Framework Titanium Research Articles

Comparative Study of Titanium-modified HMS Mesoporous Materials via Different Preparing Methods

The current work is focused on the testing of titanium modified hexagonal mesoporous silica (HMS) for catalysts in epoxidation of cyclohexene. Two samples were prepared via the chemical liquid deposition (CLD) and the chemical vapor deposition (CVD), namely Ti/HMS-L and Ti/HMS-V, respectively. HMS and Ti/HMS were characterized by XRD, N2-adsorption, ICP-AES, UV-Vis. The samples were also evaluated by the epoxidation of cyclohexene with cumene hydroperoxide (CHP) as oxidant. It is revealed that Ti/HMS samples possess typical hexagonal mesoporous structure in which most of titanium species exist in the form of framework tetracoordinated state. Meanwhile, Ti/HMS-V is more seriously affected than Ti/HMS-L since the former was prepared at higher temperature. Ti/HMS-V gives more excellent catalytic performance than Ti/HMS-L, which is likely because the former has more isolated and framework titanium species. Either Ti/HMS-V or Ti/HMS-L can be used only 1 time in epoxidation experiment.

Effect of TS-1 Treatment by Mixed Alkaline on Propylene Epoxidation

Titanium silicate-1 (TS-1) was treated with a mixed alkaline of tetrapropyl ammonium hydroxide (TPAOH) and NaOH. It was characterized by XRD, nitrogen physical adsorption, SEM, FT-IR, UV–Vis and ICP-OES, and studied in propylene epoxidation. The mixed alkaline treatment with TPAOH/NaOH solution did not destroy the MFI structure of TS-1. With increasing NaOH concentrations, the relative crystallinity and the framework titanium decreased to some extent while the mesopore volume, mesopore diameter, and extra-framework titanium increased appreciably. When NaOH concentration was 0.0333 mol L−1, the best catalytic performance was obtained.

On the synthesis and performance of hierarchical nanoporous TS-1 catalysts

Hierarchical TS-1 zeolite was successfully prepared using chitosan as a sacrificial template. The X-ray diffraction showed that the presence of chitosan with the synthesis precursor had no deleterious effect on the crystallinity and phase purity of this zeolite. X-ray absorption spectroscopy at the Ti K-edge, FTIR and Raman spectroscopies revealed the titanium ions in the zeolite structure have predominantly tetrahedral coordination. However, it appears that the higher chitosan content in the synthesis gel imparted some hydrophilic character to the TS-1 system. Furthermore, the technique adopted for the preparation of the synthesis gel – e.g partially dried or fully dried – appears to affect the amount of framework titanium in the zeolite structure. The calcined form of the chitosan templated TS-1 zeolites exhibited higher cyclohexene conversion compared to the TS-1 material synthesised without this template, but these catalysts showed lower selectivity for cyclohexene epoxide.

Structure and catalytic activity of a newly proposed titanium species in a Ti-YNU-1 zeolite: a density functional theory study

Density functional theory was applied to investigate the structure of the framework titanium (Ti) species in the Ti-YNU-1 zeolite, and to evaluate its catalytic activity for 1-hexene epoxidation with H2O2 as the oxidant.

Magnetic metal–organic framework–titanium dioxide nanocomposite as adsorbent in the magnetic solid-phase extraction of fungicides from environmental water samples

In this work, a core-shell Fe3O4@SiO2@MOF/TiO2 nanocomposite was synthesized and used to as adsorbent for magnetic solid-phase extraction (MSPE) of triazole fungicides from environmental water samples. Five triazole fungicides, namely, triadimenol, hexaconazole, diniconazole, myclobutanil, and tebuconazole, were selected as target analytes for MSPE. These analytes were quantitatively adsorbed on microspheres, and the sorbents were separated from the solution by using a magnet. The analytes were desorbed by methanol and determined through liquid-chromatography coupled with tandem mass spectrometry. The extraction parameters affecting the extraction efficiency were optimized through response surface methodology. The limits of detection and limits of quantification for the selected fungicides were 0.19–1.20ngL−1 and 0.61–3.62ngL−1, respectively. The proposed method was applied to determine the concentration of fungicides in actual environmental water samples. The accuracy of the proposed method was evaluated by measuring the recovery of the spiked samples. The satisfying recoveries of the four water samples ranged from 90.2% to 104.2%. Therefore, the magnetic metal–organic framework/TiO2 nanocomposite based MSPE is a potential approach to analyze fungicides in actual water samples.

Synthesis of mesoporous titanium silicalite-1 with high stability in cyclohexanone ammoximation

Mesoporous titanium silicalite-1(TS-1)was hydrothermally synthesized with the addition of triethanolamine( TEA)in the conventional process, and used in the cyclohexanone ammoximation in a continuous slurry reactor. The as-prepared TS-1 was characterized with X-ray diffraction(XRD), scanning electron microcopy( SEM), N2 adsorption-desorption, Fourier transform infrared (FT-IR)spectroscopy, UV-Visible(UV-Vis) diffuse reflectance spectra and UV Raman spectroscopy. The results indicated that the addition of TEA resulted in the formation of mesopores and the slight increase of framework titanium in TS-1. TS-1 synthesized with the addition of TEA exhibited a higher stability in the cyclohexanone ammoximation than that without the addition of TEA, attributing to the increase of mesopore volumes and the slight increase of the framework titanium in TS-1. However, when the addition of TEA was up to TEA/SiO2 ratio of 0.24, the crystallinity and framework titanium of TS-1 decreased markedly, and the average crystal sizes of TS-1 increased, with the catalyst stability becoming poor.

Anionic polymer as a quasi-neutral medium for low-cost synthesis of titanosilicate molecular sieves in the presence of high-concentration alkali metal ions

Titanosilicate molecular sieve (TS-1) is an important commercial catalyst for green production of oxyfunctionalized chemicals such as alcohols, ketones, epoxides, and oximes. Active TS-1 can normally be achieved in the synthetic media strictly free of alkali metal cations (e.g., Na+ and K+). This condition severely raises the purity demands of starting reagents, which greatly increases the production cost. Herein, we developed a method to achieve highly active titanosilicates in the presence of high-concentration alkali metal cations using anionic polymer-based quasi-neutral media. This synthetic route could effectively inhibit the adverse influence of alkali metal cations on the incorporation of active titanium(IV) sites into the framework of titanosilicate, directly affording the maximal framework titanium content (2.0wt.%) with almost 100% yield. This development shows a great commercial potential and opens the possibility of preparing silica-based molecular sieves/zeolites in non-fluoride neutral media.

The roles of different titanium species in TS-1 zeolite in propylene epoxidation studied by in situ UV Raman spectroscopy.

Titanium silicalite (TS-1) zeolites with different titanium species were synthesized and characterized by ultraviolet (UV)-Raman, ultraviolet visible (UV-Vis) diffuse reflectance spectroscopies and by the NH3 temperature programmed desorption (NH3-TPD) method. The roles of different titanium species in TS-1 samples have been investigated by gas chromatography-Raman spectrometry (GC-Raman) during the propylene epoxidation process. For the first time, a positive correlation was found among the concentration of framework Ti species, the amount of active intermediate Ti-OOH (η(2)) and the conversion of propylene by the in situ GC-Raman technique. The results give evidence that the framework titanium species is the active center and Ti-OOH (η(2)) is the active intermediate. The presence of extra-framework Ti species is harmful to propylene epoxidation. Furthermore, the amorphous Ti species has a more negative effect on the yield of propylene oxide (PO) than the anatase TiO2. The NH3-TPD results reveal that the amorphous Ti species are more acidic and thus should be mainly responsible for the further conversion of PO.

Rational Design of Engineered Multifunctional Heterogeneous Catalysts. The Role of Advanced EPR Techniques

The importance of surface paramagnetic species owes much to that of surface phenomena which are involved in numerous areas of chemistry and material science such as heterogeneous catalysis, photochemistry and, in general terms, nano-sciences and technology. In the present contribution the opportunities offered by the use of EPR in the field of heterogeneous catalysis, with emphasis on the application of hyperfine techniques, will be illustrated taking as an example Ti-based catalytic materials. The reductive activation of framework titanium ions in the different materials and their sub-sequent reactivity towards NH3 is followed, highlighting subtle differences in chemical reactivity related to the different matrixes.

Density functional theory investigations into the structure and spectroscopic properties of the Ti4+ species in Ti-MWW zeolite

The location and structure of the framework Ti4+ species in Ti-MWW zeolites were studied by density functional theory (DFT) based on the cluster models mimicking the Ti-MWW zeolite with a Si/Ti ratio of 35. The geometry, spectroscopic properties and energies of substitution for the Ti4+ species at different T sites were investigated. The results indicated that the Ti4+ species in form of Ti(OSi)4 prefers to locate at the T1 and T3 sites, positioning on the edge of 12-membered ring cavity and the intersection of the 10-membered ring intralayer channels in MWW, respectively. In their calculated infrared (IR) spectra, the vibrational mode at 960cm−1 band is actually a collective vibration of the antisymmetric stretching of Ti(–O–Si)4 in a deformed tetrahedral geometry. The totally symmetric Ti(OSi)4 tetrahedron at the Ti4 site is absent from the 960cm−1 band because the vibrational mode is infrared inactive. Hydrolysis of a Ti–O–Si bridge was thermodynamically favorable and resulted in a tripodal defect Ti species with an inversed Ti–OH group and a remaining Si–OH in the adjacent T site. The calculated IR spectra of the tripodal Ti species have a blue shift in the characteristic IR band to 990–1000cm−1. The UV–vis spectra of the Ti4+ species were calculated by the time-dependent DFT method. The calculated IR and UV–vis spectra are in very good agreement with the experimental measurements, and provide microscopic features for the identification of the structure of framework titanium in Ti-MWW zeolites.

Direct Hydroxylation of Benzene to Phenol Using Palladium–Titanium Silicalite Zeolite Bifunctional Membrane Reactors

A series of titanium silicalite zeolite catalysts were successfully incorporated inside a Pd membrane reactor aiming to improve the direct hydroxylation of benzene to phenol. The correlation between the membrane structure and the reaction efficiency was investigated. The influences of reactor configuration, feed mode, and catalysts on benzene conversion, product yield, hydrogen conversion, and water production rate were examined in detail. The reaction was very sensitive to the porosity of Ti-containing zeolite film and the bonding state of the titanium atom in the titanosilicates (i.e., framework and extraframework titanium). The framework titanium could adsorb active oxygen species to form Ti peroxo species which would suppress the decomposition, while the extraframework titanium promoted the decomposition of active oxygen species leading to more water production. Large inter- and intracrystalline pores as well as mesopores provided the reactive species greater opportunity to contact directly with frame...

Effect of TS-1 treatment by tetrapropyl ammonium hydroxide on cyclohexanone ammoximation

TS-1 has been treated with different concentrations of tetrapropyl ammonium hydroxide (TPAOH), characterized by a series of techniques, and studied in cyclohexanone ammoximation. The results show that the MFI structure of TS-1 is not destroyed, while mesopores are formed in TS-1 crystals and the pore diameters increase with the TPAOH concentrations; the amount of the framework Ti species decreases slightly at low concentrations of TPAOH, but decreases appreciably at the concentrations of TPAOH above 0.0125mol/L; the activity and the selectivity of TS-1 are not affected by the treatment, but the catalyst stability is markedly increased after the TPAOH treatment. The increase of the stability is attributed to the mesopore formation. But when the pore diameters are larger than 6.51nm, the catalyst stability begins decreasing due to the appreciable damage of the framework titanium by TPAOH.

Effect of sodium ions in synthesis of titanium silicalite-1 on its catalytic performance for cyclohexanone ammoximation

Titanium silicalite-1 (TS-1) has been hydrothermally synthesized with tetrapropylammonium hydroxide (TPAOH) as the template in the presence of various amounts of Na+, characterized by inductively coupled plasma, X-ray diffraction, scanning electron microscope, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and ultro-violet-visible spectroscopy and studied in cyclohexanone ammoximation. The characterization results show that with the increase of Na+ concentration in the synthesis, both the crystal sizes of TS-1 and extra framework Ti increase but framework Ti decreases. The addition of Na+ below 3 mol-% of TPAOH in the synthesis does not influence the catalytic properties with above 98% conversion of cyclohexanone and 99.5% selectivity to cyclohexanone oxime. However, at the concentrations of Na+⩾3mol-% of TPAOH in the synthesis, the catalysts are deactivated faster with the increase of Na+ addition, which can be attributed to more high molecular weight byproducts deposited in the large TS-1 particles and the loss of the frame-work titanium. The results of this work are of great importance for the industry.

Finding the “Missing Components” during the Synthesis of TS-1 Zeolite by UV Resonance Raman Spectroscopy

The catalytic performance of TS-1 zeolite greatly depends on the types of titanium species and their concentrations in the zeolite. Coupled with UV/vis spectroscopy, we present a UV resonance Raman spectroscopic investigation on the evolution of the titanium species during the crystallization of TS-1 zeolite. It is found that a small portion of Ti species leaches from the solid phase into the liquid phase. The “missing” Ti species in addition to the framework Ti species during the assembly process of TS-1 is identified as isolated “TiO6” species, due to the resonance Raman effect excited at 266 nm. The formation mechanisms of framework Ti species in TS-1 and the relation with the isolated “TiO6” species during the synthesis process are clarified. A synthetic strategy was designed to increase the concentration of the framework titanium by a factor of 1.5 for the actual synthesis of TS-1 zeolite.

Mesoporous Ti-KIT-6 molecular sieves: Their catalytic activity in the epoxidation of cyclohexene

Titanium incorporated mesoporous KIT-6 molecular sieves with Si/Ti ratios 20, 50, 100, 150 and 200 were synthesized hydrothermally using triblock copolymer (Pluronic P123) as the structure directing template. The materials were characterized using XRD, BET, SEM, HR-TEM, FT-IR and UV–vis DRS. Their BET surface areas were in the range 675–780 m 2/g. TEM images demonstrated highly porous network with well ordered arrangement of pores. Ti-KIT-6 with Si/Ti ratios 100, 150 and 200 were found to contain both free TiO 2 and framework Ti, whereas KIT-6 (20 and 50) contained only framework Ti. Their catalytic activity was evaluated in the epoxidation of cyclohexene with tert-butyl hydroperoxide (TBHP). Ti-KIT-6 (20 and 50) gave higher yield of cyclohexene oxide than others. This result revealed that the reaction is strongly influenced by framework titanium compared to non-framework one. The influence of solvent polarity on the epoxidation was also studied and it was found that the yield of cyclohexene oxide increased with the increase in the solvent polarity.

Hydrothermal Stability of Ordered Mesoporous Titanosilicate Materials Prepared at Room Temperature

A study of hydrothermal stability, performed in boiling water under static conditions, of MCM-41 materials containing different titanium content, prepared by direct synthesis at ambient temperature and pressure, using tetraethoxysilane, titanium ethoxide and octadecyltrimethyl- ammonium bromide is presented. The behaviour is compared with pure silica grades prepared by a similar procedure. The samples were characterised by X-ray diffraction, adsorption of nitrogen at 77K and diffuse reflectance UV–Vis spectroscopy. It was found that the stability improves as the amount of titanium increases and that Ti-MCM-41 samples prepared with Si/Ti≤50 are significantly stable. After 12h in boiling water the pore size uniformity was practically maintained and only a small decrease in pore volume (5-9%), total surface area (2-7%) and mesopore width (3%) and a slight increase in pore wall thickness (3-6%) occurred. In contrast, some degradation of the MCM-41 structure for the pure silica and the lower Ti content (Si/Ti=100) samples was observed with the effect being less pronounced for the latter. The higher hydrothermal stability of titanium substituted samples is probably correlated with a higher degree of polymerisation of the pore walls and with the presence of extra framework titanium.

Loading and release of ibuprofen on mesoporous molecular sieve Ti-HMS

Mesoporous molecular sieve Ti-HMS, which possesses uniform wormlike structure and high specific surface area, was used as the support of anti-inflammatory drug ibuprofen. The samples before and after charging with ibuprofen were characterized using X-ray diffraction, UV–visible and thermal gravimetry. The loading amount and release rates on Ti-HMS with different titanium content and pore size were investigated systematically. It has been shown that Ti-HMS can adsorb ibuprofen and be used in drug delivery system. With the increase of framework titanium content, the loading amount of ibuprofen increases. The larger pore size also facilitates the loading and release of ibuprofen.

Mesoporous titanosilicates with high loading of titanium synthesized in mild acidic buffer solution

Mesoporous titanosilicates with high titanium content were synthesized under mild acidic conditions (pH = 4.4, HAc–NaAc buffer solution) by co-condensation of acetylacetone-modified titanium isopropoxide (Ti(OBu n ) 3 (acac)) and mixture of sodium silicate with tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) or tetrakis(2-hydroxyethyl)orthosilicate (EGMS), using block copolymer Pluronic P123 as template. The combined results of XRD, N 2 sorption and TEM show that the highly regular structure of the mesoporous titanosilicates can still be obtained when Ti/Si molar ratio in the final product is as high as 0.059. The results of UV–vis diffuse reflectance spectra and UV resonance Raman spectra show that the framework titanium species are predominant in the mesoporous titanosilicates when Ti/Si molar ratio in the final product is less than 0.042. The mixture of sodium silicate and EGMS was proved to be the best silicon source for the synthesis of titanosilicates with ordered mesostructure and high titanium content. The efficiency of this synthetic method may be attributed to the mild acidic medium as well as the modified hydrolysis–condensation rate and hydrophility of the precursors.

Synthesis and structural investigations on aluminium-free Ti-Beta/SBA-15 composite

An aluminium-free Ti-Beta/SBA-15 composite material was prepared by the post-synthesis incipient wetness-deposition of different amounts of Ti-Beta nanoparticles solution on the SBA-15 matrix. The presence of crystalline nanoparticles in the solution, used for impregnation on SBA-15, was confirmed by HRTEM measurements. The hexagonal arrangement of the mesopores of SBA-15 was proven by XRD and TEM measurements. The presence and the deposition of Ti-incorporated zeolitic nanoparticles on the mesopore walls of SBA-15 were proven by nitrogen sorption analysis, IR spectroscopy and TG analysis. The X-ray absorption spectroscopy analysis of local environment of titanium incorporated in the new composite material showed that the product contained five fold coordinated framework titanium. These Ti sites can be oxidation titanium sites.

Deactivation and regeneration of titanium silicalite catalyst for epoxidation of propylene

Continuous epoxidation of propylene with 30-wt% hydrogen peroxide in isopropyl alcohol solvent over titanium silicalite (TS-1) zeolite was carried out and the good selectivity to propylene oxide and stability of TS-1 were obtained. After the reaction of 263 h, the deactivation of the TS-1 catalyst occurred. The spent catalysts were regenerated by calcinations at 550 °C for 5 h, washing with isopropyl alcohol solvent or washing with dilute hydrogen peroxide, and the activity evaluation of the regenerated catalysts were studied. The deactivated and regenerated catalysts were characterized by scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, 29Si magic angle spinning nuclear magnetic resonance, thermogravimetric analysis and Brunauer–Emmett–Teller method. The compositions deposited on the surface of the catalysts were collected by solvent extraction and were analyzed by gas chromatography–mass spectrometry (GC–MS). The results showed that calcination and washing with dilute hydrogen peroxide were highly effective regeneration methods and the catalytic activity of TS-1 catalysts can be mostly recovered after these treatments. Washing with isopropyl alcohol had a subtle effect in recovering the catalytic activity of the TS-1 catalyst. The major cause of the deactivation of the TS-1 catalyst for epoxidation is blocking micropores induced by bulky organic by-products formed by consecutive reactions of target reaction, such as dimerization or oligomerization of propylene oxide, etherification of propylene oxide with isopropyl alcohol. In addition, leaching of framework titanium also contributes to the deactivation of the catalyst.