J. Phys. Chem. 1989, 93, 7689-7694
7689
Structures of Dehydrated Potassium Zeolite L at 298 and 78 K and at 78 K Containing Sorbed Perdeuteriobenzene J. M. Newsam Exxon Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801 (Received: December 27, 1988; In Final Form: May 15, 1989)
The complete structures of dehydrated potassium zeolite L, K9A19Si27072, at 298 and 78 K and of the same zeolite at 78 K containing, on average, 1 molecule of perdeuteriobenzene/unit cell have been determined by powder neutron diffraction. Comparison with earlier literature data indicates that little change in structure accompanies dehydration, benzene sorption, or temperature change 298 K 1 T 1 78 K. The use of powder neutron diffraction has enabled a direct measure of the aluminum partitioning between the two inequivalent T-sites in the structure that, consistent with the complementary measure from bond length arguments, demonstrates a distinct aluminum preference for the 12-ring, Si1 (T4)sites. The nonframework K+ configuration shows only subtle changes over the range of conditions studied. Perdeuteriobenzene is observed at 78 K in "capping" positions above the channel wall (type D) K+ cations. Simple atom-atom potential modeling of the benzene site based on the structural results yields only a small activation barrier to benzene molecule reorientation.
Introduction Zeolite L (framework topology code LTL' was first described in 19682*3and is one of a relatively small number of zeolites in which access to the internal pore volume is controlled by apertures that contain 12 T atoms (T = tetrahedral species, Si, Al, Ga, etc.) and 12 shared apical oxygen atoms.' The chemical characteristics of materials adopting these various framework topologies vary quite widely, and their relative utilities reflect an interplay between accessible void volume, structural chemistry, and materials-related constraints. Zeolite L has been studied as a support for platinum metal in reforming applications.M The structure of zeolite L was determined in 19697 based on powder X-ray diffraction (PXD) measurements on a hydrated sodium-potassium form of the zeolite. The structure is hexagonal, P6/mmm (No. 191) Figure I ) , with approximate unit cell dimensions of a = 18.5 and c = 7.5 This space group allows for no distinction between Si and A1 at either of the two crystallographically inequivalent T sites. The average aluminum content at each of the two sites is determined by the overall framework Si:Al ratio, and by the partitioning of the aluminum between them. In addition to the earlier PXD studies on hydrated (K, Na)-L,7 structural results for hydrated (K,Ba)-G(L),8 dehydrated gallosilicate zeolite L,9,Io and the natural mineral perlialite" have been reported. We present here the complete structure results of powder neutron diffraction studies of a dehydrated potassium zeolite L at 298 and 78 K and of the same zeolite at 78 K containing, on average, 1 sorbed molecule of perdeuteriobenzene/unit cell. Preliminary communications
a
(1) Meier, W. M.; Olson, D. H. Atlas ofZeolite Structure Types, 2nd ed. Butterworths: Surrey, UK, 1987. (2) Breck, D. W.; Acara, N. A. U S . Patent 711,565, 1968. (3) Breck, D. W.; Flanigen, E. M. In Molecular Sieves, Barrer, R. M., Ed.; Society of Chemical Industry: London, 1968; pp 47-61. (4) Bernard, J. R. In Proceedings of the Fijth International Conference on Zeolites; Rees, L. V. C., Ed.; Heyden: London, 1980; pp 686-695. (5) Hughes, T. R.; Buss, W. C.; Tamm, P. W.; Jacobson, R. L. In New Deoelopments in Zeolite Science Technology, Murakami, Y . ; Iijima, A,; Ward, J. W., Eds.; Kodansha: Tokyo, 1986; pp 725-732. (6) Tauster, S.J.; Steger, J. J. In Microstructure and Properties of Catalysts; Treacy, M. M. J., White, J. M., Thomas, J. M., Eds.; Materials Research Society Symposium Proceedings; Materials Research Society: Pittsburgh, PA, 1988; Vol. 111, pp 419-423. (7) Barrer, R. M.; Villiger, H. 2. Kristallogr. Kristallgeom., Kristallphys., Kristallchem. 1969, 128, 352-370. (8) Baerlocher, C.; Barrer, R. M. Z . Kristallogr. Kristallgeom., Kristallphys., Kristallchem. 1972, 136, 245-254. (9) Newsam, J. M. Mater. Res. Bull. 1986, 21, 661-672. (10) Wright, P. A.; Thomas, J. M.; Cheetham, A. K.; Nowak, A. K. Nature 1985, 318, 611-614. (1 1) Rinaldi, R. Manuscript in preparation.
0022-3654/89/2093-7689$01.50/0
outlining certain of these results have already appeared.I2J3
Experimental Section Sample Preparation. A sample of potassium zeolite L was synthesized by literature p r o c e d ~ r e s . ~Analysis .~ by inductively coupled plasma emission spectroscopy (ICPES) yielded the forwith a Si:AI ratio of 2.87. mula N~,027Kg,272Si26,701A19,299072~nH20, Thermogravimetric analysis (TGA) on a Du Pont Model 5 IO/ 1090 TGA gave a value of n = 17 following extended equilibration in a 88% relative humidity atmosphere followed by an overnight purge by dry He gas. Sorption measurements on an automated Cahn system gave a n-hexane sorption capacity of 8.4 wt %. Least-squares full-profile fitting of the digitized 29Si NMR spectrum gave an adequate fit, assuming it to be the sum of only four Gaussian terms of independent positions, widths, and intensities.12J4 The framework Si:Al ratio, R , computed as R = 4CZ,,/CnZ,, with the optimized normalized intensities, I,, (n = 0-3), is 2.89,I2J4 in good agreement with the ICPES results. Powder X-ray diffraction scans on a Siemens Model D500 diffractometer (Cu K a radiation) showed the sample to be phase pure, with least-squares-optimized lattice constants of a = 18.392 (3) A and c = 7.534 (4) A (based on 33 reflection positions to 42' in 28). Interactive Molecular Graphics. In addition to viewing various framework models and considering potassium cation sites, interactive molecular graphics r ~ u t i n e s ' ~were J ~ used in modeling possible positions and orientations for the sorbed benzene molecule(s) (the hardware and software configuration have been described earlierI7). Initially, a first-order estimate of the local stability of a benzene molecule in the vicinity of the 12-ring window was examined by atom-atom potential calculations. A model of the immediate environs of the channel, 3 unit cells deep, was constructed based on the framework atom coordinates from Barrer and VilligerS7A benzene molecule was introduced coplanar with and centered in the 12-ring window. An atom-atom potential sum18was then computed as a function of translation along the center of the channel and benzene molecule rotation about the (12) Newsam, J. M. J. Chem. Soc., Chem. Comm. 1987, 123-124. (13) Newsam, J. M.; Silbernagel, B. G.; Garcia, A. R.; Hulme, R. J . Chem. Soc., Chem. Comm. 1987, 664-666. (14) Newsam, J. M.; Melchior, M. T.; Malone, H. Solid State Ionics 1988, 26, 125-131. (15) Davies, E. K. CHEM-X Program Sufte; Chemical Design Ltd.: Oxford, UK, 1982. (16) Ramdas, S.; Thomas, J. M.; Betteridge, P. W.; Cheetham, A. K.; Davies, E. K. Angew. Chem., Int. Ed. Engl. 1984, 23, 671-679. (17) Bradley, J. S.; Harris, S.; Newsam, J. M.;Hill, E. W.; Leta, S.; Modrick, M. A. Organometallics 1987, 6, 2060-2069. (18) Kiselev, A. V. J . Chem. Technol. Eiotechnol. 1979, 29, 673-685.
0 1989 American Chemical Society
7690
The Journal of Physical Chemistry, Vol. 93, No. 22, 1989
Newsam
TABLE I: Fml OveraU dI w M Parameters with Estimated Standard Deviations in Pareatheaea parameter 298 K 78 K 78 K BZb temp, K 296 (5) 78 (1) 78 (1) sample wt, g 5.22 5.22 5.65 data acquisitn time, h 24 24 24 data range: 20, deg 5.0-105.0 5.0-105.0 5.0-105.0 (d-spacing, A) (14.8-0.81) (14.8-0.81) (14.8-0.81) no. of data pt 1001 1001 1001 no. of contributg reflectns 925 905 904 [7] no. of at. variables 24 24 30 tot1 no. of variables 33 41 32 a, A
Figure 1. Representation of the framework structure of zeolite L, indicating the atom numbering scheme. Two cancrinite cages joined through a hexagonal prism are shown in ball and stick representation. The manner in which these chains are interconnected to create the lobed, 12-ring channels is illustrated by straight lines connecting adjacent T sites.
unique molecular axis. Although the full potential energy surface was not explored, nor were the effects of nonframework cations or framework Coulombic charges included, this simple approach did indicate the 12-ring window site to be a local energy minimum configuration. Subsequently, benzene was observed to occupy the 12-ring window site in sodium zeolite Y a t low loadings at 4.2 K,l9qZ0and the observed site and orientation were calculated as a local energy minimum configuration with simplistic atomatom potential calculations similar to those described here for the LTL framework.21 The 12-ring window site in zeolite L was therefore considered during the diffraction data analyses (see text below). Interactive molecular graphics techniques were also used extensively in considering other possible benzene molecule locations and for generating corresponding trial fractional atomic coordinates. The same procedures were used to produce various representations of the structural results. Powder Neutron Diffraction. A sample of the zeolite was dehydrated under vacuum. Approximately 20 g of material was dried in air at 120 OC,placed in a silica tube, and connected to a vacuum line (roughing pump; minimum line pressure, 200 mTorr). The temperature was raised over several hours to 400 OC,held for 16 h, and transferred to a dry N, glove bag. Of this dehydrated material, 5.22 g was loaded into a 7/16-in.outside diameter, 0.005-in. walled vanadium sample can that was sealed with indium wire. For the benzene loading experiments, the zeolite was dehydrated similarly and 6.8 g was sealed in a septum vial. The required volume (assuming the above composition) of perdeuteriobenzene (KOR Isotopes C6D6: nominally > 99.98% D) was injected through the septum by syringe and a maximum loading level of 1.12 molecules C&/Unit cell was then determined from the weight gain. The loose powder was allowed to homogenize over 24 h at room temperature with repeated agitation, and 5.65 g was then transferred under dry N2to a 'Il6-in. outside diameter, 0.005-in. walled vanadium sample can that was again sealed with indium wire. A series of other samples for parallel deuterium N M R measurements were made up ~ i m i l a r l y . ' ~ . ~ ~ Powder neutron diffraction (PND) data were collected on the powder diffractometer at the Missouri University Research Reactor Facilityz3 at a wavelength of 1.2892 A selected from the (220) planes of a Cu monochromator at a take-off angle of 60.6'. In each case, data from four 25' spans of the linear position sensitive detector were each accumulated over some 6 h (Table I) and combined to yield the diffraction profiles 5 5 28 I105O, rebinned in 0.1 O steps. For the low-temperature scans, the sample can was centered in a liquid N, immersion cryostat. The data (19) Fitch, A. N.; Jobic, H.; Renouprez, A. J . Chem. Soc., Chem. Comm. 1985, 284-286. (20) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 1311-1318. (21) Newsam, J. M. Mater. Sci. Forum 1987, 27/28, 385-396. (22) Silbernagel, B. G.; Garcia, A. R.; Newsam, J. M.; Hulme, R. J. fhys. Chem., in press. (23) Tompson, C. W.;Mildner, D. F. R.; Mehregany, M.; Sudol, J.; Berliner, R.; Yelon, W. 9. J. Appl. Crystallogr. 1984, 17, 385-394.
18.466 (3)
18.490 (1)
c, A 7.4763 (6) half-width parameters, deg2
7.4781 (7)
18.460 (2) [3.5877 (4)] 7.4798 (9)
1.08 (7) -1.11 (6) 0.55 ( I ) 0.35 (2) 0.07 (1) -0.256 (3) 1.15 2.10 2.94 1.21 5.90
0.73 (7) -0.73 (6) 0.45 (1) 0.01 (3) 0.08 (2) -0.031 (4) 1.76 [1.01] 2.59 3.19 1.34 5.67
0.91 (6) -0.98 (6) 0.55 ( I )
(I
V W Y"
asym zeropoint, deg RB R,
-0.119 (2) 3.54 5.69 6.44 3.12 4.26
residual (R,,/R,)*
Constant defining the relative Lorentzian contribution to the pseudo Voigt function.26 bSample containing perdeuteriobenzene. Numbers in brackets refer to a small contribution to the diffraction pattern from the stainless steel end caps of the sample can. This contribution was accounted for in biphasic refinements, modeled as Fe(meta1).
v)
I-
z 2
8 0
llllll I I . # H I I Y
50
193 3 3 6 47.9 62 1
76.4 9 0 7 1050
Two Theta (")
Figure 2. Final observed (points), calculated (continuous line connecting computed data points), and difference (lower-same scale) powder neutron diffraction profiles for dehydrated potassium zeolite L at 298 K. The positions of the contributing reflections are indicated by the vertical bars. The background has been subtracted prior to plotting.
v)
I-
z 2
8 0
Illill
50
uIYIIa-
193 33.6 47.9 62 1
764 907 105.0
Two Theta (")
Figure 3. Final observed, calculated, and difference PND profiles for dehydrated potassium zeolite L at 78 K (legend as for Figure 2).
were analyzed by full-matrix Rietveld refinement" with modified versions of Rietveld's original and of the DBW3.2 code of Wiles and Y o ~ n g . ~The > ~ background was treated by linear (24) Rietveld, H. M. J . Appl. Crystallogr. 1969, 2, 65-71. (25) Hewat, A. W. United Kingdom Atomk Energy Research Establishment, Report AERE-RRL 731239; AERE: Harwell, UK, 1973. (26) Wiles, D. B.; Young, R. A. J . Appl. Crystallogr. 1981,14, 149-151. (27) Newsam, J. M.; Leonowicz, M. E. Unpublished results.
Structures of Dehydrated Potassium Zeolite L
The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 7691
TABLE II: Fractional Atomic Coordinates with Estimated Standard Deviations in Parentheses site conditions Y z atom X 0.5 0.0943 (4) 0.3584 (4) Si1 (T4) 12q 298 K 0.0942 (4) 0.3576 (4) 78 K 78 K BZ 0.0946 (5) 0.3601 (7) 0.4989 (3) Si2 (T6) 24r 298 K 0.2121 (4) 0.1660 (3) 0.4993 (3) 78 K 0.21 14 (4) 0.1666 (3) 78 K BZ 0.2096 (7) 0.5000 (5) 0.1675 (5) 0.0 0.5 0.2754 (4) 01 6k 298 K 78 K 0.2749 (4) 78 K BZ 0.2752 (6) 02 6m 298 K 0.1659 (2) 0.5 2x 78 K 0.1659 (2) 78 K BZ 0.1648 (3) 03 120 298 K 0.2657 (2) 2x 0.2560 (6) 78 K 0.2589 (6) 0.2654 (1) 0.2576 (9) 78 K BZ 0.2649 (2) 0.3183 (4) 04 24r 298 K 0.4142 (3) 0.1022 (3) 0.3197 (4) 78 K 0.4145 (2) 0.1024 (2) 78 K BZ 0.3199 (6) 0.4151 (3) 0.1023 (3) 298 K 0.2751 (6) 2x 0.4251 (2) 05 120 78 K 0.2743 (6) 0.4251 (2) 78 K BZ 0.2765 (8) 0.4255 (2) 298 K 0.4772 (3) 0.0 0.1442 (3) 06 12P 0.4791 (3) 78 K 0.1450 (3) 78 K BZ 0.4786 (4) 0.1439 (4) 0.6667 KA 2c 298 K 0.0 0.3333 78 K 78 K BZ KB 2d 298 K 0.3333 0.6667 0.5000 78 K 78 K BZ 298 K 0.0 0.5000 0.5000 KC 3g 78 K 78 K BZ 298 K 0.0 0.3197 (9) 0.0 KD 6j 78 K 0.3213 (9) 78 K BZ 0.3208 (14) KE 3f 298 K 0.5000 0.0 0.0 78 K 78 K BZ c 3I 24r 78 K BZb 0.084 (3) 0.196 (2) -0.100 (3) 12h 78 K B Z ~ 0.0 C32 -0.168 (8) 0.142 (2) D3 1 24r 78 K BZb 0.140 (3) -0.169 (3) 0.227 (2) 12h 78 K B Z ~ 0.0 D32 -0.316 (8) 0.142 (2)
B, A2 1.4 (1) 1.4 (1) 1.8 (1) -
-
2.6 (1) 2.4 (1) 2.5 ( I )
populationa 11.37 ( 9 ) O 11.29 (9)’ 11.15 (14)’ 23.06 (9) 23.14 (9) 23.28 (14) 6.0
-
6.0
-
12.0
-
24.0
-
12.0
-
12.0
3.4 (2) 2.8 (2) 3.1 (3)
-0.01 (5) -0.06 (5) 0.49 (7) 2.0
-
~ 9 . 3( 8 ) ~
-
3.0 4.78 (9) 4.52 (IO) 4.18 (12) 0.16 (6) 0.22 (6) 0.23 (8) 3.25 (9)d 1.63 (4)d 3.25 (9)d 1.63 (4)d
“Site populations refer to the number of the associated species per unit cell. The T-site values are expressed as numbers of Si atoms (the sum constrained to equal 34.43 in each case, the value calculated based on the known scattering lengths and chemical composition) and yield aluminum partitioning factors, P (see text), of 1.3 (3) [298 K], 1.6 (3) [78 K], and 2.4 (6) [78 K BZ]. bAtomic coordinate shifts for C and bonded D constrained to be equal (see text). C B factor constrained to equal 3X that of K. dRelative occupancies constrained to generate the composition XCgD6.
interpolation between a set of estimated points that were constrained to lie on a smooth curve. Scattering lengths of 3.67, 3.449, m) for K, AI, Si, 0, 4.1491, 5.805, 6.648, and 6.674 fm ( X C, and D, respectively, were taken from Koester.** Approximate coordinates for the framework components and for the nonframework potassium cations were taken from Barrer and Villiger.’ Space group P6/mmm (No. 191%) was assumed and confirmed for each of the present samples by the subsequent analyses. The refinements of the dehydrated sample converged rapidly to the final overall and atomic parameters listed in Tables I and 11, respectively. Selected separations and angles are given in Table 111. The final observed and calculated diffraction profiles are shown in Figures 2-4. In each refinement, the partitioning of aluminum over the two T sites was treated as a variable, the total aluminum content being fixed at the known composition. The occupancies of cation sites A, D, and E were included as variables in the final refinements (Table 11). Analysis of the PND data for the benzene-loaded material comprised initial adjustment of the atomic coordinates for the framework atoms and potassium cation sites, and the evaluation (28) Koester, L.; Rauch, H.; Hcrkens, M.; Schrader, K. Jahresber.Kernforschungsanlage Jiielich, Report Jul-1755; KFA: Jiilich, FDR, 1981. (29) Hahn, Th., Ed. International Tables for Crystallography; Riedel, Dordrecht, Holland, 1983; Vol. A.
9600
w 111111 I I l l U l l l l l Y l
50
19.3 33.6 47.9 62.1 76.4 90.7 105.0
Two Theta (“)
Figure 4. Final observed, calculated, and difference PND profiles for potassium zeolite L at 78 K, containing, on average, 1 sorbed molecule of perdeuteriobenzene/unit cell (legend as for Figure 2). The upper set of reflection bars is for a stainless steel component in the pattern included in the Rietveld refinements.
of a series of models for benzene molecule location. The 12-ring window site (see text above) was described by two unique atom positions C11 at 0.024,0.085,0.5 (12q) and D11 at 0.041,0.148, 0.5 (1 2q). The space-group symmetry generates two equivalent benzene orientations, related by a 30° rotation. When refined, however, the occupancy of this site converged to -0.04 (6)
7692 The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 TABLE III: Selected Separations (A) with Estimated Standard Deviations in 298 K 78 K 78 K BZ Sil-01 1.647 (6) 1.646 (6) 1.664 (9) Si 1-02 1.625 (10) 1.617 (11) 1.651 (15) Sil-04b 1.667 (6) 1.670 (6) 1.649 (9) 1.651 (4) 1.651 (4) 1.653 (5) avg Si1 (T4)-0 Si2-03 Si2-04 Si2-052 Si2-06 avg Si2 (T6)-0
1.659 (6) 1.619 (5) 1.643 (8) 1.636 (4) 1.639 (3)
1.655 (6) 1.632 (5) 1.644 (8) 1.628 (4) 1.640 (3)
1.636 (9) 1.643 ( 8 ) 1.653 (12) 1.622 (6) 1.638 (4)
P (bond length)
1.4 (2) 2.889 (5) 3.496 (4) 2.926 (5) 3.303 (5) 2.895 (6) 3.156 (5)
1.40 (2) 2.913 (5) 3.475 (4) 2.934 (5) 3.301 (5) 2.895 (6) 3.157 (5)
1.5 (3) 2.915 (7) 3.484 (5) 2.909 (7) 3.286 (7) 2.875 (8) 3.154 (7) 3.44 (6) 3.53 ( 5 )
[Kl A-031 [Kl A-061 K3 C-052 K3 C-04 [KS E-061 [K5 E-052] K4 D C 3 1 K4 D C 3 2
Newsam Parentheses' Sil-Si1 Sil-Si19 Sil-Si2
298 K 3.136 (15) 3.015 (11) 3.113 (7)
78 K 3.128 (15) 3.017 (11) 3.131 (6)
78 K BZ 3.155 (23) 3.025 (18) 3.117 (10)
Si2-Si2I6 Si2-Si220 Si2-Si221
3.171 (7) 3.083 (13) 3.104 (13)
3.162 (7) 3.073 (13) 3.105 (13)
3.135 (11) 3.046 (19) 3.094 (19)
K2 K2 K4 K4
2.830 3.384 2.793 2.996
2.826 3.388 2.807 3.001
2.841 (7) 3.389 (7) 2.794 (15) 3.006 (8)
B-03 B-05 Do6 DO4
C3 1 C32 D3 1 D32
12p 24r 12p 24r
0.1098 0.1267 0.0751 0.1098
0.1922 0.1701 0.2267 0.1876
0.0 0.1654 0.0 0.2935
The occupancy of this second orientation was found to be low, 0.1 molecules uc-l or less, and it was excluded from the final refinements. This orientation corresponds to a computed energy maximum in the atom-atom potential modeling results (see text below), and a low refined occupancy is therefore consistent. The computed energy minimum configuration differs from the orientation defined by atoms C21, C22, D21, and D22 by a 12' rotation and can be described by the following atomic coordinates: C4 1 C42 c43 D4 1 D42 D43
24r 24r 24r 24r 24r 24r
0.9824 0.0646 0.0822 0.1137 0.9691 0.8554
0.8428 0.8839 0.8927 0.9084 0.8361 0.7793
0.1812 0.1231 0.9419 0.2165 0.3186 0.1022
Refinements based on this model yielded residuals and benzene molecule populations and temperature factors similar to those obtained for the first orientation (atoms C21, C22, D21, and D22-Table 11). The latter is chosen for its higher symmetry and smaller number of atomic parameters. The similarity between (30) Nowak, A. K.; Cheetham, A. K. In New Developments in Zeolite Science Technology:Murakami, Y . ,Iijima, A., Ward, J. W., Eds.; Kodansha: Tokyo, 1986; pp 475-419.
(5) (5) (10) (5)
1.50 (4) 1.03 (7) 1.46 (5) 1.11 (8)
C3 1-C3 1 l 6 C31-D31 C31-C32 C32-D32
Superscript numbers indicate the number of the symmetry operator that has been applied.29 factor-see text.
molecules/unit cell (uc-I), indicating that this site is not occupied to any significant extent a t the composition studied. The only sites that were found to have significant occupancy are those that correspond to benzene molecules in "capping" positions above the channel wall (type D) K+ cations. Such sites were examined in some detail. Interactive molecular graphics and atom-atom potential calculations were, as above, used to assist the development of appropriate models. No evidence was found for sites that would be asymmetric with respect to the type D K+ cations (such as those described for pyridine in gallosilicate zeolite LIo, nor for any benzene molecule orientation (such as the near-perpendicular configuration predicted by recent modeling results30) other than those with the ring normal directed toward the type D K+ cation. The orientation presented in the tables (atoms C21, C22, D21, and D22) has a C-D vector parallel with the crystallographic c direction. A second orientation that was also examined was generated by a 30' rotation about the K+ cation to the ring centroid vector and described by the following approximate coordinates:
(5) (5) (10) (5)
Multiplicity of 2. CAluminum partitioning
Figure 5. Cross-stereoview illustrating the site occupied by benzene in potassium zeolite L at 78 K. One channel lobe is drawn as straight lines connecting framework T and 0 atoms; Kt cations and C and H atoms are drawn with van der Waals radii (K+, 1.35 A; C, 1.77 A; H, 1.17 A). Only one of six equivalent benzene molecule positions is shown. TABLE IV: Selected Angles (des) with Deviations in Parentheses' 298 K angle 111.1 (6) 01-Sil-02 107.5 (3) 01-Si 1-04b 02-Si 1-04b 110.7 (3) 04-Si 1-0416 109.2 (6) 0342-04 113.0 (4) 03432-05 106.2 (4) 03-Si2-06 111.9 (4) 04-Si2-052 109.2 (4) 04-Si2-06 105.5 (3) 111.1 (4) 052-Si2-06 132.6 (8) Sil-Ol-Si19 149.7 (6) Sil-02-Si120 136.6 (5) Si2-03-Si220 142.5 (3) Sil-0442 141.7 (4) Si23-05-Si219 151.5 (4) Si2-06-Si2l6 mean Si1 (T4)-0-T mean Si2 (T6)-0-T C3116-C31-C22 C3116-C31-D31 C31-C22-C3124 C31-C32-D32
141.8 (3) 143.1 (2)
Estimated Standard 78 K 111.6 (6) 107.8 (3) 110.9 (3) 107.7 (5) 112.1 (4) 106.5 (3) 112.9 (4) 108.7 (4) 106.5 (3) 110.2 (4) 132.8 (8) 150.4 (6) 136.3 (5) 143.1 (3) 141.6 (4) 152.5 (4) 142.3 (3) 143.4 (2)
78 K BZ 108.2 (8) 107.8 (5) 111.6 (5) 109.5 (8) 111.7 (6) 107.6 (5) 114.2 (6) 106.5 (6) 105.9 (4) 110.7 (6) 130.7 (1.2) 145.7 (8) 137.1 (7) 142.5 (5) 138.8 (6) 150.3 (6) 140.4 (4) 142.2 (3) I10 (2) 120 (3) 135 (5) 110 (3)
"Superscript numbers indicate the number of the symmetry operator that has been applied.29 Multiplicity of 2.
the convergences for these two slightly different orientations partly reflects the relatively low barriers to molecule reorientation about the ring normal (see text below) but also illustrates the relatively limited precision with which hydrocarbon sorbate configurations
The Journal of Physical Chemistry, Vol. 93, No. 22, 1989 7693
Structures of Dehydrated Potassium Zeolite L
-r -37'5 l----l
c -90.0
-45.0
0.0
90.0
45.0
Benzene Rotation (")
Figure 6. Atom-atom potential energy sum for benzene in the channel lobe of zeolite L (see Figure 5) computed as a function of benzene molecule rotation about the site D K+ cation to ring centroid vector.
can be defined by the present PND methods.21 For the first orientation (atoms C21, C22, D21, and D22Figure 5 ) , the molecular geometry was initially fixed but in the final stages the atomic coordinates of the C21 D21 and C22 + D22 pairs were allowed to vary. The resulting bond lengths and angles (Tables I11 and IV) again illustrate that the definition of the sorbate structure is more limited than that of the framework components, and the slight deviations from regularity that the optimized coordinates suggest cannot be considered significant. The carbon and hydrogen atoms were assigned equivalent B factors, with relative occupancies constrained to satisfy the formula C6D6 For the results presented in the tables, the carbon/hydrogen B factor was additionally constrained to equal 3 times that of the K+ cations. With this constraint, the final optimized benzene population, P = 0.82 (2) molecules uc-' (Table 11), is somewhat lower than the maximum loading level determined gravimetrically (1.12 molecules uc-'). A lower than theoretical value is expected, partly as a result of small losses of benzene from the system during the preparation and the loading of the PND sample can and partly due to the relatively high degree of correlation between occupancy and temperature factors. A refinement in which both occupancy and temperature factors were allowed to vary without constraint converged to the values B = 25 (3) A3, P = 1.06 (5) molecules uc-' and residuals R , = 0.025 and R,, = 0.031). Following complete definition of the structure, a benzene molecule (with idealized planar geometry: C-C, 1.395 A; C-H, 1.10 A, C-C-C, 120'; C-C-H, 120') was placed at the observed position in a model constructed from the measured structure (Table 11). An atom-atom potential energy sum was then computed as a function of the benzene molecule rotation (Figure 6). The individual potential terms were computed with a LennardJones 6-12 potential with literature A and B parameter^.^'^^^ The results are consistent with the observed location being close to a minimum energy orientation but yield only a small activation barrier to rotation at
