Synthesis and characterization of new solid acid catalysts, H3PW12O40 supported on nanoparticle tin oxide: An efficient catalyst for the preparation of 7-hydroxy-4-methylcoumarin

A series of 12-tungstophosphoric acid (TPA) modified SnO2 samples (TPA–SnO2) with varying TPA content were prepared by wet impregnation technique. The TPA–SnO2 catalysts were characterized by thermal analysis, powder X-ray diffraction (XRD) patterns and N2 adsorption at −196 °C. The surface acidity was measured by potentiometric titration and FTIR spectra of chemically adsorbed pyridine. The DTA shows a peak maximum at 600 °C which was ascribed to the crystallization of tin oxyhydroxide into SnO2. The thermograms of TPA show no exothermic effect due to the decomposition of TPA up to 25 wt%. When TPA content reached 30 wt% TPA/SnO2 an exothermic effect at 618 °C indicated that the thermal stability of TPA on SnO2 seems to be comparable to that of pure TPA. XRD measurements showed that there are no diffraction patterns of TPA for the catalyst calcined at 500 °C. The presence of TPA on tin oxide inhibits sintering of the particles during calcination by reducing the crystallite size of SnO2 from 27.7 to 19.8 nm. The surface area values increase gradually with increasing TPA loading to reach a maximum limit of 25 wt% TPA. When calcination temperature is increased to 500 °C the specific surface is decreased. The acidity measurements showed that the total acidity increases with the rise of TPA content up to 30 wt%. FTIR spectra of pyridine adsorbed on the catalysts showed the presence of both Bronsted and Lewis acid sites. An optimum reaction performance (78.0% of 7-hydroxy-4-methyl coumarin formed) was achieved at 30 wt% loading of TPA calcined at 400 °C and reaction conditions of 120 °C, 2 h reaction and 1:2 (resorcinol:ethyl acetoacetate) molar ratio.

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► We prepare TPA/SnO2 by impregnating SnO2 with the appropriate amount of TPA solution to obtain 3, 7, 10, 15, 25 and 30 wt% TPA.
► We conclude that the surface area values increase gradually with increasing TPA loading.
► We observe that 30 wt% TPA/SnO2 calcined at different temperatures contains the strongest and the highest number of acid sites.
► We use the catalysts for the preparation of 7-hydroxy-4-methylcoumarin.
► We conclude that the formation of 7-hydroxy-4-methylcoumarin increases gradually with increasing the amount of TPA loaded on SnO2.