Search

A simple and recyclable homogeneous catalytic system for the hydrogenation of carbon monoxide to methanol was established. The reaction is catalyzed by a molecular manganese complex using a high-boiling alcohol as the solvent for catalyst immobilization. The CO hydrogenation is assisted by the product itself and the solvent through the formation of a methyl or dodecyl formate ester intermediate mediated by catalytic amounts of NaOMe as the base. This allows the catalytic formation of methanol in alcohols combined with facile product separation and catalyst recycling via distillation. Initial turnover frequencies (TOF) of 2250 h−1 were reached under optimized conditions in 1-dodecanol/methanol as the reaction medium (T = 160 °C, p(H2/CO) = 80/10 bar). The performance was stabilized in batch-wise recycling over 6 runs achieving a total turnover number (TTON) of >12 000 corresponding to an enhancement of more than five times compared to single batch operation under identical conditions. Minimal leaching of the components of the organometallic catalyst was observed during distillative product separation and catalyst activity could be fully restored by re-addition of the base NaOMe. [ABSTRACT FROM AUTHOR]

This work focuses on the development and investigation of a Rh/TPP catalytic system for the synthesis of saturated aliphatic carboxylic acids from readily available oxygenated substrates, H2 and non-toxic, renewable CO2. Optimization of the reaction conditions and reagents was carried out using 2-butanol and 1-butanol as typical secondary and primary alcohols. Afterwards, the reaction system was investigated in-depth with a separate investigation of the reverse water–gas-shift-reaction and the carbonylation reaction as key steps of the overall tandem transformation. Based on these results, a reaction network was established with two distinct main pathways. While the secondary alcohols are converted preferentially via acid catalysed dehydration to the corresponding olefins followed by hydroxycarbonylation, the primary alcohols react primarily via nucleophilic substitution to the iodide compound followed by a Monsanto-type carbonylation. Based on these results, a broad range of alcohols including bio-based substrates was converted to the corresponding C1 elongated carboxylic acids. Additionally, other oxygenated substrates such as aldehydes, ketones and industrially relevant substrate mixtures were applied successfully. [ABSTRACT FROM AUTHOR]

This density functional theory (DFT) study explores the efficacy of cooperative catalytic systems in enabling the ionic hydrogenation of N2 with H2, leading to NH3 formation. A set of N-heterocyclic carbene-based pincer tungsten/molybdenum metal complexes of the form [(PCP)M1(H)2] (M1 = W/Mo) were chosen to bind N2 at the respective metal centres. Simultaneously, cationic rhodium/iridium complexes of type [Cp*M2{2-(2-pyridyl)phenyl}(CH3CN)]+ (Cp* = C5(CH3)5 and M2 = Rh/Ir), are employed as cooperative coordination partners for heterolytic H2 splitting. The stepwise transfer of protons and hydrides to the bound N2 and intermediate NxHy units results in the formation of NH3. Interestingly, the calculated results reveal an encouraging low range of energy spans ranging from ∼30 to 42 kcal mol−1 depending on different combinations of ligands and metal complexes. The optimal combination of pincer ligand and metal center allowed for an energy span of unprecedented 29.7 kcal mol−1 demonstrating significant potential for molecular catalysts for the N2/H2 reaction system. While exploring obvious potential off-cycle reactions leading to catalyst deactivation, the computed results indicate that no increase in energy span would need to be expected. [ABSTRACT FROM AUTHOR]

This density functional theory (DFT) study explores the efficacy of cooperative catalytic systems in enabling the ionic hydrogenation of N2 with H2, leading to NH3 formation. A set of N-heterocyclic carbene-based pincer tungsten/molybdenum metal complexes of the form [(PCP)M1(H)2] (M1 = W/Mo) were chosen to bind N2 at the respective metal centres. Simultaneously, cationic rhodium/iridium complexes of type [Cp*M2{2-(2-pyridyl)phenyl}(CH3CN)]+ (Cp* = C5(CH3)5 and M2 = Rh/Ir), are employed as cooperative coordination partners for heterolytic H2 splitting. The stepwise transfer of protons and hydrides to the bound N2 and intermediate NxHy units results in the formation of NH3. Interestingly, the calculated results reveal an encouraging low range of energy spans ranging from ∼30 to 42 kcal mol−1 depending on different combinations of ligands and metal complexes. The optimal combination of pincer ligand and metal center allowed for an energy span of unprecedented 29.7 kcal mol−1 demonstrating significant potential for molecular catalysts for the N2/H2 reaction system. While exploring obvious potential off-cycle reactions leading to catalyst deactivation, the computed results indicate that no increase in energy span would need to be expected. [ABSTRACT FROM AUTHOR]

One approach to recycle homogeneous catalysts is through multiphase catalysis. Multiphase catalysis is not only limited to liquid–liquid multiphase systems but also includes for example solid and liquid phases. In this work, we present a catalyst recycling system based on the crystallization of the entire catalyst phase after the reaction at ambient temperature. Using the green and polar solvent ethylene carbonate, the polar Rh/sulfoXantphos catalyst is trapped in the crystallized ethylene carbonate phase. The product can be decanted under air as the catalyst is stabilized in the solid phase and the entire solid phase including the solvent is recycled. Several reactions such as the hydroformylation of hexene, octene and decene, and the hydrogenation of C14 aldol products were conducted with this system. A TTON of 8627 could be achieved in the hydroformylation of 1-octene with initial turnover frequencies up to 1460 h−1. In addition, the catalyst "pill" was switched between different reactions to show the flexibility of the system. [ABSTRACT FROM AUTHOR]

The selective hydrogenation of fluorinated arenes with polar functionalities is achieved, opening a versatile pathway for the production of valuable substituted fluorocyclohexane derivatives from readily available substrates. The use of supercritical carbon dioxide (scCO2) as a hydrophobic and green solvent in combination with Rh nanoparticles immobilized on molecularly modified silica as catalysts favours hydrogenation over hydrodefluorination. The approach compares favourably in terms of green chemistry metrics relative to traditional synthetic routes as exemplified for the useful building block 4-fluorocyclohexan-1-ol. [ABSTRACT FROM AUTHOR]

Carbon capture and utilization is considered a promising approach for introducing CO2 into the chemical value chain, especially in combination with bioenergy applications (BECCU). We report here on the catalytic hydrogenation of CO2 to formic acid in a biphasic reaction system using aqueous solutions of amino acids as the product phase and possible capture solutions for biogenic CO2. The molecular structure of the ruthenium catalyst and the catalyst phase were matched through a combined design process identifying n-dodecanol (lauryl alcohol) as the preferred "green" solvent. A total turnover number (TON) of over 100 000 mole HCOOH per mole of catalyst (46 582 g HCOOH per g of Ru) with minimal contamination of the aqueous phase with metal or organic solvent was obtained. The resulting aqueous solutions attained almost quantitative conversions with up to 0.94 mol formic acid per mol amino acid (ca. 108 g HCOOH per kg). Such solutions may find use directly, or after upgrading, in agricultural applications without the need for energy intensive and costly isolation of pure formic acid. [ABSTRACT FROM AUTHOR]

A versatile synthetic pathway for the production of valuable alkyl phenols and anilines has been developed based on the selective hydrodeoxygenation of a wide range of hydroxy-, amino-, and nitro-acetophenone derivatives as readily available substrates. Bimetallic iron ruthenium nanoparticles immobilized on an imidazolium-based supported ionic liquid phase (Fe25Ru75@SILP) act as highly active and selective catalysts for the deoxygenation of the side-chain without hydrogenation of the aromatic ring. The catalytic system allows operation under continuous flow conditions with high robustness and flexibility as demonstrated for the alternating conversion of 3′,5′-dimethoxy-4′-hydroxyacetophenone and 4′-hydroxynonanophenone as model substrates. [ABSTRACT FROM AUTHOR]

Observing and understanding the phenomena associated with the reaction mechanisms and catalyst deactivation in molecular catalysis is a very challenging task in green chemistry. This knowledge is crucial for applying and scaling catalyzed reactions as well as preventing misproduction at a very early point. Over the years, experimental arrangements have evolved towards analysis of catalysts and reaction products in the so-called operando setups. This contribution reflects on the potential of operando studies to elucidate reaction and deactivation mechanisms in homogeneous catalysis as well as the outstanding opportunities that arise from the application of operando experimental setups. Such setups mostly rely on spectroscopic analysis, optionally coupled with chromatographic techniques that monitor the reaction system. This in turn means that not only the evolution of the reaction substrates and products can be monitored, but also changes of the molecular catalyst species that may affect the catalytic performance. Therefore, this review focusses on techniques to monitor the catalyst under real conditions. In this review, different spectroscopic techniques relevant for monitoring molecular transition metal catalysts in solution are covered, followed by numerical methods used in the chemometrics literature to undertake the challenge of untangling the complex raw signals and allocating them to individual chemical species. Finally, two exemplary case studies of industrially relevant chemical reactions are presented, namely the hydroformylation and the asymmetric hydrogenation. These examples illustrate the significance of the application of both the experimental setups as well as data processing and signal resolution to have an insight into the deactivation of catalytic systems. The operando approach shows high potential for the increased use in future research to develop stable and more selective catalysts which can be applied in greener processes. [ABSTRACT FROM AUTHOR]

We present an integrated process for in situ CO2 hydrogenation during bioethanol production by combining bio- and chemocatalysis. The biphasic catalytic system used comprises an aqueous whole-cell fermentation broth and a tailored organometallic catalyst in the organic phase. Glucose is converted to ethanol, and the by-product CO2 is simultaneously upgraded to formic acid in a single reactor unit. Under the optimized conditions, 26% of the generated CO2 was hydrogenated directly. [ABSTRACT FROM AUTHOR]

Upon CO2 pressurization, 2-methyltetrahydrofuran (2-MTHF) expands. The expanded phase exerts different solvation properties, triggering fractional lignin precipitation. Depending on the actual CO2 pressure, fractions of lignin with a gradient of different sizes and chemical properties are selectively precipitated. A range of raw materials and applications may be thus reached. [ABSTRACT FROM AUTHOR]

This publication is reminiscent of the 12 principles of CO2 chemistry as formulated in the first Faraday Discussion on CO2 utilization in 2015. Their visionary significance at the time is brought into context with the current developments in society and industry. "What has changed since then?" and "is our enthusiasm still enough?" are only a few questions that are to be answered in the following from today's perspective. The synergy of the use of carbon dioxide (CCU) with the concepts of green chemistry as well as the connection to the energy sector is demonstrated using selected examples from industry and research. [ABSTRACT FROM AUTHOR]

Sheet silicates, also known as phyllosilicates, contain parallel sheets of tetrahedral silicate built up by [Si2O5]2− entities connected through intermediate metal–oxygen octahedral layers. The well-known minerals talc and pyrophyllite are belonging to this group based on magnesium and aluminium, respectively. Surprisingly, the ferric analogue rarely occurs in nature and is found in mixtures and conglomerates with other materials only. While partial incorporation of iron into pyrophyllites has been achieved, no synthetic protocol for purely iron-based pyrophyllite has been published yet. Here we report about the first artificial synthesis of ferripyrophyllite under exceptional mild conditions. A similar ultrathin two-dimensional (2D) nanosheet morphology is obtained as in talc or pyrophyllite but with iron(III) as a central metal. The high surface material exhibits a remarkably high thermostability. It shows some catalytic activity in ammonia synthesis and can serve as catalyst support material for noble metal nanoparticles. [ABSTRACT FROM AUTHOR]

Tandem reactions are of great importance to efficiently execute multiple conversions in one synthesis step. Herein we present a multiphase system for the hydroformylation/aldol condensation, which is able to recycle both optimized catalysts multiple times. The system consists of an organometallic rhodium/sulfoXantphos hydroformylation catalyst and basic NaOH as aldol condensation initiator, which are both immobilized in a polyethylene glycol phase. Under reaction conditions, NaOH is converted to sodium formate, which is still able to catalyse the aldol condensation. The reaction and recycling are demonstrated by the conversion of 1-pentene to the corresponding aldol product in a recycling experiment. During nine consecutive runs, no significant loss of activity is found with an overall TON of 8700 in regard to the rhodium catalyst and an average rhodium leaching of only of 0.07% per run is observed. [ABSTRACT FROM AUTHOR]

The oxidative carbonylation of toluene-2,4-diamine (TDA) with methyl formate (MF), which can be produced from CO2, provides a possible route for the non-phosgene production of isocyanate precursors and enables a valuable utilization of the greenhouse gas. Extensive analysis of the product spectrum has provided detailed insight into the reaction network leading to the target product toluene-2,4-dicarbamate (TDC) and the most important side products. The most prominent one has been identified as methylene-bridged tetracarbamate 5, which is also an interesting precursor for applications in polyurethane chemistry. The side products are caused by three different reaction paths: N-formylation by MF, condensation with in situ formed formaldehyde, and N-methylation by in situ formed dimethyl carbonate (DMC). The influence of the catalyst on product distribution was evaluated for PdCl2/CuCl2 and a large number of heterogeneous Pd-catalysts. The oxidic support materials ZrO2, CeO2 and SiO2 were found to partially suppress the undesired side reactions leading to higher yields of TDC and tetracarbamate 5. The ratio of TDC to 5 was demonstrated to be affected significantly by the choice of the support. The synthetic protocol was extended to the synthesis of dicarbamates from 4,4′-methylenedianiline (MDA) and 2,4-diaminomesitylene (17). These results encourage further investigations into the design of selective catalysts for the production of isocyanate precursors from CO2 as a C1 source. [ABSTRACT FROM AUTHOR]

The selective hydrodeoxygenation of hydroxyacetophenone derivatives is achieved opening a versatile pathway for the production of valuable substituted ethylphenols from readily available substrates. Bimetallic iron ruthenium nanoparticles immobilized on an imidazolium-based supported ionic liquid phase (Fe25Ru75@SILP) show high activity and stability for a broad range of substrates without acidic co-catalysts. [ABSTRACT FROM AUTHOR]

The valorization of sugars from lignocellulosic biomass has attracted much interest for the production of chemicals and fuels. From the dehydration of glucose or fructose and xylose, 5-hydroxymethylfurfural (5-HMF) and furfural can be obtained, respectively, which are highly praised chemicals used as building blocks. To increase the productivity of the process avoiding undesired side reactions that furans may undergo in the reaction phase, many authors follow a liquid–liquid approach. This way, an in situ extraction of the dehydration products occurs from the reaction phase (usually aqueous) to an organic solvent phase. Solvent selection is a matter of interest in Green Chemistry; therefore, careful consideration must be given to select the most appropriate alternatives in terms of performance, environmental, health and safety (EHS) impacts and subsequent downstream processing. For performance, the COnductor-like Screening MOdel for Real Solvents (COSMO-RS) has emerged as a tool to screen among different candidates based on structural information of the molecules. For EHS considerations, different solvent guides assist in the assessment of the most favourable alternatives. This review provides a comprehensive survey of the solvents and operational conditions employed in the biphasic dehydration of sugars to 5-HMF and furfural, followed by an account of the selection guides and methods for the evaluation of solvents, including COSMO-RS. Finally, to contrast with the most commonly selected solvents, such as methyl isobutyl ketone, a rational screening is presented here for the biphasic production of furans based on COSMO-RS predictions and the assessment of the selection guides. Predictions and further validation of the distribution coefficients show that ethyl acetate and methyl propionate are promising solvents for the in situ extraction of 5-HMF and furfural from aqueous media whilst being ranked as recommended green solvents by most solvent selection guides available in literature. [ABSTRACT FROM AUTHOR]

An integrated two-step process for the production of methylformate (MF) from CO2, H2 and MeOH was developed. In the first step, the hydrogenation of CO2 to a formate-amine adduct is carried out in a biphasic system comprising n-decane as the catalyst phase and MeOH as the product phase. In the second step, the resulting methanol solution containing the formate-amine adduct is subjected to reactive distillation for esterification and isolation of methylformate. The selection of the amine played an important role for devising the overall process. Whereas in the hydrogenation step basic amines work best, medium to low basic amines are preferred in the esterification step. 1,2-Dimethyl-imidazole (1,2-DMI) was identified as an effective compromise for the integration of both steps. In the hydrogenation step, a bis(diphenylphosphino)methane ligand tailored with long alkyl chains ensured effective retention of the Ru-catalyst in the non-polar phase allowing straightforward reuse of the catalyst phase. In a semi-continuous set-up, repetitive hydrogenation (8 cycles) led to a total turnover number (TTON) of 38 000 at an average turnover frequency (TOF) of 1400 h−1 with a cumulative catalyst leaching of only 1.4 mol% for P and 2.0 mol% for Ru. Reactive distillation was demonstrated in a continuously operated rectification unit leading to the isolation of MF at the head of the column with a purity of 91.5%. [ABSTRACT FROM AUTHOR]

In this work, we present the first continuous flow process to produce the tailored biofuel 2-butyltetrahydrofuran from renewable resources. In a two-step approach lignocellulose-derived furfuralacetone is first hydrogenated and then deoxygenated over commercial catalysts to form the desired product. Both reactions were studied independently in batch conditions. The transition to a continuous flow system was done and various parameters were tested in the miniplant. Both reactions were performed in a two-reactor-concept approach to yield the desired 2-butyltetrahydrofuran in a high yield directly from furfuralacetone. [ABSTRACT FROM AUTHOR]

A series of iron(+ III) and manganese(+ II) complexes based on the dpaqR-ligand system (dpaq = 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamide) were investigated using cyclic voltammetry (CV) to elucidate how the electronic properties of the ligands influence the overpotential and catalytic current in the context of water oxidation catalysis. For the Fe-complexes an electron withdrawing NO2 or CF3 group attached to the 5-position of the quinoline unit increased the catalytic current, but only with a simultaneous increase of the overpotential. However, when a pyrene moiety was attached to the dipicolylamine unit of the ligand, the overpotential decreased with concomitant increase of the catalytic current. Although the manganese complexes showed no reversible formation of a molecular catalytically active species for water oxidation, the variations of the ligand scaffold affected largely the same trends in their electrochemical behavior. [ABSTRACT FROM AUTHOR]

A facile complexation of formaldehyde with the water-tolerant frustrated Lewis pair (FLP) B(C6F5)3/PtBu3 and its Al-analog under ambient conditions is reported. Unprecedented formaldehyde adducts 1, 2 and 4 have been identified and crystallographically characterized. [ABSTRACT FROM AUTHOR]

Despite the rapid progress in the field of biomass fractionation and lignin valorization, no industrial process for chemical utilization of lignin has yet been established. One major step in that direction has been made with the advent of biorefineries and new biomass fractionation methods that deliver a relatively clean lignin stream, allowing a more efficient recovery and utilization of this fraction. However, the transfer of lignin from the fractionation solvent to a different medium for subsequent valorization has been largely disregarded so far. In this work, we demonstrate the use of a green liquid/liquid-extraction to transfer lignin from the organic phase of the OrganoCat process into differently concentrated alkaline solutions for further utilization. We show that alkaline solutions of pH 13 and 14 are able to almost completely extract the OrganoCat lignin from the organic phase but that this extraction might be accompanied by changes in the molecular structure of lignin, here shown by a change in the apparent molecular weight distribution. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of oligomeric poly(acetalcarbonates) obtained with the use of formaldehyde as a sustainable and renewable C1-building block is reported. Our approach involves generating monomeric formaldehyde in situ by depolymerisation of paraformaldehyde and repolymerisation to oligomeric poly(acetalcarbonates) using a bifunctional catalyst combination comprising a Lewis acid and Cs2CO3. Analysis of the chemical structure of the products revealed the presence of oligomeric polyoxymethylene sequences together with carbonate units and oxyethylene units along the polymer chain. The latter units act as zip stoppers, and lead to significant stabilisation of the polymer chains. Terminal OH groups allow for rich follow-up chemistry and render as products interesting bifunctional polymer building blocks. [ABSTRACT FROM AUTHOR]

This paper introduces a set of 12 Principles, based on the acronym CO2 CHEMISTRY, which are intended to form a set of criteria for assessing the viability of different processes or reactions for using CO2 as a feedstock for making organic chemicals. The principles aim to highlight the synergy of Carbon Dioxide Utilisation (CDU) with the components of green and sustainable chemistry as well as briefly pointing out the connection to the energy sector. [ABSTRACT FROM AUTHOR]

The fractionation of lignocellulose in its three main components, hemicellulose, lignin and cellulose pulp can be achieved in a biphasic system comprising water and bio-based 2-methyltetrahydrofuran (2-MeTHF) as solvents and oxalic acid as catalyst at mild temperatures (up to 140 °C). This so-called OrganoCat concept relies on selective hemicellulose depolymerization to form an aqueous stream of the corresponding carbohydrates, whereas solid cellulose pulp remains suspended and the disentangled lignin is to a large extent extracted in situ with the organic phase. In the present paper, it is demonstrated that biomass loadings of 100 g L−1 can be efficiently fractionated within 3 h whereby the mild conditions assure that no significant amounts of by-products (e.g. furans) are formed. Removing the solid pulp by filtration allows to re-use the water and organic phase without product separation in repetitive batch mode. In this way, (at least) 400 g L−1 biomass can be processed in 4 cycles, leading to greatly improved biomass-to-catalyst and biomass-to-solvent ratios. Economic analysis of the process reveals that the improved biomass loading significantly reduces capital and energy costs in the solvent recycle, indicating the importance of process integration for potential implementation. The procedure was successfully scaled-up from the screening on bench scale to 3 L reactor. The feedstock flexibility was assessed for biomasses containing moderate-to-high hemicellulose content. [ABSTRACT FROM AUTHOR]

In this article, the author talks about the periodical ideal features of green chemistry research project such as technology for reducing environmental impact, celebration of silver anniversary and collaboration with Martyn Poliakoff as Chair.

A recent DFT study of the ruthenium pincer benzoate complex [Ru(PNP)(PhCOO)2] I (PNP = 2,6-bis- (diphenylphosphanyl)lutidine) in its meridional form has revealed mer-I to be a promising catalyst lead structure for the direct insertion of CO2 into the C-H bonds of arenes, such as benzene. After the successful synthesis of I, its solid state structure interestingly and unexpectedly showed the pincer ligand to adopt the facial rather than the meridional coordination mode. Recalculation of the catalytic cycle with fac-I including all relevant local minima and transition states revealed (a) fac-I to be significantly more stable (6.1 kcal mol-1) than mer-I, (b) that the energetic span (ES; i.e. the effective activation barrier) for the cycle with fac-I amounts to 38.8 kcal mol-1, while the cycle with mer-I has an ES of 25.5 kcal mol-1 only. These results are a hint that fac-I is catalytically inactive. Experimental testing of fac-I showed indeed no product formation, which is in full accordance with the computations. To reduce the spatial flexibility of the pincer ligand, its CH2 groups were replaced by O atoms. The resulting complex [Ru(PONOP)(PhCOO)2] II (PONOP = 2,6-bis(diphenylphosphinito)pyridine) was used for the calculation of the catalytic cycle in benzene as the solvent. Gratifyingly, the starting complex mer-II is more stable than fac-II by 1.9 kcal mol-1 in benzene as the solvent. Consequently, mer-II should be available experimentally. As with fac-I, also fac-II generates a catalytic cycle with a high ES (37.1 kcal mol-1), while mer-II generates a cycle with a significantly lower ES (27.2 kcal mol-1) indicating mer-II to be a potentially active catalyst. A possible explanation of the much lower ES in the case of the meridionally coordinated species is found in the stronger interaction of the substrate with the metal center in the arene-s-bond complex. As a result the issue that is created by the mer/fac isomerism can be resolved by creating spatially less flexible structures. [ABSTRACT FROM AUTHOR]

The author reflects on the 15 years of the journal related to green chemistry. The author states that the journal celebrated its 15th year by publishing web collection that covered topics important to green chemistry. The author notes that the journal has provided scientific basis for developing chemical technologies. The author mentions that the journal is attracting articles related to enzymatic and cellular processing.

The activation of CO2 is highlighted for two lead-structures of cobalt- and zinc-based homogeneous catalysts used in the copolymerization of epoxides and CO2. Following the activation of CO2 with in situ ATR-IR spectroscopy, a surprisingly facile insertion of CO2 into metal–phenoxide bonds was revealed. [ABSTRACT FROM AUTHOR]

Methyl formate was used as a green and efficient carbonylating agent in the synthesis of methyl phenyl carbonate from phenol. Methyl formate showed better performance compared to toxic CO gas and the ability to produce other useful carbonylated products, e.g., dimethyl carbonate and dimethyl oxalate. [ABSTRACT FROM AUTHOR]