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Al = 1.56)
Microporous and Mesoporous Materials 264 (2018) 139–146
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Structure of a cyclohexane sorption complex of partially dehydrated, fully Mn2+-exchanged zeolite Y (FAU, Si/Al = 1.56)
T
Dae Jun Moona, Hae-Kwon Jeongb,c, Woo Taik Lima,∗, Karl Seffd,∗∗ a
Department of Applied Chemistry, Andong National University, Andong 36729, South Korea Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, USA c Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843, USA d Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, HI 96822, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Zeolite Y Manganese Cyclohexane Crystal structure Sorption
The structure of a cyclohexane sorption complex of partially dehydrated, fully Mn2+-exchanged zeolite Y has been determined at 100(1) K by single-crystal synchrotron X-ray diffraction techniques. It was refined using all intensities to the final error indices R1 = 0.052 and wR2 = 0.174. Cyclohexane molecules of symmetry 3 2/m (D3d, chair form) occupy 5.7 of the 16 12-rings per unit cell. Each cyclohexane molecule is held in place by 18 weak hydrogen bonds between its six equatorial hydrogen atoms and all 12 oxygen atoms of its 12-ring: C⋯O ca. 3.85 Å. Mn2+ ions are found at four crystallographic sites, I, I′, II′, and II. Each of the 17.7(2) Mn2+ ions per unit cell at site II (opposite 6-rings in the supercage) coordinates tetrahedrally to three framework oxygen atoms and a water molecule. The cyclohexane molecules interact neither with the Mn2+ ions nor with the water molecules.
1. Introduction Zeolites are of great interest due to their widespread industrial use as catalysts, ion exchangers, and selective sorbents [1,2]. These functions depend on the structure of zeolite and its interactions with guest molecules. Knowing how these molecules interact with the zeolite provides a structural basis for understanding the chemistry involved [3–5]. The dynamic properties of cyclohexane guest molecules in various solid host structures such as the thiourea [6], Cd(CN)2 [7], Cd(CH3NH (CH2)3NH2)Ni(CN)4 [8], and zeolite H-ZSM-5 [9] have been investigated by wide-line solid-state 2H NMR spectroscopy, a particularly sensitive probe of molecular motions with characteristic time scales in the intermediate motion regime. Poupko et al. used 2H NMR to study the thiourea-perdeuterocyclohexane inclusion complex from 93 to 333 K [6]. They identified three temperature regions involving the structure of the host thiourea lattice and the behavior of the guest cyclohexane molecules. Below 129 K, the host lattice was monoclinic and the guest molecules undergo thermally activated reorientation about the C3 axis. Between 129 and 156 K, the monoclinic host lattice gradually transformed into a hexagonal form and three nonequivalent kinds of guest molecules were found. Above 156 K, the host lattice was hexagonal and the guest molecules were highly disordered. Nishikiori et al. studied the molecular motion of cyclohexane ∗
enclathrated in Cd(CH3NH(CH2)3NH2)Ni(CN)4 using CP/MAS 13C NMR and 2H NMR [8]. They suggested that the chair form of cyclohexane was undergoing n-fold reorientation about its 3-fold axis. This motion was retained to 397 K accompanied by inversion of the ring above ca. 239 K. Aliev et al. studied the dynamic properties of C6D12 molecules in zeolite H-ZSM-5 using variable temperature wide-line solid state 2H NMR spectroscopy [9]. They reported that the C6D12 molecules underwent three types of rapid motion between 93 and 233 K: (1) reorientation about its C3 axis, (2) restricted wobbling (precession) about its C3 axis, and (3) a four-site jump motion. The sorption of the hydrocarbons benzene [10–12], xylene [13], toluene [14], mesitylene [15,16], ethylene [17], acetylene [18], and cyclopropane [19] in FAU-type zeolites were investigated by singlecrystal X-ray diffraction techniques to learn their sorption sites. From this their interactions with the cations and the zeolite framework could be described. The sorption of these hydrocarbons is governed mainly by their sizes, the identity of the cations, and the Si/Al ratio of the framework. Vitale et al. used powder neutron diffraction and energy minimization calculations to learn the locations of sorbed cyclohexane in the acid form of zeolite Y (FAU, Si/Al = 2.43) [20]. From their neutron diffraction work, they found cyclohexane in its chair conformation in 12-rings. They calculated, however, that other positions of lower
Corresponding author. Corresponding author. E-mail addresses: [email protected] (W.T. Lim), seff@hawaii.edu (K. Seff).
∗∗
https://doi.org/10.1016/j.micromeso.2018.01.016 Received 27 October 2017; Received in revised form 11 January 2018; Accepted 12 January 2018 Available online 17 January 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
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O3, and O4] in dehydrated |Tl75|[Si117Al75O384]-FAU [26]. It converged to the error indexes (agreement parameters), defined in a footnote to Table 1, R1 = 0.40 and wR2 = 0.83. The progress of structure determination as subsequent peaks were found on difference Fourier functions and identified as nonframework atoms, and hydrogen atoms were added at calculated positions, are given in Table 2. A small decrease in the error indexes was observed when the hydrogen atoms were added at calculated positions, and another small decrease was seen when the thermal parameters at C were assigned to H1 and H2 (Table 2). These decreases were too small to allow further definition to be added to the anisotropic thermal parameters assigned to the hydrogen atoms. Fixed weights were used initially. The final weights were assigned using the formula w = 1/[σ2(Fo2) + (aP)2 + bP], where P = [max (Fo2,0) + 2Fc2]/3, with a and b as refined parameters (Table 1). Neutral atomic scattering factors within SHELX-97 were used, and all were modified to account for anomalous dispersion [27,28]. The final structural parameters and selected interatomic distances and angles are presented in Tables 3 and 4.
symmetry had similar sorption energies. Of the organic molecules that can be sorbed into zeolites, polar molecules are most strongly held, unsaturated hydrocarbons are weakly held unless the zeolite contains cations to which they can π-bond, and saturated hydrocarbons are most weakly held. However, saturated hydrocarbons can be effectively sorbed if they fit features of the zeolite structure, so that they can have multiple weak interactions with a relatively small decrease in entropy. Cyclohexane in a round 12-ring is an example of this [20]. This report provides a detailed look at the geometry and thermal motion of that interaction. 2. Experimental section 2.1. Crystal preparation Large colorless octahedral single crystals of |Na75|[Si117Al75O384]FAU (Na75-Y) with diameters up to 0.19 mm were prepared by Lim et al. [21]. Mn2+ exchange was done using aqueous 0.1 M Mn (NO3)2·xH2O (Aldrich, 99.99%, Co 17.3 ppm, Fe 9.23 ppm, K 5.56 ppm, Cr 5.1 ppm, Na 2.87 ppm, Ce 2.49 ppm). Hydrated Na75-Y (10 mg) was mixed with 10 ml of 0.1 M Mn(NO3)2, a 20 fold-excess, in a 15 ml conical tube and the mixture was stirred on a shaking incubator at 343 K for 24 h. This procedure was repeated 20 times with fresh Mn (NO3)2 solution. Finally, the product was washed with distilled water (300 ml), filtered, and oven dried at 323 K for 1 d. The structures of complexes of crystals from this batch have been reported before, and the exchangeable cation and framework composition, |Mn37.5|[Si117Al75O384]-FAU (Mn37.5-Y) has been established by crystallographic and SEM-EDX methods [12–15,22]. One of these crystals, a clear light brown octahedron about 0.19 mm in cross section, was lodged in a fine Pyrex capillary. This was attached to a vacuum system, and the crystal was cautiously dehydrated under dynamic vacuum by gradually increasing its temperature (ca. 25 K/h) to 723 K. The crystal was then maintained at this temperature for 2 d at P = 1 × 10−4 Pa. Still under vacuum in its capillary, the crystal was then allowed to cool at room temperature; it remained light brown. To prepare the cyclohexane sorption complex of Mn-Y, the crystal was exposed to 1.1 × 104 Pa of zeolitically dried cyclohexane (Aldrich, 99.97%, residue on evaporation 0.0005%) vapor for 5 d at 294(1) K. This vapor was dried by allowing it to pass through a 30 cm tube filled with activated beads of zeolite 5A. The excess vapor was then evacuated from the capillary for 600 s. Finally, the crystal in its capillary was sealed off under vacuum from the line. It had become dark brown.
4. Description of the structure 4.1. Brief description of zeolite Y The framework structure of zeolite Y (FAU) is characterized by the double 6-ring (D6R, hexagonal prism), the sodalite cavity (a cubooctahedron), and the supercage (Fig. 1) [29]. Each unit cell has 8 supercages, 8 sodalite cavities, 16 D6Rs, 16 12-rings, and 32 single 6rings (S6Rs). The exchangeable cations, which balance the negative charge of the zeolite Y framework, usually occupy some or all of the sites shown with Roman numerals in Fig. 1. The orders (maximum occupancies, ions per unit cell) of the cation sites I, I′, II′, II, and III are 16, 32, 32, 32, and 48, respectively. Further description is available [30,31]. 4.2. Cyclohexane in zeolite Y Per unit cell, 5.7 cyclohexane molecules are found. Each centers a 12-ring, the best plane of each coinciding with the plane of its 12-ring (Fig. 2). Just 5.7 of the 16 12-rings per unit cell host a cyclohexane molecule. This is the amount of cyclohexane remaining in the crystal when its capillary was sealed off under vacuum at room temperature from the vacuum line. The sorbed cyclohexane molecules form weak hydrogen bonds to oxygen atoms of the zeolite framework (Table 4 and Fig. 2). They do not interact at all with the Mn2+ ions. Their symmetry is 32/m (D3d, chair form). Each equatorial hydrogen atom H1 interacts with one O4 and two O1 framework oxygen atoms to give a total of 6 × 3 = 18 interactions per molecule. The H1 hydrogen atoms are directed toward lone-pair orbitals of the framework oxygens at distances of 3.02 and 3.16 Å, respectively. The axial H2 hydrogen atoms do not participate in any interactions beyond their bonds to the C atom.
2.2. X-ray diffraction X-ray diffraction data for the resulting single crystal were collected at 100(1) K at the Pohang Light Source, Pohang, Korea. Preliminary cell constants and an orientation matrix for the crystal were determined from 36 sets of frames collected at scan intervals of 5° with an exposure time of 1 s per frame. The basic data file was prepared using the HKL2000 program [23]. The reflections were successfully indexed by the automated indexing routine of the DENZO program [23]. About 103,120 reflections were harvested for the crystal by collecting 72 sets of frames with a 5° scan and an exposure time of 1 s per frame. These highly redundant data sets were corrected for Lorentz and polarization effects; negligible corrections for crystal decay were also applied. The space group Fd3m, standard for zeolite Y, was determined by the program XPREP [24]. A summary of the experimental and crystallographic data is presented in Table 1.
4.3. Mn2+ ions Mn2+ ions occupy four different crystallographic sites: I, I′, II′, and II. Of the 40.1(4) Mn2+ ions per unit cell, 12.7(1) are found at site I (Mn1). They center D6Rs (Fig. 3(a)). The octahedral Mn1–O3a distance, 2.270(4) Å, is substantially longer than the sum of the conventional ionic radii of Mn2+ and O2−, 0.80 + 1.32 = 2.12 Å [32]. This indicates that Mn2+ is a little too small for the D6R and that each has pulled the six O3a atoms to which each bonds closer. The O3a-Mn1-O3a angles, 88.81(12)° and 91.19(12)° (Table 4), are nearly octahedral. The 6.7(2) Mn2+ ions per unit cell at Mn1′ occupy site I’. Because the neighboring positions I and I′ are too close to be occupied simultaneously because of the strong interacationic repulsion that would result, pairs of ions
3. Structure determination Least squares refinement using SHELXL-97 [25] was done on Fo2 using all 1612 unique reflections. Anisotropic refinement was initiated with the atomic parameters of the framework atoms [(Si,Al), O1, O2, 140
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Table 1 Summary of experimental and crystallographic data. crystal cross-section (mm) Mn2+ exchange T (K), t (day), V(ml) dehydration T (K), P (Pa) crystal color before sorption cyclohexane sorption T (K), t (day) crystal color after sorption data collection T (K) space group, Z X-ray source detector wavelength (Å) unit cell constant, a (Å) 2θ range in data collection (deg) total reflections harvested no. of unique reflections, m no. of reflections with Fo > 4σ(Fo) no. of variables, s data/parameter ratio, m/s weighting parameters, a/b Rinta Rsigmab final error indices R1/wR2 (Fo > 4σ(Fo))c R1/wR2 (all intensities)d goodness-of-fite
0.19 343, 20, 200 723, 1 × 10−4 pale brown 294, 5 dark brown 100(1) Fd3 m, 1 Pohang Light Source (PLS) (Beamline 2D SMC) ADSC Quantum 210 0.66999 24.6061(1) 67.36 103,120 1612 1484 66 24.4 0.083/115.4 0.0100 0.0100 0.052/0.174 0.055/0.183 1.21
a Rint = Σ|(Fo2-Fo2(mean))|/ΣFo2; Rint is calculated from the merging of equivalent data. It is a measure of the internal agreement for all reflections. b Rsigma = Σσ(Fo2)/ΣFo2. c R1 = Σ|Fo-|Fc||/ΣFo and wR2 = [Σw(Fo2-Fc2)2/Σw(Fo2)2]1/2; R1 and wR2 are calculated using only the reflections for which Fo > 4σ(Fo). d R1 and wR2 are calculated using all unique reflections measured. e Goodness-of-fit = [Σw(Fo2-Fc2)2/(m-s)]1/2, where m is the number of unique reflections and s is the number of variables.
their 6-ring planes, 1.07 Å, are large (Table 5), and the O3b–Mn1′–O3b bond angles, 98.6(8)°, are small. These latter three features can be attributed to Mn1′-Mn1′ repulsion through the D6R. Note that the occupancy at
at Mn1′ must share a D6R as shown in Fig. 3(b). Each coordinates to three O3b framework oxygens at 2.204(3) Å, somewhat longer than 2.12 Å, the sum of the conventional radii [32]. The deviations of the Mn1′ ions from
Table 2 Steps of structure determination and refinement. step
occupancya at Mn1
1b 2 3 4 5 6c 7 8d 9 10e 11f 12g 13h 14i 15j 16k
9.4(5) 13.6(2) 13.9(2) 13.2(1) 13.1(1) 12.8(1) 12.8(1) 12.6(1) 12.6(1) 12.6(1) 12.6(1) 12.6(1) 12.5(1) 12.7(1) 12.7(1)
Mn1′
5.9(4) 6.2(3) 5.4(3) 5.1(3) 5.6(3) 5.8(3) 5.9(3) 5.9(3) 5.9(3) 6.0(3) 6.7(2) 6.7(2)
Mn2′
2.1(2) 2.4(2) 2.3(2) 2.2(2) 2.5(2) 2.5(2) 2.9(2) 2.8(2) 2.8(2) 2.9(2) 3.0(2) 3.0(2) 3.0(2)
Mn2
16.7(2) 16.4(2) 17.3(2) 17.3(2) 17.4(2) 17.9(2) 17.5(2) 17.8(2) 17.9(2) 17.9(2) 17.9(2) 17.6(2) 17.7(2) 17.7(2)
O5
C
26.0(13) 17.9(2) 17.5(2) 17.8(2) 17.9(2) 17.9(2) 17.9(2) 17.6(2) 17.7(2) 17.7(2)
34.7(22) 40.0(24) 37.3(52) 34.0(20) 33.8(20) 34.8(19) 34.0(18) 34.0(18)
a
O3
O3a
O3b
R1
wR2
14.9(45) 19.7(5) 20.0
0.40 0.29 0.094 0.087 0.074 0.068 0.0620 0.0624 0.0598 0.0586 0.0585 0.0584 0.0583 0.0542 0.0522 0.0521
0.83 0.76 0.28 0.27 0.233 0.224 0.1966 0.2019 0.1881 0.1856 0.1849 0.1850 0.1848 0.1763 0.1744 0.1739
96 96 96 96 96 96 96 96 96 96 96 96 96 81.5(44) 76.3(5) 76.0
The occupancy is given as the number of atoms or ions per unit cell at each position. Only the zeolite framework positions were included in the initial structure model; all were refined anisotropically. c All Mn2+ ions were refined anisotropically. d The occupancies at Mn2 and O5 were constrained to be equal because the occupancy at O5 cannot exceed the occupancy at Mn2. O5 was refined anisotropically. e C was refined anisotropically. f The positions of the hydrogen atoms were calculated and included. g The occupancies at C, H1, and H2 were constrained to be equal. h The anisotropic U values at C were assigned to H1 and H2. i The framework O3 position was resolved to O3a and O3b. Both were refined isotropically. j O3s were refined with the constraint that the occupancies at O3a and O3b sum to 96. O3a and O3b were refined anisotropically. k The occupancy at O3a was fixed to be six times the occupancy at Mn1. The occupancy at O3b was fixed as described in footnote j. b
141
Microporous and Mesoporous Materials 264 (2018) 139–146 Table 4 Selected interatomic distances (Å) and angles (deg).a (Si,Al)-O1 (Si,Al)-O2 (Si,Al)-O3a (Si,Al)-O3b (Si,Al)-O4 Mean (Si,Al)-O
1.6469(12) 1.6771(10) 1.7125(19) 1.694(10) 1.6312(9) 1.666
O1-(Si,Al)-O2 O1-(Si,Al)-O3a O1-(Si,Al)-O3b O1-(Si,Al)-O4 O2-(Si,Al)-O3a O2-(Si,Al)-O3b O2-(Si,Al)-O4 O3a-(Si,Al)-O4 O3b-(Si,Al)-O4
113.02(9) 104.79(12) 119.5(8) 111.26(12) 106.83(14) 103.3(7) 106.70(12) 114.31(14) 101.8(6)
(Si,Al)-O1-(Si,Al) (Si,Al)-O2-(Si,Al) (Si,Al)-O3a-(Si,Al) (Si,Al)-O3b-(Si,Al) (Si,Al)-O4-(Si,Al)
129.63(16) 135.03(15) 123.29(23) 125.7(14) 157.61(19)
Mn1-O3a Mn1′-O3b Mn2′-O2 Mn2-O2 Mn2-O5
2.270(4) 2.204(3) 2.171(4) 2.1727(24) 2.488(17)
O3a-Mn1-O3a O3b-Mn1′-O3b O2-Mn2′-O2 O2-Mn2-O2 O2-Mn2-O5
88.81(12), 91.19(12) 98.6(8) 115.4(3) 115.24(5) 96.3(3), 115.38(6)
C-C C-H1b C-H2b C-C-C C-C-H1b C-C-H2b H1-C-H2b
1.50(5) 0.970 0.970 113(4) 109 109 108
C⋯O1 C⋯O4 H1⋯O1b H1⋯O4b (H2⋯O1b (H2⋯O4b C-H1⋯O1b C-H1⋯O4b
3.77(3) 3.93(3) 3.16 3.02 3.85) 4.04) 122.2 156.4
a The numbers in parentheses are the esds in the units of the least significant digit given for the corresponding parameter. b The hydrogen atom positions, H1 (equatorial) and H2 (axial), were calculated.
Fig. 1. Stylized drawing of the framework structure of zeolite Y. Near the center of the each line segment is an oxygen atom. The nonequivalent oxygen atoms are indicated by the numbers 1 to 4. There is no evidence in this work of any ordering of the silicon and aluminum atoms among the tetrahedral positions, although it is expected that Loewenstein's rule (ref. [29]) would be obeyed. Extraframework cation positions are labeled with Roman numerals.
Mn1 + (0.5 × the occupancy at Mn1′) = 16.0 (Table 3). This indicates that all D6Rs are occupied, ca. 12.7 as shown in Fig. 3(a) and ca. 3.3 as shown in Fig. 3(b). The remaining 20.7 Mn2+ ions are at sites II′ and II, at Mn2′ and Mn2, with occupancies of 3.0(2) and 17.7(2), respectively (Figs. 4 and 5). Each coordinates at 2.171(4) and 2.1727(24) Å, respectively, to three O2 framework oxygens, close to the sum of the conventional radii, 2.12 Å [32].
e
c
b
a
I I′ II′ II 192(i) 96(h) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 96(g) 96(g) 96(g) 96(g) Si,Al O1 O2 O3a O3b O4 Mn1 Mn1′ Mn2′ Mn2 O5 C H1e H2e
Fractional positional ×105 and thermal Å2 ×104 parameters are given. Numbers in parentheses are the esds in the units of the least significant figure given for the corresponding parameter. The anisotropic temperature factor is exp[−2π2a−2(U11h2 + U22k2 + U33l2 + 2U23kl + 2U13hl + 2U12hk)]. The occupancy is given as the number of atoms or ions per unit cell. The hydrogen atom coordinates were calculated from the C atom positions.
12.66(8) 6.67(17) 3.04(18) 17.70(16) 17.70(16) 34.0(18) 34.0(18) 34.0(18) ΣMn2+ = 40.1(4)
96
76.0(5) 20.0(5)
1. 1. 1. 0.792 0.208 1. 0.791 0.208 0.095 0.553 0.184 0.354 0.354 0.354 −2(1) −73(6) 59(1) −2(7) 118(53) 1(6) 20(2) 328(49) 98(28) 30(3) −673(130) 1298(271) 1298(271) 1298(271) −2(2) 2(8) 30(5) −2(7) 118(53) −1(6) 20(2) 328(49) 98(28) 30(3) −206(65) −1016(208) −1016(208) −1016(208) −15(1) −73(6) 30(5) −29(9) 61(50) −79(8) 20(2) 328(49) 98(28) 30(3) −206(65) −1016(208) −1016(208) −1016(208) 12240(2) 0 −224(7) 6118(9) 7291(51) 8192(8) 0 6590(41) 20597(32) 22837(4) 27635(51) 22138(199) 21641 19675 −5329(2) −10798(7) −224(7) −3204(15) −2311(98) −6614(11) 0 6590(41) 20597(32) 22837(4) 27635(51) 22138(199) 21641 19675
3578(2) 10798(7) 14678(9) 6118(9) 7291(51) 16808(8) 0 6590(41) 20597(32) 22837(4) 30333(94) 45673(182) 41794 47522
132(3) 233(6) 196(6) 127(11) 567(126) 308(12) 123(3) 893(46) 279(33) 193(4) 891(127) 1710(265) 1710(265) 1710(265)
99(3) 233(10) 196(6) 127(9) 171(41) 205(6) 123(3) 893(46) 279(33) 193(4) 891(127) 1710(265) 1710(265) 1710(265)
111(3) 233(10) 182(9) 127(9) 171(41) 205(6) 123(3) 893(46) 279(33) 193(4) 660(153) 1066(214) 1066(214) 1066(214)
192 96 96
varied fixed
U12 U13 U23 y x cation site wyckoff position atom
Table 3 Positional, thermal, and occupancy parameters.a
z
U11b
U22
U33
occupancyc
fraction
D.J. Moon et al.
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The 17.7(2) Mn2+ ions per unit cell at Mn2 are in the supercage. Each coordinates further to a nonframework oxygen atom at O5 (Mn2+O = 2.488(17) Å). These are presumably water molecules, but if some are hydroxide ions, that would provide change balance to the structure and would explain why too many (at ca. 6.5 sigma) Mn2+ ions (40.1(4) per unit cell instead at 37.5) were found. 5. Discussion Despite our attempts to keep the crystal studied anhydrous (section 2.1, paragraph 3), 17.7(2) water molecules were found per unit cell. This experimental error could be attributed to a failure to bake out some lengths of the vacuum system, to the use of a valve that could not be baked out, or to the quick passage of the vapor through the desiccant. It may also have resulted from a blocking of the pores of the zeolite 5A desiccant by cyclohexane, much like that seen in this report, rendering it ineffective. Fortunately, these water molecules were sequestered by Mn2+ ions and held far from the 12-rings, and thus have little effect on the positions of the sorbed cyclohexane molecules. The geometry of the cyclohexane molecule found here differs
Fig. 2. A cyclohexane molecule in a 12-ring. Each equatorial hydrogen atom, H1, hydrogen bond to three oxygen atoms of the zeolite framework.
Fig. 3. Stereoviews of double 6-rings (D6Rs): (a) 12.7(1) of the 16 D6Rs per unit cell are occupied by Mn1 and (b) the remaining 3.3(1) hold Mn1′ ions as shown. The zeolite Y framework is drawn with heavy bonds. The coordination of Mn2+ ions to oxygens of the zeolite framework is indicated by light bonds. Ellipsoids of 25% probability are shown.
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greater than that seen at 5 K, 0.062(4) Å2 [20]. Judging from their C-O distances, 3.77(3) and 3.93(3) Å, respectively, the C-H1⋯O1 and C-H1⋯O4 hydrogen bonds are very weak. Each is almost as long as the suggested upper limit, 4.0 Å, for C-H⋯O hydrogen bonds [36]. At these distances, the interaction may be viewed as nearly entirely electrostatic, between the negatively charged framework oxygen atoms and the positive charge of the H1 atoms. The oxygen atoms are negative not only because of the T-O dipoles, but also because the zeolite framework carries a formal charge of 75- per unit cell. It can be expected that the very small polarity of the C-H1 bond has been enhanced [36] by its interaction with these framework oxygen atoms.
Table 5 Displacements of atoms (Å) from 6-ring planes.
at O3aa at O3ba at O2b
atom
site
displacement
Mn1 Mn1′ Mn2′ Mn2
I I′ II′ II
−1.34 1.07 −0.47 0.48
a Site I is at the center of a D6R. Site I′ is near the plane of one 6-ring of a D6R. Displacements out of the D6R into the sodalite unit are given as positive. b Site II is in the supercage; displacements from its 6-rings are given as positive. A negative deviation indicates that the atom is at site II′ and lies within a sodalite cavity.
Fig. 4. A stereoview of a representative sodalite cavity. The Mn2+ ions at Mn1′, Mn2′, and Mn2 are shown. Each ion at Mn2 bonds to an O5 oxygen atom (a water molecule). See the caption to Fig. 3 for other details.
Fig. 5. A stereoview of a supercage. Cyclohexane molecules are shown at a higher density (four on the surface of supercage) than observed (1.4) to show that no intramolecular contacts would inhibit the sorption of up to 16 (one per 12-ring) per unit cell. See the caption to Fig. 3 for other details.
Table 6 Cyclohexane geometries determined by diffraction methods.a state
phase
beam
T (K)
pure
single crystal
X-ray
115
pure pure in zeolite Y in zeolite Y
gas gas crystalline powder single crystal
electron electron neutron X-ray
ambient ambient 5 100
a b
C-C (Å) 1.510(11) 1.521(12) 1.528(6) 1.536(2) 1.528(5) 1.540(4)b 1.50(5)
C-C-C (deg) 110.4(6) 111.3(4) 112.3(4) 111.4(2) 111.55(15) 109.8(4)b 113(4)
notes low T phase
reference a
C6D12 in H-Y C6H12 in Mn-Y
[33]
[34] [35] [20] this work
Space group C2/c. The cyclohexane molecule is distorted by a small amount by its environment in its crystal structure. The soft constraints used in least-squares refinement did not allow these values to vary.
insignificantly from those in previous determinations in C6H12(s) [33], in the gas phase [34,35], and in C6D12 sorbed onto D-Y (Table 6) [20]. It is less precisely determined within zeolites than in C6H12(s) because of its dilution and higher thermal motion (Figs. 6 and 7). The placement of the cyclohexane molecules in 12-rings is the same as previously reported in D-Y [20]. Comparing U11 values, the thermal parameter of a carbon atom in cyclohexane at 100 K, 0.17(3) Å2, is appropriately
The strength of a C-H⋯O hydrogen bond depends primarily on the acidity of the C-H bond [36]. Apparently the acidity of the aliphatic C-H bond is so low that no examples of it were included in a relatively recent review of C-H⋯O hydrogen bonding [36]. A tabulation of pKa values for hydrocarbons [37] shows that the highest values (lowest acidity) belong to the saturated hydrocarbons, and a consideration of the H/C ratio in the three compounds listed (methane, 48; propane, 51; and 2-methylpropane, 144
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with oxygen atoms of the zeolite framework. Examples involving Mn-Y are toluene [14] and the xylenes [13]. This was also seen for mesitylene in Mn-Y [15] and Ca-X [16], and for the aliphatic cyclopropane molecule in Co-A [38], Mn-A [38], Cd-A [39], and Mn-X [19]. Multiple CH⋯O interactions are seen in all cases although, where possible, the bonding involves a π interaction between the aromatic ring and the transition metal cation. Benzene, however, occupies both kinds of sites upon sorption into the faujasitic zeolites X and Y. When benzene was sorbed by Mn-Y, 18 benzene molecules were found on 3-fold axes and 6 occupied 12-rings [12]. Most of the benzene molecules sorbed into Ca-X [11] and Mn-X [40] also lie on 3-fold axes, although many, like the cyclohexane molecules in this report, are found in 12-rings. In Ca-X, 22 benzene molecules coordinated facially to Ca2+ and 16 fully occupied the 16 12rings per unit cell [11]. In Mn-X, the values 26 and 14, respectively, were observed [40]. In dehydrated Mn37.5-Y [22], the Mn2+ ions at site II nearly lie in the planes of the three framework oxygen atoms, O2, to which they coordinate; they extend only 0.19 Å into the supercage. In this structure, where each is coordinated by a water molecule, it extends 0.48 Å into the supercage (Tables 6 and 7) to a more tetrahedral position [22]. Accordingly the Mn2+-O2 bond has increased by a small amount from 2.134(3) Å in the dehydrated structure [22] to 2.1727(24) Å, and the O2-Mn2-O2 bond angle has decreased from a near trigonal planar value of 119.22(6)° to 115.24(5)°. It is clear that benzene and cyclohexane both occupy 12-rings because they fit well and can make multiple weak hydrogen bonds. The C⋯O1 distances in the present structure and in the benzene complexes of Cd-X [10], Ca-X [11], and Mn-Y [12] are 3.77(3), 3.84(6)/3.86(6), 3.78(6)/3.76(7), and 3.83(2) Å, respectively. The corresponding C⋯O4 distances in those four structures are similar: 3.93(3), 3.87, 4.20(6)/ 4.02(6), and 3.97(3) Å, respectively. The corresponding H⋯O1 distances are 3.16 (H1⋯O1), 2.81, 3.10/3.02, and 3.18 Å, and the corresponding H⋯O4 distances are: 3.02 (H1⋯O4), 2.96/2.99, 3.18/2.93, and 3.04 Å. In the C6D12 sorption complex of zeolite Y studied by powder neutron diffraction methods at 5 K, C⋯O4 = 3.81 Å and D···O4 = 2.74 Å [20]. Mn2+ ions were found at four crystallographic sites in this study, as was found in fully dehydrated Mn37.5-Y [22] and in five of its sorption complexes, those with benzene [12], the three xylenes [13], and toluene [14] (Table 6). The same is true for the mesitylene sorption complex, although there site II could be resolved into two sites, IIA and IIB; mesitylene coordinated only to those at IIA [15,16]. The occupancies at site I are less here than those in the benzene [12], xylene [13], and toluene [14] sorption complexes but higher than in fully dehydrated Mn37.5-Y [22] (Table 6). Complementarily, the occupancies at site I′ are higher than in the benzene, xylene, and toluene sorption complexes and almost same as in fully dehydrated Mn37.5-Y (Table 6).
Fig. 6. A stereoview of a cyclohexane molecule in a 12-ring. The C-H⋯O hydrogen bonds are not shown (all can be seen in Fig. 2). For clarity, the anisotropic thermal parameters assigned to the H1 and H2 positions are not shown. Ellipsoids of 25% probability are used.
Fig. 7. A cyclohexane molecule in a 12-ring with assigned thermal parameters at H1 and H2. Ellipsoids of 25% probability are used.
53) suggests that cyclohexane's pKa is even higher. Accordingly, cyclohexane, if it participates in hydrogen bonding at all, should form the weakest such bonds, and the stability of its complex with zeolite 12-rings (references in Table 5) can be attributed to the large number (18) of such weak bonds that each molecule is able to form. Cyclic organic molecules, whether aliphatic or aromatic, are more frequently found at 3-fold axis positions in the large cavities. There they can associate facially with site II cations and form weak hydrogen bonds Table 7 Comparison of structures of Mn37.5-Y and its sorption complexes.a T (K)
Mn37.5-Yc Mn37.5benzene24-Yc Mn37.5o-xylene18-Yc,d Mn37.5m-xylene18-Yc,d Mn37.5p-xylene18-Yc,d Mn37.5toluene17-Yc,d Mn40.1cyclohexane5.7H2O17.7-Ye a b c d e
294 100 100 100 100 100 100
occupancies at sites I
I′
II′
II
11 13.5 14 14 14 14 12.7
6.5 4 4 4 4 4 6.7
3 2 1.5 1.5 1.5 2.5 3.0
17 18 18 18 18 17 17.7
Mn2–O(2) (Å)
distance (Å) from 111 planeb
ref
2.134(3) 2.205(5) 2.215(5) 2.228(6) 2.210(6) 2.213(6) 2.173(2)
0.19 0.62 0.63 0.68 0.67 0.63 0.48
[22] [12] [13] [13] [13] [14] this work
The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. The Mn2+ ion at site II extends this distance from the 111 plane at O2 into the supercage. Fully dehydrated. In these sorption complexes, the sorbed molecules coordinate facially to Mn2+ ions on 3-fold axes in the supercage. They do not occupy 12-rings. Partially hydrated.
145
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6. Summary
1222–1227. [11] Y.H. Yeom, A.N. Kim, Y. Kim, S.H. Song, K. Seff, J. Phys. Chem. B 102 (1998) 6071–6077. [12] M. Shamsuzzoha, S.M. Seo, Y.H. Kim, W.T. Lim, J. Inclusion Phenom. Macrocycl. Chem. 70 (2011) 59–68. [13] M. Shamsuzzoha, Y.H. Kim, W.T. Lim, J. Phys. Chem. C 115 (2011) 17750–17760. [14] M. Shamsuzzoha, Y.H. Kim, W.T. Lim, J. Phys. Chem. C 115 (2011) 24681–24687. [15] M. Shamsuzzoha, S.M. Seo, Y.H. Kim, W.T. Lim, Microporous Mesoporous Mater. 143 (2011) 326–332. [16] E.Y. Choi, Y. Kim, K. Seff, J. Phys. Chem. B 106 (2002) 5827–5832. [17] S.B. Jang, M.S. Jeong, Y. Kim, K. Seff, J. Phys. Chem. B 101 (1997) 9041–9045. [18] M.N. Bae, Y. Kim, Bull. Kor. Chem. Soc. 19 (1998) 1095–1098. [19] E.Y. Choi, Y. Kim, Y.W. Han, K. Seff, Microporous Mesoporous Mater. 40 (2000) 247–255. [20] G. Vitale, C.F. Mellot, A.K. Cheetham, J. Phys. Chem. B 101 (1997) 9886–9891. [21] W.T. Lim, S.M. Seo, T. Okubo, M. Park, J. Porous Mater. 18 (2010) 305–317. [22] S.M. Seo, G.H. Kim, W.T. Lim, Bull. Kor. Chem. Soc. 31 (2010) 2379–2382. [23] Z. Otwinowski, W. Minor, Methods Enzymol. 276 (1997) 307–326. [24] XPREP, Program for the Automatic Space Group Determination, version 6.12 Bruker AXS Inc, Madison, WI, 2001. [25] G.M. Sheldrick, Acta Crystallogr. A64 (2008) 112–122. [26] W.T. Lim, S.M. Seo, L. Wang, G.Q. Lu, K. Seff, Microporous Mesoporous Mater. 129 (2010) 11–21. [27] D.T. Cromer, Acta Crystallogr. 18 (1965) 17–23. [28] J.A. Ibers, W.C. Hamilton (Eds.), International Tables for X-ray Crystallography, vol. IV, Kynoch Press, Birmingham, 1974, pp. 148–150. [29] W. Loewenstein, Am. Mineral. 39 (1954) 92–96. [30] L. Zhu, K. Seff, D.H. Olson, B.J. Cohen, R.B. Von Dreele, J. Phys. Chem. B 103 (1999) 10365–10372. [31] H. Van Bekkum, E.M. Flanigen, P.A. Jacobs, J.C. Jansen, Introduction to Zeolite Science and Practice, Elsevier, 2001, p. 44. [32] Handbook of Chemistry and Physics, 70th ed., CRC Press, Cleveland, OH, 1989/ 1990 F-187. [33] R. Kahn, R. Fourme, D. André, M. Renaud, Acta Crystallogr. B29 (1973) 131–138. [34] O. Bastiansen, L. Fernholt, H.M. Seip, H. Kambara, K. Kuchitsu, J. Mol. Struct. 18 (2) (1973) 163–168. [35] M. Davis, O. Hassel, Acta Chem. Scand. 17 (4) (1963) 1181–1181. (just one page). [36] G.R. Desiraju, Acc. Chem. Res. 29 (9) (1996) 441–449. [37] Chemistry and Chemical Biology, Harvard University, 2017 Evans pKa Table for the Evans Group http://evans.rc.fas.harvard.edu/pdf/evans_pKa_table.pdf/ , Accessed date: 2 June 2017. [38] W.V. Cruz, P.C.W. Leung, K. Seff, J. Am. Chem. Soc. 100 (1978) 6997–7003. [39] E.Y. Choi, Y. Kim, K. Seff, Microporous Mesoporous Mater. 41 (2000) 61–68. [40] Y. Kim, A.N. Kim, Y.W. Han, K. Seff, Proceedings of the 12th International Zeolite Conference, vol. IV, Materials Research Society, Warrendale, PA, 1999, pp. 2839–2846.
In this cyclohexane sorption complex of partially hydrated, fully Mn2+-exchanged zeolite Y, cyclohexane molecules in their chair form occupy 12-rings. The distances from each of the cyclohexane carbon atoms to their closest framework oxygens are 3.77(3) and 3.93(3) Å, indicative of very weak hydrogen bonding. Each cyclohexane molecule is held in place by 18 such hydrogen bonds. Mn2+ ions occupy four crystallographic sites: I, I′, II′, and II with occupancies of 12.7(1), 6.7(2), 3.0(2), and 17.7(2), respectively. Acknowledgements The authors are grateful to the staff at beamline 2D SMC of the Pohang Light Source, Korea, for assistance during data collection. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B03029558). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.micromeso.2018.01.016. References [1] D.W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York, 1974. [2] C. Baerlocher, W.M. Meier, D.H. Olsen, Atlas of Zeolite Framework Types, fifth ed., Elsevier, Amsterdam, 2001. [3] A. Corma, Chem. Rev. 95 (1995) 559–614. [4] R. Roque-Malherbe, R. Wendelbo, A. Mifsud, A. Corma, J. Phys. Chem. 99 (1995) 14064–14071. [5] R. Wendelbo, R. Roque-Malherbe, Microporous Mater. 10 (1997) 231–246. [6] R. Poupko, E. Furman, K. Muller, Z. Luz, J. Phys. Chem. 95 (1991) 407–413. [7] J.A. Ripmeester, C.I. Ratcliffe, Spectroscopic and Computational Studies of Supramolecular Systems, Kluwer Academic Publishers, Dordrecht, 1992, pp. 1–27. [8] S. Nishikiori, C.I. Ratcliffe, J.A. Ripmeester, J. Phys. Chem. 94 (1990) 8098–8102. [9] A.E. Aliev, K.D.M. Harris, J. Phys. Chem. A 101 (1997) 4541–4547. [10] Y. Kim, Y.H. Yeom, E.Y. Choi, A.N. Kim, Y.W. Han, Bull. Kor. Chem. Soc. 19 (1998)
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Structure of a cyclohexane sorption complex of partially dehydrated, fully Mn2+-exchanged zeolite Y (FAU, Si/Al = 1.56)
T
Dae Jun Moona, Hae-Kwon Jeongb,c, Woo Taik Lima,∗, Karl Seffd,∗∗ a
Department of Applied Chemistry, Andong National University, Andong 36729, South Korea Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, USA c Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843, USA d Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, HI 96822, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Zeolite Y Manganese Cyclohexane Crystal structure Sorption
The structure of a cyclohexane sorption complex of partially dehydrated, fully Mn2+-exchanged zeolite Y has been determined at 100(1) K by single-crystal synchrotron X-ray diffraction techniques. It was refined using all intensities to the final error indices R1 = 0.052 and wR2 = 0.174. Cyclohexane molecules of symmetry 3 2/m (D3d, chair form) occupy 5.7 of the 16 12-rings per unit cell. Each cyclohexane molecule is held in place by 18 weak hydrogen bonds between its six equatorial hydrogen atoms and all 12 oxygen atoms of its 12-ring: C⋯O ca. 3.85 Å. Mn2+ ions are found at four crystallographic sites, I, I′, II′, and II. Each of the 17.7(2) Mn2+ ions per unit cell at site II (opposite 6-rings in the supercage) coordinates tetrahedrally to three framework oxygen atoms and a water molecule. The cyclohexane molecules interact neither with the Mn2+ ions nor with the water molecules.
1. Introduction Zeolites are of great interest due to their widespread industrial use as catalysts, ion exchangers, and selective sorbents [1,2]. These functions depend on the structure of zeolite and its interactions with guest molecules. Knowing how these molecules interact with the zeolite provides a structural basis for understanding the chemistry involved [3–5]. The dynamic properties of cyclohexane guest molecules in various solid host structures such as the thiourea [6], Cd(CN)2 [7], Cd(CH3NH (CH2)3NH2)Ni(CN)4 [8], and zeolite H-ZSM-5 [9] have been investigated by wide-line solid-state 2H NMR spectroscopy, a particularly sensitive probe of molecular motions with characteristic time scales in the intermediate motion regime. Poupko et al. used 2H NMR to study the thiourea-perdeuterocyclohexane inclusion complex from 93 to 333 K [6]. They identified three temperature regions involving the structure of the host thiourea lattice and the behavior of the guest cyclohexane molecules. Below 129 K, the host lattice was monoclinic and the guest molecules undergo thermally activated reorientation about the C3 axis. Between 129 and 156 K, the monoclinic host lattice gradually transformed into a hexagonal form and three nonequivalent kinds of guest molecules were found. Above 156 K, the host lattice was hexagonal and the guest molecules were highly disordered. Nishikiori et al. studied the molecular motion of cyclohexane ∗
enclathrated in Cd(CH3NH(CH2)3NH2)Ni(CN)4 using CP/MAS 13C NMR and 2H NMR [8]. They suggested that the chair form of cyclohexane was undergoing n-fold reorientation about its 3-fold axis. This motion was retained to 397 K accompanied by inversion of the ring above ca. 239 K. Aliev et al. studied the dynamic properties of C6D12 molecules in zeolite H-ZSM-5 using variable temperature wide-line solid state 2H NMR spectroscopy [9]. They reported that the C6D12 molecules underwent three types of rapid motion between 93 and 233 K: (1) reorientation about its C3 axis, (2) restricted wobbling (precession) about its C3 axis, and (3) a four-site jump motion. The sorption of the hydrocarbons benzene [10–12], xylene [13], toluene [14], mesitylene [15,16], ethylene [17], acetylene [18], and cyclopropane [19] in FAU-type zeolites were investigated by singlecrystal X-ray diffraction techniques to learn their sorption sites. From this their interactions with the cations and the zeolite framework could be described. The sorption of these hydrocarbons is governed mainly by their sizes, the identity of the cations, and the Si/Al ratio of the framework. Vitale et al. used powder neutron diffraction and energy minimization calculations to learn the locations of sorbed cyclohexane in the acid form of zeolite Y (FAU, Si/Al = 2.43) [20]. From their neutron diffraction work, they found cyclohexane in its chair conformation in 12-rings. They calculated, however, that other positions of lower
Corresponding author. Corresponding author. E-mail addresses: [email protected] (W.T. Lim), seff@hawaii.edu (K. Seff).
∗∗
https://doi.org/10.1016/j.micromeso.2018.01.016 Received 27 October 2017; Received in revised form 11 January 2018; Accepted 12 January 2018 Available online 17 January 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
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O3, and O4] in dehydrated |Tl75|[Si117Al75O384]-FAU [26]. It converged to the error indexes (agreement parameters), defined in a footnote to Table 1, R1 = 0.40 and wR2 = 0.83. The progress of structure determination as subsequent peaks were found on difference Fourier functions and identified as nonframework atoms, and hydrogen atoms were added at calculated positions, are given in Table 2. A small decrease in the error indexes was observed when the hydrogen atoms were added at calculated positions, and another small decrease was seen when the thermal parameters at C were assigned to H1 and H2 (Table 2). These decreases were too small to allow further definition to be added to the anisotropic thermal parameters assigned to the hydrogen atoms. Fixed weights were used initially. The final weights were assigned using the formula w = 1/[σ2(Fo2) + (aP)2 + bP], where P = [max (Fo2,0) + 2Fc2]/3, with a and b as refined parameters (Table 1). Neutral atomic scattering factors within SHELX-97 were used, and all were modified to account for anomalous dispersion [27,28]. The final structural parameters and selected interatomic distances and angles are presented in Tables 3 and 4.
symmetry had similar sorption energies. Of the organic molecules that can be sorbed into zeolites, polar molecules are most strongly held, unsaturated hydrocarbons are weakly held unless the zeolite contains cations to which they can π-bond, and saturated hydrocarbons are most weakly held. However, saturated hydrocarbons can be effectively sorbed if they fit features of the zeolite structure, so that they can have multiple weak interactions with a relatively small decrease in entropy. Cyclohexane in a round 12-ring is an example of this [20]. This report provides a detailed look at the geometry and thermal motion of that interaction. 2. Experimental section 2.1. Crystal preparation Large colorless octahedral single crystals of |Na75|[Si117Al75O384]FAU (Na75-Y) with diameters up to 0.19 mm were prepared by Lim et al. [21]. Mn2+ exchange was done using aqueous 0.1 M Mn (NO3)2·xH2O (Aldrich, 99.99%, Co 17.3 ppm, Fe 9.23 ppm, K 5.56 ppm, Cr 5.1 ppm, Na 2.87 ppm, Ce 2.49 ppm). Hydrated Na75-Y (10 mg) was mixed with 10 ml of 0.1 M Mn(NO3)2, a 20 fold-excess, in a 15 ml conical tube and the mixture was stirred on a shaking incubator at 343 K for 24 h. This procedure was repeated 20 times with fresh Mn (NO3)2 solution. Finally, the product was washed with distilled water (300 ml), filtered, and oven dried at 323 K for 1 d. The structures of complexes of crystals from this batch have been reported before, and the exchangeable cation and framework composition, |Mn37.5|[Si117Al75O384]-FAU (Mn37.5-Y) has been established by crystallographic and SEM-EDX methods [12–15,22]. One of these crystals, a clear light brown octahedron about 0.19 mm in cross section, was lodged in a fine Pyrex capillary. This was attached to a vacuum system, and the crystal was cautiously dehydrated under dynamic vacuum by gradually increasing its temperature (ca. 25 K/h) to 723 K. The crystal was then maintained at this temperature for 2 d at P = 1 × 10−4 Pa. Still under vacuum in its capillary, the crystal was then allowed to cool at room temperature; it remained light brown. To prepare the cyclohexane sorption complex of Mn-Y, the crystal was exposed to 1.1 × 104 Pa of zeolitically dried cyclohexane (Aldrich, 99.97%, residue on evaporation 0.0005%) vapor for 5 d at 294(1) K. This vapor was dried by allowing it to pass through a 30 cm tube filled with activated beads of zeolite 5A. The excess vapor was then evacuated from the capillary for 600 s. Finally, the crystal in its capillary was sealed off under vacuum from the line. It had become dark brown.
4. Description of the structure 4.1. Brief description of zeolite Y The framework structure of zeolite Y (FAU) is characterized by the double 6-ring (D6R, hexagonal prism), the sodalite cavity (a cubooctahedron), and the supercage (Fig. 1) [29]. Each unit cell has 8 supercages, 8 sodalite cavities, 16 D6Rs, 16 12-rings, and 32 single 6rings (S6Rs). The exchangeable cations, which balance the negative charge of the zeolite Y framework, usually occupy some or all of the sites shown with Roman numerals in Fig. 1. The orders (maximum occupancies, ions per unit cell) of the cation sites I, I′, II′, II, and III are 16, 32, 32, 32, and 48, respectively. Further description is available [30,31]. 4.2. Cyclohexane in zeolite Y Per unit cell, 5.7 cyclohexane molecules are found. Each centers a 12-ring, the best plane of each coinciding with the plane of its 12-ring (Fig. 2). Just 5.7 of the 16 12-rings per unit cell host a cyclohexane molecule. This is the amount of cyclohexane remaining in the crystal when its capillary was sealed off under vacuum at room temperature from the vacuum line. The sorbed cyclohexane molecules form weak hydrogen bonds to oxygen atoms of the zeolite framework (Table 4 and Fig. 2). They do not interact at all with the Mn2+ ions. Their symmetry is 32/m (D3d, chair form). Each equatorial hydrogen atom H1 interacts with one O4 and two O1 framework oxygen atoms to give a total of 6 × 3 = 18 interactions per molecule. The H1 hydrogen atoms are directed toward lone-pair orbitals of the framework oxygens at distances of 3.02 and 3.16 Å, respectively. The axial H2 hydrogen atoms do not participate in any interactions beyond their bonds to the C atom.
2.2. X-ray diffraction X-ray diffraction data for the resulting single crystal were collected at 100(1) K at the Pohang Light Source, Pohang, Korea. Preliminary cell constants and an orientation matrix for the crystal were determined from 36 sets of frames collected at scan intervals of 5° with an exposure time of 1 s per frame. The basic data file was prepared using the HKL2000 program [23]. The reflections were successfully indexed by the automated indexing routine of the DENZO program [23]. About 103,120 reflections were harvested for the crystal by collecting 72 sets of frames with a 5° scan and an exposure time of 1 s per frame. These highly redundant data sets were corrected for Lorentz and polarization effects; negligible corrections for crystal decay were also applied. The space group Fd3m, standard for zeolite Y, was determined by the program XPREP [24]. A summary of the experimental and crystallographic data is presented in Table 1.
4.3. Mn2+ ions Mn2+ ions occupy four different crystallographic sites: I, I′, II′, and II. Of the 40.1(4) Mn2+ ions per unit cell, 12.7(1) are found at site I (Mn1). They center D6Rs (Fig. 3(a)). The octahedral Mn1–O3a distance, 2.270(4) Å, is substantially longer than the sum of the conventional ionic radii of Mn2+ and O2−, 0.80 + 1.32 = 2.12 Å [32]. This indicates that Mn2+ is a little too small for the D6R and that each has pulled the six O3a atoms to which each bonds closer. The O3a-Mn1-O3a angles, 88.81(12)° and 91.19(12)° (Table 4), are nearly octahedral. The 6.7(2) Mn2+ ions per unit cell at Mn1′ occupy site I’. Because the neighboring positions I and I′ are too close to be occupied simultaneously because of the strong interacationic repulsion that would result, pairs of ions
3. Structure determination Least squares refinement using SHELXL-97 [25] was done on Fo2 using all 1612 unique reflections. Anisotropic refinement was initiated with the atomic parameters of the framework atoms [(Si,Al), O1, O2, 140
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Table 1 Summary of experimental and crystallographic data. crystal cross-section (mm) Mn2+ exchange T (K), t (day), V(ml) dehydration T (K), P (Pa) crystal color before sorption cyclohexane sorption T (K), t (day) crystal color after sorption data collection T (K) space group, Z X-ray source detector wavelength (Å) unit cell constant, a (Å) 2θ range in data collection (deg) total reflections harvested no. of unique reflections, m no. of reflections with Fo > 4σ(Fo) no. of variables, s data/parameter ratio, m/s weighting parameters, a/b Rinta Rsigmab final error indices R1/wR2 (Fo > 4σ(Fo))c R1/wR2 (all intensities)d goodness-of-fite
0.19 343, 20, 200 723, 1 × 10−4 pale brown 294, 5 dark brown 100(1) Fd3 m, 1 Pohang Light Source (PLS) (Beamline 2D SMC) ADSC Quantum 210 0.66999 24.6061(1) 67.36 103,120 1612 1484 66 24.4 0.083/115.4 0.0100 0.0100 0.052/0.174 0.055/0.183 1.21
a Rint = Σ|(Fo2-Fo2(mean))|/ΣFo2; Rint is calculated from the merging of equivalent data. It is a measure of the internal agreement for all reflections. b Rsigma = Σσ(Fo2)/ΣFo2. c R1 = Σ|Fo-|Fc||/ΣFo and wR2 = [Σw(Fo2-Fc2)2/Σw(Fo2)2]1/2; R1 and wR2 are calculated using only the reflections for which Fo > 4σ(Fo). d R1 and wR2 are calculated using all unique reflections measured. e Goodness-of-fit = [Σw(Fo2-Fc2)2/(m-s)]1/2, where m is the number of unique reflections and s is the number of variables.
their 6-ring planes, 1.07 Å, are large (Table 5), and the O3b–Mn1′–O3b bond angles, 98.6(8)°, are small. These latter three features can be attributed to Mn1′-Mn1′ repulsion through the D6R. Note that the occupancy at
at Mn1′ must share a D6R as shown in Fig. 3(b). Each coordinates to three O3b framework oxygens at 2.204(3) Å, somewhat longer than 2.12 Å, the sum of the conventional radii [32]. The deviations of the Mn1′ ions from
Table 2 Steps of structure determination and refinement. step
occupancya at Mn1
1b 2 3 4 5 6c 7 8d 9 10e 11f 12g 13h 14i 15j 16k
9.4(5) 13.6(2) 13.9(2) 13.2(1) 13.1(1) 12.8(1) 12.8(1) 12.6(1) 12.6(1) 12.6(1) 12.6(1) 12.6(1) 12.5(1) 12.7(1) 12.7(1)
Mn1′
5.9(4) 6.2(3) 5.4(3) 5.1(3) 5.6(3) 5.8(3) 5.9(3) 5.9(3) 5.9(3) 6.0(3) 6.7(2) 6.7(2)
Mn2′
2.1(2) 2.4(2) 2.3(2) 2.2(2) 2.5(2) 2.5(2) 2.9(2) 2.8(2) 2.8(2) 2.9(2) 3.0(2) 3.0(2) 3.0(2)
Mn2
16.7(2) 16.4(2) 17.3(2) 17.3(2) 17.4(2) 17.9(2) 17.5(2) 17.8(2) 17.9(2) 17.9(2) 17.9(2) 17.6(2) 17.7(2) 17.7(2)
O5
C
26.0(13) 17.9(2) 17.5(2) 17.8(2) 17.9(2) 17.9(2) 17.9(2) 17.6(2) 17.7(2) 17.7(2)
34.7(22) 40.0(24) 37.3(52) 34.0(20) 33.8(20) 34.8(19) 34.0(18) 34.0(18)
a
O3
O3a
O3b
R1
wR2
14.9(45) 19.7(5) 20.0
0.40 0.29 0.094 0.087 0.074 0.068 0.0620 0.0624 0.0598 0.0586 0.0585 0.0584 0.0583 0.0542 0.0522 0.0521
0.83 0.76 0.28 0.27 0.233 0.224 0.1966 0.2019 0.1881 0.1856 0.1849 0.1850 0.1848 0.1763 0.1744 0.1739
96 96 96 96 96 96 96 96 96 96 96 96 96 81.5(44) 76.3(5) 76.0
The occupancy is given as the number of atoms or ions per unit cell at each position. Only the zeolite framework positions were included in the initial structure model; all were refined anisotropically. c All Mn2+ ions were refined anisotropically. d The occupancies at Mn2 and O5 were constrained to be equal because the occupancy at O5 cannot exceed the occupancy at Mn2. O5 was refined anisotropically. e C was refined anisotropically. f The positions of the hydrogen atoms were calculated and included. g The occupancies at C, H1, and H2 were constrained to be equal. h The anisotropic U values at C were assigned to H1 and H2. i The framework O3 position was resolved to O3a and O3b. Both were refined isotropically. j O3s were refined with the constraint that the occupancies at O3a and O3b sum to 96. O3a and O3b were refined anisotropically. k The occupancy at O3a was fixed to be six times the occupancy at Mn1. The occupancy at O3b was fixed as described in footnote j. b
141
Microporous and Mesoporous Materials 264 (2018) 139–146 Table 4 Selected interatomic distances (Å) and angles (deg).a (Si,Al)-O1 (Si,Al)-O2 (Si,Al)-O3a (Si,Al)-O3b (Si,Al)-O4 Mean (Si,Al)-O
1.6469(12) 1.6771(10) 1.7125(19) 1.694(10) 1.6312(9) 1.666
O1-(Si,Al)-O2 O1-(Si,Al)-O3a O1-(Si,Al)-O3b O1-(Si,Al)-O4 O2-(Si,Al)-O3a O2-(Si,Al)-O3b O2-(Si,Al)-O4 O3a-(Si,Al)-O4 O3b-(Si,Al)-O4
113.02(9) 104.79(12) 119.5(8) 111.26(12) 106.83(14) 103.3(7) 106.70(12) 114.31(14) 101.8(6)
(Si,Al)-O1-(Si,Al) (Si,Al)-O2-(Si,Al) (Si,Al)-O3a-(Si,Al) (Si,Al)-O3b-(Si,Al) (Si,Al)-O4-(Si,Al)
129.63(16) 135.03(15) 123.29(23) 125.7(14) 157.61(19)
Mn1-O3a Mn1′-O3b Mn2′-O2 Mn2-O2 Mn2-O5
2.270(4) 2.204(3) 2.171(4) 2.1727(24) 2.488(17)
O3a-Mn1-O3a O3b-Mn1′-O3b O2-Mn2′-O2 O2-Mn2-O2 O2-Mn2-O5
88.81(12), 91.19(12) 98.6(8) 115.4(3) 115.24(5) 96.3(3), 115.38(6)
C-C C-H1b C-H2b C-C-C C-C-H1b C-C-H2b H1-C-H2b
1.50(5) 0.970 0.970 113(4) 109 109 108
C⋯O1 C⋯O4 H1⋯O1b H1⋯O4b (H2⋯O1b (H2⋯O4b C-H1⋯O1b C-H1⋯O4b
3.77(3) 3.93(3) 3.16 3.02 3.85) 4.04) 122.2 156.4
a The numbers in parentheses are the esds in the units of the least significant digit given for the corresponding parameter. b The hydrogen atom positions, H1 (equatorial) and H2 (axial), were calculated.
Fig. 1. Stylized drawing of the framework structure of zeolite Y. Near the center of the each line segment is an oxygen atom. The nonequivalent oxygen atoms are indicated by the numbers 1 to 4. There is no evidence in this work of any ordering of the silicon and aluminum atoms among the tetrahedral positions, although it is expected that Loewenstein's rule (ref. [29]) would be obeyed. Extraframework cation positions are labeled with Roman numerals.
Mn1 + (0.5 × the occupancy at Mn1′) = 16.0 (Table 3). This indicates that all D6Rs are occupied, ca. 12.7 as shown in Fig. 3(a) and ca. 3.3 as shown in Fig. 3(b). The remaining 20.7 Mn2+ ions are at sites II′ and II, at Mn2′ and Mn2, with occupancies of 3.0(2) and 17.7(2), respectively (Figs. 4 and 5). Each coordinates at 2.171(4) and 2.1727(24) Å, respectively, to three O2 framework oxygens, close to the sum of the conventional radii, 2.12 Å [32].
e
c
b
a
I I′ II′ II 192(i) 96(h) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 96(g) 96(g) 96(g) 96(g) Si,Al O1 O2 O3a O3b O4 Mn1 Mn1′ Mn2′ Mn2 O5 C H1e H2e
Fractional positional ×105 and thermal Å2 ×104 parameters are given. Numbers in parentheses are the esds in the units of the least significant figure given for the corresponding parameter. The anisotropic temperature factor is exp[−2π2a−2(U11h2 + U22k2 + U33l2 + 2U23kl + 2U13hl + 2U12hk)]. The occupancy is given as the number of atoms or ions per unit cell. The hydrogen atom coordinates were calculated from the C atom positions.
12.66(8) 6.67(17) 3.04(18) 17.70(16) 17.70(16) 34.0(18) 34.0(18) 34.0(18) ΣMn2+ = 40.1(4)
96
76.0(5) 20.0(5)
1. 1. 1. 0.792 0.208 1. 0.791 0.208 0.095 0.553 0.184 0.354 0.354 0.354 −2(1) −73(6) 59(1) −2(7) 118(53) 1(6) 20(2) 328(49) 98(28) 30(3) −673(130) 1298(271) 1298(271) 1298(271) −2(2) 2(8) 30(5) −2(7) 118(53) −1(6) 20(2) 328(49) 98(28) 30(3) −206(65) −1016(208) −1016(208) −1016(208) −15(1) −73(6) 30(5) −29(9) 61(50) −79(8) 20(2) 328(49) 98(28) 30(3) −206(65) −1016(208) −1016(208) −1016(208) 12240(2) 0 −224(7) 6118(9) 7291(51) 8192(8) 0 6590(41) 20597(32) 22837(4) 27635(51) 22138(199) 21641 19675 −5329(2) −10798(7) −224(7) −3204(15) −2311(98) −6614(11) 0 6590(41) 20597(32) 22837(4) 27635(51) 22138(199) 21641 19675
3578(2) 10798(7) 14678(9) 6118(9) 7291(51) 16808(8) 0 6590(41) 20597(32) 22837(4) 30333(94) 45673(182) 41794 47522
132(3) 233(6) 196(6) 127(11) 567(126) 308(12) 123(3) 893(46) 279(33) 193(4) 891(127) 1710(265) 1710(265) 1710(265)
99(3) 233(10) 196(6) 127(9) 171(41) 205(6) 123(3) 893(46) 279(33) 193(4) 891(127) 1710(265) 1710(265) 1710(265)
111(3) 233(10) 182(9) 127(9) 171(41) 205(6) 123(3) 893(46) 279(33) 193(4) 660(153) 1066(214) 1066(214) 1066(214)
192 96 96
varied fixed
U12 U13 U23 y x cation site wyckoff position atom
Table 3 Positional, thermal, and occupancy parameters.a
z
U11b
U22
U33
occupancyc
fraction
D.J. Moon et al.
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The 17.7(2) Mn2+ ions per unit cell at Mn2 are in the supercage. Each coordinates further to a nonframework oxygen atom at O5 (Mn2+O = 2.488(17) Å). These are presumably water molecules, but if some are hydroxide ions, that would provide change balance to the structure and would explain why too many (at ca. 6.5 sigma) Mn2+ ions (40.1(4) per unit cell instead at 37.5) were found. 5. Discussion Despite our attempts to keep the crystal studied anhydrous (section 2.1, paragraph 3), 17.7(2) water molecules were found per unit cell. This experimental error could be attributed to a failure to bake out some lengths of the vacuum system, to the use of a valve that could not be baked out, or to the quick passage of the vapor through the desiccant. It may also have resulted from a blocking of the pores of the zeolite 5A desiccant by cyclohexane, much like that seen in this report, rendering it ineffective. Fortunately, these water molecules were sequestered by Mn2+ ions and held far from the 12-rings, and thus have little effect on the positions of the sorbed cyclohexane molecules. The geometry of the cyclohexane molecule found here differs
Fig. 2. A cyclohexane molecule in a 12-ring. Each equatorial hydrogen atom, H1, hydrogen bond to three oxygen atoms of the zeolite framework.
Fig. 3. Stereoviews of double 6-rings (D6Rs): (a) 12.7(1) of the 16 D6Rs per unit cell are occupied by Mn1 and (b) the remaining 3.3(1) hold Mn1′ ions as shown. The zeolite Y framework is drawn with heavy bonds. The coordination of Mn2+ ions to oxygens of the zeolite framework is indicated by light bonds. Ellipsoids of 25% probability are shown.
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greater than that seen at 5 K, 0.062(4) Å2 [20]. Judging from their C-O distances, 3.77(3) and 3.93(3) Å, respectively, the C-H1⋯O1 and C-H1⋯O4 hydrogen bonds are very weak. Each is almost as long as the suggested upper limit, 4.0 Å, for C-H⋯O hydrogen bonds [36]. At these distances, the interaction may be viewed as nearly entirely electrostatic, between the negatively charged framework oxygen atoms and the positive charge of the H1 atoms. The oxygen atoms are negative not only because of the T-O dipoles, but also because the zeolite framework carries a formal charge of 75- per unit cell. It can be expected that the very small polarity of the C-H1 bond has been enhanced [36] by its interaction with these framework oxygen atoms.
Table 5 Displacements of atoms (Å) from 6-ring planes.
at O3aa at O3ba at O2b
atom
site
displacement
Mn1 Mn1′ Mn2′ Mn2
I I′ II′ II
−1.34 1.07 −0.47 0.48
a Site I is at the center of a D6R. Site I′ is near the plane of one 6-ring of a D6R. Displacements out of the D6R into the sodalite unit are given as positive. b Site II is in the supercage; displacements from its 6-rings are given as positive. A negative deviation indicates that the atom is at site II′ and lies within a sodalite cavity.
Fig. 4. A stereoview of a representative sodalite cavity. The Mn2+ ions at Mn1′, Mn2′, and Mn2 are shown. Each ion at Mn2 bonds to an O5 oxygen atom (a water molecule). See the caption to Fig. 3 for other details.
Fig. 5. A stereoview of a supercage. Cyclohexane molecules are shown at a higher density (four on the surface of supercage) than observed (1.4) to show that no intramolecular contacts would inhibit the sorption of up to 16 (one per 12-ring) per unit cell. See the caption to Fig. 3 for other details.
Table 6 Cyclohexane geometries determined by diffraction methods.a state
phase
beam
T (K)
pure
single crystal
X-ray
115
pure pure in zeolite Y in zeolite Y
gas gas crystalline powder single crystal
electron electron neutron X-ray
ambient ambient 5 100
a b
C-C (Å) 1.510(11) 1.521(12) 1.528(6) 1.536(2) 1.528(5) 1.540(4)b 1.50(5)
C-C-C (deg) 110.4(6) 111.3(4) 112.3(4) 111.4(2) 111.55(15) 109.8(4)b 113(4)
notes low T phase
reference a
C6D12 in H-Y C6H12 in Mn-Y
[33]
[34] [35] [20] this work
Space group C2/c. The cyclohexane molecule is distorted by a small amount by its environment in its crystal structure. The soft constraints used in least-squares refinement did not allow these values to vary.
insignificantly from those in previous determinations in C6H12(s) [33], in the gas phase [34,35], and in C6D12 sorbed onto D-Y (Table 6) [20]. It is less precisely determined within zeolites than in C6H12(s) because of its dilution and higher thermal motion (Figs. 6 and 7). The placement of the cyclohexane molecules in 12-rings is the same as previously reported in D-Y [20]. Comparing U11 values, the thermal parameter of a carbon atom in cyclohexane at 100 K, 0.17(3) Å2, is appropriately
The strength of a C-H⋯O hydrogen bond depends primarily on the acidity of the C-H bond [36]. Apparently the acidity of the aliphatic C-H bond is so low that no examples of it were included in a relatively recent review of C-H⋯O hydrogen bonding [36]. A tabulation of pKa values for hydrocarbons [37] shows that the highest values (lowest acidity) belong to the saturated hydrocarbons, and a consideration of the H/C ratio in the three compounds listed (methane, 48; propane, 51; and 2-methylpropane, 144
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with oxygen atoms of the zeolite framework. Examples involving Mn-Y are toluene [14] and the xylenes [13]. This was also seen for mesitylene in Mn-Y [15] and Ca-X [16], and for the aliphatic cyclopropane molecule in Co-A [38], Mn-A [38], Cd-A [39], and Mn-X [19]. Multiple CH⋯O interactions are seen in all cases although, where possible, the bonding involves a π interaction between the aromatic ring and the transition metal cation. Benzene, however, occupies both kinds of sites upon sorption into the faujasitic zeolites X and Y. When benzene was sorbed by Mn-Y, 18 benzene molecules were found on 3-fold axes and 6 occupied 12-rings [12]. Most of the benzene molecules sorbed into Ca-X [11] and Mn-X [40] also lie on 3-fold axes, although many, like the cyclohexane molecules in this report, are found in 12-rings. In Ca-X, 22 benzene molecules coordinated facially to Ca2+ and 16 fully occupied the 16 12rings per unit cell [11]. In Mn-X, the values 26 and 14, respectively, were observed [40]. In dehydrated Mn37.5-Y [22], the Mn2+ ions at site II nearly lie in the planes of the three framework oxygen atoms, O2, to which they coordinate; they extend only 0.19 Å into the supercage. In this structure, where each is coordinated by a water molecule, it extends 0.48 Å into the supercage (Tables 6 and 7) to a more tetrahedral position [22]. Accordingly the Mn2+-O2 bond has increased by a small amount from 2.134(3) Å in the dehydrated structure [22] to 2.1727(24) Å, and the O2-Mn2-O2 bond angle has decreased from a near trigonal planar value of 119.22(6)° to 115.24(5)°. It is clear that benzene and cyclohexane both occupy 12-rings because they fit well and can make multiple weak hydrogen bonds. The C⋯O1 distances in the present structure and in the benzene complexes of Cd-X [10], Ca-X [11], and Mn-Y [12] are 3.77(3), 3.84(6)/3.86(6), 3.78(6)/3.76(7), and 3.83(2) Å, respectively. The corresponding C⋯O4 distances in those four structures are similar: 3.93(3), 3.87, 4.20(6)/ 4.02(6), and 3.97(3) Å, respectively. The corresponding H⋯O1 distances are 3.16 (H1⋯O1), 2.81, 3.10/3.02, and 3.18 Å, and the corresponding H⋯O4 distances are: 3.02 (H1⋯O4), 2.96/2.99, 3.18/2.93, and 3.04 Å. In the C6D12 sorption complex of zeolite Y studied by powder neutron diffraction methods at 5 K, C⋯O4 = 3.81 Å and D···O4 = 2.74 Å [20]. Mn2+ ions were found at four crystallographic sites in this study, as was found in fully dehydrated Mn37.5-Y [22] and in five of its sorption complexes, those with benzene [12], the three xylenes [13], and toluene [14] (Table 6). The same is true for the mesitylene sorption complex, although there site II could be resolved into two sites, IIA and IIB; mesitylene coordinated only to those at IIA [15,16]. The occupancies at site I are less here than those in the benzene [12], xylene [13], and toluene [14] sorption complexes but higher than in fully dehydrated Mn37.5-Y [22] (Table 6). Complementarily, the occupancies at site I′ are higher than in the benzene, xylene, and toluene sorption complexes and almost same as in fully dehydrated Mn37.5-Y (Table 6).
Fig. 6. A stereoview of a cyclohexane molecule in a 12-ring. The C-H⋯O hydrogen bonds are not shown (all can be seen in Fig. 2). For clarity, the anisotropic thermal parameters assigned to the H1 and H2 positions are not shown. Ellipsoids of 25% probability are used.
Fig. 7. A cyclohexane molecule in a 12-ring with assigned thermal parameters at H1 and H2. Ellipsoids of 25% probability are used.
53) suggests that cyclohexane's pKa is even higher. Accordingly, cyclohexane, if it participates in hydrogen bonding at all, should form the weakest such bonds, and the stability of its complex with zeolite 12-rings (references in Table 5) can be attributed to the large number (18) of such weak bonds that each molecule is able to form. Cyclic organic molecules, whether aliphatic or aromatic, are more frequently found at 3-fold axis positions in the large cavities. There they can associate facially with site II cations and form weak hydrogen bonds Table 7 Comparison of structures of Mn37.5-Y and its sorption complexes.a T (K)
Mn37.5-Yc Mn37.5benzene24-Yc Mn37.5o-xylene18-Yc,d Mn37.5m-xylene18-Yc,d Mn37.5p-xylene18-Yc,d Mn37.5toluene17-Yc,d Mn40.1cyclohexane5.7H2O17.7-Ye a b c d e
294 100 100 100 100 100 100
occupancies at sites I
I′
II′
II
11 13.5 14 14 14 14 12.7
6.5 4 4 4 4 4 6.7
3 2 1.5 1.5 1.5 2.5 3.0
17 18 18 18 18 17 17.7
Mn2–O(2) (Å)
distance (Å) from 111 planeb
ref
2.134(3) 2.205(5) 2.215(5) 2.228(6) 2.210(6) 2.213(6) 2.173(2)
0.19 0.62 0.63 0.68 0.67 0.63 0.48
[22] [12] [13] [13] [13] [14] this work
The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. The Mn2+ ion at site II extends this distance from the 111 plane at O2 into the supercage. Fully dehydrated. In these sorption complexes, the sorbed molecules coordinate facially to Mn2+ ions on 3-fold axes in the supercage. They do not occupy 12-rings. Partially hydrated.
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6. Summary
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In this cyclohexane sorption complex of partially hydrated, fully Mn2+-exchanged zeolite Y, cyclohexane molecules in their chair form occupy 12-rings. The distances from each of the cyclohexane carbon atoms to their closest framework oxygens are 3.77(3) and 3.93(3) Å, indicative of very weak hydrogen bonding. Each cyclohexane molecule is held in place by 18 such hydrogen bonds. Mn2+ ions occupy four crystallographic sites: I, I′, II′, and II with occupancies of 12.7(1), 6.7(2), 3.0(2), and 17.7(2), respectively. Acknowledgements The authors are grateful to the staff at beamline 2D SMC of the Pohang Light Source, Korea, for assistance during data collection. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B03029558). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.micromeso.2018.01.016. References [1] D.W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York, 1974. [2] C. Baerlocher, W.M. Meier, D.H. Olsen, Atlas of Zeolite Framework Types, fifth ed., Elsevier, Amsterdam, 2001. [3] A. Corma, Chem. Rev. 95 (1995) 559–614. [4] R. Roque-Malherbe, R. Wendelbo, A. Mifsud, A. Corma, J. Phys. Chem. 99 (1995) 14064–14071. [5] R. Wendelbo, R. Roque-Malherbe, Microporous Mater. 10 (1997) 231–246. [6] R. Poupko, E. Furman, K. Muller, Z. Luz, J. Phys. Chem. 95 (1991) 407–413. [7] J.A. Ripmeester, C.I. Ratcliffe, Spectroscopic and Computational Studies of Supramolecular Systems, Kluwer Academic Publishers, Dordrecht, 1992, pp. 1–27. [8] S. Nishikiori, C.I. Ratcliffe, J.A. Ripmeester, J. Phys. Chem. 94 (1990) 8098–8102. [9] A.E. Aliev, K.D.M. Harris, J. Phys. Chem. A 101 (1997) 4541–4547. [10] Y. Kim, Y.H. Yeom, E.Y. Choi, A.N. Kim, Y.W. Han, Bull. Kor. Chem. Soc. 19 (1998)
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