Ti Active Sites Research Articles

Assemblies of Ti Active Sites Over HMS Supports Containing Surface's OH or NH2Groups

Three types of HMS supports, which were as-synthesized, HMS(u), HMS(c) calcined from HMS(u), and HMS(m) prepared by the modification of HMS(c) with 3-amino propyltriethoxysilane, were first used to assemble the Ti catalytic sites through the exchange reactions between them and Ti compounds such as Ti tetraisoproxide (Ti(OiPr)4), Ti tetrachloride (TiCl4)) and their Ti tartrate complexes. The Ti-assembled HMS catalysts were investigated in detail by means of FT-IR, UV–vis reflection spectra, chemical and elemental analyses. It is found that in Ti(OiPr4 and especially TiCl4, the exchange that takes place with the surface hydroxyls of the HMS(c) is easier than with the surface NH2 groups of HMS(u) and HMS(m). Ti tartrate complex can be successfully assembled on the HMS(c) through three assembled pathways. The UV–vis reflection spectra indicates that the Ti species are highly dispersed upon the surface of HMS. In catalyzing epoxidation of styrene with TBHP, the above-described catalysts show a higher catalytic activity and selectivity for the epoxides than unloading Ti tartrate complex, and the Ti-substituted HMS catalysts. Their outstanding advantage is that they can be reused many times with little loss of activity.

Epoxidation of allylic alcohols with TiO2-SiO2: hydroxy-assisted mechanism and dynamic structural changes during reaction

Epoxidation of allylic alcohols and cyclohexene with TBHP and titania–silica aerogels containing 1 and 5 wt% TiO2 has been studied. For the oxidation of geraniol and cyclohexenol, the regio- and diastereoselectivities and kinetic data indicate an OH-assisted mechanism involving a dative bond between the OH group and the Ti site. This mechanism is disabled in the oxidation of cyclooctenol due to steric hindrance. The moderate regio- and diastereoselectivities of the aerogels, compared with those of TS-1 and the homogeneous model Ti(OSiMe3)4, are attributed to the presence of non-isolated Ti sites and to a “silanol-assisted” mechanism, according to which model the allylic alcohol is anchored to a neighboring SiOH group instead of the Ti-peroxo complex. Kinetic analysis of the initial transient period revealed rapid catalyst restructuring during the first few turnovers. A feasible explanation is the breaking of Si–O–Ti linkages of the carefully predried aerogels by water or TBHP, resulting in active Ti sites with remarkably different catalytic properties.

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.

Layered perovskites with giant spontaneous polarizations for nonvolatile memories.

A series of titanate-based layered perovskites having large values of the spontaneous polarization P(s) were developed for their applcations to nonvolatile ferroelectric random access memories. Among these, the Nd-modified bismuth titanate [Bi(4-x)Nd(x)Ti(3)O(12) (BNdT)] system exhibited the most remarkable ferroelectric properties. The c-axis oriented BNdT capacitor was characterized by a switchable remanent polarization 2P(r) of over 100 microC/cm(2) and imprinting and fatigue-free behavior. The active Ti site responsible for the giant P(s) was identified with the help of Rietveld analysis, x-ray absorption near-edge structure study, and ab initio quantum computations.

In situreal‐time environmental TEM of gas phase Ziegler–Natta catalytic polymerization of propylene

In situ gas phase polypropylene (PP) formation over heterogeneous ballmilled TiCl4‐MgCl2 Ziegler–Natta (Z–N) catalysts (containing no electron donors or selectivity control agents) has been observed for the first time in real time using environmental transmission electron microscopy (ETEM). The MgCl2 support and associated Z–N catalysts were characterized with electron diffraction and high‐resolution electron microscopy. The support was found to be composed of highly defective β‐MgCl2 nanocrystallites, 3 nm in size. Ball milling these supports in TiCl4 to create the pro‐catalyst increased the degree of disorder and resulted in a higher fraction of amorphous material. The surfaces of the catalysts consist of amorphous material, various terminations of β‐MgCl2, and other contaminant phases. The ETEM observations revealed initial nanoscale PP growth in the form of globules and continuous non‐uniform polymer layers. The growth rate was measured to be 0.2 nm s−1 at 1 Torr of C3H6 pressure. The continuous growth implies that the active Ti sites are located on both the crystalline and non‐crystalline regions of the MgCl2 surface. These active Ti sites appear to be located at the polymer/MgCl2 buried interface and there does not appear to be any significant diffusion limitation for polymer layers up to 150 nm in thickness. Our data and calculations show that only 1 in 105 collisions with the active site results in monomer insertion into the polymer chain.

Nature of Active Sites in Sol–Gel TiO2–SiO2 Epoxidation Catalysts

A series of titania–silica aerogels with 0–100 wt% TiO2 content were synthesized and characterized by N2 physisorption, DRIFT, UV-Vis, XPS, and 29Si CP/MAS NMR analysis. It is shown that kinetic analysis of the epoxidation of 2-cyclohexene-1-ol (1) with TBHP is an informative test reaction providing insight in the nature of active sites. The surface area, pore volume, hydrophobicity, and relative abundance of Ti–O–Si linkages in the aerogels decreased with increasing Ti/Si ratio. Parallel to these changes, the initial rate of epoxide formation per Ti site (TOF) and the epoxide selectivity decreased but the productivity of the catalysts went through a maximum at 10 wt% TiO2. We propose that due to kinetic effects in the sol–gel synthesis the whole range of active Ti sites may be present in the mixed oxides, spanning from tetrahedral Ti isolated by four SiO groups to octahedral Ti surrounded by six TiO groups in titania nanodomains. Ether formation from 1 was catalyzed by Brønsted sites present only on high titania-containing aerogels. Oligomerization was a major side reaction on all catalysts including Ti-free silica. Si-free titania was the most active in allylic oxidation of 1 to cyclohexenone. Silylation, or amine (Me2BuN) addition to the reaction mixture, eliminated ether formation and suppressed oligomerization.

Living-Like Polymerization of Propylene with Mixed Metallocene Catalyst Systems

Propylene polymerization was conducted with the Cp2ZrHCl/B(C6F5)3/[tBuNSiMe2Flu]TiMe2 catalyst system with AlOct3 as a scavenger at –50°C. The polymer obtained displayed a bimodal molar mass distribution. It could be confirmed that the polymer with higher M̄n was produced from Zr active sites and the polymer with lower M̄n resulted from Ti active sites. In both fractions, M̄n was increased linearly with increasing polymerization time. The MWD (M̄w/M̄n) values of each fraction were around 1.1. Thus, it could be said that propylene polymerization proceeded in a living manner even with zirconocene active species by using the mixed metallocene system. The living-like polymerization of propylene with Cp2ZrMe2/B(C6F5)3 /Cp*TiCl3 was also demonstrated at –50°C. Under the reaction between carbon monoxide (CO) and this living polypropylene (PP) at –78°C, it could be found that CO was quantitatively incorporated into living PP.

Elucidating the local environment of Ti(IV) active sites in Ti-MCM-48: a comparison between silylated and calcined catalysts

A mesoporous Ti-MCM-48 material was silylated using hexamethyldisilazane (HDMS) and a remarkable increase in the catalytic performances in the cyclohexene epoxidation with tert-butylhydroperoxide obtained. We report on a combined FTIR, diffuse reflectance UV–Vis–NIR, photoluminescence and 29Si MAS NMR study, augmented by TGA and XRD analysis, of the local environment of Ti sites in both calcined and silylated Ti-MCM-48 catalysts. Spectroscopic studies were also performed on the as-synthesised material and showed that both silanols and titanols are formed during the decomposition of the cetyltrimethylammonium hydroxide surfactant used to template the mesoporous structure. Unequivocal evidences that a further silica condensation of the MCM-48 framework structure occurs both during thermal treatments in vacuum and after calcination at 540°C are also reported. Upon silylation, a more hydrophobic surface is obtained where trimethylsilyl groups (Si(CH3)3) are anchored on to the silica surface by reaction of the silylating agent, HDMS, with silanols, and in addition, (SiO)3TiOH sites transformed into (SiO)3TiOSi(CH3)3. It is proposed that the more hydrophobic character of the silylated material prevents the deactivation of the catalyst by decreasing the amount of glycols which may be formed by reaction of the epoxide with water adsorbed on the surface of the Ti-MCM-48, and that poison the titanium catalytic centres.

Effect of potassium on the catalytic activity of TS-1

Titanium silicate-1 (TS-1) samples contained a significant amount of potassium derived from contaminants in an aqueous solution of Pr 4NOH. By washing with H 2SO 4, the alkali-exchanged sites were converted to sites with two OH groups or ‘closed’ Ti(OSi) 4 sites. Treatment of the acid-washed samples with an aqueous solution of K 2CO 3 led to the incorporation of potassium with a K/Ti ratio of around unity, suggesting that K ion exchange on TS-1 corresponds to the presence of Ti in the framework. The potassium ion exchange is considered to occur mainly on the Si–OH group adjacent to the ‘open’ Ti site. The presence of potassium retarded the oxidation of organic compounds with aqueous H 2O 2 to a different extent depending on the substrates. The retarding effect decreased in the order hexane>2-hexene>2-penten-1-ol and there was little inhibition in the oxidation of 2-penten-1-ol. This could be interpreted in terms of polarities of substrates and active sites in TS-1; non-polar hexane was less easily accessible to the Ti environment made more polar by adjacent potassium. In a similar manner, promotion of the oxidation of the alcoholic group in the reaction of 2-penten-1-ol over the K-type TS-1 might be ascribed to increased interaction of the alcoholic group with the active Ti site.

The preparation, molecular structure and catalytic relevance of Ti(OSiPh 3) 4 and Ti(OGePh 3) 4

The preparation, spectroscopic characterisation and molecular structures of the four-coordinate, tetrahedral and monomeric compounds Ti(OSiPh 3) 4 and Ti(OGePh 3) 4 are presented and discussed. The catalytic inactivity of both of these compounds may be rationalised on the basis of the steric inaccessibility of the substrates to the Ti(IV) active sites, which are effectively shrouded by the bulky triphenyl groups.

Syndiospecific polymerization of styrene using fluorinated indenyltitanium complexes

Nine new fluorinated half-sandwich titanocene complexes (1b-9b) based on substituted alkylindenes were synthesized, by reacting Me 3 SnF with the corresponding chloride species, and employed as catalyst precursors for the syndiospecific polymerization of styrene. When activated with methylaluminoxane (MAO), the new precursors 1b-9b exhibited increased activities by factors of 15-40 compared with the corresponding chlorinated compounds and provided improved syndiotacticity, enhanced melting temperature, and higher polymer molecular weights. The activities of indenyl and methyl- or phenyl-substituted indenyl complexes were found to be higher by factors of 4-12.5 than those of CpTiF 3 and Cp * TiF 3 . More importantly, the amount of MAOcan be reduced to an Al: Ti molar ratio of 300 in the temperature range of 10-90°C. It is likely that Ti-F, more polarized than the Ti-Cl bond in the half-sandwich titanocenes, allows the formation of more active and stable active sites of Ti(III) complexes needed for the syndiospecific polymerization of styrene. Evidence in this direction is brought via the electron paramagnetic resonance (EPR) spectrum and redox titration. The higher activity and syndiospecificity of the fluorinated catalysts are attributable to a greater number, more stable Ti(III) active sites, and/or higher propagation rate constant.

Structure–functionality relationships of grafted Ti-MCM41 silicas. Spectroscopic and catalytic studies

Detailed spectroscopic measurements have been used to elucidate the nature of Ti(IV)-centred active sites that were anchored to a mesoporous silica surface (MCM41) by two distinct routes from titanocene precursors. The catalyst prepared in dry argon (Ti-MCM41 [Ar]) is more active than that prepared in air (in the presence of water vapour) in its activity in the epoxidation of cyclohexene with tert-butylhydroperoxide (TBHP). The degree of loading of the titanium onto the silica support also influences the precise nature of the active sites. In Ti-MCM41 [Ar] samples containing less that 2 wt.% of Ti, the most abundant species are tetrahedrally bonded Ti(IV) active sites which absorb at 210–230 nm in diffuse reflectance (DR) UV–Vis spectra and exhibit an emission at 430 and 490 nm when excited with a 250 nm light. Samples of catalyst with greater than 4 wt.% loading are shown to contain TiO2-like microclusters. These species absorb at λ250 nm in the DR UV–Vis and emit very weakly in the 500–600 nm region. In the case of Ti-MCM41 [air] catalysts, which were prepared in the presence of atmospheric water at the MCM41 surface, even at the lowest Ti loading (e.g. ⩽0.5%) an incipient formation of oligomers occurs. It is proposed that dimers or very small oligomers which absorb at around 250 nm in the DR spectra are responsible for a very strong emission at around 500 nm in the photoluminescence spectra. The abundant presence of these species might well explain the lower catalytic performance of Ti-MCM41 [air] in comparison with that of Ti-MCM41 [Ar]. The anchoring of the Ti species at the surface silanol groups of MCM41 was in all cases followed by FTIR spectroscopy.

Probing the Titanium Sites in Ti−MCM41 by Diffuse Reflectance and Photoluminescence UV−Vis Spectroscopies

Ti−MCM41 catalysts prepared by anchoring titanocene onto the inner walls of MCM-41 exhibit high catalytic performance in the epoxidation of cyclohexene and in oxidation of bulky cyclic and aromatic compounds because of the presence, as revealed by means of XANES and EXAFS spectroscopy, of a high concentration of accessible, well-spaced, and structurally well-defined Ti tetrahedral active sites (see Maschmeyer et al. Nature, 1995, 378, 159). These results have prompted us to study the nature and the coordinative environment of Ti active sites in MCM-41 also by means of diffuse reflectance UV−visible and luminescence spectroscopy. We revealed, particularly by means of photoluminescence spectroscopy, the presence of more than one kind of tetrahedral titanium site.

Computational and EXAFS Study of the Nature of the Ti(IV) Active Sites in Mesoporous Titanosilicate Catalysts

First principles, nonlocal density functional theory (DFT) calculations within the cluster approximation have been carried out in order to model the formation of Ti active sites in the pores of mesoporous silicas by the reaction of (η5-C5H5)2TiCl2 with terminal surface silanols. In line with recent experimental observations (Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature, 1995, 378, 159), the calculations predict that, after calcination, the dominant surface Ti species is the (≡SiO)3TiOH complex. However, our results imply that there could be an appreciable concentration of the (≡SiO)2Ti(OH)2 complex present. The (≡SiO)2TiO species is energetically unfavorable. Also in line with experiment, our calculations predict that the dominant surface Ti complex is formed via a (≡SiO)3Ti(η5-C5H5) intermediate. Calculated structures of this Ti intermediate are not, however, in agreement with the experimental EXAFS structure, and it is suggested that the cause of this difference is the use of an oversimplified model during the EXAFS fit. Computational studies of the hydrolysis and hydration of Ti centers provide a possible rationale for the uncertainties in previous studies concerning the nature and coordination of such sites in materials synthesized by different methods and studied under a variety of conditions.

Ammoximation of Cyclic Ketones on TS-1 and Amorphous SiO2–TiO2

Ammoximation of cyclic ketones on TS-1 and amorphous SiO2–TiO2with H2O2and NH3was studied. The reactivity of cyclicketones over TS-1 and amorphous SiO2–TiO2showed the following order: cycloheptanone>cyclopentanone>cyclohexanone>cyclooctanone. Shape-selective poisoning studies revealed that the active sites of TS-1 for ammoximation of cycloheptanone are mostly inside the molecular sieve. The reactivity of cyclohexanone and methylcyclohexanones over TS-1 followed the order cyclohexanone>2-methylcyclohexanone=3-methylcyclohexanone>2,6-dimethylcyclohexanone, reflecting the difference in the diffusivity among the reactants or products inside TS-1. Besides the diffusivity the reactivity seemed to be affected by the difficulty in access of the carbonyl group to the Ti active site inside the zeolite; from the comparison of reactivity of dimethycyclohexanone isomers, the steric hindrance of the substituent methyl group to the access of carbonyl group was considered to decrease in the following order: β-equatorial>α-equatorial>β-axial>α-axial. Diffusion of cyclohexanone or its oxime has been found to play an important role in the ammoximation when the TS-1 crystals larger than 1.5 μm were employed as a catalyst. For the TS-1 crystals smaller than 0.3 μm, the ammoximation activity was independent of the diffusivity factor and was determined by the number of active Ti sites present in the zeolite framework.

Effect of Structure and Composition on Epoxidation of HexeneCatalyzed by Microporous and Mesoporous Ti–Si Mixed Oxides

A series of microporous titania–silica mixed oxides werecharacterized and tested as catalysts for the liquid-phaseepoxidation of 1-hexene witht-butyl hydroperoxide. Results from29Si MAS NMR spectroscopy verified results from earliercharacterization studies that indicated cohydrolysis of alkoxideprecursors produced well-mixed oxide samples. The catalyticactivity of the samples for hexene epoxidation at 353 Kincreased withincreasing silica content. Since the fraction of tetrahedral Tiatoms in the samples also increased with silica content, theactive site for the reaction is proposed to be a tetrahedrallycoordinated Ti atom in a silica matrix. Polar solvents likewater, acetone, and methanol inhibited the epoxidation reaction.To investigate the effect of pore size on activity, mesoporousTi–Si mixed oxides analogous to MCM-41 were synthesized. Themesoporous samples were the most active and selective catalystsfor epoxidation with TBHP, presumably due to the ease of accessof the reactants to the active Ti sites. Results from EXAFS andUV reflectance spectroscopy indicated that Ti atoms in themesoporous mixed oxides are tetrahedrally coordinated to oxygenatoms with the same Ti–O bond distance as TS-1. However, theactivities of our mesoporous samples are orders of magnitudelower than that of TS-1 for hexene epoxidation with aqueoushydrogen peroxide. Lower hydrophobicity of a silica mesopore(2–4 nm) compared to a TS-1 micropore (0.6 nm) may account forthe difference in activity observed in reactions with aqueoushydrogen peroxide.

Magnesium chloride supported high mileage catalysts for olefin polymerization. XII. Polymerization of ethylene

AbstractPolymerizations of ethylene by the MgCl2/ethylbenzoate/p‐cresol/AlEt3 TiCl4‐AlEt3/methyl‐p‐toluate (CW‐catalyst) have been studied. The initially formed active site concentration, [Ti] has a maximum value of 50% of total titanium at 50°C and lower values at other temperatures. The Ti decays rapidly to Ti sites with conc. ca. 10 mol %/mol Ti. The rate constants for four chain transfer processes have been obtained at 50°C: for transfer with AlEt3, k = 2.1 × 10−4 s−1 and k = 4.8 × 10−4 s−1; for transfer with monomer, k = 3.6 × 10−3 (M s)−1 and K = 8.3 × 10−3 (M s)−1; for β‐hydride transfer, k = 7.2 × 10−4 s−1 and k = 4.9 × 10−4 s−1; and transfer with hydrogen, k = 4.0 × 10−3 torr1/2 s− and k = 5.1 × 10−3 torr1/2 s−1. The rate constants for the termination assisted by hydrogen is k = 1.7 (M1/2 torr1/2 S)−1. If monomer is assisting termination as was observed for propylene polymerization, then k = 7.8 (M3/2 s)−1. Values of all the rate constants can be higher or lower at other temperatures. Detailed comparisons were made with the results of propylene polymerizations. There are more than four times as many Ti active sites for ethylene polymerization than there are for stereospecific polymerization of propylene; the difference is more than a factor of two for the Ti sites. Certain rate constants are nearly the same for both monomers while others are markedly different. Some of the differences can be explained by stereoelectronic effects.