Stability mesoporous solids

Bray KL (2001) High Pressure Probes of Electronic Structure and Luminescence Properties of Transition Metal and Lanthanide Systems. 213 1 -94 Bronstein LM (2003) Nanoparticles Made in Mesoporous Solids. 226 55-89 Briicher E (2002) Kinetic Stabilities of Gadolinium(III) Chelates Used as MRI Contrast Agents. 221 103-122... 

The catalytic properties of mesoporous materials with embedded nanoparticles are mainly determined by the type of the inclusion (particle). All catalytic reactions, which are normally known for the particular metals or alloys, can be carried out with mesoporous soHds containing nanoparticles. The important advantage of mesoporous oxides is their stability at high temperatures. Due to this feature, mesoporous oxides with nanoparticles can be successfully used as catalysts in such reactions where nanoparticles embedded in polymeric systems cannot be employed. Another probable advantage of mesoporous catalysts is an appropriate use of pores as nanoreactors of certain size. This can be applicable to large molecules or to cyclization reaction where pore size and shape will influence the reactive path [90]. However, for mesoporous solids with nanoparticles such applications are not reported so far. 

Oxides Compared to silica-based networks, nonsiliceous ordered meso-poious materials have attracted less attention, due to the relative difficulty of applying the same synthesis principles to non-sihcate species and their lower stability (227). Nonsiliceous framework compositions are more susceptible to redox reactions, hydrolysis, or phase transformations to the thermodynamically preferred denser crystalline phases. Template removal has been a major issue and calcination often resulted in the collapse of the mesostracture. This was the case for mesostractured surfactant composites of mngsten oxide, molybdenum oxide, and antimony oxide, and meso-structured materials based on vanadia that were obtained at early stages. Because of their poor thermal stability, none of these mesostructures were obtained as template-free mesoporous solids (85, 228, 229). 

Mesoporous silicas can possess intercage windows within the microporous region, but their stability and acid strength are low compared to those of zeolites. One current approach to this particular problem is the use of partially crystallised zeolite nuclei as starting materials in the synthesis of mesoporous solids, in an attempt to prepare a solid with structural features of both microporous and mesoporous solids. Such solids are said to possess hierarchical porosity, and are discussed further in Chapter 10. 

Another approach to introduce mesoporous channels to give better access of reactant molecules to the microporous regions is to assemble zeolite nanoparticles around micellar templates, in a modification of the standard route to mesoporous silicas. Reported examples include structures that possess walls made out of nanocrystals of zeolites such as Beta or ZSM-5. These composite solids possess enhanced hydrothermal stabilities and acidities compared to mesoporous solids with fully amorphous walls. The improved properties are attributed to the presence of the zeolite fragments, because zeolites are known to have higher acidity and hydrothermal stability than amorphous silica/... 

The definition for catalytic purposes of a zeolite reads as follows a crystalline material with micropores and cation-exchange capacity that is insoluble in water and common organic solvents and has sufficient thermal stability that allows removal of all pore-filling agents present in the as-synthesized materials. This definition is narrower than that of the IZA Constitution, which includes mesoporous solids, metal organic frameworks (MOFs), cationic and anionic clays [3]. 

Zeolites form a class with tremendous variety. Besides the microporous solids described in the above, mesoporous materials have been synthesized. A breakthrough were the MCM-41 mesoporous zeolites with pores of typically 3 nm. Later, many related materials have been reported allowing fine-tuning of pore sizes. A recent example is the synthesis of materials with pores in the lOnm range with satisfactory uniformity and stability (Sun etai, 2001). 

In order to overcome these problems, attention was focused on the use of heterogeneous catalysis. We have found that functionalized solid materials, e.g., ionic liquids or tin triflates immobilized into mesoporous materials, can be used in N-acylation reactions as environmentally friendly replacements for traditional homogeneous acids which are useful but environmentally unacceptable catalysts [17, 18]. They had comparable activity to homogeneous reagents but can offer greater stability, safer and easier handling and can be... 

Attia, A. Zukalova, M. Pospisil, L. Kavan, L. 2007. Electrochemical impedance spectroscopy of mesoporous Al-stabilized Ti02 (anatase) in aprotic medium. J. Solid State Electrochem. 11 1163-1169. 

Dispersion of POMs onto inert solid supports with high surface areas is very important for catalytic application because the surface areas of unsupported POMs are usually very low (—10 m2g). Another advantage of dispersion of POMs onto inert supports is improvement of the stability. Therefore, immobilization of POMs on a number of supports has been extensively studied. Silica and active carbon are the representative supports [25], Basic supports such as MgO tend to decompose POMs [101-104], Certain kinds of active carbons firmly entrap POMs [105,106], The maximum loading level of POMs on active carbons is 14 wt% [107], Dispersion of POMs onto other supports such as zeolites, mesoporous molecular sieves, and apatites, is of considerable interest because of their high surface areas, unique pore systems, and possibility to modify their compositions, morphologies, and sorption properties. However, a simple impregnation of POM compounds on inert supports often results in leaching of POMs. 

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