Crystallographic study of the temperature-dependent dehydration and migration of Mn2+ ions in Mn2+-exchanged zeolite Y (Si/Al = 1.67)

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, 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 determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group Fd \(\overline{3 }\) m 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 F_o > 4σ(F_o)) for crystals 1, 2, 3, 4, and 5, respectively. In all five crystals, about 36 Mn²⁺ ions occupy five crystallographic sites. Mn²⁺ ions are energetically preferentially located 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.