Pure Siliceous MCM-41 Research Articles

Effects of the crystallization time on the mesoporous structure, texture, morphology and styrene oxidation performances of V-MCM-41

The new V-MCM-41 molecular sieves with good ordered hexagonal mesoporous structure and crystallinity were synthesized through in-situ hydrothermal preparation method. The effects of the crystallization time were discussed. The synthesized samples were characterized by X-ray diffraction, N2 adsorption/desorption, Fourier transformed infrared and scanning electron microscopy. The different structures, textures, morphologies of V-MCM-41 obtained with different crystallization times were observed and analyzed on the basis of the characterized results. The results showed that the V-MCM-41 molecular sieve obtained at 110°C for 48h crystallization times was of good spherical morphology, ordered hexagonal structure, most uniform pore size distribution and high surface area compared with other samples. Meanwhile, the V-MCM-41 molecular sieve with the high skeleton Si condensation and the good crystallinity was obtained. The heteroatom could be incorporated into MCM-41 framework with increasing crystallization times, which was beneficial to improve the catalytic activity and selectivity of pure siliceous MCM-41. The V-MCM-41 showed the good catalytic selectivity in catalytic oxidation of styrene using hydrogen peroxide, and the selectivity of the benzaldehyde and phenylacetic acid reached 30.68% and 49.44%, respectively.

Thermal degradation of CTAB in as-synthesized MCM-41

Thermal evacuation of a surfactant template from pure siliceous MCM-41 and MCM-41 containing aluminium in hydrogen flow was investigated. Micelle templated MCM-41 were prepared using hexadecyltrimethylammonium bromide (CTAB). The products of thermal surfactant degradation outside and inside pores were identified at various temperatures using 13C solid-state nuclear magnetic resonance (NMR) spectroscopy, gas chromatography coupled with mass spectrometer (GC-MS) and temperature programmed desorption coupled with mass spectrometer (TPD-MS). The GC-MS and 13C MAS NMR results obtained from this study provide an insight into the mechanism of surfactant transformation during MCM-41 synthesis on molecular level.

The role of different parameters of synthesis in the final structure of Ti-containing mesoporous materials

Ti-containing mesoporous materials have been prepared by hydrothermal synthesis at 373 K and compared to pure siliceous MCM-41 synthesized in the same conditions. A detailed and correlative study about the effect of different synthesis parameters on the physicochemical properties of the material has been carried out. All the samples showed typical XRD patterns of a kind MCM-41 structure. It seems that the synthesis time and the surfactant chain length are critical variables to obtain good structures. Mesoporous structure allowed the incorporation of different degrees of Ti loading, although a lower degree of ordering was observed with the increase of metal content above 1%. Moreover, a high incorporation of Ti into the structure seems responsible for the development of a secondary mesoporosity confirmed by TEM and associated with a pronounced hysteresis loop in the N 2 isotherm. DRUVA-Vis and XPS studies revealed that the Ti species are mainly found isolated and tetrahedral coordinated into the framework. The superego-Ti(IV) species generated by the interaction of Ti-MCM-41 with aqueous H 2O 2 have been studied by EPR spectroscopy and correlated to DRUVA-Vis and XPS measurements.

Preparation and characterization of zinc containing MCM-41 spheres

A series of pure siliceous and Zn-modified MCM-41 spheres were synthesized at room temperature, atmospheric pressure and at short reaction times. Mesoporous molecular sieves were characterized by a number of techniques: XRD, N 2 adsorption, TEM, XPS and NH 3–TPD. Pure siliceous MCM-41 exhibits the typical hexagonal array of well-defined pores with a mean pore diameter of 3.0 nm. Addition of a small percentage of Zn (Zn/Si ratio 0.02) into the synthesis gel produces a material with increased lattice parameter ( a o), pore diameter and total adsorption capacity. These results suggest the incorporation of zinc, with a larger ionic radius, into the silica framework. Unfortunately, higher zinc contents (Zn/Si ratio 0.04) produce a deterioration of the long-range order of the material, together with a decrease in the surface area and total pore volume. In this sample, the presence of small ZnO nanoparticles embedded into the silica matrix can be envisaged. From a catalytic point of view, these Zn-modified MCM-41 mesoporous molecular sieves appear as quite interesting materials to be used as a catalyst by themselves or as a support for noble metal catalysts.

Synthesis, characterization and selective oxidation properties of Ti-containing mesoporous catalysts

Ti-containing mesoporous catalysts with MCM-41 structure have been successfully prepared by direct hydrothermal synthesis. These mesoporous materials were compared with pure siliceous MCM-41 synthesized at the same conditions. An extended and detailed study, which includes the effects of several factors such as the state and content of Ti, surfactant/Si ratio and template removal technique on the physicochemical properties and catalytic activity of Ti-MCM-41, is presented in this contribution. Various techniques including XRD, FT-IR, DRUV–vis, ICP and N 2 adsorption were employed for the materials characterization. An optimum surfactant/Si molar ratio in the synthesis gel for the highest regularity of the formed mesostructure was found. Both the structure stability and the local environment of Ti sites in mesostructures with different degrees of Ti loading (0.10–3.22 wt.%) have been carefully investigated. Based on a combined FT-IR and DRUV–vis study and ICP analysis, it was possible to conclude that Infrared spectroscopy can result a powerful tool to verify the Ti(IV) incorporation into the framework of MCM-41. Therefore, a simple and novel method for estimation of framework Ti content could be developed. Also, these materials were catalytically tested in the oxidation of cyclohexene with aqueous hydrogen peroxide. Finally, the catalysts could be regenerated without any loss in activity/selectivity.

Synthesis, Characterization and Catalysis of Ce-MCM-41

The cerium-containing MCM-41 (Ce-MCM-41) has been synthesized by direct hydrothermal method. The low-angle XRD patterns revealed the typical five major peaks of MCM-41 type hexagonal structures. The interplanar spacing d100 = 38.4 Å was obtained that can be indexed on a hexagonal unit cell parameter with ao = 44.3 Å which was larger than that of pure siliceous MCM-41 (Si-MCM-41). Transmission electron micrograph shows the regular hexagonal array of uniform channel characteristics of MCM-41. The BET surface area of Ce-MCM-41 was 840 m2/g, which is much reduced as compared to that of Si-MCM-41, with the pore size of 26.9 Å and mesopore volume of 0.78 cm3/g were measured by nitrogen adsorption-desorption isotherm at 77 K. Along with the results, the synthesized Ce-MCM-41 exhibited a well-ordered MCM-41-type mesoporous structure with the incorporation of cerium. Using Ce-MCM-41 as a support, the Rh (0.5 wt%) catalyst exhibited very high activity for the NO/CO reactions.

Investigation on the mechanism of the photoluminescence of MCM-41

Pure siliceous MCM-41 sample was synthesized in ethylenediamine (EDA) medium. MCM-41 sample calcined at 813 K showed the most strong photoluminescent (PL) effect, while those calcined at 1073 K and 1323 K only showed weak photoluminescence. The intensity of photoluminescence decreased as the calcined temperature increased. By nuclear magnetic resonance (NMR) spectroscopy and infrared (IR) investigation, it was demonstrated that both the Al-depleted defect sites and silanol contents were responsible for the strong PL effect of the investigated MCM-41 samples. It was also suggested that the mesoporous channel structure of MCM-41 influences the investigated PL effect.

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.

Photoluminescence study of mesoporous MCM-41 and Ti-grafted MCM-41

Photoluminescence combined with diffuse reflectance UV—Vis spectroscopy was used to characterise both the pure siliceous MCM-41 and Ti-MCM-41 containing tetrahedral Ti(IV) species anchored onto the inner walls of the siliceous MCM-41. Both Ti-MCM-41 and MCM-41 showed complex photoemission signals and this study allows, for the first time, the distinction of the emission signals of tetrahedral Ti9IV) ions from those due to silica surface centres.

Removal of the Template Molecules from MCM-41 with Supercritical Fluid in a Flow Apparatus

Supercritical CO2 modified with a methanol/dichloromethane mixture was used to quantitatively remove the template molecules (cetyltrimethylammonium bromide) from the pores of as-synthesized pure siliceous MCM-41 in a flow apparatus. The resulting mesoporous MCM-41 materials, characterized by powder X-ray diffraction, nitrogen adsorption, and high-resolution transmission electron microscopy, exhibited higher surface areas, larger pores, and narrower pore size distributions than those obtained by conventional calcination at high temperature. The removed template retained its structures and properties and could be reused in the synthesis of MCM-41 materials.

Mo incorporation in MCM-41 type zeolite

Mo-incorporated MCM-41 has been prepared by direct hydrothermal synthesis. XRD and N2-adsorption measurements showed the characteristics of MCM-41. IR, FT-Raman and UV-VIS DR spectroscopic analyses gave the evidences for the incorporation of Mo in the framework of MCM-41. They are found to be stable and active for cyclohexanol and cyclohexane oxidation reactions with H2O2 as oxidant. Activity of this system has been compared with that of Ti-MCM-41 and molybdena impregnated on pure siliceous MCM-41.

Supercritical fluid extraction of surfactant template from MCM-41

Supercritical fluid extraction was used to extract and recover >90% of the surfactant template from the pores of as-synthesized pure siliceous MCM-41 and the MCM-41 after supercritical fluid extraction shows more uniform pore size distribution than the calcined MCM-41.