Metal-organic frameworks for photocatalytic CO2 reduction under visible radiation: A review of strategies and applications

Metal-organic frameworks (MOFs) have emerged over the last decades as an interesting class of materials for a plethora of applications due to their flexible tunability in composition, structure and functional properties. In particular, the urgent need to convert CO2 into useful chemical substances have led to growing attention towards these catalysts as promising candidates for CO2 capture and storage with the ultimate aim of promoting sustainable pathways to address energy and environmental problems. This review provides an overview of the fundamental factors and recent studies of MOFs for CO2 photoreduction, pointing out the main strategies adopted in the design and testing of innovative MOFs-based catalysts, where MOFs serve either directly as the only photoactive materials for CO2 transformation into chemical fuels or as units of hybrid catalytic arrangements to enhance CO2 conversion. The functionalization of MOFs is a promising approach to improve their photocatalytic activity. Their absorption of visible light can be enhanced either by modification of MOFs structure or by adjusting the organic ligands to contain reactive functional groups, through the inclusion of metal nodes with higher reduction potential, or even through a combination of both strategies. MOFs can also be used as co-catalysts along with other photocatalytic entities, resulting in MOF-based composites with enhanced CO2 photoreduction. The performance of MOFs-based photocatalysts is analyzed in this review, with a view to emphasizing not only the advantages but also the limitations, over the more traditional semiconductors photocatalysts. At the current stage, the main challenges of these MOFs-based photocatalysts relate to the economic feasibility of industrial-scale processes and to the poor stability of these materials, which is often lost after some runs. It is hoped that this review will help the design of increasingly efficient MOF-based materials for CO2 reduction and create the needed awareness that more work is still needed in order to be able to actualize and explore the potentials of MOFs in addressing the key issues of environmental sustainability.