Molecular Understanding of Sorption in Mesoscale Organized Zeolites with MFI Structure
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Molecular Understanding of Sorption in Mesoscale Organized Zeolites with MFI Structure Robin Kolvenbach • Luis Francisco Gonzalez-Penˇa Andreas Jentys • Johannes A. Lercher
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Received: 28 August 2013 / Accepted: 22 September 2013 / Published online: 12 October 2013 Ó Springer Science+Business Media New York 2013
Abstract We report herein the adsorption of light hydrocarbons in small crystal ZSM-5 monitored by infrared spectroscopy including a direct monitoring of the individual concentrations (surface and intracrystalline). Accessing the individual rates reveals that the sorption enhancement upon surface modification with TEOS is selective towards molecules with a small steric demand and a high heat of adsorption within the silica overlayer. Keywords IR spectroscopy Zeolites Adsorption Time-resolved spectroscopy Transport mechanism
1 Introduction Medium pore zeolites, such as ZSM-5 (MFI structure) [1] are a key catalyst component in petroleum and petrochemical processes [2–4], including toluene alkylation [5– 8] and xylene isomerization [2, 9–12]. In addition to high acid strength and thermal stability they act as shape selective catalysts, showing a clear preference for forming products that do not exceed a certain size, by exerting steric
Electronic supplementary material The online version of this article (doi:10.1007/s10562-013-1118-z) contains supplementary material, which is available to authorized users. R. Kolvenbach L. F. Gonzalez-Penˇa A. Jentys (&) J. A. Lercher Department Chemie, Catalysis Research Center, Technische Universita¨t Mu¨nchen, Lichtenbergstr. 4, 85748 Garching, Germany e-mail: [email protected] J. A. Lercher e-mail: [email protected]
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constraints on the formation and transport of molecules. This induced selectivity, however, comes with the price of a reduced utilization of the zeolite material through diffusional constraints. Thus, understanding and controlling sorption and transport properties of reactants and products is essential to rationally develop and fully utilize the shape selective potential of zeolites. Previously, we have had identified five key steps in the transport of molecules through a zeolite particle [13, 14], forming a complex network [15] of consecutive steps as presented in Fig. 1. It involves the collision of a gas phase molecule (Fig. 1a) with the external surface, in which the sorbate molecule has a certain probability to be adsorbed into a weakly bound physisorbed state (Fig. 1b) with a high two-dimensional mobility [13]. This step is followed by the adsorption at terminal hydroxyl groups (Fig. 1c) at the external surface or the adsorption on bridging hydroxyl groups inside the pores (Fig. 1d). The latter two steps occur in parallel. The transport inside the micropores proceeds via hopping mechanism between adsorption sites [16, 17] and can be predicted with reasonable accuracy via MD simulations [16, 18, 19]. A quantitative description of the transport rate, however, that includes the surface processes has not been accessible. Assessing
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