The Effect of Co²⁺-Ion Exchange Time into Zeolite Y (FAU, Si/Al = 1.56): Their Single-Crystal Structures

Abstract

Three single crystals of fully dehydrated $Co^{2+}$-exchanged zeolite Y (Si/Al = 1.56) were prepared by the exchange of $Na_{75}$-Y ($|Na_{75}|[Si_{117}Al_{75}O_{384}]$-FAU) with aqueous streams 0.05 M in $Co(NO_3)_2$, pH = 5.1, at 294 K for 6 h, 12 h, and 18 h, respectively, followed by vacuum dehydration at 673 K. Their single-crystal structures were determined by synchrotron X-ray diffraction techniques in the cubic space group Fd3m at 100(1) K. They were refined to the final error indices $R_1/wR_2$ = 0.0437/0.1165, 0.0450/0.1228, and 0.0469/0.1278, respectively. Their unit-cell formulas are $|Co_{29.1}Na_{11.8}H_{5.0}|[Si_{117}Al_{75}O_{384}]$-FAU, $|Co_{29.8}Na_{11.0}H_{4.4}|[Si_{117}Al_{75}O_{384}]$-FAU, and $|Co_{30.3}Na_{9.5}H_{4.9}|[Si_{117}Al_{75}O_{384}]$-FAU, respectively. In all three crystals, $Co^{2+}$ ions occupy sites I, I' and II; $Na^+$ ions are also at site II. The tendency of $Co^{2+}$ exchange slightly increases with increasing contact time as $Na^+$ content and the unit cell constant of the zeolite framework decrease.

Introduction

Co2+-exchanged zeolite Y is well established efficient heterogeneous catalysts in industrial processes1 and they have been extensively studied for the following applications: the epoxidation of styrene2 and α-pinene,3 the disproportionation of CO,4 the hydrodesulfurization reaction,5 the reduction of NO,6 the oxidation of benzyl alcohol7,8 and methylene chloride and carbon tetrachloride,9 the oxidative dehydrogenation of ethanol10 and cyclohexene,11 the ammoxidation of ethane,12 the decomposition of N2O,13 and the dehydration of ethanol.14 To develop better Co2+ catalysts, the positions, occupancies, and environments of Co2+ ions in zeolites are of great interest in understanding their catalytic properties.15

Ion exchange (cation substitution) is the most important method for the modification of the physical and chemical properties of zeolites to be used as catalysts, sorbents, or molecular sieves.16 The influenceable factors of ion-exchange processes of zeolites are (1) the nature of the cation species, the cation size, both anhydrous and hydrated, and cation charge, (2) the nature of exchangeable cations such as their valency and tendency to hydrolyze, (3) the structure of framework, (4) the Si/Al ratio of framework, (5) the concentration of the cation species in solution, (6) the anion species associated with the cation in solution, (7) the solvent (the residual water in nonaqueous solvents plays an important role in facilitating the ion exchange process),17 (8) the pH of exchange solution, (9) the temperature of ion exchange, and (10) the time of ion exchange.18,19

Three structures of Co2+-exchanged zeolite X (Si/Al = 1.09) were investigated as a function of ion-exchange temperature at 296 K (crystal 1), 323 K (crystal 2), and 353 K (crystal 3), respectively.20 In all cases excess Co2+ exchange with the uptake of OH− was seen. After partial dehydration at 296 K and ca. 0.13 Pa, the numbers of Co2+, Na+, H3O+, and OH− ions in the three structures were about 38, 24, 0, 8; 38, 11, 18, 13; and 46, 8, 0, 8 per unit cell, respectively. Co2+ ions preferred site II in all three crystal structures.

Four structures of partially Co2+-exchanged zeolite X (Si/ Al = 1.09) were studied as a function of dehydration temperature at 673 K (crystal 1), 423 K (crystal 2), room temperature (crystal 3), all at P ≤ 5 × 10−4 Pa; crystal 4 was fully hydrated.21 The dehydration induced a migration of Co2+ ions from site I' to site I; the occupation at site I increased monotonically (10(2)%, 19.8(6)%, 38.4(6)%, and 94.2(6)% for crystals 4 to 1, respectively), while that at site I' correspondingly decreased (50(2)%, 24.9(6)%, 31.8(6)%, and 4.5(6)%, respectively).

Recently, two structures of fully and partially dehydrated, largely Co2+-exchanged zeolite Y (Si/Al = 1.56) were determined crystallographically.1 Partial dehydration (at 294 K and ca. 7 × 10−1 Pa) was sufficient to decompose some of H3O+ ions that had exchanged into the zeolite, leaving only H+. Upon fully dehydration Co2+ ions moved into the hexagonal prisms and Na+ ions moved from sodalite cavities to supercages.

The present study was done to investigate the Co2+-ion exchange tendency into zeolite Y (Si/Al = 1.56) with the contact time of solution by single-crystal X-ray diffraction techniques. The effect of ion-exchange time on the extent of exchange might be seen.

 

Experimental

Ion Exchange and Dehydration. Large clear colorless octahedral single crystals of sodium zeolite Y, |Na75|[Si117- Al75O384]-FAU (Si/Al = 1.56), with diameters up to 0.32 mm were prepared by Lim et al.22 by the method of Sacco et al.23 Three of these were lodged in Pyrex capillaries, one in each. Ion exchange was done by the flow method using 0.05 M aqueous Co(NO3)2 (Aldrich, 99.999%, Ca 0.54 ppm, Zn 0.43 ppm, Fe 0.29 ppm, Ni 0.21 ppm, B 0.21 ppm, Cs 0.08 ppm, V 0.05 ppm). The pH of this solution was 5.1 at 296 K. This solution was allowed to flow past crystals 1, 2, and 3 for 6 h, 12 h, and 18 h, respectively, at 294 K.

Each capillary containing its crystal, now pale pink and clear, was attached to a vacuum system. Under dynamic vacuum, the temperature was cautiously raised at a heating rate of 10 K/h to 423 K and maintained at that temperature for 5 h. It was raised further (12.5 K/h) to 673 K and was fully dehydrated at this temperature and a dynamic vacuum of 1.3 × 10−4 Pa 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. Still under vacuum in their capillaries, the crystals were only then allowed to cool to room temperature and were sealed by torch in their capillaries. Microscopic examination showed that three crystals had become deep blue.

X-ray Diffraction. Synchrotron X-ray diffraction data were collected for the three crystals using an ADSC Quantum210 detector at Beamline 6B MX I 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 scale file was prepared using the HKL2000 program.24 The reflections were successfully indexed by the automated indexing routine of the DENZO program.24 The diffraction data were harvested by collecting 72 sets of frames with 5° scans with an exposure time of 1 s per frame. These highly redundant data sets were corrected for Lorentz and polarization effects, and a very small correction for crystal decay was applied. The space group Fd3m, standard for zeolite Y, was determined by the program XPREP.25 A summary of the experimental and crystallographic data is presented in Table 1.

 

Structure Determination

Full-matrix least-squares refinement (SHELXL97)26 was done on F2 using all data for each crystal. Each refinement was initiated with the atomic parameters of the framework atoms [(Si,Al), O(1), O(2), O(3), and O(4)] in dehydrated |Tl75|[Si117Al75O384]-FAU.22 Each initial refinement used anisotropic thermal parameters and converged to the high error indices (given in step 1 of Table 2) R1/wR2 = 0.45/0.83, 0.44/0.83, and 0.44/0.83 for crystals 1, 2, and 3, respectively. The further steps of structure determination and refinement as new atomic positions were found on successive difference Fourier electron density functions are presented in Table 2.

Table 1.aRint = ∑|Fo2-Fo2(mean)|/∑[Fo2]; Rint is calculated from the merging of equivalent data for internal agreement For all reflections. bRsigma = ∑[σ(Fo2)]/∑[Fo2] cR1 = ∑|Fo-|Fc||/∑Fo and wR2 = [∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2; R1 and wR2 are calculated using only the reflections for which Fo > 4σ(Fo). dR1 and wR2 are calculated using all unique reflections measured. eGoodness-of-fit = [∑w(Fo2-Fc2)2/(m-s)]1/2, where m is the number of unique reflections and s is the number of variables, respectively.

Because of the silicon and aluminum disorder in the zeolite framework, which extends to all framework oxygen positions and is substantially compounded by the partially occupied nonfrmaework cation positions, careful consideration was given to the acceptability of the structural parameters, especially the thermal and occupancy parameters, that emerged at each stage of structure determination and least-squares refinement. For the most part, the effects of disorder have been absorbed into thermal parameters, which are therefore inflated.

Fixed weights were used initially; the final weights were assigned using the formula w = 1/[σ2(Fo 2) + (aP)2 + bP] where P = [Max(Fo 2,0) + 2Fc 2]/3; the refined parameters a and b are given in Table 1. Atomic scattering factors for Co2+, Na+, O−, and (Si,Al)1.80+ were used.27,28 The function describing (Si,Al)1.80+ is the weighted (for the composition Si/Al = 1.56) mean of the Si4+, Si0, Al3+, and Al0 functions, assuming half formal charges. All scattering factors were modified to account for anomalous dispersion.29,30 All shifts in the final cycles of refinement were less than 0.1% of their corresponding estimated standard deviations (esds). The final error indices are given in Table 1. The structural parameters are given in Table 3, and selected interatomic distances and angles can be found in Table 4.

Table 2.aThe occupancy is given as the number of ions or molecules per unit cell at each position. bOnly the atoms of zeolite framework were included in the initial structure model. They were all refined anisotropically. cAll Co2+ positions were refined anisotropically

Further details of each crystal structure investigations may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-arlsruhe. de/request_for_deposited_data.html) on quoting CSD numbers 426761 (crystal 1), 426760 (crystal 2), and 426759 (crystal 3), respectively.

 

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 Figure 1). Each unit cell has 8 supercages, 8 sodalite cavities, 16 D6Rs, 16 12-rings, and 32 single 6-rings (S6Rs).32,33

The exchangeable cations, which balance the negative charge of the zeolite Y 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, and III are 16, 32, 32, 32, and 48, respectively. Site III' in space group Fd3m is a 192-fold position. Further description is available.32,33

In all three structures, 29.1(7), 29.8(7), and 30.3(5) Co2+ ions per unit cell, respectively, occupy identically three equipoints: site I, site I', and site II (see Table 5). Unexpectedly, 11.8(22), 11.0(22), and 9.5(17) residual Na+ ions, respectively, are found at site II. The sum of the cationic charges of the Co2+ and Na+ ions per unit cell, 70.0+, 70.6+, and 70.1+ for crystals 1, 2, and 3, respectively, is insufficient to balance the negative of the zeolite framework, 75-. This difference is attributed to about 5.0(14), 4.4(14), and 4.9(11), respectively, H+ ions that exchanged into the crystal as H3O+ which were found at supercages with occupancy of 4.0(9) per unit cell in partially dehydrated largely Co2+-exchanged zeolite Y.1

The 14.8(2), 14.6(2), and 14.8(2) Co2+ ions per unit cell in crystals 1, 2, and 3, respectively, occupy site I (at the center of the D6Rs, see Figure 2(a)). Each coordinates to six O(3) framework oxygens of its D6R at a distance of 2.231(4) Å, 2.230(4) Å, and 2.227(4) Å, respectively, somewhat longer than the sum of the corresponding ion radii of Co2+ and O2−, 0.72 + 1.32 = 2.04 Å.34 Co(I) is octahedral with bond angles of 89.20(14)° and 90.80(14)°, 89.31(15)° and 90.69(15)°, and 89.17(16)° and 90.83(16)°, respectively.

In agreement with previous reports,1,21,35 Co2+ ions are preferred site I in double six-rings to better satisfy their coordination requirements upon dehydration. In two structures of fully and partially dehydrated largely Co2+-exchanged zeolite Y,1 the occupancy at site I increased from zero in partially dehydrated zeolite to 14.6(2) in fully dehydrated zeolite. In four crystal structures of fully hydrated, partially dehydrated, and fully dehydrated partially dehydrated Co2+-exchanged zeolite X,21 the number of Co2+ ions at site I increased from 1.6(2) to 15.1(6) with increasing dehydration temperature; the Co(I)-O(3) bond distances decreased monotonically from 2.75(1) to 2.206(3) Å. In zeolite Y (dehydrated |Co14Na25|[Si136Al56O384] and |Co19Na18|- [Si136Al56O384]), the Co2+ ions at site I' migrated into the hexagonal prism when the temperature increased from 473 K to 873 K.35

Table 3.aPositional and thermal parameters × 104 are given. Numbers in parentheses are the esds 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)]. cThe Occupancy factor is given as the number of atoms or ions per unit cell.

The distance between sites I and I' is only 2.43(4) Å in crystal 1 (2.45(3) Å and 2.34(4) Å in crystals 2 and 3, respectively). Intercationic electrostatic repulsion should be severe for Co2+ ions at this distance. However, the observed occupancies indicate that it is avoided: if a site I is occupied, the two I' positions of that same D6R should not be. Accordingly, only (16−14.8(2)) × 2 = 2.4(4) site-I' positions in crystals 1 and 3 (2.8(4) in crystal 2) should be available for Co2+ ions per unit cell. This agrees well with the occupancy observed at Co(I'), 1.8(2), 2.2(2), and 2.1(3), respectively, in three structures. Accordingly, it is concluded that all or very nearly all D6Rs are occupied, either by a Co2+ ion at site I or two at the two sites I' associated with each D6R.

Each ion at Co(I') coordinates to three O(3) framework oxygen atoms at 2.172(20) Å, 2.177(15) Å, and 2.122(22) Å, respectively, again a little longer than the sum of the corresponding ionic radii, 2.04 Å.34 It is recessed 1.16 Å, 1.18 Å, and 1.07 Å, respectively, into a sodalite cavity from O(3) plane, so its coordination is trigonal, far from planar (see Figures 2(b) and 3 and Table 6).

At site-II positions (opposite S6Rs in the supercages, see Figures 3 and 4), 12.5(9), 13.0(9), and 13.4(7) Co2+ ions, respectively, are found at Co(II) in crystals 1, 2, and 3, respectively. Each bonds at 2.146(5) Å, 2.142(5) Å, and 2.128(5) Å, respectively, a little longer than the sum of the ionic radii as above, to three O(2) framework oxygen atoms. Each Co(II) ion is only 0.27 Å, 0.26 Å, and 0.12 Å, respectively, from their plane, so its coordination is nearly trigonal planar (see Table 6). With increasing contact time from 6 h to 24 h1 with the ion-exchange solution, the occupancies of Co2+ ions at site II slightly increased, 12.5(9), 13.0(9), 13.4(7), and 14.2(7), respectively.

Figure 1.Stylized drawing of the framework structure of zeolite Y. Near the center of the each line segment is an oxygen atom. The nonequivalent oxygen atoms are indicated by the numbers 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 Loewenstein’ rule (ref. 31) would be obeyed. Extraframework cation positions are labeled with Roman numerals.

Figure 2.Stereoviews of the two ways that Co2+ ions occupy D6Rs in crystals 1 to 3. In all three crystals, about 15 Co2+ ions must be present at Co(I) as shown in (a). Two Co(I’) ions must be present as shown in (b) about the remaining 16 D6Rs per unit cell. Co(I’)···Co(I') = 4.864 Å, 4.897 Å, and 4.677 Å in crystals 1, 2, and 3, respectively, as shown in (b); the coordinates plotted are those of crystal 3. The zeolite Y framework is drawn with heavy bonds. The coordination of the exchangeable cations to oxygens of the zeolite framework is indicated by light bonds. Ellipsoids of 25% probability are shown.

Figure 3.Stereoview of a representive sodalite cavity in crystals 1 to 3; the coordinates plotted are those of crystal 3. See the caption to Figure 2 for other details.

Figure 4.Stereoview of a representive supercage in crystals 1 to 3; the coordinates plotted are those of crystal 3. See the caption to Figure 2 for other details.

The long bonds seen between Co2+ and framework oxygen atoms in all three structures could be inaccurate due to local distortions in the zeolite framework caused by the Co2+ ions; only averaged framework oxygen positions have been determined in this work.1

The remaining cations, 11.8(22), 11.0(22), and 9.5(17) Na+ ions per unit cell at site II in crystals 1, 2, and 3, respectively, are shown Figures 3 and 4. Each Na(II) ion is in the supercage, 0.59 Å, 0.55 Å, and 0.84 Å, respectively, from the plane of three O(2) framework oxygen atoms. The Na(II)-O(2) distances, 2.210(9) Å, 2.197(8) Å, and 2.286(12) Å, respectively, are comparable to the sum of the conventional ionic radii, 2.29 Å.34

To see the effect of ion-exchange time on the product, it is interesting to compare four structures of zeolite Y Co2+ exchanged at pH 5.1 for 6 h (crystal 1 of this work), 12 h (crystal 2 of this work), 18 h (crystal 3 of this work), and 24 h (crystal 1 in reference 1), respectively. The degree of exchange increased from 77.6% to 82.7% with the increasing contact time (see Figure 5 and Table 5). With increasing ion exchange time with the exchange solution, the Co2+ content appears to have increased slightly from 29.1(7) to 31.0(5) per unit cell as the number of Na+ ions decreased from 11.8(22) to 7.7(15). In addition, the unit cell constant of the zeolite framework decreased with this higher level of Co2+ exchange (see Figure 5 and Table 1). This is generally seen in dehydrated zeolites with an increase number of more highly charged cations.36,37 Unfortunately, the tendency of the H+ content, observed by difference, was not seen.

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

In two single-crystal structures of fully dehydrated, largely Cu2+-exchanged zeolite Y (Si/Al = 1.56), both the Cu2+ and H+ content of the zeolite increased slightly as the Na+ content and the unit cell constant decreased with increasing Cu2+-exchange time.37.

The effect of ion-exchange temperature of Co2+ into zeolite X (Si/Al = 1.09) was investigated from three structures of Co2+-exchanged zeolite X.20 The degree of Co2+ exchange increased from 74% to 88% to 91% with increasing temperature at 296 K, 323 K, and 353 K, respectively. Residual Na+ ions were found at D6R in all three structures. It mentioned that the solvation shell of Na+ ions breaks down at higher temperature so coordinatively unstable Na+ ions migrate to D6Rs to satisfy their coordination requirements.

Table 5.aNumber of ions per unit cell. bIon exchange Time. cNo. of H+ ions per unit cell required to balance the negative charge of the zeolite framework. d[∑Co2+ ions/(M+/2)] × 100, where, M is required to balance the negative charge of framework per unit cell, 75. eRef. 1, crystal 1.

Table 6.aSite I' is near the plane of one 6-ring of a D6R; displacements into the sodalite unit are given as positive. A negative deviation indicates that the atom lies within a D6R. bSite II is in the supercage; displacements from its 6-rings are given as positive.

Figure 5.Extent of Co2+ ion exchange (%) and the unit cell constant (Å) with increasing contact time.

In four structures of zeolites X (Si/Al = 1.09) and Y (Si/Al = 1.56) Li+ exchanged at 293 K and 333 K,38 the level of Li+ exchange increased from 93% to 95% in zeolite X and from 67% to 72% in zeolite Y, respectively, with the ion-exchange temperature. It explained that this can be attributed to the greater mobility of all cations at the higher ion-exchange temperature.

 

Conclusion

Three single crystals of Co2+ exchanged zeolite Y were prepared from aqueous solution depending largely on contact time and dehydrated fully. Their single-crystal structures were determined crystallographically. After Co2+ exchange, residual Na+ ions were found and H+ ions must be present for charge balance.

In all three structures, Co2+ ions occupy sites I, I' and II, preferring I and II. Not a few Na+ ions occupy a second site II in all structures. With increasing Co2+-exchange time, these structures clearly show that the degree of Co2+ slightly increased. Both Na+ content and the unit cell constant slightly decreased with increasing level of Co2+ exchange.