Stabilization of porous crystals

The reason why one zeolite structure is more stable than another is generally difficult to explain, but, at least for some types, it is connected to the presence of water in the framework. As discussed by Barrer ([4] pp. 54-60) on a thermodynamic basis, the stabilization of porous crystals is strictly related to the presence of guest molecules, i.e. water molecules, some oxygens of which share electron pairs with the extra-framework cations. When a zeolite undergoes dehydration, this involves a structure distortion, due to the shift of the cations... 

Figure 8. SEM surface images of partly crystallized sections of an activated Fe Zr alloy used for ammonia synthesis [23, 24J The main image reveals the formation of a stepped iron metal structure with a porous zirconium oxide spacer structure An almost ideal transport system for gases into the interior of the catalyst is created with a large metal-oxide interface which provides high thermal and chemical stability of this structure The edge contrast in the 200 keV backscatlered raw data image arises from the large difference in emissivity between metal and oxide It is evident that only fusion and segregation-crystallization can create such an interface structure.

Bilayer architectures formed in M2(2)3(N03)4 n (where M = Co, Ni and Zn) were one of the first systems of coordination polymers to be shown as porous materials [43]. The bilayer architectures interdigitate with each other leaving small channels in the crystal lattice which were occupied by solvated water molecules. Powder X-ray studies indicate that the water molecules can be removed from the network without causing any distortion or decomposition of the network. The adsorption studies of water removed and dried sample indicated that the material is capable of adsorbing CH4, N2 and 02. About 2.3 mmol of CH4 and 0.80 mmol of N2 or 02 are adsorbed per gram of anhydrous material. The adsorption-readsorption followed the same isotherm, indicating the stability of the network throughout the process. Further, the isotherms for the adsorption-readsorption can be classified as type I in the IUPAC classification [48]. 

A synthesis protocol of porous zirconia catalyst support, through a neutral Ci3(EO)6-Zr(OC3H7)4 assembly pathway has been developed. Our studies evidenced the role played by the surfactant. It has also been observed that the increase of hydrothermal treatment time and temperature have a benefical effect on tailoring the pore sizes. The synthesized materials will be used in preparation of Au / ZrOz, Au / VO / ZrOz catalysts, which will be tested in the benzene oxidation reaction. Thermogravimetric analysis shows that the recovered zirconia present a relatively low thermal stability. Then the structure collapses due to the crystallization to more stable tetragonal and monoclinic phase. 

The first successful preparation of micro/mesoporous or micro/macroporous molecular sieves as well as mesoporous zeolite single crystals started an intensive search of optimization procedures for their synthesis, to increase their thermal stability and to tailor their acid, base and redox properties for possible applications in heterogeneous catalysis. There is no doubt that mastering of synthesis of these hierarchic materials is an important challenge in the area of porous materials. 

The modification of BEA zeolite by surface deposition of silica and impregnation with cerium oxide was studied as a tool to improve the selectivity of the reaction. The number of acid sites, particularly the strong ones, on BEA zeolite decreases with increasing amounts of silica deposited on its surface. Moreover, there is no severe pore blocking after deposition. On the contrary, cerium oxide impregnation affords a catalyst with decreased adsorption capacity because part of the cerium oxide is deposited in the channels of the zeolite crystals and blocks the porous system. In addition, cerium oxide modification creates new weak acid sites on the zeolite surface. Silica modification decreases catalytic activity but slightly increases selectivity with respect to all ortho-HAP, para-HAP and para-acetoxyacetophenone, in comparison to the unmodified BEA zeolite, and the stability of the catalyst is also improved after modification. The best reaction results are obtained over 16% cerium-oxide-modified catalyst, the selectivity with respect to the C-acetylated products being increased to about 70% while the conversion remains 60%-80%. 

Due to their pore diameters, less than 1 nm, the application of zeolites in catalytic processes is limited. On the other hand, mesoporous molecular sieves such as MCM-41 and MCM-48 with pore diameters up to 10 nm [1], have insufficient thermal and hydrothermal stability. To overcome these restrictions many efforts were imdertaken to combine tihe catalytic activity and stability of microporous zeolites with the better accessibility on the active sites of mesoporous molecular sieves [2]. The majority of the studies have been focused to the transformation of the amorphous pore walls of mesoporous molecular sieves into crystalline microporous zeolites by secondary crystallization [3], the mesostructuration of zeolite precursors [4] or the synthesis of a zeolite using porous carbons as cast [5]. The first step to develop... 

Ren L, Tsum K, Hayakawa S, Osaka A (2001) Sol-gel preparation and in vitro deposition of apatite on porous gelatin-siloxane hybrids. J Non-Crystal Solids 285 116-122 Riboud PV (1973) Composition and stability of apatite phases in the system CaO-P205-iron oxide-H20 at high temperature. Ann Chim Fr 8 381-390 (In French)... 

In the past few decades the technological possibilities and interests have boosted research in systems in highly restricted geometries in almost every field of physics — recently down to lengthscales close to or even below the molecular level. In the field of liquid crystals, the importance of electro-optical applications which incorporate ordered liquid materials [1-3] has focused the research on LC systems with high surface-to-volume ratio [4]. In order to provide mechanically stable applications, liquid crystals are dispersed in polymers, stabilized by a polymer network, fill the cavities in porous materials, etc. [5,6]. The major technological interest concerns the scattering, reflective and bistable displays, optical switches, and others. 

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