reactivity of ti (iv) sites in ti-zeolites: an embedded cluster approach

JOURNAL OF CHEMICAL PHYSICS VOLUME 117, NUMBER 1 1 JULY 2002

Reactivity of Ti „IV… sites in Ti-zeolites: An embedded cluster approachAlessandro Damin, Silvia Bordiga,a) and Adriano ZecchinaDipartimento Chimica IFM, Via P. Giuria n. 7 10125-Torino, Universita` di Torino, Italy

Carlo LambertiDipartimento Chimica IFM, Via P. Giuria n. 7 10125-Torino and INFM UdR Universita` di Torino, Italy

~Received 30 January 2002; accepted 3 April 2002!

We report a complete cluster/embedded cluster study by means ofab initio methods in the ONIOMscheme, as implemented inGAUSSIAN 98 code, of the reactivity towards water and ammonia ofTi~IV ! centers in zeolitic frameworks. For water adsorption, we observe a remarkable increment ofthe binding energies by moving from 2.1 kJ mol21 for the unconstrained Ti~OSiH3)4 cluster to 16.9kJ mol21 for the TiSi17O26H20 cluster, obtained by cutting a portion of the MFI framework. Thesame holds for ammonia, where the binding energy increases from 17.4 to 35.4 kJ mol21, allreported values being BSSE corrected. These results underline the fundamental role played byzeolitic framework constraints, in enhancing the reactivity of Ti~IV ! centers towards both H2O andNH3 probes. On the geometrical ground the Ti–O distance of bare clusters and its modification incomplexes are in good agreement with the first shell EXAFS data analysis. ©2002 AmericanInstitute of Physics.@DOI: 10.1063/1.1481378#

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I. INTRODUCTION

Ti-silicalite ~TS-1!1 is an active and selective catalysta remarkable number of low-temperature oxidation reactiwith aqueous H2O2 as the oxidant.2–4 For its relevance inindustrial applications, it has been one of the most studmaterials in heterogeneous catalysis in the last years5–16

Quoted works have demonstrated that Ti atoms~up to 2 at-oms per unit cell! occupy framework tetrahedral positiosubstituting Si atoms. Insertion of Ti atom in a T~OSi)4 sitestrongly perturbs the T–O distance, which increases fr1.59–1.60 Å forT5Si to 1.79–1.8160.01 Å for T5Ti, asproved by EXAFS data.11–13

The determination of geometry, electronic structure, areactivity of Ti~IV ! center in TS-1 is of primary importancand consequently has been the subject of many theoreworks.16–27 The calculation of the structure and adsorptiproperties of the Ti~IV ! center has been performed on tbasis of simple cluster, periodic and MM/QM embeddcluster models.

The cluster situation can be summarized as follows:Man and Sauer17 made Hartree–Fock calculations oTi~OSi~OH)3)4 , as a model for Ti site in TS-1 and obtainea Ti–O distance of 1.790 Å. Moreover, using the same clter, they reported an interpretation of the major vibratiofeatures due to Ti~OSi)4 moiety in TS-1. Similar results havbeen obtained by Karlsenet al.18 on Ti~OSiH3)4 at RHF andB88-LYP level ~Ti–O51.798 and 1.803 Å, respectively!.Slightly longer Ti–O distance~1.802 and 1.814 Å!, at thesame two levels of theory, but on Ti~OH)4 , was found.18

Ti–O distance of 1.810 Å has been obtained by NeurockManzer19 and Sinclairet al.20 at B-P level on Ti~OSiH3)4 .Moreover in Ref. 20, in studying H2O adsorption at Ti site,

a!Corresponding author: Tel139011-6707858; Fax139011-6707855; Elec-tronic mail [email protected]

2260021-9606/2002/117(1)/226/12/$19.00

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Ti~OSiH3)4 /nH2O (n51,2) clusters have been adopted:order to mimic framework constraints effect, Si atom codinates have been fixed at the same positions obtainedTi~OSiH3)4 ~fully optimized!. The calculated binding energies for the so obtained monohydrated and bihydrated cplexes were112.0 and129.0 kJ mol21, respectively. Con-strained ~by fixing Ti–O–Si angles! and unconstrainedoptimizations on Ti~OSiH3)4 , performed by Vayssilov andvan Santen22 at B-P level, provided a Ti–O distance ranginbetween 1.830 and 1.840 Å. In the same work, bindingergies of13.5 and133.0 kJ mol21, after constrained~byfixing hydrogen atoms at their previously optimized potions! and unconstrained optimization of Ti~OSiH3)4 /H2O2

complexes, have been calculated, respectively. Converno stables structures for water adsorption are found. Vrecently, Munakataet al.28 published their results about modeling Ti site with Ti~OSi~OH)3)4 cluster. By fixing cappinghydrogen atoms during optimizations~performed at LDAand B-P levels!, they found Ti–O distances between 1.7801.840 Å and 1.810–1.870 Å, respectively. Binding energfor water, hydrogen peroxide, and ammonia adsorptionTi~OSi~OH)3)4 , calculated at B-P level on constrained opmized LDA structures, resulted in141.0,131.0, and152.0kJ mol21, respectively. Notice, that in disagreement with rsults reported in Ref. 22, Munakataet al.28 found that thecomplex with water is more stable than that with H2O2.

The periodic approach has been adopted in Refs. 25,and 27. Two different methods have been proposed tocount for framework constraints effects. In Ref. 25, a peodic Hartree–Fock~PHF! scheme, as implemented in thCRYSTAL29 code, has been employed to study the Ti4

structure in Ti-chabazite as a model for TS-1. In Ref.interaction with water was also investigated. A Ti–O distanof 1.790 Å in Ti-chabazite with a Ti/Si ratio of 1/1 per uncell is found. The calculated binding energies, basis set

© 2002 American Institute of Physics

IP license or copyright, see http://ojps.aip.org/jcpo/jcpcr.jsp

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227J. Chem. Phys., Vol. 117, No. 1, 1 July 2002 Reactivity of Ti sites in zeolites

FIG. 1. Clusters employed to modethe Ti~IV ! site in Ti-zeolites. Modelzone M in the T7-MFIIT18 clusters isrendered with ball and sticks. H linkatoms are here omitted for the sakeclarity. White: H and Si atoms. Black:O atoms. Gray: Ti atom.

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perposition error (BSSE) not accounted for, for one and twwater molecules adsorption were134.4 and 146.6kJ mol21, respectively. From this result it is inferred that thinsertion process of a second water molecule into the cdination sphere of Ti~IV ! is less favored, probably as a cosequence of large constraints, imposed by the frameworking on Ti~OSi)4 moiety. The problem of the constrainimposed by the framework has been treated by Ricchiet al.,27 by means of an hybryd periodic/embedded MM/Qcluster approach, as implemented in QMPOT compucode.30,31 In particular in Ref. 27, Ti-chabazite~Ti/Si51/11!and TS-1 ~Ti/Si51/95!, and their interaction with waterwere studied. In the optimization process of TS-1, the ebedded cluster is Ti~OSi~OH)3)4 . Calculated Ti–O distancefor the three different chosen T-sites, is 1.770–1.780 Å, vclose to that obtained by Zicovichet al.25,26on Ti–chabazite.Binding energies,123, 128, and131 kJ mol21 ~BSSEac-counted for!, with one water molecule have been calculatat Hartree–Fock level for three T-sites. Very similar val~129 kJ mol21! has been obtained for Ti–chabazite. In R27, interaction of the Ti site with water was also studiemploying, as a model of the Ti site, a three-fused fomembered rings cluster cut from chabazite framework~assuch as that reported in Fig. 1, model CHAIT8!. Calculatedbinding energy with water at Hartree–Fock level is139kJ mol21 ~119 kJ mol21 whenBSSEis accounted for27!.

In the present work, interaction of one H2O and one NH3molecule with Ti-zeolites will be studied, adopting seveclusters~see Fig. 1! as models of Ti~IV ! site. With the onlyexception of the smaller clusters~T1 and T5!, frameworkconstraints effect is auto-imposed by the structure itswithout fixing any atom. In order to clarify how constraincan modify energetic features of TiH2O and Ti NH3 ad-ducts formation, results from calculations on completely fTi~OSiH3)4 ~cluster T5! will be also presented for comparson. In fact, as will be shown in this work, geometrical costraints will play an important role in the reactivity of Ti~IV !centers toward ligand molecules. This implies that the usgeometricalhard constraints such as fixed distances andatomic positions and/or angles must be done with care sit will strongly perturb not only the geometrical featuresthe adduct but also the energetic of its formation. On


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other hand, the total absence of constraints~vide infra thediscussion on T5 results! also yields in macroscopic computational errors since Ti, O, and Si atoms of the clustercompletely free in the research of the minimum energy cfiguration for the adduct. On this basis, it is evident that mreliable models are obtained by cutting portions of zeolframework ~clusters CHAIT8 and MFIIT14-18!. In such away we introducesoft constraints on the Ti–O, O–Si distances and on the Ti–O–Si, O–Ti–O, and Si–O–Siangles:all atoms of the clusters are now free to optimize their potions upon interaction with the ligand molecules, howeveach structural arrangement has an energetic cost, beinlated to the stretching and/or bending of already optimizdistances and/or angles. Of course, periodic approaches25–27

are free from this kind of problem.The choice of H2O and NH3 molecules for probing the

reactivity of the Ti~OSi)4 centers in zeolites has been driveby the important role played by such molecules in the indtrial application of titanosilicates.2 In fact, the understandingof the interaction with water is important since the catalworks in aqueous solution. The interest in the study of N3

is twofold: ammonia is a reactant in the ammoximationcyclohexanone to give cyclohexanone ossime and it istronger base than water, thus allowing a direct comparibetween the effect induced by Lewis bases of increasstrength.

II. MODELS

As can be seen from Fig. 1, a wide variety of clustersused here to model the Ti site in TS-1 and to study its intaction towards H2O and NH3. The label defining each cluster terminates with Tn, n being the number of tetrahedral~Ti or Si! units included in the cluster. Following previoutheoretical works ~see Table I!, Ti~OH)4

18,27 andTi~OSiH3)4

18–20,22,23~hereafter T1 and T5 models, respetively! clusters have also been considered, in order to copare our new results with literature data. Of course, modT1 and T5 represent only a starting point of our work, asgain of this study is the investigation of framework costraints effects. As it is evident from the structure of TSconstraints imposed on Ti~OSi)4 center are due exclusivel


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228 J. Chem. Phys., Vol. 117, No. 1, 1 July 2002 Damin et al.

to cyclic structures. On this basis, some of the structuillustrated in Fig. 1 are obtained directly cutting the clustefrom different zeolite frameworks. As the simplest cycstructure, we have studied T7 cluster (TiSi6O8H12) whereTi~IV ! is confined into two Ti-sharing four-membered ringas far as CHAIT8 model (TiSi7O10H12) is obtained from thechabazite framework. Models MFIIT15 (TiSi14O20H20),MFIIT16 (TiSi15O22H20), and MFIIT18 (TiSi17O26H20) arederived from MFI, by cutting different portions of framework around T1 site.32 In particular, as can be seen fromodels reported in Fig. 2, MFIIT15 and MFIIT16 are ob-tained from MFIIT18, by simply clearing three and twT-centers from the model, diminishing the number of trings typical for T1 environment in the MFI structure. Additionally, moving from MFIIT15 to MFIIT14 (TiSi13O19H18),a strained structure is obtained by deleting a T-center frofive-membered ring, forming a structure which is not typicof the T1 site environment in perfect MFI framework, thumimicking the effect of a structural defect around a Ti cen

III. COMPUTATIONAL DETAILS

All calculations have been run using theGAUSSIAN 98

computer code.33 For the T7-MFIIT18 models the cluster

TABLE I. Main results from literature concerning Ti~OH!4 ~model T1!,Ti~OSiH3)4 ~model T5!, and Ti~OSi~OH3))4 cluster models. Ti–O is ex-pressed in Å.BE: binding energy~number in parenthesis areBE correctedfor the BSSE!, expressed in kJ mol21. Labels~1! and ~2! in Ref. 27 showresults obtained with different basis sets. L: ligand.

Ti~OH!4

Refs. HamiltonianTi–O in bare

cluster L BE for Ti~OH4!/L

18 RHF 1.80218 B88-LYP 1.81427 RHF~1! H2O 150.5~128.3!27 RHF~2! H2O 137.8~130.6!27 MP2~2! H2O 153.1~131.9!27 B3-LYP~2! 1.817 H2O 140.3~130.8!

Ti~OSiH3)4


cluster LBE for

Ti~OSiH3)4 /L

18 RHF 1.79818 B88-LYP 1.80319 B-P 1.81020 B-P 1.810 H2Oa 112.022 B-P 1.830 H2O2 133.022 B-P 1.830a H2O2

a 13.523 RHF 1.740

Ti~OSi~OH!3)4


cluster LBE for

Ti~OSi~OH!3)4 /L

17 RHF 1.79022 B-P ~1.82–4!a

28 LDA-VWN ~1.780;1.840!a H2Oa 141.0b

28 LDA-VWN ~1.780;1.840!a H2O2a 131.0b

28 LDA-VWN ~1.780;1.840!a NH3a 152.0b

aConstrained optimization, by fixing some atoms.bComputed at GGA level on LDA optimized geometries.


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embedded cluster ONIOM method, developed by Morokuand co-workers,34 has been adopted to limit computationdemand. The ONIOM methods allow both the integrationmolecular orbital~MO! with molecular mechanics~MM !methods~IMOMM !,35 as well as the integration of MO withMO methods~IMOMO!.36 In the present work the lattescheme is applied. The ONIOM methods~including IM-OMM and IMOMO! are extrapolation schemes. A moleculsystem can be divided up into three different layers. Evlayer can be treated at an arbitrary level of theory. Inpresent case a two-layers implementation has been adoin which the extrapolated energy~and its related quantities!E(ONIOM2) is defined as:

E~ONIOM2!5EHM1~ELR2ELM !, ~1!

whereELR is the energy of the whole cluster~R, or the realpart!, calculated at the low~L! level of theory andELM andEHM are the energies of the model system M~a subportion ofR!, determined at low and high level~H! of theory, respec-tively. Mechanical constraints are transferred to M ato~balls and sticks in Fig. 1! from the remaining part of R, bythe so-called ‘‘link atoms’’~generally H atoms, not showein Fig. 1 for the sake of clarity!: these replace the bordeatoms between M and R~oxygen atoms in T7-MFIIT18models! and are included in M. In such way, M is always thsame for the six different models and corresponds to themodel. This approach also guarantees the electrical neutrof the M zone.

Such method was previously applied by us to the stuof hydroxyls nests in defective silicalites37 and by other col-leagues of our group to investigate the interaction of N3

with isolated silanols in zeolites.38

A. Binding energy and basis set superposition error„BSSE… calculation

The binding energyBE, the BSSEand BEc ~BE cor-rected for theBSSE!, following the a posteriori counter-poise method proposed by Lendvay and Mayer,39 for theABcomplex formation fromA ~zeolite cluster! and B ~H2O orNH3! moieties, are:

BE5Ea~A!1Eb~B!2Eab~AB!, ~2!

BEc5Ea~A!1Eb~B!2@Ea~Ade f!2Eab~Ade f!

1Eb~Bde f!2Eab~Bde f!#2Eab~AB!, ~3!

BSSE5BE2BEc5@Ea~Ade f!2Eab~Ade f!1Eb~Bde f!

2Eab~Bde f!#, ~4!

where: i! a andb are theA andB basis sets, respectively; ii!Eab(AB) is the energy of the complex andEa(A) andEb(B)are the energies of the two isolated systems at their equrium geometry and with their own basis sets; iii! Eab(A

de f)andEab(B

de f) are, respectively, the energy ofA at the actualgeometry in the complex plus the basis functions only oB~at the actual geometry in the complex! and vice versa; iv!Ea(Ade f) and Eb(Bde f) are the energies ofA and B at theactual geometry in the complex with their own basis seWhen the complex formation is energetically favored,BE


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FIG. 2. Schematic representation of MFIIT14-MFIIT18 clusters employedto model Ti site in TS-1. Oxygen and hydrogen atoms are omitted in oto underline the dimension of the cyclic structures to which Ti atom belonGray sphere: Ti. White spheres: Si.


andBEc are positive quantities. TheBE andBEc have beenobtained by considering that the reaction of theAB complexdecomposition:

AB→A1B⇒BE ~5!

can be split in three reactions, each characterized by thdifferent additive energies:

AB→Ade f1Bde f⇒BEde f5Ea~Ade f!1Eb~Bde f!

2Eab~AB!, ~6!

Ade f→A⇒REA5Ea~A!2Ea~Ade f!, ~7!

Bde f→B⇒REB5Eb~B!2Eb~Bde f!, ~8!

whereREA andREB are the energy gains~which are gener-ally negative values! when deformedA and B subsystemsreach the equilibrium geometries which have been isolaBEde f is the binding energy for the complex decompositito give the two deformed, but separated,A andB moieties. IfBSSEis also accounted for, then Eq.~6! must be rewrittenas:

BEcde f5Eab~Ade f!1Eab~Bde f!2Eab~AB!. ~9!

In order to obtain the whole process~5!, the subprocesse~6–8! must be added. As a result,BE can be defined as:

BE5BEde f1REA1REB. ~10!

SubstitutingBEde f in Eq. ~10! with the correspondingBSSEcorrected value@BEcde f in Eq. ~9!#, BEc can be defined as:

BEc5BEcde f1REA1REB. ~11!

Equation~2! can so be obtained by substituting Eqs.~6–8! inEq. ~10!, while Eq. ~3! can so be obtained by substitutinEqs.~7–9! in Eq. ~11!. Finally, introducing the deformationenergies~DEA52REA and DEB52REB!, which are nor-mally positive values, Eqs.~10–11! become:

BE5BEde f2DEA2DEB, ~12!

BEc5BEcde f2DEA2DEB. ~13!

ONIOM BE0 andBE0c can be obtained applying directl

Eqs. ~2! and ~3!. Moreover, referring to Eq.~1!, the corre-sponding ONIOMBE0 and BE0

c can be split in its compo-nents and defined as:

BE05HM1@LR2LM #, ~14!

BE0c5HMc1@LRc2LMc#, ~15!

whereHM , LM , andLR are the binding energies for M anR, respectively, calculated at high~H! and low ~L! levelsfrom Eq. ~2!. HMc, LRc, and LMc are the correspectivevalues, each corrected for theBSSE, obtained from Eq.~3!.Also in ONIOM calculations, each component to the finONIOM BE0 or BE0

c in Eqs.~14! or ~15! can be obtained byreferring to Eqs.~12! or ~13!.

B. Hamiltonians Õbasis sets for bare T1 and T5 modelsand their H 2O and NH3 adducts

Calculations are performed at DFT level,40,41 using theB3-LYP scheme with the Becke’s hybrid exchange B342 and

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the Lee, Yang, and Parr LYP43 correlation functionals. Theemployed basis set is an all-electrons standard Pople 61G(d,p) for H, N, O, Si, and Ti atoms~hereafter AE basisset!. Moreover, two new basis sets have been constructewhich H, N, O, Si atoms are described by a standard Po6-3111G(d,p), being Ti core electrons@1s2,2s2,2p6#treated with effective small core pseudopotentials~ECP!; theTi valence electrons are described with a double-zeta quGaussian basis set developed by Hay and Wadt44 plus stan-dard Pople 6-3111G(d,p) ‘‘ f’’ polarization function@here-after ECP~f! basis set# or an Ahlrichs45 TZV ‘‘ p’’ polarizationfunction @hereafter ECP(p) basis-set#. When not explicitlyspecified, the adopted Hamiltonian is the B3-LYP one.

No symmetry constraints nor other constraints have bimposed during optimizations of isolated clusters and H2O orNH3 complexes.

C. Hamiltonians Õbasis sets for ONIOM calculations

T7-MFIIT18 models and their complexes with H2O andNH3 are optimized only in the ONIOM scheme. The higlevel calculations are always performed at B3-LYP levwith a standard Pople 6-3111G(d,p) basis set for the H, NO, and Si atoms and the ECP(p) or AE ~only for CHAIT8and MFIIT16 models! basis sets. The Hartree–Fock HHamiltonian is used for low-level calculations, employingstandard Pople 3-21G for H, N, O, and Si atoms and3-21G(d) for the Ti atom, leading to a~B3-LYP/AE orECP(p): HF/3-21G,3-21G(d)! ONIOM computationalscheme. The resulting optimized structures for CHAIT8model are also employed to perform single point energyculations ~hereafter labeled by SP!, treating at(B3-LYP/~ECP(p)) level the whole cluster. When not explicitly specified, the reported results are the ONIOM one

D. Geometrical variable defining the TiO 4 unit in adistorted Td symmetry

An important part of this work is devoted to the comptation of the geometrical perturbation and energetic cogains induced either by insertion of the TiO4 moiety inside acluster mimicking the zeolite framework or by adsorptiona probe molecule or both. Figure 3 reports the centraatom ~big gray sphere! and its four first neighbor oxygen~black little spheres! in a nearly tetrahedral symmetry. ForperfectTd symmetry the unique geometrical variable of tTiO4 group is the Ti–O distance, being the six O–Ti–angles equivalent~;109.5°!.

If a ligand molecule L approaches the Ti~IV ! centeralong one of the four Ti–O axes, from the Ti side~Fig. 3!,then theTd symmetry is broken and the four oxygen liganare no longer equivalent, being split into on apical and thequatorial~Oap and Oeq in Fig. 3!. The TiO4 group is nowdefined by two Ti–O distances~Ti–Oap and Ti–Oeq! and bytwo angles:a5Oeq–Ti–Oeq andb5Oeq–Ti–Oap. Of interestis also the distance between Ti and CM, defined ascenter-of-mass of the three equatorial oxygens~see little graysphere in Fig. 3!. In a perfectTd symmetrya5b;109.5°,while Ti–CM50.3343~Ti–O!. Conversely, when the pertur


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bation reaches the bipiramidal geometry~Ti in the Oeq

plane!, a5120° andb590°, while Ti–CM50.In Sec. IV, we will often refer toa, b, and Ti–CM values

to give a quantitative evaluation of the distortion fromTd

symmetry undergone by TiO4 moiety upon ligand adsorption, insertion in the zeolitic framework or both~vide infraTables II and III!. This brief description has been done undthe assumption that the LTi~Oeq)3 moiety retainsC3 axis(Ti–Oap). If this does not hold then the symmetry is evelower and the three Oeq ligands are no more equivalent, resulting in four different Ti–O distances and three differentaandb angles.

IV. RESULTS AND DISCUSSION

A. Brief overview on the experimental evidences

Before entering in the description and discussion ofcomputational results, a brief overview on the firm andthe questionable experimental evidences is mandatory tothe confidence of the results obtained with the different mels. The low Ti content~up to 3 wt% in TiO2! makes theextraction of vibrational, energetic, and geometric featuspecific of TiO4 moieties a difficult task, being all experimental data dominated by the features of the siliceoustrix. This is the reason why the local environment of Ti hbeen experimentally defined by EXAFS studies only mothan 10 years after the discover of the material.11–13 Suchstudies were limited to the first shell, showing that Ti atomare, within experimental errors, coordinated to 4 oxygenoms located in the 1.79–1.81 Å range. A second shell stappears only very recently,15 showing that the four Ti–O–Sangles are not equivalent. Two couples of angles have bfound, the narrower one of 14365°, and the broader of 162

FIG. 3. Schematic representation of the TiO4 moiety ~big gray sphere: Ti,black spheres: O!. Small gray sphere shows CM, the center-of-mass defiby the three Oeq atoms. L: H2O or NH3 .



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TABLE II. Some Ti~OSi!4 moiety geometrical features of the optimized T5-MFIIT18 bare models.Ti–O&~average Ti–O distance! and Ti–CM are expressed in Å,a andb in degrees.

Model/basis set ^Ti–O& a b Ti–CM Ti–Si Ti–O–Si

T5/ECP~p! 1.803 109.5 109.5 0.601 3.461 180.0T5/AE 1.802 109.5 109.5 0.601 3.461 180.0

Model/basis set ^Ti–O& a b Ti–CM Ti–Si Ti–O–Si

T7/ECP~p! 1.808 107.8;110.4 107.9;110.2 0.602 3.403 159.1CHAIT8/ECP~p! 1.810 108.7;113.8 105.3;110.1 0.562 3.330;3.367 148.6;153.0MFIIT14/ECP~p! 1.810 110.5;111.8 105.5;111.5 0.550 3.202;3.438 134.8;173.2MFIIT15/ECP~p! 1.808 110.1;111.9 106.7;110.5 0.562 3.267;3.437 141.9;173.2MFIIT16/ECP~p! 1.806 110.6;111.3 106.4;109.4 0.559 3.306;3.426 146.2;169.5MFIIT18/ECP~p! 1.808 110.4;112.1 105.8;109.1 0.550 3.296;3.421 145.0;170.5CHAIT8/AE 1.812 109.0;113.2 105.7;109.9 0.567 3.326;3.366 147.9;152.7MFIIT16/AE 1.807 110.0;111.0 106.7;109.9 0.568 3.303;3.424 146.0;169.2

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65°. The four Ti–Si distances lie in the 3.26–3.3860.02 Åinterval. XANES spectroscopy witnessed theTd-like geom-etry of Ti species.

Interaction with water and ammonia has also been invtigated by EXAFS technique and first shell data have breported.46,47 It has been shown that the Ti–O distance of tfour framework bonds undergo a stretching of 0.02~0.05! Åupon H2O (NH3) adsorption. On the parallel groundXANES spectroscopy has monitored a change of the logeometry from aTd-like one to more of a coordinated onsimilar to theOh type observed in anatase. Unfortunately, tlack of a higher shell analysis of the EXAFS data haspermitted the localization of the adsorbed molecule, soexperimental Ti OH2 and Ti NH3 distances~in the fol-lowing defined as Ti–Ow and Ti–Na, respectively! are still

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missing and a computational approach is mandatory~videinfra!.

On the energetic ground only few experimental papappeared. Microcalorimetry has been used to investigateinteraction between ammonia and TS-148 and between wateand Ti-b.46 In Ref. 48 some of us have used the calorimetdata obtained in two parallel experiment performed by ding NH3 on both TS-1 and Ti-free silicalite to extrapolate thformation enthalpy of the Ti NH3 adduct. That methodyields to DadsH as high as 60 kJ mol21. Concerning theTi-b/H2O system anDadsH of 50 kJ mol21 has been reportedat high coverages.46 We have obtained similar values by doing water on both TS-1~52 kJ mol21! and silicalite ~50kJ mol21!.49 The small difference obtained between the twTi-containing systems and the Ti-free one and the closen

n
TABLE III. Some geometrical features of Ti~OSi!4 moiety in (T5-MFIIT18)/H2O and (T5-MFIIT18)/NH3 complexes. Ti–Ow and Ti–Na are Ti–O~H2O)and Ti–N~NH3) distances. For the sake of comparison, also the deformation energyDEA ~A5T5-MFIIT18 models! are listed. Distances in Å, angles idegrees and energies in kJ mol21.
H2O

Model/basis set ^Ti–O& a b Ti–CM Ti–Si Ti–O–Si Ti–Ow DEA

T5/ECP~p! 1.823 116.1;116.6 100.7;102.1 0.356 3.390;3.449 151.4;170.1 2.43 130.7T7/ECP~p! 1.828 116.2;120.3 98.6;101.6 0.336 3.312;3.435 153.3;166.7 2.40 128.9CHAIT8/ECP~p! 1.829 113.8;119.0 99.1;101.6 0.337 3.332;3.406 145.2;160.5 2.40 122.0MFIIT14/ECP~p! 1.829 115.2;118.2 99.3;102.8 0.334 3.233;3.437 135.6;177.3 2.37 120.3MFIIT15/ECP~p! 1.828 115.2;117.9 100.2;102.5 0.350 3.314;3.423 144.1;167.4 2.40 120.4MFIIT16/ECP~p! 1.825 115.4;117.0 99.9;101.8 0.351 3.338;3.436 151.8;174.0 2.41 120.1MFIIT18/ECP~p! 1.826 115.0;117.5 99.2;101.6 0.342 3.330;3.437 145.8;175.2 2.40 120.4T5/AE 1.822 115.9;116.3 101.2;102.3 0.367 3.441;3.387 151.2;167.3 2.46 128.5CHAIT8/AE 1.829 114.7;118.4 99.5;101.6 0.343 3.330;3.404 145.3;158.6 2.42 121.6MFIIT16/AE 1.825 114.9;117.2 100.1;102.0 0.356 3.332;3.432 150.6;171.5 2.43 120.7

NH3

Model/basis set ^Ti–O& a b Ti–CM Ti–Si Ti–O–Si Ti–Na DEA

T5/ECP~p! 1.832 117.5;117.9 98.8;98.9 0.282 3.407;3.472 155.3;179.4 2.35 142.9T7/ECP~p! 1.841 115.5;119.1 97.2;100.6 0.268 3.269;3.448 138.8;163.3 2.33 142.3CHAIT8/ECP~p! 1.839 114.9;119.9 97.6;99.6 0.270 3.311;3.427 143.7;160.8 2.33 134.4MFIIT14/ECP~p! 1.838 115.3;119.6 97.5;97.8 0.284 3.258;3.459 138.3;174.9 2.33 129.5MFIIT15/ECP~p! 1.835 116.8;118.0 98.2;100.4 0.286 3.338;3.454 147.4;167.2 2.34 131.3MFIIT16/ECP~p! 1.833 117.1;118.2 98.1;99.8 0.292 3.359;3.457 150.7;178.4 2.35 130.5MFIIT18/ECP~p! 1.834 116.3;119.0 97.4;99.8 0.287 3.456;3.353 149.5;177.6 2.34 130.0MFIIT16/AE 1.834 116.7;118.1 98.3;100.0 0.295 3.357;3.457 150.5;178.7 2.36 131.8


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among measured values and the liquefaction enthalpy ofter (2DLH544 kJ mol21) suggests that both groups46,49

have mainly measured the condensation of water insidezeolites’ pores and that theDadsH of the Ti H2O complex isprobably much lower. Coming back to the case of ammoon the basis of a more complete calorimetric study involv~i! different TS-1 samples characterized by an increasingcontent and~ii ! different Ti-free silicalites characterized ban increasing internal defectivity,50 we have realized that aspecificDadsH value for the adducts formed on Ti sites canot be obtained with a sufficient confidence. In fact theperimental data results from the simultaneous contributicoming from the interaction between the probe moleculethe different adsorbing sites of the guest matrix, Ti ones rresenting a large minority of them. A direct comparison btween theoretical~vide infra! and experimental data is so nfeasible owing to the absence of a straightforward metable to separate, at each equilibrium pressure, the measquantities~integral heats and adsorbed moles! into the corre-sponding subquantities referred to the different adsorpsites. On this basis, the computational chemistry remainsthe moment, the only method for the investigation of tenergetic aspect of this topic.

Finally, on the vibrational ground, the insertion of Tithe MFI lattice results in the appearance of two Ti-specmodes at 960 and 1125 cm21, the latter visible in Ramanspectra only.8–10,13,16The perturbation undergone by both vbrational features upon interaction of TS-1 with ammoand water reflects the direct interaction of bases with Ti simirroring XANES and EXAFS data.8,9,12,13Vibrational pecu-liarities of TiO4 moieties have already been investigated ocomputational ground in Refs. 16, 17, and 51# and so willnot be discussed in this context.

B. Bare T1 and T5 Ti site models: optimizedgeometries

Clusters T1 and T5 have been fully optimized~see Fig. 1for the optimized structures!, without symmetry constraintsnor other constraints, using standard settings for SCFoptimization procedure inGAUSSIAN 98 code. The obtainedTi–O distances for T1 model~1.810;1.811 Å for AE,1.793;1.794 Å for ECP(f ) and 1.810 Å for ECP(p) basisset! appear slightly shorter when compared with thoseported in Refs. 18 and 27 obtained with DFT methods~seeTable I! on the same system. Moreover, the average Tidistance~^Ti–O&! are very similar to each other,@1.810,1.793, and 1.810 Å for AE, ECP(f ) and ECP(p) respec-tively# and in basic agreement with the 1.79–1.81 Å vaobtained from EXAFS measurements on TS-1.11–13 ShorterTi–O distances are obtained on model T5 with AE~1.802 Å!,ECP(f ) ~1.790 Å! and ECP(p) ~1.803 Å! basis sets:Ti–O&distances~vide infraTable II! obtained in the three cases avery similar ~1.802, 1.790, and 1.803 Å! and still in goodagreement with the experimental values.11–13This result con-firms that Ti–O bond geometry is well reproduced, evwhen ECP on Ti are employed. Notice that our Ti–O dtances are slightly shorter with respect to those reporteRefs. 19 and 20 and that the difference is enhanced wrespect to the Ti–O distance obtained by Vayssilov and


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Santen22 ~Table I!. Notice that when Hartree–Fock Hamitonian and ECP(p) basis set are used, T5 model is charaterized by Ti–O bonds shorter~1.789 Å! with respect to DFT~B3-LYP in this case! results, as in Ref. 18.

a andb angles~see Sec. III D. and Fig. 3! in T1 and T5models are only slightly affected by basis-set effects. Inmodel, two sets of O–Ti–O angles can be always disguished~two near at 108° and four near at 110°!. Conversely,T5 model, where all six O–Ti–O angles are equivale~109.5°!, retains the nearly perfectTd symmetry. Ti–CM dis-tance~see Sec. III D. and Fig. 3!, showing ‘‘planarization’’ ofTiO3 group, is very similar in T1-T5 [email protected] and0.601 Å with ECP(p)# and varies slightly with different basis sets. In T5 model, Si–O distances of the four H3Si–Ogroups are all 1.658 Å with AE, ECP(f ), and ECP(p) basissets. This distances are similar to those obtained in Ref~1.645 Å!. Conversely, 1.68 and 1.69 Å Si–O distances habeen obtained in Refs. 20 and 22. A smaller value~1.644 Å!is obtained when Ti atoms is replaced by Si: in this ca^Si–O& in SiO4 moiety is 1.622 Å~about10.02 than experi-mental value!. From this is inferred that it is very difficult toestablish if Si–O bond elongation in H3Si–O group is agenuine effect due to the presence of Ti. Finally, the hTi–O–Si group flexibility ~similar to that of Si–O–Si! andthe complete absence of framework constraints leads toO–Si angles of 180°. Since EXAFS has shown two coupof Ti–O–Si angles at 14365°, and at 16265°, we can con-clude that the unconstrained T5 model is able to reprodcorrectly the local environment of Ti in TS-1, up to the fircoordination sphere only.

C. T1 – T5ÕH2O and T1 – T5 ÕNH3 complexes

BE, calculated from Eqs.~2! and ~3! for T1/H2O com-plex formation, vary on passing from AE and ECP(f ) toECP(p) basis sets ~from 136.4 kJ mol21 and 136.2kJ mol21 to 139.7 kJ mol21!. However, whenBSSE, @16.7,18.8, and110.6 kJ mol21 with AE, ECP(f ) and ECP(p),respectively# is accounted for, only the two AE and ECP(p)BEc slightly differ from each other~129.7 and 129.1kJ mol21, respectively! and are very close to the value reported in Ref. 27~see Table I!, being ECP(f ) BEc equal to126.2 kJ mol21. Ti–Ow distance for the optimized compleslightly varies on passing from AE to ECP(f ) and ECP(p)basis sets~from 2.41 Å to 2.43 and 2.40 Å, respectively!.

BE calculated for T1/NH3 is higher than with [email protected],145.7, and150.6 kJ mol21 with AE, ECP(f ), andECP(p), respectively#, suggesting a stronger interactioBSSEis estimated to be16.4, 18.8, and110.2 kJ mol21

for AE, ECP(f ), and ECP(p) basis sets, respectively. Fromthis, AE BEc differs for 11.7 kJ mol21 and 21.8 kJ mol21

with respect to ECP(f ) and ECP(p) BEc, respectively.Ti–Na distance for the optimized complexes is equivalentpassing from AE to ECP(f ) and ECP(p) basis sets~from2.30 Å to 2.30 and 2.29 Å, respectively!.

On passing to T5/(H2O,NH3) complexes~vide infraTable III for geometrical features of TiO4 moiety in the com-plexes! treated with ECP(p) basis set, Ti–Ow and Ti–Na

distances are longer with respect to those obtained for


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model ~2.43 and 2.33 Å, respectively! and the energetic othe interaction of Ti center toward H2O and NH3 moleculesis drastically changed: indeed, the two ECP(p) BEc are ofonly 12.1 kJ mol21 and117.4 kJ mol21, respectively~27.0and 23.0 kJ mol21 smaller than in T1 model!. BSSEis 19.9and19.1 kJ mol21 for H2O and NH3 complexes. The originof this dramatically different behavior clearly emerges froresults listed in Table IV. In fact, from calculatedDEA, theenergy cost for reaching equilibrium T1-T5 geometries in(H2O, NH3) complexes appears nearly the same for thesystems; conversely, the difference inBEcde f is remarkable,suggesting a much higher acidity of the deformed T1 syswith respect to deformed T5 such as to determine the gvariance inBEc. The same results~see Table IV! are ob-tained when AE basis set is employed for the T5/(H2O)complex (Ti–Ow52.46 Å). For the sake of comparison, interaction of T5 model with water has also been studied@HF/6-3111G(d,p),ECP(p)# level: calculatedBE(BEc)are of only 112.4~11.0! kJ mol21, with DEA

5132.0 kJ mol21 and Ti–Ow52.44 Å.Because of the greatDEA accompanying T5/H2O com-

plex formation, our resulting HF and B3-LYPBE and BEc

are very small, reproducing results obtained for monodrated complex reported in Ref. 20~see Table I! and suggest-ing that water adsorption at the Ti site is not energeticafavored. This contrasts with the experimental~IR, Raman,XANES, and EXAFS! evidence that water adsorb to Ti site~vide supraSec. IV A!. This suggests that unconstrained Tmodel, even if including also the Ti second shell, is nosuitable model of perfect Ti site in TS-1.

D. Bare CHA OT8 and MFI OT18 Ti site models:optimized geometries

On passing to embedded cluster models~T7-MFIIT18,see Fig. 1 for the optimized structures!, only the CHAIT8and MFIIT18 geometrical features will be discussed heredetail, these two being the most representative of perframeworks of chabazite and silicalite, respectively. Forsake of comparison, results obtained for T7, MFIIT14,MFIIT15, and MFIIT16 are listed in Table II. The optimizestructures of clusters CHAIT8 and MFIIT18 are reported in

TABLE IV. BE andBEc calculated from the adopted reaction scheme,Eqs. ~6–10!, for the ~T1 and T5!/H2O and ~T1 and T5!/NH3 complexesformation. In this caseA5T1 or T5 andB5H2O or NH3 . All the quantitiesare expressed in kJ mol21.

H2O

Model/basis set DEA DEB BEde f(BEcde f) BE(BEc)

T1/ECP~p! 125.6 10.31 165.5~155.0! 139.6~129.1!T5/ECP~p! 130.7 10.17 142.8~133.0! 111.9~12.1!T1/AE 124.5 10.33 161.2~154.5! 136.4~129.7!T5/AE 128.5 10.18 140.3~132.9! 111.6~14.2!

NH3


T1/ECP~p! 146.8 10.47 197.9~187.7! 150.6~140.4!T5/ECP~p! 142.9 10.02 169.3~160.3! 126.4~117.4!


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ncte

Fig. 1, while the average Ti–O distance in TiO4 , calculatedfor ECP(p) basis sets, can be obtained from Table II.previously discussed in Sec. IV B., cluster T5@ECP(p) andAE basis sets# has both first and second shells around Ti innearly perfectTd symmetry ~a5b5109.5°, see Fig. 3!,which is broken by the zeolitic framework constraintsmodels CHAIT8 ~where the threea andb angles lie in the108.7–113.8° and 105.3–110.1° intervals, respectively! andMFIIT18 ~110.4°,a,112.1° and 105.8°,b,109.1°! whenECP(p) basis set is employed. Similar results are obtainfor the CHAIT8 model when AE basis set is employed, bei109.0°,a,113.2° and 105.7°,b,109.9°. On passing tooptimized ECP(p) TiO4 moiety in the three T5, CHAIT8and MFIIT18 models, theTd symmetry of model T5 resultsin an unique first shell Ti–O distance~1.803 Å!; conversely,model CHAIT8 exhibits two couples of distances, at 1.8and 1.812 Å. Finally, model MFIIT18 shows 4 different dis-tances in the 1.796–1.815 Å range. Quite similar resultsobtained when AE basis set is employed: four identical Tidistances in T5 model at 1.802 Å and two couples of dtances at 1.810 and 1.812 Å in CHAIT8 model and fourdifferent distances in the 1.797–1.812 Å range in MFIIT16model. All these values agree well with EXAFS experimetal data, reporting the Ti–O distance in the 1.79–1.81interval11–13,15 ~with a typical error of60.01 Å! and withprevious theoretical studies18–22 on the Ti~OSiH3)4 cluster~1.798–1.830 Å range!.

Coming to the higher shells, model T5@ECP(p) and AEbasis sets# shows four linear equivalent Ti–O–Si moieties,while distortions are present in CHAIT8 with both AE andECP(p) basis sets and MFIIT18 model with ECP(p) basisset, where two couples of Ti–O–Si angles are present. Inmodel CHAIT8, the two couples are rather close, differinonly of about 4° with both basis sets, while in modMFIIT18 they are significantly different@20° with ECP(p)#,being the narrower one around 149° and the largeraround 169° in rather close agreement with the valuestained from EXAFS15 ~vide supraSec. IV A!. Furthermore,a satisfactory agreement is obtained for the two couplesTi–Si distances: 3.2660.02 and 3.3860.02 Å ~exper-imental!15 and 3.296–3.421 Å in model MFIIT18 withECP(p) basis set. Also model CHAIT8 gives reasonableTi–Si [email protected]–3.367 Å and 3.326–3.366 Å witECP(p) and AE basis sets, respectively#, however it is notable to reproduce the experimental splitting between sh~narrow! and long~large! distances~Ti–O–Siangles!. Theseresults, mimicking a portion of the chabazite framework, cbe compared with those obtained by Zicovich–Wilset al.25,26 which studied Ti-chabazite~Si/Ti51:1! using a pe-riodic approach with CRYSTAL code. They found Ti–O ditances, Ti–O–Siangles and Ti–Si distances in the 1.78–1.Å, 150–157°, and 3.28–3.34 Å ranges, respectively.

On the basis of this preliminary study on the bare cluters, it emerges that the second shell around Ti~IV ! is cor-rectly reproduced only after insertion of the@Ti~OSi!4# unitinside bigger clusters mimicking the zeolitic framework.

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E. CHAOT8-MFIOT18ÕH2O and CHA OT8-MFIOT18ÕNH3complexes

1. Optimized geometries

As in Sec. IV D, only the optimized structures oCHAIT8/(H2O,NH3) and MFIIT18/(H2O,NH3) complexeswill be discussed here for the sake of brevity~see Fig. 4, forthe optimized structures!. For the sake of comparison resulobtained for T7, MFIIT14, MFIIT15 and MFIIT16 are listedin Table III. The first evidence of our calculation on all cluters @ECP(p) basis set# is that the adsorption of both moecules causes an increase of the first shell Ti–O dista~Table III versus Table II!. The^Ti–O&, averaged on the foubonds, increases on both H2O and NH3 complexes with re-spect to bare clusters. This datum is in qualitative agreemwith the first shell EXAFS data.46–49In all cases we observthat ligand adsorption causes a~further! distortion from tet-

FIG. 4. Optimized MFIIT18/H2O and MFIIT18/NH3 complexes. Modelzone M is rendered with ball and sticks. White: H and Si atoms. Blackand N atoms. Gray: Ti atom.


es

nt

rahedral towards bipiramidal symmetry, as monitored bymodification ofa andb angles~see Sec. III D! which tend to120 and 90°, respectively. In all models, NH3 is slightlymore efficient in the distortion of the bare cluster. This faagrees well with the IR evidences showing that NH3 is moreefficient than H2O in the perturbation of the 960 cm21

band.49 The same holds for the reduction ofTd pre-edge peakin XANES spectra.49 Ligand adsorption also modifies thsecond shell environment as documented by the Ti–O–Siand Ti–Si columns of Table III. Coming to the equilibriumdistance of the ligand molecules~Ti– L, L5H2O, NH3! thetwo clusters gives a definitively shorter Ti–L in the case ofammonia~2.33;2.34 versus'2.40 Å, see Table III!, reflect-ing the stronger interaction. No appreciable differencesbe observed when AE basis set is employed~see Table III!.Notice that the Ti–N distances of crystalline compounwhere titanium is simultaneously coordinated to four oxygatoms lie in the 2.38–2.05 interval.52

2. Binding energy and basis-set superposition error(BSSE) calculation

ECP(p) BE0c obtained from Eq. ~15! for

CHAIT8/(H2O,NH3) and MFIIT18/(H2O,NH3) complexes

TABLE V. Contributions to the ONIOMBE0(BE0c) for (T7-MFIIT18)/

(H2O,NH3) complexes.HM is the binding energy calculated for the higlevel model; (LR-LM ) is the difference between the low level real systeand the low level model binding energies. For the sake of comparisonBE for T5/(H2O,NH3) complexes are listed. Numbers in parentheses arerespective quantities each corrected for theBSSE. All values are expressedin kJ mol21.

H2O

Model/basis set BE(BEc)

T5/ECP~p! 112.0~12.1!T5/AE 111.6~14.2!

H2O

Model/basis set HM (HMc) LR-LM (LRc-LMc) BE0(BE0c)

T7/ECP~p! 111.8~11.9! 14.6~13.6! 116.4~15.5!CHAIT8/ECP~p! 122.8~113.0! 12.2~10.6! 125.0~113.6!MFIIT14/ECP~p! 128.3~118.3! 16.0~11.4! 134.3~119.7!MFIIT15/ECP~p! 126.4~116.0! 16.4~12.2! 132.8~118.2!MFIIT16/ECP~p! 123.0~113.0! 16.8~12.5! 129.8~115.5!MFIIT18/ECP~p! 122.7~112.6! 18.5~14.3! 131.2~116.9!CHAIT8/AE 119.9~112.5! 12.4~10.8! 122.3~113.3!MFIIT16/AE 120.4~112.8! 16.7~12.6! 127.1~115.4!

NH3

Model/basis set BE(BEc)T5/ECP~p! 126.4~117.4!

NH3

Model/basis set HM (HMc) LR-LM (LRc-LMc) BE0(BE0c)

T7/ECP~p! 126.9~117.8! 110.1~19.6! 137.0~127.4!CHAIT8/ECP~p! 138.8~129.7! 14.1~13.1! 142.9~132.8!MFIIT14/ECP~p! 145.5~136.2! 16.0~13.2! 151.5~139.4!MFIIT15/ECP~p! 142.0~132.8! 15.8~12.3! 147.8~135.1!MFIIT16/ECP~p! 138.1~129.1! 17.4~14.0! 145.5~133.1!MFIIT18/ECP~p! 138.0~129.0! 19.7~16.4! 147.7~135.4!MFIIT16/AE 134.6~127.9! 17.2~13.7! 141.8~131.6!


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TABLE VI. HM (HMc) calculated from the adopted reaction scheme,@~see Eqs. ~6–13!# for the(T7-MFT18)/(H2O,NH3) complexes formation.DEHM (A,B) are the contributions from the high level modelthe total ONIOM deformation energy. In this caseA5M part of T7-MFIIT18 models andB5H2O or NH3 . Allthe quantities are expressed in kJ mol21.

H2O


T5/ECP~p! 130.7 10.17 142.8~133.0! 111.9~12.1!T5/AE 128.5 10.18 140.3~132.9! 111.6~14.2!

H2O

Model/basis set DEHMA DEHMB HMde f(HMcde f) HM (HMc)

T7/ECP~p! 130.0 10.26 142.1~132.2! 111.8~11.9!CHAIT8/ECP~p! 119.0 10.21 142.0~132.2! 122.8~113.0!MFIIT14/ECP~p! 117.4 10.31 146.0~136.0! 128.3~118.3!MFIIT15/ECP~p! 117.9 10.28 144.6~134.2! 126.4~116.0!MFIIT16/ECP~p! 117.7 10.24 141.0~130.9! 123.0~113.0!MFIIT18/ECP~p! 119.2 10.27 142.2~132.1! 122.7~112.6!CHAIT8/AE 118.7 10.21 138.8~131.4! 119.9~112.5!MFIIT16/AE 118.2 10.26 138.9~131.3! 120.4~112.8!

NH3


T5/ECP~p! 142.9 10.02 169.3~160.3! 126.4~117.4!

NH3

Model/basis set DEHMA DEHMB HMde f(HMcde f) HM (HMc)

T7/ECP~p! 146.4 10.03 173.3~164.2! 126.9~117.8!CHAIT8/ECP~p! 130.6 10.02 169.4~160.3! 138.8~129.7!MFIIT14/ECP~p! 124.9 10.03 170.4~161.1! 145.5~136.2!MFIIT15/ECP~p! 125.5 10.04 167.5~158.3! 142.0~132.8!MFIIT16/ECP~p! 125.8 10.03 163.9~154.9! 138.1~129.1!MFIIT18/ECP~p! 126.9 10.03 164.9~155.9! 138.0~129.0!MFIIT16/AE 127.0 10.03 161.6~154.9! 134.6~127.9!

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are listed in Table V, together with their components HMc

and (LRc-LM c). For the sake of comparison the full setdata obtained for all remaining Ti site mode~T5,T7,MFIIT14,MFIIT15,MFIIT16! is also listed, togethewith AE BEc calculated for T5/(H2O) and BE0

c

CHAIT8/(H2O) and MFIIT16/(H2O,NH3) complexes. Asalready observed for T1 and T5 models, interaction wNH3 is more energetic than with H2O, in full agreement withthe higher distortion caused on TiO4 moiety ~vide supraTable III!. The same holds for AE basis set on CHAIT8 andMFIIT16 models.

As can be seen from Table V, there is a remarkaincrement on passing from T5 ECP(p) BEc to BE0

c calcu-lated for CHAIT8, MFIIT14, MFIIT15, MFIIT16, andMFIIT18 water and NH3 adducts. The higherBE0

c has beenobtained for MFIIT14/(H2O,NH3) complexes. The analysiof calculatedHMc and (LRc-LMc) @the BE0

c componentsaccording to Eq. ~15!# for CHAIT8/(H2O,NH3) andMFIIT18/(H2O,NH3) complexes shows that the major cotribute to the observed increment comes fromHMc, beingcontributions from (LRc-LMc) not negligible~in particularalong the sequence MFIIT14, MFIIT15, MFIIT16 andMFIIT18! but less important~29.1% and 35.6% of the totaincrement in MFIIT18 water and ammonia adducts, respetively!. In CHAIT8 water and ammonia complexe


h

e

-

(LRc-LMc) amounts to 5.2% and 20.1% of the total incrment, respectively. It is worth noticing thatBEc obtained bySP~see Sec. III C, for its definition! calculations on CHAIT8model at (B3-LYP/ECP(p)) level are very close to theONIOM ones~114.3 and132.6 kJ mol21 for the water andNH3 complexes formation, respectively!. This suggests thata! the observed increment on CHAIT8 and MFIIT18 modelswith respect to T5 one is genuine and not due to simplifitions introduced by the ONIOM method; b! ‘‘long range’’interactions are well reproduced by (LRc-LMc),38 even ifcalculated at a lower level of theory. Finally, notice that Tmodel gives ECP(p) BE0

c which are strongly affected by(LRc-LMc) terms, beingHMc quite similar toBEc calcu-lated on T5 for both water and ammonia complexes.

In order to rule out the origin of the observed trendHMc on passing from T7/(H2O,NH3) to CHAIT8/(H2O,NH3) and MFIIT18/(H2O,NH3) adducts~for whichBSSE is quite similar: subtractHMc from HM values inTable V!, we have decomposedHMc in its components~HMcde f, DEHMA, andDEHMB!, according to Eqs.~6–13!.The results are listed in Table VI for all ONIOM. For thsake of comparisonBEc calculated for T5/(H2O,NH3) com-plexes are also reported in Table VI. Looking at data reporhere, it is evident that the increment in HMc on passing fromT7 to CHAIT8-MFIIT18 models is mainly due to the energ



Downloaded 19 Ju

TABLE VII. BE(BEc) andBE0(BE0c) calculated from the adopted reaction scheme, see Eqs.~6–13!, for the

(T5-MFIIT18)/(H2O,NH3) complexes formation.DE(A,B) are the deformation energies ofA5T5-MFIIT18models andB5H2O or NH3 . All the quantities are expressed in kJ mol21.

H2O


T5/ECP~p! 130.7 10.17 142.8~133.0! 111.9~12.1!T5/AE 128.5 10.18 140.3~132.9! 111.6~14.2!

H2O

Model/basis set DE0A DE0

B BE0de f(BE0

cde f) BE0(BE0c)

T7/ECP~p! 128.9 10.26 145.6~134.7! 116.4~15.5!CHAIT8/ECP~p! 122.0 10.21 147.2~135.8! 125.0~113.6!MFIIT14/ECP~p! 120.3 10.31 154.9~140.3! 134.3~119.7!MFIIT15/ECP~p! 120.4 10.28 153.5~138.9! 132.8~118.2!MFIIT16/ECP~p! 120.1 10.24 150.1~135.8! 129.8~115.5!MFIIT18/ECP~p! 120.4 10.27 151.9~137.6! 131.2~116.9!CHAIT8/AE 121.6 10.21 144.1~135.1! 122.3~113.3!MFIIT16/AE 120.7 10.26 148.1~136.4! 127.1~115.4!

NH3


T5/ECP~p! 142.9 10.02 169.3~160.3! 126.4~117.4!

NH3

Model/basis set DE0A DE0

B BE0de f(BE0

cde f) BE0(BE0c)

T7/ECP~p! 142.3 10.03 179.3~169.7! 137.0~127.4!CHAIT8/ECP~p! 134.4 10.02 177.3~167.2! 142.9~132.8!MFIIT14/ECP~p! 129.5 10.03 181.0~168.9! 151.5~139.4!MFIIT15/ECP~p! 131.3 10.04 179.1~166.4! 147.8~135.1!MFIIT16/ECP~p! 130.5 10.03 176.0~163.6! 145.5~133.1!MFIIT18/ECP~p! 130.0 10.03 177.7~165.4! 147.7~135.4!MFIIT16/AE 131.8 10.03 173.6~163.4! 141.8~131.6!

T

cree

hfte

r

th

lehe

ass-

fleare

tedinTiom-

ltumealct

hesterrfect

cost ~i.e., the deformation energyDEHMA! reduction re-quested to deform bare Ti~OSi!4 equilibrium geometry inorder to allow a coordination of water and ammonia toatom. The same holds when ECP(p) BEc components, cal-culated for T5/(H2O,NH3) adducts ~see Table VI!, andECP(p) HMc components, calculated for CHAIT8-MFIIT18models, are compared. In such cases the model zone Mincides always with T5 model. So we can infer that theactivity of Ti~OSiH3)4 moiety is enhanced mainly by thgeometrical deformation~vide supraTable II! induced by theembedding structure, which destabilize the Ti~OSiH3)4 moi-ety. This could be one of the possible reasons explaining wframework Ti species in zeolites are more active than graones. Calculations with AE basis set on T5/(H2O),CHAIT8/(H2O) and MFIIT18/(H2O) ~see Table VI! leads tothe same conclusions. ECP(p) BEc calculated forT5/(H2O,NH3) and ECP(p) BE0

c obtained forCHAIT8-MFIIT18/(H2O,NH3) adducts, together with theicomponents from Eqs.~6–13! are listed in Table VII, leadingto the same conclusions previously discussed. NoticeDEA obtained by SP~see Sec. III C, for its definition! calcu-lations on CHAIT8 model at (B3-LYP/ECP(p)) level arevery close to the ONIOM ones~118.8 and130.4 kJ mol21

for the water and NH3 complexes formation, respectively!.


i

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V. CONCLUSIONS AND FUTURE DEVELOPMENTS

The results reported in this work clearly show the roplayed by zeolitic framework constraints, in enhancing treactivity of Ti~IV ! centers towards both H2O and NH3

probes. In fact a remarkable increment is observed on ping from BEc ~for free T5! to BE0

c ~for embedded modelsCHAIT8-MFIIT18! caused by framework destabilization othe Ti~OSiH3)4 moiety. This could be one of the possibreasons explaining why framework Ti species in zeolitesmore active than grafted ones. The computedBE0

c are in the14–17 and 33–35 kJ mol21 ranges for H2O and NH3, re-spectively. Both sets of values’ results are underestimawhen compared with microcalorimetric data reportedRefs. 46, 48, and 49. However, owing to the very lowcontent of the materials and to the presence of other cpetitive adsorption sites~silanols and titanols! for both probemolecules, theDadsH extrapolated from the experimentadata are to be considered with care. The experimental dacan be hardly improved, owing to the absence of an idTS-1 material without silicon vacancies, where perfeTi~IV ! centers would be the only adsorption sites for tprobe molecules. On the computational ground, the cluapproach presented here could be affected by an impe


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constraints reproduction and by an underestimation of efof ‘‘long-range’’ interactions as a consequence of finitemensions of the employed models. In order to elucidatefundamental aspect, calculations by means ofab initio peri-odic approach are planned on Ti-chabazite with Ti/Si ratio1/11. It is worth noticing that the Ti–O distance of baMFIIT18 cluster and its modification in MFIIT18/(H2O,NH3) complexes are in good agreement with the fishell EXAFS data obtained on TS-1 and on Ti-b.11–13,15,46–49

A simulation of first and second shell EXAFS data, in tframe of the multiple scattering approach, is in progreThis study will give us the experimental modification of thTi–O–Si angles and of the Ti–Si distances upon waterammonia adsorption, to be compared with the computedues reported here. Moreover, MFIIT16 model is able to re-produce theDn undergone by the TS-1 960 cm21 centeredband upon interaction with H2O and NH3, as discussed inRefs. 49 and 51. The same holds for MFIIT18 model.

ACKNOWLEDGMENTS

This study has been supported by MURST COFIN20‘‘ Structure and reactivity of catalytic centers in zeolitic mterials’’ We are indebted with G. Spano`, and M. Ricci~Polimerieuropa Novara! and with P. Ugliengo, G. Ricchiard~University of Torino! and V. Bolis~University of Piemonteorientale! for fruitful discussions.

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52Values obtained from more than 20 compounds extracted from the Cbridge Structure Data Bank.


reactivity of ti (iv) sites in ti-zeolites: an embedded cluster approach

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