| Summary: | 博士 === 國立中央大學 === 化學工程與材料工程研究所 === 96 === In this investigation, hydrotalcites MgxAlO with various Mg/Al molar ratios were prepared by co-precipitation, and gold catalysts containing 2 wt% Au (2%Au/MgxAlO) were prepared by deposition precipitation (DP). The effect of various parameters on the preparation of catalysts, including the temperature and the duration of aging, the pH and the concentration of HAuCl4 in the initial gold solution, the Mg/Al molar ratio and the calcination temperatures of the MgxAlO support, and the calcination temperatures of the Au/MgxAlO catalysts for CO oxidation, CO selective oxidation and hydrogenation of α,β-unsaturated aldehydes were systematically discussed. The catalysts were characterized by the specific surface areas analysis (SBET), inductively coupled plasma spectroscopy (ICP), X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-vis), thermogravimetry (TGA), X-ray photoelectron spectroscopy (XPS), and in situ diffuse reflection Fourier transform infrared spectroscopy (in situ DR-FTIR).
The optimal catalyst, 2%Au/Mg2AlO(100), was obtained using the following preparation parameters: 1 × 10–3 M HAuCl4, pH = 2 (without adjusting pH) in the initial solution, Mg/Al = 2 (Mg2AlO) calcined at 100°C as a support, and 2%Au/Mg2AlO catalyst calcined at 100°C for CO oxidation, CO selective oxidation and hydrogenation of α,β-unsaturated aldehydes. This investigation confirms that not only gold loading of the catalyst is important, the ratio of gold states (Au3+/Au0) is also critical in determining the activity of the catalyst, and the activity declined markedly as the Au3+/Au0 ratio decreased for CO oxidation, CO selective oxidation and hydrogenation of α,β-unsaturated aldehydes. As the 2%Au/Mg2AlO(100) catalyst had the largest Au3+/Au0 ratio, the activity of the catalyst and the selectivity of cinnammyl alcohol is also highest. The activity of this optimal catalyst was improved through CO selective oxidation pretreatment and the Au3+/Au0 ratio increased from 1.2 to 1.5.
In situ DR-FTIR were performed to investigate Mg2AlO(100) and 2%Au/Mg2AlO(100) catalyst calcined at 100 and 300°C, respectively, in the presence(CO/O2/He) or absence(CO/He) of O2. Undoubtedly, the formation of CO2 in an oxygen-free environment resulted from the reaction between CO and the active OH groups on Mg2AlO calcined at 100 °C. On the other hand, those Mg2AlO calcined at 300°C have no active OH groups to participate in the reaction. 2%Au/Mg2AlO(100) catalyst calcined at 100°C had adsorption of CO on Au0 and Au3+. These results demonstrate that the CO oxidation reaction may be related to the Au3+/Au0 ratio. Otherwise, for 2%Au/Mg2AlO catalyst during the programmed temperature reaction of selective oxidation of CO could be observed that the absorption intensity of the OH group CO2 increased with the temperature to a maximum at 45°C. These results demonstrate that the generation of CO2 may be related to the formation of the OH groups during the reaction.
Based on XPS and in situ DR-FTIR analyses, a mechanism for CO selective oxidation on 2%Au/Mg2AlO was proposed. The hydroxyl group on Mg2AlO also participated in the reaction. CO is adsorbed on Au0 and reacts with Au3+-OH to form carboxylate group. Oxygen on Au0 dissociates to form Au0-O. The carboxylate group reacts with the oxygen of Au0-O to form the bicarbonate intermediate which then dissociates into CO2 and OH radical. Simultaneously, the carboxylate group may also react with the OH group on Mg2AlO to form CO2 and H2O. The adsorption of OH groups on Au3+ and the replenishment of OH groups by H2O did not change the stability of Au3+ on the 2%Au/Mg2AlO catalyst, maintaining a Au3+/Au0 ratio that is suitable for the reaction.
The interaction forces of reactant molecules adsorbed on 2%Au/Mg2AlO(100) catalyst surface are different from that of conventional metal catalysts, and so is their selective hydrogenation behavior for α,β-unsaturated aldehydes; intrinsically more active in the hydrogenation of the C=O bond as compared to the hydrogenation of the C=C bond, but further hydrogenation of the C=C bond to 3-phenylpropanol is not likely. For complete reaction, the yield of cinnammyl alcohol is about 84% from the hydrogenation of cinnamaldehyde and a high yield of nerol/geraniol was obtained over the 2%Au/Mg2AlO catalysts. The order of hydrogenation activity for different function groups over 2%Au/Mg2AlO(100) catalysts is C=C/C=O(unsaturated aldehydes) > C=O(saturated aldehydes) >> C=C(unsaturated alcohol), which is irrelevant to the activity of the metal catalysts in Ⅷ groups. The interaction forces of reactant molecules adsorbed on the gold catalyst surface is dipole-dipole interaction and instantaneous dipole interaction (dispersion force) rather than non-covalence interaction. The delocalization instantaneous dipole Cδ+ Oδ--Cδ+ Oδ- resulted from the resonance of conjugate C=C/C=O bond has made its adsorption on the gold catalyst surface stronger than that of dipole C=O bond, and even stronger than that of dipole C=C bond.
In the hydrogenation of cinnamaldehyde by 2%Au/Mg2AlO(100) catalyst, reaction rate was improved significantly as the reaction temperature and pressure was increased. Same trend was observed for the selective hydrogenation of cinnammyl alcohol, however, this is different from that of metal catalysts in the Ⅷ group. Under solvent effect, hydrogen adsorption dissociation ability of gold catalyst is much weaker than that of metal catalyst in the Ⅷ groups. The solubility of hydrogen in the solvent has a huge influence on the activity of gold catalyst comparing with other factors, therefore, the solvent effect of gold is completely different from that of metal catalysts in the Ⅷ groups. The solubility of hydrogen in the nonpolar solvent, such as cyclohexane and n-hexane, is bigger than that in polar solvent like alcohols. As the result, the activivty of 2%Au/Mg2AlO(100) catalyst for the hydrogenation of cinnamaldehyde is in the order of cyclohexane ≈ n-hexane > alcohol.
|
|---|
