Intrinsic and Effective Acidity of Oxide Surfaces

The distinction between the intrinsic acidity of a solid and the effective acidity displayed when the surface acidic groups are screened by interaction with solvent molecules becomes a topic of prominent importance when the solid has to work in contact with liquids for its practical uses. This is the case of liquid-solid heterogeneous catalysts in which the activity of the catalyst can be modified by the presence of solvent which may establish physical or chemical interactions with the acid sites of the surface. For reactions carried out in liquid phase, the knowledge of the effective acidity (in terms of number and strength of the sites) of the catalyst in given liquids allows determining sound relationships between the catalytic activity and surface acidity. 

Adsorption calorimetry in liquid is a complex matter of study because many variables are simultaneously present solids with different acidity in terms of distributions of acid sites (nature and acid strength) liquids of different polarity, proticity, and solvating ability and probes with different basicity (pKa scale). Therefore, results permit comparing for a given solid and probe couple, the influence of the solvent for a given solid and liquid couple, the influence of the probe and for a given liquid and probe couple, the properties of different solids. 

One of the main goals of the calorimetric experiments of acid-base titrations in liquid is to evaluate the effective acid strengths of the surfaces when suspended in liquids of different nature. As for the gas-solid acid-base titrations, solutions of base probe in liquids of various natures are used for titrating the acid sites of the solid which 

Various papers have recently been published concerning the liquid-phase titration of acid solids, such as acidic polymeric resins [107-109]. Example is here presented on the study of the intrinsic and effective acidities of two catalysts based on niobium niobium oxide (NBO) and niobium phosphate (NBP) [110] which found application in reaction of acid transformation of monosaccharides (fructose, glucose, in particular) to useful chemicals, like 5-hydroxymethyl-2-furaldehyde (HMF) [111, 112]. 

When a strongly basic probe, like 2-phenylethylamine (PEA), in the decane solvent is used for titrating the NBO and NBP surfaces (Fig. 8.14), the information obtained on the two acid surfaces support the conclusions obtained from the more classical determinations of acidity using gas-solid calorimetric adsorption and thermal desorption approaches [110]. The calorimetric curve of acid-base titration in decane for NBP always lies above that of NBO, and the difference between the two curves increases as the titration progresses further. This seems confirming that the main difference between the NBO and NBP surfaces has to be ascribed to the medium and weak strength acid sites. 

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