Acid-base interactions measurement zeolites

Alternatively, deprotonation enthalpies can be evaluated from probe adsorption calorimetric data or from temperature programmed desorption (TPD) measurements. The strengths of surface Lewis acid sites and of surface basic sites can also be evaluated, in principle, by the heat of probes adsorption or desorption. In all cases, however, probes adsorption on solids can result in multiple interactions for example, van der Waals interactions can be superimposed to true acid-base interactions, which can also be multiple and finally give rise to some kind of solvation effects, in particular, in the zeolite cavities [23-25]. Thus, the pure acidity/basicity... 

Isothermal titration calorimetry is a technique that can directly measure liquid-solid or liquid-liquid interactions. This method has been used for determination of acid-base properties of solid catalysts in an inert phase (n-decane), for determination of heats of reaction in liquid phase (alkylation, anisole+benzoic anhydride on H-Beta zeolite), and for determination of heats of adsorption of different pollutants on an adsorbent [33], It is also recognized as the only technique that can directly measure the binding energies of biological processes such as protein-ligand binding and protein-protein binding [34]. 

Using a series of amines as probe molecuies, Parrilo et al. found a good correlation between heats of adsorption and gas phase proton affinities, but not with the proton transfer energies of those bases in aqueous phase [54-56]. These results indicate that the proton transfer dominates the interaction between the adsorbate and the acid sites. However, in a theoretical study published by Teraishi the fact that the heat of ammonia adsorption depends both on the proton affinity and the ammonium ion affinity was underlined [57]. With regard to the catalytic reaction in zeolites, the activity depends not only on the proton affinity but also on the stability of the cationic intermediate in the zeolite. The heat of ammonia adsorption, which includes the later effect, is thus in disagreement with proton affinity and provides a different measure of acidity, which is better suited to evaluate the acid strength of the zeolite in relation with its catalytic activity. 

Whereas the number of Brpnsted acid sites can be easily determined,146 149 their acidity may vary depending on their position in the framework and also by interaction with Lewis acid sites. For these reasons and despite the various techniques tested, there are no general and reliable methods to measure the acidity of solid acids. The synthesis of active acidic zeolites used in industry still relies on a very empirical base, the most important character being their catalytic activity. 

In other words, the medium-specific contribution is not easily measured or quantified for probe molecules in interaction with solid surfaces. Moreover, in the case of microporous solids, the short-distance interactions known as "confinement effects" are even more difficult to evaluate. In all comparisons of experimental data one should be aware that the reactivity of probe base molecules is largely influenced by the size of adsorbates and micropore dimensions. As a result, the acidity scales based on the free energy of proton transfer to a specific base are expected to depend on the choice of reference base. This fact has been confirmed experimentally, as calorimetric heats of adsorption of various bases on, e.g., zeolites, depend on the base chosen. For example, a ZH zeolite may be a stronger acid... 

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