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

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).