Zeolites material modification

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 surveys show that advances in adsorphve separahons come not only from exploring new pairings of established materials and desorbents but also through modification of a zeoHte s physicochemical characteristics and synthesis of novel zeolitic materials. Much work is devoted to seeking advancements by these last two means. This work will inevitably lead to the development of new appHcahons and establishment of new industrial liquid adsorphve separahon processes. 

Chemical Modif ic at ions of Zeol ites. Some chemical treatments may modify th zeolite material Vftholuit dealumination. The purpose of such treatments is either to dissolve some amorphous materials located within the channels or cavities as discussed above or to incorporate some chemical compound onto active sites within the channels or cavities or even to artificially introduce amorphous compound or bulky cations into the zeolite pores or channels. In the latter two cases it is purposedly desired to reduce the pore volume or the pore mouth resulting in larger diffusivity resistance and, subsequently, produce different catalytic properties. In the latter case the active sites may or may not be modified but the shape selectivity is expected to be enhanced by coating the inner walls of the pore and thereby increase resistance to diffusivity. For instance in the case of ZSM-5 type zeolite many compounds of P (53, 25), Mg (53), B have been introduced,... 

Zeobase IBM-PC AT 286 and above Crystal structure of various modifications of zeolites. Entries on as many as 1300 materials are available. X-ray patterns and molecular models of zeolitic materials can be viewed on screen, plotted on printer or plotter. 27... 

In this study a bentonite was modified to a zeolitic material by alkaline treatment in a seawater medium. The structural and textural characteristics of the clays modified in this medium were determined and compared with those of the natural clay and with those of the clays modified under the same conditions but in distilled water. The samples were characterized by XRD, IR, TG and SEM. The modifications observed in the composition of the resulting zeolitic products depend not only on the NaOH concentration and treatment time but also on the nature of the synthesis media. The zeolitic products synthesized in seawater showed higher crystallinity and less heterogeneity. The treatment can be designed according to the characteristics required for the process in which the zeolitic product will be applied. 

In order to characterize such zeolitic materials and the effect of modifications the use of chemical or physical methods are described with special emphasis on techniques not described in other presentations of this book. It is shown that the use of several physical techniques is absolutely necessary for characterizing the material in a relatively satisfactory way. 

Characterization of zeolites is primarily carried out to assess tire quality of materials obtained from syntliesis and postsyntlietic modifications. Secondly, it facilitates tire understanding of tire relation between physical and chemical properties of zeolites and tlieir behaviour in certain applications. For tliis task, especially, in situ characterization metliods have become increasingly more important, tliat is, techniques which probe tire zeolite under actual process conditions. 

Vinyl octanoate was obtained from TCI (Tokyo, Japan). All other chemicals with the exception of the zeolite beta are available from Sigma Aldrich. The synthesis of a particularly active modification of low-alumina zeolite beta has been described by us. Commercial material, available as samples from, for example, Zeolyst or Siidchemie can be used, but because of excessive acidity may result in up to 15 % of styrene formation. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Overall Milton s concept of hydrothermal crystallization of reactive gels has been followed with various additions and modifications for most of the molecular sieve, zeolite, and zeotype materials synthesis since the late 1940s. 

Most recently, significant research efforts have been focused on materials compatibility and adhesion at the zeoHte/polymer interface of the mixed-matrix membranes in order to achieve enhanced separation property relative to their corresponding polymer membranes. Modification of the surface of the zeolite particles or modification of the polymer chains to improve the interfacial adhesion provide new opportunity for making successful zeolite/polymer mixed-matrix membranes with significantly improved separation performance. 

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