A study of the highly crystalline, low-silica, fully hydrated zeolite P ion exchanged with (Mn2+, Cd2+, Pb2+, Sr2+, Ba2+) cations

An extensive study of a highly crystalline of sodium maximum aluminum zeolite P (Na-MAP) ion exchanged with Mn2+, Cd+, Pb2+, Sr2+, and Ba2+ cations and with a Si/Al ratio of 1.0, is presented. All the crystal structures were refined from conventional X-ray powder data in the monoclinic crystal system, space group P2(1)/c, and have the GIS framework topology. The Cd-MAP and Mn-MAP structures are similar to that of Ca-gismondine and the positions of the Cd and Mn atoms match the cations positions in natural Ca-gismondine. The Ba-MAP, Sr-MAP and Pb-MAP structures, however, are quite different. The unit cells of Ba-MAP and Pb-MAP are metrically orthorhombic, but their structures were refined with the monoclinic space group P2(1)/c. Sr-MAP, in contrast to the other structures, presents two sites for the extra-framework cations. The incorporation of Cd, Mn, Ba, Sr, and Pb cations produces different distortions in the GIS framework due to its high flexibility. Besides the distortions induced by the cation incorporation, the Si-29 NMR data and the Si-O and Al-O bonds show that all structures present a strict ordering scheme of Si and Al. The number of water molecules per unit cell differs between the title compounds, as does their dehydration behaviour. Dehydration of Cd-MAP and Mn-MAP induces the formation of new phases. However, under rehydration conditions, these new phases are restored to the original ones. On the other hand, dehydration of Sr-MAP and Pb-MAP induces the formation of new phases that are unchanged under rehydration conditions. Dehydration of Ba-MAP at 140-180 degreesC induces the formation of a new phase that is restored to the original one upon rehydration. However, above this temperature range the new phase is formed irreversibly. The results indicate that the unusual reversibility and irreversibility of MAP zeolites are related to the cation-dominant media, because some cations act as a precursor for obtaining different natural zeolites whose frameworks are different from the GIS framework. (C) 2002 Elsevier Science Inc. All rights reserved.