Search results for "Molecular sieve"
showing 10 items of 58 documents
ChemInform Abstract: The Synthesis of Spherical Mesoporous Molecular Sieves MCM-48 with Heteroatoms Incorporated into the Silica Framework.
2010
Mild Hydrocracking of Vacuum Gasoil over NiMo-Beta Zeolite Catalysts: The Role of the Location of the NiMo Phases and the Crystallite Size of the Zeo…
1998
Abstract Mild hydrocracking composite catalysts based on NiMo/γ-Al2O3-Beta zeolite have been prepared and the influence of the location of the NiMo phases has been studied. A good hydrocracking activity and the highest HDS conversion was found during the hydrocracking of a vacuum gasoil with the composite catalyst where the NiMo is located on the alumina component and then mixed with the zeolite in a proportion of 1 : 1 by weight. The crystal size of the beta zeolite had a significative influence on the catalytic behaviour, being better the smaller the crystallite size is. The small crystallite beta-based catalyst displayed a higher hydrocracking activity than conventional USY and silica-al…
Influence of layer structure preservation on the catalytic properties of the pillared zeolite MCM-36
2010
MCM-36 is a catalytically active material made by swelling and subsequent pillaring of a layered zeolite precursor, MCM-22(P). However, the swelling procedures lead to significant destruction of layer structure and alteration of framework silicon/aluminum (Si/Al) ratio, which are likely to influence the acidity and catalytic activity of the final materials. We report a milder swelling and pillaring process to prevent such structural destruction. The resulting pillared materials show higher crystallinity and improved retention of the zeolitic layer structure when compared to the ones prepared by the previously reported methods involving more aggressive treatments. Implications of such struct…
Evaluation of the Stability of Pure Silica MCM-41 toward Water Vapor
1999
Water vapor adsorption/desorption isotherms at 298 K and XRD measurements and nitrogen isotherms at 77 K before and after exposure to water vapor were determined on pure silica MCM-41 samples; samples had different pore widths and were prepared by different synthesis methods, including hydrothermal and room-temperature procedures. It was found that prolonged exposure to water vapor provoked structural alterations in all of the MCM-41 materials studied, the most significant effects being a loss of pore shape uniformity and a large decrease in pore size and, in consequence, pore volume. Analysis of the results suggests that these alterations are due to expansion of the pore walls as well as, …
V-containing MCM-41 and MCM-48 catalysts for the selective oxidation of propane in gas phase
2001
Well-organised V-containing MCM-41 and -48 (0.3‐1 wt.% of V content) have been synthesised by one-pot synthesis or by grafting using VOSO4 or VOCl3 as V sources, respectively. The samples before and after the calcination step have been characterised by several physicochemical techniques (Ar and N2 adsorption, XRD, diffuse reflectance-UV‐VIS (DR-UV‐VIS) spectroscopy, temperature-programmed reduction). It was found that V species in the as-prepared catalysts were mainly as vanadyl ions (VO 2C ), while highly dispersed V 5C species with tetrahedral coordination were observed in the calcined materials. The catalytic behaviour of the calcined materials for the gas phase oxidation of propane has …
Isomerization of α-pinene oxide over ZSM-5 based micro-mesoporous materials
2018
Abstract Few types of ZSM-5 based micro-mesoporous materials obtained via a dual template method, steam-assisted conversion and dual-functional templating were evaluated in α-pinene oxide isomerization. Complete conversion and the highest selectivity towards trans-carveol (ca. 40–43%) were achieved over X-ray amorphous micro-mesoporous aluminosilicates as well as mesoporous molecular sieves AlSi-SBA-15. In addition, X-ray amorphous samples containing the secondary building units of ZSM-5 zeolite demonstrated the highest rate of α-pinene oxide isomerization. The yield of the most desired product trans-carveol to a large extent depends on the accessibility of acid sites to the reagents molecu…
Characterization of MCM-48 Materials
2000
Mesoporous molecular sieves of MCM-48 type were prepared by conventional hydrothermal and novel room temperature syntheses. Scanning electron microscopy (SEM) studies have shown that nonagglomerate...
Novel synthesis of spherical MCM-48
1999
Abstract A novel synthesis route was developed for the cubic member of the M41S family, MCM-48, with a three-dimensional pore system allowing the formation of submicrometre- to micrometre-sized beads with a narrow pore-size distribution. The synthesis is based on the modified Stober method applying tetraethoxysilane, ethanol, water, ammonia and n -hexadecyltrimethylammonium bromide as template. The specific surface area, the specific pore volume and the average pore diameter were varied in the following ranges: 900–1600 m 2 g −1 , 0.5–0.9 cm 3 g −1 and 2–3 nm. Aluminium-, chromium-, gallium-, niobium- and vanadium-MCM-48 were also synthesized following this procedure.
Factors affecting the UV-Transparency of Molecular Sieves
1994
The optical properties in the UV-region of several types of molecular sieves were studied. The initial onset of absorption around 280 nm which was observed for basically all samles synthesized without special precautions is not due to an early onset of band gap absorption, but might be correlated to the crystal size as well as structural defects within the crystals, e.g. E'-center or peroxy-radical. Using template free synthesis procedures which lead to small crystals containing few defect sites, we were able to synthesize molevular sieves which were almost completely transparent down to 200 nm. Such crystals can become very important in the development of technical luminophores.
FTIR Microscopy with Polarized Radiation for the Analysis of Adsorption Processes in Molecular Sieves
1994
FTIR Microscopy with polarized light was used to study the orientation of different molecules in Silicalite I, SAPO-5 and AlPO 4 -5 molecular sieves during the adsorption process and in their equilibrium positions. While the uptake and the equilibrium positions of p-xylene are essentially not influenced by the the framework charge, large differences between AlPO 4 -5 and SAPO-5 are observed with the strongly basic p-nitroaniline molecule.