摘要
甲醇制烯烃(MTO)反应是最有希望替代常规石油路线生产低碳烯烃的新兴工艺路线。作为MTO反应的核心,催化剂的合成成为人们研究的重点。SAPO-34分子筛以其极高的甲醇转化率和优异的乙烯、丙烯选择性成为目前MTO反应最好的催化剂。然而由于SAPO-34分子筛受到其微孔的限制,孔道容易被积碳堵塞从而造成较短的催化剂寿命。因此有效的提高SAPO-34分子筛的催化剂寿命对于推进MTO反应工业化有着极其重要的意义。本论文致力于研究SAPO-34分子筛的合成和改性,从而达到提高催化剂寿命的目的。
论文考察了吗啉(MOR)、三乙胺(TEA)和四乙基氢氧化胺(TEAOH)三种模板剂以及气相转移(VPT)法对于SAPO-34分子筛合成的影响。目的是找到一种廉价,简单和稳定的SAPO-34分子筛合成配方,为其改性提供良好的基础。结果表面:(1)三种模板剂合成的催化剂晶粒大小由大到小为MOR-SAPO-34>TEA-SAPO-34>TEAOH-SAPO-34;(2)催化剂寿命由长到短为TEAOH-SAPO-34≈TEA-SAPO-34>MOR-SAPO-34;(3)尽管以TEAOH为模板合成SAPO-34分子筛寿命最长,但是由于TEAOH价格昂贵不适合应用,因此,以TEA为模板合成SAPO-34分子筛更适合于工业和实验室应用。(4)以VPT法合成的SAPO-34分子筛,因为缺少分散介质造成晶化度低,寿命短。
论文考察了Mg、Co、Ni和Mn四种金属离子对提高催化剂寿命的影响,初步探讨了多级孔道SAPO-34分子筛的合成以及在MTO反应中的应用。结果表明:(1)在Mg,Co,Ni和Mn四种金属离子改性的MeSAPO-34分子筛中Mn离子的加入可使分子筛的酸性降低,乙烯、丙烯的选择性提高并表现出最长的寿命。(2)在350,400,450和500℃四种温度下,MnSAPO-34分子筛在400℃下催化剂寿命最长为7.25小时。(3)Mn/Al_2O_3=0.01时催化剂寿命最长并且有良好的乙烯、丙烯的选择性。(4)硅烷偶联剂TPOAC可以作为导向介孔生成的模板剂,合成的多级孔道SAPO-34分子筛与常规的SAPO-34分子筛相比寿命有所增加,但乙烯、丙烯的选择性会有降低。
本论文还考察了SAPO-34分子筛的再生性能。结果表明三次再生后的SAPO-34分子筛无论是寿命还是乙烯、丙烯的选择性都与新鲜的SAPO-34分子筛非常接近,说明SAPO-34分子筛有良好的再生性能。
Methanol to olefin (MTO), as a potential method instead of conventional oil route, has received wide attention in the past years. The key issue for the MTO reaction is to develop new catalyst with higher lifetime, higher selectivity to olefin, higher conversion and stability. SAPO-34 molecular sieve is a potential catalyst for MTO reaction, because of its high conversion of methanol and selectivity to ethane and propene. However, fast deactivation has been reported due to coke formation. In this paper, the synthesis and modification of SAPO-34 molecular sieve were studied to prolong the lifetime of the catalyst.
The experiments were carried out to investigate the effect of Vapor Phase Transport (VPT) method and different templates such as MOR, TEA and TEAOH on SAPO-34 synthesis. The results show that the crystal grain size of catalyst synthesized with different templates is in the order: MOR-SAPO-34> TEA-SAPO-34>TEAOH-SAPO-34; the lifetime is in the the order: TEAOH-SAPO-34^TEA-SAPO-34>MOR-SAPO-34; The lifetime of SAPO-34 synthesized by VPT method is short due to without dispersion medium in the system.
The experiments were carried out to investigate the effect of metal ions such as Mg, Co, Ni and Mn on lifetime of catalyst. The results show that MeSAPO-34 modified by Mg, Co, Ni or Mn metal ion has low acidity of catalyst and high of the selectivity to ethane; at different temperature, the lifetime of MnSAPO-34 shows the longest lifetime, 7.25 h, at 400℃; the MnSAPO-34 with Mn/Al_2O_3 being 0.01 shows the best selectivities to ethane and propene. In the studies, a hierarchical porous SAPO-34, synthesized with silane coupling agent TPOAC as mesopore structure directing agent, was also used for MTO reaction. Compared to the conventional SAPO-34, the hierarchical porous SAPO-34 molecular sieve has a longer lifetime, while lower selectivities to ethane and propene.
The reproducibility of SAPO-34 was studied. The catalyst can be regenerated through simply calcinations in air at 550℃. The performance of regenerated SAPO-34 after 3 times is very similar to the fresh SAPO-34.
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