J Porous Mater (2013) 20:1449–1456 DOI 10.1007/s10934-013-9731-1
Complete Li+ exchange into zeolite X (FAU, Si/Al 5 1.09) from undried methanol solution Hu Sik Kim • Sik Young Choi • Woo Taik Lim
Published online: 30 July 2013 Ó Springer Science+Business Media New York 2013
Abstract Complete exchange of Li? into zeolite Na-X, |Na92|[Si100Al92O384]-FAU, was accomplished using undried methanol solvent (water concentration 0.02 M). A crystal of Na-X was treated with 0.1 M LiNO3 in the solvent at 333 K, followed by vacuum dehydration at 673 K and 1 9 10-6 Torr for 2 days. Its structure was determined by single-crystal synchrotron X-ray diffraction techniques, in the cubic space group Fd3 at 100(1) K. The 92 Li? ions per unit cell are found at three different crystallographic sites. The 32 Li? ions occupy at site I’ in the sodalite cavity: these Li? ions are ˚ into the sodalite cavity from their 3-oxygens recessed 0.28 A ˚ and O–Li–O = 117.8(3)°]. plane [Li–O = 1.903(5) A ? Another 32 Li ions are found at site II in the supercage, being ˚ into the supercage [Li–O = 1.968(5) A ˚ and recessed 0.26 A ? O–Li–O = 118.3(3)°]. The remaining 28 Li ions are located ˚ ]. at site III in the supercage [Li–O = 2.00(8) A Keywords Lithium Zeolite X Methanol Ion exchange Structure
1 Introduction Li?-exchanged zeolites are the most effective and selective adsorbents for separating nitrogen from air [1, 2]. Its characteristic adsorption properties are closely related to the amount and distribution of Li? over the available sites in zeolite [2]. When used for a variety of industrial processes such as gas adsorbents, gas separators, and catalysts,
H. S. Kim S. Y. Choi W. T. Lim (&) Department of Applied Chemistry, Andong National University, Andong 760-749, Korea e-mail:
[email protected]
it is necessary to achieve highly Li?-exchanged zeolite and to have detailed information of the location of Li?. 1.1 Ion exchange of Li? into zeolites In general, Li? exchanged zeolites have been prepared from aqueous solution. But, the result of ion-exchange experiment from aqueous solution is usually not simple. The complete exchange of Li? for Na? ion was readily achieved for LSX (Si/Al = 1.0) from aqueous solution [2, 3]. However, it was not easy to achieve complete Li?exchanged zeolite when Si/Al ratio was higher than 1.0 unlike LSX [2, 4–7]. To achieve complete ion exchange, Li? exchange was attempted into zeolites X and Y using NH4?-X(Si/Al = 1.23 and 1.25) and Y (Si/Al = 2.36), respectively [2, 5]. However, both Na? and H? (after deamination) were found in zeolites [5]. Recently, Kim et al. [6] attempted Li? exchange into single-crystals of zeolite Y (Si/Al = 1.56) using methanol– water mixed solvents (These solvent ranging in composition from as-purchased (undried) methanol (water concentration 0.02 M) to pure water were used) at 333 K to achieve complete Li? exchange and to learn the dependence of the degree of Li? exchange into zeolite Y on the water content of the methanol. The complete Li? exchange did not occur in any solvent composition; the highest degree of Li? exchange, 96 %, was achieved from methanol solution containing ca. 0.02 M H2O, and the degree of exchange from pure water solution is the least. However, these results show that the ion exchange with Li? using undried methanol (water concentration 0.02 M) is more efficient than traditional aqueous exchange. To study the effect of temperature on the degree of Li? incorporation into zeolite X (Si/Al = 1.09), we previously performed dynamic ion exchange at 293 and 333 K [7].
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Despite the high temperature employed, complete Li? exchange into zeolite X (Si/Al = 1.09) was not observed. The extent of Li? exchange improves slightly from 93 to 95 % with increasing in ion exchange temperature. 1.2 Applications of Li?-exchanged zeolites When compared with other alkali metal cations, Li? ion is a small and has a high electric field gradient [6, 7]. Its strong electric fields gradient can interact strongly with the dipoles or quadrupoles of sorbate molecules, so sorption or catalysis process may be enhanced [6–8]. Accordingly, Li?-exchanged zeolites have been studied for following applications: the separation of nitrogen from air in the PSA (pressure swing adsorption) process [1, 8, 9], purification inert fluids in particular argon or helium in the PSA or TSA (temperature swing adsorption) process [10], the adsorption of toluene, the catalysts for ring alkylation of toluene [11], and isomerization of olefins [12]. 1.3 Structure of Li?-exchanged zeolites Generally, the sites I’ and II are preferentially occupied with Li? ions and the remaining cations are located at sites III or III’ in the structures of Li?-exchanged zeolites [2, 3, 5–7]. However, it is difficult to determine the distribution of Li? ions in the supercage because of their high mobility and low occupancy of certain sites [2, 8, 11]. Sometimes, remaining Li? ions for charge balance of the zeolite framework are not found [2, 6]. Accordingly, various methods such as powder-neutron diffraction, single-crystal X-ray diffraction, and solid-state MAS NMR spectroscopy have been used to better understand the location of Li? ions in zeolite framework [2, 3, 5, 8, 11, 13]. Forano et al. [5] studied the structure of dehydrated partially Li?-exchanged zeolites X (Si/Al = 1.23) and Y (Si/Al = 2.36) from NH4?-X and Y by powder neutron diffraction techniques. In both structures, Li? ions were found at sites I’ and II and in Li-X, remaining Li? ions were located at site III’. In addition, Na? and H? (after deamination) ions were found in both structures. Plevert et al. [3] investigated the structure of Li-LSX at two different neutron diffraction temperatures of 300 and 10 K by powder neutron diffraction technique. Li? ions were found at four crystallographic sites per unit cell in 300 K: 32 were at sites I’ and II at each, 15 at site III, and 16 site III’. In addition, they observed that a phase transition was occurred at low temperature from Fd3 (cubic) to Fddd (orthorhombic). The structures of Li?-exchanged zeolite X (Si/Al = 1.0 and 1.25) were studied by Feuerstein et al. [2] using solidstate MAS NMR spectroscopy and neutron diffraction
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technique. In both structures, Li? ions were found at three different sites: I’, II, and III’. They also reported that phase transition from Fd3 to Fddd was occured in the temperature range T = 200–300 K in the structure of Li-X (Si/ Al = 1.0). Shepelev et al. [4] determined the structures of the hydrated and partially dehydrated partially Li?-exchanged zeolites X (Si/Al = 1.09) by single-crystal X-ray diffraction techniques. However, the detailed information of the structure and chemical compositions in both structures were not achieved. In addition, the structure determination of fully dehydrated Li?-exchanged zeolite X failed due to the loss of crystallinity during the dehydration process. Kim et al. [6] investigated the four single-crystal structures of fully dehydrated Li?- exchanged zeolite Y (Si/Al = 1.56) to study the degree of Li? exchange into zeolite Y as a function of solvent composition by singlecrystal X-ray diffraction techniques. Li? and Na? ions filled sites I’ II, and III’, respectively, in the resulting single-crystal structure. Recently, the structures of two dehydrated Li?exchanged single crystals of zeolite X (Si/Al = 1.09) were determined by single-crystal X-ray diffraction techniques at 100(1) K in the space group Fd3 [7]. In Li86Na6-X, only 78 Li? ions of 86 per unit cell were found at four crystallographic sites: 32, 30, and 16 at sites I’, II, and III, respectively. Remaing Li? ions for charge balance of the zeolite framework were not found. In Li87Na5-X, 87 Li? ions per unit cell occupy three crystallographic sites: 32, 30, and 25, at sites I’, II, and III’, respectively. Some Na? ions remained at sites II and III’ in both structures. 1.4 Objective of this work Li? exchange into zeolite X (Si/Al = 1.09) was attempted from methanol solution at an elevated temperature, 333 K, to achieve complete Li? exchanged zeolite and to investigate the ion exchange behavior of Li? in zeolite X. The solvent was used as-purchased (undried) methanol.
2 Experimental section 2.1 Ion exchange and dehydration Large single crystals of zeolite X (FAU), stoichiometry Na92Si100Al92O384 per unit cell, were synthesized by Petranovskii in Russia [14]. One of these colorless octahedra of about 0.25 mm in cross-section, was lodged in its own fine Pyrex capillary. Ion exchange was done by the dynamic (flow) method using undried methanol solution [Baker Analyzed HPLC Solvent, assay 100.0 %, acetone 0.0005 %, residue
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after evaporation 0.4 ppm, water content 0.04 % (0.02 M)] of 0.1 M LiNO3 (Aldrich, 99.99 %, Ca 4.82 ppm, Na 1.91 ppm, Sc 0.48 ppm, Mg 0.42 ppm, Ba 0.41 ppm, Zr 0.31 ppm, Cu 0.21 ppm, Al 0.07 ppm, La 0.05 ppm, Sr 0.04 ppm). This solution was allowed to flow past crystal for 24 h at 333 K. Although a previously unopened bottle of methanol was used, the water content of solution could have increased significantly during its preparation by sorption of moisture from the LiNO3(s) and from the atmosphere (although the (not its) humidity was low because it was winter, end of January, in Korea). The resulting clear colorless single crystal was slowly heated under dynamic vacuum to 673 K and dehydrated at 1 9 10-6 Torr for two days. While these conditions were maintained, the hot contiguous downstream lengths of the vacuum system, including a Pyrex U-tube of zeolite 5A beads fully activated in situ, were cooled to ambient temperature to prevent the later movement of water molecules to the crystal from more distant parts of the vacuum system. While still under vacuum, the crystal was allowed to cool to room temperature and was sealed in its capillary by torch. Microscopic examination showed them to be colorless.
Table 1 Summary of experimental and crystallographic data
2.2 Single-crystal X-ray diffraction
|Li92|[Si100Al92O384]-FAU Crystal cross-section (mm)
0.25
Solvent
CH3OH
[H2O]a
0.02
Ion exchange t(h),T (K)
24, 333
Crystal color
Colorless
Dehydration T (K)
673
Data collection T (K)
100(1)
Space group, Z
Fd3, 1
X-ray source
Pohang light source, beamline 6B MXI
˚) Wavelength (A
0.90000
˚) Unit cell constant, a (A
24.6623(3)
2h range in data collection (deg)
70.32
No. of unique reflections, m
1,390
No. of reflections with Fo [ 4r(Fo)
1,179
No. of variables, s
70
Data/parameter ratio, m/s
19.8
Weighting parameters, a/b
0.108/388.8
Final error indices
Synchrotron X-ray diffraction data were collected for the crystal using an ADSC Quantum 210 detector at Beamline 6B MXI at the Pohang Light Source. Their temperature was maintained at 100(1) K by a flow of cold nitrogen gas. Crystal evaluation and data collection were done with a detector-to-crystal distance of 60 mm. Preliminary cell constants and an orientation matrix were determined from 72 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 [15]. The reflections were successfully indexed by the automated indexing routine of the DENZO program [15]. The diffraction intensities were harvested by collecting 72 sets of frames with 5° scans 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 Fd3, standard for zeolite X, was determined by the program XPREP [16]. A summary of the experimental and crystallographic data are presented in Table 1.
3 Structure determination Full-matrix least-squares refinements (SHELXL97) [17] were done on F2o using all data. Refinement was initiated with the atomic parameters of the framework atoms [Si, Al,
R1/wR2 [Fo [ 4r(Fo)]b R1/wR2 (all intensities)c Goodness-of-fitd a
0.080/0.265 0.090/0.297 1.18
Water concentration (mol/L) in the exchange solution
b
R1 = R|Fo-|Fc||/RFo and R2 = [Rw(F2o-F2c )2/Rw(F2o)2]1/2; R1 and R2 are calculated using only the 1,179 reflections for which Fo [ 4r(Fo) c R1 and R2 are calculated using all unique reflections measured d
Goodness-of-fit = (Rw(F2o-F2c )2/(m-s))1/2, where m and s are the number of unique reflections and variables, respectively
O(1), O(2), O(3), and O(4)] in dehydrated |Rb71Na21| [Si100Al92O384]-FAU [18]. Initial refinement used anisotropic thermal parameters and converged to the high error indices (given in steps 1 of Table 2) R1/R2 = 0.13/0.49. The detailed progress of structure determination as subsequent peaks were found on difference Fourier functions and identified as nonframework atoms is provided in Table 2. The occupancies of Li? ions at Li(I’) and Li(II) were fixed to maximize the occupancy at sites I’ and II, respectively, as shown in steps 5 of Table 2. The final cycles of refinement were done with anisotropic temperature factors for all positions. The final error indices R1 and R2 are given in Table 1. ˚ -3 in the final difference The largest peak 0.70 e A Fourier function was found at (0.2494, 0.2494, 0.2494) in crystal. This peak was tried to refine as Na(II), but the occupancy was very low with higher esd and the thermal
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123
32(e)
48(f)
Li(II)
Li(III)
The anisotropic temperature factor is exp[-2p2a-2(U11h2 ? U22k2 ? U33l2 ? 2U23kl ? 2U13hl ? 2U12hk)] Occupancy factors are given as the number of atoms or ions per unit cell
32(e) Li(I’)
b
96(g) O(4)
a
-2,802(2,070) 0 0 1,948(1,347) 3,025(2,073) 3,859(2,595) 3,826(62) 1,250 1,250
96(g) O(3)
III
96(g) 96(g) O(1) O(2)
Positional parameters X 104 and thermal parameters X 104 are given. Numbers in parentheses are the estimated standard deviations in the units of the least significant figure given for the corresponding parameter
28 26(7)
32 41(3)
34(3) -41(46)
91(48) 91(48)
-41(46) -41(46)
91(48) 321(39)
367(42) 367(42)
321(39) 321(39)
367(42) 2,235(5)
471(5) 471(5)
2,235(5) 2,235(5)
471(5)
96(g)
I’ 96(g)
Al
II
96
96 -29(19)
33(19) 27(20)
21(19) -79(20)
5(20) 388(26)
368(25) 364(25)
354(24) 328(23)
319(24) 1,686(2)
96 96 -27(21) 20(18)
96
96 375(1)
218(2) -738(2)
781(2) -736(2)
U22 U11 or Uaiso z y
Si
The framework structure of zeolite X (FAU) is characterized by the double 6-ring (D6R, hexagonal prism), the sodalite cavity (a cubooctahedron), and the supercage (see Fig. 1). Each unit cell has 8 supercages, 8 sodalite cavities, 16 D6Rs (32 6-rings), 16 12-rings, and 32 single 6-rings (S6Rs). The exchangeable cations, which balance the negative charges of the zeolite X framework, usually occupy some or all of the sites shown with Roman numerals in Fig. 1 [23]. The maximum occupancies at the cation sites I, I’, II, II’, III, and III’ in zeolite X are 16, 32, 32, 32, 48, and 92, respectively. Further description is available [24, 25].
x
4.1 Zeolite X framework and exchangeable cation sites
Table 3 Positional, thermal, and occupancy parameters
4 Description of the structures
U33
parameters become 0.00001. According, this peak was not included in the final model. All shifts in the final cycles of refinements were less than 0.1 % of their corresponding estimated standard deviations (esds). The final structural parameters are presented in Table 3, and selected interatomic distances and angles are given in Table 4. Fixed weights were used initially; the final weights were assigned using the formula w = 1/[r2(F2o) ? (aP)2 ? bP] where P = [Max(F2o,0) ? 2F2c ]/3, with a and b as refined parameters (see Table 1). Atomic scattering factors for Li?, Na?, O-, and (Si,Al)1.75? were used [19, 20]. The function describing (Si,Al)1.75? is a weighted mean of the Si4?, Si0, Al3?, and Al0 functions. All scattering factors were modified to account for anomalous dispersion [21, 22]. Other crystallographic details are given in Table 1.
-695(2)
U23
Occupancies fixed at 32 for Li(I’) and Li(II) All Li? ions were refined anisotropically
Cation site
d
Wyckoff position
c
-36(20) -22(18)
U13
Only the atoms of zeolite framework were included in the initial structure model
-22(21) -21(18)
The occupancy is given as the number of Li? ions per unit cell
b
Atom
a
337(25) 348(24)
Isotropic temperature factors were used for all Li positions except for the last step
489(29) 301(22)
0.2645
?
346(25) 297(22)
0.0800
1,031(2) 1,529(2)
23(6)
-6(2) 11(2)
32
-983(2) -7(2)
32
-2(6)
6d
-16(7)
0.2593
-3(7)
0.2621
0.0806
-15(7)
0.0803
22(6)
-15(6)
26(7)
32
-18(7)
34(3)
32
228(9)
41(3)
5c
304(10)
0.2666
226(9)
4
0.3139
0.0813
316(10)
33(3)
253(9)
40(3)
0.4937
0.1023
347(10)
3
0.1310
1,247(1)
33(3)
364(1)
2
R2
1,235(1)
1b
R1
-497(1)
Li(III)
-5.06(1)
Li(II)
Varied
Li(I’)
Occupancyb
Occupancya at
U12
Step
Initial
Table 2 Steps of structure refinement
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˚ ) and angles (deg) Table 4 Selected interatomic distances (A Distances
4.2 Structure of |Li92|[Si100Al92O384]-FAU
Angles
Si–O(1)
1.701(5)
O(1)–Si–O(2)
114.86(24)
Si–O(2)
1.742(5)
O(1)–Si–O(3)
107.74(24)
Si–O(3)
1.755(5)
O(1)–Si–O(4)
109.88(24)
Si–O(4)
1.710(5)
O(2)–Si–O(3)
107.18(23)
Mean Si–O
1.727(10)
O(2)–Si–O(4)
107.14(22)
O(3)––Si–O(4)
109.97(23)
Al–O(1)
1.597(5)
O(1)–Al–O(2)
113.0(3)
Al–O(2)
1.645(5)
O(1)–Al–O(3)
110.0(3)
Al–O(3)
1.649(5)
O(1)–Al–O(4)
108.4(3)
Al–O(4)
1.606(5)
O(2)–Al–O(3)
105.98(24)
Mean Al–O
1.624(10)
O(2)–Al–O(4) O(3)–Al–O(4)
108.97(24) 110.5(3)
Li(I’)–O(3)
1.903(5)
Si–O(1)–Al
146.5(3)
Li(II)–O(2)
1.968(5)
Si–O(2)–Al
127.5(3)
Li(III)–O(4)
2.15(10), 2.15(10)
Si–O(3)–Al
126.5(3)
Si–O(4)–Al
138.8(3)
O(3)–Li(I’)–O(3)
117.8(3)
O(2)–Li(II)–O(2)
118.3(3)
O(4)–Li(III)–O(4)
95(6)
Values in bold indicate the mean distance of Si, Al and O The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter
Fig. 1 Stylized drawing of the framework structure of zeolite X. Near the center of the each line segment is an oxygen atom. The different oxygen atoms are indicated by the numbers 1–4. 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 Lowenstein’s rule (ref. [23]) would be obeyed. Extraframework cation positions are labeled with Roman numerals
The 92 Li? ions per unit cell are found at three crystallographic sites in this structure: I’, II, and III. Unlike the previous studies [2, 5, 6], no Li? ions were found at site III’. The 32 Li? ions per unit cell at Li(I’) occupy site I’ (opposite D6Rs in the sodalite cavity, see Fig. 2). Each coordinates to three O(3) framework oxygens of its D6R at ˚ , which are very close to the sum of distance of 1.903(5) A the corresponding conventional ionic radii of Li? and O2-, ˚ [13, 26], as was seen 0.59 ? 1.32 (respectively) = 1.91 A in previous work [2, 3, 13]. This indicates that Li? ions coordinate tightly to their three O(3) oxygens. Each Li? ion ˚ from its three extends inside the sodalite cavity 0.28 A O(3) plane (see Table 5). The O(3)–Li(I’)–O(3) bond angle, 117.8(3)°, is near trigonal planar (see Table 4). At site II (opposite S6Rs in the supercage), 32 Li? ions are found at Li(II) (see Fig. 3). Each of those lies inside the ˚ , from the plane of their O(2) framework supercage, 0.26 A oxygen atoms; the O(2)–Li(II)–O(2) angle is 118.3(3)° (see ˚ , is also in Table 4). The Li(II)–O(2) distance, 1.968(5) A agreement with the sum of the ionic radii of Li? and O2-, ˚ [13, 26]. 1.91 A The remaining 28 Li? ions are located at site III on a two-fold axis opposite a 4-ring between two 12-rings in the supercage (see Fig. 4). These Li? ions are only 2-coordi˚, nated by two oxygen atoms, O(4), at distances, 2.15(10) A respectively (see Table 3). These distances are somewhat longer than the sum of the conventional ionic radii of Li? and O2- radii. However, these distances are similar to those previously reported [3, 7, 13]. The thermal parameters of the Li? ions at site III are much larger than those at sites I’ and II (see Table 3) as observed in previous reports [2, 3, 7]. Its high thermal parameters may be a consequence of the low coordination
Fig. 2 A Stereoview of the double 6-rings (D6Rs) in dehydrated |Li92|[Si100Al92O384]-FAU. The zeolite X framework is drawn with heavy bonds. The coordination of Li? ions to oxygens of the zeolite framework is indicated by light bonds. Ellipsoids of 25 % probability are shown
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˚ ) from 6-ring planes Table 5 Displacements of atoms (A
Completely Li?-exchanged zeolite X (Si/Al = 1.09) was achieved from undried methanol solution [water content 0.04 % (0.02 M)] at 333 K. The site preference for Li? over Na? is approximately I’ [ II [ III (Table 3).
near an O(1)–Al–O(4) sequence. Unlike the above result, remaining Li? ions were found only at site III’ in the structures of Li-LSX, Li-X (Si/Al = 1.25) [2], Li-X (Si/ Al = 1.23) [5], and Li-Y (Si/Al = 1.56) [6]. Interesting, this position is quit different. In the structures of Li-LSX [2] and Li-X (Si/Al = 1.23) [5], Li(III’) is on slightly off twofold axis opposite a 4-ring between two 12-rings close to site III. However, it is close to the side of the 12-ring near an O(1)–Si–O(4) sequence and an O(1)–Al–O(4) sequence, respectively, in Li-X (Si/Al = 1.25) [2] and LiY (Si/Al = 1.56) [6]. In our previous work on dehydrated Li?-exchanged zeolites X (Si/Al = 1.09) [7], the remaining Li? ions were found at site III and III’ in Li86Na6-X [7] and Li87Na5-X [7], respectively, (Li(III’) is on close to the side of the 12-ring near an O(1)–Al–O(4) sequence). In the structure of Li92-X (this work), the remaining Li? ions are found only at site III on a twofold axis opposite a four-ring between two 12-rings in the supercage (see Fig. 4). This is in agreement with the structure of Li86Na6X [7] but differs from the structure of Li87Na5-X [7] although the zeolite composition is same. This defference can be attributed to the high mobility of Li? ion in zeolite.
5.1 Li? positions in supercage
5.2 Framework geometry
As discussed in the Introduction part, the Li? ions are preferentially fully occupy sites I’ and II and the remaining Li? ions are located at sites III or III’ in dehydrated Li?exchanged zeolite. However, the location of remaining Li? ions in the supercage become less certain because of high mobility of Li? and low fractional occupancy in these site. So, remaining Li? ions were found in variety position in the supercage. In the case of Li-LSX [3], remaining Li? ions were equally distributed at sites III and III’, respectively. Li(III) is on a twofold axis opposite a 4-ring between two 12-rings, and Li(III’) is on close to the side of the 12-ring
The T–O–T angles at O(2) and O(3), 127.5(3)° and 126.5(3)°, respectively, are much smaller than that in fully dehydrated Na-X, [27] 145.6(3)°and 141.2(4)° (Si/Al = 1.09 as in this work). The observed values are very close to those observed in fully dehydrated Li-LSX [2, 3], Li-X [2, 7], and Li-Y [6]. It occurs because Li? is smaller than Na?, it strongly pulls the oxygens atoms of its 6-rings toward the 6-ring centers as the bond strengths indicate (Li–O, 333.5 kJ/mol, Na–O, 256.1 kJ/ mol) [28]. This result indicates that zeolite framework is experienced some strain as a result of much smaller size and strong interaction of Li? ion than Na? ion, which is one of the underlying reasons for loss of the crystallinity during
Positions
Sites
Displacement At O(3)a
Li(I’)
I’
Li(II)
II
At O(2)b
0.28 0.26
a
A positive displacement indicates that the cation lies in a sodalite cavity
b
A positive displacement indicates that the cation lies in a supercage
number and high mobility of Li? ion at this site. Its thermal ellipsoid is elongated, and it seems to present of further positional disorder at Li(III), which could not be resolved in this work.
5 Discussion
Fig. 3 A Stereoview of representative sodalite unit in dehydrated |Li92|[Si100Al92O384]-FAU. See the caption to Fig. 2 for other details
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Fig. 4 A Stereoview of representative supercage in dehydrated |Li92|[Si100Al92O384]-FAU. See the caption to Fig. 2 for other details
Table 6 Numbers of Li? and Na? ions of fully dehydrated Li?-exchanged zeolites X and Y Crystal
Site I’
Site II
% IEa
Site III
Site III’
Total cations
Li?
Li?
Na?
Total Li?
Total Na?
Total cations
Li?
Na?
Li?
Na?
Li87Na5-Xb
32
–
30
2
–
25
3
87
5
92
95
Li92-Xc
32
–
32
–
28
–
–
92
–
92
100
Li72Na3-Yd
32
–
29
3
–
11
–
72
3
75
96
a
?
Percent ion exchange of Li
b
Reference [9]. Crystal prepared by flow method from aqueous solution at 333 K
c
This work. Crystal prepared by flow method from undried methanol solution at 333 K
d
Reference [6]. Crystal prepared by flow method from undried methanol solution at 333 K
dehydration [4]. Thus, Li? exchange and subsequent complete dehydtration have distorted the zeolite X frame work considerably. In the case of Li-X (Si/Al = 1.09) [4], the loss of the crystallinity was found due to the distortions of the zeolite framework during dehydration. However, the present structure shows no loss of crystallinity. 5.3 Effect of the Si/Al ratio of the zeolite framework In the previous study [6], complete Li?-exchange into zeolite Y (Si/Al = 1.56) [6] from methanol solution (water concentration 0.02 M) did not occur. While the complete exchange of Li? for Na? into zeolite X (Si/Al = 1.09, in this work) has been achieved from methanol solution, only 96 % exchange is observed in zeolite Y (Si/Al = 1.56) (see Table 6). This may be attributed to the difference of the composition of the zeolite framework (Si/Al = 1.09 and 1.56). Generally, the aluminum-rich aluminosilicate zeolites are amenable for a greater degree of ion-exchange because more cation sites have a suitably high negative charge. Considering Al content in zeolites X and Y, zeolite X is more polar than zeolite Y. Therefore, the negative charge of the zeolite framework oxygen and the resulting electric fields in zeolite X are higher than in zeolite Y, thus,
encouraging the entry of Li? ions into zeolite X, consistent with the complete exchange observed in this work. 5.4 Effect of the solvent polarity in ion exchange The structure of Li?-exchanged zeolite X (Si/Al = 1.09) from aqueous solution at 333 K was determined by singlecrystal synchrotron X-ray diffraction techniques [7]. When the structures of Li54Na21-X [7] and Li92-X (this work) are compared, the complete exchange of Li? for Na? into zeolite X is achieved from undried methanol, not from water known as a typical solvent at given ionexchange temperature (see Table 6). This is attributed to the properties of the ion-exchange solution between water and methanol. The dielectric constants of water and methanol are 78.48 and 32.66 eu at 298 K [29] and 66.13 and 26.06 eu at 333 K; the latter value was obtained by regressing published dielectric constant data [29]. Methanol with a dielectric constant lower than water would facilitate ion exchange into zeolite X because the Li? replace is likely to give up methanol easily in its coordination sphere to replace it with the framework oxygen of zeolite. This is consistent with the result of ion exchange (up to 96 %) in zeolite Y with Si/Al = 1.56 [6].
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6 Summary Single-crystal of fully Li?-exchanged zeolite X was prepared from undried methanol solution containing ca. 0.02 M H2O by dynamic ion-exchange method at 333 K and dehydrated fully. Its crystal structure was determined by single-crystal X-ray diffraction techniques. In this structure, 92 Li? ions occupy three different equipoints; 32 are at sites I’ and II, respectively, and the remaining 28 are at site III. The complete Li? exchange achieved here shows that methanol can be a solvent of choice for such ion exchange. Acknowledgments The authors are grateful to the staff at beamline 6B MXI of the Pohang Light Source, Korea, for their assistance during data collection. This work was supported by a grant from 2012 Academic research Fund of Andong National University.
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