Single-Crystal Structure of |Li₅₀Na₂₅|[Si₁₁₇Al₇₅O₃₈₄]-FAU

Abstract

The single-crystal structure of fully dehydrated partially $Li^+$-exchanged zeolite Y, ${\mid}Li_{50}Na_{25}{\mid}[Si_{117}Al_{75}O_{384}]$-FAU, was determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group $Fd\bar{3}m$ at 100(1) K. Ion exchange was accomplished by flowing stream of 0.1 M aqueous $LiNO_3$ for 2 days at 293 K, followed by vacuum dehydration at 623 K and $1{\times}10^{-6}$ Torr for 2 days. The structure was refined using all intensities to the final error indices (using only the 801 reflections with ($F_o$ > $4{\sigma}(F_o)$) $R_1/R_2=0.043/0.140$. The 50 $Li^+$ ions per unit cell are found at three different crystallographic sites. The 19 $Li^+$ ions occupy at site I' in the sodalite cavity: the $Li^+$ ions are recessed 0.30 ${\AA}$ into the sodalite cavity from their 3-oxygens plane (Li-O = 1.926(5) ${\AA}$ and $O-Li-O=117.7(3)^{\circ}$). The 20 $Li^+$ ions are found at site II in the supercage, being recessed 0.23 ${\AA}$ into the supercage (Li-O = 2.038(5) ${\AA}$ and $O-Li-O=118.7(3)^{\circ}$). Site III' positions are occupied by 11 $Li^+$ ions: these $Li^+$ ions bind strongly to one oxygen atom (Li-O = 2.00(8) ${\AA}$). About 25 $Na^+$ ions per unit cell are found at four different crystallographic sites: 4 $Na^+$ ions are at site I, 5 at site I', 12 at site II, and the remaining 4 at site III'.

INTRODUCTION

Faujasite-type zeolites (FAU) are widely used in the range of applications as gas sorption, gas separation, and effective catalysts for many chemical reactions due to the excellent stability of the crystal structure and pore system.1-4 These processes depend on the type, number, and distribution of exchangeable cations over the available sites.5 Generally, the sorption property of zeolite is related to the forces of interaction between the sorbate molecules and the zeolite. Smaller cations with higher charges will render a zeolite more effective as a sorbing agent.6

The Li+ ion is so small and it charge more concentrated. Accordingly, fully dehydrated fully Li+-exchanged zeolites are generally used in the separation of nitrogen from air in the PSA (pressure swing adsorption) process.7-9 In addition, Li+-exchanged faujasite-type zeolites are useful as microporous catalysts for industrially important reactions such as ring alkylation of toluene10 and isomerization of olefins.11

The crystal structures of Li+-exchanged faujasite-type zeolites have been widely studied by several scientists for the understanding of their properties.12-15 However, it is not easy to study local structure of Li+ ions because of their low X-ray scattering power and mobility in the supercage of zeolite.

Forano et al. investigated the dehydrated partially Li+-exchanged zeolites X (Si/Al = 1.23) and Y (Si/Al = 2.36) by powder neutron diffraction techniques.12 They tried to improve the degree of Li+ exchange into zeolites X and Y using NH4 +-X and Y because the more loosely bound NH4 + ion as compared to Na+ would be more easily replaced by Li+ ion.16 However, both Na+ and H+ ions were found in these structures.

Plevert et al. studied the fully dehydrated Li+-exchanged zeolite X (Si/Al = 1.0) at two different temperatures of 300 and 10 K by powder neutron diffraction techniques.13 They reported that a phase transition was occurred at low temperature from (cubic) to Fddd (orthorhombic) due to the large distortion of the framework by the strong interaction between Li+ ions and the framework oxygen atoms.

Lobo et al. also investigated the phase transition in fully dehydrated fully Li+-exchanged zeolites X (Si/Al = 1.0 and 1.25) by solid-state MAS NMR spectroscopy and neutron diffraction technique.14 Phase transition from to Fddd was observed in the temperature range T = 200−300 K in the structure of Li-X (Si/Al = 1.0). However, it was interesting that phase transition did not occur at low temperature (10 K) in Li-X (Si/Al = 1.25).

Smolin et al. studied that the structures of the hydrated and partially dehydrated a partially Li+-exchanged zeolites X by single-crystal X-ray diffraction techniques to explore the detailed knowledge of the structure and chemical compositions.15 However, the informations of Li+ and Na+ ions in both structures were incomplete. In addition, the structure determination of fully dehydrated Li+-exchanged zeolite X was failed due to the loss of crystallinity at high dehydration temperature.

According to the previous studies of dehydrated Li- LSX,13,14 Li-X (Si/Al = 1.23 and 1.25),12, 14 and Li-Y (Si/Al = 1.56),17 Li+ ions tend to occupy preferentially sites I’ and II and the remaining Li+ ions were located at site III or III’ in the supercage. Sometimes it was not found crystallographically, because of their low occupancies and scattering powers at sites III or III’.

In the case of Li-LSX,13 Li+ ions were equally located at sites III and III’, respectively. Unlike the Li-LSX,13 Li+ ions were found only at site III’ in Li-LSX, Li-X (Si/Al = 1.25),14 Li-X (Si/Al = 1.23),12 and Li-Y (Si/Al = 1.56),17 but were not found at site III.

Generally, the complete exchange of Li+ for Na+ ion was readily achieved for LSX (Si/Al = 1.0) from conventional aqueous solution.13, 14 However, it is not simple to achieve completely Li+-exchanged zeolite when Si/Al ratio was higher than 1.0.12, 14, 17

Composition of zeolite Y (Si/Al = 1.56) is quite different to those of the previous structures. It would show unique application in separation and catalysis due to the different distribution and coordination of Li+ ions by different the local Si/Al ordering in tetrahedral sites. Therefore, knowledge of the distribution of Li+ ions in the zeolite Y framework is a key for understanding of peoperties of the Li+ ions.

This work was carried out to investigate the precise cation position of Li+ and Na+ ions in fully dehydrated partially Li+-exchanged zeolite Y (FAU, Si/Al = 1.56) from conventional aqueous solution at room temperature by single-crystal synchrotron X-ray diffraction techniques. Previous reports for the structures of Li+-exchanged faujasitetype zeolites will be compared with that from our results.

 

EXPERIMENTAL SECTION

Ion Exchange of Zeolite Y (FAU)

Large single crystals of zeolite Y (FAU), stoichiometry Na75Si117Al75O384, were synthesized in this laboratory.18 One crystal, colorless octahedra about 0.18 mm in crosssection, was lodged in a fine Pyrex capillary. Ion exchange was accomplished at 293 K by flowing stream of 0.1 M aqueous 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). The resulting clear colorless single crystal was slowly heated under dynamic vacuum to 673 K and dehydrated at 1 × 10−6 Torr for two 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 the crystal. 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.

Single-crystal X-ray Diffraction Work

X-ray diffraction data for the crystal was collected at 100(1) K using an ADSC Quantum 210 detector at Beamline 6B MXI at The Pohang Light Source. For 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 second per frame. The basic data file was prepared using the program HKL2000.19 The reflections were successfully indexed by the auto-mated indexing routine of the DENZO program.19 The diffraction intensities were harvested by collecting 72 sets of frames with 5 scans and an exposure time of 1 second per frame. These highly redundant data sets were corrected for Lorentz and polarization effects; negligible corrections for crystal decay were also applied.

Table 1.aR1 = ∑|Fo−|Fc||/∑Fo and R2 = [∑w(Fo 2−Fc 2)2/∑w(Fo 2)2]1/2; R1 and R2 are calculated using only the 801 reflections for which Fo > 4𝜎(Fo). bR1 and R2 are calculated using all unique reflections measured. cGoodness-of-fit = (∑w(Fo 2−Fc 2)2/(m−s))1/2, where m and s are the number of unique reflections and variables, respectively

The space group , conventional for zeolite Y, was determined by the program XPREP.20 Additional experimental details are presented in Table 1.

 

STRUCTURE DETERMINATION

Full-matrix least-squares refinements (SHELXL97)21 were 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 |K71|[Si121Al71O384]-FAU.22 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: Na (I’), Li(II), and Li(III’). 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 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.

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

Table 3.aPositional 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.

Table 4.aThe numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter

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, with a and b as refined parameters (see Table 1). Atomic scattering factors for Li+, Na+, O−, and (Si,Al)1.82+ were used.23, 24 The function describing (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.25, 26 Other crystallographic details are given in Table 1.

 

RESULTS AND DISCUSSION

The framework structure of zeolite Y (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).

Fig. 1.Stylized drawing of the framework structure of zeolite Y.Near the center of the each line segment is an oxygen atom. The different oxygen atoms are indicated by the numbers 1 to 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. 26) would be obeyed. Extraframework cation positions are labeled with Roman numerals.

The exchangeable cations, which balance the negative charges of the zeolite Y framework, usually occupy some or all of the sites shown with Roman numerals in Fig. 1. The maximum occupancies at the cation sites I, I’, II, II’, and III in zeolite Y are 16, 32, 32, 32, and 48, respectively. Site III’ in zeolite Y studied using space group is a 192-fold position. Further description is available.28, 29

In this structure, 50 Li+ ions per unit cell are found at sites I’, II, and III’ and 25 Na+ ions at sites I, I’, II, and II’ (see Table 3). The degree of Li+-ion exchange in this structure is therefore about 67％.

Four Na+ ions per unit cell at Na(I) occupy site I (at the center of the D6Rs, see Fig. 2(a)). Each coordinates to six O(3) framework oxygens of its D6R at distance of 2.556(3) Å, which is much longer than the sum of the corresponding ionic radii, 0.97 + 1.32 = 2.29 Å,30 indicating a reasonably good fit: this was seen before in the structure of |Li54Na21|[Si117Al75O384]-FAU.17 The Na(I) is nearly octahedral with O(3)−Na(I)−O(3) bond angles at 80.3(1)° and 99.7(1)° (see Table 4). In this position, Li+ ions do not fit in octahedral coordination at the center of the hexagonal prism due to small size of Li+ ion.

Fig. 2.Stereoviews of the four possible ways that cations occupy double 6-rings (D6Rs) in dehydrated |Li50Na25|[Si117Al75O384]-FAU. Of the 16 D6Rs per unit cell in dehydrated |Li50Na25| [Si117Al75O384]-FAU, four are occupied as shown in (a), nine as shown in (b), five as shown in (c), and two as shown in (d). 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.

The 19 Li+ and 5 Na+ ions at Li(I’) and Na(I’), respectively, lie at two site I’ positions, on the 3-fold axis in the sodalite unit opposite D6Rs (see Fig. 3).

Fig. 3.A stereoview of representative sodalite unit in dehydrated |Li50Na25|[Si117Al75O384]-FAU.See the caption to Fig. 2 for other details.

The distances between sites I and two I’ are 2.01 and 2.58 Å. To avoid the very short distances where intercationic electrostatic repulsion should be severe, the two I’ positions of the same D6R should not be occupied if its site I is filled. Accordingly, only (16−4) × 2 = 24 site I’ positions should be available for cations per unit cell. This is in agreement with the number found at site I’, 24.7(4).

The Li+ ions at Li(I’) are bonded by three O(3) oxygen atoms of the D6R at 1.926(5) Å, which are almost the same as the sum of the conventional ionic radii of Li+ and O2−, 0.59 + 1.32 (respectively) = 1.91 Å.31, 32

The Na+ ions at site I’ are 2.083(12) Å from three O(3) framework oxygens. This distance is somewhat shorter than the sum of the corresponding ionic radii of Na+ and O2−, 2.29 Å.30 However, it is in agreement with previous results, Na(I’)−O(3) = 2.12(7), 2.14(6), and 2.128(21) Å in fully dehydrated partially Li+-exchanged zeolites Y.17 This is attributed to the low occupancy number of Na+ at site I’.

Table 5.aA 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.

Fig. 4.A stereoview of representative supercage in dehydrated |Li50Na25|[Si117Al75O384]-FAU.See the caption to Fig. 2 for other details.

Each ion at Li(I’) and Na(I’) lies inside the sodalite cavity, 0.30 and 0.87 Å, respectively, from its three O(3) plane (see Fig. 3 and Table 5). The corresponding O(3)–Li(I’)–O(3) and O(3)–Na(I’)–O(3) bond angles are 117.7(3)° and 104.6(8)°, respectively.

About 20 Li+ and 12 Na+ ions per unit cell at Li(II) and Na(II), respectively, are located at site II opposite S6Rs in the supercage (see Fig. 4). These 32-fold positions are fully occupied by the Li+ and Na+ ions.

The Li+ and Na+ ions at Li(II) and Na(II), respectively, are coordinated by three O(2) oxygen atoms of the S6Rs at distances of 2.038(5) and 2.211(7) Å, respectively, which are in close agreement with the sum of the ionic radii of Li+ and O2−, 1.91 Å and Na+ and O2−, 2.29 Å.30 While Li(II) extends 0.23 Å into supercage from its three O(2) plane, Na(II) extends, 0.89 Å, which is 0.66 Å much further than Li(II) (see Fig. 4 and Table 5).

The 11 Li+ ions per unit cell are located at site III’ in the supercages (see Fig. 4). These Li+ ions bind strongly only to one oxygen atom, O(4), at 2.00(8) Å (see Table 4). This position was not found in previous studies. No Li+ ions were found at site III as observed in the structure of fully dehydrated Li72Na3-Y.17

The thermal parameters of the Li+ ions at site III’ are much higher than sites I’ and II, as in previous work (see Table 3),17 suggesting the presence of further positional disorder at Li(III’), which could not be resolved in this work. This high thermal parameters may be a consequence of the combined effects of the low coordination number, one, and high mobility of Li+ ion at this site.

The remaining 4 Na+ ions in the supercage at site III’ are 4-coordinated by framework oxygen atoms, two O(1) atoms at 2.77(4) Å and two O(4) atoms at 2.60(3) Å (see Table 4). These distances are much longer than the sum of the Na+ and O2− radii, 2.29 Å.30 However, these long interaction distances were also observed in the structures of fully dehydrated Na75-Y,33 Li63Na12-Y, Li60Na15-Y, and Li54Na21-Y.17

Generally, the zeolite framework adapts itself to the size of the cation, leading to remarkable distortions of the 6-rings windows in the case of Li+-exchanged zeolites due to small size and strong electric filed of Li+ ion.12-1417

In this structure, T-O-T angles at O(2) and O(3), 132.19(18)° and 126.79(18)°, respectively, are much smaller than that in fully dehydrated Na-Y,33 145.3(2)°and 140.4(2)°. This observation is an excellent agreement with the result of previous works.12-14, 17 It seems clear that zeolite framework is under some strain as a result of much smaller size and higher interaction of Li+ ion than Na+ ion: it strongly pulls the oxygens atoms of its 6-rings toward the 6-ring centers.

In the case of single-crystal structure of Li+-exchanged zeolite X (Si/Al = 1.09),15 the loss of the crystallinity was found due to the distortions of the zeolite framework during dehydration. However, crystallographic evidence for indication of the loss of crystallininty is not observed in this structure. It is strongly correlated with relatively higher structural stability of Li+-exchanged zeolite Y than X at high temperature.

In this structure, Li+ ions are preferentially located at sites I’ and II and the remaining Li+ ions occupy site III’ as found in previous structural studies in fully dehydrated Li-LSX,13, 14 Li-X,12, 14 and Li-Y.17 However, Li(III’) position is quite different from previous studies.12-14, 17 Li(III’) is close to the side of a 12-ring in the supercage by the closest oxygen atom O(4) as shown in Fig. 4. These Li+ ions coordinate only to one oxygen atoms, O(4). This position is similar to the Na+ ions in the structure of fully dehydrated Na71-Y.34 This is attributed to the high mobility of Li+ ion in the supercage and local Si-Al order among T atoms in zeolite framework.

Table 6.aPercent ion exchange of Li+. bReference 17. Crystal prepared from aqueous solution at 333 K. cThese Li+ ions were not found crystallographically, presumably because of their low occupancies and scattering powers. They are needed for charge balance. dThis work. Crystal prepared from aqueous solution at 293 K.

Recently, the structure of fully dehydrated partially Li+-exchanged zeolite Y from aqueous solution at 333 K was determined by single-crystal synchrotron X-ray diffraction techniques.17 In this structure, 48 Li+ ions were found at sites I’ and II, 24 at each, and 21 Na+ ions at sites I, I’, II, and III’.

When the structures of fully dehydrated Li54Na21-Y17 and Li50Na25-Y (this work) are compared, it can be seen that the site II is full and sites I/I’ are full according to the rule nI + nI’/2 = 16 by Li+ and Na+ ions and remaining Na+ ions are located at sites III’ in both structures (see Table 6). In the structure of Li50Na25-Y, remaining Li+ ions of the expected 75 cations per unit cell to balance the negative charge of the framework were found at site III’. These cations were not found at either of the sites, III or III’ in the structure of Li54Na21-Y.

Generally, the extent of cation exchange increases with increasing ion-exchange temperature due to the greater mobility of cation at higher ion-exchange temperature. A similar trend is seen when the structures of Li54Na21-Y17 and Li50Na25-Y are compared.

With increasing ion-exchange temperature from 293 K (this work) to 333 K (Li54Na21-Y),17 the Li+ exchange level increases from 67 to 72％. The number of Na+ ions at sites I, I’, II, and III’ decreased with increasing Li+-exchanged level in both structures of dehydrated Li+-exchanged zeolite Y.

As mention above, the complete Li+-exchange from conventional aqueous solution did not occur in the zeolite Y. This is attributed to the large polarity of the ion-exchange solution relative to polarity of zeolite Y. Water is high polar. Therefore, Li+ ion exchange into zeolite Y from aqueous solution is discouraged due to the higher Si/Al ratio of the zeolite framework, the smaller negative charge density of the framework, and the resulting electric fields.17

 

SUMMARY

Li+-exchanged zeolite Y was prepared from aqueous 0.1 M Li(NO3) solution by dynamic ion-exchange method at 293 K, followed by vacuum dehydration at 673 K and 1.0 × 10−6 Torr. The crystal structure was determined by singlecrystal synchrotron X-ray diffraction techniques. The complete exchange of Li+ into zeolite Y was not achieved because Li+ ion has high hydration energy in water. The degree of Li+ ion exchange in this structure is 67%. In the crystallographic study, 50 Li+ ions per unit cell are found at sites I’, II, and III’ and 25 Na+ ions at sites I, I’, II, and III’. The Li+ for Na+ ions fill preferentially at sites I’ and II and the remaining Li+ ions occupy sites III’. Li(III’) is close to the side of a 12-ring in the supercage by the closest oxygen atom O(4).