Mesoporous Mixed Oxide Catalysts

Mesoporous mixed oxide catalysts via non-hydrolytic sol-gel a review. Appl. Catal. A, 451, 192-206. 

Zhi et al. [31] reported the effect of the precipitator concentration on the activity of mesoporous Cu-Ce-La mixed oxide catalyst. They concluded that the precipitator concentration influences the activity of catalyst via the stability of crystal structure and mesoporous structure. 

Luo, J. Meng, M. Qian, Y. et al. Mesoporous Mixed Oxide La-Co-Ce-0 Catalysts Prepared by Citric Acid Complexation-Organic Template Decomposition Method. Chin J. Catal. 2006, 27(6), 471-473. 

The objective of this work is to study the potential of modified ZSM5 zeolite, MCM41 mesoporous silica, hydrotalcites (HD) and HD originated mixed oxides as catalysts for degradation of PE, PP PS and PVC using thermal analytical measurements and laboratory reactor experiments. 

One could find that physical mixing is a simple way to prepare molybdenum oxide loaded mesoporous or silica oxide catalysts. The rate of catalyst deactivation is expressed in terms of the percentage decrease in initial conversion after 2 h of reaction. Initial and steady state activities were taken after 0.5 and 24 h of reaction on stream, respectively. Generally, the selectivity of styrene, which is the major and desired product, is at least 96% at steady state. 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

Although hydrotalcites are relahvely stable (up to circa 500 °C), they are also of potential applicahon as precursors of mixed metal oxide catalysts, for example Reference [66]. Dehydrahon-rehydration equilibria account for the switching between hydrotalcites and mixed/supported metal oxides, which is somehmes termed the memory effect [67-69]. Recent advances have seen attempts to prepare highly dispersed LDH systems, such as those dispersed within mesoporous carbon [70]. Owing to widespread interest in their application, hydrotalcite catalysts have been the subject of a number of reviews, for example References [71-75]. Other layered-based systems have also attracted attention for application in catalysis, for example Reference [76]. 

Amorphous Sn-, Si-, and Al-containing mixed oxides with homogeneous elemental distribution, elemental domains, and well-characterized pore architecture, including micropores and mesopores, can be prepared under controlled conditions by use of two different sol-gel processes. Sn-Si mixed oxides with low Sn content are very active and selective mild acid catalysts which are useful for esterification and etherification reactions [121]. These materials have large surface areas, and their catalytic activity and selectivity are excellent. In the esterification reaction of pentaerythritol and stearic acid catalytic activity can be correlated with surface area and decreasing tin content. The trend of decreasing tin content points to the potential importance of isolated Sn centers as active sites. 

These mesoporous mixed titania-silica oxides are hydrophilic materials and are excellent catalysts for epoxidations of olefins, allylic alcohols and a,jff-unsaturated ketones with alkyl hydroperoxides in non-aqueous media [37]. Their performance can be improved even further by adding organic or inorganic bases to neutralize acid sites present on the surface [38,39], The latter cause side-reactions, especially with acid sensitive epoxides. Amine addition was particularly effective and led to the development of a mesoporous Ti-Si mixed oxide containing surface-tethered tertiary amino groups as an active, selective, and recyclable catalyst for the epoxidation of allylic alcohols [38]. 

Both microporous [23] and mesoporous [24] hydrophobic Ti-Si mixed oxides have been synthesized but their activities as epoxidation catalysts with aqueous hydrogen peroxide are, as yet, disappointingly low compared with the corresponding reactions with TBHP in organic media or with TS-1 (Section 9.1.6). 

Mesoporous titania-silica mixed oxides containing different amount of covalently boimd methyl groups were prepared in a sol-gel process, followed by low-temperature supercritical extraction with CO2. The catalysts were used for the epoxidation of various olefins and allylic alcohols [49]. The olefin epoxidation activity was generally lowered by the methyl modification, whereas in the epoxidation of allylic alcohols a maximum in activity was observed with increasing methyl content. 

The most efficient catalysts in liquid-phase oxidation of organic compoimds were crystalline mked oxides [1]. They are ionic mixed oxides or mixed oxides containing oxides supported on oxides. In the latter case, the catalytic activity of the oxide support is increased by adding one or more metal components or is obtained by immobilization of metal oxides on inactive oxide support. Metal ions were isomorphously substituted in framework positions of molecular sieves, for example, zeolites, silicalites, silica, aluminosilicate, aluminophosphates, silico-aluminophosphates, and so on, via hydrothermal synthesis or postsynthesis modification. Among these many mixed oxides with crystalline microporous or mesoporous structure, perovskites were also used as catalysts in liquid-phase oxidation. 

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