Chemical modifications zeolites

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 catalysts are predominantly modified ZSM-5 zeolite. In general, the modifications are intended to restrict pore mouth size to promote the shape selective production of para-xylene within the microporous structure. The same modifications also serve to remove external acid sites and eliminate the consecutive isomerization of para-xylene. Methods used to modify the zeolite pore openings have included silation [50], incorporation of metal oxides such as MgO, ZnO and P2O5 [51, 52], steaming and the combination of steaming and chemical modification [53]. 

Extensive studies of the acidity and basicity of zeolites by adsorption calorimetry have been carried out over the past decades, and many reviews have been published [62,64,103,118,120,121,145,146,153,154]. For a given zeolite, different factors can modify its acidity and acid strength the size and strength of the probe molecule, the adsorption temperature, the morphology and crystallinity, the synthesis mode, the effect of pretreatment, the effect of the proton exchange level, the Si/Al ratio and dealumination, the isomorphous substitution, chemical modifications, aging, and coke deposits. 

It is noteworthy that, in general, the host framework does not retain its p, porous phase upon guest release, but collapses to the structure of the apohost - the a-phase. There are a few exceptions, in which the host behaves as a zeolite retaining its structural integrity [4-6], and a copper-containing porous open-framework structure has been reported which also withstands partial chemical modification while maintaining its structure [7],... 

Zeolite membranes are amenable by surface modification with a variety of chemical functional groups using simple silane chemistry, which may provide alternative surface chemistry pathways for enzyme immobilization. In this context, Shukla et al. [238] have recently used a chemically modified zeolite-clay composite membrane for the immobilization of porcine lipase using glutaraldehyde to provide a chemical linkage between the enzyme and the membrane. The effects of pH, temperature, and solvent on the performance of such biphasic zeohte-membrane reactors have been evaluated in the hydrolysis of olive oil to fatty acids. 

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,... 

The present world rcscr es of natural gas that contains mainly methane are still underutilized due to high cost of transportation. Considerable interest is therefore presently shown in the conversion of methane to transportable liquids and feedstocks in addition to its previous sole use for heating purposes by combustion. One possible new route for the utilization of methane derived from natural gas or other sources for conversion to more valuable higher hydrocarbons is the methylation of aromatic hydrocarbons. This chapter provides a general overview of the work that has been done so far on the use of methane for catalytic methylation of model aromatic compounds and for direct liquefaction of coal for the production of liquid hydrocarbons. The review is especially focused on the use of both acidic and basic zeolites in acid-catalyzed and base-catalyzed methylation reactions, respectively. The base-catalyzed methylation reaction covered in this discussion is mainly the oxidative methylation of toluene to produce ethylbenzene and styrene. This reaction has been found to occur over basic sites incorporated into zeolites by chemical modification or by changing the electronegative charge of the zeolite framework. 

Chemical modification of zeolitic tuff and bentonite with octadecyldimethylbenzyl ammonim ion (ODMBA) increased adsorption of zearaienone (ZEN) as the levels of ODMBA increased. The organozeolite required lOX less ODMBA than the organobentonite for maximum ZEN adsorption (100%). In addition, changing the pH from 3 to 9 had little effect on adsorption of ZEN on organozeolites. 

A short review has been published by Tsitsishvili in which the structures of porous zeolites, their porosities, surfaces, types of lattice, and the effect of chemical modification on their adsorption, c omatographic, spectroscopic, and catalytic properties have been examined. The effect of the nature of the cations on the properties of the zeolites has also been discussed. 

Lastly, when the available zeolite cannot be elfective for a desired reaction, chemical modifications of the zeolite is effected to offer the required chemical and physical characteristics of the catalyst. 

Research on developing better adsorbents for H2 PSA applications is an on going effort. Structural and chemical modifications of activated carbons and synthesis of mixed-cation exchanged zeolite frameworks are two active areas of research [16]. Increasing impurity mass transfer coefficients into the adsorbent particles is another important goal needed for reducing the adsorption time of the PSA cycle, and thus reduce adsorbent inventory or increase H2 productivity. 

In terms of chemical modifications, treatment of ZSM-5 with phosphorus compounds [42,43] seems to be an interesting route to enhance the selectivity to light olefins in cracking reactions. It has been demonstrated that after phosphorus treatment, the strong acid sites of the original zeolite are replaced by an increased number of weaker acid sites, whose concentration increases after steam treatment. Finally, the combined treatment phosphorus/ REs [44] results in an improvement in both stability and activity. Apparently, REs reduce aromatics formation on the external surface area of the zeolite, whereas phosphorus reduces the loss in activity caused by dealumination. 

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