Source: IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change
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. This has led to release huge amounts of CO2 in the atmosphere at an unprecedented rate, causing climate change. The recently established Paris Agreement sets the goal of limiting the rise in the average global temperature to 2 degrees by 2100. If global carbon emissions continue to grow as they have in the last decade, it is projected that the 2 degrees carbon budget will be spent by 2035.
Carbon capture, storage and utilisation is regarded as one of the key technologies to reduce CO2 emissions. 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. Large point sources, such as coal- or gas-fired power plants and industrial facilities, are responsible for about half of the global CO2 emissions and generate concentrated CO2 streams, Carbon capture from these sources is easier than from thin air and can greatly contribute to reach the target set by the Paris Agreement.
Metal-organic frameworks (MOFs) are crystalline and highly porous coordination polymers built from the connection of metal ions or clusters and organic linkers. They offer virtually unlimited possibilities of customisation of their crystal structure and physical-chemical properties. This is an extremely attractive feature for application in several fields, including gas sorption/separation. MOFs display high CO2 adsorption capacity and selectivity and have attracted interest as potential sorbents for carbon capture. However, most of them suffer from stability issues that have so far hindered their application in industrial contexts.
Zirconium-based MOFs (Zr-MOFs) are a subclass of MOFs known for their remarkable stability, especially in the presence of water. It was recently discovered that Zr-MOFs can contain large amounts of structural defects without suffering from significant loss of stability and that defects are reactive sites towards exchange of terminal groups. The goal is to take advantage of the dynamic nature of defects in Zr-MOFs to introduce functional groups of various natures, allowing to explore a chemical variety that is non accessible through classical functionalisation routes that involve modification of the organic linkers.
Most MOFs are based on carboxylates as organic linkers, whereas use of phosphonate linkers for construction of MOFs is not yet well established, due to synthetic and crystallographic challenges. However, the excellent robustness of metal phosphonates represents a significant advantage, if compared to conventional carboxylate-based MOFs, for practical applications and this fact alone makes metal phosphonates an attractive class of materials. The primary goal is to develop new phosphonate-based MOFs using linkers with high rigidity and non-linear shape. More recently, interest is also directed at "rediscovering" known metal phosphonates as sorbents for CO2 capture.
MOFs with highly polar fluorine atoms exposed in the pores are known to display enhanced affinity for CO2, thanks to favourable interactions between the Lewis basic fluorines and the Lewis acidic carbon in CO2. Using perfluorinated analogues of common organic linkers is likely to produce perfluorinated versions of known frameworks and to obtain better sorbents for CO2 capture. In addition, perfluorinated MOFs can display high hydrophobicity and maintain their CO2 capture performance also in humid conditions. The recent discovery of a perfluorinated Ce-based MOF with MIL-140A topology that displays excellent CO2 capture properties kickstarted our research in this area, which is currently expanding in several directions.