Rs. Drago et al., CALORIMETRIC AND SPECTROSCOPIC INVESTIGATION OF THE ACIDITY OF HZSM-5, Journal of the American Chemical Society, 119(19), 1997, pp. 4444-4452
The cal-ad analysis of HZSM-5 provides a novel characterization of the
donor-acceptor properties of this solid acid showing 0.0415 mmol per
g of a strong Bronsted site and 0.53 mmol per g of a weaker site. Of a
ll studies to date, only cal-ad provides a measure of the extent, K-i;
strength, -Delta H-i; and quantity of sites. The relative K values sh
ow why gas phase calorimetry, TPD, TGA, DSC, and isopropylamine decomp
osition do not resolve the two sites found by cal-ad and incorrectly s
uggest 0.6 mmol.g(-1) of strong acid sites. Reaction with pyridine giv
es an enthalpy of 42.1 and 8.6 kcal mol(-1), respectively, for the two
sites suggesting the latter is a hydrogen bonding site. Other acidity
measures average these sites, and their enthalpies contain dispersion
as well as donor-acceptor components. Comparison of these results wit
h other cal-ad results suggests HZSM-5 is a strong solid acid but not
a super acid. Titrations with 2,6-lutidine and 2,6-di-tert-butylpyridi
ne indicate that the strong Bronsted sites are located in the 5.5 a ch
annels with no detectable amounts on the exterior surface.