J.V. Smith

Positions of cations and molecules in zeolites with the mordenite-type framework II dehydrated hydrogen-ptilolite

The crystal structure of dehydrated Ca-exchanged ptilolite has 4 sites for Ca: one each in twisted and near-circular 8-rings, one on the wall of the main channel, and one in a boat-shaped pocket in a zig-zag channel. Hindrance but not blocking of molecules could occur. The TO distances correlate inversely with TOT angles, consistent with observations and molecular-orbital calculations for feldspar. Tetrahedral mean TO distances adjusted for this effect indicate strong but not complete Si, A|l disorder. The Ca occupancy is consistent with simple electrostatic and geometrical arguments.

Positions of cations and molecules in zeolites with the chabazite framework III. Dehydrated Na-exchanged chabazite

Chabazite from Bozen, Tyrol was ion-exchanged with NaCl solution to Na15.2Al15.2Si32.8O96.nH2O. After vacuum dehydration at 320°C, the crystal structure was determined at room temperature. Reduction of the ideal rhombohedral symmetry to monoclinic (C2/m or lower; a 19.319(3) b 13.833(2) c 11.849(2)A β 11.348(3)°) is interpreted to result from near-elliptical distortion of most 8-rings to accommodate 57% of the Na+ cations. All these cations have 4 framework oxygens as nearest neighbors, but positional disorder complicates interpretation of the distances which range from 2.34 to 2.92A. Most other Na+ cations (38%) are displaced into the large cavity from 6-rings with 3 framework oxygens as nearest neighbors at 2.32 to 2.49A. Diameters of three types of 8-rings range from 4.5 to 9.4A, but the fourth type with very low occupancy of Na+ is less distorted with diameters ranging from 6.0 to 7.0A. Correlation between T-O distances and T-O-T angles is semi-quantitatively similar to that in mordenite.

Positions of cations and molecules in zeolites with the chabazite framework. IV Hydrated and dehydrated Cu2+-exchanged chabazite

Chabazite from Bozen, Tyrol was ion-exchanged with KCl and further exchanged with CuCl2 solution to Cu1.8K0.2Al3.9Si8.1O24.nH2O. Both the hydrated and dehydrated structures were determined at room temperature. Cell dimensions in R3m are: hydrated a 9.411(2)A α 95.31(1)°; vacuum-dehydrated at 350°C a 9.310(2)A α 92.01(2)°. All Cu2+ ions of d-Cu-chabazite were located near the center of the 6-ring, 1.97A from three 0(4) and 2.83A from three 0(3). Displacement of Cu2+ by 0.15A into the large cage from a position for triangular coordination to three 0(4) may result from either an attempt to reach a Jahn-Teller distorted tetrahedral coordination, or electrostatic repulsion from another Cu2+ across the ditrigonal prism, or both. The T-O distances increase as the T-O-T angles decrease. Interpretation of the electron density of h-Cu-chabazite is uncertain, but all Cu2+ ions and water molecules are in the large cage, perhaps as hydration complexes with Jahn-Teller distortion.

Positions of cations and molecules in zeolites with the chabazite framework I. Dehydrated ca-exchanged chabazite

Chabazite from Bozen, Tyrol, was ion-exchanged with CaCl2 solution to Ca1.9Al3.8Si8.2O24.nH2O. After dehydration at 320°C for 1 day, the crystal structure (R3m a 9.442(2) A α 93.09(1)°) was determined at room temperature. Most Ca atoms (refined to 1.88) are displaced into the large cavity from the center of a 6-ring and lie at 2.33A to three O(4) and 2.78 to three O(3). The center of the ditrigonal prism contains 0.1 Ca apparently at 2.8A to six O(4). All cations have O(4) as nearest neighbors leaving the other framework oxygens undersaturated. The TO bond lengths increase ∼0.08A per unit charge of undersaturation. The cation distribution and cell dimensions differ from those determined earlier for Ca-exchanged chabazite from Benton County, Oregon, which was dehydrated at 360°C for 4 hr.

Positions of cations and molecules in zeolites with the mordenite-type framework VII dehydrated cesium mordenite

Mordenite from Challis Valley, Idaho, was ion-exchanged with RbCl2 solution to ∼Rb8Al8Si40O96·nH2O. After dehydration at 340°C for 1 day, the crystal structure was determined at room temperature. Earlier attempts to dehydrate crystals at higher temperature led to disintegration, and even the present crystal fractured. Although this resulted in poor X-ray data, refinement in Cmcm (a 18.127(7) b 20.408(6) c 7.463(3)A) yielded atomic positions similar to those for dehydrated K-mordenite. Rb atoms in sites II (3.7Rb), IV (3.1Rb) and VI (0.7Rb) block most of the small channels (“side pockets”) and may hinder diffusion along the main channels. A few diffractions violating the C lattice were ignored in the refinement but indicate structural distortion similar to that in dehyrated K-mordenite which was refined in Pbcn. However the presence of a few weak diffractions violating the b glide indicates even lower symmetry.

Positions of cations and molecules in zeolites with the mordenite framework IX dehydrated H-mordenite via acid exchange

Abstract Nearly complete removal of cations was accomplished for natural mordenite by preliminary Na-exchange followed by exposure to 2M HCl for 1 month at 90°C. The electron microprobe analysis (atomic Si/Al 4.7) indicates no detectable extraction of Al. However the cell dimensions after dehydration at 300°C followed by cooling to room temperature ( a 18.178(7) b 20.394(6) c 7.488(4)A) are lower than for “hydrogen-mordenite” prepared via an ammonium-exchanged intermediary ( a 18.223(7) b 20.465(9) c 7.531(4)A). Furthermore the tetrahedral distances are smaller (T4 down 0.017Ȧ, T3 0.010, T1 0.008, T2 0.002). Diffractions from the HCl-treated mordenite are sharp, and the positional and population parameters are close to those for the de-ammoniated mordenite. The simplest interpretation is that treatment with HCl results in extraction of Al from tetrahedral sites into other positions in the crystal coupled with migration of Si so that a crystalline structure is retained: however, many subtle complications require cautious interpretation.

Positions of cations and molecules in zeolites with the faujasite-type framework V. La-exchanged Type X zeolite at 25, 425 and 735°C

Abstract La-exchanged Type X zeolite activated at 500° and 600° shows three OH infra-red bands which are not completely removed even after vacuum activation at 730°C. At room temperature, the La atoms occupy sites I′, II and V. At 425 and 735°C in a helium stream the La atoms occupy sites I, I′ and II. The simplest interpretation is that La atoms tend to occupy sites I and II when X zeolite is strictly dehydrated and dehydroxylated, and that they prefer site I′ when some residual molecules are available for bonding.

Positions of cations and molecules in zeolites with the chabazite framework II Adsorption of carbon monoxide on dehydrated Ca-exchanged chabazite

Abstract Adsorption of CO onto the single crystal of dehydrated Ca-chabazite described in Pt. I reduced slightly the perturbation of TO distances ( ∼ 0.01 A ) and perhaps the OTO angles (∼ 0.2°). The distances between Ca and framework oxygens increased about 0.02A. The CO molecule is adsorbed onto the Ca(II) cation which is displaced into the large cavity from the 6-ring. Although the structure refinement is uncertain because of large “temperature” factors for C and O, it is certain that the CO molecule is canted off the triad axis. The oxygen is apparently at ∼ 3.7 A to framework oxygens, the Ca(II)C and CO distances are 2.74(5) and 1.31(8)A, and the Ca(II)CO angle is 134(5)°, but the errors may be unrealistically low.

Positions of cations and molecules in zeolites with the mordenite-type framework X dehydrated calcium hydrogen mordenite

Abstract Prolonged acid treatment of natural mordenite (2M Hc1, 90°C, 2 months) did not remove 1.4Ca atoms which persisted in site I of the final product after dehydration at 300°C. Electron microprobe analysis showed no detectable reduction of Al/Si ratio, but the cell parameters at room temperature ( a 18.058(3) b 20.297(3) c 7.484(2)A) are lower than for the dehydrated-deammoniated variety, and the mean T-O distance is lower (1.606 vs. 1.617A). The O-T-O and T-O-T angles are closer generally to those of dehydrated Ca-mordenite than to dehydrated-deammoniated mordenite. Population refinement produced no evidence in favor of vacant tetrahedral sites, but technical considerations cause complications. The present data would not be inconsistent with earlier suggestions that Al is replaced by Si in tetrahedral sites.