Highly Efficient Dissolution of Lignin by Eutectic Molecular Liquids

Exploring the mechanism of solubilization and release of isoliquiritigenin in deep eutectic solvents

Isoliquiritigenin (ISL) is a natural medicinal product with extensive pharmacological activities. However, its low solubility limits its application. Therefore, this study aimed to explore the solubilization and release mechanism of the ISL using deep eutectic solvents (DESs). The choline chloride (ChCl) and oxalic acid (OA)/malic acid (MA)/gallic acid (GA) were used to synthesize ChCl-OA/MA/GA DESs, and the solubility of ISL in these DESs was studied to explore the solubilization mechanism of ISL. The thermodynamic properties of DESs were characterized using differential scanning calorimetry (DSC). The molecular interactions in DESs were studied using spectroscopy and molecular dynamics (MD) simulations. The relative density of DESs was measured using a pycnometric method, its accuracy was validated by comparing it with the MD simulation. The release of ISL from ChCl-OA/MA/GA eutectogels was studied using Carbomer 940 as the thickener, and the release mechanism of ISL in the eutectogels was explored by the drug release kinetic model. The solubility study found that the solubility of ISL in ChCl-OA/MA/GA DESs is 30073, 5055, and 68,103 times higher than that in an aqueous solution. In addition, further studies using MD simulations revealed that enhancing the interactions between ISL and solvent molecules can improve the solubility of ISL in DESs. In vitro release studies showed that the release of ISL in ChCl-OA/MA/GA eutectogels followed a first-order release model, with correlation coefficients of 0.9812, 0.9916, and 0.9961, respectively. In conclusion, the study of the solubilization and release mechanism of ISL in DESs provides new ideas and methods for the study of poorly soluble drugs, which is expected to improve the efficacy and clinical application value of drugs.

Everything You Wanted to Know about Deep Eutectic Solvents but Were Afraid to Be Told

Are deep eutectic solvents (DESs) a promising alternative to conventional solvents? Perhaps, but their development is hindered by a plethora of misconceptions. These are carefully analyzed here, beginning with the very meaning of DESs, which has strayed far beyond its original scope of eutectic mixtures of Lewis or Brønsted acids and bases. Instead, a definition that is grounded on thermodynamic principles and distinguishes between eutectic and deep eutectic is encouraged, and the types of precursors that can be used to prepare DESs are reviewed. Landmark works surrounding the sustainability, stability, toxicity, and biodegradability of these solvents are also discussed, revealing piling evidence that numerous DESs reported thus far, particularly those that are choline based, lack sufficient sustainability-related traits to be considered green solvents. Finally, emerging DES applications are reviewed, emphasizing their most remarkable feature: the ability to liquefy a solid compound with a target property, allowing its use as a liquid solvent.

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.

Process intensification strategies for green solvent mediated biomass pretreatment

Demonstrated to be highly effective for lignocellulosic biomass pretreatment, deep eutectic solvent (DES) has attracted increasing attention owing to its advantages of simple synthesis, relatively low chemical cost, and better biocompatibility as compared to certain ionic liquids. Here we provide a critical review of the status of the design/selection of DES for the pretreatment of biomass feedstocks with an emphasis on the process intensification strategies: 1) integration of microwave, ultrasound, and high solid extrusion for pretreating biomass, 2) one-pot DES pretreatment, enzymatic hydrolysis, and fermentation, 3) strategies for DES recycling and product recovery; and 4) recent progress on molecular simulations toward understanding the interactions between DES and biomass compounds such as lignin and cellulose. Lastly, we provide perspectives toward cost-effective, continuous, high-solid, environmental-benign, and industrial-relevant applications and point to future research directions to address the challenges associated with DES pretreatment.

Solvent effects on the wood delignification with sustainable solvents

Solutions of a pure organic solvent acidified with 1% sulfuric acid, and some of their aqueous mixtures were used for lignin extraction in the Pinus radiata sawdust delignification. Organic acid solvents including acetic, lactic and citric acids as well as non-acidic compounds such as γ-valerolactone, 2-methyltetrahydrofuran, glycerol and ethylene glycol were studied. Crude lignin extractions yields (%) ranging between ca. 5-50% were obtained, from which ethylene glycol (33%), γ-valerolactone (48%) and propylene carbonate (52%) showed the greatest effectiveness. The effect of added water on the lignin extraction was investigated in mixtures of an organic solvent with a variable water content (75%w/w, 50% w/w, 25%w/w and 10% w/w) where it was observed that the yield of extraction decreased with the increased water content. Moreover, the purity of extracted lignins were analyzed by spectroscopic methods (UV and IR). Kamlet-Taft solvent polarity parameters, were determined with the solvatochromic probes 4-nitroaniline, N,N-diethyl-4-nitroaniline, Nile Red and 6-propionyl-2-N,N-dimethylaminonaphthalene (PRODAN), and then correlated to lignin extraction yields to explain the influence of the solute-solvent interactions on biomass delignification. A reasonable correlation was found between the medium polarizability-dipolarity π* and the effectiveness of the solvent mixture on the extraction of lignin wood.

Liquefying Flavonoids with Terpenoids through Deep Eutectic Solvent Formation

The formation of deep eutectic solvents (DES) is tied to negative deviations to ideality caused by the establishment of stronger interactions in the mixture than in the pure DES precursors. This work tested thymol and menthol as hydrogen bond donors when combined with different flavonoids. Negative deviations from ideality were observed upon mixing thymol with either flavone or flavanone, two parent flavonoids that only have hydrogen bond acceptor (HBA) groups, thus forming non-ionic DES (Type V). On the other hand, the menthol systems with the same compounds generally showed positive deviations from ideality. That was also the case with the mixtures containing the more complex hydroxylated flavonoid, hesperetin, which resulted in positive deviations when mixed with either thymol or menthol. COSMO-RS successfully predicted the behavior of the solid-liquid phase diagram of the studied systems, allowing for evaluation of the impact of the different contributions to the intermolecular interactions, and proving to be a good tool for the design of DES.

Robust superbase-based emerging solvents for highly efficient dissolution of cellulose

The development of robust solvent systems for cellulose dissolution is of significant importance for cellulose utilization and transformation. Herein, six kinds of novel superbase-based solvents were designed by a combination of 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) with pyridine N-oxide (PyO) or 2-picoline-N-oxide (PiO) for dissolution of cellulose. It was observed that the prepared superbase-based solvents (denoted as DBN-PyO-x and DBN-PiO-4) could efficiently dissolve cellulose at mild temperatures (<80 °C). The chemical structure of the prepared superbase-based solvents and the molar ratio of the components significantly affected the solubility of cellulose, and DBN-PyO-4 showed the best performance with a cellulose solubility of 14.1 wt% 70 °C. The systematic study revealed that the good performance of the prepared superbase-based solvents on cellulose dissolution resulted from the synergistic effect of their ability to form hydrogen bonds and their polarizability.

Pretreatment of lignocellulosic biomass: A review on recent advances

Depleting fossil reserves and growing energy needs have raised the demand for an alternative and clean energy source. The use of ubiquitously available lignocellulosic biomass for developing economic and eco-friendly large scale biorefinery applications has provided the much-needed impetus in this regard. The pretreatment process is a vital step for biomass transformation into added value products such as sugars, biofuels, etc. Different pretreatment approaches are employed to overcome the recalcitrance of lignocellulosic biomass and expedite its disintegration into individual components- cellulose, hemicellulose, and lignin. The conventional pretreatment methods lack sustainability and practicability for industrial scale up. The review encompasses the recent advances in selective physical and chemical pretreatment approaches such as milling, extrusion, microwave, ammonia fibre explosion, eutectic solvents etc. The study will allow a deeper understanding of these pretreatment processes and increase their scope as sustainable technologies for developing modern biorefineries.