Zintl-Ions as Molecular Analogues of Liquid Alloys for C-H Activation

甲烷是天然气的主要成分，并且可以大量获得。尽管天然资源丰富，但除了其简单的热值之外，由于其惰性、低能量密度和高昂的运输成本，甲烷作为原材料开发不足。目前最先进的气转液技术是甲烷增值的一种解决方案，但这首先需要转化为合成气（CO和H2的混合物），这是一个能源和经济成本高的过程。因此，大量过量的天然气被炼油公司燃烧。例如，2012年，全球3.5%的甲烷供应被燃烧，产生了3.5亿吨二氧化碳。这相当于当年二氧化碳排放总量的10%。因此，可持续地利用过剩的天然气将对环境产生重大的积极影响，减少不必要的二氧化碳排放。最近，麦克法兰等人报道了甲烷直接转化为氢气和石墨的令人兴奋的例子。在这项工作中，熔融（~1000 ℃）的主族过渡金属（MGTM）合金被用作催化剂。Ni：Bi MGTM合金（27%Ni：73%Bi）是最成功的。据认为，Bi稳定的“裸”Ni（0）反应中心与烷烃反应。类似的液相Pd：Ga MGTM合金也被报道用于催化丁烷脱氢。尽管有这种重大的技术兴趣，在撰写本概述时，使用MGTM合金的明确定义的分子类似物，二元Zintl离子的C-H活化化学是未知的。Zintl离子是二元合金的分子类似物，目前已知五种“Ni：Bi”Zintl离子。基于碱金属主族元素组合的已知Zintl相和离子，例如K5 Bi 4、[K-crypt 222]2[Bi 2]、[K-crypt 222]2[Bi 4]，将提供阴离子主族离子的可溶源。这种阴离子可以与过渡金属试剂反应，以提供新的定义明确的MGTM合金。Zintl等人对液氨中的碱金属和主族元素进行了电位滴定，报告了Na：Bi相Na 3Bi、Na 3Bi 3和Na 3Bi 5。Ni：Bi簇的拟议路线受到这些反应的启发，并且可以在低得多的温度下形成前体。在这些合成和结构研究的同时，MGTM合金中C-H活化的基本模式将在溶液和分子固态中进行研究。催化过程（热，受体，或光化学驱动）将被探测，特别是那些密切相关的烷烃的直接转化，并与MGTM合金的类比。这些明确定义的体系也将在较高温度（即熔融催化剂）烷烃活化中用作直接前体。这个DTG DPhil项目提出了四个主要目标：1）合成各种新颖的定义明确的二元Zintl离子，具有可变的主族/过渡金属组成，最初选择以反映已知的成功的液体合金催化剂。Goicoechea小组是合成方面的专家。2)探索基础研究，详细说明Zintl离子和工业相关碳氢化合物中的C-H键之间的相互作用：甲烷，乙烷，丙烷和丁烷。Weller集团在这一化学领域处于世界领先地位。3)通过利用不同的方法，包括Weller小组开发的方法，在具有挑战性的C-H键活化化学中开发此类系统。4)在阿姆斯特丹壳牌技术中心为期3个月的实习中，将重点放在甲烷和丁烷脱氢方面。壳牌对利用这些碳氢化合物的新方法感兴趣。该项目福尔斯EPSRC“能源”研究领域，并与EPSRC的优先领域（催化，清洁化石能源，能源储存，氢和燃料电池，新型化学合成）保持一致。

Methane is the principle component of natural gas and available in vast quantities. Despite this natural abundance, outside of its simple calorific value, methane is underexploited as a raw material due to its inertness, low-energy density and significant transportation costs. The current state of the art gas to liquid technologies are one solution for methane's valorisation, but this first requires conversion to syn gas (a mixture of CO and H2), an energy and financially costly process. Therefore, a significant amount of excess natural gas is flared by oil refinement companies. In 2012, for example, 3.5% of the world's methane supply was flared, producing 350 million tonnes of CO2. This amounted to 10% of the total annual CO2 emissions that year. Therefore, providing a sustainable use for excess natural gas will impose a significant positive impact on the environment, reducing unnecessary CO2 emissions. Exciting recent examples of the direct conversion of methane to dihydrogen and graphite were reported by McFarland et al. In this work, molten (~1000 degrees C) main-group-transition metal (MGTM) alloys were employed as catalysts. The Ni:Bi MGTM alloy (27% Ni : 73% Bi), was most successful. It is thought that the Bi stabilised "naked" Ni(0) reactive centres react with the alkane. A similar liquid-phase Pd:Ga MGTM alloy has also been reported for catalytic butane dehydrogenation. Despite this significant technological interest, at the time of writing this summary, C-H activation chemistry employing well-defined molecular analogues of MGTM alloys, the binary Zintl ions, is unknown. Zintl ions are molecular analogues of binary alloys and five "Ni:Bi" Zintl ions are currently known. Known Zintl phases and ions based on alkali metal main-group element combinations, such as K5Bi4, [K-crypt222]2[Bi2], [K-crypt222]2[Bi4], will provide a soluble source of anionic main-group ions. Such anions can be reacted with transition-metal reagents to afford novel well-defined MGTM alloys. Zintl et al. performed potentiometric titrations of alkali metals and main group elements in liquid ammonia, reporting the Na:Bi phases Na3Bi, Na3Bi3 and Na3Bi5. A proposed route to Ni:Bi clusters is inspired by these reactions and may allow precursor formation at much lower temperatures. In parallel with these synthetic and structural studies, the fundamental modes of C-H activation at MGTM-alloys will be investigated in both solution and the molecular solid-state. Catalytic processes (thermal, acceptor, or photochemically driven) will be probed, especially those closely related to direct conversion of alkanes, and analogies made with MGTM-alloys. These well-defined systems will also be used as direct pre-cursors in higher-temperature (i.e. molten catalyst) alkane activation. This DTG DPhil project proposes four main objectives: 1) Synthesis of a variety of novel well defined binary Zintl ions with variable main group/transition metal composition, initially chosen to reflect the known successful liquid alloy catalysts. The Goicoechea group are experts in their synthesis. 2) Exploration of fundamental studies detailing the interaction between Zintl ions and C-H bonds featured in industrially relevant hydrocarbons: methane, ethane, propane and butane. The Weller group are world leading in this chemistry. 3) Development of such systems in challenging C-H bond activation chemistry, by taking advantage of different methodologies, including those developed by the Weller group. 4) Deployment in catalysis, focussing on methane and butane dehydrogenation as part of a 3-month placement at the Shell Technology Centre in Amsterdam. Shell are interested in novel methodologies for utilizing these hydrocarbons. This project falls within the EPSRC "Energy" research area and aligns well with EPSRC priority areas (Catalysis, Clean Fossil Energy, Energy Storage, Hydrogen & Fuel Cells, Novel Chemical Synthesis).