Ga–Al Mixed‐Oxide‐Supported Gold Nanoparticles with Enhanced Activity for Aerobic Alcohol Oxidation

Abstract

The selective oxidation of alcohols is one of the most challenging reactions in green chemistry. Although a number of methods have been developed, the search for new, facile, cost-effective, and environmentally benign procedures that avoid the use of a large excess of toxic and expensive stoichiometric metal oxidants has attracted substantial interest. An attractive method is the direct oxidation of alcohols—promoted by reusable heterogeneous catalysts—using air or molecular oxygen (O2) under solventfree conditions or (in the case of solid alcohols) in green organic solvents. Ideally, the reaction should also be performed under mild conditions (preferably at room temperature) for the synthesis of complex, thermolabile compounds, which are typical in fine chemistry. Satisfactory results were attained in only very few cases, in which a large excess of base additives was required, and this was usually achieved at the expense of selectivity. Therefore, the development of excellent reusable catalysts for liquid-phase aerobic oxidation of alcohols under mild conditions would constitute a breakthrough in both green chemistry and organic synthesis. Recently, supported gold nanoparticles have attracted considerable attention because of their extraordinarily high activity and selectivity. The outstanding catalytic ability of gold is related to the size and shape of the nanoparticles, the degree of coordinative unsaturation of the gold atoms, and the interactions between gold and the oxide support. Although several gold systems have been reported for the catalysis of alcohol oxidation reactions, in most cases they have been applied at temperatures above 100 8C. Dehydrogenation is known to be the rate-limiting step in the oxidation of alcohols on various noble metals. Therefore, the combination of gold nanoparticles with a suitable support (characterized by an exceptional alcohol-dehydrogenation activity) may allow the fabrication of new, versatile gold catalysts that could be used for liquid-phase organic synthesis under mild conditions. Herein, we demonstrate for the first time that mesostructured Ga–Al mixed-oxide solid solutions are highly promising supports for the fabrication of exceptionally effective gold catalysts for aerobic alcohol oxidation under mild conditions. A series of binary mesostructured Ga–Al mixed-oxide supports (denoted as GaxAl6 xO9; x= 2, 3, 4), along with unitary oxides of g-Ga2O3 and g-Al2O3, was prepared through an alcoholic sol–gel pathway. The X-ray diffraction (XRD) patterns of all as-synthesized binary substrates are characteristic of g-Ga2O3/Al2O3 solid solutions with a spinel-type structure. When gold nanoparticles were deposited onto these high-surface-area materials, no gold diffraction line was detected, and the pattern showed no significant differences relative to that of the support, thus indicating that the structure of the catalyst was maintained. A representative transmission electron microscopy (TEM) image of the Au/ GaxAl6 xO9 sample confirms that the gold particles were evenly deposited on the Ga–Al mixed-oxide support, with most particles being smaller than about 6 nm (see the Supporting Information for TEM and XRD data). To check the possible alcohol-dehydrogenation capability of the Au/GaxAl6 xO9 materials, we adsorbed 2-propanol on their surface and performed temperature-programmed surface reaction (TPSR) measurements of the desorbed H2 molecules (see the Supporting Information). Ga-containing mixed-oxide supports were found to be indispensable for attaining highly active alcohol-dehydrogenation materials (Figure 1A). Furthermore, the dehydrogenation activity of the catalysts was observed to be strongly dependent on the composition of these supports. A strongly enhanced hydrogen signal was identified in the case of a Ga3Al3O9 solid solution containing a Ga/Al molar ratio of 1:1—in sharp contrast to what was observed for the reference gold catalysts Au/TiO2 and Au/Fe2O3 (provided by the World Gold Council), where no H2 species were detected. These results can be rationalized by assuming that the formation of Ga–Al mixed-oxide solid solutions may favor the creation of specific dehydrogenation sites as a consequence of the presence of Ga atoms at the surface atomic sites (Td and Oh) of Al2O3 and highly dispersed GaO4 tetrahedra in the surface spinels. [16] These sites are responsible for the considerably enhanced dehydrogenation activity observed for the Ga–Al mixed-oxide-supported Au catalysts. Our initial aerobic-oxidation studies focused on the case of benzyl alcohol (Figure 1B), with the aim to understand the effect of the composition of the support on the catalytic performance of the gold catalysts. The reactions were performed in a magnetically stirred glass batch reactor in the presence of a solvent (at 90 8C) under O2 and at [*] F. Z. Su, Dr. Y. M. Liu, L. C. Wang, Prof. Y. Cao, Prof. H. Y. He, Prof. K. N. Fan Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials Fudan University, Shanghai 200433 (P. R. China) Fax: (+86)21-6564-2978 E-mail: yongcao@fudan.edu.cn