Mesoporous Ti Research Articles

Synthesis and characterization of V- and Ti-substituted mesoporous materials

Mesoporous Ti- and V-containing materials have been synthesized with different degree of loadings via incorporation of titanium (IV) isopropoxide and vanadium (V) oxide in the initial precursor gels. These mesoporous materials were compared with pure siliceous MCM-41 synthesized at the same conditions without adding metal precursors. The UV–vis and Raman measurements confirmed that the vanadium and titanium were mainly tetrahedrally coordinated and their coordination is not changing after calcinations. At high amount of Ti and V, the regular mesoporous long-range order decreases, however, the V- and Ti-mesoporous samples do not collapse upon calcinations.

Removal of fluoride from water through ion exchange by mesoporous Ti oxohydroxide

In our previous study, we found that Ti(OH) 4 exhibited fluoride ion exchange properties. In order to improve the ion exchange capacity, mesoporous Ti oxohydroxide (TiO x (OH) y ) had been prepared by using dodecylamine as template. Zirconia and silica had been introduced into the mesoporous Ti oxohydroxide to enhance the ion exchange capacity. The mesoporous structure and the morphology of the mesoporous materials obtained were confirmed using XRD and SEM, respectively. A fluoride ion exchange study was done on each sample. Results showed that mesoporous Ti oxohydroxide containing zirconia exhibited the highest fluoride ion exchange capacity, as it has the smallest particle size, with high uniformity among the mesoporous materials prepared.

Direct Formation of Pinacols from Olefins over Various Titano–Silicates

The epoxidation and successive pinacol formation of tri- and tetraalkyl-substituted olefins using Ti-β/H2O2/H2O as the catalytic system has been investigated. Aluminum-free Ti-β exhibits better activity and pinacol selectivity than TS-1, TS-2, Ti–MCM-22, and mesoporous Ti–MCM-41. Pinacol (vic-diol) is obtained as the major product with small amounts of the side products pinacolone, alcohol (via hydration), and oligomers. The conversion and pinacol selectivity increase with an increase in reaction temperature and time. The change in product distribution with reaction time over Ti-β shows that the epoxide is the initial product which undergoes a secondary reaction to give pinacol as the major product. The conversion and H2O2 selectivity decrease with the bulkiness of the substituents at the CC bond but the selectivity of pinacol is not significantly affected. The reactivity of cyclic 1-methyl-1-cyclohexene is considerably lower than that of the corresponding open-chain analogue 2-methyl-2-butene. The symmetrical tetramethyl-substituted 2,3-dimethyl-2-butene led to the formation of small amount of dimers over medium-pore titanium silicates TS-1, TS-2, and Ti–MCM-22. The epoxidation of these substituted olefins proceeded more rapidly when using acetonitrile as a cosolvent than under triphase conditions. Mechanistically, the primary epoxide product undergoes acid-catalyzed nucleophilic ring opening by H2O molecules to give pinacol.

A Novel Titanosilicate with MWW Structure: II. Catalytic Properties in the Selective Oxidation of Alkenes

The catalytic properties of Ti–MWW catalysts have been studied for the oxidation of alkenes using hydrogen peroxide or tert-butyl hydroperoxide (TBHP) as an oxidant. Ti–MWW shows superior activity independent of the nature of oxidant when compared to the other Ti-containing catalysts both of microporous TS-1 and Ti-MOR and of mesoporous Ti–MCM-41 in the oxidation of cyclohexene. Compared to large-pore Ti–Beta, Ti–MWW is less active than in the oxidation of cyclohexene using H2O2, but its shows comparable activity in the case of TBHP. Ti–MWW is very stable in both the structure and the states of Ti species in the actual cycle of reaction–regeneration. The framework B species remaining within the Ti–MWW catalysts seem to have little influence on the intrinsic activity of framework Ti species, but they act as Brønsted acid sites contributing to the hydrolysis of the oxide product to form glycols. The turnover number (TON) of Ti–MWW for the cyclohexene conversion decreases sharply with increasing Ti content, while the TON for the 1-hexene conversion shows volcanic behavior when related to the Ti content. It is clarified that the bulky reaction is catalyzed mainly by the Ti species within the supercages and the exterior pockets of the MWW structure, while in addition to these highly accessible Ti species, those within the channels of a 10-membered ring greatly contribute to the oxidation reactions not significantly sizedemanding.

Synthesis and characterization of a new family of electroactive alkali metal doped mesoporous Nb, Ta, and Ti oxides and evidence for an Anderson transition in reduced mesoporous titanium oxide.

Recently we reported that mesoporous niobium oxide can be chemically reduced by Na-naphthalene while fully retaining its mesostructure. This was the first report of a molecular sieve acting as a stoichiometric electron acceptor. Herein we expand on the initial work by presenting a detailed study on Li-, Na-, K-, Rb-, and Cs-reduced samples of mesoporous Nb oxide, as well as Li-reduced mesoporous Ta and Ti oxides. While the Nb- and Ta-based materials fully retained their structure on reduction as determined by X-ray diffraction (XRD) and nitrogen adsorption, the Li-reduced Ti material retained high surface area and narrow pore size distribution, but lost its diffraction pattern, indicating an increased level of disorder in this material. X-ray photoelectron spectroscopy (XPS) and UV-visible reflectance spectroscopy revealed that all reduced mesoporous oxides studied have a similar electronic structure, corresponding to the presence of a disordered impurity band in the material lying between the valence band and the conduction band. Electron paramagnetic resonance (EPR) studies suggest that the electron in this impurity level is unpaired and best described as a free electron, only loosely bound to the alkali or transition metal. SQUID magnetometry showed that all reduced materials are paramagnetic, further confirming the presence of unpaired electrons in the structure. All materials in this study were insulating with the exception of the Li-reduced mesoporous Ti material, which was highly conducting, possibly due to an Anderson transition. Electrochemical studies on the unreduced mesoporous oxides demonstrated that while the Ta and Nb materials are capacitors with only a small degree of reversible electrochemical behavior in the bulk sample, the Ti material was an electrical conductor with fully reversible redox behavior.

X-ray photoelectron spectroscopy and magnetic studies on the effect of pore size, wall thickness, and wall composition on superparamagnetic cobaltocene mesoporous Nb, Ta, and Ti composites.

Recent results in our group demonstrated that mixed oxidation state mesoporous niobium oxide cobaltocene composites display superparamagnetism at certain composition ratios. This was the first report of superparamagnetism in nanoscale molecular ensembles. A series of mesoporous niobium oxide materials were synthesized in order to understand the role of pore size and thickness of the walls in the mesostructure on the magnetic properties. Mesoporous Ti oxide and Ta oxide composites were also synthesized in order to investigate the effect of changing the wall composition on the magnetic properties of this new series of materials. All samples were characterized by X-ray diffraction, nitrogen adsorption, ultraviolet spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and superconducting quantum interference device magnetometry. The results of this study showed that variation of wall thickness or pore size in the Nb system had little effect on the properties and that superparamagnetism most likely arises from mixed oxidation state cobaltocene grains residing in the individual pores and not from the free electrons in the mesostructure or much larger domains. The Langevin function was applied to the isothermal magnetic data from the Nb composites and gave mean superparamagnetic particle sizes of ca. 14 nm in each system. The Co(II) to Co(III) ratios in these materials were approximately 1:1. The Ti and Ta materials showed no sign of superparamagnetism and only very low levels of neutral cobaltocene in the pores. This suggests that a critical amount of cobaltocene is required to bring about superparamagnetic behavior.

Characterisation of microporous and mesoporous materials by the adsorption of molecular probes: FTIR and UV–Vis studies

This contribution describes with a few, properly selected, examples the spectroscopic characterisation of acid, base and redox centres in microporous and mesoporous molecular sieves. FTIR studies of the adsorption of molecular probes that lead to weak H-bonding (N 2 and CO), medium–strong H-bonding (H 2O) and proton-transfer reactions (NH 3) with the Brønsted acid hydroxyl groups of zeolites and aluminophosphates (HSAPO-34 and CoAPO-18) are illustrated. The acid Lewis sites (countercations) and the acid–base couples in NaX and NaY zeolites are monitored by CO and CO 2 adsorption respectively. Finally, FTIR and UV–Vis studies of the H 2O and NH 3 adsorption that probe the transition metal cation centres in mesoporous Ti–MCM41 and microporous CoAPO-18 catalysts are described.

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.

Transition metal substituted derivatives of cubic MCM-48 mesoporous molecular sieves

Mesoporous Ti-, Cr- and V-substituted derivatives of the cubic silica molecular sieve MCM-48 have been hydrothermally synthesized at 373 K by an electrostatic assembly pathway using cetyltrimethylammonium cation as the templating agent. The transition metal substituted products all exhibit improved crystallinity and a narrower pore size distribution relative to the pristine molecular sieve. Transition metal substitution caused an enlargement of the cubic unit cell as well as an increase in the degree of crosslinking in the mesopore walls. The coordination of V-sites in the meso-structure is exclusively tetrahedral, as judged by NMR spectroscopy. These sites as well as those of Ti and Cr sites are active and selective for the peroxide oxidation of styrene and methyl methacrylate to benzaldehyde and methyl pyruvate, respectively.