| Popis: |
Mesoporous MCM-41 materials are silica-based systems with hexagonally arranged mesopores with long-range order, which were initially developed in 1992. Generally, the ordered mesopores are formed through the condensation of silicates around the self-assembled micelles by surfactant molecules or through liquid-crystal phases influenced by the silicate. During the past decades, numerous routes for the synthesis of MCM-41 materials have been developed. With modification of the synthesis conditions, such as the type of solvents, silica sources, addition of other metal species (e.g. , Al), surfactant type and relative concentration, pH value, synthesis temperature, aging and drying conditions, etc. , the morphology and catalytic properties of the obtained MCM-41 materials can be controlled. Among them, the direct room-temperature synthesis of siliceous MCM-41 f, h,4] has drawn great interest as it is a rapid method under mild conditions with less energy and time consumed compared with classic hydrothermal synthesis (heating normally required at 60–150 8C for 1–6 d). 5] Huo et al. synthesized mesoporous silica spheres by using different silica sources under basic conditions with stirring for 15–30 h at room temperature. They found that lower alkoxysilanes, such as tetramethyl orthosilicate (TMOS) and tetraethyl orthosilicate (TEOS), yielded only small MCM-41 particles, whereas higher alkoxysilanes, such as tetrabutyl orthosilicate (TBOS), led to larger MCM-41 spheres. These spheres were obtained at a low stirring speed ( 450 rpm). Cai et al. and Grun et al. synthesized siliceous MCM-41 particles in a diluted solution of surfactants with tetraethoxysilane under basic conditions at room temperature with stirring for 2 h. The resulting particle size was in the range of micrometers (1 mm) to nanometers (100 nm) controlled by using different basic additives (NaOH/NH4OH) [3c] and surfactants (n-hexadecyltrimethylammonium bromide and nhexadecylpyridiniumchloride monohydrate). Recently, Pescarmona and co-workers synthesized [Ti]MCM-41 materials with particle sizes of 80–160 nm by using TEOS, cetyltrimethylammonium bromide (CTAB), NaOH, and titanium(IV) isopropoxide with stirring at room temperature for 2 h and found that Mesoporous [Al]MCM-41 materials with nSi/nAl ratios of 15 to 50 suitable for the direct catalytic conversion of phenylglyoxal to ethylmandelate have been successfully synthesized at room temperature within 1 h. The surface areas and pore sizes of the obtained [Al]MCM-41 materials are in the ranges of 1005– 1246 mg 1 and 3.44–3.99 nm, respectively, for the different nSi/nAl ratios. For all [Al]MCM-41 catalysts, most of the Al species were tetrahedrally coordinated with Si in the next coordination sphere of atoms. H and C magic-angle spinning NMR spectroscopic investigations indicated that the acid strength of the SiOH groups on these [Al]MCM-41 catalysts and the density of these surface sites are enhanced with increasing Al content in the synthesis gels. These surface sites with enhanced acid strength were found to be catalytically active sites for phenylglyoxal conversion. The [Al]MCM-41 material with nSi/ nAl=15 showed the highest phenylglyoxal conversion (93.4%) and selectivity to ethylmandelate (96.9%), whereas the [Al]MCM-41 material with nSi/nAl=50 reached the highest turnover frequency (TOF=99.3 h ). This is a much better catalytic performance than that of a dealuminated zeolite Y (TOF= 1.7 h ) used as a reference catalyst, which is explained by lower reactant transport limitations in mesoporous materials than that in the microporous zeolite. |
