Titanosilicate catalysts are highly active for the selective liquid-phase oxidation of various organic compounds using an aqueous solution of H2O2 [1, 2]. The well-known TS-1 catalyst is composed of 10-membered ring (10-MR) micropores and is only suitable for the oxidation of substrates and oxidants with small molecular size [3, 4]. Ti-MWW zeolite possesses a unique crystalline structure and an attractive pore system consisting of two independent 10-MR channels, 12-MR supercages, and side cups [5, 6]. Unlike other zeolites, Ti-MWW exhibits excellent catalytic activity for the selective oxidation of both bulky and small organic molecules [5, 6, 7, 8, 9]. The ultraviolet-visible (UV-vis) and Fourier transform infrared (FTIR) spectra of Ti-MWW samples show characteristic bands at 220 nm and 960 cm−1, respectively [5], which are assigned to the tetrahedral Ti species isolated in the zeolite framework [10, 11, 12, 13]. In the presence of H2O2 and H2O, the titanium hydroperoxo complex forms, which is the actual oxidation active center [14, 15, 16, 17, 18]. The coordination of Ti(IV) with H2O and reagent is an essential process for catalysis. The coordination of H2O and NH3 with Ti(IV) in TS-1 has been investigated by the extended X-ray absorption fine structure (EXAFS) technique [19, 20], which revealed that adsorption of H2O or NH3 causes the T-O distances to lengthen by 0.02 or 0.05 Å, respectively, and changes the symmetry of the Ti center from tetrahedral to octahedral. Damin et al. [21, 22] studied the reactivity of TS-1 towards H2O and NH3, and investigated the geometry of the complexes by the ONIOM method. They found that H2O or NH3 attaches to tetrahedral Ti(OSi)4 at the Ti site and the structure finally reached a bipyramidal geometry. Ricchiardi et al. [23] studied the adsorption of TS-1 and Ti-chabazite with water by a hybrid periodic/embedded molecular mechanical/quantum mechanical cluster approach. They found that one or two H2O molecules form stable complexes with Ti sites, the oxygen coordination increases from four to five or six, and the binding energies strongly depend on the location of the Ti atom. Bordiga et al. [24] experimentally and theoretically investigated the effect of H2O and NH3 adsorption on the vibrational, electronic, and energetic features of Ti(IV) centers in TS-1. They calculated the vibrational frequencies of the tetrapodal Ti(IV) and confirmed that the Ti-specific band at 960 cm−1 undergoes a blue shift to 970 cm−1 upon contact with H2O. Zhanpeisov et al. [25] investigated the adsorption features of both “defective” and “nondefective” Ti sites in TS-1 using DFT and 5T (tetrahedron) cluster models. They calculated the energies upon water, methanol, and NH3 adsorption through either a coordination mechanism or hydrogen-bonding (H-bonding) process, indicating that interaction of ammonia with the Ti species might occur either via H-bonding or coordination mechanisms. Recently, Gallo et al. [26] reported a non-resonant valence-to-core X-ray emission spectroscopic (vtc-XES) investigation and DFT calculations of TS-1 upon molecular adsorption, and confirmed that only one H2O or NH3 molecule adsorbs to the Ti center to form a five- coordination structure. Although a number of experimental and theoretical studies have been reported on coordination properties of Ti(IV), most of them are only relevant to TS-1 zeolites.
Ti-MWW has unique channel systems with the Ti active centers distributed in both 12-MR supercage and 10-MR intralayer channels. Experimentally, it is difficult to discriminate the coordination features of Ti(IV) at different T site locations. Yang et al. [27] reported a DFT study of the adsorption of NH3 to Ti(IV) species at the T3 site, and obtained an adsorption energy of 34 kJ/mol, which is comparable with that for TS-1 zeolites [24]. In our previous work [28], DFT calculations of 35T cluster models revealed that Ti(IV) prefers to be located at the T1 and T3 sites in Ti-MWW. The calculated vibrational frequencies include the 960 cm−1 band for the tetrapodal Ti(OSi)4 species. It is believed that the Ti(IV) active centers in both the 10-MR channel and 12-MR supercage in the Ti-MWW catalyst show different catalytic selectivity. Thus, it is important to investigate the interaction of H2O and NH3 molecules with Ti(IV) at different framework positions.
In this work, we investigate the coordination of H2O and NH3 to tetrapodal and tripodal Ti species located at the T1 and T3 sites in Ti-MWW zeolite using DFT calculations. In the first part of this work, we determine the optimized structures of the coordination complexes of Ti(OSi)4 and Ti(OSi)3OH species located at the T1 and T3 sites, respectively. Their coordination and H-bonded complexes with H2O and NH3 are analyzed. In the second part, we compare the binding energies of H2O and NH3 to different Ti species. In the final part, we calculate the IR spectra of the coordination complexes, and provide comprehensive and precise insight into the vibrational features of different Ti species.
There are eight crystallographically distinct tetrahedral sites in the unit cell of the MWW zeolite lattice [29, 30]. The Ti(IV) atoms prefer to be located at the T1 and T3 sites [28]. The cluster models at the T1 and T3 sites (T1@35T and T3@36T, respectively) were truncated from the superlattice of MWW (Fig. 1). The tetrapodal Ti(OSi)4 and tripodal Ti(OSi)3OH species were constructed and optimized, and the optimized structures were used as the starting models of the bare clusters for the coordination of H2O and NH3 molecules. During the calculations, the Ti atom, the four surrounding layers of framework atoms, and the ligand molecule were relaxed, while the Cartesian coordinates of the rest of the framework atoms were fixed [28].
In this work, we used the B3LYP hybrid functional [31, 32] and polarized 6-31G(d,p) basis set, which have been widely used in the studies of Ti-containing zeolites [21, 22, 23, 24, 25, 26, 27, 28]. For all of the geometry optimizations, the energy minima were verified by frequency calculations, and no imaginary vibrational frequencies existed for the optimized parts, except for contamination of the imaginary frequencies because of the constrained atoms. For all of the coordination complexes considered in this study, the binding energies were calculated as the total energy difference between the coordination complex and the sum of the isolated initial cluster model and ligand molecule(s):
Eb = EC − (EL + EZ)
where the subscripts C, L, and Z stand for coordination complex, ligand, and zeolite cluster model, respectively. Note that the energies of C, L, and Z are those estimated at their respective equilibrium geometries. The binding energies thus obtained were then corrected for basis set superposition error (BSSE) using the full Boys-Bernardi counterpoise correction scheme [33]. Negative values of the binding energy mean a favorable interaction. To compare with the experimental IR spectra, the calculated vibrational frequencies were scaled by a factor of 0.961 [34].
To investigate the electronic properties of the adsorption systems, we performed natural bond orbital (NBO) calculation and analysis [35, 36, 37, 38]. The second-order perturbation theory analysis can provide an estimate of the donor-acceptor (bond-antibond) interactions in the NBO basis. This analysis is carried out by examining all of the possible interactions between “filled” (donor) Lewis-type NBOs and “empty” (acceptor) non-Lewis NBOs, and estimating their energetic importance by second-order perturbation theory. The larger the second-order perturbation stabilization energy E(2), the stronger the donor-acceptor interaction. All of the calculations were performed with the Gaussian 09 program and included NBO version 3.1 [39].
In Ti-MWW, the Ti(OSi)4 and Ti(OSi)3OH species have tetrahedral symmetry. A ligand molecule will approach the tetrahedral Ti(IV) center along one of the four Ti-O bond axes from the channel or the cavity direction. In view of the spatial environment, H2O and NH3 molecules attack the Ti site along the Ti1-O4 and Ti3-O12 bond axes of the Ti(OSi)4 species at the T1 and T3 site, respectively. The optimized adducts are shown in Fig. 2, and the geometric parameters and binding energies are summarized in Table 1. For clarity, the local structures of the Ti(OSi)4 unit are shown in Fig.3.
Adsorption of H2O and NH3 induces the average Ti-O distance to increase by 0.015-0.028 Å, which qualitatively agrees with the EXAFS and theoretical results of H2O and NH3 adsorption to TS-1 zeolite [18, 19, 24]. The second shell of the Ti(OSi)4 unit is also affected by the ligand, as indicated by the <O-Si>, <Ti-O-Si>, and <Ti-Si> columns in Table 1, which represent the average bond lengths and bond angles. The <O-Si> bond lengths decrease, and the <Ti-O-Si> bond angles are slightly distorted (<2°), while the <Ti-Si> distances are essentially constant. The distance between the ligand molecules and the Ti atom (Ti-L, L denotes the O and N atoms in H2O and NH3, respectively) is between 2.35 and 2.42 Å, reflecting the strong interaction between the ligand and Ti. The perturbation effect of NH3 is larger than that of H2O because of its stronger basicity. In addition, the perturbation effect is more prominent at the T1 site. In all cases, ligand adsorption causes a distortion of TiO4 from tetrahedral to distorted triangular bipyramidal symmetry, as indicated by the change of the O-O-Ti-O dihedral angles and L-Ti-O bond angles, which are about 150°-160° and 165°-174° (Table 1), respectively, showing the trend toward the ideal value of 180°. Apparently, NH3 adsorption is more effective to modify the tetrahedral symmetry.
The Ti(OSi)3OH species has distorted tetrahedral symmetry. It can interact with H2O and NH3 either via coordination to the central Ti or via H-bonding with the Ti-OH group. The optimized coordination complexes and the local structures of [Ti(OSi)3OH]-H2O are shown in Figs. 4 and 5, respectively. Table 2 summarizes the selected geometric parameters and binding energies of all of the adducts.
At the T1 site, H2O and NH3 can attack the Ti site either along the Ti1-O2 or Ti1-O3 bond axes. Compared with the L···Ti1-O2 adduct, the L···Ti1-O3 adduct gives rise to longer Ti-O bond lengths and more obvious distortion of the tetrahedral symmetry of TiO4, as characterized by the larger O-O-Ti-O dihedral angles of 167° (vs. 145° for L···Ti1-O2). The Ti-L distances are also relatively short, indicating that H2O and NH3 prefer to approach along Ti1-O3. The structures of the five-coordinated complexes are more analogous to a perfect triangular bipyramid, with L-Ti-O bond angles of 173°-179° and O-Ti-O-O dihedral angles of 167°-168°. This further confirms that the tripodal Ti(OSi)3OH species favors the change of the coordination number of Ti(IV) from four to five.
H2O and NH3 can also form H-bonds with the Ti-OH group. From Table 2, the Ti-O distances in the H-bonded complexes are less influenced by adsorption than in the coordination complexes. The L-H distances are 1.671-1.760 Å for O-H and 1.609-1.650 Å for N-H, and are longer on Ti1-OH than on Ti3-OH. The O-H distances obtained in this work are considerably shorter than the value (1.817 Å) in Ref. [25], where 5T(oh) models were used, and the terminal Si-OH groups formed H-bonds with H atoms of H2O, which restrains the oxygen of H2O to interact with the Ti-OH group. Moreover, the H-bonded complex shows no perturbation of the tetrahedral symmetry of TiO4.
The BSSE-corrected binding energies for all of the coordinated and H-bonded complexes are listed in Tables 1 and 2, respectively. For the Ti(OSi)4 species, the binding energies of NH3 are significantly higher (more than 100%) than those of H2O. This is not unexpected because of the higher basicity of NH3. The Ti(OSi)4 species show slightly higher binding energies for H2O and NH3 at the T1 site than at the T3 site. For the Ti(OSi)3OH species, the Ti-OH group noticeably enhances the Lewis acidity of Ti(IV), resulting in higher binding energies. The H-bonding strength is weaker than the coordination interaction, suggesting that H2O and NH3 molecules prefer to adsorb to the central Ti atom to form coordination complexes. The energetics are generally consistent with the geometric features that the shorter the L-Ti distances, the higher the binding energies. It is noteworthy that the binding energies of the tripodal Ti(IV) species are 2-3 times higher than the tetrapodal Ti(IV) species, indicating that the Ti(OSi)3OH species favors the formation of a coordination complex with the ligand. This is in agreement with the hypothesis for the change of the coordination number of Ti(IV). It has been assumed that the coordination expansion takes place with partial hydrolysis of the Ti-O-Si bridges and that hydrolysis increases the accessibility of the Ti center for additional ligands. This assumption is supported by IR and 17O NMR spectroscopic observations [40].
It is commonly believed that the Lewis acidity of Ti-zeolite can be defined considering the lowest unoccupied molecular orbital (LUMO) energies of the clusters. Based on the molecular orbital analysis, it was found that the LUMO and LUMO +1 in Ti-zeolite clusters have degenerate energy levels and belong to the unoccupied d orbitals of Ti, which are used to coordinate with the framework oxygens through electron density donation of p(O)→d*(Ti). The energy levels of the LUMOs in the Ti-clusters have no definite relationship with the Lewis acidity. To investigate the Lewis acid-base interaction, NBO analyses were carried out. It was found that the bonding between the Ti center and the H2O molecule comes from electron density donation of sp2(Ow)→4p*(Ti) (Fig. 6), indicating that the Lewis acidity of the Ti site is related to the LUMO + 2 and LUMO + 3 but not to the LUMO of the Ti-clusters. Similar electronic properties were found in a theoretical study of TS-1 zeolite [41]. The E(2) energies for H2O adsorption decrease in the order Ti1OH > Ti3OH > Ti1 > Ti3 species, which is consistent with the adsorption energies of H2O.
The calculated vibrational frequencies of Ti(IV) species upon H2O adsorption are summarized in Table 3. For the bare tetrapodal Ti species, there are three vibrational modes related to tetrahedral Ti(OSi)4, among these the strongest ones are at 963 cm−1 (at the Ti1 site) and 958 cm−1 (at the Ti3 site), which are attributed to a combination of the antisymmetric stretching vibrations of four Ti-O-Si [19, 28]. Coordination of the Ti(OSi)4 species with H2O induces the characteristic Ti band to undergo a slight hypsochromic shift from 963 to 971 cm−1 (at the Ti1 site) and from 958 to 960 cm−1 (at the Ti3 site). The O-H vibrational frequencies of the adsorbed H2O show a bathochromic shift of 20-50 cm−1 to the 3600-3730 cm−1 region, indicating that the adsorption of H2O to the Ti(OSi)4 species is quite weak, which is in agreement with the low binding energies of H2O.
For the coordination complexes of Ti(OSi)3OH species with H2O, the Ti-specific bands for Ti1OH (990 cm−1) and Ti3OH species (970 cm−1) both shift to 980 cm−1. For the H-bonded complexes, the Ti-specific bands blue shift by about 5-9 cm−1, and the TiO-H stretching vibrational frequencies show a bathochromic shift of about 525-570 cm−1. Furthermore, when H2O adsorbs to the Ti center, the adjacent Si-OH forms a H-bond with one of the bridging oxygen atoms, resulting in the SiO-H stretching vibration showing a bathochromic shift of about 110-160 cm−1. Fig. 7 shows the calculated vibrational spectra of Ti(OSi)4 and Ti(OSi)3OH species (at the T1 site) before and after coordination with H2O.
Many experimental studies have reported the IR spectra of Ti-silicate zeolites upon H2O adsorption. Bordiga et al. [24] reported that at low pH2O the formation of the Ti(IV)-H2O adduct is not able to cause a significant perturbation of the 960 cm−1 band. The Ti-specific band only undergoes a hypsochromic shift at higher pH2O when more than one H2O molecule is adsorbed per Ti site (shifting from 959 to 973 cm−1). If H2O is dosed directly from the liquid phase, the 960 cm−1 band undergoes a further shift to 990 cm−1. They also performed DFT calculations and confirmed that adsorption of one H2O molecule only slightly affects the vibrational spectrum of the Ti(OSi)4 species. Nevertheless, a reasonable interpretation of the 973 and 990 cm−1 bands is still absent. Although Ti-MWW is structurally different from TS-1, isolated Ti(IV) is also characterized by the 960 cm−1 band. Therefore, we might compare the calculated results with the experimental data for TS-1. In Ti-MWW, the Ti species at the T1 and T3 sites showed different vibrational features. For the Ti3 site, when the Ti(OSi)4 species adsorbs one H2O molecule, hydrolyzes, and then adsorbs another H2O, the Ti-specific band changes from 958 to 960, then to 970, and finally to 981 cm−1. Whereas, for the Ti1 site, the Ti-specific band changes from 963 to 971, then to 990, and finally to 982 cm−1, corresponding to the Ti(OSi)4 species, [Ti(OSi)4]-H2O adduct, Ti(OSi)3OH species, and [Ti(OSi)3OH]-H2O adduct, respectively. Our results indicate that if the Ti(OSi)4 species adsorbs one H2O molecule, the Ti-specific frequency shows minor changes (2-8 cm−1). When the tetrapodal Ti(OSi)4 species hydrolyzes to the tripodal Ti(OSi)3OH species, the Ti-specific frequency shows a clear shift (12-27 cm−1).
In the hydroxyl stretching vibration region, the ν(O-H) stretching modes of gaseous H2O, the internal silanol groups, and the Ti-OH groups appear in the 3600-3760 cm−1 region. Upon H2O adsorption, the bands shift to the 3150-3460 cm−1 region, where the H-bonded Ti-OH groups in the H2O-TiOH adduct show a vibration at around 3150 cm−1, and the H-bonded silanol groups as well as H2O molecules in the H2O-TiOH adduct show a vibration at around 3460 cm−1. All of the results are comparable with the experimental IR spectra of TS-1 [24].
In the presence of a protonic acid, NH3 can easily form NH4+, which shows a characteristic IR absorption band of the N-H bending vibration at 1450 cm−1. With a Lewis acid ligand, it can also form the L···NH3 adduct, which shows an IR absorption band near 1630 cm−1. Therefore, it is possible to distinguish between protonic and Lewis acids: upon NH3 adsorption, the absorption bands at 1450 and 1630 cm−1 are characterized as protonic and Lewis acids, respectively.
The calculated vibrational frequencies of the Ti species upon NH3 adsorption are summarized in Table 4. Coordination of NH3 to the Ti(OSi)4 species induces the Ti-specific band to undergo a slight hypochromatic shift from 963 to 972 cm−1 (at the Ti1 site) and from 958 to 964 cm−1 (at the Ti3 site). These shifts can be attributed to framework distortion from tetrahedral to triangular bipyramidal symmetry, which leads to lengthening of the Ti-O distances and shortening of the O-Si distances. Regarding the vibrational features of the adsorbed NH3 molecule, the N-H asymmetric and symmetric stretching vibrational frequencies vary by ±30 cm−1, and the N-H bending vibrational frequency appears near 1630 cm−1, indicating that the Ti(OSi)4 species has Lewis acidity.
For the coordination complexes of the Ti(OSi)3OH species with NH3, the Ti-specific band shifts by <15 cm−1 for all of the models, with the exception of the Ti1OH-NH3 adduct where a H-bond forms between NH3 and the bridging oxygen in Ti-O-Si, resulting in a blue shift of 29 cm−1 for ν(Ti-O-Si). The most remarkable variations are the hydroxyl stretching vibrational frequencies of the Ti-OH group and the adjacent Si-OH group. When the Ti-OH group interacts with NH3 through H-bonding, the TiO-H stretching vibration shows a bathochromic shift of >1000 cm−1, which is stronger than the change induced by H-bonded H2O. Furthermore, with adsorption of NH3 to the Ti center, the adjacent Si-OH forms a H-bond with one of the bridging oxygen atoms, resulting in the SiO-H stretching vibration undergoing a bathochromic shift of about 100-200 cm−1.
In this work, we report theoretical calculations of the geometric and vibrational features of tetrapodal and tripodal Ti(IV) species in Ti-MWW with coordination of H2O and NH3. The DFT method was used with large cluster models, in which four shells of framework atoms surrounding the central Ti were relaxed. The computational results confirm that NH3 interacts with the central Ti(IV) atom to a greater extent than H2O. Adsorption of one H2O or NH3 molecule to the tetrahedral Ti(IV) center induces formation of a five-coordinated complex with triangular bipyramidal symmetry. The binding energy to Ti(OSi)3OH species is larger than to Ti(OSi)4 species, and the Ti(IV) species at the T1 site show slightly stronger Lewis acidity than at the T3 site. The tripodal Ti(OSi)3OH species can also form H-bonded complexes with H2O and NH3 through Ti-OH. The calculated vibrational frequencies of the adsorption complexes are divided into two regions: the hydroxyl group vibrational region and the Ti-specific region. The ν(O-H) and ν(N-H) vibrational modes show slight blue shifts, reflecting the Lewis acidity of the Ti species. The H-bonded Ti-OH group shows an obvious red shift of 500-1100 cm−1 and appears in the 2700-3200 cm−1 region. The Ti-specific band of the [Ti(OSi)4]-H2O adduct is at 960-970 cm−1, and that of the [Ti(OSi)3OH]-H2O adduct is at 980 cm−1. The calculated results are comparable with the experimental IR spectra of TS-1, indicating that Ti-MWW zeolites have similar coordination properties to TS-1 zeolites.
2015, Vol. 36 Issue (10): 1733-1741
PDF (944 KB)
2015, Vol. 36 Issue (10): 1733-1741
PDF (944 KB)