Distinctive signatures and ultrafast dynamics of Brønsted sites, silanol nests and adsorbed water in zeolites revealed by 2D-IR spectroscopy

Characterising hydroxyl groups in zeolites and other amorphous solids often relies on methods such as IR and NMR spectroscopy. Their power to distinguish different types of hydroxyl groups diminishes when band broadening from hydrogen bonding and structural heterogeneity occurs. In support of this problem, we report in situ femtosecond 2D-IR spectroscopy of some of the different types of hydroxyl groups present in zeolites. Despite the samples studied being optically scattering pellets, we show that their structural and rotational dynamics can be determined. We show that the hydroxyl groups of Brønsted acid sites, silanol defects and water of hydration display distinct features in their 2D-IR spectra. Brønsted site hydroxyl group structural distributions have characteristic inhomogeneously broadened 2D-IR bandshapes. Water of hydration and partially hydrogen bonded silanol groups give unique 2D-IR cross peak signatures off-diagonal. Hydrogen bonded silanol groups arising from vacancy defects (silanol nests) show a distinctive 2D-IR signature with unique ultrafast dynamics observed to be identical between ZSM-5 and silicalite-1. 2D-IR spectroscopy makes IR measurements quantitative, and we use this property to estimate the concentration of ZSM-5 silanol nest hydroxyl groups relative to the number of Brønsted sites. Overlapping silanol nest spectral features are revealed by frequency dependence of their vibrational lifetime. In contrast to other framework hydroxyls, the silanol nest band shows picosecond 2D-IR anisotropy decay and spectral diffusion. The signatures of nest structural mobility revealed here presents new opportunities to understand these hitherto elusive structural defects.