Mesoporous hydrothermal method

The mechanical incorporation of active nanoparticles into the silica pore structure is very promising for the general synthesis of supported catalysts, although particles larger than the support s pore diameter cannot be incorporated into the mesopore structure. To overcome this limitation, pre-defined Pt particles were mixed with silica precursors, and the mesoporous silica structures were grown by a hydrothermal method. This process is referred to as nanoparticle encapsulation (NE) (Scheme 2) [16] because the resulting silica encapsulates metal nanoparticles inside the pore structure. 

A good-quality CeMCM-41 material with Si/Ce=50 was synthesized by hydrothermal method. For the purpose of comparison a pure siliceous MCM-41 was prepared using the same composition without cerium. Thermogravimetric curves for the synthesized uncalcined samples exhibit shape characteristic for the MCM-41-type materials. The specific surface area of CeMCM-41 evaluated from nitrogen adsorption was equal to 850 m2/g, whereas the pore width and mesopore volume of this material were equal to 3.8 nm and 0.8 cm3/g, respectively. In contrast to the pure silica MCM-41, the CeMCM-41 material exhibits medium and strong acid sites as revealed by thermogravimetric studies of n-butylamine thermodesorption. 

Although crystallization does occur at low temperatures, hydrothermal methods are often used to speed up the reaction by raising the temperature to 160-250 C. Numerous templating agents have been used in the formulation of zeolite materials. Microporous materials (highly crystalline, pores <12 A) often use organic bases such as TEA (tetraethyl-ammonium), while mesoporous materials (larger pores but lower crystallinity, pores >12 A) often use liquid crystal templates ... 

S. Yoshikawa, J. Jitputti, S. Pavasupree, Y. Suzuki, Synthesis and photocatal3dic activity for water-splitting reaction of nanocrystaUine mesoporous titania prepared by hydrothermal method . Journal of Solid State Chemistry, 180,1743-1749, (2007). 

J. Yu, H. Yu, B. Cheng, X. Zhao, Q. Zhang, Preparation and photocatalytic activity of mesoporous anatase Ti02 nanofibers by a hydrothermal method . Journal of Photochemistry and Photobiology A Chemistry, 182, 121-127,... 

A new nanocrystaUine Ga—Al—Zn complex-oxide (designated here as nano-GAZ) has been synthesized by a hydrothermal method, with uniform nanocrystalline particles of size around 5-lOnm and very uniform 3.8nm mesopores [123]. This new material was designed for the elimination of N-containing polycyclic aromatic compounds (NPACs) present in diesel-engine emissions, which are known to be carcinogenic and need now to be removed. For instance, the concentration of 1-nitropyrene, which is the most abundant NPAC in diesel emission extracts, is... 

The use of ordered mesoporous supports has been investigated in parallel as an alternative strategy to obtain the high dispersion of chromium oxidic species for the ODH with CO2. Bi et al. investigated the catalytic behavior for the CO2-ODH of ethane using transition metal-doped M-MCM-41 (M = Ni, Co, Cr) mesoporous materials, prepared by the direct hydrothermal method. Cr-containing catalysts... 

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

The NiO phase could be reduced to metallic Ni by hydrogen treatment (723 K, 1 h). The crystallite size of Ni metal (estimated from XRD pattern of the reduced sample) is similar to that of the NiO in the unreduced sample (e.g., for 7B, the Ni metal crystallite size is 8.2 nm). The textural characterization studies reveal that for the samples prepared by coprecipitation + digestion and hydrothermal synthesis (methods B and C) the pore sizes are in the mesopore range (2.9 to 6.8 nm) (Table 11.2). 

As indicated by XRD patterns, there exist just 2-3 broad peaks in the calcined acid-made materials (Fig. 3A). Moreover, the N2 adsorption/desorption isotherm shown in Fig. 3B, the calcined acid-made mesoporous silica indeed possesses a broad capillary condensation at the partial pressure p/p0 of ca. 0.2-0.4, indicating a broad pore size distribution with a FWHM ca. 1.0 nm calculated from the BJH method. This is attributed to the occurrence of partial collapse of the mesostructure during the high temperature calcination. The hexagonal structure completely collapsed when subjected to further hydrothermal treatment in water at 100 °C for 3 h. Mesoporous silica materials synthesized from the acid route are commonly believed to be less stable than those from the alkaline route [6,7]. 

Substitution of silicon by other atoms like Ti or Al was reported to improve the thermal and hydrothermal stability to some extent [6]. It was also reported that improved hydrothermal stability could be achieved by adjusting the gel pH several times during crystallization process [7], Post-synthesis silylation technique has also been reported to enhance the hydrothermal stability of mesoporous materials by increasing the hydrophobicity of the samples [8,9]. However, it is most desirable to develop a method for preparing hydrother-mally stable mesoporous material by direct synthesis route. 

Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction (SFE) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. 

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