Synthesis of 7-hydroxy-4-methyl coumarin over Lewis acid metal ion-exchanged ZAPO-5 molecular sieves

Zinc aluminophosphate (ZAPO-5) molecular sieve was synthesized hydrothermally and ion-exchanged with Lewis acid metal ions such as Ga3+, In3+, La3+ and Ce3+ by wet method. The materials were characterized by XRD, FT-IR, BET, ICP-AES, 27Al and 31P MAS NMR, SEM, TPD (ammonia) and TGA. XRD patterns of ion-exchanged molecular sieves appeared almost the same as that of parent ZAPO-5 thus illustrating absence of structural degradation during ion-exchange. However, BET surface area and ICP-AES analysis indicate a small amount of non-framework oxide present in the ion-exchanged catalysts. 27Al and 31P NMR account for the incorporation of zinc in the framework of AlPO-5. The thermograms of metal ion-exchanged ZAPO-5 showed a weight loss between 550 °C and 600 °C for all ion-exchanged materials. This is assigned to the decomposition of M(OH)2+ into MO+ species. The total acidity of the materials decreased after ion-exchange as revealed from TPD (ammonia) study. It also showed selective ion-exchange of strong acid sites by Ga3+, In3+, La3+ and Ce3+ ions. Their catalytic activity was tested in the solvent free synthesis of coumarin using resorcinol and ethyl acetoacetate in the liquid phase. Lewis acid ion-exchanged catalysts showed higher activity than the parent ZAPO-5. The Lewis acid metal ion could act as a template which brings the reactants to its co-ordination sphere and subsequently reacts to form coumarin.