Periodic Reporting for period 1 - CuZnSyn (Understanding Copper–Zinc Synergy for Carbon Dioxide Hydrogenation)
Reporting period: 2020-10-01 to 2022-09-30
Carbon dioxide (CO2) is a greenhouse gas that is significantly contributing to climate change. In tandem with advances in sequestering carbon, beneficial uses for CO2 are of high societal importance for developing a sustainable future. One attractive use of CO2 is its conversion to energy dense fuels (green energy vectors). One such fuel is methanol, made from CO2 via hydrogenation in conjunction with a multimetallic catalyst. The current best industrial (heterogeneous) catalyst incorporates copper and zinc-oxide nanoparticles with an alumina support. A special synergy is observed between the copper (active site) and zinc (reaction promoter), but these species and their connection is poorly defined and remains debated.
This project aims to isolate proximal copper and zinc centres, the fundamental building block for the construction of critical copper--zinc interfaces, within a well-defined, and highly tuneable ligand framework. Once isolated, the binding, activation and interconversion of key intermediates along the CO2-to-methanol hydrogenation pathway will be analysed.
The climate crisis is perhaps that biggest societal issue of our generation. Alarming trends were identified at the First World Climate Conference in 1979 in Geneva, and scientists from 50 nations agreed that urgent action was required to curb climate change. Since then, similar alarms have been made through the 1992 Rio Summit, the 1997 Kyoto Protocol, and the 2015 Paris Agreement. Yet greenhouse gas emissions continue to rise rapidly, with increasingly damaging effects on the Earth’s climate. In 2018 the Intergovernmental Panel on Climate Change (IPCC) stated “an immense increase of scale in endeavours to conserve out biosphere is needed to avoid untold suffering due to the climate crisis”. Despite over 40 years of global climate negotiations, with few exceptions, we have generally conducted business as usual and have largely failed to address this predicament.
The emission of CO2, one of the three most abundant greenhouse gases (along with methane and nitrous oxide) continues to rise, including an ominous spike in 2019. This, combined with other contributing factors, has led to increased surface temperatures, the disappearance of global ice, and increased ocean heat content, acidity and sea levels. Climate change is predicted to greatly affect marine, freshwater and terrestrial life, from plankton and corals to fishes and forests. Sequestering CO2 out of the atmosphere in useful forms is vitally important for the survival of life on Earth.
According to an IPCC special report, one of the most attractive uses of CO2 as a C1 feedstock is in the production of liquid fuels such as methanol. However, this process requires energy, and if this comes from fossil-fuel based electricity, there is no net reduction as the total CO2 generated during fuel synthesis tends to exceed the CO2 converted. The development of more efficient and selective catalysts for the conversion of CO2 to methanol will have far-reaching global benefits.
The overall objectives of the action were to use homogeneous organometallic molecules to model the reactions at the surface of the heterogeneous Cu/ZnO/Al2O3 catalyst. This was done through the design and investigation of new compounds that contain Cu and Zn in close proximity. These new Cu--Zn species were studied in reactions with key intermediates in CO2-to-methanol catalysis pathway.
In summary, the objectives of the proposal are as follows:
• Development of a series of ligands and associated Cu--Zn complexes, and their detailed characterisation that must include an understanding of the Cu--Zn interaction (work package 1).
• Build a detailed mechanistic picture of the potential steps in CO2 hydrogenation and an understanding of the role Cu--Zn cooperativity plays via stoichiometric reactivity and computational modelling (work package 2).
• As extension work, use the available Cu--Zn species and test for catalytic activity for direct CO2 hydrogenation with H2 to methanol (work package 3).
This project aims to isolate proximal copper and zinc centres, the fundamental building block for the construction of critical copper--zinc interfaces, within a well-defined, and highly tuneable ligand framework. Once isolated, the binding, activation and interconversion of key intermediates along the CO2-to-methanol hydrogenation pathway will be analysed.
The climate crisis is perhaps that biggest societal issue of our generation. Alarming trends were identified at the First World Climate Conference in 1979 in Geneva, and scientists from 50 nations agreed that urgent action was required to curb climate change. Since then, similar alarms have been made through the 1992 Rio Summit, the 1997 Kyoto Protocol, and the 2015 Paris Agreement. Yet greenhouse gas emissions continue to rise rapidly, with increasingly damaging effects on the Earth’s climate. In 2018 the Intergovernmental Panel on Climate Change (IPCC) stated “an immense increase of scale in endeavours to conserve out biosphere is needed to avoid untold suffering due to the climate crisis”. Despite over 40 years of global climate negotiations, with few exceptions, we have generally conducted business as usual and have largely failed to address this predicament.
The emission of CO2, one of the three most abundant greenhouse gases (along with methane and nitrous oxide) continues to rise, including an ominous spike in 2019. This, combined with other contributing factors, has led to increased surface temperatures, the disappearance of global ice, and increased ocean heat content, acidity and sea levels. Climate change is predicted to greatly affect marine, freshwater and terrestrial life, from plankton and corals to fishes and forests. Sequestering CO2 out of the atmosphere in useful forms is vitally important for the survival of life on Earth.
According to an IPCC special report, one of the most attractive uses of CO2 as a C1 feedstock is in the production of liquid fuels such as methanol. However, this process requires energy, and if this comes from fossil-fuel based electricity, there is no net reduction as the total CO2 generated during fuel synthesis tends to exceed the CO2 converted. The development of more efficient and selective catalysts for the conversion of CO2 to methanol will have far-reaching global benefits.
The overall objectives of the action were to use homogeneous organometallic molecules to model the reactions at the surface of the heterogeneous Cu/ZnO/Al2O3 catalyst. This was done through the design and investigation of new compounds that contain Cu and Zn in close proximity. These new Cu--Zn species were studied in reactions with key intermediates in CO2-to-methanol catalysis pathway.
In summary, the objectives of the proposal are as follows:
• Development of a series of ligands and associated Cu--Zn complexes, and their detailed characterisation that must include an understanding of the Cu--Zn interaction (work package 1).
• Build a detailed mechanistic picture of the potential steps in CO2 hydrogenation and an understanding of the role Cu--Zn cooperativity plays via stoichiometric reactivity and computational modelling (work package 2).
• As extension work, use the available Cu--Zn species and test for catalytic activity for direct CO2 hydrogenation with H2 to methanol (work package 3).
Work package 1. The synthesis of a series of ligands that are constructed from a central linker group and two terminating groups was undertaken. Six of the proposed combinations were synthesised but only one could be isolated in sufficient purity and in high enough yields to be of synthetic use for subsequent studies. This ligand was used to synthesis a variety of Cu--Zn complexes, which were isolated and characterised using a suite of techniques including single crystal X-ray diffraction and computational bonding analysis. Reduction of copper from Cu(I) to Cu(0) was attempted using chemical reductants, but no stable complex could be isolated.
Work package 2. A complex featuring proximal Cu--Zn centres and incorporating bridging hydride ligands between consecutive metal centres was successfully synthesised, isolated and characterised. X-ray diffraction studies showed a short intermetallic distance, that approached the sum of the covalent radii of the metals, suggesting metal--metal interactions may be operable. Computational bonding analysis suggests any metal--metal interaction will be primarily electrostatic, as no formal covalency could be found between the metals. The stoichiometric reduction of CO2 using a heterometallic Cu--Zn hydride complex was successfully achieved, yielding formate (CO2H–) ligands that remained bound to the metal centres. This represents the first step in the pathway of CO2-to-methanol hydrogenation.
Work package 3. Initial test to use a Cu--Zn hydride complex as a putative catalyst for the hydrogenation of CO2 were conducted, but unfortunately only stoichiometric reactivity was observed. The goals described in this work package were extension goals, any catalytic turnover would have been a remarkable discovery.
Exploitation and dissemination. This work will primarily be disseminated through publication of two high-impact papers. The first being a structural analysis of a variety of complexes featuring proximal copper and zinc centres, while the second will be a detailed mechanistic paper that describes the observed stoichiometric reactivity, as well as in depth bonding descriptions. Exploitation of the results are on-going, through future grant and lectureship applications, as well as translation through existing networks at Imperial College and the host group.
Work package 2. A complex featuring proximal Cu--Zn centres and incorporating bridging hydride ligands between consecutive metal centres was successfully synthesised, isolated and characterised. X-ray diffraction studies showed a short intermetallic distance, that approached the sum of the covalent radii of the metals, suggesting metal--metal interactions may be operable. Computational bonding analysis suggests any metal--metal interaction will be primarily electrostatic, as no formal covalency could be found between the metals. The stoichiometric reduction of CO2 using a heterometallic Cu--Zn hydride complex was successfully achieved, yielding formate (CO2H–) ligands that remained bound to the metal centres. This represents the first step in the pathway of CO2-to-methanol hydrogenation.
Work package 3. Initial test to use a Cu--Zn hydride complex as a putative catalyst for the hydrogenation of CO2 were conducted, but unfortunately only stoichiometric reactivity was observed. The goals described in this work package were extension goals, any catalytic turnover would have been a remarkable discovery.
Exploitation and dissemination. This work will primarily be disseminated through publication of two high-impact papers. The first being a structural analysis of a variety of complexes featuring proximal copper and zinc centres, while the second will be a detailed mechanistic paper that describes the observed stoichiometric reactivity, as well as in depth bonding descriptions. Exploitation of the results are on-going, through future grant and lectureship applications, as well as translation through existing networks at Imperial College and the host group.
There are a limited number of reported complexes that contain proximal copper and zinc centres. None of these have been studied for the reduction of CO2. The results from this fellowship represent the first example of Cu(I) and Zn(II) atoms being combined and precisely orientated within one ligand framework, as well as their use in the reduction of CO2 to formate. Good progress has been made to building a putative model for the poorly defined and highly debated mechanism by which CO2 is hydrogenated to methanol on the surface of the Cu/ZnO/Al2O3 industrial catalyst. The socio-economic impact and societal implications of developing better catalysts for the production of methanol from CO2 cannot be overstated. The urgent need to find useful ways to valorise CO2 is vital for developing a sustainable future. Methanol represents a prime candidate for future green energy, and any research into its sustainable synthesis, while reducing its effects on anthropogenic climate change, could be generation-defining.