Surface Transport Processes and Sticking Probability of Aromatic Molecules in HZSM-5

Abstract

The elementary steps of the sorption of aromatic molecules such as benzene, toluene, p-xylene, and o-xylene on nonporous amorphous SiO2 (Aerosil) and microporous silicas using HZSM-5 as an example are studied by time-resolved rapid scan IR spectroscopy. Sorption into the zeolite pores proceeds via a weakly bound physisorbed and nonlocalized state on the external surface as the dominating reaction pathway. The weak interaction leads to very low sticking probabilities on the order of 10-7 for porous and nonporous materials alike. Within the molecules investigated, the sticking coefficient increases in the series toluene, o-xylene, benzene, and p-xylene. Using a statistical thermodynamic analysis, this sequence is attributed to the symmetry of the sorbate molecule, the sorption enthalpy of the sorbate increasing with the molar mass, and the space the sorbate molecule occupies on the surface.

Probing the Nature of Surface Barriers on ZSM‐5 by Surface Modification

“…Similar or related explanations on surface barriers were also given in other literature [22 -27]. During an uptake process, the entropic barrier could also be a source of surface barriers, since molecular rearrangement is required before entering into surface pores and this rearrangement could reduce the pre-exponential for diffusion [11,28].…”

“…The desorption of molecules from zeolite interior to gas phase includes three consecutive steps [11,12]: (1) diffusing from the interior to a pore mouth, (2) hopping out of a pore mouth and physically adsorbing on the external surface; (3) desorbing into the gas phase.…”

“…Moreover, desorbing molecules from external surface to gas phase is relatively easy, and the sticking coefficients are generally very low (10 -6~1 0 -7 ) [11,12,28,38]. Therefore, hopping out of a pore mouth (step 2) should be the rate determining step of surface transport; the surface properties affect this step and, subsequently, the surface barriers.…”

“…The surface transport can be further divided into a series of elementary steps (including molecules adsorbing on the external surface of zeolites and entering into or leaving surface pores) on the surface layer that separates the gas-phase region from the core zeolite space, as revealed by Lercher et al [11,12] using time-resolved rapid-scan IR spectroscopy. Thus, the surface barriers should be tightly linked to the properties of this surface layer.…”

Enhancement of Sorption Processes in the Zeolite H‐ZSM5 by Postsynthetic Surface Modification

“…Such low values are the consequence of the difference between the gas-phase and surface degrees of freedom in rigid molecules. [30] Primarily, the sticking of the aromatic molecules is governed by the sorbate mass and the decrease of entropy in the sorption process. The latter contribution is critically influenced by factors such as the geometrical sorbate dimensions, symmetry, and the space occupied on the surface as well as by the external morphology.…”