Chapter 3 - Applications of heteropoly acids as heterogeneous catalysts

Abstract

Homogenous catalysts are being replaced by solid (heterogeneous) catalysts because of the drawbacks of homogeneous catalysts to conduct organic transformations such as unwanted saponification, difficulty in separation of the products and/or catalyst recovery. The mentioned problems can be resolved by employing heterogeneous (solid) acid catalysts. In addition, solid acid catalysts are more environmentally friendly than other catalysts applied in organic reactions. Heteropoly acids have advantages in comparison with other Brønsted acids, including higher acidity strengths, thermal stability, water tolerance, and easy handling. Therefore, they have proven to be relatively efficient catalysts, but there are restrictions such as high polarity and low surface area for using them in catalytic systems. On the other hand, the most attractive feature of the heteropoly acids is that they can be used as homogeneous or heterogeneous catalysts. To resolve such restrictions, heteropoly acids are supported on suitable supports to serve as heterogeneous catalysts. The use of such heterogeneous catalysts makes the processes environmentally friendly owing to the lower effluent generation, easy recovery, and catalyst reuse. This chapter describes various reactions catalyzed by heterogeneous heteropoly acid catalysts, including the syntheses of heterocyclic compounds, multicomponent reactions, oxidation reactions, and photocatalytic reactions.