The controversy over whether Cal-Ad measure-measures the enthalpy change for the reaction of vapor-phase pyridine [Py (vap) ] with the zeolite acid ments are consistent with vapor-phase calorimetric measurements is quite important since the question sites [ZOH(s)], at elevated temperatures, to form the pyridinium-ion complex [ HPy+ZO-(s)] shown cuts to the heart of our microscopic understanding of zeolite acidity [1,2]. Based on vapor-phase below: measurements and other spectroscopic informa-Py (vap) +ZOH(s) HPy+Z(O)-(s) tion, we have concluded that all of the Brønsted acid sites associated with framework Al in H-MFI -DH py =200 kJ/mol; T=480 K. are, within experimental error, energetically equiv-That one actually achieves the pyridinium-ion alent [3-7]. This conclusion is consistent with reacfinal state under the temperature and pressure contion studies, which show that rates for a number of ditions of these calorimetric measurements has different reactions increase linearly with Al content been demonstrated by combined IR spectroscopy [8]. By contrast, results from the Cal-Ad method and temperature-programmed desorption ( TPD) suggest there is large, energetic heterogeneity in the [7,10]. The enthalpy change for this reaction is sites in H-MFI [9]. Based on their Cal-Ad results, 200 kJ/mol of pyridine for all pyridine coverages Drago et al. have stated that less than 10% of less than 1/Brønsted site, independent of the Al framework Al sites in H-MFI are able to protonate content or synthesis conditions of the H-MFI pyridine and that the remaining 90% of the sites sample [3-7]. interact only weakly with pyridine through By contrast, the Cal-Ad measurements expose a hydrogen bonds. Drago et al. further claim that dilute pyridine solution [Py (hex)
] to the zeolite satuthere is an ''… incorrect assessment of the acidity rated with n-hexane [ZOH(hex)] to form a product and number of strong acid sites with gas-solid cal-[Py-ZOH(hex)], saturated in n-hexane, which has orimetry, TPD, and isopropylamine decomposinot been adequately characterized and may or may tion'' [9]. Obviously, the claims of Drago et al.
not have pyridine associated with the Brønsted have important consequences for the characterizasites [9]: tion and preparation of materials and need to be checked.
Py (hex) +ZOH(hex) Py-ZOH(hex) In vapor-phase calorimetric measurements, one -DH cal-ad,av <50 kJ/mol; T=298 K.