H5CoW12O40 supported on nano silica from rice husk ash: A green bifunctional catalyst for the reaction of alcohols with cyclic and acyclic 1,3-dicarbonyl compounds

Rice husk ash (RHA) is an abundant agricultural by-product. The present research work deals with the production of nano silica powders, with high surface area and in amorphous form, from RHA using optimized technique. 12-Tungestocobaltic acid, H5CoW12O40 (CoW), was supported on silica from RHA to produce silica supported CoW (CoW/NSiO2) as a nano catalyst. The characterization data derived from FT-IR reveal that the Keggin structure of CoW remains intact in CoW/NSiO2. TEM image showed that the catalyst had spherical shape with an average particle size of 10 nm. The acidity of the catalyst was measured by potentiometric titration with n-butylamine. To our surprise, this very strong solid acid catalyst showed an excellent distribution of acid sites, suggesting that the catalyst possesses a higher number of surface active sites compared to CoW supported on commercial silica (CoW/SiO2), CoW and K5CoW12O40. A high catalytic activity was found over CoW/NSiO2. Finally, CoW/NSiO2 has been used as a highly effective catalyst for benzylation of linear 1,3-dicarbonyl compounds with benzylic alcohols and synthesis of β-keto enol ethers from cyclic 1,3-dicarbonyl compounds. The present methodology offers a practical, simple, mild, environmentally friendly, and timesaving method under solvent-free conditions.

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► Nano silica with high surface area has been produced from rice husk.
► Preparation of supported 12-tungestocobaltic acid as a bifunctional catalyst.
► Acidic properties, TEM, and FTIR of the supported catalyst have been investigated.
► Different products were obtained with linear or cyclic 1,3-diketons.
► Reactions have been catalyzed by Brönsted acidity or electron transfer properties.