Improved Bronsted acidity of mesoporous [Al]MCM-41 material treated with ammonium fluoride

Mesoporous [Al]MCM-41 material (n(Si)/n(Al) = 15) has been modified by an aqueous solution of ammonium fluoride and characterized by XRD, N-2 adsorption, H-1, F-19, Al-27, and Si-29 NMR spectroscopies and tested in the conversion of 2-propanol. By H-1{Al-27} TRAPDOR MAS NMR it is shown that a significant number of SiOH groups (1.8 ppm) in the dehydrated parent [Al]MCM-41 material is located in the vicinity of aluminum atoms. After a weak treatment of the [Al]MCM-41 material with NH4F, a F-19 NMR signal of Si-F species at - 156 ppm and a transformation of tetrahedrally coordinated framework aluminum (Al-27 NMR signal at 53 ppm) to octahedrally coordinated aluminum (Al-27 NMR signal at 0 ppm) were observed in the spectra of hydrated samples. In addition, an enhanced catalytic activity was found accompanied by a low-field shift and an intensity decrease of the H-1 NMR signal of SiOH groups in dehydrated samples, A model is proposed that explains the enhanced catalytic activity and the improved number of hydroxyl groups acting as Bronsted acid sites in weakly modified and dehydrated [Al]MCM-41 materials by the formation of Si(O2F)OH species in the vicinity of strongly disturbed framework aluminum atoms. Simultaneously, an incorporation of fluorine atoms in the local structure of framework aluminum occurs. A strong treatment with NH4F leads to a damage of the mesoporous structure.