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Nanopores for New Molecular Nitrogen Chemistry

Periodic Reporting for period 2 - NANOCHEM (Nanopores for New Molecular Nitrogen Chemistry)

Reporting period: 2018-12-01 to 2020-05-31

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 (X. Han, S, Yang and M. Schröder, Nature Rev. Chem., 2019, 3, 108-118).

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, underdevelopment of economically-viable storage/transportation methods, and potential competition with the food supply chain.

The objectives of the research programme are to synthesise novel metal-organic framework (MOF) materials to address three key issues on the roadmap for the implementation of cleaner, low-carbon-emission energy resources. These include: (i) selective capture of toxic gases from automobile emissions; (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, in this study typically carboxylates. The project aims to develop new porous ultra-stable MOF materials for capture and conversion of NH3, NO2 and SO2.
Work has focussed on 5 Workpackages:
WP1.Design and synthesis of decorated porous MOFs with emphasis on their rapid and efficient scale-up, coupled to post-synthetic materials engineering;
• WP2.Characterisation of host and substrate-loaded materials by state-of-the-art in situ structural, dynamic and spectroscopic methods for the construction of structure-function relationships, supplemented by computational analysis and modelling;
• WP3.Adsorption binding, release and separation of targeted nitrogen-containing gas molecules (NH3, N2H4, N2O, NO, NO2 and N2O4) via both static and dynamic experiments;
• WP4.Testing for degradation/selective reduction of captured NOx with NH3, urea and hydrocarbons using nanoporous MOFs as hosts;
• WP5.Assembly of MOF-based (i) catalytic deNOx reactor and (ii) NH3 storage system for portable applications.

We have successfully prepared a range of new ultra-stable porous MOFs that are sufficiently stable and robust to reversibly capture toxic and fuel gases over many cycles. Of particular note is their ability to capture SO2, NO2 and NH3 over multiple cycles in the presence and absence of water. It should be emphasised that porous MOF materials are often unstable to exposure of water and especially to corrosive and caustic gases such as SO2, NO2 and NH3. The materials that we have prepared are unique and are the first examples of MOF materials to be stable with these substrates under both wet and dry conditions. We have also undertaken a range of state of the art diffraction, scattering and spectroscopic studies at National Facilities to characterise our materials and to determine precisely at a molecular level the mechanisms by which they operate. We have successfully achieved separation and purification of these gases by passing them through a column loaded with our porous materials using breakthrough experiments under both dry and wet conditions with flows simulating exhaust mixtures. We can also take the trapped NO2 and convert it in air to a nitric acid which can then be used as a commodity chemical.

Although our focus has been mainly on NOx, N2O4 and NH3 capture as detailed in the original proposal, we have also studied our materials with other fuel and emission gases, notably SO2 (Nature Chem., 2019, 11, 1085-1090; J. Am. Chem. Soc., 2018, 140, 15564-15567), CO2 (Chem. Sci., 2019, 10, 1472-1482), CH4 (J. Am. Chem. Soc., 2017, 139, 13349-13360), hydrogen (Inorg. Chem., 2018, 57, 12050-12055) and hydrocarbons (Chem. Sci., 2019, 10, 1098-1106; J. Am. Chem. Soc., 2018, 140, 16006-16009; Chem. Sci., 2018, 9, 3401-3408).

We have also initiated studies on catalysis using MOFs, and have found a new Cu(II) system that is effective for alcohol oxidation (Nature Comm., 2019, 10, 4466). A key feature of this has been a new route to the very rapid synthesis of the target materials in minutes using electrosynthesis templated by ionic liquid molecules. This generates a highly porous materials containing structural defects and mesopores that enhance catalysis.
The research has progressed as planned and has thus far delivered the following results that are beyond the previous state of the art:

(a) The first MOF materials that are stable to NO2, SO2 and NH3 under wet and dry conditions over multiple cycles (up to 50 cycles) have been designed, prepared and characterised.
(b) Successful breakthrough experiments to remove NO2 and SO2 from flow mixtures that simulate flue exhaust gases have been achieved.
(c) Trapped NO2 within MFM-520 has been converted to HNO3 (nitric acid), a useful commodity chemical that can used in further chemical processes.
(d) State of the art characterisation of how our materials trap these gases has been undertaken using X-ray and neutron scattering and diffraction and advanced in situ spectroscopy at National Facilities (Diamond Light Source, ISIS, Advanced Light Source in Berkeley, ORNL in Oak Ridge, USA). Key experiments confirm a range of supramolecular contacts between the gas substrate (NO2, SO2 and NH3) and the host material via interaction with C-H and O-H bonds of the framework, thus explaining why these materials work so well.
(e) Observation that NO2 can be stabilised and trapped within a MOF via dimerization to N2O4.

The project will continue to develop these and other new materials for capture and conversion of NO2, SO2 CO2 and NH3. A particular goal for the future is to use not only our porous materials as stores and traps for these substrates, but also to convert these substrates catalytically to environmentally benign and/or chemically useful products. Thus, can we convert NO2 or NH3 to N2, CO2 to other C1, C2 and C3 products catalytically for further chemical use? This work is now underway.