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J. Phys. Chem. B 2005, 109, 2254-2261
Surface Processes during Sorption of Aromatic Molecules on Medium Pore Zeolites† A. Jentys, H. Tanaka, and J. A. Lercher* Department for Chemistry, Technische UniVersita¨t Mu¨nchen, Lichtenbergstraβe 4, D-85747 Garching, Germany ReceiVed: March 15, 2004; In Final Form: May 11, 2004
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.
Introduction Zeolites and related molecular sieves are the key element in a wide variety of catalytic and sorptive processes in the petroleum and petrochemical industries.1-6 The materials combine a well-defined crystalline structure that includes regular void spaces (pores and cavities) with well-defined functional groups generated by the controlled substitution of elements into silicate or aluminumphosphate lattices.7,8 When the formal charge at the metal cation is lower than the average charge of the metal cations, a localized charge is generated that needs to be balanced by a proton, metal, or organic cation.9,10 For hydrocarbon catalysis, protons are the main functional groups controlling catalytic activity.11 Although all sites in the interior of a zeolite crystal (and hence also the void space, i.e., the pores) are part of the lattice structure, the termination of the periodicity at the outer surface requires the presence of strained bridging oxygen atoms or hydroxy groups, which are, in the case of zeolites, silanol groups. For illustration, a fraction of such a lattice with the corresponding hydroxy groups is depicted in Figure 1. Separation and catalytic production of aromatic molecules is one of the large groups of applications of zeolites amounting to volumes of 400.000 t per year p-xylene production via separation of xylene isomers by sorption (e.g., UOP-PAREX process) or via direct selective catalytic processes such as the shape selective alkylation of toluene. As catalysis and sorption uses primarily the chemical functionality of sites inside the micropores, the transport of the sorbing/reacting molecules (whose size is similar to that of the pores and cages) into the micropores is a critical issue,12,13 which has been intensely studied using experimental 14-19 and theoretical methods.19-25 Due to the complexity of the problem, either the mass transfer process of the molecules from the gas phase to sites inside the pores under nonequilibrium conditions (transport diffusivity) or the mobility of the molecules inside the pores at equilibrium †
Part of the special issue “Michel Boudart Festschrift”. * To whom correspondence should be addressed.
Figure 1. Schematic presentation of bridging hydroxy groups and silanol groups on zeolites.
(self-diffusivity)18,26 is described. These processes differ markedly in nature, but using continuum models, the quantitative rate constants obtained from each type of measurement can be converted.27 From the description of the molecular sieve crystals, it is intuitively clear that the movement of molecules in the micropores has to be preceded by a step in which the molecules enter these pores. Therefore, transport through a whole crystal may follow different reaction rate laws than transport within micropores. As it is commonly agreed that the transport processes of molecules occur via hopping mechanisms between local minima of the sorbate on the surface or in the pores of a sorbent,28,29 understanding the specific interactions of molecules
10.1021/jp0488362 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/08/2004
Surface Processes during Aromatic Sorption with the surface is a key to understand the overall complex transport process. Three principal models can be sketched for the overall transport process through a zeolite crystal: (i) The molecules adsorb directly on the hydroxy groups inside the pores and on the outer surface. (ii) They adsorb first on the SiOH groups at the outer surface and diffuse in a subsequent step into the pores. (iii) The reactant molecules adsorb on the outer surface unspecifically and successfully adsorb on SiOH groups or on sites in the pores after surface diffusion. The latter two models imply that the rates of sorption on the different sites are kinetically coupled, whereas the first mechanism suggests completely independent pathways. Experimentally, it has been observed that the equilibration with aromatic molecules is significantly slower than expected from the pressures applied, the heat of adsorption, and the mobility under equilibrium conditions. This hindrance is typically referred to as “surface barriers”.30 Starting with the work of Ka¨rger et al.30 and Bu¨low et al.,31 several models and methods have been applied to analyze these observations.32-35 Additionally, molecular dynamics simulations have been employed to describe transport processes to and through the outer surface.36,37 Here, we report the first direct spectroscopic evidence of the routes of the adsorption of aromatic molecules on a medium pore zeolite, i.e., H/ZSM-5. The kinetics of the uptake has been directly followed during dynamic pressure changes using rapid scan time-resolved IR spectroscopy. Experimental Section Materials. H/ZSM-5 with a Si/Al ratio of 45 and a particle size of 0.5 µm was used for the experiments. The concentration of Brønsted acid bridging hydroxy groups (SiOHAl groups) and silanol groups (SiOH groups) determined by H/MAS NMR was 0.21 and 0.27 mmol g-1, respectively. For comparative measurements, micropore free amorphous SiO2 (Aerosil 200) was used. The specific surface area and the concentration of OH groups of this material were 200 m2 g-1 and 0.54 mmol g-1, respectively. Thermogravimetry. The sorption isotherms for the aromatic molecules on H/ZSM-5 were measured on a Setaram TG-DSC 111 thermoanalyzer. The sample (18-20 mg) was activated at 823 K for 1 h (heating rate 10 K min-1) under vacuum ( benzene . o-xylene). The estimation of the diffusivity from the uptakes of benzene, toluene, and p-xylene on the SiOHAl groups during the pressure modulation using the square root law18 leads to diffusivities that are in the same order of magnitude (i.e., 1 × 10-14 m2 s-1) as the values determined on this material by single step pressure uptake experiments and by the frequency response technique.54 It is important to note that the increase of the uptake rates observed with increasing molecular weight of the adsorbate molecules is fully compatible with a higher concentration of the heavier molecule in the precursor state resulting in a higher availability of the molecules for diffusion into the zeolite pores. Reaction Pathways for Adsorption on Zeolites. In summary, three transport pathways for the molecules to sorption sites on the surface and in the pores can be envisaged, i.e., (i) the direct adsorption on SiOH and SiOHAl groups (parallel reaction pathway), (ii) sequential adsorption via external SiOH followed by the transport of the molecules into the pores (consecutive reaction pathway), and (iii) adsorption via a weakly adsorbed surface state and subsequent transport of the molecules to the hydroxy groups (pathway via common precursor state). The direct transport of the molecules from the gas phase to sites inside the pores was ruled out for two reasons. (i) Given the low apparent sticking coefficient of the molecules investigated, the differences of the uptake rates of benzene, toluene, and p-xylene on the SiOH groups of SiO2 and H/ZSM-5 strongly suggests the coupling of this sorption process with another process. Note that the equivalent acid strength of the SiOH groups has been thoroughly determined by the heat of adsorption. (ii) The rate of the adsorption of o-xylene, which cannot enter the pores of H/ZSM-5 in the time scale of the transient experiment, on SiOH groups of SiO2 and H/ZSM-5 was identical, which indicates that the coupled kinetic step is the adsorption into the zeolite pores. The consecutive sorption of the aromatic molecules from the gas phase on external SiOH groups followed by surface transport to SiOHAl groups is ruled out, because the rate of equilibration of the molecules on the SiOHAl groups was significantly faster compared to the equilibration on the SiOH groups. A simple microkinetic model clearly shows that for a consecutive adsorption the rate of the second step, i.e., the equilibration of the SiOHAl groups, cannot exceed the rate of equilibration of the SiOH groups. Thus, it is concluded that the sorption proceeds via a joint weak adsorption state (precursor state) in which the molecules are mobile on the outer surface and migrate to a SiOH group or enter a micropore of the zeolite. The sorption in this state is dominated by dispersion forces (i.e., nondirected interactions) to lattice oxygen atoms. It is important to note that for alkane
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Figure 7. Kinetic model describing the transport process.
sorption in zeolites this type of interaction was found to dominate over the localized interactions with the acidic hydroxy groups.49 In the preadsorbed state, the molecules are mobile in two dimensions; that is, the molecules loose only one degree of translational freedom and have reduced rotational degrees of freedom. The loss of entropy in that step is, therefore, significantly lower than that associated with a direct localized adsorption. It can be speculated that the main reason for adsorbing first into this state is the reduction of the rotational degrees of freedom allowing a better suited state for the subsequent sorption on the SiOH groups and in the pores. The weak bonding of the aromatic molecules to the Brønsted acid sites (SiOH and SiOHAl groups) appear to have a very low transition entropy making it difficult to reach the best approach for the adsorption step. The precursor also plays an important role in the reversibility of the adsorption and desorption step. As the uptake and desorption are completely symmetrical, we propose that these processes are controlled by the change in the rate of the adsorption and desorption process from and into the physisorbed state. It is, however, unclear at the moment whether this is achieved via a mechanism that forces all molecules desorbing to leave the surface via the same channel (the physisorbed state) or via a more rapid adjustment of the surface concentration acting as a buffer for more direct changes in the individual sorption sites. As it has been outlined above, the first step of the microkinetic sequence, i.e., the adsorption of the aromatic molecules on the external surface by van der Waals forces, is the step with the largest reaction rate constant in the forward direction. At 403 K also, the rate constant for desorption is large, and hence, the reaction step rapidly reaches equilibrium. Thus, the adsorption rate on the SiOH and SiOHAl groups can be formulated as the product of the respective rate constants, the concentration of the aromatic molecule in the physisorbed state, and the concentration of the SiOH groups and the pore openings, respectively. A simplified microkinetic model for the process is shown in Figure 7. The kinetics of the sorption on SiOHAl and SiOH groups can be well described with this model when the rate constants of the individual steps decrease in the order k1,k-1 . k3,k-3 > k2,k-2.
Jentys et al. The concentration of free SiOH groups and the concentration of pore openings (calculated from the geometry of the ZSM-5 surface) were 0.24 mmol g-1, whereas the concentration of unoccupied SiAlOH groups at a partial pressure of 6 × 10-2 mbar p-xylene was 0.082 mmol g-1. The significantly lower concentration of the free SiOHAl shows that the main driving force to sorb into the pores is not related to the fraction of unoccupied SiOHAl groups. The similar concentration of the sorption sites and the higher rate of sorption on SiOHAl groups suggests that significant differences in the rate constants exist, i.e., that the rate constant for adsorption on the SiOHAl groups (or better in the pores) is higher than the rate constant for adsorption on SiOH groups. As the transport process of the molecules into the pores should have only a very small energetic barrier (activation energy), the main differences for the two pathways must be related to the transition entropy. In this context, we speculate that the transition state leading to localized sorption on SiOH groups is more constrained (lower transition entropy) than the transition state allowing the aromatic molecule entering the channel of the microporous material. The speculation is based on the Polany relationship between the final states in the two sorption structures applied to the entropy term of the rate constant. Although for molecules adsorbed on SiOH groups only rotational degrees of freedom remain, molecules entering into the pores have rotational and some translational degrees of freedom. This leads to a larger loss of entropy for the molecules adsorbed on the SiOH groups compared to nonlocalized (unspecific) sorption inside the zeolite pores. Thus, based on the assumption that the material with the higher entropy in the final state also has the higher entropy in the transition state, we attribute the faster uptake into the pores to a higher transition state entropy for molecules entering the pores. The enthalpy of the molecules in the final state (i.e., the heat of adsorption) does not appear to be decisive for the adsorption process. If the sorption of the molecules into the pores is sterically hindered, such as for o-xylene, this faster adsorption channel is blocked which leads to a higher concentration of available physisorbed molecules. Consequently, the probability to adsorb onto a SiOH group increases to the value also found for nonporous SiO2. Conclusions Time-resolved IR spectroscopy was used to study the reversible sorption processes of aromatic molecules (benzene, toluene, p-xylene, and o-xylene) from the gas phase on the acidic sites of H/ZSM-5. The results show unexpectedly that the SiOH groups at outside of the zeolite crystal reach the equilibrium slower than the bridging hydroxy groups in the zeolite pores. To explain these results, the transport process is described as a sequence of several consecutive elementary steps. The data presented give first evidence that after collision of the molecules with the surface a weakly bound surface state (precursor state) is formed from which the aromatic molecules migrate to the acidic sites and that direct adsorption onto the acidic groups does not take place. The formation of this state is a nonactivated process and has only a low heat of adsorption. The rate of sorption into this state is the fastest of all reactions occurring during the studied equilibrations. The relative rates for the transport of the molecules from the preadsorbed state to the hydroxy groups are controlled by the availability of the sorption sites with an additional contribution of the transition entropies of these processes. If the sorption of the molecules into the pores is sterically hindered, such as for o-xylene on H/ZSM-5, the
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