Clays modified with Heteropoly acids (HPAs) have attracted much interest as green catalysts for organic synthesis. Heteropoly acids (HPAs) have several advantages over liquid acid catalysts such as noncorrosive, environmentally benign and due to their strong acidity HPAs have attracted much interest as the catalyst when used with montmorillonite and bentonite clays. Green chemistry focuses on advencement in techniques to prevent pollution and reduce consumption of hazardous chemicals. The development in this area may involve clean synthesis, ecofriendly methods, solvent free synthesis, green catalysts and avoiding the use of hazardous reagents in chemical methods. For the the present study, heteropoly acid (HPA) modified montmorillonite & bentonite clays have been synthesized by swelling and pillaring of clays. Modified clays possess qualities such as good thermal stability, high acidity and high oxidising ability. These modified clays can be used as solid acid catalysts in reactions such as Deoximation of oximes of aldehydes and ketones, synthesis of acetal derivatives of aldehydes and ketones, synthesis of coumarin derivatives etc. For coumarin synthesis, these clays have been found to be efficient and reusable catalysts.