Zeolites redox catalysis

Zeolites also provide convenient framework sites for activating transition metal ions for redox catalysis. Iwamolo30 has described a CutlI)/Cu(I) exchanged zeolite that holds promise for the high-temperature conversion of NO, tin diesel and auto exhaust) to N and O, ... 

B. Wichterlova, J. Dedeak, and Z. Sobalik, Redox Catalysis over Molecular Sieves. Structure and Function-active Site. Proceedings of the 12th International Zeolite Conference, Part II, ed. M.M.J. Treacy, B.K. Marcus, M.E. Bisher, and J.B., Higgins MRS, Warrendale, PA, 1998 941-973. 

We have seen that it is possible to design active sites with acidic or basic strength better adapted to the needs of a particular reaction. In comparison with progress made in acidic or basic catalysis with zeolites, redox catalysis with zeolites is in its infancy. We are just now understanding that properties such as zeolite hydrophobicity, pore dimensions, and active site location are determinant factors for this type of catalyst. Several reviews are recommended for those interested in organic chemistry [111-113]. 

In this section, we discuss the general aspects of chemical bonding in zeolites and the zeolite O H bond. Brpnsted and Lewis acid catalysis by zeolites is presented in Section 4.2. Section 4.3 covers redox catalysis by zeolites. The final three sections describe the catalytic cycle and the role of adsorption and diffusion on catalytic performance. An important question that arises in each of these sections is the relation between the micropore structure of the zeolite and its activity and selectivity. 

Redox reactions can be catalyzed by reducible cations substituted into the framework of zeolitic systems as well as polymorphic AIPO4 systems or by cations not located in the framework but in the micropores. In Chapter 8 we will discuss more extensively catalysis by Tia,Si(i 2,)02 systems using peroxides. Here we will initiate the discussion on redox catalysis with Coa Al(i 2.)P04 oxidation catalysts where reducible ions such as Co + substitute for AP+. Catalytic oxidation carried out with oxygen provides an opportunity to discuss radical-type chemistry. A second system that we will discuss is photochemical oxidation induced by the strong electrostatic field of ion-exchanged cations. We will subsequently discuss catalysis by Fe " " and Fe " " ion exchanged zeolites with comparisons to Zn + systems and the important role of the corresponding oxycation. 

Zeolites are very useful catalysts for a large variety of reactions, from acid to base and redox catalysis [27]. Hutchings et al. reported that bis(oxazoline)-modified Cu (II)-HY catalysts are effective for asymmetric carbonyl- and imino-ene reactions and aziridination of styrene [28, 29]. Recently Djakovitch and Kohler [30-34] found that Pd(II)-NaY zeolite activates aryl halides towards Heck olefination, a-arylation of malonate, and amination reactions. It is well known that alkali-exchanged faujasite zeolites are solid base catalysts [35]. Owing to the usefulness of zeolites in organic chemistry, and our interest, we recently reported the use of modified alkali-exchanged zeolite Y, NaY zeolite [36] with electron rich copper catalyst in the Y-arylation of nitrogen heterocycles with aryl halides to afford A -arylheterocycles in excellent yields under mild conditions without the use of any additive. 

The rich variety of active sites that can be present in zeolites (i) protonic acidic sites, which catalyze acid reactions (ii) Lewis-acid sites, which often act in association with basic sites (acid-base catalysis) (iii) basic sites (iv) redox sites, incorporated either in the zeolite framework (e.g., Ti of titanosHicates) or in the channels or cages (e.g., Pt clusters, metal complexes). Moreover, redox and acidic or basic sites can act in a concerted way for catalyzing bifunctional processes. 

Porous oxide catalytic materials are commonly subdivided into microporous (pore diameter <2nm) and mesoporous (2-50 nm) materials. Zeolites are aluminosilicates with pore sizes in the range of 0.3-1.2 nm. Their high acidic strength, which is the consequence of the presence of aluminium atoms in the framework, combined with a high surface area and small pore-size distribution, has made them valuable in applications such as shape-selective catalysis and separation technology. The introduction of redox-active heteroatoms has broadened the applicability of crystalline microporous materials towards reactions other than acid-catalysed ones. 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

Each zeolite type can be easily obtained over a wide range of compositions directly by synthesis and/or after various post synthesis treatments. Moreover, various compounds can be introduced or even synthesized within the zeolite pores (ship in a bottle synthesis). This explains why zeolites can be used as acid, base, acid-base, redox and bifunctional catalysts, most of the applications being however in acid and in bifunctional catalysis. 

Further variation of the stmctural and catalytic properties of four-coimected tetrahedral frameworks is obtained by the substitution of silicon or metal cations,giving materials known as SAPO s and MeAPO s, respectively. More than twenty metal aluminophosphate frameworks have been identified with Mg, Mn, Fe, Co, or Zn substituents. These give the possibility of framework redox activity (e.g. Fe +/Fe +) in catalysis as well as the usual Bronsted acidity. For further information about zeolitic and microporous phosphate frameworks see Porous Inorganic Materials and Zeolites) and recent reviews. ... 

Catalytic oxidation-reduction (redox) reactions in zeolites are generally limited to reactions of molecules for which total oxidation products are desired. One important class of such reactions falls under the category of emission control catalysis. This encompasses a broad range of potential reactions and applications for zeolite catalysts. As potential catalysts one may consider the entire spectrum of zeolitic structural types combined with the broad range of base exchange cations which are known to carry out redox reactions. 

Volume 4 is dedicated to three important topics Catalysis (Part 4.1), Heterogeneous Systems (Part 4.2), and Gas Phase Systems (Part 4.3). The six chapters of Part 4.1 cover the most important aspects of electron transfer catalysis, from fundamental concepts to organic synthesis, from carbon dioxide fixation to protein catalysis, from redox modulation to biomimetic catalysis. Part 4.2 deals with the basic aspects and the latest developments in electron transfer on semiconductors, dye-sensitized electrodes, mono- and multilayers, intercalated compounds, zeolites, micelles and related systems. Part 4.3 covers gas phase systems, from atoms to small molecules, exciplexes, and supermolecules. 

---