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

Abstract

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 N2 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 WOx species on the catalyst surfaces, but all WOx species are homogeneously dispersed as small particles on the surfaces. At higher ratio of W (ZrW3 and SnW3) the excess WOx 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 N2 adsorption- desorption study. Raman spectra showed that the tungstate species has WO4 distorted tetrahedral structure. The surface acidity measurement showed that the catalysts contain mixture of Lewis and Bronsted 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.