Chromium modified silica from rice husk as an oxidative catalyst

RH-Cr and RH-Cr500 were synthesized from rice husk by solvent extraction and gel precipitation technique. The specific surface area of RH-Cr and RH-Cr500 were found to be 0.542 and 1.20 m² g⁻¹ respectively. Energy dispersive X-ray (EDX) analysis showed that Cr(III) was homogenously incorporated in the matrix of both samples to a maximum of ca. 16% in RH-Cr500. Elemental analysis showed that RH-Cr contained ca. 15% carbon, while RH-Cr500 contained negligible amounts. FTIR analysis showed the extracted solid contained silanol (Si–OH) and siloxane (Si–O–Si) groups. Catalytic oxidation of cyclohexane with H₂O₂ using RH-Cr as the catalyst showed a 27.13% conversion to cyclohexanone and cyclohexanol with a selectivity of 57.37% and 42.63% respectively. However, RH-Cr500 showed only 14.01% conversion but with a selectivity of 64.83% of cyclohexanone and 35.17% of cyclohexanol. Epoxidation of cyclohexene using H₂O₂ with RH-Cr as the catalyst gave a conversion rate of 30.17% with a selectivity of 11.51% towards cyclohexene oxide, 63.21% 2-cyclohexen-1-one and 25.29% 2-cyclohexen-1-ol. The same reaction with RH-Cr500 as the catalyst showed 21.28% conversion with 14.65% cyclohexene oxide, 68.71% 2-cyclohexen-1-one and 16.64% 2-cyclohexen-1-ol. In the catalytic oxidation of cyclohexanol to cyclohexanone, RH-Cr showed a conversion of 12.25% while RH-Cr500 showed a conversion of 13.52%. No others products were detected in the conversion. Comparison with published catalytic systems showed that RH-Cr and RH-Cr500 to be a better catalyst even though the surface area of these catalysts were low.