A theoretical and NMR study of p-xylene sorption into ZSM-5 - The

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J. Phys. Chem. 1988, 92, 5165-5169

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singlet aromatic carbonyls with amines, larger kiscof aromatic carbonyls (for example, ki, = 10" SKIfor b e n ~ o p h e n o n e would )~~ make the reaction from the singlet states negligible as compared with the case of aliphatic carbonyls in which ki, is relatively small (for example, kisc= 5 X lo8 s-l for a ~ e t o n e ) . ~ J ~

ketyl radical in TEA is roughly estimated as 5 X M, which is acceptable as the initial concentration of the transient radical produced by the excimer laser excitation. Finally we here mention a recent report about the similar time change of the CIDEP spectra reported by Depew and Wan.2c They discussed the time evolution of the CIDEP signal obtained by the photoreduction of camphorquinone in i-PrOH in terms of the changes of the relative contributions among the SToM of the singlet and triplet geminate pairs, free pairs, and TM. 4.5. Other Carbonyls. On the basis of the results for other carbonyls (result f), we conclude that the A*/E pattern in the tertiary and secondary amines and the E/A* pattern in the primary amines at room temperature are general phenomena for aliphatic carbonyls. Therefore, aliphatic carbonyls in the excited singlet states react with the tertiary and secondary amines effectively. As for aromatic carbonyls in amines, however, only the net emission due to T M was always observed. It is concluded that no measurable reaction occurs from SI and the fast reaction (k: lo9 s-l M-1)22occurs predominantly from T I . This difference is understood in view of the difference in the magnitudes of k,,,. Although we do not know the exact reaction rate constants of the

5. Summary We have investigated the photochemical reaction of aliphatic carbonyls with amines by means of time-resolved EPR and fluorescence measurements. We observed the reactions from both the singlet and triplet states simultaneously. Various dependences of the CIDEP spectra were rationalized on the basis of the two reactions: The reactions from SI and TI give rise to the A/E and E/A* patterns due to SToM and SToM and TM, respectively, and the spectral pattern is determined by the relative contributions of the two reactions. From the detailed investigations of the CIDEP spectra, the reaction from SI was found to be dependent on the type of amine. However, the main quenching process in SI has the rate constant of (1-3) X lo9 M-' regardless the type of amine and is determined by a deactivation process different from the hydrogen abstraction reaction.

(22) Cohen, S. G.; Parola, A,; Parsons, G. H., Jr. Chem. Reu. 1973, 73, 141.

(23) Anderson, R. W.; Hochstrasser, R. M.; Lutz, H.; Scott, G. W. J . Chem. Phys. 1974, 61, 2500.

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A Theoretlcal and NMR Study of p-Xylene Sorption into ZSM-5 P. Thomas Reischman,* Kirk D. Schmitt, and David H. Olson Mobil Research and Development Corporation, P. 0. Box 1025, Princeton, New Jersey 08540 (Received: May 7, 1987; In Final Form: March 2, 1988)

Equilibrium sorption sites for p-xylene in the channels of ZSM-5 were estimated based on calculated van der Waals interactions between the hydrocarbon and zeolite and between the hydrocarbon molecules themselves. These calculations indicate that the most energetically favored sorption sites are the channel intersections for p-xylene loadings 5 4 molecules per unit cell of zeolite. At p-xylene loadings >4 molecules per unit cell, molecules begin to preferentially fill the sinusoidal channels in addition to channel intersections. Variable-temperature MAS carbon NMR confirms this higher loading packing arrangement at temperatures 1 2 0 O C .

Introduction Siting and mobility of hydrocarbon molecules within zeolite channel systems are intrinsically related to channel geometry and size. Adsorption of p-xylene in ZSM-5 is particularly interesting. Its adsorption isotherm at 70 O C ' indicates that four molecules of p-xylene will readily sorb into a unit cell of ZSM-5, but additional molecules can sorb only by increasing the partial pressure of p-xylene above a threshold of PIPo= 0.03. Above this threshold a phase transition occurs resulting in a packing arrangement which contains about seven molecules per unit cell. This phase transition possibly indicates the onset of molecule-molecule interactions.] We have combined theoretical calculations and variable-temperature MAS carbon N M R to study the packing arrangements and mobility of p-xylene in ZSM-5. Previous to our work, two N M R studies of p-xylene in ZSM-5 have appeared which were attempts to determine the mobility of the hydrocarbon in the zeolite. In the first, Nagy2 interpreted static carbon cross polarization powder spectra to imply rapid rotational and translational motion under conditions where the packing was about 70% maximum at 35 O C . Rotation about the long molecular axis was rapid enough to average the 3.6-kHz (1) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. J . Phys. Chem. 1981, 85, 2238-2243. (2) Nagy, J. B.; Derouane, E. G.; Resing, H. A,; Miller, G. R. J . Phys. Chem. 1983, 87, 833-837.

0022-3654/88/2092-5165$01.50/0

chemical shift anisotropy to a 1.6-kHz axially symmetric pattern. Translational reorientation of the long axis between perpendicular sites in the straight and sinusoidal channels was sufficient to attain nearly isotropic averaging of this 1.6-kHz interaction. Eckman and Vega3 used N M R of the ring deuterons of p-xylene-dlo at about 60% maximum packing to determine that rotation about the long molecular axis was insufficiently rapid below room temperature to reduce a quadrupolar interaction of 126 kHz, but at 100 OC produced narrowing to 17 kHz. Samples containing from 20 to 100% maximum packing ofp-xylene-d6 (methyl groups deuteriated) showed insufficient reorientation (nutation) of the long axis to average the 36-kHz methyl quadrupole interaction over the temperature range -130 to +150 "C. Mentzen and Vigne-Maeder4 located p-xylene molecules in a pentad-type zeolite at low loadings using X-ray diffraction. Their data indicate that at low loadings p-xylene molecules lie in the channel intersections with methyl groups parallel to the straight channel. Only a few molecular modelling studies based on van der Waals interactions of hydrocarbons in zeolites have been (3) Eckman, R.; Vega, A. J. J . Am. Chem. SOC.1983, 105,4841-4842; J . Phys. Chem. 1986, 90,4679-4683. (4) Mentzen, B. F.; Vigne-Maeder, F. Mater. Res. Bull. 1987, 22, 309-321. ( 5 ) Ramdas, S.; Thomas, .I. M.; Betteridge, P. W.; Cheetham, A. K.; D a w s , E. K. Angew. Chem., Int. Ed. Engl. 1984, 23, 611-679.

0 1988 American Chemical Society

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The Journal of Physical Chemistry, Vol. 92, No. 18, 1988

Reischman et al.

Figure 1. Stereographicviews of the high-loading packing arrangement of p-xylene in ZSM-5. (Top) View down [OOl], straight channels run vertical and sinusoidal channels run horizontal. (Bottom) View down the straight channels [OlO]; sinusoidal channels run left and right.

In general, these studies considered the interactions of a hydrocarbon molecule at infinite dilution in a zeolite channel system. Higher loadings have not been considered. In our study, theoretical calculations reveal the most probable sorption sites as p-xylene molecules pack into the pores of ZSM-5 at both low and high loadings. Our VT-MAS observations taken with the earlier results allow reasonable limits to be placed on long molecular axis reorientation. Our observation of discrete species allows limits to be placed on the rate of long axis reorientation by translation between sites in the straight and sinusoidal channels, at least for fully loaded samples.

Experimental Section Energy Minimization Program, PCK6. Williams and Starr’ showed that crystal packing of various hydrocarbons can be theoretically determined by calculating intermolecular nonbonded interactions. To perform these calculations, Williamsg developed an energy minimization program, PCK6. P C K ~does a pairwise summation of intermolecular nonbonded energies using the equation

where rjk is a nonbonded interatomic distance, A,@is the coefficient of the London dispersion attraction term between atoms of type a and b, B , and C, characterize the short-range repulsive energy, and qa is the electrostatic charge on the atom of type cy. Subscript j runs over the atoms of the molecule and k runs over all the atoms in different molecules. The program minimizes E by a “steepest descent” procedure. The adjustable parameters are three translational parameters specifying the coordinates of the center of the molecule and three rotational parameters specifying the angular orientation of the molecule. The program obtains the derivatives of E with respect to a particular parameter by increasing that parameter and reevaluating E; the derivative is the difference in E divided by the increment of the parameter. The gradient of E is thus obtained, and shifts are made along the gradient to minimize E. Williams et al. have derived the A, B, and C potential energy parameters for elements H, C, N, 0, F, and Cl.831“-13 In addition (6) Kiselev, A . V.; Lopatkin, A. A,; Shulga, A. A. Zeolites 1985, 5 , 261-267. ( 7 ) Derouane, E. G.; Andre, J.; Lucas, A. A. Chem. Phys. Lett. 1987,137, 336-340. (8) Williams, D. E.; Starr, T. L. Comput. Chem. 1977, 1 , 173-177. (9) Williams, D. E. Acta Crystallogr.,Sect. A 1972, 28, 629-635; 1969, 25, 464-470

TABLE I: Potential Energy Parameters Used in

atomic interaction H-H H-C H-0 H-Si

c-c c-0 C-Si

0-0

0-Si Si-Si

PCK6

potential energy coeff A. kJ.A6

B, kJ

c. A-’

136 573 387 903 2414 1623 3806 1103 2573 6000

11 677 65 485 57 943 53 268 367 250 324 949 298 734 287 520 264 324 243 000

3.74 3.67 3.85 3.28 3.60 3.78 3.21 3.96 3.39 2.82

to these, Williams also derived a tentative set of potential energy parameters for Si14which allow the nonbonded interactions between hydrocarbons and high silica zeolites to be calculated. Theoretical Calculations on p-Xylene in ZSM-5. We obtained PCK6 from the Quantum Chemistry Program Exchange (Indiana University, Bloomington, IN) for use in our study of p-xylene in ZSM-5. In our calculations we used Williams’ parameters for H, C, 0, and Si. We further modified the A parameter for Si so that the calculated global minimum for p-xylene in high silica ZSM-5 was reasonably close to its empirically determined isosteric heat of adsorption, -76 kJ/mol.15 The repulsion parameters for Si were not changed because Si is “buried” in the zeolite structure and this term should have little effect on the total energy. The potential energy parameters used in all calculations are listed in Table I. The ZSM-5 atomic coordinates and cell parameters used in this study were those of Olson et al.‘ The following restrictions were made in all calculations. (1) Calculations were limited to rjk I10 A. van der Waals (vdW) interactions are negligible beyond 10 A. (2) A1 was not considered in any of the calculations because the amount of A1 in ZSM-5 is small. The ZSM-5 sample used to obtain thep-xylene sorption data’ had a Si02/A1203= 226 which is 4 p-xylene molecules per unit cell, we considered four possible arrangements: (1) one molecule in each intersection and one molecule in each straight channel double IO-ring, (2) one molecule in each intersection and one molecule in each sinusoidal channel elbow 10-ring, (3) two molecules in each intersection, and (4) one molecule in each ring system. Each of these arrangements represents a loading limit of eight p-xylene molecules per unit cell, which is just slightly higher than the empirically observed limit of seven molecules. The calculated energies for these four packing arrangements are given in Table 11. The most favored of these high-loading arrangements is one p-xylene molecule in each intersection with its methyls along the straight channel and one p-xylene molecule in each of the sinusoidal elbow rings. The calculated potential energy for this arrangement is -57 kJ/mol which is 25 kJ/mol higher in energy than for a p-xylene loading 5 4 molecules per unit cell. But, the energy for this high-loading arrangement is still significantly large and negative which indicates that it is favorable. Two stereographic views of this high-loading packing arrangement are shown in Figure 1. Zeolite atoms were left out for purposes of clarity. Figure 1 clearly shows the close interactions of the p-xylene molecules along the sinusoidal channel. One methyl group of the molecule in the elbow ring has close contact with the ring hydrogens of an intersection molecule and the elbow ring system of the zeolite, while the other methyl group has close contact with the ring carbons of a second intersection molecule. For this packing arrangement the minimum H-H, H-C, and H-0 interactions are 2.3,2.7, and 2.2 A, respectively. Atomic coordinates of the p-xylene molecules shown in Figure 1 are listed in Table 111. Molecules in the intersections are labeled “I”, and those in the elbow rings are labeled “R”. The calculated energy (-57 kJ/mol) for the most favored high-loading arrangement suggests that hydrocarbon-hydrocarbon interactions do not contribute to the energy of the arrangement, since this energy is essentially the average of the energies for p-xylene in the intersections (-82 kJ/mol) and for p-xylene in the sinusoidal channel elbow rings (-30 kJ/mol). However, recalculating the energy for this arrangement by including a Coulombic term for only the p-xylene molecules indicates that hydrocarbon-hydrocarbon interactions do lower the total energy by several kilojoules per mole. Recalculating low-loading arrangements in a similar manner result in little change in the total energy. These results indicate that the phase transition for p-xylene above PIPo = 0.03 may indeed be the result of hydrocarbon-hydrocarbon Coulombic interactions. V T MAS Carbon N M R . CP-MAS carbon spectra show that p-xylene motion is essentially frozen even at 20 OC. The serendipitous splitting of the methyl resonance into three peaks places quite a low limit on translational motion. Figures 2 and 3 show both the aliphatic and aromatic carbons of the p-xylene at 20 and -14 “C, respectively. Splitting of the methyl carbons into three distinct peaks (22.0, 21.1, and 20.0 ppm) is just visible in Figure 2 and very distinct in Figure 3. Evidently a slow exchange of sites ( k < 5 Hz)I8 begins at 20 “C which is entirely frozen out at -14 ( 1 8) Sandstrom, J. Dynamic N M R Spectroscopy; Academic: New York, 1982; p 86. A complete band-shape analysis could yield the rate constant and indicate whether two- or three-site exchange is involved but was not justified by the data since the change in peak positions between the extreme temperatures was on the order of the data point resolution.

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The Journal of Physical Chemistry, Vol. 92, No. 18, 1988

C"3

-iw.oo

13n.00

1m.m

FW

30.00

20.00

m

Figure 2. CP-MAS carbon N M R spectrum of p-xylene in ZSM-5 at 20 OC: (left) aromatic carbons; (right) methyls.

n 7 - r iT T

Ir

,LI[I.uu

I

r

I 1 1 I,I

1

I3U O b

rT-n-

m

y p " 7 " 1 %.on

L'o.00

m

Figure 3. CP-MAS carbon N M R spectrum of p-xylene In ZSM-5 at -14 OC. (left) aromatic carbons: (right) methyls.

OC. Lowering the temperature further results in no more changes in the spectra, even at -150 OC. The different methyls are present in ratio 1:1.9:1.1 (1:2:1). The quaternary aromatic carbons share a symmetry axis with the methyls, but do not project as far. They are, thus, somewhat less sensitive to changes in environment. Only two peaks are seen (136.5 and 135.4 ppm), but their 3.4:l (3:l) ratio is consistent with the number of environments sampled by the methyls. The methine aromatic carbons show at least four peaks, indicating either that opposite sides (carbons 2 and 3 versus 5 and 6) or opposite ends (carbons 2 and 6 versus 3 and 5) are different. The dipolar dephasing e ~ p e r i m e n t ' ~puts ~ * ~a lower limit on the rate of rotational reorientation about the long axis. High-power decoupling is ordinarily used to eliminate the effect of proton dipolar splitting, but in the dipolar dephasing experiment the decoupler is switched off for a short time (40-200 ~ s before ) data acquisition. Resonances due to carbons which have directly attached protons and which are not undergoing rapid motion are dramatically broadened and disappear from the spectrum. For

(19) Opella, S.J.; Frey, M. H. J . Am. Chem. SOC.1979,101,5854-5856. (20) Alemany, L. B.: Grant, D. M.; Alger, T. D.; Pugmire, R. J. J . Am. Chem. SOC.1983, 105, 6697-6704 and references therein.

Reischman et al. TABLE III: Atomic Coordinates for Eight p-Xylene Molecules Which Completely Fill One Unit Cell of ZSM-5 sinusoidal channel 2 sinusoidal channel 1 X Y Z X Y Z atom 14.888 1.683 8.392 -3.426 Cl-R -6.605 4.928 2.946 15.222 9.659 -3.922 C2-R -6.109 5.248 3.163 15.276 9.877 -5.301 C3-R -4.730 5.297 2.117 14.996 8.830 -6.184 C4-R -3.847 5.026 14.662 0.854 7.563 -5.688 C5-R -4.343 4.706 14.608 0.637 7.345 -4.309 C6-R -5.722 4.657 14.830 1.450 S.158 -1.945 -8.086 4.876 C7-R 15.054 2.350 9.064 -7.665 -2.366 5.078 C8-R 3.714 15.428 H2-R -6.759 5.448 10.429 -3.272 4.091 15.521 H3-R -4.366 5.532 10.808 -5.665 14.456 0.086 6.793 -6.338 H5-R -3.693 4.506 14.363 -0.291 6.414 -3.945 H6-R -6.086 4.422 1.660 13.805 8.358 -1.576 -8.457 3.850 H7a-R 0.393 15.091 7.104 -1.726 -8.304 5.149 H7b-R 2.122 15.552 8.838 -1.435 H ~ c - R -8.594 5.592 2.140 16.079 8.864 -8.034 -1.995 6.104 H8a-R 3.407 14.793 H8b-R -2.148 4.805 10.118 -7.884 1.678 14.332 8.384 -8.175 H ~ c - R -1.858 4.362 6.373 6.978 -0.304 16.389 0.264 6.352 Cl-I 7.470 0.238 15.715 8.078 0.831 5.702 c2-I 7.504 0.233 14.318 8.1 17 0.878 4.307 C3-I 6.441 7.056 -0.3 14 13.595 C4-I 0.360 3.560 5.344 5.956 -0.856 14.269 -0.207 4.210 c5-I 5.310 15.666 5.917 -0.851 -0.254 5.605 C6-I 6.336 17.888 6.937 -0.299 0.213 7.850 C7-I 6.478 7.097 -0.3 19 12.096 0.41 1 2.062 C8-I 0.641 16.246 8.251 8.858 1.211 6.251 H2-I 0.632 13.823 8.310 8.925 1.295 3.829 H3-I 4.563 5.176 -1.259 13.738 -0.587 3.661 H5-I 4.504 16.161 5.109 -1.250 -0.671 6.083 H6-I 6.798 18.274 7.396 -1.231 -0.734 8.202 H7a-I 5.282 18.233 5.882 -0.239 H7b-I 0.261 8.194 6.901 0.578 18.270 7.501 H~c-I 1.073 8.267 6.016 0.613 11.710 6.638 H8a-I 1.358 1.710 7.532 11.751 8.152 -0.379 H8b-I 0.363 1.718 5.913 11.714 1.645 6.533 -1.196 H ~ c - I -0.449 5.027 6.609 14.888 3.408 4.933 11.737 CI-R 6.073 5.726 14.608 4.289 4.641 12.781 C2-R 5.856 4.347 14.662 5.669 4.698 12.567 C3-R 4.593 3.851 14.996 6.166 5.047 11.309 C4-R 3.547 4.734 15.276 5.285 5.339 10.265 C5-R 3.764 6.113 15.222 3.905 5.282 10.479 C6-R 5.260 8.090 14.831 1.927 4.873 1 1.968 C7-R 4.360 2.370 15.053 7.647 5.107 11.078 C8-R 7.001 6.090 14.363 3.923 4.385 13.705 H2-R 6.624 3.698 14.456 6.3 17 4.483 13.333 H3-R 9.341 2.619 4.370 15.521 5.651 5.595 H5-R 2.996 9.7 13 6.762 15.428 3.257 5.497 H6-R H7a-R 1.558 3.850 11.744 8.459 13.805 5.050 4.588 8.600 15.552 H7b-R 1.418 5.602 11.302 8.309 15.092 6.317 H~c-R 1.705 5.121 13.027 4.570 2.001 16.079 H8a-R 8.016 6.130 11.302 1.860 14.332 5.032 H8b-R 8.156 4.378 11.744 3.303 2.151 14.792 H~c-R 7.869 4.859 10.019 9.731 16.391 0.337 C1-I 10.338 6.429 13.081 1.400 9.184 15.668 C2-I 10.881 5.706 12.016 9.179 14.271 1.366 C3-I 10.884 4.310 12.048 9.721 13.597 0.269 C4-I 10.342 3.635 13.145 C5-I 9.799 4.358 14.210 10.268 14.320 -0.794 C6-I 9.796 5.754 14.178 10.273 15.717 -0.760 0.374 9.736 17.890 C7-I 10.335 7.928 13.046 9.716 12.098 0.232 C8-I 10.345 2.136 13.180 2.206 8.785 16.163 H2-I 11.279 6.202 11.209 2.147 8.776 13.740 H3-I 11.283 3.779 11.265 H5-I 9.401 3.862 15.017 10.667 13.825 -1.600 H6-I 9.397 6.285 14.961 10.676 16.248 -1.541 H7a-I 11.269 8.312 13.507 8.804 18.276 -0.088 9.460 8.311 13.614 10.613 18.272 -0.191 H7b-I 1.428 9.796 18.235 H~c-I 10.273 8.275 11.993 H8a-I 9.411 1.752 12.719 10.648 11.712 0.694 0.797 8.839 11.716 H8b-I 11.220 1.753 12.612 9.656 11.753 -0.822 H ~ c - I 10.407 1.789 14.233

a given molecule with fixed carbon-proton distances, the rate at which the signal disappears during this dephasing period is a

The Journal of Physical Chemistry, Vol. 92, No. 18, 1988 5169

p-Xylene Sorption into ZSM-5 C-1,4

C-2,3,5,6

500,

C-l,4

I

I

1

I

1

V

1

26

28

I

3

I

I

1

32

34

36

1I-r

=7v lS0.Oii

I3li.bli

PPpl

I'iO.00

m

taoo

Figure 4. CP-MAS carbon N M R spectrum showing the aromatic carbons of p-xylene in ZSM-5: (left) normal spectrum; (right) spectrum obtained with a 41-psdephasing delay.

TABLE I V Rotation about Long Axis jump loading as approx freq. kHz temp, OC % of max