We outline two methodologies to selectively characterize the Brønsted acidity of the external surface of FAU-type zeolites by IR and NMR spectroscopy of adsorbed basic probe molecules. The challenge and goal are to develop reliable and quantitative IR and NMR methodologies to investigate the accessibility of acidic sites in the large pore FAU-type zeolite Y and its mesoporous derivatives often referred to as ultra-stable Y (USY). The accessibility of their Brønsted acid sites to probe molecules (n-alkylamines, n-alkylpyridines, n-alkylphosphine- and phenylphosphine-oxides) of different molecular sizes is quantitatively monitored either by IR or 31 P NMR spectroscopy. It is now possible, for the first time to quantitatively discriminate between the Brønsted acidity located in the microporosity and on the external surface of large pore zeolites. For instance, the number of external acid sites on a Y (LZY-64) zeolite represents 2 % of its total acid sites while that of a USY (CBV760) represents 4 % while the latter has a much lower framework Si/Al ratio.
Keywords: 31P NMR spectroscopy; Faujasite; infrared spectroscopy; phosphine oxide; tri-tert-butylpyridine.
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