Abstract
Many industrial applications of protonic zeolites as solid-acid heterogeneous catalysts rely on the strength or their Brønsted acidity, which (together with zeolite topology) affects both, catalytic activity and selectivity. Therefore, the convenience to have an accurate (and simple) experimental technique for measuring Brønsted acid strength. The enthalpy change, ΔH0, corresponding to the hydrogen bonding interaction of a weak base (such as CO or dinitrogen) with their Brønsted acid [Si(OH)Al] hydroxyl groups should directly correlate with the zeolite acid strength. Nevertheless, because of simplicity, the bathochromic shift of the O–H stretching frequency, Δν(OH), is usually measured by IR spectroscopy at a low temperature, and correlated with acid strength, for ranking zeolite acidity. Herein the use of variable-temperature IR (VTIR) spectroscopy to determine simultaneously ΔH0 and Δν(OH) is demonstrated; followed by an abridged overview showing that direct correlation between Δν(OH) and Brønsted acid strength can be misleading when ranking zeolite acidity.
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