| 摘要: | 鹵素硝基芳香族(Aromatic halo-amine)是有機合成、精細化工中間體的重要化工原料，廣泛地應用於染料、農藥、殺蟲藥等各領域中，其主要製程以利用硝基化合物在金屬觸媒上進行非均相氫化反應為主。以對-氯硝基苯(p-Chloronitrobenzene)為例，它所含的硝基會被氫化，所含的-Cl也有可能被去鹵化(Dehalohenation)，造成一些副產物的產生。 奈米非晶態物質結合了非晶結構與奈米尺寸的優點，他們含有較多的表面原子與高度配位不飽和鍵；此外，奈米非晶態合金粒子因為具有特殊的化學催化性質，而引起廣泛的注意。本研究以硼氫化鈉為還原劑，醋酸鎳為前驅鹽類與次磷酸鈉和醋酸鈷，以化學還原法製備一系列的NiPB及NiCoB奈米非晶形合金觸媒，並探討其物理特性及其在液相對-氯硝基苯選擇性氫化反應上之影響。將此一系列製備而成的NiPB及NiCoB奈米鎳觸媒，以氮氣吸附儀、X-光繞射儀、穿透式電子顯微鏡、X-光光電子能譜儀等儀器鑑定其物理及表面性質。不同的Ni/P或Ni/Co製備比例，會影響在觸媒製備過程中硼的沉澱量，進而影響觸媒的型態、電子組態的改變及氫化反應活性的不同。由XPS的結果得知，硼會將一部分的電子給鎳，而磷則是會部分拉鎳的電子，在鎳硼觸媒中加入少量的磷，對於觸媒粒子的表面積及氫化活性均有提升的作用。 加入鈷後可鎳硼觸媒的粒子變小，NiCoB觸媒的粒子直徑均小於5nm，因此表面積增加，此效應對觸媒的氫化活性有很大的幫助，且發現加入鈷可增加NiB觸媒的熱穩定性，觸媒粒子結晶的現象因而遭到抑制。由於鄰近的元素態鎳與元素態硼會有微量的電子轉移情形發生，以致鎳有較多的電子，當電子轉移的情形愈明顯就愈容易吸引對-氯硝基苯吸附至觸媒表面以進行反應，換言之，電子轉移情形較明顯的觸媒，其活性會較高。由XPS鑑定可看出，NiCoB(Ni/Co molar ratio=1:0.1)擁有最明顯的電子轉換情形，因此其擁有最高的氫化活性。雖然NiCoB(Ni/Co molar ratio=1:0.1)擁有最高的氫化活性，但是它卻失去了對對-氯苯胺的選擇性。 水溶性高分子PVP是由乙烯吡咯酮為單體聚合而成，其分子鏈上有N與O原子，其未共用電子對易與過渡金屬之空軌域產生某種程度之配位作用力，對金屬形成一高分子保護層，避免製備過程聚集成長，本研究嘗試在製備過程添加水溶性PVP高分子為穩定劑，期能將NiCoB觸媒顆粒更進一步奈米化、均一化。高分子穩定劑PVP，分子量以10,000為主，結果顯示能有效的促使觸媒粒徑更小且均一化，並為耐熱性與結構穩定性較佳的非晶形奈米合金觸媒。 高壓二氧化碳的引進，確實造成的甲醇體積的膨脹，除了可增加氫氣溶解度外，亦因甲醇體積膨脹導致液體密度與黏度的降低，而減少了質傳阻力。然而因為在操作壓力為3.54 MPa及反應溫度為343 K的環境下，濃度1.5 M的對-氯硝基苯的氫化反應過程中，同時受到反應控制及氣-液質傳阻力互相影響，而且是以反應控制為主，因此，即使加入高壓二氧化碳可以減少質傳阻力，對整個氫化反應的活性卻沒有太明顯的影響。 Aromatic halo-amines are used extensively in industrial applications for the production of fine chemicals, i.e., dyes, herbicides, pesticides, etc. The main route for their production is the selective hydrogenation of the corresponding nitrocompounds over heterogeneous metal catalysts. Considering p-chloronitrobenzene as an example, its N=O group may be hydrogenated or its -Cl may be dehalogenated. Besides the desired product p-chloroaniline, many byproducts such as aniline, nitrobenzene, p-chlorophenylhydroxylamine, p-chloronitrosobenzene, azo- and azoxy-dichlorobenzenes, and chlorobenzene were formed at the same time. The nanomaterials, combining the features of amorphous and nanometer powers, have more surface atoms and a higher concentration of coordinately highly unsaturated sites. Nanometer amorphous alloy powders have attracted extensive attention due to their unique isotropic structural and chemical properties. A series of nickel catalysts were prepared by chemical reduction method. Nickel acetate and sodium hypophosphite or cobalt acetate were mixing in methan, the solution of sodium borohydride in excess amount was then added dropwise into the mixture to ensure full reduction of cations. The as-prepared catalysts were characterized with X-ray diffraction, nitrogen sorption, transmission electron microscopy, and X-ray photoelectron spectroscopy. The initial molar ratios of starting materials affected the concentration of boron bounded to the nickel metals, resulting in the change of surface area, electron structures of the metals and catalytic activities of the catalysts. The XPS results revealed that boron combined with nickel metal in the NiPB powder donates electrons to nickel metal and that phosphorus withdraws electrons from nickel metal. Small amounts of phosphorus in the NiB catalyst can increase the surface area and turnover frequency. Both are beneficial for promoting the reaction. The addition of cobalt into NiB catalyst could reduce the particle of nickel catalyst, improve the particle dispersion, and suppress the growth of crystalline structure of NiB and help the NiB catalyst to maintain its amorphous state. Based on the electron transference between elemental nickel and boron, NiCoB(1:0.1) had the most d-band electrons and the highest electron density, therefore it also has the highest activities of hydrogenation of nitro-group and dehalogenation reactions. Although it had the highest activity of hydrogenation of p-CNB, it lost the selectivity of p-CAN at the same time. The water-soluble polymer of polyvinylpyrolidone was polymerized by vinylpyrolidone, there are –N and –O atom on the molecular chain. Because of the lone pair electrons of functional groups, the multi-coordination produces a considerable strength of chemisorption of polymer on the nano-particle. The polymer adsorbed on the nano-particles might prevent the aggregation of nano-particles by steric stabilization. The PVP polymers effectively had significant effect on the thermal stability. It is demonstrated that adding high pressure carbon dioxide into methanol really can expand the volume of liquid thus the solubility of hydrogen in the solvent will be increased and the resistance of gas-liquid mass transfer will be reduce. Although the pressure is increased up to 3.54 MPa and the concentration of p-CNB is increased to 1.5 M, the hydrogenation of p-CNB is influenced by the effect of gas-liquid mass transfer, but it is mainly controlled by the resistance of the reaction. So even the resistance of gas-liquid mass transfer is reduced, it is unable to cause too great influence on the hydrogenation of p-CNB. |
