CO2MOF network

The last two decades have seen the explosive growth of a new class of materials. Variously known as metal-organic frameworks (MOFs) or porous coordination-polymers (PCPs), these microporous crystalline solids are composed of organic bridging ligands coordinated to metal-based units to form a three-dimensional extended network with uniform pore diameters typically in the range 3 to 50 Å. The intense current research effort towards industrial applications of MOFs in gas storage, separation and catalysis is attributed to their unique structural properties, including: robustness, high thermal and chemical stabilities, unprecedented internal surface areas (up to 10000 m²/g), high void volumes (up to 90%) and low densities (down to 0.21 g/cm³). The regular monodisperse nature of the crystalline array of micropores is a key feature that distinguishes these systems from other porous materials (for example polymers, mesoporous silicas and carbons, etc.). In addition, the ability to modulate systematically the pore dimensions and surface chemistry within MOFs, providing enormous structural and chemical diversity, is a feature that was previously largely absent in zeolite materials.

The high surface area-to-weight ratio of MOFs is such that they have enhanced capacities for CO₂ capture at moderate pressures compared with zeolites. While zeolites possess higher storage capacities at pressures of less than 10 bar, it has been projected that their maximum capacities are limited to one third those of MOFs at pressures greater than 10 bar. The capacities of metal–organic frameworks up to high pressures scale with the amount of active area per unit weight: activated carbon has an active area of 400–1000 m²/g, zeolites of up to 1500 m²/g, and frameworks of 1500–4500 m²/g. The peak gravimetric adsorption for CO₂ has been reported in frameworks with high surface areas and pore diameters of greater than 15 Å.

MOFs offer an unprecedented opportunity to target new materials with exceptional efficiencies for CO₂ capture. Over the past decade a largely scattergun approach to materials discovery in this area has led to a broad array of highly porous materials displaying very impressive CO₂ selectivities and capacities, with such properties in most cases requiring careful moderation of conditions (i.e., to prevent thermal decomposition and/or chemical decomposition with exposure for example to water vapour) but in a number of noteworthy cases occurring under industrially relevant conditions.

In contrast to the exploration of selective CO₂ adsorption, the possibility of integrating multiple functionalities such as catalytic capabilities into MOFs that would enable the conversion of CO₂ into viable commodities has been almost entirely unexplored. This is despite a wealth of literature spanning several decades on molecular approaches to CO₂ conversion, and despite numerous chemical and technological advantages that are inherent in the use of solid-based approach to CO₂ transformation. The opportunity therefore exists to couple the exciting CO₂ utilisation found in molecular species with the molecular sieving selectivity, high capacity and improved exposure of reactive metal sites found in MOFs. For example, a molecular mercaptopyridyl copper species was recently reported as exhibiting rapid conversion of CO₂ to oxalate, an important component in the deposition of complex metal oxide thin films for use in computer memory and solar cell electrodes. Incorporation of a similar reactive molecular unit into a MOF topology could deliver massive increases in the turnover rate of CO₂ to oxalate, and very likely deliver this conversion from industrially relevant gas mixtures.

Carbon (dioxide) capture and storage (CCS) schemes embody a group of technologies for the capture of CO₂ from large point sources such as power plants and natural gas wells, followed by compression, transport and permanent storage. Indeed, conventional technologies for large scale CO₂ capture are already commercially available and are focused on postcombustion separation of CO₂ from flue gases of power plants by the use of amine absorbers (‘scrubbers’) and cryogenic coolers. However, it is estimated that CO₂ capture using conventional scrubbing increases the energy requirements of a plant by 25-35% owing to the dilute concentrations of CO₂ (typically 10-15%) that are present at low pressures in the flue gas stream (at 1 atmosphere).

Clearly, the existing methods of capture are energy intensive and are not cost-effective for carbon emissions reduction. Indeed, one explanation for the slow deployment of fully-integrated commercial CCS schemes is the considerable cost of the capture phase, which represents approximately two thirds of the total cost for CCS. These considerations underscore the immense opportunities and incentives that exist for improved CO₂ capture processes and materials.

A diverse range of promising methods and materials have been proposed as alternatives to conventional chemical absorption. These include the use of physical absorbents, membranes, cryogenic distillation, hydrate formation, chemical-looping combustion and adsorption on solids using pressure and/or temperature swing adsorption processes. One key challenge for gas separations materials is that the differences in properties between the gases that have to be separated are relatively small. Novel concepts for capture therefore require a molecular level of control that can take advantage of differences in the chemical reactivity of the gas molecules.

More detailed background information about CCS is published by the CRC for Greenhouse Gas Technologies.

t is possible that the carbon dioxide created by burning fossil fuels could be ‘recycled’ into fuels or other useful chemical commodities. There is a wealth of literature, spanning several decades, on conversion of CO₂ using transition metal complex catalysts. While the catalytic reduction of CO₂ is thermodynamically challenging, these reactions have been demonstrated on the laboratory scale.

However, there has been little progress in the development of solid-based catalysts for CO₂ conversion, despite numerous chemical and technological advantages that are inherent in this approach. Furthermore, the possibility of integrating these catalytic capabilities into MOFs has been almost entirely unexplored. The opportunity therefore exists to couple the exciting CO₂ utilisation found in molecular species with the molecular sieving selectivity, high capacity and improved exposure of reactive metal sites found in MOFs.

Key technical challenges include the creation of frameworks that are stable at moderately elevated temperatures and pressures in the absence of solvent, and which are catalytically active for CO₂ conversion in both gas/solid and liquid phase reactions.