Brønsted acidity of H-MCM-22 as probed by variable-temperature infrared spectroscopy of adsorbed CO and N2

• Is the OH frequency shift on hydrogen bonding a reliable measure of zeolite acidity?
• Does frequency shift correlate with interaction energy of IR probe molecules?
• Variable temperature IR spectroscopy gives new insights on such relevant questions.

Brønsted acidity is a key factor controlling catalytic performance of zeolites in many technological processes, and hence the need to find a reliable means for measuring acidity strength. The enthalpy change, ΔH0, involved in the hydrogen bonding interaction between a weak base, such as carbon monoxide or dinitrogen, and the Brønsted acid [Si(OH)Al] hydroxyl groups should correlate with the zeolite acid strength. However, on account of simplicity, the red shift of the OH stretching frequency, ΔνOH, in the hydrogen-bonded OH⋯CO or OH⋯N2 complexes is frequently measured (instead of ΔH0) and correlated with acid strength; and in fact it is often found that ΔH0 and ΔνOH correlate among themselves, but this might not always be the case. We report herein on a variable-temperature IR (VTIR) spectroscopic study of the interaction of CO and N2 with the protonic zeolite H-MCM-22 (structure type MWW). From series of infrared spectra taken over a temperature range, the standard enthalpy of formation of the OH⋯CO and OH⋯N2 complexes was found to be −23.5(±2) and −14.5(±2) kJ mol−1, respectively. Corresponding ΔνOH values are −321 and −123 cm−1, respectively. Comparison with corresponding ΔH0 and ΔνOH values reported in the literature for other Brønsted-acid zeolites shows strong evidence of a limited validity of the rule correlating ΔνOH with ΔH0 for both probe molecules, which gives a clear warning about the risk of using the magnitude of ΔνOH as a measure of zeolite acidity.

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