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

Lead Research Organisation: Imperial College London
Department Name: Dept of Chemistry

Abstract

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'.

Planned Impact

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.

Publications

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Bennett EL (2019) A New Mode of Chemical Reactivity for Metal-Free Hydrogen Activation by Lewis Acidic Boranes. in Angewandte Chemie (International ed. in English)

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Cooper RT (2017) Hydrogen activation using a novel tribenzyltin Lewis acid. in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences

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Scott DJ (2017) Designing effective 'frustrated Lewis pair' hydrogenation catalysts. in Chemical Society reviews

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Turnell-Ritson RC (2018) Base-induced reversible H addition to a single Sn(ii) centre. in Chemical science

 
Description We have so far discovered that carbon dioxide can be reduced at significantly lower temperatures than expected (50 degrees Celsius) using a specific combination of tin-based catalysts. We did not anticipate that [R2Sn]2+ species were critically involved in the reduction of formate to methoxide, which necessitates further research beyond the lifetime of this award. Nevertheless, much exciting chemistry (developing water-tolerant hydrogenation catalysts) has been uncovered. This work is being refined in preparation for publication. Additionally, we have revealed a novel H2 activation pathway for divalent Sn (R2Sn - stannylenes) using frustrated Lewis pair methodology; this was published in the high-impact RSC journal Chemical Science in 2018. We have also shown that such chemistry can be utilised for catalytic hydrogenation (of organic compounds e.g. imines )- a long sought after goal for a Sn(II)/Sn(IV) oxidative-addition/reductive elimination cycle - this will be published imminently as soon as the activity has been improved via ligand modification within the Ar2Sn unit.
Exploitation Route We are currently in discussion with BASF about the use of these catalysts for other related chemical transformations that they have interest in. This will hopefully lead to further funding opportunities.
Sectors Chemicals,Energy,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

 
Description Royal Society Carbon Capture and Use Workshop - 25 January 2017. Consultation/advisory committee to guide policy on formulation of UK government white paper.
Geographic Reach National 
Policy Influence Type Participation in a advisory committee
 
Description FLP hydrogenation catalysts - probing the speciation in next generation cationic Lewis acid systems 
Organisation Hungarian Academy of Sciences (MTA)
Country Hungary 
Sector Academic/University 
PI Contribution We provided experimental information about our hydrogen-activating FLP systems based on R3Sn-OTf, which display complex behaviour in comparison with more widely studied BR3-based compounds. This is a synergistic and iterative collaboration - we receive initial computational data from the collaborator (Imre Papai, Budapest) and we test hypotheses, feeding back to the computational team.
Collaborator Contribution Detailed high-level computational assistance in studying complex Sn-based FLP systems for hydrogenation catalysis.
Impact Chemical Science publication 2018. More manuscripts in draft form.
Start Year 2017
 
Description Presentation at BASF 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Invited presentation to BASF employees in the catalysis division at company HQ in Ludwigshafen, Germany. The purpose was to stimulate future collaborative activities regarding Sn-based homogeneous catalysis in areas of mutual interest.
Year(s) Of Engagement Activity 2017