多层镍铝催化剂的合成及其CO甲烷化性能研究

发布时间：2018-08-12 09:19

【摘要】：“富煤、贫油、缺气”的能源特点使我国长期以来以煤炭作为一次能源的现状将不会改变,但由于煤炭直接燃烧排放到大气中,导致当下我国空气污染严重,雾霾情况令人担忧。因此改变煤炭在能源结构中所占比重,实现煤炭的清洁利用迫在眉睫。开发以煤经合成气制甲烷工艺,不仅可以调整我国的能源结构,弥补天然气需求的缺口,而且可以实现煤炭的清洁利用,在环境保护方面做出特殊的贡献。在甲烷化反应过程中,一氧化碳歧化和甲烷分解等副反应的发生,容易导致催化剂积碳;强放热效应容易导致催化剂烧结。故设计制备一种低温活性高、稳定好的催化剂是甲烷化技术的关键。本论文采用MIL-53(Al)作为铝源和模板,借助等体积浸渍法将Ni~(2+)浸渍到MIL-53(Al)的孔道中,通过不同温度焙烧,分别形成以氧化铝和镍铝尖晶石为载体,镍为活性金属的多层镍铝甲烷化催化剂,Ni/Al_2O_3-m和Ni/NiAl_2O_4-m。采用扫描电子显微镜对催化剂形貌进行观察,发现催化剂保留了MIL-53(Al)的三维层状结构;透射电子显微镜结果表明活性金属镍均匀分散到催化剂载体层间,与载体形成两个有效的作用面;氢气程序升温还原结果证明镍与镍铝尖晶石载体间形成强的相互作用力;N_2吸附-脱附表明催化剂具有大的比表面积(206和185 m2·g-1)和丰富的孔道结构,保证了甲烷化反应过程中物质传递和热量扩散;氢气升温脱附结果表明在催化剂金属-载体接触面存在明显的氢溢流现象;一氧化碳漫反射原位红外表征结果表明,一氧化碳主要以线式吸附在镍颗粒表面,催化剂镍颗粒含有大量缺陷位。以煤基合成气为原料,研究了多层镍铝催化剂的甲烷化催化性能。结果表明,在0.1 MPa,空速15 L?gcat-1?h-1反应条件下,Ni/Al_2O_3-m催化剂在400℃一氧化碳转化率最高达到99.6%,甲烷选择性为54.7%;Ni/NiAl_2O_4-m催化剂在350℃一氧化碳转化率最高达到99.8%,甲烷选择性为53.8%。在400℃,0.1 MPa,空速15 L?gcat-1?h-1反应条件下,两者均具有良好的反应稳定性,反应100 h催化剂活性没有明显下降,通过透射电子显微镜观察催化剂稳定性测试后的样品没有发现积碳和镍颗粒的团聚。在400℃,0.1 MPa,空速240L?gcat-1?h-1反应条件下,Ni/Al_2O_3-m催化剂的一氧化碳转化频率达到最高为2.79 s-1,一氧化碳转化率为15%;Ni/NiAl_2O_4-m催化剂的一氧化碳转化频率达到最高为3.26 s-1,一氧化碳转化率为24%。通过改变催化剂质量和催化剂颗粒粒径并根据Mears和Weisz-Prater判据计算,发现在400℃,0.1 MPa,空速240 L?gcat-1?h-1反应条件下,有效消除了内外扩散对催化反应的影响,反应过程不受物质传递和热量扩散限制。在CO加氢甲烷化反应的过程中,CO首先在镍活性位上解离(CO→Cα+O)形成活性物种。形成的碳物种在后续的加氢反应中有着至关重要的作用。值得注意的是,CO在镍缺陷位上解离为非常活泼地Cα物种的过程是快速的。层状结构镍铝催化剂中镍颗粒与载体界面产生的氢溢流的碳物种会很快被带走,使得镍颗粒表面一直处在干净的状态,这样大大的抑制了包埋碳和无定型碳的生成。镍颗粒与载体间强的相互作用力避免了催化反应过程中镍颗粒烧结;有效避免了催化反应积碳;大的比表面积和丰富的孔道结构有利于催化反应过程中物质传递和热量扩散,使催化剂具有好的稳定性。
[Abstract]:The energy characteristics of "rich coal, poor oil, lack of gas" make the status quo of coal as a primary energy in China will not change for a long time. However, the direct combustion of coal into the atmosphere, resulting in serious air pollution, haze situation is worrying. It is imminent to develop the technology of syngas from coal to methane, which can not only adjust the energy structure of our country and make up for the shortage of natural gas demand, but also realize the clean utilization of coal and make a special contribution to environmental protection. In this paper, MIL-53 (Al) was used as the aluminum source and template, and Ni~ (2+) was impregnated into the pores of MIL-53 (Al) by means of isovolumetric impregnation method, and then calcined at different temperatures to form the catalysts respectively. The multi-layer nickel-aluminum methanation catalysts, Ni/Al_2O_3-m and Ni/NiAl_2O_4-m, supported by alumina and nickel-aluminum spinel and nickel as active metal, were observed by scanning electron microscopy (SEM). It was found that the catalyst retained the three-dimensional layered structure of MIL-53 (Al). The results of hydrogen temperature programmed reduction showed that there was a strong interaction between nickel and nickel-aluminum spinel support. The adsorption-desorption of N_2 showed that the catalyst had a large specific surface area (206 and 185 m2.g-1) and a rich pore structure, which ensured the mass transfer during methanation. Diffusive heat diffusion; hydrogen desorption at elevated temperature showed that there was an obvious hydrogen overflow at the metal-support interface of the catalyst; diffuse reflectance in situ infrared spectroscopy of carbon monoxide showed that carbon monoxide was mainly adsorbed on the surface of nickel particles, and nickel particles of the catalyst contained a large number of defective sites. The results showed that under the conditions of 0.1 MPa and 15 L?Gcat-1?H-1 space velocity, the highest conversion of CO and methane selectivity of Ni/Al_2O_3-m catalyst were 99.6% and 54.7% at 400 C, 99.8% and 53.8% at 350 C, respectively. Under the conditions of temperature, 0.1 MPa and space velocity of 15 L?Gcat-1?H-1, both of them had good reaction stability, and the activity of catalyst did not decrease obviously after 100 h reaction. No agglomeration of carbon and nickel particles was found in the samples after the stability test by transmission electron microscopy. I/Al_2O_3-m catalyst has the highest CO conversion frequency of 2.79 S-1 and carbon monoxide conversion rate of 15%; Ni/NiAl_2O_4-m catalyst has the highest CO conversion frequency of 3.26 S-1 and carbon monoxide conversion rate of 24%. At present, the effect of internal and external diffusion on the catalytic reaction has been effectively eliminated under the conditions of 400, 0.1 MPa and 240 L? Gcat-1? H-1. The reaction process is not limited by material transfer and heat diffusion. It is noteworthy that the dissociation of CO from nickel defect sites into very active C alpha species is rapid. Carbon species produced by hydrogen spillover at the interface between nickel particles and support in layered nickel-aluminum catalysts are quickly removed, leaving the surface of nickel particles in a clean state all the time, thus greatly improving the catalytic performance. The strong interaction between the nickel particles and the supports avoided the sintering of nickel particles in the catalytic reaction, effectively avoided the deposition of carbon in the catalytic reaction, and the large specific surface area and rich pore structure were conducive to material transfer and heat diffusion in the catalytic reaction process, so that the catalyst had good stability. Sex.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ221.11;O643.36

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