Page 180 - Principles of Catalyst Development

168 CHAPTER 7
sites. The most successful approach is to adsorb a strong base, such as
pyridine, and measure the intensity and wavelength of resulting infrared
adsorption bonds. (247) Table 7.11, for example, shows the results of a study
on Faujasite Y catalysts ion exchanged with different cations. (248) Lewis
acidity increases dramatically as the polarizing power of the exchanged
cation increases. Bronsted acidity shows a moderate increase, except with
hydrogen exchange, which is almost completely Bronsted.
Similar studies have shown that Bronsted acidity is dominant in most
acidic solids. Differences in acid strengths are observable, but quantitative
determinations are difficult and have not resulted in accepted methods for
acid distribution measurement. Methods that have gained wide acceptance
are (1) nonaqueous titration and (2) base chemisorption.

7.4.4.1. Nonaqueous Titration
The most direct method is to adsorb an indicator on the catalyst in
suspension with a nonpolar solvent. The indictor has a known pKa for the
acid-base color change. A base is then added until the acid-base end point
is observed. The amount of base indicates the number of acid sites with
strengths less than the pKa of the indicator. By using a range of indicators
with different pKa's, the distribution of acid strengths is determined. End
points are detected either by visible color changes or with spec-
trophotometry. For visible detection, the Hammett indicators used are given
in Table 7.12.
Typical results are shown in Fig. 7.28 for fresh and steamed silica-
alumina cracking catalyst.
Some workers report that Hammett indicators give unreliable results
at higher values of p K". Better data are obtained with fluorescent indicators

TABLE 7.11. Acidity Type by I. R. Pyridine Measurement

1545 em I 1490 em I 1438 em I
Faujasite (8)" (8 + L)" (L)'
-----_. __
.,
Na l 0 0.27 1.2
Li 0.29 2.1
Ca'- 0.33 1.0 4.1
Mg'~ 0.41 1.3 3.0
Cd' • 0.57 I.R 6.8
H' 7.0 0.8

" 1540 cm'l, pyridinium ions I Bronsted, R).
h 14'10 em I, pyridinium + coordinately bound pyridine.
c 1438 em-- I • coordinately bound pyridine (Lewis, L).