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
2012
Rafiee, Ezzat | Khodayari, Maryam | Kahrizi, Masoud | Tayebee, Reza
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 10nm. 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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