Porous zeolites

Different synthetic methodologies can be pursued to prepare hierarchical porous zeolites, which can be discriminated as bottom-up and top-down approaches. Whereas bottom-up approaches frequently make use of additional templates, top-down routes employ preformed zeolites that are modified by preferential extraction of one constituent via a postsynthesis treatment For the sake of conciseness, we restrict ourselves here to the discussion of the latter route. Regarding bottom-up approaches, recently published reviews provide state-of-the-art information on these methodologies [8, 9,17-19]. 

The search for better catalysts has been facilitated in recent years by molecular modeling. We are seeing here a step change. This is the subject of Chapter 1 (Molecular Catalytic Kinetics Concepts). New types of catalysts appeared to be more selective and active than conventional ones. Tuned mesoporous catalysts, gold catalysts, and metal organic frameworks (MOFs) that are discussed in Chapter 2 (Hierarchical Porous Zeolites by Demetallation, 3 (Preparation of Nanosized Gold Catalysts and Oxidation at Room Temperature), and 4 (The Fascinating Structure... 

The FPI principle can also be used to develop thin-film-coating-based chemical sensors. For example, a thin layer of zeolite film has been coated to a cleaved endface of a single-mode fiber to form a low-finesse FPI sensor for chemical detection. Zeolite presents a group of crystalline aluminosilicate materials with uniform subnanometer or nanometer scale pores. Traditionally, porous zeolite materials have been used as adsorbents, catalysts, and molecular sieves for molecular or ionic separation, electrode modification, and selectivity enhancement for chemical sensors. Recently, it has been revealed that zeolites possess a unique combination of chemical and optical properties. When properly integrated with a photonic device, these unique properties may be fully utilized to develop miniaturized optical chemical sensors with high sensitivity and potentially high selectivity for various in situ monitoring applications. 

Transition metal complexes encapsulated in the channel of zeolites have received a lot of attention, due to their high catalytic activity, selectivity and stability in field of oxidation reactions. Generally, transition metal complex have only been immobilized in the classical large porous zeolites, such as X, Y[l-4], But the restricted sizes of the pores and cavities of the zeolites not only limit the maximum size of the complex which can be accommodated, but also impose resistance on the diffusion of substrates and products. Mesoporous molecular sieves, due to their high surface area and ordered pore structure, offer the potentiality as a good host for immobilizing transition complexes[5-7]. The previous reports are mainly about molecular sieves encapsulated mononuclear metal complex, whereas the reports about immobilization of heteronuclear metal complex in the host material are few. Here, we try to prepare MCM-41 loaded with binuclear Co(II)-La(III) complex with bis-salicylaldehyde ethylenediamine schiff base. 

Multiple hydrogen bonding interactions in tetrahedral building blocks have been employed by the group of Wuest to produce a remarkable tailored porous zeolite mimic, dubbed an organic zeolite because it does not contain any inorganic components at all.104 The pore-forming material is based on... 

The role of the template in the synthesis is not merely as a porogen on the contrary, it is also responsible for many key functions [5, 9, 10]. The template (typically cationic) balances the negative charge that characterizes zeolitic framework, due to the isomorphic substitution of Si(IV) by Al(III), prearranges the secondary building units (SBUs) toward the zeolitic framework, improves the gel synthesis conditions, especially the solubility of the silica precursors, and favors the thermodynamics of the reaction by stabilizing the porous zeolite framework. 

Unlike traditional porous zeolites, whose pores are confined by tetrahedral oxide skeletons, and thus, are difficult to tune, the pores within PCPs can be systematically... 

A water pump can reach pressures of 1 Torr. An oil vacuum pump can reach 20 mTorr. A turbomolecular pump can reach pressures of 10 10 Torr (10-8 Pa). A sorption pump can reach pressures of 10-2 Torr by exposing the system to a porous zeolite cooled to liquid nitrogen temperature with a Dewar flask placed on the outside. 

The aluminosilicate zeolites may be regarded as the most important and well-established members of a special class of microporous adsorbents in which the porosity is intra-crystalline. Although zeolites have been known for over 200 years, their potential value as highly selective adsorbents was first realized about 50 years ago (Barrer, 1945, 1978). Interest was further stimulated by die announcement by Breck et al. (1956) of the synthesis of the hitherto unknown zeolite A (i.e. Linde sieve A). Since then several hundred new porous zeolites have been synthesized. 

One of the early problems with catalytic control of automobile exhaust emissions was during the few minutes immediately after starting the engine when the cold catalytic systems did not function. This was solved by developing a porous zeolite which traps the unburned hydrocarbons while the catalysts are still cold [15]. Once the catalysts have warmed up, the zeolite canister also warms, releasing the trapped hydrocarbons to the catalytic systems to perform their important control reactions. 

Finally, Vroon et al. [82,97] reported the synthesis of continuous porous films of ZSM5 on top of y-alumina supported membranes (pore diameter 4 nm) by slip-casting with a zeolite crystal suspension. The porous zeolite layers (thickness 1-2.5 pm) consist of densely packed zeolite crystals with a diameter of 70-80 nm and with micropores in the zeolite and mesopores (diameter 8-24 nm) between the zeolite particles. This zeolite layer can be used as a support for further processing, e.g., pore filling of the mesopores or deposition of catalysts. First experiments by Vroon et al. to fill the mesopores by in situ crystallisation of MFI in the pores did not result in gas-tight membranes... 

One approach to improving selectivity is to use a porous zeolite on the electrode [168, 281, 341, 342]. In addition to using the controlled porosity to preferentially control the relative diffusion rates of different species, the zeolite can be loaded with catalysts to promote the desired chemical reaction. 

So far, only few studies have been reported on the use of porous zeolite films for optical applications. While significant efforts have been extended towards the encapsulation of various dye molecules into zeolite crystals or powders (see section 2.2), integration of such systems into thin films has not been pursued by many groups yet. As one of the few examples reported so far, we discuss the inclusion of oriented hemicyanine dyes into thin zeolite films aimed at Second Harmonic Generation (SHG).[106] The zeolite film plays the important role... 

Sensors based on adsorption of species onto or into lattice structures have been reported for molecules besides water. For example, devices based on the detection of carbon dioxide adsorption onto semiconductor materials have been developed [10]. In other cases, dielectric materials that have some degree of chemical specificity have been used for making chemically-sensitive layers. One such application is the use of the highly porous zeolite lattice to detect adsorbed hydrocarbons [11]. The specific dimensions and shape of the zeolite pores allows for size and chemical selectivity in the lattice. As in the case of the humidity devices, the adsorbed molecules dipoles cause a local change in the electric fields that can be detected through a capacitive effect. 

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