Silica/titania heterogeneous catalyst

Active heterogeneous catalysts have been obtained. Examples include titania-, vanadia-, silica-, and ceria-based catalysts. A survey of catalytic materials prepared in flames can be found in [20]. Recent advances include nanocrystalline Ti02 [24], one-step synthesis of noble metal Ti02 [25], Ru-doped cobalt-zirconia [26], vanadia-titania [27], Rh-Al203 for chemoselective hydrogenations [28], and alumina-supported noble metal particles via high-throughput experimentation [29]. 

We are developing a new method for preparing heterogeneous catalysts utilizing polyamidoamine (PAMAM) dendrimers to template metal nanoparticles. (1) In this study, generation 4 PAMAM dendrimers were used to template Pt or Au Dendrimer Encapsulated Nanoparticles (DENs) in solution. For Au nanoparticles prepared by this route, particle sizes and distributions are particularly small and narrow, with average sizes of 1.3 + 0.3 nm.(2) For Pt DENs, particle sizes were around 2 nm.(3) The DENs were deposited onto silica and Degussa P-25 titania, and conditions for dendrimer removal were examined. 

Heterogeneous catalysts that exhibit good characteristics are silica-supported mixed Mo-V heteropoly acids and their Pd salts,1317 Pd on titania,1318 supported H3PMO12O40, and heteropoly acids and salts with Pd(OAc)21320 or PdCl2.1321... 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

In general, the ammoxidation reactions are carried out in the presence of a heterogeneous catalysts at elevated temperatures, slightly increased pressure and in the gas phase. Multicomponent oxidic catalysts, mainly containing transition metal oxides are in use. Most catalysts for olefin conversions consist of bismuth and molybdenum oxides. They can be doped by a wide variety of further transition metals. In addition, oxide components like silica, alumina and titania are incorporated as inert diluents or active constituents. The catalysts for the reactions of aromatics and hetero-aromatics mainly contain vanadium, molybdenum and other transition metals. In addition, the reaction can also be carried out in liquid phase. The reported literature on this subject is rather scanty. 

For example, titania-modified mesoporous silicates was synthesized using titanium tetrabutoxide by an impregnation method with variable Si/Ti ratios and applied to the degradation of a dye. A basic dye such as methylene blue was photo-catalytically degraded by titania-modified mesoporous silica under UV irradiation. And these heterogeneous catalysts were also applied to the degradation of organic pollutants such as phenol and toluene [96]. 

As already published in the literature, sol-gel associated to supercritical drying always gives highly developed and divided materials which can have many applications in heterogeneous catalysis. Addition of a small part of Y2O3 to silica, alumina, zirconia, and titania revealed a beneficial effect on the thermal resistance of these materials. However, it is difficult to compare our results to those already published because in our work, the volumetric ratio of precursor to solvent (1/1) was much higher than the one generally used by other authors on the one hand, and on the other hand, we never added any catalyst, either acidic or basic, contrary to most of the works recently described in the literature. 

A disadvantage of HPA as catalysts lies in their relatively low thermal stability. It has been tried to stabilize them by supporting the HPA on several carriers as silica, alumina, titania [2, 3] and functionalized silica [4]. Nevertheless, the leaching of supported heteropolycompounds cannot beforehand be excluded when catalysts are used in heterogeneous liquid reactions. 

Shell subsequently developed a heterogeneous, silica-supported titania catalyst [11,12] which forms the basis of the commercial process for the epoxidation of propylene with ethylbenzene hydroperoxide. The co-product alcohol is dehydrated, in a separate step, to styrene. Ti(IV)Si02 was the first truly heterogeneous epoxidation catalyst useful for continuous operation in the liquid phase. 

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