Polyoxometalate-based porphyrinic metal-organic frameworks as heterogeneous catalysts

Abstract

The ever-increasing appearance of inorganic–organic hybrid materials incorporating organic metalloporphyrins (MPs) and inorganic polyoxometalates (POMs) – separately, two groups of effective molecular catalysts – which combine to form the ground-breaking class of metal–organic frameworks (MOFs), has allowed this class to participate in numerous kinds of chemical reactions and mimic the activity of heterogeneous bio-, photo- and electrocatalysts. Recent results have shown that the integration of POMs and MPs in a porous structure not only maintains the characteristic functionalities of both components, but can also generate novel and unparalleled features including augmented stabilities, remarkable selectivity, improvable porosity, and novel and more favorable topologies. Furthermore, the immobilization of these component species (POMs and MPs) within a unified framework can prevent their catalytic deactivation, which mostly arises from suicidal self-oxidation of porphyrin moieties. It has also been established that the introduction of POMs – owing to their broad range of size, structure, and highly negative charge distribution throughout the cluster – can further expand their structural diversity and greatly influence the stability and catalytic performance of porous porphyrinic MOFs. What is more, the incorporation of POMs inside the pores or their use as secondary building units (SBUs) within the framework can give rise to an increase in their original reusability, perpetuity, and adsorption ability. Moreover, their incorporation can also boost the transfer of electrons (e.g., photoexcited charge carriers) through the network. Therefore, POM-based porphyrinic MOFs (porphyrinic POMMOFs) are materials that can simultaneously take advantage of the combined uniqueness of the individual traits of POMs, porphyrins and MOFs, which display rich redox photochemistry, biomimetic ability, and porosity, respectively. This review aims to represent the latest progress in porous porphyrinic POMMOFs, encompassing their categorization, fabrication and exploitation in the catalysis field. To the best of our knowledge, hitherto no specific attempt has been made to cover this topic, and our review would be the first systematic scrutiny of these porous materials. Last but not least, the POMs that have been templated to synthesize porphyrinic POMMOFs are still mostly limited to a few types of POMs. Moreover, the porphyrin linkers which were used to create such MOFs have also been confined to only a few kinds. Thus, we suggest that other POMs ((iso-) heteropolyoxometalates) and porphyrin-based linkers could be explored, with an eye to creating new state-of-the-art porphyrinic POMMOFs. In the short term, it is anticipated that this review will encourage scientists to substantially extend the frontiers of knowledge about these systems, not only in the area of catalysis, but also in such versatile and underexplored domains as magnetism, gas sorption, sensing, biomedicine, and electrochemical and photocatalytic utilizations. For these reasons, we strongly believe that this comprehensive review will not only provide a better overview of the subject, but also open up promising directions of research and stimulate scientific interest and enthusiastic curiosity. It will also pave the way to generating important new ideas for the future of the synthesis and applications of these invaluable types of MOFs.