Defect-engineered metal-organic frameworks for carbon dioxide capture

Since the Industrial Revolution, mankind has started to heavily interfere with the natural carbon cycle by extracting and burning increasingly larger amounts of fossil fuels, which has led to release huge amounts of CO2 in the atmosphere at an unprecedented rate, causing climate change. In order to mitigate the effects of climate change, the recently established Paris Agreement sets the goal of limiting the rise in the average global temperature to 2 degrees by 2100. This will require keeping cumulative CO2 emissions from all anthropogenic sources since year 1860 to less than 840 gigatons of carbon. If global carbon emissions continue to grow as they have in the last decade, the 2 degrees carbon budget will be spent by year 2035. This dictates to look for alternative energy sources and sustainable processes to enable the transition to a low-carbon economy.

CO2 capture, storage and utilisation (CCSU) is regarded as one of the key technologies to reduce CO2 emissions while fossil fuels are progressively phased out. Adoption of this technology on a large scale depends on its efficiency and economic viability, demanding the constant development of new materials able to combine excellent performances with long-term stability and affordability. The ideal sorbent for CO2 capture (CC) should have high mass uptake capacity, be selective towards CO2 over other gases, be able to be regenerated with a low energy penalty and be stable over various working cycles. CC from large point sources, such as coal- or gas-fired power plants and industrial facilities, is the most attractive option. These sources are responsible for about half of the global emissions and they generate concentrated CO2 streams that are easier to treat, if compared with direct air CO2 capture.

This project aims at developing new solid sorbents for CC by exploiting defects in zirconium-based metal-organic frameworks (Zr-MOFs) to functionalise them with a wide range of amino groups. Zr-MOFs are a class of crystalline and highly porous materials constructed from the connection of hexanuclear zirconium oxide-hydroxide clusters and carboxylate linkers. They are attractive for their remarkable stability, especially in the presence of water, which makes them suitable for practical applications. The CO2 adsorption capacity of bare Zr-MOFs is moderate, if compared to that of other sorbents. Functionalisation of Zr-MOFs using organic linkers with pending amino groups or through grafting of ethanolamine to the metal clusters has been demonstrated to increase their affinity for CO2. However, these methods are rather limited in scope. Defects in Zr-MOFs are reactive sites and can be exploited to introduce functional groups that cannot be otherwise inserted in the porous structure. Functionalisation of defective Zr-MOFs with amino groups of different nature (aliphatic, aromatic, heterocyclic) will allow to investigate and evaluate the influence of a large set of parameters on their CC performances. The resulting defect-engineered MOFs will be a library of novel, stable and versatile solid sorbents with tuneable physical-chemical properties for application in CC.

Tata Steel will be part of this project as an industrial partner. This will provide an excellent case study for the proposed research, because the steelworks in Port Talbot are the largest industrial CO2 emitter in the UK and Tata Steel is committed to address this issue. The materials developed during this project will be tested in conditions relevant to CC from blast furnace gas. This gas is mainly composed of N2 (45-50%), CO (20-25%), CO2 (20-25%) and H2 (0-5%) and is normally flared, due to its low calorific value. Removal of CO2 would allow to recycle the CO-rich stream in the blast furnace for reduction of iron ore and to convert the captured CO2 into useful chemicals.