Hierarchically porous monolithic MOFs: An ongoing challenge for industrial-scale effluent treatment

Metal-Organic Frameworks (MOFs) have attracted a lot of attention over the past two decades mainly because their unique set of properties, notably their high porosity and surface area, are useful for gas storage and separation, but these materials are also well suited to the decontamination of liquid effluents. They are synthesized as crystalline solid powders but, for a broad applicability, producing MOFs as handleable materials is crucial. Furthermore, the main challenge for their use in fix bed processes consists in creating and controlling a meso- and macroporous network in the body of the material while ensuring the MOF's micropores remain accessible. Two techniques have recently been proposed, mechanical shaping and sol-gel synthesis, with the main difficulty in both cases being to retain the properties of the MOFs in the final product, because of the pressure (for mechanical shaping) or the additives (for sol-gel synthesis) required to pack the powder. The focus of recent developments has therefore been on using mild synthesis conditions (sol-gel processes, in situ synthesis, templating). Although relatively few studies have been published to date, the field is expanding fast in terms of the techniques proposed and the understanding of the stability of MOFs and their performance as hierarchically porous materials rather than powders. This review will describe current developments in the shaping of MOFs with an emphasis on the design and control of the final porous structure for effluent treatment applications.