Protons, zeolitic

For a monomolecular reaction, such as the cracking of hydrocarbons by protonic zeolites, the rate expression is very similar to the one in Eq. (1.5). The rate of the reaction is now proportional to the concentration of molecules at the reaction center, the proton of the zeolite, Eq. (1.22a). 

Previously, we have developed several techniques for platinum supported zeolite catalysts to improve the benzene product purity, including on-line sulfiding [3], precoking [6], and dual-bed catalyst system [7]. We report herein an in-depth investigation on the synergism of proton zeolite and platinum supported ZSM-12 catalyst (Pt/Z12) in a cascade dual-catalyst system. 

Considering all we know up to now, the specific properties of zeolites can be summarized as follows. Zeolites are aluminosilicates with defined microporous channels or cages. They have excellent ion-exchange properties and can thus be used as water softeners and to remove heavy metal cations from solutions. Furthermore, zeolites have molecular sieve properties, making them very useful for gas separation and adsorption processes, e.g., they can be used as desiccants or for separation of product gas streams in chemical processes. Protonated zeolites are efficient solid-state acids, which are used in catalysis and metal-impregnated zeolites are useful catalysts as well. 

The ammonia is released and the protons remain in the zeolite, which then can be used as acidic catalysts. Applying this method, all extra-framework cations can be replaced by protons. Protonated zeolites with a low Si/Al ratio are not very stable. Their framework structure decomposes even upon moderate thermal treatment [8-10], A framework stabilization of Zeolite X or Y can be achieved by introducing rare earth (RE) cations in the Sodalite cages of these zeolites. Acidic sites are obtained by exchanging the zeolites with RE cations and subsequent heat treatment. During the heating, protons are formed due to the autoprotolysis of water molecules in the presence of the RE cations as follows ... 

Figure 12.6 Mechanism of anisole acetylation over protonic zeolites.

The acetylation over protonic zeolites of aromatic substrates with acetic anhydride was widely investigated. Essentially HFAU, HBEA, and HMFI were used as catalysts, most of the reactions being carried out in batch reactors, often in the presence of solvent. Owing to the deactivation effect of the acetyl group, acetylation is limited to monoacetylated products. As could be expected in electrophilic substitution, the reactivity of the aromatic substrates is strongly influenced by the substituents, for example, anisole > m-xylene > toluene > fluorobenzene. Moreover, with the poorly activated substrates (m-xylene, toluene, and fluoroben-zene) there is a quasi-immediate inhibition of the reaction. It is not the case with activated substrates such as anisole and more generally aromatic ethers. It is why we have chosen the acetylation of anisole and 2-methoxynaphtalene as an example. 

In the same experimental conditions (lh, 160°C), the protonic zeolite NaHY gives rise to quantitative percentages of conversion, being the rate 2/3 = 33 65, while at 130°C, 3h, the conversion is of a 15% and the main product the ketone 3. 

The gas phase isomerisation of o-dichlorobenzene (odCB) was studied over protonic zeolites HZSM5, HMOR, HMAZ, HOFF, HBETA and a pillared clay HPILC. All the catalysts deactivate. The deactivation rate is the highest when dry air is used as carrier gas, and the lowest with nitrogen... 

The shape selectivity in products is evidenced mainly by the mdCB/pdCB ratio at a given conversion (Table 4). The behaviour of HPILC cannot be taken into account since the majority of products are formed by radical mechanism. On the protonic zeolites the isomerisation of dichlorobenzenes follows a consecutive reaction scheme for the main part, and the mdCB/pdCB ratio depends on the odCB conversion. However, this ratio can be modified when limitation to diffusion occurs, and the final product pdCB will be then favoured. 

The catalysis by protonated zeolites, used in industrial cracking, isomerization and alkylation of hydrocarbons, involves proton transfer and formation of carbenium or car-bonium ions as reactive intermediates488,489. To understand the function of the zeolite, the reactions between CD4 and acidic hydrogens of OH groups of two zeolite samples have been studied recently490. 

The liquid phase acetylation with AA of benzenic ethers, especially of anisole and veratrole [Reactions (3.1) and (3.2)], was investigated over various protonic zeolites with 12- (large pores) or 10- (medium pores) membered ring openings (Table 3.1). 

Figure 3.7 Reaction scheme of the liquid phase transformation of phenyl acetate over protonic zeolite...

Whilst the majority of the discussion thus far has been concerned with metallo-substituted redox molecular sieves, it is important to note that proto-nated zeolite forms can also be employed for selective oxidation with aqueous hydrogen peroxide. An excellent example of this is the study conducted by the Mobil Oil Corporation.52 Their work has shown that a number of protonated zeolites such as H-ZSM-5 or zeolite-/ can be used with hydrogen peroxide to catalyse the oxidation of cyclic ketones to lactones or the co-hydroxycarboxylic acids (Figure 4.12). 

Figure 4.12 Oxidation of cyclic ketones with aqueous hydrogen peroxide in the presence of protonated zeolites.

B.2. Olefinic Hydrocarbons Formed in Various Protonic Zeolites... 

In aU three cases the protonated zeolite, HZ, is created containing Bronsted sites, which, when heated above 550 °C, lose water to prodnce Lewis sites, viz. 

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