EPR and NMR Studies on Lewis Acids in HZSM-5 Zeolites

The formation and the existent form of the Lewis acidic centers in HZSM-5 zeolite were investigated. HZSM-5 zeolite samples were calcined to dehydroxyl at a series of temperatures. It was found that during the calcination the Bronsted acids were transformed into the Lewis acids and there was a sudden change in the temperature range of 600-700℃. The HZSM-5 zeolite samples which had been calcined under the series of temperatures were then impregnated with a solution of 38 Vol.% acetylacetone in ethanol and observed in terms of ²⁷A1 NMR. Only a few nonframework aluminium existed in those calcined samples according to the ²⁷A1 NMR observations. On the other hand the ²⁹Si MAS NMR observations and X-ray diffraction analysis indicated that there was no obviously change of the aluminium quantity on the framework of the calcined HZSM-5 samples. Therefore there wasn't any indications to provide that the dehydroxyl process and the formation of the L-acids for HZSM-5 zeolites would accompany simutancously with the dealuminization process. The facts have refuted the connection between L-acids and extra-lattice aluminium. So it can be concluded that aluminium ions which concerned with L-acid centers were atill sitting on the framework of HZSM-5 zeolites. ²⁷Al MAS NMR observations have shown that the coordination states of the framework aluminium concerned with those formed L-acids were changed significantly.