Aluminum in mesoporous silica–alumina

Quantitative 27Al MAS NMR spectral decomposition, employing a Gaussian distribution of anisotropic parameters, along with triple-quantum spectroscopy were used to examine the aluminum nuclei in mesoporous silica–alumina (MSA). Only half of the Al nuclei in H-MSA were detected by MAS NMR analysis at 7 Tesla field strength. Complete hydration provided full detection and revealed large amounts of AlO6 sites (ca. 40% mole). All these sites were converted to AlO4 sites, with NH3 protonation, using a pressurized ammonia–water treatment. 27Al and 29Si MAS NMR spectral changes indicated this occurred by partial reformation of the aluminosilicate.

Undetectable aluminum in H-MSA, a high-silica mesoporous silica–alumina with ample AlO6 nuclei, can be made fully “visible” by hydration, or by a hydrothermal treatment with ammonia where all the six-coordinate aluminum is converted into AlO4 during partial reformation of the aluminosilicate.Figure optionsDownload as PowerPoint slideHighlights
► Aluminum nuclei in mesoporous silica–alumina examined by NMR spectroscopy.
► Hydration relaxes coordination strain of aluminum allowing full detection.
► Hydrothermal ammonia treatment re-introduces AlO6 sites into the aluminosilicate as AlO4.