| The catalytic potential of Au has long been ignored due to its high chemical inertness. However, in the1980’s, there was a renewed interest in studying supported gold catalysts when Haruta et al. accidently discovered that these catalysts have ultrahigh catalytic activity in the low temperature CO oxidation and that the activity is even higher in humid atmosphere. After that, intensive and extensive research efforts have been devoted to the subject of Au catalysis. It has been found that besides CO oxidation, nitrogen oxide elimination and water-gas shift (WGS) reaction supported gold catalysts also exhibit unique performance in many other types of organic synthesis reactions, such as selective reduction and oxidation. Compared with supported Pd and Pt, gold catalyst is distinguished by its low-temperature reactivity and high chemo-selectivity. Thus, till now, supported gold catalyst has been successfully employed in alcohol oxidation, amine oxidation, C-H oxidation, carbonyl hydrogenation, nitro reduction and olefin hydrogenation.In order to unravel the origin of the unique catalytic properties of gold catalysts, several aspects of gold catalysts have been widely investigated, including the particles size effect of gold, nature of support materials, preparation methods and chemical state of active gold species, etc. There is a general consensus that the activity of supported gold catalysts largely depends on the gold particle size, the type of the oxide support as well as the preparation method. Generally, for most organic reduction and oxidation reactions, metal oxides is the first option for the catalyst’s support owing to its redox ability and Lewis acid/base sites on the surface. As for the preparation method, NaOH deposition-precipitation (DP) method is wildly used in gold catalyst preparation. Through this method, gold catalyst with high gold dispersion and strong metal-support interaction can be facilely obtained.Nowadays, one great challenge for the current sustainable chemistry is the development of new green catalytic technologies that can afford resource-saving, environmentally benign, mild and atom-economic synthesis of fine chemicals. However, the diversity of the gold catalyst use in selective reduction and oxidation as well as the novel approach for the multi-functional gold catalyst preparation offer great opportunity for the catalysis research. In this dissertation, we study the catalytic behavior of metal-oxide-supported nano-gold catalysts in transfer-hydrogen deoxygenation of epoxides and selective oxidation of alcohols. Moreover, against the transfer-hydrogen strategy, we also develop the novel integrated gold-catalytic system for organic reactions, including epoxidation of olefins and ammoximation of cyclohexanone. The main conclusions are described as follows:1. Deoxygenation of epoxides over supported gold catalysts with CO and H2OThe catalytic activities of a series of gold catalysts with different supports (Au/TiO2, Au/Fe2O3, Au/C, Au/CeO2) and other supported noble metal catalysts (Pt, Pd, Ru, Ag, Ir) for the deoxygenation of styrene epoxide under CO/H2O assisted transfer-hydrogenation condition was studied. Out of all the noble metals, only gold catalysts exhibit potential in this reaction condition. As for the support, the catalytic activity of TiO2-supported gold was much higher than other gold catalysts. Through modified-deposition-precipitation method using special gold precursor, a TiO2supported gold catalyst with very small gold particle size was obtained. It was found that the decrease in gold particle size strongly enhanced the catalytic activity in olefin deoxygenation. With this catalyst, styrene epoxide could be fully converted to styrene in only1.2h even under room temperature and very low CO pressure (2atm).With the optimized catalyst, we explored the scope of the deoxygenation of structure-diverse epoxides. It was found that Au/TiO2-CO/H2O transfer hydrogenation system could afford excellent yield for all the substrates. No dehalohydrination reaction occurred in the deoxygenation of halogen-substituted epoxide. Compared to aromatic epoxides, aliphatic epoxides required higher temperature and longer reaction time. Moreover, the Au/TiO2also showed good stability in successive five runs.Controlled experiments showed that no H2was produced in the reaction process, the substitution of H2with CO/H2O largely lowered the epoxide conversion and the reaction cannot proceed without the addition of water. Based on these results, we assumed that the Au-H species, which took away the oxygen in epoxide, was directly formed through the reaction of CO and H2O over gold catalysts, but not via the WGS pathway.2. Gold-catalyzed selective alcohol oxidation using H2O2as green oxidantThe catalytic activities of several commercial supported gold catalysts for selective oxidation of1-phenylethanol in water were examined. Under same conditions, TiO2performed much better than other supports. When using the same support (TiO2), Au/TiO2(Mintek) offered higher conversion than Au/TiO2(WGC). Transmission electron microscope showed that the gold particle size in Au/TiO2(Mintek) was smaller than in Au/TiO2(WGC). Then, we prepared a series of Au/TiO2with different gold particle size through treating Au/TiO2(Mintek) in elevated temperature. The difference of these catalysts in the alcohol oxidation reactivity further convinced that smaller the gold particle size, better the catalytic performance.The Au/TiO2(Mintek)-H2O2system afforded excellent conversion in the oxidation of many kinds of alcohols. The oxidation of secondary alcohol or primary alcohol gave ketone or aldehyde and acid as main product, respectively. Especially, for aliphatic alcohols, the Au/TiO2(Mintek)-H2O2system also showed great catalytic reactivity.3. Selective oxidation with supported gold catalysts via O2reductive activationUsing formate salts as hydrogen source, supported gold catalysts could convert O2into H2O2. And with this in-situ formed H2O2, the titanosilicate could afford various organic oxidation reactions. First, the H2O2productivity of several supported gold catalysts in CH3OH/H2O with formate salts was examined. It was found that Au/CeO2together with HCOOK gave the best activity in H2O2synthesis. Then, we studied the catalytic ability of various titanosilicates in the epoxidation of styrene with commercial H2O2. Among all the titanosilicates, the titanosilicate with MFI structure (TS-1) afford the highest epoxide yields. Based on these results, we integrated these two processes into one pot, in which the H2O2was in-situ formed over Au/CeO2, and then interact with TS-1to afford the epoxidation reaction. In this condition, the binary catalysts system (Au/CeO2&TS-1) could smoothly convert styrene to corresponding styrene epoxide with34%conversion at93%selectivity. The kinetic study of the binary catalyst system showed that the H2O2formation was the rate-determined step, and the mechanical-mixing state of the catalyst system limited the styrene conversion. In order to overcome this obstacle, we designed a novel integrated supported gold catalyst, in which the CeO2was supported on the TS-1and the gold particles selectively deposited on the surface of CeO2. This strategy largely promoted the conversion of styrene from34%to98%, with the epoxide selectivity at87%. The integrated catalyst was highly stable and efficient for epoxidation of many kinds of olefins.Based on the results above, we further extended this O2reductive activation strategy into the ammoximation of cyclohexanone. In this study, water was chosen as the solvent. And Au/Al2O3together with HCOONH3gave the best activity in H2O2synthesis. Additionally, HCOONH3could act as not only the hydrogen donor but also the ammonium source, which made it prior to other formate salts (HCOOK and HCOONa). In the titanosilicate screen, among all the catalysts, the titanosilicate with MWW structure (Ti-MWW) afford the highest oxime yields. The kinetic study also implied the same results, that the H2O2generation is the rate-determined step. Following the previous strategy, we prepared the Au/Al2O3/Ti-MWW integrated catalyst, and this catalyst enhanced the oxime yield from31%(for binary catalysts system) to98%(for integrated catalyst). |
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