Local Structure and Surface Acidity of Overlayers Prepared by Atomic Layer Deposition of Silica on Alumina

Abstract

Understanding the structure of the silica–alumina interface and the reactivity of such interfacial sites in amorphous aluminosilicate (ASA) materials is essential due to their industrial utilization as solid acid catalysts. Here, we link the structure of silica layers grown on alumina by atomic layer deposition (ALD) to the acidic and catalytic properties of ASA. In particular, we study the local structure of silica overlayers as a function of the number of ALD cycles applied and reveal how the coordination environment of Al and Si sites governs the Lewis and Bro̷nsted acidity and catalytic activity using methanol dehydration as a model structure-sensitive reaction. Relying on advanced solid-state NMR characterization, including ²⁷Al{²⁹Si} dipolar heteronuclear multiple-quantum coherence (D-HMQC) and ²⁹Si{²⁷Al} dipolar-mediated refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT) experiments, dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS), and infrared spectroscopy using probe molecules (CO, pyridine), we demonstrate that the atomic-scale mixing of silica and alumina generates strong Bro̷nsted acidity and increases the strength of the Lewis acid sites. Our findings indicate that the density of acid sites is closely related to the coverage of the alumina surface by silica and can be controlled by the number of ALD cycles applied. This study advances our understanding of the relationship between the local environment of Si and Al sites, the abundance and strength of acid sites, and the superior high-temperature selectivity of SiO_x-Al₂O₃-based catalysts in methanol dehydration when compared to unmodified alumina.