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Using the dihydroxylation of alkenes as a benchmark, the reactivities of fresh and aged solutions of (NH4)2S2O8 (electrochemically generated) were compared to commercially-procured peroxydisulfate and Oxone®. The study revealed that peroxymonosulfate (Caro’s acid, PMS) is the active oxidant in such reactions. Using complementary redox colorimetry and in situ IR spectroscopy to monitor the oxidants, the competitive decomposition of peroxydisulfate in an acidic solution into PMS and H2O2 can be quantified for the first time. The new insight enabled the design and implementation of both batch and flow processes to maximise the concentration of active PMS oxidant. The utility of these oxidants for organic synthesis is demonstrated by the dihydroxylation of eight styrenes and seven alkyl alkenes, where the ammonium PMS solutions performed better than Oxone® (counterion effect). Last but not least, a non-chromatographic method for isolating and purifying the water-soluble diol product was developed.

Electrolytic formic acid synthesis delivers green formic acid at the right concentration for emerging applications.

Depolymerization of lignin, a key component of lignocellulose, is a sustainable approach to provide value-added aromatics. Selective cleavage of recalcitrant C−C linkages is a key challenge for lignin depolymerization. Photocatalysis, which can provide highly reactive species under mild conditions, represents a potential strategy for such C−C bond cleavage. Here, we report an efficient approach to break lignin C−C bonds using a Z-scheme Ag3PO4−polymeric carbon nitride (PCN) nanocomposite under simulated solar light at room temperature. Various lignin model compounds were converted into aromatic aldehydes with high yields of 66%-95%. Our mechanistic study reveals that the C−C bond cleavage mainly involves an electron-hole coupled photoredox mechanism. The construction of Z-scheme Ag3PO4−PCN heterojunction structure is essential to simultaneously preserve strong reduction and oxidation capability of its components, which is key for the electron-hole coupled Cα−Cβ bond cleavage. The present work offers useful guidance for the design of efficient semiconductors for C−C bond cleavage through band structure engineering.

Muconic acid (MA) is a high value-added dicarboxylic acid with conjugated double bonds, presenting three isomeric forms, i.e., cis,cis-MA, cis,trans-MA and trans,trans-MA. Its production is garnering increased interest owing to its potential as a starting material for the synthesis of value-added products as well as by being a versatile monomer for the production of specialty polymers. This review presents a systematic overview of production and synthesis routes to MA isomers as well as the routes for its final valorization. The production of MA through chemical pathways and the progress developed in the biotechnological pathways will be discussed. Traditional and new processes for achieving successful isomerization into the value-added trans,trans form of MA are also discussed and compared, with their constraints and possible solutions. The valorization of the three different isomers of MA into value-added chemicals such as adipic or terephthalic acids and MA polymers are summarized. Finally, the review concludes with a thorough summary, a practical insights section and an outlook. This work thus offers new perspectives and guidelines to tackle the challenges in MA chemistry, especially when aiming to combine an efficient biotechnological production of MA with its valorization.

In recent years, the work of various research groups has allowed the substitution of the hazardous solvents most widely used in solid-phase peptide synthesis, namely DMF, NMP, DCM, DEE, among others, by several much less hazardous solvents. Indeed, greener alternatives have been found for almost all steps of the process, with the exception of the cleavage of protected peptides from 2-chlorotrityl chloride resin. Here, after careful screening of several of the so-called green solvents, we propose 2% TFA in either anisole or 1,3-dimethoxybenzene as optimal for the cleavage step. The higher boiling point of these solvents compared with the DCM allows the preparation of protected peptides with less risk of premature removal of the most labile protecting groups, such as the Trt of His. Our findings once again evidence the value/versatility of green solvents in strict chemical terms., The work was funded in part by the following: the National Research Foundation (NRF) (# 105892 and Blue Sky’s Research Programme # 120386) and the University of KwaZulu-Natal (South Africa); and the Spanish Ministry of Science, Innovation, and Universities (RTI2018-093831- B-100) and the Generalitat de Catalunya (2017 SGR 1439) (Spain).

Phenol and its derivatives, which naturally occur in lignocellulose, can be considered as a renewable feedstock not only for aromatic, but also for alicyclic compounds, such as primary and N-substituted cyclohexylamines. So far, the latter are mostly produced from non-renewable starting materials like benzene via problematic nitration/reduction or cross-coupling routes. Herein, an efficient reductive amination of phenol with ammonia or amines is demonstrated, for the first time without the need for rare and expensive noble metals and without using any additives. Various supported Ni catalysts were screened and we elucidated the influence of the key parameters, including the acid–base properties of the supporting material. Acquired knowledge was then applied to different phenol–ammonia/amine combinations, resulting in the synthesis of various primary, secondary and tertiary cyclohexylamines in fair to very high yields.

The hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) using the η4-(2,3,4,5-tetraphenylcyclopentadienone) ruthenium tricarbonyl precursor of the well-known Shvo catalyst and H2 pressure was established under solvent-free conditions to achieve 100% conversion and 100% selectivity within 5 hours. Kinetic reaction curves were measured in order to deduce the optimal reaction conditions, which were further evaluated by means of density functional theory (DFT) calculations, and compared with catalytic concepts that consume formic acid or isopropyl alcohol as hydrogen donor. Ultimately, this alternative reaction procedure was subjected to a life cycle assessment (LCA) in comparison with the transfer hydrogenation methodologies, in order to verify its contribution towards a practice of environmentally benign chemistry.

A split-anion solvent extraction process was developed for the separation of precious metal ions Au(III), Pt(IV), Pd(II) and Rh(III) from aqueous chloride media using water-saturated ionic liquids. The metal extraction and stripping behavior of the chloride form [A336][Cl], bromide form [A336][Br] and the iodide form [A336][I] of the quaternary ammonium ionic liquid Aliquat 336 were compared. The three ionic liquids extracted Au(III), Pd(II) and Pt(IV) quantitatively in most cases, whereas the co-extraction of Rh(III) was strongly dependent on the acidity and the chloride concentration. Among the studied ionic liquids, [A336][I] achieved the highest separation factors between Pd(II)/Rh(III), Pt(IV)/Rh(III), and Au(III)/Rh(III) at 6 mol L−1 Cl−. Additionally, the selective stripping of the individual metal ions Pd(II), Au(III), and Pt(IV) was only possible from loaded [A336][I] using ammonia solution (NH4OH), sodium thiosulfate (Na2S2O3), and thiourea ((NH2)2CS), respectively. A closed-loop flow sheet was designed for the recovery of the precious metals from chloride media using split-anion extraction with [A336][I]. The integrated process was demonstrated to be suitable for the purification of Rh(III), Pt(IV) and Pd(II) from a complex metal feed such as the leachate of spent automotive catalysts. The ionic liquid-based split-anion extraction process is simple, selective and effective for the sustainable separation of the precious metals, using only one green extractant [A336][I], which can be regenerated for consecutive extraction-stripping cycles.

The hydrogenation of amides is a straightforward method to produce (possibly bio-based) amines. However current amide hydrogenation catalysts have only been validated in a rather limited range of toxic solvents and the hydrogenation of aliphatic (acyclic) primary amides has rarely been investigated. Here, we report the use of a new and relatively cheap ruthenium–tungsten bimetallic catalyst in the green and benign solvent cyclopentyl methyl ether (CPME). Besides the effect of the Lewis acid promotor, NH3 partial pressure is identified as the key parameter leading to high primary amine yields. In our model reaction with hexanamide, yields of up to 83% hexylamine could be achieved. Beside the NH3 partial pressure, we investigated the effect of the catalyst support, PGM-Lewis acid ratio, H2 pressure, temperature, solvent tolerance and product stability. Finally, the catalyst was characterized and proven to be very stable and highly suitable for the hydrogenation of a broad range of amides.

Recycling of cobalt from end-of-life lithium-ion batteries (LIBs) is gaining interest because they are increasingly used in commercial applications such as electrical vehicles. A common LIB cathode material is lithium cobalt oxide (LiCoO2). Besides the cathode, LIBs contain other components, such as metallic aluminium and copper as current collectors, which are often separated at initial hydrometallurgical recycling stages. Leaching of cobalt from LiCoO2 is mainly driven by reducing cobalt(III) in LiCoO2 to cobalt(II) via adding reducing agents. In this work, a green, cheap and safe approach is proposed by using a choline chloride–citric acid deep-eutectic solvent (DES) as lixiviant. Aluminium and copper were evaluated as reducing agents for cobalt(III). After optimisation, lithium and cobalt were quantitatively leached from LiCoO2 in the presence of aluminium and copper. Copper was the most effective reducing agent for cobalt(III), so that no additional reducing agents or a pre-separation step were required. A speciation study of the pregnant leach solution (PLS) confirmed the dominance of chloro complexes. DES leaching was compared with conventional hydrochloric acid leaching, whereby the DES avoided the formation of toxic chlorine gas. Finally, the DES PLS was used as the more polar phase in a non-aqueous solvent extraction process. This process consisted of a copper(I/II) extraction step with the extractant LIX 984, followed by selective extraction of cobalt(II) with the extractant Aliquat 336. Both metals were completely stripped from the loaded organic phases by oxalic acid. The total recovery yield of cobalt was 81%, as a 99.9% pure oxalate precipitate.

Difficulties in the recycling of soluble acid catalysts within the lignocellulosic biorefinery constitute a serious issue to the sustainability of cellulose hydrolysis and several other transformations. Herein, we demonstrate a simple and effective method for the removal and recovery of p-toluenesulfonic acid (p-TSA) employed as a replacement of H2SO4 in the saccharification of cellulose by a mechanocatalytic route. p-TSA is recovered from its diluted aqueous solutions using a non-expensive hypercrosslinked polymer adsorbent. In a batch process, around 97% p-TSA acid was removed from a 5 mM solution after exposure to the hypercrosslinked polymer. Notably, a flow-through process was able to selectively remove the catalyst completely from even lower initial concentrations. Importantly, the efficient recovery of the p-TSA is achieved by washing the spent polymer with methanol. In the purification of a stream derived from the saccharification of cellulose, the extraction of p-TSA catalyst in addition to 5-hydroxymethylfurfural (HMF) and furfural was also achieved. With the rational choice of solvents, p-TSA and furfurals were separately recovered. In the broader context, the current findings represent a step forward towards the acid management from the acid-catalyzed saccharification of cellulose. Moreover, this approach is conducive for the purification of sugars for bioconversion in which even low levels of furfurals may exert a strong inhibitory effect upon yeast cultures.

© 2018 The Royal Society of Chemistry. Lignocellulose constitutes an alluring renewable feedstock for the production of bio-based chemicals. In this contribution, we propose a chemocatalytic biorefinery concept that aims to convert lignocellulosic biomass (Eucalyptus sawdust) into (i) lignin-derived (mono)phenolics, (ii) hemicellulose-derived polyols, and (iii) a cellulose pulp. This is achieved by processing biomass in an equivolumetric mixture of n-butanol and water at elevated temperature (200 °C), in the presence of Ru/C and pressurised hydrogen (30 bar). During this one-pot Reductive Catalytic Fractionation (RCF) process, the hot liquor enables the extraction and solvolytic depolymerisation of both lignin and hemicellulose, while the catalyst and reductive environment are essential to hydrogenate reactive intermediates (coniferyl/sinapyl alcohol and sugars) toward stable target products (phenolics and polyols, respectively). After the catalytic reaction, the solid carbohydrate pulp (mainly cellulose) is easily retrieved upon filtration. Phase separation of n-butanol and water occurs upon cooling the liquor (

The catalytic activity of a series of neutral and cationic, homo- and heteroleptic, mono- and bimetallic palladium(II) compounds based on dithiocarbamate and dithiooxamide S,S-donor ligands is described. High activity was observed in the regio- and chemo-selective C–H functionalization of benzo[h]quinoline to 10-alkoxybenzo[h]quinoline and 8-methylquinoline to 8-(methoxymethyl)quinoline in the presence of the oxidant PhI(OAc)2. The best performance was found for [Pd(Me2dazdt)2]I6 (Me2dazdt = N,N′-dimethyl-perhydrodiazepine-2,3-dithione), [PdI2(Me2dazdt)] and [Pd(Cy2DTO)2]I8 (Cy2DTO = N,N′-dicyclohexyl-dithiooxamide) which are all obtained directly as products of sustainable Pd-metal leaching processes used to recover palladium from scrap metal. These compounds provided almost quantitative yields under milder conditions (50 °C, 1–3 mol% Pd loading) and much shorter reaction times (1–3 h) than reported previously. These results illustrate how the complexes obtained from the selective and sustainable recovery of Pd from automotive heterogeneous Three Way Catalysts (TWC) can be employed directly in homogeneous catalysis, avoiding further metal recovery steps and valorising the metal complex itself in a ‘circular economy’ model.

Acidic anions of ionic liquids have been demonstrated as efficient catalysts for the cleavage of the β-O-4 ether linkage prevalent in the lignin superstructure. Through the use of lignin model compounds with varying functionality and by monitoring reaction kinetics, a full mechanistic investigation into the hydrolysis of the β-O-4 linkage in acidic ionic liquid solutions is reported. Hammett acidities are reported for different 1-butyl-3-methylimidazolium hydrogen sulfate [C4C1im][HSO4] ionic liquid systems with varying acid and water concentrations and were correlated to substrate reactivity. Results show that the rate of ether cleavage increases with an increase in acidity and the initial dehydration of the model compound is the rate-determining step of the reaction. The Eyring activation parameters of the reaction in hydrogen sulfate ionic liquids with a variety of cations are reported, indicating a consistent E1 dehydration mechanism. Hydrogen bonding in protic ionic liquids was shown to significantly influence anion–cation interactions, consequently altering the solvation of the protonated starting material and therefore the overall rate of reaction. Comparison of reaction rates in these ionic liquids with results within aqueous or aqueous/organic media indicate that the ionic liquids facilitate more rapid cleavage of the β-O-4 ether linkage even under less acidic conditions. All the reported results give a complete overview of both the mechanistic and solvation effects of acidic ionic liquids on lignin model compounds and provide scope for the appropriate selection and design of ionic liquids for lignin processing.

Although excellent candidate solvents for cellulose, capa-ble of dissolving 20 wt% of the carbohydrate for electro-spinning processes, dialky-limidazolium carboxylate ionic liquids undergo unde-sirable side reactions with the reducing end of saccharides, terminating in an equilibrium concentration of a 2-(hydroxymethyl)-substituted imidazolium ‘adduct’. The addition of small molar quantities of a benign, non-toxic and inexpensive co-solvent, e.g. glycerol, mini-mises the rate of adduct ac-cumulation, thereby enhanc-ing the long-term thermal stability and recyclability of the expensive ionic liquid component. NMR, UV-vis and mass spectrometry ex-periments reveal that the im-proved stability is likely at-tributable to suppression of the transient dialkylimidazol-2-ylidene carbene, via hy-drogen-donation by the pro-tic co-solvent, rather than by cyclic acetal protection of the carbohydrate. The incor-poration of (up to) 10 wt% of glycerol into the solvent mix-ture does not exacerbate the rate of cellulose depolymeri-sation compared to in the neat ionic liquid, and high solubility of cellulose is main-tained. Furthermore, a col-ourimetric comparison of the recovered solvents, following cellulose re-precipitation, demonstrates that glycerol does not increase the concen-tration of contaminant re-ducing sugars in the organic electrolyte.

Recently, acidic ionic liquid water mixtures based on the hydrogen sulfate anion have been shown to effectively extract lignin from lignocellulosic biomass. This study analyses Miscanthus giganteus lignin isolated after extraction with the protic ionic liquid 1-butylimidazolium hydrogen sulfate ([HC4im][HSO4]) followed by precipitation with the antisolvent water. Several analytical techniques were employed, such as quantitative 13C-NMR, 1H–13C HSQC NMR, 31P-NMR, Py-GC-MS, GPC and elemental analysis. The analysis shows that the ionic liquid pretreatment breaks lignin-hemicellulose linkages and depolymerizes the lignin through the cleavage of glycosidic, ester and β-O-4 ether bonds. This is accompanied by solubilization of the newly generated lignin fragments. At longer pretreatment times, repolymerization of lignin fragments through condensation reactions occurs. The isolated lignins were carbohydrate-free, had low sulfur contents, low molecular weights. Early stage lignins were structurally similar to ball-milled lignin, while more treated lignins were enriched in p-hydroxyphenyl and guaiacyl units and had a high phenolic hydroxyl group content. We conclude that, depending on the treatment conditions, lignins with a variety of characteristics can be isolated using this type of ionic liquid solution.

The application of the protic ionic liquid 1-butylimidazolium hydrogen sulfate in the deconstruction (aka pretreatment) and fractionation of lignocellulosic biomass has been investigated. A cellulose rich pulp and a lignin fraction were produced. The pulp was subjected to enzymatic saccharification which allowed recovery of up to 90% of the glucan as fermentable glucose. The influence of the solution acidity on the deconstruction of Miscanthus giganteus was examined by varying the 1-butylimidazole to sulfuric acid ratio. Increased acidity led to shorter pretreatment times and resulted in reduced hemicellulose content in the pulp. Addition of water to the ionic liquid resulted in enhanced saccharification yields. The ability to tune acidity through the use of protic ionic liquids offers a significant advantage in flexibility over dialkylimidazolium analogues.

The electrochemical behavior of indium in the ionic liquid trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101) was studied. Cyphos IL 101 first had to be purified, as the impurities present in commercial Cyphos IL 101 interfered with the electrochemical measurements. Electrochemical deposition of indium metal from this electrolyte occurs without hydrogen evolution, increasing the cathodic current efficiency compared to deposition from water and avoiding porosity within the deposited metal. Indium(iii) is the most stable oxidation state in the ionic liquid. This ion is reduced in two steps, first from indium(iii) to indium(i) and subsequently to indium(0). The high thermal stability of Cyphos IL 101 allowed the electrodeposition of indium at 120 °C and 180 °C. At 180 °C indium was deposited as liquid indium which allows for the easy separation of the indium and the possibility to design a continuous electrowinning process. On molybdenum, indium deposits as liquid droplets even below the melting point of indium. This was explained by the combination of melting point depression and undercooling. The possibility to separate indium from iron and zinc by electrodeposition was tested. It is possible to separate indium from zinc by electrodeposition, but iron deposits together with indium. ispartof: GREEN CHEMISTRY vol:21 issue:6 pages:1517-1530 ispartof: location:England status: published

Comprehensive HT-GC × GC FID/MS enables reliable detection and quantification of RCF lignin monomers, dimers and, trimers.

Extraction of copper from sufidic ores, either by pyrometallurgy or hydrometallurgy, has various limitations. In this study, a solvometallurgical process for the extraction of copper from sulfidic ore minerals (chalcopyrite, bornite, chalcocite and digenite) was developed by using an organic lixiviant (FeCl3 as oxidizing agent and ethylene glycol (EG) as organic solvent). All the studied copper sulfide minerals could be leached efficiently with a FeCl3–EG solution. Other lixiviant systems, namely CuCl2–EG, FeCl3–ethanol and FeCl3–propylene glycol could also extract copper, but they did not perform as well as the FeCl3–EG solutions. The mechanistic study of chalcopyrite leaching in FeCl3–EG solutions confirmed that the leaching products of chalcopyrite were FeCl2, CuCl and solid elemental sulfur, where the Fe(II) and Cu(I) were quantified by UV-Vis absorption spectroscopy and solid sulfur was identified by powder XRD. A kinetic study showed that the leaching process was a surface chemical controlled process and the apparent activation energy was calculated to be 60.1 kJ mol−1. Subsequently, electrodeposition of copper from the pregnant leachate was investigated, and SEM-EDX analysis showed that uniform cubic crystalline deposits of pure copper were produced. Meanwhile, the Fe(III) was regenerated by oxidizing Fe(II) at the anode, with a Morgane membrane in between two electrode compartments to prevent the transfer of Fe(III) to the cathode. Finally, a closed-loop solvometallurgical process was designed with three operational steps: leaching, electrodeposition and removal of Fe(II). The regeneration of the FeCl3–EG solution and the use of EG contribute to the sustainability and the greenness of the process.