Molecular Competition Effects in Liquid‐Phase Adsorption of Long‐Chain n‐Alkane Mixtures in ZSM‐5 Zeolite Pores

Abstract

Zeolite adsorbents play a vital role in molecular separation technology, industrial catalysis and pollution control. In spite of its relevance to these areas, few fundamental studies have thus far been devoted to the adsorption of mixtures. The complexity of the molecular filling patterns of zeolite micropores directed earlier investigations, rather, into the adsorption of a single compound. Herein, we report the investigation of the adsorption of binary mixtures of n-alkanes with chain lengths up to C 22 from the liquid phase on ZSM-5 zeolite, and the discovery of unexpected selectivities depending on subtle differences in the number of carbon atoms. In some n-alkane mixtures, the uptake of the lighter molecule was favored, whereas in other mixtures the heavier molecule was preferred. With other alkane combinations, azeotropic behavior of the adsorbate phase was also encountered. Systematic exploration of binary mixtures in the carbon range C 5 -C 22 led to the identification of the underlying mechanisms.Intriguing effects in the adsorption of single components in microporous environments, such as zeolites were found recently. For example, ferrierite behaves as a 1D pore system for long-chain n-alkane molecules (> C 5 ), whereas only shorter molecules can exploit its full 2D pore system. [1][2][3][4] With ferrierite and ZSM-22, the siting of dibranched alkane seems to depend subtly on their size. At low coverage the dibranched alkanes adsorb in parallel to the outer surface of the zeolite and as coverage increases, additional molecules adsorb in such a manner that propyl and butyl groups point into the pores.[5] Other studies focused on the relationship between pore size and energy of interaction of pure substances. [6,7] Efforts have been made in the study of adsorption of hydrocarbons on ZSM-5 and Silicalite-1. These zeolites have the MFI framework topology are among the most studied zeolites in the field of adsorption, diffusion and catalysis.[8] The pore system of these zeolites comprises linear channels, with a free pore diameter of 5.6 5.3 , that are

Dependence of surface properties of silylated silica on the length of silane arms

“…All this shows that on adsorption the n-alkanes lose one degree of freedom and a number of conformations depending on the chain length of the hydrocarbon molecule with respect to the pore configuration of silica (Eder and Lercher 1997;Daems et al 2007). In the crystalline zeolites the straight and sinusoidal pore segments are narrow and impose stretching of the alkylchains along the pore axis, and then significant loss of entropy (Denayer et al 2003). In such a case, however, strong interactions with the pore walls implies high adsorption enthalpy too.…”

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

“…At low loading the selectivity is, for most zeolites, toward the linear molecule; for some zeolites a slight preference for the branched isomers is found. Molecular simulations show that at low pressures the energy difference between the linear and branched isomers is very small for those zeolites in which the diameter is sufficiently large to accommodate the 242 ) and simulation (sim from Chempath et al 235 ) selectivities of mixutures of n-alkanes (C 6 /C 7 and C 8 /C 9 ) in MFI. These reduced mole fractions are calculated on the basis of the number of molecules of adsorbing alkanes, where the x-axis is the mole fraction in the liquid and the y-axis is the corresponding mole fraction in the zeolite.…”

Understanding cage effects in the n-alkane conversion on zeolites

“…High pressure can induce this selectivity inversion because it enhances the importance of intermolecular interactions at the expense of molecule-wall interactions. When the intermolecular interactions dominate the Gibbs free energy of adsorption, short-chain n-alkanes can exhibit a lower Gibbs free energy of adsorption than long-chain n-alkanes, because the former can pack more efficiently (in more conformations, with more mobility) and therefore lose less entropy on adsorption [42][43][44][45][46][47][48]. This pressure-induced selectivity inversion is beyond the scope of this paper.…”

“…These studies have clearly established that reactivity increases with increasing n-alkane chain length, mainly because the Gibbs free energy of adsorption decreases linearly with n-alkane chain length, and-by the same token-the adsorption constant increases exponentially [6-19, 21,37-40]. At pressures above the n-alkane saturation pressure, this steady decrease in Gibbs free energy of adsorption with increasing n-alkane chain length can inverse, resulting in the preferential adsorption and conversion of the shorter-chain instead of the longer-chain n-alkane [41][42][43][44][45][46][47][48]. High pressure can induce this selectivity inversion because it enhances the importance of intermolecular interactions at the expense of molecule-wall interactions.…”