Interfacial Brønsted Acid Site Architectures on Amorphous Silica-Alumina Resolved by Heteronuclear-Filtered...

Area of Science:

• Materials Science
• Catalysis
• Solid-State NMR Spectroscopy

Background:

• Amorphous silica-aluminas (ASAs) are crucial industrial catalysts due to their Brønsted acid sites (BAS).
• Characterizing interfacial sites in ASAs is challenging due to spectral complexity and signal interference.
• Understanding these sites is vital for rational catalyst design and performance optimization.

Purpose of the Study:

• To develop and apply a novel NMR method for selectively probing hydroxyls at solid-state interfaces in ASAs.
• To investigate the effect of hydrothermal post-treatment on BAS density and interfacial structure.
• To elucidate the atomic-scale origin of enhanced acidity in hydrothermally treated ASAs.

Main Methods:

• Utilized ²⁷Al-filtered ¹H-¹H double-quantum/single-quantum (f-DQ/SQ) NMR spectroscopy.
• Integrated NMR with Density Functional Theory (DFT) calculations and probe molecule adsorption (acetone, TMP).
• Employed dynamic-nuclear-polarization-enhanced ²⁹Si-{²⁷Al} D/J-based correlation and time-dependent f-DQ/SQ NMR experiments.

Main Results:

• Hydrothermal post-treatment significantly increases BAS density in ASAs.
• The enhanced acidity originates from the formation of specific pseudobridged silanol (PBS) pairs, a feature not detectable by conventional NMR.
• Identified a vicinal-silanol-derived configuration as the most stable PBS structure, resolving the atomic-scale origin of augmented acidity.

Conclusions:

• The developed ²⁷Al-filtered f-DQ/SQ NMR method provides unprecedented atomic-level insight into interfacial hydroxyls in ASAs.
• Hydrothermal treatment promotes the formation of specific PBS pairs, leading to increased BAS density and catalytic activity.
• This study resolves the atomic-scale origin of enhanced acidity in ASAs, paving the way for tailored catalyst design.