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Nanopores for New Molecular Nitrogen Chemistry

Periodic Reporting for period 4 - NANOCHEM (Nanopores for New Molecular Nitrogen Chemistry)

Reporting period: 2021-12-01 to 2022-11-30

Increased consumption of fossil and bio-fuels leads to excessive emissions of CO2, SOx, NOx, toxins and heavy metals, and is a major cause of environmental pollution, poor air quality and global warming. The implementation of increasing numbers of diesel engines and widespread use of bio diesel in transport reduces carbon emissions in operation, but significantly increases the emission of NOx. This is particularly the case in developing countries with heavy and ageing industry and high population densities, where the poor air quality is responsible for over 1 million premature deaths as stated in reports from the World Health Organisation and International Energy Agency. Capture and elimination of toxic gases are, therefore, important targets for a sustainable society.

NH3 is a major chemical commodity as a fertilizer for food production, and is widely regarded a cleaner energy resource owing to its potential zero-carbon-emission at the point of use via conversion to N2 and H2 or within a fuel cell. In comparison to H2, other major advantages of using NH3 as an energy carrier include high volumetric energy density, facile liquefaction for transport, and high octane number. Indeed, it has been proposed that NH3 can act as an excellent H2 store for use in the hydrogen economy. NH3, urea and hydrocarbons can be used to reduce and degrade NOx from combustion engines via the selective catalytic reduction (SCR) based upon metal oxide catalysts, preferably to form N2. However, practical and domestic applications of NH3 are restricted by its corrosive nature, the underdevelopment of economically-viable storage/transportation methods, and potential competition with the food supply chain.

The objectives of the research programme were to synthesise novel metal-organic framework (MOF) materials to address for:
(i) selective capture of toxic gases such as SO2, NO2 and related species
(ii) development of reversible high capacity NH3 stores for portable applications;
(iii) selective catalytic reduction (SCR) of captured NOx within confined nanopores.

MOFs are crystalline porous materials that comprise of metal or metal cluster nodes bridged by polydentate ligands, typically polycarboxylates. The project aimed to develop new porous ultra-stable MOF materials for capture and conversion of toxic and corrosive gases and to generate real understanding of the fundamental chemistry underpinning how these advanced materials operate. It should be emphasised that at the start of this project there were no MOF materials reported that were sufficiently stable or robust to tackle the storage or capture of these highly toxic and corrosive chemicals over multiple cycles, let alone to apply them to any catalytic conversion of NH3 or NO2.
The project proceeded to plan and excellent progress was made on all objectives, most notably in the design, synthesis and characterisation of a range of MOF materials that are sufficiently stable and robust to reversibly capture and separate SO2, NO2 and NH3 over multiple cycles in the presence and absence of water. The materials that we have prepared are unique and are the first examples of MOF materials to show such stability with these substrates under both wet and dry conditions. We can take the trapped NO2 and convert it in air to nitric acid, or capture the NO2 impurities and use them for the nitration of organic substrates. This opens up routes to utilising toxic emissions to produce new chemicals of value. Catalytic reduction of NO2 has been achieved using atomically-dispersed single-site metal doped MOFs to afford N2 at high conversion and efficiency.

We have applied our new materials to a range of related applications including CO2 sorption and reduction, and have developed materials showing mesopores, active defects and/or dispersed single sites within their structures for catalysis. We can also apply these materials to organic oxidation chemistry and the selective oxidation of hydrocarbons and for the catalytic formation of C-C bonds.

Our new porous MOF materials can be applied to the other important separations such as of xylenes isomers, propylene vs ethylene, and of other light hydrocarbons. We can control the chemistry of the pores of our materials to generate systems that can efficiently capture and separate benzene from cyclohexane. The separation of these hydrocarbons is economically and environmentally very important, but highly challenging to achieve. Current technologies are highly energy inefficient and expensive.

Methane (CH4) is highly abundant but is a major greenhouse gas. Its conversion to methanol (CH3OH) and other carbon C-H and Cx values represents an important step in sustainable use of fossil fuel resources. We have achieved the photocatalytic conversion of CH4 to CH3OH using a multi-component MOF material. The material incorporates an [Fe-OH] site for binding and activation of CH4 to CH3• radical under ambient conditions, thus mimicking monoxygenase enzyme activity.

Throughout the project the use of National Facilities in the UK (Diamond Light Source, ISIS Neutron and Muon Centre), USA (Advanced Light Source, Berkeley, Oak Ridge National Laboratories) and France (ESRF and ILL) to study in situ the binding of substrates within these porous have been vitally important. By using advanced diffraction, scattering and spectroscopy we have determined and visualised the precise locations of substrates within the pores of these materials and identified the precise host to guest supramolecular contacts. Thus by appropriate pore design and functionality we can enhance interactions and binding of the selected substrate by cooperative effects within the channels.
The research has delivered the following results that are beyond the previous state of the art:
• the first MOF materials that are stable to NO2, SO2 and NH3 under wet and dry conditions over multiple cycles (up to 100 cycles) and for up to 4 years under static conditions;
• successful breakthrough experiments to remove NO2 and SO2 from flow mixtures that simulate flue exhaust gases;
• conversion of trapped NO2 to useful commodity chemicals thus converting toxic impurities to chemicals of value;
• reduction of CO2 and NO2 to carbon values and to N2, respectively;
• new separations of hydrocarbon substrates;
• a unique multi-functional MOF for the catalytic photoreduction of CH4 to CH3OH at high efficiency;
• state of the art characterisation to elucidate how these materials interact with substrates thus giving a deep understanding of how these advanced materials function at the molecular level.
We are currently under active negotiation with a number of companies regarding the commercialisation, spin out and licencing for our new technologies.
NO2/N2O4 captured within a porous zinc material
SO2 captured within a stable porous copper material