Zeolite acidity strength

Utilization of zeolite acidic strength in Cg-aromatics (xylens) systems is illustrated next. In the presence of strong acids, xylene isomers have varying basicity (Table 6.5), with m-xylene being the most basic and p-xylene the least basic among the Cg-aromatics [27]. Based on the basicity of the xylenes, the acidity of each zeolite can be properly adjusted to selectively adsorb m-xylene or p-xylene. As demonstrated in Figure 6.4, a more acidic zeolite such as NaY will selectively adsorb m-xylene from other Cg-aromatics [28, 29], In contrast. Figure 6.5 shows that a weaker acidic zeolite such as KY will selectively adsorb p-xylene from other Cg-aromatics [30, 31]. In both systems, toluene was used as the desorbent. 

M. V. Frash, M. A. Makarova, and A. M. Rigby, J. Phys. Chem. B, 101, 2116 (1997). Quantum-Chemical Justification of the Zeolite Acid Strength Measurement by Infrared Spectroscopy. 

Owing to the emphasis on zeolite acid strength made in the literature, it is not surprising that reactivity of zeolites, when not shape selectivity phenomena are involved, has been mostly explained on the bases of the ionic terms of equation (1). 

J. F. Haw, Zeolite acid strength and reaction mechanisms in catalysis, Phys. Chem. Chem. Phys., 2002, 4, 5431-5441. 

The probe was either neutralizing hydroxyl groups or was shifting m OH frequency from an amount as high as the zeolite acid strength was high. Let us connder some examples not presented in this book by other contributors. 

Framework substitution of silicon by various trivalent elements proved to be an elegant means to shape out the zeolite acid strength. It was only after the discovery of TS-1 that framework elements were considered as potential catalytic centres on their own. The reactions catalyzed by TS-1 fall into a few categories ... 

Moscou, L. and Mon6, R., "Structure and catalytic properties of thermally and hydrothermally treated zeolites. Acid strength distribution on REX and REY", J. Catal., 30, 471 (1973). 

Besides stmctural variety, chemical diversity has also increased. Pure silicon fonns of zeolite ZSM-5 and ZSM-11, designated silicalite-l [19] and silicahte-2 [20], have been synthesised. A number of other pure silicon analogues of zeolites, called porosils, are known [21]. Various chemical elements other than silicon or aluminium have been incoriDorated into zeolite lattice stmctures [22, 23]. Most important among those from an applications point of view are the incoriDoration of titanium, cobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reactions. In some cases it remains questionable, however, whether incoriDoration into the zeolite lattice stmcture has really occurred. 

Fig. 1. a) Standard protonation enthalpy in secondary carbenium ion formation on H-(US)Y-zeolites with a varying Si/Al ratio, b) Effect of the average acid strength for a series of H-(US)Y zeolites experimental (symbols) versus calculated results based on the parameter values obtained in [11] (lines) for n-nonane conversion as a function of the space time at 506 K, 0.45 MPa, Hj/HC = 13.13 (Si/Al-ratios 2.6, 18, 60)... 

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. 

Elanany, M., Koyama, M., Kubo, M. el al. (2005) Periodic density functional investigation of Lewis acid sites in zeolites Relative strength order as revealed from NH3 adsorption, Appl. Surf. Sci., 246, 96. 

The formation of protonated H+(H20)n species can affect the acidity of the non-solvated protonic sites. Therefore, as the acid strength of the protonic sites in zeolites plays a key role in the hydrocarbon transformation reactions, driving the rate of the hydrocarbon protonation [4-6], the presence of water vapor among the reactants can modify reaction rates of the individual reactions involving in the hydrocarbon transformations. 

XL30). Mossbauer spectroscopy (KFKIj was applied to follow the state of Fe species in the zeolites. Carbon monoxide and ammonia adsorption (monitored with FTIR) (EQUINOX 55) was used to determine the nature, concentration and acid strength of the active sites in the Fe-TON zeolites. 

---