Carbon capture Using Metal–Organic Frameworks

There is an urgent need to reduce emissions of the greenhouse gas CO2 to decrease the threat and consequences of global warming. Carbon capture, the separation and concentration of CO2 from gas streams produced during power generation using fossil fuels and also coming from industrial sources and methane-rich natural gas and biogas, can play an important part in the medium term. Furthermore, the direct capture of CO2 from air can find an increasing role. Absorption by amine solutions is in current use for many of these gas streams, but there is a pressing need for more environmentally friendly techniques with lower energy costs. The two major candidate technologies for these applications are separation and concentration using columns of solid adsorbents and membrane technologies. Nanoporous and chemically versatile metal–organic frameworks (MOFs) can find important application in each, once issues of cost and stability are addressed. Here, we discuss the use of MOFs as adsorbents for CO2 at atmospheric pressure and at levels varying from 400 ppm (in air) to 3–15% (flue gases from fossil fuel-fired power plants) and 1–40% (natural gas, biogas) or at elevated pressures (and partial pressures >1 bar) in precombustion or hydrogen generation gas streams. More than a decade of research into MOFs for selective CO2 uptake has yielded a myriad of adsorbents, but relatively few satisfy all the requirements of capacity, selectivity, stability and performance in moist gases. Nevertheless, several highly promising candidates have emerged that can operate at a few percent of CO2 or even down to levels in the air. These include ‘ultramicroporous’ MOFs with reversible physisorption on sites ideally matched to the quadrupole of CO2 that have working capacities of ca. 3 mmol g−1 and biomimetic amine-appended Mg carboxylate MOFs which reversibly chemisorb similarly high working capacities at low pCO2 and possess stepped isotherms. There have also been significant developments in MOF-containing membranes for CO2 separation, where CO2 is either the permeating component of gases containing mainly N2 or CH4 or the non-permeating component of gases including H2 as the permeant. Here, the role of the MOF is as the filler in polymer membranes, where its role is to enhance permeability while maintaining or enhancing the selectivity and longevity of the membranes. While research remains strongly empirical, due at least in part to difficulties in predicting the microstructure of the MOF–polymer interface, the ‘Robeson upper bound’ that represents the trade-off between permeability and selectivity in polymers has been exceeded in many laboratory experiments. The outstanding challenge for MOF-based mixed matrix membranes is to transfer this promise into real-world applications.