Hexagonal Mesoporous Molecular Sieves Research Articles

The catalytic oxidation of isoamyl alcohol on modified hexagonal mesoporous silicas (HMS)

Hexagonal mesoporous molecular sieves HMS and Al, Co doped HMS were synthesized by sol-gel method and subsequently, ZnO supported catalysts were prepared via wetness impregnation method. The samples were characterized by XRD, FT-IR and N2 adsorption-desorption. Then, they were used for selective oxidation of isoamyl alcohol to isovaleric acid. The results showed that HMS was successfully modified and ZnO supported Co doped HMS with a ZnO loading of 3wt% exhibited excellent catalytic performance.

Superior Performance of Gold Supported on Titanium-Containing Hexagonal Mesoporous Molecular Sieves for Gas-Phase Epoxidation of Propylene with Use of H2 and O2

Titanium-containing hexagonal mesoporous silicas (Ti-HMS) with wormhole structure and Si/Ti molar ratios ranging from 10 to 40 have been prepared by using long-chain alkyl primary amines as template agents. The Ti-HMS supported Au catalyst (Au/Ti-HMS) was obtained by a deposition−precipitation method for direct gas-phase epoxidation of propylene with use of O2 and H2. The structures of Ti-HMS and Au/Ti-HMS samples were characterized by X-ray diffraction, N2-physisorption, scanning electron microscopy, transmission electron microscopy, UV−vis diffuse reflectance spectroscopy, UV Raman spectroscopy, ammonia-temperature-programmed desorption, and atomic adsorption spectroscopy. The results showed that the Au/Ti-HMS catalyst exhibited superior performance in terms of propylene conversion, propylene oxide (PO) selectivity, and H2 efficiency in comparison with the Au catalysts supported on the conventional Ti-containing mesoporous materials. Besides the Si/Ti molar ratio, the chain length of alkylamine for the Ti-HMS preparation was crucial for the enhancement of catalytic performance. Specifically, 9.0% of propylene conversion, 97.3% of PO selectivity, and 30.4% of H2 efficiency can be obtained at 373 K in the initial 30 min of time-on-stream on the Au/Ti-HMS catalyst, where the Ti-HMS having a Si/Ti molar ratio at 20 was prepared by using tetradecylamine as the template agent. Regeneration of the spent catalyst by calcination in air gave almost no change in the PO selectivity but about 25% loss in the propylene conversion. The enhanced catalytic performance of Au/Ti-HMS catalyst may be essentially attributed to the homogeneous dispersion and uniformity of titanium species in combination with accessible pore structure.

Epoxidation of Styrene by Fe, Mn, and V Metalloporphyrins Encapsulated Si, Al, Ti And V- Mcm-41

Iron(III), Mn(III) and VO(IV) metalloporphyrin complexes have been encapsulated into ordered hexagonal mesoporous molecular sieves. The encapsulation is effective due to the electrostatic attraction of quaternary ammonium sites present in the porphyrin and the Bronsted/Lewis acid sites present in the mesoporous molecular sieves. These heterogeneous catalysts were catalytically active for the epoxidation of styrene with PhIO exhibiting different selectivity and negligible leaching of active species. The heterogeneous catalysts were reused three times with a slighter decrease in activity but the selectivity patterns were maintained during the recycling experiments.

Characterization and catalytic activity of cobalt containing MCM-41 prepared by direct hydrothermal, grafting and immobilization methods

Cobalt containing hexagonal mesoporous molecular sieves (MCM-41) were prepared by different methods viz., direct hydrothermal synthesis, grafting and immobilization. The calcined material was characterized by various spectroscopic tools such as powder X-ray diffraction (XRD), N2 adsorption–desorption isotherms, Fourier transformed infra-red (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance UV–vis and X-ray photoelectron spectroscopy (XPS). The XRD patterns show the highly intense (100) reflection peak in the range 2–3° (2θ angle), characteristic of mesoporous material. Higher order reflection peaks suggest highly ordered mesoporous structure. N2 adsorption–desorption isotherms are of type IV according to IUPAC classification and the steep rise in the isotherms in the narrow range 3.5–4.5 of relative pressure (P/P0) is due to the condensation of N2 gas molecules in the pores (characteristic of mesoporous structure). In FT-IR increase in the intensity of band at 960cm−1 with the increase of the cobalt content in Co-MCM-41 samples indicate the incorporation of cobalt ions in the framework of MCM-41. SEM and TEM reveal spherical morphology for the cobalt substituted MCM-41. UV–vis spectra demonstrate the characteristic features of framework and extraframework cobalt in MCM-41. Cobalt is in +2 oxidation state as evidenced from UV–vis and XPS. The catalysts were tested for the side chain oxidation of ethylbenzene using 70wt.% tert-butyl hydrogen peroxide as an oxidant with and without the use of solvent. Solvents have effect in the catalytic activity and selectivity. In the absence of solvent, substituted cobalt [Co-MCM-41 (50)] in the framework of MCM-41 gives maximum ethylbenzene conversion while higher selectivity to acetophenone was achieved over immobilized cobalt on the MCM-41. Leaching data shows, immobilized cobalt catalyst is well hetrogenized.

Cobalt-containing hexagonal mesoporous molecular sieves (Co-HMS): Synthesis, characterization and catalytic activity in the oxidation reaction of ethylbenzene

Cobalt-containing hexagonal mesoporous materials (Co-HMS and Co/HMS) with different cobalt content were synthesized for the first time by direct hydrothermal and post-synthesis (grafting) method. The materials were characterized in detail by X-ray diffraction, BET surface area, N 2 sorption isotherms, SEM, TEM, UV–vis and XPS techniques. Powder X-ray diffraction pattern, N 2 adsorption–desorption analysis and TEM analysis show the presence of hexagonal mesoporous structure, having Type IV isotherms and reveals the typical wormhole-like morphology. Spectroscopic techniques like UV–vis and XPS reveal cobalt in +2 oxidation state and tetrahedrally coordinated. Liquid phase oxidation of ethylbenzene using TBHP (70 wt%) as an oxidant shows that the catalysts are highly active, under solvent free conditions as well as under lower cobalt concentrations. Leaching studies performed by hot filtration experiments show that the cobalt catalysts prepared by hydrothermal methods are stable, while the grafted catalysts show the leaching of cobalt under the reaction conditions.

Mesoporous microcapsules with noble metal or noble metal oxide shells and their application in electrocatalysis

A versatile peripheral-pore-nanocasting method is employed to fabricate a series of novel microcapsules with mesoporous shell of noble metal or noble metal oxides, including Pt, PdO, RuO2 and IrO2. The shell thickness of these hollow microcapsules can be adjusted by controlling the depth of 3-amino-propyltriethoxysilane (APS) modification. By pre-filling the guest species or their precursors into the 3-D pores of HMS (hexagonal mesoporous molecular sieves) spheres, an interesting material with a “sphere in shell” structure could be fabricated via the same method. Both the mesoporous platinum hollow microcapsules and carbon spheres encapsulated in platinum microcapsules have shown high mass-normalized activities as a catalyst in the oxidation of methanol, a fundemantal reaction in direct methanol fuel cells (DMFC).

Catalytic growth of carbon nanotubes from the internal surface of Fe-loading mesoporous molecular sieves materials

Catalytic formation of carbon nanotubes on Fe-loading hexagonal mesoporous molecular sieve Fe/SiHMS and microporous molecular sieve Fe/NaY has been investigated. TEM pictures, together with the different porous properties of SiHMS and NaY, show us that the carbon nanotubes probably grow inside the channels of Fe/SiHMS and on the external surface of Fe/NaY. The Fe(2p) binding energy (BE) suggests a stronger interaction between SiHMS and the deposited iron oxide species than that between NaY and iron species. The BE of Si, O and Fe elements for Fe/SiHMS vary much before and after the catalytic synthesis of carbon nanotubes, whereas the BE for NaY changes little. The above XPS information can serve as a supplement to TEM analysis.

ChemInform Abstract: Deprotection of Acetals and Ketals over Hexagonal Mesoporous Molecular Sieve as a Reusable, Heterogeneous Catalyst.

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Growth of Carbon Nanotubules on Fe/HMS Mesoporous Molecular Sieve Materials

Hexagonal mesoporous molecular sieve materials (HMS) with different pore sizes were applied as supports of Fe-loading catalysts to catalytically synthesize carbon nanotubules. The larger the pore size of HMS is, not only the larger the pore size of the formed carbon nanotubes is, but also the longer the reaction time is. It was observed that the growth of carbon nanotubes can be orientated by the one-dimensional mesoporous structure of HMS.

Effect of Structure and Pore Size of Mesoporous Molecular Sieve Materials on the Growth of Carbon Nanotubes

Microporous and mesoporous molecular sieve materials with different channel structures and pore sizes were applied as supports for Fe-loaded catalysts to catalytically synthesize carbon nanotubes. The deposited carbon materials were characterized by the TEM technique. It was shown that the structures and pore sizes of supports greatly influence the forms and quality of the deposited carbon materials. The larger the pore size of the support used, the larger the diameter and pore size of the carbon nanotubes formed. It seems that the growth of carbon nanotubes can be orientated by the one-dimensional mesoporous structure of hexagonal mesoporous molecular sieve materials.

Room-temperature synthesis of an Fe-containing mesoporous molecular sieve

An Fe-containing hexagonal mesoporous silica molecular sieve material was synthesized at room temperature for the first time. X-ray powder diffraction, infrared spectroscopy, electron spin resonance (ESR) and Mossbauer spectroscopy confirm the incorporation of Fe(III) in the structural framework. The tetrahedrally coordinated Fe(III) in this material is thermally more stable compared to that in another kind of hexagonal mesoporous molecular sieve containing Fe. It also exhibits better hydrothermal stability than Fe-free and Al-containing hexagonal molecular sieves.