Mesoporous Titanosilicates Research Articles

XPS characterization of a synthetic Ti‐containing MFI zeolite framework: the titanosilicalites, TS‐1

AbstractFollowing a previous investigation on micro‐ and mesoporous titanosilicates, we have here deepened the XPS and X‐ray‐excited Auger electron spectroscopy (XAES) studies on the surface properties of microporous titanosilicalites‐1 (TS‐1) and reference silicalites‐1 (S‐1), both characterized by the MFI framework type. The aim was that of complementing the information already obtained on these compounds with other techniques such as X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), dielectric relaxation spectroscopy (DRS), electron spin resonance (ESR), and quantomechanical calculations. Notwithstanding the expected low photoelectron intensity due to the very low solubility of Ti (IV) ions in the MFI framework, an accurate XPS curve‐fitting procedure and the simultaneous use of the XPS and Auger signals (Wagner plot) have made possible, both qualitatively and quantitatively, the distinction of two Ti (IV) sites: the framework tetrahedral species—Ti (IV) substituting Si (IV)—and the Ti(IV) tetrahedral species grafted to surface hydroxyl groups. Both species are prone to convert into octahedrally coordinated Ti (IV) species as a consequence of interaction with water and other ligands. Copyright © 2008 John Wiley & Sons, Ltd.

Synthesis and characterization of visible-light absorbing ordered mesoporous titanosilicate incorporated with vanadium oxide

Ordered mesoporous titanosilicates (OMTs) showing UV–visible-light absorption were synthesized using SBA-15 ordered mesoporous silica as a host framework, where the incorporated vanadium oxide worked as a visible-light receptor. The structures and the chemical composition of the OMTs were investigated with powder X-ray diffraction, nitrogen physisorption isotherm, transmission electron microscope and X-ray photoelectron spectroscopy. Vanadium oxide nanoparticles are incorporated into the interconnecting mesopores of SBA-15 structure and the most of titanium dioxide is uniformly coated on SBA-15 as well as vanadium oxide surfaces. The presence of vanadium oxide nanoparticles in OMTs induced visible-light absorption of titanium oxide at the wavelength above 400 nm.

Evaluation of the thermal and mechanical stability of Si-MCM-41 and Ti-MCM-41 synthesised at room temperature

A study of thermal and mechanical stability of MCM-41 materials containing different titanium content, prepared by direct synthesis at ambient temperature and pressure, using tetraethoxysilane, titanium ethoxide and cationic surfactants (hexa- or octadecyltrimethylammonium bromide) is presented. The behaviour is compared with pure silica grades prepared by a similar procedure. The evolution of pore structure was investigated from 823 to 1373 K and it was found that all these materials have high thermal stability in air. Up to 1073 K the structural changes are not significant and the MCM-41 structure is still observed at 1173 K. The thermal stability is practically independent of the metal content (5 ⩽ Si/Ti ⩽ 100) and of the surfactant used in the synthesis, but, in comparison with pure silica grades, the mesoporous titanosilicates have a superior heat resistance. At all temperatures the structural changes are less pronounced for the Ti-MCM-41 and the complete collapse of the ordered mesopore structure occurs only at 1373 K while for the pure silica it occurs at 1273 K. Tetracoordinated titanium incorporated in the walls remains stable up to 1073 K and seems to be responsible for the enhancement of thermal stability, although extra walls titanium species may also contribute. Concerning mechanical stability in air, it was found that for all freshly calcined pure silica and titanium containing samples tested, the MCM-41 structure is still observed after the application of a unidirectional external pressure of 518 MPa and the collapse of the pore structure occurs at around 814 MPa.

Improvement in thermal stability and catalytic activity of titanium species in mesoporous titanosilicates by addition of ammonium salts

Mesoporous titanosilicates (MTS-9A) with thermally stable titanium species were synthesized by adding ammonium salts in the preparation procedure. Characterization results show that hexagonal mesoporous titanosilicates (MTS-9A) contain primary and secondary structural building units of zeolite in the framework, in a similar manner to MTS-9. The promoted condensation of Ti-containing silica network in MTS-9A leads to higher thermal stability of four-coordinated titanium species compared with MTS-9, as evidenced by UV–vis and 29Si NMR spectra. Catalytic tests indicate that MTS-9A samples show high activity in hydroxylation of both small (phenol) and bulky reactants (2,3,6-trimethylphenol) before and after calcination. Moreover, the recycled catalyst retains high conversion, confirming the high thermal stability of titanium species.

Epoxidation of unsaturated FAMEs obtained from vegetable source over Ti(IV)-grafted silica catalysts: A comparison between ordered and non-ordered mesoporous materials

The liquid-phase epoxidation of mixtures of fatty acid methyl esters (FAMEs) over titanium-containing silica materials, using tert-butylhydroperoxide (TBHP) as oxidant, is here reported. The mixtures were obtained from vegetable renewable source, i.e. from high-oleic sunflower oil, coriander oil, castor oil and soya-bean oil. The influence of the nature and the position of functional groups on the C-18 chain of the FAMEs was studied. Very high activity and selectivity were obtained in the epoxidation of castor and soya-bean oil methyl esters in a reaction medium free from organic acids. Ti–MCM-41 (an ordered mesoporous titanium-grafted silica) displayed in this case, for the first time, superior performances, from a synthetic point of view, with respect to non-ordered mesoporous titanosilicates.

Ordered Mesoporous Materials with Improved Stability and Catalytic Activity

Microporous crystals of zeolites such as Y, Beta, and ZSM-5 are widely used commercial catalysts, but their applications are strongly limited by their small pore sizes. Recent progress in solving this is used to ordered mesoporous materials such as MCM-41, HMS, and SBA-15. These mesoporous materials have pore diameters of 2.0–30 nm and exhibit good catalytic properties for the catalytic conversion of bulky reactants. However, when compared with microporous crystals of zeolites, the catalytic activity and hydrothermal stability are relatively low, which severely hinders their practical applications in industrial catalytic reactions such as petroleum cracking. The relatively low catalytic activity and hydrothermal stability can be attributed to the amorphous nature of the mesoporous walls. In this account, we systemically review the routes for improving catalytic activity and hydrothermal stability of mesoporous materials, which include (1) acidic sulfated zirconia supported in mesoporous materials; (2) strongly acidic and thermally stable mesostructured sulfated zirconia with tetragonal crystalline phase; (3) strongly acidic and hydrothermally stable mesoporous aluminosilicates synthesized in alkaline media; (4) strongly acidic and hydrothermally stable mesoporous aluminosilicates synthesized in strongly acidic media; (5) hydrothermally stable mesoporous titanosilicates with catalytically active titanium species in oxidations; (6) high-temperature generalized synthesis of ultrastable ordered mesoporous silica-based materials by using fluorocarbonhydrocarbon surfactant mixtures.

Synthesis and characterization of Al-, Bi-, and Fe-incorporated mesoporous titanosilicate (MPTS) materials and their hydrophilic properties

Mesoporous titanosilicates (MPTS) including Al, Bi, and Fe ions of three positive oxidation states (Metal 3+) were prepared using a common hydrothermal method. The XRD result indicated that these metals (10.0 wt.% per titania amount) are stably incorporated in a titanosilicate framework. The mesopore sizes were distributed in the range of 1.0–6.0 nm in TEM images, and the surface areas obtained were above 300 m 2/g in all metal-incorporated MPTSs. The dehydrated amount decreased in the order of Al- > Bi- > Fe- > non-metal MPTS, and the activation energy needed for H 2O desorption from the surface of photocatalyst decreased in the order of Al- > Fe- > Bi- > non-metal MPTS. The XPS result showed that the special peak for Ti2p in metal-MPTSs shifted to stronger binding energy than in non-metal MPTS. The O1s (Ti–O, Ti–OH) peaks for metal-MPTSs also shifted to stronger binding energy than those for non-metal MPTS. In particular, the shift to the larger binding energy for the second peak (Ti–OH) was remarkable in Al-MPTS. In H 2-TPR, Ti 3+ could be more easily transferred into Ti 0, which affected the photoreaction, over Al- and Fe-MPTSs compared with non-metal MPTS. The photoluminescence intensity from the contribution of emitted electron decreased in the order: non-metal, Al-, Fe-, and finally Bi-MPTSs. Consequently, these findings indicate that the super-hydrophilicity of Al-MPTS was the best, and that the contact angle for water droplets was below 3° under 120 min UV-radiation.

Three-Dimensional Mesoporous Titanosilicates Prepared by Modified Sol−Gel Method: Ideal Gold Catalyst Supports for Enhanced Propene Epoxidation

Mesoporous titanosilicates with 1-12 mol % Ti content and with three-dimensional wormhole-like mesoporosity are prepared by a modified sol-gel technique. Sorption analysis shows that there is little change in the surface properties with increasing Ti concentration in the samples, implying that Ti atoms either are well-dispersed on the walls of the silica matrix or are present inside the framework with no pore blocking effect. Spectroscopic analysis shows that the Ti atoms are atomically dispersed in the silica matrix even at very high Ti concentration and there is no observable Ti aggregate (anatase) present in the samples. These titanosilicate samples after Au deposition followed by trimethylsilylation (for enhanced hydrophobicity) are highly efficient catalysts for vapor-phase propene epoxidation using O2 and H2. It was possible to achieve commercially desirable performance with about 7% propene conversion, >90% propene oxide selectivity, and about 40% hydrogen efficiency.

Hydrothermally Stable and Catalytically Active Ordered Mesoporous Materials Assembled from Preformed Zeolite Nanoclusters

AbstractFor Abstract see ChemInform Abstract in Full Text.

An XPS study of microporous and mesoporous titanosilicates

AbstractIn this contribution we report on an XPS study of microporous and mesoporous titanosilicates, in particular microporous titanium silicalite TS‐1, ordered mesoporous Ti‐MCM‐41 and [Ti]‐MCM‐41 and amorphous mesoporous silica–titania (MST) catalysts.Our aim was to obtain both photoemission and x‐ray‐excited Auger data for Ti species on these catalysts and use them in a Ti Wagner plot to rationalize the dependence of the local electronic structure on the atomic environment. Isolated Ti(IV) species coordinated to four and six oxygen anions and segregated TiO2 clusters were detected on all catalysts by a curve‐fitting procedure of Ti 2p, O 1s and related peaks.The presence of the Si 2p peak excited by an O Kα ghost makes the detection of Ti LMM Auger transitions in mesoporous samples impossible due to the low Ti loadings and its homogeneous distribution in the silica matrix. Small TiO2 clusters are eventually segregated within the mesopores of the catalysts and not at their external surface. On TS‐1 microporous catalysts with similar Ti loadings to the mesoporous catalysts we were able to detect Ti LMM Auger transitions, and by the Ti Wagner plot we clearly identify the presence of octahedrally coordinated Ti(IV) species. Thus, it is suggested that on TS‐1 the in‐framework (O)4Ti species are easily changed to (O)4(H2O)2Ti species by insertion of water molecules from the atmosphere. Small TiO2 clusters (diameter <5 nm), eventually present on samples with Ti loading >2 wt.%, are segregated at their external surface and present spectroscopic features similar to (O)4(H2O)2Ti species. Copyright © 2004 John Wiley & Sons, Ltd.

Hydrothermally Stable and Catalytically Active Ordered Mesoporous Materials Assembled from Preformed Zeolite Nanoclusters

Microporous zeolites are widely used commercial catalysts, but their applications are intrinsically limited by their small channel diameters. Recent progress in solving this is used to ordered mesoporous materials such as MCM-41, HMS and SBA-15. These mesoporous materials have pore diameters of 30–60 A and exhibit catalytic properties for the catalytic conversion of bulky reactants, but unfortunately, when compared with microporous zeolites, the catalytic activity and hydrothermal stability are relatively low, which severely hinders their practical applications. The relatively low catalytic activity and hydrothermal stability can be attributed to the amorphous nature of the mesoporous walls. We review here that the assembly of preformed zeolite precursors with surfactants can synthesize a series of ordered mesoporous materials, which include (1) strongly acidic and hydrothermally stable mesoporous aluminosilicates synthesized in alkaline media; (2) strongly acidic and hydrothermally stable mesoporous aluminosilicates synthesized in strongly acidic media; (3) hydrothermally stable mesoporous titanosilicates with catalytically active titanium species in oxidations; (4) hydrothermally stable mesoporous ferrisilicates. This work would open a door for the industrial application of mesoporous materials as catalysts for large molecules.

Ordered mesoporous titanosilicates with better catalytically active titanium sites assembled from preformed titanosilicate precursors with zeolite building units in alkaline media

Mesostructured titanosilicates have been successfully synthesized from the assembly of preformed titanosilicate precursors with CTAB surfactant micelle in both alkaline media (MTS-5) and strongly acidic media (MTS-8). The samples were characterized by powder X-ray diffraction (XRD), nitrogen isotherms, 29Si MAS NMR, IR, and UV–visible spectroscopy. These materials exhibit highly hydrothermal stability, as compared with Ti-MCM-41. Characterization results indicate that these mesostructured titanosilicates of MTS-5 and MTS-8 have TS-1-like titanium species, and that the mesoporous walls contain the primary and secondary building units similar to those in microporous crystal of TS-1 zeolite. In catalytic hydroxylation of 2,3,6-trimethylphenol, MTS-5 and MTS-8 show higher activities in comparison with microporous crystal of TS-1 and mesoporous material of Ti-MCM-41. More importantly, MTS-5 assembled from preformed titanosilicate precursors in alkaline media has higher silica condensation degree, compared with MTS-8 synthesized in strongly acidic media. After heating at 550 °C for 4 h, MTS-5 basically remains its catalytic activities in hydroxylation of phenol and 2,3,6-trimethylphenol, indicating its higher stability of Ti species for calcination. In contrast, calcined MTS-8 shows much lower conversion than as-synthesized MTS-8.

Ordered mesoporous titanosilicates with catalytically stable and active four-coordinated titanium sites

The stable ordered mesoporous titanosilicate (Ti-JLU-20) has been successfully synthesized from an assembly of mixed surfactants (fluorocarbon and triblock copolymer surfactants) with preformed titanosilicate zeolite precursors at high temperature (180-220 degrees C), and catalytic tests show that Ti-JLU-20 has highly stable and active four-coordinated titanium sites in oxidations.

Effect of surface chemical properties and texture of mesoporous titanosilicates on direct vapor-phase epoxidation of propylene over Au catalysts at high reaction temperature

Two kinds of organically modified mesoporous titanosilicate supports, Ti-MCM-48- and Ti-MCM-41-type, with different surface chemical properties and texture were prepared by two-step and one-step synthesis, respectively. The direct vapor-phase epoxidation of propylene with O 2 and H 2 was performed over Au nanoparticles deposited on the above supports. Both the supports and the Au catalysts were characterized by XRD, UV-Vis, N 2 adsorption–desorption isotherms, FT-IR, ICP, TEM, and ADF-EELS. Over Au-deposited on trimethylsilylated Ti-MCM-48 supports, an optimum reaction temperature for the highest yield of propylene oxide (PO) while maintaining selectivity above 80% was 523 K. This temperature was 100 K higher than over the non-silylated Au catalyst. Catalyst deactivation with time-on-stream was appreciably depressed for the silylated ones, which could be explained by the change of surface property from hydrophilic to hydrophobic. Over Au-deposited on Ti-MCM-41-type amorphous supports, but not on pure Ti-MCM-41, excellent catalyst stability with time-on-stream was observed from the very early stage of reaction at 473 K. In terms of PO formation rate per unit weight of Au, Au/Ti-MCM-41-type amorphous support with a very low Au loading of 0.015 wt.% and Ti/Si molar ratio of 1/100 was found to be more active by one order of magnitude than Au/Ti-MCM-48 with a high Au loading of 0.57 wt.% and Ti/Si molar ratio of 2/100; the former also presented higher H 2 efficiency. Organic modification did not change this feature. FT-IR spectra showed strong adsorption of water even on the Au catalyst composed of trimethyl silylated Ti-MCM-48 and the partial regeneration of hydroxylated surfaces during reaction at 523 K. This may partly explain why the silylated catalysts still deactivate in the reaction mixture.

Role and effect of supercritical fluid extraction of template on the Ti(IV) active sites of Ti-MCM-41

Supercritical fluid (supercritical carbon dioxide mixed with methanol) has been used as an extraction medium for the removal of template for a series of titanosilicates. Mesoporous titanosilicates Ti-MCM-41 with variable Si/Ti ratio (200–25) have been prepared by hydrothermal method. The location and coordination of titanium in the calcined and extracted titanosilicate mesoporous molecular sieve was studied by FTIR, UV–Vis spectroscopy, photoluminescence spectra, 29Si NMR, combined with XRD and TG/DTA. Up to 97% template can be removed by extraction and the extracted material are hydrophobic in character. Removal of template by supercritical fluid extraction have no effect what so ever on the elemental composition of Ti-MCM-41 as well as it improves the structural ordering which was confirmed by XRD. Unequivocal evidences are presented to confirm the presence of same chemical environment of active titanium site after extraction, as present in as-synthesized material. It was also reported that, the entire extracted solid exhibited catalytic activity superior to that of the calcined product for epoxidation of cyclohexene because of the hydrophobic character and unchanged active framework titanium site.

ChemInform Abstract: Hydrothermally Stable Ordered Mesoporous Titanosilicates with Highly Active Catalytic Sites.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Titanium coordination in microporous and mesoporous oxide materials by monochromated X-ray photoelectron spectroscopy and X-ray Auger electron spectroscopy

In photoemission, the Auger electron energy, combined with the respective photoelectron energy, provides the so-called Auger parameter α′ which in the case of 3d transition metals, has been demonstrated to be influenced by the coordination state of the emitting atom. In the literature of electron spectroscopy for chemical analysis studies on titanosilicates, the Ti Auger parameter has typically not been reported. In this work, the X-ray photoelectron and the X-ray induced Auger electron spectra of a Ti silicalite (TS-1) and other crystalline mesoporous titanosilicates such as TiSBA-15 and TiMCM-41 have been measured. An α′ value of 882.3 eV, ∼10 eV higher than in TiO 2, has been found for part of the titanium in TS-1 and TiSBA-15 samples. Such a shift in the Auger parameter is attributed to tetrahedral coordination of Ti, substituting for Si atoms in the framework.

Hydrothermally stable ordered mesoporous titanosilicates with highly active catalytic sites.

Mesoporous titanosilicates (MTS-9) are successfully prepared in strong acidic media by a two-step synthesis. MTS-9 has an ordered hexagonal structure and exhibits superior hydrothermal stability and high catalytic activity for the oxidation of the small molecules of phenol and styrene and also of the bulky molecule of trimethylphenol.

Synthesis and Characterization of Titanium-Containing Mesoporous Silica by a Non-Hydrothermal Microwave Method

Abstract We report on a new method for preparing mesoporous silica and titanium-containing mesoporous silica using microwave (MW) synthesis under non-hydrothermal conditions. The materials were characterized by XRD, nitrogen adsorption, UV-vis and FTIR techniques, and compared with a sample prepared by the conventional method. A single peak at 2θ = 2.4° in the XRD pattern showed the formation of mesoporous silica. A typical isotherm, characteristic of a mesoporous material where pore filling occurred at p/p0 in the range of 0.2—0.3 was obtained for nitrogen sorption. UV-vis diffused reflectance spectra showed bands at 210—220 nm and a shoulder at 270 nm, indicating the presence of Si-O-Ti bonds for titanium-containing mesoporous silica. This new method has been observed to have inherent advantages over reported methods, such as instant start up, a short heating time and low cost. These mesoporous titanosilicates prepared using a microwave heat energy source are active in the oxidation of 2,6-diisopropylphenol (DIPP) and 2,6-di-t-butylphenol (DTBP) with hydrogen peroxide as an oxidant.

Crystallization of hydrophobic mesoporous titano-silicates useful as epoxidation catalysts

Hydrophobic mesoporous titano-silicates (HyMTS) have been prepared at ambient temperature by a sol–gel technique using hexadecyl-trimethyl-ammonium chloride in acid medium as templating surfactant. The gellation time increases for low reaction temperatures and low H +/Si ratios. The amorphous materials thus obtained at long gellation times show hydrophobic properties and a lower number of OH bearing Si atoms as determined from 29Si MAS NMR. In the epoxidation of norbornene with H 2O 2 or of cyclohexene with tert-butyl hydroperoxide, HyMTS reaches selectivities for the epoxide of 95% or higher. The epoxidation of cyclohexene by H 2O 2 also gives better selectivities to the epoxide on these hydrophobic samples, compared to hydrophilic solids. It is suggested that hydrophobicity results from the slow kinetics of gellation.