Acid-base interactions measurement catalytic properties

As discussed above, there have been few systematic studies in which the acid or basic strength of materials relevant to catalysis has been correlated on a quantitative scale. The utility of microcalorimetric measurements of the heats of adsorption of various molecules is evident. These measurements can be used to determine the acid or basic strength of surfaces and establish their effect on the catalytic behavior of the materials. If we desire to control these acid-base properties to tailor and improve catalysts for existing processes and to design improved catalysts for new catalytic processes, a quantitative scale of the acid-base interactions is required. Appropriate correlations, perhaps involving electronegativity scales, would allow the prediction of the acid-base strength of the surface sites which can then be related to the catalytic activity of those sites. Additional research in this area is required. 

Formic acid is a popular molecule for probing the catalytic properties of metal oxides [23-28], The selectivity of its decomposition has frequently been used as a measure of the acid-base properties of oxides. This is a tempting generalization to make oxides that produce dehydration products (H2O and CO) are described as acidic oxides, while their basic counterparts produce dehydrogenation products (H2 + CO2). It has been shown that in many cases the product selectivity is better connected to the surface redox behavior of the oxide [29], Thus, more reducible surfaces produce higher yields of CO2, Consequently, particular attention has been paid in surface science studies to the interaction between adsorbed formate ions (the primary reaction intermediate) and surface metal cations, as well as to the participation of lattice oxygen anions in the surface reaction mechanism,... 

Attempts have been made of correlating the oxide acid-base properties, as determined by calorimetric techniques, and the corresponding catalytic activity and selectivity for the decomposition of propan-2-ol [126] and 4-methylpentan-2-ol [125]. As discussed in ref [127], however, this sort of correlations, in addition to the limitations inherent to the cleaning procedure applied in the calorimetric measurements, has a second difficulty under reaction conditions, the interaction of alcohols with the surface of rare earth sesquioxides leads to the formation of carboxilate and carbonate species which certainly will modify the acid-base properties of the actual catalytic surface. 

The liquid-solid interactions of physical (essentially van der Waals) and chemical (essentially acid-base) type can quench or put out some surface sites, so modifying their reactivity towards reagents. Therefore, in order to understand and predict the catalytic ability of given surfaces in different liquids, it is very important to measure the effective surface properties, besides the knowledge of the intrinsic properties. 

Abstract Attention should be devoted to the measurements of the adsorption properties of catalytic surfaces when they have to work in liquid-solid heterogeneous conditions. The mutual characteristics of the surface and the liquid affect the reagent interactions with the surface sites which could be engaged with the liquid interaction and then not-available for the reagent coordination. This leads to observe effective adsorption properties that could be different from the intrinsic properties of the surface. The possibility to quantitatively determine the effective acid properties of catalytic surfaces by base adsorption is here showed. The adsorption can proceed in any type of liquid of various characteristics (apolar, polar, aprotic, protic) with dynamic (pulse liquid chromatographic method) or equilibrium (liquid recirculation chromatographic method) methods. The measurements of effective acidity allows finding more sound relations with the catalytic activity for a better comprehension of the catalyst work and for a more correct determination of the turnover numbers in liquid-solid catalysis. 

---