化學還原法製備Ag/Mg2AlO觸媒之研究-α,β-不飽和醛選擇性氫化反應;Preparation of Ag/Mg2AlO catalyst by chemical reduction method-selective hydrogenation of α, β-unsaturated aldehyde

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题名:	化學還原法製備Ag/Mg2AlO觸媒之研究-α,β-不飽和醛選擇性氫化反應;Preparation of Ag/Mg2AlO catalyst by chemical reduction method-selective hydrogenation of α, β-unsaturated aldehyde
作者:	莊依蕙;Juang,Yi-Huei
贡献者:	化學工程與材料工程研究所
关键词:	α;β-不飽和醛;氫化反應;硼氫化鈉;鎂鋁氧;銀觸媒;hydrogenation;β-unsaturated aldehyde;α;Ag;Mg2AlO;NaBH4
日期:	2012-07-06
上传时间:	2012-09-11 18:11:14 (UTC+8)
出版者:	國立中央大學
摘要:	本實驗室曾以傳統臨濕含浸法經含浸、煅燒、還原製備Ag/Mg2AlO觸媒，應用於檸檬醛選擇性氫化反應，發現活性不及金觸媒，但選擇率不亞於金觸媒。本研究以硝酸銀為前驅鹽，並依過去經驗採經100°C煅燒的Mg2AlO-hydrotalcite為擔體，採臨濕含浸法擬將硝酸銀含浸並乾燥固著於Mg2AlO擔體上，以NaBH4化學還原製備觸媒，避免發生燒結。研究過程中發現以100~150°C乾燥固著時，觸媒顏色明顯由白色轉變為黑色，且具有媲美金觸媒的活性與選擇率。本研究將觸媒製備分為兩大類加以探討，一為乾燥固著，未經NaBH4還原的Ag/Mg2AlO觸媒，另一為乾燥固著後再以NaBH4進行還原的Ag/Mg2AlO觸媒。 10wt%Ag/Mg2AlO觸媒，其擔體Mg2AlO-hydrotalcite的鹼性扮演重要的角色，Mg2AlO擔體的強鹼性基能促進硝酸銀部分分解，即使以低溫的50°C及25°C為乾燥固著溫度，10wt%Ag/Mg2AlO觸媒就具有相當的活性，隨乾燥固著溫度上升，觸媒顏色由白變黑，硝酸銀部分分解成Ag0，於150°C有最佳活性與選擇率。乾燥固著溫度大於150°C，則銀顆粒聚集，使活性明顯下降。 10wt%Ag/Mg2AlO觸媒為乾燥固著之10wt%Ag/Mg2AlO再經NaBH4還原程序所得，NaBH4還原程序可將尚未分解的硝酸銀還原成Ag0，但10wt%Ag/Mg2AlO觸媒活性遠不及10wt%Ag/Mg2AlO未還原觸媒。150°C乾燥固著溫度不足以將硝酸銀有效固著於擔體上，經NaBH4還原程序的銀顆粒容易聚集，且Na離子的殘留會削弱銀觸媒活性。乾燥固著溫度不足也使一部分硝酸銀來不及在擔體上被還原，就溶出至還原液中，在還原液中被還原成奈米銀，使得還原液具有高於觸媒的活性。化學還原法不適用於Mg2AlO擔體，Mg2AlO擔體的強鹼性基在乾燥固著時會促進硝酸銀分解。SiO2及γ-Al2O3則可藉提高乾燥固著溫度強化硝酸銀固著力，以NaBH4還原製得Ag/SiO2及Ag/γ-Al2O3觸媒，但反應活性仍低於Ag/Mg2AlO還原觸媒，更遠不及Ag/Mg2AlO未還原觸媒。本研究發現硝酸銀採簡單的含浸與乾燥固著，不經NaBH4還原程序，即可得到具有高活性及選擇率的氫化觸媒Ag/Mg2AlO，用於α,β-不飽和醛選擇性氫化反應。反應物分子於銀觸媒表面之吸附作用力為類似物理作用力，不同於一般金屬觸媒之共價作用力，使得10wt%Ag/Mg2AlO觸媒於α,β-不飽和醛反應不同於一般金屬觸媒催化行為，優先氫化共軛C=C/C=O鍵的C=O鍵，且不會繼續氫化單一C=C鍵；不飽和醛分子愈大，催化活性愈好；C=C鍵旁較大立體障礙的肉桂醛，不飽和醇選擇率反不及檸檬醛。Our research in the past, silver was dispersed on a solid base of Mg2AlO-hydrotalicite using the traditional wet impregnation method via impregnating, calcinating and reducing to preparation Ag/Mg2AlO catalyst for selective hydrogenation of citral. The investigation confirms the activity of Ag/Mg2AlO inferior to gold catalyst, but selectivity as good as gold catalyst. To avoid sintering, in the present study, silver nitrate as precursor salts, and the Mg2AlO-hydrotalcite calcined at 100°C as a support by past experience, using the wet impregnation method to be containing silver nitrate impregnating, drying and fixating on Mg2AlO to preparation of Ag/Mg2AlO catalyst by chemical reduction method with NaBH4. This study found the catalyst drying and fixating at 100~150°C, the color evident changes from white to black, and have a comparable gold catalyst activity and selectivity. The study will take catalyst preparation are divided into two categories, one is Ag/Mg2AlO catalyst by drying and fixating without NaBH4 reducing, and another is Ag/Mg2AlO catalyst by drying and fixating with NaBH4 reducing. The basic of Mg2AlO-hydrotalcite as a support plays an important role on 10wt%Ag/Mg2AlO catalyst, the strong basic sites of Mg2AlO can promote silver nitrate to decompose even at low temperatures 50°C or 25°C as drying and fixating temperature, the 10wt%Ag/Mg2AlO catalyst still has considerable activity. The color changes from white to black, silver nitrate partially decomposed into Ag0 when the drying and fixating temperature raise, a selection of the best activity and selectivity at 150°C. Drying and fixating temperature greater than 150°C, silver is sintering then activity decreased significantly. Preparation 10wt%Ag/Mg2AlO catalyst by 10wt%Ag/Mg2AlO drying and fixating with NaBH4 reducing, the NaBH4 reducing procedures will making silver nitrate decomposed into Ag0, but 10wt%Ag/Mg2AlO catalyst activity is far less 10wt%Ag/Mg2AlO catalyst without NaBH4 reducing. Drying and fixating temperature at 150°C not enough to be effective fixed silver nitrate on the support, using NaBH4 reducing procedures, silver easily sintering and Na ion residues weakens the activity of silver catalyst. In addition, drying and fixating temperature not enough, a portion of the silver nitrate too late to reduce on the support, it will dissolving to reducing liquid and reducing to the Nano silver in the reducing liquid, the reducing liquid caused higher activity than the catalyst. Chemical reduction method does not apply to Mg2AlO support, the strong basic sites of Mg2AlO can promote silver nitrate to decompose when drying and fixating. SiO2 and γ-Al2o3 can strengthening silver nitrate fixed on the support by increase drying and fixating temperature, and using NaBH4 reducing procedures to preparation Ag/SiO2 and Ag/γ-Al2O3 catalysts, but the activity remains below Ag/Mg2AlO catalyst, more far less Ag/Mg2AlO catalyst. This study found that simple method containing silver nitrate impregnating, drying and fixating without NaBH4 reducing procedures, can get a hydrogenation catalyst Ag/Mg2AlO with high activity and selectivity for selective hydrogenation of α,β- unsaturated aldehyde. The interaction force of reactant molecule adsorbed on the surface of the silver metal catalyst was similar of the dipole-dipole interaction and dispersion force which was such as physical interaction not as the covalent interaction of gerenal hydrogenation metal catalyst, making the 10wt%Ag/Mg2AlO catalyst in α,β- unsaturated aldehyde reaction different from general catalytic behavior of metal catalyst. The C=O bond of conjugated C=C/C=O bond was hydrogenated priority, the remained C=C bond from conjugated C=C/C=O bond would not be hydrogenated continuously. The longer fuctional groups of unsaturated aldehyde molecule, the greater catalytic activity. Besides, cinnamaldehyde have the larger steric hindrance of the phenyl group around the conjugated C=C bond, but the selectivity of cinnamyl alcohol from cinnamaldehyde reduction is smaller than that of nerol/geraniol from citral.
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