Unraveling the Dehydration Temperature Dependent of Mn 2+ -exchanged Zeolite Y (FAU, Si/Al = 1.67) through Crystallographic Study

Abstract To study the behavior of Mn2+ ions and water molecules in Mn2+-exchanged zeolite Y (Si/Al = 1.67) at different temperatures during dehydration, the single crystals of Mn2+-exchanged zeolite Y were prepared by batch method at room temperature. Five single crystals of Mn2+-exchanged zeolite Y were dehydrated at 297 K (crystal 1), 523 K (crystal 2), 573 K (crystal 3), 623 K (crystal 4), and 673 K (crystal 5), respectively, under dynamic vacuum for 48 h. Their crystal structures were completely determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group Fdm at 100(1) K. They were refined to the final error indices R1/wR2 = 0.0594/0.1615, 0.0450/0.1229, 0.0445/0.1108, 0.0447/0.1145, and 0.0418/0.1084 (for Fo > 4σ(Fo)) for crystals 1, 2, 3, 4, and 5, respectively. In all five crystals, about 36 Mn2+ ions occupy five crystallographic sites. Mn2+ ion was energetically preferred and the first to be filled at site I in all structures. The Mn2+ ions migrated from one site (sites I’ or II’) to another available site (sites I or II) to better satisfy their coordination requirements upon dehydration. Finally, in the completely dehydrated crystal 5, 36 Mn2+ ions occupy the sites I, I’, II’, IIa, and IIb with the fractional occupancies 15, 2, 2, 12, and 5, respectively. All water molecules associated with Mn2+ ions in the incompletely dehydrated crystals 1 ~ 4 were located in the sodalite cavities. In the structure of crystal 1, about 10.5 water molecules were found per unit cell, each coordinating to Mn2+ ions at Mn(1’b). These water molecules formed clusters as [Mn4(H2O)4]8+. Only 5, 3, and 2 water molecules were found in the structures of crystals 2 ~ 4, respectively, with increasing temperature. Each of these water molecules was bonded to one Mn2+ ion at Mn(2’), forming [MnH2O]2+. The unit cell constant of the zeolite framework decreased, as the number of water molecules decreased with the increasing dehydration temperature.