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Crystal Structure of a Methylamine Sorption Complex of Fully Dehydrated Fully Ca2+-Exchanged Zeolite X, |Ca46(CH3NH2)16|[Si100Al92O384]-FAU Gyoung Hwa Jeong and Yang Kim* Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Pusan 609-735, Korea
Karl Seff* Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822 Received May 10, 2004. In Final Form: July 14, 2004 The structure of a methylamine sorption complex of fully dehydrated fully Ca2+-exchanged zeolite X, |Ca46(CH3NH2)16|[Si100Al92O384]-FAU, has been determined in the cubic space group Fd3 h at 21(1) °C (a ) 24.994(4) Å) by single-crystal X-ray diffraction techniques. The crystal was prepared by ion exchange in a flowing stream of 0.05 M aqueous Ca(NO3)2 for 3 days, followed by dehydration at 480 °C and 2 × 10-6 Torr for 2 days, and exposure to 160 Torr of zeolitically dry methylamine gas at 21(1) °C. The structure was determined in this atmosphere and was refined, using the 739 reflections for which I > 0, to the final error indices R1 ) 0.152 and R2 ) 0.061. In this structure, Ca2+ ions occupy three crystallographic sites. Sixteen Ca2+ ions fill the octahedral site at the centers of hexagonal prisms (Ca-O ) 2.429(7) Å). The remaining 30 Ca2+ ions are found at two nonequivalent sites II (in the supercages) with occupancies of 14 and 16 ions. Each of these Ca2+ ions coordinates to three framework oxygens, either at 2.296(7) or 2.334(7) Å, respectively. Sixteen methylamine molecules have been sorbed per unit cell, two per supercage. Each coordinates to one of the latter 16 site-II Ca2+ ions: N-Ca ) 2.30(9) Å. The imprecisely determined N-C bond length, 1.48(23) Å, differs insignificantly from that in methylamine(g), 1.474(5) Å. The positions of the hydrogen atoms were calculated. One of the amino hydrogen atoms hydrogen bonds to a 6-ring oxygen, and the other forms a bifurcated hydrogen bond to two other 6-ring oxygens. The methyl group does not hydrogen bond to anything.
Introduction Zeolites are high-capacity sorbents because of their high intracrystalline surface area and the strong interactions that they can have with sorbates, especially polar sorbates. A knowledge of molecule-zeolite and molecule-molecule interactions is crucial to understanding the diffusion and sorption processes. These interactions depend on many factors such as the kinds of cations present, their distribution over the available sites, and their interactions with the sorbed molecules.1 The interaction between sorbed methylamine and |Na31|[Si161Al31O384]-FAU was investigated with powder X-ray diffraction (XRD), 29Si and 13C magic-angle spinning (MAS) NMR, and Fourier-transform infrared (FTIR) spectroscopy.2 As methylamine was loaded into the zeolite, one peak split into four peaks in the 29Si MAS NMR spectrum and the high-resolution IR spectrum became simpler. This indicated that the crystal structure of the zeolite had changed significantly and that the interaction between sorbed methylamine and the zeolite was strong.2 Recently Su et al.,3,4 using IR methods, reported that there are three types of interactions between methylamine and a series of alkaline-earth cation exchanged faujasite (FAU) zeolites. The primary one is the electrostatic (1) Breck, D. W. Zeolite Molecular Sieves; Robert E. Krieger: Malabar, FL, 1974. (2) Long, Y. C.; Yang, H.; Sun, Y. J.; Ping, Z. H. Gaodeng Xuexiao Huaxue Xuebao 2001, 22, 365. (3) Docquir, F.; Toufar, H.; Su, B. L. Langmuir 2001, 17, 6282. (4) Docquir, F.; Norberg, V.; Toufar, H.; Paillaud, J. L.; Su, B. L. Langmuir 2002, 18, 5963.
interaction between the lone electron pair on the nitrogen atom and the alkaline-earth cations. The other two involved hydrogen bonding between the oxygen atoms of the zeolite and the hydrogen atoms of the methylamine molecules; these were assigned to NH‚‚‚O and to CH‚‚‚O interactions. They found that the interaction between methylamine and the zeolite depends primarily on the Lewis acidity (size) of the alkaline-earth cation, with a lesser dependency on the Si/Al ratio. A deformation of the zeolite framework due to the sorption of methylamine was observed. Their work also showed clearly that methylamine can be an efficient probe molecule for the characterization of the acid-base properties of zeolites. This work was done to locate the methylamine molecules sorbed in Ca46-X at ambient temperature, to observe their interactions with the zeolite framework and cations, and to determine the framework distortions and cation shifts upon sorption. Experimental Section Large single crystals of zeolite Na-X, with stoichiometry |Na92|[Si100Al92O384]-FAU, were prepared in St. Petersburg, Russia.5 One of these, a colorless octahedron about 0.2 mm in cross section, was lodged in a fine Pyrex capillary. Aqueous 0.05 M Ca(NO3)2 (Aldrich, 99.997%) with pH > 6.0 was allowed to flow past the crystal at a velocity of 1.0 cm/s for 3 days. This procedure had previously been shown to give complete Ca2+ exchange.6 In that work, 45.4(2) Ca2+ ions were (5) Bogomolov, V. N.; Petranovskii, V. P. Zeolites 1986, 6, 418. (6) Yeom, Y. H.; Jang, S. B.; Song, S. H.; Kim, Y.; Seff, K. J. Phys. Chem. B 1997, 101, 6914.
10.1021/la040073m CCC: $27.50 © 2004 American Chemical Society Published on Web 09/11/2004
Structure of a Methylamine Sorption Complex
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Table 1. Positional, Thermal, and Occupancy Parametersa Wyckoff cation atom position site Si Al O(1) O(2) O(3) O(4) Ca(1) Ca(2) Ca(3) N C H(1)e H(2)e H(3)e H(4)e H(5)e
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)
I II II
x
y
-540(1) -567(1) -1131(3) -42(4) -360(3) -597(3) 0 2206(3) 2274(2) 3008(37) 3486(80) 2951 3064 3425 3546 3836
1229(2) 366(1) 5(3) -46(3) 664(3) 797(3) 0 2206(3) 2274(2) 2475(38) 2776(85) 2154 2351 2909 3122 2517
z
U11b or Uisod
337(1) 27(6) 1216(2) 33(6) 1071(3) 759(54) 1422(3) 782(48) 611(3) 622(47) 1741(3) 945(57) 0 23(13) 2206(3) 135(25) 2274(2) 192(25) 2793(40) 982(402) 2976(91) 1028(1185) 3032 165f 2413 165f 3387 165f 2718 165f 2957 165f
occupancyc U22
685(48) 746(47) 663(51) 749(51) 23(13)
U33
U12
U13
U23
varied constr fixed
600(48) 57(49) 148(38) -59(44) 545(46) 43(47) 20(44) -23(44) 669(50) -10(45) 51(45) 55(43) 663(49) -47(53) 181(53) -176(46) 23(13) -2(16) -2(16) -2(16) 14.1(4) 17.2(7) 17(2) 19(3)
13.4(3) 16.6(3) 16.6(3) 16.6(3)
96 96 96 96 96 96 16 14 16 16 16
a a ) 24.994(4) Å, space group Fd3 h , origin at center of symmetry. Fractional (fractions of the unit-cell edge length) positional and thermal parameters are given ×104. Numbers in parentheses are the estimated standard deviations (esd’s) in the units of the least significant digit given for the corresponding parameter. b The anisotropic temperature factor ) exp[(-2π2/a2)(U11h2 + U22k2 + U33l2 + U12hk + U13hl + U23kl)]; units of Uij are Å2. c Occupancy factors are given as the number of atoms or ions per unit cell. d Biso ) 8π2Uiso; units of Uiso are Å2. e Hydrogen atom positions were calculated. f Recommended by MolEN (ref 10).
found in the final refinement, consistent with complete Ca2+ exchange (46 Ca2+ ions). The capillary containing the crystal was attached to a vacuum system, and the crystal was cautiously dehydrated by gradually increasing its temperature (ca. 30 °C/h) to 480 °C under a dynamic vacuum. Finally, the system was maintained at 480 °C and 2 × 10-6 Torr for 2 days. After cooling to room temperature, the crystal remained colorless. During this process, the crystal was protected from rehydration from the adjacent (not baked out) section of the vacuum system by an in-series 8-cm tube of zeolite 5A beads that had been fully dehydrated in situ. To prepare the methylamine complex, the crystal was treated with 160 Torr of zeolitically dry methylamine gas (Aldrich, 99.995%) 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 Fd3 h was used throughout this work. This choice appeared to be appropriate because most crystals made from this synthesis batch have been successfully refined in Fd3 h with mean Al-O distances correctly longer than mean Si-O distances.7 That is true for this crystal also, verifying that Fd3 h is correct. Diffraction data were collected with an automated EnrafNonius four-circle computer-controlled CAD-4 diffractometer equipped with a pulse-height analyzer and a graphite monochromator, using Mo radiation (KR1, λ ) 0.70930 Å; KR2, λ ) 0.71359 Å). The cubic unit cell constant at 21(1) °C, determined by least-squares refinement of 25 intense reflections for which 14° < 2θ < 22°, is a ) 24.994(4) Å. All unique reflections in the positive octant of an F-centered unit cell for which 2θ < 50°, l > h, and k > h were recorded. Of the 1241 unique reflections examined, only 739 reflections for I > 0 were used in subsequent structure determination and refinement. An absorption correction (µ ) 1.97 mm-1, F ) 1.455 g/cm3, and F(000) ) 7184) was made empirically using a ψ scan.8 The calculated transmission coefficients ranged from 0.985 to 0.995. This correction had little effect on the final R indices. Other details are the same as previously reported.9
Structure Determination Full-matrix least-squares refinement was initiated by using the atomic parameters of the framework atoms [Si, Al, O(1), O(2), O(3), and O(4)] in dehydrated Ca46-X.6 Isotropic refinement converged to an unweighted R1 in(7) Bae, D.; Seff, K. Microporous Mesoporous Mater. 2001, 42, 299. (8) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1944; Vol. II, p 302. (9) Yeom, Y. H.; Kim, Y.; Seff, K. J. Phys. Chem. B 1997, 101, 5314.
dex, (∑(Fo - |Fc|)/∑Fo), of 0.37 and a weighted R2 index, (∑w(Fo - |Fc|)2/∑wFo2)1/2, of 0.35. A difference Fourier function revealed two large peaks at (0.0, 0.0, 0.0) and (0.223, 0.223, 0.223) with heights of 19.4 and 14.5 eÅ-3, respectively. Isotropic refinement including these as Ca2+ ions at Ca(1) and Ca(3) converged to R1 ) 0.161 and R2 ) 0.070 with occupancies of 16.3(2) and 30.9(3), respectively. The thermal ellipsoids at Ca(3) became very elongated in subsequent anisotropic refinement, indicating the presence of two nonequivalent Ca2+ ions at this position. It was allowed to refine as two adjacent 3-fold-axis positions, Ca(2) and Ca(3), which refined to (0.220, 0.220, 0.220) and (0.227, 0.227, 0.227) with occupancies of 14.3(2) and 16.6(3), respectively. The occupancy at Ca(1) was fixed at 16 ions, its maximum value, and the error indices converged to R1 ) 0.159 and R2 ) 0.070. An ensuing Fourier function revealed two more peaks at the general positions (0.300, 0.252, 0.271) and (0.338, 0.264, 0.315) which were stable in least-squares refinement. The position closer to the Ca2+ ions at Ca(3) was refined as nitrogen, and the other as carbon. In support of this assignment, the occupancies at N and C were nearly equal in subsequent least-squares refinement. With the occupancies at Ca(3), N, and C constrained to be equal, the refinement converged with 16.3(3) ions or atoms at each position. Isotropic refinement of the framework atoms, all Ca2+ ions, N, and C converged to R1 ) 0.154 and R2 ) 0.064. Finally, the occupancies at Ca(2) and Ca(3), and therefore N and C, were fixed at the values shown in Table 1 so that the sum of the cationic charges would be 92+ per unit cell. When the anisotropic refinement of the framework atoms was attempted, the Si and Al positions became nonpositive definite. Accordingly, Si and Al were refined only isotropically. The anisotropic refinement of O(1), O(2), O(3), O(4), and Ca(1) and isotropic refinement of Si, Al, Ca(2), Ca(3), N, and C converged to R1 ) 0.150 and R2 ) 0.057. The positions of the five hydrogen atoms of the methylamine molecule were calculated using N-H ) 1.01 Å and C-H ) 1.09 Å. With these positions fixed, further isotropic refinement of N and C was unsuccessful, whether the hydrogen Uiso values were set equal to those of the atoms to which they are bound (N and C) or to much smaller values as suggested by MolEN.10 (The nitrogen
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Table 2. Selected Interatomic Distances (Å) and Angles (deg)a Si-O(1) Si-O(2) Si-O(3) Si-O(4) mean Al-O(1) Al-O(2) Al-O(3) Al-O(4) mean Ca(1)-O(3) Ca(2)-O(2) Ca(3)-O(2) Ca(3)-N N-C
1.597(8) 1.630(9) 1.633(8) 1.594(9) 1.614 1.712(8) 1.747(9) 1.763(8) 1.700(9) 1.731 2.429(7) 2.296(7) 2.334(7) 2.30(9) 1.48(23)
N‚‚‚O(2) N‚‚‚O(2)′ c N‚‚‚O(4) H(1)‚‚‚O(2) H(2)‚‚‚O(2)′ c H(2)‚‚‚O(4)
2.94(9) 3.62(10) 3.27(10) 2.42b 2.84b 2.34b
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) 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) Si-O(1)-Al Si-O(2)-Al Si-O(3)-Al Si-O(4)-Al
111.3(5) 108.7(4) 113.2(4) 107.0(4) 103.4(4) 113.0(4) 111.7(4) 106.4(4) 117.6(4) 106.3(4) 100.3(4) 114.0(4) 125.8(5) 142.8(5) 130.1(5) 168.3(6)
O(3)-Ca(1)-O(3) O(2)-Ca(2)-O(2) O(2)-Ca(3)-O(2)
93.3(2)/86.8(2) 119.6(3) 116.4(3)
Ca(3)-N-C
111(7)
a
Numbers in parentheses are in the units of the least significant digit given for the corresponding value. b Hydrogen positions were calculated. N-H ) 1.01 Å; C-H ) 1.09 Å. c Related by symmetry 3 to O(2).
position was stable in least-squares refinement, but the carbon position was not.) The final error indices converged to R1 ) 0.152 and R2 ) 0.061. (For comparison, the final error indices for the 329 reflections for which I > 3σ(I) were R1 ) 0.060 and R2 ) 0.066.) The shifts in the final cycles of least-squares refinement were all less than 0.1% of their corresponding standard deviations. All crystallographic calculations were done using MolEN,10 a structure determination program package supplied by Enraf-Nonius. Atomic scattering factors11,12 for Si, Al, O-, Ca2+, N, and C were used. All scattering factors were modified to account for anomalous dispersion.13 The final structural parameters are listed in Table 1, and selected interatomic distances and angles are presented in Table 2. Discussion Zeolite X is a synthetic Al-rich analogue of the naturally occurring mineral faujasite. The sodalite cavity (or β cage), a 14-hedron with 24 vertexes, may be viewed as the principal building block of the aluminosilicate framework of the zeolite (see Figure 1). These β cages are connected tetrahedrally at 6-rings by bridging oxygens to give double 6-rings (D6Rs, hexagonal prisms) and, concomitantly, an interconnected set of even larger cavities (supercages) accessible in three dimensions through 12-ring (24membered) windows. (An n-ring consists of n oxygen atoms and n T (Si or Al) atoms.) The T atoms occupy the vertexes of these polyhedra. The oxygen atoms lie approximately midway between each pair of T atoms but are displaced from those points to be closer to Si (farther from Al) and to give near tetrahedral angles about the T atoms. Single 6-rings (S6Rs) are shared by sodalite and supercages and may be viewed as the entrances to the sodalite cavities. (10) MolEN, a structure determination package supplied by EnrafNonius: Netherlands, 1990. (11) Cromer, D. T. Acta Crystallogr. 1965, 18, 17. (12) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV, pp 73-87. (13) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV, p 149.
Figure 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 a silicon atom substitutes for aluminum at about 4% of the Al positions. Extraframework cation positions are labeled with Roman numerals.
Figure 2. Stereoview of a double 6-ring. The Ca2+ ion at Ca(1) is at site I. Each D6R contains a Ca2+ ion. Ellipsoids of 20% probability are used.
Each unit cell has 8 sodalite cavities, 8 supercages, 16 D6Rs, 16 12-rings, and 32 S6Rs. The exchangeable cations that balance the negative charge of the aluminosilicate framework are found within the zeolite’s windows and cavities. They are usually found at the following sites shown in Figure 1: site I at the center of a D6R, site I′ in the sodalite (β) cavity on the opposite side of either of the D6R’s 6-rings from site I, site II′ inside the sodalite cavity near a single 6-ring (S6R) entrance to the supercage, site II in the supercage adjacent to a S6R, site III in the supercage opposite a sodalite 4-ring (on a 2-fold axis), and site III′ near the inner surface of the supercage somewhat or substantially off that 2-fold axis. The ordering of the Si and Al atoms in the lattice (and hence the choice of space group) is confirmed by the average values of the Si-O (1.614 Å) and Al-O (1.731 Å) distances. These averaged values are not sensitive to ion exchange or dehydration. The individual bond lengths, however, show marked variations: Si-O from 1.597(8) to 1.633(8) Å and Al-O from 1.700(9) to 1.763(8) Å (see Table 2). The individual Si-O and Al-O distances depend on Ca2+ coordination to framework oxygen. O(1) and O(4) are not involved in coordination; the Ca2+ ions (at sites I and II) coordinate only to O(2) and O(3) in this structure and, as a consequence of these interactions, the Al-O(2), Si-O(2), Al-O(3), and Si-O(3) bonds are somewhat lengthened (see Table 2). This effect is commonly seen in fully divalentcation-exchanged zeolite X.6,14 (14) Jang, S. B.; Jeong, M. S.; Kim, Y.; Seff, K. J. Phys. Chem. B 1997, 101, 9041.
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Figure 3. Stereoview of a sodalite cavity. Two Ca2+ ions at Ca(2) (site II) and two Ca2+ ions at Ca(3) (site II) are shown. Each Ca2+ ion at Ca(3) coordinates to a methylamine molecule. Six of the eight sodalite cavities per unit cell have this arrangement. The remaining two have only one Ca2+ ion at Ca(2) (site II). To simplify this drawing, artificially small values of U, 0.0200 Å2, are used for the five hydrogen atoms. Otherwise ellipsoids of 20% probability are used.
Figure 4. Stereoview of a supercage. Two Ca2+ ions at Ca(2) (site II) are shown. The two Ca2+ ions shown at Ca(3) (another site II position) each coordinate to a methylamine molecule. Two of the eight supercages per unit cell have only one Ca2+ ion at Ca(2). To simplify this drawing, artificially small values of U, 0.0200 Å2, are used for the five hydrogen atoms. Otherwise ellipsoids of 20% probability are used.
In |Ca46(CH3NH2)16|[Si100Al92O384]-FAU, all Ca2+ ions are found at three crystallographic sites, all with high occupancy. Sixteen Ca2+ ions at Ca(1) fill site I at the center of the D6Rs as they did in empty Ca46-X6 (see Figure 2). The octahedral Ca(1)-O(3) distance, 2.429(7) Å, is a little longer than the sum of the conventional ionic radii of Ca2+ and O2-, 0.99 + 1.32 ) 2.31 Å,15 indicating a reasonably good fit. In empty Ca46-X, the Ca(1)-O(3) distance is the same, 2.429(8) Å.6 The 14 cations at Ca(2) and the 16 at Ca(3) are located at two different sites II in the supercage (see Figures 3 and 4); each Ca2+ ion coordinates at 2.296(7) and 2.334(7) Å, respectively, to three O(2) framework oxygens. The O(2)-Ca(2)-O(2) angle, 119.6(3)°, is essentially trigonal planar, while the O(2)-Ca(3)-O(2) angle, 116.4(3)°, is somewhat shifted toward tetrahedral, a result of coordination to CH3NH2 (see Table 2). Because different O(2) positions for these situations have not been resolved, only the average O(2) coordinates were determined and used. Each of the 16 Ca2+ ions at Ca(3) complexes to a CH3NH2 molecule (see Figures 3 and 4). To do so, each Ca(3) ion has moved 0.15 Å further into the supercage, further from the planes of their triads of three O(2) oxygens (from 0.30 to 0.45 Å) as compared to the corresponding Ca2+ position in dehydrated Ca46-X6 (see Tables 3 and 4). Similarly, in the crystal structures of the ethylene,16 acetylene,16 cyclopropane,17 benzene,18 mesitylene,19 and (15) Handbook of Chemistry and Physics, 70th ed.; The Chemical Rubber Co.: Cleveland, OH, 1989/1990; p F-187. (16) Jang, S. B.; Jeong, M. S.; Kim, Y. J. Phys. Chem. B 1997, 101, 3091. (17) Choi, E. Y.; Kim, Y.; Song, S. H. Bull. Korean Chem. Soc. 1999, 20, 791.
Table 3. Deviations (Å) of Cations and Atoms from 6-Ring Planes at O(3)a at O(2)b
cation or atom
cation site
deviation (Å)
Ca(1) Ca(2) Ca(3) N C
I II II
1.321 0.154 0.450 2.55 3.94
a Ca(1) is at the center of a D6R. b A positive deviation indicates that the cation or atom is in the supercage. Only a single site-II Ca2+ position was seen in fully dehydrated Ca46-X (ref 6); its deviation from its 6-ring plane at O(2) is 0.30 Å.
ammonia20 sorption complexes of fully dehydrated fully Ca2+-exchanged zeolite X, the Ca2+ ions at site II had shifted 0.11, 0.12, 0.17, 0.25, 0.28, and 0.57 Å, respectively, further into the supercage as compared to the corresponding position in dehydrated Ca46-X6 (see Table 4). As the Ca(3) ions moved further away, each Ca(2) ion moved 0.15 Å, the same distance, closer to the plane of its three O(2) oxygens as compared to the site-II position in dehydrated Ca46-X6 (see Tables 3 and 4). This movement tends to preserve a balance of charge with respect to the surface of the sodalite cavity. It is this action that may be responsible for the limited sorption of methylamine; further sorption would require an unreasonable relocation of Ca2+ charge into the supercage and away from the surface of the (anionic) sodalite cavity. (18) Yeom, Y. H.; Kim, A. N.; Kim, Y.; Song, S. H.; Seff, K. J. Phys. Chem. B 1998, 102, 6071. (19) Choi, E. Y.; Kim, Y.; Seff, K. J. Phys. Chem. B 2002, 106, 5827. (20) Jang, S. B.; Jeong, M. S.; Kim, Y.; Seff, K. Microporous Mesoporous Mater. 1999, 28, 173.
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Figure 5. A Ca(NH2CH3)2+ complex on the inner surface of a supercage is shown. Each of the 16 Ca2+ ions per unit cell (2 per supercage) at Ca(3) coordinates to a methylamine molecule in this manner. H(1) hydrogen bonds to O(2). H(2) hydrogen bonds primarily to O(4), but it has a longer (weaker) interaction with O(2)′ to give a bifurcated hydrogen bond. (The hydrogen positions were not determined crystallographically; they were only calculated.) To simplify this drawing, artificially small values of U, 0.0200 Å2, are used for the five hydrogen atoms. Otherwise ellipsoids of 20% probability are used. Table 4. Deviations (Å) of Ca2+ Ionsa and Carbon or Nitrogen Atoms from 6-Ring Planes at O(2) and Ca2+-O(2) Distances zeolite X
Ca2+ b
C or Nb
Ca46-X (dehydrated) Ca46-X‚30C2H4 Ca46-X‚30C2H2 Ca46-X‚16CH3NH2 Ca46-X‚30C3H6 Ca46-X‚28C6H6 Ca46-X‚8C9H12 Ca46-X‚135NH3
0.30 0.41 0.42 0.45 0.47 0.55 0.58 0.87c
3.28 3.19 2.55, 3.94 3.42 3.29, 3.31 3.10, 3.47, 3.77 2.94c
Ca2+-O(2) reference 2.276 2.316 2.318 2.334 2.311 2.318 2.332 2.382
6 16 16 this work 17 18 19 20
a Only those Ca2+ ions which coordinate to guest molecules, except for Ca46-X(dehydrated) which has none. b The ion or atom extends this distance into the supercage. c Ca2+ is octahedrally coordinated; three ammonia molecules coordinate to each Ca2+.
The Ca-N coordination bond length, 2.30(9) Å, is reasonable. It can be compared with the eight values, ranging from 2.57 to 2.63 Å, reported for the primary amine nitrogen atoms in some 8-coordinate complexes of Ca2+ containing the ligand bis(2-aminoethyl)amine (dien).21 By comparison with other cations (Mg2+, Mn2+, and Pb2+, but also see Ag+, Cu+, and Ni2+), the amine coordination distance should be about 0.3 Å shorter for 4-coordinate Ca2+ than 2.60 Å for 8-coordinate Ca2+.22 The low coordination number 4 is rarely seen in complexes of Ca2+. The imprecisely determined C-N bond distance, 1.48(23) Å, is insignificantly different from that in methylamine(g), 1.474(5) Å,23 and the Ca(3)-N-C angle, 111(7)°, is essentially tetrahedral, as would be expected because of the nitrogen lone pair. The closest approach of methylamine to the zeolite framework involves N‚‚‚O(2) at distances of 2.94(9) and 3.00(10) Å and N‚‚‚O(4) at 3.27(10) Å. H(1)‚‚‚O(2) ) 2.42 Å, H(2)‚‚‚O(2)′ ) 2.84 Å, and H(2)‚‚‚O(4) ) 2.34 Å. (The hydrogen atom positions were not determined crystallographically but were calculated from the Ca(3), N, and C positions.) These distances and the angles at H(1) and H(2) (see Figure 5) indicate the presence of two nonequivalent hydrogen bonds, a relatively direct hydrogen bond between one hydrogen atom of the NH2 group and a 6-ring oxygen and a bifurcated hydrogen bond between the other amine hydrogen atom and two 6-ring oxygens. These H‚‚‚O guest-to-framework contacts are also found in many organic sorption complexes of zeolites.24 (21) Waters, A. F.; White, A. H. Aust. J. Chem. 1996, 49, 87-98. (22) Handbook of Chemistry and Physics, 80th ed.; CRC Press: Boca Raton, FL, 1999-2000; p 12-14. (23) Handbook of Chemistry and Physics, 70th ed.; CRC Press: Boca Raton, FL, 1989-1990; p F-156. (24) Zhen, S.; Seff, K. Microporous Mesoporous Mater. 2000, 39, 1.
As reported by Su et al.,3,4 methylamine interacts not only with a cation via the lone electron pair on the nitrogen atom but also with the anionic oxygen atoms of the zeolite framework by hydrogen bonding. However, contrary to Su et al.’s interpretation of their experimental measurements,3,4 methyl hydrogen atoms are far from framework oxygens; the shortest methyl-H to O distance, H(5) to O(4), is 4.1 Å (see Figure 5), so there are no CH‚‚‚O interactions. Instead, consistent with their measurements, there are two kinds of NH‚‚‚O hydrogen bonds, one bifurcated and the other not. A simple explanation for the nonequivalence of the two NH‚‚‚O hydrogen bonds can be seen in Figure 5. N-H(1) hydrogen bonds to O(2) at 2.42 Å. N-H(2) cannot reach O(2)′ to make an equivalently short hydrogen bond; this distance is 2.84 Å. Instead, N-H(2) hydrogen bonds primarily to O(4) at 2.34 Å. Some concerted change in the geometry of the zeolite framework is to be expected upon the sorption of methylamine. However, the small differences between the framework geometry of dehydrated Ca46-X6 and the complex reported here cannot be considered significant. The largest differences between the two structures, respectively, are as follows: Si-O(2), 1.654(7) and 1.630(9) Å; Si-O(4), 1.623(7) and 1.594(9) Å; O(1)-Si-O(2), 112.7(4)° and 111.3(5)°; and Si-O(2)-Al, 140.8(4)° and 142.8(5)°. More significant changes are seen in the Ca2+to-oxygen distances, including Ca(3)-O(2) which increased from 2.276(5) to 2.334(7) Å. The Ca(3) ions and CH3NH2 molecules have been distributed evenly among the supercages, two of each per supercage, in Figure 4. This allows for a more locally balanced distribution of Ca2+ charge with respect to the surface of the sodalite cavity (and the supercage). These crystallographic results do not, however, require that. It remains possible that some supercages have up to 4 Ca(3) ions and methylamine molecules and others fewer or none, averaging to exactly 2 each. The most reasonable alternative structure, also consistent with the sorption of exactly 16 CH3NH2 molecules per unit cell, would have 4 Ca(3) ions and 4 CH3NH2 molecules in alternating supercages and none in the remainder. However, the supercages with 4 CH3NH2 molecules would not represent a “condensation” of methylamines; the C atoms would be too far apart (5.4 Å; the van der Waals radius of a methyl group is ca. 2.0 Å25). Closer approaches between pairs of methyl groups are possible with either model but are not expected. Recently the structures of Ca46-X‚135NH3 20 and Sr46-X‚102NH3 26 were determined by single-crystal dif(25) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1980; p 260.
Structure of a Methylamine Sorption Complex
Langmuir, Vol. 20, No. 21, 2004 9359
fraction techniques. Each of 30 M2+ (M ) Ca or Sr) ions per unit cell at site II coordinates octahedrally to 3 framework oxygens and to 3 ammonia molecules. Each of 4 Sr2+ ions per unit cell at site I′ also coordinates octahedrally to 3 framework oxygens and to 3 ammonia molecules. Each of the remaining 45(3) NH3 molecules in the structure of Ca46-X‚135NH3 hydrogen bonds via its lone pair to 2 coordinating NH3 molecules and is in position to associate weakly with 2 or more framework oxygens. In the structure reported here, only 1 methylamine molecule (not 3 as with ammonia) coordinates to each coordinating Ca2+ ion, and only 16 of the 30 site-II Ca2+ ions participate in coordination. Although methylamine should be more basic than ammonia,27 the zeolite sorbs much less of it, presumably for steric and hydrogenbonding reasons.
In summary, all Ca2+ ions are found at sites I and II: 16 per unit cell at site I and 30 per unit cell at site II. There are two types of site-II Ca2+ ions: 16 coordinate to methylamine molecules and 14 do not. The two hydrogen atoms of each NH2 group hydrogen bond differently to framework oxygen atoms.
(26) Kim, M. J.; Jeong, M. S.; Kim, Y.; Seff, K. Microporous Mesoporous Mater. 1999, 30, 233.
(27) Huheey, J. E. Inorganic Chemistry, 2nd ed.; Harper & Row: New York, 1978; p 265.
Acknowledgment. This work was supported by a Pusan National University Research Grant (April 1, 2003February 28, 2006) and also by the Brain Korea 21 Project, 2003. Supporting Information Available: A table of calculated and observed structure factors. This material is available free of charge via the Internet at http://pubs.acs.org. LA040073M
