Effect of the Methanol-to-Olefin Conversion on the PFG NMR Self-Diffusivities of Ethane and Ethene in Large-Crystalline SAPO-34

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• 作者：Weili Dai ; Matthias Scheibe ; Landong Li ; Naijia Guan ; Michael Hunger
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：January 26, 2012
• 年：2012
• 卷：116
• 期：3
• 页码：2469-2476
• 全文大小：451K
• 年卷期：v.116,no.3(January 26, 2012)
• ISSN：1932-7455

文摘

SAPO-34 catalysts obtained after MTO conversion times of 0鈥?0 min were investigated by PFG NMR spectroscopy for studying the self-diffusivities of ethane and ethene. Furthermore, the catalysts were investigated by in situ UV/vis, ¹³C MAS NMR, and ¹H MAS NMR spectroscopy, giving insights into the organic deposits and acid sites in different periods of the catalyst lifetime. The ratio of the self-diffusivities of ethene and ethane, D_ethene/D_ethane, used as a measure of the diffusion selectivity, was found to increase to the benefit of the olefin compared to the alkane for higher adsorbate loadings and increasing MTO conversion times. This change of the diffusion selectivity is one of the reasons for the variation of the product selectivity during the catalyst lifetime. Variable-temperature PFG NMR studies gave a significantly larger apparent activation energy for the self-diffusivity of ethane (E_a = 7.6 kJ/mol) compared with ethene (E_a = 4.2 kJ/mol).