MOF/POM hybrids as catalysts for organic transformations

Insertion of molecular metal oxides, e.g. polyoxometalates (POMs), into metal-organic frameworks (MOFs) opens up new research opportunities in various fields, particularly in catalysis. POM/MOF composites have strong acidity, oxygen-rich surface, and redox capacity due to typical characteristics of POMs and the large surface area, highly organized structures, tunable pore size, and shape are due to MOFs. Such hybrid materials have gained a lot of attention due to astonishing structural features, and hence have potential applications in organic catalysis, sorption and separation, proton conduction, magnetism, lithium-ion batteries, supercapacitors, electrochemistry, medicine, bio-fuel, and so on. The exceptional chemical and physical characteristics of POMOFs make them useful as catalysts in simple organic transformations with high capacity and selectivity. Here, the thorough catalytic study starts with a brief introduction related to POMs and MOFs, and is followed by the synthetic strategies and applications of these materials in several catalytic organic transformations. Furthermore, catalytic conversions like oxidation, condensation, esterification, and some other types of catalytic reactions including photocatalytic reactions are discussed in length with their plausible catalytic mechanisms. The disadvantages of the POMOFs and difficulties faced in the field have also been explored briefly from our perspectives.

Heterogenisation of polyoxometalates and other metal-based complexes in metal–organic frameworks: from synthesis to characterisation and applications in catalysis

This review provides a thorough overview of composites with molecular catalysts (polyoxometalates, or organometallic or coordination complexes) immobilised into MOFs via non-covalent interactions.

POM@MOF Hybrids: Synthesis and Applications

The hybrid materials that are created by supporting or incorporating polyoxometalates (POMs) into/onto metal–organic frameworks (MOFs) have a unique set of properties. They combine the strong acidity, oxygen-rich surface, and redox capability of POMs, while overcoming their drawbacks, such as difficult handling, a low surface area, and a high solubility. MOFs are ideal hosts because of their high surface area, long-range ordered structure, and high tunability in terms of the pore size and channels. In some cases, MOFs add an extra dimension to the functionality of hybrids. This review summarizes the recent developments in the field of POM@MOF hybrids. The most common applied synthesis strategies are discussed, together with major applications, such as their use in catalysis (organocatalysis, electrocatalysis, and photocatalysis). The more than 100 papers on this topic have been systematically summarized in a handy table, which covers almost all of the work conducted in this field up to now.

Strategies for Incorporating Catalytically Active Polyoxometalates in Metal-Organic Frameworks for Organic Transformations

Polyoxometalates (POMs) can benefit from immobilization on solid supports to overcome their difficulty in processability and stability. Among the reported solid supports, metal-organic frameworks (MOFs) offer a crystalline, versatile platform for depositing highly active POMs. The combination of these structures can at times benefit from the combined reactivity of both the POM and MOF, sometimes synergistically, to improve catalysis while balancing desirable properties like porosity, substrate diffusion, or stability. In this Review, we survey the strategies for immobilizing POMs within MOF structures, with an emphasis on how physical and catalytic properties of the parent materials are affected in the composite when employed in organic transformations.

A Polyoxometalate-Based Microfluidic Device for Liquid-Phase Oxidation of Glycerol

Peroxidation of glycerol has been carried out in a polyoxometalate (POM)-based microfluidic reactor, which was fabricated on a capillary by using a layer-by-layer strategy. Lactic acid (LA) is produced selectively in high yield with a TOF as high as 20 000 h^-1 , compared to a TOF of 200 h^-1 in batch mode. This POM microfluidic reactor is readily prepared, scalable, highly stable, reusable, and also potentially applicable to selective oxidation of other bio-wastes.

Efficient Catalytic Upgradation of Bio-Based Furfuryl Alcohol to Ethyl Levulinate Using Mesoporous Acidic MIL-101(Cr)

Enlarging the dispersed micropores of metal-organic frameworks to the well-ordered mesostructure can enhance the accessibility of acidic sites for reactants resulting in the improvement of selectivity. With the regulation of cetyltrimethylammonium bromide/Cr³⁺ and ClSO₃H sulfonation, a series of mesoporous MIL-101(Cr)-SO₃H (MMSs) with multiple pore sizes and acidic properties were synthesized and explored to further facilitate the furfuryl alcohol (FA) ethanolization to ethyl levulinate (EL) for the first time. The optimal catalytic activity of 83.8% EL yield at full FA conversion demonstrated that the appropriate mesopore size, acidic density, and accessible -SO₃H sites contributed to the superior performance of the MMS(0.3)-0.15. The turnover frequency value (14.8 h^-1) obtained with MMS(0.3)-0.15 was comparable to the commercial Nafion NR50 (18.3 h^-1) and much higher than the classical Amberlyst-15 (1.9 h^-1), confirming the predominant catalytic performance. Furthermore, two coexistent reaction paths with 2-ethoxymethylfuran and 4,5,5-triethoxy-pentan-2-one as intermediates indicated the possibility of producing EL from FA.

Recent advances of polyoxometalate-catalyzed selective oxidation based on structural classification

The structural diversity and tenability observed in POMs has encouraged extensive investigations into their catalytic activity. Based on the structural classification of POMs, this review summarizes recent advances relating to POM-catalyzed selective oxidation and places most emphasis on dynamic developments from 2015 onwards. Work which contributes to comparing the catalytic performance of POMs with delicate structural differences (e.g. the same type of POM structure with differences of the heteroatom, addenda, protonated state or counter-ion) and in elucidating the origin/distinction of catalytic activity, as well as reasonable mechanisms, are especially highlighted.

Synthesis of SO42−/TiO2–ZnAl2O4 composite solid acids as the esterification catalysts

A SO₄²⁻/TiO₂–ZnAl₂O₄ composite catalyst with superior stability and acidic properties was synthesized and applied in esterification.