Heteroelements in polyoxometalates: a study on the influence of different group 15 elements on polyoxometalate formation.

In the field of polyoxometalate (POM) chemistry, different heteroelements are integrated into the cage-like structures, to obtain different structural types of so-called heteropolyanions (HPAs). While it is generally accepted, that some elements favor certain types of structure, a systematic study is still missing. In this article, we present a systematic investigation of the influence of the group 15 elements nitrogen, phosphorous, arsenic, and antimony on the formation of different POM structure types. Our study is comprised of DFT calculations and corresponding experimental structural analysis. In this context, the DFT study establishes the thermodynamics of formation of different coordination geometries with various heteroelements on two POM structure types, the Keggin and the Anderson-Evans structures. Our POM synthesis experiments were performed at two different pH values (1 and 5) and resulted in a variety of heteropolytungstates, which were identified and characterized by elemental analysis as well as single crystal X-ray diffraction and vibrational spectroscopy. With these methods, we were able to establish a clear trend, showing that heavier elements lead to formation of different structure types than lighter elements. These results signify a large step towards a better understanding of POM formation specifically with respect to the choice of heteroelement.

Effect of Conversion, Temperature and Feed Ratio on In2 O3 /In(OH)3 Phase Transitions in Methanol Synthesis Catalysts: A Combined Experimental and Computational Study.

Catalytic hydrogenation of CO2 to methanol has attracted lots of attention as it makes CO2 useable as a sustainable carbon source. This study combines theoretical calculations based on the dummy catalytic cycle model with experimental studies on the performance and degradation of indium-based model catalysts for methanol synthesis. In detail, the reversibility of phase transitions in the In2 O3 /In(OH)3 system under industrial methanol synthesis conditions are investigated depending on conversion, temperature and feed ratio. The dummy catalytic cycle model predicts a peculiar degradation behavior of In(OH)3 at 275 °C depending on the water formed either by methanol synthesis or the competing reverse water-gas-shift reaction. These results were validated by dedicated experimental studies confirming the predicted trends. Moreover, X-ray diffraction and thermogravimetric analysis proved the ensuing phase transition between the indium species. Finally, the validated model is used to predict how hydrogen drop out will affect the stability of the catalyst and derive practical strategies to prevent irreversible catalyst degradation.

H8 [PV5 Mo7 O40 ] - A Unique Polyoxometalate for Acid and RedOx Catalysis: Synthesis, Characterization, and Modern Applications in Green Chemical Processes.

Polyoxometalates (POMs) are a fascinating group of anionic metal-oxide clusters with a broad variety of structural properties and several catalytic applications, especially in the conversion of bio-derived platform chemicals. H8 [PV5 Mo7 O40 ] (HPA-5) is a unique POM catalyst that ideally links numerous fascinating research fields for the following reasons: a) HPA-5 can be synthesized by rational design approaches; b) HPA-5 can be well characterized using multiple analytical tools explaining its catalytic properties; and c) HPA-5 is suitable for multiple important catalytic transformations of bio-based feedstock. This Review combines the fields of synthesis, spectroscopic, electrochemical, and crystallographic characterization of HPA-5 with those of sustainable catalysis and green chemistry. Selected catalytic applications include esterification, dehydration, and delignification of biomass as well as selective oxidation and fractionation of bio-based feedstock. The unique HPA-5 is a fascinating POM that has a broad application scope for biomass valorization.

Investigation of the Formation, Characterization, and Oxidative Catalytic Valorization of Humins.

The industrial use of biomass, e.g., for the production of platform chemicals such as levulinic acid, became increasingly important in recent years. However, the efficiency of these processes was reduced by the formation of insoluble solid waste products called humins. Herein, the formation of humins from various carbohydrates was investigated under different process conditions, in order to obtain information about the structure and the formation mechanism. During this process, new potential structural fragments of humins were identified. Subsequently, the produced humins were oxidatively converted to low-molecular-weight carboxylic acids with the use of polyoxometalate catalysts. The experiments showed that the use of sugars in acetic acid and ethanol only lead to the formation of a small amount of humins, which were also structurally most suitable for conversion to carboxylic acids. The main products of the oxidative valorisation of these humins were acetic acid, formic acid, and CO2, respectively, and our results indicate that certain functional groups were converted preferentially. These findings will help to improve processes for the valorisation of biomass by enabling an overall more efficient use of thermo-sensitive feedstock such as carbohydrates.

Revealing the nitrogen reaction pathway for the catalytic oxidative denitrification of fuels.

Aside from the desulfurisation, the denitrogenation of fuels is of great importance to minimze the environmental impact of transport emissions. The oxidative reaction pathway of organic nitrogen in the catalytic oxidative denitrogenation could be successfully elucidated. This is the first time such a pathway could be traced in detail in non-microbial systems. It was found that the organic nitrogen is first oxidized to nitrate, which is subsequently reduced to molecular nitrogen via nitrous oxide. Hereby, the organic substrate serves as a reducing agent. The discovery of this pathway is an important milestone for the further development of fuel denitrogenation technologies.

Spectroscopic, Crystallographic, and Electrochemical Study of Different Manganese(II)-Substituted Keggin-Type Phosphomolybdates.

Adjusting the RedOx activity of polyoxometalate catalysts is a key challenge for the catalysis of selective oxidation reactions. For this purpose, the possibility of influencing the RedOx potential by the introduction of an additional RedOx-active element was investigated. Thereby, Keggin-type polyoxometalates (POMs) with up to three different elements in the metal framework were created. An advanced and reproducible synthetic procedure to incorporate MnII and additionally VV into Keggin-type heteropolyacids alongside comprehensive characterization of the new molecules is presented. The success of our syntheses was confirmed by vibrational spectroscopy (IR and Raman) and elemental analysis. Furthermore, the new compounds were analyzed by NMR spectroscopy to investigate the characteristics of the POMs in solution. The structures of successfully crystalized compounds were determined by single-crystal X-ray diffraction. Moreover, all synthesized compounds were characterized using UV/Vis spectroscopy and electrochemical analysis to get further insights into the electronic transfer processes and redox potentials.

Switchable Catalytic Polyoxometalate-Based Systems for Biomass Conversion to Carboxylic Acids.

We present the Keggin-type polyoxometalate H6[PV3Mo9O40] as a switchable catalyst being able to catalyze the transformation of both glucose and glyceraldehyde to formic acid (42%) and lactic acid (40%), respectively, within 1 h reaction time by simply changing the reaction atmosphere at 160 °C from oxygen to nitrogen in one reactor setup. In detail, we report the influence of different gas atmospheres and reaction temperatures on various vanadium-containing catalysts in the selective transformation of several biogenic substrates to carboxylic acids with a special emphasis on reaction pathways and switchability of the catalyst systems. All investigations were carried out in parallel using either an oxygen or a nitrogen atmosphere of 20 bar performing time-resolved experiments between 0.25 and 5 h and a temperature variation from 160 to 200 °C. Furthermore, a catalyst and a substrate variation led to the reaction system consisting of glyceraldehyde and the Keggin-type polyoxometalates (POM) H6[PV3Mo9O40] as the best switchable reaction system under the applied conditions. This study shows interesting potential for using both Keggin-type and Lindqvist-type POMs as switchable catalysts for selective biomass conversion to platform chemicals.

Combining Cost-Efficient Cellulose and Short-Chain Carboxylic Acid Production: The Polyoxometalate (POM)-Ionosolv Concept.

Full cost-effective exploitation of all wood components is key to growing a commercially successful biorefining industry. An innovative process is reported that combines fractionation of lignocellulosic biomass using a low-cost ionic liquid (Ionosolv) and production of bio-derived formic acid using polyoxometalates and molecular oxygen (OxFA process). We show that the hemicellulose and part of the lignin were selectively dissolved into the ionic liquid triethylammonium hydrogen sulfate and oxidised in situ to short-chain, distillable carboxylic acids by a Keggin-type polyoxometalate with high yields and selectivities. Characterization by several techniques, including ICP-OES, FTIR, GC, HPLC and NMR spectroscopy confirmed stability of the catalyst over three consecutive POM-Ionosolv recycles and stable formic acid yields.High formic acid yields of 26 % (pine chips), 23 % (beech chips), and 18 % (Miscanthus) were obtained with respect to the initial carbon content of the biomass, with unprecedented oxidation selectivities for formic acid of 54-62 % depending on vanadium substitution in the polyoxometalate, the processing temperature and the water content in the reaction mixture.. We also demonstrate that the cellulose rich pulp is a suitable source of glucose via enzymatic saccharification. We report cellulose yields of 37% for Miscanthus (from originally 48% glucan content), 33% for pine (from originally 49%) and 31% for beech (from originally 41%) were achieved, and a saccharification yield of up to 25% without optimisation. With further optimisation, this concept has the potential to generate two chemical products directly from lignocellulose in high yields and selectivities and hence a novel avenue for full utilisation of cellulose, hemicellulose and lignin.

Catalytic Low-Temperature Dehydration of Fructose to 5-Hydroxymethylfurfural Using Acidic Deep Eutectic Solvents and Polyoxometalate Catalysts.

HMF synthesis typically requires high temperature and is carried out in aqueous solutions. In this work, the low-temperature dehydration of fructose to HMF in different deep eutectic solvents (DES) was investigated. We found a very active and selective reaction system consisting of the DES tetraethyl ammonium chloride as hydrogen bond acceptor (HBA) and levulinic acid as hydrogen bond donor (HBD) in a molar ratio of 1:2 leading to a maximum HMF yield of 68% after 120 h at 323 K. The DES still contained a low amount of water at the initial reaction, and water was also produced during the reaction. Considering the DES properties, neither the molar ratio in the DES nor the reaction temperature had a significant influence on the overall performance of the reaction system. However, the nature of the HBA as well as the acidity of the HBD play an important role for the maximum achievable HMF yield. This was validated by measured yields in a DES with different combinations of HBD (levulinic acid and lactic acid) and HBA (choline chloride and tetra-n-alkyl ammonium chlorides). Moreover, addition of vanadium containing catalysts, especially the polyoxometalate HPA-5 (H8PV5Mo7O40) leads to drastically increased reaction kinetics. Using HPA-5 and the DES tetraethyl ammonium chloride-levulinic acid we could reach a maximum HMF yield of 57% after only 5 h reaction time without decreasing the very high product selectivity.

Ru-Doped Wells-Dawson Polyoxometalate as Efficient Catalyst for Glycerol Hydrogenolysis to Propanediols.

A Ru-doped phospho-tungstic Wells-Dawson polyoxometalate (POM) was successfully applied as homogeneous catalyst for glycerol hydrogenolysis in aqueous media. The synthesized compound showed superior catalytic activity compared to classical homogeneous/heterogeneous Ru catalysts like RuCl3 and Ru/C under identical reaction conditions, whereas the analogous POM doped with Pd or Pt proved far less activity. Detailed characterization of the POMs was performed using 31P-NMR to identify characteristic phosphorous peaks of the heteroatoms, infrared spectroscopy (ATR-FTIR) to confirm characteristic P-O and W-O-W vibrations, powder XRD for comparison of crystal structures, and X-ray fluorescence (XRF) and inductive-coupled plasma (ICP) analysis to determine elemental composition. Variation of the reaction parameters for the best performing Ru-doped POM catalyst showed that substrate concentration played an important role for both product selectivity and conversion. Moreover, medium hydrogen pressure and high stirring speed were key factors to obtain highly selective conversion of glycerol to 1,2-propanediol.

Combining autoclave and LCWM reactor studies to shed light on the kinetics of glucose oxidation catalyzed by doped molybdenum-based heteropoly acids.

In this work we combined kinetic studies for aqueous-phase glucose oxidation in a high-pressure autoclave setup with catalyst reoxidation studies in a liquid-core waveguide membrane reactor. Hereby, we investigated the influence of Nb- and Ta-doping on Mo-based Keggin-polyoxometalates for both reaction steps independently. Most importantly, we could demonstrate a significant increase of glucose oxidation kinetics by Ta- and especially Nb-doping by factors of 1.1 and 1.5 compared to the classical HPA-Mo. Moreover, activation energies for the substrate oxidation step could be significantly reduced from around 80 kJ mol-1 for the classical HPA-Mo to 61 kJ mol-1 for the Ta- and 55 kJ mol-1 for the Nb-doped species, respectively. Regarding catalyst reoxidation kinetics, the doping did not show significant differences between the different catalysts.

Highly Selective Synthesis of Acrylic Acid from Lactide in the Liquid Phase.

A new reaction system for the highly selective, hydrobromic acid catalyzed conversion of lactide into acrylic acid under mild conditions is reported. The applied liquid reaction system consists of a temperature-stable bromide-containing ionic liquid and 2-bromopropionic acid as a source of dry HBr, with no volatile organic solvent being used. This allows for the in situ removal of the formed acrylic acid, leading to an unmatched acrylic acid selectivity of over 72 % at full lactide conversion. Accounting for leftover reaction intermediates on the way to acrylic acid, which could be recycled in an elaborate continuous process, the proposed reaction system shows potential for acrylic acid yields well above 85 % in the liquid phase. This opens new avenues for the effective conversion of biogenic lactic acid (e.g., obtained by fermentation from starch) to acrylic acid. The resulting bio-acrylic acid is a highly attractive product for, for example, the diaper industry, where we expect consumers to be especially sensitive to aspects of sustainability.

Zwitterionic Hydrobromic Acid Carriers for the Synthesis of 2-Bromopropionic Acid from Lactide.

A convenient and highly efficient way of synthesizing 2-bromopropionic acid (2-BrPA) from lactide is presented. The procedure uses ionic liquids obtained from the addition of HBr to ammonium-based zwitterions as the solvent and bromination agent. The buffered HBr acidity, high polarity, and charge stabilizing character of the ionic liquid (IL) enable the synthesis of 2-BrPA with excellent selectivity. The best results are obtained with an imidazolium-based IL, that is, 1-(4-butanesulfonic acid)-3-methylimidazolium bromide ([MIMBS]Br). The HBr loading and water content of the IL are crucial parameters for the bromination reaction. The formed 2-BrPA product can be selectively isolated by extraction from the IL, and the unconverted substrate remains in the [MIMBS]Br IL for the next run. Successful recycling of the IL over four cycles is demonstrated.

Selective oxidation of lignocellulosic biomass to formic acid and high-grade cellulose using tailor-made polyoxometalate catalysts.

The main goal of this project was to identify and optimize tailor-made polyoxometalate catalysts for a fractionated oxidation of lignocellulosic biomass (i.e. wood and residues from sugar or paper industries) to produce formic acid (FA) and high-grade cellulose for further processing e.g. in white biotechnology to provide bio-ethanol. Homogeneous vanadium precursors like sodium metavanadate and vanadyl sulfate as well as Keggin-type polyoxometalates (POMs) and more exotic structures like Anderson-, Wells-Dawson- and Lindqvist-type POMs were screened for the desired catalytic performance. The most promising behaviour was found using the Lindqvist-type POM K5V3W3O19, showing for the first time in the literature a selective oxidation of only hemicellulose and lignin to formic acid, while the cellulose fraction was untrapped. However, this can only be a first step towards the project goal as low product yields were obtained.

Measuring and Predicting the Extraction Behavior of Biogenic Formic Acid in Biphasic Aqueous/Organic Reaction Mixtures.

The distribution coefficients and selectivities required for extraction purposes were predicted with a thermodynamic equation of state for the ternary system formic acid/water/extraction solvent. These predictions were validated with experimental data from the literature and experimental data from the oxidation of biomass to formic acid process measured in this work. Extraction solvents discussed in this work are 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-decanol, ethyl n-butyl ether, diisopropyl ether, di-n-butyl ether, benzyl formate, and heptyl formate. The considered temperature ranged from 273 to 363 K under atmospheric pressure. Perturbed-chain statistical associating fluid theory (PC-SAFT) was used for prediction purposes applying an approach as simple as possible and as complex as necessary to achieve trustworthy data for selecting the best extraction solvent. Using PC-SAFT allowed identifying 1-hexanol as the most promising solvent out of the 11 extraction agents. The predicted data were in good agreement with the experimental distribution coefficients and the selectivities, which are very sensitive to experimental uncertainties.

The effect of Community Based Education and Service (COBES) on medical graduates' choice of specialty and willingness to work in rural communities in Ghana.

BACKGROUND: Career choices and placements of healthcare professionals in rural areas are a major problem worldwide, and their recruitment and retention to these areas have become a challenge to the health sector. The purpose of this study was to investigate the effect of Community Based Education and Service (COBES) on medical graduates' choice of specialty and willingness to work in a rural area. METHOD: This cross sectional survey was conducted among 56 pioneering graduates that followed a Problem Based Learning/Community Based Education and Service (PBL/COBES) curriculum. Using a mixed methods approach, open-and closed-ended questionnaire was administered to 56 graduates. Cross tabulation using Chi-square test were used to compare findings of the quantitative data. All qualitative data analysis was performed using the principles of primary, secondary and tertiary coding. RESULTS: All 56 graduates answered and returned the questionnaire giving a 100 % response rate. 57.1 % (32) of them were male. Majority of them lived in towns (41.1 %) and cities (50 %) prior to medical school. A significant number of graduates (53.6 %,) from the cities, without any female or male predominance said COBES had influenced their choice of specialty. Again, a significant proportion of graduates from the towns (60.9 %,) and cities (67.8 %,), indicated that COBES had influenced them to work in the rural area. However, there was no significant difference between males and females from the towns and cities regarding the influence of COBES to work in the rural area. Qualitative data supported the finding that COBES will influence graduates willingness to work in the rural area CONCLUSION: The majority of graduates from the towns and cities in Ghana, with a male predominance, indicated that COBES may have influenced their choice of specialty and willingness to practice in the rural areas despite their town or city based upbringing.