新型非晶态合金的制备、结构表征及其催化糠醛加氢的研究
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摘要
提高催化效率和降低环境污染是目前化学和化工工作者研究的热点,也是绿色化学的两个基本要求,为此开发新型的催化材料是摆在研究者面前的关键问题,而非晶态合金是21世纪最具开发前景的高效环境友好催化新材料之一。
     非晶态合金是一类介于晶态和无定型物质之间的特殊材料,在结构上表现为长程无序而短程有序,这种独特结构使其具有优良的催化性能。如在α,β不饱和醛加氢反应中表现出高活性和高选择性,并具有良好的抗中毒能力。目前,在糠醛催化加氢反应中所用的催化剂主要有Cu-Cr和修饰的Cu-Cr以及Raney Ni等,Cu-Cr催化剂毒性较高,对环境构成严重的危害,而且易阻塞反应器。Raney Ni催化剂虽然活性高,但容易产生副产物,且催化剂制备过程中涉及碱抽铝,造成一定的环境污染。非晶态合金材料在许多催化反应中表现出优良的催化性能,可用于制备和开发高效的、环境友好的、新型的糠醛加氢制备糠醇的催化剂。
     本论文采用化学还原法制备了超细Ni-Ce-B、Co-Fe-B非晶态合金以及负载型Co-B/CeO_2-SiO_2非晶态合金催化剂,考察了其在糠醛加氢反应中的催化性能。本文采用一系列的表征方法(XRD、XPS、TEM、BET、氢吸附、TPD等)进一步研究非晶态合金催化剂的表面结构、表面电子态和催化性能之间的关系,探索电子效应和几何效应对催化剂活性中心结构的影响,探索非晶态合金的晶化失活和载体对非晶态结构的稳定化作用,以及修饰剂对催化性能的改善作用等。
     (一)催化剂的制备
     (1)超细Ni-Ce-B非晶态合金催化剂的制备:用化学还原法将适量的KBH_4溶液逐滴加入到含有镍盐和铈盐的混合的溶液中,得到黑色沉淀,即为Ni-Ce-B非晶态合金催化剂,经多次洗涤后存放在无水乙醇中备用。
     (2)Co-Fe-B的制备方法同上。只是用铁盐代替铈盐。
     (3)负载型Co-B/CeO_2-SiO_2非晶态合金催化剂的制备:先用适量的Ce(NO_3)_3溶液浸渍40~60目SiO_2载体过夜,在红外灯下干燥后置入管式炉中,于773K焙烧2.0h,再用适量的CoCl_2溶液浸渍过夜,用红外灯干燥,进一步在管式炉中于573~623K下烘干,冷却到室温。用适量的KBH_4逐滴加入还原,得到黑色沉淀,即为Co-B/CeO_2-SiO_2非晶态合金催化剂,经多次洗涤后存放在无水乙醇中备用。
    
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     (二)催化性能评价
     在高压釜中加入一定量所制备的非晶态合金催化剂和糠醛乙醇溶液,在1.0
    MPaH:以及适当温度下进行催化加氢,观察釜内压力变化或采用气相色谱测定催
    化反应速率、糠醛的转化率以及对目标产物糠醇的选择性。结果表明,所有非晶
    态合金催化剂在上述反应中的选择性均非常高,超细Ni一Ce一B、CO一Fe一B非晶态
    合金的催化活性明显的高于Cu一Cr催化剂;负载型C。一B的不仅活性高、而且热
    稳定性高,具有广泛的工业前景。
     (三)催化性能与结构的关系的研究
    根据催化剂的系统表征和糠醛加氢动力学的研究,对下列问题进行了探索:
     (l)非晶态合金比对应的晶态金属催化剂具有优良的催化活性,一方面归因
    于其短程有序而长程无序的独特非晶态结构、均匀分布和高度配位不饱和性的活
    性位;另一方面,归因于金属和类金属(B)之间的相互电子作用,导致金属呈富电
    子态,而类金属归)为缺电子态。
     (2)非晶态合金经高温处理后发生晶化失活,主要归因于非晶态结构转化为
    晶态结构以及金属一类金属合金的分解,同时与催化剂粒子团聚,导致活性比表
    面积下降有关。
     (3)将非晶态合金负载于适当的载体上可显著提高热稳定性及催化活性,主
    要归因于其与载体的相互作用以及载体对非晶态合金颗粒的分散作用。考虑到催
    化剂的还原度和分散度,不同负载型非晶态催化剂具有不同的最佳负载量和最佳
    焙烧温度。
     (4)Ce对Ni一B非晶态合金催化活性的促进作用归因于其对糠醛C=O的活化;
    Ce的氧化物的另一作用类似于载体提高了催化剂的分散度。Fe对C。一B合金活性
    的促进作用主要归因于Fe的氧化物的分散性和还原态Fe的供电子。
Chemists and chemical engineers are now focusing on researching new materials, improving catalytic efficiency and eliminating none environmental pollution. The latter two makes fundamental essentials for green chemistry. Therefore, it became crucial for scientists to develop novel catalytic materials.
    Amorphous alloys represent a variety of special materials with short-range ordering while long-range disordering structure which may provide pathways to the excellent catalytic properties, i.e., good anti-poisoning ability, the higher catalytic activity and selectivity in homogeneous hydrogenation of a,p-unsaturated aldehyde, the low and even none environment pollution
    Up to now, in hydrogenation of furfural to furfuryl alcohol, the most commonly used catalysts are Cu-Cr dopant Cu-Cr, and Raney Ni et al. However, highly toxicity and reactor-blockage restrained wide application to Cu-Cr catalyst. No matter how high activity Raney Ni has, it could not play an important role on the mentioned industry, because of its various undesired products and environmental contamination. Therefore, the amorphous alloy, possessing excellent catalytic properties in different reaction, would be applied in hydrogenation of furfural
    In the present thesis, the ultrafine Ni-Ce-B, Co-Fe-B amorphous alloys and supported Co-B have been prepared by chemical reduction. The catalytic performance of all the amorphous alloy catalysts mentioned above was measured during liquid phase hydrogenation of furfural to furfuryl alcohol, together with the kinetic studies. The relationship between the catalyst performance and the structure of catalyst, surface electronic state of the amorphous alloy has been systematically studied based on a series of characterization of catalysts (XRD, XPS, TEM, BET, TPD) . Meanwhile, the promoting effects of the support on the thermal stability and catalytic activity of the amorphous alloy catalysts have been discussed. Furthermore, by alternating the kinds of the additives and their amount in the amorphous alloys, the modification of several admixtures on the structure of the amorphous alloy has been investigated, which could account for their promoting effect on the catalytic behaviors.
    
    
    The researching work in the present thesis could be summarized as follows.
    1. Catalyst preparation. By adding KBH4 into a solution containing (1) NiCl2 and
    Ce(SO4)2, (2) both CoCl24H2Oand FeCl3, the ultrafme amorphous alloys of Ni-Ce-B, Co-Fe-B were prepared, which were thoroughly washed with distilled water and stored in alcohol until the time of use.
    Meanwhile, the Co-B/CeO2-SiO2 amorphous catalyst was prepared by impregnating the SiO2 support with Ce(NO3)3 solution, and the drying, calcinations at 773 K for 2.0 h, and adding CoCl2 aqueous into prepared SiO2, followed by drying, calcinations, and KBRt reducing. The resulting Co-B/CeO2-SiO2 sample was washed in the similar way to that mentioned above and also kept in alcohol until the time of use.
    2. Activity test. All the furfural hydrogenation reactions were carried out in a autoclave containing certain amount of the as-prepared catalysts, furfural and alcohol. RH2m was determined by monitoring the drop of the hydrogen pressure inside the autoclave. The conversion of the furfural and the selectivity to furfuryl alcohol was obtained according to the product analysis by gas chromatograph. All the amorphous alloy catalysts exhibited much higher catalytic activity than Cu-Cr, which was widely used in industrial process of furfural hydrogenation, showing a good potential in industrial application.
    3. Correlation of the catalytic performance to the structure of the catalysts.
    (1) The amorphous alloys usually exhibited much higher activity than their corresponding crystalline counterpart. On one hand, in viewpoint of the structural effect, this could be attributed to the unique amorphous structure (i.e., the short-range ordering but long range disordering structure), the homogeneous distribution of the active sites, and the highly coordinative installation of these acti
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