Base metal catalysis of acceptorless alcohol dehydrogenation for hydrogen storage

无受体催化醇脱氢是一种不需要氧化剂的原子经济的醇氧化方法。来自该反应的可逆脱氢/氢化催化提供了使用衍生自生物质的有机分子作为液体有机氢载体（LOHC）的途径。乙二醇、甘油和C4-C6类似物木糖醇、木糖醇和山梨糖醇等醇类被认为是潜在有用的生物质衍生原料，因为它们可以衍生自农业或木材资源，包括废物流，并且它们的重量储氢能力（图1）符合欧盟和美国能源部设定的目标。这种化学长期以来一直由铂族金属（PGMs）主导，最近的一个优雅的例子是使用乙二醇和钌钳络合物的可逆液体到液体有机氢载体系统的报道。然而，PGMs的低丰度导致高的经济和环境成本，并且它们的高毒性意味着通常需要将它们从产品中去除，从而产生大量的废物流。因此，研究人员必须寻找工业过程的其他催化剂，明显的候选者是低成本、高自然丰度、全球分布均匀和低毒性的贱金属。本项目将研究一系列第一行过渡金属的低配位和钳形络合物，以实现无受体脱氢反应，并与适当的候选物一起研究通过加入H2进行逆反应的可能性。具有低金属-配体键强度的第一行过渡金属是实现醇脱氢反应的优异候选物，因为净氧化需要从金属损失二氢。钳形配体已被证明可以促进在无受体醇脱氢反应中与第一行过渡金属络合物的优异稳定性和反应性，以及使用PGM催化剂成功的金属-配体协同性，随着项目的进展，也可以对贱金属进行研究。我们最近的研究表明，廉价、无毒、地球资源丰富的贱金属配合物可以催化一系列反应，包括脱氢和氢元素化反应。就提出的醇底物而言，我们设想了挑战，例如羟基残基取代的反应性不同以及与H2反应的催化（逆反应），从而允许LOHC技术的封闭循环。催化剂（或预催化剂）（包括金属和配体）和反应条件的明智选择将促进具有效率和产物选择性的无受体醇脱氢反应。通过光谱、结构和动力学研究确定反应机理可以优化反应。设想研究还将允许改进氢气释放所需的温度、溶剂（包括一系列生物质衍生的溶剂）和催化剂负载，以及选择性（例如避免任何不期望的副产物）。学生将受益于化学技术（例如有机金属化学，光谱学，晶体学和动力学研究）和可持续化学原理的培训。

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.