Surface Processes during Sorption of Aromatic Molecules on Medium Pore Zeolites

Abstract

The elementary steps of sorption and transport of benzene, toluene, and o- and p-xylene from the gas phase to hydroxy groups of zeolite H/ZSM-5 on the outer surface (SiOH groups) and in the pores (SiOHAl groups) were studied using pressure modulations followed by fast time-resolved IR spectroscopy. Sorption on these acid sites occurs via a common physisorbed state on the outer surface. The equilibration of the molecules in this state is fast compared to the sorption rates on SiOH and SiOHAl groups. The relative rates of equilibration of functional groups with the aromatic molecules suggest that the aromatic molecules move freely on the surface of the outer surface before reversibly binding to OH groups, entering the micropores or desorbing. Molecules able to enter into the pores (benzene, toluene, p-xylene) adsorb faster on SiOHAl groups than on SiOH groups. If the access of the molecules into the pores is sterically constrained (o-xylene), the rate of adsorption on the remaining accessible SiOH groups is strongly enhanced.

Complete oxidation of o-xylene over Pd/Al2O3 catalyst at low temperature

“…Bands due to the vibrations of bicarbonate species rose at 3630, 1653, and 1427 cm À1 [32,33]. When 100 ppm o-xylene mixture gas was introduced to Pd/ Al 2 O 3 at 30 8C, the adsorbed o-xylene (1610, 1587, 1489 and 1465 cm À1 ) was detected immediately, and we also noticed that a negative peak of n(OH) (at 3745 cm À1 ) appeared, indicating the oxylene molecules may be adsorbed on the catalyst surface through an interaction with the surface OH of the g-Al 2 O 3 support [34].…”

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

“…A complete description of the full instrumental setup, including the applied measurement principle, can be found elsewhere. [14,36] TEM images were obtained on a JEOL-2011 electron microscope operated at 200 kV. The transport diffusivities of benzene were measured using the frequency response method on powdered zeolite samples dispersed on several layers of glass wool.…”

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

“…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.…”