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2007, 111, 5561-5567 Published on Web 03/23/2007
Masked Lewis Sites in Proton-Exchanged Zeolites: A Computational and Microcalorimetric Investigation Claudia Busco,† Vera Bolis,† and Piero Ugliengo*,‡ Dept. DiSCAFF, UniVersity of Eastern Piedmont “A. AVogadro” Via G. BoVio 6, I-28100 NoVara, Italy, and NIS-Nanostructured Interfaces and Surfaces Centre of Excellence, INSTM-Materials Science and Technology National Consortium, UdR Eastern Piedmont, and Dept. of Chemistry IFM, UniVersity of Torino, Via P. Giuria 7, I-10125 Torino, Italy, and NIS-Nanostructured Interfaces and Surfaces Centre of Excellence, INSTM-Materials Science and Technology National Consortium, UdR UniVersity of Torino ReceiVed: January 22, 2007; In Final Form: March 3, 2007
Lewis sites, in both proton-exchanged zeolites and silico-alumina systems, exhibit different coordination states, depending on the thermal and/or chemical treatments undergone by the material as well as on the experimental conditions. A new modulating effect of the Lewis acidity, similar to that reported for sulfated zirconia, vanadia, and tungstena materials, is addressed here for alumino-silicates, showing that the Lewis acidity of a formal Al(III) site may become “masked” when the local topology around the Al(III) ions allows the latter to expand their coordination by making an extra bond to the aluminosilicate-framework oxygen atoms. Their intrinsic Lewis acidic character is, however, not irreversibly lost because adsorbed molecules of sufficiently high basicity can unhook the Al atom from the framework. Although the “masking” effect has been discovered by ab initio modeling on the edingtonite framework, microcalorimetric data for the adsorption of N2, CO, and NH3 on a Lewis-rich H-BEA zeolite provides experimental evidence that the “demasking” is an endothermic process occurring only with strong bases like NH3. The resulting enthalpy of adsorption is thus a compromise between the endothermic demasking process and the exothermic interaction with the restored Al(III) site.
Introduction In the literature, it has been recognized that the Al atoms in both crystalline and amorphous aluminosilicates (protonexchanged zeolites and silico-alumina systems, respectively) can exhibit different coordination states, depending on the thermal and/or chemical treatments undergone by the material as well as on the experimental conditions, in particular on the presence of ligands of different strengths.1-5 Proton-exchanged zeolites have been investigated extensively in the past few decades in virtue of their unique catalytic properties. Such materials are indeed used widely as heterogeneous solid acid catalysts in several relevant industrial processes.6-9 It is well known that the proton-exchanged zeolite’s activity is related to the presence of the Si(OH)+Al- species (Brønsted acidic sites), which are located within the zeolite nanocavities and are able to adsorb guest molecules by strong H-bonding interactions, often leading to the formation of protonated species. Much about the features of such acidic species is known.10-12 The ability to donate protons, however, is not the only property of acidic zeolites relevant to catalysis. Several experimental studies13-15 have shown that some proton-exchanged zeolites are also strong electron acceptors; that is, they behave as Lewis acids. At variance with the detailed knowledge of the structural features of the Brønsted acidic sites, very little is known about the nature and structure of the Lewis ones and only hypotheses have been * Corresponding author. E-mail:
[email protected]. † University of Eastern Piedmont. ‡ University of Torino.
10.1021/jp0705471 CCC: $37.00
suggested in the literature. In particular, some authors associate zeolites’ Lewis acidity to the presence of framework trigonal Al(III) atoms,16 whereas some other groups suggest that Lewis sites are made of extraframework Al species (EFAL) within the zeolite pores.17-19 In previous papers,20-23 we have simulated the structure of the Al(III) Lewis site by ab initio calculations in a cluster approach, in which the Lewis acidic strength of the coordinatively unsaturated Al(III) atoms was modulated by varying the local geometrical strain around the Al(III) atom. The Lewis acidity of the Al(III) models of modulated strain has been studied ab initio by adsorbing molecular probes of increasing basic strength in order to evaluate the ability of the sites to accept electron transfer. The binding energy values (BE, kJ/mol) obtained for the probe adsorbed onto the individual sites have shown that the larger the local strain around the Al(III) atom, the larger the Lewis acidic strength of the site. The validity of the models proposed was also confirmed by comparing the calculated binding energies with the enthalpy of adsorption of the same probes measured by microcalorimetry (qdiff ) -∆adsH, kJ/mol). The two sets of calculated and experimental data were found to be in reasonably good agreement, taking into account that the experimental enthalpy values are average values including the contribution of different processes occurring simultaneously in the zeolite nanocavities.21 The result that the Lewis acidic strength of the site depends so strongly on the local strain around the Al(III) atom does suggest that (i) different framework topologies impart different acidic strengths and (ii) © 2007 American Chemical Society
5562 J. Phys. Chem. C, Vol. 111, No. 15, 2007
Letters
Figure 1. (a) Construction of surface Lewis acidic site periodic model EDISLAB. A slab parallel to the (100) crystal plane of the bulk edingtonite (EDIBULK) has been cut out, and the resulting surface SiS atoms have been substituted by AlS atoms. The Al-Al distances of the surface Al(III) sites before the energy minimization are reported in angstroms. (b) LLC and LSC cluster models adopted for mimicking different structural strain around the cus Al(III) site.
defective and/or amorphous materials may sport a variety of Lewis acidic sites of different strengths (as indicated by the enthalpy of adsorption vs coverage plots).20-23 The Lewis acidity can also be modulated by the interaction of the surface acidic site with the basic groups belonging to the material structure in close spatial proximity, as has been reported for sulfated zirconia24 in which the surface sulfate groups (basic) can interact with the nearby coordinatively unsaturated surface Zr ions, rendering the latter less prone to show up as a strong Lewis site when probes of moderate strength, such as CO and N2, are adsorbed. Only when very strong bases like pyridine or ammonia are employed will the hidden Lewis character of these sites be revealed. Similar effects have also been reported for vanadia and tungstena materials.25 In the present work, it will be shown that the same effect happens on silico-aluminas and zeolites as well. Here, the Lewis acidic properties of the Al(III) sites become “masked” when the local topology around the Al(III) ions allows the latter to expand their coordination by making an extra bond to a zeolite framework-O atom (Of). When this occurs, the Al(III) atoms increase their coordination and NMR26 and other spectroscopic techniques25 will no longer detect them as Lewis acidic Al(III) sites. However, their Lewis acidic character will not be irreversibly lost. For this reason, we will refer to these species as masked Al(MS) sites. The extra coordination of the Al atoms of the Al(MS) sites is, in fact, somehow labile in that, on one hand, the Of atoms are not so strongly basic and, on the other hand, their mobility is low despite the rather flexible silica matrix. For both of these reasons, the masked Al(MS) sites restore their full Lewis acidic strength when interacting with molecules of basicity sufficiently high to pull out the Al(MS) atoms from their extra coordination with the Of atoms similarly to the pyridine adsorption on sulfated zirconia.24 Interestingly, the extra coordination is a feature of the material topology and is not intrinsically affected by thermal treatments of the sample.
Our approach is to use ab initio molecular modeling to highlight, at atomic detail, the process that brings an Al(III) in its masked state. As can be easily imagined, the masking is a rather peculiar event, which requires a special local surrounding of the Al(III) site. We discovered that the edingtonite microporous material (EDI framework) has a topology (vide infra) that allows this process to occur readily. Unfortunately, direct comparison between ab initio results and experiments run on edingtonite cannot be carried out, due to the difficulty in handling the edingtonite zeolite (presence of a rich variety of cations and water in the natural zeolite). Alternatively, it is well known that Lewis sites in microporous materials are invariably formed at framework defects (broken Si-O-Al bonds, stacking faults, missing Si atoms, etc.), which are precursors for the formation of aluminum extraframework particles.27,28 This means that the local geometry around a Lewis site cannot, straightforwardly, be connected with the zeolitic framework topology to which it belongs. The ab initio results on edingtonite are then adopted in the following in a euristic manner, to verify whether the masking effect can be experimentally revealed on a H-BEA zeolite specimen that is known to be particularly defective29 and, consequently, rich in Lewis sites.27,28 Methods In this work, we address the variability of coordination of the framework three-coordinated Al(III) Lewis acidic sites typical of proton-exchanged zeolites, which unfortunately are difficult to observe directly and characterize.26 To this purpose, we resort to ab initio modeling, by adopting the (001) edingtonite surface as a model for an aluminosilicate, the structural and energetic details of which will be described in the following (see Figure 1a). Edingtonite is a microporous material and has been proposed by some of us as a successful model of the hydroxylated amorphous silica surface,30,31 so it is expected to be suited for simulating an aluminosilicate surface as well.
Letters Interestingly, this material possesses a topology that allows the Al(III) masking process to occur rather easily. To probe the acidity of the resulting Al(MS) Lewis site, so as to investigate its reversibility toward the Al(III) coordination, the binding energies (BE, kJ/mol) of the edingtonite model with the same set of molecular probes adopted in the past (from the very weak N2 up to CO and to the moderately strong NH3) have been computed. The obtained results have been compared with the previous ones obtained by us on cluster models (see Figure 1b) mimicking the acidity as a sole function of the geometrical strain.20-23 The resulting BE values of the probes with the Al(MS) Lewis acidic sites will be also compared with the experimental enthalpy of the adsorption (at 303 K) of the same molecular probes on a H-BEA zeolite specimen, the richness of which in Lewis acidic sites (due to a relevant fraction of lattice defects) is well-known.29 Molecular Modeling. Two cluster models (LLC and LSC, Figure 1b), already adopted in the past,20-23 have been considered as references for mimicking different geometrical strains around the cus Al(III). Conversely, the Al(MS) site has been mimicked by a periodic bidimensional slab model (EDISLAB, Figure 1a) derived from the all-silica edingtonite zeolite framework30 (EDIBULK, Figure 1a). Edingtonite framework (space group P4hm2, 8 symmetry operators, 5 atoms/asymmetric unit, 15 atoms/cell) is characterized by a small asymmetric cell unit and structural rigidity. The slab has been cut parallel to the (100) crystal plane family, and the resulting surface SiS atoms have been substituted by AlS atoms (see Figure 1a for details). As a consequence of these operations, the resulting bidimensional model has a lower symmetry (layer group P2mm, 4 symmetry operators, 11 atoms/ asymmetric unit, 29 atoms/cell) than the bulk edingtonite. The Al sites are sufficiently dispersed (d(Al-Al) g 6.5 Å, see Figure 1a) to ensure negligible mutual interference. Calculations on the EDISLAB periodic model (both isolated or in interaction with the adopted probe molecules) have been performed at the DFT level on a standard Linux PC by the CRYSTAL03 computer code.32 The Becke three-parameter hybrid exchange functional33 in combination with the gradientcorrected correlation functional of Lee, Yang, and Parr (B3LYP),34 used widely and successfully in solid-state calculations,35-38 has been adopted for all of the geometry optimizations. In the CRYSTAL code, the crystalline orbitals are expanded in terms of a Gaussian-type basis set. For the present case, the 31G(p) for H, the 6-21G(d) for Si, the 8-511G(d) for Al, and the 6-31G(d) for O, C, and N contractions have been adopted. The exponents (in bohr-2 units) of the most diffuse sp shells are 0.16 (H), 0.13 (Si), 0.28 (Al), 0.27 (O), 0.17 (C), and 0.12 (N); the exponents of the polarization functions (p on H and d on Si, Al, O, C, and N) are 1.1 (H), 0.5 (Si), 0.47 (Al), 0.6 (O), and 0.8 (C and N). Calculations on the LLC and LSC cluster models have been performed with the same B3LYP functional by the GAUSSIAN03 computer code,39 adopting the standard Gaussian 6-31+G(d,p) basis set. For both the periodic (EDISLAB) and cluster (LLC, LSC) models, the binding energies (BE) between the Lewis site models and probe molecules were corrected for the basis set superposition error using the standard procedure.40,41 The interaction with the probe molecules leads to a distortion of the adsorbent so that the deformation-corrected binding energies (BEdef) have been worked out as BEdef ) BE + ∆E, in which ∆E is the difference between the energy of the relaxed free Lewis site model (cluster or periodic) and the energy of the deformed model as due to the interaction with the probe molecule.
J. Phys. Chem. C, Vol. 111, No. 15, 2007 5563 Adsorption Microcalorimetry. The enthalpy change associated with the adsorption on a H-BEA zeolite specimen of the chosen probe molecules (N2, CO, and NH3, all by Praxair) have been measured at 303 K by means of a heat-flow microcalorimeter (Calvet C80, Setaram) connected to a high vacuum gas volumetric glass apparatus (residual pressure p e 10-5 Torr, 1 Torr ) 133.3 Pa), which allowed us to determine during the same experiment both adsorbed amounts and heats evolved for small increments of the adsorptive, following a well-established procedure.21 The H-BEA zeolite specimen (kindly supplied by Polimeri Europa srl. Centro Ricerche Novara Istituto G. Donegani Novara, Italy)20,21 is a proton-exchanged BEA zeolite having a SiO2/Al2O3 ratio of 4.9, which corresponds to a distribution of 5.9 Al atoms per unit cell. The sample was vacuum activated by a prolonged outgassing (2 h at a residual pressure of ∼10-5 Torr) at a temperature (T ) 873 K) that ensured the achievement of the maximum dehydration of the surface, that is, the maximum density of Lewis acidic sites, compatible with the stability of the structure. The heats of adsorption were measured as integral heats evolved during the stepwise adsorption of molar quantities of the adsorbate. The integral heats evolved, Qint (J), at a chosen equilibrium pressure, peq (Torr), are often normalized to the adsorbed amounts, nads (mmol), so generating the molar heats of adsorption [q ) Qint/nads]p (kJ/mol), which are comprehensive of the whole thermal contributions from the variety of interactions that the molecular probe experiences at that equilibrium pressure. By making the derivative of the function that describes the evolution of the integral heat upon the increasing coverage, the differential heats of adsorption are generated (qdiff ) -∆adsH), which are still intrinsically average quantities but quantify with a better accuracy the interaction between the molecular probe and the individual sites. Briefly, this is the quantity than can be properly compared to the calculated binding energies (BE) and in the present work will be reported as a function of the increasing coverage, giving useful information about the heterogeneity of the surface investigated. In Figure 2a and b, the enthalpy of adsorption (-∆adsH) of N2 and CO and of NH3 are, respectively, shown as a function of the number of molecules adsorbed per Al atom. It is worth noting that the abscissa scale in the two plots is dramatically different for the weak N2 and CO probes and the moderately strong NH3, according to the large difference in the basic strength of NH3 with respect to N2 and CO (proton affinities PA in kJ/mol are 494, 594 for the latter and 854 for the former, respectively42). In the inset of Figure 2a and b, the volumetric isotherms (adsorbed molecules vs equilibrium pressure) are also reported for the two sets of probes. In both (-∆adsH vs adsorbed molecules and adsorbed molecules vs peq) plots, data obtained for the first run (ads. I) and second run (ads. II) of adsorption are shown. Ads. I was performed by contacting the activated sample with the probe, whereas ads. II was performed by contacting the sample with the probe after outgassing overnight (at 303 K) the reversible component adsorbed during the first run. This was done to check the reversibility/irreversibility features of the process. Results and Discussion The EDISLAB structures, before and after geometry relaxation, are reported in Figure 3. As a consequence of the energy minimization, the final geometry of the periodic model is largely distorted. The distortion is due to the capture of a nearby framework oxygen atom (Of) by the Al(III) strong Lewis site.
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Figure 2. Enthalpy of adsorption (-∆adsH ) qdiff, kJ/mol) of basic probes on H-BEA zeolite vs number of adsorbed molecules per Al atom, N/Al. (a): N2 (up triangles) and CO (circles). (b): NH3 (squares). Inset of the figures: volumetric isotherms (number of adsorbed molecules per Al atom, N/Al vs equilibrium pressure, peq). Solid symbols, first run; open symbols, second run of adsorption at T ) 303 K. Dashed lines, enthalpy of liquefaction -∆liqH of N2, CO, and NH3 (5.5, 6.0, and 21 kJ/mol, respectively, by “Gas Encyclopaedia” by Air liquide, Ed Elsevier).
Figure 3. Effect of energy minimization on the EDISLAB structure and on the corresponding surface electrostatic potential maps. The AlAl and Al-Of distances are reported in angstroms. The electrostatic potential maps are plotted in the -0.1/+0.1 au interval. Each positive (solid) and negative (dashed) isoline corresponds to 0.01 au.
During the energy minimization, the Al-Of distance is greatly reduced (starting structure: Al-Of ) 3.3 Å; relaxed structure: Al-Of ) 2.0 Å) and the initial three-coordinated Al(III) atom
Letters
Figure 4. B3LYP optimal structures of different Lewis acidic site models interacting with N2, CO, and NH3 probe molecules. The computed binding energies, BE, and the deformation corrected binding energies, BEdef (in parenthesis), are reported in kJ/mol and the d(AlOf) distances in Å. In the last column, the experimental zero-coverage enthalpy of adsorption [(-∆adsH)0 ) q0, kJ/mol] of the three probes on H-ΒΕΑ zeolite are reported for comparison purposes.
increases its coordination to become the Al(MS) site, in which the Lewis character has become masked. The Al(MS) electrostatic polarizing power is reduced if compared with that of Al(III), as shown by the electrostatic potential maps in proximity of the Al site (Figure 3). The positive potential in the region above the Al(III) (where adsorption will take place) of the starting structure is reduced, whereas its topology is reshaped in the relaxed structure, as a consequence of the increased coordination of Al(MS). Looking carefully at Figure 3, one can figure out what the general structural requests are for the Al(III) masking to occur: (i) the Al(III) belongs to a fourmembered ring so that the most distant Of is no further than 3.5 Å; (ii) the Of should be in the cis position with respect to the Al(III)-O-Si-Of sequence; and (iii) the local topology around the Al(MS) should be flexible enough to allow for the large distortion occurring in the four-membered ring during the masking process (see the large deformation of the Si-O-Si pillar angle). Careful study of the topology of the most common framework topologies (BEA, MFI, FAU, MOR) reveals that the above-reported conditions cannot be fulfiled for a regular Al(III) framework site. However, one has to consider that Lewis sites are always associated to defective zeolites (vide supra)29 usually obtained after harsh thermal treatment and/or steaming processes at high temperatures so that the above conditions for the Al(MS) to occur may be feasible for such a case. The residual Lewis acidity of the Al(MS) site has been gauged by computing its interaction with probe molecules of increasing basic strength (N2, CO, and NH3). Figure 4 shows the resulting optimal geometries and the BSSE-corrected binding energies, together with the deformation corrected binding energies (BEdef in parenthesis, vide supra for the definition). It is interesting to compare the Al(MS) BE results with the corresponding ones computed previously for both the LLC (low strained Al(III) site) and LSC (high strained Al(III) site) cluster models, as illustrated in Figure 4. The last column of Figure 4 also shows the experimental zero-coverage enthalpy of adsorp-
Letters tion [(-∆adsH)0 ) (qdiff)0] obtained by extrapolating the -∆adsH versus adsorbed molecules curves to the vanishing coverage (see Figure 2a and b). Such (-∆adsH)0 values represent the enthalpy of adsorption of the probe with the strongest fraction of the coordinatively unsaturated Al(III) atoms, which are expected to be active in the early stage of the process. The (-∆adsH)0 values are in good agreement with the BE values for LLC and the LSC cluster models only for the weak basic probes (N2 and CO, hereafter referred to as WB, weak bases), as shown in our previous work.20 The experimental values (43 and 70 kJ/mol for N2 and CO, respectively) are indeed reasonably located in between the LLC and the LSC cluster models’ values (56 and 38 kJ/mol, and 64 and 84 kJ/mol, for N2 and CO, respectively). By contrast, the calculated BE values for NH3 interacting with the LLC (175 kJ/mol) and LSC (212 kJ/mol) cluster models are overestimated with respect to the experimental (-∆adsH)0 of 140 kJ/mol (as anticipated in refs 22 and 23). This deserves a comment that goes in the direction of the existence of the masked Lewis sites. We will come back to this result after discussing the computed results, that is, the comparison between the LLC/LSC clusters and edingtonite models’ data (vide infra). The BE values computed for the EDISLAB/WB case are considerably lower than those resulting even for a low strained Al(III) site (LLC). The Al-Of distances are 2.1 Å (EDISLAB/ N2) and 2.2 Å (EDISLAB/CO), only slightly longer than the corresponding Al-Of distance computed for the relaxed free structure (Al-Of ) 2.0 Å, see Figure 3). This result strongly suggests that the WB probes do not compete with the basicity of the Of framework so that the Al(MS) in contact with WB molecules remains in its masked state. The cost of restoring the three-coordinative status of Al(MS) can be quantified by the difference, ∆E, between BE and BEdef. ∆E values are 5 and 13 kJ/mol for N2 and CO, respectively, showing that the hypothetical final structure is still close to the starting one. In the simulation of the interaction, it appears that the basic strength of the N2 and CO probes is not sufficiently high to compensate for the rearrangement cost from the masked Al(MS) site to the coordinatively unsaturated trigonal Al(III) one. From the experimental point of view, it is expected that the hypothetical masked Al(MS) sites remain silent with respect to the WB probe, and mostly in the case of N2. Indeed, in such a latter case we have measured that ∼2 N2 molecules per 100 Al atoms (see Figure 2a) are adsorbed with an average constant heat of adsorption of ∼40 kJ/mol, a value close to the LLC/N2 computed BE (38 kJ/mol). In the case of CO, a similar behavior is observed (see Figure 2a): ∼5 CO molecules per 100 Al atoms are adsorbed with an average constant heat of adsorption of ∼65 kJ/mol, again close to the LLC cluster model value (64 kJ/mol) and in agreement with the larger basic strength of CO with respect to N2. These energy values can be reasonably associated to the adsorption on the fraction of the most energetic Al(III) sites. This result indicates that the highly coordinative unsaturated Al(III) sites interacting with WB molecules, such as N2 and CO, are fairly well simulated by the LLC and LSC cluster models. Afterward, in both cases the adsorption process goes toward the saturation (see the volumetric isotherms shown in the inset of Figure 2a) and the enthalpy of adsorption starts to progressively decrease to very low values, compatible with the weak interaction with the acidic Brønsted sites (see ref 20) and/or to a mere physical adsorption (confinement effects).20,22,23 The BE computed for the EDISLAB/NH3 case is still considerably lower than the LLC/LSC corresponding ones, similar to what was observed for the WB probes (see Figure 4). However, as anticipated above, at variance with the BE
J. Phys. Chem. C, Vol. 111, No. 15, 2007 5565 resulting from the WB set, the LLC/LSC BE values (175 and 212 kJ/mol, respectively) are always overestimated with respect to the experimental (-∆adsH)0 value (∼140 kJ/mol). In our previous work,21 we have suggested that the discrepancy between the LLC/LSC calculated and the experimental energy values for H2O interaction was due to the simultaneous adsorption of the moderately strong basic probe also on the Brønsted acidic sites, which are much less strong but intrinsically much more abundant than Lewis acidic sites. So, the measured heat of adsorption is found to be lower than that expected because already in the early stage of the process the Brønsted sites compete for the adsorption of moderately strongly basic molecules, such as NH3 in the present case. It is worth noting that the BE for NH3 on the Brønsted site is 62 kJ/mol (see ref 23). This interpretation is certainly valuable, but on the basis of what reported in the present letter it also has to be taken into account that NH3 is sufficiently strong to cause relevant restructuring in the complexes. For instance, the Al-Of distance for the EDISLAB/NH3 models is significantly longer (Al-Of ) 2.6 Å) than the corresponding Al-Of distance of the relaxed free structure (Al-Of ) 2.0 Å) as shown in Figure 3. At variance with the WB case, the ∆E ) (BE - BEdef) is now 38 kJ/mol (see Figure 4), showing a large cost associated to the “demasking” process. This fact implies that only probes of enough basicity will be able to reveal the nature of the Al(MS) Lewis sites. So, in the NH3 case, the measured heat of adsorption is the combination of an endothermic effect due to the withdrawing of the Al site from its extra coordination, and the exothermic effect of the NH3/Al site interaction. Indeed, the EDISLAB BE value(112 kJ/mol) is not so distant from the zerocoverage experimental (-∆adsH)0 value (∼140 kJ/mol), which is still likely comprehensive of the energy of interaction of NH3 with the strong Al(III) sites (the fraction detected by N2 and CO adsorption, vide supra). Then, the strong sites being rapidly saturated, the heat of adsorption decreases abruptly with the increasing coverage (opposite to what is observed for WB probes, see Figure 2), confirming that the measured heat is a combination of endothermic and exothermic effects. Afterward, the heat values decrease toward values typical of the physical adsorption (close to the latent heat of liquefaction of NH3, -∆liqH ) 21 kJ/mol). Indeed, by the inspection of the enthalpy of adsorption versus coverage plot of Figure 2b, it is quite evident that in the case of NH3 the curve is typical of a highly heterogeneous surface. This fact does suggest that the NH3/HBEA zeolite interaction is due to a variety of different kinds of sites, all contributing to the measured heat of adsorption. In particular, the existence of Al(MS) sites implies that the dissection of the experimental heat of adsorption in different contributions can be biased by the presence of such sites. It is worth noting that for NH3 the number of sites interacting with the probe is 2 orders of magnitude larger than that for WB probes (at comparable equilibrium pressure of the adsorptive), as clearly evident by comparing sections a and b of Figure 2. Furthermore, it is worth noticing that in the case of the WB probes the interaction is fully reversible (as evidenced by the coincidence of the first and second run of adsorption), whereas in the case of NH3 some 20% of the adsorbed phase is not removed by simply pumping off at 303 K. Figure 5a summarizes the correlation between experimental (-∆adsH)0 values and either the BE (solid symbols) or the BEdef (open symbols) computed for the EDISLAB case. In the case of WB probes for which ∆E ) (BE - BEdef) is very low, the experimental enthalpy of adsorption is larger than both BE and BEdef. For NH3, for which ∆E is quite large (38 kJ/mol) the
5566 J. Phys. Chem. C, Vol. 111, No. 15, 2007
Figure 5. (a) Experimental zero-coverage enthalpy of adsorption [(-∆adsH)0 ) q0, kJ/mol] of N2, CO, and NH3 on H-BEA zeolite reported as a function of the corresponding binding energies (BE, kJ/ mol) computed for the EDISLAB periodic model. Solid symbols, data not corrected (BE); open symbols, data corrected for the deformation energy contribution (BEdef). (b) Experimental zero coverage enthalpy of adsorption [(-∆adsH)0 ) q0, kJ/mol] of N2, CO, and NH3 on H-BEA zeolite reported as a function of the corresponding binding energies (BE) computed for LSC (up triangles) and LLC (down triangles) cluster models.
(-∆adsH)0 value correlates quite well with the BEdef value. This result does confirm that the experimental heat of adsorption of basic probes may be surprisingly small in the presence of Al(MS) Lewis sites, if the probe is sufficiently strong, in agreement with the calculated BE, which is a compromise between the demasking cost (always negative) and the energy of the acid/ base interaction (always positive). In Figure 5b, the correlation between experimental (-∆adsH)0 values and the BE values computed for LLC and LSC models is shown. It is clearly evident that LSC is a good model for strong Lewis acidic sites, and it is representative of only a small fraction of highly coordinative unsaturated Al(III)sites, active in the very early stage of the interaction (see the good agreement between the experimental and computed energy of interaction only for the WB probes). Alternatively, the LLC model likely represents fairly well-coordinative unsaturated Al(III) sites whose contribution is averaged on the contribution of Al(MS) sites, when moderately strong basic molecules are considered (see the lack of agreement between the experimental and computed energy of interaction in the case of NH3). Relevance of the Al(MS) in Previous Experimental Studies. The general relevance of the masking process is extremely difficult to assess from previous experimental studies for many reasons. First, as the present experimental results on a H-BEA specimen have clearly shown, data has to be analyzed very carefully due to the small amount of the Al(MS) present in a given aluminosilicate. Second, one has to adopt molecular
Letters probes of different basic strengths with some well-defined spectral features specific of the interaction with the Lewis site in order to be able to follow their evolution with the different sample treatments. In the literature it has emerged that both CO and pyridine are able to provide evidence on the nature of the Lewis site because the former suffers an hypsochromic shift of the C-O stretching frequency larger than that observed with other sites (Brønsted site, surface silanols, etc.), whereas the latter suffers from a bathochromic shift in the 1445 cm-1 ring frequency.43,44 A spectroscopic study, in which both CO and pyridine are adsorbed on a zeolite material such as H-BEA would be an interesting experiment to check if the two probes will spot different Lewis centers, as was the case for sulfated zirconia.24 One must anticipate that the experimental data will be of delicate interpretation because of the co-presence of some fully tricoordinated Al(III) sites together with a fraction of Al(MS) ones. A number of interesting papers have appeared about the role of Lewis sites as catalysts for some important industrial reactions.27,45-48 However, the target was to establish the reversibility of the Brønsted site from the EFAl particles more than characterize the features of Lewis sites. The only paper devoted to the study the existence of tricoordinated Al in fully dehydrated samples of H-BEA and H-MOR has been published recently by van Bokhoven et al.26 They resort to in situ X-ray absorption near-edge spectroscopy (XANES) at the Al K-edge to reveal the amount of octahedral, tetrahedral, and, eventually, trigonal Al sites. That was achieved on samples that were pretreated at 725 K in flux of dry nitrogen (H-BEA) or in air at 725 K (H-MOR). The Al K-edge XANES spectra were recorded in situ with the sample (either H-MOR or H-BEA) kept at 298, 675, and 975 K in high vacuum (P < 10-5 Torr), and a weak signal due to Al(III) was defenitely found when the sample was treated at 675 and 975 K, whereas disapparence of the specific signal was observed at RT. An intriguing result, not completely clarified with the proposed mechanisms (see Schemes 1-3 of reference 26 for details) was that after increasing the temperature from 675 to 975 K an increase in the amount of Al(III) was detected, which, however, decreases sligthly when the temperature was set back to 298 K. This dynamic behavior of the Al(III) content with the temperature can be explained by assuming the existence of a fraction of the total Al(III) population in a masked Al(MS) state. As explained above in the present work, a molecular probe of enough basicity must be adsorbed in order to demask the Al(MS). An alternative way to demask the Al(MS) is to heat the free sample at sufficiently high temperature: the kinetic energy will then be converted to demask some of the Al(MS) site, which then contributes to the Al K-edge XANES spectra (more at 975 K then at 675 K). By using our ab initio data, we have estimated some 25 kJ/mol as the average cost of demasking the Al(MS) site. This will bring ∼1% at 675 K and ∼5% at 975 K of the total population of masked Al(MS) (the latter contributing to the tetrahedrally coordinated band in the XANES) to become Al(III), contributing to the highenergy feature in the XANES spectra. Interestingly, when the sample is cooled again this fraction of Al(III) sites will become masked (almost tetrahedral coordination) with the disapparence of the high-energy feature in the XANES spectra. Concluding Remarks In the present work, the concept of flexible coordination of a typical Al(III) Lewis acidic site, typical of proton-exchanged zeolites, has been addressed. By adopting as a surface rich in Al(III) ions the (001) surface of edingtonite (EDISLAB), it has been shown that the Al(III) ions may expand their coordination by extra bonding with a nearby framework oxygen (Of). This
Letters is allowed by their exceedingly large Lewis character and the peculiar topolgy of the EDI frramework. The resulting Al(MS) site masks its pristine Lewis acidic character so that techniques able to assign their valence status, like NMR, will not be able to detect them as a Al(III) Lewis site.26 The extra coordination is, however, rather weak so that the hidden Lewis acidity of Al(MS) shows up when adsorption of molecules of moderate basic strength such as NH3 (or CH3CN, pyridine, etc.) takes place, in that they are sufficiently basic to “restore” the full Lewis character hidden in the Al(MS) sites. The resulting energy of interaction, both computed and calorimetrically measured on a Lewis-rich H-BEA specimen, is, however, rather small because of the cost associated with the structural changes needed to convert Al(MS) into Al(III). This effect has been put here as evidence in the case of a crystalline aluminosilicate (i.e., a proton-exchanged zeolite), whereas similar evidence has been reported already for sulfated zirconia24 and vanadia.25 However, we believe that it could be even more evident in the case of amorphous aluminosilicate (i.e., silico-alumina systems), the structural flexibility of which has been demonstrated by us to play a role in lowering the measured heat adsorption of water.21 It is clear that the adopted EDISLAB model has an euristic value in that, its main feature (the Al(MS) site), occurs because of the particular topology of the edingtonite framework as explained thoroughly in the text. However, it supports the idea that the masking process will have important consequences on (i) the measured heat of adsorption; (ii) the assessment of the Al valence state as has been proposed for the interpretation of the Al XANES experiments;26 and (iii) the interpretation of vibration frequency shifts of adsorbed molecules. Together with the evidence that different local strain around Al atoms imparts different Lewis acidity,20-23 the masking effect adds richness to the true nature of Lewis acidic sites and suggests that we interpret experimental data with some extra care. Acknowledgment. The staff of the Theoretical Chemistry Research Group of Torino University is kindly acknowledged for providing the development versions of the CRYSTAL03 code. CINECA supercomputing centre is also acknowledged for allowance of computer time. Financial support from INSTM (PRISMA 2002 “Nanostructured oxidic materials for the adsorption and the catalysis”) is gratefully acknowledged. Fruitful discussion with Prof. Barbara Onida and Prof. Edoardo Garrone (Politecnico of Torino, Italy) is also acknowledged. References and Notes (1) Bourgeat-Lami, E.; Massiani, P.; Rienzo, F. D.; Espiau, P.; Fajula, F.; Courieres, T. D. Appl. Catal. 1991, 72, 139. (2) Woolery, G. L.; Kuehl, G. H.; Timken, H. C.; Chester, A. W.; Vartulli, J. C. Zeolites 1997, 19, 288. (3) Hitz, S.; Prins, R. J. Catal. 1997, 168, 194. (4) Omegna, A.; Bokhoven, J. A. v.; Prins, R. J. Phys. Chem. B 2003, 107, 8854. (5) Trombetta, M.; Busca, G.; Storaro, L.; Lenarda, M.; Casagrande, M.; Zambon, A. Phys. Chem. Chem. Phys. 2000, 2, 3529. (6) Corma, A. Chem. ReV. 1995, 95, 559. (7) Corma, A.; Martinez, A. AdV. Mater. 1995, 7, 137. (8) Farneth, W. E.; Gorte, R. J. Chem. ReV. 1995, 95, 615. (9) Auroux, A. Top. Catal. 2002, 19, 205. (10) Zecchina, A.; Lamberti, C.; Bordiga, S. Catal. Today 1998, 41, 169. (11) Gorte, R. J. Catal. Lett. 1999, 62, 1. (12) Garrone, E.; Rodriguez Delgado, M.; Otero Arean, C. Trends Inorg. Chem. 2001, 7, 119. (13) Huang, M.; Auroux, A.; Kaliaguine, S. Microporous Mesoporous Mater. 1995, 5, 17.
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