Yh. Yeom et al., 3 CRYSTAL-STRUCTURES OF VACUUM-DEHYDRATED ZEOLITE-X, M46SI100AL92O384, M=MG2+, CA2+, AND BA2+, JOURNAL OF PHYSICAL CHEMISTRY B, 101(35), 1997, pp. 6914-6920
The crystal structures of vacuum-dehydrated, fully Mg2+-, Ca2+-, and B
a2+-exchanged zeolite X (Mg-46(H2O)(4)-, Ca-46-X, and Ba-46-X; X = Si1
00Al92O384) have been determined by single-crystal X-ray diffraction t
echniques in the cubic space group Fd (3) over bar at 21 degrees C (a
= 24.671(5), 25.024(4), and 25.266(5) Angstrom, respectively). Their s
tructures were refined to the final error indices R-w = 0.046 with 439
reflections, 0.037 with 434 reflections, and 0.049 with 485 reflectio
ns, respectively, for which I > 3 sigma(I). In Mg-46(H2O)(4)-X, Mg2+ i
ons are found at four crystallographic sites: 14 per unit cell are loc
ated at site I at the center of the hexagonal prism (Mg-O 2.262(6) Ang
strom), only four are at site I' in the sodalite cavity near the hexag
onal prism (Mg-O 2.221(5) Angstrom), four, each coordinated to an H2O
molecule, are located at site II' in the sodalite cavity (Mg-O 2.223(1
2) Angstrom), and the remaining 24 are at site II near single six-oxyg
en rings in the supercage (Mg-O = 2.184(5) Angstrom). In Ca-46-X, Ca2 ions are found at only two sites: 16 per unit cell fill site I (Ca-O
= 2.429(8) Angstrom) and the remaining 30 at site II (Ca-O = 2.276(5)
Angstrom). In Ba-46-X, Ba2+ ions are again found at three sites: 14.5
per unit cell are at site I (Ba-O = 2.778(11) Angstrom), only 1.5 are
at site I' (Ba-O = 2.44(3) Angstrom), and 30 are at site II (Ba-O = 2.
667(8)Angstrom). In the three crystals, sites I and II are the most po
pulated; sites I' and II', if they were used, are only sparsely occupi
ed. Ca2+ appears to fit the octahedral site I best. No cations are fou
nd at sites III or III', all of which are clearly less favorable for a
lkaline-earth cations in dehydrated zeolite X.