Determination of Acid-Base Strength Distribution
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A New Determination of Acid-Base Strength Distribution of a Common Scale on Solid Surfaces Tohr Yamanaka Central Research Laboratory, Takasago Perfumery Company Limited, Kamata, Ohta-ku, Tokyo, Japan
and Kozo Tanabe* Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Japan (Received February 19, 1975) Publication costs assisted by the Takasago Perfumery Company Limited
A method of determining the basicity at various basic strengths by titrating a solid suspended in benzene with trichloroacetic acid using a series of Hammett indicators is proposed. The method makes it possible to determine the basic strength expressed by a Hammett acidity function and, hence, the acid-base strength distribution of a solid surface on a common scale. It has been found by this method that the basicity of selected solids at basic strengths for which Ho 2 1.5 has the order ZnO > Ti02 > y-Al203 > BaO > activated A1203 > B203 > ZrO2 > MgS04 > MOOS,the basicity of ZnO being as high as 0.62 mmol/g. MgS04 and MOOSexhibited both acidity and basicity. The present method permits the determination of basicity at relatively weak basic strength, which has not previously been measured.
Introduction Since it was reported 20 years ago by Johnson’ and Benesi,2 determination of the acidity (the number of acid sites) and acid strength (the ability of acid sites to convert an adsorbed base into its conjugate acid) on a solid surface by titration with n-butylamine, using a series of Hammett indicators, has been widely used to characterize the nature of solid acid catalysts.3 It was relatively recent that a method of determining the basicity (the number of basic sites) and basic strength (the ability of basic sites to convert an adsorbed acid into its conjugate base) on a solid surface by titration with benzoic acid or acetic acid, using Bromothymol Blue and a series of nitroaniline derivatives as indicators, was r e p ~ r t e d . ~However, -~ since the indicators used for the basicity measurement are different from those used for the acidity measurement, it is impossible to determine the acid-base strength distribution on a common scale. In order to solve this problem, we attempted to measure the basicity at various basic strengths by titrating solid samples with trichloroacetic acid using a series of Hammett indicators. By this method, the basicity at relatively weak basic strength, which has not previously been measured, can also be determined. The present paper reports the acid-base strength distributions of various oxides and magnesium sulfate determined by the new method. Principle of the Method The acid strength of a solid is defined as the ability of its surface to convert adsorbed neutral base B into its conjugate acid BH+ and can be expressed by the Hammett acidity function Ho7. This function is given by B
+ H+ = B H +
(1)
HO = -log(aH+fB/fBH+) = PKBH+ - log(CBH+/CB) (2) where a is the proton activity, CB and CBH+are respectively the concentrations of B and BH+, and fB and fBH+ are their corresponding activity coefficients. If acid sites of HOIPKBH+exist on a solid surface, the color of the basic indicator having the ~ K B H value +
changes to the color of its conjugate acid. Thus, the acidity and acid strength of a solid can be determined by titrating the solid suspended in benzene with n-butylamine using the series of Hammett basic indicators, I, listed in Table 1 . l When ~ ~ a solid has no acid sites for which Ho I ~ K I H + , the color of the basic indicator does not change. In this case, if a standard solution of Br4nsted acid in benzene is added gradually, the color of the basic indicator on the surface will change to the color of its conjugate acid. The color change is taken as the endpoint of the titration. At the endpoint, the acid strength Ho of the resultant solid, which was formed by the addition of Br4nsted acid to the original solid, is equal to the ~ K I Hof + the indicator used. As basic sites, if they exist on the surface, are neutralized by Br4nsted acid at the endpoint, the titres of Br4nsted acid required for the neutralization should give a measure of the number of basic sites (basicity) on the surface. During the titration, stronger basic sites are neutralized earlier and weaker ones later and weaker basic sites require stronger acids for the neutralization. Therefore, it can be assumed that the weakest basic sites have been finally neutralized by an acid having an acid strength for which Ho = ~ K B H + . The proton-donating ability of the solid at the endpoint of titration is considered to be either due to the conjugate acids which were formed by the proton transfer from Br4nsted acid solution to the original solid or due to the Br4nsted acid which was physically adsorbed on the surface during the titration. The proton donating ability of both the conjugate acid and the Br4nsted acid used for titration is assumed to be equal. Since the weakest basic sites form the strongest conjugate acids, the acid strength, Ho, of the conjugate acid of the weakest basic sites should be equal to or greater than the ~ K I Hof+ the indicator used. Thus, we define “basic strength Ho” of basic sites as the acid strength, Ho,of the conjugate acids. We shall express the function Ho used previously by “acid strength Ho” in the case where it is necessary to distinguish between this and “basic strength Ho”. As the basicity at “basic strength Ho” = ~ K I His+ easily determined by using a series of basic indicators as described above, the distribution of basic The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
TohrYamanakaandKozoTanabe
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TABLE I: Color and PKIH+of Indicators Color Indicators Neutral red Phenylazonaphthylamine p-Dimethylaminoazobenzene Benzeneazodiphenylamine
pKrH+ Basic (r) Acidic (I"? +4.0
Yellow Yellow
Red Red
+3.3
Yellow
Red
+1.5
Yellow
Purple
+6.8
strength of a solid as well as that of acid strength can be expressed by a common scale of acid-base strength. It should be mentioned that the use of the function Ho for basic strength is neither surprising nor curious, because the basic strengths of the organic compounds in homogeneous solution are usually expressed by the pK,'s of the conjugate acids. It should be noticed that the measurement of the basicity when the basic strength Ho is equal to or greater than a ~ K I Hvalue + is possible only when there are no acid sites whose acid strength Ho is equal to or less than the same ~ K I Hvalue. + Experimental Section Solid Materials and Reagents. Commercially available TiO2, B2O3, ZnO, MOOS, ZrO2, PbO, MgO, CaO, BaO, A1203, Ca(OH)2 (guaranteed reagents of Kanto Chemical Co.), 7-A1203 (Nikki Chemicals Co. Ltd.), and activated A1203 (Junsei Pure Chemicals Co. Ltd.) were ground and sieved to prepare powders of 100 to 200 mesh. A guaranteed reagent grade MgS04 was calcined in a dry nitrogen stream at 4OOOC for 4 hr and ground and sieved similarly. No particular caution was taken to exclude traces of moisture. The indicators as well as n-butylamine, trichloroacetic acid, and benzene were all guaranteed reagents of Wako Pure Chemical Co. Measurement of the Distribution of Acid and Basic Strength. The acidity at various acid strengths of a solid was measured by titrating the solid suspended in benzene with a 0.1 N solution of n-butylamine in benzene using the indicators listed in Table I, as reported by J0hnson.l The basicity at various basic strengths of a solid was measured by titrating the solid suspended in benzene with a 0.1 N solution of trichloroacetic acid in benzene using the same indicators as those used for acidity measurement.8 The colors of the indicators on the surface a t the endpoints of the titrations were the same as the colors which appeared by the adsorption of the respective indicators on acid sites. The color of the benzene solution was the basic color of the indicator at the endpoint, but it turned to its acidic color by adding an excess of Br4nsted acid. As the results for a titration lasting 1 hr were the same as those for a titration lasting 20 hr, 1 hr was taken for the titration. Results a n d Discussion Figures 1 and 2 show the results of acid-base strength distributions of various solids. The acidity at an Ho value shows the number of acid sites whose acid strength is equal to or less than the Ho value and the basicity at an Ho value shows the number of basic sites whose basic strength is equal to or greater than the Ho value. In the present method, if the acid sites for which Ho is The Journal of Physical Chemistry, Val. 79, No. 22, 1975
0.8
'
0
2
i 8
6
4
ACID-BASE STRENGTH (Ho)
Figure 1. Acid-base strength distribution of A1203 (0),activated and y-AI2O3 (0). A1203 (A),
-2 0.21 m
B 0
2
4
6
8
ACID-BASE STRENGTH (Ho)
Figure 2. Acid-base strength distribution of ZnO (O),Ti02 (0),BaO
(A),6 2 0 3 (n),ZrO2 (W), Moo3 (A),and MgS04 (X).
less than or equal to the ~ K I Hof+ an indicator exist on the surface together with basic sites for which Ho is greater than or equal to the ~ K I Hof+ the same indicator, the basicity cannot be determined as described in the foregoing section. Therefore, it is necessary to determine whether both acidic and basic sites of equal strength exist. For this purpose, the following experiment has been done. Moo3 and MgS04, which, as shown in Figure 2, have mainly acid sites were titrated with n-butylamine using phenylazonaphthylamine and p-dimethylaminoazobenzene, respectively, as indicators. An excess of n-butylamine was added, and the solutions were then back-titrated with trichloroacetic acid. The millimoles of trichloroacetic acid required for the back-titration was found to be the same as the number of moles of excess n-butylamine. This indicates that, on the + not same solid surface, basic sites for which Ho 1 ~ K B Hdo + .they exist together with acid sites for which Ho 5 ~ K B H If did, the millimoles of trichloroacetic acid would be larger than the number of moles of excess n-butylamine added. Thus, the present method is sufficiently significant. Three kinds of aluminas tested showed basicity at basic strengths, Ho 1 1.5, but not acidity at acid strengths, Ho I 6.8, as seen in Figure 1. The basicity of 7-A1203was higher than that of the other aluminas at every basic strength, this basicity being 0.43 mmol when Ho 2 1.5. The presence of basic sites on alumina is suggested also by its adsorption of boron t r i f l ~ o r i d e . ~ ZnO, TiO2, BaO, and B2O3 exhibited high basicity at
Acid-Base Properties of Phenylazo-2-naphthols basic strength Ho 2 1.5 (Figure 2). ZrOz showed a low but definite basicity. The presence of basic sites on ZrOz is confirmed by the adsorption of carbon dioxide.1° MgS04 and Moos showed both acidity and basicity. Except for BaO and MgS047 the basicity Of the Oxides tested not be measured bv the methods of Tanabe and Yamaeuchi4 and Take et al.5"The present method makes it possi
