Efficient Approach for the Computational Study of Alcohol and Nitrile Adsorption in H-ZSM-5

• Published in: Journal of physical chemistry. C Vol. 116; no. 9; pp. 5499 - 5508
• Main Authors: Van der Mynsbrugge, Jeroen, Hemelsoet, Karen, Vandichel, Matthias, Waroquier, Michel, Van Speybroeck, Veronique
• Format: Journal Article
• Language: English
• Published: Columbus, OH American Chemical Society 08.03.2012
• Subjects: Catalysis; Catalysts: preparations and properties; Chemistry; Exact sciences and technology; General and physical chemistry; Ion-exchange; Surface physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Nitrile; Adsorption; Enthalpy; Van der Waals interaction; Density functional method; Alcohol; Ab initio calculations; Zeolite; Heat of adsorption; Catalyst; Hydrogen bond
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp2123828

Since many industrially important processes start with the adsorption of guest molecules inside the pores of an acidic zeolite catalyst, a proper estimate of the adsorption enthalpy is of paramount importance. In this contribution, we report ab initio calculations on the adsorption of water, alcohols, and nitriles at the bridging Brønsted sites of H-ZSM-5, using both cluster and periodic models to account for the zeolite environment. Stabilization of the adsorption complexes results from hydrogen bonding between the guest molecule and the framework, as well as from embedding, i.e., van der Waals interactions with the pore walls. Large-cluster calculations with different DFT methods, in particular B3LYP(-D), PBE(-D), M062X(-D), and ωB97X-D, are tested for their ability to reproduce the experimental heats of adsorption available in the literature (J. Phys. Chem. B 1997, 101, 3811–3817). A proper account of dispersion interactions is found to be crucial to describe the experimental trend across a series of adsorbates of increasing size, i.e., an increase in adsorption enthalpy by 10–15 kJ/mol for each additional carbon atom. The extended-cluster model is shown to offer an attractive alternative to periodic simulations on the entire H-ZSM-5 unit cell, resulting in virtually identical final adsorption enthalpies. Comparing calculated stretch frequencies of the zeolite acid sites and the adsorbate functional groups with experimental IR data additionally confirms that the cluster approach provides an appropriate representation of the adsorption complexes.