Silver modified porous silica from rice husk and its catalytic potential

Rice husk silica modified with Ag (RH-Ag) and its calcined form, RH-Ag(C) are heterogeneous catalysts prepared from rice husk. The sodium silicate was extracted from rice husk using a simple solvent extraction technique. Silver was added during the precipitation of the gel to form RH-Ag. Calcination of RH-Ag yielded RH-Ag(C). RH-Ag and RH-Ag(C) was found to have a well defined amorphous phase and a crystalline phase. TEM analysis showed that silver was encapsulated into the silica matrix. Well defined mesoporous channels were found in RH-Ag(C). The EDX analysis showed that silver was indeed incorporated in the matrix of RH-Ag but it was not homogenously distributed. The specific surface area of RH-Ag and RH-Ag(C) was found to be 447 and 419 m² g⁻¹ respectively. The cation exchange capacity (CEC) for RH-Ag was found to be 0.470 mmol g⁻¹ which was higher than that of RH-Ag(C) with 0.273 mmol g⁻¹. Both the CEC and the BET values for RH-Ag(C) was consistent with the calcination process. Catalysis of benzyl alcohol using RH-Ag and RH-Ag(C) gave two products which were identified as benzaldehyde and dibenzyl ether. Percentage conversion was higher when the reaction was conducted with pure oxygen (14.0%) than in open air (0.60%). Overall, RH-Ag(C) was found to be a better catalyst for the formation of dibenzyl ether. The optimum weight and time of reaction for the oxidation was found to be 0.3 g and 3 h respectively for both catalysts. It was found that even in an inert atmosphere, 13% of benzaldehyde was obtained with RH-Ag(C) as catalyst. Mechanisms for the formation of benzaldehyde under oxygen and inert atmosphere and the condensation mechanism assisted by the presence of the Ag on the surface of the catalyst have been suggested.