Zeolites cation exchange

Panov, A.G., Larsen, R.G., Totah, N.I., Larsen, S.C. and Grassian, V.H. (2000). Photooxidation of toluene and p-xylene in cation-exchanged zeolites X, Y, ZSM-5, and beta the role of zeolite physicochemical properties in product yield and selectivity. J. Phys. Chem. B 104, 5706-5714... 

A 0.5-gram mass of either the organo-treated or inorganic cation exchanged zeolite and 50 mL of 10 mM/L arsenate or chromate aqueous solutions were placed into Erlenmeyer flasks and mechanically shaken in reciprocating mode to attain equilibrium. Different equilibrium periods for individual zeolite modifications and both aqueous oxyanions species have been established. The adsorption isotherm experiments were conducted using above mass/ volume ratio of samples with an initial metal concentrations ranged from 0.5 to 100 mM/L at laboratory temperature. The... 

Kulprathipanja, S., Sherman,)., Napolitano, A., and Markovs, J. (2002) Method for treating a liquid stream contaminated with an iodine-containing compound using a cation-exchanged zeolite. U.S. Patent 5,380,428. 

Figure 6.6 Acidity (sint) versus Si/AI ratio for mono-valence cation exchanged zeolites.

Yashina, T., Sato, K., Hayasaka, T., and Hara, N. (1972) Alkylation on synthetic zeolites III. Alkylation of toluene with methanol and formaldehyde on alkali cation exchange zeolite.. Catal, 26, 303. 

Even if the metal cation exchanged zeolite is not dehydrated, the introduction of the ligand or its precursors frequently requires thermal treatment, which can redistribute the cations within the zeolite. Not all metal ions will become com-plexed and their presence can complicate the characterization of the materials as well as the interpretations of catalytic reactions. In principle, the uncomplexed metal ions should be removed by back-extraction. Unfortunately, however, this is a requirement more frequently honored in the breach, rather than the observance. 

The replacement of Si4+ by Al3+ ions in the tetrahedra generates a deficit of one positive charge per aluminum ion, which must be compensated by the incorporation of extrinsic cations in the zeolite structure. The sodium or calcium ions which are most commonly found in natural or synthetic zeolites can be exchanged with other alkali, alkaline-earth, rare-earth, or transition metal ions. The zeolite open structure can accommodate not only the extraframework cations, but also various molecules provided that their size is smaller than the zeolite apertures. A key feature of cation-exchanged zeolites is the local electrostatic field associated with the cations. This has led to the view of zeolites as solid solvents (258 and references therein). 

Side-Chain Alkylation. There is continued interest in the alkylation of toluene with methanol because of the potential of the process in practical application to produce styrene.430 Basic catalysts, specifically, alkali cation-exchanged zeolites, were tested in the transformation. The alkali cation acts as weak Lewis acid site, and the basic sites are the framework oxygen atoms. The base strength and catalytic activity of these materials can be significantly increased by incorporating alkali metal or alkali metal oxide clusters in the zeolite supercages. Results up to 1995 are summarized in a review.430... 

For ethylene hydration [290], a correlation between the catalytic activity of cation-exchanged zeolites and the electronegativities of the cations was established. 

The temperatures reported in the kinetic studies range from 260 to 350°C. In most of the investigations, the hydration rates were found to be of the first order with respect to acetylene [300,302—304]. With zinc phosphate [303], cadmium—calcium phosphate [300] and cation-exchanged zeolites [304], the rates were independent of the concentration of water. Thus the simple kinetic equation... 

With cation-exchanged zeolites [304], the first-order kinetics [eqn. (18)] is explained by the degeneration of the Langmuir—Hinshelwood equation for monomolecular transformation of adsorbed acetylene in the rate-determining step... 

The degree of cation exchange was determined by direct analysis of the exchanged solid material. The cation-exchanged zeolites were dried under vacuum and hydrated by equilibrating over a saturated ammonium chloride solution. 

Figure 1. Thermal stability (°C) of cation-exchanged zeolites as a function of the degree of exchange (a) (a,b) and modul (c)...

One of the most promising methods for controlling the intensity and selectivity of processes is the introduction of various substances into the reaction mixture. Venuto et al. (58) attained a highly selective dehydrogenation of hydrocarbons over cation exchanged zeolite X by conducting the reaction in the presence of NH3. It is also well known that the addition of small amounts of water increases the activity of zeolites for carbo-nium-ion type reactions cracking (59), alkylation (58), isomerization (56,60), disproportionation (60,61,62) and others (56). 

A central feature of the mechanism that accounts for the catalytic cracking of hydrocarbons by appropriately cation exchanged zeolites is the formation of carbonium ions (also designated carbocations and alkylcarbenium ions) as intermediates. Many other reactions for which aluminosilicates, be they clay-or zeolite-based, also predicate (320) the existence of carbonium ion intermediates, formed usually by proton donation from Bronsted acid sites, have been discussed earlier (Section III,K). 

Sodium Cation Exchanger (Zeolite) Process. This is the most widely used water-softening process in industrial, commercial, institutional and household applications. Hard water is softened by flowing it, usually downward, through a bed (2 feet to over 8 feet in thickness) of a granular... 

We have shown that the isomerization of glucose into fructose (Scheme 7.2) was easily achieved in the presence of cation-exchanged zeolites and hydrotalcites, in water as the solvent, at higher temperatures and higher glucose concentrations than in the case of ion-exchange resins and enzymes as catalysts.[20]... 

Li, X. and Ramamurthy, V. (1996) Selective oxidation of olefins within organic dye cation-exchanged zeolites. Journal of the American Chemical Society, 118 (43), 10666-10667. 

Ramamurthy and coworkers reported the enantiodifferentiating photorearrangement (Scheme 5) of achiral benzonorbornadiene (16) to chiral benzonortricyclene (17) in chirally modified cation-exchanged zeolites [56]. 

Cation exchanged zeolites are successfully applied as catalysts or selective sorbents in separation technologies. " For both catalytic and sorption processes a concerted action of polarizing cations and basic oxygen atoms is important. In addition, transition metal cation embedded in zeolites exhibit peculiar redox properties because of the lower coordination in zeolite cavities compared to other supports." " Therefore, it is important to establish the strength and properties of active centers and their positions in the zeolite structure. Various experimental methods and simulation techniques have been applied to study the positions of cations in the zeolite framework and the interaction of the cations with guest molecules.Here, some of the most recent theoretical studies of cation exchanged zeolites are summarized. 

External versus Internal Cationic Sites and Complex Interactions in Cation Exchanged Zeolites... 

Repeatedly from the 1980s onwards, and especially when I was grappling with many of the then unknown aspects of zeolite science, I often consulted Rabo et al. The paper gave authoritative accounts of certain features pertaining to structure, adsorbability and reactivity of various kinds of cation-exchanged zeolites. It also highlighted some of the properties of these solids that were enigmatic or poorly understood. 

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