Successive C1-C2 bond cleavage: the mechanism of vanadium(v)-catalyzed aerobic oxidation of d-glucose to formic acid in aqueous solution.

Vanadium(v)-catalyzed aerobic oxidation in aqueous solution shows high selectivity in the field of C-C bond cleavage of carbohydrates for chemicals with less carbon atoms. However, the pathway of C-C bond cleavage from carbohydrates and the conversion mechanism are unclear. In this work, we studied the pathway and the mechanism of d-glucose oxidation to formic acid (FA) in NaVO3-H2SO4 aqueous solution using isotope-labeled glucoses as substrates. d-Glucose is first transformed to FA and d-arabinose via C1-C2 bond cleavage. d-Arabinose undergoes similar C1-C2 bond cleavage to form FA and the corresponding d-erythrose, which can be further degraded by C1-C2 bond cleavage. Dimerization and aldol condensation between carbohydrates can also proceed to make the reaction a much more complicated mixture. However, the fundamental reaction, C1-C2 bond cleavage, can drive all the intermediates to form the common product FA. Based on the detected intermediates, isotope-labelling experiments, the kinetic isotope effect study and kinetic analysis, this mechanism is proposed. d-Glucose first reacts with a vanadium(v) species to form a five-membered-ring complex. Then, electron transfer occurs and the C1-C2 bond weakens, followed by C1-C2 bond cleavage (with no C-H bond cleavage), to generate the H3COO˙-vanadium(iv) complex and d-arabinose. FA is generated from H3COO˙ that is oxidized by another vanadium(v) species. The reduced vanadium species is oxidized by O2 to regenerate to its oxidation state. This finding will provide a deeper insight into the process of C-C bond cleavage of carbohydrates for chemical synthesis and provide guidance for screening and synthesizing new highly-efficient catalyst systems for FA production.

High-Selectivity Recycling of Valuable Metals from Spent Lithium-Ion Batteries Using Recyclable Deep Eutectic Solvents.

The recovery of valuable metals from spent lithium-ion batteries using deep eutectic solvents (DESs) is an environmentally and economically beneficial process. In this study, a method has been developed for recovering LiNi0.33Co0.33Mn0.33O2. Our process operates under mild conditions and with a little oxalic acid as a reducing agent, dissolving lithium, cobalt, manganese, and nickel completely utilizing a DES that is composed of tetrabutylammonium chloride and of monochloroacetic acid. Lithium and nickel were selectively precipitated using oxalic acid. Cobalt and manganese were precipitated as oxalates by adding an oxalic acid aqueous solution. Finally, the DES can be regenerated by evaporating the water. Importantly, valuable metals can be recovered with a 100 % yield through the process of DES recycling. This environmentally friendly and recyclable process is suitable for the recycling of spent lithium-ion batteries industry.

Absorption and Conversion of SO2 in Functional Ionic Liquids: Effect of Water on the Claus Reaction.

The absorption of SO2 from flue gas and its conversion to chemicals is important in the industry. Functional ionic liquids (ILs) have been broadly used to absorb SO2 in flue gas, but seldom convert it to chemicals. As we know, water is inevitable in a desulfurization process. In this work, three functional ILs (monoethanolaminium lactate-[MEA][Lac], 1,1,3,3-tetramethylguanidinium lactate-[TMG][Lac], tetraethylammonium lactate-[N2222][Lac]) with or without water were used as absorbents to absorb SO2 in flue gas, and then the absorbed SO2 in the absorbents was converted to sulfur via a Claus reaction. The result shows that the three ILs can efficiently absorb SO2 and convert it to sulfur. But the addition of water in the ILs can reduce the conversion of absorbed SO2, and the conversion increases with increasing the acidity of absorbents. To explain this phenomenon, we studied the Claus reaction in H2SO3, NaHSO3 and Na2SO3 aqueous solutions. It turns out that the conversion of the Claus reaction is related to the species of S (IV) in the order of the oxidability: H2SO3 > HSO3 - > SO3 2-, and their proportions dependent on the pH of solutions. On the basis of the absorption mechanism of SO2 in functional ILs aqueous solution, H2S reacts with HSO3 - and SO3 2- with weaker oxidability, resulting in the lower conversion. Importantly, we found that the addition of lactic acid could increase the conversion of SO2 via the Claus reaction.

A New Structural Model of Enzymatic Lignin with Multiring Aromatic Clusters.

Lignin is a natural aromatic compound in plants. Several lignin structural models have been proposed in the past years, but all the models cannot be converted to benzene carboxylic acids (BCAs) for all aromatic rings connected to oxygen. This inspired us to explore the structures of lignin. Based on the yields of BCAs, the results of 13C NMR and ethanolysis residues, and gas chromatography-mass spectrometry and electrospray ionization mass spectrometry of ethanolysis of lignin, we have constructed a structural model of lignin with a formula C6407H6736O2590N147S3. The model not only satisfies the results of analyses, but also explains the generation of BCAs from lignin oxidation and the ethanolysis products. Importantly, double-ring and triple-ring aromatic clusters are found in lignin, and some of them are connected by alkyl bridges, which results in conventional low conversions of lignin. Our findings in the structures of lignin may significantly influence the structures and applications of lignin.

Polycyclic Aromatics Observed in Enzymatic Lignin by Spectral Characterization and Ruthenium Ion-Catalyzed Oxidation.

It is generally considered that lignin is a three-dimensional amorphous polymer consisting of methoxylated phenylpropane structures. However, high yields of monomer structural units of lignin cannot be obtained through various ways, which inspired us to gain insights into the structures of lignin. Herein, enzymatic lignin (EL) was directly characterized by a solid-state 13C nuclear magnetic resonance spectrometer and Fourier transform infrared spectrometer and then subjected to ruthenium ion-catalyzed oxidation. According to the spectral characterization, it can be inferred that multi-ring aromatic clusters exist in EL because of the aromatic bridgehead carbon ratio of 0.136. Based on the results of ruthenium ion-catalyzed oxidation of the EL, it can be deduced that (1) double- and triple-aromatic ring clusters exist in the EL besides the traditional phenylpropane single-aromatic ring clusters, and (2) some aromatic rings with long-alkyl chain substituents exist in the EL, which is quite different from the traditional cognition of lignin. This investigation provides a new insight into the structure of EL.

Hydrophobic Functional Deep Eutectic Solvents Used for Efficient and Reversible Capture of CO2.

CO2 emission from flue gas is an important issue threatening human survival. Deep eutectic solvents (DESs), which have many unique properties, have been studied for CO2 capture. However, water can be absorbed by DESs during the absorption of CO2, which may increase the energy cost during the desorption of CO2. In this work, a new kind of hydrophobic functional DES formed by polyamine hydrochloride and thymol was synthesized and used for CO2 capture. It had been found that these DESs could efficiently capture CO2 even at low partial pressures. The CO2 capacity of [TEPA]Cl-thymol (n [TEPA]Cl/n thymol = 1:3) was high up to 1.355 mol CO2/mol DES at 40 °C and 101.3 kPa. Interestingly, these DESs were still hydrophobic after saturated with CO2. The CO2 absorption capacity increased with a decrease of temperature and an increase of CO2 partial pressure. Regeneration results showed that no obvious loss in the capacity could be found after five absorption/desorption cycles of these DESs. The Fourier transform infrared (FT-IR) spectra indicated that CO2 could interact with amino in the DESs by the formation of carboxylate. Moreover, the equilibrium constant and Henry's law constant in chemical absorption and physical absorption were studied.

Improvement of the selectivity of isophorone hydrogenation by Lewis acids.

The selective hydrogenation of isophorone (3,5,5-trimethyl-2- cyclohexen-1-one) to produce 3,3,5-trimethylcyclohexanone (TMCH), an important organic solvent and pharmaceutical intermediate, is of significance in industry. However, the over-hydrogenation to produce the by-product 3,3,5-trimethylcyclohexanol causes issues. Up to now, it is still a challenge to hydrogenate isophorone to TMCH with high selectivity. In this work, we found that Lewis acids could inhibit the hydrogenation of C=O bond on isophorone, thus greatly improving the selectivity towards TMCH. In addition, added solvents like supercritical CO2 also had a positive impact on the selectivity. Both the conversion and selectivity could be increased to more than 99% when suitable Lewis acid and solvent were employed. Nevertheless, Lewis acid also exhibited some inhibition on the hydrogenation of the C=C bond of isophorone. Hence, a relatively weak Lewis acid, ZnCl2, is suitable for the selective hydrogenation.

Conversion of Cellulose into Formic Acid by Iron(III)-Catalyzed Oxidation with O2 in Acidic Aqueous Solutions.

The conversion of abundant renewable cellulose into versatile formic acid (FA) is a potential process for efficient energy storage and application. Vanadium(V)-catalyzed oxidation with O2 in acidic aqueous media now is the most common method to realize the FA production from cellulose with both high yields and high purity. However, vanadium-based catalysts are difficult to synthesize and expensive. Thus, the seeking for cheaper catalysts with the same high efficiency is expected. In this work, after testing a variety of metal salts in acidic aqueous solution for the conversion of cellulose under O2, iron(III) was found as a cheaper and readily available catalyst for FA formation, with a comparable yield (51.2%, based on carbon) with that of vanadium(V). The effect of reaction parameters was studied. The competition between oxidation and hydrolysis was found and discussed in detail. FeCl3 and H2SO4 can accelerate oxidation and hydrolysis, respectively, whereas suppress the other. The effects can reflect on the product distribution. Intermediates were found and the pathway from cellulose to products was reasonably proposed. The reusability of the catalytic system shows good performance after four runs. The mechanism study suggests a catalytic ability by a mutual transformation between iron(III) and iron(II), where iron(III) oxidizes substrates to iron(II) that is reoxidized by O2.

Efficient absorption of SO2 with low-partial pressures by environmentally benign functional deep eutectic solvents.

Sulfur dioxide (SO2) emitted from the burning of fossil fuels is one of the main air contaminants. In this work, we found that environmentally benign solvents, deep eutectic solvents (DESs) could be designed with a function to absorb low-partial pressure SO2 from simulated flue gas. Two kinds of biodegradable functional DESs based on betaine (Bet) and l-carnitine (L-car) as hydrogen bond accepters (HBA) and ethylene glycol (EG) as a hydrogen bond donor (HBD) were prepared with mole ratios of HBA to HBD from 1:3 to 1:5, and they were investigated to absorb SO2 with different partial pressures at various temperatures. The results showed that the two DESs could absorb low-partial pressure SO2 efficiently. SO2 absorption capacities of the DESs with HBA/HBD mole ratio of 1:3 were 0.332mol SO2/mol HBA for Bet+EG DES and 0.820mol SO2/mol HBA for L-car+EG DES at 40°C with a SO2 partial pressure of 0.02atm. In addition, the regeneration experiments demonstrated that the absorption capacities of DESs did not change after five absorption and desorption cycles. Furthermore, the absorption mechanism of SO2 by DESs was studied by FT-IR, 1H NMR and 13C NMR spectra. It was found that there are strong acid-base interactions between SO2 and -COO- on HBA.

Hydrophobic task-specific ionic liquids: synthesis, properties and application for the capture of SO2.

The capture of SO2 by ionic liquids (ILs) has drawn much attention all over the world. However, ILs can absorb not only SO2 but also water from flue gas. The removal of water from ILs is necessary for reusing the absorbent. In order to reduce the energy costs of removing water, it would be helpful to weaken the interactions between ILs and water. In this work, two kinds of hydrophobic task-specific ILs, 1-(2-diethyl-aminoethyl)-3-methylimidazolium hexafluorophosphate ([Et2NEmim] [PF6]) and 1-(2-diethyl-aminoethyl)-1-methylpyrrolidinium hexafluorophosphate ([Et2NEmpyr][PF6]), were designed and synthesized. Thermal stability and physical properties of the ILs were studied. Furthermore, the application of the ILs for the capture of SO2 and the absorption mechanism were systematically investigated. It has been found that both of the ILs are immiscible with water, and [Et2NEmim][PF6] has much lower viscosity, much higher thermal stability and much higher SO2 absorption rate than [Et2NEmpyr][PF6]. [Et2NEmim][PF6] shows high SO2 absorption capacities up to 2.11 mol SO2 per mole IL (pure SO2) and 0.94 mol SO2 per mole IL (3% SO2) under hydrous conditions at 30 °C. The result suggests that [Et2NEmim][PF6] is a promising recyclable absorbent for the capture of SO2.

Separation of the isomers of benzene poly(carboxylic acid)s by quaternary ammonium salt via formation of deep eutectic solvents.

Because of similar properties and very low volatility, isomers of benzene poly(carboxylic acid)s (BPCAs) are very difficult to separate. In this work, we found that isomers of BPCAs could be separated efficiently by quaternary ammonium salts (QASs) via formation of deep eutectic solvents (DESs). Three kinds of QASs were used to separate the isomers of BPCAs, including the isomers of benzene tricarboxylic acids (trimellitic acid, trimesic acid, and hemimellitic acid) and the isomers of benzene dicarboxylic acids (phthalic acid and isophthalic acid). Among the QASs, tetraethylammonium chloride was found to have the best performance, which could completely separate BPCA isomers in methyl ethyl ketone solutions. It was found that the hydrogen bond forming between QAS and BPCA results in the selective separation of BPCA isomers. QAS in DES was regenerated effectively by the antisolvent method, and the regenerated QAS was reused four times with the same high efficiency.

What are functional ionic liquids for the absorption of acidic gases?

As a kind of novel and efficient material, ionic liquids (ILs) are used for capture of acidic gases including SO2 and CO2 from flue gas. Due to very low content of acidic gases in flue gas, it is important to find functional ILs to absorb the acidic gases. However, up to now, there is no criterion to distinguish if the ILs are functional or not before use, which greatly influences the design of functional ILs. In this work, a series of ILs were synthesized and used to determine functional or normal ILs for the capture of acidic gases. It has been found that the pKa of organic acids forming the anion of ILs can be used to differentiate functional ILs from normal ILs for the capture of acidic gases from flue gas. If the pKa of an organic acid is larger than that of sulfurous acid (or carbonic acid), the ILs formed by the organic acid can be called functional ILs for SO2 (or CO2) capture, and it can have a high absorption capacity of SO2 (or CO2) with low SO2 (or CO2) concentrations. If not, the IL is just a normal IL. The pKa of organic acids can also be used to explain the absorption mechanism and guide the synthesis of functional ILs.

Solubilities and thermodynamic properties of SO2 in ionic liquids.

Task-specific ionic liquids (TSILs) have been experimentally demonstrated to absorb more sulfur dioxide (SO(2)) than normal ILs from gas mixtures with low SO(2) concentrations; however, the differences of SO(2) solubilities in the two kinds of ILs at given temperatures and pressures have not been studied systematically. Moreover, the mechanism of the interaction between SO(2) and ILs still remains unclear. In this work, the solubilities of SO(2) in TSILs (1,1,3,3-tetramethylguanidinium lactate and monoethanolaminium lactate) and normal ILs (1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate) were determined. The solubilities of SO(2) are correlated by a modified Redlich-Kwong equation of state (RK EoS). The chemical absorption and physical absorption are differentiated, and the absorption mechanism has been proposed with the aid of the modified RK EoS. SO(2) absorption capacity in TSILs is contributed from both chemical interaction and physical interaction. Two TSIL molecules chemically absorb one SO(2) molecule, and the chemical absorption amount follows the chemical equilibrium. Normal ILs only physically absorb SO(2) following Henry's law. The chemical equilibrium constant, reaction enthalpy, Gibbs energy of reaction, reaction entropy, and Henry's law constant of SO(2) absorbed in ILs have been calculated. The present model can predict SO(2) absorption capacity for capture and SO(2) equilibrium concentration in IL for recovery.

Properties of ionic liquids absorbing SO2 and the mechanism of the absorption.

Room-temperature ionic liquids (ILs) have been demonstrated to absorb SO(2) efficiently. However, after absorbing a large amount of SO(2), the viscosity, the conductivity, and the density of the ILs have not been studied systematically, and the mechanism of the interaction between SO(2) and ILs is still being disputed. In this work, two kinds of ILs (task-specific ILs and normal ILs) have been studied to absorb pure SO(2) at atmospheric pressure. It is found that the viscosity, the conductivity, and the density show different behaviors between task-specific ILs and normal ILs. For the task-specific ILs to absorb SO(2), before a 0.5 mol ratio of SO(2) to IL, the viscosity and density increase, and the conductivity decreases with an increase of the mole ratio of SO(2) to IL. After that, the conductivity and density increase, and the viscosity decreases with further increasing the mole ratio of SO(2) to IL. However, for the normal ILs, the conductivity and density increase and the viscosity decreases with an increase of the mole ratio of SO(2) to IL. A new mechanism of ILs absorbing SO(2) has been proposed. Task-specific ILs can chemically absorb SO(2) when the mole ratio of SO(2) to IL is not more than 0.5, and they can physically absorb SO(2) when the mole ratio is more than 0.5. The normal ILs can only physically absorb SO(2).