Zeolites catalytic chemistry

Scheme 33. Click chemistry in carbohydrate derivatives using a heterogeneous Cu(I)-zeolite catalytic system.

Debra R. Rolison is head of Advanced Electrochemical Materials at the Naval Research Laboratory (NRL). She received a B.S. in chemistry from Florida Atlantic University in 1975 and a Ph.D. in chemistry from the University of North Carolina at Chapel Hill in 1980 under the direction of Royce W. Murray. Dr. Rolison joined the Naval Research Laboratory as a research chemist in 1980. Her research at NRL focuses on the influence of nanoscale domains on electron- and charge-transfer reactions, with special emphasis on the surface and materials science of aerogels, electrocatalysts, and zeolites. Her program creates new nano structured materials and composites for catalytic chemistries, energy storage and conversion (fuel cells, supercapacitors, batteries, thermoelectric devices), and sensors. 

Venuto P.B., Organic Catalysis Over Zeolites A Perspective On Reaction Paths Within Micropores, Review presented at Symposium On New Catalytic Chemistry Utilizing Molecular Sieves, ACS National Meeting, Chicago, 1993 And Symposium Proceedings Microporous Materials. 2 (1994) 297-411. 

The use of molecular sieves as catalysts or catalyst components for synthesis of intermediates and fine chemical has increased impressively over the last two decades. A large number of reactions has been explored over a growing number of microporous materials. Also the level of imderstanding of the catalytic chemistry and the structure-activity relationships has greatly improved. Since the first review of Venuto and Landis in 1968 [1] and the one of Venuto in 1994 [2] the discovery of medium pore zeolites such as ZSM-5 [3] and of phosphate based molecular sieves [4] had the largest impact on the field. 

What has to be noted first is that the number of applications for a few molecular sieves is high, however there exist many more stmctures that currently have no application. The widely employed molecular sieves include various forms of faujasites, mordenites, zeolite BETA, ZSM5, TS-1, zeolite L and to a lesser degree S(Me)AP05. For the other molecular sieves examples of utilization are quite scattered and are mainly confined to comparative studies. This suggests that a move to more catalytic chemistry and less material oriented approaches is required. It has to be critically noted that the quality of the materials used often varies quite substantially and this makes it difficult to derive genuine structure-activity correlations. 

The above three are the official publications of the International Zeolite Association. Besides, lots of papers on molecular sieves and porous materials have been frequently published in some journals on inorganic chemistry (Inorg. Chem., J. Chem. Soc., Dalton Trans., etc.), physical chemistry (J. Phys. Chem. and Langmuir, etc.), material chemistry (Chem. Mater, and /. Mater. Chem. etc.), solid-state chemistry (J. Solid State Chem. and Solid, State, Sci., etc.), catalytic chemistry (J. Catal. Appl. Catal., A, Curr. Opin. Colloid. Interface Sci., etc.), and some famous communication journals, such as Chem. Commun. and Angew. Chem., etc. Some important creative letters and reviews have also been published in Nature, Science, Chem. Rev., Chem. Soc. Rev., and Acc. Chem. Res., etc. 

It is not proposed to review the properties of zeolites in general. Structural aspects related to catalysis are, however, discussed in detail. Accessibility of sites and other related factors basic to an understanding of zeolite catalytic behavior are also examined. Considerable emphasis is given to the chemistry of activation processes, the characterization of acid zeolite catalysts, and the physicochemical interactions of adsorbed molecules with zeolite surfaces. 

The catalytic chemistry of chiral complexes entrapped in zeolite cages has been the subject of recent reviews. [75-78]. Much attention has been devoted to the encapsulation of the Jacobsen... 

Whereas the appropriate forms of zeolites and related solids are widely used in acid-catalysed industrial processes, microporous solids are not currently of importance in commercial base-catalysed conversions. Instead, high-surface-area forms of alkali metal and alkaline earth metal oxides and hydroxides, often supported on alumina, fulfil the need for solid base catalysts. Nevertheless, interest remains in characterising basic sites in cationic zeolites and in developing routes to more strongly basic sites in microporous solids." Routes to the latter include the introduction of metallic forms of alkali metals or nanoparticles of metal oxides and the partial replacement of amine groups at the sites of framework oxygen atoms. Porous solid bases have been shown to exhibit a varied catalytic chemistry, particularly for reactions such as dehydrogenations,... 

Catalytic centers can be easily isolated in the intracrystalline space of zeolites. As discussed above, the Br0nsted acid sites are isolated from a certain concentration, viz. critical Si/Al ratio, onwards. As a result it is possible to promote/suppress intramolecular versus intermolecular reactions depending on the overall chemical composition of the intracrystalline zeolite space [90]. Also, occluded metal complexes can be isolated and give rise to new catalytic chemistry [91, 92]. 

So far we have proven that not only nitrates are stored onto Fe- (and Cu-) zeolite catalysts in the presence of NO2, but also that they do participate effectively in the NH3-SCR catalytic chemistry, being indeed responsible for the very high DeNOx activity associated with the Fast SCR reaction. In the next paragraph we make use of transient reaction analysis to elucidate in more detail the reactivity of surface nitrates with NO and NH3, i.e., the SCR reactants in so doing, we will also explore the individual steps of the Fast SCR mechanism. 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

Zeolite chemistry is an excellent example of how a three-dimensional surface can alter the course of chemical reactions, selecting for one product out of a host of potential candidates. In addition to the many commercial applications that they have found, shape-selective zeolites have provided the basis for a rich new area of catalytic science and technology, one expected to spawn yet more materials, knowledge, and applications. 

Acidic, high area silica-almnina had received substantial attention in ICC 1, 52-58). Perhaps the most dramatic change in the subsequent catalytic literature was the debut of zeolites. Why acid catalyzed reactions are so much faster on zeolites than on silica-alumina has been extensively discussed but probably not conclusively. One should be able to know the exact structures of catalytic sites in zeolites, but initial hopes that this would do wonders for mechanistic imderstanding have not been fully realized. Super acids and carbonium ions came into heterogeneous catalysis from homogeneous chemistry and in special cases reaction via carbonium ions seems to occur. 

In this chapter, we demonstrate the potential of such agents as catalysts/promoters in key steps for the derivatization of sugars. The most significant catalytic approaches in carbohydrate chemistry that use aluminosilicate porous materials, namely zeolites and montmorillonite clays, are reviewed and discussed. Silica gel is a porous solid silicate that has also been used for heterogeneous catalysis of organic reactions in general. We include here its usefulness as promoter and reagent support for the reactions under consideration. 

The presence in carbohydrates of multiple hydroxyl groups of similar reactivity makes the chemo- and regio-selective manipulation frequently required quite difficult. For this reason, multistep protection-deprotection approaches are regularly employed in carbohydrate chemistry, and versatile techniques for these transformations are particularly helpful. The following section addresses this aspect, concentrating on the catalytic procedures that have been developed employing zeolites and related siliceous materials. 

---