Calorimetric and Spectroscopic Investigation of the Acidity of HZSM-5

Abstract

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 Brönsted site and 0.53 mmol per g of a weaker site. Of all studies to date, only cal-ad provides a measure of the extent, K i; strength, −Δ H i; and quantity of sites. The relative K values show why gas phase calorimetry, TPD, TGA, DSC, and isopropylamine decomposition do not resolve the two sites found by cal-ad and incorrectly suggest 0.6 mmol g -1 of strong acid sites. Reaction with pyridine gives 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 with 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 -butylpyridine indicate that the strong Brönsted sites are located in the 5.5 Å channels with no detectable amounts on the exterior surface.