Catalytic activity of silica

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

Boreskov and co-workers (45) point out that on y irradiation the specific catalytic activity of silica gel with respect to the hydrogen-deuterium exchange reaction first increases with increasing radiation dose and then attains saturation at a sufficiently large dose. 

By pillaring the layered oxides, it is possible to increase the interlayer space to make such space available for guest molecules. Ebina et al. [136] investigated the catalytic activity of silica-pillared Ca2Nb3O 0. Effective pillaring was achieved by intercalation of TEOS into HCa2Nb30io. The BET surface area of the pillared material was 200 m g. Since the interlayer space is now accessible for the alcohol with longer alkyl chains as well, the activity with these alcohols improves to a... 

At least a large fraction of the radiation effect on catalytic activity of silica is connected with aluminum impurity. The addition of 0.06 atom of aluminum per 100 atoms of silicon to a commercial gel containing 10 to 100 ppm A1 gave a sample which was increased in activity (at room temperature) by 2.0 X 10 (V/M) cm gm i min- by a given dose. The same dose of radiation increased the rate constant of the undoped sample by 0.4 x 10 2. After the same samples were reevacuated above 500°, they were again irradiated, and the increases were 2.4 x 10"2 and 0.4 x 10 2 for the doped and undoped samples (69). In a different... 

This hypothesis has been expanded to explain the catalytic activity of silica-magnesia, silica-zirconia, alumina-boria, and titania-boria. In addition, a generalization has been presented which states that ... 

In this manuscript, the preparation, characterization, and catalytic activity of silica or alumina pillared CT are shown. The catalytic activity of sulfided NiMo supported catalysts was evaluated by using the model reaction, hydrogenation of pyrene at 573 K. 

Parmahana, A., Sokolovskii, V., Miceli, D., Arena, F, and Giordano, N. On the nature of the catalytic activity of silica-based oxide catalysts in the partial oxidation of methane to formaldehyde with O2. J. Catal 1994, 148, 514-523. 

ParmaUana, A., Arena, R, Frusteri, R, Martinez-Arias, A., Lopez-Granados, M., and Fierro, J.L.G. Effect of Fe-addition on the catalytic activity of silicas in the partial oxidation of methane to formaldehyde. Appl. Catal. A Gen. 2002, 226, 163-174. 

This paper describes the catalytic activity of nickel phosphide supported on silica, alumina, and carbon-coated alumina in the hydrodesulfurization of 4,6-dimethyldibenzothiophene. The catalysts are made by the reduction of phosphate precursors. On the silica support the phosphate is reduced easily to form nickel phosphide with hi catalytic activity, but on the alumina support interactions between the phosphate and the alumina hinder the reduction. The addition of a carbon overlayer on alumina decreases the interactions and leads to the formation of an active phosphide phase. 

Catalytic activities of the silica-supported vanadium oxides in either 3% methanol or in 5% O2 and 3% ethanol. Oxygen uptake was measured at 625 K. O2 and 3%... 

Specific catalytic activities of a number of silica-supported metals have been determined for the hydrogenolysis of ethane to methane (16, 29-31,... 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

Chem. Soc., Vol. 125, M. Iwamoto, Y. Tanaka, N. Sawamura, S. Namba, Remarkable Effect of Pore Size on the Catalytic Activity of Mesoporous Silica for the Acetalization of Cyclohexanone with Methanol, pp. 13032-13033, Copyright 2003. With permission from American Chemical Society.)... 

The key effect of oxide supports on the catalytic activities of metal particles is exerted through the interface between oxides and metal particles. The key objective of this study is to develop synthesis methodologies for tailoring this interface. Here, an SSG approach was introduced to modify the surface of mesoporous silica materials with ultrathin films of titanium oxide so that the uniform deposition of gold precursors on ordered mesoporous silica materials by DP could be achieved without the constraint of the low lEP of silica. The surface sol-gel process was originally developed by Kunitake and coworkers.This novel technology enables molecular-scale control of film thickness over a large 2-D substrate area and can be viewed as a solution-based... 

Rhodium immobilized complexes were also found to be effective catalysts of the addition of HSiMe(OSiMe3)2 and HSi(OEt)3 to various allyl ethers. The data presented in Table 7.4 confirm a high catalytic activity of catalysts 1, 3 and 5 in the conversion of allyl ethers into the corresponding silyl derivahves, but, unfortunately, only in the case of allyl phenyl ether did the catalytic achvity remained unchanged up to 10 cycles. ICP analysis of the rhodium solid catalysts after hydrosilylation tests revealed a high concentration of rhodium. Therefore, the decrease in catalytic activity of 1 does not depend only on leaching of rhodium from the silica surface. 

The catalytic activity of a lanthanum (R)-BINOL complex tethered either on silica (62a) or MCM-41 (62b) was evaluated for the enantioselective nitroaldol reaction of cyclohexanecarboxaldehyde (Se), hexanal (Sf), iso-butyraldehyde (Sg) and hydro-cinnamaldehyde (Sh) with nitromethane inTHF (Scheme 12.22) [166]. The silica-anchored lanthanum catalyst 62a gave 55-76% e.e. and yields up to 87%, while the PMS-immobilized catalyst 62b revealed slightly higher e.e.s (57-84%) for the same aldehydes. The homogeneous counterparts showed similar catalytic performance, albeit within a shorter reaction time. The increased enantioselectivity observed for the MCM-41 hybrid catalyst 62b was explained by transformations inside the channels, which is also reflected by lower yields due to hindered diffusion. The recyclability of the immobilized catalysts 62b was checked with hydrocin-namaldehyde (Ph). It was found that the reused catalyst gave nearly the same enantioselectivities after the fourth catalytic run, although the time period for achieving similar conversion increased from initially 30 to 42 h. 

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