Preparation, characterization of highly active recyclable zirconium and tin tungstate catalysts and their application in Pechmann condensation reaction

摘要

In this contribution, a facile and efficient one-step method for the preparation of mesoporous zirconium and tin tungstate with different Zr or Sn:W molar ratios. The prepared catalysts were characterized by different textural and spectroscopic techniques such as XRD, Raman spectra, SEM, EDX, TEM and N~(2)adsorption–desorption at ??196?°C. The surface acidity was measured by nonaqueous potentiometric titration of n-butylamine and FTIR spectra of chemisorbed pyridine. XRD results showed that all the prepared samples calcined at 400?°C are amorphous, no evidence for formation of big aggregates of WO~(x)species on the catalyst surfaces, but all WO~(x)species are homogeneously dispersed as small particles on the surfaces. At higher ratio of W (ZrW3 and SnW3) the excess WO~(x)species are distributed inside the pores or block the pores by covering the necks of the pores, which leads to decrease in the specific area as evident from N~(2)adsorption- desorption study. Raman spectra showed that the tungstate species has WO~(4)distorted tetrahedral structure. The surface acidity measurement showed that the catalysts contain mixture of Lewis and Br?nsted acid sites with different ratios, the maximum B/L ratio reach maximum value at molar ratio 1:2 (ZrW2 and SnW2). The results showed that zirconium tungstate samples have higher surface acidities than tin tungstate samples. The prepared catalysts showed high catalytic activities for the synthesis of 7-hydroxy-4-methyl coumarin via the Pechmann condensation reaction of resorcinol with ethyl acetoacetate. High yield (~?94%) with 100% selectivity of 7-hydroxy-4-methyl coumarin was obtained under solvent free conditions.