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Crystal structure of an ethylene sorption complex of fully dehydrated, fully oxidized, fully Ag+-exchanged zeolite X
Microporous and Mesoporous Materials 62 (2003) 201–210 www.elsevier.com/locate/micromeso
Crystal structure of an ethylene sorption complex of fully dehydrated, fully oxidized, fully Agþ-exchanged zeolite X Eun Young Choi a, Soo Yeon Kim a, Yang Kim a
a,*
, Karl Seff
b,*
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Pusan 609-735, Republic of Korea b Department of Chemistry, University of Hawaii at Manoa, 2545 The Mall, Honolulu, HI 96822-2275, USA Received 20 January 2003; received in revised form 7 May 2003; accepted 7 May 2003
Abstract The crystal structure of an ethylene sorption complex of fully dehydrated, fully oxidized, fully Agþ -exchanged zeolite ), jAg92 (C2 H4 )27 j [Al92 Si100 O384 ]-FAU, has been determined by singleX, (Agþ )92 Si100 Al92 O384 Æ 27C2 H4 (a ¼ 24:981(9) A crystal X-ray diffraction techniques in the cubic space group Fd3 at 21 C. Ion exchange was accomplished by allowing 0.05 M aqueous AgNO3 to flow past a crystal for 3 days to give hydrated Ag92 Si100 Al92 O384 . This crystal was dehydrated at 400 C for 2 days followed by cooling to 21 C, both in a flowing stream of zeolitically dry oxygen gas (790 Torr), followed by evacuation at 2 · 106 Torr for 20 min. To prepare the ethylene sorption complex, this colorless crystal was treated at 21 C with 300 Torr of zeolitically dry ethylene gas for 2 h. The structure of the resulting dark yellow crystal was determined in this atmosphere and was refined to the final error index wR2 based on F 2 and all data (1155) of 0.293; R1 ¼ 0:069 for the 221 reflections for which F0 > 4r ðF0 Þ. In this structure, per unit cell, 12 Agþ ions were found at the ) and 16 Agþ ions occupy the nearby site-I0 positions (Ag–O ¼ 2.47(3) A ). A sigoctahedral site I (Ag–O ¼ 2.63(2) A interaction must occur between the Agþ ions at sites I and I0 ; linear (Ag3 )3þ is proposed. Thirty-two nificant 2.951-A ) and 5 at site II0 (Ag–O ¼ 2.30(4) A ). Each of the 27 site-II Agþ ions fill the single 6-rings, 27 at site II (Ag–O ¼ 2.41(2) A þ Ag ions extends 1.08 A into the supercage to form a strong lateral p-complex with an ethylene molecule. In turn, each C2 H4 molecule forms two electrostatic hydrogen bonds to framework oxygens. The remaining 32 Agþ ions occupy four different III0 sites. The Agþ site occupancies changed substantially upon the sorption of ethylene. 2003 Elsevier Inc. All rights reserved. Keywords: Silver; Ethylene; Zeolite X; Structure; Sorption; Complex
1. Introduction Dispersions of silver ions and atoms are effective as catalysts. For example, catalysts containing * Corresponding authors. Tel.: +1-808-956-7480; fax: +1808-956-5908. E-mail addresses: [email protected] (Y. Kim), seff@ hawaii.edu (K. Seff).
both Ag0 and Agþ are important in partial oxidation processes such as the formation of ethylene oxide from ethylene and oxygen [1]. As another example, Agþ -exchanged zeolite Y can cleave water into hydrogen and oxygen [2] by a photochemically induced reduction of Agþ , followed by the oxidative thermal desorption of hydrogen. Silver ions can be reduced intrazeolitically by heating [3,4], by reaction with reducing agents [5],
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or by sorption of metal atoms [6]. Tsutsumi and Takahashi reported that the Agþ ions in zeolite Y can be reduced to bulk clusters of Ag0 after treatment with alcohol and alkylbenzene above 300 C [7]. Agþ ions in Ag–X and Ag–Y were also reduced after treatment with carbon monoxide at 350 C [8], perhaps by the reaction 2Agþ þ H2 O þ CO ! CO2 þ 2Hþ þ 2Ag Carter et al. studied, by infrared spectroscopy and microcalorimetry, the sorption of ethylene at room temperature onto a series of transition metal ionexchanged synthetic near-faujasites [9]. They reported that C2 H4 molecules form laterally held complexes of symmetry C2 . Of the transition metal ions examined, Agþ and Cd2þ were found to hold C2 H4 most strongly; furthermore, the sorbed ethylene molecules were reported to be rotating freely in all cases, except in their complexes with Agþ and Cd2þ . In the crystal structure of an ethylene sorption complex of anhydrous fully Cd2þ -exchanged zeolite X [10], about 29.5 Cd2þ ions are found at site II ). Each of these Cd2þ ions ex(Cd–O ¼ 2.221(6) A tends 0.50(1) A into the supercage from the plane of the three oxygens to which it is bound. To complete its distorted tetrahedral coordination, each coordinates laterally (symmetrically) with an ethylene molecule (counted as monodentate) in the and C@C ¼ 1.26(5) supercage (Cd–C ¼ 2.70(4) A A). The center of the ethylene molecule does not lie on the 3-fold axis of the zeolite. Such sorption structures had been seen in other intrazeolitic ethylene complexes [11–16]. Kim and Seff found octahedral hexasilver molecules, each coordinated facially to eight silver ions, (Agþ )8 (Ag6 ), in the sodalite units of vacuumdehydrated fully Agþ -exchanged zeolite A [3,4]. They also found (Agþ )6 (Ag6 ) clusters in the sodalite cavities of an ethylene sorption complex of Ag–A and ethylene coordination to Agþ ions in the large cavities [11]. Upon the introduction of ethylene, two Agþ ions per unit cell moved from hexasilver to coordinate instead to ethylene molecules, lowering the number of Agþ ions about each hexasilver cluster from eight to six. The redox reactions of Ag–Y with oxygen and hydrogen were studied using powder X-ray diffrac-
tion techniques [17]. When Ag–Y was dehydrated, Ag2þ 3 clusters were seen to have formed by ‘‘autoreduction’’, the reaction of Agþ ions with zeolite oxygen atoms to release oxygen gas: 4Agþ +2O2 ! 4Ag0 + O2 , followed by 2Agþ + Ag0 ! Ag2þ [18]. 3 When this zeolite was treated with oxygen at 600 C, Agþ ions were seen to migrate into the sodalite units and the D6Rs from general (reduced) silver positions in the supercages [18]. A review of silver chemistry and structure in zeolites is available [19]. Lee et al. [20] studied the crystal structure of Ag92 –X fully dehydrated at 360 C in flowing oxygen followed by the removal of O2 (g) by evacuation at 400 C before subsequent cooling. This removal of O2 (g) at 400 C appears to have led to the formation of eight Ag atoms per unit cell by ‘‘autoreduction’’. These eight Ag atoms associated with seven Agþ ions to give three Agþ 3 and two Ag2þ clusters. 3 Recently we succeeded in preparing (Agþ )92 Si100 Al92 O384 , fully dehydrated, fully Agþ -exchanged zeolite X with no reduction of Agþ ions and therefore no concommitant loss of framework oxygen atoms [21]. This was accomplished by keeping the crystal in an oxygen atmosphere while at elevated temperature, both during dehydration and while cooling to ambient temperature. Having prepared this uncomplicated zeolite, we wished next to determine the structures of some of its sorption complexes. This structure was determined to locate the sorbed ethylene molecules within the zeolite and to observe the effect of sorption on the Agþ positions.
2. Experimental section 2.1. Crystal preparation Large single crystals of zeolite Na–X, stoichiometry Na92 Si100 Al92 O384 , were prepared in St. Petersburg, Russia [22]. One of these, a colorless octahedron about 0.2 mm in cross-section, was lodged in a fine Pyrex capillary. Aqueous 0.05 M AgNO3 (Aldrich, 99.9999%) was allowed to flow past the crystal at a velocity of 1.0 cm/s for 3 days. The capillary containing the crystal was attached to a vacuum system, and the crystal was dehy-
E.Y. Choi et al. / Microporous and Mesoporous Materials 62 (2003) 201–210
drated at 400 C for 2 days followed by cooling to 21 C, both in a flowing stream of zeolitically dry oxygen gas (790 Torr), followed by evacuation at 2 · 106 Torr for 20 min. To prepare the ethylene complex, the crystal was treated with 300 Torr of zeolitically dry ethylene gas (Aldrich, 99.5%) for 2 h at 21 C. After seal-off in its capillary while still in its ethylene atmosphere, the crystal was observed to be dark yellow. 2.2. X-ray data collection The cubic space group Fd3 was used. This choice is supported by the low Si/Al ratio which requires, at least in the short range, alternation of Si and Al. It is also supported by the final structure in which the mean Si–O bond length is less than that of Al– O. The unit cell constant at 21(1) C is a ¼ . All unique reflections in the positive 24:981ð9Þ A octant of an F -centered unit cell for which 2h < 50, l > h, and k > h were recorded at 21(1) C. An absorption correction, which had little effect on the final R indices, was made empirically using a w scan. Other details are given in Table 1 or are the same as previously reported [23,24].
Table 1 Summary of crystallographic data Data collection T (C) 21 Scan technique h–2h Radiation (MoKa) ) 0.70930 k1 (A ) k2 (A 0.71359 ) Unit cell constant, a (A 24.981(9) 2h range for a (deg) 14–22 No. of reflections for a 25 2h range in data collection (deg) 3 < 2h < 50 No. of reflections gathered 1426 No. of unique reflections (m) 1155 No. of reflections with Fo > 4r (Fo ) 221 No. of parameters (s) 68 Data/parameter ratio (m=s) 17.0 Weighting parameters: a=b 0.0927/0.00 R1 a /wR2 b (Fo > 4r (Fo ÞÞ 0.069/0.169 wR2 b (all data) 0.293 0.987 Goodness-of-fitc P P a R1 ¼ jFo jFc k= Fo . P P b wR2 ¼ ½ wðFo2 Fc2 Þ2 = wðFo2 Þ2 1=2 . P c Goodness-of-fit ¼ ½ wðFo2 Fc2 Þ2 =ðm sÞ 1=2 .
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3. Structure determination Full-matrix least-squares refinement [25] was done on F 2 using all 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 30C3 H6 [26]. These positions when refined isotropically yielded R1 (based on F , Fo > 4h ðFo ÞÞ ¼ 0:40 and wR2 (based on F 2 all data) ¼ 0.80. See the footnotes in Table 1. A Fourier difference electron-density function yielded two strong peaks at (0.0, 0.0, 0.0) and (0.241, 0.241, 0.241), positions near framework oxygens normally occupied by cations. Isotropic refinement of the framework atoms and anisotropic refinement of these two Agþ positions converged to R1 ¼ 0:21 and wR2 ¼ 0:57. The occupancies refined to 12.1(7) ions at Ag(1) and 26.5(3) at Ag(4). A difference Fourier function based on the above model revealed two large peaks at (0.067, 0.067, 0.067) and (0.185, 0.185, 0.185). Including these peaks as Ag(2) and Ag(3), respectively, and allowing them to refine anisotropically, led to convergence with R1 ¼ 0:13 and wR2 ¼ 0:41 with occupancies of 16.0(9) and 5.5(4), respectively. A subsequent difference Fourier function revealed three additional peaks at (0.167, 0.091, 0.410), (0.210, 0.165, 0.420), and (0.278, 0.167, 0.422). Allowing them to refine isotropically as ions at Ag(5), Ag(6), and Ag(7), respectively, lowered the error indices to R1 ¼ 0:083 and wR2 ¼ 0:323. The thermal ellipsoids of Ag(7) became elongated in subsequent (anisotropic) refinements, indicating the presence of two nonequivalent Agþ ions at this position. This position was split into Ag(7) and Ag(8) which refined to (0.270, 0.175, 0.421) and (0.165, 0.185, 0.414), respectively. Isotropic refinement of framework atoms and Ag(5), Ag(6), Ag(7), and Ag(8), with anisotropic refinement of Ag(1), Ag(2), Ag(3), and Ag(4), converged to R1 ¼ 0:073 and wR2 ¼ 0:307. The occupancy numbers at Ag(5), Ag(6), Ag(7), and Ag(8) refined to 8.4(19), 9.6(20), 11.4(14), and 5.6(17), respectively. The carbon position (0.280, 0.285, 0.318) with a 3 was seen on next differpeak height of 1.65 e A ence Fourier function. Allowing it to refine isotropically at C lowered the error indices to R1 ¼ 0:068
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and wR2 ¼ 0:302. Attempts to refine other peaks that might show carbon positions associated with Agþ ions at Ag(5), Ag(6), Ag(7), and Ag(8) were unsuccessful. The occupancy numbers at Ag(1), Ag(2), Ag(3), Ag(4), Ag(5), Ag(6), Ag(7), Ag(8), and C were fixed as shown in Table 1 by the assumption of stoichiometry, the requirement of neutrality, and the observation that the occupancies at Ag(4) and C were refining in the ratio of 1:2. The final error indices converged to R1 ¼ 0:069 and wR2 ¼ 0:293. Fixed weights were used initially; the final weights were assigned using the formula w ¼ q=½r2 ðFo2 Þ þ ðaP 2 Þ þ bp þ d þ e sinðhÞ , where p ¼ fFo2 þ ð1 f ÞFc2 to give w ¼ 1=½r2 ðFo2 Þ þ ðaP 2 Þ þ bp , where p ¼ ðFo2 þ 2Fc2 Þ=3, with a and b as refinable parameters (Table 1). The final difference function was featureless; in particular, no electron density was seen near the Agþ ions at the III0 sites. Atomic scattering factors [27] for Si, Al, O, Agþ , and C were used. All scattering factors were modified to account for anomalous dispersion [28,29]. The final structural parameters, and selected interatomic distances and angles, are presented in Tables 2 and 3, respectively.
4. Discussion Zeolite X is a synthetic Al-rich analogue of the naturally occurring mineral faujasite. The 14hedron with 24 vertices known as the sodalite cavity or b cage may be viewed as the principal building block of the aluminosilicate framework of the zeolite (see Fig. 1). These b 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 (24-membered) windows. (An nring consists of n oxygen atoms and nT (Si or Al) atoms.) The Si and Al atoms occupy the vertices of these polyhedra. The oxygen atoms lie approximately midway between each pair of Si and Al atoms, but are displaced from those points to be closer to Si and to give near tetrahedral angles about Si and Al. Exchangeable cations that balance the negative charge of the aluminosilicate framework are found
within the zeolites cavities at the sites shown in Fig. 1. Note that site III is in the supercage opposite a 4-ring between two 12-rings, and that site III0 is somewhat or substantially off III (off the 2-fold axis) but still on the inner surface of the supercage. Each unit cell has 8 sodalite units, 8 supercages, 16 D6Rs, 16 12-rings, and 32 S6Rs. In Ag92 –X Æ 27C2 H4 , jAg92 (C2 H4 )27 j[Al92 Si100 O384 ]FAU, the Agþ ions occupy eight different crystallographic sites. In contrast, Agþ ions occupied only four sites (I, I0 , II, and III0 ) in fully dehydrated (Agþ )92 –X (see Table 4). As hoped for, there are no Ag0 atoms in this structure; no Agþ ions were reduced during crystal dehydration, as had been demonstrated previously [21], and none resulted from the sorption of ethylene at 21 C. As a result of ethylene sorption, the 32 site-II Agþ ions in the dehydrated structure redistributed to two different 3-fold axis Agþ positions: 27 Agþ ions at site II coordinate to ethylene molecules and five Agþ ions at site II0 do not. To coordinate to ethylene, each of the 27 site-II Agþ ions has moved along their 3-fold axes more 1:08 0:28 ¼ 0:80 A deeply into the supercage from their three-O(2) plane (see Fig. 2 and Table 5). In this way, these Agþ ions are able to coordinate more tetrahedrally to ethylene (considering ethylene to be monodentate). Also the 32 Agþ ions at site III0 in the dehydrated structure have redistributed to four different III0 sites. The 12 Agþ ions at Ag(1) are octahedrally coordinated at site I, at the centers of the D6Rs (see Figs. 3 and 4). This 16-fold position is only 75% , is occupied. The Ag(1)–O(3) distance, 2.63(2) A approximately equal to the sum of the corre, indicative of a reasponding ionic radii, 2.58 A from sonably good fit. Each Ag(1) ion is 1.62 A two O(3)-oxygen planes. The 16 Agþ ions at Ag(2) are at site I0 , in the sodalite cavities opposite the D6Rs (see Fig. 4). This 32-fold position is only half occupied. Each Agþ ion lies relatively far inside the sodalite cavity, from the plane of the three O(3) framework 1.33 A oxygens of the D6R to which it is bound. The , is a little shorter Ag(2)–O(3) distance, 2.47(3) A . than the sum of the ionic radii, 2.58 A The distance between Ag(1) and Ag(2) (see Fig. , substantially shorter than the 4) is 2.951 A
Atom Si Al O(1) O(2) O(3) O(4) Ag(1) Ag(2) Ag(3) Ag(4) Ag(5) Ag(6) Ag(7) Ag(8) C H1e H2e
Wyc. Pos. 96(g) 96(g) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 96(g) 96(g) 96(g) 96(g) 96(g) 96(g) 96(g)
Site
x
y
z
U11 b or Uiso c
U22
U33
U12
U13
U23
Occupancyd Varied
I I0 II0 II III0 III0 III0 III0
)528(4) )526(4) )1082(10) )15(12) )305(11) )744(12) 0 682(5) 1981(22) 2410(2) 1551(34) 2084(27) 2688(30) 1651(31) 2798(44) 2881 2503
1242(7) 364(4) )4(13) )14(12) 723(12) 774(11) 0 682(5) 1981(22) 2410(2) 973(34) 1621(27) 1749(28) 1852(34) 2857(46) 2983 2605
356(4) 1254(7) 1070(10) 1455(9) 703(12) 1751(12) 0 682(5) 1981(22) 2410(2) 4093(36) 4199(26) 4208(25) 4137(29) 3175(38) 3533 3155
130(27) 84(26) 206(87) 138(75) 215(88) 148(77) 340(44) 706(67) 872(237) 441(32) 1105(261) 668(187) 1203(240) 254(203) 708(376)
340(44) 706(67) 872(237) 441(32)
340(44) 706(67) 872(237) 441(32)
156(44) 281(73) 600(303) 171(30)
156(44) 281(73) 600(303) 171(30)
156(44) 281(73) 600(303) 171(30)
12.1(7) 16.0(9) 5.5(4) 26.5(4) 8.4(19) 9.6(20) 11.4(14) 5.6(17) 53.0(7)
Fixed 96 96 96 96 96 96 12 16 5 27 8 8 11 5 54 54 54
a . The space group is Fd3 with origin chosen at the center of symmetry. Positional and anisotropic thermal parameters are given · 104 . Numbers in a ¼ 24:981ð9Þ A parentheses are the esds in the units of the least significant digit given for the corresponding parameter. b The anisotropic temperature factor ¼ exp[()2p2 /a2 )(U11 h2 +U22 k 2 +U33 l2 +U12 hk+U13 hl+U23 kl)]. c Biso ¼ 8p2 Uiso . d Occupancy factors are given as the number of atoms or ions per unit cell. Four or more (4 þ x) Si atoms [30] are present at the Al position per unit cell; x Al atoms may therefore be at the Si position. e . Hydrogen atom positions were calculated [25] with a C–H distance of 0.97 A
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Table 2 Positional, thermal, and occupancy parametersa for Ag92 –X Æ 27C2 H4
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Table 3 ) and angles (deg)a Selected interatomic distances (A Distance 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 Ag(1)–O(3) Ag(2)–O(3) Ag(3)–O(2) Ag(4)–O(2) Ag(5)–O(1) Ag(6)–O(1) Ag(6)–O(4) Ag(7)–O(1) Ag(7)–O(4) Ag(8)–O(1) Ag(8)–O(4) Ag(1)–Ag(2) Ag(4)–C C–C C–H(1) C–H(2) H(2)–O(4)
Angle 1.66(3) 1.67(3) 1.65(3) 1.65(3) 1.66 1.73(3) 1.67(3) 1.73(3) 1.70(3) 1.71 2.63(2) 2.47(3) 2.30(4) 2.41(2) 2.72(5) 2.32(7) 2.57(7) 2.35(7) 2.88(5) 2.57(8) 2.25(8) 2.951(6) 2.42(10) 1.24(15) 0.97b 0.97b 2.89b
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
114.1(15) 111.3(16) 103.0(16) 106.3(14) 110.3(14) 111.9(15) 113.3(15) 108.7(16) 104.9(16) 106.8(14) 111.5(14) 111.7(14) 133.7(17) 139.9(18) 135.5(18) 140.6(21)
O(3)–Ag(1)–O(3) O(3)–Ag(2)–O(3) O(2)–Ag(3)–O(2) O(2)–Ag(4)–O(2) O(1)–Ag(5)–O(1) O(1)–Ag(6)–O(4) O(1)–Ag(7)–O(4) O(1)–Ag(8)–O(4)
86.1(10)/93.9(10) 93.5(11) 109.0(23) 101.7(8) 87.8(8) 67.4(20) 58.4(10) 64.6(21)
a Numbers in parentheses are estimated standard deviations in the units of the least significant digit given for the corresponding value. b Hydrogen positions were calculated [25].
distance in the dehycorresponding 3.157(4)-A drated structure (empty (Agþ )92 Si100 Al92 O384 ) [21]. This is indicative, as before, of a weak Agþ –Agþ interaction [20,21]. Distances ranging from have been reported for 2.655(2) to 3.058(1) A similar Agþ –Agþ interactions in seven organic complexes of Agþ [31]. Perhaps the most sensible placement of 12 Ag(1) ions and 16 Ag(2) ions in the 16 D6Rs is 8 D6Rs with Agþ –Agþ –Agþ as shown in Fig. 4, 4 D6Rs with one Agþ ion at their center as shown in Fig. 3, and 4 D6Rs with no Agþ ions at either position. D6Rs with two Ag(2) ions (with no Ag(1) cation) are not reasonable because the two Agþ ions should be much closer together and have a weak Agþ –Agþ interaction as was seen in the crystal structure of empty (Agþ )92 Si100 Al92 O384 [20]. D6Rs with one ion at Ag(1) and one at Ag(2) are unlikely
because Ag(2) does not coordinate to a ligand [20]; it should relax to give two equivalent Agþ ions (as above) with a weak Agþ –Agþ interaction. Multiple Agþ weak interactions (nets) are common [32], and frequently contain linear (Agþ )3 sequences. Thirty-two Agþ ions fill the single 6-rings, 27 at site II and 5 at site II0 . The five Agþ ions at site II0 (at Ag(3)) lie inside the sodalite cavities opposite S6Rs. Each Agþ ion at Ag(3) coordinates at to three O(2) framework oxygens, and is 2.30(4) A into the sodalite cavity from their recessed 0.78 A plane (see Fig. 5). The 27 Agþ ions at Ag(4) lie at site II; each of these coordinates to three frame and extends 1.08 A into work oxygens at 2.41(2) A the supercage where it forms a lateral p-complex with an ethylene molecule. The 54 carbon atoms in this structure (27 molecules of C2 H4 ) are equivalent at Wyckoff position
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Fig. 1. A stylized drawing of the framework structure of zeolite X. Near the center of each line segment is an oxygen atom. The nonequivalent oxygen atoms are indicated by the numbers 1–4. Silicon and aluminum atoms alternate at the tetrahedral intersections, except that Si must substitute for at least 4% of the Al atoms. Extra framework cation positions are labeled with Roman numerals.
Table 4 Distribution of Agþ cations among sites Sites I I0 II0 II III0
a; b
a
Ag96 –X
Ag92 –X
Ag92 –X Æ 27C2 H4
32
4 24
12 16 5 27 32d
32 32
32 32
c
a
Ref. [21]. Predicted (for LSX). c This work. d At four nonequivalent III0 sites. b
96(g). The two carbon atoms of each ethylene molecule are equidistant from Ag(4). The center of the C@C bond does not lie on a 3-fold axis of the crystal structure. The bonding between the silver ion at Ag(4) and ethylene can be described in terms of two components according to the Chatt–Dewar model [33]. A r component arises from the overlap of a filled ethylene p orbital and a vacant Agþ 5s orbital, and a p component occurs by the overlap of the filled 4d orbitals of Agþ with the vacant antibonding p orbital of ethylene. By the latter interaction, the
207
Fig. 2. A Ag(C2 H4 )þ complex on the inner surface of a supercage. Twenty-seven Agþ ions per unit cell at Ag(4) coordinate to ethylene molecules as shown. Two hydrogen atoms of each ethylene molecule are calculated by SHELX97 to interact weakly with O(2) oxygens. Ellipsoids of 20% probability are used.
Table 5 ) of atoms from 6-ring oxygen planes Deviations (A Position
Cation site
Displacement
At O(2)a; b
Ag(3) Ag(4) C
II0 II
)0.779 1.077 3.46
At O(3)b;c
Ag(1) Ag(2)
I I0
)1.618 1.333
a A positive displacement indicates that the atom or ion lies in the supercage. b Sites I0 and II0 are in the sodalite units. c The negative displacement indicates that the ion lies within a double 6-ring.
C@C bond order may be diminished by back-donation. Carter et al. [9] studied the sorption of ethylene onto a series of transition–metal exchanged zeolite X samples using calorimetric and IR methods. They found that ethylene is relatively weakly held and that it can readily be removed by evacuation at room temperature, unless the cation involved is Agþ or Cd2þ . The Ag(4)–C bond length herein
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Fig. 3. Stereoview of a double 6-ring in Ag92 –X Æ 27C2 H4 . A Agþ ion at Ag(1) (site I) is at its center. Although this occurs in 12 of the 16 D6Rs per unit cell, it is proposed that only 4 of the 16 D6Rs are occupied as shown, with no additional Agþ ions nearby. Ellipsoids of 20% probability are used.
Fig. 4. Stereoview of a double 6-ring in Ag92 –X Æ 27C2 H4 . Two Agþ ions at Ag(2) (site I0 ) and one at Ag(1) (site I) are shown. It is ) interactions are shown with dashed proposed that 8 of the 16 D6Rs per unit cell have this arrangement. Two weak Agþ –Agþ (2.951 A lines. Ellipsoids of 20% probability are used.
Fig. 5. Stereoview of a sodalite cavity in Ag92 –X Æ 27C2 H4 . One Agþ ion at Ag(3) (site II0 ) and three Agþ ions at Ag(4) (site II) are shown. Each Agþ ion at Ag(4) coordinates to an ethylene molecule. About five-eights of the sodalite cavities may have this arrangement. The remaining three-eights may have four Agþ ions at Ag(4). Ellipsoids of 20% probability are used.
, indicates that the ethylene is reported, 2.42(10) A relatively firmly held by the Agþ ion at Ag(4). This bond length can be compared with distances of in acenaphthene AgClO4 [34], 2.45– 2.48–2.51 A 2.56 A in anthracene [AgClO4 ]4 Æ 4H2 O [35], 2.50– in benzene AgClO4 [35], 2.60–2.63 in 2.63 A
naphthalene [AgClO4 ]4 Æ 4H2 O [35] and 2.47–2.92 in benzene AgAlCl4 [36]. The Agþ –C distance A in zeolite X is insignificantly different from that in an ethylene complex of zeolite A [11] (Agþ –C ¼ ). For comparison, the corresponding M– 2.54(8) A C distances in other intrazeolitic cation–ethylene
E.Y. Choi et al. / Microporous and Mesoporous Materials 62 (2003) 201–210 Table 6 Comparison of ethylene–cation approach distances in zeolites A and X ) ) Structure M–C (A Cation radius (A Ag92 –X Æ 27C2 H4 Co4 Na4 –A Æ 4C2 H4 Ag12 –A Æ 3.6C2 H4 Cd6 –A Æ 4C2 H4 Cd46 –X Æ 29.5C2 H4 Mn46 –X Æ 30C2 H4 Ca6 –A Æ 4C2 H4 Ca46 –X Æ 30C2 H4
2.42(10) 2.51(6) 2.54(8) 2.67(6) 2.70(4) 2.76(6) 2.87(5) 2.98(4)
1.26 0.72 1.26 0.97 0.97 0.80 0.99 0.99
complexes are given in Table 6. Metal cation to ethylene carbon distances in zeolites range from (this work) to 2.98(4) A . 2.42(10) A , is imprecisely The C@C distance, 1.24(15) A determined and is likely to suffer from foreshortening. As such, this work cannot say that it differs significantly from the C@C bond length in ethyl. In nonzeolitic transition–metal ene gas, 1.344 A complexes, a broad range of C@C distances (from ) can be observed [37]. The 1.354(15) to 1.46(2) A C@C distances in other zeolite structures are also imprecisely determined due to the combined effects of disorder and high thermal motion of the carbon in Mn46 – atoms. They range widely: 1.10(8) A X Æ 30C2 H4 [14], 1.19(12) A in Ag12 –A Æ 3.6C2 H4 in Co4 Na4 –A Æ 4C2 H4 [12], 1.26(5) [11], 1.21(11) A in Ca6 – A in Cd46 –X Æ 29.5C2 H4 [10], and 1.48(7) A A Æ 4C2 H4 [15].
209
Effective ethylene ) radius (A
Ref.
1.16 1.79 1.28 1.70 1.73 1.96 1.88 1.99
This work [12] [11] [13] [10] [14] [15] [16]
The positions of the four hydrogen atoms of each ethylene molecule were calculated with a C–H using the software system bond length of 0.97 A SHELX97 [25]. In each molecule, two cis hydro from an O(4) gen atoms can each be about 2.89 A framework oxygen (see Fig. 2). This indicates that they can interact weakly (electrostatically) with the anionic zeolite framework; the sum of the van der Waals radii of oxygen and hydrogen is 1:4 þ 1:2 ¼ [38]. 2:6 A The remaining 32 Agþ ions occupy four different III0 sites (Ag(5), Ag(6), Ag(7), and Ag(8)) in the supercage with occupancies of 8, 8, 11, and 5, respectively (see Fig. 6). Each of these Agþ ions coordinates to only two framework oxygens. The great diversity of III0 positions (there was only one III0 position in dehydrated Ag92 –X [21]) suggests that some may coordinate further to ethylene
Fig. 6. Stereoview of a supercage in Ag92 –X Æ 27C2 H4 . One Agþ ion at Ag(5) (site III0 ), one at Ag(6) (site III0 ), one at Ag(7) (site III0 ), and one at Ag(8) (site III0 ) are shown. About five-eights of the supercages may have this arrangement. The remaining three-eights may have one Agþ ion at Ag(5), one at Ag(6), and two at Ag(7). Ellipsoids of 20% probability are used.
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molecules that could not be located in this work. Alternatively, the III0 positions may have adjusted to avoid sorbed ethylene molecules and because there are now two S6R positions, II and II0 . 5. Summary Of the 92 Agþ ions per unit cell of Ag92 –X, 27 (of the 32 at site II in the empty zeolite) have moved along their 3-fold axes more deeply into the 0.80 A supercage to coordinate strongly (laterally) to ethylene molecules. The centers of these ethylene molecules lie off these 3-fold axes. Each ethylene molecule makes two electrostatic hydrogen bonds to framework oxygen atoms. The remaining Agþ site occupancies have changed substantially as a result of ethylene sorption. Weak Agþ –Agþ interactions are again seen. Acknowledgements This work was supported in part by a grant from the Korean Research Foundation made in the program year 2001 (Grant No. 2000-015-DP0190).
References [1] P.A. Kilty, W.M.H. Sachtler, Cat. Rev. 10 (1) (1974) 1. [2] P.A. Jacobs, J.B. Uytterhoeven, H.K. Beyer, J. Chem. Soc. Chem. Commun. 128 (1977). [3] Y. Kim, K. Seff, J. Am. Chem. Soc. 99 (1977) 7055. [4] Y. Kim, K. Seff, J. Am. Chem. Soc. 100 (1978) 6989. [5] Y. Kim, K. Seff, Bull. Korean Chem. Soc. 5 (1984) 135. [6] L.B. McCusker, Ph.D. Thesis, University of Hawaii, 1980. [7] K. Tsutsumi, H. Takahashi, Bull. Chem. Soc. Jpn. 45 (1972) 2332. [8] Y.Y. Huang, J. Catal. 32 (1974) 482. [9] J.L. Carter, J.C. Yates, P.J. Lucchesi, J.J. Elliott, V. Kevorkian, J. Phys. Chem. 70 (1966) 1126. [10] Y.H. Yeom, S.H. Song, Y. Kim, K. Seff, J. Phys. Chem. B 101 (1966) 2138. [11] Y. Kim, K. Seff, J. Am. Chem. Soc. 100 (1978) 175.
[12] P.E. Riley, K. Seff, K.B. Kunz, J. Am. Chem. Soc. 97 (1975) 537. [13] K.N. Koh, U.S. Kim, D.S. Kim, Y. Kim, Bull. Korean Chem. Soc. 12 (1991) 178. [14] S.B. Jang, M.S. Jeong, Y. Kim, K. Seff, J. Phys. Chem. B 101 (1997) 9041. [15] S.B. Jang, S.D. Moon, J.Y. Park, U.S. Kim, Y. Kim, Bull. Korean Chem. Soc. 13 (1992) 70. [16] S.B. Jang, M.S. Jeong, Y. Kim, K. Seff, J. Phys. Chem. B 101 (1997) 3091. [17] L.R. Gellens, W.J. Mortier, J.B. Uytterhoeven, Zeolites 1 (1981) 85. [18] L. Kevan, N. Narayana, ACS Symp. Ser 218 (1983) 283. [19] T. Sun, K. Seff, Chem. Rev. 94 (1994) 859. [20] (a) S.H. Lee, Y. Kim, K. Seff, Micropor. Mesopor. Mater. 41 (2000) 49; (b) Errata S.H. Lee, Y. Kim, K. Seff, Micropor. Mesopor. Mater 52 (2002) 61. [21] S.Y. Kim, Y. Kim, K. Seff, J. Phys. Chem. B 107 (2003) in press. [22] V.N. Bogomolov, V.P. Petranovskii, Zeolites 6 (1986) 418. [23] V.R. Choudhary, A.P. Singh, Zeolites 6 (1986) 206. [24] J.H. Kwon, S.B. Jang, Y. Kim, K. Seff, J. Phys. Chem. 100 (1996) 13720. [25] G.M. Sheldrick, SHELX97, Program for the Refinement of Crystal Structures, University of G€ ottingen, Germany, 1997. [26] E.Y. Choi, Y. Kim, S.H. Song, Bull. Korean Chem. Soc. 20 (1999) 791. [27] International Tables for X-ray Crystallography, vol. IV, Kynoch Press, Birmingham, England, 1974, p. 73. [28] International Tables for X-ray Crystallography, vol. IV, Kynoch Press, Birmingham, England, 1974, p. 149. [29] D.T. Cromer, Acta Crystallogr. 18 (1965) 17. [30] D. Bae, K. Seff, Micropor. Mesopor. Mater. 42 (2001) 299. [31] M.A. Romero, J.M. Salas, M. Quiros, J. Molina, J. Mol. Struct. 354 (1995) 189, and references therein. [32] M. Jansen, Angew. Chem. Int. Ed. Engl. 26 (1987) 1098. [33] (a) J. Chatt, J. Chem. Soc. 3340 (1949); (b) J. Chatt, R.G. Wilkins, J. Chem. Soc. 2622 (1952); (c) J. Chatt, L.A. Duncanson, J. Chem. Soc. 2939 (1953); (d) M.J.S. Dewar, Bull. Soc. Chim. Fr. 18 (1971) C71. [34] P.F. Rodesiler, E.L. Amma, Inorg. Chem. 11 (1972) 388. [35] W.E. Silverthorn, Adv. Organomet. Chem. 13 (1975) 125. [36] R.W. Turner, E.L. Amma, J. Am. Chem. Soc. 88 (1966) 3243. [37] L. Guggenberger, J. Inorg. Chem. 12 (1973) 499. [38] Handbook of Chemistry and Physics, 70th ed., The Chemical Rubber Company, Cleveland, OH, 1990, p. D190.
Crystal structure of an ethylene sorption complex of fully dehydrated, fully oxidized, fully Agþ-exchanged zeolite X Eun Young Choi a, Soo Yeon Kim a, Yang Kim a
a,*
, Karl Seff
b,*
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Pusan 609-735, Republic of Korea b Department of Chemistry, University of Hawaii at Manoa, 2545 The Mall, Honolulu, HI 96822-2275, USA Received 20 January 2003; received in revised form 7 May 2003; accepted 7 May 2003
Abstract The crystal structure of an ethylene sorption complex of fully dehydrated, fully oxidized, fully Agþ -exchanged zeolite ), jAg92 (C2 H4 )27 j [Al92 Si100 O384 ]-FAU, has been determined by singleX, (Agþ )92 Si100 Al92 O384 Æ 27C2 H4 (a ¼ 24:981(9) A crystal X-ray diffraction techniques in the cubic space group Fd3 at 21 C. Ion exchange was accomplished by allowing 0.05 M aqueous AgNO3 to flow past a crystal for 3 days to give hydrated Ag92 Si100 Al92 O384 . This crystal was dehydrated at 400 C for 2 days followed by cooling to 21 C, both in a flowing stream of zeolitically dry oxygen gas (790 Torr), followed by evacuation at 2 · 106 Torr for 20 min. To prepare the ethylene sorption complex, this colorless crystal was treated at 21 C with 300 Torr of zeolitically dry ethylene gas for 2 h. The structure of the resulting dark yellow crystal was determined in this atmosphere and was refined to the final error index wR2 based on F 2 and all data (1155) of 0.293; R1 ¼ 0:069 for the 221 reflections for which F0 > 4r ðF0 Þ. In this structure, per unit cell, 12 Agþ ions were found at the ) and 16 Agþ ions occupy the nearby site-I0 positions (Ag–O ¼ 2.47(3) A ). A sigoctahedral site I (Ag–O ¼ 2.63(2) A interaction must occur between the Agþ ions at sites I and I0 ; linear (Ag3 )3þ is proposed. Thirty-two nificant 2.951-A ) and 5 at site II0 (Ag–O ¼ 2.30(4) A ). Each of the 27 site-II Agþ ions fill the single 6-rings, 27 at site II (Ag–O ¼ 2.41(2) A þ Ag ions extends 1.08 A into the supercage to form a strong lateral p-complex with an ethylene molecule. In turn, each C2 H4 molecule forms two electrostatic hydrogen bonds to framework oxygens. The remaining 32 Agþ ions occupy four different III0 sites. The Agþ site occupancies changed substantially upon the sorption of ethylene. 2003 Elsevier Inc. All rights reserved. Keywords: Silver; Ethylene; Zeolite X; Structure; Sorption; Complex
1. Introduction Dispersions of silver ions and atoms are effective as catalysts. For example, catalysts containing * Corresponding authors. Tel.: +1-808-956-7480; fax: +1808-956-5908. E-mail addresses: [email protected] (Y. Kim), seff@ hawaii.edu (K. Seff).
both Ag0 and Agþ are important in partial oxidation processes such as the formation of ethylene oxide from ethylene and oxygen [1]. As another example, Agþ -exchanged zeolite Y can cleave water into hydrogen and oxygen [2] by a photochemically induced reduction of Agþ , followed by the oxidative thermal desorption of hydrogen. Silver ions can be reduced intrazeolitically by heating [3,4], by reaction with reducing agents [5],
1387-1811/$ - see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1387-1811(03)00406-2
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or by sorption of metal atoms [6]. Tsutsumi and Takahashi reported that the Agþ ions in zeolite Y can be reduced to bulk clusters of Ag0 after treatment with alcohol and alkylbenzene above 300 C [7]. Agþ ions in Ag–X and Ag–Y were also reduced after treatment with carbon monoxide at 350 C [8], perhaps by the reaction 2Agþ þ H2 O þ CO ! CO2 þ 2Hþ þ 2Ag Carter et al. studied, by infrared spectroscopy and microcalorimetry, the sorption of ethylene at room temperature onto a series of transition metal ionexchanged synthetic near-faujasites [9]. They reported that C2 H4 molecules form laterally held complexes of symmetry C2 . Of the transition metal ions examined, Agþ and Cd2þ were found to hold C2 H4 most strongly; furthermore, the sorbed ethylene molecules were reported to be rotating freely in all cases, except in their complexes with Agþ and Cd2þ . In the crystal structure of an ethylene sorption complex of anhydrous fully Cd2þ -exchanged zeolite X [10], about 29.5 Cd2þ ions are found at site II ). Each of these Cd2þ ions ex(Cd–O ¼ 2.221(6) A tends 0.50(1) A into the supercage from the plane of the three oxygens to which it is bound. To complete its distorted tetrahedral coordination, each coordinates laterally (symmetrically) with an ethylene molecule (counted as monodentate) in the and C@C ¼ 1.26(5) supercage (Cd–C ¼ 2.70(4) A A). The center of the ethylene molecule does not lie on the 3-fold axis of the zeolite. Such sorption structures had been seen in other intrazeolitic ethylene complexes [11–16]. Kim and Seff found octahedral hexasilver molecules, each coordinated facially to eight silver ions, (Agþ )8 (Ag6 ), in the sodalite units of vacuumdehydrated fully Agþ -exchanged zeolite A [3,4]. They also found (Agþ )6 (Ag6 ) clusters in the sodalite cavities of an ethylene sorption complex of Ag–A and ethylene coordination to Agþ ions in the large cavities [11]. Upon the introduction of ethylene, two Agþ ions per unit cell moved from hexasilver to coordinate instead to ethylene molecules, lowering the number of Agþ ions about each hexasilver cluster from eight to six. The redox reactions of Ag–Y with oxygen and hydrogen were studied using powder X-ray diffrac-
tion techniques [17]. When Ag–Y was dehydrated, Ag2þ 3 clusters were seen to have formed by ‘‘autoreduction’’, the reaction of Agþ ions with zeolite oxygen atoms to release oxygen gas: 4Agþ +2O2 ! 4Ag0 + O2 , followed by 2Agþ + Ag0 ! Ag2þ [18]. 3 When this zeolite was treated with oxygen at 600 C, Agþ ions were seen to migrate into the sodalite units and the D6Rs from general (reduced) silver positions in the supercages [18]. A review of silver chemistry and structure in zeolites is available [19]. Lee et al. [20] studied the crystal structure of Ag92 –X fully dehydrated at 360 C in flowing oxygen followed by the removal of O2 (g) by evacuation at 400 C before subsequent cooling. This removal of O2 (g) at 400 C appears to have led to the formation of eight Ag atoms per unit cell by ‘‘autoreduction’’. These eight Ag atoms associated with seven Agþ ions to give three Agþ 3 and two Ag2þ clusters. 3 Recently we succeeded in preparing (Agþ )92 Si100 Al92 O384 , fully dehydrated, fully Agþ -exchanged zeolite X with no reduction of Agþ ions and therefore no concommitant loss of framework oxygen atoms [21]. This was accomplished by keeping the crystal in an oxygen atmosphere while at elevated temperature, both during dehydration and while cooling to ambient temperature. Having prepared this uncomplicated zeolite, we wished next to determine the structures of some of its sorption complexes. This structure was determined to locate the sorbed ethylene molecules within the zeolite and to observe the effect of sorption on the Agþ positions.
2. Experimental section 2.1. Crystal preparation Large single crystals of zeolite Na–X, stoichiometry Na92 Si100 Al92 O384 , were prepared in St. Petersburg, Russia [22]. One of these, a colorless octahedron about 0.2 mm in cross-section, was lodged in a fine Pyrex capillary. Aqueous 0.05 M AgNO3 (Aldrich, 99.9999%) was allowed to flow past the crystal at a velocity of 1.0 cm/s for 3 days. The capillary containing the crystal was attached to a vacuum system, and the crystal was dehy-
E.Y. Choi et al. / Microporous and Mesoporous Materials 62 (2003) 201–210
drated at 400 C for 2 days followed by cooling to 21 C, both in a flowing stream of zeolitically dry oxygen gas (790 Torr), followed by evacuation at 2 · 106 Torr for 20 min. To prepare the ethylene complex, the crystal was treated with 300 Torr of zeolitically dry ethylene gas (Aldrich, 99.5%) for 2 h at 21 C. After seal-off in its capillary while still in its ethylene atmosphere, the crystal was observed to be dark yellow. 2.2. X-ray data collection The cubic space group Fd3 was used. This choice is supported by the low Si/Al ratio which requires, at least in the short range, alternation of Si and Al. It is also supported by the final structure in which the mean Si–O bond length is less than that of Al– O. The unit cell constant at 21(1) C is a ¼ . All unique reflections in the positive 24:981ð9Þ A octant of an F -centered unit cell for which 2h < 50, l > h, and k > h were recorded at 21(1) C. An absorption correction, which had little effect on the final R indices, was made empirically using a w scan. Other details are given in Table 1 or are the same as previously reported [23,24].
Table 1 Summary of crystallographic data Data collection T (C) 21 Scan technique h–2h Radiation (MoKa) ) 0.70930 k1 (A ) k2 (A 0.71359 ) Unit cell constant, a (A 24.981(9) 2h range for a (deg) 14–22 No. of reflections for a 25 2h range in data collection (deg) 3 < 2h < 50 No. of reflections gathered 1426 No. of unique reflections (m) 1155 No. of reflections with Fo > 4r (Fo ) 221 No. of parameters (s) 68 Data/parameter ratio (m=s) 17.0 Weighting parameters: a=b 0.0927/0.00 R1 a /wR2 b (Fo > 4r (Fo ÞÞ 0.069/0.169 wR2 b (all data) 0.293 0.987 Goodness-of-fitc P P a R1 ¼ jFo jFc k= Fo . P P b wR2 ¼ ½ wðFo2 Fc2 Þ2 = wðFo2 Þ2 1=2 . P c Goodness-of-fit ¼ ½ wðFo2 Fc2 Þ2 =ðm sÞ 1=2 .
203
3. Structure determination Full-matrix least-squares refinement [25] was done on F 2 using all 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 30C3 H6 [26]. These positions when refined isotropically yielded R1 (based on F , Fo > 4h ðFo ÞÞ ¼ 0:40 and wR2 (based on F 2 all data) ¼ 0.80. See the footnotes in Table 1. A Fourier difference electron-density function yielded two strong peaks at (0.0, 0.0, 0.0) and (0.241, 0.241, 0.241), positions near framework oxygens normally occupied by cations. Isotropic refinement of the framework atoms and anisotropic refinement of these two Agþ positions converged to R1 ¼ 0:21 and wR2 ¼ 0:57. The occupancies refined to 12.1(7) ions at Ag(1) and 26.5(3) at Ag(4). A difference Fourier function based on the above model revealed two large peaks at (0.067, 0.067, 0.067) and (0.185, 0.185, 0.185). Including these peaks as Ag(2) and Ag(3), respectively, and allowing them to refine anisotropically, led to convergence with R1 ¼ 0:13 and wR2 ¼ 0:41 with occupancies of 16.0(9) and 5.5(4), respectively. A subsequent difference Fourier function revealed three additional peaks at (0.167, 0.091, 0.410), (0.210, 0.165, 0.420), and (0.278, 0.167, 0.422). Allowing them to refine isotropically as ions at Ag(5), Ag(6), and Ag(7), respectively, lowered the error indices to R1 ¼ 0:083 and wR2 ¼ 0:323. The thermal ellipsoids of Ag(7) became elongated in subsequent (anisotropic) refinements, indicating the presence of two nonequivalent Agþ ions at this position. This position was split into Ag(7) and Ag(8) which refined to (0.270, 0.175, 0.421) and (0.165, 0.185, 0.414), respectively. Isotropic refinement of framework atoms and Ag(5), Ag(6), Ag(7), and Ag(8), with anisotropic refinement of Ag(1), Ag(2), Ag(3), and Ag(4), converged to R1 ¼ 0:073 and wR2 ¼ 0:307. The occupancy numbers at Ag(5), Ag(6), Ag(7), and Ag(8) refined to 8.4(19), 9.6(20), 11.4(14), and 5.6(17), respectively. The carbon position (0.280, 0.285, 0.318) with a 3 was seen on next differpeak height of 1.65 e A ence Fourier function. Allowing it to refine isotropically at C lowered the error indices to R1 ¼ 0:068
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and wR2 ¼ 0:302. Attempts to refine other peaks that might show carbon positions associated with Agþ ions at Ag(5), Ag(6), Ag(7), and Ag(8) were unsuccessful. The occupancy numbers at Ag(1), Ag(2), Ag(3), Ag(4), Ag(5), Ag(6), Ag(7), Ag(8), and C were fixed as shown in Table 1 by the assumption of stoichiometry, the requirement of neutrality, and the observation that the occupancies at Ag(4) and C were refining in the ratio of 1:2. The final error indices converged to R1 ¼ 0:069 and wR2 ¼ 0:293. Fixed weights were used initially; the final weights were assigned using the formula w ¼ q=½r2 ðFo2 Þ þ ðaP 2 Þ þ bp þ d þ e sinðhÞ , where p ¼ fFo2 þ ð1 f ÞFc2 to give w ¼ 1=½r2 ðFo2 Þ þ ðaP 2 Þ þ bp , where p ¼ ðFo2 þ 2Fc2 Þ=3, with a and b as refinable parameters (Table 1). The final difference function was featureless; in particular, no electron density was seen near the Agþ ions at the III0 sites. Atomic scattering factors [27] for Si, Al, O, Agþ , and C were used. All scattering factors were modified to account for anomalous dispersion [28,29]. The final structural parameters, and selected interatomic distances and angles, are presented in Tables 2 and 3, respectively.
4. Discussion Zeolite X is a synthetic Al-rich analogue of the naturally occurring mineral faujasite. The 14hedron with 24 vertices known as the sodalite cavity or b cage may be viewed as the principal building block of the aluminosilicate framework of the zeolite (see Fig. 1). These b 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 (24-membered) windows. (An nring consists of n oxygen atoms and nT (Si or Al) atoms.) The Si and Al atoms occupy the vertices of these polyhedra. The oxygen atoms lie approximately midway between each pair of Si and Al atoms, but are displaced from those points to be closer to Si and to give near tetrahedral angles about Si and Al. Exchangeable cations that balance the negative charge of the aluminosilicate framework are found
within the zeolites cavities at the sites shown in Fig. 1. Note that site III is in the supercage opposite a 4-ring between two 12-rings, and that site III0 is somewhat or substantially off III (off the 2-fold axis) but still on the inner surface of the supercage. Each unit cell has 8 sodalite units, 8 supercages, 16 D6Rs, 16 12-rings, and 32 S6Rs. In Ag92 –X Æ 27C2 H4 , jAg92 (C2 H4 )27 j[Al92 Si100 O384 ]FAU, the Agþ ions occupy eight different crystallographic sites. In contrast, Agþ ions occupied only four sites (I, I0 , II, and III0 ) in fully dehydrated (Agþ )92 –X (see Table 4). As hoped for, there are no Ag0 atoms in this structure; no Agþ ions were reduced during crystal dehydration, as had been demonstrated previously [21], and none resulted from the sorption of ethylene at 21 C. As a result of ethylene sorption, the 32 site-II Agþ ions in the dehydrated structure redistributed to two different 3-fold axis Agþ positions: 27 Agþ ions at site II coordinate to ethylene molecules and five Agþ ions at site II0 do not. To coordinate to ethylene, each of the 27 site-II Agþ ions has moved along their 3-fold axes more 1:08 0:28 ¼ 0:80 A deeply into the supercage from their three-O(2) plane (see Fig. 2 and Table 5). In this way, these Agþ ions are able to coordinate more tetrahedrally to ethylene (considering ethylene to be monodentate). Also the 32 Agþ ions at site III0 in the dehydrated structure have redistributed to four different III0 sites. The 12 Agþ ions at Ag(1) are octahedrally coordinated at site I, at the centers of the D6Rs (see Figs. 3 and 4). This 16-fold position is only 75% , is occupied. The Ag(1)–O(3) distance, 2.63(2) A approximately equal to the sum of the corre, indicative of a reasponding ionic radii, 2.58 A from sonably good fit. Each Ag(1) ion is 1.62 A two O(3)-oxygen planes. The 16 Agþ ions at Ag(2) are at site I0 , in the sodalite cavities opposite the D6Rs (see Fig. 4). This 32-fold position is only half occupied. Each Agþ ion lies relatively far inside the sodalite cavity, from the plane of the three O(3) framework 1.33 A oxygens of the D6R to which it is bound. The , is a little shorter Ag(2)–O(3) distance, 2.47(3) A . than the sum of the ionic radii, 2.58 A The distance between Ag(1) and Ag(2) (see Fig. , substantially shorter than the 4) is 2.951 A
Atom Si Al O(1) O(2) O(3) O(4) Ag(1) Ag(2) Ag(3) Ag(4) Ag(5) Ag(6) Ag(7) Ag(8) C H1e H2e
Wyc. Pos. 96(g) 96(g) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 96(g) 96(g) 96(g) 96(g) 96(g) 96(g) 96(g)
Site
x
y
z
U11 b or Uiso c
U22
U33
U12
U13
U23
Occupancyd Varied
I I0 II0 II III0 III0 III0 III0
)528(4) )526(4) )1082(10) )15(12) )305(11) )744(12) 0 682(5) 1981(22) 2410(2) 1551(34) 2084(27) 2688(30) 1651(31) 2798(44) 2881 2503
1242(7) 364(4) )4(13) )14(12) 723(12) 774(11) 0 682(5) 1981(22) 2410(2) 973(34) 1621(27) 1749(28) 1852(34) 2857(46) 2983 2605
356(4) 1254(7) 1070(10) 1455(9) 703(12) 1751(12) 0 682(5) 1981(22) 2410(2) 4093(36) 4199(26) 4208(25) 4137(29) 3175(38) 3533 3155
130(27) 84(26) 206(87) 138(75) 215(88) 148(77) 340(44) 706(67) 872(237) 441(32) 1105(261) 668(187) 1203(240) 254(203) 708(376)
340(44) 706(67) 872(237) 441(32)
340(44) 706(67) 872(237) 441(32)
156(44) 281(73) 600(303) 171(30)
156(44) 281(73) 600(303) 171(30)
156(44) 281(73) 600(303) 171(30)
12.1(7) 16.0(9) 5.5(4) 26.5(4) 8.4(19) 9.6(20) 11.4(14) 5.6(17) 53.0(7)
Fixed 96 96 96 96 96 96 12 16 5 27 8 8 11 5 54 54 54
a . The space group is Fd3 with origin chosen at the center of symmetry. Positional and anisotropic thermal parameters are given · 104 . Numbers in a ¼ 24:981ð9Þ A parentheses are the esds in the units of the least significant digit given for the corresponding parameter. b The anisotropic temperature factor ¼ exp[()2p2 /a2 )(U11 h2 +U22 k 2 +U33 l2 +U12 hk+U13 hl+U23 kl)]. c Biso ¼ 8p2 Uiso . d Occupancy factors are given as the number of atoms or ions per unit cell. Four or more (4 þ x) Si atoms [30] are present at the Al position per unit cell; x Al atoms may therefore be at the Si position. e . Hydrogen atom positions were calculated [25] with a C–H distance of 0.97 A
E.Y. Choi et al. / Microporous and Mesoporous Materials 62 (2003) 201–210
Table 2 Positional, thermal, and occupancy parametersa for Ag92 –X Æ 27C2 H4
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Table 3 ) and angles (deg)a Selected interatomic distances (A Distance 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 Ag(1)–O(3) Ag(2)–O(3) Ag(3)–O(2) Ag(4)–O(2) Ag(5)–O(1) Ag(6)–O(1) Ag(6)–O(4) Ag(7)–O(1) Ag(7)–O(4) Ag(8)–O(1) Ag(8)–O(4) Ag(1)–Ag(2) Ag(4)–C C–C C–H(1) C–H(2) H(2)–O(4)
Angle 1.66(3) 1.67(3) 1.65(3) 1.65(3) 1.66 1.73(3) 1.67(3) 1.73(3) 1.70(3) 1.71 2.63(2) 2.47(3) 2.30(4) 2.41(2) 2.72(5) 2.32(7) 2.57(7) 2.35(7) 2.88(5) 2.57(8) 2.25(8) 2.951(6) 2.42(10) 1.24(15) 0.97b 0.97b 2.89b
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
114.1(15) 111.3(16) 103.0(16) 106.3(14) 110.3(14) 111.9(15) 113.3(15) 108.7(16) 104.9(16) 106.8(14) 111.5(14) 111.7(14) 133.7(17) 139.9(18) 135.5(18) 140.6(21)
O(3)–Ag(1)–O(3) O(3)–Ag(2)–O(3) O(2)–Ag(3)–O(2) O(2)–Ag(4)–O(2) O(1)–Ag(5)–O(1) O(1)–Ag(6)–O(4) O(1)–Ag(7)–O(4) O(1)–Ag(8)–O(4)
86.1(10)/93.9(10) 93.5(11) 109.0(23) 101.7(8) 87.8(8) 67.4(20) 58.4(10) 64.6(21)
a Numbers in parentheses are estimated standard deviations in the units of the least significant digit given for the corresponding value. b Hydrogen positions were calculated [25].
distance in the dehycorresponding 3.157(4)-A drated structure (empty (Agþ )92 Si100 Al92 O384 ) [21]. This is indicative, as before, of a weak Agþ –Agþ interaction [20,21]. Distances ranging from have been reported for 2.655(2) to 3.058(1) A similar Agþ –Agþ interactions in seven organic complexes of Agþ [31]. Perhaps the most sensible placement of 12 Ag(1) ions and 16 Ag(2) ions in the 16 D6Rs is 8 D6Rs with Agþ –Agþ –Agþ as shown in Fig. 4, 4 D6Rs with one Agþ ion at their center as shown in Fig. 3, and 4 D6Rs with no Agþ ions at either position. D6Rs with two Ag(2) ions (with no Ag(1) cation) are not reasonable because the two Agþ ions should be much closer together and have a weak Agþ –Agþ interaction as was seen in the crystal structure of empty (Agþ )92 Si100 Al92 O384 [20]. D6Rs with one ion at Ag(1) and one at Ag(2) are unlikely
because Ag(2) does not coordinate to a ligand [20]; it should relax to give two equivalent Agþ ions (as above) with a weak Agþ –Agþ interaction. Multiple Agþ weak interactions (nets) are common [32], and frequently contain linear (Agþ )3 sequences. Thirty-two Agþ ions fill the single 6-rings, 27 at site II and 5 at site II0 . The five Agþ ions at site II0 (at Ag(3)) lie inside the sodalite cavities opposite S6Rs. Each Agþ ion at Ag(3) coordinates at to three O(2) framework oxygens, and is 2.30(4) A into the sodalite cavity from their recessed 0.78 A plane (see Fig. 5). The 27 Agþ ions at Ag(4) lie at site II; each of these coordinates to three frame and extends 1.08 A into work oxygens at 2.41(2) A the supercage where it forms a lateral p-complex with an ethylene molecule. The 54 carbon atoms in this structure (27 molecules of C2 H4 ) are equivalent at Wyckoff position
E.Y. Choi et al. / Microporous and Mesoporous Materials 62 (2003) 201–210
Fig. 1. A stylized drawing of the framework structure of zeolite X. Near the center of each line segment is an oxygen atom. The nonequivalent oxygen atoms are indicated by the numbers 1–4. Silicon and aluminum atoms alternate at the tetrahedral intersections, except that Si must substitute for at least 4% of the Al atoms. Extra framework cation positions are labeled with Roman numerals.
Table 4 Distribution of Agþ cations among sites Sites I I0 II0 II III0
a; b
a
Ag96 –X
Ag92 –X
Ag92 –X Æ 27C2 H4
32
4 24
12 16 5 27 32d
32 32
32 32
c
a
Ref. [21]. Predicted (for LSX). c This work. d At four nonequivalent III0 sites. b
96(g). The two carbon atoms of each ethylene molecule are equidistant from Ag(4). The center of the C@C bond does not lie on a 3-fold axis of the crystal structure. The bonding between the silver ion at Ag(4) and ethylene can be described in terms of two components according to the Chatt–Dewar model [33]. A r component arises from the overlap of a filled ethylene p orbital and a vacant Agþ 5s orbital, and a p component occurs by the overlap of the filled 4d orbitals of Agþ with the vacant antibonding p orbital of ethylene. By the latter interaction, the
207
Fig. 2. A Ag(C2 H4 )þ complex on the inner surface of a supercage. Twenty-seven Agþ ions per unit cell at Ag(4) coordinate to ethylene molecules as shown. Two hydrogen atoms of each ethylene molecule are calculated by SHELX97 to interact weakly with O(2) oxygens. Ellipsoids of 20% probability are used.
Table 5 ) of atoms from 6-ring oxygen planes Deviations (A Position
Cation site
Displacement
At O(2)a; b
Ag(3) Ag(4) C
II0 II
)0.779 1.077 3.46
At O(3)b;c
Ag(1) Ag(2)
I I0
)1.618 1.333
a A positive displacement indicates that the atom or ion lies in the supercage. b Sites I0 and II0 are in the sodalite units. c The negative displacement indicates that the ion lies within a double 6-ring.
C@C bond order may be diminished by back-donation. Carter et al. [9] studied the sorption of ethylene onto a series of transition–metal exchanged zeolite X samples using calorimetric and IR methods. They found that ethylene is relatively weakly held and that it can readily be removed by evacuation at room temperature, unless the cation involved is Agþ or Cd2þ . The Ag(4)–C bond length herein
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Fig. 3. Stereoview of a double 6-ring in Ag92 –X Æ 27C2 H4 . A Agþ ion at Ag(1) (site I) is at its center. Although this occurs in 12 of the 16 D6Rs per unit cell, it is proposed that only 4 of the 16 D6Rs are occupied as shown, with no additional Agþ ions nearby. Ellipsoids of 20% probability are used.
Fig. 4. Stereoview of a double 6-ring in Ag92 –X Æ 27C2 H4 . Two Agþ ions at Ag(2) (site I0 ) and one at Ag(1) (site I) are shown. It is ) interactions are shown with dashed proposed that 8 of the 16 D6Rs per unit cell have this arrangement. Two weak Agþ –Agþ (2.951 A lines. Ellipsoids of 20% probability are used.
Fig. 5. Stereoview of a sodalite cavity in Ag92 –X Æ 27C2 H4 . One Agþ ion at Ag(3) (site II0 ) and three Agþ ions at Ag(4) (site II) are shown. Each Agþ ion at Ag(4) coordinates to an ethylene molecule. About five-eights of the sodalite cavities may have this arrangement. The remaining three-eights may have four Agþ ions at Ag(4). Ellipsoids of 20% probability are used.
, indicates that the ethylene is reported, 2.42(10) A relatively firmly held by the Agþ ion at Ag(4). This bond length can be compared with distances of in acenaphthene AgClO4 [34], 2.45– 2.48–2.51 A 2.56 A in anthracene [AgClO4 ]4 Æ 4H2 O [35], 2.50– in benzene AgClO4 [35], 2.60–2.63 in 2.63 A
naphthalene [AgClO4 ]4 Æ 4H2 O [35] and 2.47–2.92 in benzene AgAlCl4 [36]. The Agþ –C distance A in zeolite X is insignificantly different from that in an ethylene complex of zeolite A [11] (Agþ –C ¼ ). For comparison, the corresponding M– 2.54(8) A C distances in other intrazeolitic cation–ethylene
E.Y. Choi et al. / Microporous and Mesoporous Materials 62 (2003) 201–210 Table 6 Comparison of ethylene–cation approach distances in zeolites A and X ) ) Structure M–C (A Cation radius (A Ag92 –X Æ 27C2 H4 Co4 Na4 –A Æ 4C2 H4 Ag12 –A Æ 3.6C2 H4 Cd6 –A Æ 4C2 H4 Cd46 –X Æ 29.5C2 H4 Mn46 –X Æ 30C2 H4 Ca6 –A Æ 4C2 H4 Ca46 –X Æ 30C2 H4
2.42(10) 2.51(6) 2.54(8) 2.67(6) 2.70(4) 2.76(6) 2.87(5) 2.98(4)
1.26 0.72 1.26 0.97 0.97 0.80 0.99 0.99
complexes are given in Table 6. Metal cation to ethylene carbon distances in zeolites range from (this work) to 2.98(4) A . 2.42(10) A , is imprecisely The C@C distance, 1.24(15) A determined and is likely to suffer from foreshortening. As such, this work cannot say that it differs significantly from the C@C bond length in ethyl. In nonzeolitic transition–metal ene gas, 1.344 A complexes, a broad range of C@C distances (from ) can be observed [37]. The 1.354(15) to 1.46(2) A C@C distances in other zeolite structures are also imprecisely determined due to the combined effects of disorder and high thermal motion of the carbon in Mn46 – atoms. They range widely: 1.10(8) A X Æ 30C2 H4 [14], 1.19(12) A in Ag12 –A Æ 3.6C2 H4 in Co4 Na4 –A Æ 4C2 H4 [12], 1.26(5) [11], 1.21(11) A in Ca6 – A in Cd46 –X Æ 29.5C2 H4 [10], and 1.48(7) A A Æ 4C2 H4 [15].
209
Effective ethylene ) radius (A
Ref.
1.16 1.79 1.28 1.70 1.73 1.96 1.88 1.99
This work [12] [11] [13] [10] [14] [15] [16]
The positions of the four hydrogen atoms of each ethylene molecule were calculated with a C–H using the software system bond length of 0.97 A SHELX97 [25]. In each molecule, two cis hydro from an O(4) gen atoms can each be about 2.89 A framework oxygen (see Fig. 2). This indicates that they can interact weakly (electrostatically) with the anionic zeolite framework; the sum of the van der Waals radii of oxygen and hydrogen is 1:4 þ 1:2 ¼ [38]. 2:6 A The remaining 32 Agþ ions occupy four different III0 sites (Ag(5), Ag(6), Ag(7), and Ag(8)) in the supercage with occupancies of 8, 8, 11, and 5, respectively (see Fig. 6). Each of these Agþ ions coordinates to only two framework oxygens. The great diversity of III0 positions (there was only one III0 position in dehydrated Ag92 –X [21]) suggests that some may coordinate further to ethylene
Fig. 6. Stereoview of a supercage in Ag92 –X Æ 27C2 H4 . One Agþ ion at Ag(5) (site III0 ), one at Ag(6) (site III0 ), one at Ag(7) (site III0 ), and one at Ag(8) (site III0 ) are shown. About five-eights of the supercages may have this arrangement. The remaining three-eights may have one Agþ ion at Ag(5), one at Ag(6), and two at Ag(7). Ellipsoids of 20% probability are used.
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molecules that could not be located in this work. Alternatively, the III0 positions may have adjusted to avoid sorbed ethylene molecules and because there are now two S6R positions, II and II0 . 5. Summary Of the 92 Agþ ions per unit cell of Ag92 –X, 27 (of the 32 at site II in the empty zeolite) have moved along their 3-fold axes more deeply into the 0.80 A supercage to coordinate strongly (laterally) to ethylene molecules. The centers of these ethylene molecules lie off these 3-fold axes. Each ethylene molecule makes two electrostatic hydrogen bonds to framework oxygen atoms. The remaining Agþ site occupancies have changed substantially as a result of ethylene sorption. Weak Agþ –Agþ interactions are again seen. Acknowledgements This work was supported in part by a grant from the Korean Research Foundation made in the program year 2001 (Grant No. 2000-015-DP0190).
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