Promoting effect of ceria on the physicochemical and catalytic properties of CeO2–ZnO composite oxide catalysts

CeO2–ZnO composite catalysts prepared by amorphous citrate method have been investigated for cyclohexanol dehydrogenation and hydrogen transfer reactions. The precursors and catalysts have been characterized by TGA, CHN analysis, XRD, UV–vis–NIR diffuse reflectance, SEM and acid–base measurements. The amorphous precursors in citrate process contain one molecule of citric acid per Ce4+ or Zn2+ ions. Structural studies of composite oxides indicate the presence of individual oxide phases along with non-equilibrium solid solutions in a limited composition range. The composite oxides contain low coordination Ce3+ and Ce4+ sites. Cyclohexanone was obtained as main product for cyclohexanol transformation reaction carried out over these mixed oxide catalysts due to dehydrogenation on basic sites. The presence of ceria in the composite oxide enhances the surface area and acid–base properties facilitating the dehydrogenation process. At low ceria content, the CeO2–ZnO composite oxide catalysts show higher catalytic activity for both cyclohexanol dehydrogenation and hydrogen transfer reactions due to higher basicity, surface area and smaller crystallite sizes. Hydrogen transfer activity is found to be higher on CeO2(10%)–ZnO catalyst prepared by citrate method compared to the catalyst prepared by decomposition from acetate precursor. This study demonstrates the promoting effect of ceria in CeO2–ZnO catalysts for reactions involving acid–base sites.

The activities of CeO2–ZnO composite catalysts prepared by amorphous citrate method are reported for cyclohexanol dehydrogenation and hydrogen transfer reactions in vapour phase. CeO2 in ZnO promotes cyclohexanol conversion with >90% selectivity to cyclohexanone. The Ce4+ Lewis acidic sites in CeO2–ZnO catalysts enhance the cyclohexanone selectivity. Hydrogen transfer reaction of cyclohexanone using 2-propanol as hydrogen donor produces cyclohexanol with selectivity >98%. The acid–base properties of CeO2–ZnO are the key factors for higher activities and selectivities which are influenced by the dispersion of CeO2 phase. Figure optionsDownload as PowerPoint slide