| 摘要: | 非晶相鎳鈷硼合金觸媒於對-氯硝基苯氫化反應上有很好的催化活性和選擇性，此研究中，主要分為質傳探討及活性探討兩大部分。質傳方面，引用了氣體滯留、外部質傳與內部質傳等觀念來評估在溫度為353 K，壓力為1.2 MPa，與轉速為500 rpm的條件下，鎳鈷硼觸媒應用在對氯硝基苯的反應性質，計算結果證明在設定條件下，質傳阻力已破除，為反應控制狀態。在活性探討方面，以硼氫化鈉為還原劑，利用醋酸鎳、醋酸鈷和偏鎢酸銨為前驅鹽類，以化學還原法製備改質後的非晶相鎳硼和鎳鈷硼合金觸媒，製備條件為：鎳/鈷/硼之莫耳比固定為1: 0.1: 3，鎳/鎢之莫耳比為0、0.1、0.3、0.5、0.7、0.9與1.0；製備溶劑使用50 vol %甲醇之混合溶劑；為避免製備過程中，觸媒被溶氧氧化使得活性降低，因此在製備環境中通以氮氣流，並探討其物理與化學特性於對-氯硝基苯氫化反應上的影響。以X光繞射儀、穿透式電子顯微鏡、高解析度穿透式電子顯微鏡、X光能譜散佈分析儀和X光光電子能譜儀等儀器鑑定其物理、化學特性和表面性質；以液相選擇性對-氯硝基苯氫化反應來測試觸媒的活性與選擇性，反應條件設定在：反應器為半批式反應器 (Parr Reactor Model 4842)；反應溫度為353 K；壓力為1.2 MPa；攪拌速率500 rpm；反應溶劑為甲醇；結果顯示添加劑的加入，能強化觸媒的熱穩定度、活性位置的分散性，提升反應的活性和選擇率，在p-CNB轉化率為100%時，對主產物對-氯苯胺的選擇率皆大於90 %，其中以鎳/鎢之莫耳比為0.7之觸媒活性表現最佳。而後針對未改質、鎢改質與鉬改質之鎳鈷硼觸媒進行觸媒壽命測試，在相同的反應條件下，連續進行3次批次反應測試；結果顯示鎢雖能改善觸媒的穩定性，卻無法延長鎳鈷硼的觸媒壽命，鉬改質之鎳鈷硼觸媒則能有效地使觸媒壽命延長。最後我們做了金觸媒與白金蜂巢狀觸媒應用在對氯硝基苯氫化反應的活性測試，金觸媒表現出在選擇率之優勢，蜂巢狀觸媒則顯示了在異相氫化反應上分離的方便性。 Nanosized NiCoB alloy catalyst has been reported to be a good catalyst for the liquid phase hydrogenation reactions due to their excellent activity and selectivity. In this research, Two investigations were carried out on the NiCoB catalyst. First, the effects of gas-holdup, external mass transfer, intra-particle mass transfer were studied based on the theoretical calculations. The results demonstrated that the hydrogenation reaction under the experimental conditions we adopted was reaction-controlled. Second, W was used as the additive to modify the properties of NiCoB catalyst. A series of W–doped NiCoB catalysts with various W/Ni molar ratios were synthesized by chemical reduction of nickel acetate, cobalt acetate and ammonium metatungstate. Sodium borohydride in 50 vol. % methanol/water mixed solvent was used as the reducing agent. The preparation was carried out at room temperature under vigorous stirring and nitrogen stream was used as the curtain gas to remove dissolved oxygen in solvent. The atomic ratio of Ni/Co was fixed at 10; and W/Ni ratios were 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0, respectively. In order to fully reduce Ni and Co cations, excess amount of NaBH4 was added (B/Ni atomic ratio was 3). These catalysts were characterized by X–ray diffraction, transmission electron microscopy, high resolution transmission electron microscopy, energy dispersive spectra, X-ray photoelectron spectroscopy and differential scanning calorimetry. The catalytic properties of W-NiCoB catalysts were studied on hydrogenation of p–chloronitrobenzene (p-CNB). The conditions for hydrogenation reaction were 1.2 MPa H2 pressure, 353 K reaction temperature and 500 rpm stirring speed. Methanol was used as the solvent, the concentration of p-CNB was 0.2 M (2.54g p–CNB in 80 ml methanol) and the amount of W-NiCoB catalyst was 0.002 mol. The results showed that the increase of W content made the particle size of NiCoB smaller. W was mainly in the form of hydroxide and acted as a spacer, which could keep NiCoB in amorphous state, and would suppress sintering in reaction. The sample with W/Ni atomic ratio of 0.7 had the highest activity for p–CNB hydrogenation. The selectivities of p-chloroaniline (p-CAN) for W-NiCoB catalysts were also very high. It demonstrated that W-doped NiCoB catalysts were suitable for liquid phase hydrogenation catalyst. Then, the deactivation tests to compare the catalytic life of NiCoB, W-NiCoB and Mo-NiCoB (the best result done in our lab) were also studied. The results showed that only Mo additive could extend the life of NiCoB. After the hydrogenation reaction, the NiCoB catalyst sintered to numerous large grains, whereas, W-NiCoB turned into the form of milkly-white powder. However, Mo-NiCoB retained the original black nano-clusters, the catalytic activities were maintained at high level after 3 batch runs. Additionally, gold catalyst for p-CNB hydrogenation with high selectivity and the monolith catalyst on this kind of reaction were also elementary studied. |
