1594
Paul E. Riley and Karl Seff
Crystallographic Evidence for Hydrolysis in Zeolites. The Structure of Hydrated Partially Cobalt(l1)-Exchanged Zeolite A Paul E. Riley and Karl Seff Chemistry Department, University of Hawaii, Honolulu, Ha wail 96822 (Received May 13, 1974; Revised Manuscript Received March 7, 1975) Publication costs assisted by the National Science Foundation
The crystal structure of a fully hydrated, partially Co(I1)-exchanged form of the synthetic molecular sieve zeolite A, C O O . ~ ~ N ~ O . ~ ~ [ Astoichiometry I S ~ O ~ ] - A , Co4Na4A11pSi1~04g-xH20 ( x ca. 35) per unit cell, has been determined from three-dimensional X-ray diffraction data gathered by counter methods. The structure was solved and refined in the cubic space group Pm3m: a = 12.267 ( 5 ) 8, at 19 (1)O. Cobalt(I1) ions are located at two distinct crystallographic sites. One Co(I1) ion (Co(1)) resides in the sodalite unit where it is coordinated by a regular octahedron of water molecules (Co(1) to HzO = 2.11 ( 3 ) A): the other three Co(I1) ions (Co(2)) are distributed about equivalent sites on unit cell threefold axes. These Co(2) ions apparently promote an extensive hydrolysis of the aluminosilicate framework, and stoichiometrically produce Brdnsted acid sites. Structural evidence suggests that each Co(2) ion is responsible for the addition of three water molecules to the zeolite framework, probably with the dissociation of one proton per water molecule, to form three Co(2)-O-(Si,Al) bridges to each Co(2). T o achieve each Co(2)-0-(Si,Al) linkage, one (Si,Al) atom, probably Al, has increased its coordination number from four to five. The Co(2)-0 bonds of the resultant Co(B)-O-(Si,Al) bridges are long (2.36 ( 3 ) &, probably a virtual result due to Co(2) disorder about the threefold axis position, but the (Si,Al)-0 bonds, probably A1-0 bonds, are normal (1.74 ( 3 ) A). Each Co(2) ion is coordinated at a fourth position by a water molecule which projects well into the large cage. Three of the four Na+ ions per unit cell are located at two sites of threefold symmetry, close to triads of framework oxygen atoms. A fourth Na+ ion apparently has selected a position in the large cage, a t a site of fourfold symmetry. Altogether, the positions of 35 water molecules per unit cell have been determined. Full-matrix least-squares refinement using 291 reflections for which IO > 30(Io) has converged to a conventional R index (on F ) of 0.072.
Introduction Successful exploitation of the well-known selective sorptive and catalytic properties1 of crystalline aluminosilicate zeolites relies heavily upon an appreciation of the structural features of these molecular sieves; Le., the dimensions of zeolitic channels and cages, the locations of the exchangeable cations and their interactions with the zeolitic framework at those locations, and the nature of the active sites that may have been formed by modification of the aluminosilicate framework. T o this end, a number of first row transition-metal-exchanged zeolite A systems (zeolite nomenclature and structure are described in ref 2) of stoichiometry M,Na12-zZAl12Si1204s.xHz0 per unit cell, where M = Mn(II), Co(II), Ni(II), or Zn(I1) and 2.5 5 z 5 5.5, have been examined by single-crystal X-ray diffraction techniques in our laboratory. Diffraction experiments have been carried out with these materials in both their hydrated and dehydrated as well as following sorption of a variety of small molecules (principally with the Mn(I1)and Co( 11)-containing zeolite^^*^-^). The results attained from the hydrated work alone suggest a diverse zeolite “aqueous” chemistry that varies as the exchangeable cation. For example, Mn(I1) and Zn(I1) ions adopt quite different coordination environments within the zeolite; the Mn(I1) ions3 favor trigonal bipyramidal coordination, but the two crystallographically distinct kinds of Zn(I1) ions5 select tetrahedral coordination. In both of these structures, however, these ions share a common feature-they are tightly bound t o zeolite framework Tha Journal of Physical Chemistry, Vol. 79, No. 15, 1975
oxygen atoms. (Actually, this is true for 4.5 of the 5.5 Zn(I1) ions per unit cell; the remaining Zn(I1) ion, at the unit cell origin, is far from the framework.) By coordinating directly with framework oxygen atoms, the Mn(I1) and Zn(I1) ions act much like the Na+ ions of unexchanged zeolite 4A’O and the Tl(1) ions of fully Tl(1)-exchanged zeolite A.l’ However, preliminary studies quickly revealed that the Co(1I) and Ni(I1) ions of their respective zeolite systems depart radically from this behavior. These ions are too far (2.8 to 2.9 A) from the framework oxygen atoms, as usually located, to coordinate to them. To explain this anomaly, the crystal structure of the Co(I1) system, CodNarA112Si12048ccH20 (x ca. 35) per unit cell, or Co0.33Na0.33[AlSi04]-A (exclusive of water molecules) in conventional nomenclature,2b was studied. (The structure of the Ni(11)-exchanged form of zeolite A, Ni2.5Na7A112Si1204g yHzO, is available elsewheree6) Experimental Section Single crystals of zeolite 4A, Na[AlSiOd]-A (exclusive of water molecules), or Na12Al12Si1204r27H20 per unit cell, were grown as colorless cubes by the method of Charnell.12 Ion exchange with aqueous 0.1 M C0(N03h as previously described4 yielded pink-tan colored cubes of appropriate composition as established by elemental analysis. A relatively large single crystal, about 0.08 mm along an edge, was selected for X-ray diffraction studies. It was mounted a t the tip of a glass fiber and maintained a t 19 (1)’ and 30% relative humidity during the X-ray experiments.
1595
Structure of Hydrated Partially Co(ll)-Exchanged Zeolite A
Previous crystallographic studies in this laboratoryZaJ1 have indicated that the space group Pm3m (cubic system; no reflections systematically absent) is suitable for our crystals, although it cannot distinguish between ordered Si and A1 atoms, and is therefore only approximately correct. Gramlich and MeierlO have reported that for the hydrated unexchanged form of sodium zeolite 4A, the cubic space group Fm3c is appropriate. However, only a few very weak superlattice reflections (indicative of an F-centered unit cell with a doubled lattice constant) were noted in our work with this Co(I1)-containing material, and most of these violated the systematic absence rules of Fm3c. All diffraction experiments were performed with an automated, four-circle Syntex Pi diffractometer, equipped with a graphite monochromator and a pulse-height analyzer. Molybdenum radiation (Kal, X 0.70926 A; Kap, X 0.71354 A) was employed for preliminary experiments and for data collection as well. The cubic unit cell constant a t 19 (l)',as determined by least-squares refinement of 15 intense reflections for which 20' < 20 < 24', is 12.267 (5) A. Reflections were examined by the 0 - 20 technique at a constant scan speed of 0.5 deg min-I. Initially each reflection was scanned symmetrically, i.e., from 1.0' (in 20) below the calculated K a l peak to LOo above the calculated Kap peak. However, after about two-thirds of the data had been gathered, an asymmetric scan (1.0' below the Kal peak to 1.3' above the Koip peak) was utilized to compensate for a minor misorientation of the crystal. Background intensity was examined at each end of the scan range for a time equal to half the time necessary to measure the reflection. As a check on crystal and instrument stability, the intensities of three reflections in diverse regions of reciprocal space were recorded after every 100 reflections initially; when the asymmetric scan was used, these check reflections were remeasured after every 25 reflections. 30 systematic variations in intensity were observed for the check reflections. All reciprocal lattice points within the sphere defined by 20 < 70' were examined. Although few reflections are significantly greater than background for high 20 values, this limit was chosen to maximize the size of the relatively small data set. Standard deviations were assigned in accordance with the expression
$d
sg
-,.
where (3 is the scan rate, CT is the total integrated count, B1 and Bp are the background counts, and the intensity IO is computed as IO = w(CT - B1 - Bp). A value of 0.02 has been found satisfactory for the empirical parameter p . 1 1 p 1 3 The data were corrected for Lorentz and polarization eff e c t ~including ,~~ that due to reflection from the monochromator crystal, which was assumed to be half perfect and half mosaic in character. An absorption correction was not made; the linear absorption coefficient is 14.0 cm-l, and the transmission factors varied only slightly, from 0.855 to 0.870. Of the 883 reflections measured, the 291 with intensities Io for which Io > 3.0a(Zo)15 were used in subsequent structural analysis. Structure Determination and Solution
Full-matrix least-squares refinement was commenced using the zeolite framework ((Si,Al), O ( l ) , 0 ( 2 ) , and O(3); see Figure 1) atomic parameters of hydrated Tl(1)-exchangedll zeolite A. (Because of the indistinguishability of The Journal of Physical Chemistry, Vo/. 79, No. IS, 1976
1596
si04 and A104 tetrahedra (vide supra), only the average species, (Si,Al), is considered in this work.) From a subsequent difference Fourier function, the four Co(I1) ions were located; one at the unit cell origin (Co(1)) and the remaining three (Co(2)) in the large central cage on the unit cell threefold axes at x = y = z = 0.25 (equipoint Wyckoff 8(g)).Inclusion of these positions in isotropic least-squares refinement led to convergence with error indices R1 = Z(F, - (F,/(/ZF, = 0.16 and Rz = (Zw(F, - (Fd)2/ZwF,2)1/2 = 0.15. In the least-squares treatment, the quantity minimized and the weights (w) are the reciprocal is ( Z u ( F , - IFd)2)14 squares of u(P,), the estimated standard deviation of each observation. Atomic scattering factors for 0- and (Si,A1)1.75+for the zeolite framework, Co2+ and Na+ for the exchangeable cations, and 0 0 for water oxygen atoms were used.16 (The function describing (Si,A1)1.75+is the mean of the Sio, Si4+, AlO,and AI3+ scattering functions.) The scattering factors for (Si,A1)1.75+and Co2+ were modified to account for the real parts (Af’) of the anomalous dispersion correction.17 A difference Fourier synthesis using framework atomic and Co(I1) positions indicated a variety of positions which could be attributed to Na+ ions and HzO molecules. The most significant of these had densities between 1.0 and 3.0 e A-3. Several criteria were to be fulfilled before the conclusions suggested by the subsequent least-squares treatment were accepted. First, the peaks were to refine close to their initially estimated positions; also the occupancies of these sites should be chemically meaningful and the corresponding thermal parameters should be realistic; finally, the resultant positions should make suitable approaches to well-established parts of the structure (e.g., framework atoms, exchangeable ions, or water molecules). Application of these conditions resulted in the acceptance of a few positions. Six water molecules (HzO(l)’s, see Table I) which describe a regular octahedron about the Co(1) ion were located in the small cage (see Figure 2). Two Na+ ions (Na(1)’s) were located at the surface of the small cage, along threefold axes near the centers of the oxygen six-windows, a position selected by Na+ ions in the dehydrated form of this Co(I1)-containing eol lite.^ (An oxygen six-window, as shown in Figure 3, is composed of six alternating (Si,Al) atoms and six oxygen atoms conjoined so as to form a puckered twelve-membered aluminosilicate ring.) A third Na+ ion (Na(2)) was also observed at a threefold axis site, but in the large cage. The remaining Na+ ion (Na(3)) apparently occupies a position in the large cage, along a fourfold axis passing through the center of a unit cell face. The closest approaches to the three equivalent Co(I1) ions (Co(2)’s) in the large cage are made by three sets of three equivalent oxygen atoms (0(4)’s), located at general positions. These O(4) atoms act as bridges and are in turn bonded to (Si,Al) cations (probably to Al) of the framework. Although some water molecules in the large cage may coordinate the Co(2) ions so that more complete coordination spheres are achieved, only one other position (HzO(2)) refined a t a satisfactory distance from the Co(2) ions, indicating that the Co(2) ions are only four coordinate. Of the remaining Fourier peaks (all with densities between 1 and 2 e only those designated as water molecules &0(3), -(4),- ( 5 ) ,and -(6) in Table I behaved properly in least-squares refinement, and also fulfilled the conditions listed above. From consideration of thermal parameThe Journal of Physical Chemistry, Vol. 79, No. 15, 1975
Paul E. Riley and Karl Seff
Figure 3. Coordination of a Co(2) ion in the large cage. Ellipsoids of 50% probability are used. ters (Table I) and spatial limitations, we conclude that altogether these peaks correspond to approxirnately 17 water molecules. In the last cycle of least-squares refinement, the framework atoms, the three threefold axis Co(2) ions, and the HzO(1) molecules were treated anisotropically, and all remaining species of Table I were treated isotropically. Convergence was attained with R1 = 0.072, Rz = 0.073, and a goodness-of-fit, (Zw(F, - IFd)2/(m - s))llZof 1.33, where m (291) is the number of observations and s (60) is the number of variables in least-squares refinement. In this final cycle of refinement, shifts in positional and thermal parameters were all less than 1%of their corresponding estimated standard deviations except for some H20(5) and HzO(6) parameters; for these all shifts were less than 2% of their corresponding esd’s. A subsequent difference Fourier function revealed five peaks of densities 0.7-1.7 e Upon least-squares refinement, the three shallowest peaks shifted greatly in position, and therefore were not regarded as meaningful features of the structure. The two remaining, more significant positions (1.0 and 1.7 e A-3), located near the center of the large cage and separated from one another by about 1.8 A, diverged in least-squares refinement, requiring thermal parameters of 20-40 A2 for low occupancies. These two peaks are reasonably far from other parts of the structure, and perhaps as suggested by Gramlich and MeierlO who observed similar residual electron density in their work with sodium zeolite A, this may be due to a small number of unresolvable water molecules. Accordingly, these two peaks along with the three mentioned above were not included in final structural calculations. On this final difference Fourier map the standard deviation of the electron density was calculated to be 0.10 e A-3. Final position, thermal, and occupancy parameters are presented in Table I; bond lengths and bond angles in Table 11. A listing of 10F, and lOP, is available.l* Discussion The four Co(I1) ions per unit cell are located at two sites (see Figures 1 and 2, and Table I): one cation, Co(l), is a t the unit cell origin (at the center of the sodalite unit), and the remaining three, C0(2)’s, are on the unit cell body diagonals (sites of threefold symmetry) in the large cavity. The Co(1) ion is coordinated by a regular octahedron of water molecules, HzO(l)’s, at distances of 2.11 (3) 8, (see Table 11), in agreement with the mean bond length reported for CoS04.6HzO (2.11 A).19 Each HzO(1) oxygen atom lies on a
Structure of Hydrated Partially Co(ll)-Exchanged Zeolite A
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TABLE I: Positional, Thermal, and Occupancy Parameters for Hydrated Coo.33Nao.33[A1Si04]-Aa Wyckoff
ocpZ3 cupancy factor
posi -
tion
X
z
3'
P i 1 OF Biso
022
P3 3
Pi2
013
(%AI)
24(k) 0 1833(2) 3706(2) 23(2) 23(1) 14(1) 0 0 6(2) lb 12(h) 0 2204(10) 1/2 llO(14) 31(8) 15(6) 0 0 0 1 O(2) 12(i) 0 2963(5) 2963(5) 54(9) 17(4) 17(4) 0 0 11(9) 1 O(3) 24(m) 1106(6) 1106(6) 3418(8) 45(4) 45(4) 77(9) 55(10) 57(9) 57(9) 1 Co(2) 8(g) 2665(24) 2665(24) 2665(24) 262(20) 262(20) 262(20) 85(56) 85(56) 85(56) 3/8 H20(1) 24(m) 595(29) 1144(16) 1144(16) 92(34) 55(19) 55(19) -73(29) -73(29) -91(41) 1/4 0 0 0 0.8(1) 1 Co(1) l(a) Na(1) 8(g) 1575(29) 1575(29) 1575(29) 5(1) 1/4 Na(2) 8(g) 2106(75) 2106(75) 2106(75) 6(3) 1/8 1/6 Na(3) 6(f) 2361(81) 1/2 1/2 5(2) O(4) 48(n) 1195(26) 2566(23) 3897(24) 2.3(5) 3/16 HzO(2) 48(n) 3006(77) 3804(78) 4173(75) 3(2) 1/16 HzO(3) 48(n) 2387(56) 2898(63) 4710(68) lO(3) 3/16 H20(4) 24(m) 382(151) 4544(99) 4544(99) 8(7) 1/12 HzO(5) 48(n) 1909(158) 3594(171) 4797(254) 9(6) 1/16 HzO(6) 48(n) 1468(111) 4021(130) 4317(133) 6(4) 1/16 a Positional and anisotropic thermal parameters are given X lo4; isotropic thermal parameters are given in (%ngstrom)2. Numbers in parentheses are the estimated standard deviations in the last significant digits. See Figure 1 for the identities of the atoms. The anisotropic Occupancy for (Si) = 1/2; occupancy for (Al) = 1/2. temperature factor = e x p [ - ( 8 d 2 + h k 2 + P3312 + P&k + P13hl + &&)I.
o(1)
TABLE 11: Selected Interatomic Distances (A) and Angles (deg)a ~~
~~~~~
~~~
(Si, A1)-0(1) 1.652(4) (Si, A1)-0(2) 1.658(3)
O(l)-(Si, A1)-0(2) 107.3(4) O(l)-(Si, A1)-0(3) 110.6(4) (Si, A1)-0(3) 1.661(3) O(1)-(Si, A1)-0(4) 74(1) (Si, A1)-0(4) 1.736(31) 0(2)
