(FAU) from Aqueous Methanol. Single-Crystal Structures of Fully

Apr 6, 2012 - ABSTRACT: The ion exchange of Li+ into zeolite Na−Y, |Na75|[Si117Al75O384]-FAU, was attempted using methanol−water mixed solvents...
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Li+ Exchange into Zeolite Na−Y (FAU) from Aqueous Methanol. Single-Crystal Structures of Fully Dehydrated Li,Na−Y Hu Sik Kim,† Donghan Bae,† Woo Taik Lim,*,† and Karl Seff*,‡ †

Department of Applied Chemistry, Andong National University, Andong 760-749, Korea Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822-2275, United States

‡

S Supporting Information *

ABSTRACT: The ion exchange of Li+ into zeolite Na−Y, |Na75|[Si117Al75O384]-FAU, was attempted using methanol−water mixed solvents. Four solvents ranging in composition from as-purchased (undried) methanol (water concentration 0.02 M) to pure water were used. Four single crystals of Na−Y were treated with 0.1 M LiNO3 in each of the four solvents at 333 K, followed by vacuum dehydration at 673 K. Their structures were determined by single-crystal synchrotron X-ray diffraction techniques at 100(1) K in the space group Fd3m ̅ . In the four structures, Li+ and Na+ fill sites I′ and II; the remaining cations, when they can be found, occupy III′ sites. The site preference for Li+ over Na+ is approximately I′ > II > III′. The greatest degree of exchange of Li+ for Na+, 96%, was achieved from undried methanol under the conditions employed. The extent of Li+ exchange decreases with increasing water content and was least, 72%, from pure water. The low water content of the undried methanol appears to play an essential role in the ion exchange process.

1. INTRODUCTION Zeolites are of great economic importance; their principle applications are in catalysis, selective sorption, and ion exchange. Their characteristic properties can be divided into two classes: (i) physicochemical properties derived from the natures of the exchangeable cations (including their size, covalency, electronegativity, and polarizability) and (ii) structural properties dictated by the zeolite framework (pore size and shape, void space, and spacial charge distribution).1 Exchangeable cations with smaller charges, e.g., 1+, can locally balance the anionic charge of a zeolite framework better than those with higher charges.2 This is because those with smaller charges must be more numerous and can approach more oxygen atoms of the zeolite framework. The zeolite framework experiences less strain from the electric fields at the surfaces of these cations and is less distorted. Furthermore, these monopositive cations are generally less able to attract, polarize, and sorb guest molecules, so both sorption and catalysis are less likely.2 Li+, however, is an exception because it is so small and its charge more concentrated. It can interact relatively strongly with sorbed molecules, including with their dipoles and quadrupoles, and it can appreciably distort its zeolite host. Quite generally, a greater degree of cation exchange, as a percentage of capacity, can be achieved in aluminum-rich aluminosilicate zeolites because a greater percentage of the cation sites have a suitably high negative charge. Thus the complete exchange of Li+ for Na+ is readily achieved for LSX from aqueous solution.3,4 Fully dehydrated, fully or partially Li+-exchanged zeolite LSX has found application in the separation of nitrogen from air in the PSA (pressure swing adsorption) process.5−7 © 2012 American Chemical Society

It may be expected that similarly exchanged zeolite Y (with more than 64 Li+ ions per unit cell, so that site III′ is occupied) would find its own unique applications in separation, under acidic conditions where LSX lacks stability, for example. However, unlike LSX whose cation sites are all very anionic, the cation sites in zeolite Y (FAU with Si/Al > 1.25; 2.5 is commonly used in applications) have various anionic charges. A 6-ring site, for example, may have three (most anionic, nearly the same as the corresponding sites in LSX), two, one, or zero Al atoms (least anionic). Cation exchange into the less anionic sites is disfavored, making the complete ion exchange into high silica zeolites more difficult to achieve. Contrary to earlier reports,8−10 the complete exchange of Li+ into zeolite Y has been elusive.11 An attempt was made to increase the degree of Li+ exchange into zeolite Y using NH4+exchanged zeolite Y (Si/Al = 2.36),12 but both Na+ and H+ (after deamination) were found in the products. Na+ ions were to be expected because the complete exchange of NH4+ for Na+ is difficult, especially for high silica zeolites.13,14 In an attempt to avoid these complexities and to prepare fully Li+-exchanged zeolite Y, methanol was selected for study as an alternative ion exchange medium. A solvent less polar than water could facilitate ion exchange into zeolite Y, which is less polar than high Al zeolites like X and LSX; it should be easier to exchange Li+ into zeolite Y from a solvent that does not solvate it as strongly as water. Like water, methanol can hydrogen bond to the zeolite framework and to other solvent molecules within Received: January 11, 2012 Revised: April 5, 2012 Published: April 6, 2012 9009

dx.doi.org/10.1021/jp300321x | J. Phys. Chem. C 2012, 116, 9009−9018

The Journal of Physical Chemistry C

Article

Table 1. Summary of Experimental and Crystallographic Data crystal 1 crystal cross-section (mm) ion exchange t (h), T (K) [H2O]a crystal color dehydration T (K) data collection T (K) space group, Z X-ray source wavelength (Å) unit cell constant, a (Å) 2θ range in data collection (deg) no. of unique reflections, m no. of reflections with Fo > 4σ(Fo) no. of variables, s data/parameter ratio, m/s weighting parameters, a/b final error indices R1/wR2 (Fo > 4σ(Fo))c R1/wR2 (all intensities)d goodness-of-fite

crystal 2

0.18 48, 333 0.02 colorless 673 100(1) Fd3m ̅ ,1

crystal 3

crystal 4 0.19 48, 333 55.5b colorless 673 100(1) Fd3m ̅ ,1

0.90000 24.6359(5) 60.60 571 515 54 10.6 0.149/165.0

0.19 0.18 48, 333 48, 333 1.02 5.02 colorless colorless 673 673 100(1) 100(1) Fd3m Fd3m ̅ ,1 ̅ ,1 Pohang Light Source, Beamline 6B MXI 0.90000 0.90000 24.6363(1) 24.6560(1) 60.60 60.55 571 570 563 563 54 54 10.6 10.6 0.062/194.0 0.047/471.7

0.078/0.253 0.081/0.263 1.22

0.042/0.148 0.043/0.152 1.35

0.040/0.130 0.041/0.133 1.29

0.041/0.137 0.041/0.137 1.35

0.90000 24.6708(1) 60.73 572 559 58 9.9 0.052/188.5

Water concentration (mol/L) in the exchange solution. bDeionized water. cR1 = Σ||Fo| − |Fc||/ΣFo and R2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2; R1 and R2 are calculated using only those reflections for which Fo > 4σ(Fo). dR1 and R2 are calculated using all unique reflections measured. eGoodness-of-fit = [Σw(Fo2 − Fc2)2/(m − s)]1/2, where m is the number of unique reflections and s is the number of variables. a

the LiNO3(s) and from the atmosphere (although its humidity was low because it was winter, end of November, in Korea). The compositions of the solvents in solutions 2 and 3 are given in Table 1. Solution 4 was prepared without methanol using deionized water. All four crystals were Li+-exchanged in their capillaries by dynamic methods at 333 K. Each of the resulting clear colorless crystals was slowly heated under dynamic vacuum to 673 K and dehydrated at 1 × 10−6 Torr for 2 days. While these conditions were maintained, the hot contiguous downstream lengths of the vacuum system, including a sequential 17 cm U-tube of zeolite 5A beads fully activated in situ, were allowed to cool to ambient temperature to prevent the movement of water molecules from more distant parts of the vacuum system to each crystal. While still under vacuum, each crystal was allowed to cool to room temperature and was sealed in its capillary by a torch. Microscopic examination showed them all to be colorless. 2.2. Single-Crystal X-ray Diffraction Work. Synchrotron X-ray diffraction data for the four crystals were collected at 100(1) K using an ADSC Quantum210 detector at Beamline 6B MXI at The Pohang Light Source. For each crystal, the preliminary cell constants and an orientation matrix 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 program HKL2000.20 The reflections were successfully indexed by the automated indexing routine of the DENZO program.20 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; very small corrections for crystal decay were also applied. The space group Fd3̅m, conventional for zeolite Y, was determined by the program XPREP.21 Additional experimental details are presented in Table 1.

the zeolite. In addition, it has a relatively high polarity and is small enough to pass readily through the windows of zeolite Y. Anhydrous methanol, however, was not used in this work because ion exchange into zeolites A and X from various anhydrous (zeolitically dry) solvents, including methanol, had been shown to fail for Cu+, Cu2+, Hg2+, Pb2+, and K+.15 No ion exchange occurred, none at all. This can be expected to extend to zeolite Y. There are, however, reports of ion exchange into zeolites from organic solvents that had not been carefully dried.16−18 These observations indicate that the residual water in nonaqueous solvents plays an important role in facilitating the ion exchange process. Li+ exchange into zeolite Y (Si/Al = 1.56) was performed using methanol and water mixtures at an elevated temperature, 333 K, in an attempt 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 solvent compositions ranged from as-purchased (undried) methanol to pure water.

2. EXPERIMENTAL SECTION 2.1. Ion Exchange of Zeolite Y (FAU). Large single crystals of zeolite Y (FAU), stoichiometry Na75Si117Al75O384, were synthesized in this laboratory.19 Each of four crystals, colorless octahedra about 0.20 mm in cross-section, was lodged in its own fine Pyrex capillary. Four ion-exchange solutions 0.1 M in 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) with variable water/ methanol content were prepared as follows. Solution 1 was prepared by dissolving LiNO3 in undried methanol (Baker Analyzed HPLC Solvent, assay 100.0%, acetone 0.0005%, residue after evaporation 0.4 ppm, water content 0.04% (0.02 M)). Although a previously unopened bottle of methanol was used, the water content of solution 1 could have increased significantly during its preparation by sorption of moisture from 9010

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The Journal of Physical Chemistry C

Article

Table 2. Steps of Structure Refinementa occupancyb at step

Na(I)

Li(I′)

Na(I′)

Li(II)

Na(II)

Li(III′)

R1

R2

0.13 0.104 0.084 0.0801 0.0774 0.0778 0.0782 0.0781

0.49 0.271 0.263 0.2541 0.2538 0.2547 0.2539 0.2530

7.2(8) 7.4(7) 7.0(7) 7.0(7) 7.0(7)

0.11 0.087 0.064 0.051 0.0437 0.0425 0.0427 0.0422

0.50 0.230 0.177 0.157 0.1491 0.1486 0.1498 0.1479

7.4(9) 7(4) 7.1(8) 7.1(8) 7.1(8)

0.11 0.09 0.064 0.055 0.0447 0.0407 0.0409 0.0405

0.53 0.24 0.169 0.148 0.1480 0.1382 0.1384 0.1373

7.5(8) 7.4(8) 7.2(8)

0.12 0.09 0.073 0.070 0.0592 0.0516 0.0413 0.0416 0.0401

0.53 0.25 0.194 0.190 0.1681 0.1522 0.1319 0.1322 0.1299

Na(III′)

Crystal 1, |Li72Na3|[Si117Al75O384]-FAU 1c 2 3 4 5 6d 7e 8f 1c 2 3 4 5 6 7g 8f 1c 2 3 4 5 6 7g 8f 1c 2 3 4 5 6 7 8e,h 9i

40(4) 39(3) 38(3) 32 32 32

50(4) 50(3) 30(5) 4.0(11) 32(6) 3.9(11) 22(7) 30(5) 3.9(11) 27(8) 28.8(11) 3.2(5) 33(8) 28.7(6) 3.3(6) 33(8) Crystal 2, |Li63Na12|[Si117Al75O384]-FAU

36(3) 36(3) 34.0(23) 31(5) 30(5) 30.4(6)

31(4) 47(3) 41.5(24) 31(4) 3.5(8) 30(4) 3.0(7) 29(4) 3.0(7) 28.8(5) 3.2(5) Crystal 3, |Li60Na15|[Si117Al75O384]-FAU

38(3) 37(3) 37(3) 31(4) 29.9(5) 29.8(5)

1.8(3) 1.9(2) 2.1(2) 2.1(2) 2.1(2) 2.2(2)

44(4) 44(4) 42(3) 26(3) 27(3) 24.7(4) 24.3(5)

1.9(11) 2.0(11) 1.6(6)

2.0(8) 2.1(5) 2.2(5)

5.7(8) 3.9(6) 4.1(4) 4.5(5)

9.6(9) 12.1(7) 11.9(6) 27(4) 8.0(9) 27(4) 5.9(9) 26.0(5) 6.0(5) 25.7(5) 6.3(5) Crystal 4, |Li54Na21|[Si117Al75O384]-FAU 46(4) 56(3) 57(3) 28(4) 28(4) 30(4) 24.8(4) 24.0(5)

6.3(9) 6.6(8) 6.4(7) 7.2(4) 8.0(5)

a

Isotropic temperature factors were used for all Li+ and Na+ positions except in the last step. bThe occupancy is given as the number of Li+ and Na+ ions per unit cell. cOnly the atoms of zeolite framework, refined anisotropically, were included in the initial structure model. dOccupancy fixed at 32 for Li(I′). eConstrained to sum to 32, the maximum occupancy at site II. f Li+ and Na+ ions were refined anisotropically, except Na(III′). gConstrained to sum to 32, the maximum occupancy at sites I′ and II, respectively. hOccupancies at sites I and I′ constrained so that nI + nI′/2 ≤ 16. iLi+ and Na+ ions were refined anisotropically, except Na(I).

3. STRUCTURE DETERMINATION Full-matrix least-squares refinements (SHELXL97)22 were done on Fo2 using all data for the four crystals. 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 |K71|[Si121Al71O384]-FAU.23 Each initial refinement used anisotropic thermal parameters and converged to the error indices given in steps 1 of Table 2. 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 final cycles of refinement were done with anisotropic temperature factors for all positions except those that were not stable in anisotropic refinement: Li(III′) in crystal 1, Na(III′) in crystals 2 and 3, and Na(I) in crystal 4. Attempts to refine Li(III′) in crystals 2 and 3 at the position

found in crystal 1 were unsuccessful. The final error indices, R1 and R2, are given in Table 1. The largest peaks on the final difference Fourier function were not included in the final model because they were too far from framework oxygen atoms to be cations and were not otherwise within bonding distance of any other position. All shifts in the final cycles of the 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/[σ2(Fo2) + (aP)2 + bP], where P = [Max(Fo2,0) + 2Fc2]/3; a and b were refined parameters (Table 1). Atomic scattering factors for Li+, Na+, O−, and (Si,Al)1.82+ were used.24,25 The function describing 9011

dx.doi.org/10.1021/jp300321x | J. Phys. Chem. C 2012, 116, 9009−9018

Wyckoff position

192(i) 96(h) 96(g) 96(g) 96(g) 32(e) 32(e) 32(e) 192(i)

192(i) 96(h) 96(g) 96(g) 96(g) 32(e) 32(e) 32(e) 32(e) 192(i)

192(i) 96(h) 96(g) 96(g) 96(g) 32(e) 32(e) 32(e) 32(e) 192(i)

192(i) 96(h) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e)

atom

Si,Al O(1) O(2) O(3) O(4) Li(I′) Li(II) Na(II) Li(III′)

Si,Al O(1) O(2) O(3) O(4) Li(I′) Na(I′) Li(II) Na(II) Na(III′)

Si,Al O(1) O(2) O(3) O(4) Li(I′) Na(I′) Li(II) Na(II) Na(III′)

9012

Si,Al O(1) O(2) O(3) O(4) Na(I) Li(I′) Na(I′) Li(II)

I I′ I′ II

I′ I′ II II III′

I′ I′ II II III′

I′ II II III′

cation site

y 1240(1) 0 1(2) 723(2) 782(2) 474(7) 2252(13) 2403(17) 799(40) 1241(1) 0 2(1) 724(1) 784(1) 465(6) 629(38) 2241(7) 2390(19) 1475(6) 1240(1) 0 1(1) 722(1) 783(1) 458(6) 633(29) 2236(9) 2382(10) 1470(8) 1240(1) 0 1(1) 721(1) 782(1) 0 455(7) 623(11) 2221(8)

x −507(1) −1009(2) 1(2) −227(2) 782(2) 474(7) 2252(13) 2403(17) 346(40) −505(1) −1003(1) 2(1) −221(1) 784(1) 465(6) 629(38) 2241(7) 2390(19) 1025(6) −507(1) −1006(1) 1(1) −224(1) 783(1) 458(6) 633(29) 2236(9) 2382(10) 1030(8) −509(1) −1007(1) 1(1) −229(2) 782(1) 0 455(7) 623(11) 2221(8)

Table 3. Positional, Thermal, and Occupancy Parametersa U11 or Uiso

U22

U33

Crystal 1, |Li72Na3|[Si117Al75O384]-FAU 370(1) 358(13) 336(13) 339(13) 1009(2) 410(22) 460(34) 410(22) 1505(2) 369(21) 369(21) 420(35) 723(2) 420(32) 366(20) 366(20) 3221(2) 368(21) 368(21) 451(34) 474(7) 540(64) 540(64) 540(64) 2252(13) 526(125) 526(125) 526(125) 2403(17) 225(159) 225(159) 225(159) 4012(39) 970(382) Crystal 2, |Li63Na12|[Si117Al75O384]-FAU 372(1) 65(7) 51(7) 50(7) 1003(1) 126(13) 144(20) 126(13) 1510(1) 85(12) 85(12) 103(19) 724(1) 107(18) 75(12) 75(12) 3224(1) 97(13) 97(13) 111(19) 465(6) 156(44) 156(44) 156(44) 629(38) 551(467) 551(467) 551(467) 2241(7) 216(75) 216(75) 216(75) 2390(19) 249(186) 249(186) 249(186) 3954(9) 45(83) Crystal 3, |Li60Na15|[Si117Al75O384]-FAU 371(1) 66(7) 51(7) 54(7) 1006(1) 128(13) 171(21) 129(13) 1507(1) 97(12) 97(12) 120(20) 722(1) 109(19) 84(12) 84(12) 3222(1) 108(13) 108(13) 111(20) 458(6) 141(47) 141(47) 141(47) 633(29) 423(309) 423(309) 423(309) 2236(9) 210(86) 210(86) 210(86) 2382(10) 280(104) 280(104) 280(104) 3955(11) 125(95) Crystal 4, |Li54Na21|[Si117Al75O384]-FAU 370(1) 92(7) 71(6) 74(6) 1007(1) 174(12) 178(19) 174(12) 1502(1) 125(11) 125(11) 149(18) 721(1) 161(18) 123(11) 123(11) 3217(2) 138(12) 138(12) 149(18) 0 200d 455(7) 129(52) 129(52) 129(52) 623(11) 263(108) 263(108) 263(108) 2221(8) 246(80) 246(80) 246(80)

z

b

10(55) 115(108) 85(78)

−15(3) −50(10) 13(10) 31(13) 27(10)

−13(3) −39(11) 15(11) 13(14) 20(10) 33(52) 129(316) 97(88) 154(104)

−11(3) −36(11) 5(10) 13(13) 17(10) 41(51) 23(424) 80(69) 219(198)

−14(5) −28(18) 15(18) 33(23) 9(17) 112(82) 93(129) 107(165)

U23

10(55) 115(108) 85(78)

−2(3) −24(14) 13(10) 39(10) 27(10)

−2(4) 1(15) 15(11) 23(10) 20(10) 33(52) 129(316) 97(88) 154(104)

−3(4) −9(14) 5(10) 23(10) 17(10) 41(51) 23(424) 80(69) 219(198)

3(6) −25(25) 15(18) 33(18) 9(17) 112(82) 93(129) 107(165)

U13

10(55) 115(108) 85(78)

−5(3) −50(10) 50(14) 39(10) 53(14)

−5(4) −39(11) 56(15) 23(10) 39(15) 33(52) 129(316) 97(88) 154(104)

−4(3) −36(11) 48(14) 23(10) 33(14) 41(51) 23(424) 80(69) 219(198)

1(6) −28(18) 62(25) 33(18) 67(24) 112(82) 93(129) 107(165)

U12

192 96 96 96 96

192 96 96 96 96

192 96 96 96 96

192 96 96 96 96

initial

2.2(2) 24.3(5) 4.5(5) 24.0(5)

29.8(5) 2.2(5) 25.7(5) 6.3(5) 7.1(8)

30.4(6) 1.6(6) 28.8(5) 3.2(5) 7.0(7)

32 28.7(6) 3.3(6) 33(8)

varied

occupancyc

2 24 4 24

30 2 26 6 7

30 2 29 3 7

32 29 3 11

fixed

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp300321x | J. Phys. Chem. C 2012, 116, 9009−9018

Table 4. Selected Interatomic Distances (Å) and Angles (deg)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. bThe anisotropic temperature factor is exp[−2π2a−2(U11h2 + U22k2 + U33l2 + 2U23kl + 2U13hl + 2U12hk)]. cOccupancy factors are given as the number of atoms or ions per unit cell. dTo achieve convergence, the isotropic thermal parameter was fixed.

217(79) −216(125) 217(79) 97(93) 217(79) −97(93) 2374(7) 1459(9) 2374(7) 1041(9) II III′ 32(e) 192(i) Na(II) Na(III′)

Article

(Si,Al)−O(1) (Si,Al)−O(2) (Si,Al)−O(3) (Si,Al)−O(4) mean (Si,Al) Na(I)−O(3) Li(I′)−O(3) Na(I′)−O(3) Li(II)−O(2) Na(II)−O(2) Li(III′)−O(1) Li(III′)−O(4) Na(III′)−O(1) Na(III′)−O(4) O(1)−(Si,Al)−O(2) O(1)−(Si,Al)−O(3) O(1)−(Si,Al)−O(4) O(2)−(Si,Al)−O(3) O(2)−(Si,Al)−O(4) O(3)−(Si,Al)−O(4) (Si,Al)−O(1)−(Si,Al) (Si,Al)−O(2)−(Si,Al) (Si,Al)−O(3)−(Si,Al) (Si,Al)−O(4)−(Si,Al) O(3)−Na(I)−O(3) O(3)−Li(I′)−O(3) O(3)−Na(I′)−O(3) O(2)−Li(II)−O(2) O(2)−Na(II)−O(2) O(1)−Li(III′)−O(4) O(4)−Na(III′)−O(4)

crystal 1

crystal 2

crystal 3

1.6387(21) 1.679(3) 1.690(3) 1.6402(21) 1.662(3)

1.6381(12) 1.6826(15) 1.6926(16) 1.6416(13) 1.664(3)

1.6388(13) 1.6818(16) 1.6929(17) 1.6422(14) 1.664(3)

1.931(7)

1.915(4) 2.12(7) 2.011(6) 2.20(3)

1.918(5) 2.14(6) 2.019(6) 2.196(17)

2.760(18) 2.547(17) 113.25(12) 109.13(14) 110.84(15) 105.45(15) 107.28(15) 110.77(16) 147.95(23) 130.37(20) 126.85(19) 140.88(22)

2.769(21) 2.548(21) 113.19(13) 109.20(16) 110.78(16) 105.48(16) 107.25(16) 110.82(17) 147.2(3) 130.85(22) 127.11(21) 141.46(24)

118.5(3) 102(5) 117.7(5) 103.0(22)

118.7(3) 101(4) 118.1(5) 104.0(11)

2.80(3) 2.551(20) 113.20(12) 109.01(14) 111.28(15) 105.26(14) 107.07(15) 110.88(16) 147.11(23) 131.53(20) 127.37(19) 142.50(22) 80.0(1), 100.0(1) 118.8(3) 102.2(14) 118.7(4) 104.9(8)

79.2(7)

79.4(8)

79.7(8)

2.032(11) 2.24(3) 2.27(9) 2.23(10)

113.23(20) 109.34(23) 110.6(3) 105.55(24) 107.22(24) 110.8(3) 146.7(4) 131.1(3) 127.5(3) 141.6(4)

117.8(5) 117.0(9) 101.5(21) 74(3)

crystal 4 1.6358(12) 1.6811(15) 1.6933(15) 1.6393(12) 1.662(3) 2.577(3) 1.925(5) 2.128(21) 2.023(5) 2.196(13)

a

The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter.

(Si,Al)1.82+ is a weighted (for composition) mean of the Si4+, Si0, Al3+, and Al0 functions (Si/Al = 1.56). All scattering factors were modified to account for anomalous dispersion.26,27 Other crystallographic details are given in Table 1.

4. DESCRIPTION OF THE STRUCTURES 4.1. Brief Description of FAU. The framework structure of faujasite is characterized by the double 6-ring (D6R, hexagonal prism), the sodalite cavity (a cuboctahedron), and the supercage (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 usually occupy some or all of the sites shown with Roman numerals in Figure 1. The maximum occupancies at the cation sites I, I′, II, II′, III, and III′ in faujasite are 16, 32, 32, 32, 48, and (in Fd3̅m) 192, respectively. Further description is available.29,30 4.2. Crystal 1, |Li72Na3|[Si117Al75O384]-FAU. Per unit cell, 72 Li+ ions were found at three crystallographic sites (Table 3). Thirty-two fill site I′, 29 nearly fill site II, and the remaining 11 occupy site III′. Unlike Li-LSX,3 no Li+ ions were found at site III. Finally, three Na+ ions complete the filling of site II. The degree of Li+ exchange in this crystal is therefore about 96%.

a

8 7 8.0(5) 7.2(8)

varied U12 U33 U22 Wyckoff position atom

cation site

x

y

z

U11 or Uiso

b

Table 3. continued

Crystal 4, |Li54Na21|[Si117Al75O384]-FAU 2374(7) 344(76) 344(76) 344(76) 3954(11) 369(134) 369(134) 192(168)

U23

U13

initial

occupancyc

fixed

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of H3O+ exchange; if so, the degree of exchange would be less. The distances of Li(I′) and Li(II) to the framework oxygens at O(3) and O(2), 1.915(4) and 2.011(6) Å, respectively, and of Na(II) to O(2), 2.20(3) Å, are almost the same as in crystal 1 (Figure 3b and Table 4). Na(I′) is 0.94 Å from the plane of the three O(3) oxygen atoms to which it bonds (Figure 3b and Table 5). The Na(I′)− O(3) distance is 2.12(7) Å, a little shorter than the sum of the ionic radii, 2.29 Å.33 The Na+ ions in the supercage at site III′ are 4-coordinated by framework oxygen atoms, two O(1) atoms at 2.760(18) Å and two O(4) atoms at 2.547(17) Å (Figure 4b). These distances are much longer than the sum of the Na+ and O2− radii.33 However, these long distances were also seen in fully dehydrated Na−X.34,35 Local distortions may allow the actual distances to be less. 4.4. Crystal 3, |Li60Na15|[Si117Al75O384]-FAU. About 56 Li+ and 15 Na+ ions per unit cell are found at two equipoints. As in crystal 2, it is assumed that four Li+ ions at site III′ were not found, so the extent of Li+ exchange is 80%, less if H+ ions were present. At sites I′ and II, 30 and 26 Li+ ions are found per unit cell, respectively. Also at sites I′ and II, 2 and 6 Na+ ions, respectively, complete the filling of the 6-rings. The remaining seven Na+ ions are at site III′. Their placements may be seen in Figures 2a,b, 3b, and 4b. The geometry about them is reasonable (Tables 4 and 5), much like that described for crystals 1 and 2. 4.5. Crystal 4, |Li54Na21|[Si117Al75O384]-FAU. About 48 Li+ ions per unit cell occupy sites I′ and II, 24 at each. At sites I, I′, II, and III′, 21 Na+ ions are found (Table 3). As in crystals 2 and 3, it is assumed that additional Li+ ions are present, in this crystal six of them, at III′ (sites I/I′ and II are full), so the extent of Li+ exchange is 72%, less if H+ ions were present. The bonding distances of Li+ (at I′ and II) and Na+ (at I′, II, and III′) to framework oxygens are similar to those in crystals 1−3 (Table 4). Unlike crystals 1−3, Na+ ions occupy site I in this structure (Figure 2c). The octahedral Na(I)−O(3) bond distance, 2.577(3) Å, is much longer than the sum of the corresponding ionic radii, 2.29 Å.33 This distance is likely to be virtual; only two of the 16 D6Rs contain Na+ ions; the actual distance is likely to be substantially shorter. To avoid the very short distances between site I and the two occupied I′ sites (1.96 and 2.65 Å) where intercationic electrostatic repulsion should be severe, the two I′ sites of a D6R should not be occupied if its site I is. Accordingly, there are only (16−2) × 2 = 28 I′ sites available for cations. These are fully occupied by 24 ions at Li(I′) and 4 at Na(I′).

Figure 1. Stylized drawing of the framework structure of zeolite Y. Near the center of 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 Loewenstein’s rule (ref 28) would be obeyed. Extraframework cation positions are labeled with Roman numerals.

Each Li(I′) ion coordinates to three O(3) oxygen atoms at 1.931(7) Å, and each Li(II) ion coordinates to three O(2)s at 2.032(11) Å (see Figure 3a and Table 4). Considering the similar chemical environments for both Li(I′) and Li(II), the difference, 0.10 Å, is quite large. However, similar results were reported for Li-LSX.3 For comparison, the sum of the ionic radii of Li+ and O2− is 0.59 + 1.32 = 1.91 Å.31,32 The remaining 11 Li+ ions are located at site III′ in the supercages near 12-rings and triple 4-rings (Figure 4a). These Li+ ions coordinate only to two oxygen atoms, O(1) and O(4), at 2.27(9) and 2.23(10) Å, respectively. The Na(II)−O(2) distance is 2.24(3) Å (Table 4). For comparison, the sum of the ionic radii of Na+ and O2− is 2.29 Å.33 Though Li(II) extends 0.36 Å into the supercage from its three O(2) plane, Na(II) extends much further, 1.01 Å (Figure 4a and Table 5). Correspondingly, the O(2)−Li(II)− O(2) and O(2)−Na(II)−O(2) angles are 117.0(9)° and 101.5(21)°, respectively. As observed in previous reports,3,4,12 the T−O−T angles at O(2) and O(3), 131.1(3)° and 127.5(3)°, respectively, are much smaller than those seen in fully dehydrated Na−X,34 145.6(3)° and 141.2(4)°, respectively. It seems clear that, because Li+ is smaller than Na+, it pulls the oxygen atoms of its 6-rings inward toward the 6-ring centers, imparting significant strain to the zeolite framework. 4.3. Crystal 2, |Li63Na12|[Si117Al75O384]-FAU. At two crystallographic sites, 59 Li+ ions are found per unit cell. Nearly filling sites I′ and II are 30 and 29 Li+ ions, respectively, and two and three Na+ ions, respectively, complete the filling of the 6-rings involved (Table 3). Finally, seven Na+ ions are found at site III′. Only 71+ cations were found, where 75 are needed per unit cell to balance the negative charge of the zeolite framework. Because Na+ ions scatter X-rays much better than Li+, and should therefore have all been found, and because sites I′ and II are all full, it is assumed that four Li+ ions, not located in this work, are present at III′ sites, as seen in crystal 1. Thus the degree of Li+ exchange is taken to be 63/75, about 84%. Some or all of these missing ions may be H+, the result

5. DISCUSSION In all four structures, Li+ and Na+ ions fill site II and, according to the nI + 0.5nI′ = 16 rule, sites I and I′. The remaining cations, when they can be found, occupy III′ sites. The site preference for Li+ over Na+ is approximately I′ > II > III′ (Table 6). Na+ ions were found in all four structures, so complete Li+ exchange was not achieved in any crystal. Of the expected 75 cations per unit cell, four in crystals 2 and 3, and six in crystal 4 were not found. It has been assumed in this report that all are Li+, and that they were not found because of the low X-ray scattering power of Li+ and their very low expected occupancies at site III′ (or III). It is possible, however, that some or all are H+ ions resulting from H3O+ exchange into crystals 2, 3, and 4. 9014

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Figure 2. Stereoviews of representative D6Rs in crystals 1−4. Of the 16 D6Rs per unit cell in crystal 1, all are occupied as shown in (a). For crystals 2 and 3, 15 are occupied as shown in (a) and two as shown in (b). For crystal 4, 12 are occupied as shown in (a), four as shown in (b), and two as shown in (c). The zeolite Y framework is drawn with heavy bonds. The coordination of Li+ and Na+ ions to oxygens of the zeolite framework is indicated by light bonds. Ellipsoids of 25% probability are shown.

It is expected that the Na+ ions, because of their substantially higher scattering power, were all found. Figure 5 shows the degree of Li+ exchange into zeolite Y as a function of solvent composition. The highest degree of Li+ exchange was achieved from methanol solution containing ca. 0.02 M H2O, and the degree of exchange from pure water solution is the least (Figure 5 and Table 6). The extent of Li+ exchange increases with decreasing water concentration. Figure 5 indicates that complete Li+ exchange may be possible from methanol with 0 < [H2O] < 0.005 M, other conditions being the same as those used here (section 2.1). As discussed in the Introduction, the complete exchange of Li+ for Na+ is readily achieved for zeolite LSX from aqueous

solution. Both media, the LSX framework and water, are highly polar. Zeolite Y is less polar than zeolite X, so it may be expected that Li+ ions would be at a significantly higher potential energy (chemical potential) in Y than in X, thus discouraging the entry of Li+ into zeolite Y from aqueous solution. This is consistent with the result of ion exchange (up to 81%) in zeolite Y with Si/Al = 2.36.12 (This argument could be extended to consider the polarity of the specific positions that the exchangeable cations occupy, that is, the number of framework Al atoms near each.) Lowering the polarity (the dielectric constant) of the ion-exchange solution, by choosing a solvent with a dielectric constant lower than that of water, would increase the chemical potential of the Li+ ions in 9015

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Figure 3. Stereoviews of representative sodalite units in crystals 1−4; (a) crystal 1; (b) crystals 2−4. (The actual coordinates plotted in (b) are those of crystal 2.) See the caption to Figure 2 for other details.

Figure 4. Stereoviews of representative supercages in crystals 1−4; (a) crystal 1; (b) crystals 2−4. (The actual coordinates plotted in (b) are those of crystal 2.) See the caption to Figure 2 for other details.

dielectric constant to the same effect. The above discussion has omitted consideration of Na+ because Li+ is smaller and thus must form stronger coordination bonds to oxygen atoms, as

solution, bringing the two chemical potentials closer together again, thus facilitating the ion exchange of Li+ into zeolite Y. Raising the temperature of the solvent also decreases its 9016

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Table 5. Displacements of Atoms (Å) from 6-Ring Planes crystal 1 positions

sites

at O(3)a

Na(I) Li(I′) Na(I′) Li(II) Na(II)

I′ I′ II II

0.28

crystal 2

at O(2)b

at O(3)a

crystal 3

at O(2)b

at O(3)a

0.24 0.94

crystal 4

at O(2)b

at O(2)b

−1.73 0.23 0.92

0.22 0.97

0.36 1.01

at O(3)a

0.31 0.94

0.28 0.91

0.24 0.88

a

A positive displacement indicates that the cation lies in a sodalite cavity; a negative displacement indicates that the cation lies in a D6R. (Na(I) centers D6Rs.) bA positive displacement indicates that the cation lies in a supercage.

Table 6. Numbers of Li+ and Na+ Ions Per Unit Cell at Their Sites site I′a

site I crystal no.

Na

1 2 3 4

0 0 0 2

+

+

32 30 30 24

Li

site IIa +

Na

+

Li

0 2 2 4

29 29 26 24

site III′ +

total cations

Na

+

+

Li

Na

total Li

3 3 6 8

11 4c 4c 6c

0 7 7 7

72 59 56 48

+

total Na+

total cations

% IEb

3 12 15 21

75 71 71 69

96 84 80 72

a

Note that sites I′ and II are each fully occupied with 32 cations in crystals 1−3. In crystal 4, site II is full and sites I/I′ are full according to the rule nI + nI′/2 = 16. bPercent ion exchange of Li+. cThese Li+ ions were not found crystallographically, presumably because of their low occupancies and scattering powers. They are needed for charge balance. As discussed in sections 4 and 5, some or all of those unlocated Li+ ions may be H+. If so, the % IE for crystals 2, 3, and 4 would be less.

the undried methanol solution appears to have played a vital kinetic role in the Li+-exchange process. Because this water concentration is less than the Li+ concentration, 0.1 M, most Li+ ions must be coordinated only by methanol molecules, and a monoaquo coordination sphere is likely to predominate for the rest. Such a monohydrated Li+ ion can approach the zeolite framework more readily (by hydrogen bonding to framework oxygen atoms) than a Li+ ion that is fully coordinated by methanol molecules. The latter would be expected to present a more hydrophobic face to the hydrophilic zeolite, discouraging ion exchange. Li+ exchange is further discouraged by the strain that Li+ imparts to the zeolite framework. Zeolite Y is synthesized from aqueous Na+ solution, so Na+ can be expected to fit its sites well to give a hydrated zeolite structure with little strain. Because Li+ is smaller than Na+, it must pull the oxygen atoms of the 6-rings that it occupies in toward the 6-ring centers. As discussed in section 4.2, the angles at O(2) and O(3) in crystal 1, 131.1(3)° and 127.5(3)°, respectively, are much smaller than those in fully dehydrated Na−X,34 145.6(3)° and 141.2(4)°, respectively. Also, whereas the corresponding Na−O distances 2.343(5) and 2.242(7) Å,34 respectively, in Na−X are close to the sum of the corresponding ionic radii, 2.29 Å,33 the first of the corresponding Li−O distances in crystal 1, 2.032(11) and 1.931(7) Å, respectively, is noticeably larger than the corresponding sum of radii, 1.91 Å.31,32 Li+, therefore, does not fit into the principle cation sites in zeolite Y as well as Na+, and this discourages its exchange into the zeolite. The geometry given above for dehydrated crystals is expected to be indicative of the situation of the solvated crystals during ion exchange.

Figure 5. Extent of Li+ exchange (●) and the dielectric constant of the solvent at 333 K (▼) as a function of the water concentration in the ion exchange solution. If H3O+ had exchanged into crystals 2, 3, and 4, the negative slope of the straight line would be greater.

the bond strengths indicate (Li−O, 333.5 kJ/mol, Na−O, 256.1 kJ/mol).36 (The dielectric constants of water and methanol are 80.1 and 33.0 εΓ, respectively, at 293.2 K.37 The dielectric constant of methanol is 32.66 εΓ at 298 K38 and 26.06 εΓ at 333 K; the latter value was obtained by regressing published dielectric constant data.38 The dielectric constants of water/ methanol mixtures are nearly linear functions of composition, so there are no abrupt changes at low water content to which the increased exchange of Li+ can be attributed; see Figure 5.) Other considerations may have been important for achieving 96% Li+ exchange (crystal 1). (1) The relatively high ionexchange temperature employed, 333 K, just 5 K below the boiling point of methanol, can be expected to have kinetically facilitated the ion-exchange process. (2) It is likely that the size of the methanol molecule, small enough to easily enter and leave the zeolite, has been important. (3) The water content (ca. 0.02 M) of

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ASSOCIATED CONTENT

S Supporting Information *

Tables of calculated and observed structure factors (27 pages for four crystals). This material is available free of charge via the Internet at http://pubs.acs.org. 9017

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(31) Wozniak, A.; Marler, B.; Angermund, K.; Gies, H. Chem. Mater. 2008, 20, 5968. (32) Handbook of Chemistry and Physics, 77th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1996/1997; p 12−14. (33) Handbook of Chemistry and Physics, 70th ed.; Weast, R. C., Ed.; CRC Press: Cleveland, OH, 1989/1990; p F−187. (34) Olson, D. H. Zeolites 1995, 15, 439. (35) Zhu, L.; Seff, K. J. Phys. Chem. B 1999, 103, 9512. (36) Handbook of Chemistry and Physics, 77th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1996/1997; p 9−55. (37) Handbook of Chemistry and Physics, 77th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1996/1997; p 6−152. (38) Albright, P. S.; Gosting, L. J. J. Am. Chem. Soc. 1946, 68, 1061.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.T.L.); seff@hawaii.edu (K.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the staff at beamline 6B MXI of the Pohang Light Source, Korea, for their assistance during data collection. 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 (2011-0026300).

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REFERENCES

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