Interfacial Brønsted Acid Site Architectures on Amorphous Silica–Alumina Resolved by Heteronuclear-Filtered Two-Dimensional Solid-State NMR Correlation Spectroscopy

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.