Calorimetric Investigation of CO and N₂ for Characterization of Acidity in Zeolite H−MFI

Abstract

Microcalorimetric measurements were performed at 195 K for CO and N2 on zeolite H−MFI and on a siliceous MFI in order to characterize the interaction of these molecules with the Brønsted 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 N2 were slightly higher on H−MFI at coverages below one per site, the additional interaction with the Brønsted sites is less than 3 kJ/mol. For CO on H−MFI, 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 Brønsted acid sites in H−MFI are essentially identical in regard to how they interact with CO.

Thermodynamics of Hydrogen Bonding between CO and the Supercage Brønsted Acid Sites of the H-Y Zeolite − Studies from Variable Temperature IR Spectrometry

“…Concerning the standard enthalpy change, the present value of ∆H°ϭ Ϫ25.6 kJ mol Ϫ1 for the CO/H-Y system compares well with the calorimetrically determined heat of interaction of CO with the protonic form of ZSM-5, which was found to be [13] ∆H°ϭ Ϫ27 kJ mol Ϫ1 . Concerning the standard enthalpy change, the present value of ∆H°ϭ Ϫ25.6 kJ mol Ϫ1 for the CO/H-Y system compares well with the calorimetrically determined heat of interaction of CO with the protonic form of ZSM-5, which was found to be [13] ∆H°ϭ Ϫ27 kJ mol Ϫ1 .…”

“…[5Ϫ10] For a detailed characterisation of active solids, which is relevant to the general field of surface chemistry and (in particular) to the more specialised field of heterogeneous catalysis, it is desirable to have a detailed knowledge of the energy involved in the adsorbent-adsorbate interaction, and not only of the spectroscopic changes involving the probe molecule. [8Ϫ12] The same applies to calorimetric studies, [13] although measurements at room temperature are sometimes possible. Interactions with weaker centres, such as alkali metal ions or protons, usually require IR measurements at a lower temperature, typically around the boiling point of nitrogen.…”

Probing zeolites by vibrational spectroscopies

“…Usually the adsorption enthalpy is measured by the isosteric method 717,[722][723][724][725] or by microcalorimetry, 168,170,266,336,405,537,600,716,722,[726][727][728][729][730][731][732][733] both are widely employed in the investigation of molecular adsorption in zeolites. More peculiar bands are obtained in the event that the probe molecule is more complex than a simple diatomic molecule.…”

Theoretical investigation of site-specific characteristics of CO adsorption complexes in the Li+-FER zeolite

“…Reported experimental results relevant for Li + /zeolites are briefly summarized below. The interaction of CO and N 2 probe molecules with Li + , Na + , and K + ions in MFI was investigated by Savitz et al using microcalorimetry [9,10]. Amari et al used microcalorimetry for characterization of CO 2 interaction with Li + ions in zeolites A, X, and MOR [11].…”