Zeolite chemistry exchange

R. von Ballmoos, The °0-exchange method in zeolite chemistry, Salle Sauerlander, Frankfurt am Main,... 

Rabo, J. A., Zeolite Chemistry and Catalysis, ACS Monograph Series 171,332 (1976). Karge, H. G. and Beyer, H. K. Solid-state ion exchange in microporous and meso-porous materials, Molecular Sieves 3(Post-Synthesis Modification I, 2002), 43-201 (2002). 

In many cases it is necessary to post-treat the prepared samples in order to obtain accessible pore systems or to exchange one guest species for another one. Here the techniques known from zeolite chemistry can be employed as well. Calcination is not so generally useful as in zeolite science, since the compounds are quite frequently susceptible to oxidation or reduction. Therefore, ion exchange or extraction methods are more important in the field of non-conven-tional open framework structures. 

Since the development of zeolite, chemistry transalkylation has been studied mainly using zeolite catalysts. Frilette used natural mordenite treated by acid. The activity was much hi er than amorophous Si02 —AI2O3, but the activity could not be maintained. Benesi reported that mordenite was about 8 times more active than Y-type zeolites and that the active centers were Brensted acid sites. Various efforts including dealumination and cation exchange have been made to improve the aging. 

There are only a few Mossbauer nuclei which are interesting in zeolite chemistry and, thus, candidates for application of Mossbauer spectroscopy in soUd-state ion exchange. However, among them is one of the most important elements, viz., iron, which has also attracted much attention in zeoUte chemistry as a key component of possible catalyst formulations. Mossbauer spectroscopy proved to be exceptionally successful in discriminating Fe + and Fe + cations residing on extra-framework sites after introduction of iron via soHd-state ion exchange. Moreover, Mossbauer spectroscopy provides information about the various coordinations of Fe + and Fe in zeoUte lattices (cf. Sect 5.3.4). 

Engelhard G, Lohse U, Samoson A, Magi M, Tarmak M, Lippmaa E (1982) Zeolites 2 59 von Ballmoos R (1981) The 0-exchange method in zeolite chemistry synthesis, characterization and dealumination of high sUica zeolites. Dalle and Sauerlander, Frankfurt/Aarau,p 185... 

We discuss here a combined process including detemplation and Fe incorporation by ion-exchange in the zeolite framework [147]. To achieve this, oxidants to decompose the organic template and Fe-cations for exchange are needed. Both requirements are in harmony with Fenton chemistry. The OH radicals can oxidize the template and the Fe-cations be exchanged simultaneously. 

Cation binding, in supramolecular chemistry, 24 40-43 Cation bridging, 11 634-635 Cation complexation, routes to, 24 41 Cation exchange, in zeolites, 16 826 Cation-exchange catalysts, 10 477-478 12 191... 

In 1962 Mobil Oil introduced the use of synthetic zeolite X as a hydrocarbon cracking catalyst In 1969 Grace described the first modification chemistry based on steaming zeolite Y to form an ultrastable Y. In 1967-1969 Mobil Oil reported the synthesis of the high silica zeolites beta and ZSM-5. In 1974 Henkel introduced zeolite A in detergents as a replacement for the environmentally suspect phosphates. By 2008 industry-wide approximately 367 0001 of zeolite Y were in use in catalytic cracking [22]. In 1977 Union Carbide introduced zeolites for ion-exchange separations. 

The discrepancy in numbers between natural and synthetic varieties is an expression of the usefulness of zeolitic materials in industry, a reflection of their unique physicochemical properties. The crystal chemistry of these aluminosilicates provides selective absorbtion and exchange of a remarkably wide range of molecules. Some zeolites have been called molecular sieves. This property is exploited in the purification and separation of various chemicals, such as in obtaining gasoline from crude petroleum, pollution control, or radioactive waste disposal (Mumpton, 1978). The synthesis of zeolites with a particular crystal structure, and thus specific absorbtion characteristics, has become very competitive (Fox, 1985). Small, often barely detectable, changes in composition and structure are now covered by patents. A brief review of the crystal chemistry of this mineral group illustrates their potential and introduces those that occur as fibers. 

The behavior of transition metal ions exchanged in zeolites is very similar to that in a homogeneous medium CuPdY zeolites are efficient substitutes for Wacker chemistry in absence of chloride ions. 

The adaptation of zeolites to a particular purpose can be done by ion exchange and by different chemical and physical treatments. Physicochemical characteristics of zeolites often reflect the modifications introduced in the structure. Different methods are used to study the modifications and their correlations with sorption properties and catalytic activity. In this section G. T. Kerr reviews the chemistry involved in the thermal activation of NH4Y zeolites. 

Whereas the synthesis of zeolite occurs in nature and in the laboratory under strongly basic conditions (pH 9-11), they are widely used as catalysts in hydrocarbon chemistry under their acidic form. In order to obtain acidic zeolites, the alkali cations (K, Li, Na, Ca, etc.) are first exchanged by NH4+C1 followed by heating which, after release of ammonia, leaves the proton loosely attached to the framework on the Si-O-Al bridging group (Figure 2.19). 

---