Anharmonicity and Confinement in Zeolites: Structure, Spectroscopy, and Adsorption Free Energy of Ethanol in H-ZSM-5

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• 作者：Konstantinos Alexopoulos ; Mal-Soon Lee ; Yue Liu ; Yuchun Zhi ; Yuanshuai Liu ; Marie-Françoise Reyniers ; Guy B. Marin ; Vassiliki-Alexandra Glezakou ; Roger Rousseau ; Johannes A. Lercher
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：April 7, 2016
• 年：2016
• 卷：120
• 期：13
• 页码：7172-7182
• 全文大小：606K
• ISSN：1932-7455

文摘

To account for thermal and entropic effects caused by the dynamics of the motion of the reaction intermediates, ethanol adsorption on the Brønsted acid site of the H-ZSM-5 catalyst has been studied at different temperatures and ethanol loadings using ab initio molecular dynamics (AIMD) simulations, infrared (IR) spectroscopy, and calorimetric measurements. At low temperatures (T ≤ 400 K) and ethanol loading, a single ethanol molecule adsorbed in H-ZSM-5 forms a Zundel-like structure where the proton is equally shared between the oxygen of the zeolite and the oxygen of the alcohol. At higher ethanol loading, a second ethanol molecule helps to stabilize the protonated ethanol at all temperatures by acting as a solvating agent. The vibrational density of states (VDOS), as calculated from the AIMD simulations, are in excellent agreement with measured IR spectra for Cb>2b>Hb>5b>OH, Cb>2b>Hb>5b>OD, and Cb>2b>Db>5b>OH isotopomers and support the existence of both monomers and dimers. A quasi-harmonic approximation (QHA), applied to the VDOS obtained from the AIMD simulations, provides estimates of adsorption free energy within ∼10 kJ/mol of the experimentally determined quantities, whereas the traditional approach, employing harmonic frequencies from a single ground state minimum, strongly overestimates the adsorption free energy by at least 20∼50 kJ/mol. This discrepancy is traced back to the inability of the harmonic approximation to represent the contributions to the vibrational motions of the ethanol molecule upon confinement in the zeolite.