Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

The effect of the framework Al content and synthesis procedure on the number of framework Al atoms located on the surface of (Al)MCM-41 channels and the distribution of the Al atoms between single Al atoms and various Al-O-(SiO)_1,2-Al species was investigated. The total number of accessible framework Al atoms on the channel surface was obtained from the ion exchange capacity for Na⁺ ions and pyridine adsorption. The number of Al pairs on the surface and their distribution between different local arrangements of the channel surface was estimated using VIS spectroscopy of Co²⁺ ions. At low framework Al contents, Al atoms occupy only accessible T sites on the surface of the channel walls. In aluminium-rich (Al)MCM-41, a significant part of Al atoms is incorporated inside the channel walls and these Al atoms do not form cationic sites. Al atoms on the wall surface preferentially form Al-O-(SiO)_1,2-Al sequences, enabling siting of divalent cations. Different synthesis procedures resulted in differences in Al distribution. The channel surface of the (Al)MCM-41 is formed by two regular local structures containing two types of deformed six-membered rings with Al-O-(SiO)_1,2-Al sequences forming cationic sites.

Keywords: Molecular sieves; (Al)MCM-41; Mesoporous materials; VIS spectroscopy; Local ordering; Cationic sites; Heterogeneous catalysis.

References: 43 live references.