Zeolites and Catalysts: A Physical Chemistry perspective

By Adam Harder

If a zeolite is strongly heated the absorbed water escapes just as if the stone is boiling. Hence the term zeolite, which is formed from Greek words meaning boiling stone. Zeolites are crystalline, hydrated aluminosilicates with a framework structure. Their three-dimensional polyanionic networks are constructed of SiO₂ and AlO₄ tetrahedra linked through oxygen atoms. Depending on the structure type, they have regular voids containing cations and water molecules which are mobile and can be exchanged[1]. This ability to absorb and exchange ions and other molecules is what makes zeolites such a big part of the chemical industry. The purpose of this paper is to briefly summarize the chemistry and applications of zeolites used in industry today.

The structure of zeolite crystals can be represented as a plurality of tetrahedra, each of which consists of four oxygen atoms surrounding a smaller silicon or aluminum atom. Each oxygen atom of a tetrahedron can combine with another silicon or aluminum atom, which, in turn forms a tetrahedron with four oxygen atoms. Thus a solid lattice is built up from multiple tetrahedra[1]. The (-Si-O-Al-) linkages form surface pores of uniform diameter and enclose regular internal cavities and channels of discrete sizes and shapes, depending on the chemical composition and crystal structure of the specific zeolite involved. The enclosed cavities contain both metal cations and water molecules. The cations are loosely bound to the lattice and thus can engage in ion exchange. The water molecules can also be reversibly driven off in most zeolites. The regular nature of the pores and their apertures enables zeolites to function as molecular sieves. This is the outstanding property of zeolites that gives them their value as selective adsorbents for separating substances and as shape-selective catalysts. Depending on the zeolite type and its pore system, molecules can enter into the cavity system or be excluded from it. Most of the chemical and physical properties of the zeolites, and hence their areas of use, are essentially determined by the aluminum content of their frameworks. In the literature, this is usually expressed by the Si/Al or SiO₂/Al₂O₃ ratio. The surface selectivity of the zeolites as adsorbents depends on this ratio. Aluminum-rich zeolites adsorb strongly polar molecules, while increasing the silicon content leads to increasingly hydrophobic character[2].

Many synthetic zeolites also occur naturally as minerals. However, zeolites only became of industrial importance in the 1950’s, when synthetic examples became available on an industrial scale. When talking about the industrial use of zeolites it is useful to break them down into three basic types: type A, type X, and type Y. Typical chemical compositions of these types are shown below.

Type A Na₁₂(AlO₂)₁₂(SiO₂)₁₂ · 27H₂O

Type X Na₈₆(AlO₂)₈₆(SiO₂)₁₀₆ · 264H₂O

Type Y Na₆₆(AlO₂)₅₆(SiO₂)₁₃₆ · 250H₂O

_______________

[3]

The zeolite’s pore size and shape determine the size of the molecules that can enter the zeolite interior. Type A zeolites have a pore diameter such that it is an excellent exchanger of calcium ions and are consequently used as detergent builders. Types X and Y zeolites have larger pore sizes than type A. In addition to the effect of the surface pores in restricting the size and shape of molecules entering and leaving the zeolite, the size and shape of the internal cavities determine which transition states are allowed in a reaction and which products can be formed[2]. Consequently the type X and Y zeolites are used as catalysts in the catalytic cracking of petroleum, the largest zeolite catalyst market. Today the type Y zeolites have taken over this market due to their higher thermal stability caused by their relatively higher silica/alumina ratio.

Although there are many processes for zeolite synthesis, most commercial zeolite production can be categorized into two main types of processes:

¨The formation and crystallization of the zeolite from basic raw materials, via sols or an aluminosilicate hydrogel.

¨The crystallization of the zeolite in situ from calcined kaolin clay.

Hydrogel/Sol Processes

Typical sodium zeolites are formed by the crystallization of sodium aluminosilicate prepared from pure sodium aluminate, sodium silicate, and sodium hydroxide solutions.

NaOH + NaAl(OH)₄ + Na₂SiO₃ + H₂O

¯

[(Na)_a(AlO₂)_b(SiO₂)_c · NaOH · H₂O]gel

¯

(Na)_m[(AlO₂)_m(SiO₂)_n] · pH₂O + mother liquor

Factors affecting the type and structure of the zeolite formed include the crystallization temperature and length of time that the gel is maintained at that temperature, the silica/alumina ratio of the reaction mixture, and the size and type of cation present[3].

Kaolin Conversion Process

Kaolin clay can also be used as a source of alumina and silica for zeolite synthesis. For example, meta-kaolin is produced by calcining kaolin at 500-600°C. Zeolite A is then formed by the reaction of meta-kaolin with aqueous sodium hydroxide. A source of additional silica, such as sodium silicate, is required for production of type X and Y zeolites with this method[3].

The applications of synthetic zeolites then, can be broken down into three major categories: zeolites as detergent builders, zeolites used for adsorption and separations, and zeolites as catalysts. For the purposes of this paper I will focus on the use of zeolites as catalysts.

The use of zeolites as heterogeneous catalysts is most important in there use in oil-refining processes. The most important is fluid catalytic cracking (FCC), which converts vacuum distillates and residues into gaseous alkenes, gasolines, and diesel fuel. Type Y zeolites have been used as active components in the FCC process since 1964. These materials constitute 5-40% of the catalyst. Zeolite catalysts have higher activity and give higher gasoline yields and less coke formation than the amorphous alumina-silica and high-alumina catalysts formerly used. The presence of sodium ions in the zeolite would deactivate the catalyst, so most of the sodium is removed by exchange with ammonium and/or rare earth cations. These catalysts can then be calcined to drive off ammonia, producing a hydrogen ion-exchanged zeolite with a smaller unit cell volume than conventional type Y zeolites. The rare earth elements provide stability and high catalytic efficiency. The framework structure of these zeolites creates intracrystalline pores called supercages, each with a diameter of about 1.2 nm. The pore structure is three dimensional, and the supercages are connected by apertures with diameters of about 0.74 nm[4]. These cages and apertures allow a zeolite catalyst to be size selective, so it can “choose” which sized molecules it wants to catalyze. Some rather large molecules can fit through these apertures and undergo catalytic reaction in the cages.

-Figure 1: Diagram of a Zeolite Supercage[4].

The zeolite frame is made up of SiO₄ tetrahedra, which are neutral, and AlO₄ tetrahedra, which have a charge of –1. The charge of the AlO₄ tetrahedra is balanced by the charges of additional cations that exist at various crystallographically defined positions within the zeolite. Zeolites are thus ion exchangers, and the cations may be catalytically active. Analyzing catalytic activity requires the computation of factors controlling reactions at the catalytic sites in the zeolite. The electronic properties of the system are thus critical.

Zeolite catalysts effect reactions in a number of ways. For reactions with big activation energies, such as in the catalytic cracking of petroleum, you would normally need to add a large amount of energy to make the reaction proceed at any reasonable rate. Introducing a catalyst to the same reaction reduces the activation energy and allows the reaction to go at a much faster rate without the need for the addition of extensive amounts of energy. For then a higher proportion of molecules are able to pass over the activation barrier.

-Figure 2: Potential Energy vs. Time[5].

Although the new route is faster, the initial reactants and final products are the same. We know that G is a state function, so ΔG_r^Ө has the same value however the reaction is brought about. Therefore, an alternative pathway between reactants and products leaves ΔG_r^Ө, and therefore K, unchanged[5]. That is the presence of a zeolite catalyst does not change the equilibrium constant of the reaction.

Overall, zeolites are very useful in the process of separating petroleum into its different C fractions. They are size selective catalysts, which gives them the advantage of being able to choose which C fractions to separate. By changing the cations present in their structure zeolites can be used to catalyze many other reactions as well.

Bibliography

[1] L.W. Cod, K. Dijkhoff, C.J. van Oss, H.G. Roebersen, E.G. Stanford (eds.): Chemical Technology: An Encyclopedic Treatment, Vol 1, Barnes & Noble Inc., New York (1968) 136-137

[2] E. Roland, P. Kleinschmit, A.G. Degussa, Z.N. Wolfgang, et al, “Zeolites,” Ullmann’s Encyclopedia of Industrial Chemistry, Vol A28, VCH (1996) 475-500

[3] M. Smart, T Esker, A. Leder, K. Sakota, “CEH Marketing Report: Zeolites,” Chemical Economics Handbook, SRI International (1999) 599.1000A-599.1002U

[4] J. I. Kroschwitz, M. Howe-Grant, et al, Encyclopedia of Chemical Technology: 4^th Ed, Vol 5, John Wiley and Sons Inc., (1993) 358-445

[5] P. Atkins, The Elements of Physical Chemistry: with Applications in Biology, 3^rd Ed, W. H. Freeman and Co., New York, (2000) 158-238