Three Crystal Structures of Vacuum-Dehydrated Zeolite X, M₄₆Si₁₀₀Al₉₂O₃₈₄, M = Mg²⁺, Ca²⁺, and Ba²⁺

The crystal structures of vacuum-dehydrated, fully Mg²⁺-, Ca²⁺-, and Ba²⁺-exchanged zeolite X (Mg₄₆(H₂O)₄−X, Ca₄₆−X, and Ba₄₆−X; X = Si₁₀₀Al₉₂O₃₈₄) have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3̄ at 21 °C (a = 24.671(5), 25.024(4), and 25.266(5) Å, respectively). Their structures were refined to the final error indices R_w = 0.046 with 439 reflections, 0.037 with 434 reflections, and 0.049 with 485 reflections, respectively, for which I > 3σ(I). In Mg₄₆(H₂O)₄−X, Mg²⁺ ions are found at four crystallographic sites:  14 per unit cell are located at site I at the center of the hexagonal prism (Mg−O = 2.262(6) Å), only four are at site I‘ in the sodalite cavity near the hexagonal prism (Mg−O = 2.221(5) Å), four, each coordinated to an H₂O molecule, are located at site II‘ in the sodalite cavity (Mg−O = 2.223(12) Å), and the remaining 24 are at site II near single six-oxygen rings in the supercage (Mg−O = 2.184(5) Å). In Ca₄₆−X, Ca²⁺ ions are found at only two sites:  16 per unit cell fill site I (Ca−O = 2.429(8) Å) and the remaining 30 at site II (Ca−O = 2.276(5) Å). In Ba₄₆−X, Ba²⁺ ions are again found at three sites:  14.5 per unit cell are at site I (Ba−O = 2.778(11) Å), only 1.5 are at site I‘ (Ba−O = 2.44(3) Å), and 30 are at site II (Ba−O = 2.667(8) Å). In the three crystals, sites I and II are the most populated; sites I‘ and II‘, if they were used, are only sparsely occupied. Ca²⁺ appears to fit the octahedral site I best. No cations are found at sites III or III‘, all of which are clearly less favorable for alkaline-earth cations in dehydrated zeolite X.