Heterogeneous catalysis by tungsten-based heteropoly compounds

Tungsten, a fascinating metal, has found a variety of catalytic applications in the form of tungsten sulfides, simple tungsten oxides (e.g. tungsten trioxide, tungstate, tungstic acid, tungstite), and polyoxotungstates (POTs). The latter, which have been less industrialized compared to the others, have attracted a great deal of interest recently stemming from reinforcement of uniquely interesting catalytic properties of polyoxometalates (POMs), such as strong acidity, redox capability, and water tolerance, by distinct inherent properties of tungsten such as having very strong BrOnsted acid sites. Additionally, the physical and chemical properties of POMs are widely and readily tunable at the molecular level, holding promise for their application in different reactions. However, the water-solubility property of POTs, resulting in lack of recovery in water-involving reactions, is a controversial challenge. To tackle this obstacle, homogeneous POTs have been heterogenized via different strategies, classified here into three groups: inorganic cation-substituted solid POTs, organo-solidified POTs, and POTs immobilized onto supports. These strategies have occasionally led to the fabrication of even more efficient catalysts compared to the parent homogeneous POT. A large number of heterogeneous POT-based catalysts have been developed so far, which intriguingly have adjustable catalytically important features such as porosity, hydrophobicity, compatibility toward organic species, chemical composition, admissibility to other elements (with tunable host-guest interactions), and magnetic properties. Such adjustments have enabled size-selective catalysis, enhanced catalytic activity in organic media, prevented poisoning of acid sites by water, rendered bifunctional catalysts, and/or provided facile recovery. We review these breakthroughs in a critical and comparative fashion along with highlighting the most interesting achievements of the reported works. Herein, we have tried to list all the recent works on the heterogeneous catalysis applications of POTs in liquid organic reactions. In doing so, photocatalytic applications of POTs and homogeneous POTs with high recoverability have been excluded.