Characterization of extraframework aluminum in H-mordenite dealuminated with ammonium hexafluorosilicate

Abstract

The status and nature of extraframework Al (EFAl) species in H-mordenite dealuminated by ammonium hexafluorosilicate (i.e., (NH 4) 2SiF 6, AHFS) was investigated. Considerable difference arising from the experimental conditions of AHFS dealumination, that is, in the presence and absence of ammonium acetate (NH 4OAc), was observed. Both treatments were not very effective in removing framework aluminum. 27Al and 19F solid-state NMR was used to detect different extraframework aluminum fluoro-complexes resulting from the AHFS dealumination. In the presence of NH 4OAc, the essential part of the dealumination process carried on with AHFS was that most of the extracted Al 3+ reacted with F − to form (NH 4) 3AlF 6. In the absence of NH 4OAc, on the other hand, at least two different forms of aluminum fluoro-complexes other than (NH 4) 3AlF 6 were formed after dealumination. These aluminum fluoro-complexes showed multiple lines located in the range of −144 to −156 ppm in the 19F NMR spectrum. An ab initio method that used both Hartree-Fock and hybrid Hartree-Fock density functional methods at the 6-31+G ∗ basis set level was used to calculate the 19F NMR shifts for discriminating possible aluminum fluoro-complexes. Our results indicated that a combination of NMR and ab initio calculations provided a basis for using them to study EFAl species of unknown compositions and structures.