| 摘要: | 本研究採臨濕含浸法製備Ag/Mg2AlO觸媒，以檸檬醛模式反應篩選5 wt% Ag/Mg2AlO觸媒之最佳製備條件。以最佳5wt% Ag/Mg2AlO觸媒探討溶劑效應、全氫化現象、反應物分子結構、擔體效應等實驗變因之影響。並以TGA、BET、TEM、XRD、TPR、TPD等分析探討觸媒之物理特性與表面性質。此外，本研究另選用γ-Al2O3、MgO、CeO2、SiO2等不同擔體製得5wt% Ag/support觸媒，探討擔體效應之影響。 5wt% Ag/Mg2AlO觸媒之最佳製備條件為Mg2AlO擔體經100 °C煅燒、觸媒經250 °C煅燒，反應前以400 oC還原。5wt% Ag/Mg2AlO觸媒可有效將檸檬醛選擇性氫化成不飽和醇，選擇率高達97 %。 x% Ag/Mg2AlO觸媒之銀負載量小於5wt%，銀粒徑隨負載量增加而變大，檸檬醛氫化反應活性隨負載量提高而增加，且不飽和醇選擇率亦增加，屬結構敏感性。銀負載量大於5wt%，負載量提高對反應活性提升效果趨緩，且不飽和醇選擇率無明顯影響。 反應物分子於銀觸媒表面之吸附作用力不同於一般金屬觸媒，使得5wt% Ag/Mg2AlO觸媒於檸檬醛反應也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，不飽和醇選擇率卻不及檸檬醛。 5wt% Ag/Mg2AlO觸媒於極性溶劑乙醇有較佳的檸檬醛氫化反應活性，且醇類溶劑的碳數愈高，反應活性愈佳，不飽和醇選擇率則略下降。 Ag/support觸媒之銀顆粒大小並非影響活性的單一因素，擔體的酸鹼性也扮演重要的角色。弱鹼性基促進氫於銀觸媒表面進行不均化解離，強鹼性基則可協助氫於銀表面進行均化或不均化解離。Mg2AlO-hydrotalcite鹼性擔體於α,β-不飽和醛選擇性氫化反應的優勢，可製得具高活性的銀觸媒。 Silver is dispersed on a solid base of Mg2AlO-hydrotalicite using a wet impregnation method to obtain a effective catalyst for selective hydrogenation of alpha,beta-unsaturated aldehydes. In the present study, 5wt% Ag/Mg2AlO catalysis on liquid-phase hydrogenation of a citral mode reactant. We disussed the experiment factors such as solvent effect, the whole phenomenon of hydrogenation, the molecular structure of reactants, steric effect and other impact from the experimental changes. The silver catalyst is characteristized by TGA, BET, TEM, XRD, TPR, TPD. In addition, we used γ-Al2O3, MgO, CeO2, SiO2 as support to observe the impact of support effect. The optimal catalyst, 5wt% Ag/Mg2AlO, was prepared by following process: Mg2AlO calcined at 100 °C as a support , 5wt% Ag/Mg2AlO catalyst calcined at 250 °C, and reducted at 400 oC before the catalysis. 5wt% Ag/Mg2AlO catalyst display effectively chemoselective hydrogenation of citral into unsaturated alcohols (Nerol and Geriol) for selectivity of 97%. The interaction force of unsaturated aldehyde molecule adsorbed on the surface of the silver metal catalyst, is different from the convalent force of conventional hydrogenation catalyst (Pt, Ni, etc), making the 5wt% Ag/Mg2AlO catalyst in citral reaction from a general catalytic behavior of metal catalysts. The C=O bond of conjugated C=C/C=O bond was hydrogenated prioritily; the remained C=C bond from conjugated C=C/C=O bond would not be hydrogenated continuously. The longer fuctional group of unsaturated aldehyde molecule, the greater catalytic activity.The 5wt% Ag/Mg2AlO has better citral hydrogenation activity in polar solvents than in nonpolar solvent. Higher alcohol carbon number can offer the catalytic better reactivity, but the unsaturated alcohol selectivity is slightly decreased. In the discussion of support effect, we conclude the activity of silver catalyst is proportional the relations with the base site concentration of support. The particle size of the silver catalyst is not affect the activity obviously. The base sites of support can promote hydrogen dissociated on the surface of silver-based catalyst. Among the supports in study, Mg2AlO-hydrotalcite in α,β-unsaturated aldehyde hydrogenation can be obtained the high potency silver catalyst. |
