Mechanisms of the AlCl3 Modification of Siliceous Microporous and Mesoporous Catalysts Investigated by Multi-Nuclear Solid-State NMR

Abstract

Solid-state NMR spectroscopy was utilized for investigating the post-synthetic formation of surface acid sites via AlCl3 modification of dealuminated zeolite Y (DeaY) and siliceous mesoporous SBA-15 and subsequent thermal treatment. Upon calcination at 723 K, aluminum atoms introduced by the AlCl3 modification coordinate at Q2 (Si(2Si,2OH)) and Q3 (Si(3Si,1OH)) sites and form tetrahedrally coordinated (AlIV) framework aluminum species. Most of the spectroscopically observed pentacoordinated (AlV) and octahedrally coordinated (AlVI) aluminum atoms are extra-framework species, partially acting as Lewis acid sites (LAC). The aluminum coordination at the framework is accompanied by the formation of Bronsted acidic SiOH groups with weak acid strength. The weak Bronsted acidity of these SiOH groups is explained by neighboring framework aluminum atoms with strongly disturbed tetrahedral oxygen coordination. For zeolite DeaY, in addition to the aluminum incorporation into silanol nests, also the aluminum incorporation via surface reactions of AlCl3 with the intact SiO2 framework occurs. In the case of mesoporous SBA-15, with a fivefold higher density of SiOH groups of the parent material compared with that of the parent zeolite DeaY, the aluminum incorporation into silanol nests is the dominating mechanism. By solid-state NMR spectroscopy of the dehydrated samples loaded with probe molecules, significantly larger densities (factor of 3–8) of LAC compared with those of Bronsted acid sites were determined for the AlCl3-modified and calcined zeolite DeaY and mesoporous SBA-15.