Contact-Induced Ion Exchange

Employing an interesting experiment, Kokotailo et al. [19] and Fyfe et al. [20] were able to show that even at room temperature cation exchange occurred between, e.g., Li-A and Na-A simply upon intimate contact beween the crystalhtes of the dry powders of both different cationic forms of zeolite A. This phenomenon was demonstrated by Si MAS NMR and XRD measurements. As expected, the mixture of Li-A and Na-A exhibited initially two weU-separated Si MAS NMR signals, viz., at -85.1 and -88.9 ppm (referenced to TMS). This reflected the different local environments of Si in Li-A and Na-A. Similarly, the XRD patterns showed separated reflections (cf. Fig. 65). 

Contact-induced ion exchange was also studied by Fraissard et al. [242]. These authors used the system Rb,Na-X/Na-Y and monitored the exchange by Xe NMR. Interestingly, the exchange was not observed upon mixing at 300 K, but only after heat-treating the Rb,Na-X/Na-Y mixture under vacuum to 673 K. 

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Upon grinding the mixture of iron(ll) chloride and NH4-Y (Fe/AI 0.5) at ambient temperature, partial contact-induced ion exchange proceeds according to... 

For various reasons iron acetate seems to be a promising reactant for solid-state incorporation of iron into zeolites i/ ammonium acetate, to be expected as reaction product, decomposes at relatively low temperatures to volatile compounds ii/ even if acedic acid is formed by hydrolysis of the iron salt no substantial damage of the zeolite lattice may occur because of the low acid strength of this compound iii/ iron acetate is soluble in water that may favour the contact-induced ion exchange during grinding. 

Earlier reviews of the field were somewhat limited and focused on the introduction of transition metal cations [23] or alkaU metal and rare earth cations into zeohtes [24,25]. The present contribution intends to provide a more extended and almost complete overview of the state of the art and, also, to address more recent developments such as reductive solid-state ion exchange, related processes and questions concerning the role of water in contact-induced ion exchange, comparison of conventional and solid-state exchange, and incorporation of noble metals into narrow-pore zeolites. 

Fourier transform (FT) Raman spectroscopy was appHed by Huang et al. [33] to study the solid-state ion exchange between liCl or CaCl2 and NH4-Y or Na-Y as well as contact-induced ion exchange in the systems Li-A/Na-Y, Li-A/Na-X and Li-A/Ca-A (cf. Sect. 6.2). 

Fig. 21. IR spectra of the pyridine ring deformation frequency region after pyridine adsorption and subsequent degassing at 475 K and 10 Pa of La,Na-Y obtained A by conventional and B contact-induced ion exchange (after [35], with permission)...

Fig. 65. Contact-induced ion exchange between crystallites of Li-A and Na-A zeolites Si MAS NMR and XRD patterns of Li-A/Na-A mixtures immediately after mixing (initial) and after equilibrating at ambient conditions (equilibrium) (after [20], with permission)...

DRIFT spectroscopy was used to determine Av0h shifts, induced by adsorption of N2 and hexane for zeolite H-ZSM-5 (ZSM-a and ZSM-b, Si/Al=15.5 and 26), H-mordenite (Mor-a and Mor-b, Si/AI— 6.8 and 10) and H-Y (Y-a and Y-b, Si/Al=2.5 and 10.4) samples. Catalysts were activated in 02 flow at 773 K in situ in the DRIFTS cell and contacted than with N2 at pressures up to 9 bar at 298 K or with 6.1% hexane/He mixture at 553 K, i.e., under reaction conditions. Catalytic activities of the solids were measured in a flow-through microreactor and kapp was obtained as slope of -ln(l-X0) vs. W/F plots. The concentration of Bronsted acid sites was determined by measuring the NH4+ ion-exchange capacity of the zeolite. The site specific apparent rate constant, TOFBapp, was obtained as the ratio of kapp and the concentration of Bronsted acid sites. 

It may be suggested that water adsorbed in the pore space of zeolites is involved in the mechanism of contact-induced and solid-state ion exchange in such a way that it mediates the transport of the applied salt into the pores by solvation. Thus, solubility of the Figure 2. Sequential 300 K salt applied should be a prerequisite for the... 

Reactions of atomic carbon, produced by nuclear reactions, with a number of hydrocarbons have been studied by Wolfgang and his collaborators (69). To minimize radiation induced secondary reactions which occur when use is made of C14, a technique has been developed using short-lived C11 produced by a neutron exchange reaction between a platinum foil and a C12 ion beam from a heavy ion accelerator. Part of the scattered Cu atoms has been allowed to penetrate through the thin brass foil wall of a brass vessel and come in contact with the compound wrhose reaction is studied. Products have been analyzed by gas chromatography using a technique of simultaneous mass and radioactivity determination. 

A measure of the range of influence of the source and drain is the depletion-layer width of the associated p-n junctions. The depletion layer of such a junction is the region in which all carriers have been depleted, or pushed away, due to the potential drop across the junction. This potential drop includes the applied bias across the junction and a spontaneous built-in potential drop induced by spontaneous charge exchange when p- and -regions are brought into contact. The depletion-layer width VT of an abrupt junction is related to potential drop V and dopant-ion concentration/unit volume N by... 

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