Zeolites alkaline exchanged

The electrochemical potential for redox reaction controls the situation where atoms of one element are available to be sorbed by a zeolite containing exchangeable cations of another element. Within the zeolite and even in the absence of water, aqueous reduction potentials are usually capable of deciding whether reaction will occur, with an error due to the difference between the zeolitic environment and aqueous solution of no more than 0.1 (or perhaps 0.2) V. Accordingly there is no question that alkali-metal vapors will reduce transition-metal ions within a zeolite, and that vapors of zinc, mercury, or sulfur will not reduce the cations of the alkali or alkaline-earth metals. 

The characteristic curves of zeolites Y exchanged with alkaline cations are shown in Figure 8. In these cases, the same concentration of cations exists in the zeolite but with quite different polarising power (i.e., ratio between charge and cation radius). It is observed that by increasing the polarising power of the cations (i.e., from K to Li ), which results in an increase of the acidity of the zeolites, the negative deviation of the characteristic curve starts at lower adsorption potentials. 

Santos, J.C., Contreras, R., Chamorro, E. and Fuentealba, P. (2002) Local reactivity index defined through the density of states describes the basicity of alkaline-exchanged zeolites./. Chim. Phys., 116, 4311-4316. 

Alkali-exchanged zeolites can be used for selective condensation reactions [32-37], double bond isomerization, alkylation of toluene by methanol, mono-methylation of phenylacetonitrile with methanol and dimethyl carbonate (DMC) [38], and selective N-alkylation with methanol, ethanol, and 2-propanol as alkylating agents [39-44]. However, the basicity of alkaline exchanged zeolites is rather low. 

Despite the existence of basic sites in zeolites, the possibility of using zeolites as basic catalysts was forgotten for many years and it was realized only recently that they can also be successful in this field [56,57]. Two approaches have been used to prepare basic zeolites ion-exchange with alkali metal ions and generation within the pores of small clusters of alkali metals or oxides and alkaline earth oxides. Whereas simple ion-exchange with alkali metal ions produces relatively weakly basic sites, the presence of such clusters results in strongly basic sites. 

Table 1. Kinetic rate constants K) for the condensation of benzaldehyde with ethyl cyanoacetate (pXa 9.0), ethyl acetoacetate (pATj 10.7), and ethyl malonate (pK 13.2) on alkaline-exchanged zeolites, NaGeX, cesium-exchanged sepiolite, and calcined Mg-Al hydrotalcite.

Mori et al. (1993) studied the liquid reaction of acetaldehyde over various ZSM-5 zeolites. Although cfr-paraldehyde and trans-paraldehyde were the main reaction products, rare-earth (La, Ce,Nd,Y) exchanged H-ZSM-5 or Na-ZSM-5 zeolites exhibited high activities for the formation of trans-paraldehyde while neither cis-paraldehyde nor trans-paraldehyde were formed over alkaline-exchanged zeolite. As for other reactions it is not surprising that the catalytic behaviour depends on the zeolite used. Thus, catalysts prepared from H-ZSM-5 exhibited a higher activity than those prepared from Na-ZSM-5. This particular effect has been explained on the basis of the presence of Bronsted acid sites in the neighbourhood of a Mn+ site in the ZSM-5 zeolite pore. 

Zeolites are naturally occurring hydrous aluminum-sodium silicates in porous granule form. They are capable of exchanging their sodium base for calcium or magnesium and of expelling these alkaline earth metals for sodium by treatment with salt. Thus, they are a type of ion-exchange media. (Some zeolites act as molecular sieves by adsorption of water and polar compounds.)... 

Reactions with clay minerals can neutralize both low-pH and high-pH solutions. Neutralization of acids occurs when hydrogen ions replace Al, Mg, and Fe. In alkaline solutions, neutralization is more complex and may involve cation exchange, clay dissolution, and reaction of cations with hydroxide ions to form new minerals called zeolites.39... 

Kwak, J.H., Szanyi, J. and Peden, C.H.F. (2004) Non-thermal plasma-assisted NOx reduction over alkali and alkaline earth ion exchanged Y, FAU zeolites, Catal. Today 89, 135—41. 

Desilicated samples were obtained by alkaline treatment of calcined H-MCM-22 (Si/Al = 15). For this purpose the zeolite was suspended in 0.05-0.50 M NaOH solutions for 45 min at 323 K, followed by filtering and washing and an additional ion-exchange step to regain the H-form of the modified zeolite. 

The category of builders consists predominantly of several types of materials -specific precipitating alkaline materials such as sodium carbonate and sodium silicate complexing agents like sodium triphosphate or nitrilotriacetic acid (NTA) and ion exchangers, such as water-soluble polycarboxylic acids and zeolites (e.g., zeolite A). 

Li, P., Xiang, Y., Grassian, Y.H. and Larsen, S.C. (1999). CO adsorption as a probe of acid sites and the electric field in alkaline earth exchanged zeolite beta using FT-IR and ab initio quantum calculations. J. Phys. Chem. B 103(24), 5058-5062... 

Exchange in zeolites of alkali, alkaline earth, transition metal ions and small organic ammonium ions, has been reviewed (111), and in general, the exchange is characterized by small AG values comparable to those found in clay minerals. Althoufft identical selectivity orders for alkali and alkaline earth metal ions are obtained, as in montmorillonite, the opposite variation of AG with charge density is found. 

Sherry, H.S. (1968) The ion-exchange properties of zeolites. IV. Alkaline earth ion exchange in the synthetic zeolites linde X and Y./. Phys. Chem., 72 (12), 4086-4094. 

Martra, G., Ocule, R., Marchses, L, Centi, G., and Coluccia, S. (2002) Alkali and alkaline-earth exchanged faujasites strength of lewis base and add centres and cation site occupancy in Na- and BaY and Na- and BaX zeolites. Catal. Today, 73, 83-93. 

Kraikul, N., Rangsunvigit, P., and Kulprathipanja, S. (2005) Study on the adsorption of 1,5-1, 5- and 2,5-dimethyl-naphthalene on a series of alkaline and alkaline earth ion-exchanged faujasite zeolites. Adsorption, 12, 317. 

Ward, J.W. (1968) The nature of active sites on zeolites. 111. The alkali and alkaline earth ion-exchanged forms. 

Zeolites are crystalline aluminosilicates with porous, framework structures made up of linked [Si04] and [A104] tetrahedra that form channels and cages of discrete size [24]. The framework structures of zeolites bear a net negative charge, which must be balanced by positively charged species, typically alkali or alkaline earth metal cations these cations maybe exchanged for one another under appropriate experimental conditions. Zeolites are capable of... 

Alkali metal ion-exchanged zeolites and occluded alkali metal oxide zeolites have been investigated extensively and applied as basic catalysts for a variety of organic transformations (1,41,221,222). Zeolites modified with alkaline earth compounds have been applied much less frequently as base catalysts for organic reactions. 

UOP molecular sieves (UOP) has developed the lonsiv family of ion exchange resins for the extraction of radionuclides from wastewater. lonsiv TIE-96 is composed of a titanium-coated zeolite (Ti-zeolite) and is used to separate plutonium, strontium, and cesium from alkaline supernatant and sludge wash solutions. The technology was developed by Pacific Northwest Laboratory (PNL) for use at the West Valley, New York, nuclear waste facility. The technology is commercially available. 

---