Titanium mesoporous materials

UV-vis DRS spectra of the calcined titanium mesoporous materials are shown in Figure 2. An absorption band centered at ca. 220 nm is observed in all the samples, and this band is usually assigned to a low-energy charge-transfer transition between tetrahedral oxygen ligands and central Ti4 ions [9, 10]. The shoulder at 270 nm probably corresponds to partially polymerized hexa-coordinated Ti species [10], and some polymeric species are suspected to co-exist with the isolated Ti sites in all the mesoporous samples prepared. However, anatase-like TiO, phase at 330 nm was absent in these samples. 

Synthesis, eharacterization and catalytic activity of titanium containing mesoporous materials with TS-1 wall structure... 

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

Kapoor, M.P., Inagaki, S.,and Yoshida, H. (2005) Novel zirconium-titanium phosphates mesoporous materials for hydrogen production by photoinduced water splitting. Journal of Physical Chemistry B, 109 (19), 9231-9238. 

The higher conversion in the presence of Ti-beta is probably a result of the higher temperature (343 v.v. 323 K). Diffusional constraints cannot account for the observed differences in selectivity. Ti-beta and TS-1 are distinctly more selective than the mesoporous material. Recalling that tetrapodal titanium sites are more predominant in the former two molecular sieves although tripodal titanium sites are the major surface species over the latter mesoporous material (Section II), we infer that the data indicate that high epoxidation selectivity is probably correlated with the presence of tetrapodal structures in these two molecular sieves. This correlation is discussed in Section VI. 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

Such detailed structural information about surface Ti species is not available for other Ti-Si02 mesoporous materials. The results of Guidotti et al. (189) (Section V.C.5) indicate that catalytic reactions on these materials involving peroxide are complex processes and other titanium oxo species may also be involved. 

Polyacrylic acid Hybrid organic/inorganic network polymers were formed via the reaction of polyacrylic acid with tin(IV). titanium(IV), and silicon (IV) alkoxides and subsequent hydrolysis to form mesoporous materials. Treatment by nitric acid removed the polyacrylate template and produced microporous inorganic hydrous metal oxides Surface areas characterized by BET measurements 130... 

Figure 1 shows the FT-1R spectra of samples dispersed in KBr. All the spectra display a strong band at 960 cm 1. This band has been assigned to Si-O-Ti bonds [14] or to Si-OH groups [15, 16]. It is usually taken as the evidence for isomorphous substitution of Si by Ti, but it cannot be used to determine quantitatively the content of titanium into the framework of mesoporous materials [17]. In addition, the broad pattern between 3700 and 3000 cm 1, originated from hydrogen-bonded surface OH groups as well as from adsorbed H20 [18], decreases dramatically in the silylated samples. 

Since discovery of MCM-41 materials [1,2] many researchers have been concentrated on the improvement of their quality and properties by incorporating heteroatoms such as titanium [3-5], boron [6,7], vanadium [8], gallium [9], and recently lanthanides (mainly La and Ce) [10-13]. Incorporation of these elements into the MCM-41 structure influences its stability as well as adsorption and catalytic properties [13]. The presence of silanol groups on the surface of these mesoporous materials allows for bonding of organic and inorganic ligands [14-16]. 

In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties. 

Composition and preparation conditions of mesoporous materials containing titanium are screened to optimise the catalytic activity and selectivity in the epoxidation of cyclohexene using tert-butyl hydroperoxide as oxidant. Important parame-... 

Chen H. R., Shi J. L., Yu J., Wang L. Z. and Yang D. S., Synthesis of titanium-doped ordered porous zirconium oxide with high-surface-area, Microporous and Mesoporous Materials 39 (2000) pp. 171-176. 

For redox catalysis, efforts have been spent on preparing transition metal modified mesoporous materials. These materials are capable of extending the catalytic oxidation chemistry to large molecules. The selective catalytic activity has also been demonstrated, for example, in the oxidation of aromatic compounds by using titanium-containing mesoporous silica (Ti-MCM-41 and Ti-HMS). ... 

Incorporation of titanium oxide species within the framework of mesoporous silicas has been shown to produce highly efficient photocatalytic materials. Extremely careful preparation conditions [84] leads to highly structured materials comprising anatase nanoparticles of dimension between 5 and 10 run. The channeled structure, together with the hydrophobic/hydrophilic character, are also key features controUing their enhanced photoreactivity. The photocatalytic activity of such mesoporous catalysts has been studied for the degradation of phenol in aqueous solutions [85]. It was observed that for structured mesoporous materials with low Ti content, the turnover frequency was four times greater than that for standard P25. 

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