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

Abstract

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.

Single-Crystal Structures of Li+-exchanged Zeolite X (FAU, Si/Al = 1.09) from Aqueous Solution Depends on Ion-exchange Temperatures at 293 and 333 K

“…These sodalite units are connected tetrahedrally at 6-rings by bridging oxygens to give double 6-rings (D6Rs, hexagonal prism) and, concomitantly, an interconnected set of even larger cavities (supercages) accessible in three dimensions through 12-ring (24-membered) windows. [27][28][29] The Si and Al atoms occupy the vertices of these polyhedra. The oxygen atoms lie approximately midway between each pair of Si and Al atoms but are displaced from those points to give near tetrahedral angles about Si and Al.…”

Towards Operando Characterisation by Powder Diffraction Techniques of Molecular Sieves

“…On the contrary, 14 potassium cations (per unit cell) are located on this site in zeolite KX [19]. When the charge compensating cation is an alkaline-earth one, cation distributions are less affected by its nature: the cations are located mainly in sites I and II (exclusively for zeolite CaX [8] [20]). The presence of residual water molecules often adsorbed in sodalite cages leads to an increase of site SI' population at the cost of site SI (zeolite CaX [21] and zeolite SrX [22]) or an increase of site SII' population (zeolite MgX [20]).…”

“…When the charge compensating cation is an alkaline-earth one, cation distributions are less affected by its nature: the cations are located mainly in sites I and II (exclusively for zeolite CaX [8] [20]). The presence of residual water molecules often adsorbed in sodalite cages leads to an increase of site SI' population at the cost of site SI (zeolite CaX [21] and zeolite SrX [22]) or an increase of site SII' population (zeolite MgX [20]). At high water content, alkaline-earth cation distribution is characterised by: -a low site SI population; -and the occupancy of low symmetry sites in the supercage in a complex arrangement with adsorbed water molecules.…”

Synthesis and Single-crystal Structure of Fully Dehydrated Fully Ca2+exchanged Zeolite Y (FAU), |Ca35.5|[Si121Al71O384]-FAU

“…So numerous investigations of the positions of the exchangeable cations in zeolite X (Si/Al = 1.09) have been conducted by single-crystal X-ray diffraction technique. [2][3][4] And the structures of sorption complexes that show how various guest molecules are held on the inner surfaces of zeolites are being obtained crystallographically. [5][6][7][8][9][10][11] Smolin et al 2 determined the crystal structures of Ca-X at various dehydration temperatures.…”