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Surface Transport Processes and Sticking Probability of Aromatic Molecules in HZSM-5 S. J. Reitmeier, R. R. Mukti, A. Jentys,* and J. A. Lercher Department of Chemistry, Technische UniVersita¨t Mu¨nchen, Lichtenbergstrasse 4, 85747 Garching, Germany ReceiVed: September 12, 2007; In Final Form: NoVember 6, 2007
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.
Introduction Sorption and transport of reactants into the pores of microporous materials, where sorption and reaction sites are located, is a critical issue to understand catalytic processes and has been controversially discussed for a long time span. The understanding of the surface processes including the adsorption on the outer surface and the diffusion to the active sites is essential when tailoring the shape-selective properties of catalytically active materials, for example, by the modification of the surface and the pore access.1,2 The diffusion of aromatic molecules in zeolites has been addressed in great detail by experimental2-16 and theoretical17-22 methods, while the elementary steps prior to the diffusion inside the pores have been the focus of our recent investigations.23-26 Because molecules with a kinetic diameter similar to that of the pore aperture do not directly enter into the pores,27 processes on the outer surface will control the concentration gradients of the reactants and, thus, the subsequent diffusion processes. Closely related to this point is the presence of surface barriers, which were reported to directly influence the transport diffusion.9,28 Because these processes are fast and only a small fraction of the molecules are typically involved, a fast spectroscopic technique with an excellent signal-to-noise ratio is required. We have described previously the sequence of transport steps of aromatic molecules in HZSM-5 using fast time-resolved (rapid scan) IR spectroscopy,26 a method being able to monitor the microkinetic processes on the surface within a time scale of milliseconds. The overall sorption process has been identified to consist of a series of consecutive steps including the collision of molecules with the surface, the sorption in a (weakly bound) physisorbed surface state with a high two-dimensional mobility of molecules on the outer surface, and finally the parallel transport to silanol groups at the surface and to bridging hydroxy groups inside the pores. The sorption of a molecule on a surface can be described with the sticking probability, which is the probability that a molecule is captured on the (particle) surface after the collision from the gas phase. From the collision frequencies and the uptake rates of aromatic molecules on the silanol groups of * Corresponding author. E-mail:
[email protected].
HZSM-5 and Aerosil sticking probabilities on the order of 10-7 were roughly estimated.26 Simon et al.29 challenged those numbers by reporting a sticking probability close to one for n-butane on Silicalite-1 determined from PFG-NMR and molecular dynamics simulations. It has been suggested that the low sticking coefficients measured are related to the presence of mass transport limitations of the molecules through the bed of zeolite particles (i.e., external diffusion limitations). We confirmed experimentally the absence of such limitations and showed by statistical thermodynamics that such low sticking probabilities are expected when the molecules lose (rotational) entropy during the sorption within the comment.23 In a reply to these arguments, the sticking probability for benzene in aluminum-free ZSM-5 (Silicalite-1) was re-estimated by Ka¨rger et al.30 based on Fick’s first law and was reported to be on the order of 10-4, whereas for ethane in zeolite NaX the number was on the order of 10-2. The reason for the lower sticking probability of aromatic molecules in Silicalite-1 compared to n-alkanes in zeolite NaX results from the gas-phase geometry of the smaller n-alkane, which leads to a faster and entropically less-demanding reorientation when entering the pores.31 A further confirmation of the order of the sticking probability was presented in a theoretical study by Schu¨ring,32 who reported a correlation between the diffusion coefficient and the probability for a molecule to enter into the pores (entering probability). Although this study was carried out for ethane in LTA zeolites, an extrapolation based on the diffusion coefficients for benzene in ZSM-5 leads to a sticking probability on the order of 10-6 to 10-7. The present study explores the mass transport processes of aromatic molecules from the gas phase to the sites inside the pores by studying the impact of different substitutions on the benzene ring. This should allow us to better understand the properties of different molecules during sorption. Experimentally, this is addressed by combining single-step and periodic pressure change measurements using IR spectroscopy as well as pressure modulation to follow the processes. Experimental Section Materials. The surface processes of aromatic molecules (i.e., benzene, toluene, and o- and p-xylene) were studied on HZSM-5 with a Si/Al ratio of 45. The average particle size of 0.5 µm
10.1021/jp077339t CCC: $40.75 © 2008 American Chemical Society Published on Web 01/29/2008
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was determined by SEM. 27Al MAS-MNR indicated the absence of octahedral Al. The concentrations of the terminal silanol groups (SiOH) and the bridging hydroxyl groups (SiOHAl groups, Brønsted acid sites) of the HZSM-5 zeolite were 0.27 and 0.21 mmol/g, determined by 1H MASNMR spectroscopy, respectively. Amorphous, micropore-free SiO2 (Aerosil 200) was used to compare the sorption on HZSM-5 with a material that does not have the additional transport limitations in micropores. The specific surface area and the concentration of OH groups of the amorphous SiO2 were 200 m2/g and 0.54 mmol/g. Aromatic sorbate molecules, benzene, toluene and o-/p-xylene in spectroscopic grade (for GC standard >99.8%), obtained from Sigma-Aldrich, were used without further purification. Fast Time-Resolved Rapid Scan IR Spectroscopy. The detailed measurement principle, instrument setup, and signalto-noise ratio requirements of fast time-resolved rapid scan IR spectroscopy are already described in ref 26. The samples were prepared as self-supporting wafers with a weight of approximately 25 mg/cm2 and inserted in a vacuum cell with a geometry optimized for transmission IR spectroscopy. All IR spectra were recorded with a resolution of 8 cm-1. The samples were activated under vacuum (
