| 摘要: | 本研究利用三種硼氫化鈉化學還原法製備奈米均一化NiB觸媒，在傳統化學還原法製備過程中導入水溶性高分子PVP穩定劑製得PVP-NiB觸媒；以H2O/CTAB/n-hexanol逆微胞技術配合化學還原法製得ME-NiB觸媒；另以含浸法配合中溫煅燒將先驅鹽固著，再以化學還原法將NiB觸媒負載於SiO2擔體上製得NiB/SiO2觸媒。藉由丁醛液相氫化為模式反應，尋找三種奈米化NiB觸媒製備之最適條件，探討糠醛、巴豆醛及檸檬醛的選擇性氫化反應性質。 非負載式觸媒以醋酸鎳鹽為佳，負載式觸媒由於需經中溫煅燒固著，則以不易分解的氯化鎳鹽為佳。在添加硼氫化鈉的還原過程中，NiB與PVP-NiB觸媒都有一最適添加速率，可是ME-NiB與NiB/SiO2觸媒是以一次瞬間加入為佳。 PVP-NiB、ME-NiB及NiB/SiO2觸媒為非晶態結構，粒徑分布2~5 nm明顯小於NiB觸媒的7.7~40 nm。經奈米化之PVP-NiB、ME-NiB及NiB/SiO2觸媒於丁醛、巴豆醛、糠醛及檸檬醛等反應都有優越的催化活性，約為NiB觸媒的3~10倍，因此，可於較低的溫度下進行反應，其活性不但媲美貴金屬觸媒，亦可以得到較多主產物之產率。 奈米化PVP-NiB、ME-NiB與NiB/SiO2觸媒可有效應用於檸檬醛選擇性氫化反應，檸檬醛(citral)為具有一共軛C=C/C=O鍵及孤立C=C鍵之多官能基不飽和醛，可經由選擇氫化成不同的產物，奈米化NiB觸媒於檸檬醛氫化反應中，主產物是先氫化共軛C=C鍵得到香茅醛，爾後繼續氫化C=O鍵得到香茅醇，香茅醛與香茅醇都是具經濟價值的香料產物。若以環己烷為反應溶劑、80°C下，PVP-NiB觸媒活性為NiB觸媒的4.9倍，ME-NiB觸媒為9.8倍，NiB/SiO2觸媒更高達10倍以上，遠優於商用Raney Ni 觸媒。 NiB與PVP-NiB觸媒中添加鉻、釷、鉬及鎢均能促進反應活性，其中又以鉻的促進效果最佳，這些促進劑反不利ME-NiB與NiB/SiO2觸媒。反應溶劑明顯影響觸媒活性，且因觸媒而不同，NiB與PVP-NiB觸媒以極性溶劑為佳，活性大小影響依序為甲醇>乙醇>環己烷>正己烷，ME-NiB與NiB/SiO2觸媒以非極性溶劑為佳，溶劑效應則相反。若香茅醛與香茅醇同為主產物，選擇高活性觸媒與溶劑，可得98%以上的高產率。若以香茅醛為單一主產物，以PVP-NiB為觸媒，50°C下可得最大產率為92%；以ME-NiB與NiB/SiO2為觸媒，30°C下可得最大產率分別為88%與90%。 動力學探討中發現，NiB系列觸媒於檸檬醛選擇氫化成香茅醛初活性隨H2壓力上升而上升，表觀反應級數分別為0.53 (NiB)、0.2 (PVP-NiB)、0.36 (ME-NiB) 與0.41 (NiB/SiO2)。初活性亦隨檸檬醛濃度上升而上升，表觀反應級數分別為0.45 (NiB)、0.2 (ME-NiB)與0.23 (NiB/SiO2)；PVP-NiB觸媒例外，初活性幾乎不受濃度影響。檸檬醛選擇氫化成香茅醛表觀活化能，NiB觸媒(70~100°C)為9.7 kJ/mol，PVP-NiB觸媒(50~100°C)為3.2 kJ/mol，ME-NiB觸媒(30~60°C)為3.1 kJ/mol，NiB/SiO2觸媒(30~50°C)為2.4 kJ/mol。 The PVP-stabilized NiB catalysts were prepared using the chemical reduction method with NaBH4, dissolving the water-soluble polymer of polyvinylpyrrolidone (PVP) in the precursor salt solution as a protective reagent. The PVP-NiB catalysts were characterized and examined for their catalysis on the hydrogenation of furfural, crotonaldehyde and citral. PVP polymer could adsorb on the nano-particles of NiB via a weak coordination bonding and stabilize it; the molecular weight of PVP about 10,000 was suitable, and the optimal quantity of PVP (PVP/Ni) in the salt solution for preparing catalysts was around 20. The PVP-NiB samples were characterized by XRD as an amorphous structure and by TEM with a particle size distribution in the range of 2.5–7.7 nm. On catalysis, the PVP-NiB catalyst was significantly more active and slightly more selective than NiB for hydrogenating furfural to furfuryl alcohol and crotonaldehyde to butyraldehyde. A good yield of citronellal about 92% could be obtained by reducing citral in cyclohexane at a low reaction temperature of 50ºC over the PVP-NiB catalyst. Surfactant-stabilized NiB catalysts (ME-NiB) were prepared using the chemical reduction method in the ternary microemulsion system of water/CTAB/n-hexanol. The surfactant molecules could adsorb on the surface of the formed particles; they act as a protective agent and restrict the growth of nano-particles. The size of nano-particles was not completely determined by the size of the microemulsion droplets, but also depended on the composition of the solution. Additionally, the concentration of nickel salt, the amount and speed of addition of NaBH4, and the temperature influenced the sizes of the particles and the reactivity of the ME-NiB nano-particles. The ME-NiB catalyst was characterized and examined for its catalysis on the hydrogenation of furfural, crotonaldehyde and citral. It was thus compared with the NiB and PVP polymer-stabilized NiB catalysts. The ME-NiB sample was characterized by XRD as an amorphous structure and by TEM with a particle size distribution in the range 1.2–5.0 nm. The ME-NiB catalyst was markedly more active and slightly more selective than NiB or PVP polymer-stabilized NiB in the hydrogenation of furfural to furfuryl alcohol and crotonaldehyde to butyraldehyde. A good yield of citronellal, around 88%, was obtained by reducing citral in cyclohexane at a room temperature of 30ºC over the ME-NiB catalyst. A super-active supported nickel catalyst-NiB/SiO2 could be obtained with chemical reduction method for liquid-phase hydrogenation. The precursor salt of nickel was mounted on SiO2 by impregnating, drying and calcination at an appropriate temperature without being decomposed, and then reduced with aqueous NaBH4 solution. The influential factors for preparation NiB/SiO2 catalysts were examined by the hydrogenation of butyraldehyde. The NiB/SiO2 catalysts were characterized as an ultrafine and amorphous structure, which are much more active than NiB and Ni/SiO2 a conventional supported nickel catalyst reduced by H2. The optimal catalyst of 5%NiB/SiO2 was used for hydrogenating citral to citronellal and cironellol, which was about fourteen times as active as NiB, but less selective than it. Nevertheless, the reaction could be performed at room temperature to promote the selectivity. A good yield of citronellal/cironellol about 90% and a yield of citronellal about 81% over 5%NiB/SiO2 could be obtained at a low temperature of 30ºC. |
