Heterogenization of heteropoly compounds: a review of their structure and synthesis

The application of catalysis to reduced toxicity systems and benign and renewable energy systems is a central focus area for green chemistry research. It is possible to prepare heterogeneous analogues of the most commonly used homogeneous soluble catalysts by immobilizing them on various insoluble supports. The use of heterogeneous catalysts in chemical processes would simplify catalyst removal and minimize the amount of waste. Therefore, to maintain economic viability, a suitable heterogeneous system should not only minimize the production of waste, but should also exhibit activities and selectivities comparable or superior to the existing homogeneous route. Accordingly, catalysis by heteropoly (and related) compounds (HPCs) is a field of increasing importance, particularly concerning nanocatalysts. Furthermore, the heterogenization of bulk HPCs is an interesting area of research from an industrial point of view. As a rapidly growing and increasing field, HPC catalysis exhibits three main merits: (1) HPCs not only possess a strong acidic property, but also an oxidative property, which can support fast reversible multi-electron redox transformations under mild conditions. (2) HPCs exhibit fairly high thermal stability in supported or salt forms. (3) Their catalytic properties can be tuned in a wide range by changing their chemical compositions. A number of important heterogenization methods will be discussed in this review. These methods can be classified into two major categories: exchange of the HPC protons with cations (precipitation and hybridization) and immobilization of the HPCs on a suitable solid support (encapsulation, grafting, tethering and dispersion). Although these compounds have been known well for over a century, only in the last few years has scientific interest in these materials begun to increase dramatically. Therefore, in this review, we aim to describe the different methods of heterogenization of these compounds developed and reported over the past 15 years.