ARTICLE pubs.acs.org/JPCC
Single-Crystal Structure of a Toluene Sorption Complex of Fully Dehydrated, Fully Mn2+-Exchanged Zeolite Y (FAU), |Mn37.5(C7H8)17|[Si117Al75O384]-FAU Md. Shamsuzzoha,† Young Hun Kim,‡ and Woo Taik Lim*,† †
Department of Applied Chemistry and ‡Department of Environmental Engineering, Andong National University, Andong 760-749, Korea
bS Supporting Information ABSTRACT: A single crystal of zeolite Y, 0.30 mm in diameter, fully exchanged with Mn2+ and fully dehydrated, was treated with zeolitically dry toluene at 298(1) K and a pressure of 1.3 104 Pa and evacuated for 2 h. The crystal structure, |Mn37.5(C7H8)17|[Si117Al75O384]-FAU (a = 24.5923(1) Å), was determined by synchrotron X-ray diffraction techniques in the space group Fd3m at 100(1) K and was refined to the final error indices R1 = 0.083 and wR2= 0.230. Mn2+ ions occupy sites I, I0 , II0 , and II with occupancies at 14, 4, 2.5, and 17, respectively. The 17 Mn2+ ions per unit cell at site II each coordinates to three framework oxygen atoms at 2.213(6) Å and extends into the supercage by 0.63 Å from their plane. Each of these Mn2+ ions also interacts facially with a toluene molecule (Mn2+toluene center = 2.62 Å). The methyl group is somewhat off the plane of the ring, indicative of a net repulsive interaction with the zeolite framework.
1. INTRODUCTION Faujasite (FAU) zeolites including zeolites X and Y are of great industrial interest in separation and catalytic processes such as alkylation, transalkylation, and isomerization of xylenes. The studies of diffusion, sorption, and desorption of hydrocarbons into zeolites are of special interest because several petrochemical processes involve the transformation of organic molecules within the pores of zeolites.1,2 This migration could be used for the further characterization of zeolites.3,4 Sorption is a well-known separation process for the high efficient removal of organic compounds from wastewater.5 The sorbents may have a mineral, organic, or biological origin: activated carbons, polymeric resins, clays, agricultural wastes, mesoporous materials, and zeolites. Zeolites have been recently noted as a practical alternative to activated carbon because of their high surface area, their high selectivity, and their ease of regeneration after saturation.6 A large number of studies have been carried out on the sorption of aromatic molecules in FAU. For understanding their sorption and diffusion properties, microscopic and macroscopic measurements (crystallographic,79 IR studies,10,11 thermodynamic,12,13 diffusion,14,15 and grant canonical Monte Carlo (GCMC) simulations16) have been reported. The interaction of molecules with zeolites provides information about properties that are important in catalytic reactions such as sorption rates, capacities, and energies as well as rates and activation energies of desorption and diffusion.24,17 r 2011 American Chemical Society
The exchangeable cations with zeolites are important in determining the sorption, separation, and catalytic properties of zeolites. Alkali metal exchanged zeolites can catalyze the sidechain alkylation of aromatic compounds,18 whereas the acidic forms can selectively catalyze ring alkylation.19 The mechanism of the side-chain alkylation of toluene with methanol over basic FAU type zeolites has been studied by NMR20,21 and IR,22 which indicated that the selectivity of the catalysts depends on the number and nature of the cations in the zeolites. Single-crystal X-ray diffraction studies were performed on the sorption of benzene,9,23 mesitylene,24,25 and xylene26 to better understand the locations of the different cations, the sorption sites, and the interactions between cations and aromatic molecules. There are also studies of toluene sorption in FAU zeolites by IR,27 NMR,21 and simulation studies.28 Zhu et al.29 investigated the sorption of toluene inside partially Li+exchanged zeolite Y using solid-state NMR spectroscopy. They reported that toluene molecules are located in the supercage and interacted facially with the cations at site II, forming πcomplexes. The sorption of toluene was studied in zeolite NaY by solidstate NMR spectroscopy.30 23Na magic-angle spinning (MAS) Received: October 6, 2011 Revised: November 5, 2011 Published: November 07, 2011 24681
dx.doi.org/10.1021/jp209618s | J. Phys. Chem. C 2011, 115, 24681–24687
The Journal of Physical Chemistry C
ARTICLE
and 23Na rotational-echo double resonance (REDOR) experiments showed that toluene molecules were facially coordinated to Na+ ions at site II in the supercages: π-complexation. Oliveira et al.31 studied the sorption capacity of toluene on transition-metal exchanged zeolite Y. They showed the Table 1. Summary of Experimental and Crystallographic Data |Mn37.5(C7H8)17|[Si117Al75O384]-FAU crystal cross-section (mm)
0.30
ion exchange T (K)
343
ion exchange for Mn2+ (day, mL)
20, 200
dehydration T (K)
723
crystal color before sorption
pale brown
sorption T (K), t (days), P (Pa)
298(1), 3, 1.3 104
crystal color after sorption data collection T (K)
black 100(1)
space group, Z
Fd3m, 1
X-ray source
PLS (6B MXI BL)
wavelength (Å)
0.90000
unit cell constant, a (Å)
24.5923(1)
2θ range in data collection (deg)
60.72
no. of unique reflections, m
570
no. of reflections with Fo > 4σ(Fo) no. of variables, s
567 55
data/parameter ratio, m/s
10.4
weighting parameters, a/b
0.132/311.8
final error indices R1/wR2 (Fo > 4σ(Fo))a
0.083/0.230
R1/wR2 (all intensities)b
0.084/0.233
goodness-of-fitc
1.278
R1 = Σ|Fo |Fc||/ΣFo and wR2 = [Σw(Fo2 Fc2)2/Σw(Fo2)2]1/2; R1 and wR2 are calculated using only the 567 reflections for which Fo > 4σ(Fo). b R1 and wR2 are calculated using all 570 unique reflections measured. c Goodness-of-fit = [Σw(Fo2 Fc2)2/(m s)]1/2, where m and s are the number of unique reflections and variables, respectively. a
importance of inserting transition metals into the zeolite structure to enhance the sorption of both aromatic and sulfur containing compounds from organic liquid mixtures, which showed promise for meeting environmental standards in transportation fuels. All sorbents were moderately selective for toluene. In this study, we investigated the sorption of toluene in fully dehydrated fully Mn2+-exchanged zeolite Y by single-crystal synchrotron X-ray diffraction techniques. This was done to observe the positions of the sorbed toluene molecules, the cation shifts upon toluene sorption, and the cation-sorbate interactions. Mn2+ was chosen as a representative transition-metal cation for this study because complete Mn2+-exchange into FAU zeolites is readily achieved without the complications of hydrolysis and it has good reproducibility.
2. EXPERIMENTAL SECTION 2.1. Single-Crystal Preparation. Large colorless single crystals of Na75Y (stoichiometry Na75Si117Al75O384) with diameters up to 0.32 mm were prepared by Lim et al.32 Ion exchange was accomplished to prepare crystals of hydrated Mn37.5-Y, stoichiometry Mn37.5Si117Al75O384 by allowing static ion-exchange of Na75Y with aqueous 0.1 M Mn(NO3)2 3 xH2O (Aldrich, 99.99%). Hydrated sodium zeolite 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 shaken in a shaking incubator at 343 K for 24 h. This was repeated 20 times with the fresh Mn(NO3)2 solution. The product was then washed with distilled water (300 mL) and filtered. This procedure had previously been applied to prepare fully Mn2+-exchanged zeolite Y (MnY).9,24,26 One of these crystals, a clear brown octahedron ∼0.30 mm in cross section, was lodged in a fine Pyrex capillary. This was attached to a vacuum system and was cautiously dehydrated by gradually increasing its temperature (ca. 25 K/h) under dynamic vacuum to 723 K, followed by 2 days at this temperature and a pressure of 1.3 104 Pa. Whereas these conditions were maintained, the hot contiguous downstream lengths of the
Table 2. Steps of Structure Determination As Atom Positions Are Found occupancy ata step
Mn(1)
Mn(10 )
Mn(20 )
Mn(2)
C(1)
C(2)
C(3)
b
1 2c
9.1(9)
3c
12.2(3)
4c
12.5(3)
3.4(4)
5c
12.5(3)
3.2(4)
6c
13.7(3)
7c
13.9(3)
8c
R1
wR2
0.45 0.36
0.81 0.79 0.52
18.9(6)
0.15
16.5(4)
0.130
0.461
1.2(4)
16.3(4)
0.129
0.444
4.0(5)
2.2(4)
16.8(4)
92(7)
0.0922
0.2590
3.6(4)
1.8(3)
16.4(4)
51(8)
63(8)
0.0909
0.2482
13.1(3)
4.3(4)
3.1(4)
17.7(4)
90(12)
66(11)
86(12)
0.0832
0.2246
9d 10e
13.6(3) 14
3.5(4) 4
2.1(4) 2.5
17.1(4) 17
51.2(11) 51
51.2(11) 51
17.1(4) 17
0.0877 0.0875
0.2340 0.2335
11f
14
4
2.5
17
51
51
17
0.0843
0.2267
12g
14
4
2.5
17
51
51
17
0.0834
0.2301
a
Occupancy is given as the number of Mn2+ ions and C atoms per unit cell. b Only the atoms of 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 Occupancies at Mn(2), C(1), C(2), and C(3) were constrained to refine in the ratio 1:3:3:1. e Occupancies of all Mn and C atoms were fixed. f All Mn atoms were refined anisotropically. g Hydrogen atoms were added at calculated positions. 24682
dx.doi.org/10.1021/jp209618s |J. Phys. Chem. C 2011, 115, 24681–24687
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Table 3. Positional, Thermal, and Occupancy Parametersa occupancyc Wyckoff cation atom
position
site
x
y 1225(1)
z
U11b
U22
U33
U23
U13
U12
initial
Si,Al
192(i)
531(1)
359(1)
175(12)
123(12) 145(12) 16(5)
2(6)
5(5)
192
O(1)
96(h)
1072(2) 0
1072(2)
277(23)
218(33) 277(23) 67(17)
1(27)
67(17)
96
O(2)
96(g)
23(2)
23(2)
1467(2)
253(21)
253(21) 210(33) 24(17)
24(17)
40(25)
96
O(3)
96(g)
310(2)
625(2)
625(2)
217(32)
268(23) 268(23) 62(26)
53(18)
53(18)
96 96
O(4)
96(g)
821(2)
821(2)
3169(2)
215(22)
215(22) 298(34) 17(17)
17(17)
59(25)
Mn(1)
16(c)
I
0
0
0
192(12)
192(12) 192(12) 36(8)
36(8)
36(8)
Mn(10 ) Mn(20 )
32(e) 32(e)
I0 II0
660(7) 2075(10)
660(7) 660(7) 479(60) 2075(10) 2075(10) 300(72)
479(60) 479(60) 75(69) 75(69) 75(69) 300(72) 300(72) 126(112) 126(112) 126(112)
II
202(13) 202(13) 42(13)
2320(1)
202(13)
42(13)
42(13)
varied
fixed
13.6(3)
14
3.5(4) 2.1(4)
4 2.5
Mn(2)
32(e)
2320(1)
2320(1)
17.1(4)
17
C(1)
96(g)
2718(16)
2718(16) 3395(24) 1541(181)
51.2(11)
51
C(2)
96(g)
2477(24)
3148(15) 3148(15) 1523(187)
51.2(11)
51
C(3)
96(g)
2598(81)
2598(81) 3983(41) 3708(1814)
17.1(4)
17
H(1)d
2590
2590
3727
1849
34
H(2)d
2144
3275
3275
1828
51
H(3a)d H(3b)d
2784 2197
2266 2500
4089 4006
4415 4415
17 17
H(3c)d
2667
2882
4205
4415
17 ΣMn2+ = 36.3(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 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 SHELXL-97.
vacuum system, including two sequential 10 cm 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 crystal. Still under vacuum in the capillary, the crystal, still pale brown, was then allowed to cool at room temperature. To prepare the toluene sorption complex, we treated the MnY crystal with zeolitically dried toluene (Aldrich, anhydrous g99%) for 3 days at 298(1) K and evacuated at that temperature for 2 h. It was then sealed in its capillary by torch and removed from the vacuum line. Microscopic examination showed that the crystal had become black. 2.2. Single-Crystal X-ray Diffraction. X-ray diffraction data for the resulting crystal were collected at 100(1) K with a flow of cold nitrogen gas using an ADSC Quantum210 detector at Beamline 6B MXI of the Pohang Light Source. 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 the crystal were determined from 36 sets of frames collected at scan intervals of 5° with an exposure time of 1 s per frame. The basic data file was prepared using the HKL2000 program.33 The reflections were successfully indexed by the automated indexing routine of the DENZO program.33 About 39 522 reflections were harvested for the crystal by collecting 72 sets of frames with a 5° scan and exposure time of 1 s of per frame. These highly redundant data sets were corrected for Lorentz and polarization effects; negligible corrections for crystal decay were also applied. The space group Fd3m standard for zeolite Y, was determined by the program XPREP.34 A summary of the experimental and crystallographic data are presented in Table 1.
3. STRUCTURE DETERMINATION Full-matrix least-squares refinement using SHELXL-9735 was done on Fo2 using all data. 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.36 Anisotropic refinement of the framework atoms converged to an unweighted R1 index of 0.45 and a weighted R2 index of 0.81. These R values, defined in footnote a of Table 1, were calculated using only the 567 reflections for which Fo > 4σ(Fo). The progress of structure determination as subsequent peaks were found on difference Fourier functions and identified as nonframework atoms is given in Table 2. The occupancies at Mn(1), Mn(10 ), Mn(20 ), Mn(2), C(1), C(2), and C(3) atoms were fixed as shown in Tables 2 and 3 by the assumption of stoichiometry, the requirement of neutrality, and the observation that the occupancy numbers at Mn(2), C(1), C(2), and C(3) were refined in the ratio 1:3:3:1. The anisotropic refinement of Mn2+ ions converged to R1 = 0.084 and wR2 = 0.227. The positions of the hydrogen atoms were calculated using SHELXL-97.35 All shifts in the final cycles of refinement were
