Calorimetric investigation of CO and N2 for characterization of acidity in zeolite H-MFI

Microcalorimetric measurements were performed at 195 K for CO and N-2 On zeolite H-MFI and on a siliceous MFI in order to characterize the interaction of these molecules with the Bronsted acid sites. On the siliceous MFI, the differential enthalpies of adsorption (isosteric heats) for CO and N2 were found to be essentially constant with coverage at a value of 16 +/- 1 kJ/mol. While the isosteric heats for Nz were slightly higher on H-MFI at coverages below one per site, the additional interaction with the Bronsted sites is less than 3 kJ/mol. For CO on H-MFl, microcalorimetry provides evidence for a 1:1 adsorption complex with the acid sites in that the isosteric heats are higher on H-MFI at low coverages and approach the values for the siliceous material at higher coverages. Because the calorimetric data can be fit to a two-site, Langmuir model in which the hydrogen-bonded CO interacts with an additional 10.5 KJ/mol, the results suggest that the Bronsted acid sites in H-MFI are essentially identical in regard to how they interact with CO.