Improved Brønsted Acidity of Mesoporous [Al]MCM-41 Material Treated with Ammonium Fluoride

Abstract

Mesoporous [Al]MCM-41 material (nSi/nAl = 15) has been modified by an aqueous solution of ammonium fluoride and characterized by XRD, N2 adsorption, 1H, 19F, 27Al, and 29Si NMR spectroscopies and tested in the conversion of 2-propanol. By 1H{27Al} 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 19F NMR signal of Si−F species at −156 ppm and a transformation of tetrahedrally coordinated framework aluminum (27Al NMR signal at 53 ppm) to octahedrally coordinated aluminum (27Al 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 1H 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 Brønsted 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.