An Acidity Scale for Brönsted Acids Including H₃PW₁₂O₄₀

Abstract

The calorimetric titration of H3PW12O40 (H3PW) with a series of bases in CH3CN gives equilibrium constants and enthalpies for the three deprotonation steps. These results show that solutions of H3PW in any solvent whose basicity is equal to or weaker than CH3CN will only furnish one strong proton per molecule of acid for stoichiometric or catalytic reactions. In more strongly basic solvents (S), more S2H+ species are available, but their strength is less being leveled by the solvent basicity. In drying H3PW, it is critical to avoid the weaker acids formed by reducing the material or forming the anhydride. Calorimetric titration of a series of samples subjected to different drying procedures establishes the best procedure for preparing the anhydrous acid. For most donors, the enthalpies of reaction for the first protonation step fit the ECW model. The W term provides a measure of the dissociation energy of the conjugate base in acetonitrile. E A* and C A* permit calculation of the enthalpies for the reaction in acetonitrile of (CH3CN)H+ and H3PW with the >100 bases in the ECW correlation. The C A*/E A* ratio of 2.88 indicates significant covalency in the interactions of this acceptor. This fitting of the enthalpies by using solvation minimized, donor parameters for neutral adducts indicates that the choice of enthalpies in acetonitrile as the solvent provides a temperature-independent, solution, Brönsted acidity scale that permits comparisons with neutral acceptors. The enthalpies of reactions of pyridine with a series of Brönsted acids in acetonitrile solvent is offered, leading to the following acidity order: H3PW12O40 > CF3SO3H > p-CH3C6H4SO3H ≅ H2SO4 > CF3COOH > ClC6H4COOH.

Acidic Characterization of Copper Oxide and Niobium Pentoxide Supported on Silica–Alumina

“…Slurry microcalorimetric experiments are more sensitive than the TG-DTG technique being able to detect much lower concentrations of the strongest Brönsted sites [24,25]. For the samples with 2 and 5 mass% CuO, the amount of the strongest sites is quite similar, considering the qualitative approximation obtained by the calorimetric method.…”

Efficient Protection of Alcohols with Carboxylic Acids Using a Variety of Heteropolyoxometallates as Catalysts, Studying Effective Reaction Parameters

“…[11][12][13][14] Due to their acidic nature, redox properties, catalytic activity, selectivity, thermal resistance, and reusability, heteropoly compounds are useful and versatile catalysts in a number of transformations. [15][16] These interesting compounds generally exhibit higher catalytic activities than conventional catalysts, such as mineral acids, ion-exchange resins, zeolites, etc. in both heterogeneous and homogeneous systems.…”

Styrene and styrene dimer derivatives, cyclohexene, tert-butylbenzene and cumene as test reactions for acid strength measurements of crystalline and amorphous silica-aluminas, sulfated oxides and Amberlyst

“…It is therefore evident that carrier sulfonation increased acid strength, but both values are close or higher than that observed for the most acidic zeolite (mazzite 190-200 kJ mol -1 ) [26]. The values of the heat of adsorption of two very different probe molecules like argon [23] and pyridine [24] have also been used as a measure of acid strength:…”

“…For example, the heat effects which accompany the interaction of probe molecules with surface sites have been applied for acid strength approximation. Microcalorimetry [21], TPD [22], adsorption [23] or calorimetric titration [24,25] have been commonly used. The initial heat of ammonia adsorption at 423 K [21,26], activation energy of ammonia desorption [16] and differential heat of ammonia adsorption [22] can be applied in the classification of solids, especially zeolites, with respect to their acid strength:…”