Effect of Heteroatom Concentration in SSZ-13 on the Methanol-to-Olefins Reaction
Effect of Heteroatom Concentration in SSZ-13 on the Methanol-to-Olefins Reaction
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DOI：
10.1021/acscatal.5b01450
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发表时间：
2016-02-01
期刊：
影响因子：
12.9
通讯作者：
Davis, Mark E.
中科院分区：
文献类型：
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作者：
Deimund, Mark A.;Harrison, Luke;Davis, Mark E.
SSZ-13材料中含有不同数量的Al，可以得到具有不同浓度Bronsted酸位点的样品，因此，随着Al含量的增加，SSZ-13材料中成对Al杂原子的数量也在增加。然后对这些材料进行了表征并测试了它们作为催化剂在400℃和大气压下100%甲醇转化率的甲醇制烯烃（MTO）反应。还合成了SAPO-34样品并进行了比较测试。SSZ-13材料的MTO反应性随Si/Al比的变化而有显著差异。降低Al含量（更高的Si/Al比），从而减少配对Al位，导致更稳定的轻烯烃选择性，减少初始瞬态时间，降低初始丙烷选择性，延长催化剂寿命。为了进一步支持在初始瞬态阶段成对Al位点在丙烷形成中的重要性，进行了一系列实验，其中将H- ssz -13样品与Cu2+交换，蒸熟，然后回交换为H形式。H-SSZ-13样品表现出较高的初始丙烷选择性，而蒸制的H-SSZ-13、Cu2+交换的SSZ-13样品和蒸制的Cu-SSZ-13样品则没有，因为蒸制选择性地去除成对的Al位点和Cu2+交换到这些位点上。然而，当Cu-SSZ-13蒸煮后回交换为质子形式时，样品仍然表现出高Al含量材料的高初始烷烃选择性和瞬态周期。这是由于在蒸煮过程中通过Cu2+阳离子保护了成对的Al位点。反应后焦炭分析表明，SSZ-13中各芳香物质的甲基化程度随着Si/Al的增加而增加。此外，SAPO-34比SSZ-13样品产生更多的多环物质。从这些数据来看，配对的AI位点含量似乎与SSZ-13样品的MTO反应行为和焦炭物种形成有关。
SSZ-13 materials have been synthesized with varying amounts of Al to produce samples with different concentrations of Bronsted acid sites, and consequently, these SSZ-13 materials contain increasing numbers of paired Al heteroatoms with increasing Al content. These materials were then characterized and tested as catalysts for the methanol-to olefins (MTO) reaction at 400 degrees C and 100% methanol conversion under atmospheric pressure. A SAPO-34 sample was also synthesized and tested for comparison. SSZ-13 materials exhibited significant differences in MTO reactivity as Si/Al ratios varied. Reduced Al content (higher Si/Al ratio) and, consequently, fewer paired Al sites led to more stable light olefin selectivities, with a reduced initial transient period, lower initial propane selectivities, and longer catalyst lifetime. To further support the importance of paired Al sites in the formation of propane during this initial transient period, a series of experiments was conducted wherein an H-SSZ-13 sample was exchanged with Cu2+, steamed, and then back-exchanged to the H form. The H-SSZ-13 sample exhibited high initial propane selectivity, while the steamed H-SSZ-13, the Cu2+-exchanged SSZ-13 sample, and the steamed Cu-SSZ-13 sample did not, as expected since steaming selectively removes paired Al sites and Cu2+ exchanges onto these sites. However, when it was back-exchanged to the proton form, the steamed Cu-SSZ-13 sample still exhibited the high initial alkane selectivity and transient period typical of the higher Al content materials. This is attributed to protection of paired Al sites during steaming via the Cu2+ cation. Post-reaction coke analyses reveal that the degree of methylation for each aromatic species increases with increasing Si/Al in SSZ-13. Further, SAPO-34 produces more polycyclic species than SSZ-13 samples. From these data, the paired AI site content appears to be correlated with both MTO reaction behavior and coke species formation in SSZ-13 samples.