| 摘要: | 本研究以共沉澱法製備不同Mg/Al比例的MgxAlO-hydrotalcite，經不同溫度煅燒後做為製備金觸媒的擔體，並以沉澱沉積法製備2%Au/MgxAlO觸媒。本研究有系統地以CO氧化反應、富氫中CO選擇性氧化及α、β-不飽和醛選擇性氫化反應探討製備變因對金觸媒活性的影響，藉以篩選出最佳製備條件，製備變因包括熟化溫度、熟化時間、Au溶液pH、Au溶液濃度、Mg/Al比、擔體煅燒溫度及觸媒煅燒溫度，另以BET、ICP、XRD、UV-vis、TGA、TEM、XPS及in situ DR-FTIR等分析，瞭解其物理與表面性質。 不論CO氧化反應、富氫中CO選擇性氧化或α、β-不飽和醛選擇性氫化反應，2%Au/Mg2AlO(100)觸媒最適製備條件是不預先調整金溶液的起始pHi值(pHi值為2)、金溶液濃度在1×10–3 M、Mg/Al = 2 (Mg2AlO)經100°C乾燥後做為製備金觸媒的擔體及2%Au/Mg2AlO(100)觸媒經100°C乾燥。在製備變因的探討中，吾人發現除了金負載量外，Au3+/Au0比值扮演重要角色，隨著2%Au/Mg2AlO(100)觸媒表面Au3+/Au0比值遞減，觸媒於CO氧化反應、富氫中CO選擇性氧化及肉桂醛選擇性氫化反應活性/選擇率也隨之遞減。2%Au/Mg2AlO(100)觸媒經100°C煅燒，表面有最高Au3+/Au0比值，於CO氧化反應、富氫中CO選擇性氧化及肉桂醛選擇性氫化反應中皆有最佳活性，且有最佳肉桂醇選擇率。2%Au/Mg2AlO(100)觸媒以升溫方式進行CO選擇性氧化後，Au3+/Au0比值由原來的1.2提升至1.5，反向降溫進行反應，觸媒維持高活性至室溫。 Mg2AlO(100)擔體與2%Au/Mg2AlO(100)觸媒經100°C及300°C煅燒後，分別在無氧(CO/He)及有氧(CO/O2/He)的氣氛下進行in situ DR-FTIR光譜分析，間接證明經100°C煅燒擔體表面有較活潑的OH基可與CO反應產生CO2，經300°C煅燒之擔體表面，則失去活潑OH基可參與反應；於100°C煅燒觸媒，觀察到CO吸附於Au3+及Au0，間接證明CO氧化反應與Au3+/Au0比值有關。此外，將2%Au/Mg2AlO(100)觸媒於程溫選擇性氧化反應下進行in-situ DR-FTIR動態分析，觀察到OH基與CO2的吸收帶同時增強，間接證明反應過程產生的OH基與CO氧化反應有關。 本研究藉CO氧化反應及選擇性氧化反應，與XPS及FTIR分析提出2%Au/Mg2AlO(100)觸媒於CO氧化反應機制。CO吸附在Au0上，與Au3+-OH反應產生Au-COOH；氧在Au0上解離吸附成Au0-O；Au-COOH與Au0-O的氧原子反應生成Au-CO3H中間體，並分解成CO2與OH基。同時Au-COOH也可能與Mg2AlO擔體的OH基反應，產生CO2和H2O，H2O回補OH基至擔體上，OH基也能溢流至Au3+上，維持Au3+的價態。 反應物分子於2%Au/Mg2AlO(100)觸媒表面之吸附作用力不同於一般金屬觸媒，於肉桂醛選擇性氫化行為也不同，本質上優先選擇性氫化共軛C=C/C=O中C=O鍵，但不會繼續氫化C=C鍵成全氫化產物。達到完全反應，肉桂醇產率為84%。2%Au/Mg2AlO(100)觸媒於不同官能基之氫化活性依序為C=C/C=O(不飽合醛) > C=O(飽合醛) >> C=C(不飽合醇)，此與第Ⅷ族金屬觸媒催化性質迴異。反應物分子於金觸媒表面吸附作用力為偶極作用力與瞬間偶極作用力，而非共價作用力。共軛C=C/C=O鍵因共振產生非定域化(delocalization)的瞬間偶極Cδ+ Oδ--Cδ+ Oδ-，其在金觸媒表面吸附強度大於具偶極的C=O鍵，且遠大於具偶極的C=C鍵。 2%Au/Mg2AlO(100)觸媒於肉桂醛氫化反應中，隨著反應溫度及反應壓力提升，反應速率明顯提升，但不同於第Ⅷ族金屬觸媒，肉桂醇選擇率隨溫度升高而提升，且肉桂醇選擇率不因提升反應壓力而降低。溶劑效應中，不同於第Ⅷ族金屬觸媒，金觸媒對於氫氣吸附解離能力遠不及第Ⅷ族金屬觸媒，氫氣於溶劑中的溶解度對於金觸媒催化活性影響勝於其他因素，所以溶劑效應完全不同於第Ⅷ族金屬觸媒。氫氣於非極性溶劑環己烷及正己烷中的溶解度大於醇類極性溶劑，2%Au/Mg2AlO(100)觸媒於肉桂醛氫化反應活性，活性大小依序為環已烷 ≈ 正己烷 > 乙醇。 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. |
