (from 6.3% to 81.9%) in moso bamboo at 120 °C and ratio of 1:7 (arginine: lactic acid). Results showed degradation of lignin molecules together with release of phenolic hydroxyl units after DESs pretreatment, which is conducive to subsequent utilization. Meanwhile, DES-pretreated cellulose exhibited unique structural characteristics including destroyed crystalline region of cellulose (Crystallinity Index from 67.2% to 53.0%), decreased crystallite size (from 3.41 nm to 3.14 nm) and roughened fiber surface. Thus, arginine-based DES pretreatment has excellent potential in bamboo lignocellulose pretreatment.

Mechanistic insights into the lignin dissolution behavior in amino acid based deep eutectic solvents

Deep eutectic solvents (DESs) composed by amino acids (L-arginine, L-proline, L-alanine) as the hydrogen bond acceptors (HBAs) and carboxylic acids (formic acid, acetic acid, lactic acid, levulinic acid) as hydrogen bond donors (HBDs) were prepared and used for the dissolution of dealkaline lignin (DAL). The mechanism of lignin dissolution in DESs was explored at molecular level by combining the analysis of Kamlet-Taft (K-T) solvatochromic parameters, FTIR spectrum and density functional theory (DFT) calculations of DESs. Firstly, it was found that the formation of new hydrogen bonds between lignin and DESs mainly drove the dissolution of lignin, which were accompanied by the erosion of hydrogen bond networks in both lignin and DESs. The nature of hydrogen bond network within DESs was fundamentally determined by the type and number of functional groups in both HBA and HBD, which affected its ability to form hydrogen bond with lignin. One hydroxyl group and carboxyl group in HBDs provided active protons, which facilitated proton-catalyzed cleavage of β-O-4, thus enhancing the dissolution of DESs. The superfluous functional group resulted in more extensive and stronger hydrogen bond network in the DESs, thus decreasing the lignin dissolving ability. Moreover, it was found that lignin solubility had a closed positive correlation with the subtraction value of α and β (net hydrogen donating ability) of DESs. Among all the investigated DESs, L-alanine/formic acid (1:3) with the strong hydrogen-bond donating ability (acidity), weak hydrogen-bond accepting ability (basicity) and small steric-hindrance effect showed the best lignin dissolving ability (23.99 wt%, 60 °C). On top of that, the value of α and β of L-proline/carboxylic acids DESs showed some positive correlation with the global electrostatic potential (ESP) maxima and minima of the corresponding DESs respectively, indicating the analysis of ESP quantitative distributions of DESs could be an effective tool for DESs screening and design for lignin dissolution as well as other applications.

Proficient bioconversion of rice straw biomass to bioethanol using a novel combinatorial pretreatment approach based on deep eutectic solvent, microwave irradiation and laccase

Current study is the first report of the combined application of chemical (deep eutectic solvent), physical (microwave irradiation) and biological (laccase) pretreatment strategies for enhancing the enzymatic digestibility of rice straw biomass. Pretreated rice straw biomass was saccharified by cellulase/xylanase from Aspergillus japonicus DSB2 to get a sugar yield of 252.36 mg/g biomass. Design of Experiment based optimization of pretreatment and saccharification variables increased the total sugar yield by 1.67 times (421.5 mg/g biomass, saccharification efficiency 72.6%). Sugary hydrolysate was ethanol-fermented by Saccharomyces cerevisiae and Pichia stipitis to achieve an ethanol yield of 214 mg/g biomass (bioconversion efficiency 72.5%). Structural/chemical aberrations induced in the biomass due to pretreatment were elucidated by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, and ¹H nuclear magnetic resonance techniques to unravel the pretreatment mechanisms. Combined application of various physico-chemical/biological pretreatment may be a promising approach for proficient bioconversion of rice straw biomass.

Deep eutectic solvents for improved biomass pretreatment: Current status and future prospective towards sustainable processes

Pretreatment processes - recognized as critical steps for efficient biomass refining - have received much attention over the last two decades. In this context, deep eutectic solvents (DES) have emerged as a novel alternative to conventional solvents representing a step forward in achieving more sustainable processes with both environmental and economic benefits. This paper presents an updated review of the state-of-the-art of DES-based applications in biorefinery schemes. Besides describing the fundamentals of DES composition, synthesis, and recycling, this study presents a comprehensive review of existing techno-economic and life cycle assessment studies. Challenges, barriers, and perspectives for the scale-up of DES-based processes are also discussed.

Can deep eutectic solvents be the best alternatives to ionic liquids and organic solvents: A perspective in enzyme catalytic reactions

As a new generation of green solvents, deep eutectic solvents (DESs) have been considered as a promising alternative to classical organic solvents and ionic liquids (ILs). DESs are normally formed by two or more components via various h-bonds interactions. Up to date, four types of DESs are found, namely, type I DESs (formed by MClx, namely FeCl₂, AlCl₃, ZnCl₂, CuCl₂ and AgCl et al., and quaternary ammonium salts); type II DESs (formed by metal chloride hydrates and quaternary ammonium salts); type III DESs (formed by choline chlorides and different kinds of HBDs) and type IV DESs (formed by salts of transition metals and urea). DESs share many advantages, such as low vapor pressure, good substrate solubility and thermal stability, with ILs, and offering a high potential to be the medium of biocatalysis reactions. In this case, this paper reviews the applications of DESs in enzymatic reactions. Lipases are the most widely used enzyme in DESs systems as their versatile applications in various reactions and robustness. Interestingly, DESs can improve the efficiency of these reactions via enhancing the substrates solubility and the activity and stability of enzymes. Therefore, the directed engineering of DESs for special reactions such as degradation of polymers in high temperature or strong acid-base conditions will be one of the future perspectives of the investigation DESs.

Efficient fractionation of woody biomass hemicelluloses using cholinium amino acids-based deep eutectic solvents and their aqueous mixtures

Novel and green cholinium amino acids-based deep eutectic solvents (DESs) and their aqueous mixtures were synthesized and employed in deconstructing poplar for hemicellulose fractionation. The effects of water content in DESs on hemicellulose dissociation and structural features were comprehensively investigated, along with the reusability of DESs for treatment. The integration of water into DESs could facilitate hemicellulose fractionation, and the cholinium lysine: urea with 5 wt% water (CL: U-5) demonstrated the best performance with a hemicellulose yield of 59.2%. Further structure analysis revealed that hemicelluloses with various branching degrees and molecular weights were obtained with varying water content of DESs. Furthermore, the CL: U-5 had recyclability and reusability with a 40.5% hemicellulose yield obtained after reused three times. The novel and eco-friendly cholinium amino acids-based DESs treatment provides an effective and sustainable strategy for hemicellulose fractionation from woody biomass.

Assessing the availability of two bamboo species for fermentable sugars by alkaline hydrogen peroxide pretreatment

This study comprehensively investigated two bamboo species (i.e. Neosinocalamus affinis and Phyllostachys edulis) in terms of their cell wall ultrastructure, chemical compositions, enzymatic saccharification, and lignin structure before and after alkaline hydrogen peroxide pretreatment (AHP). During AHP, Neosinocalamus affinis (NAB) had higher delignification than Phyllostachys edulis (PEB), and thus showed better enzymatic digestibility (93.05% vs 53.57% for glucan). The fundamental chemical behavior of the bamboo lignins was analyzed by fluorescence microscope (FM), confocal Raman microscope (CRM), molecular weight analysis, and 2D HSQC-NMR. Results indicated that the PEB has thicker cell wall and more concentrated lignin in its compound middle lamella and cell corner middle lamella than NAB. Moreover, PEB lignin contains more G units (S/G of 0.95), in evident contrast to that of NAB lignin (S/G of 1.30), which favor the formation of C-C linkages, thus impeding its degradation during the AHP.

Novel, recyclable Brønsted acidic deep eutectic solvent for mild fractionation of hemicelluloses

Acidic deep eutectic solvents (DESs) are promising media for lignin valorization and cellulose conversion due to their good ability in efficient deconstruction of plant cell wall. However, hemicellulose extraction from lignocellulose using acidic DESs remains a challenge. Herein, novel and green Brønsted acidic DESs (BDESs) were synthesized from natural organic acids and common polyols and successively adopted to deconstruct corncob for mild fractionation of hemicelluloses. Oxalic acid (OA)-based BDESs were preferred for corncob processing due to the high solubility of xylan. The results revealed that the suitable acidity of DESs and mild temperature effectively avoided the over-degradation of hemicelluloses. The chemical composition and structural features of the recovered hemicelluloses were investigated systematically. Moreover, after ethylene glycol (EG)-OA BDES was recycled and reused three times, the extraction still resulted in a satisfactory hemicellulose yield. The novel and eco-friendly processing offers a practical and sustainable route for hemicellulose extraction in acidic condition.

Industrial lignins: the potential for efficient removal of Cr(VI) from wastewater

Cr(VI), a serious threat to human health, widely exists in the effluents of various industrial processes. In this paper, the potential of industrial lignin for efficient removal of Cr(VI) from wastewater was systematically investigated, including pulping black liquor lignin (BLN), enzymolysis lignin (ELN), and SPORL pretreatment spent liquor (FS). The structure characterizations of three lignins were investigated by thermogravimetry (TG), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET) surface area measurement, scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS). Among these three lignins, BLN showed the highest adsorption amount of Cr(VI) and good selectivity in wastewater simulation. According to the Langmuir model, the calculated maximum adsorption amount of Cr(VI) on ELN, BLN, and FS was 801.57, 864.30, and 642.26 mg g^-1, respectively. The adsorption of Cr(VI) by industrial lignins was a chemisorption process, during which Cr(VI) was reduced to low-toxic Cr(III). This paper provided a promising application for the effective utilization of industrial lignins.

A review on physico-chemical delignification as a pretreatment of lignocellulosic biomass for enhanced bioconversion

Effective pretreatment of lignocellulosic biomass (LCB) is one of the most important steps in biorefinery, ensuring the quality and commercial viability of the overall bioprocess. Lignin recalcitrance in LCB is a major bottleneck in biological conversion as the polymerization of lignin with hemicellulose hinders enzyme accessibility and further bioconversion to fuels and chemicals. Therefore, there is a need to delignify LCB to ease further bioprocessing. The efficiency of delignification, quality and quantity of the desired products, and generation of inhibitors depend upon the type of pretreatment employed. This review summarizes different single and integrated physicochemical pretreatments for delignification. Additionally, conditions required for effective delignification and the advantages and drawbacks of each method were evaluated. Advances in overcoming the recalcitrance of residual lignin to saccharification and the methods to recover lignin after delignification are also discussed. Efficient lignin recovery and valorization strategies provide an avenue for the sustainable lignocellulose biorefinery.

Alkaline deep eutectic solvents as novel and effective pretreatment media for hemicellulose dissociation and enzymatic hydrolysis enhancement

In recent years, deep eutectic solvents (DESs) are used for enhancing the enzymatic digestibility and lignin fractionation in pretreatment, while hemicellulosic fraction receives scant attention. Herein, we report a novel approach of applying alkaline deep eutectic solvents (ADESs) for dissociating hemicelluloses from woody biomass. Among these ADESs, choline chloride-monoethanolamine (C-M) was the most efficacious medium for deconstructing the recalcitrant structure of poplar and 63.3% of hemicelluloses was obtained at 80 °C. Structure analysis showed that the ADESs-extracted hemicelluloses retained partial of O-acetyl groups. Different ADESs could be used to obtain hemicelluloses with various degrees of branching. Furthermore, the enzymatic digestibility of cellulose was significantly increased by 6.6 times compared to that of the untreated poplar under the optimum conditions (C-M, 140 °C). This work provides a view on the dissociation behavior of hemicelluloses during ADESs pretreatment, which would be beneficial for devising DESs toward effective fractionation and comprehensive utilization of biomass.

Preparation, characterization, and efficient chromium (VI) adsorption of phosphoric acid activated carbon from furfural residue: an industrial waste

Furfural residue (FR) is an inevitable by-product of industrial furfural production. If FR is not managed properly, it will result in environmental problems. In this study, FR was used as a novel precursor for activated carbon (AC) production by H₃PO₄ activation under different conditions. Under optimum conditions, the prepared FRAC had high BET surface area (1,316.7 m²/g) and micro-mesoporous structures. The prepared FRAC was then used for the adsorption of Cr(VI). The effect of solution pH, contact time, initial Cr(VI) concentration, and temperature was systematically studied. Characterization of the adsorption process indicated that the experimental data were well-fitted by the Langmuir isotherm model and pseudo-second-order kinetics model. The maximum adsorption capacity of 454.6 mg/g was achieved at pH 2.0, which was highly comparable to the other ACs reported in the literatures. The preparation of FRAC using H₃PO₄ activation can make use of FR's characteristic acidity, which could make it preferable in practical industrial production.

A review of sustainable lignocellulose biorefining applying (natural) deep eutectic solvents (DESs) for separations, catalysis and enzymatic biotransformation processes

Conventional biorefinery processes are complex, engineered and energy-intensive, where biomass fractionation, a key functional step for the production of biomass-derived chemical substances, demands industrial organic solvents and harsh, environmentally harmful reaction conditions. There is a timely, clear and unmet economic need for a systematic, robust and affordable conversion method technology to become greener, sustainable and cost-effective. In this perspective, deep eutectic solvents (DESs) have been envisaged as the most advanced novel polar liquids that are entirely made of natural, molecular compounds that are capable of an association via hydrogen bonding interactions. DES has quickly emerged in various application functions thanks to a formulations’ simple preparation. These molecules themselves are biobased, renewable, biodegradable and eco-friendly. The present experimental review is providing the state of the art topical overview of trends regarding the employment of DESs in investigated biorefinery-related techniques. This review covers DESs for lignocellulosic component isolation, applications as (co)catalysts and their functionality range in biocatalysis. Furthermore, a special section of the DESs recyclability is included. For DESs to unlock numerous new (reactive) possibilities in future biorefineries, the critical estimation of its complexity in the reaction, separation, or fractionation medium should be addressed more in future studies.

Facilely reducing recalcitrance of lignocellulosic biomass by a newly developed ethylamine-based deep eutectic solvent for biobutanol fermentation

BACKGROUND

Biobutanol is promising and renewable alternative to traditional fossil fuels and could be produced by Clostridium species from lignocellulosic biomass. However, biomass is recalcitrant to be hydrolyzed into fermentable sugars attributed to the densely packed structure by layers of lignin. Development of pretreatment reagents and processes for increasing surface area, removing hemicellulose and lignin, and enhancing the relative content of cellulose is currently an area of great interest. Deep eutectic solvents (DESs), a new class of green solvents, are effective in the pretreatment of lignocellulosic biomass. However, it remains challenging to achieve high titers of total sugars and usually requires combinatorial pretreatment with other reagents. In this study, we aim to develop novel DESs with high application potential in biomass pretreatment and high biocompatibility for biobutanol fermentation.

RESULTS

Several DESs with betaine chloride and ethylamine chloride (EaCl) as hydrogen bond acceptors were synthesized. Among them, EaCl:LAC with lactic acid as hydrogen bond donor displayed the best performance in the pretreatment of corncob. Only by single pretreatment with EaCl:LAC, total sugars as high as 53.5 g L^-1 could be reached. Consecutive batches for pretreatment of corncob were performed using gradiently decreased cellulase by 5 FPU g^-1. At the end of the sixth batch, the concentration and specific yield of total sugars were 58.8 g L^-1 and 706 g kg^-1 pretreated corncob, saving a total of 50% cellulase. Utilizing hydrolysate as carbon source, butanol titer of 10.4 g L^-1 was achieved with butanol yield of 137 g kg^-1 pretreated corncob by Clostridium saccharobutylicum DSM13864.

CONCLUSIONS

Ethylamine and lactic acid-based deep eutectic solvent is promising in pretreatment of corncob with high total sugar concentrations and compatible for biobutanol fermentation. This study provides an efficient pretreatment reagent for facilely reducing recalcitrance of lignocellulosic materials and a promising process for biobutanol fermentation from renewable biomass.

Use of ammonium salts or binary mixtures derived from amino acids, glycine betaine, choline and indole-3-butyric acid as plant regulators

Natural origin ammonium salts or binary mixtures including indole-3-butyric acid as novel plant growth regulators.