Active Sites of M(IV)-incorporated Zeolites (M = Sn, Ti, Ge, Zr)

M(IV)-incorporated zeolites have recently aroused wide interest due to outstanding catalytic effects while their active sites remain largely elusive. Here periodic density functional theory calculations are conducted finding that active sites are determined jointly by identity of M(IV) ions, topology of zeolites, type of framework species and choice of T sites. All M2(IV) active sites in BEA zeolites are penta-coordinated with chemisorption of one water while subsequent water molecules that form only H-bonds promote chemisorption of the first water, especially the second water possessing comparable or even higher adsorption strengths as the first water; Ti(IV) and Ge(IV) active sites at the intersection remain penta-coordinated and Sn(IV) and Zr(IV) active sites prefer to hexa-coordination although potentially expanded to hepta-coordination. Different from other zeolites, Ti(IV) active sites in FER zeolites are hexa-coordinated as Sn(IV) active sites, due to the promoting effect of the first water. Lewis acidic defects expand Ti(IV) active sites to hexa-coordination while inhibit the formation of hepta-coordinated Sn(IV) species. Two forms of Brϕnsted acidic defects exist for Sn(IV) sites instead of only one for Ti(IV) sites, and all M(IV) Brϕnsted acidic defects, regardless of different acidic forms and M(IV) ions, can chemisorb only one water.

being hexa-coordinated as Sn(IV) active sites; (5) Lewis acidic defects in M(IV)-BEA zeolites, which produce significantly beneficial effects for water adsorption. The coordination number of Ti(IV) sites is expanded to six while the hepta-coordinated Sn(IV) species is inhibited; (6) Brφnsted acidic defects, which promote chemisorption for the first water while prevents the second water from chemisorption. A second form of Brφnsted acidic defects with higher stability was detected in Sn-BEA zeolite that shows distinct adsorption properties. Results obtained thus far are beneficial to understand the structural, adsorption and coordination aspects of M(IV)-incorporated zeolites and to decipher the active sites that are critical to adsorption and catalytic processes.

Computational Details
Models. Atomic coordinates of zeolites were downloaded from the International Zeolite Association (IZA) website 63 . The periodic models of M(IV)-BEA, M(IV)-FER and M(IV)-CHA zeolites as used previously 39 were displayed in Fig. 2, wherein M(IV)-FER and M(IV)-CHA consist of two and four unit cells along the c (1 × 1 × 2) and a × b (1 × 2 × 2) lattice vectors, respectively. Different from CHA and FER zeolites where all T sites are indistinguishable (referred to as T1), BEA possesses nine crystallographically distinct T sites, and as recommended elsewhere [28][29][30]33,35,38,39,[53][54][55][56][57][58][59][60][61] , T2 site that is the most energetically favorable and T9 and T6 sites that are situated at the intersection of two channels were investigated.  Lewis and Brφnsted acidic defects in M(IV)-incorporated zeolites were given in Figs 1 and 3. Lewis acidic defects (M L ) were constructed by removing a neighboring Si atom as well as its first-shell three Si atoms 26,34,[48][49][50]55 , while Brφnsted acidic defects (M B ) were created with the formation of the ≡M(OH) 2 Si≡ linkage 31,53,58,64 . Methods. The Perdew, Burke and Ernzrhof (PBE) exchange-correlation functional 65,66 supplemented with the damped C 6 dispersion term 67 (referred to as DFT-D2, implemented in the VASP software) was used. The standard PAW (projected augmented wave) psuedopotentials were used for all elements, while M(IV) ions are exceptions that were handled by the highest electronic PAW pseudopotentials (Sn_d for Sn, Ti_pv for Ti, Zr_sv for Zr and Ge_d for Ge), in that their semi-core s, p or d states should be regarded as valence electrons 33,37,39,55,68 . The energy cutoff was 400.0 eV, and the Brillouin zone sampling was restricted to Γ-point. Structural optimizations finished when the forces on each atom are below 0.05 eV Å −1 . The adsorption energies of the N th water within M(IV)-incorporated zeolites were defined as, where MZeo-nH 2 O stands for M(IV)-incorporated zeolites respectively adsorbed with N-numbered water (n = 0, 1, 2, 3, 4). Noting that the E ad2 calculations were based on the lower-energy MZeo-H 2 O configurations (e.g., Sn9 P b rather than Sn9 P a due to the larger E ad1 value and higher stability, see Table 1).

Results and Discussion
As illustrated in Fig. 4, water can approach some T sites of zeolites via different directions that are represented by "a", "b" and "c". Nomenclature of adsorption configurations includes such information as M(IV) species (M P , M L , M B , see Fig. 1), number of T sites (1∼9) and direction of water adsorption (a, b, c); e.g., M9 P ab (Fig. 5) in BEA zeolite stands for the adsorption configuration where two water molecules approach the perfectly tetrahedral M9(IV) site via "a" and "b" directions (M = Sn, Ti, Zr, Ge). Adsorption configurations where water is assumed to form H-bonds with other water molecules and framework-O atoms (referred to as O F ) are suffixed by hi (i = 1, 2, …), see M2 P h1 in Fig. 5 for instance.       (Table 1). As a synergy of the two effects, the adsorption energies of the second water in M2 P h1 (E ad2 ) are substantial and amount to −58.     Table 1). When two water molecules via "a" and "b" directions approach the M9(IV) sites of M9(IV)-BEA zeolites at the same time, both of them construct direct bonds with Sn9(IV) and Zr9(IV) sites while only one remains chemisorbed for Ti9(IV) and Ge9(IV) sites, see Fig. 5 (Sn9 P ab and Ti9 P ab ). That is, the coordination numbers of Sn9(IV), Zr9(IV) and Ti9(IV), Ge9(IV) active sites are expanded respectively to six and five, in line with the results of dynamic nuclear polarization surface enhanced NMR spectra of Sn-BEA zeolite and (resonant) valence-to-core X-ray emission spectra of TS-1 zeolite 61,62 . The adsorption energies of the second water (E ad2 ) in M9 P ab are calculated to be −19.0, −42.6, −20.5 and −61.0 kJ/mol respectively for M = Ti, Sn, Ge and Zr, consistent with the changing trends of M-O W distances (Table 1). Owing to competition of two chemisorbed water, the E ad2 values in Sn9 P ab and Zr9 P ab reduce to some extent as compared to the E ad1 values, while the E ad2 value of Ti9 P ab due to mainly H-bonding interactions descends substantially and is close to those of all-siliceous zeolite ( Table 1). M(IV)6 sites at the intersection of BEA zeolite are also investigated that substantialize the results of M9(IV) sites: Sn6(IV) and Ti6(IV) sites can chemisorb at most two and one water molecules, respectively, see their local structures in Fig. 5 (Sn6 P ab and Ti6 P ab ) and geometric and energetic data in Table 1. That is, Sn6(IV) active sites adopt the octahedral geometry with adsorption of two water molecules while Ti6(IV) active sites are penta-coordinated with adsorption of one water.
Adsorption with More Water Molecules. Figure 6 depicts the local configurations of Sn2(IV)-BEA zeolite adsorbed with three (Sn2 P h2 ) and four (Sn2 P h3 ) water molecules. In Sn2 P h2 , the second and third water molecules are located at either side of the chemisorbed one and their chemical environments resemble each other. shows considerable reductions albeit the fourth water also constructs H-bonds with the chemisorbed water, due to the obviously weaker H-bonding interactions; e.g., the O W4 H•••O Wa distance equals 1.971 Å and is apparently longer than those of the second and third water molecules. Subsequent adsorption constructs the second, third and higher-order water shells around the chemisorbed one, which will not be discussed here.
H-bonded adsorption configurations of Sn9(IV) site are shown in Fig. 7 (Sn9 P h1 and Sn9 P h2 ) that resemble the condition of Sn2(IV) site. The third water in Sn9 h1 rather than in Sn9 h2 is more stabilized by H-bonds, and the           70 , see Sn9 P abi in Fig. 7 and Table 2. However, the E ad3 value of Sn9 P abi equals −26.6 kJ/mol and is apparently less than that of Sn9 h1 . That is, the coordination number of Sn9(IV) site in BEA zeolite should predominate as six although potentially expanded to seven. Parallel p-DFT calculations are conducted for Zr9(IV)-BEA zeolite and it indicates that its coordination number can also be expanded to seven with the Zr-O w distances of 2.518, 2.518 and 2.454 Å ( Table 2). The E ad3 difference of Zr9 P abi vs. Zr9 P h1 (−36.5 vs. −59.8 kJ/mol) is less than that of Sn9 P abi vs. Sn9 P h1 , suggesting that Zr9(IV)-BEA zeolite has a greater possibility of developing the hepta-coordination mode. Figure 8 and Table 3 indicate that water can approach M(IV) sites of CHA zeolite from three different directions ("a", "b" and "c" as shown in Fig. 4) with Ti-O w distances of 3.699 and 2.347 Å). Chemisorption of three water molecules in Sn(IV)-CHA zeolite seems impossible, see the optimized structure (Sn1 P abc ) in Fig. 8. In consequence, Sn(IV) and Ti(IV) active sites of CHA zeolites are respectively hexa-and penta-coordinated that resemble the condition of BEA zeolites. Figure 9 shows the adsorption configurations of water within M(IV)-FER zeolites. Water can be chemisorbed at the M(IV) sites of FER zeolite from two different directions referred to as "a" and "b", and the Sn-O wa , Ti-O wa , Sn-O wb and Ti-O wb distances in Sn1 P a , Ti1 P a , Sn1 P b and Ti1 P b are respectively 2.398, 2.392, 2.398 and 2.449 Å. Chemisorption of two water molecules is viable in Sn(IV)-FER zeolite as in Sn(IV)-BEA and Sn(IV)-CHA zeolites (Sn-O w : 2.379 and 2.392 Å), implying that chemisorption of the first water facilitates the interaction of the second water as verified subsequently. It is surprising to find that two water molecules can be chemisorbed at the Ti(IV) sites of FER zeolite, and the Ti-O wa and Ti-O wb distances in Ti1 P ab are 2.365 and 2.419 Å that are also shorter than those adsorbed with one water (Table 3). Accordingly, both Sn(IV) and Ti(IV) active sites in FER zeolites adopt the octahedral geometry with adsorption of two water molecules. As indicated in Table 3, the adsorption energies of the second water (E ad2 ) are comparable to those of the first water (E ad1 ) from the same direction (e.g., "b" direction for Ti1 P ab and Ti1 P b ). Figure 4A shows that "a" and "b" directions each have three different O F O F O F M dihedrals, and the accessibility of water to M(IV) sites is estimated by the minimal dihedral 33 , Chemisorption of one water transforms the M(IV) geometry from tetrahedral to bipiramidal 21,23,33 , and the dihedral of direction with water adsorption reduces substantially while that of the other direction ascends considerably. Chemisorption of the second water causes the Ω min and Ψ min values to again get close to each other and reduce pronouncedly due to the formation of octahedral geometry 61 , see the Ω min and Ψ min values for Sn9(IV) sites of BEA zeolite in Table 4. In M(IV)-FER zeolites, however, both Ω min and Ψ min decline due to adsorption of the first water although to an apparently less extent for the other direction with no water (Table 4), and the smaller dihedral of the other direction indicates that adsorption of the first water facilitates the accessibility of the second water towards M(IV) sites, which is distinct from the condition of other zeolites. It thus deciphers why two water molecules can be chemisorbed at Ti(IV) sites of FER rather than other zeolites.

Lewis Acidic Defects.
Defects have been implicated to be critical for catalytic reactions [54][55][56][57][58][59][60] , and is this effect associated, at least in part, with alteration of the active sites? The adsorption configurations of water at M9(IV) Lewis acidic defects (M9 L ) of BEA zeolites are shown in Fig. 10. Water can approach M9(IV) Lewis acidic defects from "a" and "b" directions as in the condition of perfectly tetrahedral sites (M9 P ) while interactions are reinforced (Table 5); e.g., the adsorption energies (E ad1 ) are −85.1 and −89.1 kJ/mol respectively for Sn9 L a and Sn9 L b and surpass those of Sn9 P a and Sn9 P b (Table 1) 49,55 . More significant promotion effects of Lewis acidic defects are detected during adsorption of the second water: The E ad2 value in Sn9 L ab equals −78.6 kJ/mol and is even larger than the E ad1 values of perfectly tetrahedral sites. Two water molecules can be chemisorbed at Ti9(IV) Lewis acidic defects (Ti-O W distances of Ti9 L ab : 2.278 and 2.233 Å), and the E ad2 value (−55.0 kJ/mol) surpasses the E ad1 values of perfectly tetrahedral Ti9(IV) sites (Table 1), which agree with the results of Sn9(IV) Lewis acidic defects. The adsorption configurations of three water molecules at Sn9(IV) Lewis acidic defects (Fig. 11) are close to those of perfectly tetrahedral sites, and the E ad3 values are calculated at −16.6 and −66.0 kJ/mol for Sn9 L abi and Sn9 L h1 , respectively (Table 2). Accordingly, Lewis acidic defects inhibit somewhat the formation of hepta-coordinated Sn9(IV) species (Chemisorption of three water molecules is also tried for Ti9(IV) Lewis acidic defects, while only two water molecules remain bonded and the third water forms H-bonds with other water molecules and O F atoms, see Fig. 11 (Ti9Labi)). Table 6 lists the root-mean-square deviations (RMSD) of local M(IV) sites during water adsorption. For each M(IV) site, RMSDs generally increase with the number of chemisorbed water molecules, implying larger structural perturbations; in addition, at a specific chemisorbed water content, Lewis acidic defects always result in obviously higher RMSDs than corresponding perfectly tetrahedral sites; e.g., the RMSDs are calculated to be 0.20 and 0.24 Å in Sn9 P a and Sn9 P ab while are enlarged to 0.34 and 0.47 Å in Sn9 L a and Sn9 L ab , respectively. Accordingly, the more structural flexibility of Lewis acidic defects facilitates the interaction with water and allows the formation of hexa-coordinated Ti(IV) species in BEA zeolite.
Brφnsted Acidic Defects. The adsorption configurations of NH 3 at defect M9 B of BEA zeolite with the ≡M(OH) 2 Si≡ linkage (Figs 1 and 3) are given in Fig. 12. Defect M9 B in BEA zeolite transfers the proton to NH 3 automatically forming NH 4 + and thus shows Brφnsted acidity, which are in line with the results of M(IV)-incorporated MFI zeolites 32,64 . The bridging hydroxyls of defect M B significantly accelerate the isomerization reaction of glucose to fructose 58 . Figure 13 depicts the adsorption configurations of water at M9(IV)  (Fig. 3)  , one water is chemisorbed while the other is H-bonded. The Si-O B2 bond in Sn9 B a is broken, and such defect referred to as Sn9 B ′ also exists in Sn(IV)-BEA zeolite. In addition, Sn9 B ′ is more stable than Sn9 B and their energy difference equals −22.8 kJ/mol; however, Ti9 B ′ is non-existent and spontaneously transforms to Ti9 B . The O B2 H B2 group in Sn9 B ′ is flexible and allows water chemisorption from either "a" or "b" direction, see Sn9 B ′ a and Sn9 B ′ b in Fig. 13. Chemisorption of water from "b" direction causes structural reconstruction to resemble Sn9 B b . The adsorption energies (E ad1 ) in Sn9 B ′ a and Sn9 B ′ b are respectively −8.5 and −68.5 kJ/mol, and the particularly small value in Sn9 B ′ a is caused by the serious structural distortion in order to accommodate the chemisorbed water. Accordingly, water from "a" direction of Sn9 B ′ should be preferentially the H-bonded adsorption configuration (i.e., Sn9 B a ), and the E ad1 value calculated this way amounts to −78.5 kJ/mol (Table 7). Similar to the condition of Sn9 B , only one water can be chemisorbed at Sn9 B ′, see Fig. 13 (Sn9 B ′ ab and Sn9 B ′ ba where water is chemisorbed from "a" "b" directions, respectively). Accordingly, M(IV) Brφnsted acidic defects, for both acidic forms and different M(IV) ions, result in only the hexa-coordination mode with chemisorption of one water.

Conclusions
Periodic density functional theory (p-DFT) calculations have been used to comprehensively investigate the active sites of M(IV)-incorporated zeolites, considering the identity of M(IV) ions, topology of zeolites, type of framework species and choice of T sites.
With regard to defect-free BEA zeolites, all M(IV) active sites are penta-coordinated with chemisorption of one water when situated at the straight channel while divergences arise when situated at the intersection: Sn(IV) and Zr(IV) active sites predominate as hexa-coordination while Ti(IV) and Ge(IV) active sites remain penta-coordinated; in addition, it is surprising to find that Sn(IV) and Zr(IV) are potentially expanded to hepta-coordination although with relatively small probabilities. For M2(IV) sites, the second and third water molecules form strong H-bonds with chemisorbed water and framework-O atoms and promote the interaction of chemisorbed water. The adsorption energies of the second water, irrelevant of the identity of M(IV) ions, are comparable to, if not larger than, those of chemisorbed water, while those of the third and fourth water molecules, especially the latter, show reduction.
Results of CHA zeolites, where water can approach M(IV) sites from three directions, are similar to those of BEA zeolites: Sn(IV) active sites are hexa-coordinated while Ti(IV) active sites are penta-coordinated. Sn(IV) active sites in FER zeolite adopt the hexa-coordination mode as in the condition of other zeolites, while it is surprising to find that Ti(IV) active sites are also presented as hexa-coordination. Chemisorption of the first water at M(IV) sites of FER zeolites facilitates the interaction with the second water, as verified by the dihedral analyses.
Owing to enhanced structural flexibility, Lewis acidic defects reinforce the adsorption of water and the promoting effects are more obvious during chemisorption of the second water; in addition, Ti(IV) Lewis acidic   The adsorption energy based on Sn9 B a is calculated to be −78.5 kJ/mol. c Data for Sn9 B ′ ba are given in brackets. d The adsorption energies of the second water (E ad2 ) in Sn9 B ′ ab and Sn9 B ′ ba are calculated on basis of Sn9 B ′ a and Sn9 B ′ b , respectively. defects can be expanded to hexa-coordination, while hepta-coordinated Sn9(IV) species is somewhat inhibited. M(IV) Brφnsted acidic defects also facilitate the interaction of the first water while present the second water from chemisorption. A second form of Brφnsted acidic defects that has higher stability exists in Sn(IV)-rather than Ti(IV)-BEA zeolites. Two forms of Sn(IV) Brφnsted acidic defects show divergent adsorption properties and can be inter-converted during water adsorption. Despite that, all M(IV) Brφnsted acidic defects are hexa-coordinated, irrespective of different M(IV) ions or acidic forms, due to limited space available for the second water.