Selective oxidation of arabinose to arabinonic acid over Pd–Au catalysts supported on alumina and ceria

Abstract

Selective oxidation of arabinose to arabinonic acid was carried out isothermally in a shaker reactor at moderate conditions of 60°C and pH 8 on Pd–Au catalysts supported on nanosized alumina and ceria. The gold (4wt.%) was supported by deposition–precipitation (DP) using HAuCl4 and urea with subsequent chemisorption of palladium (1wt.%) using PdCl2. Before catalytic tests, samples were treated in hydrogen or oxygen at 300°C, or by aqueous solution of formaldehyde at room temperature. Synthesized bimetallic Pd–Au catalysts are characterized with higher activity in the selective aqueous phase oxidation of arabinose to arabinonic acid by molecular oxygen in comparison with monometallic ones. Activity and selectivity of catalysts depend on the sample treatment and the nature of the support used. Pd–Au/CeO2 catalyst reduced by formaldehyde manifests the highest activity and selectivity. The synthesized bimetallic catalysts have been characterized by TEM, XPS and UV–vis techniques under different stages of sample activation. Mutual interactions of supported gold and palladium species depending on the support nature and sample treatment were observed. A more pronounced interaction between gold and palladium species was observed for Pd–Au/Al2O3 catalyst, while interactions of Au and Pd with reducible ceria coexist with the mutual interaction between these metals. Gold metallic species seem to be responsible for activation of arabinose while an easy redox transformation of Pd species can provoke oxygen activation. Thus, gold species or Au–Pd alloy covered with thin PdO film manifest a synergetic effect in the selective arabinose oxidation by molecular oxygen.