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Formation of Heteroatom Active Sites in Zeolites by Hydrolysis and Inversion.

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DOI: 10.1002/ange.200503006
Formation of Heteroatom Active Sites in Zeolites
by Hydrolysis and Inversion**
Judy To,* Alexey A. Sokol, Samuel A. French,
C. Richard A. Catlow, Paul Sherwood, and
Huub J. J. van Dam
Following the discovery of the titanosilicalite zeolite TS-1 and
its remarkable catalytic properties in selective oxidation
reactions with aqueous H2O2,[1] there have been several
reports on the possible use of other heteroatom-substituted
zeolites. Of particular interest are tin and zirconium silicalites,
which are efficient catalysts in the hydroxylation of phenol
with aqueous H2O2.[2] Germanium-containing silicalites, as
well as a number of Ti–Ge–Si zeolites, showing catalytic
activity toward oxidation with H2O2, have also been successfully synthesized.[3] Other important examples are leadcontaining zeolites with framework PbII species; these compounds are promising photocatalysts in denitrification (deNOx) reactions.[4, 5] Despite the considerable effort that has
been devoted to the synthesis and characterization of these
interesting materials, there is still no well-defined model to
explain the observed catalytic properties. As the heteroatoms are postulated to be the active sites for the oxidation
reactions, accurate information on their coordination environment and electronic structure is of fundamental importance for understanding the catalytic behavior of the heteroatom-substituted zeolites and for designing new materials
with predetermined properties.
Herein, we propose a new model for the formation of
active sites in silicalite, which is based on the hydrolysis and
inversion of tetrahedral sites in the zeolitic framework. We
demonstrate that the inversion mechanism can help to
stabilize the active sites, as well as increase their accessibility
to guest molecules in the zeolite pores. This model is in
agreement with early experimental studies on TS-1,[6] which
suggested that hydrolysis of a TiOSi bridge forces the
titanium atom to move away from its tetrahedral configuration to a new more “external” relaxed position. In contrast,
[*] J. To, Dr. A. A. Sokol, Dr. S. A. French, Prof. C. R. A. Catlow
Davy Faraday Research Laboratory
The Royal Institution of Great Britain
21 Albemarle Street, London, W1S 4BS (UK)
Fax: (+ 44) 20-7670-2920
P. Sherwood, H. J. J. van Dam
CCLRC Daresbury Laboratory
Warrington, WA4 4AD (UK)
[**] For the useful discussions, we thank Dr. J. M. Garc@s, Dr. A.
Kuperman, Dr. P. E. Sinclair, Dr. C. M. Barker, Prof. R. J. Davis, Dr. G.
Berlier, Dr. A. M. Beale, Prof. G. Sankar, Dr. N. R. Shiju, and Prof. Sir
J. M. Thomas. J.T. gratefully acknowledges EPSRC for the financial
support and HPCx Materials Chemistry Consortium for the
computational resources from grant GR/S13422.
Angew. Chem. 2006, 118, 1663 –1668
current models make a strong distinction between the modes
of incorporation of titanium atoms: in zeolites they occupy
tetrapodal sites within the framework, while in mesoporous
materials they are grafted onto the silica surface and have a
tripodal coordination environment.[7] Our calculations show
that both configurations are realized within zeolites and that
the hydrolysis-plus-inversion mechanism is the missing link
between them.
The characteristic features of this model are closely
related to a number of structures in silica. For example, it
has been postulated that the hydrolysis step during silica
synthesis occurs through an SN2 mechanism with inversion of
a silicon tetrahedron.[8] A similar mechanism was reported for
the formation of positively charged oxygen-vacancy defects
(E’ centers) in a-quartz and amorphous silica.[9] It was
suggested that the E’ center undergoes a distortion to a
stable puckered configuration, which is accompanied by a
large relaxation of the silicon atoms adjacent to the oxygen
vacancy. Recently, Sokol et al. advanced the hydrolysis-plusinversion model for the stabilization of intrinsic defects, for
example, vicinal disilanols and trigonal aluminum species, in
hydrated siliceous and aluminosilicate zeolites.[10]
The early computational work on titanium sites was based
on generic cluster models with no real distinction between
micro- and mesoporous silicas. In particular, Sinclair et al.
provided evidence of a tripodal configuration for titanium
sites on a mesoporous support.[11] Although the hydrolysis of
one or two of the TiOSi bridges was calculated to be an
endothermic process, it was argued that the formation of
hydrogen bonds helps to stabilize the hydrolyzed sites further.
Therefore, both tetra- and tripodal species should co-exist in
the material. Ricchiardi et al. applied an embedded-cluster
approach in a study of the hydrolysis of the titaniumcontaining zeolite chabazite and drew similar conclusions.[12]
More recent computational studies followed the experimental
lead and concentrated on tetrapodal models for the titanium
sites in zeolites[13] and on tripodal models for the sites in
mesoporous materials.[14] Herein, for the first time, we apply
both types of model to zeolites and explore further related
structures of the active sites.
During synthesis or post-synthetic treatments, the oxidation state of the heteroatoms can also change. Upon exposure
of TS-1 to radiation, or H2 or CO gas, the TiIV centers can be
reduced to TiIII.[15, 16] The chemistry of titanium, zirconium,
and hafnium is dominated by the tetravalent oxidation state
(although a few compounds containing the elements in the
trivalent oxidation state are known).[17] Lower oxidation
states of tin and lead are, of course, well characterized, with
PbII being the dominant valence state of this element.
We first explored the structure and stability of sites
substituted by elements from Groups 4 and 14 in their main
oxidation states. The structures we consider have four, three,
or two TOSi bridges linking the metal T site with the
framework; these configurations are referred to as tetrapodal,
tripodal, and bipodal, respectively.
We employed the hybrid quantum mechanical/molecular
mechanical (QM/MM) embedded-cluster approach, which
was developed to treat localized states in extended systems.
Herein, the zeolite system is represented by a QM/MM
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
cluster embedded in an array of point charges that models the
electrostatic environment. The QM/MM model can be loosely
partitioned into a QM and a MM region. The QM region,
which contains the proposed active site, is treated with
ab initio methods, whereas the MM region, which includes the
rest of the system, is described using classical interatomic
potentials. As part of the embedding procedure, the QM
region is further terminated by hydrogen atoms to satisfy the
valence of the SiO bonds that are cut when defining the QM
and MM regions. The methodology described is implemented
in the computational-chemistry environment code ChemShell.[18]
Based on neutron diffraction and extended X-ray absorption fine structure (EXAFS) studies of TS-1,[19, 20] the T(8) site
of the structure was chosen for substitution and was placed at
the centre of a QM/MM cluster of 18 H radius. The atoms
within three coordination spheres of the T(8) site (that is,
shell-3 atoms), and those that are part of the connected fiveand ten-membered rings (ring sizes are defined by the number
of tetrahedral atoms) were all included in the QM region
(Figure 1). Only the heteroatoms, shell-3 atoms, and atoms
that are part of the five-membered ring were used for
geometry optimization.
This error arises from the embedding procedure, the limited
size of the quantum mechanical and active regions, the
accuracy of the BB1K density functional, and the basis set
superposition error.
Let us first consider a tetravalent heteroatom occupying a
tetrapodal framework site, which is expected to be its ground
state under anhydrous conditions. Upon interaction with
water, one of the TOSi bridges opens, and a stable twomembered ring is formed (Figure 2 a, left). A reaction path to
Figure 2. a) The tripodal structure with (right) and without (left)
inverted tetrahedral sites. b) The bipodal structure with (right) and
without (left) inverted tetrahedral sites. Si black, O gray, H small white
circles, heteroatom large white circles.
Figure 1. A 2D view of the QM/MM cluster, highlighting the QM
region. The hydrogen terminating atoms are omitted for clarity.
Si black, O gray, heteroatom white.
For the QM treatment, we used the density functional
theory (DFT) as implemented in the GAMESS-UK code.[21]
We employed the BB1K exchange correlation functional,[22]
which was shown in our cluster calculations to provide a very
accurate description of the electronic structure of localized
and paramagnetic defects in silica materials.[23] The heteroatoms and the other optimized atoms were described with
TZVP and DZP basis sets, respectively, while the remaining
QM atoms were treated with SV basis sets.[24] Interactions in
the MM region were dealt with using the aluminosilicate
interatomic potentials of Hill and Sauer[25] with modified
charges,[18] using routines from DL_POLY[26] as incorporated
in the ChemShell code. To include relativistic effects, smallcore pseudopotentials were used for the heavier elements
(tin, lead, zirconium, and hafnium).[27] Based on our calculations on selected cluster models of larger size and with more
extensive basis sets, we estimate the error of our calculations
to be within 10–20 kJ mol1, which is in line with state-of-theart quantum-mechanical calculations in solid-state chemistry.
this structure can be achieved by transferring one proton from
water to a bridging oxygen atom to yield a TOH group, and
then forming a SiOH group with the remaining hydroxide
ion. As the two hydroxy groups strongly repel each other in
the confinement of the small space between the two framework T sites, this configuration is further transformed by
inversion, resulting in two tripodal sites with terminal hydroxy
groups [Figure 2 a, right; Eq. (1)].[*]
ðSiOÞ4 TIV þ H2 O ðgÞ ! ðSiOÞ3 TIV
tri OH þ Sitri OH
The mechanism of inversion should be fully analogous
with that reported for vicinal disilanols.[10] A proton hopping
to a neighboring bridging oxygen site leads to rotation of the
oxygen tetrahedron surrounding the T atom. The T atom
linked to the new hydroxy group moves from its central
position into the channel of the zeolite. The deprotonated
oxygen atom subsequently takes the place of the leaving
bridging hydroxy group, thus restoring the TOSi bridge.
The mechanism of hydrolysis and inversion of the TOSi
bridges is, of course, not exclusive to tetrapodal sites, but can
also involve tripodal sites, leading to the bipodal configuration [Figure 2 b; Eq. (2)]. Alternatively, the hydrolysis may
[*] For all reactions, active sites are represented by all atoms in the first
two coordination shells of the T atoms. Subscripts tri and bi refer to
tripodal and bipodal sites, respectively; T atoms without subscripts
occupy tetrapodal sites.
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Angew. Chem. 2006, 118, 1663 –1668
proceed as a concerted process; combining Equations (1) and
(2) gives Equation (3).
tri OH þ H2 O ðgÞ ! ðSiOÞ2 Tbi ðOHÞ2 þ Sitri OH
ðSiOÞ4 TIV þ 2 H2 O ðgÞ ! ðSiOÞ2 TIV
bi ðOHÞ2 þ 2 Sitri OH
Table 1 lists the energies of the reactions in Equations (1)–
(3) and the ionic radii of the tetravalent heteroatoms.[28] The
Table 1: Calculated energies [kJ mol1] for the hydrolysis reactions at TIV
and TIV
tri sites given in Equations (1)–(3).
Ionic radius [H][a]
[Eq. (1)]
[Eq. (2)]
[Eq. (3)]
[a] Data taken from Ref. [28].
opening of the TOSi bridges and the rearrangement of the
local structure through inversion help to release the lattice
strain associated with the heteroatoms and, hence, to stabilize
the structure. The large negative energies of the reaction in
Equation (1) reveal that the formation of all of the tripodal
structures is energetically favorable. In contrast, the formation of all of the bipodal structures from their corresponding
tripodal precursors is endothermic, except in the cases of
carbon and lead sites. The stability of the bipodal structure is
clearly dependent on the ionic size of the heteroatom. Being
at the two size extremes of the series studied, the carbon and
lead atoms are associated with the largest defect strain, which
is released in the less-constrained bipodal configuration. As
heteroatoms may be introduced into zeolites, not only in their
synthesis, but also by impregnation, ion exchange, and/or
grafting on pretreated silica matrices, we assessed the
feasibility of these substitution methods. Figure 3 displays
the substitution energies with respect to the pure-silica system
for two ion-exchange routes [Eq. (4a) and (4b)].
ðSiOÞ4 SiIV þ TCl4 ðgÞ ! ðSiOÞ4 TIV þ SiCl4 ðgÞ
ðSiOÞ4 SiIV þ TðOHÞ4 ðgÞ ! ðSiOÞ4 TIV þ SiðOHÞ4 ðgÞ
In agreement with our previous observations for tetrapodal models,[29] the energies calculated for the reaction in
Equation (4a) show that the larger the heteroatom, the
greater the substitution energy for all of the configurations
considered. For the larger atoms, the substitution energies are
lower for the heteroatoms in the tri- and bipodal configurations; this effect can be attributed to the increased ability of
the heteroatoms to enter a less-constrained framework. The
large positive energies, which indicate highly endothermic
processes, suggest that grafting may be preferred over ion
Angew. Chem. 2006, 118, 1663 –1668
Figure 3. Plot of the substitution energies for C, Si, Ge, Ti, Sn, Zr, Hf,
and Pb atoms calculated from Equations (4a) (DECl ~) and
(4b) (DEOH &) as a function of ionic radius (r) for tetrapodal (top),
tripodal (middle), and bipodal (bottom) sites.
exchange as a preparation route. The substitution energies for
the reaction in Equation (4b), apart from those of the carbon
and silicon sites, are lower than the energies for the reaction in
Equation (4a) for all configurations. Furthermore, the energies for the reaction in Equation (4b) are less dependent on
the ionic radius of the heteroatom. They also span a much
smaller range, which makes identification of the preferred
degree of hydrolysis more difficult. In a recent review of TS1,[7] it was suggested that tetra- and tripodal structures could
be distinguished by their X-ray absorption, UV/Vis, Raman,
and EPR spectroscopic signatures. Our initial calculations do
not support the distinctive nature of the vibrational modes
reported for these species. Similarly, Chaudhari et al. were
unable to definitively assign the vibrational bands in the FTIR spectra of mesoporous titano- and zirconosilicates.[30]
However, in an attempt to determine the dominant titanium
species in TS-1, an accurate refinement of EXAFS data based
on our calculated structures is in progress.
We would expect that under aqueous conditions the
heteroatom sites in silicalite would hydrolyze to a mixture of
tri- and bipodal structures. Note that these heteroatom sites
can undergo further hydrolysis, leading first to unipodal
structures, and finally to the removal of the heteroatom from
the framework and the formation of hydroxy-nest defects.
The bipodal sites, which have fewer TiOSi bridges, are
more susceptible to such degradation. Therefore, the tripodal
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
structures are considered to be the dominant and more stable
species under severe post-synthetic conditions.
Based on these results, the tripodal model was chosen as
the starting structure for the reduction reactions of tetravalent heteroatoms. When substituted into a pure silica framework, heteroatoms with oxidation states less than iv will have
negative effective charges, which must be compensated.
Herein, the charge is neutralized either by adding protons
or by removing hydroxy groups bonded to the heteroatom.
The results of our calculations on the structure and
stability of TiIII, ZrIII, and HfIII sites are summarized in
Table 2, and the reaction scheme considered is outlined in
Table 2: Calculated energies [kJ mol1] for the reduction of TIV
tri centers to
tri, and the structural reorganization of the Ttri sites, as depicted in
Figure 4 and given in Equations (5)–(7).
III [a]
[Eq. (5a)]
[Eq. (5b)]
III [a]
[Eq. (6)]
III [a]
[Eq. (7)]
[a] For steps A–C see Figure 4.
Figure 4. The reduction step A in Figure 4 can be effected
with either H2 [Eq. (5a)] or CO [Eq. (5b)] as reducing
agent.[31, 32]
tri OH þ Sitri OH þ 1=2 H2 ðgÞ Ð
tri þ Sitri OH þ H2 O ðgÞ
tri þ Sitri OH þ CO2 ðgÞ þ 1=2 H2 ðgÞ
Treatment with H2 or CO reduces the diamagnetic TIV
(nd ) transition-metal center to a paramagnetic, EPR-active
tri (nd ) center. The formation of the trivalent-heteroatom
site upon interaction with H2 is calculated to be an endothermic process. Reduction with CO is also endothermic,
although more energetically favorable (by approximately
55 kJ mol1). Characterization of titanium and zirconium
species in micro- and mesoporous materials by EPR spectroscopy has confirmed the presence of trivalent metal
centers in the framework structure after reduction.[31, 32] In
particular, Prakesh and Kevan investigated the formation of
trivalent titanium centers in TS-1 upon treatment with H2 or
CO, and reported that both reduction processes occur above
673 K.[31] This observation indicates the presence of unidentified activation processes preceding reduction, which should
shift the equilibrium to the right in Equations (5a) and (5b).
To probe the activation mechanisms of the trivalentheteroatom sites, we further explored the structure and
stability of the triply coordinated bipodal TIII
bi site formed after
hydrolysis step B in Figure 4 [Eq. (6)], and of the tetrapodal
TIII site formed after back-inversion step C [Eq. (7)].
tri þ Sitri OH þ H2 O ðgÞ ! ðSiOÞ2 Tbi OH þ 2 Sitri OH
tri þ Sitri OH ! ðSiOÞ4 T OHSi
The calculated formation energies of the TiIII, ZrIII, and
Hf centers in the different structural environments indicate
the order of stability of these species as follows: bipodal >
tripodal > tetrapodal. Interestingly, Bal et al.,[16] and Chaudhari et al.[30] gave EPR spectroscopic evidence for the
presence of two different types of TiIII sites in both TS-1
and Ti-MCM-41, which were assigned as tetra- and tripodal
structures by Ratnasamy et al.[7a] However, our results do not
support the spectroscopic signatures, because we predict that
TiIII centers are most stable in the less-constrained bipodal
structure. The two different results can be attributed to the
preparation procedures of the samples. Under dry conditions,
we expect calcined anhydrous samples to contain a high
proportion of tetrapodal TiIV sites, which would first be
reduced to the tetrapodal TiIII species, and then undergo
inversion to the tripodal configuration, as observed in the
published experiments. In the presence of water, however, we
expect the sample to contain a large number of tripodal TiIV
sites, which would first be reduced to tripodal TiIII species, and
then undergo hydrolysis and inversion to form the bipodal
structure, as predicted by our calculations. Note that the
overall formation processes of all bipodal TIII
bi sites are
endothermic, which explains the necessity of the hightemperature treatments mentioned above.
While the reduction of the Group 4 elements can be
expected to proceed by the occupation of d shells, with
trivalent species being of prime interest, lone pairs in s shells
are known to define the chemistry of tin and lead; hence, we
focus on oxidation state ii for these elements. The results of
ðSiOÞ3 T OH þ Sitri OH þ CO ðgÞ Ð
Figure 4. Reduction of TIV
tri centers to Ttri, and structural reorganization
of the TIII
for full details.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 1663 –1668
our calculations are summarized in Table 3, and the reaction
scheme considered is outlined in Figure 5. As before, we
consider two reactions for the reduction step D in Figure 5
[Eqs. (8a) and (8b)].
shown in step E [Eq. (9)], whilst back-inversion leads to a
tetrapodal structure, as shown in step F [Eq. (10)].
tri OH þ Sitri OH þ H2 ðgÞ !
ðSiOÞ2 TIItriOHSi þ Sitri OH ! ðSiOÞ2 TII ðOHSiÞ2
ðSiOÞ2 T OHSi þ Sitri OH þ H2 O ðgÞ
tri OH þ Sitri OH þ CO ðgÞ !
ðSiOÞ2 TIItriOHSi þ Sitri OH þ CO2 ðgÞ
Table 3: Calculated energies [kJ mol1] for the reduction of TIV
tri centers to
TIItri, and the structural reorganization of the TIItri sites, as depicted in
Figure 5 and given in Equations (8)–(10).
II [a]
[Eq. (8a)]
[Eq. (8b)]
E = TIItri!TIIbi [a]
[Eq. (9)]
F = TIItri!TII [a]
[Eq. (10)]
[a] For steps D–F see Figure 5. [b] TIItri (noninverted).
ðSiOÞ2 TIItriOHSi þ Sitri OH ! ðSiOÞ2 TIIbi þ 2 Sitri OH
The formation of a SnII center in a tetrapodal configuration is highly endothermic, as it is for the trivalent centers.
However, the order of stability of the different structures
containing SnII centers is different: tripodal > bipodal > tetrapodal.
The PbII center appears to be a special case. In agreement
with X-ray absorption near-edge structure (XANES) and FTEXAFS measurements on PbII/ZSM-5, our results indicate
that triply coordinated PbII species are preferred within the
zeolite framework.[5] In fact, a PbII center in a tetrapodal
configuration was not found, because the atom is too big to
retain the tetrahedral coordination. Instead, optimization
starting from a tetrapodal configuration produced a structure
similar to the tripodal configuration, but without silanol
inversion. When we compared the energies of the two
structurally different tripodal configurations, we found that
the structure containing the inverted silanol group was more
stable by 23 kJ mol1. This result provides further evidence
that the inversion of silanol groups is significant in helping to
relieve the structural strain associated with larger heteroatoms.
In summary, we predict that the tetravalent-heteroatom
sites preferentially adopt tripodal configurations, particularly
in the presence of water. The hydrolysis of TOSi bridges,
which occurs with rearrangement of the local structure, helps
to relieve lattice strain and to stabilize the structure. On the
other hand, we find that the trivalent-heteroatom sites are
most stable in bipodal configurations, whereas SnII and PbII
sites prefer to adopt tripodal structures. Thus, hydrolysis and
inversion play fundamental roles in the stabilization of
heteroatom-based active sites in zeolites. The present study
has concentrated on Group 4 and 14 elements, but the
mechanisms identified are expected to be general. Studies
in progress on zeolites containing other early transition
metals will be reported elsewhere.
Received: August 23, 2005
Revised: September 27, 2005
Published online: January 30, 2006
Figure 5. Reduction of TIV
tri centers to Ttri, and structural reorganization
of the TIItri sites; T = Sn, Pb. See text for full details.
Both reduction reactions lead to the formation of a
divalent center in a distorted trigonal-pyramidal configuration (which is tripodal, with two TOSi and one TOHSi
bridges) and are calculated to be exothermic.
Considering alternative configurations, silanol inversion
in the tripodal structure results in a bipodal structure, as
Angew. Chem. 2006, 118, 1663 –1668
Keywords: computer chemistry · hydrolysis · reduction ·
titanium · zeolites
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site, activ, formation, zeolites, inversion, hydrolysis, heteroatom
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