Solid acids Hammett acidity value

Sulfated tin oxide (STO) is classified as one of the strongest solid acids (STO calcined at 550°C ranks first among sulfated metal oxides according to the Hammett function scale, Hq value = 18). However, the use of STO has been more limited than that of SZ (calcined at 650°C, Hq value = —16.1) due to preparation difficulties and poor yields. However, new preparation routes are making this catalyst more accessible, and recently its use has become more widespread. In a recent study by Furuta et al., STO was compared to SZ in the esterification of n-octanoic acid with methanol. The STO catalyst showed superior activity compared to SZ at temperatures below 150°C. For instance, STO approached a 100% ester yield at 100°C, while SZ required temperatures as high as 150°C to reached similar yields. 

In 1950, Walling (I) discovered that the solid surface of some metal sulfates changes the color of Hammett indicators, suggesting a new series of solid acids, but did not consider their usage as a possible catalyst. In 1957, Tanabe et al. 2,3) were able to affirm that the solid acidity of the metal sulfates is an intrinsic one, not arising firom any impurities, and they found that it increased remarkably on heat treatment, attained a maximum value, and then decreased at higher temperatures. Since then, many of the metal sulfates heat-treated at optimum temperatures have been used as solid catalysts for various acid-catalyzed reactions. Among those reactions are the depolymerization of paraldehyde, the polymerization of propylene and of aldehydes, the hydration of propylene, the formation of formaldehyde from methylene chloride,... 

What makes Nafion so useful for catalytic purposes is the superacidity of the acidic form of the polymers. Because the sulfonic acid group is attached to a highly electron-withdrawing perfluoroalkyl backbone, a relatively high polarization of the -OH bond results. Because methods for measuring solution acidity cannot be directly applied to heterogeneous solid materials, determination of the acidity of Nafion-H is difficult. Nevertheless, studies indicate that the acidic character of Nafion-H resin is comparable with that of 100% sulfuric acid, because the estimated Hammett Hq acidity function value of Nafion-H, approximately —12, is comparable with that of 100% sulfuric acid [1]. 

J. V. Sinisterra, J. M. Marinas, and A. Llobura [Can. J. Chem., 61, 230 (1983)] employed an extended form of the Hammett equation to correlate data for esterification of ethanol with substituted benzoic acids in the gas phase over a solid AIPO4 catalyst. The focus of their efforts was correlation of values of the adsorption equilibrium constants and apparent rate constants. Use the information tabulated below to prepare a Hammett plot of experimental absorption equilibrium constants versus the corresponding values of the a parameters for the substituted benzoic acids indicated. Employ the a values in Table 7.2 of the text. In principle the intercept of the best straight line through these data should correspond to the adsorption equilibrium constant for benzoic acid. By how much does this value differ from the experimental value below What value of the parameter p characterizes the chemisorption of substituted benzoic acids ... 

The essential feature of sulphonated polystyrene resins is that the sulphonic acid group offers a structure analogous to /7-toluene sulphonic acid (TSA), and hence in principle should be able to replace this acid in catalytic applications (Figure 6.23). In fact, typical acid resins are somewhat more acidic than TSA as shown by the values of the Hammett acidity function gathered together by Sharma [108] (Table 6.2) and correlates better with sulphuric acid ( 40%). The reason for this has been discussed before [99], and cooperative effects of adjacent sulphonic acid groups on the resin network probably play a role. Overall, however, the acidity of conventional resins is less than sulphuric acid (100%) and certainly lower than some of the solid inorganic acids. This automatically limits the useful application of these materials. However, acid resins do display remarkable selectivities and they have found some important niche applications (see below). Replacement of a mineral acid catalysed process by a polymer-... 

Fig. 1. Extrapolation of indicator data for various types of weak bases to standard state of infinite dilution in water to give (supposedly) thermodynamic pKa s. (1) Theoretical plot for ideal Hammett Base with pK =—6.0. Solid lines represent observable region of indicator change and dotted lines represent extrapolation. All ideal Hammett indicators should lie on line 1 of this plot with appropriate change of left-hand scale, to correspond to their pKa s. (2) Theoretical plot of ideal weak base that gives slope of 1.10. Note that it does not extrapolate to standard state in water (pH = 7) and so observed value of pHa does not refer to same standard state as ideal Hammett base. (3) Actual behavior of benzoic acid (half-protonated at Ht, = —7.38). Note that although it follows ideal Hammett base behavior in observable region of indicator change, it departs sharply from it at lower acidities as shown by activity measurements. At present there is no way of telling how many supposedly ideal Hammett bases behave in this way over part of the acidity region and what standard state they finally extrapolate to.

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