【作者】 邱业君；

【导师】 张继炎；

【作者基本信息】 天津大学 ， 工业催化， 2007， 博士

【摘要】 与CH₄水蒸气重整和CO₂重整过程相比,CH₄催化部分氧化制备合成气具有反应速度快、能耗低、设备投资小、产品中H2/CO摩尔比接近2/1（很适于合成甲醇、高碳醇等）等优点,是极具开发前景的CH₄化工转化利用的课题。本文以负载型Ni基催化剂为研究对象,采用BET、XRD、H2-TPR、TEM、EDS、IR、CH₄-TPSR、TGA、程序升温分解和活性评价等多种研究方法,系统地考察了助剂（MgO、CeO₂、CaO等）、催化剂颗粒度、Ni组分分布情况、焙烧条件等因素对Ni/γ-Al₂O₃催化剂体系的物化性质和催化CH₄部分氧化反应性能的影响。在Ni/γ-Al₂O₃催化剂中,NiO的分散度较差,并且易与γ-Al₂O₃载体反应生成难以还原的NiAl2O4尖晶石,因此Ni/γ-Al₂O₃催化剂的CH₄部分氧化活性较差。适量的MgO助剂有利于改善Ni/γ-Al₂O₃催化剂中NiO物种的分散性能,同时抑制了NiAl2O4尖晶石生成,从而提高了催化剂的催化反应性能。但是,过量的MgO助剂不利于NiO物种的分散。研究表明,适宜的MgO助剂含量为7wt.%。通过研究发现,采用CeO₂与CaO复合助剂能够进一步提高Ni/MgO（7wt.%）-γ-Al₂O₃催化剂的催化性能,这可能与CeO₂和CaO复合助剂之间形成固溶体,并且在二者间存在明显的协同效应有关。CeO₂-CaO固溶体与Ni物种间可能存在较强的相互作用,可以提高活性Ni物种的分散度、减小Ni晶粒尺寸、促进Ce4+至Ce3+转变、增强催化剂的储氧能力以及晶格氧的流动性,从而改善催化剂的CH₄部分氧化催化反应性能。采用CeO₂或CaO单独作为助剂时,虽然可以提高Ni/MgO（7wt.%）-γ-Al₂O₃催化剂中Ni物种的还原性能,但Ni/MgO（7wt.%） -γ-Al₂O₃催化剂的综合反应性能没有得到明显的改善。在上述小颗粒催化剂体系（内扩散的影响在很大程度上被消除）研究工作的基础上,对于毫米级球形Ni/γ-Al₂O₃催化剂在CH₄部分氧化过程中的催化作用进行了研究。结果表明,采用毫米级球形催化剂时,甲烷部分氧化反应受内扩散过程限制。蛋壳型Ni基催化剂因其活性组分Ni较集中分布在催化剂的表层区域,提高了催化剂的有效因子。因此,与均匀型Ni/γ-Al₂O₃催化剂相比,蛋壳型Ni/γ-Al₂O₃催化剂显示了良好的CH₄部分氧化催化反应性能,其中CH₄转化率和H2选择性明显提高。除了Ni组分分布情况外,催化剂的颗粒度、活性组分Ni含量、反应温度及气体空速等因素均对CH₄部分氧化反应进行有不同程度的影响。O₂比CH₄的分子量大,因而具有较大的内扩散阻力,使得O₂在催化剂孔内的扩散过程成为CH₄氧化过程的速率控制步骤;同时,还会导致CH₄/O₂摩尔比沿催化剂孔道轴向长度逐渐增加,促进了CH₄进行直接部分氧化过程,避免CH₄深度氧化反应的发生。MgO助剂对毫米级蛋壳型Ni/γ-Al₂O₃球形催化剂在甲烷部分氧化反应中的性能同样具有一定的改善作用。研究表明,蛋壳型Ni/MgO-γ-Al₂O₃催化剂的活性受Ni含量的影响较为显著,且在2wt.%含量处有一个明显的临界点。当Ni含量低于2wt.%时,在773～1073 K范围内CH₄部分氧化反应不能被引发,CH₄的转化率几乎为0;Ni含量达到2wt.%后,则催化剂显示很好的引发性能和催化POM反应活性,CH₄部分氧化反应在低于773 K的温度下即可被引发。随着Ni含量增加,催化剂的反应活性有所提高;当Ni含量达到12wt.%时,催化剂的POM活性已趋于稳定。CeO₂和CaO复合助剂明显改善了1wt.%Ni/MgO-γ-Al₂O₃催化剂的引发性能和催化反应性能,此与CeO₂和CaO复合助剂的表面氧和晶格氧的浓度及活性密切相关。CeO₂和CaO复合助剂也能够进一步提高毫米级蛋壳型10wt.%Ni/MgO-γ-Al₂O₃球形催化剂的催化POM反应性能,主要原因在于添加适量的CeO₂和CaO复合助剂后,能够削弱催化剂表面NiO物种的氧化能力,有利于CH₄直接部分氧化反应进行。然而,过量的助剂会导致CeAlO3生成和表面NiO物种的氧化能力增强,对CH₄部分氧化反应进行不利。研究表明,复合助剂CeO₂和CaO的含量以1wt.%为宜。对于Ni/CeO₂-CaO-MgO-γ-Al₂O₃催化剂,γ-Al₂O₃载体焙烧温度对于催化剂的性能有显著的影响。γ-Al₂O₃经过1273 K高温焙烧预处理一方面导致催化剂的比表面积严重下降和Ni晶粒烧结长大,另一方面使催化剂的平均孔径增加,从而有利于反应物分子在孔内的扩散,提高了催化剂的有效因子。对于毫米级颗粒催化剂而言,后者是主要影响因素,因此对γ-Al₂O₃载体进行1273 K高温焙烧预处理有利于提高催化剂的反应性能。研究发现,浸渍Ni组分后未经焙烧处理制备的催化剂还原后具有较小的Ni晶粒,因而具有较好的CH₄部分氧化反应性能,当反应温度为1073 K时,该催化剂（其γ-Al₂O₃载体经过高温焙烧预处理）的CH₄转化率、CO选择性和H2选择性可分别达到97.5%、94.3%和94.3%。在催化剂稳定性实验中,前20 h催化剂基本无失活现象,随后催化剂的活性稍有降低,当反应100 h时,CH₄转化率、CO选择性与H2选择性分别下降了3%、1.3%和1.2%。研究表明,引起催化剂失活的主要原因可能不是催化剂表面上的积碳,而很可能是生成了难于还原的NiO-MgO固溶体或/和NiAl₂O₄尖晶石物种;此外,反应过程中Al₂O₃发生物相变化及镍晶粒长大等因素也会导致催化剂的活性下降。本论文采用了两种不同的方法制备了蛋壳型镍基催化剂:一种是以丙酮作为浸渍溶剂直接制备蛋壳型镍基催化剂;另一种是采用含MgO物种修饰的γ-Al₂O₃载体,以水作浸渍溶剂制备蛋壳型镍基催化剂。两种制备方法分别利用不同原理降低了溶液在载体上的浸渍速率,从而实现对浸渍深度（即蛋壳层厚度）的有效控制。由于γ-Al₂O₃表面富含O-H基,有利于硝酸镍水溶液的浸渍,其浸渍速率较高(> 0.097 mm.s^-1/2),因而易于制得均匀型催化剂。而丙酮属于弱极性溶剂,它的硝酸镍溶液在γ-Al₂O₃载体上的浸渍速率很低(仅为0.0058 mm.s^-1/2);当浸渍30 min时,浸渍深度约为0.20 mm,所制得的催化剂中Ni组分呈蛋壳型分布。采用Mg（NO₃）₂的水溶液先浸渍γ-Al₂O₃载体,然后在723 K下焙烧,可以得到适宜平均孔径与表面性能（表面O-H基较少）的复合载体。硝酸镍的水溶液在该复合载体上的浸渍速率远小于它在γ-Al₂O₃载体上的浸渍速率,故而同样可以制得蛋壳型镍基催化剂,并且通过调节浸渍时间、干燥温度及系统压力,可以调控蛋壳层的厚度。更多还原

【Abstract】 Compared with methane reforming with steam or with carbon dioxide, the catalytic partial oxidation of methane （POM） to synthesis gas is a better approach for methane utilization, since it has many advantages such as higher space velocity, lower power energy consumption, smaller equipment investment and proper H2/CO molar ratio in the products about 2/1, which is favorable to the synthesis of methanol or other higher alcohols. Therefore, the POM becomes one of the most attractive and challenging research tasks in the field of chemical conversion of nature gas.In this work, the supported nickel-based catalyst was selected as the research object, and the effects of some promoters （including MgO, CeO₂ and CaO）, catalyst particle size, nickel component distribution and calcination temperature on the physico-chemical properties and the POM reactivity of Ni/γ-Al₂O₃ catalyst were systematically investigated by means of BET, XRD, H2-TPR, TEM, EDS, IR, CH₄-TPSR, TGA, temperature-programmed decomposition and catalytic activity evaluation.Ni/γ-Al₂O₃ catalyst had some shortcomings, such as the poor dispersity of nickel species and easy formation of NiAl2O4 spinel, all of which were not beneficial to its POM reactivity. Using the proper amount of MgO promoter, the homogeneity of nickel species in Ni/γ-Al₂O₃ catalyst could be improved and the formation of NiAl2O4 spinel was inhibited, and thus the performance of Ni/γ-Al₂O₃ catalyst modified MgO for POM were enhanced. However, the excessive MgO promoter might be unfavorable to the dispersity of nickel species. The suitable content of MgO promoter in Ni/γ-Al₂O₃ catalyst was about 7wt.%.When CeO₂ and CaO were used as composite promoters to modify Ni/（7wt.%）MgO-γ-Al₂O₃ catalyst, they could form the solid solution, and their remarkable synergetic effect was detected. It was deduced that the formation of CeO₂-CaO solid solution could not only result in higher dispersity of active nickel species, easier conversion of Ce4+ to Ce3+ species, and smaller size of nickel crystallites, but also lead to the better oxygen adsorption ability, more oxygen-vacant sites and easier migration of lattice oxygen anions, all of which benefited the improvement of the catalytic POM performances. When Ni/MgO（7wt.%）-γ-Al₂O₃ catalyst was promoted with CeO₂ or CaO alone, no obvious improvement of the catalyst reactivity was observed. The inner diffusion was eliminated over the catalyst with small particle size to a large extent. For the aim of application, the spherical catalysts with ca. 1.5 mm diameter were applied and investigated for POM reaction process, where the intra-particle mass transport became the rate-limiting step. Compared with the homogeneous catalysts, the eggshell-typed catalysts had higher effectiveness factors because the active nickel component was distributed in the outer surface layer, and thus owned better catalytic POM performance, especially on the methane conversion and hydrogen selectivity. Besides nickel component distribution, catalyst particle size, nickel loadings, reaction temperature, and space velocity also affected the POM reactivity to some extent. Since oxygen molecule encountered higher diffusion resistance than methane molecule due to its twice higher molecular weight, the inner diffusion of oxygen became the rate determine step in POM process. The CH₄/O₂ molar ratio could be increased along the axial length of pores, which might promote the direct partial oxidation and prevent the deep oxidation of methane.MgO promoter could also improve the catalytic performance of eggshell-typed Ni/γ-Al₂O₃ catalyst with larger particle size （in millimeter grade） to some extent. As for eggshell-typed Ni/MgO-γ-Al₂O₃ catalysts, CH₄ conversion increased with the increase of nickel loadings. There existed an obvious turning point of nickel loading at 2wt.%. If Ni loadings were lower than 2wt.%, the catalysts exhibited rather poor catalytic performance at the temperature range between 773 K and 1073 K, and the methane conversion was nearly zero because POM could not be ignited. However, after Ni loading reached 2wt.%, the catalysts showed good ignition property and catalytic performance in POM reaction, and the reaction could be ignited at the temperature of about 773 K. With the increase of nickel loadings, the POM activity increased, and it changed very slightly at the nickel loading of higher than 12 wt.%.It was found that by means of CeO₂ and CaO composite promoters, the ignition and reactivity of 1wt.%Ni /MgO-γ-Al₂O₃ catalyst could be improved, which was closely related to the concentration and oxidizability of surface oxygen as well as lattice oxygen. Furthermore, suitable amount of CeO₂ and CaO composite promoters could also improve the catalytic POM reactivity of millimeter grade eggshell-typed 10wt.%Ni/ MgO-γ-Al₂O₃ spherical catalyst furtherly. The addition of CeO₂ and CaO composite promoters would weaken the oxidizability of surface NiO species, which favored the direct partial oxidation of methane and avoided some side reactions, such as CH₄ deep oxidation （complete combustion）. However, excessive promoters caused the formation of CeAlO3, which was unfavorable to POM reaction. It was found the suitable content of CeO₂ and CaO composite promoters was about 1wt.%.The properties of Ni/CeO₂-CaO-MgO-γ-Al₂O₃ catalyst were greatly affected by the calcination conditions. Although the calcination ofγ-Al₂O₃ support at 1273 K would cause the serious decrease of specific surface area and the sintering of nickel crystallites, which might result in negative effect to the catalytic performance, however, it could also result in the increase of average pore radius, which was beneficial to the inner diffusion of CH₄ and O₂ molecules, thus the effectiveness factor of the catalysts was enhanced. For the catalysts with larger particle size, the latter effect was more important, so the catalytic performance was improved after the calcination ofγ-Al₂O₃ support at 1273 K. It was found that the catalyst impregnated with Ni（NO3）2 aqueous solution without calcination showed good catalytic reactivity. Based on theγ-Al₂O₃ supports calcined at 1273 K, the catalyst Ni/CeO₂-CaO-MgO-γ-Al₂O₃, which was prepared by the impregnation with Ni（NO3）2 aqueous solution but without calcination, showed the best catalytic POM performance. At the temperature of 1073 K, CH₄ conversion, CO selectivity and H2 selectivity could reach 97.5%, 94.3% and 94.3%, respectively. In the stability investigation, it showed no deactivation during first 20 h, thereafter, its reactivity decreased slightly. In comparison to the initial activity, the CH₄ conversion, CO selectivity and H2 selectivity after 100 h test decreased by 3%, 1.3% and 1.2%, respectively. The characterization results of the deactivated catalyst sample suggested that NiO-MgO solid solution and/or NiAl2O4 spinel formed during reaction process caused the catalyst deactivation to a great extent due to their reduction difficulty. Additionally, the phase transformation ofγ-Al₂O₃ and the aggregation of Ni species could also lead to the catalyst deactivation. Coke deposition might not be the main reason for the catalyst deactivation.In this work, two different methods were used to prepare the eggshell-typed nickel-based catalysts. In the first one,γ-Al₂O₃ was impregnated with acetone solution of nickel nitrate, which directly led to the nickel eggshell-typed catalysts; In the second one,γ-Al₂O₃ support was firstly modified with MgO promoter, and then impregnated with the water solution of nickel nitrate, and the eggshell catalyst was also prepared. These two approaches were based on different principles to lower the impregnation rate and thus to control the impregnation depth effectively, i.e. the thickness of eggshell layer. There were abundant O-H groups on the surface of γ-Al₂O₃ support, which was beneficial to the impregnation of aqueous solution, thus the impregnation rate of aqueous solution of nickel nitrate onγ-Al₂O₃ support was over 0.097 mm?s-1/2, so the prepared catalyst showed the homogeneous nickel distribution. Owing to the weak polarity of acetone, the impregnation rate of acetone solution of nickel nitrate onγ-Al₂O₃ was only about 0.0058 mm?s-1/2, and the impregnation depth was about 0.20 mm at the impregnation time of 30 minutes. When acetone was used as impregnation solvent, the formation of eggshell-typed distribution of nickel component was attributed to the slower impregnation rate. The composite support with proper average pore radius and less surface O-H groups could be obtained by impregnatingγ-Al₂O₃ with aqueous solution of magnesium nitrate and then calcining at 723 K. On this composite support, the impregnation rate of aqueous solution of nickel nitrate was much slower than that onγ-Al₂O₃ support, and so the eggshell-typed supported nickel-based catalyst could be prepared. By the adjustment of impregnation time, drying temperature and system pressure, the eggshell thickness could be controlled effectively.更多还原