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Structure of a methylamine sorption complex of fully dehydrated Cd2+-exchanged zeolite X, ∣Cd46(CH3NH2)16∣[Si100Al92O384]-FAU
Microporous and Mesoporous Materials 90 (2006) 16–22 www.elsevier.com/locate/micromeso
Structure of a methylamine sorption complex of fully dehydrated Cd2+-exchanged zeolite X, jCd46(CH3NH2)16j[Si100Al92O384]-FAU Gyoung Hwa Jeong a, Yang Kim a
a,*
, Karl Seff
b,*
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea b Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, HI 96822, USA Received 3 December 2004; accepted 24 August 2005
Dedicated to the late Denise Barthomeuf, George Kokotailo and Sergey P. Zhdanov in appreciation of their outstanding contributions to zeolite science
Abstract The structure of a methylamine sorption complex of fully dehydrated, fully Cd2+-exchanged zeolite X, jCd46(CH3NH2)16j˚ ), has been determined by single-crystal X-ray diffraction techniques in the cubic space group [Si100Al92O384]-FAU (a = 24.863(4) A Fd 3 at 21(1) C. An aqueous exchange solution 0.05 M in Cd2+ was allowed to flow past the crystal for 5 days. The crystal was then dehydrated at 480 C and 2 · 106 Torr for 2 days (colorless), and exposed to 160 Torr of methylamine gas at 21(1) C for 2 h (yellow). Diffraction data were then gathered in this atmosphere and were refined using all data to the final error indices (based upon the 524 reflections for which Fo > 4r(Fo)) of R1 = 0.069 and wR2 = 0.200. In this structure, Cd2+ ions occupy three crystallographic sites. ˚ ). The remaining The octahedral sites I at the centers of the hexagonal prisms are filled with 16 Cd2+ ions per unit cell (Cd–O = 2.369(8) A 30 Cd2+ ions are located at two non-equivalent sites II with occupancies of 14 and 16. The 16 methylamine molecules per unit cell lie in ˚ . The imprecisely determined N–C bond the supercage where each interacts with one of the latter 16 site-II Cd2+ ions: N–Cd = 2.11(8) A ˚ , agrees with that in gaseous methylamine, 1.474 A ˚ . The positions of the hydrogen atoms were calculated. It appears length, 1.49(22) A that one of the amino hydrogen atoms hydrogen bonds to a 6-ring oxygen, and that the other forms a bifurcated hydrogen bond to this and another 6-ring oxygen. The methyl group is not involved in hydrogen bonding. 2006 Elsevier Inc. All rights reserved. Keywords: Structure; Methylamine; Zeolite X; Sorption; Cadmium
1. Introduction A zeolite framework can be considered a relatively rigid polydentate ligand [1]. The exchangeable cations in fully dehydrated zeolites must select their coordination environments from the limited number of sites available [1]. These coordination environments are often far less suited to the cation than those commonly found elsewhere in chemistry, e.g. in solution. Transition-metal ions that have been ion exchanged into zeolites are generally, after dehydration, *
Corresponding authors. Tel.: +1 808 956 7480; fax: +1 808 956 5908. E-mail addresses: [email protected] (Y. Kim), seff@hawaii.edu (K. Seff). 1387-1811/$ - see front matter 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.08.041
unusually coordinated or coordinatively unsaturated [1]. To relieve their coordinative unsaturation in zeolites A and X, these cations complex readily with a variety of guest molecules such as C2H2 [2], C3H6 [3,4], NO2 [5], H2S [6], CS2 [7], and NH3 [8,9]. The interaction between sorbed methylamine and jNa31j [Si161Al31O384]-FAU was investigated by Long et al. using powder X-ray diffraction (XRD), 29Si and 13C MAS NMR, and FTIR [10]. As methylamine was loaded into the zeolite, one peak in the 29Si MAS NMR spectrum split into four peaks and the high resolution IR spectrum became simpler. They concluded that the sorbed methylamine interacted strongly with the zeolite, and therefore that the zeolite has a high affinity for methylamine.
G.H. Jeong et al. / Microporous and Mesoporous Materials 90 (2006) 16–22
Using IR, Su et al. [11,12] studied the interactions between methylamine and a series of alkaline-earth-cation exchanged faujasite zeolites. They reported the effects that the size of the alkaline-earth cation, and the Si/Al ratio of the zeolite framework, had on their results. They found that the size of the cations (their Lewis acidity) was the dominating factor in the interaction between methylamine and the zeolites. They concluded that methylamine interacts with the alkali cations via the lone electron pair on its nitrogen atom, and that it hydrogen bonds to the anionic oxygen atoms of the zeolite framework via the hydrogen atoms of both the –NH2 and the –CH3 groups. They detected a deformation (otherwise undefined) of the zeolite framework upon the sorption of methylamine. Their work showed clearly that methylamine can be an efficient probe molecule for the characterization of the acid–base properities of zeolites. Morishige et al. reported that methylamine is selectively adsorbed on the site-II cations of zeolite Y (FAU) in such a configuration that the dipole axis of the molecule coincides with the line connecting the site-II cation and the center of the supercage, optimizing the electrostatic-field–dipole interaction [13]. The structure of Ca46–X Æ 16CH3NH2 was recently reported [14]. There each methylamine molecule coordinated to one of the site-II Ca2+ ions: N–Ca2+ = ˚ . One of the amino hydrogen atoms hydrogen2.30(9) A bonded to a 6-ring oxygen, and the other formed a bifurcated hydrogen bond to two other 6-ring oxygens. As expected, the methyl group did not participate hydrogen bonding. The present work was undertaken as part of our structural study of the sorption of methylamine onto zeolites at ambient temperature. We wished to determine the positions of the methylamine molecules sorbed by Cd46–X to observe the coordination and hydrogen bonding of methylamine at the sorption site, and also to observe the shifts in the cation and framework positions upon sorption. 2. Experimental section Large single crystals of zeolite Na–X, stoichiometry Na92Si100Al92O384, were prepared in St. Petersburg, Russia [15]. One of these, a colorless octahedron about 0.2 mm in cross-section, was lodged in a fine Pyrex capillary. An ion-exchange solution of Cd(NO3)2 (Aldrich, 99.999%) and Cd(O2CCH3)2 (Aldrich, 99.99%) in the mole ratio of 1:1 with a total Cd2+ concentration of 0.05 M was allowed to flow past the crystal for 5 days. The capillary containing the crystal was attached to a vacuum system, and the crystal was cautiously dehydrated by gradually increasing its temperature (ca. 29 C/h) under dynamic vacuum to 480 C. Finally, the system was maintained at 480 C and 2 · 106 Torr for 2 days. After cooling to room temperature (without the possibility of water reabsorption because adjacent portions of the vacuum system, including a 15-cm in-series tube of zeolite beads, had been baked out
17
in situ), the crystal remained colorless. To prepare the methylamine complex, the crystal was treated with 160 Torr of methylamine gas (Aldrich, 99.995%, zeolitically dried in situ) for 2 h at 21(1) C. The resulting yellow crystal, still in its methylamine atmosphere, was sealed in its capillary by torch. The cubic space group Fd 3 was used throughout this work. This choice is supported by (a) the low Si/Al ratio which in turn requires, at least in the short range, alternation of Si and Al [16], and (b) the observation that this crystal, like most crystals from the same batch [17], does not have intensity symmetry across (1 1 0) and therefore lacks that mirror plane. Diffraction data were collected with an automated Enraf–Nonius four-circle computer-controlled CAD-4 diffractometer equipped with a pulse-height analyzer and a graphite monochromator, using Mo radiation ˚ ). The cubic unit cell (Ka1, k = 0.70930; Ka2, k = 0.71359 A constant at 21(1) C, determined by least-squares refinement of 25 intense reflections for which 14 < 2h < 22, is ˚ . All unique reflections in the positive a = 24.863(4) A octant of an F-centered unit cell for which 2h < 50, l P h, and l P k were recorded. An absorption correction (l = 1.97 mm1) was made empirically using a w scan [18]. This correction had little effect on the final error (R) indices. Other details are the same as those previously reported [19]. 2.1. Structure determination Full-matrix least-squares refinement [20] was done on F2 using all 1139 unique data. It was initiated with the atomic parameters of the framework atoms (Si, Al, O(1), O(2), O(3), and O(4)) in dehydrated Ca46–X Æ 8C9H12 [21]. These positions when refined anisotropically yielded R1 = 0.43 and wR2 = 0.74; R1 and R2 are defined in footnotes of Table 1. R1 (based on F) and wR2 (based on F2) are calculated using only the 524 reflections for which Fo > 4r(Fo). A Fourier difference electron-density function yielded two strong peaks at (0.0, 0.0, 0.0) and (0.230, 0.230, 0.230), positions near framework oxygens normally occupied by cations. Anisotropic refinement of the framework atoms and Cd2+ ions at these two positions converged to R1 = 0.14 and wR2 = 0.33 with occupancies at these two positions of 16.4(4) at Cd(1) and 29.5(7) at Cd(3), respectively. The thermal ellipsoid at Cd(3), by being very elongated, indicated the presence of two non-equivalent Cd2+ ions at this position. Accordingly, this position was split into two that refined isotropically to convergence as Cd(2) at (0.220, 0.220, 0.220) and Cd(3) at (0.232, 0.232, 0.232) with occupancies of 13.4(3) and 16.3(5), respectively. The occupancy at Cd(1) was then fixed at 16, the maximum value at this position, and refinement converged with R1 = 0.088 and wR2 = 0.269. An ensuing Fourier function revealed two more peaks at the general positions (0.305, 0.284, 0.268) and (0.320, 0.295, 0.317). These two positions ˚ , respectively, from the 3-fold axis. The are 0.48 and 0.65 A first, the one closer to Cd(3), was refined as nitrogen and
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G.H. Jeong et al. / Microporous and Mesoporous Materials 90 (2006) 16–22
Table 1 Experimental and structure refinement data Crystal color Ion exchange T (C)/t (days) Data collection T (C) Scan technique ˚) Radiation (Mo Ka)k1/k2 (A ˚) Unit cell constant, a (A 2h range for a (deg) No. of reflections for a No. range in data collection (deg) 2h range in data collection (deg) No. of reflections gathered No. of unique reflections (m) No. of reflections with Fo > 4r(Fo) No. of parameters (s) Data/parameter ratio (m/s) Weighting parameters: a/b R1a/wR2b (Fo > 4r(Fo)) R1a/wR2b (all data) Goodness-of-fitc P P a R1 = jFo jFck/ Fo. hP i1=2 P b wR2 ¼ wðF 2o F 2c Þ2 = wðF 2o Þ2 . hP i1=2 c 2 2 2 Goodness-of-fit ¼ wðF o F c Þ =ðm sÞ .
Yellow 21/5 21 h2h 0.70930/0.71359 24.863(4) 14–22 25 20 3 < 2h < 50 1368 1121 524 72 15.6 0.1353/1132 0.069/0.200 0.213/0.259 1.05
All data were used.
the other as carbon. Because the occupancy values at Cd(3), N, and C were refining in the ratio of 1:1:1 (see Table 2), they were constrained to be equal; this value converged to 16.6(3) ions or atoms at each position with R1 = 0.078 and wR2 = 0.210. Anisotropic refinement
of Cd(2) and Cd(3) converged to R1 = 0.070 and wR2 = 0.208. The positions of the five hydrogen atoms of each methylamine molecule (C–N) were then calculated ˚ and C–H = 1.09 A ˚ . With these posiusing N–H = 1.01 A tions fixed, N and C were again refined isotropically and new hydrogen positions were calculated. This process was repeated until the N and C positions had converged with R1 = 0.068 and wR2 = 0.200. Finally, simultaneous positional, occupancy (except for the atoms of the zeolite framework), and anisotropic refinement (except N and C which were refined isotropically) was done for all non-hydrogen atoms. When the occupancies at Cd(3), N, and C were constrained to be equal, they again refined to 16.6(3). The occupancy factors for all were then fixed at the values given in Table 2, and the above refinement was repeated to give the final structural parameters (Table 2). The final error indices converged to R1 = 0.069 and wR2 = 0.200. The goodness-of-fit, defined in a footnote to Table 1, is 1.15. All crystallographic calculations were done using SHELX-97 [20]. Fixed weights were used initially; the final weights were assigned using the formula w ¼ q=½r2 ðF 2o Þ þ ðaP 2 Þ þ bP þ d þ e sinðhÞ, where p ¼ fF 2o þ ð1 f ÞF 2c to give w ¼ q=½r2 ðF 2o Þ þ ðaP 2 Þ þ bP , where p ¼ ðF 2o þ 2F 2c Þ=3, with a and b as refinable parameters (see Table 1). Additional refinement data are given in Table 1. Atomic scattering factors [22] for Si, Al, O, Cd2+, N, C, and H were used. All scattering factors were modified to account for anomalous dispersion [23]. The
Table 2 Positional, thermal, and occupancy parametersa Atom
Wyc. Pos.
x
531(1) 547(2) 1113(4) 30(4) 324(4) 649(3) 0 2262(6) 2305(5) 2950(32) 3147(130) 3239 2861 2836 3244 3506 ˚ , space a = 24.863(3) A
y
z
U11bor Uisod
U22
U33
U12
U13
U23
Occupancyc Varied
Constrained
Fixed
1228(2) 338(1) 87(17) 58(17) 81(17) 27(15) 12(14) 23(15) 96 374(1) 1215(2) 80(18) 61(19) 57(18) 8(14) 15(16) 12(16) 96 13(4) 1057(4) 193(55) 221(52) 160(51) 53(43) 25(37) 80(44) 96 36(4) 1448(4) 195(50) 186(51) 152(47) 73(43) 44(42) 2(42) 96 656(3) 611(4) 170(49) 66(46) 112(47) 64(39) 6(41) 55(36) 96 830(4) 1714(4) 150(49) 133(49) 133(49) 17(40) 24(40) 67(39) 96 0 0 115(7) 115(7) 115(7) 8(6) 8(6) 8(6) 16.4(4) 16 2262(6) 2262(6) 798(93) 798(93) 798(93) 842(85) 842(85) 842(85) 13.4(3) 14e 2305(5) 2305(5) 795(59) 795(59) 795(59) 234(45) 234(45) 234(45) 16.3(5) 16.6(3) 16e 2705(54) 2674(50) 646(357) 15(5) 16.6(3) 16e 2763(101) 3241(69) 1839(1105) 16(4) 16.6(3) 16e 2532 2448 3071 2524 2949 3486 2367 3403 3014 3246 a group Fd 3 with origin at center of symmetry. Positional and anisotropic thermal parameters are given · 104. Numbers in parentheses are the esds in the units of the least significant digit given for the corresponding parameter. b The anisotropic temperature factor = exp[(2p2/a2) (U11h2 + U22k2 + U33l2 + U12hk + U13hl + U23kl)]. c Occupancy factors are given as the number of atoms or ions per unit cell. d Biso = 8p2Uiso. e Only for purposes of discussion. These four values (14, 16, 16, 16) were chosen instead of (13, 17, 17, 17) because, as in jCa46(CH3NH2)16j[Si100Al92O384]-FAU [14], this places exactly two methylamine molecules in each supercage. Methylamine crowding is not the reason for this limit (see Fig. 3). f Hydrogen atom positions were calculated. Si Al O(1) O(2) O(3) O(4) Cd(1) Cd(2) Cd(3) N C H(1)f H(2)f H(3)f H(4)f H(5)f
96(g) 96(g) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 96(g) 96(g) 96(g) 96(g) 96(g) 96(g) 96(g)
G.H. Jeong et al. / Microporous and Mesoporous Materials 90 (2006) 16–22
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Table 3 ˚ ) and angles (deg)a Selected interatomic distances (A Si–O(1) Si–O(2) Si–O(3) Si–O(4) Mean Al–O(1)
1.599(9) 1.628(10) 1.656(9) 1.592(10) 1.619 1.713(10)
O(1)–Si–O(2) O(1)–Si–O(3) O(1)–Si–O(4) O(2)–Si–O(3) O(2)–Si–O(4) O(3)–Si–O(4)
111.7(5) 108.9(5) 110.8(5) 106.5(5) 107.5(5) 111.3(5)
Al–O(2) Al–O(3) Al–O(4) Mean Cd(1)–O(3) Cd(2)–O(2)
1.739(10) 1.750(10) 1.700(10) 1.726 2.369(8) 2.237(10)
O(1)–Al–O(2) O(1)–Al–O(3) O(1)–Al–O(4) O(2)–Al–O(3) O(2)–Al–O(4) O(3)–Al–O(4)
112.1(5) 105.8(5) 113.3(5) 106.6(5) 105.0(5) 114.0(5)
Cd(3)–O(2) Cd(3)–N N–C
2.283(11) 2.11(8) 1.49(22)
Si–O(1)–Al Si–O(2)–Al Si–O(3)–Al Si–O(4)–Al
128.3(6) 138.3(6) 126.7(5) 160.7(6)
O(3)–Cd(1)–O(3) O(2)–Cd(2)–O(2) O(2)–Cd(3)–O(2)
89.4(3)/90.6(3)c 117.0(4) 113.9(5)
Cd(3)–N–C
135(10)
H(1) O(4)b H(1) O(2)b H(2) O(2)b
2.82(17) 3.05(19) 3.10(19)
a Numbers in parentheses are estimated standard deviations in the units of the least significant digit given for the corresponding value. b ˚ and C– Hydrogen positions were calculated using N–H = 1.01 A ˚ . Because of the especially large esds at the C position, the H = 1.09 A H positions and their approach distances to framework oxygens may be somewhat inaccurate. These esds are underestimated. c Cd(1) has an octahedral coordination sphere slightly shortened along the 3 axis.
final structural parameters are given in Table 2, and selected interatomic distances and angles are presented in Table 3, respectively. 3. Discussion Zeolite X is a synthetic Al-rich analogue of the naturally occurring mineral faujasite (FAU). Its structure, including the sites that cations usually occupy, are shown in Fig. 1. Further description is available [19,21,24–26]. In this structure, the mean values of the Si–O and Al–O ˚ , respecbond lengths are normal, ca. 1.619 and 1.726 A tively. The individual bond lengths, however, show marked ˚ and Al–O variations: Si–O from 1.592(10) to 1.656(9) A ˚ (see Table 3). The individual from 1.700(10) to 1.750(10) A Si–O and Al–O distances depend on Cd2+ coordination to framework oxygen. O(1) and O(4) are not involved in coordination; Cd2+ ions at sites I and II coordinate only to O(2) and O(3) in this structure. As a consequence, the Al–O(2), Si–O(2), Al–O(3), and Si–O(3) bonds are somewhat lengthened (see Table 3). This effect is commonly seen in fully divalent-cation exchanged zeolite X [21,24,25]. In Cd46–X Æ 16CH3NH2, the Cd2+ ions occupy three crystallographic sites, each with high occupancy. Sixteen Cd2+ ions at Cd(1) fill site I at the centers of the D6Rs (see Fig. 2). The octahedral Cd(1)–O(3) distance, ˚ , is a little longer than the sum of the ionic radii 2.369(8) A
Fig. 1. A stylized drawing of the framework structure of zeolite X (FAU). Near the center of each line segment is an oxygen atom. The different oxygen atoms are indicated by the numbers 1–4. Silicon and aluminum atoms alternate at the tetrahedral intersections, except that (in this work) a silicon atom substitutes for aluminum at about 4% of the Al positions. Extraframework cation positions are labeled with Roman numerals.
˚ [27], indicating a of Cd2+ and O2, 0.97 + 1.32 = 2.29 A reasonably good fit. In empty Cd46–X, the Cd(1)–O(3) dis˚ [28]. The 14 cations at Cd(2) tance is the same, 2.35(1) A and the 16 at Cd(3) occupy two sites II in the supercage (see Figs. 2 and 3); each Cd2+ ion coordinates at ˚ , respectively, to three O(2) 2.237(10) and 2.283(11) A framework oxygens. The O(2)–Cd(2)–O(2) angle is 117.0(4), and the O(2)–Cd(3)–O(2) angle is 113.9(5) (see Table 3). To coordinate to a methylamine molecule, each Cd2+ ˚ further into the supercage, ion at Cd(3) has moved 0.39 A further from their triads of three O(2) oxygens as compared to the corresponding Cd2+ position in dehydrated Cd46–X [28] (see Table 4 and Figs. 2 and 3). For comparison, the ˚ further into corresponding Ca2+ ion, Ca(3), moved 0.15 A the supercage in the methylamine complex of Ca46–X [14] (Table 5). Similar shifts, larger and smaller, have been seen in a series of sorption complexes of dehydrated fully Cd2+exchanged zeolite X (see Table 4). Cd(2), the site-II Cd2+ position that has not participated in coordination to sorbed molecules, lies closer than Cd(3) to the planes of their triads of three O(2) oxygens (Table 5), but it is still further from that plane than the site-II Cd2+ ions in dehydrated Cd46–X [28] (Tables 4 and 5). (The positions of the centers of the 6-ring planes vary a little amongst these structures. In addition, only an average 6-ring center has been determined for each structure; the structures of individual 6-rings should depend upon their contents.) Each of the 16 (or 17, see footnote e of Table 2) Cd2+ ions at Cd(3) complexes to a CH3NH2 molecule. The ˚ , is in agreement with that N–C bond distance, 1.49(22) A
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G.H. Jeong et al. / Microporous and Mesoporous Materials 90 (2006) 16–22
Fig. 2. Stereoview of a sodalite cavity with an attached D6R in Cd46–X Æ 16CH3NH2. One Cd2+ ion at Cd(1) is shown at site I. Two Cd2+ ions at Cd(2) and two Cd2+ ions at Cd(3) are shown. Only the Cd2+ ions at Cd(3) coordinate to methylamine. To simplify this and subsequent figures, artificially small ˚ 2, were used at the five hydrogen positions. Ellipsoids of 20% probability are used throughout. values of Uiso, 0.0200 A
Fig. 3. Stereoview of a supercage. Two Cd2+ ions at Cd(2) (site II) are shown. The two Cd2+ ions at Cd(3) (site II) each coordinate to a methylamine molecule. Two of the eight supercages per unit cell have only one Cd2+ ion at Cd(2).
Table 4 ˚ ) of coordinating Cd2+ ions and carbon or nitrogen atoms Deviations (A from 6-ring planes at O(2), and Cd2+–O(2) distancesa Zeolite X
Cd2+b
Cd46–X (dehydrated) Cd46–X Æ 28CO Cd46–X Æ 28C2H2 Cd46–X Æ 16NO Cd46–X Æ 29.5C2H4 Cd46–X Æ 16CH3NH2 Cd46–X Æ 43C6H6 Cd46–X Æ 25.5N2O4
0.19 0.33 0.46 0.49 0.50 0.58 0.60 1.05
C or Nc
Cd2+–O(2)
Reference
2.71 3.19 2.91 3.28 2.60 3.24, 3.41 2.81
2.16 2.16 2.19 2.224 2.221 2.283 2.224 2.346
[28] [29] [2] [32] [30] This work [31] [32]
a Numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. b The Cd2+ ion extends this distance into the supercage. c The coordinating atom(s) extend this distance into the supercage.
˚ [33]. Similarly, the N–C in methylamine(g), 1.474(5) A ˚. bond distance in Ca46–X Æ 16CH3NH2 [14], is 1.48(23) A 2+ Even the corresponding M –N–C angles, 135(10) and 111(7), respectively, are not significantly different. The deviations of the site-II Cd2+ ions and the N and C atoms from their 3-oxygen planes are given in Table 5. The high
Table 5 ˚ ) of cations and atoms from 6-ring planes at O(2)a Deviations (A Ca46–X Æ 16CH3NH2 Ca(2) Ca(3) Nb C
Cd46–X Æ 16CH3NH2 0.15 0.45 2.55 3.94
Cd(2) Cd(3) Nb C
0.39 0.58 2.60 3.79
a
A positive deviation indicates that the cation or atom is in the supercage. b Methylamine coordinates via N to Ca(3) and Cd(3).
esds in the methylamine geometry are a consequence of the small scattering factors of C and N, their relatively low occupancy as compared to framework positions, and especially their high thermal motion (see Figs. 2, 3 or 4). The –NH2 group of each methylamine molecule makes three weak non-equivalent hydrogen bonds to oxygen atoms of the zeolite framework; see Fig. 4. H(1) hydrogen ˚ and to O(2) at 3.05(19) A ˚ , and bonds to O(4) at 2.82(17) A ˚. H(2) hydrogen bonds to the same O(2) atom at 3.10(19) A (The hydrogen atom positions were not determined crystallographically but were calculated from the N, C, and Cd(3)
G.H. Jeong et al. / Microporous and Mesoporous Materials 90 (2006) 16–22
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Fig. 4. A Cd(CH3NH2)2+ complex on the inner surface of a supercage is shown. Each of the 16 Cd2+ ions per unit cell (two per supercage) at Cd(3) coordinates to a methylamine molecule in this manner. H(1) approaches O(2) and O(4) to give a weak bifurcated hydrogen bond. H(2) hydrogen bonds weakly to the same O(2) atom. Hydrogen positions were not determined crystallographically; they were only calculated. The H positions and their approach distances to framework oxygens may be somewhat inaccurate because of the especially large esds at the C position.
positions.) These H O hydrogen bonds are longer than those in the Ca46–X Æ 16CH3NH2 complex: H(1) O(2), ˚ H(2) O(2) 0 , and H(2) O(4) are 2.42, 2.84, and 2.34 A in Ca46–X Æ 16CH3NH2, respectively. Also the topology of the hydrogen bonding appears to be different: it involved three framework oxygens for each –NH2 group in Ca46– X Æ 16CH3NH2, but only two here. Similar H O guestto-framework contacts are found in many organic sorption complexes of zeolites [34]. For each nitrogen position, three carbon positions are ˚ in possible. One of these gives a C–N bond 1.83(22) A length with unacceptable hydrogen bonding. A second choice of orientation for the methylamine molecule, however, is not easily dismissed; for this the C–N bond length ˚ . One amine hydrogen would participate in is 1.33(22) A ˚ to O(2), 2.7 A ˚ three electrostatic hydrogen bonds (2.9 A ˚ to another O(2)), while the other would to O(4), and 3.1 A ˚ to the first O(2). This make only one long approach, 3.3 A would be very different from the result seen for the methylamine complex of Ca–X, where only one orientation was sensible. The selection made (see Fig. 4) is similar to that reported for the methylamine complex of Ca–X, where one amine hydrogen atom hydrogen bonds to one framework oxygen atom and the other forms a bifurcated hydrogen bond to two. Kustanovich et al. [35] studied mono-, di-, and trimethylamine sorption onto ZK-5 and Y zeolites using deuterium NMR. They also observed hydrogen bonding between N–H and oxygens of the zeolite framework. (They also found alkylammonium ion formation at the zeolite’s Brønsted acid sites.) The structures of Ca46–X Æ 135NH3 [36] and Sr46– X Æ 102NH3 [37] were determined by single-crystal diffraction techniques. Each of 30 M2+ (M = Ca or Sr) ions per unit cell at site II coordinates octahedrally to three framework oxygens and to three ammonia molecules. Each of four Sr2+ ions per unit cell at site I 0 also coordinates octahedrally to three framework oxygens and to three ammonia molecules. Each of the remaining 45(3) NH3 molecules in the structure of Ca46–X Æ 135NH3 hydrogen bonds via its
lone pair to two coordinating NH3 molecules and is in position to hydrogen bond further to two or more framework oxygens. In the structure reported here and in Ca46–X Æ 16CH3NH2 [14], only one methylamine molecule (not three as with ammonia) coordinates to each coordinating cation, and only 16 of the 30 site-II cations participate in coordination. Although methylamine should be more basic than ammonia [38], the zeolite sorbs much less of it, presumably for steric and hydrogen-bonding reasons. The structure of the methylamine sorption complex of fully dehydrated, fully Ca2+-exchanged zeolite X [14] is very similar to that of the present structure. In each there are 16 cations per unit cell at site I and 30 per unit cell at site II. In each there are two types of site-II ions: 16 coordinate to methylamine molecules and 14 do not. In each the two hydrogen atoms of each –NH2 group hydrogen bond, albeit differently, to oxygen atoms of the zeolite frame˚ , appears to be work. The Cd–N bond length, 2.13(9) A ˚ , although the shorter than the Ca–N distance, 2.30(9) A difference is not significant. The Cd2+ and Ca2+ radii, ˚ , respectively, are nearly the same. 0.97 and 0.99 A Together, these values suggest, although again without significance, that the Cd2+–NH2CH3 interaction is stronger than Ca2+–NH2CH3. However neither the shorter distance nor the stronger bond should be believed; a tabulation of the bond strengths of Cd and Ca with a diverse series of other elements indicates that diatomic bonds involving Cd atoms are always much weaker than those involving Ca2+ [39]. Consistent with this expectation, the N position refined to a 3-fold-axis position in Ca46–X Æ 16CH3NH2 [14], although it should be at least somewhat off this position because CH3NH2 coordinates asymmetrically. As with Ca46–X Æ 16CH3NH2, no significant changes in the geometry of the zeolite framework, as compared to dehydrated Cd46–X [28], were seen upon sorption. It is clear from an inspection of Fig. 3 that enough volume is available in each supercage for a third and perhaps a fourth methylamine molecule to be sorbed in the same manner as the first two. This is, of course, a function of T, P, and the energy of formation of the sorption bond,
22
G.H. Jeong et al. / Microporous and Mesoporous Materials 90 (2006) 16–22
Table 6 Number of Cd2+ ions at sites in zeolite X Zeolite X
Site I
Cd46–X (dehydrated) Cd46–X Æ 16CH3NH2 Cd46–X Æ 16NO Cd46–X Æ 25.5N2O4 Cd46–X Æ 43C6H6 Cd46–X Æ 28CO Cd46–X Æ 28C2H2 Cd46–X Æ 29.5C2H4
16 16 16 11.5 11 13 13 15.5
a b
Site I 0
Site IIa
9 6 5 5 1
16 16 25.5 27 28 28 29.5
Site IIb
Reference
30 14 14
[28] This work [32] [32] [31] [29] [2] [30]
2
Coordinates to a guest molecule. Does not coordinate to a guest molecule.
according to Irving Langmuir’s theory. In addition, however, a cooperative effect appears to be operating in zeolites. When a basic ligand coordinates to a cation, that cation moves away from its nearest framework oxygens (M–O bond lengths increase). That cation, which receives electron density from the ligand, then withdraws electron density less effectively from the basic zeolite framework. The increasingly basic zeolite framework then bonds more strongly to the remaining cations (M–O bond lengths decrease). These cations then become less acidic and less able to coordinate to additional ligands. This is generally true for intrazeolitic sorption complexes (see Table 6). Acknowledgements This work was supported by a Pusan National University Research Grant (April 1, 2003–February 28, 2006) and also by Brain Korea 21 Project, 2003. Appendix A. Supplementary data Tables of calculated structure factors squared, and observed structure factors squared with esds (14 pages). The supporting materials will be given upon your request to a corresponding author (fax: + 82 51 516 7421; e-mail: [email protected]; seff@hawaii.edu), and can also be accessed with the electronic version of this paper on ScienceDirect. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2005.08.041. References [1] K. Seff, Acc. Chem. Res. 9 (1976) 121. [2] S.B. Jang, M.S. Jeong, Y. Kim, K. Seff, Zeolites 19 (1997) 228. [3] E.Y. Choi, Y. Kim, K. Seff, Micropor. Mesopor. Mater. 40 (2000) 247.
[4] E.Y. Choi, Y. Kim, S.H. Song, Bull. Korean Chem. Soc. 20 (1999) 791. [5] W.M. Cruz, P.C.W. Leung, K. Seff, Inorg. Chem. 18 (1979) 230. [6] Y.H. Yeom, Y. Kim, K. Seff, J. Phys. Chem. 100 (1996) 8373. [7] S.B. Jang, Y. Kim, K. Seff, Zeolites 14 (1994) 262. [8] R.Y. Yanagida, A.A. Amaro, K. Seff, J. Phys. Chem. 77 (1973) 805. [9] Y. Kim, K. Seff, J. Am. Chem. Soc. 97 (1977) 7057. [10] Y.C. Long, H. Yang, Y.J. Sun, Z.H. Ping, Peop. Rep. China. Gaodeng Xuexiao Huaxue Xuebao 22 (2001) 365. [11] F. Docquir, H. Toufar, B.L. Su, Langmuir 17 (2001) 6282. [12] F. Docquir, V. Norberg, H. Toufar, J.L. Paillaud, B.L. Su, Langmuir 18 (2002) 5963. [13] K. Morishige, S. Kittaka, S. Ihara, J. Chem. Soc. Faraday Trans. 1 (18) (1985) 2525. [14] G.H. Jeong, Y. Kim, K. Seff, Langmuir 20 (2004) 9354. [15] V.N. Bogomolov, V.P. Petranovskii, Zeolites 6 (1986) 418. [16] W. Loewenstein, Am. Miner. 39 (1954) 92. [17] D. Bae, K. Seff, Micropor. Mesopor. Mater. 40 (2000) 233. [18] International Tables for X-ray Crystallography, Kynoch Press, Birmingham, UK, vol. II 1944, p. 302. [19] Y.H. Yeom, Y. Kim, K. Seff, J. Phys. Chem. B 101 (1997) 5314. [20] G.M. Sheldrick, SHELX-97: Program for the Refinement of Crystal Structures, University of Go¨ttingen, Go¨ttingen, Germany, 1997. [21] E.Y. Choi, Y. Kim, K. Seff, J. Phys. Chem. B 106 (2002) 5827. [22] International Tables for X-ray Crystallography, Kynoch Press, Birmingham, UK, vol. IV 1974, pp. 73–87. [23] International Tables for X-ray Crystallography, Kynoch Press, Birmingham, UK, vol. IV 1974, pp. 149–150. [24] J.V. Smith, in: E.M. Flanigen, L.B. Sand (Eds.), Molecular Sieve Zeolites-I, Advances in Chemistry Series, 101, American Chemical Society, Washington, DC, 1971, pp. 171–200. [25] Y.H. Yeom, S.B. Jang, S.H. Song, Y. Kim, K. Seff, J. Phys. Chem. B 101 (1997) 6914. [26] M.K. Song, Y. Kim, K. Seff, J. Phys. Chem. B 107 (2003) 3117. [27] Handbook of Chemistry and Physics, 70th ed., The Chemical Rubber Co., Cleveland, Ohio, 1989/1990, p. F-187. [28] J.H. Kwon, S.B. Jang, Y. Kim, K. Seff, J. Phys. Chem. B 100 (1996) 13720. [29] S.B. Jang, J.H. Kwon, S.H. Song, Y. Kim, K. Seff, in: H. Chon, S.K. Ihm, Y.S. Uh (Eds.), Progress in Zeolite and Microporous Materials: Studies in Surface Science and Catalysis, 105, Elsevier, Amsterdam, 1997, p. 1819. [30] Y.H. Yeom, S.H. Song, Y. Kim, K. Seff, J. Phys. Chem. B 101 (1997) 2138. [31] Y. Kim, Y.H. Yeom, E.Y. Choi, A.N. Kim, Y.W. Han, Bull. Korean Chem. Soc. 19 (1998) 1222. [32] Y.M. Lee, Y. Kim, K. Seff, J. Phys. Chem. B 109 (2005) 4900. [33] Handbook of Chemistry and Physics, 70th ed., The Chemical Rubber Co., Cleveland, Ohio, 1989/1990, p. F-156. [34] S. Zhen, K. Seff, Micropor. Mesopor. Mater. 39 (2000) 1. [35] I. Kustanovich, Z. Luz, S. Vega, A.J. Vega, J. Phys. Chem. 94 (1990) 3138. [36] S.B. Jang, M.S. Jeong, Y. Kim, K. Seff, Micropor. Mesopor. Mater. 28 (1999) 173. [37] M.J. Kim, M.S. Jeong, Y. Kim, K. Seff, Micropor. Mesopor. Mater. 30 (1999) 233. [38] J.E. Huheey, Inorganic Chemistry, second ed., Harper & Row, New York, 1978, p. 265. [39] Handbook of Chemistry and Physics, 77th ed., The Chemical Rubber Co., Cleveland, Ohio, 1996/1997, Section 12, p. 14.
Structure of a methylamine sorption complex of fully dehydrated Cd2+-exchanged zeolite X, jCd46(CH3NH2)16j[Si100Al92O384]-FAU Gyoung Hwa Jeong a, Yang Kim a
a,*
, Karl Seff
b,*
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea b Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, HI 96822, USA Received 3 December 2004; accepted 24 August 2005
Dedicated to the late Denise Barthomeuf, George Kokotailo and Sergey P. Zhdanov in appreciation of their outstanding contributions to zeolite science
Abstract The structure of a methylamine sorption complex of fully dehydrated, fully Cd2+-exchanged zeolite X, jCd46(CH3NH2)16j˚ ), has been determined by single-crystal X-ray diffraction techniques in the cubic space group [Si100Al92O384]-FAU (a = 24.863(4) A Fd 3 at 21(1) C. An aqueous exchange solution 0.05 M in Cd2+ was allowed to flow past the crystal for 5 days. The crystal was then dehydrated at 480 C and 2 · 106 Torr for 2 days (colorless), and exposed to 160 Torr of methylamine gas at 21(1) C for 2 h (yellow). Diffraction data were then gathered in this atmosphere and were refined using all data to the final error indices (based upon the 524 reflections for which Fo > 4r(Fo)) of R1 = 0.069 and wR2 = 0.200. In this structure, Cd2+ ions occupy three crystallographic sites. ˚ ). The remaining The octahedral sites I at the centers of the hexagonal prisms are filled with 16 Cd2+ ions per unit cell (Cd–O = 2.369(8) A 30 Cd2+ ions are located at two non-equivalent sites II with occupancies of 14 and 16. The 16 methylamine molecules per unit cell lie in ˚ . The imprecisely determined N–C bond the supercage where each interacts with one of the latter 16 site-II Cd2+ ions: N–Cd = 2.11(8) A ˚ , agrees with that in gaseous methylamine, 1.474 A ˚ . The positions of the hydrogen atoms were calculated. It appears length, 1.49(22) A that one of the amino hydrogen atoms hydrogen bonds to a 6-ring oxygen, and that the other forms a bifurcated hydrogen bond to this and another 6-ring oxygen. The methyl group is not involved in hydrogen bonding. 2006 Elsevier Inc. All rights reserved. Keywords: Structure; Methylamine; Zeolite X; Sorption; Cadmium
1. Introduction A zeolite framework can be considered a relatively rigid polydentate ligand [1]. The exchangeable cations in fully dehydrated zeolites must select their coordination environments from the limited number of sites available [1]. These coordination environments are often far less suited to the cation than those commonly found elsewhere in chemistry, e.g. in solution. Transition-metal ions that have been ion exchanged into zeolites are generally, after dehydration, *
Corresponding authors. Tel.: +1 808 956 7480; fax: +1 808 956 5908. E-mail addresses: [email protected] (Y. Kim), seff@hawaii.edu (K. Seff). 1387-1811/$ - see front matter 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.08.041
unusually coordinated or coordinatively unsaturated [1]. To relieve their coordinative unsaturation in zeolites A and X, these cations complex readily with a variety of guest molecules such as C2H2 [2], C3H6 [3,4], NO2 [5], H2S [6], CS2 [7], and NH3 [8,9]. The interaction between sorbed methylamine and jNa31j [Si161Al31O384]-FAU was investigated by Long et al. using powder X-ray diffraction (XRD), 29Si and 13C MAS NMR, and FTIR [10]. As methylamine was loaded into the zeolite, one peak in the 29Si MAS NMR spectrum split into four peaks and the high resolution IR spectrum became simpler. They concluded that the sorbed methylamine interacted strongly with the zeolite, and therefore that the zeolite has a high affinity for methylamine.
G.H. Jeong et al. / Microporous and Mesoporous Materials 90 (2006) 16–22
Using IR, Su et al. [11,12] studied the interactions between methylamine and a series of alkaline-earth-cation exchanged faujasite zeolites. They reported the effects that the size of the alkaline-earth cation, and the Si/Al ratio of the zeolite framework, had on their results. They found that the size of the cations (their Lewis acidity) was the dominating factor in the interaction between methylamine and the zeolites. They concluded that methylamine interacts with the alkali cations via the lone electron pair on its nitrogen atom, and that it hydrogen bonds to the anionic oxygen atoms of the zeolite framework via the hydrogen atoms of both the –NH2 and the –CH3 groups. They detected a deformation (otherwise undefined) of the zeolite framework upon the sorption of methylamine. Their work showed clearly that methylamine can be an efficient probe molecule for the characterization of the acid–base properities of zeolites. Morishige et al. reported that methylamine is selectively adsorbed on the site-II cations of zeolite Y (FAU) in such a configuration that the dipole axis of the molecule coincides with the line connecting the site-II cation and the center of the supercage, optimizing the electrostatic-field–dipole interaction [13]. The structure of Ca46–X Æ 16CH3NH2 was recently reported [14]. There each methylamine molecule coordinated to one of the site-II Ca2+ ions: N–Ca2+ = ˚ . One of the amino hydrogen atoms hydrogen2.30(9) A bonded to a 6-ring oxygen, and the other formed a bifurcated hydrogen bond to two other 6-ring oxygens. As expected, the methyl group did not participate hydrogen bonding. The present work was undertaken as part of our structural study of the sorption of methylamine onto zeolites at ambient temperature. We wished to determine the positions of the methylamine molecules sorbed by Cd46–X to observe the coordination and hydrogen bonding of methylamine at the sorption site, and also to observe the shifts in the cation and framework positions upon sorption. 2. Experimental section Large single crystals of zeolite Na–X, stoichiometry Na92Si100Al92O384, were prepared in St. Petersburg, Russia [15]. One of these, a colorless octahedron about 0.2 mm in cross-section, was lodged in a fine Pyrex capillary. An ion-exchange solution of Cd(NO3)2 (Aldrich, 99.999%) and Cd(O2CCH3)2 (Aldrich, 99.99%) in the mole ratio of 1:1 with a total Cd2+ concentration of 0.05 M was allowed to flow past the crystal for 5 days. The capillary containing the crystal was attached to a vacuum system, and the crystal was cautiously dehydrated by gradually increasing its temperature (ca. 29 C/h) under dynamic vacuum to 480 C. Finally, the system was maintained at 480 C and 2 · 106 Torr for 2 days. After cooling to room temperature (without the possibility of water reabsorption because adjacent portions of the vacuum system, including a 15-cm in-series tube of zeolite beads, had been baked out
17
in situ), the crystal remained colorless. To prepare the methylamine complex, the crystal was treated with 160 Torr of methylamine gas (Aldrich, 99.995%, zeolitically dried in situ) for 2 h at 21(1) C. The resulting yellow crystal, still in its methylamine atmosphere, was sealed in its capillary by torch. The cubic space group Fd 3 was used throughout this work. This choice is supported by (a) the low Si/Al ratio which in turn requires, at least in the short range, alternation of Si and Al [16], and (b) the observation that this crystal, like most crystals from the same batch [17], does not have intensity symmetry across (1 1 0) and therefore lacks that mirror plane. Diffraction data were collected with an automated Enraf–Nonius four-circle computer-controlled CAD-4 diffractometer equipped with a pulse-height analyzer and a graphite monochromator, using Mo radiation ˚ ). The cubic unit cell (Ka1, k = 0.70930; Ka2, k = 0.71359 A constant at 21(1) C, determined by least-squares refinement of 25 intense reflections for which 14 < 2h < 22, is ˚ . All unique reflections in the positive a = 24.863(4) A octant of an F-centered unit cell for which 2h < 50, l P h, and l P k were recorded. An absorption correction (l = 1.97 mm1) was made empirically using a w scan [18]. This correction had little effect on the final error (R) indices. Other details are the same as those previously reported [19]. 2.1. Structure determination Full-matrix least-squares refinement [20] was done on F2 using all 1139 unique data. It was initiated with the atomic parameters of the framework atoms (Si, Al, O(1), O(2), O(3), and O(4)) in dehydrated Ca46–X Æ 8C9H12 [21]. These positions when refined anisotropically yielded R1 = 0.43 and wR2 = 0.74; R1 and R2 are defined in footnotes of Table 1. R1 (based on F) and wR2 (based on F2) are calculated using only the 524 reflections for which Fo > 4r(Fo). A Fourier difference electron-density function yielded two strong peaks at (0.0, 0.0, 0.0) and (0.230, 0.230, 0.230), positions near framework oxygens normally occupied by cations. Anisotropic refinement of the framework atoms and Cd2+ ions at these two positions converged to R1 = 0.14 and wR2 = 0.33 with occupancies at these two positions of 16.4(4) at Cd(1) and 29.5(7) at Cd(3), respectively. The thermal ellipsoid at Cd(3), by being very elongated, indicated the presence of two non-equivalent Cd2+ ions at this position. Accordingly, this position was split into two that refined isotropically to convergence as Cd(2) at (0.220, 0.220, 0.220) and Cd(3) at (0.232, 0.232, 0.232) with occupancies of 13.4(3) and 16.3(5), respectively. The occupancy at Cd(1) was then fixed at 16, the maximum value at this position, and refinement converged with R1 = 0.088 and wR2 = 0.269. An ensuing Fourier function revealed two more peaks at the general positions (0.305, 0.284, 0.268) and (0.320, 0.295, 0.317). These two positions ˚ , respectively, from the 3-fold axis. The are 0.48 and 0.65 A first, the one closer to Cd(3), was refined as nitrogen and
18
G.H. Jeong et al. / Microporous and Mesoporous Materials 90 (2006) 16–22
Table 1 Experimental and structure refinement data Crystal color Ion exchange T (C)/t (days) Data collection T (C) Scan technique ˚) Radiation (Mo Ka)k1/k2 (A ˚) Unit cell constant, a (A 2h range for a (deg) No. of reflections for a No. range in data collection (deg) 2h range in data collection (deg) No. of reflections gathered No. of unique reflections (m) No. of reflections with Fo > 4r(Fo) No. of parameters (s) Data/parameter ratio (m/s) Weighting parameters: a/b R1a/wR2b (Fo > 4r(Fo)) R1a/wR2b (all data) Goodness-of-fitc P P a R1 = jFo jFck/ Fo. hP i1=2 P b wR2 ¼ wðF 2o F 2c Þ2 = wðF 2o Þ2 . hP i1=2 c 2 2 2 Goodness-of-fit ¼ wðF o F c Þ =ðm sÞ .
Yellow 21/5 21 h2h 0.70930/0.71359 24.863(4) 14–22 25 20 3 < 2h < 50 1368 1121 524 72 15.6 0.1353/1132 0.069/0.200 0.213/0.259 1.05
All data were used.
the other as carbon. Because the occupancy values at Cd(3), N, and C were refining in the ratio of 1:1:1 (see Table 2), they were constrained to be equal; this value converged to 16.6(3) ions or atoms at each position with R1 = 0.078 and wR2 = 0.210. Anisotropic refinement
of Cd(2) and Cd(3) converged to R1 = 0.070 and wR2 = 0.208. The positions of the five hydrogen atoms of each methylamine molecule (C–N) were then calculated ˚ and C–H = 1.09 A ˚ . With these posiusing N–H = 1.01 A tions fixed, N and C were again refined isotropically and new hydrogen positions were calculated. This process was repeated until the N and C positions had converged with R1 = 0.068 and wR2 = 0.200. Finally, simultaneous positional, occupancy (except for the atoms of the zeolite framework), and anisotropic refinement (except N and C which were refined isotropically) was done for all non-hydrogen atoms. When the occupancies at Cd(3), N, and C were constrained to be equal, they again refined to 16.6(3). The occupancy factors for all were then fixed at the values given in Table 2, and the above refinement was repeated to give the final structural parameters (Table 2). The final error indices converged to R1 = 0.069 and wR2 = 0.200. The goodness-of-fit, defined in a footnote to Table 1, is 1.15. All crystallographic calculations were done using SHELX-97 [20]. Fixed weights were used initially; the final weights were assigned using the formula w ¼ q=½r2 ðF 2o Þ þ ðaP 2 Þ þ bP þ d þ e sinðhÞ, where p ¼ fF 2o þ ð1 f ÞF 2c to give w ¼ q=½r2 ðF 2o Þ þ ðaP 2 Þ þ bP , where p ¼ ðF 2o þ 2F 2c Þ=3, with a and b as refinable parameters (see Table 1). Additional refinement data are given in Table 1. Atomic scattering factors [22] for Si, Al, O, Cd2+, N, C, and H were used. All scattering factors were modified to account for anomalous dispersion [23]. The
Table 2 Positional, thermal, and occupancy parametersa Atom
Wyc. Pos.
x
531(1) 547(2) 1113(4) 30(4) 324(4) 649(3) 0 2262(6) 2305(5) 2950(32) 3147(130) 3239 2861 2836 3244 3506 ˚ , space a = 24.863(3) A
y
z
U11bor Uisod
U22
U33
U12
U13
U23
Occupancyc Varied
Constrained
Fixed
1228(2) 338(1) 87(17) 58(17) 81(17) 27(15) 12(14) 23(15) 96 374(1) 1215(2) 80(18) 61(19) 57(18) 8(14) 15(16) 12(16) 96 13(4) 1057(4) 193(55) 221(52) 160(51) 53(43) 25(37) 80(44) 96 36(4) 1448(4) 195(50) 186(51) 152(47) 73(43) 44(42) 2(42) 96 656(3) 611(4) 170(49) 66(46) 112(47) 64(39) 6(41) 55(36) 96 830(4) 1714(4) 150(49) 133(49) 133(49) 17(40) 24(40) 67(39) 96 0 0 115(7) 115(7) 115(7) 8(6) 8(6) 8(6) 16.4(4) 16 2262(6) 2262(6) 798(93) 798(93) 798(93) 842(85) 842(85) 842(85) 13.4(3) 14e 2305(5) 2305(5) 795(59) 795(59) 795(59) 234(45) 234(45) 234(45) 16.3(5) 16.6(3) 16e 2705(54) 2674(50) 646(357) 15(5) 16.6(3) 16e 2763(101) 3241(69) 1839(1105) 16(4) 16.6(3) 16e 2532 2448 3071 2524 2949 3486 2367 3403 3014 3246 a group Fd 3 with origin at center of symmetry. Positional and anisotropic thermal parameters are given · 104. Numbers in parentheses are the esds in the units of the least significant digit given for the corresponding parameter. b The anisotropic temperature factor = exp[(2p2/a2) (U11h2 + U22k2 + U33l2 + U12hk + U13hl + U23kl)]. c Occupancy factors are given as the number of atoms or ions per unit cell. d Biso = 8p2Uiso. e Only for purposes of discussion. These four values (14, 16, 16, 16) were chosen instead of (13, 17, 17, 17) because, as in jCa46(CH3NH2)16j[Si100Al92O384]-FAU [14], this places exactly two methylamine molecules in each supercage. Methylamine crowding is not the reason for this limit (see Fig. 3). f Hydrogen atom positions were calculated. Si Al O(1) O(2) O(3) O(4) Cd(1) Cd(2) Cd(3) N C H(1)f H(2)f H(3)f H(4)f H(5)f
96(g) 96(g) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 96(g) 96(g) 96(g) 96(g) 96(g) 96(g) 96(g)
G.H. Jeong et al. / Microporous and Mesoporous Materials 90 (2006) 16–22
19
Table 3 ˚ ) and angles (deg)a Selected interatomic distances (A Si–O(1) Si–O(2) Si–O(3) Si–O(4) Mean Al–O(1)
1.599(9) 1.628(10) 1.656(9) 1.592(10) 1.619 1.713(10)
O(1)–Si–O(2) O(1)–Si–O(3) O(1)–Si–O(4) O(2)–Si–O(3) O(2)–Si–O(4) O(3)–Si–O(4)
111.7(5) 108.9(5) 110.8(5) 106.5(5) 107.5(5) 111.3(5)
Al–O(2) Al–O(3) Al–O(4) Mean Cd(1)–O(3) Cd(2)–O(2)
1.739(10) 1.750(10) 1.700(10) 1.726 2.369(8) 2.237(10)
O(1)–Al–O(2) O(1)–Al–O(3) O(1)–Al–O(4) O(2)–Al–O(3) O(2)–Al–O(4) O(3)–Al–O(4)
112.1(5) 105.8(5) 113.3(5) 106.6(5) 105.0(5) 114.0(5)
Cd(3)–O(2) Cd(3)–N N–C
2.283(11) 2.11(8) 1.49(22)
Si–O(1)–Al Si–O(2)–Al Si–O(3)–Al Si–O(4)–Al
128.3(6) 138.3(6) 126.7(5) 160.7(6)
O(3)–Cd(1)–O(3) O(2)–Cd(2)–O(2) O(2)–Cd(3)–O(2)
89.4(3)/90.6(3)c 117.0(4) 113.9(5)
Cd(3)–N–C
135(10)
H(1) O(4)b H(1) O(2)b H(2) O(2)b
2.82(17) 3.05(19) 3.10(19)
a Numbers in parentheses are estimated standard deviations in the units of the least significant digit given for the corresponding value. b ˚ and C– Hydrogen positions were calculated using N–H = 1.01 A ˚ . Because of the especially large esds at the C position, the H = 1.09 A H positions and their approach distances to framework oxygens may be somewhat inaccurate. These esds are underestimated. c Cd(1) has an octahedral coordination sphere slightly shortened along the 3 axis.
final structural parameters are given in Table 2, and selected interatomic distances and angles are presented in Table 3, respectively. 3. Discussion Zeolite X is a synthetic Al-rich analogue of the naturally occurring mineral faujasite (FAU). Its structure, including the sites that cations usually occupy, are shown in Fig. 1. Further description is available [19,21,24–26]. In this structure, the mean values of the Si–O and Al–O ˚ , respecbond lengths are normal, ca. 1.619 and 1.726 A tively. The individual bond lengths, however, show marked ˚ and Al–O variations: Si–O from 1.592(10) to 1.656(9) A ˚ (see Table 3). The individual from 1.700(10) to 1.750(10) A Si–O and Al–O distances depend on Cd2+ coordination to framework oxygen. O(1) and O(4) are not involved in coordination; Cd2+ ions at sites I and II coordinate only to O(2) and O(3) in this structure. As a consequence, the Al–O(2), Si–O(2), Al–O(3), and Si–O(3) bonds are somewhat lengthened (see Table 3). This effect is commonly seen in fully divalent-cation exchanged zeolite X [21,24,25]. In Cd46–X Æ 16CH3NH2, the Cd2+ ions occupy three crystallographic sites, each with high occupancy. Sixteen Cd2+ ions at Cd(1) fill site I at the centers of the D6Rs (see Fig. 2). The octahedral Cd(1)–O(3) distance, ˚ , is a little longer than the sum of the ionic radii 2.369(8) A
Fig. 1. A stylized drawing of the framework structure of zeolite X (FAU). Near the center of each line segment is an oxygen atom. The different oxygen atoms are indicated by the numbers 1–4. Silicon and aluminum atoms alternate at the tetrahedral intersections, except that (in this work) a silicon atom substitutes for aluminum at about 4% of the Al positions. Extraframework cation positions are labeled with Roman numerals.
˚ [27], indicating a of Cd2+ and O2, 0.97 + 1.32 = 2.29 A reasonably good fit. In empty Cd46–X, the Cd(1)–O(3) dis˚ [28]. The 14 cations at Cd(2) tance is the same, 2.35(1) A and the 16 at Cd(3) occupy two sites II in the supercage (see Figs. 2 and 3); each Cd2+ ion coordinates at ˚ , respectively, to three O(2) 2.237(10) and 2.283(11) A framework oxygens. The O(2)–Cd(2)–O(2) angle is 117.0(4), and the O(2)–Cd(3)–O(2) angle is 113.9(5) (see Table 3). To coordinate to a methylamine molecule, each Cd2+ ˚ further into the supercage, ion at Cd(3) has moved 0.39 A further from their triads of three O(2) oxygens as compared to the corresponding Cd2+ position in dehydrated Cd46–X [28] (see Table 4 and Figs. 2 and 3). For comparison, the ˚ further into corresponding Ca2+ ion, Ca(3), moved 0.15 A the supercage in the methylamine complex of Ca46–X [14] (Table 5). Similar shifts, larger and smaller, have been seen in a series of sorption complexes of dehydrated fully Cd2+exchanged zeolite X (see Table 4). Cd(2), the site-II Cd2+ position that has not participated in coordination to sorbed molecules, lies closer than Cd(3) to the planes of their triads of three O(2) oxygens (Table 5), but it is still further from that plane than the site-II Cd2+ ions in dehydrated Cd46–X [28] (Tables 4 and 5). (The positions of the centers of the 6-ring planes vary a little amongst these structures. In addition, only an average 6-ring center has been determined for each structure; the structures of individual 6-rings should depend upon their contents.) Each of the 16 (or 17, see footnote e of Table 2) Cd2+ ions at Cd(3) complexes to a CH3NH2 molecule. The ˚ , is in agreement with that N–C bond distance, 1.49(22) A
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G.H. Jeong et al. / Microporous and Mesoporous Materials 90 (2006) 16–22
Fig. 2. Stereoview of a sodalite cavity with an attached D6R in Cd46–X Æ 16CH3NH2. One Cd2+ ion at Cd(1) is shown at site I. Two Cd2+ ions at Cd(2) and two Cd2+ ions at Cd(3) are shown. Only the Cd2+ ions at Cd(3) coordinate to methylamine. To simplify this and subsequent figures, artificially small ˚ 2, were used at the five hydrogen positions. Ellipsoids of 20% probability are used throughout. values of Uiso, 0.0200 A
Fig. 3. Stereoview of a supercage. Two Cd2+ ions at Cd(2) (site II) are shown. The two Cd2+ ions at Cd(3) (site II) each coordinate to a methylamine molecule. Two of the eight supercages per unit cell have only one Cd2+ ion at Cd(2).
Table 4 ˚ ) of coordinating Cd2+ ions and carbon or nitrogen atoms Deviations (A from 6-ring planes at O(2), and Cd2+–O(2) distancesa Zeolite X
Cd2+b
Cd46–X (dehydrated) Cd46–X Æ 28CO Cd46–X Æ 28C2H2 Cd46–X Æ 16NO Cd46–X Æ 29.5C2H4 Cd46–X Æ 16CH3NH2 Cd46–X Æ 43C6H6 Cd46–X Æ 25.5N2O4
0.19 0.33 0.46 0.49 0.50 0.58 0.60 1.05
C or Nc
Cd2+–O(2)
Reference
2.71 3.19 2.91 3.28 2.60 3.24, 3.41 2.81
2.16 2.16 2.19 2.224 2.221 2.283 2.224 2.346
[28] [29] [2] [32] [30] This work [31] [32]
a Numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. b The Cd2+ ion extends this distance into the supercage. c The coordinating atom(s) extend this distance into the supercage.
˚ [33]. Similarly, the N–C in methylamine(g), 1.474(5) A ˚. bond distance in Ca46–X Æ 16CH3NH2 [14], is 1.48(23) A 2+ Even the corresponding M –N–C angles, 135(10) and 111(7), respectively, are not significantly different. The deviations of the site-II Cd2+ ions and the N and C atoms from their 3-oxygen planes are given in Table 5. The high
Table 5 ˚ ) of cations and atoms from 6-ring planes at O(2)a Deviations (A Ca46–X Æ 16CH3NH2 Ca(2) Ca(3) Nb C
Cd46–X Æ 16CH3NH2 0.15 0.45 2.55 3.94
Cd(2) Cd(3) Nb C
0.39 0.58 2.60 3.79
a
A positive deviation indicates that the cation or atom is in the supercage. b Methylamine coordinates via N to Ca(3) and Cd(3).
esds in the methylamine geometry are a consequence of the small scattering factors of C and N, their relatively low occupancy as compared to framework positions, and especially their high thermal motion (see Figs. 2, 3 or 4). The –NH2 group of each methylamine molecule makes three weak non-equivalent hydrogen bonds to oxygen atoms of the zeolite framework; see Fig. 4. H(1) hydrogen ˚ and to O(2) at 3.05(19) A ˚ , and bonds to O(4) at 2.82(17) A ˚. H(2) hydrogen bonds to the same O(2) atom at 3.10(19) A (The hydrogen atom positions were not determined crystallographically but were calculated from the N, C, and Cd(3)
G.H. Jeong et al. / Microporous and Mesoporous Materials 90 (2006) 16–22
21
Fig. 4. A Cd(CH3NH2)2+ complex on the inner surface of a supercage is shown. Each of the 16 Cd2+ ions per unit cell (two per supercage) at Cd(3) coordinates to a methylamine molecule in this manner. H(1) approaches O(2) and O(4) to give a weak bifurcated hydrogen bond. H(2) hydrogen bonds weakly to the same O(2) atom. Hydrogen positions were not determined crystallographically; they were only calculated. The H positions and their approach distances to framework oxygens may be somewhat inaccurate because of the especially large esds at the C position.
positions.) These H O hydrogen bonds are longer than those in the Ca46–X Æ 16CH3NH2 complex: H(1) O(2), ˚ H(2) O(2) 0 , and H(2) O(4) are 2.42, 2.84, and 2.34 A in Ca46–X Æ 16CH3NH2, respectively. Also the topology of the hydrogen bonding appears to be different: it involved three framework oxygens for each –NH2 group in Ca46– X Æ 16CH3NH2, but only two here. Similar H O guestto-framework contacts are found in many organic sorption complexes of zeolites [34]. For each nitrogen position, three carbon positions are ˚ in possible. One of these gives a C–N bond 1.83(22) A length with unacceptable hydrogen bonding. A second choice of orientation for the methylamine molecule, however, is not easily dismissed; for this the C–N bond length ˚ . One amine hydrogen would participate in is 1.33(22) A ˚ to O(2), 2.7 A ˚ three electrostatic hydrogen bonds (2.9 A ˚ to another O(2)), while the other would to O(4), and 3.1 A ˚ to the first O(2). This make only one long approach, 3.3 A would be very different from the result seen for the methylamine complex of Ca–X, where only one orientation was sensible. The selection made (see Fig. 4) is similar to that reported for the methylamine complex of Ca–X, where one amine hydrogen atom hydrogen bonds to one framework oxygen atom and the other forms a bifurcated hydrogen bond to two. Kustanovich et al. [35] studied mono-, di-, and trimethylamine sorption onto ZK-5 and Y zeolites using deuterium NMR. They also observed hydrogen bonding between N–H and oxygens of the zeolite framework. (They also found alkylammonium ion formation at the zeolite’s Brønsted acid sites.) The structures of Ca46–X Æ 135NH3 [36] and Sr46– X Æ 102NH3 [37] were determined by single-crystal diffraction techniques. Each of 30 M2+ (M = Ca or Sr) ions per unit cell at site II coordinates octahedrally to three framework oxygens and to three ammonia molecules. Each of four Sr2+ ions per unit cell at site I 0 also coordinates octahedrally to three framework oxygens and to three ammonia molecules. Each of the remaining 45(3) NH3 molecules in the structure of Ca46–X Æ 135NH3 hydrogen bonds via its
lone pair to two coordinating NH3 molecules and is in position to hydrogen bond further to two or more framework oxygens. In the structure reported here and in Ca46–X Æ 16CH3NH2 [14], only one methylamine molecule (not three as with ammonia) coordinates to each coordinating cation, and only 16 of the 30 site-II cations participate in coordination. Although methylamine should be more basic than ammonia [38], the zeolite sorbs much less of it, presumably for steric and hydrogen-bonding reasons. The structure of the methylamine sorption complex of fully dehydrated, fully Ca2+-exchanged zeolite X [14] is very similar to that of the present structure. In each there are 16 cations per unit cell at site I and 30 per unit cell at site II. In each there are two types of site-II ions: 16 coordinate to methylamine molecules and 14 do not. In each the two hydrogen atoms of each –NH2 group hydrogen bond, albeit differently, to oxygen atoms of the zeolite frame˚ , appears to be work. The Cd–N bond length, 2.13(9) A ˚ , although the shorter than the Ca–N distance, 2.30(9) A difference is not significant. The Cd2+ and Ca2+ radii, ˚ , respectively, are nearly the same. 0.97 and 0.99 A Together, these values suggest, although again without significance, that the Cd2+–NH2CH3 interaction is stronger than Ca2+–NH2CH3. However neither the shorter distance nor the stronger bond should be believed; a tabulation of the bond strengths of Cd and Ca with a diverse series of other elements indicates that diatomic bonds involving Cd atoms are always much weaker than those involving Ca2+ [39]. Consistent with this expectation, the N position refined to a 3-fold-axis position in Ca46–X Æ 16CH3NH2 [14], although it should be at least somewhat off this position because CH3NH2 coordinates asymmetrically. As with Ca46–X Æ 16CH3NH2, no significant changes in the geometry of the zeolite framework, as compared to dehydrated Cd46–X [28], were seen upon sorption. It is clear from an inspection of Fig. 3 that enough volume is available in each supercage for a third and perhaps a fourth methylamine molecule to be sorbed in the same manner as the first two. This is, of course, a function of T, P, and the energy of formation of the sorption bond,
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Table 6 Number of Cd2+ ions at sites in zeolite X Zeolite X
Site I
Cd46–X (dehydrated) Cd46–X Æ 16CH3NH2 Cd46–X Æ 16NO Cd46–X Æ 25.5N2O4 Cd46–X Æ 43C6H6 Cd46–X Æ 28CO Cd46–X Æ 28C2H2 Cd46–X Æ 29.5C2H4
16 16 16 11.5 11 13 13 15.5
a b
Site I 0
Site IIa
9 6 5 5 1
16 16 25.5 27 28 28 29.5
Site IIb
Reference
30 14 14
[28] This work [32] [32] [31] [29] [2] [30]
2
Coordinates to a guest molecule. Does not coordinate to a guest molecule.
according to Irving Langmuir’s theory. In addition, however, a cooperative effect appears to be operating in zeolites. When a basic ligand coordinates to a cation, that cation moves away from its nearest framework oxygens (M–O bond lengths increase). That cation, which receives electron density from the ligand, then withdraws electron density less effectively from the basic zeolite framework. The increasingly basic zeolite framework then bonds more strongly to the remaining cations (M–O bond lengths decrease). These cations then become less acidic and less able to coordinate to additional ligands. This is generally true for intrazeolitic sorption complexes (see Table 6). Acknowledgements This work was supported by a Pusan National University Research Grant (April 1, 2003–February 28, 2006) and also by Brain Korea 21 Project, 2003. Appendix A. Supplementary data Tables of calculated structure factors squared, and observed structure factors squared with esds (14 pages). The supporting materials will be given upon your request to a corresponding author (fax: + 82 51 516 7421; e-mail: [email protected]; seff@hawaii.edu), and can also be accessed with the electronic version of this paper on ScienceDirect. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2005.08.041. References [1] K. Seff, Acc. Chem. Res. 9 (1976) 121. [2] S.B. Jang, M.S. Jeong, Y. Kim, K. Seff, Zeolites 19 (1997) 228. [3] E.Y. Choi, Y. Kim, K. Seff, Micropor. Mesopor. Mater. 40 (2000) 247.
[4] E.Y. Choi, Y. Kim, S.H. Song, Bull. Korean Chem. Soc. 20 (1999) 791. [5] W.M. Cruz, P.C.W. Leung, K. Seff, Inorg. Chem. 18 (1979) 230. [6] Y.H. Yeom, Y. Kim, K. Seff, J. Phys. Chem. 100 (1996) 8373. [7] S.B. Jang, Y. Kim, K. Seff, Zeolites 14 (1994) 262. [8] R.Y. Yanagida, A.A. Amaro, K. Seff, J. Phys. Chem. 77 (1973) 805. [9] Y. Kim, K. Seff, J. Am. Chem. Soc. 97 (1977) 7057. [10] Y.C. Long, H. Yang, Y.J. Sun, Z.H. Ping, Peop. Rep. China. Gaodeng Xuexiao Huaxue Xuebao 22 (2001) 365. [11] F. Docquir, H. Toufar, B.L. Su, Langmuir 17 (2001) 6282. [12] F. Docquir, V. Norberg, H. Toufar, J.L. Paillaud, B.L. Su, Langmuir 18 (2002) 5963. [13] K. Morishige, S. Kittaka, S. Ihara, J. Chem. Soc. Faraday Trans. 1 (18) (1985) 2525. [14] G.H. Jeong, Y. Kim, K. Seff, Langmuir 20 (2004) 9354. [15] V.N. Bogomolov, V.P. Petranovskii, Zeolites 6 (1986) 418. [16] W. Loewenstein, Am. Miner. 39 (1954) 92. [17] D. Bae, K. Seff, Micropor. Mesopor. Mater. 40 (2000) 233. [18] International Tables for X-ray Crystallography, Kynoch Press, Birmingham, UK, vol. II 1944, p. 302. [19] Y.H. Yeom, Y. Kim, K. Seff, J. Phys. Chem. B 101 (1997) 5314. [20] G.M. Sheldrick, SHELX-97: Program for the Refinement of Crystal Structures, University of Go¨ttingen, Go¨ttingen, Germany, 1997. [21] E.Y. Choi, Y. Kim, K. Seff, J. Phys. Chem. B 106 (2002) 5827. [22] International Tables for X-ray Crystallography, Kynoch Press, Birmingham, UK, vol. IV 1974, pp. 73–87. [23] International Tables for X-ray Crystallography, Kynoch Press, Birmingham, UK, vol. IV 1974, pp. 149–150. [24] J.V. Smith, in: E.M. Flanigen, L.B. Sand (Eds.), Molecular Sieve Zeolites-I, Advances in Chemistry Series, 101, American Chemical Society, Washington, DC, 1971, pp. 171–200. [25] Y.H. Yeom, S.B. Jang, S.H. Song, Y. Kim, K. Seff, J. Phys. Chem. B 101 (1997) 6914. [26] M.K. Song, Y. Kim, K. Seff, J. Phys. Chem. B 107 (2003) 3117. [27] Handbook of Chemistry and Physics, 70th ed., The Chemical Rubber Co., Cleveland, Ohio, 1989/1990, p. F-187. [28] J.H. Kwon, S.B. Jang, Y. Kim, K. Seff, J. Phys. Chem. B 100 (1996) 13720. [29] S.B. Jang, J.H. Kwon, S.H. Song, Y. Kim, K. Seff, in: H. Chon, S.K. Ihm, Y.S. Uh (Eds.), Progress in Zeolite and Microporous Materials: Studies in Surface Science and Catalysis, 105, Elsevier, Amsterdam, 1997, p. 1819. [30] Y.H. Yeom, S.H. Song, Y. Kim, K. Seff, J. Phys. Chem. B 101 (1997) 2138. [31] Y. Kim, Y.H. Yeom, E.Y. Choi, A.N. Kim, Y.W. Han, Bull. Korean Chem. Soc. 19 (1998) 1222. [32] Y.M. Lee, Y. Kim, K. Seff, J. Phys. Chem. B 109 (2005) 4900. [33] Handbook of Chemistry and Physics, 70th ed., The Chemical Rubber Co., Cleveland, Ohio, 1989/1990, p. F-156. [34] S. Zhen, K. Seff, Micropor. Mesopor. Mater. 39 (2000) 1. [35] I. Kustanovich, Z. Luz, S. Vega, A.J. Vega, J. Phys. Chem. 94 (1990) 3138. [36] S.B. Jang, M.S. Jeong, Y. Kim, K. Seff, Micropor. Mesopor. Mater. 28 (1999) 173. [37] M.J. Kim, M.S. Jeong, Y. Kim, K. Seff, Micropor. Mesopor. Mater. 30 (1999) 233. [38] J.E. Huheey, Inorganic Chemistry, second ed., Harper & Row, New York, 1978, p. 265. [39] Handbook of Chemistry and Physics, 77th ed., The Chemical Rubber Co., Cleveland, Ohio, 1996/1997, Section 12, p. 14.