Faujasite base exchanged

A new dimension to acid-base systems has been developed with the use of zeolites. As illustrated in Fig. XVIII-21, the alumino-silicate faujasite has an open structure of interconnected cavities. By exchanging for alkali metal (or NH4 and then driving off ammonia), acid zeolites can be obtained whose acidity is comparable to that of sulfuric acid and having excellent catalytic properties (see Section XVIII-9D). Using spectral shifts, zeolites can be put on a relative acidity scale [195]. An important added feature is that the size of the channels and cavities, which can be controlled, gives selectivity in that only... 

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. 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

The protons released are presumably available to compensate for the loss of the charge balancing cations within the zeolite. In conventional syntheses, the phtha-lonitrile condensation normally requires the nucleophilic attack of a strong base on the phthalonitrile cyano group [176, 177]. This function is presumably accommodated by the Si-O-Al (cation) basic sites within the ion-exchanged faujasite zeolites [178, 179]. The importance of this role is perhaps emphasized by the widespread use of alkali metal exchanged faujasites, particularly the more basic NaX materials of higher aluminium content [180, 181] as hosts for encapsulated phthalocyanine complexes. 

Co2 + -exchanged faujasite zeolite is a unique heterogeneous catalyst for liquid-phase epoxidation using 02 [45]. This catalyst is active only for styrene, and the conversion and yield of styrene oxide were 65 and 45%, respectively. The TON, based on Co ions, reached 12. The Co2+ ions, located in supercages, are thought to cause activation of 02 for epoxidation. 

Table 1 summarizes the data on the thermal stability of the hydroxyl groups in faujasites and mordenites. The table contains the results derived from the measurements of the relative intensity of the hydroxyl bands at 3640 cm l and 3610 cm l as a function of the calcination temperature for faujasites and mordenites, respectively, vrith different amounts of the framework Al. Included also are the data calculated from the concentration of OH groups foimd by H-D exchange and from the high temperature weight loss based on thermogravimet-ric analysis. 

It is evident that the presence of water (or perhaps of other proton donors) introduces ambiguities into the interpretation of catalytic results based on a series of cation-exchanged zeolites with different calculated (40,48) electrostatic field strengths. Although the water source could be an added promoter (49-51), it could also arise via elimination in alcohol dehydration, a reaction which is known (6,7,5i,5< ,56) to proceed smoothly over mono- and divalent cation-exchanged faujasites. 

The major products were -( ) and y- (II) picolines traces of pyridine and small amounts of ethylpyridines and higher molecular weight bases were also formed. Similar products have been observed when the same reactants were passed over silica-alumina 148). Zeolite catalysts, notably silver-exchanged X-type faujasites, have also proven effective in the synthesis of methylpyridines from acetylene and NHs, and methylacetylene and NHs, at temperatures ranging from 100° to 300° 149). 

In the first method the metal complex is assembled in the zeolite cavities by allowing the metal-exchanged zeolite to react with ligands that are small enough to access the micropores. The metal complex, once formed, is too large to diffuse out. For example, bis- or tris-bipyridyl complexes of Fe", Ru", Mn", Co" and Cu" have been encapsulated in zeolite Y (FAU) [12-15, 36], Metal-Salen and related SchifFs base complexes have been similarly encapsulated in faujasites [12-15, 37, 38]. However, in this case there is virtually no difference in kinetic diameter between the complex and the free ligand and metal-Salen complexes are readily leached by protic solvents, such as ethanol [12]. 

The history of the synthesis of zeolites has been the object of recent review articles f 1, 4,18] and some landmarks arc cited in Table 1. The story of the development of zeolites from a mineralogical oddity to a commodity subtly present in the everyday life of each of us truly began with the discovery of zeolite A and synthetic faujasites by Breck and Milton in 1949 [3, 4]. A cheap and reliable synthesis method was available for new materials, original by their structure or their composition. The stage was set for the development of industrial processes based on the outstanding properties of the zeolites microporosity, cation exchange and reactivity. 

The ship-in-a-bottle technique is perhaps the most common method for encapsulation of transition metal complexes. In this way the tetradentate Schiff base ligand SALEN (bis-salicylidene) ethylenediamine can diffuse through the 12 MR windows of faujasite. Then, when complexed with a previously exchanged metal ion, nearly square planar coordination geometry is formed inside the a-cages [97-100], Mn complexes with a chiral ligand, prepared by the ship-in-a-bottle technique inside Y and EMT zeolites, have enantioselectively carried at the epoxidation of olefins [101,102]. 

---