Supported Nano-gold Catalysts For Selective Hydrogenation Of ¦Á In The Liquid Phase Of ¦Á, ¦Â-unsaturated Aldehydes, Beta-unsaturated Alcohol Reaction Studies

α,β-unsaturated alcohols are important intermediates in the production of fragrance, flavors and pharmaceuticals etc. and widely used in the organic synthesis. Currently,α,β-unsaturated alcohols are commercially achieved by homogeneous stoichiometric reduction using NaBH4 or LiAlH4 as reductant. Although this method is beneficial to the production ofα,β-unsaturated alcohols at high yield, yet this process makes the separation of products difficult, is in need of a large amount of solvent, involves a lot of pollutants, and results in the environment destruction. Therefore, development and study of the highly efficient catalysts and alternative green processes for the synthesis ofα,β-unsaturated alcohols has attracted great attentions. However, according to the published researches, the majority of the metal-based catalyst is prone to hydrogenate C=C bond instead of C=O. Recent years, gold catalysts have been paid more and more attention, and proven to be much more active and selective than the tranditional noble metal catalysts towards some reactions in gas or liquid phase. For the selective hydrogenation of a,(3-unsaturated aldehyde, crotonaldehyde can be reduced to crotyl alcohol at high selectivity over gold catalyst in the presence of clean molecular H2. More importantly, the high selectivity can be maintained until the end of the reaction. These discoveries, at the same time, greatly develop the application of gold catalysts in the area of liquid phase hydrogenation.1. Chemoselective hydrogenation ofα,β-unsaturated aldehyde over silica supported gold catalystsA unique deposition-precipitation method mediated via a coordination complex precursor was used to immobilize the cataionic precursor on the negatively charged surface of silica. The highly dispersed gold nanoparticles in mesoporous silica were subsequently generated by low-temperature reduction under H2 atmosphere followed by high-temperature oxidation under air atmosphere. On one hand, the catalyst precursor was treated under different temperature and different atmosphere. On the other hand, we studied the effect of silica structure and compared preparation method of catalysts by Au(en)2Cl3 with that by direct HAuCl4·4H20.According to the catalytic results, we find that the catalyst which is reduced at 150℃under Hb/Ar atmosphere followed by oxidized at 400℃under air atmosphere presented the best catalytic performance. When the conversion of crotonaldehyde reaches the maximum value (96%), the selectivity to the crotyl alcohol is up to 42% and the yield is 40%. The N2-physisorption results show that the surface area of all the catalysts after gold deposition is smaller than the original silica support. From the TEM images, it can be clearly seen that the gold particles are highly dispersed in the mesopores of SBA-15, and the average particle size is 4.9 nm, which indicates the high stability of gold nanoparticles prepared by cationic adsorption. XPS results tell the electronic structure of the catalyst surface. It is shown that there is a negative shift of Au 4f peak by 1.0 eV. We assume that the shift results from the strong metal-support interaction by Au-Si or Au-O-Si dangling bond which results in electron transfer from silica to gold and increase the electron density of surface gold atoms. A higher density on gold surface atoms decreases the binding energy of the C=C bond via an increase of the repulsive four-electron interaction and favors the back-bonding interaction with theπco*, so that the hydrogenation of the C=O group should be favored over that of the C=C group, thus the high selectivity to crotyl alcohol.2. Chemoselective hydrogenation ofα,β-unsaturated aldehyde over magnetite supported gold catalysts and its mechanism studyWe synthesized 3D flowerlike hematite nanostructures by an ethylene glycol-mediated self-assembly process using ferric chloride or ferric nitrate. Gold precursor was deposited onto the as-synthesized hematite materials by the homogeneous deposition-precipitation using urea as the precipitant, followed by being reduced under H2/Ar at 350℃. On the other hand, two particulate hematite materials, as reference supporting materials, were synthesized via ferric salt precipitation by base, NH4OH or Na2CO3, in an aqueous solution. According to the catalytic results, all the Au/Fe3O4 catalysts reduced at 350℃showed excellent selectivities to crotyl alcohol (> 70%). More importantly, the flowerlike Au/Fe3O4 catalysts feature high selectivity (≥76%) in the entire reaction course. Moreover, the flowerlike Au/Fe3O4 catalysts were ca.1.5 times more active than the particulate counterparts. Transmission electron microscopy showed that the average gold particle size on the flower-like magnetite (2.6 nm) is much smaller than that on the particulate one (3.8 nm). High-resolution transmission electron microscopy revealed the heteroepitaxial growth of gold crystallites on flowerlike magnetite materials along the{111} planes, which can be the underlying reason for the smaller and more uniform gold nanoparticles (AuNPs) in these catalysts. Thus, the smaller gold nanoparticles showed the higher catalytic activity. We compared the selectivity to crotyl alcohol over the Au/Fe3O4 catalyst with those over literature oxide-supported gold catalysts for the same reaction. It is apparent that gold supported on magnetite is more selective than other oxide-supported gold catalysts at the beginning or end of the reaction. A perimeter interface mechanism involving the hydroxyl groups on magnetite and the positively charged gold atoms linked to the lattice oxygen of magnetite substantiated by X-ray photoelectron spectroscopy and extended X-ray absorption fine structure spectroscopy is established to account for the remarkably high selectivity over the flowerlike catalysts.