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J . Phys. Chem. 1989, 93, 7294-7298
cyclic ether ethylene oxide (ETO) and that, for C 0 2 as a dilute solvate within such clusters, the q band structure collapses to a singlet with a peak frequency that matches the value for dilute solid solutions of C 0 2 in amorphous ice (Le., 2340 cm-1).6 The u3 band for the same ratio of the components within the mixture, but with no ether present, resembles curve c of Figure 1 . In summary, the fact that the cluster spectra are sensitive to both cluster molecule concentrations and the presence of small amounts of other chemicals implies that an interesting range of cluster sizes, structures, and compositions are accessible by this sampling approach. Secondly, there is strong evidence that the larger C 0 2 clusters have a crystalline structure. Some of this evidence was described previously’J and is noted above, but a more conclusive argument can be made from the positions and fwhm
of the u2 and vj bands of isotopically decoupled 13C02. Recently published infrared spectra of crystalline and amorphous thin films of COz at 77 K show that the crystal-phase decoupled bands are at 2282.5 and 638.9 cm-’ with a fwhm of -2.5 cm-’ while the amorphous-phase bands are at 2280.3 and 643.0 cm-’ with a fwhm greater than 5 cm-l.’ Clearly, the cluster values noted above, 2282 and 638 cm-’ and fwhm of -3 cm-I, identify the cluster structure with the crystalline rather than the amorphous phase.
Acknowledgment. This research was funded by the National Science Foundation under Grant CHE-87 19998. The assistance of Matthew S. Devlin in sampling, and the O.S.U. Physics Instrument Shop (Mike Lucas, manager, and Wes Cash, builder) in fashioning the cluster cell, is also acknowledged. (7) Falk, M.; Seto, P. F. Can. J . Spectrosc. 1986, 31, 134.
(6) Fleyfel, F.; Devlin, J. P. Manuscript in preparation.
Heterogeneous Adsorption Equitibria with Comparable Molecule and Pore Sizes Orhan Talu,* Chang-Jie Guo, and David T. Hayhurst Department of Chemical Engineering, Cleveland State University, Cleveland, Ohio 441 15 (Received: March 13, 1989; In Final Form: August 28, 1989)
Isotherms have been measured at temperatures ranging from 0 to 150 OC for the adsorption of benzene, toluene, and p-xylene on silicalite. Results show that the shape of the isotherms changes from type I to type IV with decreasing temperature. This unusual behavior is consistent with a two-patch heterogeneous model with surface-phase transition. Although silicalite pore walls are homogeneous, “structural heterogeneity”results because the adsorbate molecules are comparable in size to channels in silicalite, thereby restricting the motion of the molecules in the surface phase. Isotherms have been modeled by considering that adsorption takes place on a high-energy patch and a low-energy patch. Local equilibria on both patches are described by the Hill-deBoer model. The heat of adsorption is found to go through a maximum as predicted by this model. The twepatch model is effective in correlating the adsorption isotherm for the three test gases. The simulations and data are also in agreement with results obtained by other researchers utilizing different experimental techniques.
Introduction Several studies on adsorption of aromatic compounds on silicalite have been published in recent years. A number of different experimental methods have been used to measure adsorption. These methods include gas chromatography,’ isotherm measurement~:-~ isostere measurements,* and calorimetric studies.” For the adsorption of aromatics on silicalite, researchers have observed unusual equilibrium behavior; most notable are the change in isotherm shape from type I to type IV with decreasing temperature and extremes in the heat of adsorption at intermediate loadings. Some of this behavior has been attributed to defects in the silicalite crystal caused by variation in sample preparation.”
’
(1) Lechert, H.; Schweitzer, W. In Proceedings of Sixth International Conference on Zeolites; Bisio, A., Olson, D H., Eds.; Butterworths: London, 1984. (2) Anderson, J. R.; Foger, K.; Mole, T.; Rajadyaksha, R. A,; Snaders, J. V. J . Caral. 1979, 58, 114. (3) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. J . Phys. Chem. 1981,85, 2238. (4) Jacobs, P. A.; Beyer, H. K.; Valyon, J. Zeolites 1981, 1 , 161. ( 5 ) Wu, P.; Debebe, A.; Ma, Y. H. Zeolites 1983, 3, 118. (6) Lohse, U.; Fahlke, B. Chem. Tech. (Leipzig) 1983,35, 350. (7) Guo, C. J.; Talu, 0.; Hayhurst, D. T. AIChE J. 1989, 35, 573. (8) Pope, C . G . J . Phys. Chem. 1986, 90, 835. (9) Thamm, H.; Regent, N. I. Z . Chem. 1982, 22, 232. (10) Stach, H.; Thamm, H.; Janchen, J.; Fiedler, K.;Schimer, W. In Proceedings of the Sixth International Conference on Zeolites; Bisio, A,, Olson, D. H., Eds.; Butterworths: London, 1984. (1 1) Thamm, H. J. Phys. Chem. 1987, 91, 8 .
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Substantial variations have also been found between batches supplied by the same commercial source. Although the quality of silicalite samples from different sources varies considerably, the unusual adsorption characteristics are common in all published data. Several attempts have been made to explain these systems both qualitatively and quantitatively. Adsorption as dimers,” surface-phase transition,’ and pore filling with energy and entropy of different sites determined by Monte Carlo simulations12 are among several suggested schemes. It is generally agreed that the well-known isotherm models such as Langmuir are inadequate for the aromatic/silicalite system. The theoretical efforts have been based on highly fragmented data covering a relatively small range of properties. Considering the additional confusion introduced by the variation in silicalite samples, it was deemed necessary to accurately measure equilibria of several aromatics over a range of temperatures. The complete and accurate data set presented in this paper has provided a firm basis for the development of an adsorption model. Developing a fundamental understanding of the adsorption behavior of aromatic compounds in silicalite is important for the understanding of shape selective systems where the adsorbent must distinguish between molecules with minute differences in size and shape. This is only possible if the pore size is very close to the (12) Stach, H.; Wendt, R.; Fiedler, K.; Grauret, B.; Janchen, J.; Spindler, H. In Characterization of Porous Solids; Unger, K. K., et al., Eds.; Elsevier: Amsterdam. 1988.
0 1989 American Chemical Society
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The Journal of Physical Chemistry, Vol. 93, No. 21, 1989 7295
1.4
I
--
.
1
benzene/silicalite - 0%
--
ooc
toluene/ silicalite
IJ 1.o U
g P
e
e
z
- I
a
0
4
data model
o.vvm
1
1
I
I
2
4
6
8
0.6
aE m
e
I-
0.2
0
10
1
Pressure (kPa)
Pressure (kPa)
Figure 1. Data and model predictions of benzene adsorption isotherms
on silicalite. molecular size. Aromatic compounds adsorbed in silicalite provide an excellent system for studying shape selectivity since homogeneous silicalite pores are essentially the same size as the aromatic ring. Anomalous behavior is directly attributable to structural effects.
Experimental Section Large silicalite crystals were prepared in our laboratory following a procedure detailed e1~ewhere.l~Large crystals were used to minimize experimental variabilities such as any contribution to adsorption capacity caused by capillary condensation between particles, external surface adsorption, etc. Two different batches were used with sizes 350 X 105 X 105 pm and 270 X 70 X 70 pm. These samples were extensively analyzed to assure the quality of silicalite. The two batches were found to have identical isotherms within experimental error. The accuracy of isothermal data was f0.003 mol/kg, f0.13 Pa, and f0.5 K. Details of the gravimetric experimental procedure are given elsewhere.' The sample weights were continuously monitored during experiments; equilibrium was assumed when sample weight changed less than hO.01 mg in a 1-h period. The total time of each experimental point was about 12 h. Data presented in Table I include both adsorption and desorption experiments; hysteresis was not observed. Results and Discussions Molecular Dimensions and Silicalite Pore System. The silicalite pore system contains (1) circular zigzag channels of 5.4-A diameter, (2) elliptical straight channels of 5.7 X 5.1 A cross section, and (3) their intersection^.'^ Benzene kinetic diameter is 5.85 A.15 The width of the aromatic ring is about 5 A as calculated from molecular bond lengths and angles.16 These dimensions indicate that the aromatic molecules can only be oriented with the ring parallel to the pore axis without overlap. As the aromatic ring is highly restricted in the pores, it can only rotate around the pore axis and translate parallel to the pore axis. Even the rotation of the ring is restricted since the length of benzene (smallest of the three adsorbates) is 6.6 A.ls Rotation on all three principal axes is possible for benzene at the pore intersections, while translational motion at the intersections is restricted to two dimensions. Toluene and p-xylene are more restricted even at intersections. It is, therefore, not surprising that these systems exhibit unusual adsorption behavior. (13) Hayhurst, D. T.; Lee, J. C. In New Development in Zeolire Science and Techtwlogv; Murkami, Y . ,Iijima, A., Ward, J. W., Us.Kodanska ; Ltd.: Japan, 1986. (14) Flanigen, E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.;Kirchner, R. M.; Smith, J. V. Nature 1978, 272, 512. (15) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974. (16) Bowen, H . J. M. Tables of InterafomicDisrances and Configuration in Molecules and Ions; Burlington House: London, 1964.
3
2
Figure 2. Data and model predictions of toluene adsorption isotherms on silicalite. 1.4
p-xylenelsilicalite
0
-g
1.0
.o
e 8 p
0.6
e
O O A A V V ~ Dexperimental
-model
0.2 m
0
15OC'
:
I
0.2
0.4
I
data
1
0.6
I
0.0
Pressure (kPa)
Figure 3. Data and model predictions of p-xylene adsorption isotherms on silicalite.
Isotherm Data. Isotherms for benzene, toluene, and p-xylene were measured at temperatures ranging from 0 to 150 OC;data are shown in Figures 1-3. Benzene and p-xylene isotherms contain desorption and adsorption runs which coincide; there is no hysteresis observed in these systems. The unusual adsorption behavior is immediately apparent in these figures. Isotherms change shape from type I to type IV with decreasing temperature. The convex region at intermediate loadings in isotherms indicates cooperative interactions between adsorbate molecules. The cooperative interactions are also consistent with the experimentally observed increase in heat of adsorption with surface converage at moderate loading." The molecules pack more densely due to cooperative interactions at low temperatures. As temperature is increased, these cooperative interactions are overwhelmed by the rotational and/or translational energy of the surface molecules. This increase in translational energy results in a transition from type IV to type I isotherms. The rotational and translational mobility of molecules is highly restricted in the pores. It is, however, surprising that a small change in mobility of molecules with temperature can overwhelm the cooperative interactions. Conversely, the cooperative interactions are relatively weak since the molecules are heavily oriented in the pores and interaction area between molecules is very small as they can only fit lengthwise in the pores. It is, therefore, postulated that the adsorption of aromatic compounds in silicalite is controlled by a delicate balance between cooperative interactions and the mobility of molecules in the pores. Similar behavior is expected to occur for all systems where the pore diameter is very close to molecular diameter as is common in shape selective systems. Such adsorptive interactions do not occur in large cavities
7296 The Journal of Physical Chemistry, Vol. 93, No. 21, 1989
Letters
TABLE I: Isotherm Data for Adsorption of Aromatic Compounds on Silicalitea P N P N P N P Benzene, 0 O C 115.3 0.976 1.211 5.3 0.524 506.5 7.3 120.0 1.064 12.0 0.697 1.222 859.7 30.7 1.217 993.0 127.3 1.023 19.4 0.742 116.5 144.8 1.054 1.232 1286.0 30.5 0.754 1.235 1437.7 44.7 0.767 203.9 1.116 70.1 228.9 1.117 1.227 1510.2 76.0 0.792 133.3 1.25 1 2465.9 87.0 0.814 459.8 1.180 102.1 0.838 2.0 p-Xylene, 20 O C 11.4 196.0 1.240 9.3 0.897 433.3 1.254 21.2 699.9 1.255 29.3 1.208 342.6 1.250 57.7 93.3 1.230 102.9 p-Xylene, 30 O C 130.0 1.154 8.0 0.538 440.0 1.178 8.7 742.6 1.185 22.7 0.787 191.9 1.164 30.0 313.3 1.177 61.3 1.136 96.1 p-Xylene, 40 OC 193.9 140.0 1.082 513.3 9.3 0.466 1.100 246.6 1.088 24.7 0.521 1.104 666.6 7.7 1.110 56.0 1.045 406.6 1.096 799.9 30.0 101.3 1.072 112.6 p-Xylene, 50 O C 199.9 5.0 0.346 56.5 0.502 313.4 1.048 12.0 0.442 75.1 0.598 450.0 1.058 10.7 100.0 0.974 20.7 0.473 646.5 1.067 20.7 134.1 1.025 54.7 0.500 64.6 p-Xylene, 60 OC 79.1 0.466 4.3 0.156 284.1 0.953 125.8 142.7 0.517 8.7 0.312 300.6 1.002 271.9 145.6 0.521 22.5 0.419 420.0 1.012 679.8 197.1 0.667 37.3 0.427 586.5 1.022 252.0 0.949 53.6 0.440 705.9 1.026 5.3 13.9 36.0 66.3 98.0
0.074 0.201 0.317 0.386 0.409
p-Xylene, 70 O C 129.3 0.428 133.3 0.436 214.6 0.457 269.3 0.470 322.4 0.506
435.4 463.7 600.6 702.6
N 0.084 0.248 0.384 0.018 0.036 0.250 0.738 0.823 1.097 1.171 0.550 0.61 1 0.669 0.711 0.381 0.525 0.613 0.635 0.121 0.248 0.374 0.038 0.073 0.160
P N p-Xylene, 80 O C 293.7 0.412 513.3 0.422
P
N
653.3 813.3
0.427 0.436
p-Xylene, 150 "C 304.0 0.064 464.0 0.099
906.6 1126.6
0.153 0.209
470.5 567.8 712.4 831.7
1.264 1.277 1.296 1.310
1366.2 1932.7 2439.2
0.992 1.011 1.033
Toluene, 148.0 230.6 287.9 387.9
0 OC 1.198 1.227 1.243 1.258
Toluene, 411.9 649.1 786.4 1073.0
30 O C 0.837 0.910 0.934 0.971
Toluene, 413.2 666.4 1066.3
50 "C 0.674 0.697 0.720
1466.2 1999.3 2399.2
0.732 0.742 0.746
Toluene, 111.8 161.3 255.4
70 O C 0.437 0.492 0.516
418.3 821.5 2665.8
0.538 0.596 0.615
Toluene, 150 "C 1150.3 0.216 1531.5 0.248
1890.0 2199.3
0.271 0.275
0.793 0.833 0.945 0.963
" P is pressure in Pa, and N is amount adsorbed in mol/kg. Benzene data at 10, 20, 30, 50, and 70 "C are tabulated in ref 7.
such as those found in type A zeolites. When the bulk of adsorption space is in cavities interconnected with windows, the shape selectivity is mainly determined by the window opening while the surface-phase behavior is dictated by the lateral interactions of more than two molecules contained in the large cavities. Two-Patch Model. Cooperative interactions that determine adsorption behavior can be likened to phase transition. Geometrical considerations may suggest a one-dimensional surface phase which contradicts a phase transition postulate; phase transition cannot occur in a tnily one-dimensional phase. On the other hand, aromatic molecules in silicalite are not literally one-dimensional due to two reasons. First, pore intersections provide enough space for molecules to overlap. Second, the length of molecules depends on their orientation. Even if the molecular centers are aligned on a straight line in a pore, the systems do not constitute a onedimensional phase since the length occupied by a molecule depends on the orientation of the molecule whereas the length of the molecules is invariable in a truly one-dimensional phase. The convex isotherm characteristics can be captured by formulating a phase transition mechanism. On the other hand, phase transition alone cannot explain the concave region of the isotherms at low coverage. Phase transition results in type V isotherm which is convex at the origin. The experimental data measured are clearly concave a t the origin, becoming convex at some intermediate surface coverage. The step in the isotherms can be explained by a combination of surface heterogeneity and phase transition. Specifically, there must be at least two surface patches to account for the single step in the isotherm. Essentially, all
characteristics of observed equilibrium behavior may be captured by a two-patch heterogeneous model which allows for phase transition. The total amount adsorbed is given by Nt = N,
+ NI
(1)
where N, is the total "observed" amount adsorbed, and N, and NIare amounts adsorbed on individual patches. The simultaneous equilibria of each surface patch with the vapor phase is described by the Hill-deBoer model.
P = exp(Aio - Uio/RT)Ni/(Ni" - N i ) exp(Ni/(N; - Ni) kNi/TN,") (2) The amount adsorbed on each patch (Ni) is calculated by solving eq 2 implicitly for the specified pressure and temperature. The total amount adsorbed is then calculated by eq 1. The combined van der Waals constant, k , contains the interaction ( a ) and size (8) parameters:
k = 2a/R@
(3)
The values of van der Waals constants ideally are a property of the adsorbed molecule only. If the molecules on the surface are not specifically oriented, the "ideal" van der Waals constants can be calculated from vapor-hase properties. The orientation of molecules by the surface causes complicated perturbations in the value of k." As discussed, the aromatic molecules adsorbed in
Letters
The Journal of Physical Chemistry, Vol. 93, No. 21, 1989 7297
TABLE I t Regression Results for Adsorption of Aromatic Compounds on SilicaUte -
temperatures, O C no. of data points overall regression R-square F-statistic F-table (at 0.01)
regressed parameters combined vdW constant, k, K s-patch N,", mol/kg
uso, K
A,'?, In (kPa)
benzene
toluene
p-xylene
0, 10, 20. 30, 50. 70
0, 30, 50, 70, 150
88
52
20, 30, 40, 50, 60, 70, 80, 150 78
0.9857 933.27 3.06
0.9751 288.41 3.25
0.9809 521.28 3.10
1240 f 90
683 f 266
2233 f 81
0.668 f 0.019 9049 f 246 32.47 f 0.80
0.801 f 0.059 8851 f 614 29.86 f 1.9
0.862 f 0.024 8260 f 229 25.60 f 0.67
0.797 f 0.01 1 6411 f 379 17.71 f 1.05
0.747 f 0.032 7484 f 490 18.80 f 1.09
0.496 f 0.014 7745 f 182 20.22 f 0.55
1.465 8.24
1.548 8.71
1.358 7.64
562.1 281.1 183.7 f 13.3
591.7 295.9 101.2 f 39.4
616.2 308.1 330.8 f 12.1
I-patch Nlm,mol/kg
Yo,K
A I 0 ,In (kPa) calculated values
Nlm,mol/kg
N1", molecules/uc
critical temperatures, K bulk phase ideal 2-dimensional calculated, k/6.75
silicalite are oriented, but the effect of orientation on the k value should be approximately equal for the two patches since the orientation of molecules is about the same in both t p of channels in silicalite. As a first approximation, the k value is assumed to be the same for the two patches although this value is expected to be substantially different from the "ideal" value. The model parameters are determined from experimental isotherms. There are seven parameters for each adsorbate. The parameter values are listed in Table 11. In Figures 1-3, the model correlations as solid curves are plotted along with experimental data. The model performs very well considering the complexity of the systems analyzed. Although there are a large number of parameters involved in the model, all parameters are significant as evidenced by their standard deviations listed in Table 11. From a statistical point of view the model is certainly not overspecified.
Physical Significance of Parameters The single most important parameter in analyzing the regression results is the energy parameter, specifically, the limiting heat of adsorption at zero coverage (the vertical interactions). The regression results for the three aromatic compounds consistently indicate a high-energy location denoted as "s-patch" and a lowenergy region denoted as "1-patch". Following general arguments for heterogeneous surfaces,I9the high-energy s-patch should always have a higher loading than the 1-patch. Conversely, equilibria is controlled by the Gibbs energy, which has both enthalpic and entropic contributions. AGadS = APdS - TmadS (4) The high-energy patch is denser only if the entropy difference does not overwhelm the difference in the enthalpies of the individual patches. The entropy contribution is very seldom addressed in the literature. This results in the oversimplified axiom that the highenergy patches are filled earlier during heterogeneous adsorption and, therefore, the heat of adsorption always decreases with surface coverage. This statement is only true if the entropy of adsorption is the same for all patches or if it decreases with increasing vertical interactions. Heterogeneous surfaces containing flat, homotatic patches satisfy either or both of these qualifications, but microporous heterogeneous surfaces may not. This is evidenced in the (17) deBoer, J. H. The Dynamical Character of Adsorption; Clarendon: Oxford, 1968. (18) Linsen, B. G. Physical and Chemical Aspects of Adsorbents and Catalysts; Academic Press: London, 1970. (19) Ross, S.;Olivier, J. P. On Physical Adsorption; Wiley: New York, 1964.
11OO0
silicalite adsorption
loo00
0
0.2
0.4
0.6
0.8
1.o
Fractional Coverage
Figure 4. Isosteric heat of adsorption calculated from two-patch heter-
ogeneous model for benzene, toluene, and p-xylene adsorption on silicalite.
present case of aromatic compounds adsorbed in silicalite. The term in Henry's law constant, Aio related to the entropy of adsorption is higher for the high-energy s-patch (Table 11). Although the adsorption potential is higher on the s-patch, the molecules may not be attached strongly to this site because of the higher restrictions on their mobility. The relative loading of the two patches is, therefore, determined by the temperature which dictates the weight of enthalpic and entropic contributions to Gibbs free energy through eq 4. The isosteric heat of adsorption predicted by the model is shown in Figure 4. The isosteric heat displays a maximum at intermediate loadings for all three adsorbates. The maximum is a consequence of the change in relative loading of individual patches. At low overall loading, the 1-patch is heavily occupied although it has a lower adsorption potential. As the overall loading increases, the coverage on the s-patch relative to 1-patch increases. This results in maxima in the overall isosteric heat although the isosteric heat on each individual patch increases linearly with coverage on that patch as a result of cooperative interactions. Resulting curves are strikingly different than commonly observed for heterogeneous flat or large pore surfaces. The trends in isosteric heat curves derived by the model are in good qualitative agreement with literature data for both adsorption heats measured directly" and calculated from isosteres.* These studies also show extremes in heat of adsorption at intermediate loading. The exact locations of these extremes are different in the two references.
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J. Phys. Chem. 1989,93, 7298-7300
The model predictions presented in Figure 4 are in better agreement with the calorimetric data by Thamm." These heat curves clearly indicate that the systems are heterogeneous. Since the isosteric heats do not follow the generalized decreasing trend with increasing loading, entropic contributions to equilibria are considered significant. The parameter values for the individual patches are consistent among different adsorbates. High-energy s-patch also has high entropy of adsorption for the three species examined. It is not possible to identify positively the physical location of each patch among the three distinct adsorption environments in the silicalite crystal, namely, the two types of channels (circular and elliptical) and their intersections. The adsorption potential is a continuous function of location, and the three locations should correspond to extremes in this potential. Qualitatively, the highest potential should occur in the small pores because of the small interaction distance between the pore walls and molecules. In addition, the molecules in the small pores should be more restricted, resulting in higher entropy values. It is, therefore, tempting to attribute the s-patch, high-energy and high-entropy patch, to the smaller elliptical pores. The location of the 1-patch would then correspond to the straight large channels or the intersections. To date, there is insufficient experimental evidence to make an unequivocal assignment.
the gravimetric technique utilized in this study; therefore, phase transition calculations are performed with the model. Only the p-xylene critical temperature is within experimental range. The coexistence curve and critical point determined by the model for p-xylene are shown in Figure 3.
Surface Critical Temperatures The van der Waals model includes phase transition. The apparent surface critical temperatures can be calculated from the combined constant, k. These values are shown in Table I1 along with the bulk and "ideal" two-dimensional critical temperature. The ideal critical temperature is calculated from the bulk values as described in the literature.'* The apparent critical temperatures are all much lower than bulk critical temperatures. There is no trend in comparing apparent critical temperatures with ideal values. The isotherms below the critical temperature show a vertical increase in the amount adsorbed. The existence of vertical regions in the isotherms is conclusive evidence of phase transitions. Isotherms within the phase boundary could not be measured with
Nomenclature
Conclusions The adsorption of aromatic compounds in silicalite exhibits highly unusual behavior. Although silicalite pore walls are homogeneous, the systems exhibit heterogeneous behavior because of the comparable sizes of the molecules and pores. Furthermore, the systems do not follow the generalized trends of heterogeneous adsorption because of the highly restricted mobility of molecules in the pores. A model is developed to explain the observed behavior by utilizing surface-phase transition and heterogeneity concepts. The anomalies are explained by arguments based on structural effects within the context of the model. Similar behavior caused by "structural heterogeneity" should be common in other shape selective systems where adsorption occurs primarily in the channels rather than in cavities.
Acknowledgment. The authors acknowledge the financial support provided for the project under the State of Ohio's Academic Challenge program.
entropy parameter of i-patch (eq 2), In (kPa) Gibbs free energy of adsorption (eq 4), J/mol enthalpy of adsorption (eq 4), J/mol combined van der Waals constant (eq 3), K amount adsorbed, t-total, s-patch, I-patch (eq l), mol/kg Ni saturation capacity, t-total, s-patch, I-patch (eq 2), mol/kg Ni" P pressure (eq 2), kPa R universal gas constant (eq 2) unds entropy of adsorption (eq 4), J/(mol.K) T temperature (eq 2), K UiO energy parameter, adsorption potential (eq 2), K Greek Letters CY van der Waals interaction parameter van der Waals size parameter P
AiO
AG~& APb k
Energy Barrler for Cyckpropylchlorocarbene Rearrangement Measured by Direct Observatlon of the Carbene in Laser Flash Spectroscopy Michael T. H. Liu*st and Roland Bonneau UA 348 du CNRS, Laboratoire de Chimie Physique, Universiti de Bordeaux I , 33405 Talence, France (Received: April 27, 1989; In Final Form: June 12, 1989)
A transient absorption in the 250-265-nm range is assigned to cyclopropylchlorocarbenegenerated from the laser flash photolysis of 3-chloro-3-cyclopropyldiazirine.Confidence in this assignment was obtained from the following: (a) the decay of this absorption matches the growth of the pyridinium ylide; (b) the values for the rate constant for reaction of the carbene with I-hexene obtained by monitoring the carbene at 250 nm and the ylide at 370 nm are similar. The kinetic parameters for the rearrangement of cyclopropylchlorocarbene to 1-chlorocyclobutene are log A = 11.1 and E, = 7.4 kcal/mol.
Introduction Cyclopropylcarbene has a unique position in the chemistry of divalent carbon. Much has been learned about the chemistry of ring expansion to yield cyclobutene' although the absolute rate for this process is not available. Moss and Fantina2reported that cyclopropylchlorocarbene generated from the corresponding diazirine added to a variety of alkenes in a stereospecific manner. 'On leave from the University of Prince Edwards Island, Canada.
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Accompanying the cyclopropane adducts was a minor product, 1-chlorocyclobutene, which became a major product when the diazirine was photolyzed in pentane. Apparently, the chloro substitution at the carbene center affects its stability and its reactivity enabling intermolecular reactions to compete with the (1) Shevlin, P. B.; McKee, M. L. J . Am. Chem. SOC.1989,121, 519, and references therein. (2) Moss, R. A,; Fantina, M. E. J . Am. Chem. SOC.1978, 100, 6788.
0 1989 American Chemical Society
