ARTICLE pubs.acs.org/JPCC
Single-Crystal Structures of the o-, m-, and p-Xylene Sorption Complexes of Fully Dehydrated, Fully Mn2+-Exchanged Zeolite Y (FAU) Md. Shamsuzzoha,† Young Hun Kim,‡ and Woo Taik Lim*,† † ‡
Department of Applied Chemistry, Andong National University, Andong 760-749, Korea Department of Environmental Engineering, Andong National University, Andong 760-749, Korea
bS Supporting Information ABSTRACT: Three single crystals of zeolite Y, each 0.3 mm in diameter, fully exchanged with Mn2+ and fully dehydrated, were treated with zeolitically dry o-, m-, and p-xylene, respectively, at 297(1) K, followed by evacuation. Their crystal structures were determined by synchrotron X-ray diffraction techniques in the space group Fd3m at 100(1) K and were refined using all intensities to the final ̅ error indices R1/wR2 = 0.056/0.160, 0.080/ 0.217, and 0.066/0.177, respectively. In each structure, Mn2+ ions occupy sites I, I0 , II0 , and II. In each structure, 18 Mn2+ ions per unit cell at site II (on the 3-fold axes of the single 6-rings) each extend 0.63, 0.68, and 0.67 Å into the supercage to coordinate facially to an o-, m-, and p-xylene molecule, respectively. The corresponding distances from Mn2+ to the centers of the xylene rings are 2.67, 2.65, and 2.63 Å. The methyl groups are all somewhat off the planes of their rings, indicative of a net repulsive interaction with oxygen atoms of the zeolite framework. The hydrogen atoms of the xylene rings all have 3.3 Å interactions with oxygen atoms of the single 6-rings.
1. INTRODUCTION Zeolites are microporous crystalline solids of great importance in industrial processes; a major one of those is catalysis. To become active catalysts, they are often ion-exchanged with transition-metal cations and dehydrated.1,2 Some of these zeolites catalyze the isomerization/disproportionation of xylenes,3 the disproportionation of trimethylbenzenes,4 and the side chain alkylation of aromatic compounds, such as toluene and ethylbenzene.5 Acidic forms can selectively catalyze ring alkylation.6 In addition, faujasite zeolites have been used to separate the isomers of xylene, utilizing the difference in their heats of sorption onto the exchanged cations.5 Mn2+ was selected as a representative transition-metal cation for this study because complete Mn2+-exchange into faujasite zeolites is readily achieved without the complications of hydrolysis. Several structural studies have been performed on the sorption of aromatic molecules, such as benzene,79 mesitylene,10,11 and toluene,12 in faujasite-type zeolites. Fewer1316 have been carried out to obtain this information for xylene sorption; these were done using neutron powder diffraction techniques. Mellot et al.13 investigated the sorption and the cosorption of xylenes in fully dehydrated Ba2+-exchanged zeolite X at two different low temperatures by powder neutron diffraction (PND). They reported the sorption sites for m-xylene and found that the locations of the sorbed molecules depended upon the extent of sorption. They concluded that the crystallographic arrangement of the sorbate was strongly dependent on intermolecular interactions. Mellot et al.14 also studied the sorption of m- and p-xylene in dehydrated Ba2+-exchanged zeolite X by thermogravimetry, r 2011 American Chemical Society
isothermal differential calorimetry, and PND. They showed that Ba2+-exchanged zeolite X sorbed more m-xylene than p-xylene, 3.0 m-xylene versus 2.75 p-xylene per supercage. Czjzek et al.15 studied the sorption of perdeuterated xylenes in Yb3+,Na+-exchanged zeolite Y at 5 K by PND. The xylene molecules were located near and parallel to the single 6-rings in the supercage, where they interacted facially with the Na+ ions at site II0 . They reported that the o- and p-xylenes were statistically disordered with respect to the “6-fold axis” of the ring and equally distributed over these orientations, whereas m-xylene molecules occupied only three of these orientations due to interactions of the methyl groups with the framework oxygens. Pichon et al.16 determined the structures of the m- or p-xylene sorption complexes of partially hydrated Ba2+-exchanged zeolite X by PND. m-Xylene was found near a Ba2+ ion at site II in the supercage; some Ba2+ ions, up to two per supercage, moved to site II to coordinate to m-xylene molecules. p-Xylene was found in the 12-ring windows (a sorption site for benzene79,17); m-xylene was not found there: this was attributed to the orientation of its methyl groups. As a result, more p-xylene than m-xylene could be sorbed despite a higher population of m-xylene at the site-II Ba2+ ion. Pichon et al.18 also studied the sorption of m-xylene on partially hydrated Ba2+-exchanged zeolite X by thermogravimetry, mass Received: June 21, 2011 Revised: July 30, 2011 Published: August 02, 2011 17750
dx.doi.org/10.1021/jp205813k | J. Phys. Chem. C 2011, 115, 17750–17760
The Journal of Physical Chemistry C
ARTICLE
Table 1. Summary of Experimental and Crystallographic Data crystal 1
crystal 2
crystal 3
crystal cross section (mm)
0.30
0.30
Mn2+ exchange (day, mL, T (K))
20, 200, 343
20, 200, 343
0.31 20, 200, 343
dehydration T (K)
723
723
723
crystal color after dehydration
pale brown
pale brown
pale brown
sorption T (K), t (day), P (torr)
297(1), 3, 6.2
297(1), 3, 7.8
297(1), 3, 8.3
evacuation T (K), t (h)
297(1), 1
297(1), 1
297(1), 1
crystal color after sorption/evac.
black
dark gray
black
data collection T (K) space group, Z
100(1) Fd3m, 1
100(1) Fd3m, 1
100(1) Fd3m, 1
X-ray source
Pohang Light Source (PLS),
wavelength (Å)
0.90000
0.90000
0.90000
unit cell constant, a (Å)
24.5943(1)
24.5969(1)
24.5860(2)
Beamline 6B MXI BL
2θ range in data collection (deg)
60.72
60.71
60.74
no. of unique reflections, m
574
566
574
no. of reflections with Fo > 4σ(Fo) no. of variables, s
566 58
565 55
572 58
data/parameter ratio, m/s
9.9
10.3
9.9
weighting parameters, a/b
0.0726/433.4
0.1184/289.1
0.0746/751.8
final error indices
a b
R1/wR2 (Fo > 4σ(Fo))a
0.056/0.160
0.080/0.217
0.066/0.177
goodness-of-fitb
1.23
1.30
1.15
R1 = ∑|Fo |Fc||/∑Fo and wR2 = [∑w(Fo2 Fc2)2/∑w(Fo2)2]1/2; R1 and wR2 are calculated using only those reflections for which Fo > 4σ(Fo). Goodness-of-fit = (∑w(Fo2 Fc2)2/(m s))1/2, where m and s are the number of unique reflections and variables, respectively.
spectroscopy, temperature-programmed desorption, and PND at different m-xylene loadings. They found relationships between the crystallographic positions of the sorbed molecules and macroscopic properties, such as the number and the area of the peaks in the desorption spectra for m-xylene. The crystallographic position of the sorbed m-xylene was seen at low and high fillings: the center of the aromatic ring was located on the 3-fold axis at a low filling (0.7 m-xylene molecules per supercage), but the center of the aromatic ring moved away from the cation and left the 3-fold axis at a high filling (2.7 molecules per supercage). The sorbatesorbent interaction between o-, m-, and p-xylene and zeolite Y was investigated by FT-Raman spectroscopy.19 A weakening of the CC bonds in the aromatic ring was observed, indicating the formation of a π complex between the xylenes and the cations employed, Na+, K+, and Ba2+. The methyl groups were, therefore, required to interact with the local framework oxygens, members of the 6-ring that played as the host to the cation. We sought to determine the structures of the o-, m-, and p-xylene complexes of fully dehydrated, fully Mn2+-exchanged zeolite Y to learn the positions of the sorbed molecules with some precision and other details of the cationsorbate interactions, such as the shifts of the Mn2+ ions upon xylene sorption and the interactions of the methyl groups and ring hydrogen atoms with the zeolite framework.
2. EXPERIMENTAL SECTION 2.1. Single-Crystal Preparation. Large colorless single crystals of sodium zeolite Y, stoichiometry of Na75Si117Al75O384 (Na75-Y), with diameters up to 0.32 mm were prepared by Lim
et al.20 Crystals of hydrated Mn2+-exchanged zeolite Y, stoichiometry of Mn37.5Si117Al75O384 (Mn37.5-Y), were prepared by static ion exchange of Na75-Y with aqueous 0.1 M Mn(NO3)2 (Aldrich, 99.99%). Hydrated Na75-Y (10 mg) was mixed with 10 mL of 0.1 M Mn(NO3)2, a 20-fold excess, in a 15 mL conical tube, and the mixture was stirred on a shaking incubator at 343 K for 24 h. This procedure was repeated 20 times with the fresh Mn(NO3)2 solution. Finally, the product was oven-dried at 323 K for 1 day. Three of these crystals, clear pale brown octahedra about 0.30 mm in cross section, were lodged in separate fine Pyrex capillaries. These were attached to a vacuum system, and the crystals were cautiously dehydrated by gradually increasing their temperatures (ca. 25 K/h) under dynamic vacuum to 723 K, followed by 2 days at this temperature and 1.3 104 Pa. While these conditions were maintained, the hot contiguous downstream lengths of the vacuum system, including two sequential U-tubes of zeolite 5A beads fully activated in situ, were cooled to ambient temperature to prevent the movement of water molecules from more distant parts of the vacuum system to the crystals. Still under vacuum in their capillaries, the crystals were then allowed to cool at room temperature; they remained pale brown in color. To prepare the o-, m-, and p-xylene sorption complexes of zeolite Y, the crystals were separately exposed to 6.2, 7.8, and 8.3 Torr of zeolitically dried o-xylene (Aldrich, anhydrous 97%), m-xylene (Aldrich, anhydrous g99%), and p-xylene (Aldrich, anhydrous g99%), respectively, for 3 days at 297(1) K, and then evacuated for 1 h at this temperature. The crystals were then sealed under vacuum in their capillaries by torch and removed from the vacuum line. Microscopic examination showed that the crystals had become black, dark gray, and black, respectively. 17751
dx.doi.org/10.1021/jp205813k |J. Phys. Chem. C 2011, 115, 17750–17760
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Table 2. Steps of Structure Determination as Nonframework Atomic Positions step
occupancya at Mn(1)
Mn(10 )
Mn(20 )
R1
wR2
0.4367
0.8290
0.3507
0.7895
16.4(5)
0.1006
0.3035
0.0825
0.2520
0.0728
0.2007
0.0673
0.1898
0.0628
0.1755
0.0615 0.0558
0.1721 0.1518
Mn(2)
C(1)
C(2)
C(3)
C(4)
crystal 1 1b 2c
12.0(9)
3c
14.2(4)
4c
13.9(3)
7.3(8)
17.6(4)
5c
14.5(3)
3.8(5)
18.5(4)
98(8)
6c
14.6(2)
3.7(4)
1.6(4)
17.8(4)
95(7)
7c
14.6(2)
3.4(4)
1.4(4)
17.5(4)
71(7)
44(7)
8c 9c
14.7(2) 14.2(2)
3.7(4) 3.8(3)
1.2(3) 1.9(4)
17.4(3) 18.2(3)
76(9) 90(9)
46(8) 58(9)
10c,d
25(6) 33(7)
91(12)
14.4(2)
3.5(4)
1.3(3)
17.9(3)
53.8(10)
53.8(10)
17.9(3)
17.9(3)
0.0594
0.1645
11e
14
4
1.5
18
54
54
18
18
0.0597
0.1663
12f
14
4
1.5
18
54
54
18
18
0.0575
0.1632
14g
14
4
1.5
18
54
54
18
18
0.0564
0.1600
0.4476
0.8228
0.3594 0.1469
0.7686 0.5307
crystal 2 1b 2c 3c
8.9(9) 14.5(4)
4c
13.4(3)
5.9(5)
17.2(5)
5c
13.4(3)
6.1(5)
16.4(5)
21(7)
6c
14.3(3)
3.1(4)
17.8(4)
80(9)
92(10)
0.0934
0.2456
7c
14.3(3)
3.0(4)
1.8(4)
17.5(4)
86(9)
74(9)
0.0848
0.2328
8c
14.4(3)
2.5(3)
2.6(4)
17.9(4)
97(11)
66(9)
0.0809
0.2177
cd
9,
14.5(3)
2.6(3)
2.4(4)
18.3(4)
54.8(11)
54.8(11)
36.6(7)
0.0847
0.2313
10e 11f
14 14
4 4
1.5 1.5
18 18
54 54
54 54
36 36
0.0848 0.0817
0.2310 0.2244
12g
14
4
1.5
18
54
54
36
0.0799
0.216
0.4456
0.8121
0.3498
0.7661
15.6(5)
0.1135
0.3298
0.0937
0.2753
0.0873
0.2523
0.0771 0.0701
0.2024 0.1977
0.0682
0.1841
0.0659
0.1753
13.7(5)
81(13)
0.1324
0.4722
0.1287
0.4637
crystal 3 1b 2c
6.2(7)
3c
14.0(4)
4c
13.5(3)
13.0(12)
16.7(4)
5c
13.8(3)
11.7(9)
17.0(4)
52(8)
6c 7c
13.9(3) 13.8(3)
4.7(5) 4.9(5)
2.1(4)
18.1(4) 17.8(4)
106(9) 101(9)
60(8) 60(8)
8c
13.6(3)
4.9(4)
2.6(4)
18.3(4)
82(9)
92(11)
78(15)
9c
13.7(2)
4.8(4)
2.4(4)
18.2(4)
68(12)
46(17)
78(15)
10c,d
13.7(3)
5.2(5)
2.1(4)
18.4(4)
55.1(11)
55.1(11)
18.4(4)
18.4(4)
0.0673
0.1768
11e
14
4
1.5
18
54
54
18
18
0.0693
0.1820
12f
14
4
1.5
18
54
54
18
18
0.0681
0.1843
14g
14
4
1.5
18
54
54
18
18
0.0658
0.1766
66(14)
a
Occupancy is given as the number of Mn2+ ions and C atoms per unit cell. b Only the atoms of the zeolite framework were included in the initial structure model. Framework atoms were allowed to refine anisotropically. c Isotropic temperature factors were used for all Mn2+ and carbon positions. d Constraints were introduced for Mn(2), C(1), C(2), C(3), and C(4). e The occupancies of the Mn and C atoms were fixed. f All Mn atoms were refined anisotropically. g The positions of the hydrogen atoms were calculated.
2.2. Single-Crystal X-ray Diffraction. Synchrotron X-ray
diffraction data for the three crystals were collected at 100(1) K using an ADSC Quantum210 detector at Beamline 6B MXI at the Pohang Light Source (Korea). Crystal evaluation and data collection were done with a detector-to-crystal distance of 6.0 cm. Preliminary cell constants and an orientation matrix for each crystal were determined from 36 sets of frames collected at scan intervals of 5° with an exposure time of 1 s per frame. The
basic data file was prepared using the HKL2000 program.21 The reflections were successfully indexed by the automated indexing routine of the DENZO program.21 The total of 39 583, 39 312, and 39 735 reflections were harvested for crystals 1, 2, and 3, respectively, by collecting 72 sets of frames with a 5° scan and an exposure time of 1 s per frame. These highly redundant data sets were corrected for Lorentz and polarization effects; negligible corrections for crystal decay were also applied. The space group 17752
dx.doi.org/10.1021/jp205813k |J. Phys. Chem. C 2011, 115, 17750–17760
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Table 3. Positional, Thermal, and Occupancy Parametersa occupancyc Wyckoff cation atoms
position
site
x
y
1225(1)
z
b
U11
U22
358(1)
106(9)
66(9)
U33
U23
U13
U12
initial
varied
fixed
crystal 1 Si,Al
192(i)
532(1)
85(9)
16(5)
1(5)
4(5)
192
O(1)
96(h)
1076(1) 0
1076(1)
188(18)
169(27)
188(18)
62(15)
11(21)
62(15)
96
O(2)
96(g)
25(2)
25(2)
1469(2)
174(17)
174(17)
153(27)
24(15)
24(15)
62(21)
96
O(3)
96(g)
311(2)
623(2)
623(2)
140(27)
177(18)
177(18)
51(21)
38(15)
38(15)
96
O(4)
96(g)
822(1)
822(1)
3167(2)
147(17)
147(17)
222(29)
5(15)
5(15)
59(21)
96
0
98(8)
98(8)
98(8)
20(7)
Mn(1)
16(c)
I
0
0
20(7)
20(7)
14.4(2)
Mn(10 ) Mn(20 )
32(e) 32(e)
I0 II0
644(6) 2086(18)
644(6) 644(6) 413(57) 2086(18) 2086(18) 528(147)
413(57) 413(57) 18(65) 528(147) 528(147) 345(217)
18(65) 345(217)
18(65) 345(217)
3.5(4) 1.3(3)
Mn(2)
32(e)
II
2321(1)
2321(1)
149(10)
34(9)
34(9)
17.9(3)
18
C(1)
96(g)
2695(13)
2695(13) 3453(18) 1519(154)
53.8(10)
54
C(2)
96(g)
2507(19)
3168(13) 3168(13) 1573(167)
53.8(10)
54
C(3)
96(g)
2103(43)
3493(26) 3493(26) 1599(492)
17.9(3)
18
C(4)
96(g)
2545(38)
2545(38) 4031(28) 1731(560)
17.9(3)
H(1)d
2540
2540
3761
1823
36
H(2)d H(3a)d
2199 1973
3323 3808
3323 3289
1887 2206
36 18
H(3b) d
2240
3608
3833
2206
18
H(3c) d
1769
3271
3565
2206
18
H(4a) d
2205
2719
4113
2066
18
H(4b) d
2820
2637
4262
2066
18
H(4c) d
2480
2150
4031
2066
2321(1)
149(10)
149(10)
34(9)
14 4 1.5
18
18 ∑Mn2+ = 37.1(6)
37.5
crystal 2 Si,Al
192(i)
532(1)
358(1)
170(12)
134(12)
O(1)
96(h)
151(12)
17(5)
2(6)
7(5)
192
1076(2) 0
1076(2)
263(22)
223(31)
263(22)
57(16)
18(25)
57(16)
O(2)
96
96(g)
25(2)
25(2)
1468(2)
265(21)
265(21)
197(30)
23(17)
23(17)
29(24)
96
O(3)
96(g)
310(2)
624(2)
624(2)
221(31)
236(21)
236(21)
28(24)
30(16)
30(16)
96
O(4)
96(g)
821(2)
821(2)
3163(2)
208(21)
208(21)
272(32)
23(16)
23(16)
63(24)
96
1225(1)
Mn(1)
16(c)
I
0
0
0
160(11)
160(11)
160(11)
26(8)
26(8)
26(8)
14.5(3)
14
Mn(10 )
32(e)
I0
634(7)
634(7)
634(7)
490(65)
490(65)
490(65)
3(71)
3(71)
3(71)
2.6(3)
4
Mn(20 ) Mn(2)
32(e) 32(e)
II0 II
2063(8) 2331(1)
2063(8) 2331(1)
2063(8) 2331(1)
206(69) 263(13)
206(69) 263(13)
206(69) 263(13)
45(86) 61(12)
45(86) 61(12)
45(86) 61(12)
2.4(4) 18.3(4)
1.5 18
C(1)
96(g)
2755(11)
2755(11) 3359(17) 1201(116)
54.8(11)
54
C(2)
96(g)
2485(16)
3188(11) 3188(11) 1113(110)
54.8(11)
54
C(3)
96(g)
2537(31)
2537(31) 3960(44) 2683(532)
36.6(7)
H(1)d
2605
2605
3672
1441
18
H(2)d
2174
3339
3339
1335
54
H(3a)d
2234
2754
4008
3442
36
H(3b)d H(3c)d
2830 2459
2596 2172
4172 3874
3442 3442
36 36 ∑Mn2+ = 37.8(7)
36
37.5
crystal 3 Si,Al
192(i)
529(1)
359(1)
126(10)
75(10)
O(1)
96(h)
92(10)
17(6)
1(6)
8(6)
192
1069(2) 0
1069(2)
228(22)
181(32)
228(22)
62(18)
14(26)
62(18)
O(2)
96
96(g)
22(2)
22(2)
1474(2)
183(20)
183(20)
183(32)
22(18)
22(18)
48(26)
96
O(3)
96(g)
304(2)
626(2)
626(2)
196(34)
221(23)
221(23)
74(27)
63(19)
63(19)
96
O(4)
96(g)
820(2)
820(2)
3174(2)
135(20)
135(20)
226(33)
19(18)
19(18)
76(25)
96
Mn(1) Mn(10 )
16(c) 32(e)
0 663(7)
0 663(7)
0 663(7)
135(10) 367(65)
135(10) 367(65)
135(10) 367(65)
21(9) 137(78)
21(9) 137(78)
21(9) 137(78)
I I0
1226(1)
17753
13.7(3) 5.2(5)
14 4
dx.doi.org/10.1021/jp205813k |J. Phys. Chem. C 2011, 115, 17750–17760
The Journal of Physical Chemistry C
ARTICLE
Table 3. Continued occupancyc Wyckoff cation position site
U33
250(88)
250(88)
18(118) 18(118) 18(118)
2.1(4)
2330(1)
2330(1)
164(11)
164(11)
39(12)
18.4(4)
18
96(g)
2721(13)
2721(13) 3392(19) 1229(136)
55.1(11)
54
96(g)
2512(23)
3162(15) 3162(15) 1586(196)
55.1(11)
54
C(3)
96(g)
2534(52)
2534(52) 3943(41) 2227(877)
18.4(4)
18
C(4)
96(g)
2511(42)
2511(42) 3936(37) 1727(630)
18.4(4)
H(1)d
2565
2565
3700
1474
36
H(2)d H(3a)d
2205 2190
3317 2709
3317 4037
1903 2590
36 18
H(3b)d
2786
2560
4209
2590
18
H(3c)d
2424
2131
3905
2590
18
H(4a)d
1927
3742
3221
1895
18
H(4b)d
2128
3534
3790
1895
18
H(4c)d
1734
3175
3440
1895
Mn(2 )
32(e)
II
Mn(2)
32(e)
II
C(1) C(2)
0
y
z 2330(1)
U11
164(11)
U23
U13 39(12)
U12
fixed
U22
2074(10) 2074(10) 250(88)
0
x
b
2074(10)
atoms
39(12)
initial
varied
1.5
18
18 ∑Mn2+ = 39.4(8)
37.5
a Positional parameters 104 and thermal parameters 104 are given. Numbers in parentheses are the estimated standard deviations in the units of the least significant figure given for the corresponding parameter. b The anisotropic temperature factor is exp[2π2a2(U11h2 + U22k2 + U33l2 + 2U23kl + 2U13hl + 2U12hk)]. c Occupancy factors are given as the number of atoms or ions per unit cell. d Hydrogen atom positions were calculated by the suite of computer programs SHELX-97.
Table 4. Fixed Occupancy Possibilities and Selectionsa occupancies at sites I
I0
II0
II
1
14
4
1.5
18
2
14.5 14
3 4
2 1.5
18 18
more than 2 esd's at II0
13.5
5
1
18
almost 3 esd's at II0
14
4
1.5
18
more than 2 esd's at I0
crystal
3
changesb
a
The occupancies selected for all three crystals are 14, 4, 1.5, and 18. To reach the occupancies given above, the largest changes from the least-squares values are given.
b
Fd3m, standard for zeolite Y, was determined by the program XPREP.22 A summary of the experimental and crystallographic data is presented in Table 1.
3. STRUCTURE DETERMINATION Full-matrix least-squares refinement using SHELXL9723 was done on Fo2 using all data for each crystal. Each refinement was initiated with the atomic parameters of the framework atoms [(Si,Al), O(1), O(2), O(3), and O(4)] in dehydrated |Mn37.5|[Si117Al75O384]-FAU.24 Anisotropic refinement of the framework atoms converged to the high unweighted R1 indices, 0.44, 0.45, and 0.45, and weighted R2 indices, 0.83, 0.82, and 0.81, for the o-, m-, and p-xylene sorption complexes, respectively. These R values are defined in a footnote to Table 1. The positions of the hydrogen atoms in the xylene molecules were calculated using SHELXL97.23 The progress of structure determination as subsequent peaks were found on difference Fourier functions and identified as nonframework atoms is given in Table 2. 3.1. Occupancy Constraints. It became apparent in the final cycles of refinement that the occupancies of the atoms of the
xylene molecules and those at Mn(2) were related (see Table 2). Accordingly, the occupancies at Mn(2), C(1), C(2), C(3), and C(4) were constrained in the ratios of 1:3:3:1:1, 1:3:3:2, and 1:3:3:1:1 for o-, m-, and p-xylene, respectively, indicating that one xylene molecule interacts facially with each Mn2+ ion at site II. The effect of these constraints can be seen in Table 2. 3.2. Final Refinement and Other Crystallographic Details. The number of Mn2+ ions (the sum of their occupancies) per unit cell in crystals 1, 2, and 3 had refined to 37.1(6), 37.8(7), and 39.4(8), respectively. Two of these values are within 1 esd, and the third is within 2.4 esd's of 37.5, the number of Mn2+ ions needed to balance the anionic charge, 75, of the zeolite framework. Considering sites I and I0 , it is clear that site I is preferred in all three crystals. It is also clear that the D6Rs are all fully occupied: each either holds a Mn2+ ion at its center (Figure 2a) or hosts two at I0 sites as, shown in Figure 2b, satisfying the rule for maximum occupancy, (no. at I) + (no. at I0 )/2 = 16. In crystal 1, 14.4 Mn2+ ions occupy site I, so the remaining 1.6 D6Rs can hold 3.2 Mn2+ ions, in agreement with the refined value of 3.5(4). In crystal 2, these values are 14.5, leading to 3.0, which is in agreement with 2.6(3). In crystal 3, these values are 13.7, leading to 4.6, which is in agreement with 5.2(5). Note that, of the three crystals, crystal 3 has the lowest occupancy at site I and the highest at site I0 , indicating that this shift in Mn2+ occupancy has occurred while satisfying the above rule. However, the excess of Mn2+ ions in crystal 3, 39.4(8), albeit significant only at the 2.4 esd level, argues against this result because decreasing the occupancy at site I and increasing that at I0 , while obeying the rule, increases the total number of Mn2+ ions per unit cell further. This is contrary to the need to select fixed occupancies that sum to 37.5 Mn2+ ions per unit cell. The occupancy at site II refines to within 1 esd of 18 for all three structures. It may be that unfavorable intermolecular interactions, which increase geometrically with the number of 17754
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Table 5. Selected Interatomic Distances (Å) and Angles (deg)a crystal 1
crystal 2
crystal 3
(Si,Al)O(1)
1.6430(24)
1.643(3)
1.639(3)
(Si,Al)O(2)
1.6740(22)
1.673(3)
1.674(3) 1.707(3)
(Si,Al)O(3)
1.706(3)
1.707(3)
(Si,Al)O(4)
1.6297(16)
1.6281(17)
1.6305(19)
Mean (Si,Al)O
1.663(3)
1.663(3)
1.663(3)
Mn(1)O(3)
2.297(5)
2.299(5)
2.301(6)
Mn(10 )O(3)
2.349(17)
2.322(18)
2.380(20)
Mn(20 )O(2) Mn(2)O(2)
2.152(14) 2.212(5)
2.173(9) 2.228(6)
2.148(11) 2.210(6)
Mn(2)C(1)
3.07(4)
2.93(4)
2.94(4)
Mn(2)C(2)
2.98(5)
3.00(4)
2.93(5)
Mn(2)centerb
2.67
2.65
2.63
C(1)C(2)
1.43(3)
1.323(23)
1.33(3)
1.45(11)
1.499(21)
C(1)C(3) C(1)C(4)
1.509(21)
C(2)C(3) C(2)C(4)
1.501(21) 1.493(21)
H(3a)O(1)
3.10
H(3b)O(1)
3.00
H(3c)O(1)
2.55
2.89
2.80
2.40
H(4a)O(1) H(4b)O(1)
2.84 3.12
H(4c)O(1)
2.89
H(1)O(4) H(2)O(2)
3.33 3.30
2.60 3.45 3.32
3.34 3.29
O(1)(Si,Al)O(2)
112.66(19)
112.71(21)
O(1)(Si,Al)O(3)
105.70(22)
105.86(24)
112.77(23) 105.4(3)
O(1)(Si,Al)O(4)
111.18(23)
111.5(3)
111.1(3)
O(2)(Si,Al)O(4)
107.43(24)
107.3(3)
107.6(3)
O(3)(Si,Al)O(4)
113.1(3)
112.9(3)
113.2(3)
(Si,Al)O(1)(Si,Al)
130.5(3)
130.5(4)
132.1(4)
(Si,Al)O(2)(Si,Al) (Si,Al)O(3)(Si,Al)
135.2(3) 124.1(3)
135.3(4) 124.1(3)
134.6(4) 124.0(4)
(Si,Al)O(4)(Si,Al)
156.4(4)
157.1(4)
154.8(4)
O(3)Mn(1)O(3)
89.95(18), 90.05(18)
89.84(20), 90.16(20)
O(3)Mn(10 )O(3)
87.4(8)
88.7(8)
85.5(9)
O(2)Mn(20 )O(2)
117.1(12)
115.5(6)
116.3(8)
O(2)Mn(2)O(2)
112.24(15)
111.21(17)
111.22(18)
C(2)C(1)C(2)
107(6)
135(5)
117(7)
C(1)C(2)C(1) C(2)C(1)C(3)
133(7)
105(5) 111.3(23)
123(7) 122(3)
C(1)C(2)C(3)
115(3)
C(2)C(1)C(4)
123(3)
C(1)C(2)C(4)
89.25(23), 90.75(23)
118(3)
a
The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. b The center of the xylene ring.
molecules sorbed (see Figure 4), have limited the number of sorbed molecules to 18, leaving 1.5 Mn2+ ions at site II0 (not drawn into the supercage to complex) and, therefore, uncomplexed per unit cell. Perhaps also, after 18 Mn2+ ions have been pulled into the supercage to complex to xylene molecules, this residue of 1.5 Mn2+ ions is needed to better locally balance the charge of the zeolite framework. It is also possible that these
1.5 Mn2+ ions did participate in complexation before the xylene vapor was evacuated from the samples in the final step of crystal preparation. To select fixed occupancies that sum to 37.5 Mn2+ ions per unit cell, the refined occupancies are modified from their leastsquares values by as few esd's as possible. Table 4 presents the integral or half-integral occupancies that have emerged at sites I, 17755
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Figure 1. Stylized drawing of the framework structure of zeolite Y. Near the center of the each line segment is an oxygen atom. The nonequivalent oxygen atoms are indicated by the numbers 14. There is no evidence in this work of any ordering of the silicon and aluminum atoms among the tetrahedral positions, although it is expected that Loewenstein’s rule30 would be obeyed. Extraframework cation positions are labeled with Roman numerals.
I0 , II0 , and II. The occupancies of the Mn and C atoms were fixed, as indicated in Tables 2 and 3. All atoms were refined anisotropically, except the carbon atoms, which were refined isotropically, and the final refinement converged to R1/wR2 = 0.056/ 0.160, 0.080/0.217, and 0.066/0.177 for crystals 1, 2, and 3, respectively. The final structural parameters and selected interatomic distances and angles are presented in Tables 3 and 5, respectively. Fixed weights were used initially for each crystal. The final weights were assigned using the formula w = 1/[σ2(Fo2) + (aP)2 + bP], where P = [max(Fo2,0) + 2Fc2]/3, with a and b as refined parameters (see Table 1). Atomic scattering factors for Mn2+, O, C, H, and (Si,Al) were used.25,26 The function describing (Si,Al) is the weighted mean of the Si4+, Si0, Al3+, and Al0 functions (Si/Al = 1.56). All scattering factors were modified to account for anomalous dispersion.27,28
4. RESULTS AND DISCUSSION The framework structure of faujasite consists of sodalite cages (cubooctahedra) linked together via double 6-rings (D6Rs, hexagonal prisms) to create an open three-dimensional pore system with supercages, large enough to host hydrocarbons, accessible via 12-membered ring windows (12-rings)29 (see Figure 1). Each unit cell has 8 supercages, 8 sodalite cavities, 16 D6Rs, 16 12-rings, and 32 single 6-rings (S6Rs). The exchangeable cations that balance the negative charge of the faujasite framework are found within the zeolite’s windows and cavities. They are usually found at the following sites shown in Figure 1. In these structures, the Mn2+ ions occupy only four sites, site I at the center of a D6R, site I0 in the sodalite cavity on the opposite side of either of the D6R’s 6-rings from site I, site II0 inside the sodalite cavity near an S6R entrance to a supercage, and site II in the supercage adjacent to an S6R. The maximum
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Figure 2. Stereoviews of double 6-rings (D6Rs) of crystals 1, 2, and 3: (a) 14 of the 16 D6Rs per unit cell are occupied by Mn(1) and (b) the remaining two hold Mn(10 ) ions as shown. This is true for all three structures. (The actual coordinates plotted are those of crystal 1.) The zeolite Y framework is drawn with heavy bonds. The coordination of Mn2+ ions to oxygens of the zeolite framework is indicated by light bonds. Ellipsoids of 25% probability are shown.
occupancies at these sites are 16 for site I and 32 for each of the others. A further description is available.20 4.1. Crystal 1. In crystal 1, the Mn2+ ions are found at four sites: I, I0 , II0 , and II. Of the 37.5 Mn2+ ions per unit cell, 14 at Mn(1) occupy the site I at the center of the D6R (see Figure 2a). The Mn(1)O(3) bond distance, 2.297(5) Å, is substantially longer than the sum of the conventional ionic radii of Mn2+ and O2, 0.80 + 1.32 = 2.12 Å,31 as seen before in the structures of |Mn37.5|[Si117Al75O384]-FAU,24 |Mn37.5(C6H6)24|[Si117Al75O384]FAU,7 and |Mn37.5(C9H12)15|[Si117Al75O384]-FAU.10 Using the crystal ionic radius for 6-coordinate Mn2+, 0.83 Å, changes little.32 It appears that Mn2+ is a little too small to fit nicely into a D6R, which, in turn, lacks the flexibility to allow its O(3) oxygen atoms to approach Mn(1) more closely. The 4 and 1.5 Mn2+ ions per unit cell at Mn(10 ) and Mn(20 ), respectively, are found at sites I0 and II0 in the sodalite cavity (see Figures 2b and 3a). These ions are 2.349(17) and 2.152(14) Å from three O(3) and O(2) framework oxygens, respectively, in close agreement with the sum of the conventional ionic radii.31 Because of Mn2+ 3 3 3 Mn2+ repulsion (see Figure 2b), the Mn(10 )O(3) distance is to be longer than the sum of the conventional radii30 and the O(3)Mn(10 )O(3) bond angle is small, 87.4(8)°. For comparison, the O(2)Mn(20 )O(2) bond angles are 117.1(12)°. The remaining 18 Mn2+ ions per unit cell at Mn(2) fill the site II in the supercage (see Figure 3a). Each Mn2+ ion at Mn(2) coordinates at 2.212(5) Å to three O(2) framework oxygens, and the O(2)Mn(2)O(2) angle, 112.24(15)°, is far from a 17756
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Figure 3. Stereoviews of representative supercages of crystals 1 (a), 2 (b), and 3 (c). Four Mn2+ ions at Mn(20 ) and Mn(2) are shown. Each of the three Mn2+ ions at Mn(2) coordinates to an o-xylene, a m-xylene, or a p-xylene molecule, respectively. The Mn2+ ion at Mn(20 ) does not coordinate to a xylene molecule. See the caption to Figure 2 for other details.
near trigonal planar value due to coordination (see the next paragraph). Each of the 18 o-xylene molecules per unit cell has been sorbed by a Mn2+ ion, Mn(2), in the supercage. The plane of the aromatic ring is perpendicular to a 3-fold axis. Each o-xylene molecule interacts facially with Mn2+ (Mn(2)center of o-xylene = 2.67 Å) with its aromatic ring plane parallel to a 6-ring of the zeolite framework (see Table 5 and Figure 3a). The closest approach of a hydrogen atom of one methyl group to a framework oxygen is 2.55 Å; for the other group, it is 2.89 Å (see Figures 4a and 5a). The four hydrogen atoms of the central ring are also near framework oxygens (see Table 5 and Figure 4a). The sum of the van der Waals radii of oxygen and hydrogen is 1.4 + 1.2 = 2.6 Å.33 4.2. Crystal 2. Crystal 2 is quite similar to crystal 1; 37.5 Mn2+ ions per unit cell are also located at the four different crystallographic sites: I, I0 , II0 , and II. At site I, the Mn(1)O(3)
distance is 2.299(5) Å and O(3)Mn(1)O(3) angles are 89.84(20)° (see Table 5 and Figure 2a). Site I0 is too close to site I for simultaneous occupancy due to electrostatic repulsion. Therefore, the remaining D6Rs per unit cell each hosts two Mn(10 ) ions (see Figure 2b). The Mn(10 )O(3) distance is 2.322(18) Å and the O(3)Mn(10 )O(3) angle is small (see Table 5), and the deviation of Mn(10 ) from framework threeO(3) plane is large (see Table 6). The 18 Mn2+ ions at Mn(2) each coordinate at 2.228(6) Å to three O(2) framework oxygens and also interact facially with a mxylene molecule that lies on a 3-fold axis. A strong interaction between the Mn(2) ions and the aromatic ring is observed (Mn(2)center of m-xylene = 2.65 Å) (see Table 5 and Figures 3b and 4b). The methyl groups of m-xylene extend toward the 12-ring windows. No H 3 3 3 O distance shorter than the sum of the corresponding van der Waals radii is seen (see Table 5 and Figures 4b and 5b). 17757
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Figure 4. Stereoviews of Mn(C8H10)2+ complexes on the inner surfaces of the supercages are shown. Each o-xylene (a), m-xylene (b), and p-xylene (c) molecule has eight H 3 3 3 O interactions. Four are between hydrogen atoms of its methyl groups and oxygen atoms of the zeolite framework, and another four are between ring hydrogen atoms and framework oxygens. Ellipsoids of 25% probability are used. Eighteen Mn2+ ions per unit cell (18/8 = 2.25 per supercage) at Mn(2) coordinate to xylene molecules as shown.
4.3. Crystal 3. All Mn2+ ions per unit cell are found at the four
different crystallographic sites like crystals 1 and 2: I, I0 , II0 , and II. The octahedral Mn(1)O(3) distance at site I is 2.301(6) Å and the angle differs insignificantly from 90° (see Table 5). The 4 and 1.5 as Mn(10 ) and Mn(20 ) are in the sodalite cavity opposite of the D6Rs (site I0 ) and S6R (site II0 ), respectively. The 18 Mn2+ ions at Mn(2) each coordinate at 2.210(6) Å to three O(2) framework oxygens and facially to a p-xylene molecule. The center of the aromatic ring is on a mirror plane (Mn(2)center of p-xylene = 2.63 Å) (see Table 5 and Figures 3c and 4c). The methyl groups are directed toward the 4-ring and 12-ring windows (see Figures 4c and 5c). A strong interaction between the aromatic ring and the Mn2+ ion is indicated by the ca. 0.48 Å greater deviation of the Mn(2) ions from its S6R plane compared to dehydrated Mn37.5-Y.24 In addition, hydrogen atoms of the methyl groups and of the central ring interact with framework oxygens (see Figure 4c). The closest approach of a hydrogen atom of one methyl group to a framework oxygen is 2.40 Å; for the other methyl group, it is
2.60 Å. The hydrogen atoms of the central ring are substantially farther away (see Table 5). 4.4. Comparison among the Sorption Complexes of o-, m-, and p-Xylene in Zeolite Y. The locations of the sorbed o-, m-, and p-xylene molecules in Mn2+-exchanged zeolites Y are very similar in all three Mn2+-exchanged zeolites Y and with those of benzene7 and mesitylene10 sorption into Mn2+-exchanged zeolite Y. However, the interaction of xylene molecules with the cation varies with the nature of the adsorbed xylene. About 2.3 xylene molecules (o-, m-, and p-xylene) per supercage are found in a single sortion site II in the supercage. None of the xylenes are found in the plane of the 12-ring window, unlike the benzene sorption complex of Mn2+-exchanged zeolite Y7 and p-xylene sorption into prehydrated Ba2+-exchanged zeolite X.16 The unit cell constant has decreased upon sorption of xylenes (o-, m-, and p-xylene) from a = 24.659(1) Å for the fully dehydrated Mn37.5-Y23 to 24.5943(1), 24.5969(1), and 24.5860(2) Å for crystals 1, 2, and 3, respectively, which indicates that there is not only a frameworkcation interaction but also a frameworkmolecule 17758
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Figure 5. Side stereoviews of sorbed o-xylene (a), m-xylene (b), and p-xylene (c) molecules. The deviations of the methyl groups from the planes of the central rings are easily seen. Ellipsoids of 25% probability are used.
interaction. Among the three xylenes, the unit cell constant of p-xylene has decreased more than those of the other two, which suggests more interaction than the others. Upon the sorption of the xylenes into Mn2+-exchanged zeolite Y, the Mn2+ ions at Mn(2) have moved ca. 0.63, 0.68, and 0.67 Å for o-, m-, and p-xylene, respectively, further into the supercage as compared to the corresponding Mn2+ position in dehydrated Mn37.5-Y24 (see Table 6 and Figure 3ac). These movements indicate that p- and m-xylene interact more strongly than o-xylene. Similarly, the distances between Mn2+ and the p- and m-xylene aromatic centers are shorter, 2.65 and 2.63 Å, than the corresponding o-xylene distance, 2.67 Å (see Table 5). The xylenes are stabilized by a strong π-type interaction between the aromatic ring and the Mn2+ ion and by electrostatic and van der Waals forces among framework oxygens and hydrogens of the methyl groups and the central ring (see Figure 4ac). The distances between the hydrogen atoms of
methyl groups and framework oxygens are at 2.55/3.00 and 2.89/3.12, 2.80/3.10 and 2.40/2.89, and 2.60/2.84 Å and other distances of hydrogens of the central ring are at 3.30/3.33, 3.32/ 3.45, and 3.29/3.34 Å for o-, m-, and p-xylene, respectively (see Table 5)). These H 3 3 3 O interactions are found in many organic sorption complexes of zeolites.711 The hydrogen of the methyl group of p-xylene is closer to the framework oxygen than o- and p-xylene. This also indicates that the p-xylene molecule interacts more strongly than the others. The intramolecular carbon carbon bonds and angles of o-, m-, and p-xylene are observed as shown in Table 4, which is almost similar with the original values.34 Pichon et al.16 observed that the center of the aromatic ring moves away from the cation and leaves the 3-fold axis at high filling of p- and m-xylene (2.7 molecules per supercage). In this work, about 2.3 xylenes per supercage are found and they lie on the 3-fold axis. Furthermore, the distance between the center of 17759
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Table 6. Displacements of Atoms (Å) from 6-Ring Planes displacement atoms a
at O(3)
at O(2)b
site
crystal 1
crystal 2
crystal 3
Mn(1)
I
1.33
1.33
1.35
Mn(10 )
I0
1.42
1.38
1.49
Mn(20 )
II0
0.41
0.47
0.44
Mn(2)
II
0.63
0.68
0.67
C(1)
3.37
3.37
3.34
C(2)
3.38
3.35
3.33
C(3)
3.72
3.55
3.41
C(4)
3.74
3.51
a
The positive deviation indicates that the cation lies in a sodalite cavity; the negative deviation indicates that the cation lies in a D6R (Mn(1) lies at the center of D6Rs). b The positive displacement indicates that the cation lies in a supercage; the negative deviation indicates that the cation lies in a sodalite cavity.
the aromatic ring and the Mn2+ ion is almost similar, as shown in the sorption complexes of benzene (Mn2+benzene center = 2.53 Å)7 and mesitylene (Mn2+mesitylene center = 2.65 Å)10 in Mn2+-exchanged zeolite Y. The Mn2+ ions at site II in the mesitylene sorption complex into Mn2+-exchanged zeolite Y10 are split into two nonequivalent positions as compared with the dehydrated Mn37.5-Y.24 In these crystal structures, the Mn2+ ions at site II are free from splitting but move more to the supercage than the mesitylene sorption complex of Mn2+-exchanged zeolite Y.10 Because none of these xylene molecules have a 3-fold axis and, therefore, do not fit the symmetry of the sites at which they have been found, each must be displaced by some amount from their reported positions. These displacements are not apparent in the results, so they must be quite small. The interactions of the methyl groups with the zeolite framework must, therefore, be quite weak in comparison to the π interactions between Mn2+ and each ring. In addition and for similar reasons, the elongated 1,2 CC bond length in o-xylene has not been observed.
’ ASSOCIATED CONTENT
bS
Supporting Information. Tables of calculated and observed structure factors (18 pages for three crystals). This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel: +82 54 820 5454. Fax: +82 54 822 5452.
’ ACKNOWLEDGMENT The authors are grateful to the staff at Beamline 6B MXI of the Pohang Light Source, Korea, for their assistance during data collection. We also gratefully acknowledge Prof. Karl Seff at the University of Hawaii, USA, for beneficial discussions and assistance in manuscript preparation. This research was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2010-0024479).
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