Catalyst materials molecular sieves

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. 

The development of new porous materials that could be used as adsorbents, catalysts, catalyst supports, molecular sieves, etc. [1], are very well discussed by several authors [2-9], describing interesting properties and characteristics of materials such as MCM-41, MCM-48, M41S, FSM16, lamellar phases, intercalation products, special CMS (carbon molecular sieves), fullerenes, carbon nanotubes, etc. being some of them silica based materials, and carbon based the others. 

A square. These materials with tunnels bigger than or equal to (2 X 2) have counter cations because the oxidation state of some manganese is lower than +4.0. The materials have cation-exchange properties [2-4]. Like zeolites, the materials might be used as shape selective catalysts and molecular sieves because of their tunnel structures and ion-exchange properties [5]. 

In addition to studies focusing exclusively on the catalyst surface, the catalyst support (when employed) can play a major role in enhacing the activity/selectivity via morphologic, electronic, and physico-chemical effects. These factors have been extensively explored in the case of thermochemical heterogeneous reactions where a variety of compounds and structures have been successfully used on an industrial scale as catalyst supports (e.g., oxides, sulfides, meso- and microporous materials (molecular sieves), polymers, carbons [251-256]). In electrocatalysis, on the other hand, the practical choice of support in gas diffusion electrodes has been largely limited thus far to carbon black particles. The high electronic conductivity requirement, combined wifli electrochemical stability and cost effectivness, imposes serious restrictions on the type of materials that could be employed as supports in electrocatalysis. 

Zeolites are an important class of materials because of their industrial use as catalysts, adsorbents ( molecular sieves ), and ion-exchangers. They can also serve as a nanoporous matrix for studying host-guest interactions. Originally the term zeolite was used for naturally occurring aluminosilicate minerals of the composition... 

Sodium alumiaate is widely used in the preparation of alumina-based catalysts. Aluminosilicate [1327-36-2] can be prepared by impregnating siHca gel with alumiaa obtained from sodium alumiaate and aluminum sulfate (41,42). Reaction of sodium alumiaate with siHca or siHcates has produced porous crystalline alumiaosiHcates which are useful as adsorbents and catalyst support materials, ie, molecular sieves (qv) (43,44). 

The surfaces of porous materials, e. g. catalysts, molecular sieves, or adsorbents, are much more readily accessible than smooth surfaces to Raman spectroscopy, because larger amounts of adsorbed substance can be placed within the laser focus, thus contributing to the scattering process. 

The second method used to reduce exliaust emissions incorporates postcombustion devices in the form of soot and/or ceramic catalytic converters. Some catalysts currently employ zeolite-based hydrocarbon-trapping materials acting as molecular sieves that can adsorb hydrocarbons at low temperatures and release them at high temperatures, when the catalyst operates with higher efficiency. Advances have been made in soot reduction through adoption of soot filters that chemically convert CO and unburned hydrocarbons into harmless CO, and water vapor, while trapping carbon particles in their ceramic honeycomb walls. Both soot filters and diesel catalysts remove more than 80 percent of carbon particulates from the exliatist, and reduce by more than 90 percent emissions of CO and hydrocarbons. 

Thermal treatments can be applied to modify the properties of a material, for example, dealumination and optimization of crystalHne phases. These techniques do not require oxidants. Oxidative thermal treatments are generally employed to activate molecular sieves, by removing the organic templates employed during synthesis. This is one of the key steps when preparing porous catalysts or adsorbents. In air-atmosphere calcination, the templates are typically combusted between 400... 

In this work we present results obtained both with batch and continuous flow operation of the gas-recycle reactor-separator utilizing Ag and Ag-Sm203 electrocatalysts and Sr(lwt%) La203 catalysts, in conjunction with Linde molecular sieve 5A as the trapping material, and discuss the synergy between the catalytic and adsorption units in view of the OCM reaction network. 

The method outUned above was initially investigated for the introduction of isolated Ti(IV) sites onto a sihca substrate for use in selective oxidation catalysis. Since the development of a silica-supported Ti(lV) epoxida-tion catalyst by Shell in the 1970s, titania-sihca materials have attracted considerable attention [135,136]. Many other titania-sihca materials have been studied in this context including, but not hmited to, TSl and TS2 (titanium-substituted molecular sieves), Ti-/i (titanium-substituted zeolite). 

An extremely versatile catalyst for a variety of synthetically useful oxidations with aqueous hydrogen peroxide is obtained by isomorphous substitution of Si by Ti in molecular sieve materials such as silicalite (the all-silica analogue of zeolite ZSM-5) and zeolite beta. Titanium(IV) silicalite (TS-1), developed by Enichem (Notari, 1988), was the progenitor of this class of materials, which have become known as redox molecular sieves (Arends et al., 1997). 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

The development of composite micro/mesoporous materials opens new perspectives for the improvement of zeolytic catalysts. These materials combine the advantages of both zeolites and mesoporous molecular sieves, in particular, strong acidity, high thermal and hydrothermal stability and improved diffusivity of bulky molecules due to reduction of the intracrystalline diffusion path length, resulting from creation of secondary mesoporous structure. It can be expected that the creation of secondary mesoporous structure in zeolitic crystals, on the one hand, will result in the improvement of the effectiveness factor in hydroisomerization process and, on the other hand, will lead to the decrease of the residence time of products and minimization of secondary reactions, such as cracking. This will result in an increase of both the conversion and the selectivity to isomerization products. 

Significant progress has been achieved in the preceding few years in the study of titanosilicate molecular sieves, especially TS-1, TS-2, Ti-beta, and Ti-MCM-41. In the dehydrated, pristine state most of the Ti4+ ions on the surfaces of these materials are tetrahedrally coordinated, being present in either one of two structures a tetrapodal (Ti(OSi)4) or a tripodal (Ti(OSi)3OH) structure. The former predominates in TS-1, TS-2, and Ti-beta, and the latter is prominent in Ti-MCM-41. The Ti ions are coordinatively unsaturated and act as Lewis acid sites that coordinatively bind molecules such as H20, NH3, CH3CN, and H202. Upon interaction with H202 or H2 + 02, the Ti ions form titanium oxo species. Spectroscopic techniques have been used to identify side-bound hydroperoxo species such as Ti(02H) and superoxo structures such as Ti(02 ) on these catalysts. 

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