【作者】 冯俊婷；

【导师】 李殿卿；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2010， 博士

【摘要】 负载型金属催化剂具有较高的活性、选择性和稳定性,且容易回收重复利用,被广泛应用于加氢、脱氢及重整等催化反应中,是石油炼制及石油化工过程中最重要的一类催化剂。该类催化剂的传统制备方法为溶液浸渍法。采用浸渍法制备负载型金属催化剂时,活性金属组分的前驱体受浸渍溶液表面张力及溶剂化效应的影响,在干燥阶段容易以聚集形式沉积于载体表面。浸渍后金属盐物种与催化剂载体之间难以形成较强的相互作用,在随后的焙烧和还原过程中活性金属组分容易发生迁移并进一步团聚,从而使得到的催化剂金属分散度较差,活性比表面较低,催化性能不佳。本论文在Pd/Al2O3催化剂的制备过程中引入第二种金属组分Ag,以期达到抑制活性组分Pd在热处理及还原过程中团聚的目的,从而提高催化剂的金属分散度,并将得到的双金属Pd-Ag/Al2O3催化剂应用于碳二加氢反应中。实验结果表明,Ag的引入未能使催化剂中Pd的金属分散度明显提高,但却使Pd的电子状态发生改变,形成了一部分Pd-Ag合金或固溶体,降低了催化剂中Pd对氢的吸附能力。吸附氢数量的减小和钯与氢原子结合能力的加强,减少了体相溢出氢参与乙烯反应的机会,因此加入Ag助剂之后,催化剂的乙烯选择性有较显著的提高。但与此同时,体相吸附氢数量的减少和向表面扩散能力的下降却降低了催化剂的加氢活性。水滑石(Layered Double Hydroxides,简称LDHs)是一类具有特殊结构的无机层状材料。由于静电排斥作用的影响,LDHs层板上的二价、三价金属阳离子彼此远离。另外,由于层板的静电平衡作用以及阴离子之间的静电排斥作用,LDHs层间的阴离子也呈高度分散状态有序排列。本论文将利用LDHs的结构特点,采用多种方法制备高分散负载型金属催化剂。首先,以尿素为沉淀剂,在球形氧化铝载体表面原位生长PdCl42-/MgAl-CO3-LDHs,焙烧后得到PdO/MgO-Al2O3催化剂前驱体。SEM、TPR和TPD结果表明,与浸渍法制备的Pd/Al2O3催化剂相比,原位法制备的Pd/MgO-Al2O3具有较高的金属Pd分散度和较小的颗粒尺寸,此外,均匀分散在氧化铝载体表面的MgO微晶降低了载体表面的酸性。在反应温度为50℃至100℃的条件下,Pd/MgO-Al2O3催化剂在碳二加氢反应中表现出较高的反应活性、选择性和稳定性。采用原位沉淀-还原法在球形氧化铝表面原位生长Pd/MgAl-LDHs,得到的产物Pd/MgAl-LDHs/Al2O3可以直接作为碳二加氢反应的催化剂。该催化剂比表面积较大、Pd颗粒尺寸均一、分散度较高、且催化剂中Pd微晶具有特殊的形貌,因此暴露出来的催化活性位(边、角和面)较多。Pd/MgAl-LDHs/Al2O3催化剂经焙烧和还原后得到另一种催化剂Pd/MgO-Al2O3。由于LDHs的存在,在焙烧和还原的过程中Pd微晶的团聚现象得到抑制,因此Pd/MgO-Al2O3催化剂仍然具有较高的金属分散度,且比表面较焙烧前有所增加。在相同的反应条件下,Pd/MgO-Al2O3和Pd/MgAl-LDHs-Al2O3催化剂在碳二加氢反应中的活性和稳定性明显高于浸渍法制备的Pd/Al2O3催化剂。以孔结构优良的γ-Al2O3为催化剂载体,利用其表面的Al源,采用原位法在氧化铝载体内孔表面原位合成NiAl-LDHs,焙烧及还原后得到高分散Ni/Al2O3催化剂。结果表明,载体孔内原位合成的NiAl-LDHs层状结构较好,且LDHs微晶并不是简单的负载在载体表面,LDHs微晶中与Ni配位的O与AlO4四面体发生键合,由于这种较强相互作用力的存在以及受到构成LDHs的Al的氧化物的隔离作用,Ni原子在焙烧和还原后仍能均匀分散在催化剂表面。与浸渍法制备的Ni催化剂相比,原位法制备的Ni/Al2O3的金属分散度提高了28%,因此催化剂在氯苯加氢脱氯反应中表现出了较高的活性和稳定性。考察了碳二加氢及氯苯加氢反应中工艺条件(如反应温度、压力、空速等)对催化剂加氢性能的影响,并对其原因进行了探讨。更多还原

【Abstract】 Supported metal catalysts have been widely used in the hydrogenation, dehydrogenation and reforming catalytic reactions because of their high activity, selectivity and stability. The conventional preparation method is solution impregnation method. However, supported metal catalysts prepared by this method have an inhomogeneity in metal distribution over the support, due to the surface tension of the impregnating solution and other solvent effects. Futhermore, the weak interactions between the support and metal ion species lead to the migration and aggregation of metal ions during subsequent calcinations and reduction processes. Thus, the catalysts prepared by this method possess low metal dispersion, active surface and catalytic properties.On the purpose of increasing Pd dispersion, Ag was introduced onto the surface of catalyst during the preparation of Pd catalyst. The obtained Pd-Ag/Al2O3 was then used as a catalyst for the selective hydrogenation of acetylene. The results indicated that the addition of Ag has no significant effect on increasing Pd dispersion but alters the electronic state of Pd supported on the surface of alumina. The formation of Pd-Ag alloy or sosolid strengthened the interaction between Pd atom and H atom and decreased the amount of adsorbed hydrogen. In comparsion of Pd/Al2O3 catalyst, Pd-Ag/Al2O3 exhibited higher ethylene sectivity but lower acetylene conversion in the selective hydrogenation of acetylene.The layered double hydroxides (LDHs) are a group of anionic clays with layered structure. Metal cations are uniformly dispersed within the layers without the formation of’lakes’of like cations. Moreover, the anions in the interlays are uniformly dispersed due to the electrostatic balance of the layer and the electrostatic repulsion among ions. Thus, these materials were used as the precursor to make high dispersed metal catalysts in this work.PdCl42-/MgAl-CO3-LDH crystallites have been synthesized in-situ on the surface of micro-spherical A12O3 using urea as the precipitant. After calcination and reduction, Pd/MgO-Al2O3 catalyst was obtained. SEM and TPR results indicated that Pd/MgO-Al2O3 prepared by in-situ method has higher Pd dispersion, smaller particle size and larger surface area than Pd/Al2O3 prepared by impergantion method. In addition, MgO microcrystallites decreased the surface acidity of alumina support. The obtained Pd/MgO-Al2O3 exhibited higher catalytic activity and selectivity than Pd/Al2O3 prepared by the impregnation method over the temperature range 50-100℃in the selective hydrogenation of actelene.Pd/MgAl-LDHs crystallites were synthesized on the surface of A12O3 by in-situ precipitation-reduction method. The obtained Pd/MgAl-LDHs/Al2O3, with high specific surface area, uniform Pd particle size, high dispersion and more active sites, can be used as a catalyst for the selective hydrogenation of actelene. After calcination and reduction, another catalyst Pd/MgO-Al2O3 was obtained. Due to the existence of LDHs crystallites grown on the surface of A12O3, the congregation of Pd2+ during calcination process was prevented. Thus, the calcinated product Pd/MgO-Al2O3 still has high dispersion and surface area. In addition, the LDHs and MgO microcrystallines decreased the surface acidity of alumina. Pd/MgO-Al2O3 and Pd/MgAl-LDHs-Al2O3 exhibited much higher activity and stability than Pd/Al2O3 prepared by the impregnation method in the selective hydrogenation of actelene.Moreover, micro-sphericalγ-Al2O3 was used as a support for the synthesis of NiAl-CO3-LDHs in the pores of the material. Formation of the LDH resulted from decomposition of urea dissolved in an aqueous solution of Ni2+ impregnated into theγ-Al2O3. After calcination and reduction, Ni/γ-Al2O3 catalyst was obtained. Due to the existence of LDHs and strong interaction between Ni and alumina, the migration and aggregation of Ni species during calcination was suspressed. In comparison of the metal dispersion of Ni catalyst prepared by impregation method, that of Ni/γ-Al2O3 prepared by in-situ method increase by 28%. Thus, Ni/γ-Al2O3 prepared by in-situ method exhibited much higher acitivity and stability than the catalyst obtained by impregation method in the hydrodechlorination of chlorobenzene. In this work, the influence of procress conditions, including reaction temperature, pressure and velocity on the catalytic properties, was also studied in the selective hydrogenation of actelene and hydrodechlorination of chlorobenzene.更多还原