Realumination of dealuminated HZSM-5 zeolites by acid treatment: a reexamination

The feasibility of reinsertion of non-framework aluminum into the lattice o f dealuminated HZSM-5 zeolites upon acid treatment was investigated by powd er X-ray diffraction, nitrogen adsorption, elemental analysis, solid-state NMR and Fourier transform infrared (FTIR) spectroscopy. XRD and nitrogen ad sorption revealed that no structural degradation of the HZSM-5 zeolite matr ix took place during dealumination and acid treatment. FTIR and multinuclea r NMR spectroscopy showed that hydrothermal treatment was more effective in dealumination than the mere calcination treatment. In the dealuminated sam ples, a fraction of the tetrahedral aluminum was present as extra-lattice a luminum. No spectroscopic evidence of reinsertion of aluminum into the fram ework was observed after acid treatment. On the contrary, in addition to le aching of the octahedral extra-framework aluminum, part of the framework al uminum was extracted as well. A fraction of the tetrahedral aluminum did no t belong to the zeolite lattice. Elemental analysis revealed that silicon w as present in the mother liquor of the acid treatments. It is proposed that an amorphous phase of silico-aluminate, with aluminum in tetrahedral coord ination, is formed as a result of the reaction between this silicon and the aluminum in the extracted solution. (C) 2001 Elsevier Science B.V. All rig hts reserved.