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

Abstract

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.

Introduction

In the past few decades, M(IV)-incorporated zeolites have gained enough attention^1,2,3. The discovery of Titanium silicate-1 (TS-1) in 1983⁴ has been regarded as a milestone in heterogeneous catalysis⁵ and other Ti(IV)-containing zeolites such as Ti-MWW, SSZ-24 and Ti-UTL were subsequently developed that also exhibit satisfying catalytic effects for a wide spectrum of substrates^6,7,8,9,10. Corma, Přech and their collaborators^11,12 reported that Sn-BEA zeolite is an efficient catalyst for the Baeyer-Villiger oxidation reactions. As a matter of fact, Sn- and Zr-BEA zeolites are also known to exhibit superior catalytic performances for the transformation of cellulosic biomass to downstream chemicals and fuels^3,13,14,15. Albeit less used for catalytic applications, Ge(IV) ions introduced into the framework of zeolites have the directing effect that results in the formation of ITQ-24, ICP-2 and other new structures, and the ADOR (assembly-disassembly-organization-reassembly) mechanism was posed for the synthesis processes^16,17.

Isolated and well-defined M(IV) sites have been acknowledged as the active sites of M(IV)-incorporated zeolites^{1,2,3,5,13,14,15,18,19,20}. As testified by density functional and two-layer ONIOM calculations, M(IV) ions exhibit outstanding adsorption performances for a variety of probe molecules such as H₂O, NH₃ and amino acids^{21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40}. The Ti-peroxo species in TS-1 zeolite produced from the interaction of Ti(IV) sites and H₂O₂ are the active sites for alkene epoxidation^{41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57}, and Wells et al.⁴⁸ demonstrated that Ti(IV) sites adjacent to Si vacancies [(OSiO₃)₃TiOH] are more reactive; that is, the epoxidation processes are significantly accelerated by defects. The two Ti(IV) species [i.e., Ti(OSiO₃)₄ and (OSiO₃)₃TiOH] were identified in TS-1 zeolite^49,50 and Sn(IV) analogues were proposed for Sn-BEA zeolite^51,52,53. Defects in Sn-BEA zeolite were reported to play a significant promoting effect during the transformation of cellulosic biomass^{54,55,56,57,58,59,60}.

Active sites are one of the central topics for adsorption and catalysis, while a number of key issues remain enigmatic for M(IV)-incorporated zeolites; e.g., we are not clear which causes the differences of active sites in Sn-BEA and TS-1 zeolites, topology of zeolites or identity of M(IV) ions. The active sites of Sn-BEA zeolite were assigned to be octahedral Sn(IV) species composed by tetrahedral Sn(IV) sites and two water molecules⁶¹, while the Ti(IV) active sites in TS-1 zeolite prefer to being five-coordinated upon water adsorption⁶². In this work, periodic density functional theory (p-DFT) calculations were conducted to have a comprehensive understanding of M(IV) active sites and their coordination numbers in M(IV)-incorporated zeolites, considering the effects such as topology of zeolites (BEA, FER, CHA), identity of M(IV) ions (M = Sn, Ti, Zr, Ge), type of framework species (M _P , M _L , M _B , see Fig. 1) and choice of crystallographically distinct T sites: (1) M2(IV) active sites in BEA zeolites containing the various M(IV) ions (M = Sn, Ti, Zr, Ge). The second water that forms only H-bonds promotes chemisorption of the first water and has comparable or even larger adsorption energies; (2) M9(IV) and M6(IV) active sites in BEA zeolites, where Sn(IV) and Zr(IV) active sites are hexa-coordinated while Ti(IV) and Ge(IV) active sites remain penta-coordinated; (3) M(IV)-BEA zeolites with adsorption of three and four water molecules. The third water at M2(IV) sites continues to promote chemisorption of the first water while subsequent adsorption may play an adverse effect. Sn(IV) and Zr(IV) active sites are potentially expanded to hepta-coordination; (4) M(IV)-CHA and M(IV)-FER zeolites. Different from other zeolites, Ti(IV) active sites in FER zeolites prefer to 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.

Figure 1

Different framework M(IV) species within zeolites that are referred to as (A) perfectly tetrahedral M(IV) sites (M _P ), (B) defects with Lewis acidity (M _L ) and (C) defects with Brϕnsted acidity (M _B ).

Computational Details

Models

Atomic coordinates of zeolites were downloaded from the International Zeolite Association (IZA) website⁶³. The periodic models of M(IV)-BEA, M(IV)-FER and M(IV)-CHA zeolites as used previously³⁹ 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.

Figure 2

Periodic models of (A) BEA, (B) CHA and (C) FER zeolites labeling the T sites presently 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)₂Si≡ linkage^31,53,58,64.

Figure 3

Local structures for defects within M(IV)-BEA zeolites that display (A) Lewis acidity (M _L ) and (B) Brϕnsted acidity (M _B ).

Methods

The Perdew, Burke and Ernzrhof (PBE) exchange-correlation functional^65,66 supplemented with the damped C₆ dispersion term⁶⁷ (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,

$${E}_{{\rm{adN}}}=E({\rm{MZeo}}-{{\rm{nH}}}_{2}{\rm{O}})\mbox{--}E({\rm{MZeo}}\,-\,({\rm{n}}-1){{\rm{H}}}_{2}{\rm{O}})-{E}({{\rm{H}}}_{2}{\rm{O}})$$

(1)

where MZeo-nH₂O 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-H₂O configurations (e.g., Sn9 _P ^b rather than Sn9 _P ^a due to the larger E _ad1 value and higher stability, see Table 1).

Table 1 M-O_W distances (Å) and adsorption energies of the n^th water (E _adn, kJ/mol) for defect-free M(IV)-BEA zeolites (M = Sn, Ti, Zr, Ge; n = 1, 2).

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.

Figure 4

(A) Two and (B) potential directions present for water molecules to approach the M(IV) sites within zeolites. Different directions are referred to as “a”, “b” and “c” that are colored in semitransparent pink, yellow and green, respectively.

Figure 5

Local configurations of water adsorption at defect-free M2(IV), M9(IV) and M6(IV) sites of BEA zeolites, where M = Sn, Ti, Zr, Ge. O_W2 in M2 _P ^h1 refers to the water molecule that is stabilized mainly by H-bonds.

M(IV)2 Sites in Defect-free BEA Zeolite

The local adsorption configurations of water at defect-free M2(IV)-BEA zeolites are shown in Fig. 5 (M2 _P ^a), and the M-O_Wa distances and adsorption energies (E _ad1) are given in Table 1. The M-O_Wa distances are 2.356, 2.365, 2.542 and 2.425 Å respectively for M = Ti, Sn, Ge and Zr, in line with previous reports^{21,22,23,25,26,27,28,29,30,31,32,33,34,35,36,37,38} and indicative of chemisorption. Although with a smaller radius than Sn(IV) (0.53 Å vs. 0.71 Å), Ge(IV) corresponds to larger M-O_Wa distances suggesting inferior interactions. The adsorption energies of water (E _ad1) are calculated at −44.9, −64.4, −27.9 and −64.8 kJ/mol respectively for M = Ti, Sn, Ge and Zr, and the interaction strengths within Ge2(IV)-BEA zeolite are almost at the level of all-siliceous zeolite (−20.3 kJ/mol). The adsorption performances of M(IV)-BEA zeolites are in line with their catalytic effects (Zr ≈ Sn > Ti > Ge)^2,3,13,14,15, including the results of two-dimensional zeolites available⁶⁹. The thermodynamic changes for the hydrolysis of M-O bonds fall within the range of 63.8∼73.9 kJ/mol, which are obviously lower than that of all-siliceous zeolite (106.2 kJ/mol) and indicate the less stability and more likelihood of forming defects due to M(IV) incorporation.

M(IV)2 sites are situated at the straight channel of BEA zeolite and it is assumed that only one water can be chemisorbed, as testified by adsorption of the second water that forms H-bonds with the first water and O_F atoms, see M2 _P ^h1 in Fig. 5. Accordingly, all M(IV)2 active sites are penta-coordinated with adsorption of one water. Strong H-bonds are constructed between two adsorbed water molecules and the O_WaH•••O_W2 distances are 1.684, 1.607, 1.664 and 1.658 Å respectively for M = Ti, Sn, Ge and Zr. The second water is further stabilized by three H-bonds with O_F atoms; e.g., the O_W2H•••O_F H-bonds in Sn2 _P ^h1 are 2.005, 2.327 and 2.765 Å. Interestingly, its presence promotes the interaction of M2(IV) sites with the first water, since the M-O_Wa distances are respectively 2.255, 2.267, 2.212 and 2.352 Å for M = Ti, Sn, Ge and Zr and all are shorter than those of M2 _P ^a (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 −58.6, −67.1, −53.0 and −62.1 kJ/mol respectively for M = Ti, Sn, Ge and Zr, which are comparable to, if not larger than, the E _ad1 values of M2 _P ^a where water is chemisorbed; in addition, the E _ad2 values are close for all M(IV) ions and are not so dependent on the identify of M(IV) ions as those of chemisorbed water (E _ad1).

M(IV)9 and M(IV)6 Sites in Defect-free BEA Zeolite

M(IV)9 sites in BEA zeolite are at the intersection of two channels and can be approached by water via either “a” or “b” direction, see Figs 4 and 5 (M9 _P ^a and M9 _P ^b)³³. For a specific M(IV) ion, the M-O_Wa and M-O_Wb distances are close to each other suggesting comparable interactions from two directions, as corroborated by the calculated adsorption energies (E _ad1, 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. The O_WaH•••O_W3 and three O_W3H•••O_F H-bonds are 1.603 Å and 2.134, 2.163, 2.961 Å, respectively. The Sn2-Ow_a distance in Sn2 _P ^h2 equals 2.186 Å and is further shortened than in Sn2 _P ^h1 (2.267 Å); nonetheless, E _ad3 (−64.2 kJ/mol) decreases slightly as compared to E _ad2, due to the inferior H-bonding interactions as reflected by their distances. The E _ad4 value (−33.5 kJ/mol) of Sn2 _P ^h3 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_W4H•••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.

Figure 6

Local adsorption configurations for three and four water molecules within defect-free Sn2(IV)-BEA zeolite. O_W3 and O_W4 refer to the water molecules that are stabilized mainly by H-bonds.

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 O_WaH•••O_W3 and O_WbH•••O_W3 distances are optimized at 1.677 and 2.151 Å, respectively. The third water in Sn9 ^h1 facilitates the chemisorption of associated water (Sn9-O_Wa: 2.289 Å in Sn9 ^h1 vs. 2.402 Å in Sn9 _P ^ab) while in Sn9 _P ^h2 plays an adverse effect for the chemisorption of associated water (Sn9-O_Wb: 2.425 Å in Sn9 _P ^h2 vs. 2.354 Å in Sn9 _P ^ab). As a result, the adsorption energies of the third water (E _ad3) differ markedly for Sn9 _P ^h1 and Sn9 _P ^h2 that amount to −61.2 and −26.9 kJ/mol, respectively. It is interesting to find that three water molecules can also be simultaneously chemisorbed at Sn9(IV) site with elongated Sn-O_W distances (2.665, 2.558 and 2.409 Å)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 7

Local adsorption configurations for three water molecules within defect-free Sn9(IV)-BEA zeolite. O_Wi in Sn9 _P ^abi refers to the water molecule in-between O_Wa and O_Wb, and O_W3 in Sn9 _P ^h1 and Sn9 _P ^h2 stands for the water molecule that is stabilized mainly by H-bonds.

Table 2 M-O_W distances (Å), adsorption energies of the 3^th water (E _adn, kJ/mol) for M9(IV)-BEA zeolites (M = Sn, Zr).

Defect-free CHA and FER Zeolites

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). Each direction in Sn(IV)-CHA zeolite results in the formation of direct Sn-O_W bonds (Sn-O_Wa: 2.492 Å; Sn-O_Wb: 2.395 Å; Sn-O_Wc: 2.443 Å) while only two directions in Ti(IV)-CHA zeolite can be chemisorbed with water (Ti-O_Wa: 2.905 Å; Ti-O_Wb: 2.371 Å; Ti-O_Wc: 2.414 Å)^{21,22,23,25,26,27,28,31,32,33}, where the Ti-O_Wa distance is apparently longer. The adsorption energies of three directions (E _ad1) differ remarkably and are maximal for M1 _P ^b with the values being −72.3 and −57.1 kJ/mol for M = Sn and Ti, respectively (Table 3). For adsorption of two water molecules in Sn(IV)-CHA zeolite, Sn1 _P ^ab remains hexa-coordinated (Sn-O_w: 2.457 and 2.261 Å) while Sn1 _P ^ac (Sn-O_w: 2.565 and 3.033 Å) and Sn1 _P ^bc (Sn-O_w: 2.307 and 3.524 Å) transform to penta-coordination. Instead, all adsorption configurations of Ti(IV)-CHA zeolite are presented as penta-coordination including the one corresponding to Sn1 _P ^ab (i.e., Ti1 _P ^ab 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 8

Local configurations for water adsorption within defect-free M(IV)-CHA zeolites, where M = Sn, Ti.

Table 3 M-O_W distances (Å), adsorption energies of the n^th water (E _adn, kJ/mol) for defect-free M(IV)-CHA and M(IV)-FER zeolites (M = Sn, Ti; n = 1, 2).

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 9

Local configurations for water adsorption within defect-free M(IV)-FER zeolites, where M = Sn, Ti.

Figure 4A shows that “a” and “b” directions each have three different O_FO_FO_FM dihedrals, and the accessibility of water to M(IV) sites is estimated by the minimal dihedral³³,

$${{\rm{\Omega }}}_{{\rm{\min }}}=\,{\rm{\min }}\,[{\rm{\Omega }}({{\rm{O}}}_{{\rm{F}}1}{{\rm{O}}}_{{\rm{F}}2}{{\rm{O}}}_{{\rm{F}}3}{\rm{M}}),{\rm{\Omega }}({{\rm{O}}}_{{\rm{F}}2}{{\rm{O}}}_{{\rm{F}}3}{{\rm{O}}}_{{\rm{F}}1}{\rm{M}}),{\rm{\Omega }}({{\rm{O}}}_{F3}{{\rm{O}}}_{{\rm{F}}1}{{\rm{O}}}_{{\rm{F}}2}{\rm{M}})]$$

(2)

$${{\rm{\Psi }}}_{{\rm{\min }}}=\,{\rm{\min }}\,[{\rm{\Psi }}({{\rm{O}}}_{{\rm{F}}1}{{\rm{O}}}_{{\rm{F}}4}{{\rm{O}}}_{{\rm{F}}2}{\rm{M}}),{\rm{\Psi }}({{\rm{O}}}_{{\rm{F}}4}{{\rm{O}}}_{{\rm{F}}2}{{\rm{O}}}_{{\rm{F}}1}{\rm{M}}),{\rm{\Psi }}({{\rm{O}}}_{{\rm{F}}2}{{\rm{O}}}_{{\rm{F}}1}{{\rm{O}}}_{{\rm{F}}4}{\rm{M}})]$$

(3)

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 geometry⁶¹, 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.

Table 4 The O_FO_FO_FM dihedrals corresponding to the “a” (Ω_min, degrees) and “b” (Ω_min, degrees) directions for defect-free M(IV)-incorporated zeolites adsorbed with water molecules (M = Sn, Ti; n = 0, 1, 2)^a.

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)).

Figure 10

Local adsorption configurations for one and two water molecules at M9(IV) Lewis acidic defects (M9 _L ) of BEA zeolites, where M = Sn, Ti.

Table 5 M-O_W distances (Å) and adsorption energies of the n^th water (E _adn, kJ/mol) for M9(IV) Lewis acidic defects of BEA zeolites (M = Sn, Ti; n = 1, 2).

Figure 11

Local adsorption configurations for three water molecules M9(IV) Lewis acidic defects (M9 _L ) of BEA zeolites, where M = Sn, Ti. O_Wi in Sn9 _L ^abi refers to the water molecule in-between O_Wa and O_Wb, and O_W3 in Sn9 _L ^h1 and Sn9 _L ^h2 stands for the water molecule that is stabilized mainly by H-bonds. In Ti9 _L ^abi, three water molecules are assumed to form direct bonds as in Sn9 _L ^abi.

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.

Table 6 Root-mean-square deviations (RMSD, Å) for M9(IV) active sites of BEA zeolites due to water adsorption (M = Sn, Ti)^a,b.

Brϕnsted Acidic Defects

The adsorption configurations of NH₃ at defect M9 _B of BEA zeolite with the ≡M(OH)₂Si≡ linkage (Figs 1 and 3) are given in Fig. 12. Defect M9 _B in BEA zeolite transfers the proton to NH₃ automatically forming NH₄ ⁺ 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 fructose⁵⁸. Figure 13 depicts the adsorption configurations of water at M9(IV) Brϕnsted acidic defects of BEA zeolites, with structural parameters and adsorption energies (E _adn) being listed in Table 7. The M-O_B and Si-O_B distances are altered but not markedly during water adsorption. The Si-O_B distances are obviously elongated than in perfectly tetrahedral Si(IV) sites (ca. 1.62 Å) while M(IV) sites remain tightly bonded with two bridging O_B atoms. Sn9 _B ^a is an exception where the Si-O_B2 bond is ruptured and the Sn-O_B distances reduce considerably. The M-O_W distances in M9 _B ^b are shorter than in perfectly tetrahedral M(IV) sites and hence interactions with water are enhanced as verified by adsorption energies (e.g., E _ad1 = −91.3 kJ/mol for Sn9 _B ^b vs. −74.2 kJ/mol for Sn9 _P ^b). Formation of Brϕnsted acidic defects (M _B ) facilitates the accessibility of water to M(IV) sites as reflected by the significantly smaller dihedrals than in perfectly tetrahedral sites: The O_F3O_B1O_B2M dihedrals (Fig. 3) are 10.06° and 10.78° respectively for Sn9 _B and Ti9 _B . However, limited space is available for the second water and as a result, water from “a” direction forms only H-bonds instead of being chemisorbed: The O_B1H_B1•••O_Wa and O_WaH•••O_F H-bonds are respectively 1.499 and 1.918, 2.836, 2.670 Å in Sn9 _B ^a while 1.662 and 2.139, 2.573 Å in Ti9 _B ^a, and stronger O_B1H_B1•••O_Wa H-bond in Sn9 _B ^a results from higher Brϕnsted acidity³² that agrees with larger O_B1-H_B1 distances (1.053 vs. 1.011 Å). The E _ad1 values are −101.3 and −69.2 kJ/mol respectively for Sn9 _B ^a and Ti9 _B ^a, and structural distortions in Sn9 _B ^a may facilitate the interactions as testified subsequently. For both Sn9 _B ^ab and Ti9 _B ^ab, one water is chemisorbed while the other is H-bonded.

Figure 12

Local configurations for NH₃ adsorption at two types of Sn9(IV) Brϕnsted acidic defects (Sn9 _B and Sn9 _B ′) and of Sn-BEA zeolite.

Figure 13

Local configurations for water adsorption at M9(IV) Brϕnsted acidic defects (M9 _B ) of BEA zeolites, where M = Sn, Ti. There emerges a second Sn(IV) Brϕnsted acidic defect named Sn9 _B ′, where O_B2 forms direct bonds only with the Sn(IV) sites. Both Sn9 _B ′ ^ab and Sn9 _B ′ ^ba are adsorption configurations with two water molecules while the chemisorbed water are from the “a” and “b” directions, respectively.

Table 7 M-O_W, M-O_B and Si-O_B distances (Å) and adsorption energies of the n^th water (E _adn, kJ/mol) for M9(IV) Brϕnsted acidic defects (M9 _B) of BEA zeolites (M = Sn, Ti; n = 1, 2)^a.

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_B2H_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 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.