Sodalite Cavities Research Articles

Single-crystal structures of highly [formula omitted]-exchanged, fully deaminated, and fully Tl +-exchanged zeolite Y (FAU, Si/Al = 1.56), all fully dehydrated

Single crystals of zeolite Y, |Na 75|[Si 117Al 75O 384]-FAU, with diameters up to 0.30 mm were grown from gels; Si/Al = 1.56. They were ion exchanged and fully dehydrated to generate |(NH 4) 67H 5Na 3|[Si 117Al 75O 384]-FAU (crystal 1, partially deaminated), |H 72Na 3|[Si 117Al 75O 384]-FAU (crystal 2, fully deaminated at 473 K) and |Tl 75|[Si 117Al 75O 384]-FAU (crystal 3). Their structures were determined crystallographically at 294 K using synchrotron X-radiation in the cubic space group Fd 3 ¯ m . They were refined to the final error indices R 1/ wR 2 = 0.037/0.126, 0.055/0.190, and 0.044/0.121 using 884, 723, and 851 reflections with F o > 4 σ( F o) for crystals 1, 2, and 3, respectively. In crystal 1, 67 NH 4 + and 3 Na + ions per unit cell are found at four different crystallographic sites. Six NH 4 + ions were found at the centers of double 6-rings (site I; N···O = 2.888(3) Å). Filling site I′ are 32 NH 4 + ions (N···O = 2.774(5) Å); each is recessed 1.59 Å into the sodalite cavity from the plane of the three oxygens to which it bonds. Filling site II in the supercages are 29 NH 4 + and 3 Na + ions, each of which extends 1.69 Å and 1.10 Å, respectively, into the supercage from its 3-oxygen plane (N···O = 2.818(5) Å, Na–O = 2.512(19) Å). The remaining 5 of the 72 NH 4 + ions originally present per unit cell have apparently been deaminated by vacuum dehydration at 294 K. NH 4 + ions simultaneously occupy sites I and I′ as K + ions did in |K 71|[Si 121Al 71O 384]-FAU, |K 92|[Si 100Al 92O 384]-FAU, and |K 96|[Si 96Al 96O 384]-FAU; the internitrogen distance is 3.36 Å. In crystal 2, three Na + ions per unit cell are found at site I′ (Na–O = 2.207(13) Å); no other cations were found so the deamination of NH 4 + is complete at 473 K. In crystal 3, 30, 30, 9, 4, and 2 Tl + ions were found at sites I′, II, and three sites III′ per unit cell, respectively. The total, 75, agrees with the results of EDX analysis to establish the framework composition (the Si/Al ratio) of these crystals.

Single-crystal Structure of Fully Dehydrated and Largely NH4+-exchanged Zeolite Y (FAU, Si/Al = 1.70), │(NH4)60Na11│[Si121Al71O384]-FAU

The single-crystal structure of largely ammonium-exchanged zeolite Y dehydrated at room temperature (293 K) and 1 <TEX>${\times}\;10^{-6}$</TEX> Torr. has been determined using synchrotron X-radiation in the cubic space group <TEX>$Fd\overline{3}m\;(a=24.9639(2)\AA)$</TEX> at 294 K. The structure was refined to the final error index <TEX>$R_1$</TEX> = 0.0429 with 926 reflections where <TEX>$F_o>4\sigma(F_o)$</TEX>; the composition (best integers) was identified as |<TEX>$(NH_4)_{60}Na_{11}$</TEX>|[<TEX>$Si_{121}Al_{71}O_{384}$</TEX>]-FAU. The 11 <TEX>$Na^{+}$</TEX> ions per unit cell were found at three different crystallographic sites and 60 <TEX>${NH_4}^{+}$</TEX> ions were distributed over three sites. The 3 <TEX>$Na^{+}$</TEX> ions were located at site I, the center of the hexagonal prism (<TEX>$Na-O\;=\;2.842(5)\;\AA\;and\;O-Na-O\;=\;85.98(12)^{\circ}$</TEX>). The 4 <TEX>$Na^{+}$</TEX> and 22 <TEX>${NH_4}^{+}$</TEX> ions were found at site I' in the sodalite cavity opposite the double 6-rings, respectively (<TEX>$Na-O\;=\;2.53(13)\;\AA,\;O-Na-O\;=\;99.9(7)^{\circ},\;N-O\;=\;2.762(11)\;\AA,\;and\;O-N-O =\;89.1(5)^{\circ}$</TEX>). About 4 <TEX>$Na^{+}$</TEX> ions occupied site II (<TEX>$(Na-O\;=\;2.40(4)\;\AA\;and\;O-Na-O\;=\;108.9(3)^{\circ}$</TEX>) and 29 <TEX>${NH_4}^{+}$</TEX> ions occupy site II (<TEX>$N-O\;=\;2.824(9)\;\AA\;and\;O-N-O\;=\;87.3(3)^{\circ}$</TEX>) opposite to the single 6-rings in the supercage. The remaining 9 <TEX>${NH_4}^{+}$</TEX> ions were distributed over site III' (<TEX>$N-O\;=\;2.55(3),\;2.725(13)\;\AA\;and\;O-N-O\;=\;94.1(13),\;62.16(15),\;155.7(14)^{\circ}$</TEX>).

Crystal Structures of Vacuum-Dehydrated Ni2+-Exchanged Zeolite Y (FAU, Si/Al = 1.69) Containing Three-Coordinate Ni2+, Ni8O4·xH2O8+, x ≤ 4, Clusters with Near Cubic Ni4O4 Cores, and H+

Five single crystals of vacuum-dehydrated Ni2+-exchanged zeolite Y (Ni−Y) were variously prepared by the exchange of Na−Y or K−Y (Na71− or K71−Si121Al71O384, Si/Al = 1.69) with Ni2+ using flowing aqueous 0.05 M Ni(NO3)2 at 294 or 353 K, followed by vacuum dehydration at 2.0 × 10−6 Torr and 623 or 723 K. Their crystal structures and chemical compositions were determined using synchrotron X-radiation and energy dispersive X-ray (EDX) analyses to give Nin−Y, where 25.3 ≤ n ≤ 34.1 per unit cell: |NiuMvHw[Ni8O4·xH2O]y[Al(OH)4]z|[Si121+zAl71−zO384]−FAU, where 21.4 ≤ u ≤ 24.2, M = Na and/or Ca, 3.3 ≤ v ≤ 13.1, 9.5 ≤ w ≤ 21.6, 2 ≤ x ≤ 4, 0.3 ≤ y ≤ 3.2, and 0 ≤ z ≤ 2.5. In each of the five crystal structures (space group Fd3m; mean a = 24.47 A), Ni2+ is found at sites I, I′, a second I′, II′, and sometimes II; unexpectedly, Na+ and Ca2+ are found at another site II. Two extra-framework oxygen positions (Oe), one in the sodalite cavity and the other nearby in the supercage, are seen on 3-fold axes in all crystals....

What do vanadium framework sites look like in redox model silicate zeolites?

Ab initio periodic DFT calculations on the structural and energy properties of different model vanadium framework sites in vanadium-substituted zeolites containing sodalite cavities are presented and related to experimental FTIR measurements on V containing SiBEA zeolite. The vanadium framework sites are characterized by their calculated geometrical parameters, vibrational frequencies and deprotonation energies. Vanadium(V) is stabilized in the zeolite framework in two different tetrahedral sites: one in the form of a very stable vanadyl group and the other possessing a hydroxyl group. Tetrahedral vanadium(IV) sites, although much less favorable, are also present with a acidic hydroxyl group. Hydration of vanadium-substituted zeolites is an athermic process. Water does not affect the coordination of vanadium and remains in the center of the cavity. These results are fully consistent with experimental data and allow to identify the molecular structure of vanadium sites in a zeolite framework.

Single Crystal Structure of Zeolite A (LTA) Containing Ag4Cl4Nanoclusters and Reduced 1,3,5-Tripyrylium Dimers with Remarkably Short 2.43 Å Interplanar Spacings

A single crystal of Ag 12−A (zeolite LTA) was prepared by the dynamic ion-exchange of Na 12−A with aqueous 0.05 M AgNO 3. It was washed with CH 3OH and allowed to react with a stream of 0.05 M KCl in CH 3OH at 294 K. The crystal structure of the product |K 2.35Ag 1.1(Ag 4Cl 4) 0.45(C 3H 3O 3) 1.1(K 3Cl) 3.45(K 3(OH) 2) 0.55(H 2O) ≥3.0|[Si 12Al 12O 48]− LTA ( a = 12.292(1) Å) was determined by single-crystal X-ray diffraction in the cubic space group Pm3̅ m at 294 K. It was refined to the final error index, R 1 = 0.052, based on the 371 reflections for which F o > 4σ( F o). Ag 4Cl 4 nanoclusters were found in about 45% of the sodalite cavities. Each Ag 4Cl 4 cluster (interpenetrating tetrahedra; symmetry T d, Ag−Cl = 3.105(17) Å) is held in place by the coordination of each of its four Ag + ions to three oxygens of the zeolite framework (Ag−O = 2.493(5) Å) and by the coordination of each of its four Cl − ions to a K + ion through a 6-ring (Cl−K = 2.70(3) Å). In each of the remaining 55% of the sodalite cavities, two reduced planar 1,3,5-tripyrylium cations, [(CH) 3O 3] 2 2+ (C−O = 1.52(3) Å), are found. These parallel eclipsed rings (symmetry D 3d) have an interplanar distance of only 2.43 Å due to σ double bonding between the rings, the result of four electrons in 12-center bonding π* orbitals, and to polar attraction. Each ring makes three strong hydrogen bonds (CH···O = 2.84(3) Å) to oxygens of the zeolite framework, and a Ag + ion coordinates to the three oxygens of one ring (Ag−O = 2.68(9) Å). The large cavities are filled with K +, Cl −, Ag +, OH −, and H 2O; the K 3Cl 2+ unit predominates. The 1,3,5-tripyrylium ring, isoelectronic with benzene, had not been reported before.

Determination of Si/Al Ratio of Faujasite-type Zeolite by Single-crystal X-ray Diffraction Technique. Single-crystal Structures of Fully Tl+- and Partially K+-exchanged Zeolites Y (FAU), |Tl71|[Si121Al71O384]-FAU and |K53Na18|[Si121Al71O384]-FAU

Large colorless single crystals of faujasite-type zeolite with diameters up to 200 /an have been synthesized from gels with the composition of 3.58SiO 2 :2.08NaAlO 2 :7.59NaOH:455H 2 O:5.06TEA:1.23TCl. Two of these, colorless octahedron about 200 μm in cross-section have been treated with aqueous 0.1 M TlC 2 H 3 O 2 and KNO 3 in order to prepare Tl + - and K + -exchanged faujasite-type zeolites, respectively, and then determined the Si/Al ratio of the zeolite framework. The crystal structures of |Tl 71 |[Si 121 Al 71 O 384 ]-FAU and |K 53 Na 18 |[Si 121 Al 71 O 384 ]-FAU per unit cell, a = 24.9463(2) and 24.9211(16) A, respectively, dehydrated at 673 K and 1 x 10 -6 Torr, have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 m at 294 K. The two single-crystal structures were refined using all intensities to the final error indices (using only the 905 and 429 reflections for which F o > 4σ(F o )) R 1 /R 2 = 0.059/0.153 and 0.066/0.290, respectively. In the structure of fully Tl + -exchanged faujasite-type zeolite, 71 Tl + ions per unit cell are located at four different crystallographic sites. Twenty-nine Tl + ions fill site I' in the sodalite cavities on 3-fold axes opposite double 6-rings (Tl-O = 2.631(12) A and O-Tl-O = 93.8(4)°). Another 31 Tl + ions fill site II opposite single 6-rings in the supercage (Tl-O = 2.782(12) A and O-Tl-O = 87.9(4)°). About 3 Tl + ions are found at site III in the supercage (Tl-O = 2.91(6) and 3.44(3) A), and the remaining 8 occupy another site III (Tl-O = 2.49(5) and 3.06(3) A). In the structure of partially K + -exchanged faujasite-type zeolite, 53 K + ions per unit cell are found at five different crystallographic sites and 18 Na + ions per unit cell are found at two different crystallographic sites. The 4 K + ions are located at site I, the center of the hexagonal prism (K-O = 2.796(8) A and O-K-O = 89.0(3)°). The 10 K + ions are found at site I' in the sodalite cavity (K-O = 2.570(19) A and O-KO = 99.4(9)°). Twenty-two K + ions are found at site II in the supercage (K-O = 2.711(9) A and O-K-O = 94.7(3)°). The 5 K + ions are found at site III deep in the supercage (K-O = 2.90(5) and 3.36(3) A), and 12 K + ions are found at another site III' (K-O = 2.55(3) and 2.968(18) A). Twelve Na + ions also lie at site I' (Na-O = 2.292(10) and O-Na-O = 117.5(5)°). The 6 Na + ions are found at site II in the supercage (Na-O = 2.390(17) A and O-Na-O = 113.1(11)°). The Si/Al ratio of synthetic faujasite-type zeolite is 1.70 determined by the occupations of cations, 71, in two single-crystal structures.

Synthesis and Crystal Structure of Zinc Iodide in the Sodalite Cavities of Zeolite A (LTA).

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Synthesis and Crystal Structure of Zinc Iodide in the Sodalite Cavities of Zeolite A (LTA)

The crystal structure of ZnI2 molecule synthesized in zeolite A (LTA) has been studied by single-crystal X-ray diffraction techniques. A single crystal of |Zn6|[Si12Al12O48]-LTA, synthesized by the dynamic ion-exchange of |Na12|[Si12Al12O48]-LTA with aqueous 0.05 M Zn(NO3)2 and washed with deionized water, was placed in a stream of flowing 0.05 M KI in CH3OH at 294 K for four days. The resulting crystal structure of the product (|K6Zn3(KI)3(ZnI2)0.5|[Si12Al12O48]-LTA, a = 12.1690(10) A) was determined at 294 K by single-crystal X-ray diffraction in the space group Pm3m. It was refined with all measured reflections to the final error index R1 = 0.078 for 431 reflections which Fo > 4σ (Fo). At four crystallographically distinct positions, 3.5 Zn2+ and nine K+ ions per unit cell are found: three Zn2+ and five K+ ions lie on the 3-fold axes opposite 6-rings in the large cavity, two K+ ions are off the plane of the 8-rings, two K+ ions are recessed deeply off the plane of the 8-rings, and the remaining a half Zn2+ ion lie on the 3-fold axes opposite 6-rings in the sodalite cavity. A half Zn2+ ion and an I- ion per unit cell are found in the sodalite units, indicating the formation of a ZnI2 molecule in 50% of the sodalite cavities. Each ZnI2 (Zn-I = 3.35(5) A) is held in place by the coordination of its one Zn2+ ion to the zeolite framework oxygens and by the coordination of its two I- ions to K+ ions through 6-rings (I-K = 3.33(8) A). Three additional I- ions per unit cell are found opposite a 4-ring in the large cavity and form a K3I2+ and two K2ZnI3+ ionic clusters, respectively.

Crystal Structure of Fully Dehydrated Partially Cs+-Exchanged Zeolite X, Cs52Na40-X (The Highest Cs+-Exchanged Level Achieved by Conventional Method and Confirmation of Special Site Selectivity)

24.9765(10) A, has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd at 21 °C. The crystal was prepared by flow method for 5 days using exchange solution in which mole ratio of CsOH and CsNO3 was 1 : 1 with total concentration of 0.05 M. The crystal was then dehydrated at 400 °C and 2 × 10 −6 Torr for 2 days. The structure was refined to the final error indices, R1 = 0.051 and wR2 (based on F 2 ) = 0.094 with 247 reflections for which Fo > 4σ (Fo). In this structure, about fifty-two Cs + ions per unit cell are located at six different crystallographic sites with special selectivity; about one Cs + ion is located at site I, at the centers of double oxygen-rings (D6Rs), two Cs + ions are located at site I', and six Cs + ions are found at site II'. This is contrary to common view that Cs + ions cannot pass sodalite cavities nor D6Rs because six-ring entrances are too small. Ring-opening by the formation of –OH groups and ring-flexing make Cs + ions at sites I, I', and II' enter six-oxygen rings. The defects of zeolite frameworks also give enough mobility to Cs + ions to enter sodalite cavities and D6Rs. Another six Cs + ions are found at site II, thirty-six are located at site III, and one is located at site III' in the supercage, respectively. Forty Na + ions per unit cell are located at two different crystallographic sites; about fourteen are located at site I, the centers of D6Rs and twenty-six are also located at site II in the supercage. Cs + ions and Na + ions at site II are recessed ca. 0.34(1) A and 1.91(1) A into the supercage, respectively. In this work, the highest exchange level of Cs + ions per unit cell was achieved in zeolite X by conventional aqueous solution methods and it was also shown that Cs + ion could pass through the six

Oxidation of carbon monoxide over Cu- and Ag-NaY catalysts with aqueous hydrogen peroxide

A novel oxidation reaction of CO with aqueous H 2O 2 over Cu-NaY (2–15 wt%) and Ag-NaY (5–15 wt%) catalysts has been achieved at low temperatures (55–70 °C) using a flow mode system. The employed catalysts were prepared by the incipient wetness impregnation of NaY zeolite (Si/Al = 5.6, surface area = 910 m 2/g) with an aqueous solution of known concentrations of copper acetate and silver nitrate. Solids were subjected to thermal treatment at 300–450 °C prior to catalytic measurements unless subjected to subsequent reduction with hydrogen at 350 °C. The physicochemical characterization of the catalysts was probed using X-ray diffraction (XRD), FT-IR and combined thermal analyses TGA–DrTGA. The XRD data indicated that, the Ag particles have an ordered location in the sodalite cavity and the center of a single six-ring. The FT-IR data also proved the presence of a new peak at 1385 cm −1 that is assigned to Ag-coordinated with the framework. A slow induced oxidation of CO (induction period, t ind) took place at the initial stage of the CO oxidation reaction after which the reaction obeyed first-order kinetics. The utilized metal ions are proposed to be reduced to lower oxidation states such as Cu + and Ag 0 during the first period of reaction, t ind, where the reaction proceeded favorably on such sites. Such argument was evidenced by carrying out the oxidation reaction over H 2-reduced Cu10-NaY and Ag10-NaY catalysts. The reduction caused a decrease in the t ind, giving an evidence that the lower oxidation states Cu + and Ag 0 are the active sites in the studied oxidation reaction. The enhancement in catalytic activity was interpreted in terms of the facile adsorption of CO on the low oxidation state species.

Preparation, Crystal Structure, and Thermal Stability of the Cadmium Sulfide Nanoclusters Cd6S44+ and Cd2Na2S4+ in the Sodalite Cavities of Zeolite A (LTA)

The crystal structure and thermal stability of two cadmium sulfide nanoclusters prepared in zeolite A (LTA) have been studied by XPS, TGA, and single-crystal and powder XRD. The crystal structures of Cd2.4Na3.2(Cd6S4)0.4(Cd2Na2S)0.6(H2O)> or =5.8[Si12Al12O48]-LTA (a = 12.2919(7) A, crystal 1 (hydrated)) and /Cd4Na2(Cd2O)(Na2O)/[Si12Al12O48]-LTA (a = 12.2617(4) A, crystal 2 (dehydrated)) were determined by single-crystal methods in the cubic space group Pm3m at 294(1) K. Crystal 1 was prepared by ion exchange of Na12-LTA in an aqueous stream 0.05 M in Cd2+, followed by washing in a stream of water, followed by reaction in an aqueous stream 0.05 M in Na2S. Crystal 2 was made by dehydrating crystal 1 at 623 K and 1 x 10(-6) Torr for 3 days. In crystal 1, Cd6S4(4+) nanoclusters were found in and extending out of about 40% of the sodalite cavities. Central to each Cd6S4(4+) cluster is a Cd4S4 unit (interpenetrating Cd2+ and S2- tetrahedra with near Td symmetry, Cd-S = 2.997(24) A, Cd-S-Cd = 113.8(12) degrees, and S-Cd-S = 58.1(24) degrees). Each of the two remaining Cd2+ ions bonds radially through a 6-ring of the zeolite framework to a sulfide ion of this Cd4S4 unit (Cd-S = 2.90(8) A). In each of the remaining 60% of the sodalite cavities of crystal 1, a planar Cd2Na2S4+ cluster was found (Cd-S/Na-S = 2.35(5)/2.56(14) A and Cd-S-Cd/Na-S-Na = 122(5)/92(7) degrees). Cd6S4(4+) and Cd2Na2S4+ are stable within the zeolite up to about 700 K in air. Upon vacuum dehydration at 623 K, all sulfur was lost (crystal 2). Instead as anions, only two oxide ions remain per sodalite unit. One bridges between two Cd2+ ions (Cd2O2+, Cd-O = 2.28(3) A) and the other between two Na+ ions (Na2O, Na-O = 2.21(10) A).

Single crystal structure of fully dehydrated fully Tl +-exchanged zeolite Y, ∣Tl 71∣[Si 121Al 71O 384]- FAU

The structure of fully dehydrated, fully Tl +-exchanged zeolite Y, ∣Tl 71∣[Si 121Al 71O 384]- FAU ( a = 24.948(2) Å), has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd 3 ¯ m at 21(1) °C. An aqueous exchange solution 0.05 M in Tl + with pH = 7.2 was allowed to flow past the crystal for 7 days. The resulting colorless crystal became black after being dehydrated at 400 °C and 2 × 10 −6 Torr for 2 days. Diffraction data were then gathered and were refined using all 702 unique reflections to the final error indices (based upon the 365 reflections for which F o > 4 σ( F o)) R 1 = 0.058 and wR 2 = 0.130. In this structure, Tl + ions occupy four crystallographic sites. About 30.4(6) Tl + ions fill or nearly fill site I′ in the sodalite cavities on 3-fold axes opposite double 6-rings; each is 1.37 Å from the plane of three oxygens (Tl–O = 2.568(17) Å, O–Tl–O = 93.9(5)°). Each is also 3.673(4) Å from three (perhaps sometimes only two) equivalent site-I′ Tl + ions. Thirty-two Tl + ions fill site II opposite single 6-rings in the supercage; each is 1.63 Å from the plane of three oxygens (Tl–O = 2.724(15) Å, O–Tl–O = 87.9(6)°). The remaining Tl + ions are located at two nonequivalent sites III′ with occupancies of 5.7(8) and 3.0(4) per unit cell; each coordinates to three framework oxygens. It appears that TlOH was imbibed during aqueous ion exchange at pH = 7.2, and that it decomposed upon vacuum dehydration at 400 °C to deposit as Tl 2O (black) on the surface of the zeolite crystal.

Synthesis and Crystal Structure of Lead Iodide in the Sodalite Cavities of Zeolite A (LTA)

The positions of PbI 2 molecule synthesized into the molecular-dimensioned cavities of |K 6 (Pb 4 I 2 )(PbI 2 ) 0.67 -(H 2 O) 2 |[Si 12 Al 12 O 48 ]-LTA have been determined. A single crystal of |Pb 6 |[Si 12 Al 12 O 48 ]-LTA, prepared by the dynamic ion-exchange of |Na 12 |Si 12 Al 12 O 48 -LTA with aqueous 0.05 M Pb(NO 3 ) 2 and washed with deionized water, was placed in a stream of flowing aqueous 0.05 M KI at 294 K for three days. The resulting crystal structure of the product (|K 6 (Pb 4 I 2 )(PbI 2 ) 0.67 (H 2 O) 2 |[Si 12 Al 12 O 48 ]-LTA, a = 12.353(1) A) was determined at 294 K by single-crystal X-ray diffraction in the space group Pm 3m. It was refined with all measured reflections to the final error index R 1 = 0.062 for 623 reflections which F o > 4σ(F o ). 4.67 Pb 2+ and six K + ions per unit cell are found at three crystallographically distinct positions: 3.67 Pb 2+ and three K + ions on the 3-fold axes opposite six-rings in the large cavity, three K + ions off the plane of the eight-rings, and the remaining one Pb 2+ ion lies opposite four-ring in the large cavity. 0.67 Pb 2+ ions and 1.34 I - ions per unit cell are found in the sodalite units, indicating the formation of a PbI 2 molecule in 67% of the sodalite units. Each PbI 2 (Pb-I = 3.392(7) A) is held in place by the coordination of its one Pb 2+ ion to the zeolite framework (a Pb 2+ cation is 0.74 A from a six-ring oxygens) and by the coordination of its two I - ions to K + ions through six-rings (I-K = 3.63(4) A). Two additional I - ions per unit cell are found opposite a four-ring in the large cavity and form Pb 2 K 2 I 5+ and PbK 2 I 3+ moieties, respectively, and two water molecules per unit cell are also found on the 3-fold axes in the large cavity.

Single crystal structure of fully dehydrated fully K +-exchanged zeolite Y (FAU), K 71Si 121Al 71O 384

The crystal structure of K 71-Y (FAU), K 71Si 121Al 71O 384 per unit cell, a = 24.9976(7) Å, dehydrated at 653 K and 8 × 10 −6 Torr, has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd 3 ¯ m at 294 K. The structure was refined using all intensities to the final error indices (using only the 695 reflections for which F o > 4 σ( F o)) R 1 = 0.045 (based on F) and R 2 = 0.136 (based on F 2). About 71 K + ions per unit cell are found at an unusually large number of crystallographically distinct positions, eight. They exhibit the same degree of cation crowding seen in K 92-X (FAU) as K + ions continue to crowd into sites I and I′. Six K + ions are at the centers of double 6-rings (D6Rs, site I; K–O = 2.733(5) Å and O–K–O (octahedral) = 89.90(14)° and 90.10(14)°). Five additional K + ions are near site I (K–O = 2.530(11) and 2.972(15) Å); each is pushed off center by a K + ion at a site-I′ position. Two site-I′ positions (in the sodalite cavities opposite D6Rs) are occupied by ten and five K + ions, respectively, per unit cell; these K + ions are recessed 1.30 Å and 1.68 Å into the sodalite cavities from their 3-oxygen planes (K–O = 2.581(15) and 2.69(3) Å, and O–K–O = 96.9(7)° and 84.9(14)°, respectively). These five site-I′ K + ions push the five site-I K + ions off center (K + ⋯ K + = 3.65(4) Å), and, in turn, are themselves pushed deeper into the sodalite cavities. Nearly filling site II in the supercages are 30.3(3) K + ions, each of which extends 1.36 Å into the supercage from its 3-oxygen plane (K–O = 2.688(4) Å and O–K–O = 96.86(13)°). The 9.4(4) K + ions at site III in the supercages each lie opposite the central ring of a triple 4-ring (K–O = 2.894(11) Å twice and O–K–O = 81.2(4)°). The remaining 2.0(7) and 1.5(6) K + ions occupy two sites III′ near triple 4-rings in the supercage (K–O = 2.41(10), 2.89(5), 2.57(8), and 2.97(4) Å). K 71-Y has 20 fewer K + ions at III′ sites than K 92-X; other much smaller occupancy differences are seen.

Single-crystal Structure of Fully Dehydrated Sodium Zeolite Y (FAU), ||Na71||[Si121Al71O384]-FAU

The crystal structure of ¦Na71¦[Si121Al71O384]-FAU, a = 24.946(3)A, dehydrated at 673 K and 1 × 10-6 Torr, was determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3m at 294 K. About 71 Na+ ions per unit cell are found at an unusually large number of crystallographically distinct positions, seven. Five Na+ ions are at the centers of double 6-rings (D6Rs, site I). Two site-I′ positions (in the sodalite cavities opposite D6Rs) are occupied by eight and fourteen Na+ ions, respectively, per unit cell. Thirteen and fifteen Na+ ions per unit cell are found at two site-II positions, in the supercages opposite S6Rs. The remaining twelve and four Na+ ions occupy two sites III′ near triple 4-rings in the supercage.

Single crystal structure of fully dehydrated, excessively Cd 2+-exchanged zeolite Y, ∣Cd 27.5(Cd 8O 4) 2∣[Si 121Al 71O 384]-FAU, containing [formula omitted] clusters

The structure of fully dehydrated, excessively Cd 2+-exchanged zeolite Y, ∣Cd 27.5(Cd 8O 4) 2∣[Si 121Al 71O 384]-FAU, has been determined using single-crystal X-ray diffraction techniques in the cubic space group Fd 3 ¯ m at 21(1) °C. Ion exchange was accomplished by allowing an aqueous stream 0.05 M in Cd 2+, pH 7, to flow past the crystal for 5 days. The crystal was then dehydrated at 350 °C and 2 × 10 −6 Torr for 2 days. The structure was determined in vacuum and was refined using all 690 unique reflections to the final error indices (based only upon the 357 reflections for which F o > 4 σ( F o)), R 1 = 0.060 and wR 2 = 0.151. In this structure, Cd 2+ ions occupy five crystallographic sites. Eight Cd 2+ ions half fill site I at the centers of the double six-rings (Cd–O (octahedral) = 2.498(11) Å), eight Cd 2+ ions one-quarter fill a site-I′ position deep in the sodalite cavities (Cd–O (octahedral) = 2.559(17) Å) and another eight one-quarter fill a more conventional site-I′ position (Cd–O = 2.278(14) Å), eight Cd 2+ ions one-quarter fill site II′ inside the sodalite cavities (Cd–O = 2.192(11) Å), and the remaining 11.5 Cd 2+ ions are found at site II in the supercages (Cd–O = 2.179(9) Å). The Cd 2+ ions at the first I′ position form, with non-framework oxide ions, Cd 4O 4 clusters (interpenetrating tetrahedra, symmetry 4 ¯ 3 m ( T d ) , Cd–O = 2.33(4) Å) in two of the eight sodalite cavities per unit cell. Each Cd 4O 4 unit coordinates further via its oxygen atoms to four more Cd 2+ ions at site II′, Cd–O = 1.86(11) Å, to give the larger cationic Cd 8 O 4 8 + clusters (symmetry still 4 ¯ 3 m ( T d ) ) . During ion exchange at pH 7, zeolite Y had apparently imbibed eight Cd(OH) 2 molecules per unit cell; these decomposed upon vacuum dehydration to give, with eight additional Cd 2+ ions, two Cd 8 O 4 8 + clusters.

Crystal Structure of an Ethylene Sorption Complex of Fully Vacuum-Dehydrated Fully Ag+-Exchanged Zeolite X (FAU). Silver Atoms Have Reduced Ethylene To Give CH22- Carbanions at Framework Oxide Vacancies

The crystal structure of an ethylene sorption complex of fully vacuum-dehydrated fully Ag(+)-exchanged zeolite X (FAU), a = 24.865(2) A, has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd at 21 degrees C. It is very different from the ethylene complex of Ag(92)-X that had been dehydrated at 400 degrees C in flowing oxygen, as were the two dehydrated structures. The crystal was prepared by ion exchange in a flowing stream of aqueous 0.05 M AgNO(3) for 3 days, followed by dehydration at 400 degrees C and 2 x 10(-6) Torr for 2 days, followed by exposure to 300 Torr of zeolitically dry ethylene gas for 2 h at 21 degrees C. The structure was determined in this atmosphere and was refined using all data to the final error indices (based upon the 534 reflections for which F(o) > 4sigma(F(o))) R(1) = 0.062 and wR(2) = 0.135. In this structure, per unit cell, 14 Ag(+) ions were found at the octahedral site I (Ag-O = 2.611(9) A), and 32 partially reduced Ag(+) ions fill two different site I' positions deep in the sodalite cavities (Ag-O = 2.601(13) and 2.618(12) A). The sodalite cavities host two different cationic silver clusters. In about 47% of sodalite units, eight silver atoms form interpenetrating tetrahedra, Ag(8)(n+) (n = 4 is suggested), with T(d)() symmetry. The other 53% of the sodalite units host cyclo-Ag(4)(m+) (m = 2 is suggested) cations with near S(4) symmetry. These clusters are very similar to those in vacuum-dehydrated Ag(92)-X. Thirty-two Ag(+) ions fill the single 6-rings, 15 at site II' (Ag-O = 2.492(10) A), and 17 at site II (Ag-O = 2.460(9) A). The latter 17 lie in supercages where each forms a lateral pi-complex with an ethylene molecule. In turn, each C(2)H(4) molecule forms two cis electrostatic hydrogen bonds to framework oxygens. The remaining 14 Ag+ ions occupy three different II' sites. Vacuum dehydration had caused substantial decomposition: per unit cell, 30 of the 92 Ag(+) ions were reduced and 15 of the 384 framework oxide ions were oxidized to O2(g), leaving lattice vacancies. The sorption of C(2)H(4) at 21 degrees C reoxidized about 7 of the 30 Ag(0) atoms to Ag(+) and reduced 1.75 ethylene molecules to give CH(2)(2-) groups which refilled 3.5 of these 15 lattice vacancies. The remaining vacancies may have been filled with H(2)C=C(2-) ions. The unit cell formula, which originally contained 384 oxygen atoms, may be |Ag(92)(C2H4)17|[Si(100)Al(92)O(369)(CH2)3.5] or |Ag(92)H(23)(C2H4)17|[Si(100)Al(92)O(369)(CH2)3.5(C2H2)11.5].

Cationic zinc clusters with mean formula [formula omitted] in the sodalite cavities of zeolite Y (FAU)

Now that the occurrence of monatomic Zn + at extraframework positions in zeolites has been established, a reevaluation of a crystal structure reported in 1992 indicates that cationic zinc clusters of mean formula Zn 5 . 4 6 . 9 + had formed in the sodalite cavities of zeolite Y (FAU), Zn ca.43Si 137Al 55O 384 per unit cell. Their mean Zn–Zn bond length is 3.14(2) Å, and their two mean Zn–Zn–Zn angles are 77.0(4)° and 101.6(4)°. To minimize the number of Zn–Zn intercationic approaches, and to distribute positive charge relatively evenly over the anionic zeolite framework, these clusters are likely to be cyclic (Zn 6) and a segment of a cycle (Zn 5). The even, four-electron clusters cyclo- Zn 6 8 + and Zn 5 6 + are proposed. Each sodalite cavity may host one of these clusters. cyclo- Zn 6 8 + would have three atoms at site I′ and three at site II′, alternating, with symmetry 3 m. Zn 5 6 + would be like cyclo- Zn 6 8 + but with one atom at site I′ missing, and symmetry m.

Gadolinium(III)‐Loaded Nanoparticulate Zeolites as Potential High‐Field MRI Contrast Agents: Relationship Between Structure and Relaxivity

The effects of dealumination, pore size, and calcination on the efficiency (as expressed in the relaxivity) of Gd3+-loaded zeolites for potential application as magnetic resonance imaging (MRI) contrast agents were studied. Partial dealumination of zeolites NaY or NaA by treatment with (NH4)2SiF6 or diluted HCl resulted in materials that, upon loading with Gd3+, had a much higher relaxivity than the corresponding non-dealuminated materials. Analysis of the 1H NMR dispersion profiles of the various zeolites showed that this can be mainly ascribed to an increase of the amount of water inside the zeolite cavities as a result of the destruction of walls between cavities. However, the average residence time of water inside the Gd3+-loaded cavities did not change significantly, which suggests that the windows of the Gd3+-loaded cavities are not affected by the dealumination. Upon calcination, the Gd3+ ions moved to the small sodalite cavities and became less accessible for water, resulting in a decrease in relaxivity. The important role of diffusion for the relaxivity was demonstrated by a comparison of the relaxivity of Gd3+-loaded zeolite NaY and NaA samples. NaA had much lower relaxivities due to the smaller pore sizes. The transversal relaxivities of the Gd3+-doped zeolites are comparable in magnitude to the longitudinal ones at low magnetic fields (<60 MHz). However at higher fields, the transversal relaxivities steeply increased, whereas the longitudinal relaxivities decreased as field strength increased. Therefore, these materials have potential as T1 MRI contrast agents at low field, and as T2 agents at higher fields.

Synthesis and Crystal Structure of Ag4Br4Nanoclusters in the Sodalite Cavities of Fully K+-Exchanged Zeolite A (LTA)

<TEX>$Ag_4Br_4$</TEX> nanoclusters have been synthesized in about 75% of the sodalite cavities of fully <TEX>$K^+$</TEX>-exchanged zeolite A (LTA). An additional KBr molecule is retained in each large cavity as part of a near square-planar <TEX>$K_4Br^{3+}$</TEX> cation. A single crystal of <TEX>$Ag_{12}$</TEX>-A, prepared by the dynamic ion-exchange of <TEX>$Na_{12}$</TEX>-A with aqueous 0.05 M <TEX>$AgNO_3$</TEX> and washed with <TEX>$CH_3OH$</TEX>, was placed in a stream of flowing 0.05 M KBr in <TEX>$CH_3OH$</TEX> for two days. The crystal structure of the product (<TEX>$K_9(K_4Br)Si_{12}Al_{12}O_{48}{\cdot}0.75Ag_4Br_4$</TEX>, a = 12.186(1) <TEX>$\AA$</TEX>) was determined at 294 K by single-crystal X-ray diffraction in the space group Pm m. It was refined with all measured reflections to the final error index <TEX>$R_1$</TEX> = 0.080 for the 99 reflections for which <TEX>$F_o\;{\gt}\;4_{\sigma}\;(F_o)$</TEX>. The thirteen <TEX>$K^+$</TEX> ions per unit cell are found at three crystallographically distinct positions: eight <TEX>$K^+$</TEX> ions in the large cavity fill the six-ring site, three <TEX>$K^+$</TEX> ions fill the eight-rings, and two <TEX>$K^+$</TEX> ions are opposite four-rings in the large cavity. One bromide ion per unit cell lies opposite a four-ring in the large cavity, held there by two eight-ring and two six-ring <TEX>$K^+$</TEX> ions (<TEX>$K_4Br^{3+}$</TEX>). Three <TEX>$Ag^+$</TEX> and three <TEX>$Br^-$</TEX>ions per unit cell are found on 3-fold axes in the sodalite unit, indicating the formation of nano-sized <TEX>$Ag_4Br_4$</TEX> clusters (interpenetrating tetrahedra; symmetry <TEX>$T_d$</TEX>; diameter ca. 7.9 <TEX>$\AA$</TEX>) in 75% of the sodalite units. Each cluster (Ag-Br = 2.93(3) <TEX>$\AA$</TEX>) is held in place by the coordination of its four <TEX>$Ag^+$</TEX> ions to the zeolite framework (each <TEX>$Ag^+$</TEX> cation is 2.52(3) <TEX>$\AA$</TEX> from three six-ring oxygens) and by the coordination of its four <TEX>$Br^-$</TEX> ions to <TEX>$K^+$</TEX> ions through six-rings (Br-K = 3.00(4) <TEX>$\AA$</TEX>).