【作者】 闪媛媛；

【导师】 李金林；

【作者基本信息】 中南民族大学 ， 物理化学， 2009， 硕士

【摘要】 费-托合成是将合成气(CO+H2)转化为液体燃料和化学品的过程。钴基催化剂因其具有高的活性、高的重质烃选择性、较低的水煤气变换反应活性被广泛应用于费-托合成。中孔分子筛具有大的比表面积和良好的水热稳定性,以其作为催化剂的载体可以获得高度分散的钴催化剂。硅材料负载的钴催化剂的反应活性主要与催化剂的还原度密切相关。钴氧化物(四氧化三钴,无定形钴氧化物,钴硅氧化物,钴铝氧化物等)的形成依赖于载体表面的性质和前处理条件,如焙烧温度、前驱体溶液的pH值等。在高温下还原后,钴氧化物与载体之间相互作用生成难还原的化合物如硅酸钴仍有部分存在,有研究报道载体表面羟基(-OH)的存在导致了这些难还原化合物的生成。本文分别采用内表面氨基化修饰和硅烷化的方法改性SBA-15表面的性质,以浆态浸渍法和满孔浸渍法制备Co/SBA-15催化剂;以KIT-6为载体,制备不同钴负载量的钴基催化剂。使用元素分析、N2吸附-脱附、X-射线衍射、透射电子显微镜、程序升温还原、程序升温脱附及氧滴定等手段表征催化剂。并在微型固定床反应器上考察催化剂的费-托合成反应性能。主要研究结果如下:1. SBA-15经内表面氨基化修饰和负载钴后,其二维六方有序结构仍然保持;氨基化修饰后的与未修饰的催化剂相比,钴在载体表面的分散度增加,形成的Co3O4晶粒平均直径减小;钴更易负载到SBA-15孔道内部,催化剂较难还原。2.对于硅烷化后的Co/SBA-15催化剂,金属钴与载体之间的相互作用降低,氧化钴的还原度增加。随着三甲基氯硅烷用量的增加,三甲基硅基的表面覆盖度增加,钴的晶粒直径增大。硅烷化后的10Co/SBA-15催化剂的高活性归因于其高还原度,C5+选择性的增加归因于钴晶粒直径的增加。载体的硅烷化可以得到具有更高CO转化率和更好的重质烃选择性的费-托合成催化剂。3. KIT-6分子筛的水热稳定良好,是一种很好的催化剂载体。随钴负载量的增加,催化剂的比表面积和孔体积下降约50%左右,而平均孔径没有什么变化。对于Co/KIT-6系列催化剂来说,金属与载体之间的相互作用小。费-托合成反应研究发现,随着钴含量的增加,催化剂的活性、C5+选择性均增加,这归因于催化剂高的还原度以及大的钴颗粒直径。更多还原

【Abstract】 Fischer-Tropsch synthesis (FTS) can convert syngas (CO+H2) into transportation fuels and chemicals. Cobalt-based FTS catalysts have been widely studied because of its high CO conversion, high selectivity for heavy hydrocarbons, low water-gas shift activity. Mesoporous molecularsieves are the most attractive catalyst supports with large surface area and high hydrothermal stability, allowing for the highly dispersed cobalt catalyst. The activity for FTS was strongly dependent on the catalyst reducibility. The formation of cobalt oxide species (crystalline Co3O4, amorphous cobalt oxides and cobalt silicates) depends on the surface nature of the support and the treatment conditions such as calcination temperature, pH of the precursor solution, etc. After reduction at high temperature, the strong interaction species between cobalt species and surface SiOH groups remain unreduced, some researchs reported that surface hydroxyl (-OH) led to the existence of these compounds hard to be reduced.In this paper, amino-modified within the surface and silylation of the SBA-15 modified the nature of the surface, cobalt catalysts supported on modified and silylated SBA-15 have been prepared by slurry and incipient wetness impregnation.Using KIT-6 as support,cobalt-based catalysts with different cobalt loading have also been prepared. The catalysts were characterized using different techniques such as elemental analysis, N2 adsorption-desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature programmed reduction (TPR), temperature programmed desorption (TPD) and oxygen titration. The catalytic properties for the Fischer-Tropsch synthesis reaction were evaluated in a fixed-bed reactor. The main results are as follows:1. The 2-D hexagonal structure of the original support was retained in the prepared amino-modified SBA-15 and catalyst. Compared to the unmodified cobalt catalyst, the modified catalyst exhibited higher dispersion and smaller cobalt cluster size. Some cobalt oxide crystallites entered the inside of the SBA-15 pore and then led to the catalyst could not be reduced easily. 2. It was found that the interaction between cobalt and support was decreased and the reducibility of cobalt oxides species increased on the silylated Co/SBA-15 catalyst. With increasing trimethylchlorosilane (TMCS) loading, the surface coverage of trimethylsilyl group increased and the cobalt cluster size was larger. The higher activity of silylated 10Co/SBA-15 catalysts is ascribed to the higher reducibility. The increase in selectivity of C5+ hydrocarbons is attributed to the increase of cobalt cluster size. Improved catalysts with higher CO conversion and better C5+ hydrocarbon selectivity for FTS were obtained.3. KIT-6 molecular sieve is a good catalyst support because of its high hydrothermal stability. With increasing cobalt loading, catalyst surface area and pore volume decreased significantly about 50%, while average pore size remained. For Co/KIT-6 catalyst, the interaction between cobalt and support was very low. With increasing the cobalt loading, catalyst activity and C5+ selectivity for FTS increased due to the high degree of reduction as well as the large cobalt particles size.更多还原