Main Zeolites Features

12.1.1 Description and Denomination of Zeolites

Zeolites are crystalline aluminosilicates based on a three-dimensional arrangement of TO4 tetrahedra (SiO4 or AlO4) connected through their O atoms to form sub-units and then large lattices by repeating identical building blocks (unit cells, u.c.). The structure formula of zeolites (i.e., the u.c. composition) is the following: Mx/n (AlO2)x (SiO2)y, where n is the valence of M (cation or proton), x + y the number of tetrahedra per u.c., and y/x the atomic Si/Al ratio, which may vary from a minimal value of 1 (Lowenstein rule) to infinite.

Most of the zeolites can be classified into three categories according to the number of O (or T) atoms in the largest micropore apertures: 8-member-ring (8-MR) or small-pore zeolites with 8 O atoms apertures with free diameters of 0.3-0.45nm; -10-MR or medium-pore zeolites, with free diameters 0.450.60 nm; -12-MR or large-pore zeolites, with free diameters of 0.6-0.8 nm. A comparison between these pore openings and the kinetic diameter of organic molecules clearly shows that zeolites can be used for molecular sieving.

Zeolite structures are designated by a three capital-letter code,5 for example, FAU stands for the faujasite structure, to which the well-known X and Y zeolites belong. A very useful short notation is used for the description of the pore system(s): each pore network is characterized by the channel directions, the number of atoms (in bold type) in the apertures, the crystallographic free diameter of the aperture (in A), asterisks (1, 2, or 3) indicating whether the systems is one-, two-, or three-dimensional. To completely specify the pore system, the eventual presence of cages (or channel intersections) should be indicated, along with their

Figure 12.1 Pore structure of three zeolites largely used in catalytic processes.

TABLE 12.1 Commercially Used Zeolites

Structure Code (other names)

Channels

Applications

LTL MOR

AEL (SAPO-11) FER

36 48

36 96

72 24

(a) Large Pore Zeolites <100> 12 6, 6 x 6,7** $[001] 12 5,6x5,6*

(b) Medium Pore Zeolites

[001] 10 4,0 x 6,5* [001] 10 4,2 x 5,4* $[010] 8 3,5 x 4,8*

? [001] 10 4,0 x 5,5**| ? [001] 10 4,1 x 5,1** [001] 10 4,6 x 5,7

(c) Small Pore Zeolites

Catal: cumene synthesis, anisole acetylation X-Drying, separation (p-xylene) Y-Separation, catalysis (FCC, Hydrocracking, etc.) Catal: aromatization Catal: isomerization of C5-C6 alkanes, of C8 aromatics

Catal: isodewaxing Catal: isomerization of n-butenes Catal: MTO, FCC, selective synthesis of alkylbenzene, (ethylbenzene, paraxylene, etc.)

Catal: synthesis of ethylbenzene, of cumene Catal: isodewaxing

Catal: MTO

Ion exchange: detergents separation (n, isoalkanes) Drying

Note: NT/u.c: number of T(Si + Al) atoms per unit cell.

dimensions. The International Zeolite Association (IZA) coding of the main commercially used zeolites is given in Table 12.1. Figure 12.1 shows as an example the pore structure of the three zeolites that are the most used in catalysis.

12.1.2 Acid and Bifunctional Metal/Acid Catalysis: Active Sites

Most of the commercial zeolite catalyzed processes occur either through acid catalysis: fluid catalytic cracking (FCC), aromatic alkylation, methanol to olefins (MTO), acetylation, and so on; or through hydrogenating/acid bifunctional catalysis: hydroi-somerization of alkanes, hydrocracking, dewaxing, hydroisomerization of the C8 aromatic cut, and so on. Protonic zeolites are generally used, the active sites being the protons that are associated with the bridging hydroxyl groups Al (OH) Si. Therefore, their maximum concentration is equal to that of the framework Al atoms; however, the actual concentration is always smaller due to the remaining cations or to dehydroxylation and dealumination during the catalyst activation.

The rate of acidic reactions depends on the concentration of accessible protonic acid sites, on their strength, and for certain bimolecular reactions, such as hydrogen transfer, on their proximity. The concentration (and inversely the proximity) of protonic sites can be adjusted either during the synthesis (choice of the Si/Al ratio) or through postsynthesis treatment of the zeolite: ion exchange, dealumination by various methods, that is, by steaming, acid leaching, or isomorphic substitution by silicon compounds. These postsynthesis treatments may also modify the acid strength of the zeolites, thus dealumination can have a positive effect on the acid strength, the "isolated" protonic sites (no next nearest neighbours) having the maximum acid strength,6 and extraframework Al species created by steaming (which are Lewis-acid sites) increasing the acid strength of neighboring protonic sites.7 These post-synthesis treatments which are well- mastered allowed the adequate adjustment of the zeolite acidity to the catalysis of the desired reactions.

Noble metals (e.g., Pt) can be introduced within the micropores of zeolites by exchange with a complex cation (e.g., Pt(NH3)4+) followed by calcination and reduction. This mode of introduction generally leads to very small clusters of Pt (high Pt dispersion) located within the micropores. Pt supported on acid zeolites are used as bifunctional catalysts in many commercial processes. The desired transformations involve a series of catalytic and diffusion (D) steps,8 as shown in n-hexane isomerization over Pt acidic zeolite (Equation 12.1).

Under the operating conditions, the reaction intermediates (n-hexenes and i-hexenes in n-hexane isomerization) are thermodynamically very adverse, hence appear only as traces in the products. These intermediates (which are generally olefinic) are highly reactive in acid catalysis, which explains that the rates of bifunctional catalysis transformations are relatively high. The activity, stability, and selectivity of bifunctional zeolite catalysts depend mainly on three parameters: the zeolite pore structure, the balance between hydrogenating and acid functions, and their intimacy.9 In most of the commercial processes, the balance is in favor of the hydrogenation function, that is, the transformations are limited by the acid function.

Bifunctional catalysis is one of the most important routes to green (more economical and more environmentally friendly) processes.10 Indeed, the deactivation of bifunctional catalysts by coking is much slower than that of monofunctional catalysts and their selectivity generally higher (e.g., hydrocracking compared to

Reactant selectivity Product selectivity

Reactant selectivity Product selectivity

Restricted transition-state selectivity

Restricted transition-state selectivity

Selective xylene isomerization over HZSM5 zeolites (no disproportionation)

Selective xylene isomerization over HZSM5 zeolites (no disproportionation)

Aluminosilicate Type Ltl
Figure 12.2 Shape selectivity of zeolites.

cracking). Moreover, bifunctional catalysts can be used in one-pot multistep synthesis with therefore a reduction in the number of chemical and separation steps, hence, of waste production.

12.1.3 Shape Selectivity

There are four widely accepted theories of shape selectivity:11-13 reactant shape selectivity (RSS), product shape selectivity (PSS), transition state selectivity (TSS) (Figure 12.2), and concentration effect; all of them are based on the hypothesis that the reactions occur within the zeolite micropores only. As indicated earlier, this hypothesis is often verified, the external surface area of the commonly used zeolites being much lower (one to two orders of magnitude) than their internal "surface area."

12.1.3.1 Shape Selectivity Due to Molecular Sieving. The simplest types of shape selectivity are related to the impossibility for certain molecules of a reactant mixture to enter the micropores (RSS) or for certain product molecules to exit from these pores (PSS). In practice, RSS and PSS are observed not only when the molecule size is larger than the pore openings (size exclusion) but also when their diffusion rate is significantly lower (by two orders of magnitude) than that of the other molecules.

12.1.3.2 Spatioselectivity or Transition State Selectivity. TSS occurs when the formation of reaction intermediates (and/or transition states) is sterically limited by the space available near the active sites. Contrary to molecular sieving, TSS does not depend on the size of pore openings, but depends on the size and shape of cages, channels, and channel intersections. TSS often plays a significant role when the reactant(s) may undergo both monomolecular and bimolecular reactions, the latter involving much bulkier intermediates and/or transition states than the former.

12.1.3.3 Shape Selectivity Related to Molecular Concentration in Zeolite Micropores. The interaction between organic molecules and the walls of molecular-size micropores is very strong and zeolites may be considered as solid solvents.14 Therefore, the concentration of reactants in the zeolite micropores is considerably higher than in the gas phase with a significant positive effect on the reaction rates, this effect being more pronounced on bimolecular than on monomolecular reactions. This concentration of molecules in zeolite micropores is largely responsible for the observation that zeolite catalysts are much more active (10 to 10,000 times) than conventional catalysts. It is also responsible for the completely different distribution of hydrocarbons in the gasoline FCC over zeolites and amorphous silica alumina catalysts: more aromatics and alkanes with zeolites at the expense of naphthenes and alkenes owing to a higher ratio between the two major reactions in FCC, that is, hydrogen transfer (bimolecular) and cracking (monomolecular).

12.1.3.4 Other Types of Shape Selectivity. Various other types of shape selectivity have been proposed, some of them requiring additional demonstration. This is not the case for the shape selectivity of the external surface of zeolite crystallites: nest effect, pore mouth, and key lock catalysis, which is discussed in the examples in the next section.

12.2 RECENT DEVELOPMENTS 12.2.1 Isodewaxing

To meet the cold flow requirements, the high molecular-weight linear paraffins (waxes) have to be removed from distillates and base oils. This removal, called dewaxing, might be achieved either by solvent extraction or by selective catalytic conversion: hydrocracking or hydroisomerization over zeolite catalysts.15 Hydroi-somerization, which decreases the pour point without decreasing the product yield, is the most advantageous. However, whereas it is very easy to selectively isomerize C5-C6 n-alkanes over bifunctional zeolite catalysts (such as Pt HMOR), this is not generally the case for heavier hydrocarbons, because of a fast secondary cracking of branched alkanes. Indeed, with C6 alkanes, only the very slow C- type acid cracking (Equation 12.2) can occur

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Responses

  • michael
    What is large,medium and small pore zeolites?
    9 years ago
  • J
    Why zeolites are being used as a catalyst?
    9 years ago
  • Edilio
    What is size exclusion in zeolite chemistry?
    9 years ago
  • heidi
    How are zeolites green?
    9 years ago

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