Abstract
Selective oxidation of benzyl alcohol to benzaldehyde with molecular oxygen was studied over sol-derived alumina supported bimetallic Au-Ru and Au-Ir (nominal atomic ratio 1:1) with respect to Au, Ru and Ir monometallic catalysts in both calcined and subsequently reduced states with and without equimolar K2CO3 in toluene at 80 °C. The Au and after an induction period also Au-Ru and Au-Ir samples had good activity and selectivity in the presence of K2CO3, the monometallic Ir and Ru ones showed negligible activity. The different pretreatments did not cause large difference in the catalytic performance, except the Ir containing samples, which were more active in reduced state.The samples were characterized by TEM, STEM-EDS, UV-Vis spectroscopy, XRD and TPR. The surface of the catalysts was investigated by XPS and CO adsorption followed by infrared spectroscopy. We confirmed the interaction between Au and both second metals, and a strongly decreased CO adsorption on gold in the bimetallic samples compared to the monometallic one. Based on the reaction rates related to the estimated Au and second metal surfaces in the bimetallic and corresponding monometallic catalysts synergetic effect is suggested for the calcined and reduced Au-Ru and reduced Au-Ir catalysts in reaction with base addition.
Highlights
One of the prime requirements of a sustainable world is the development of “green” ways for the production of commodity products
The gold loading determined by promptgamma neutron activation analysis (PGAA) well met the nominal values in all catalysts, but the ruthenium and iridium loading was much less than the nominal one in case of the sol-derived samples
We believe that the reduction of ruthenium and iridium by NaBH4 was not complete, gold enhanced the efficiency of their reduction, as in ref
Summary
One of the prime requirements of a sustainable world is the development of “green” ways for the production of commodity products. Nowadays the industry still uses conventional and very harmful stoichiometric oxidizing reagents (such as nitric acid and metal oxides) to produce fine chemicals. In the case of benzyl alcohol oxidation to benzaldehyde, the over oxidation of the substrate is a problem (Scheme 1) This reaction has two typical by-products; benzoic acid and benzyl benzoate. Skupien and co-workers made a profound study about the poison effect of benzoic acid [13]. For example Li and co-workers have found a synergistic effect of AuPd/MgO catalyst in toluene (two different preparation methods were used where ∼80 and ∼250 % increase in catalytic activity was observed compared to the corresponding monometallic gold catalysts and the selectivity was ∼100 % in both cases) [15]. Toluene (Fisher); benzyl alcohol (Lach-ner); K2CO3 (Lach-ner) were used in the catalytic reactions and dodecane (Fisher) was the internal reference for GC analysis
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