J. Phys. Chem. C 2010, 114, 9923–9930
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What Do Tantalum Framework Sites Look Like in Zeolites? A Combined Theoretical and Experimental Investigation Frederik Tielens,*,†,‡ Tetsuya Shishido,§ and Stanislaw Dzwigaj†,‡ UPMC UniV Paris 06, UMR 7197, Laboratoire de Re´actiVite´ de Surface, Tour 54-55, 2e`me e´tage - Casier 178, 4, Place Jussieu, F-75005 Paris, France, CNRS, UMR 7197, Laboratoire de Re´actiVite´ de Surface, Tour 54-55, 2e`me e´tage - Casier 178, 4, Place Jussieu, F-75005 Paris, France, and Department of Molecular Engineering, Kyoto UniVersity, Kyoto, 615-8510, Japan ReceiVed: March 10, 2010; ReVised Manuscript ReceiVed: April 17, 2010
Ab initio periodic DFT calculations on the structural and energy properties of different model tantalum framework sites in tantalum-substituted sodalite cavities are presented and related to experimental FTIR measurements on Ta-containing SiBEA zeolites. The tantalum framework sites are characterized by their calculated geometrical parameters, vibrational frequencies, and (de)protonation energies. Ta(V) is stabilized in the zeolite framework as a pentacoordinated site possessing one hydroxyl group. The Ta(V)-OH group is not acidic. It is found that hydration of this site is easier than dehydration. This site is found to be only slightly hydrophilic. Dehydration of the site leads to the formation of Lewis acidic and Lewis basic sites. These results are in agreement with experimental data and allow determining the molecular structure of tantalum sites in the zeolite framework. 1. Introduction Zeolite frameworks can be modified by substitution of their tetrahedral sites by transition-metal cations. These modified materials have revealed remarkable physical and chemical properties and, in particular, catalytic and photocatalytic properties, for example, in CO2 and NO2 reduction reactions1-3 and oxidation, epoxidation, and hydroxylation of hydrocarbons.4-7 Different cations belonging to groups III, IV, V, VI, and VII of the periodic table have been introduced in a zeolite framework. Nevertheless, the incorporation of these cations in the framework is not always straightforward, and only a few transition-metal-substituted zeolites have been characterized. Especially group V transition metals have received particular interest.8-14 Theoretically, it has been concluded that the acid-base properties of group V-substituted zeolites increases going down the periodic table, whereas the opposite trend is observed for the redox properties.15,16 Overall, one can say that very few studies exist on tantalum oxide systems. Some studies are available discussing TaxOy clusters or Ta2O5.17-21 To our knowledge no joint theoretical/experimental characterization studies are available on tantalum oxide clusters supported or inserted on/in metal oxides. In our group, zeolitic materials have been studied theoretically,22-27 in line with the present research topic. In this work, framework Si substituted by Ta is characterized experimentally on BEA zeolite and theoretically using periodic DFT. Different framework site models are proposed after a systematic theoretical study. The sites are characterized calculating structural parameters, vibrational frequencies, and (de)protonation energies. The results are related to those obtained * To whom correspondence should be addressed. E-mail: frederik.tielens@ upmc.fr. Tel: +33 1 44276004. Fax: + 33 1 44276033. † UPMC Univ Paris 06, UMR 7197. ‡ CNRS, UMR 7197. § Kyoto University.
experimentally. This study is a logic continuation of our former characterizations of group V zeolite framework sites.15,16 2. Experimental Details 2.1. Material Preparation. TaSiBEA zeolite (with 1.3 wt % Ta) is prepared by the two-step postsynthesis method, similar to the one reported earlier for VSiBEA.28-30 To obtain a sample with 1.3 wt % Ta, first, a siliceous BEA zeolite (Si/Al > 1300) was obtained by treatment of a tetraethyl ammonium BEA zeolite (Si/Al ) 11) in a 13 mol L-1 HNO3 solution (4 h, 353 K, pH ) 1), and then, the resulting SiBEA solid was washed in distilled water and dried in air at 353 K for 24 h. A 2 g portion of dried SiBEA solid was then put into 100 mL of isopropanol solutions containing 3.5 × 10-3 mol L-1 Ta(OC2H5)5 (Alfa Aesar, 99.999%) solution and stirred for 3 h at 353 K. After that, the suspension (pH ) 6.8) was stirred for 1 h in air at 353 K until complete evaporation of isopropanol. The resulting solid, washed three times in distilled water and dried in air at 353 K for 24 h, was calcined at 723 K in flowing air for 3 h. The sample is white and contains 1.3 wt % Ta and is labeled TaSiBEA. 2.2. Methods of Characterization. The chemical analysis of the samples was performed with inductively coupled plasma atom emission spectroscopy at the CNRS Centre of Chemical Analysis (Vernaison, France). Powder X-ray diffractograms (XRDs) of as-prepared samples were recorded at ambient atmosphere on a Siemens D5000 using Cu KR radiation (λ ) 154.05 pm). DR UV-vis spectra were recorded at ambient atmosphere on a Cary 5000 Varian spectrometer equipped with a double integrator with polytetrafluoroethylene as the reference. Transmission FT-IR spectra of self-supported wafers were recorded at room temperature on a PerkinElmer Spectrum One spectrometer with a resolution of 2 cm-1 after calcination at 773 K for 3 h in flowing dry air and then outgassing to 10-3 Pa at 573 K. Dehydrated wafers were contacted with gaseous pyridine via a separate cell containing liquid pyridine. Phys-
10.1021/jp102181m 2010 American Chemical Society Published on Web 05/10/2010
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Figure 1. Interconnection between the studied active sites showing the most pertinent structures.
isorbed pyridine was outgassed (10-3 Pa) for 2 h at 373, 423, 473, 523, and 573 K. 2.3. Computational Details. 2.3.1. Methods. All calculations were performed using ab initio plane-wave pseudopotential calculations implemented in VASP.31,32 The Perdew-BurkeErnzerhof (PBE) functional33-36 has been chosen to perform the periodic DFT calculations. The accuracy of the method has been tested elsewhere.37-40 The valence electrons are treated explicitly, and their interactions with the ionic cores are described by the projector augmented-wave method (PAW),41,42 which allows the use of a low-energy cutoff for the plane-wave basis. The cutoff used in the calculations is set equal to 500 eV. A (3 × 3 × 3) k-point grid is used in the Brillouin-zone sums, and the partial occupancies are set for each wave function using the tetrahedron method with Blo¨ch corrections.41 The positions of all the atoms in the supercell as well as the cell parameters are relaxed, in the potential energy determined by the full quantum mechanical electronic structure until the total energy differences between the loops are less than 10-4 eV. The systems having unpaired electrons were calculated, taking into account their spin state. To calculate the Hessian matrix, finite differences were used. That is, each ion is displaced in the direction of each Cartesian coordinate, and the Hessian matrix is determined from the forces. All atoms are displaced in all three Cartesian directions. The frequency calculations are performed considering only the Gamma point. 2.3.2. Description of the Model. The sodalite structure (SOD) is chosen to mimic a zeolitic environment in our periodic calculations. This choice is done on the basis of the quality/ cost ratio. The sodalite cage, commonly referred as the β cage, contains a regular network of tetrahedral sites (T sites) and resembles a truncated octahedron with each vertex corresponding
to a Si atom. In total, the cluster counts 36 atoms (Si12O24) from which one Si is substituted by TaOxHy (with x ) 0 or 1 and y ) 0, 1, and 2) and is discussed in detail in our former studies on vanadium- and niobium-substituted zeolites.16,43-45 To study the effect of the Ta site environment as a function of its oxidation state and hydration on the Ta/Si isomorphous substitution of the silica sodalite structure, several geometrical configurations are systematically investigated (Figures 1 and 2). We start from a structure in which one silicon atom of the pure SiO2 framework is isomorphously substituted by one tantalum atom (TaSOD structure). In this case, the Ta site has neither hydroxyl nor tantalyl TadO groups and the oxidation state of tantalum is +IV. To this structure, one can add H atoms and OH groups or consider the TadO group. The structures are named using the following notation: B (bridging hydroxyl), T (terminal OH group), Ta (TadO group), and W (addition of physisorbed H2O). The number next to the geometry code indicates the oxidation state of tantalum. Adding to TaSOD a H gives site B3, adding an OH group gives T5, adding a dissociated water molecule gives BT4, the hydrolysis of the Si-O-Ta linkage gives T4, and the physisorption of H2O gives W4. These sites can be further oxidized, reduced, hydrated, or isomerized. The relationships between the sites are shown in Figure 1. In summary, we have studied 16 tantalum structures obtained by adding H, 1/2O, OH, H2O, H2O + OH, and 2H2O. It should be noted that the series presented in this work is only a selection of the different possibilities and only the energetically most stable ones are retained. 3. Results and Discussion 3.1. XRD and FTIR Evidence for Incorporation of Tantalum into the Framework of Dealuminated BEA. The significant increase of the d302 spacing from 3.920 (SiBEA, 2θ
Tantalum Framework Sites in Zeolites
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Figure 2. Environments of the other reactive sites studied in this work.
) 22.68°) to 3.949 (TaSiBEA, 2θ ) 22.50°) upon incorporation of tantalum into SiBEA (results not shown) indicates expansion of the matrix due to the longer Ta-O bond distance (1.90 Å for mononuclear Ta(V) species in BEA zeolite)46 as compared with that of Si-O (typically 1.60-1.65 Å in zeolites).47 The XRD patterns of SiBEA and TaSiBEA samples do not show any evidence of extra framework crystalline compounds or longrange amorphization of the zeolite. After treatment of AlBEA by HNO3 solution, the bands at 3781, 3665, and 3609 cm-1 assigned to the OH stretching modes of AlO-H groups (3781 and 3665 cm-1) and Si-O(H)-Al groups (3609 cm-1)28,30,48,49 are not seen, evidencing the removal of Al atoms from the structure, leading to a fully dealuminated SiBEA (Figure 3). The appearance of narrow bands at 3736 and 3705 cm-1, related to isolated SiO-H groups, and of a broad band at 3516 cm-1 due to H-bonded SiO-H groups in SiBEA reveals the presence of vacant T-atom sites associated with silanol groups, as reported earlier.50 The incorporation of tantalum in SiBEA induces a reduction of intensity of all these bands (Figure 3), suggesting that SiO-H groups present in vacant T sites are consumed in the reaction with the tantalum precursor. For TaSiBEA, only the IR narrow band at 3744 cm-1 might be related to the presence of the Ta(V)O-H group. This will be discussed in section 3.5.1. 3.2. Probing Pentahedral Tantalum(V) Species in TaSiBEA by DR UV-vis (Diffuse Reflectance UV-vis). The spectrum of TaSiBEA exhibits a high-energy ligand-to-metal charge transfer transition centered at 216 nm (Figure 4), very similar to that observed for pentahedral Ta in TaSBA15 material.51 The absence of a band at higher wavelengths (235-280 nm) characteristic of extra framework octahedral Ta(V) and/or TaOx confirms the absence of such species, in
line with earlier work on TaSBA1551 and TaMFI systems.52 Moreover, the absence of (d-d) transitions in the UV-vis spectra of this sample in the range of 600-800 nm (Figure 4) indicates that the Ta(IV) species are not present and suggests that tantalum is most probably in a d° configuration corresponding to Ta(V). 3.3. Acidity of TaSiBEA Zeolite. To check the acidity of TaSiBEA, pyridine was adsorbed on the TaSiBEA sample after calcination at 773 K for 3 h in flowing dry air and outgassed at 573 K (10-3 Pa) for 2 h. The FTIR spectra recorded after desorption of pyridine at different temperatures are given in Figure 5, spectra a-e. The absence of the band at 1545 cm-1 suggests that Brønsted acidic sites are not present in dehydrated TaSiBEA, evidencing that all Al atoms are removed from BEA zeolite. The bands at 1615, 1597, 1492 and 1446-1450 cm-1 (Figure 5, spectrum a) correspond to pyridine interacting with Lewis acidic sites, in line with earlier data.53,54 The intensities of these bands decrease with increasing desorption temperature (Figure 5, spectra b-e). 3.4. Geometry and Energetics of the Ta Framework Sites. The geometrical parameters of the substituted framework sites are obtained after full optimization of the unit cell parameters and atom positions. The spin state of the different models has been respected, that is, doublet for the Ta(IV) and singlet for the Ta(V) structure. The Ta(III) structure B3 was found to be a singlet. The interatomic distances (Table 1) of the optimized framework sites are used to investigate the type of Ta-O bond present. The Ta-O distance is a direct indicator of the oxidation state as well. The Ta geometrical environment in zeolites can be compared with the results of Ta oxide clusters; nevertheless, very few theoretical studies on TanOy clusters are present in the literature.18,55
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Figure 4. DR UV-vis spectrum of TaSiBEA as prepared, recorded at room temperature and ambient atmosphere. Figure 3. FTIR spectra recorded at room temperature of AlBEA, SiBEA, and TaSiBEA calcined at 773 K for 3 h in flowing dry air and outgassed at 573 K (10-3 Pa) for 2 h.
In the studied models, one finds the following bond distances: for the TadO group, a bond distance of 1.747 Å, for the Ta-O bond, 1.90-1.99 Å, in agreement with experiment,46 and for Ta-O dative or coordinative bond, 2.05-2.30 Å (see Table 1). The introduction of a tantalum atom into the SOD framework has a direct influence on the volume of the crystal unit cell. The unit cell parameter a increases upon introduction of one tantalum into the unit cell by 0.6 up to 1.2%, depending on the tantalum framework site (for cell parameters, see Table 1). The isomorphous substitution of Si by Ta(IV) increases the cell parameter 0.6%. As was found for vanadium15 and niobium,16 the addition of bridging hydrogen atoms decreases the unit cell parameter. The presence of TadO groups expands the unit cell more than a Ta-OH group. This trend is similar to the one found in the case of the other group V-substituted zeolites.15,16 In summary, the presence of a bridging hydrogen contracts the unit cell, whereas the presence of a Ta-OH and a TadO group expands the unit cell, however, the first to a lesser extent than the second. Silica frameworks are known to be very flexible due to the flexible Si-O-Si angle.56 Analyzing the Ta-O-Si and the O-Ta-O angles give us information on the strain of the structure. In Table 2, we have tabulated the angles for some model framework sites of different coordinations, going for 4 to 6 (tetrahedral to octahedral). An isomorphous substitution of a Si atom by a Ta atom distorts the tetrahedral configuration with a mean value of 16° for the O-Ta-O and more than 18° for Ta-O-Si angles. The structures showing the least deviation
to the perfect environments and thus having the least amount of stress (tetrahedral, 109.5°; hexahedral, 120° (equatorial) and 180° (axial); and octahedral, 90° (equatorial) and 180° (axial)) are the T5 and WT4, which are pentacoordinated, and the Ta5 and WTa5 model sites, which are tetracoordinated. The WTa5 model corresponds to the hydrated Ta5 (having a tetragonal structure) model with a water molecule 3.72 Å from the Ta atom. These models are also among the most stable structures of their isomeric groups (see Table 3). The total energies can be used to compare the stability between different “isomeric” sites, that is, in particular groups with the same oxidation state and hydration. One can divide the model framework sites in four groups, depending on their oxidation state and hydration, and structure B3, which does not belong to one of the following groups: Group I ) W4, Ta4, T4, BV4, and BTa4; Group II ) Ta5, T5, TaT5, and BTa5; Group III ) TW4 and BTT4; and Group IV ) TaT5, TW5, BTTa5, TT5, and BTT5 (see Figures 1 and 2). The most stable structures of each group are W4 (the hydrated form of TaSOD), T5, TW4 (the hydrated form of T4), and TaW5, as can be seen in Tables 1 and 3. The isomorphous substitution of Si by Ta into the SOD framework is calculated as the reaction energy of the following reaction:
SOD + Taatom f TaSOD + Siatom
(1)
The reaction energy can give, in first approximation, an idea of the stability for the atom substitution in the zeolite framework. This process is endothermic and needs 1.55 eV; this is almost 2 eV lower compared with vanadium15 and 1 eV lower compared with niobium.16 This result predicts, in first ap-
Tantalum Framework Sites in Zeolites
Figure 5. FTIR spectra of (a) TaSiBEA after calcination at 773 K for 3 h in flowing dry air and outgassing at 573 K (10-3 Pa) for 2 h and adsorption of pyridine at room temperature, followed by desorption for 2 h at 373, (b) 423, (c) 473, (d) 523, and (e) 573 K recorded at room temperature.
proximation, that tantalum is energetically much more favored to be inserted in a zeolite framework compared with the two other group V elements, a result that is confirmed by the experiments. It is interesting to note that this result cannot be explained via the ionic radii. Indeed, the group V element with the most similar atomic or ionic radius compared with silicium is vanadium, which is the most difficult element of the group V elements to introduce in the zeolite framework. A possible explanation would be that it is not the atom or ion size that dominates the process but the nature of the chemical bond between the substituted group V atom and an oxygen. The bond between vanadium and oxygen is more ionic than with niobium or tantalum.57,58 To keep the covalent character of the zeolite structure, the insertion of tantalum, which shows the least ionic character of the three elements, is preferred. This conclusion can be related to the reactivity properties, in the sense that vanadium-substituted zeolites show redox properties, whereas niobium and tantalum show more acid-base proprieties. The comparison of the stability between the sites can be in first approximation, considering the following “theoretical” reaction simulating the hydration and oxidation:
SiSOD + Ta + nH2O + mO2 f TaSOD(H2nOn+2m) + Si (2) The associated reaction energies ∆Er of the most pertinent sites, generally written as, TaSOD(H2nOn+2m), are presented in Table
J. Phys. Chem. C, Vol. 114, No. 21, 2010 9927 1. From these energies, it is clear that the most stable oxidation state for tantalum inserted in the zeolite framework is +V. This result will be discussed in more detail together with the calculated properties below. 3.5. Calculated Properties and Comparison with the Experiment. 3.5.1. Vibrational Frequencies. On these fully geometrically optimized structures, the vibrational frequencies are calculated in order to obtain not only spectroscopic properties but also to confirm the stability of the geometry. All structures were checked to be minima in the potential energy surface. The theoretical frequencies (Table 4) are compared with experimental results for Ta-loaded BEA using FTIR in situ measurement. The experimental silanol frequency is found at 3736 and 3705 cm-1. The latter value can be used as a reference in order to estimate the deviation due to the anharmonicity and the description of the OH vibrator in our calculations. Doing so, one can propose an anharmonicity scaling factor of 0.979, which is very close to those used in general.59 In our study on niobium zeolites, we had to use a different scaling factor (0.984) for the Nb-OH groups than for the silanol groups, if we wanted to fit the theoretical results to the experimental ones. The Nb-OH groups are indeed not chemically equivalent to Si-OH groups, and nor are Ta-OH groups. Using the original scaling factor (0.979), it can be seen from Table 4 that the best correspondence between experimental hydroxyl stretching frequencies and the theoretical ones is found for models WT5 and TT5. Nevertheless, the most stable model is T5, with a Ta-OH stretching frequency of 3785 cm-1, 40 cm-1 higher than the band observed in the experimental spectrum. Because this structure is the most stable one and thus the most abundant, one would expect to observe this band in the spectrum. As will be seen below, the Ta sites are more hydrophilic than the vanadium and niobium sites and possible explanations, for this shift, would be that the calcined zeolites got hydrated by exposure to ambient moisture or that the T5 sites are H bonded to neighboring silanols. The more simple explanation is that no TaOH groups are present in the sample, as is suggested in the following paragraph. From pyridine adsorption discussed in section 3.3, dehydration of the Ta zeolite leads to Ta(V)+ Lewis acid sites. These sites are probably similar to those found in the equivalent niobium materials16,60,61 originating from the dehydration of Ta-OH sites:
2Ta-OH site f Ta+ site + Ta-O- site + H2O
(3)
The reaction energy (-3.78 eV) calculated following eq 2 gives an idea of the stability of this double site. This value is close to the energy of the most stable model site T5 and can thus be considered as a valuable candidate for the site having Lewis acid character. Considering the zeolite being dehydrated and knowing that the double Lewis acidsbase pair site is very stable (TaT5 site), it can be possible that no Ta-OH sites are left over after calcinations, which could also explain the experimental FTIR spectrum because the 3744 cm-1 band is particularly close to the silanol vibration (3736 cm-1) and probably corresponds to isolated SiO-H groups. In that case, one might have to revise the results obtained for the Nb zeolites in which a similar FTIR spectrum has been recorded. In summary, taking into account that (i) the energetics (most stable site being T5), (ii) the particular high stability of the double Lewis acid-base pair site, and (iii) the difficulty to assign
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TABLE 1: Reaction Energy (eq 4) and First Neighbor Ta-O Distances of the Different Reactive Sites Considered in This Work (Distances in Å, Energies in eV) distance SOD
B3
T4
W4
∆Er 1.95 –1.34 a (mean) 8.935 8.975 9.008 D(Ta–O) 1.905 1.909 D(Ta–O) 1.970 1.923 D(Ta–O) 1.985 1.931 D(Ta–O) 2.287 1.937
TW4
–1.57 9.001 1.922 1.927 1.927 1.946 2.866 (H2O)
BT4
BTT4
T5
–1.01 –0.76 –0.12 –4.39 9.005 8.980 9.002 9.017 1.904 1.910 1.887 1.897 1.917 1.931 1.976 1.906 1.939 1.986 1.994 1.908 1.972 2.076 1.994 1.944 2.530 (H2O) 2.121 2.143 1.961
D(Ta–O)
TW5
Ta5
WTa5
–4.34 –3.71 9.068 9.015 1.887 1.747 1.899 1.896 1.946 1.900 1.949 1.911 1.959 2.529 (H2O)
BTT5
TT5
BTa5 BTTa5 TaSOD TaT5
–3.85 –3.64 –3.79 –2.72 –2.94 –1.55 –3.78 9.039 9.043 9.026 8.985 8.965 8.991 1.749 1.863 1.876 1.766 1.741 1.922 1.896 1.955 1.923 1.896 1.923 1.911 1.959 1.925 1.912 1.964 1.915 1.965 1.963 2.017 1.981 3.720 (H2O) 1.968 1.981 2.287 2.320 2.261
TABLE 2: O-Ta-O and T-O-Ta Angles of the Most Pertinent Active Site Models (Angles in Degrees) angle
TaSOD
W4
T4
coordination O-Ta-O dev. from theoretical angles (90°, 109.5°, 120°, 180°) Ta-O-Si dev (Si-O-Si) calc (152.5°)43
4 99.8.0-131.3 ((16.1)
4 96.0-133.4 ((18.1)
4 93.0-148.3 ((22.0)
134.0 ((18.5)
130.0-139.7 ((16.0)
128.4-160.3 ((16.0)
TABLE 3: Energy Differences between the Different “Isomer” Sites of the Four Groups Studied (Energies in eV) group
site
I
W4 0.00 T5 0.00 TW4 0.00 TW5 0.00
II III IV
site
site
T4 0.23 Ta5 0.72 BTT4 0.89 TaW5 0.49
site
Ta5
WTa5
T5
TW5
5 110.9-123.4 ((6.0) 169.7 ((10.1) 130.1-143.8 ((10.3)
4 105.3-115.3 ((4.8)
4 107.4-111.8 ((2.2)
142.3-164.4 ((11.0)
127.9-139.4 ((17.8)
5 116.3-125.4 ((4.6) 177.8 ((2.2) 132.5-165.4 ((16.4)
6 79.7-99.9 ((10.1) 176.5 ((3.5) 139.4-158.3 ((7.0)
TABLE 5: Deprotonation and Protonation Energies (∆Edeprot.) Calculated for Different Ta Model Sites
site
∆Edeprot. (eV)
BT4 0.81 BTa5 1.67 TT5 0.55
TW4
B3 (SiO(H)Ta) T4 (TaO-H) BTa4 (SiO(H)Ta) T5 (TaO-H) Silanol (Si-OH) BTT5 0.70
BTTa5 1.40
TABLE 4: OH Vibrational Frequencies in cm-1, Calculated for Different Ta Model Sites site
OH vibrational frequencies
scaled OH vibrational frequencies
B3 (SiO(H)Ta) T4 (SiO-H) T4 (TaO-H) TW4 (SiO-H) TW4 (TaO-H) Nb5 (SiO-H) T5 (TaO-H) TW5(TaO-H) TT5 (TaO-H) TT5 (TaO-H) TT5 (SiO-H) BTT5 (TaO-H) BTT5 (TaO-H) BTT5 (SiO(H)Ta)
3814 3860 3817 3810 3553 3818 3866 3822 3827 3791 3813 3844 3833 3786
3732 3777 3735 3728 3477 3736 3783 3740 3745 3710 3731 3761 3751 3705
T5 (Ta-O-H2 ) T5 (Ta-O(H+)-Si) Ta5 (TadO-H+) Ta5 (Ta-O(H+)-Si)
(4)
The larger the ∆Edeprot., the weaker the acidity of the considered hydroxyl group.
4.83 4.35 4.68 3.59
Table 5 shows these energies for different Ta framework sites having a OH group. Analyzing the acidity, one can observe that the Ta(V and IV)-OH groups are slightly more acidic than the silanol groups, but less acidic than V-OH and Nb-OH groups.16,43 The most acidic hydroxyl group is the bridging hydroxyl group Ta-O(H)-Si found in the B3 site and in the BTa4 site, which are also expected to be the least stable structures. Looking at the most stable sites of each oxidation state, one can say that the acidity of the Ta sites decreases with increasing oxidation state; that is, the tantalum structures with oxidation state +V are the least acidic structures. Similarly, one can calculate the protonation energy, that is, the energy needed to add a H+ to the zeolite structure. The protonation energy was calculated as the following:
∆Eprot. ) E(SOD) - E(SOD - H+)
the vibrational frequencies, we suggest that, in the calcined TaSiBEA zeolite, no, or very few, Ta-OH groups are present. 3.5.2. Acid-Base Properties. To characterize the Brønsted acidity of the different sites, the deprotonation energy has been calculated as a difference between the energy of the conjugated base and the acid.43,62 Indeed, this property is not directly measurable experimentally, but the data can easily be obtained from QM calculations in which the deprotonation energy is considered as the energy to remove the acidic proton from the zeolite structure and is calculated as a positive value by
∆Edeprot. ) E(SOD-) - E(SOD - H)
4.71 4.85 5.27 5.40 5.95-6.00
∆Eprot. (eV) +
(5)
This property was calculated for the Ta5 structure. On this structure, which contains a TadO group, one can add a H+ to the TadO group to form a TadO-H+ group or protonate the bridging oxygen (Ta-O-Si), forming Ta-O(H+)-Si. For the T5 site, one can also protonate two sites: the Ta-OH, forming Ta-OH2+, and the bridging oxygen like in the Ta5 case. The results for the protonation energies, tabulated in Table 5, predict that the most basic site is the bridging oxygen of site Ta5. On comparison now of the protonation and deprotonation energies, it seems energetically more favorable to protonate than deprotonate a Ta site in a zeolite, which indicates the Brønsted basic character of these materials. From our calculations, we predict that the Ta zeolites have a more Brønsted basic than acidic character, similar to Nb zeolites, which is in line with experimental data. Indeed, after pyridine adsorption, the band of pyridinium ions at 1545 cm-1 (Figure 5), corresponding to pyridine chemisorption on Brønsted acidic sites, is not observed. As has been discussed in section 3.5.1,
Tantalum Framework Sites in Zeolites the dehydration of site T5 can lead to the formation of two different sites with equivalent concentrations (double Lewis acidsbase pair site). The presence of this site explains the Lewis acidity of the zeolites observed experimentally (Figure 5, the bands at 1615, 1597, 1492, and 1446-1450 cm-1). 3.5.3. Effect of Hydration. Considering now the hydrated Ta(IV) and Ta(V) sites, we find that the TaSOD, Ta5, T4, and T5 sites (the most stable sites from their group) are in a hydrophilic environment. Indeed, the lowest total energy of these Ta sites is found for the W4, TW4, TW5, and TaW5 sites. In the W sites, the water molecules are found to be almost in the center of the cage and not very close to the Ta site. The total energies of these four sites are lower than those of the other corresponding “isomer” sites, independent of the different oxidation states and hydration combinations. The adsorption of water is a slightly exothermic process for the sites TaSOD, T4, Ta5, and T5, with -0.02, -0.33, -0.18, and -0.05 eV, respectively. Hence, the Ta zeolite is more hydrophilic than the V- and Nb-substituted zeolites. Water molecules interact weakly with the Ta site in our model cage. This can be seen from the geometries having a physisorbed water molecule in their sodalite cage. The distance between the water molecule and the tantalum atom site in the model is about 2.7 Å, which is in agreement with a slightly hydrophilic character. 4. Conclusions In this work, different possible molecular models for active sites in tantalum-substituted silicate zeolites are investigated. It was found that the isomorphous substitution of tantalum into the zeolite structure is exothermic. The most favorable Ta(V) structure is a pentahedral Ta(V) having a Ta(V)O-H group and Ta linked by four Ta-OSi bonds to the zeolitic walls (site T5), which is in line with the appearance of the UV-vis band at 216 nm for TaSiBEA assigned to an oxygen pentahedral Ta(V) ligand-to-metal charge transfer transition. Theoretical calculation shows that TaO-H groups could vibrate in the range of 3740-3783 cm-1. In tantalum-containing zeolites, Brønsted acidic sites are not present, as shown by pyridine adsorption. The deprotonation and protonation energies of the different sites agree with this finding. Tantalum-containing zeolites have more Brønsted basic than acidic character. Nevertheless, in the dehydrated form, Lewis acidic sites are present. Both Lewis acidic and basic sites appear simultaneously in the Ta-containing zeolites (TaT5 site). From the vibrational frequency analysis, we expect that calcined Ta zeolites do not contain Ta-OH groups. These groups are transformed in double Lewis acidsbase pair sites. Finally, we predict that the Ta-containing zeolites are more hydrophilic than the other group V-substituted zeolites. Acknowledgment. This work was performed using HPC resources from GENCI-[CCRT/CINES/IDRIS] (Grant No. 2009[x2009082022]) and the CCRE of Universite´ Pierre et Marie Curie. References and Notes (1) Anpo, M.; Che, M. AdV. Catal. 1999, 44, 119. (2) Dzwigaj, S. Curr. Opin. Solid State Mater. Sci. 2003, 7, 461. (3) Dzwigaj, S.; Matsuoka, M.; Anpo, M.; Che, M. Res. Chem. Intermed. 2003, 29, 665. (4) Corma, A. Chem. ReV. 1997, 97, 2373. (5) Bellussi, G.; Rigutto, M. S. Stud. Surf. Sci. Catal. 1991, 85, 177. (6) Notari, B. AdV. Catal. 1996, 41, 253. (7) Hoelderich, W. F.; van Bekkum, H. Stud. Surf. Sci. Catal. 1991, 58, 631.
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