纳米ZSM-5碱脱硅构建多孔结构及其MTH反应性能研究

发布时间：2018-06-04 02:50

本文选题：纳米ZSM-5 + 甲醇制烃　；参考：《太原理工大学》2017年硕士论文

【摘要】：逐年递增的能源需求和日趋紧张的石油供给使中国能源安全面临严峻挑战。结合我国能源结构特点,充分发挥我国煤炭资源相对丰富的优势,发展新型煤化工技术变得愈加重要。其中以煤基甲醇为基础的甲醇制烃反应(MTH),是在酸性分子筛催化下可将甲醇转化为烷烃、低碳烯烃和芳烃等多种烃类的过程。通过该技术可实现煤炭的清洁高效利用,是未来替代石油路线的重要途径。作为MTH反应关键技术的催化剂,ZSM-5分子筛由于其良好的热稳定性、合适的酸性和有序微孔结构,可以高活性、高选择性地催化甲醇转化生成烃类。然而,也正是其规整有序微孔结构限制了反应物和产物的扩散,使积炭前驱体容易在微孔内积累,促进积炭的生成,同时覆盖活性位并最终引起孔道堵塞,致使催化剂失活。诸多研究表明,减小晶粒尺寸和引入介孔是解决分子筛微孔扩散限制的重要方法。它们均可缩短催化剂扩散路径,增加其外比表面积,促进反应大分子扩散。本论文主要围绕纳米ZSM-5分子筛不同孔结构构建进行研究。首先选取具有较好催化性能的纳米ZSM-5为对象,通过调控其NaOH碱处理时间,实现了纳米ZSM-5介孔结构的构建。结合MTH反应评价结果和XRD、NH3-TPD、BET、TEM、ICP-AES等系列表征,考察了纳米ZSM-5孔构建过程对其催化性能的影响;然后,在上述纳米ZSM-5介孔构建研究基础上,我们基于ZSM-5分子筛上存在的铝分布梯度和骨架铝对脱硅的抑制作用,通过调控NaOH碱处理时间并结合季铵碱四丙基氢氧化铵(TPAOH)的引入,实现了不同孔结构纳米ZSM-5的定向制备。结合MTH反应评价结果和系列表征,系统研究了这些特殊孔结构和酸性微观因素与其催化性能之间的构效关系。基于上述研究,以期实现纳米ZSM-5扩散性能的可控调变,找到影响其催化性能的关键孔结构和酸性特征,为高性能催化剂的设计提供理论依据。本论文的主要结论如下:(1)研究纳米ZSM-5碱脱硅介孔构建过程及其对MTH反应性能的影响发现,延长碱脱硅时间,分子筛晶粒依次经历小介孔形成、小介孔连通形成大介孔和大介孔融合为空心结构的成孔过程。分子筛外比表面积、总酸量和酸强度随脱硅时间的增加先增加后降低。当碱处理5 h,外比表面积和总酸量分别达到最大的115 m~2 g~(-1)和0.71 mmol g~(-1),酸性也最强。活性评价结果显示,催化剂寿命也随碱脱硅时间先增加后降低,且均明显长于母粉。其中15 h碱处理制备的空心ZSM-5,由于其良好的扩散性能和适宜的酸性,在4.7 h~(-1)下寿命可达150 h。另外,研究发现催化剂扩散性能和酸性显著影响产物选择性:催化剂良好的扩散性能可提供充足的空间结构,有利于异构烷烃的生成;而强的酸性则可加速芳构化反应,促进芳烃的生成。(2)定向制备不同孔结构纳米ZSM-5催化剂的研究发现,通过延长碱处理时间发现其外比表面积增加至一定程度后很难再继续提升,只能形成空心结构。然而当以NaOH和TPAOH混合碱二次处理初步具备空心结构的纳米ZSM-5时,其壳层进行保护性深度脱硅形成丰富介孔,酸量也显著提升。对比不同孔结构催化剂反应性能发现,介孔分布均匀ZSM-5孔扩散性能良好,内部弱酸位的利用,体现出长的催化寿命;规整空心ZSM-5尽管孔扩散性能优异,但强酸性导致其寿命略短于介孔分布均匀的纳米ZSM-5;壳层富孔空心ZSM-5,由于壳层丰富介孔的引入,外比表面积可达母粉的2倍,催化寿命最长,在9.4 h~(-1)空速下可达149 h。此外,发现催化剂孔结构也显著影响产物选择性。规整的纳米空心结构更易于芳烃和异构烷的生成;体相介孔均匀分布的结构和壳层富孔的纳米空心结构利于异构烷烃和C6以下低碳烃的生成。
[Abstract]:With the increasing demand of energy and the increasing supply of petroleum, China's energy security is facing a severe challenge. Combining the characteristics of China's energy structure and giving full play to the abundant coal resources in China, the development of new coal chemical technology becomes more and more important. The methanol based hydrocarbon generation reaction (MTH) based on coal based methanol is an acid fraction. Under the catalysis of sieves, methanol can be converted into hydrocarbons, such as alkanes, low carbon olefin and aromatics. Through this technology, the clean and efficient use of coal can be realized. It is an important way to replace the petroleum route in the future. As a catalyst for the key technology of MTH reaction, ZSM-5 molecular sieve has good thermal stability, suitable acidic and ordered micropores. The structure, which can catalyze the conversion of methanol to a high selectivity, catalyzes the conversion of methanol to hydrocarbons. However, its orderly and orderly microporous structure restricts the diffusion of the reactants and products, which makes the carbon precursors accumulate easily in the micropores and promotes the formation of the carbon deposits. At the same time, it covers the active sites and causes the clogging of the pores at the end of the pore. Many studies have been made. It is shown that reducing the size of grain and introducing mesoporous pores is an important method to solve the limiting diffusion of microporous molecular sieves. They can shorten the diffusion path of the catalyst, increase the surface area and promote the diffusion of macromolecules. This paper mainly focuses on the construction of different pore structure of nano ZSM-5 molecular sieve. First, the better catalytic performance is selected. Nanoscale ZSM-5 is the object. By regulating the time of NaOH alkali treatment, the mesoporous structure of nano ZSM-5 is constructed. Combined with the evaluation results of MTH reaction and the series characterization of XRD, NH3-TPD, BET, TEM and ICP-AES, the effect of nano ZSM-5 pore construction process on its catalytic performance is investigated. The distribution gradient of aluminum and the inhibition of skeleton aluminum on desilication on ZSM-5 molecular sieve. By regulating the treatment time of NaOH base and combining quaternary ammonium four propyl ammonium hydroxide (TPAOH), the directional preparation of nano ZSM-5 with different pore structure was realized. The special pore structure was systematically studied in combination with the evaluation results and series characterization of MTH reaction. The structure-activity relationship between the acid micro factors and their catalytic properties. Based on the above study, the controllable modulation of the nano ZSM-5 diffusion properties is realized, and the key pore structure and acid characteristics that affect its catalytic performance can be found, which provide the theoretical basis for the design of high performance catalysts. The main conclusions of this paper are as follows: (1) study on the nanoscale ZSM-5 alkali removal. The construction process of mesoporous silicon and its effect on the reaction performance of MTH found that the time of alkali desilication was extended, and the small mesoporous grains were formed in turn. Small mesoporous pores were connected to form large mesoporous and large mesoporous pores, and the specific surface area, total acid content and acid strength increased first and then decreased with the increase of the time of desilication. The alkali treatment was 5 h, and the outer surface area and total acid content reached the maximum 115 m~2 g~ (-1) and 0.71 mmol g~ (-1), and the acidity was the strongest. The activity evaluation showed that the catalyst life also increased first and then decreased with the alkali desilication time, and all were obviously longer than the mother powder. The hollow ZSM-5 prepared by 15 h alkali treatment was due to its good diffusion properties and suitable The acid, under 4.7 h~ (-1), can reach 150 h.. The study found that the catalyst diffusivity and acidity significantly affect the selectivity of the product: the good diffusion properties of the catalyst can provide sufficient space structure and be beneficial to the formation of isomeric alkanes, while strong acidity can accelerate aromatization and promote the formation of aromatics. (2) directional preparation of different pores. The study of nanostructured ZSM-5 catalyst shows that it is difficult to continue to enhance the outer surface area to a certain extent by prolonging the alkali treatment time to a certain extent and only form a hollow structure. However, the protective depth of deilication is rich in the shell layer when the nano ZSM-5 is initially treated with NaOH and TPAOH mixed alkali two times. Compared to the pore structure, the reaction performance of different pore structure catalysts shows that the diffusion property of the mesoporous ZSM-5 hole is good and the internal weak acid position is used, which shows the long catalytic life. Although the porous ZSM-5 has excellent pore diffusion properties, the strong acidity leads to the short life of the nano ZSM-5 with uniform mesoporous distribution; the shell is rich. Core ZSM-5, with the introduction of rich mesoporous shell, the outer surface area can reach 2 times that of the mother powder, the longest catalytic life and up to 149 h. at the speed of 9.4 h~ (-1). It is found that the pore structure of the catalyst also affects the selectivity of the product significantly. The shell hollow nano hollow structure is favorable for the generation of low carbon hydrocarbons under ISO alkanes and below C6.
【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】：O643.36;TQ211

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