Nanostructure sulfated tin oxide as an efficient catalyst for the preparation of 7-hydroxy-4-methyl coumarin by Pechmann condensation reaction

Nanocrystalline sulfated tin oxide with different sulfate contents (5-30 wt%) were prepared from hydroxylated tin oxide obtained by the precipitation method, followed by wet impregnation with SO₄^2- species using a sulfuric acid solution. In samples calcined at 400, 500 and 600°C the characterization of the solids was made by DTA, XRD and nitrogen adsorption. The strength and number of acid sites were determined by nonaqueous titration of n-butylamine in acetonitrile. Both Bronsted and Lewis acid sites were determined by FTIR spectra of pyridine adsorbed. Sulfated tin oxides were tested in the synthesis of 7-hydroxy-4-methyl coumarin via solvent free Pechmann reaction of resorcinol:ethylacetoacetate (molar ratio 1:2), at 120°C. DTA measurements showed that SO₄^2- addition tends to slow down the formation of SnO₂ crystallites. XRD profiles showed that the sulfating inhibits the SnO₂ crystal growth. The SO₄ species remained strongly bonded at the SnO₂ surface stabilizing its crystallite size against sintering and it acts as a structure porogen director mediating nanoparticle growth and assembly yielding a mesostructured form of SnO₂ with wormhole morphology and high thermal stability. The surface areas of the investigated samples were influenced with the sulphate content and calcination temperature. The acidity measurements showed that the total acidity increases with the rise of sulfate content up to 25 wt%. FTIR spectra of pyridine adsorbed on the catalysts showed the presence of both Bronsted and Lewis acid sites. The formation of 7-hydroxy-4-methyl coumarin increases with the increase of surface acidity showing a maximum when the sulfate content and calcination temperature were 25 wt% and 400°C, respectively.