Gold supported on nanocrystalline beta-Ga2O3 as a versatile bifunctional catalyst for facile oxidative transformation of alcohols, aldehydes, and acetals into esters.

Esterification is one of the most fundamentally important reactions in organic synthesis. Although a number of methods have been developed, the search for new, facile, cost-effective, and environmentally friendly procedures that avoid the use of large excess of reagents and expensive activators has attracted substantial interest. An attractive alternative is the direct catalytic transformation of alcohols or aldehydes to esters, without the use of the corresponding acid or acid derivative. In particular, the direct oxidative conversion of alcohols or aldehydes under mild conditions is an attractive goal. As opposed to the traditional esterification method, in which a two-step synthetic procedure first involving the synthesis of carboxylic acids or activated carboxylic acid derivatives, such as acid anhydrides or chloACHTUNGTRENNUNGrides, is required, the single-step nature of the oxidative esterification procedure has economic and environmental benefits in the synthesis of esters. However, relevant reports are limited to a few nonselective heterogeneous reactions or homogeneous systems, which utilize stoichiometric amount of oxygen donors and long reaction times. In general, heterogeneous systems capable of catalyzing oxidative esterification of alcohols or aldehydes using molecular oxygen (O2) as oxidant are relatively scarce. One notable exception is the ethylene glycol to methyl glycolate (MGC) process based on a specific Au-based catalyst system with proprietary formulations recently developed by the company Nippon Shokubai; this result represents a milestone towards greener commercial process for clean and efficient production of carboxylic esters. Solid catalysts based on supported gold nanoparticles have attracted tremendous recent attention owing to their unique catalytic properties for a broad spectrum of organic transformations, especially for aerobic oxidation of alcohols under mild conditions. Over the last few years, our group, Hutchings et al., Baiker et al., and Corma et al. reported that gold nanoclusters deposited on TiO2, CeO2, and Cu-Mg-Al or Ga-Al mixed-metal oxides are highly effective for aerobic alcohol oxidation under solventfree conditions. One critical issue associated with the goldcatalyzed primary-alcohol oxidation process is the selectivity toward aldehydes. Most recently, several studies have revealed that, depending on the substrate or acidic nature of the supports, the yielding of esters may severely reduce the selectivity toward target products of aldehyde, with hemiacetal being identified as the key intermediate for ester formation. Taking into account that inorganic oxides contain some appropriate acid sites that are able to facilitate the hemiacetal formation, it appeared to us that a new concept of catalyst could be brought forward if the gold nanoparticles in the presence of these acidic sites can cooperatively work together by introducing a solid bifunctional catalyst that facilitates the generation and consecutive oxidation of the intermediate hemiacetals to the corresponding esters. To explore this possibility, we chose a solid catalyst formed by gold supported on nanocrystalline b-Ga2O3 (denoted as Au/b-Ga2O3). The reason for this choice is that gallium oxide has emerged as an exceptional catalytic or supporting material that is highly efficient for a wide range of acid-catalyzed reactions; it is also known that significantly increased surface Lewis acidity can be achieved on nanocrystalline b-Ga2O3. [16] The nanocrystalline b-Ga2O3 support was prepared by an alcoholic gel-precipitation method. The X-ray diffraction (XRD) pattern of the as synthesized nanocrystalline support shows well-defined diffraction features characteristic of bGa2O3 (Figure S1 in the Supporting Information). Transmission electron microscopy (TEM) shows that the support is highly porous in nature; it consists of interconnected parti[a] F.-Z. Su, J. Ni, H. Sun, Prof. Dr. Y. Cao, Prof. Dr. H.-Y. He, Prof. K.-N. Fan Department of Chemistry & Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials Fudan University, Shanghai 200433 (P. R. China) Fax: (+86)21-6564-2978 E-mail : [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200800982.