Shedding light on the atomic-scale structure of amorphous silica–alumina and its Brønsted acid sites

Abstract

Advanced solid-state NMR methods, using dynamic nuclear polarization (DNP), are applied to probe the atomic-scale bulk structure of amorphous silica–alumina catalysts prepared by flame-spray pyrolysis, and the structure of their Brønsted acid sites.

Probing Interactions of γ‐Alumina with Water via Multinuclear Solid‐State NMR Spectroscopy

“…Li Shen + , [a, b] Yang Wang + , [a] Jia-Huan Du, [a] Kuizhi Chen, [c] Zhiye Lin, [a] Yujie Wen, [a] Ivan Hung, [c] Zhehong Gan, [c] and Luming Peng* [a] Interactions of γ-alumina with water are important in controlling its structure and catalytic properties. We apply solid-state multinuclear NMR spectroscopy to investigate these interactions by monitoring 1 H and 17 O spectra in real-time. Surfaceselective detection is made possible by adsorbing 17 O-enriched water on γ-alumina nanorods.…”

“…We apply solid-state multinuclear NMR spectroscopy to investigate these interactions by monitoring 1 H and 17 O spectra in real-time. Surfaceselective detection is made possible by adsorbing 17 O-enriched water on γ-alumina nanorods. Structural evolution on the surface was selectively probed by 1 H/ 17 O double resonance NMR and 27 Al NMR at ultrahigh 35.2 T magnetic field.…”

“…Surfaceselective detection is made possible by adsorbing 17 O-enriched water on γ-alumina nanorods. Structural evolution on the surface was selectively probed by 1 H/ 17 O double resonance NMR and 27 Al NMR at ultrahigh 35.2 T magnetic field. Formation of hydroxyl species on the surface of nanorods is rapid upon the exposure of water, which involves low coordinated aluminum ions with doubly bridging and isolated hydroxyl species being generated first.…”

“…Previous studies of γ-alumina based on 27 Al or 17 O NMR spectroscopy gave little surface structural information, because most of the 27 Al ions are in the bulk part of the material which dominate the 27 Al NMR spectra. In addition, 17 O NMR studies require expensive isotopic enrichment at high temperature prior to the measurement due to the low natural abundance of 17 O (0.037 %) and this process is usually not surface-selective, either. Therefore, most of NMR studies focus on 1 H, which uniquely originate from the surface as hydroxyl groups and / or adsorbed water.…”

NMR Spectroscopic Characterization of Flame‐Made Amorphous Silica‐Alumina for Cyclohexanol and Glyceraldehyde Conversion

“…Recently, ASAs before and after ammonia loading has been characterized by 27 Al‐{ 1 H} dipolar‐mediated heteronuclear multiple quantum correlation (D‐HMQC) two‐dimensional (2D) NMR experiments . Upon loading with ammonia, the correlation between the occurrence of Al IV /Al V species and NH 4 + ions revealed the formation of BAS based on the interaction of Al IV /Al V with neighboring SiOH groups, which has been evidenced by various DNP‐enhanced homonuclear and heteronuclear correlation NMR experiments . Moreover, correlations between the occurrence of Al IV /Al V species and 1 H MAS NMR signals at δ 1H =2.2–2.6 ppm are also detected in ammonia‐loaded ASAs.…”