Base metal catalysis of acceptorless alcohol dehydrogenation for hydrogen storage

点击翻译按钮获取中文摘要

Catalytic acceptorless alcohol dehydrogenation is an atom-economical approach for alcohol oxidation without the need for an oxidant. Reversible dehydrogenation/hydrogenation catalysis from this reaction provides a route to the use of organic molecules derived from biomass as liquid organic hydrogen carriers (LOHCs). Alcohols such as ethylene glycol, glycerol and the C4-C6 analogues erythritol, xylitol, and sorbitol are considered to be potentially useful biomass-derived feedstocks since they can be derived from agricultural or lumber resources, including waste streams and their gravimetric hydrogen storage capacities (Figure 1) meet targets set by the EU and the US Department of Energy. This chemistry has long been dominated by the platinum group metals (PGMs), with an elegant recent example being the report of a reversible liquid to liquid organic hydrogen carrier system using ethylene glycol and a ruthenium pincer complex. However, the low abundance of PGMs leads to high economic and environmental cost, and their high toxicity means that their removal from products often is required, producing significant waste streams. It is therefore essential that researchers look to other catalysts for industrial processes, with obvious candidates being base metals that exhibit low cost, high natural abundance, uniform global distribution and low toxicity. This project will investigate a range of low-coordinate and pincer complexes of the first-row transition metals in order to achieve the acceptorless dehydrogenation reactions, and, with appropriate candidates, investigate the possibility of undertaking the reverse reaction with addition of H2. The first- row transition metals, with their low metal-ligand bond strengths are excellent candidates to achieve alcohol dehydrogenation reactions, as net oxidation requires dihydrogen loss from the metal. Pincer ligands have been shown to promote excellent stability and reactivity in acceptorless alcohol dehydrogenation reactions with first-row transition metal complexes, and metal-ligand cooperativity that has been successful using PGM catalysts, which may also be investigated for the base metals as the project progresses. Our recent research has revealed that complexes featuring cheap, non-toxic and earth abundant base metals can catalyse a range of reactions, including dehydrogenation and hydroelementation reactions. In terms of the proposed alcohol substrates, we envisage challenges such as differing reactivity with substitution at the hydroxyl residues and the catalysis of the reaction with H2 (reverse reaction), allowing a closed cycle for LOHC technology. Judicious choice of catalyst (or pre-catalyst), including the metal and ligand, and reaction conditions will facilitate the acceptorless alcohol dehydrogenation reactions with efficiency and product selectivity. Determination of reaction mechanisms through spectroscopic, structural and kinetic investigations allows the optimisation of the reactions. It is envisaged that investigations will also allow improvements in the temperatures, solvents (including a range of biomass-derived solvents) and catalyst loadings required for hydrogen release, but also the selectivity (e.g. avoidance of any undesirable by-products). The student will benefit from training in chemical techniques (e.g. organometallic chemistry, spectroscopy, crystallography and kinetic investigations) and principles of sustainable chemistry.