Zeolite metal-catalyzed shape selectivity

The types of shape selective catalysis that occur in zeolites and molecular sieves are reviewed. Specifically, primary and secondary acid catalyzed shape selectivity and encapsulated metal ion and zero valent metal particle catalyzed shape selectivity are discussed. Future trends in shape selective catalysis, such as the use of large pore zeolites and electro- and photo-chemically driven reactions, are outlined. Finally, the possibility of using zeolites as chiral shape selective catalysts is discussed. 

Acetaldehyde decomposition, reaction pathway control, 14-15 Acetylene, continuous catalytic conversion over metal-modified shape-selective zeolite catalyst, 355-370 Acid-catalyzed shape selectivity in zeolites primary shape selectivity, 209-211 secondary shape selectivity, 211-213 Acid molecular sieves, reactions of m-diisopropylbenzene, 222-230 Activation of C-H, C-C, and C-0 bonds of oxygenates on Rh(l 11) bond-activation sequences, 350-353 divergence of alcohol and aldehyde decarbonylation pathways, 347-351 experimental procedure, 347 Additives, selectivity, 7,8r Adsorption of benzene on NaX and NaY zeolites, homogeneous, See Homogeneous adsorption of benzene on NaX and NaY zeolites... 

Isomerization of olefins or paraffins is an acid-catalyzed reaction that can be carried out with any number of strong acids, including mineral acids, sulfated metal oxides, zeolites and precious metal-modified catalysts [10]. Often the catalyst contains both an acid function and a metal function. The two most prevalent catalysts are Pt/chlorided AI2O3 and Pt-loaded zeolites. The power of zeoHtes in this reaction type is due to their shape selectivity [11] and decreased sensitivity to water or other oxygenates versus AICI3. It is possible to control the selectivity of the reaction to the desired product by using a zeoHte with the proper characteristics [12]. These reactions are covered in more detail in Chapter 14. 

The zeolite framework may impose shape selectivity in metal-catalyzed reactions due to the limitation of size against in-coming reactant molecules [4]. 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

Ruthenium is known to catalyze a number of reactions, including the Fischer-Tropsch synthesis of hydrocarbons (7) and the polymerization of ethylene (2). The higher metal dispersions and the shape selectivity that a zeolite provides has led to the study of ruthenium containing zeolites as catalytic materials (3). A number of factors affect the product distribution in Fischer-Tropsch chemistry when zeolites containing ruthenium are used as the catalyst, including the location of the metal (4) and the method of introducing ruthenium into the zeolite (3). 

The gas phase acid-catalyzed synthesis of pyridines from formaldehyde, ammonia and an alkanal is a complex reaction sequence, comprising at least two aldol condensations, an imine formation, a cyclization and a dehydrogenation (9). With acetaldehyde as the alkanal, a mixture of pyridine and picolines (methylpyridines) is formed. In comparison with amorphous catalysts, zeolites display superior performance, particularly those with MFI or BEA topology. Because formation of higher alkylpyridines is impeded in the shape-selective environment, the lifetime of zeolites is much improved in comparison with that of amorphous materials. Moreover, the catalytic performance can be enhanced by doping the structure with metals such as Pb, Co or Tl, which assist in the dehydrogenation. 

When the hydrogenation function is embedded in the crystal voids of an MFI topology, the formation of trans-isomers is strongly reduced. After partial reduction of soy bean oil with such catalyst from an iodine value of 140 to 80, virtually no trans-isomers are obtained (56). This is the result of pore mouth catalysis combined with zeolite shape selectivity. Due to the bent character of the cts-isomer chains in triglycerides, trans-configured chains preferentially enter the pore mouths for hydrogenation. In this environment, metal-catalyzed cis-trans isomerization is restricted for steric reasons as multiple readsorption is minimal. 

We also mentioned stereospecificity of metal-catalyzed reactions inside zeolite cavities. In acid catalysis by zeolites it is well known that shape selectivity can be imposed by (1) selective admission of reactants fitting into zeolite pores, (2) selective release of products able to diffuse through zeolite channels, while larger molecules are retained, and (3) transition state selectivity, favoring, e.g., a monomolecular transition state over a bimolecular state in a narrow cavity. New tools that have conceptually been added to this arsenal include the collimation of molecules diffusing through well-defined pores, which then hit an active site preferentially via one particular atom or group. 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

The use of microporous solid catalysts such as zeolites and related molecular sieves has an additional benefit in organic synthesis. The highly precise organization and discrimination between molecules by molecular sieves endows them with shape-selective properties [12] reminiscent of enzyme catalysis. The scope of molecular sieve catalysis has been considerably extended by the discovery of ordered mesoporous materials of the M41S type by Mobil scientists [13,14]. Furthermore, the incorporation of transition metal ions and complexes into molecular sieves extends their catalytic scope to redox reactions and a variety of other transition metal-catalyzed processes [15,16]. 

This chapter presents a comprehensive overview of heterogeneously catalyzed MPVO reactions. It includes, apart from the use of various metal oxides, the more recent application of chemically anchored co-ordination compounds, hydro-talcites, mesoporous materials, and zeolites as recyclable solid catalysts. Some remarkable examples of shape-selective conversions resulting in high stereoselectivities illustrate the progress made in this field. 

Evidence for transition state shape selectivity in metal containing zeolites is less persuasive. One example involves Pd clusters in the zeolite ZSM-5, which is a medium pore zeolite (Tkble 4-1) with a set of straight parallel pores intersected by a set of zig-zag pores. The addic form of the Pd containing zeolite is selective for the conversion of acetone to methylisobutyl ketone. This reaction occurs by an Aldol condensation which is catalyzed by the addic groups followed by a hydrogenation which is catalyzed by the metal clusters. [189] Pt in zeolite Y, however, is much less selective, presumably because the larger cages do not impose a shape selectivity. 

The application of zeolite encapsulated metal chelate complexes in catalysis is a promising area of research. In particular shape selective oxidations catalyzed by metallophthalocyanines (MPc), shown in Figure 1, included in synthetic faujasite (FAU) type zeolites (2-10) appear to be competitive with other molecular sieve based catalysts that may have commercial potential. The restricted apertures ( 7.4 A) to the supercages (12A) in FAU type zeolites precludes removal of the large MPc complex unless the zeolite lattice is destroyed. Such physically trapped complexes have been termed ship-in-a-bottle complexes as well as zeozymes (to reflect the biomimetic reactivity that is often associated with these catalysts). 

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