State of aluminium in hydrothermally dealuminated MFI zeolite

Abstract

²⁷ Al Magic-angle spinning (MAS) NMR, ²⁹Si MAS NMR, X-ray diffraction (XRD), FTIR and water adsorption have been used to investigate the state of aluminium after steam treatment of MFI zeolite. Dealumination and removal of silanol groups, as well as a transformation of the zeolite symmetry, have been observed. The signal (50 ppm) of tetrahedrally coordinated framework aluminium disappears at a lower temperature (< 1040 K) steam treatment and then reappears at a higher temperature. The framework aluminium is first observed to be extracted by steam to form extraframework aluminium species (with 0 ppm chemical shift) at a high temperature. With further steam treatment, the aluminium is changed to another form, which has the same chemical shift as that of framework aluminium. XRD, ²⁹Si MAS NMR and FTIR confirm that the framework symmetry of the sample is monoclinic with high crystallinity, and the signals of framework aluminium and silanol groups are removed. From water adsorption data, two increases in hydrophobicity can be found in the sample treated with steam at different temperature, the first at 770 K and the second at 1030 K. This result is consistent with the change in the ²⁷Al MAS NMR spectra. The ²⁷Al MAS NMR spectrum shows that the newly formed aluminium species, after high-temperature steam treatment, is not Al₂O₃, based on the different chemical shift. Aluminium on MFI zeolite and on amorphous silica, after impregnation with Al³⁺, acts like framework aluminium during hydrothermal treatment. This result indicates that the newly formed aluminium species is in the form of Si—O—Al dispersed on the outer surface of the zeolite.