Softer Frustrated Lewis Pair Catalysis for Harder Substrates: Stannyl Cations for the Hydrogenation of Carbon Dioxide to Methanol and Methyl Formate

By lowering the energy barrier required for reactions to proceed, catalysts enable chemical transformations to be conducted at faster rates and with greater energy efficiency than would otherwise be possible. As such, around 90% of all processes in chemical manufacturing rely upon catalysis for effective production. Catalytic hydrogenations (the reaction of compounds with hydrogen, H2) are routinely employed in all areas of chemical production, yet the catalysts are predominantly based on precious metals (e.g. Rh, Ru, Pd, Pt) which are both expensive and of limited supply; there is therefore a strong motivation to develop new catalysts which do not incorporate such elements.

In the last decade a new and exciting chemical methodology using catalysts based on inexpensive and abundant main group elements has been discovered. Known as 'frustrated Lewis pairs' (FLPs), these consist of a Lewis acid and base which (for steric and/or electronic reasons) cannot interact strongly with one another, leading to unquenched reactivity that can be exploited for the reaction with small molecules, most notably H2. When H2 reacts with FLPs it is converted into a much more reactive ionic form (protic H+ and hydridic H-) which can subsequently be delivered to substrates, hence effecting catalytic hydrogenation. To date, FLP hydrogenation catalysts almost exclusively use boron at the Lewis acidic centre, which is not optimal for the reduction of compounds containing oxygen, since the products (alcohols, water: hard bases) bind too strongly to the hard Lewis acid, which has a potent inhibitory effect on the overall rate of reaction.

This proposal aims to develop new FLP-hydrogenation catalyst protocols using stannyl cations (based on [R3Sn]+ fragments), explicitly for the catalytic conversion of CO2 (carbon dioxide, a greenhouse gas) and H2 to two important commodity platform chemicals: methanol (CH3OH) and methyl formate (HCO2CH3). These can be used as feedstocks for upgrading to added-value products, or as liquid fuels. In the latter case, assuming the H2 is obtained by renewable means (e.g. photo-splitting of water using solar energy) these would represent sustainable sources of storable energy, with the potential to impact positively on the global carbon balance.

This transformative approach stems from extremely encouraging initial results which show that a Bu3SnH/catalytic [Bu3Sn]+ (Bu = C4H9) system is competent for the reduction of CO2 under mild conditions, thereafter reaction with H2 liberates CH3OH, HCO2CH3 and water, in addition to the regeneration of Bu3SnH. Taken together, these results demonstrate that all stages of a catalytic cycle for CO2 hydrogenation can be achieved. Notably, the [Bu3Sn]+ catalysts are thermally stable to water, demonstrating a considerable advantage over boron-based FLPs. Currently the rate of reaction with H2 is too slow to be comparable with CO2 conversion; this will be addressed by increasing the bulk of the stannyl cations, and hence increasing their reactivity via augmented 'frustration'.

The ultimate target of this proposal is to develop new Lewis acids based on tin (Sn) for use as catalysts for the hydrogenation of carbon dioxide into the useful products methanol (CH3OH) and methyl formate (HCO2CH3), which capitalises from our highly promising preliminary results. In recognition of the importance of catalysis to the UK, this research area has been earmarked for growth; indeed, the research activities to be undertaken in this proposal align exactly within the definition of catalysis by the EPSRC ('Structural and kinetic studies to understand the molecular mechanisms involved in catalytic reactions, preparation of novel or improved catalysts and the development of new catalytic processes'), as can be seen in the Programme and Methodology section of the Case for Support. Furthermore, the aim to chemically sequester CO2 into platform feedstocks that can be used either as energy-dense liquid fuels or upgraded to added-value chemicals, fits perfectly within the EPSRC priority themes Energy and Manufacturing the Future, in addition to responding to the EPSRC grand challenge 'Utilising Carbon Dioxide (CO2) in Synthesis and Transforming the Chemicals Industry'.

Currently, limited technologies exist for the conversion of CO2 into useful products, especially for the efficient transformation with hydrogen (H2) to liquid, energy-dense fuels. In conjunction with the use of renewably sourced H2 in our proposed research goal, success in our endeavour will maintain the UK as a leader in the discovery and implementation of sustainable manufacturing technologies, with the attendant ecological and environmental benefits afforded by switching from fossil fuel derived C1 sources (e.g. carbon monoxide) to CO2. Furthermore, these renewable fuels and feedstock chemicals avoid the security-of-supply issues which can affect petroleum-based commodities.

More generally, catalytic hydrogenations are highly prevalent throughout the chemical manufacturing sector and are currently performed using rare and precious transition metals (e.g. Rh, Ru, Pd, Pt); these are difficult to source and are vulnerable to supply chain issues. Because our goal is to utilise Sn as the platform element in our hydrogenation catalyst system, which is both inexpensive and abundant, advances resulting from this research proposal would lead to cost savings for myriad related industrial hydrogenations (e.g. aldehydes/ketones to alcohols, amides to amines). This will likely lower the barrier to wide-scale adoption in the chemical sector, leading to more efficient processes (financially and energetically), which will ultimately filter down to benefit the consumer through lower prices, driven by market force competitiveness. Since the chemical industry contributes > 20 % of UK GDP, we firmly believe that the uptake of even one new industrial process using heavier p-block elements for catalytic hydrogenation over the next 20 years, as a direct consequence of the research activities of this project, could clearly provide a substantial economic impact.

The impact of our research outcomes will also be keenly felt in the scientific community; the knowledge that heavier p-block elements can participate in FLP chemistry, in particular the activation of H2, is highly likely to lead to a new era of exploration for the FLP community, who have thus far almost exclusively focused on catalysts comprised solely of the 2nd and 3rd row elements. This will uncover new small molecule transformations and expand the knowledge base of this fascinating and timely area of research.

In order to help guarantee the skill-set for future generations of catalysis researchers, this proposal will supply a highly trained researcher for UK industry or academia. They will be equipped with essential advanced skills in synthesis and catalysis of particular relevance to many areas in chemical manufacturing, where the UK has a significant presence.