The Siting of Ti in TS-1 Is Non-Random. Powder Neutron Diffraction Studies and Theoretical Calculations of TS-1 and FeS-1

Quantitative localisation of titanium in the framework of titanium silicalite-1 using anomalous X-ray powder diffraction

One of the biggest obstacles to developing better zeolite-based catalysts is the lack of methods for quantitatively locating light heteroatoms on the T-sites in zeolites. Titanium silicalite-1 (TS-1) is a Ti-bearing zeolite-type catalyst commonly used in partial oxidation reactions with H2O2, such as aromatic hydroxylation and olefin epoxidation. The reaction mechanism is controlled by the configuration of titanium sites replacing silicon in the zeolite framework, but these sites remain unknown, hindering a fundamental understanding of the reaction. This study quantitatively determines heteroatoms within the zeolite-type framework using anomalous X-ray powder diffraction (AXRD) and the changes in the titanium X-ray scattering factor near the Ti K-edge (4.96 keV). Two TS-1 samples, each with approximately 2 Ti atoms per unit cell, were examined. Half of the titanium atoms are primarily split between sites T3 and T9, with the remainder dispersed among various T-sites within both MFI-type frameworks. One structure showed significant non-framework titanium in the micropores of a more distorted lattice. In both samples, isolated titanium atoms were more prevalent than dinuclear species, which could only potentially arise at site T9, but with a significant energy penalty and were not detected.

Insights into the mechanism of the solvolysis of propylene oxide over titanium silicalite-1: a theoretical study

In order to probe into the mechanism of solvolysis (alcoholysis/hydrolysis) of propylene oxide (PO), the formation of propylene glycol (PG), 1-methoxy-2-propanol (PPM) and 2-methoxy-1-propanol (SPM) over the TS-1 catalyst with tetrahedral Ti and Ti/defect sites was systematically discussed using an embedded quantum mechanical/molecular mechanics (QM/MM) approach. The results showed that the activity of PO solvolysis is closely related to the ring-opening ability of active substances, and the ring-opening ability is in the following order: Si-O(H)-Ti > Ti-OH > 5MR Ti-OOH > Ti-OCH3 (tetrahedral Ti site); 3MR Ti-OOH > Ti-OH > 5MR Ti-OOH > Ti-OCH3 (Ti/defect site). At the tetrahedral site, the concerted mechanism is the dominant pathway for PO ring opening to form PPM, while a competitive relationship exists between stepwise and concerted mechanisms to form PG and SPM. Si-O(H)-Ti exhibits excellent PO ring-opening activity because of its strong Brønsted acidity, but it is difficult to form. At the Ti/defect site, the stepwise mechanism via PO ring opening with 3MR Ti-OOH and then successive hydrolysis/alcoholysis to form product is the dominant pathway. The overall energy barrier of the optimal route is relatively lower as compared to the tetrahedral Ti site. This work opens up a new path for providing more information on the detailed mechanism in the solvolysis of PO over the TS-1 catalyst from a theoretical point of view.

NMR Signatures and Electronic Structure of Ti Sites in Titanosilicalite-1 from Solid-State 47/49Ti NMR Spectroscopy

Although titanosilicalite-1 (TS-1) is among the most successful oxidation catalysts used in industry, its active site structure is still debated. Recent efforts have mostly focused on understanding the role of defect sites and extraframework Ti. Here, we report the ^47/49Ti signature of TS-1 and molecular analogues [Ti(OTBOS)₄] and [Ti(OTBOS)₃(OⁱPr)] using novel MAS CryoProbe to enhance the sensitivity. While the dehydrated TS-1 displays chemical shifts similar to those of molecular homologues, confirming the tetrahedral environment of Ti consistent with X-ray absorption spectroscopy, it is associated with a distribution of larger quadrupolar coupling constants, indicating an asymmetric environment. Detailed computational studies on cluster models highlights the high sensitivity of the NMR signatures (chemical shift and quadrupolar coupling constant) to small local structural changes. These calculations show that, while it will be difficult to distinguish mono- vs dinuclear sites, the sensitivity of the ^47/49Ti NMR signature should enable distinguishing the Ti location among specific T site positions.

Mechanistic Insight into Propylene Epoxidation with H2O2 over Titanium Silicalite-1: Effects of Zeolite Confinement and Solvent

Density functional theory (DFT) calculations were performed to investigate the effects of zeolite confinement and solvent on propylene epoxidation with H₂O₂ over the titanium silicalite-1 (TS-1) catalyst. The 144T and 143T cluster models containing typical 10MR channels of TS-1 were constructed to represent the tripodal(2I) and Ti/defect sites. It was found that the confinement of the zeolite pore channel not only impacts the adsorption stability of guest molecules but also alters reaction barriers, as compared to the results obtained based on small cluster models. When dispersion corrections were considered, an enhancement of the adsorption stability of guest molecules was observed because of the important contribution from van der Waals interactions, especially for propylene adsorption. An explicit protic methanol molecule was introduced into the catalytic system to probe the influence of the solvent on propylene epoxidation, based on which a significant enhancement of CH₃OH-H₂O₂ co-adsorption was obtained owing to H-bond formation. More importantly, the energy barrier for H₂O₂ dissociation was largely reduced by ∼13 kcal/mol because of the participation of the methanol in the H-transfer process and the formation of H-bond network, resulting in an alteration of the rate-limiting step. By comparison, adding an aprotic acetonitrile solvent did not have substantial effect on reaction path and kinetics. The calculation results clearly demonstrate the important role of the protic methanol solvent, which not only strengthens the adsorption of guest molecules but also promotes the kinetics for propylene epoxidation with H₂O₂ over TS-1 catalyst.

Ti-Based Catalysts and Photocatalysts: Characterization and Modeling

This perspective article aims to underline how cutting-edge synchrotron radiation spectroscopies such as extended X-ray absorption spectroscopy (EXAFS), X-ray absorption near edge structure (XANES), high resolution fluorescence detected (HRFD) XANES, X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) have played a key role in the structural and electronic characterization of Ti-based catalysts and photocatalysts, representing an important additional value to the outcomes of conventional laboratory spectroscopies (UV-Vis, IR, Raman, EPR, NMR etc.). Selected examples are taken from the authors research activity in the last two decades, covering both band-gap and shape engineered TiO₂ materials and microporous titanosilicates (ETS-10, TS-1 and Ti-AlPO-5). The relevance of the state of the art simulation techniques as a support for experiments interpretation is underlined for all the reported examples.

Porous metallosilicates for heterogeneous, liquid-phase catalysis: perspectives and pertaining challenges

Porous silicates containing dilute amounts of tri-, tetra- and penta-valent metal sites, such as TS-1, Sn-β and Fe-ZSM-5, have recently emerged as state of the art catalysts for a variety of sustainable chemical transformations. In contrast with their aluminosilicate cousins, which are widely employed throughout the refinery industry for gas-phase catalytic transformations, such metallosilicates have exhibited unprecedented levels of performance for a variety of liquid-phase catalytic processes, including the conversion of biomass to chemicals, and sustainable oxidation technologies with H2O2. However, despite their unique levels of performance for these new types of chemical transformations, increased utilization of these promising materials is complicated by several factors. For example, their utilization in a liquid, and often polar, medium hinders process intensification (scale-up, catalyst deactivation). Moreover, such materials do not generally exhibit the active-site homogeneity of conventional aluminosilicates, and they typically possess a wide variety of active-site ensembles, only some of which may be directly involved in the catalytic chemistry of interest. Consequently, mechanistic understanding of these catalysts remains relatively low, and competitive reactions are commonly observed. Accordingly, unified approaches towards developing more active, selective and stable porous metallosilicates have not yet been achieved. Drawing on some of the most recent literature in the field, the purpose of this mini review is both to highlight the breakthroughs made with regard to the use of porous metallosilicates as heterogeneous catalysts for liquid-phase processing, and to highlight the pertaining challenges that we, and others, aim to overcome during the forthcoming years.

Insight into the stereoselectivity of TS-1 in epoxidation ofcis/trans-2-hexene: a computational study

The mechanism of the stereoselectivity forcis/trans-2-hexene epoxidation in TS-1 zeolite was studied using density functional theory and the ONIOM scheme.

Modeling short-range substitution order and disorder in crystals: Application to the Ga/Si distribution in a natrolite zeolite

Atomic substitutions are a central feature of the physicochemical properties of an increasing number of solid-state materials. The complexity that this chemical disorder locally generates in otherwise crystalline solids poses a major challenge to the understanding of the relationships between the structure and properties of materials at the atomic and molecular level. Strategies designed to efficiently explore the ensemble of local chemical environments present in disordered crystals and predict their signatures in local spectroscopies such as solid-state nuclear magnetic resonance (NMR) are therefore essential. Focusing on the Ga/Si disorder in the framework of rubidium-exchanged gallosilicate natrolite zeolite (Rb-PST-1) with a high Ga content (SiGa=1.28), we show how the structure-generation approach implemented in the new program supercell (Okhotnikov et al. [26]) provides an excellent basis for the understanding of complex experimental spectroscopic data. Furthermore, we describe how exhaustive explorations of atomic configurations can be performed to seek local structural ordering and/or disordering factors. In the case of Rb-PST-1, we more specifically explore the possibility to form and to detect the presence of thermodynamically unfavorable Ga-O-Ga connectivities. While particularly adapted to the description of dense materials, we demonstrate that this approach may successfully be used to reproduce and interpret the distributions of local structural distortions (i.e., the geometrical disorder) resulting from the chemical disorder in systems as complex as microporous zeolites.

Lewis Acid Zeolites for Biomass Conversion: Perspectives and Challenges on Reactivity, Synthesis, and Stability

Zeolites containing Sn, Ti, Zr, Hf, Nb, or Ta heteroatoms are versatile catalysts for the activation and conversion of oxygenated molecules owing to the unique Lewis acid character of their tetrahedral metal sites. Through fluoride-mediated synthesis, hydrophobic Lewis acid zeolites can behave as water-tolerant catalysts, which has resulted in a recent surge of experimental and computational studies in the field of biomass conversion. However, many open questions still surround these materials, especially relating to the nature of their active sites. This lack of fundamental understanding is exemplified by the many dissonant results that have been described in recent literature reports. In this review, we use a molecular-based approach to provide insight into the relationship between the structure of the metal center and its reactivity toward different substrates, with the ultimate goal of providing a robust framework to understand the properties that have the strongest influence on catalytic performance for the conversion of oxygenates.

Local environments of boron heteroatoms in non-crystalline layered borosilicates

Boron heteroatom distributions are shown to be significantly different in two closely related layered borosilicates synthesized with subtly different alkylammonium surfactant species. The complicated order and disorder near framework boron sites in both borosilicates were characterized at the molecular level by using a combination of multi-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy techniques and first-principles calculations. Specifically, two-dimensional (2D) solid-state J-mediated (through-bond) (11)B{(29)Si} NMR analyses provide direct and local information on framework boron sites that are covalently bonded to silicon sites through bridging oxygen atoms. The resolution and identification of correlated signals from distinct (11)B-O-(29)Si site pairs reveal distinct distributions of boron heteroatoms in layered borosilicate frameworks synthesized with the different C16H33N(+)Me3 and C16H33N(+)Me2Et structure-directing surfactant species. The analyses establish that boron atoms are distributed non-selectively among different types of silicon sites in the layered C16H33N(+)Me3-directed borosilicate framework, whereas boron atoms are preferentially incorporated into incompletely condensed Q(3)-type sites in the C16H33N(+)Me2Et-directed borosilicate material. Interestingly, framework boron species appear to induce framework condensation of their next-nearest-neighbor silicon sites in the C16H33N(+)Me3-directed borosilicate. By comparison, the incorporation of boron atoms is found to preserve the topology of the C16H33N(+)Me2Et-directed borosilicate frameworks. The differences in boron site distributions and local boron-induced structural transformations for the two surfactant-directed borosilicates appear to be due to different extents of cross-linking of the siliceous frameworks. The molecular-level insights are supported by density functional theory (DFT) calculations, which show the distinct influences of boron atoms on the C16H33N(+)Me3- and C16H33N(+)Me2Et-directed borosilicate frameworks, consistent with the experimental observations.

Design of Lewis-acid centres in zeolitic matrices for the conversion of renewables

We review preparative approaches to introduce Lewis-acid centres in zeolites, underlining the structure–performance relationship and highlighting directions to design improved materials of practical relevance.

Valence to core X-ray emission spectroscopy

This Progress Report discusses the chemical sensitivity of Kβ valence to core X-ray emission spectroscopy (vtc-XES) and its applications for investigating 3d-transition-metal based materials. Vtc-XES can be used for ligand identification and for the characterization of the valence electronic levels. The technique provides information that is similar to valence band photoemission spectroscopy but the sample environment can be chosen freely and thus allows measurements in presence of gases and liquids and it can be applied for measurements under in situ/operando or extreme conditions. The theoretical basis of the technique is presented using a one-electron approach and the vtc-XES spectral features are interpreted using ground state density functional theory calculations. Some recent results obtained by vtc-XES in various scientific fields are discussed to demonstrate the potential and future applications of this technique. Resonant X-ray emission spectroscopy is briefly introduced with some applications for the study of 3d and 5d-transition-metal based systems.

Metal site-mediated, thermally induced structural changes in Cr6+-silicalite-2 (MEL) molecular sieves

Cr(6+) ions were incorporated into the lattice sites of phase-pure silicalite-2 made using 3,5-dimethylpiperidinium as a structure-directing agent. The materials exhibited a remarkably well-resolved vibronic emission consisting of a high frequency progression of 987 cm(-1), which was assigned to the fundamental symmetric stretching mode of the (Si-O-)(2)Cr(═O)(2) group dominated by the terminal Cr═O stretch. A low frequency progression at 214 cm(-1), which was assigned to a symmetric O-Cr-O bending mode, was built on each band of the 987 cm(-1) progression. Studies of the vibronic structure of the emission spectrum as a function of temperature and Cr ion concentration reveal an abrupt change in the Franck-Condon factor of the emission at 20 K for samples with very low Cr concentrations (0.03 mol %). The change in the Franck-Condon factor is attributed to a temperature-induced structural change in the coordination sphere of the metal ion. This structural change was found to be accompanied by a concomitant structural change in the lattice structure of the silicalite-2. This structural change, as studied by temperature-dependent X-ray diffraction, did not involve a crystallographic phase change but an abrupt decrease in the unit cell volume, caused specifically by a decrease in the c-axis. This structural change was not observed in pure silicalite-2, indicating that it is not intrinsic to the silicalite lattice. Moreover, no similar structural change was observed at higher Cr loading (1 mol %). This suggests that the presence of the Cr ions and the changes in the coordination geometry they undergo at low temperature induced the observed contraction in the silicalite-2 lattice, in effect acting as a thermal switch that decreases the unit cell volume.

Investigation of the valence electronic states of Ti(IV) in Ti silicalite-1 coupling X-ray emission spectroscopy and density functional calculations

We present an application of valence to core X-ray emission spectroscopy to understand the electronic structure of the industrially relevant catalyst titanium silicalite-1. The experimental spectrum was modelled within density functional theory, adopting a one electron approach, investigating the effects of different basis sets, density functionals and cluster sizes. The description of titanium silicalite-1 valence states follows the Kohn-Sham evaluation of the molecular orbitals involved in the computed transitions.

Location of Si vacancies and [Ti(OSi)4] and [Ti(OSi)3OH] sites in the MFI framework: a large cluster and full ab initio study

Titanium silicalite-1 (TS-1) is an important catalyst for selective oxidation reactions. However, the nature and structure of the active sites and the mechanistic details of the catalytic reactions over TS-1 have not been well-understood, leaving a continuous debate on the genesis of active sites on the TS-1 surface in the literature. In this work, the location of Si vacancies and [Ti(OSi)(4)] and [Ti(OSi)(3)OH] sites in the MFI (Framework Type Code of ZSM-5 (Zeolite Socony Mobile-Five)) framework has been studied using a full ab initio method with 40T clusters with a Si:Ti molar ratio of 39:1. It was shown that the former four energetically favorable sites for Si vacancies are T6, T12, T4, and T8 and for Ti centers of [Ti(OSi)(4)] are T10, T4, T8 and T11, being partially the same sites. Whether by replacing Si vacancies or substituting the fully coordinated Si sites, the most preferential site for Ti is T10, which indicates that the insertion mechanism does not affect the favorable sites of Ti in the MFI lattice. For the defective [Ti(OSi)(3)OH] sites, it was found that the Si vacancy at T6 with a Ti at its neighboring T9 site (T6-def-T9-Ti pair) is the most energetically favorable one, followed by a T6-def-T5-Ti pair with a small energy gap. These findings are significant to elucidate the nature of the active sites and the mechanism of reactions catalyzed by TS-1 and to design the TS-1 catalyst.

Density functional calculations on the distribution, acidity, and catalysis of Ti(IV) and Ti(III) ions in MCM-22 zeolite

Isolated Ti species in zeolites show unique catalytic activities for a variety of chemical reactions. In this work, density functional calculations were used to explore three current concerns: 1) the distributions of Ti(IV) and Ti(III) ions in the MCM-22 zeolite; 2) the Lewis acidity of the Ti(IV) and Ti(III) sites; and 3) activation of alkane C-H bonds by photocatalysis with Ti-doped zeolites. Neither the Ti(IV) nor Ti(III) ions are randomly distributed in the MCM-22 zeolite. The orders of relative stability are very close for the eight Ti(IV) and Ti(III) sites, and the T3 site is the most probable in both cases. The wavelengths for Ti(IV)-Ti(III) excitations were calculated to lie in the range λ=246.9-290.2 nm. The Ti3(IV) site shows Lewis acidity toward NH(3) in two different modes, and these two modes can coexist with each other. The calculated Ti(IV) coordination numbers, Ti(IV)-O bond elongations, and charge transfers caused by NH(3) adsorption are in good agreement with previous results. Similarly, two different NH(3) adsorption modes exist for the Ti3(III) site; the site that exhibits radical transfer from the lattice O to N atoms is preferred due to the higher adsorption energy. This indicates that the Ti3(III) site does not show Lewis acidity, in contrast to the Ti3(IV) site. At the Ti3(III) site, the energy barrier for activating the methane C-H bond was calculated to be 33.3 kJ mol(-1) and is greatly reduced by replacing the hydrogen atoms with methyl groups. In addition, the reactivity is improved when switching from MCM-22 to TS-1 zeolite. The studies on the various Ti species reveal that lattice O atoms rather than Ti(III) radicals are crucial to the activation of alkane C-H bonds. This work provides new insights into and aids understanding of the catalysis by isolated Ti species in zeolites.

TS-1 from first principles

First principles studies on periodic TS-1 models at Ti content corresponding to 1.35% and 2.7% in weight of TiO(2) are presented. The problem of Ti preferential siting is addressed by using realistic models corresponding to the TS-1 unit cell [TiSi(95)O(192)] and adopting for the first time a periodic DFT approach, thus providing an energy scale for Ti in the different crystallographic sites in nondefective TS-1. The structure with Ti in site T3 is the most stable, followed by T4 (+0.3 kcal/mol); the less stable structure, corresponding to Ti in T1, is 5.6 kcal/mol higher in energy. The work has been extended to investigate models with two Ti's per unit cell [Ti(2)Si(94)O(192)] (2.7%). The possible existence of Ti-O-Ti bridges, formed by two corner-sharing TiO(4) tetrahedra, is discussed. By using cluster models cut from the optimized periodic DFT structures, both vibrational (DFT) and electronic excitation spectra (TDDFT) have been calculated and favorably compared with the experimental data available on TS-1. Interesting features emerged from excitation spectra: (i) Isolated tetrahedral Ti sites show a Beer-Lambert behavior, with absorption intensity proportional to concentration. Such a behavior is gradually lost when two Ti's occupy sites close to each other. (ii) The UV-vis absorption in the 200-250 nm region can be associated with transitions from occupied states delocalized on the framework oxygens to empty d states localized on Ti. Such extended-states-to-local-states transitions may help the interpretation of the photovoltaic activity recently detected in Ti zeolites.

Determining the aluminium occupancy on the active T-sites in zeolites using X-ray standing waves

Zeolites are microporous crystalline materials that find wide application in industry, for example, as catalysts and gas separators, and in our daily life, for example, as adsorbents or as ion exchangers in laundry detergents. The tetrahedrally coordinated silicon and aluminium atoms in the zeolite unit cell occupy the so-called crystallographic T-sites. Besides their pore size, the occupation of specific T-sites by the aluminium atoms determines the performance of the zeolites. Despite its importance, the distribution of aluminium over the crystallographic T-sites remains one of the most challenging, unresolved issues in zeolite science. Here, we report how to determine unambiguously and directly the distribution of aluminium in zeolites by means of the X-ray standing wave technique using brilliant, focused X-rays from a third-generation synchrotron source. We report in detail the analysis of the aluminium distribution in scolecite, which demonstrates how the aluminium occupancy in zeolites can systematically be determined.

Reactivity of Ti(iv) species hosted in TS-1 towards H2O2–H2O solutions investigated by ab initio cluster and periodic approaches combined with experimental XANES and EXAFS data: a review and new highlights

This work is intended to underline how the most-advanced experimental and theoretical physical chemistry tools must be used synergistically to understand the reactivity of Ti-silicalite-1 (TS-1) in an important number of low-temperature oxidation reactions with aqueous H(2)O(2) as the oxidant. Literature results are carefully reviewed and accompanied with new, unpublished highlights of both experimental and computational origin. The first part of this work is devoted to a discussion of the defective nature of the material and to the synergic role played by Si vacancies and Ti insertion in the framework. A summary of the experimental Ti-K-edge EXAFS and XANES literature concerning the activated material in vacuo conditions is then presented and compared to the corresponding Ti geometries obtained by ab initio calculations. From such a comparison, the excellent agreement between experiment and theory is evident. A very good agreement is also obtained for the interaction with water and ammonia. For both H(2)O and NH(3), the agreement is due to the possibility to perform experiments in which the probe molecule is dosed from the gas phase, thus allowing to reach the 1 : 1 (or 1 : 2) ratio between the adsorbing Ti sites and the adsorbed molecule. Then, interaction with hydrogen peroxide is discussed, underlining the problems faced in reaching a common view between experimental and theoretical results, owing to the difficulties both in performing experiments where anhydrous H(2)O(2) is dosed on TS-1, and in taking into account the role played by the aqueous medium in the reactivity of Ti(iv) centres toward H(2)O(2) using ab initio calculations. Only once such difficulties have been overcome, by increasing the complexities of both experiments and ab initio models, is a joint-view finally obtained. Where needed, comparison with other experimental results (X-ray and neutron diffraction, NMR, IR, Raman, UV-vis and resonant Raman) is made.

Comparison of all sites for Ti substitution in zeolite TS-1 by an accurate embedded-cluster method

We studied the preferential location of Ti centers in the framework of the Ti-containing MFI zeolite TS-1 using a hybrid DFT/MM embedding method developed recently. This "covalent elastic polarizable environment" (covEPE) cluster embedding allows a complete and self-consistent treatment of solid covalent systems such as zeolites. For the present study, we used a gradient-corrected density functional approach. The resulting structural features of both Si- and Ti-substituted forms of the zeolite framework fit well with available experimental information. The calculated substitution energy of Ti at the 12 crystallographically different tetrahedral sites of the MFI structure vary within 19 kJ/mol with T12 and T2 as most and least preferred sites, respectively. On the basis of these computational results and the preferential sites for Ti substitution reported from different experimental investigations, we concluded that the Ti distribution in the TS-1 framework is not governed by the thermodynamic stability of the pure material.

Ti Coordination in Titanium Silicalite-1

Progressive isomorphous incorporation of TiIV (or BIII) heteroatoms into the MFI structure of as-synthesized silicalite-1 caused a decrease in the amount of siloxy groups (anions), requisite for counter-balancing the structural directing agent (cation), as determined using 1H MAS NMR to quantify the silanol protons H-bonded to the siloxy oxygen. This revealed the negative charge on the incorporated heteroatoms, identifying them as TiO5 (or BO4) sites.

Uniform catalytic site in Sn-beta-zeolite determined using X-ray absorption fine structure

The Sn silicate zeolite, Sn-beta, has been shown to be an efficient, selective heterogeneous catalyst for Baeyer-Villiger oxidations. Using primarily a multishell fit to extended X-ray absorption fine structure (EXAFS) data, we show that the Sn does not randomly insert into the beta-zeolite structure but rather occupies identical, specific, crystallographic sites. These sites are the T5/T6 sites in the six-membered rings. Moreover, the Sn is substituted in pairs on opposite sides of these six-membered rings. We believe that it is the specific, uniform crystallographic location of the Sn in the beta crystal structure that leads to sites with uniform catalytic activity, and consequently to the high chemical selectivity demonstrated for this catalyst. This manifests itself in the almost enzyme-like selectivity of this catalyst in Baeyer-Villiger oxidations. This uniform site distribution of the Sn suggests that there is likely a symbiotic relationship between the structure-directing agent in the zeolite synthesis and the Sn heteroatoms during the framework formation.

Template burning inside TS-1 and Fe-MFI molecular sieves: an in situ XRPD study

The high X-ray flux available at the European Synchrotron Radiation Facility (ESRF), combined with the use of a suitably designed area detector setup, allowed us to follow in real time the structural changes occurring during the template burning processes inside TS-1 and Fe-silicalite MFI zeolites with a X-ray powder diffraction technique (XRPD). Rietveld analysis of the XRPD patterns collected in the 350-1000 K interval, integrated each 15 K, yields to the determination of the template overall occupancy factor versus T with an accuracy comparable with that obtained by thermogravimetric measurements, routinely employed for this purpose. The evolution of the structural parameters (V, a, b, c, site occupancy factor of the template molecule) vs T has been obtained. These data allow us to have, for the first time, a complete view of the structural rearrangements induced by the template burning process on the zeolitic framework. The differences caused by the different heteroatom inserted in the MFI lattice (Ti or Fe) are discussed. For both TS-1 and Fe-MFI, the kinetics of the reaction were investigated, to obtain the activation energy of the calcinations process employing the nonisothermal data according to the theory recently proposed by Kennedy and Clark [Thermochim. Acta, 1997, 307, 27-35]. For TS-1 only, the time-resolved template burning experiment has been repeated in isothermal conditions at four different temperatures, to obtain the activation energy from isothermal data, according to the standard procedure. Comparison between Arrhenius plots obtained from isothermal and nonisothermal data demonstrates that the Kennedy and Clark method can be also applied to complex materials such as the MFI zeolites. This approach, when applied to time-resolved XRPD studies, is much less time consuming (requesting, in principle, one single nonisothermal run) with respect to the classic approach, which requests at least three isothermal runs. Moreover, it allows a remarkably lower associated error (151 +/- 11 versus 146 +/- 30 kJ mol(-)(1)) due to the much higher number of experimental points employed to perform the linear fit.