J . Phys. Chem. 1986, 90, 1311-1318
1311
Localization of Benzene In Sodlum-Y Zeolite by Powder Neutron Dlffractlon A. N. Fitch,* Institut Laue- Langevin, 38042 Grenoble Cedex, France
H. Jobic, and A. Renouprez Institut de Recherches sur la Catalyse, 69626 Villeurbanne Cedex, France (Received: February 13, 1985; I n Final Form: September 12, 1985)
The structure of sodium-Y zeolite, Na56Si1360384A156, containing chemisorbed deuterated benzene has been studied for two different benzene coverages at 4 K and room temperature by powder neutron diffraction. The bare dehydrated zeolite has also been examined. The space group is Fd3m with cell parameters, at room temperature, of a = 24.85 (3) A, 24.83 (3) A, and 24.81 (3) 8, for the bare zeolite, low-coverage, and high-coverage samples, respectively. At 4 K a = 24.85 (3) 8, at low coverage and a = 24.81 (3) 8, for the high-coverage sample. The sodium ions are distributed over three sites whose positions at room temperature for the bare zeolite are Na(1) at 32e (x,x,x),x = 0.2345 (2); Na(2) at 32e, x = 0.0507 (4), and Na(3) at 16c (O,O,O), with fractional occupancies of 100 (2)%, 58 (2)%, and 44 (2)%, respectively. At 4 K, the benzene molecules are localized on two distinct sites within the cavities of the zeolite. Benzene(1) is in the supercage with the center of the molecule on the cube diagonal at 32e (x,x,x) at an average distance from Na(1) of 2.70 (1) 8, (with x = 0.2970 (2)) at low coverage and 2.64 ( I ) 8, ( x = 0.2982 (1)) at high coverage. There are four of these sites available per supercage. Benzene(2) is centered at 16d (‘/z,’/2,’/z) in the window between adjacent supercages. This window, formed by 12 Si/AI atoms and 12 bridging 0 atoms, has a free diameter of about 7.5 8, and provides a well-tailored environment for the benzene molecule, which is presumably stabilized in this position by van der Waals forces. There is an average of two of these sites available per supercage. At 4 K the average separation between adjacent benzene(1) sites decreases on increasing the benzene coverage, and Na(l) appears to migrate toward the center of the supercage. This may be associated with clustering of benzene molecules, which has been observed in earlier small-angle neutron scattering studies. At room temperature, the benzene molecules are largely delocalized but are still confined to the supercages in the region of their positions at 4 K.
Introduction
The catalytic properties of synthetic zeolites stem from their capacity to adsorb a variety of molecular species into the interconnecting chambers and channels within the framework structure in which the cations are also 1ocated.I To understand the catalytic behavior of zeolites, knowledge is required not only of the distribution of the cations, but also of the interaction between the cations and the adsorbed molecules, between the framework and the adsorbed molecules, and between the adsorbed molecules themselves. There is at present very little direct structural information concerning the nature of adsorbed molecules in zeolites, although previous studies have located adsorbed acetylene or methane in zeolite A by single-crystal X-ray diffractionZor powder neutron diffraction3 The rotational and translational motions of adsorbed molecules have been investigated by NMR4 and by quasi-elastic neutron s ~ a t t e r i n g . ~Studies *~ at room temperature by small-angle neutron scattering on benzene adsorbed into sodium-Y zeolite have suggested that, at a benzene coverage of 2.5 molecules per supercage, there is a tendency for the benzene molecules to aggregate in a limited number of cavities. At a lower benzene coverage, 1.0 molecules per supercage, a more even distribution of the benzene molecules is observed.’ The structure of Y zeolite, which is similar to that of the mineral faujasite, has been previously determined by X-ray d i f f r a c t i ~ n . ~ . ~ The space group is Fd3m with a lattice parameter typically in High-resolution powder neutron the range 24.60-25.12 .&Io diffraction has proved to be a very useful technique for the investigation of the cation distribution and framework structure of and has also been successful in locating xenon atoms adsorbed into zeolite-rho.” We report here the use of this method to locate the benzene molecules adsorbed into sodium-Y zeolite. A preliminary report has already been published.Is Experimental Section
The Linde sodium-Y zeolite (SK 40), with a Si/Al ratio of 2.43, was exchanged in D,O and then carefully dehydrated at 450 O C for 15 h to give a nominal composition of Nas6Si1360384A156.
* Present address: Department of Chemistry, University of Keele, Staffordshire. U.K. 0022-3654/86/2090-13 1 1$01.50/0
The zeolite was sealed in a standard 16-mm thin-walled vanadium sample can in a glovebox. For the two samples containing C6D6, the adsorption of a known amount of hydrocarbon was performed over a period of 24 h to give benzene contents, to an accuracy of about lo%, of 1.1 (low coverage) and 2.6 (high coverage) molecules per supercage, respectively. The samples were allowed to attain equilibrium for several days before attempting the neutron diffraction measurements. Powder neutron diffraction patterns were obtained at room temperature and at 4 K at a wavelength of 1.909 %, (and 2.98 A) on the diffractometer D1A at the ILL Grenoble. The patterns were measured between 6 O and 158’ in 28 in steps of 0.05’. Each run took about 18 h.
(1) Barrer, R. M. “Zeolites and Clay Minerals as Sorbents and Molecular Sieves”; Academic Press: London, 1978. (2) Amaro, A.; Seff, K. J . Phys. Chem. 1973, 77, 906. (3) Kahn, R.; Cohen de Lara, E.; Thorel, P.; Ginoux, J. L. Zeolites 1982, 2, 260. (4) Karger, J.; Ruthven, D. M. J . Chem. SOC., Faraday Trans. 1 1981, 77, 1485. ( 5 ) Stockmeyer, R. Zeolites 1984, 4 , 81. (6) Jobic, H.; Bee, M.; Renouprez, A. J. Surf. Sci. 1984, 140, 307. (7) Renouprez, A. J.; Jobic, H.; Oberthur, R. C. Zeolites 1985, 5 , 222. (8) Eulenberger, G. R.; Shoemaker, D. P.; Keil, J. G. J . Phys. Chem. 1967, 71, 1812. (9) Olsen, D. H. J. Phys. Chem. 1968, 72, 4366. (10) Klinowski, J.; Ramdas, S.; Thomas, J. M.; Fyfe, C. A,; Hartman, J. S. J . Chem. SOC.,Faraday Trans. 2 1982, 78, 1025. (11) Cheetham, A. K.; Eddy, M. M.; Jefferson, D. A,; Thomas, J. M. Nature 1982, 299, 24. (12) Adams, J. M.; Haselden, D. A,; Hewat, A. W. J . Solid State Chem. 1982, 44, 245. (13) Parise, J. B.; Shannon, R. D.; Prince, E.; Cox, D. E. Z . Kristallogr. 1983, 165, 175. (14) Cheetham, A. K.; Eddy, M. M.; Thomas, J. M. J. Chem. SOC.,Chem. Commun. 1984, 1337. (15) Adams, J. M.; Haselden, D. A. J . Solid State Chem. 1984, 55, 209. (16) Parise, J. B.; Gier, T. E.; Corbin, D. R.; Cox, D. E. J . Phys. Chem. 1984, 88, 1635. (17) Wright, P. A,; Thomas, J. M.; Ramdas, S.; Cheetham, A. K. J . Chem. SOC.,Chem. Commun. 1984, 1338. (18) Fitch, A. N.; Jobic, H.; Renouprez, A. J . Chem. SOC.,Chem. Commun. 1985, 284.
0 1986 American Chemical Society
1312
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986
Fitch et al.
TABLE I: Final Parameters and R Factors for the Bare Sodium-Y Zeolite at Room Temperature in atom %/AI O(l) O(2) o(3) o(4) Na( I ) Na(2) Na(3)
nosition 192i 96h 96g 96g 96g 32e 32e I6c
x
-0.0543 0 -0.0021 0.1764 0.1786 0.2345 0.0507 0
(I) (1)
(I) (I) (2) i4j
0.0354 4.1061 -0.0021 0.1764 0.1786 0.2345 0.0507 0
Space Croup Fd3m.
z
Y
0.1247 0.1061 0.1418 -0.0335 0.3182 0.2345 0.0507 0
(I) (I) (I) (I) (I) (2) (4)
‘Cell parameter: (I = 24.8536 (3) A; error due to wavelength = +0.03 Estimated standard deviations (esd) in parentheses.
A.
R
factors:” R,
(I) (I) (I) (I) (I) (2) (4j
Origin st j m *
B. Ai
N
0.8 ( I ) 1.9 ( I ) 2.3 ( I ) 2.4 ( I ) 2.6 ( I ) 3.3 (21 3.3 (2j 3.3 (2)
192 96 96 96 96 32.2 (8) 18.6 (6) 7.1 (4)
= 7.1%. R,, = 10.1%. R, = I1.4%, RE = 6.9%.
TABLE II: Average Bond Distances (A) and Angles (deg) for the Framework Atoms and the Sodium Ions in Sodium-Y Zeolite‘ n Y ~
(Si/Al)-O(l) (Si/AI\-OI2\ average
O(I)-O(2) x 2 O(1)-O(3) X2 0 ( 1 ) - 0 ( 4 ) X2 0(2j-o(3j x 2 O(2)-0(4) X2 O(3)-0(4) X2 average O( I)-(Si/AI)-O(2) O(l)-(Si/Al)4(3) O(l)-(Si/Al)-0(4) 0(2)-(Si/AI)-0(3) 0(2)-(Si/AI)-0(4) 0(3)-(Si/AI)-O(4) average (Si/AI)-O( I )-(Si/Al) (Si/AI)-0(2)-(SikAI) (Si/AI)-0(3)-(Si/AI) (Si/A1)-0(4)-(Si/AI) average
Figure 1. A view of the structure of Y zeolite showing the Si/AI atoms a t the vertices of the polyhedra. The data were analyzed by the Rietveld method’q using the following scattering lengths,’0 Si = 0.4149, AI = 0.3449, 0 = 0.5805, Na = 0.363, D = 0.6674, C = 0.66484 X IO-‘’ cm. Results Bare Zeolite at Room Temperaiure. The structure of the bare Y zeolite was found to be consistent with that obtained from earlier X-ray diffraction studies?’ The space group is Fd3m. and there is no long-range order associated with the Si and AI atoms. Three positions for the sodium ions have previously been found on the three-fold axis of the unit cell. In the refinement separate isotropic temperature factors were allowed to vary for the framework Si/AI and 0 atoms, whereas a single isotropic temperature factor was refined for all three sodium ions. There were 21 positional, thermal, and occupancy parameters in addition to a scale factor, cell parameter, zero point, three halfwidth parameters, and Rietvelds asymmetry correction. There was a total of 450 Bragg reflections included in the refinement, of which 206 were at unique values of 28. Three peaks occurring below 16’ in 28 were excluded owing to their severe asymmetry, which was beyond the capabilities of the correction employed. In addition the ( I I I ) reflection was omitted, since it was partially obscured by the beamstop. (The intensities of these reflections for the 1.909- or 2.98-t% data were later checked with those calculated from the final model and were found to be satisfactory.) The refinement converged to give the values shown in Table I, and the following R factors:” R, = 7.1%. Rwp= 10.1%. (19) Rietveld, H. M. J . Appl. Crystollogr. 1969, 2, 65. (20) Koester. L.: Rauch. H. IAEA Report 2517/RH, 1981
Na(l)-0(2) X3 Na(l)-0(4) X3 Na(2)-0(3) X3 NaiZi4i2j x3 Na(3)-0(3) X6 Na(2)-Na(3)
L
H
1.641 (4) 1.675 (4) 1.645 (4) 1.637 (5) 1.649 2.735 (4) 2.681 (4) 2.702 (4) 2.672 (4) 2.613 (5) 2.749 (5) 2.692 111.1 (2) 109.4 (2) 1 11.0 (2) 107.2 (2) 104.2 (2) 113.8 (3) 109.5 137.2 (2) 143.8 (3) 138.6 (2) 147.4 (3) 141.8 2.35 ( I ) 2.87 ( I ) 2.24 ( I ) 2.96 ( I ) 2.677 (3) 2.24 ( I )
1.633 (6) 1.660 (6) 1.656 (6) 1.648 (7) 1.649 2.739 (6) 2.709 ( 5 ) 2.683 ( 5 ) 2.661 (6) 2.645 (6) 2.718 (6) 2.692 112.5 (4) 110.9 (3) 109.7 (3) 106.7 (3) 106.2 (4) 110.7 (4) 109.5 133.9 (3) 143.0 (4) 138.4 (4) 150.3 (4) 141.4 2.40 ( I ) 2.95 ( I ) 2.23 (2) 2.96 (2) 2.657 (4) 2.23 (2)
~
1.627 (4) 1.653 (4) 1.666 (4) 1.648 (4) 1.649 2.733 (4) 2.693 (4) 2.699 (4) 2.650 (4) 2.621 (4) 2.750 ( 5 ) 2.691 112.9 (2) 109.7 (2) IIl.0(2) 105.9 (2) 105.1 (2) 112.1 (2) 109.5 137.9 (2) 145.0 (2) 140.5 (2) 146.9 (3) 142.6 2.39 ( I ) 2.86 ( I ) 2.24 ( I ) 2.93 ( I ) 2.718 (3) 2.18 ( I )
‘(B) bare zeolite at room temperature, (L) low benzene coverage at 4 K. (H) high benzene coverage at 4 K.
R , = 11.4%. RE = 6.9%. The observed and calculated profiles are shown in Figure 2. No residual deuterium atoms could be found attached to the oxygen atoms of the zeolite framework. The likely positions for these have been previously determined by powder neutron diffraction studies on partially decationated sodium Y zeolite?’ The estimated standard deviation (esd) on the lattice parameter a = 25.8536 (3) 8, does not account for uncertainty in the neutron wavelength. This would typically give an uncertainty of f0.03 A. The framework of Y zeolite consists of cuboctahedral sodalite cages composed from 24 Si/AI and 36 0 atoms, which are linked together tetrahedrally by six-membered rings of O(I) atoms to form hexagonal prisms and large cavities or “supercages”, ca. 12.5 A in diameter, Figure 1. There are eight sodalite cages (centered (21) The R factors arc defined in ref 19. R, is the weighted profile R factor and is directly related to the value of the feast-squares minimization function. It is a measure of the agreement between the observed and tabulated profiles. R, refen to the agreement between the observed and calculated integrated intensities of the Hragg peaks and i s therefore less sensitive than R,, to inaccuracies in the assumed Gaussian peak-shape function or the variation of peak width with scattering angle. R, is the ““weighted profile R factor and REthe value of R,, to be expected were differences between the observed and calculated profiles purely statistical in origin. (22) Jirik, 2.: Vralislav, S . : HosBPek, V. Phys. Chem. Solids 1980, 41, 1089.
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1313
Localization of Benzene in Sodium-Y Zeolite Counts
1000
800
600
400
200
0
, 20
40
60
80
100
I
,
120
140
28 in degrees Figure 2. Observed, calculated, and difference profiles for bare sodium-Y zeolite at room temperature and 1.909 A. The profile below 16O in 28 was excluded from the refinement owing to the very marked asymmetry of these peaks.
at 8a
and eight supercages (centered at 8b ( 3 / 8 , 3/8,3/8)) per unit cell. Each supercage is joined to four others via circular windows that are formed by 12 Si/Al and 12 0 atoms and have a free diameter of about 7.5 A. For the sodium ions the three positions found correspond to (i) the SI1 site, of which there are four in each supercage, at 32e (x,x,x) with x = 0.2345 (2); (ii) the SI' site, of which there are four in each sodalite cage, also at 32e with x = 0.0507 (4); and (iii) the SI site at the center of the hexagonal prism at 16c (O,O,O). The fractional occupancy of these sites is 100 (2)%, 58 (2)%, and 44 (2)%, respectively. The various sodium ion occupancies, with a sum of 57.9 (1 .l), are in very good agreement with the chemical formula and are similar to those obtained in earlier s t ~ d i e s . * , ~ ~ - ~ ~ They are consistent with the notion that, owing to electrostatic repulsion, two adjacent SI and SI' sites cannot be simultaneously of 2.18 (1) A. occupied because of their small Bond lengths and angles are given in Table 11. The values obtained are largely in agreement with those of Eulenberger et a1.,8 although the spread in our values is less and the Na(2) ion is shifted by about 0.44 A toward the hexagonal prism. This results in a rather short Na(2)-O(3) distance of 2.24 (1) 8, for the three-coordinate Na(2) ion, although Na-O distances as short as 2.27 A have been reported for fivecoordinate N a in Na2Si03.25 Zeolite with Adsorbed Benzene at 4 K . Initial refinements involving only the framework and the sodium ions were attempted on the data collected at 4 K for the zeolite samples with low and high benzene coverage. These were unsatisfactory, yielding R, = 17.7% and R,, = 19.5% for low benzene coverage and RI = 29.5% and R,, = 31.8% for high benzene coverage. Fourierdifference sections were then computed perpendicular t o the threefold axis of the cubic [ l , l , l ] direction, which also contains three intersecting mirror planes. Two features arise which strongly ('/8,'/8,'/8))
(23) BosiEek, V.; Beran, S.; Jirik, Z. J . Phys. Chem. 1981, 85, 3856. (24) Cheetham, A. K.; Eddy, M. M.; Klinowski, J.; Thomas, J. M. J . Chem. Soc., Chem. Commun. 1983, 23. ( 2 5 ) Grund, A,; Pizy, M. Acta Crystallogr. 1952, 5 , 837.
Y
. 2b
Y
2A
Figure 3. Fourier difference maps perpendicular to [ l , l , l ] ; x is parallel to [-1,1,0] and y is parallel to [-1,0,1]. The sections are (a) about (0.295,0.295,0.295) showing the first benzene site for the sample with lower benzene coverage, and (b) about ( 1 / 2 , 1 / 2 , ' / 2 ) showing the second benzene site for the sample with higher benzene coverage.
1314 The Journal of Physical Chemistry, Vol. 90, No. 7, 1986
Fitch et al.
TABLE 111: Final Parameters and R Factors for the Zeoliteti at 4 K with L o w Benzene Coverage (L) and High Benzene Coverage (H) in Space Grow Fd3m. Origin at 3m
atom
position
Si/AI
192i
O(1)
96h
O(2)
96g
a31
96g
o(4)
96g
Na(l)
32e
Na(2)
32e
Na(3)
16c
C(1)
192i
D(1)
192i
C(2)
9%
D(2)
9%
L H L H L H L H L H L H L H L H L H L H L H L H
X
Y
Z
-0.0541 (1) -0.0543 (2) 0 0 -0.0025 ( I ) -0.0027 (1) 0.1776 (1) 0.1780 (1) 0.1776 (1) 0.1757 (2) 0.2342 (2) 0.2367 (3) 0.0520 (4) 0.0520 (7) 0 0 0.2571 (2) 0.2583 (1) 0.2262 (2) 0.2274 (1) 0.47696 0.47696 0.4591 1 0.4591 1
0.0365 (1) 0.0355 (2) -0.1061 (1) -0.1072 (1) -0.0025 (1) -0.0027 (1) 0.1776 (1) 0.1780 (1) 0.1776 ( I ) 0.1757 (2) 0.2342 (2) 0.2367 (3) 0.0520 (4) 0.0520 (7) 0 0 0.3369 (2) 0.3381 (1) 0.3678 (2) 0.3690 (1) 0.47696 0.47696 0.4591 1 0.4591 1
0.1241 (1) 0.1237 (2) 0.1061 (1) 0.1072 (1) 0.1431 (1) 0.1427 (2) -0.0336 (1) -0.0331 (2) 0.3177 (2) 0.3186 (2) 0.2342 (2) 0.2367 (3) 0.0520 (4) 0.0520 (7) 0 0 0.2970 (2) 0.2982 (1) 0.2970 (2) 0.2982 (1) 0.54608 0.54608 0.58178 0.58178
B, A2
N
0.7 (1) 1.1 (1) 1.4 (1) 1.1 (1) 1.5 (1) 1.7 (1) 1.5 (1) 1.4 (1) 2.1 (1) 1.9 (1) 2.2 (2) 1.7 (2) 2.2 (2) 1.7 (2) 2.2 (2) 1.7 (2)
192 192 96 96 96 96 96 96 96 96 32 31.8 (1.2) 16.5 (6) 14.6 (8) 7.0 (4) 5.8 (5) 39.9 (7) 119.2 (1.5) 39.9 (7) 119.2 (1.5) 6.3 (7) 29.4 (7) 6.3 (7) 29.4 (7)
ldeal benzene geometries, esd’s in parentheses. Cell parameter: L, a = 24.8484 (4) A, H, a = 24.8073 (7) A; error due to wavelength = f0.03 10.3%, R, = 11.5%, RE = 7.4%; H, RI = 8.5%, R,, = 13.3%, R, = 13.9%, RE = 7.9%.
A. R factors:*, L, R, = 7.5%, R,, =
suggest the presence of localized benzene molecules. The first of these, which may be seen for the low-coverage sample in Figure 3a (a comparable feature also occurring for the high-coverage sample), occurs centered about (x,x,x), x i= 0.295. The insertion of a benzene molecule into the refinement in this position causes a considerable improvement to the fit, with RI = 7.9% and R,, = 10.7% for low coverage, and RI = 16.2% and R,, = 19.9% for high coverage. Further Fourier maps were then computed for the high-coverage data, and these reveal the second site for benzene centered about (‘/2,1/2,1/2), also in the plane perpendicular to the cube diagonal, Figure 3b. Crystallographically the two types of benzene molecule can each be described by a single carbon atom and a single deuterium atom, the rest of the molecule being generated by the space group symmetry. The molecular symmetry resulting in each case is lower than that required for a regular, hexagonal, planar molecule, so that distortions from the ideal molecular geometry are possible. For the first benzene molecule, whose center lies on a site of 3m symmetry, alternating C-C bond lengths are obtained around the six-membered ring unless the C atoms lie midway between the intersecting mirror planes. The same considerations apply to the D atom and there is no requirement that the C and D atoms should be coplanar. A regular hexagonal geometry may be achieved for the carbon or deuterium atoms by imposing the constraint 22 = x y , with z(C) = z(D) for a planar molecule. The second benzene molecule is rotated by 30’ with respect to the first so that the C and D atoms lie on the mirror planes, Le., at 96g (x,x,z). The molecular center is on a site of 3m symmetry so that all C-C bond distances are equivalent. However, puckering of the ring is possible by displacement of the C or D atoms alternately above and below the plane of the ideal molecule. A planar molecule can be obtained by enforcing the constraint 2x z = 1.5. In addition to constraining the two types of benzene molecule to their ideal geometry, Le., perfectly hexagonal with all the C and D atoms coplanar, the C-C and C-D bond lengths were fKed at the values of 1.40 and 1.085 A, respectively. These were considered to be reasonable values, considering the distances found in gaseous benzene (1.397 (3) and 1.084 ( 6 ) A)26and in C6H,Cr(C0)3 (mean values, uncorrected for thermal motion, of 1.412 and 1.087 A).27 Thus, for the benzene(1) molecule there is only
+
+
(26) Stoicheff, B. P. Can. J . Phys. 1964, 32, 339. (27) Rees, B.; Coppens, P. Acta Crystallogr. Sect. E 1973, E29, 2515.
one extra positional variable introduced-the distance of the molecular center along the cube diagonal. For benzene(2), there are no extra positional variables, since the molecular center is fixed at (1/291/291/2). Using the strict constraints of the Rietveld program the temperature factors of the carbon and deuterium atoms were constrained to describe rigid-body translational and librational motion of the benzene molecules and thereby simulate the method of TLS described by Schomaker and Trueblood,28which is equivalent to the method of T L of C r ~ i c k s h a n k since , ~ ~ the S matrix is zero for the centrosymmetric benzene molecules of ideal geometry. This is described in Appendix 1. For each benzene molecule there was therefore an occupation factor, two variables corresponding to the two parameters of the T matrix, and a further variable corresponding to a uniaxial L matrix representing libration of the benzene molecule about its “sixfold” axis, Le., about the cube diagonal. Refinements incorporating the full L matrix gave nonphysical results. During the analysis the occupancy of Na( 1) for the low-coverage sample refined to a value of 34.7 (9), which is slightly more than fully occupied. The value was therefore fixed at fully occupied. For the high-coverage sample the region 18.30-20.30° was excluded from the refinement owing to the presence of what appeared to be an unidentified spurious peak at about 19.1O , which, though close to the expected position of the (331) reflection (19.2O), could not be fitted whatever models were attempted. This peak was also observed in the diffraction pattern at room temperature. Fourier-difference maps computed whilst including this peak revealed no significant features. The final refinements converged to give RI = 7.5% and R,, = 10.3% for low coverage and RI = 8.5% and R,, = 13.3% for high coverage. Further refinements were performed to check the validity of the constraints imposed on the geometry of the benzene molecules. For low coverage, relaxation of the geometry of the benzene( 1) molecule (an extra five variables) led to R, = 7.4% and R,? = 10.2%, and relaxation of both benzene molecules (an extra nine variables) produced R1 = 7.4% and R,, = 10.1%. For high coverage, the relaxation of both benzene molecules produced RI = 7.8% and R,, = 12.9%. It would appear, therefore, that the refinements are not very sensitive to deviation of the benzene (28) Schomaker, V.; Trueblood, K. N. Acta Crystallogr., Sect. B 1968, B24, 6 3 . (29) Cruickshank, D. W. J. Acta Crystallogr. 1956, 9, 7 5 4 .
Localization of Benzene in Sodium-Y Zeolite CO
The Journal of Physical Chemistry, Val. 90, No. 7, 1986 1315
s
800
600
400
200
i I II I I I I Ill 111 I1 II IIIIIIIIIIII 1111111111111111111llllll1lllll III11111111IIIIIIIIIIIIIIII1111IIIIIII1III11111111 11111111111IIIIIII1IIIIIIIIIII111111111111 I I1 I
0
20
40
60
BO
100
120
140 28 ~n degrees
Figure 4. Observed, calculated, and difference profiles for the high-coverage sample at 4 K and 1.909 A. The profile below 1 6 O in 28 was excluded from the refinement. In addition the region 18.3-20.3' was omitted owing to the presence of the spurious peak at 19.1°, which may also be seen in the diffraction pattern at rmm temperature. molecules from their ideal geometry, so that such deviations that may actually occur are probably small and are unlikely to be reliably determined by the present study. The final structural parameters for the constrained refinements are given in Tables 111 and IV, and the observed and calculated profiles for the high-coverage sample are illustrated in Figure 4. The bond distances and angles for the framework atoms are in Table I1 and the average environment of the ideal, constrained benzene molecules in Table V.
Discussion Introduction of benzene into the cavities of the zeolite bas generally only a small effect on the framework structure. The average (Si/Al)-O bond lengths and angles are hardly modified, and the sodium ion site occupancies are consistent for all three refinements. The first benzene molecule is situated in the supercage and is bonded via its r electron density to N a ( l ) a t a n average distance of 2.70 ( I ) A at low coverage and 2.64 ( I ) A a t high coverage. The introduction of the benzene appears to cause, a t least a t high coverage, the displacement of N a ( l ) along the cube diagonal by ca. 0.09 A toward the center of the supercage. This suggests that, in binding to the benzene(1) molecule, the interaction between Na(l) and the framework oxygen atoms O(2) and O(4) is reduced. The closest approach between benzene(1) and the zeolite framework is between the deuterium atom and O(1) at a distance of 3.22 ( I ) A. There are four sodium ions and four benzene(1) sites per supercage, disposed tetrahedrally about the center, Figure 5. The smallest distance between adjacent benzene(1) molecules is between deuterium atoms a t 2.91 ( I ) A. The second benzene molecule is situated in the window that joins together neighboring supercages, which is formed from six O(1) atoms and six O(4) atoms linked together by %/AI, Figure 6. The deuterium atoms are generally directed toward the O(4) atoms and lie between the O( I ) atoms, which are coplanar with the ideal benzene(2) position. The O(4) atoms lie in two planes
Figure 5. The four benzene(l) positions (van der Waals radii) and the SI1 sodium ions (ionic radii) within the supercage. One benzene molecule is shaded to suggest occupancy of this site.
of three atoms each displaced above and below the plane of the benzene(2) molecule by a distance of 0.47 A, a t high coverage. This distance increases with decreasing benzene content, being 0.54 A a t low coverage and 0.56 A in the bare zeolite. The distance from the center of the window to the O(1) atoms decreases from 5.06 A in the bare and low-coverage samples to 5.01 8, a t high coverage. This is accompanied by a steady decrease
1316 The Journal of Physical Chemistry, Vol. 90, No. 7 , 1986 TABLE 1%’: Refined Anisotropic Temperature Factors (in Samples“
Fitch et al.
A2)for the Benzene Molecules at 4 K for the Low-Coverage and the High-Coverage (a) Low Coverage
B22
Bll
C(I) D(1) TLd 1)
LD(1) C(2) D(:) TB(2)
L,( 2)
3.4 (3)
3.4 (3)
3.6 (2) 3.3 ( 3 ) 0.3 (1) 4.3 (1.6) 5.9 (1.7) 3.5 (1.6) 2.3 ( 3 )
3.6 (2) 3.3 (3) 0.3 ( I ) 4.3 (1.6) 5.9 (1.7) 3.5 (1.6) 2.3 ( 3 )
0.039 (5) T(l) = 0 IO
T(l) =
0.077 (4) 0
lo
-0,010 (6) T(2) =
0 I O
x ’ = [-1,-1,2],y’=
0 0.039 (5) 0 0 0.03 (2) 0
0
0.077 (4) 0 0 -0.010 (6) 0
B33
3.6 (2) 4.4 (3) 3.3 (3) 1.1 (4) 3.5 (1.6) 3.5 (1.6) 3.5 (1.6) 0 0 0
BIZ
0.3 (2) 0.4 (2) 0.2 (2) 0.3 (1) 0.7 (1.2) -0.9 (1.4) 1.4 (1.2) -2.3 (3)
0
L(1) = 0
0.046 (6) 0 0 0.08 (4)
0 0 0.069 (3) 0 0 0.019 (7)
lo 0 L(2) = 0 10
L(I) =
0 0
l o L(2) =
0 0
Io
B13
B23
0.0 (2) -0.4 (3) 0.2 (2) -0.6 ( 2 ) 1.4 (1.2) 1.4 (1.2) 1.4 (1.2) 0
0.0 (2) -0.4 (3) 0.2 (2) -0.6 (2) 1.4 (1.2) 1.4 (1.2) 1.4 (1.2) 0
0 0 0 0 0
0
0 0 0 0 0 O
: I : I
1 1 (4)
31 (4)
:I
8 (3) 0
O6 2 ( 1 ) I
[1.-1,0], z’= [ l , I , l ] .
in the (Si/AI)-O( 1)-(Si/Al) angle and an increase in the (Si/ Al)-O(4)-(Si/Al) angle, indicating that the introduction of the benzene causes some contraction and flattening of the window. The 12 oxygen atoms provide a close-fitting environment for the benzene molecule in the window, where its position is probably stabilized by van der Waals forces. The distance between the benzene(2) molecules and benzene( 1) molecules is less than the distance between the benzene( 1) molecules themselves. Each D(2) atom is separated from two adjacent D ( l ) atoms on a single benzene( 1) molecule by a distance of 2.56 (1) A, and from a D( 1) atom on each of two neighboring benzene( 1) molecules on the opposite side of the window by 2.68 (1) A. Every supercage is associated with four windows, which are equally shared with four neighboring supercages. Thus, there is an average of two benzene(2) sites per supercage, and, including benzene( l), a maximum capacity for the zeolite of six benzene molecules per supercage. At low coverage, the refined benzene occupancies are 0.83 (2) and 0.13 (2) molecules per supercage for benzene( 1) and benzene(2), respectively, corresponding to fractional site occupancies of 20.8 (4)% and 6.5 (8)%. The total occupancy of 0.96 (3) molecules per supercage is in good agreement with the experimentally determined value of 1.1 10%. At high coverage, the occupancies are 2.48 (3) and 0.61 (2) molecules per supercage, (fractional occupancies of 62.1 (8)% and 30.6 (8)%), with a total occupancy of 3.10 (4). This compares rather less favorably with the experimental value of 2.6 & IO%, though it is probably on the limits of an acceptable value.
The main contribution to the temperature factors of benzene( 1) is from the translational displacement of the molecule, which appears to be nearly isotropic. Librational motion is not pronounced. The temperature factors are somewhat higher at high coverage, which could perhaps reflect more static disorder in the position of the benzene( 1) molecule, depending on the number of occupied neighboring benzene( 1) and benzene(2) sites that a given molecule may have within a supercage. For the benzene(2) molecules the translational displacements appear more anisotropic, with the largest amplitude along the cubic [ l , l , 11 axis perpendicular to the plane of the molecule. This clearly must reflect the effects of the confines of the window site. For T(2) at high coverage the diagonal elements become slightly negative in the directions perpendicular to the cube diagonal, but remain within two esd’s of zero. Translational motion is reduced at high coverage as compared to low coverage for benzene(2), probably due to the greater stabilization of this position as the number of occupied neighboring benzene( 1) sites increases. This is also reflected in the relative increase in the occupancy of the benzene(2) site at high coverage. Librational motion is considerably more pronounced for benzene(2). It can be seen from Table IV that at high coverage the average separation between the center of the benzene( 1) sites decreases by about 0.09 A, as compared to low coverage, owing to a translation of the molecules by ca. 0.05 A along the cube diagonal toward the center of the supercage. This suggests that there is a mutual attraction between adjacent occupied benzene sites.
Localization of Benzene in Sodium-Y Zeolite
The Journal of Physical Chemisfry, Val. 90, No. 7, 1986
1317
TABLE V Average Environment of the Benzene Molecules of Ideal, Constrained Geometry at 4 K a t L o w Coverage (L) and at High Coverage (Hp
L
H
L
.~,
H
fa) Ions le) BEniene(li-Renienel1i . , Benrenefl)-FrameworklNa ,, I C(1)-0(4) 3.59 (I) 3.70 (1) C(l)-C(l)’ 4.08 ( I ) 3:9$ ( I ) C(1)-0(2) 3.69 (1) 3.74 (1) C(l)-D(l)’ 3.66 ( I ) 3.58 (I) ~~
~
~~~~~~~~
~~I
C(I)-O(I) 3.96(1) C(I)-Na(l) 3.04 (1) D(I)-0(1) 3.22 (1) D(I)-0(4) 3.43 (1) D(I)-~(z) 3.70 (1) M(I)-Na(l) 2.70 (I) M(1)-0(2) X3 4.13 (1) M(I)-0(4) X3 4.23 ( I ) M(1)-0(1) X6 5.10 ( I )
3.96 (1) D(I)-D(I)’ 3.00 ( I ) 2.91 (I) 2.99 (1) M(I)-C(l)’x6 4.94 (I) 4.85 (I) 3.22 ( I ) M(I)-D(l)’X6 4.76 (I) 4.67 (1) 3.53 (1) M(l)-Mfl)‘ . , . . X3 5.49 f.I .\ 5.39 I I ) 3.74 i i j (d) Benzene(2)-Benzene(l) 2.64 (I) 3.47 (I) 3.46 ( I ) 4.17 ( I ) C(2)-C(I) XZ 2.76 (1) 2.75 ( I ) 4.33 (1) C(2)-D(I) X2 3.20 ( I ) 3.21 S . l O ( 1 ) D(2)-C(1) X2 D(2)-D(I) X2 2.55 ( I ) 2.56 (b) Benzcnc(Z)-Framework C(2)-C(I)’ 3.77 ( I ) 3.75 C(2)-0(4) 3.80 ( I ) 3.81 (I) C(2)-D(1)’ 2.83 ( I ) 2.81 C(2)-0(1) X2 3.90 ( I ) 3.86 (I) D(2)-C(1)’ 3.76 ( I ) 3.75 C(2)-0(4) X 2 4.66 ( I ) 4.66 ( I ) D(2)-D(I)’ 2.69 ( I ) 2.68 D(2)-0(4) 2.73 ( I ) 2.74 (I) C(Z)-C(I)” 3.77 ( I ) 3.75 D(2)-0(1) X2 3.16 ( I ) 3.12 ( I ) C(Z)-D(IY 2.83 ( I ) 2.81 D(2)-0(4) X2 4.51 ( I ) 4.51 ( I ) D(2)-C(lY 3.76 ( I ) 3.75 M(Z)-O(l) X6 5.06 (I) 5.01 ( I ) D(2)-D(lY’ 2.69 (I) 2.68 M(2)-0(4) X6 5.19 ( I ) 5.20 ( I ) M(Z)-C(I) XI2 4.22 ( I ) 4.20 M(Z)-D(I) XI2 3.54 (I)3.51 M W M ( I ) X6 5.31 (I)5.28
‘Distances are in A. Primes arc used to denote atoms on different benzene molecules. M represents the middle of a benzene molecule.
Small-angle neutron scattering studies at room temperature’ reveal that a t a coverage of ca. I .O molecule per supercage most of the benzene molecules are rather evenly distributed throughout the channels of the zeolite, whereas a t higher coverage, 2.5 molecules per supercage, aggregation of the molecules in a limited number of the cavities occurs to form clusters with a typical diameter of 10-12 A, or more. It is attractive to consider that the formation of clusters and the observed decrease in the separation between adjacent benzene(1) molecules should be closely related. In addition, as was noted above, the Na(1) ion moves by a distance of 0.09 A, owing to its association with benzene(1). However, since the benzene(1) site is considerably less than fully occupied, not all the Na(1) ions will be bound to a benzene molecule and therefore displaced. The average displacement observed of 0.09 8,is calculated from an N a ( l ) position that is the average position of both the displaced and undisplaced ions in the unit cell. The actual displacement that occurs, therefore, is likely to be somewhat more than 0.09 A, and the real average distance between Na( 1) and benzene(1) rather less than 2.64 ( I ) 8,. At low coverage the averagedistance between N a ( l ) and benzene(1) is longer at 2.70 ( I ) A, and the Na(1) ion is largely unshifted with respect to its position in the bare zeolite. It appears, therefore, that a t high coverage there may be some form of cooperative interaction between adjacent benzene molecules assisting the shift in the Na( l ) ions towards the center of the supercage. This may help in the formation of the benzene clusters. Zeolite with Adsorbed Benzene ai Room Temperature. Initial refinements without benzene were again poor, with R,= 20.0% and R,, = 21.7% at low coverage, and R,= 26.8% and R,, = 26.3%a t high coverage. Insertion of benzene molecules in the same positions as found at 4 K and performing similar refinements produced a marked improvement in the fits. The refinements converged to give R , = 10.2%and R,, = 12.2%at low coverage and R, = 8.9% and R,, = 11.6% a t high coverage. However, although all the various parameters appeared to refine satisfactorily with acceptable esd‘s, the temperature factors on the benzene molecules became very large, particularly for the T matrix with mean-square displacements up to 0.41 (4) A’ for T(1) and 0.21 (7) 8,’ for T(2). Mean-square librational amplitudes varied from 53 (31) to 194 (5)”. The occupancies were reasonably sensible, nevertheless, being 1.26 (3) and 0.20 (3) molecules per supercage at low coverage and 2.79 (3) and 0.45 (2) a t high coverage. It is clear that a t room temperature the benzene molecules are
Figure 6. Molecular packing diagram (van der Waals radii) showing the benzene(2) molccule in the window of the zeolite.
performing molecular motions of very large amplitude, or are subject to substantial positional disorder. The refined atomic positions suggest that the benzene molecules are still located in the supercage close to their positions a t 4 K. Thus the Na(l)-M(I) distance refines to 2.83 ( I ) 8, a t low coverage and 2.73 ( I ) 8, at high coverage, the decrease again being due to a migration of the sodium ion by about 0.10 A toward the center of the supercage, althou h the position of the center of benzene( I ) changes by only 0.01 between low and high coverage. At 4 K a displacement of about 0.05 8, is observed in the benzene(l) position. However, since the benzene molecules are clearly very mobile or highly disordered. the temperature-factor description used is probably only a relatively crude model of the resulting distribution. It is unlikely, therefore, that a comparison between the benzene parameters obtained a t room temperature and at 4 K would be very meaningful, even though the parameters themselves appear to be reasonably well-defined. All that can really be said is that at room temperature the benzene molecules occupy sites close to their positions a t 4 K, where they are likely to be performing vigorous molecular motion. The refined cell parameters are a = 24.8305 (4) 8, for low coverage and a = 24.8100 (5) 8,at high coverage, showing that, a t room temperature, there is a steady decrease in the cell parameter from that of the bare zeolite with increasing benzene content. A similar decrease is observed between the low-coverage and high-coverage samples at 4 K.
$:
Conclusions Powder neutron diffraction has been very successful in elucidating the nature of benzene adsorbed into sodium-Y zeolite. Two samples have been studied with benzene coverages of I . I and 2.6 molecules per supercage. At room temperature the benzene molecules are largely delocalized within the supercages of the zeolite, whereas at 4 K the benzene molecules are found to be localized on two distinct sites, each centered on, and with its plane perpendicular to, the threefold axes of the unit cell. The first benzene molecule is found in the supercage where it is bonded via its m electron density to the SI1 sodium ion. There are four of these sites available per supercage. The second benzene molecule is held by van der Waals forces in the circular window that links together neighboring supercages. There are, on average, two of these sites available per supercage. At high benzene coverage at 4 K the benzene( I ) molecules and the SI1 sodium ions are seen to have moved toward the center of
1318 The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 r
r
Fitch et al.
r
X=
W
-112‘12 1/6’12 1/3’12
112’12 1/6‘12 1/3‘12
0 -2 16 1’
’
1/3‘12
W
Figure 7. Cross-section through the interconnecting cavities of the Y zeolite, showing the Si/AI framework, the SI1 sodium ions (shaded circles, ionic radii), and the benzene molecules (van der Waals radii). Two groups of four benzene( 1) molecules (fourth molecule obscured) each completely filling a supercage are linked through the benzene(2) molecule in the window site to form a larger benzene cluster This may be propagated further in three dimensions.
the supercage, as compared to their positions at low benzene coverage or in the bare zeolite. This suggests the formation of clusters of benzene molecules within the cavities of the zeolite, as has been observed by small-angle neutron scattering studies at room t e m p e r a t ~ r e .The ~ structure of such benzene clusters can be easily understood from the two benzene positions found in the present study at 4 K. Groups of four benzene(1) molecules completely filling a supercage are linked together via occupied window sites to form larger clusters. This is illustrated in Figure 7. Small deviations of the benzene molecules from their ideal, planar, hexagonal geometry could not be determined from the current data. Such information is likely to be more readily asscessible from data collected on neutron powder diffractometers with higher resolution to greater values of sin B / X , and may give important clues toward the full understanding of the chemical reactivity between a hydrocarbon molecule and a given zeolite.
Appendix 1 To constrain the B matrices of the carbon and deuterium atoms to simulate rigid-body translational and librational motion, we must express both the translational and librational components of the molecular motion, (Le., the T and L matrices referred to an orthogonal set of molecular axes), in terms of the B matrices related to the unit-cell axes. Firstly we must define an orthogonal set of molecular axes about the center of mass of the benzene molecule. For benzene( 1) a suitable choice is with x’along the direction from the center of the molecule to one of the carbon and deuterium atoms, i.e., the [-1,l ,O] direction, z’along the direction perpendicular to the plane of the molecule, Le., the [ l , l , I ] direction; and y ’ orthogonal to x’ and z’, Le., the [1,1,-21 direction, which bisects a C-C bond in the plane of the molecule. The benzene( 1) molecule has 3m symmetry so that the T matrix and L matrix have the form (from Willis and Pryor,’O p 184):
1;
A
0
0
1;
P
O
This corresponds to displacements of the deuterium atom in the y ’direction and the z’direction, as expected from rotations about the z’axis and the x’and y’axes, respectively. If we set P = 0, so that only rotations about z’are considered, and transform LD‘ to the unit-cell coordinate system by LD( 1) = X-]LD’X,we obtain
1 E’
LD(1) =
The matrix relating the unit-cell coordinate system and the molecular coordinate system is (30) Willis, B. T. M.; Pryor, A. W. “Thermal Vibrations in Crystallography”; Cambridge University Press: Cambridge, U.K., 1975.
Q’
-2Q‘
-29‘
-2Q’ -2Q’l 4Q‘
where Q’ = Q(xD)’/6. Likewise for the carbon atom we obtain a similar matrix whose elements are related to those of LD( 1) by the relation Lc( 1) = (xC/xD)’LD(1). In the Rietveld refinement we may use two dummy atoms whose occupancy is zero to refine, using the normal codeword constraints, both the matrices TB( 1) and LD( l ) , and set the elements of the B matrix of the carbon and the deuterium atoms to be the sum of these, using strict linear constraints, Le., Bc( 1) = TB(1) L,-( 1) and BD(1) = TB(1) LD(1). For benzene(2), which has 3m symmetry, the T(2) and L(2) matrices have the same form as for benzene(1). The molecular coordinate system chosen has x’along [-1,-1,2], z’ along [l,l,l], and y’along [ 1,-1,0]. Following an identical procedure as above we may show that
+
+
T,(2) =
where A’ = (’/’A
+
0
U1)=
Q’
LD(2) =
I:: B’
I/$)
:1
B‘ A’
B’
and B’ = (-1/3A
-Q’
-Q/ 0 Q’
1 :I
:’ A
+ ‘ / ’ B ) , and 0
where Q’ = Q ( x ~ ) ~Thus, / ~ . by inserting two more dummy atoms and constraining, the B matrices of C(2) and D(2) may be forced to simulate rigid-bcdy translational and librational motion for the benzene(2) molecule. Registry No. Benzene, 71-43-2.
