Zeolites as Cation Exchangers

Water that contains significant amounts of Ca or is said to be 

Alternatively, finely powdered zeolites may be used in detergent powders as a builder since zeolite particles smaller than 10 pm do not stick to clothing. Formerly, sodium polyphosphates (Section 7.7) were used extensively as detergent builders to tie up the Ca + and Mg in hard water as soluble complexes (or as a precipitate that washes away). However, many communities ban phosphate detergents because of pollution problems (Sections 7.7 and 9.6), thus creating a major new market for zeolites. 

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The next section will give a brief account of the commercial and environmental uses of zeolites as cation exchangers. Further information can be found in recent reviews by Sherry [111] and by Townsend and Coker [112],... 

The zeolite Si/Al—O framework is rigid, but the cations are not an integral part of this framework and are often called exchangeable cations they are fairly mobile and readily replaced by other cations (hence their use as cation exchange materials). 

Zeolites have many uses, most importantly as cation exchangers (e.g., in water softening), as desiccants (i.e., drying agents), and as solid acid catalysts. 

Zeolites display a number of interesting physical and chemical properties. The three classes of phenomena of greatest practical importance are the ability to sorb organic and inorganic substances, to act as cation exchangers, and to catalyze a wide variety of reactions. The most important aspects of these are described below. 

Early studies, reviewed by Malquori (M81), showed that natural pozzolanas take up CH, including that produced by Portland cement, with the formation of products similar to those formed on hydration of the latter material. They also showed that the zeolites present in many of them were at least as reactive in this respect as the glassy constituents. Zeolites are cation exchangers, but the amounts of CaO they take up are much greater than can be thus explained moreover, cation exchange could not explain the develop-... 

Ca zeolite to about 53% cation exchange and thereafter changes only very slightly as cation exchange proceeds further (Figure 3). 

Hydration of olefins to alcohols is equilibrium limited and hence CD is potentially suitable for such applications. The catalysts used for the process are acidic catalysts such as cation-exchange resins or zeolites. The hydration of isobutylene to produce tert-h ity alcohol via CD results in a higher conversion and there is no need to recycle the water. The hydration process is catalyzed by acidic ion-exchanged resins at 85°C and about 1200 kPa. The CD process configuration involves feeding the isobutylene below the catalyst zone and the water is fed above the catalyst zone. Flooding of the reaction zone is introduced in the process to improve the contact between the catalyst and the liquid and to ensure that the water is in constant contact with the catalyst sites. Flooding of the catalyst zone apparently improves the catalyst lifetime and performance because catalyst deactivation is caused by mass transfer and liquid distribution problems. Some recent publications on the hydration of isobutylene include a patent and a study of the kinetics of the hydration process and discussions on the merits of the application of CD for hydration. 

Invention and application of synthetic zeolites 11,2> have led to a most peculiar process of innovation in chemical industry, initiated by the prospects of the three main fields of zeolite application as cation exchanger, molecular sieves and catalysts. In all these case the relevant processes in the adsorbate-adsorbent system are inevitably accompanied by a succession of transport phenomena, maintaining the molecular exchange between the active sites and the surrounding fluid. The investigation of mass transfer in zeolitic adsorbate-adsorbent systems has become therefore a major objective of both fundamental research and industrial application. 

Dichlorobenzenes are commercially synthesized in the consecutive chlorination of benzene in the presence of Lewis acid such as FeCU in liquid phase. Similar to proton-exchanged zeolites, alkali-cation exchanged zeolites can catalyze the chlorination of monochlorobenzene and zeolite catalysts tend to prevent the consecutive reactions. Furthermore, the product ratio of dichlorobenzene is varied that is the selectivity of para-dichlorobenzene ( PDCB) among DCB isomers (para-selectivity) is promoted. The selectivity to para-dichlorobenzene to more than 85 % is achieved at 80.5 wt. % conversion level of chlorobenzene in zeolite K-L catalyzed reaction. The catalyst K-L shows its superiority over conventional homogeneous catalyst, FeCh. 

The use of natural zeolites of sedimentary origin (namely zeolitized tuffs) as cation-exchangers in the treatment of waters polluted by Pb has been recommended on the basis of... 

Zeolites find widespread commercial applications and new opportunities emerge as new structures and compositions are attained. In terms of bulk quantities, synthetic zeolites are most commonly sold as cation exchangers builders) in the detergent industry zeolites are able to soften water... 

The former base-catalyzed aldose-ketose isomerization is named the Lobry de Bruyn-van Ekenstein transformation (Scheme 6.25). Deprotonation of the a-carbonyl carbon of aldose (glucose) requires a base, and results in the form of a series of enolate intermediates. Solid bases such as cation-exchanged zeolites and Mg-Al HT catalyze glucose isomerization in water [176-178]. 

Capturing of the Fe + ions in the zeolite a-cation-exchange positions was first considered theoretically in [81], The Fe + ion grafted to the zeolite framework, or captured by a zeolite lattice defect =Si-0-Fe-0-Si= was proposed in [82] as an active center with low coordination of Fe +. Such structure could be emerged as the result of iron immobilization on vicinal hydroxyl zeolite groups. Also, it was found that addition of trimethylaluminum to Fe-silicalite drastically improves the catalyst activity in the process (20.10). Based on these data a conclusion about the formation of FeAlO active species was made [83],... 

The introduction of monovalent and bivalent transition metal cations into zeolites is also possible and introduces in zeolites sites with redox activity. Several of these systems have wide application in catalysis. In particular, Co-zeolites, such as Co-MFI and Co-FER, have been deeply investigated for their activity in the CH4-SCR reaction [246]. In this case the adsorption of bases such as nitriles and ammonia, followed by IR and by TPD technique, show that they act as medium-strong Lewis acid sites. The current opinion is that these sites are catalytically active for the DeNO c reaction just when they are isolated in the zeolite cavities. A recent investigation provided evidence for the deposition of part of Co ions also at the external surface of the zeolite upon cation exchanging [85] and to their likely nonnegligible catalytic activity [247]. The deposition of Co species at the external cavities can be a reason for only apparent over-exchanging (i.e., production of zeolites with Co +/AP+ atomic ratios >0.5). 

Ojha et al. [43] have synthesized zeolites from fly ash by fusion. The authors have worked for calcined fly ash of fineness more than 80 p (the mesh size). The fly ash has been pre-treated with HCl to dealuminate and to remove its iron contents to increase the reactivity. The pre-treated fly ash and sodium hydroxide mixture in ratio of 1 1.5 has been analyzed after fusion at temperature of 500-650 °C for 1 h. The fusion product containing sodium silicates and sodium aluminates has been converted to slurry with water in 1 10 (ash water) ratio and observed for several hours under ageing. It has been demonstrated that the resulting mixture kept for crystallization at 90 °C for 6 h, finally can exhibit the presence of Na-X type zeolites with high crystallinity and excellent performance as cation exchanger. 

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