Application of Zeolites in Catalysis

Zeolites also been used in ion-exchange products, both synthetic and natural, were used extensively in nuclear waste cleanup after the Three Mile Island and Chernobyl nuclear accidents. New appUcations emerged for zeolites powders in two potentially major areas, odor removal and as plastic additives. There have been several reports where environment 

---

At present, the major applications of zeolites in catalysis are in the oil refinery. They find increasing application in petrochemical processes. When considering the enormous success of the application of zeolites in bulk chemistry, it is to be expected that the same trend will be seen in fine chemicals production. For this sector, it is fortunate that in bulk chemistry so much development work is being done in catalysis and in particular in zeolite synthesis and application. 

G.K. Boreskov and IQi. Minachev, Applications of Zeolites in Catalysis, AKADEMIAI KIADO, Budapest, 1979. 

An excellent comprehensive review was published by Leach on the application of zeolites to catalysis 34). Its coverage is much broader than that attempted here. However, in addition to the highlights of aluminum-deficient zeolites covered by Leach, recent significant findings will be reviewed. 

N. Y. Chen (Mobil Research Development Corp., Princeton, N. J. 08540) It might be of interest to the audience, particularly to those who are not familiar with the application of zeolites in industrial catalytic processes, to mention that since the discovery of catalysis over shape-selective zeolite first published by Weisz and Frilette in I960, a commercial process based on selective hydrocracking reactions similar to that reported in this paper has been in operation on a large scale in more than four of our refineries since 1967. A technical paper describing this process, known as the Selectoforming process, was published in 1968. 

Crystalline aluminosilicates (zeolites) have pores with diameters of the order of 1 nm. The smallness and regularity of these pores account for shape-selectivity and many of the important applications of zeolites in acid catalysis. Zeolite frameworks consist of linked TO4 tetrahedra (T = Si, Al). The zeolites that have been most often investigated as supports for metal clusters are faujasites (zeolites X and Y), which have three-dimensional pore structures incorporating nearly spherical cages with diameters of about 1.2 nm connected by apertures that are 12-membered oxygen rings, with diameters of about 0.75 nm. Zeolite LTL, which is used as the support for industrial aromatization catalysts, has a two-dimensional pore structure consisting... 

Mass transfer from the void space between the individual pellets of a bed of granulated zeolites into the crystallites proceeds in a sequence of three transport processes (1) mass transfer through the intercrystalline space, (2) penetration through the crystallite surface, and (3) intracrystalline diffusion 11-5]. Depending on the given system, each of these processes may be rate determining for the overall adsorption/desorption process. An analysis of the relative importance of each of these processes may therefore be of substantial relevance for ensuring optimum transport conditions for the application of zeolites in both fluid separation and catalysis. 

Zeolites are crystalline aluminosiUcates characterized by a structure comprising a three-dimensional pore system and regular framework formed by linked TO4 tetrahedral (T = Si, Al) with different morphological and physico-chemical properties. Due to their impressive selectivity and uniform pore structure, they have very efficient molecular sieving properties, and are able to separate molecules based on size and shape. Zeolite powders, films and membranes are widely used in catalysis, adsorption and separation applications (McLeary et al, 2006 Pina et al., 2011). Zeolites are cheap and widely available due to their abundance in both natural and synthetic forms. The application of zeolites in the membrane field is growing very fast, and has been the subject of increased research focus during the last few decades (McLeary et al., 2006). 

Schollner R, Notzel P, Herden H, Korner G (1978) In Fejes P (ed) Application of Zeolites in Heterogeneous Catalysis and Related Fields. Proc Symposium on Zeolites, Szeged, Hungary, Sept 11 -14,1978. Acta Universitas Szegediensis, Acta Pysica et Chemica, Nova Series, 24 293... 

The applications of zeolites can be divided into tliree major categories ion exchange, adsorjDtion and catalysis. The largest amount of zeolites is used in ion exchange applications while tire largest value is derived from catalytic applications [1, 33]. 

In a recent paper Pijpers et al. [2.42] have reviewed the application of XPS in the field of catalysis and polymers. Other recent applications of XPS to catalytic problems deal with the selective catalytic reduction of using Pt- and Co-loaded zeolites. Although the Al 2p line (Al from zeolite) and Pt 4/ line interfere strongly, the two oxidation states Pt and Pt " can be distinguished after careful curve-fitting [2.43]. 

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

The discovery of the new class of high-silica zeolites "pentasil" during the last decade has attracted considerable interest due to the important applications of these zeolites in catalysis. The best known member of this family of zeolites is ZSM-5, developed in the Mobil laboratories. The unusual properties of pentasil zeolites have rekindled the interest in other high-silica zeolites, prepared by dea-lumination of low-silica zeolites. In this paper we shall review the preparation methods of aluminum-deficient zeolites, and shall discuss the properties of these materials, with emphasis on recent advances in their characterization. 

There are, however, two limitations associated with preparation and application of zeolite based catalysts. First, hydrothermal syntheses Umit the extent to which zeolites can be tailored with respect to intended appUcation. Many recipes involving metals that are interesting in terms of catalysis lead to disruption of the balance needed for template-directed pore formation rather than phase separation that produces macroscopic domains of zeoUte and metal oxide without incorporating the metal into the zeohte. When this happens, the benefits of catalysis in confined chambers are lost. Second, hydrothermal synthesis of zeoHtic, silicate based soHds is also currently Hmited to microporous materials. While the wonderfully useful molecular sieving abihty is derived precisely from this property, it also Hmits the sizes of substrates that can access catalyst sites as weU as mass transfer rates of substrates and products to and from internal active sites. 

In the first chapter, Bates and van Santen summarize the theoretical foundations of catalysis in acidic zeolites. Being the most important crystalline materials used as catalysts, zeolites have been the obvious starting point for applications of theory to catalysis by solids and surfaces. Impressive progress has been made in the application of theory to account for transport, sorption, and reaction in zeolites, and the comparisons with experimental results indicate some marked successes as well as opportunities for improving both the theoretical and experimental foundations. 

A rather interesting application of zeolite-based alkene oxidation catalysis has been demonstrated by Japanese workers (46, 47). In particular, a Pd2 +, Cu2 +Y zeolite was shown to be an active and stable heterogeneous oxidation catalyst which is analogous to the well-known homogeneous Wacker catalyst system containing PdCl2 and CuCl2 (48). Under Wacker conditions (i.e., alkene/02/H20) the zeolite Y catalyst was shown to convert ethylene to acetaldehyde and propylene to acetone with selectivities in excess of 90% with C02 as the major by-product. 

Tn most applications of zeolites, it is necessary for molecules to be able to diffuse into or out of their fine pore structure, and in many of these applications, particularly catalysis, the counterdiffusion of at least 2 different kinds of molecules occurs. The rates of these diffusion processes can have a profound effect upon the apparent activity and selectivity of zeolitic catalysts (21) and upon such characteristics as dispersion and sharpness of separation in the use of zeolites in separation and purification processes. The state of knowledge of intracrystalline diffusion in zeolites is reviewed by Barrer in a paper for this symposium (4). Little is known about unidirectional diffusion in zeolites of substances of indus-... 

---