The Role of Cyclopentadienium Ions in Methanol-to-Hydrocarbons Chemistry
The Role of Cyclopentadienium Ions in Methanol-to-Hydrocarbons Chemistry
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DOI：
10.1021/cs500722m
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发表时间：
2014-10-01
期刊：
影响因子：
12.9
通讯作者：
Jentoft, Friederike C.
中科院分区：
文献类型：
--
作者：
Wulfers, Matthew J.;Jentoft, Friederike C.
H型分子筛上甲醇转化为碳氢化合物的“碳氢化合物池”机制是在二十多年前首次提出的，但其细节仍然是一个有争议的话题。利用原位紫外可见光谱(UV-Vis)研究了H-ZSM-5和H-β沸石上的油气藏。将诱导期内生长的吸收带强度与气相产物的生成率进行比较。两种催化剂的谱图都显示了烷基苯和烷基取代的环戊二茂离子的存在；在H-β上，还形成了更大的多环芳香化合物。H-ZSM-5上物种在295 nm处的吸收带归属于具有四个或五个烷基的烷基取代的环戊二烯离子，与诱导期内乙烯生成速率的增加平行，因此暗示这些阳离子参与了限速步骤。H-ZSM-5上的烷基苯离子平均含有3~4个烷基，而H-β上生成的烷基苯类化合物在反应7min后已完全烷基化。在这两种情况下，烷基苯离子的浓度随时间几乎呈线性增加，与气相产物的生成速率无关。通过对六甲苯和1，2，3，4，5-五甲基环戊二烯的吸附实验，为UV-Vis光谱的详细解释提供了参考。后者的光谱首次报道了沸石上烷基取代的环戊二阳离子。
The "hydrocarbon pool" mechanism for the conversion of methanol to hydrocarbons on H-form zeolites was first introduced more than two decades ago, but the details continue to be a topic of debate. In this contribution, the hydrocarbon pool on zeolites H-ZSM-5 and H-beta was investigated by applying in situ ultraviolet visible (UV-vis) spectroscopy. The intensity of absorption bands that grew during an induction period were compared with formation rates of gas-phase products. Spectra of both catalysts revealed the presence of alkylbenzenium and alkyl-substituted cyclopentadienium ions; on H-beta, larger polycyclic aromatic compounds were also formed. An absorption band at 295 nm of species on H-ZSM-5, assigned to an alkyl-substituted cydopentadienium ion with four or five alkyl groups, paralleled the increase in ethene formation rates during an induction period, thus implicating the participation of such cations in the rate-limiting step. The alkylbenzenium ions on H-ZSM-5 were characterized by an average of three to four alkyl groups, whereas the alkylbenzene compounds formed on H-beta were fully alkylated already after 7 min on stream. In both cases, the concentration of alkylbenzenium ions increased almost linearly with time and was not correlated with rates of formation of gas-phase products. Adsorption experiments with hexamethylbenzene and 1,2,3,4,5-pentamethylcyclopentadiene were used to provide a reference for detailed interpretation of UV-vis spectra. Spectra of the latter are the first reported of an alkyl-substituted cyclopentadienium cation on a zeolite.