Heat-treatment of isomorphously substituted ZSM-5 zeolites and its structural consequences -: An X-ray diffraction, 29Si MAS-NMR, XPS and FT-IR spectroscopy study

The structural consequences of template removal by calcination, and (under non-caring conditions) those caused by overheating have been studied by X-ray diffraction, Si-29 MAS-NMR, FT-IR and XPS spectroscopies using Fe-ZSM-5 (Si/Fe=15.0) and Ti-ZSM-5 (Si/Ti=29.8) samples. The heat-treatments bring about in either case dislodgement of Fe and Ti framework species, respectively; causing thereby a linear/quasi-linear decrease of isomorphously substituted framework Fe3+ and Ti4+ sites as the temperature of treatments increases. The ejected materials seem to have extremely high dispersity. The heat-treatments increase the Si/T ratio and concomitantly lattice shrinkage and a slight (less than or equal to 9 cm-l) blue-shift of the respective IR spectra takes place. The framework contraction occurs almost exclusively along the sinusoidal channels (i.e. vertical to the [1 0 0] crystallographic plane). Though the differences in the binding energies are slight (less than or equal to 1.9 eV) it seems to be possible to estimate the framework/extra-framework T ratio by deconvolution of the respective XPS spectra. The catalytic consequences caused by non-caring calcination in the quality and quantity of potential active sites are briefly discussed. (C) 1998 Elsevier Science B.V. All rights reserved.