Zeolites adsorption agents

Ba-Modenite s selectivity to MX is higher than OX, but the opposite is true for BaY. This reversal in selectivity is a result of differences in adsorbent framework characteristics mordenite has higher acid strength compared to Y zeolite. Adsorption and desorption rates of xylenes are expected to be faster in BaY compared to Ba-Mordenite because Mordenite is a one-dimensional channel system while Y zeoUte is a three-dimensional channel. With the reason stated, a three-dimensional channel ZeoUte is the preferred mass separating agent of choice compared to one-or two-dimensional channels for the liquid adsorption separation. 

In a relatively few years zeolites were promoted from simple adsorption agents to catalysts of wide spread use in all fields of chemistry. Apart from their acidic properties generated by exchanging their Na+ or K+ starting forms by ammonium ions and subsequent decomposition of the latter, their unique properties as supports for various precious metals and their solution behaviour attracted much of the attention devoted to catalysis. 

Prior to their use as adsorption agents or after industrial utilization for the adsorption of water, zeolites have to be dehydrated. This is carried out at 450 to 650°C e.g. in a rotary tube furnace or a similar unit. Industrially zeolites charged with water or other compounds are regenerated directly by passing hot dry inert gas through the absorber. 

In the Parex process, the p-xylene is separated at 120 to 175 °C by selective adsorption. Separation of the Cg-aromatics is effected by an adsorbing solid material (adsorbant) and a suitable liquid. The process is based on the fact that the various components, adsorbed to a different degree, are recovered from the active surface of the adsorbant. A synthetic zeolite is used as the adsorption agent, the active centres of which are formed by cations from the first and second group (K, Ca) of the periodic table. A hydrocarbon with a low adsorption capacity, such as toluene or p-diethylbenzene is used for desoiption, and can easily be separated from p-xylene by distillation. In this process, the liquid phases are fed through a... 

The methods which involve complexation of the metal ion with chelating agents followed by adsorption on a solid phase such as octadecylsilane (ODS) [2], Zeolite [3] or activated cai bons [4] have been reported. 

Consequently, adsorption phenomena of the acylating agent on the zeolitic catalyst plays a major role in determining the course of the reaction. 

Nitrogen dioxide, N02, is a fairly small molecule with an unpaired electron and may be expected to be a selective molecule for electron-deficient or Lewis acid sites. Nevertheless, only very little spectroscopic information on the nature of surface species formed on adsorption of N02 is available. Naccache and Ben Taarit (242) have shown by infrared spectroscopy and ESR that N02 forms Cr+N02+ and Ni+N02+ complexes on chromium and nickel zeolites. Thus, N02 behaves as an electron donor and reducing agent in these zeolites. Boehm (243) has studied the adsorption of N02 on anatase and on tj-A1203, which were pretreated at very low temperatures of only 100°-150°C. At 1380 cm-1, a band which is to be attributed to a free nitrate ion, was observed. Boehm (243) has explained the formation of the nitrate ion by the disproportionation by basic OH ions ... 

Since the mordenite type zeolite has two dimensional pore structure with nonintersecting parallel channels, the internal surface area of the catalyst may be easily blocked by the adsorption of reactants as well as by the deposition of deactivating agents. To confirm the speculation that the adsorbed reactants can block the pores, the change in surface... 

Chemical vapor deposition of silanes, along with a subsequent calcination using steam, can be utilized to deposit silica (Si(>2) inside the pore system. By variation oF the temperature, the partial pressure of the silane and the duration of the treatment, location and amount of the deposited material can be controlled [104]. When, for example, tetraethoxy- or tetrame-thoxysilane are used as reacting agents on a mordenite, ZSM-5, or /1-zeolite, then a controlled deactivation of only the external cristallite surface is possible [23, 44]. This is because these are rather bulky molecules which are not able to diffuse into the pore system of the crystallite. Alternatively, an irreversible adsorption of bulky bases may serve to destroy the undesired external acidity. Suitable basic compounds are 4-methylquino-line for ZSM-5 [2] and tributylphosphite for mordenites [71]. 

Based on the observation that the best ee is obtained with bifunctional chiral agents (ephedrine, pseudoephedrine, norephedrine, and valinol see Scheme 43), we tentatively conclude that a multipoint interaction between the reactant molecule, the chiral inductor, and the zeolite interior is necessary to induce preferential adsorption of tropolone alkyl ether from a single enantiotopic face. The dependence of chiral induction (% ee) on the nature of cations (Scheme 45) suggests a crucial role of the cation present in the supercages in the chiral induction process. This is further strengthened by the results observed with wet and dry zeolites. The presence of water decreases chiral selectivity (Scheme 45). Water molecules... 

Almost all of the studies of zeolite-catalyzed FC acylations have been conducted with electron-rich substrates. There is clearly a commercial need, therefore, for systems that are effective with electron-poor aromatics. In this context, the reports [52] on the acylation of benzene with acetic acid over H-ZSM-5 in the gas phase are particularly interesting. These results suggest that the adsorption ratios of substrate, acylating agent and product are more favorable in the gas phase than in the liquid phase. 

In the present paper the change of cry stalline structure and adsorption ability of NaY zeolite is investigated with the purpose of obtaining from it the ultra stable form. An aqueous solution hexafluorosilicate is used as the agent of dealumination. 

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