Sustain. Chem., Volume 1, Issue 3 (December 2020) – 10 articles

Lignin is a unique renewable aromatic resource in nature. In the past decades, researchers have attempted to breakdown the linkage bonds in lignin to provide aromatic platform chemicals that used to come from the petrochemical industry. In recent years, electrochemical lignin degradation under mild conditions has drawn much attention from the scientific community owing to its potential to scale up and its environmental friendliness. Sustainable electrochemical degradation of lignin consumes less energy and usually requires mild conditions, but low degradation efficiency and insufficient product selectivity are still significant challenges. The method for lignin degradation by reactive oxygen species (ROS) generated through the water oxidation reaction (WOR) at the anode and oxygen reduction reaction (ORR) at the cathode are more attractive for sustainable electrochemical degradation. The present contribution aims to review advancements in electrochemical degradation of lignin in aqueous or non-aqueous supporting electrolytes, focusing on the regulation of ROS in situ generated on the electrode. Full article

The eco-sustainable use of materials derived from agricultural and food processing waste will represent one of the most stimulating challenges shortly. Chitin and chitosan are two remarkable examples of how molecules with high added value can be extracted from food waste, such as crustaceans’ shells, fungi, mollusks, etc. This Perspective summarizes the current state of knowledge about chitin extraction, chitosan production, and hydrogel formation, highlighting the environmental critical steps in the common route (use of strong acids and basis, toxic solvents, and not eco-friendly crosslinkers). At the same time, promising green alternatives are described and analyzed. Examples are the employment of NADESs or DESs (such as choline chloride: urea or choline chloride: organic acids mixtures) for chitin extraction and dissolution, use of citric acid both in chitin extraction and hydrogel formation or utilization of natural extracts, like genipin, as green cross-linkers under mild conditions (heating at 37 °C for 12 h). In particular, this perspective aims to provide a stimulating basis for the development of processes based on the recycling and reusing of chemicals, during the different preparation steps, in line with “system chemistry” and “circular economy” principles. Full article

Bio-oil (pyrolysis oil) is an essential feedstock for the production of renewable fossil-free fuels and valuable chemicals. Enhancement of the pyrolysis oil yield and its quality are significant challenges for an efficient and sustainable biorefinery. Here, we report the microwave (MW)-assisted noncatalytic pyrolysis of cellulose, as a green and controllable alternative to conventional heating, in the presence of eco-friendly Li-, Na-, K-, Mg-, Ca- and Ba-bentonites. The detailed analysis of the MV heating traces demonstrates that the bentonite MW activity significantly depends on the presence of internal water. The intensity of this interaction is controlled by the cation nature reduced in the order: Li⁺ > Na⁺ > K⁺ and Mg²⁺ > Ca²⁺ > Ba²⁺. A unique experimental design for the MW-assisted pyrolysis of cellulose in the presence of Li-doped clays helps to increase the bio-oil yield to 37.8% with high selectivity towards the commercially useful levoglucosan (purity: 39.36%). The combination of an alternative green heating method and environmentally friendly bentonites can be used many times without recycling. We believe that the improved yields of bio-oil are due to: (i) high MW activity of bentonites, which conventionally increases the heating rates of cellulose; and (ii) production of water by hydrophilic clay minerals, favouring depolymerisation of cellulose. Full article

Herein, we report on the metal-free, one-pot synthesis of industrially important dimethyl carbonate (DMC) from molecular CO₂ under ambient conditions. In this process, initially the CO₂ was chemisorbed through the formation of a switchable ionic liquid (SIL), [DBUH] [CH₃CO₃], by the interaction of CO₂ with an equivalent mixture of organic superbase 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) and methanol. The obtained SIL further reacted with methyl iodide (CH₃I) to form DMC. The synthesis was carried out in both dimethyl sulfoxide (DMSO) and methanol. Methanol is preferred, as it not only served as a reagent and solvent in CO₂ capture and DMC synthesis, but it also assisted in controlling the side reactions between chemical species such as CH₃I and [DBUH]⁺ cation and increased the yield of DMC. Hence, the use of methanol avoided the loss of captured CO₂ and favored the formation of DMC with high selectivity. Under the applied reaction conditions, 89% of the captured CO₂ was converted to DMC. DBU was obtained, achieving 86% recovery of its salts formed during the synthesis. Most importantly, in this report we describe a simple and renewable solvent-based process for a metal-free approach to DMC synthesis under industrially feasible reaction conditions. Full article

The flavin-containing monooxygenase from Methylophaga sp. strain SK1 (mFMO) is a valuable biocatalyst for the preparation of optically active sulfoxides, among other valuable compounds. In this study, we explored to benefits of using Natural Deep Eutectic Solvents (NADESs) when doing oxidation with this biocatalyst, fused to phosphite dehydrogenase for cofactor regeneration (PTDH-mFMO). It was found that optically active sulfoxides could be obtained with slightly higher conversions in 10% v/v NADES when working at substrate concentrations of 50–200 mM, whereas there was no loss in the enantioselectivity. With these results, it is demonstrated for the first time that flavin-containing monooxygenases can be employed as biocatalysts in presence of NADESs. Full article

Currently, investigations of polymer-building blocks made from biorenewable feedstocks such as, for example, fatty acids, are of high interest for the chemical industry. An alternative synthesis of nitrile-substituted aliphatic carboxylic acids as precursors for ω-amino acids, which are useful to produce polymers, was investigated starting from biorenewable fatty acids. By hydroformylation of unsaturated fatty acids or unsaturated acids being accessible from unsaturated fatty acids by cross-metathesis reactions, aldehydes are formed. In this work, the hydroformylation of such unsaturated acids led to the formation of the corresponding aldehydes, which were afterwards converted with hydroxylamine to aldoximes. Subsequent dehydration by an aldoxime dehydratase as a biocatalyst or by Cu^II acetate led to the desired nitriles. Within this work, C7-, C9- and C11-carboxylic acids with a terminal nitrile functionality as well as a branched nitrile-functionalized stearate derivative were synthesized by means of this approach. As these nitriles serve as precursors for amino acids being suitable for polymerization, this work represents an alternative synthetic access to polyamide precursors, which starts directly from unsaturated fatty acids as biorenewable resources and avoids harsh reaction conditions as well as and by-product formation. Full article

There has been increasing interest in recent years in the use of agro-based raw materials for the production of bio-friendly and sustainable products. Plant-based proteins are among the popular materials being studied. In particular, cottonseed protein (a byproduct of cotton fiber production) is widely available and has useful properties. Although not as well-known as soy protein, cottonseed protein has been shown to be a potentially valuable raw material for numerous applications. In this review, the latest developments in isolation, composition and molecular weight, chemical and enzymatic modifications, and non-food applications are delineated. Among these applications, films and coatings, interfacial and emulsifying applications, adhesives, and bioplastics seem to attract the most attention. A particular effort has been made to cover the literature on these topics in the past 10 years. Full article

Deep eutectic solvents (DESs) appeared recently as a new class of green designer solvents. The recovery of metals using hydrometallurgy is of major importance with the growth in metal demand. Several authors used these solvents for the hydrometallurgical recovery of metals from primary and secondary resources, and these studies are reviewed in the present work. Hydrophilic DESs can be used for the leaching of metals and have great potential to replace mineral acids, and even to reduce water consumption. Efficient and selective leaching of metals from minerals or wastes is feasible by using DESs. However, the kinetics of leaching as well as the physicochemical properties of DESs are still limiting their large-scale application. Electrochemical recovery from DES is also possible but deserves further investigation. Finally, the recovery of metals from aqueous solutions using hydrophobic DESs was studied in several works. For the solvent extraction of metals, hydrophobic DESs constitute credible alternative ionic liquids. Full article

This paper is review with 119 references. Approaches to supplant currently used plastics with materials made from more sustainably-sourced monomers is one of the great contemporary challenges in sustainable chemistry. Fatty acids are attractive candidates as polymer precursors because they can be affordably produced on all inhabited continents, and they are also abundant as underutilized by-products of other industries. In surveying the array of synthetic approaches to convert fatty acids into polymers, those routes that produce organosulfur polymers stand out as being especially attractive from a sustainability standpoint. The first well-explored synthetic approach to fatty acid-derived organosulfur polymers employs the thiol-ene click reaction or the closely-related thiol-yne variation. This approach is high-yielding under mild conditions with up to 100% atom economy and high functional group tolerance. More recently, inverse vulcanization has been employed to access high sulfur-content polymers by the reaction of fatty acid-derived olefins with elemental sulfur. This approach is attractive not only because it is theoretically 100% atom economical but also because elemental sulfur is itself an underutilized by-product of fossil fuel refining. The thiol-ene, inverse vulcanization, and mechanistically-related thiol-yne and classic vulcanization are therefore discussed as promising routes to access polymers and composites from fatty acid-derived precursors. Full article

Films formed from cellulose nanofibrils (CNFs) are known to be good barrier materials against oxygen, but they lose this feature once placed in humid conditions. To tackle this issue, we applied an optimized pressing condition under elevated temperature to increase the films’ density and improve their barrier performance. Furthermore, a water barrier coating was employed on the surfaces to control the moisture uptake at high relative humidity (RH). Neat self-standing films of CNF with the basis weight of 70 g/m² were made through a filtration technique and pressed for 1 hour at 130 °C. The resulting nanostructures were covered on both sides using a water-borne barrier layer. Hot-pressing resulted in a significant reduction in oxygen transmission rate (OTR) values, from 516.7 to 3.6 (cm³/(m²·day)) and to some degree, helped preserve the reduced oxygen transmission at high relative humidity. Introducing 35 g/m² of latex coating layer on both sides limited the films’ swelling at 90% RH for about 4 h and maintained the OTR at the same level. A finite element model was used to predict the dynamic uptake of water into the systems. The model was found to over-predict the rate of water uptake for uncoated samples but gave the correct order of magnitude results for samples that were coated. The obtained data confirmed the positive effect of hot-pressing combined with coating to produce a film with low oxygen transmission rate and potential to maintain its oxygen barrier feature at high relative humidity. Full article