Abstract

Amorphous silica–aluminas (ASAs) are widely used catalysts, with a distribution of Brønsted acid sites (BAS), that yield unique catalytic properties exploited in numerous industrial settings. While having atomic-level insight into their structures, in particular their interfacial sites, would be key to enable rational design, these sites are notoriously difficult to characterize due to spectral complexity arising from a diversity of hydroxyls and the overwhelming interference from noninterfacial signals. Herein, we introduce a 27Al-filtered 1H–1H double-quantum/single-quantum NMR (f-DQ/SQ) spectroscopy method integrated with DFT calculations to probe hydroxyls at solid-state interfaces selectively. Combined with probe molecule (acetone and TMP) adsorption experiments, this approach unequivocally demonstrates that hydrothermal post-treatment increases BAS density in ASA. Dynamic-nuclear-polarization-enhanced 29Si-{27Al} D/J-based correlation experiments corroborate the rearrangement process at the silica–alumina interface, while the heteronuclear-filtered 1H–1H DQ/SQ NMR reveals that the increase in BAS density originates from the formation of specific pseudobridged silanol (PBS) pairs─a distinction imperceptible in conventional 1H or 1H-{27Al} correlation NMR. PBS pairs constitute ca. 73% of the total PBS population, estimated by a semiquantitative analysis combining 27Al-filtered experiments and spin-dynamics simulations. Through a time-dependent 1H–1H f-DQ/SQ variant, we quantified key interatomic distances (∼2 Å for H–H and ∼4.1 Å for H–Al) within these PBS motifs. Constrained DFT calculations ultimately identify a vicinal-silanol-derived configuration as the most stable PBS structure, being 294 kJ/mol lower in relative energy, thereby resolving the atomic-scale origin of augmented acidity in hydrothermally treated ASAs.

Link: https://pubs.acs.org/doi/full/10.1021/jacs.5c20339