Methane Interaction with Zn2+-Exchanged Zeolite H-ZSM-5: Study of Adsorption and Mobility by One- and Two-Dimensional Variable-Temperature 1H Solid-State NMR

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• 作者：Mikhail V. Luzgin ; Dieter Freude ; J眉rgen Haase ; Alexander G. Stepanov
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：June 25, 2015
• 年：2015
• 卷：119
• 期：25
• 页码：14255-14261
• 全文大小：312K
• ISSN：1932-7455

文摘

Acidic zeolites in which protons are substituted (completely or in part) by metal cations demonstrate higher activity in catalytic reaction of light alkanes conversion in comparison with parent zeolites. The study of alkane adsorption on metal cationic species as the first stage of alkane interaction with the active sites of zeolites is crucial for understanding the mechanism of light alkane activation on metal-containing zeolites. By using variable-temperature ¹H magic-angle spinning nuclear magnetic resonance (¹H MAS NMR), the adsorption of methane on Zn²⁺-exchanged H-ZSM-5 zeolites (Zn²⁺/H-ZSM-5) with different Zn²⁺ content, as well as on source H-ZSM-5, has been studied at 333鈥?63 K. The enthalpy of adsorption (螖H_ads) on different centers, acid-bridged OH groups and Zn²⁺ cations, has been determined to be 鈭?8.6 卤 0.4 kJ mol^鈥? for OH groups and from 鈭?0.9 to 鈭?1.7 kJ mol^鈥? for Zn²⁺ cations of both Zn²⁺/H-ZSM-5 zeolite samples studied. Two-dimensional ¹H MAS exchange NMR experiments revealed the chemical exchange between the adsorbed and gas-phase methane molecules at 353鈥?63 K with the effective rate constant of 10鈥?0 s^鈥?. The increase of Zn²⁺ content leads to a decrease of the exchange rate, whereas the apparent activation energy (E_a) of the exchange increases from 13 kJ mol^鈥? for H-ZSM-5 zeolite to 20 and 23 kJ mol^鈥? for the samples, in which 50% and 85% of acid protons, respectively, were substituted by Zn²⁺cations. Variation of E_a for samples with and without Zn²⁺ cations was interpreted in terms of internal transport barriers that can exist at the intersections between the elementary building blocks of the crystals and/or between the crystal intergrowths in crystals of our zeolite samples. Additional transport barriers can arise from the bulky Zn²⁺ cations, located at the entrances of the zeolite pore system on the crystal surface.