Activation of Molecules in Confined Spaces: An Approach to Zeolite

Departamento de Quı´mica Fı´sica, UniVersidad de Valencia, Doctor Moliner ... Ab initio calculations showing frontier molecular orbital energy...
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J. Phys. Chem. B 1997, 101, 4575-4582

4575

Activation of Molecules in Confined Spaces: An Approach to Zeolite-Guest Supramolecular Systems Avelino Corma,* Hermenegildo Garcı´a, German Sastre, and Pedro M. Viruela† Instituto de Tecnologı´a Quı´mica UPV-CSIC, UniVersidad Politecnica de Valencia, AVenida de los Naranjos s/n, 46071 Valencia, Spain, and Departamento de Quı´mica Fı´sica, UniVersidad de Valencia, Doctor Moliner 50, 46100 Burjasot, Valencia, Spain ReceiVed: July 26, 1996; In Final Form: March 21, 1997X

Ab initio calculations showing frontier molecular orbital energy modifications of a guest molecule when located inside a microporous zeolitic cavity are presented. Micropores of zeolites Beta, ZSM-12, and ZSM-5 have been modeled using all silica clusters from which it is found that the highest occupied molecular orbital (HOMO) energy of toluene increases when going from the gas phase state to restricted microporous environments. The influence of the zeolite cavity size and its chemical composition on the toluene HOMO energy are discussed. Regarding the size of the confining environment, it is found that the smaller the zeolite cavity the higher is the rise in toluene HOMO energy. The effect of the chemical composition on the toluene HOMO energy was tested in the ZSM-5 zeolite by varying the Al content. The results obtained showed that the frontier orbital energy increases upon decreasing the Al content of the cluster. As a consequence of the confinement effect, toluene reactivity in zeolite catalyzed reactions is expected to change toward a more covalent behavior in which electronic transference from the toluene molecule to an electronic acceptor will be more favored than in gas phase reactions.

1. Introduction Zeolites are among the most important heterogeneous catalysts from an economical and industrial point of view.1-3 Many petrochemical processes, including cracking, selective toluene alkylation, and xylene isomerization, take advantage of the high activity of the protonic form of these microporous aluminosilicates.4 Furthermore, the application of zeolites to the production of fine chemicals is growing rapidly,5,6 and their use as selective catalysts for general organic reactions at the laboratory scale is currently an active area of research.7-10 Over the last three decades, significant advances have been made concerning: (i) the synthesis of novel aluminosilicates including the isomorphous framework substitution of silicon or aluminum by other heteroatoms and the production of new extra large pore size materials;11,12 (ii) the tailoring of zeolite physicochemical parameters by postsynthesis solid state treatments to enable them as host structures to better suit the specific requirements of each process;13 and (iii) the structural characterization of these solids by means of recently available instrumental techniques such as solid state NMR, X-ray photoelectron spectroscopy (XPS), and both neutron- and synchroton-based diffraction.14 However, despite the considerable volume of information obtained recently and the central role presently played by zeolites in catalysis, there are still many facets of their behavior that are poorly understood. Firstly, regarding the acidic properties, it is found that the intrinsic strength of the sites depends not only on the next neighbor Al coordination sphere (short range effect) but also on the crystalline structure of the zeolite (long range effect). The reasons why, even with identical physicochemical parameters, medium pore ZSM-5 possesses stronger acid sites than large pore zeolites are still not fully understood. Furthermore, no differences in the chemical * To whom correspondence should be addressed. Tel: 34-6-3877800. Fax: 34-6-3877809. E-mail: [email protected]. † Departamento de Qui´mica Fi´sica, Universidad de Valencia. X Abstract published in AdVance ACS Abstracts, May 1, 1997.

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composition of the sites can be called upon to explain the higher acid strength of microporous zeolites compared to amorphous silica-alumina or novel mesoporous aluminosilicates such as MCM-41.15 Only minor refinements of the actual paradigm, such as of T-O-T bond angle distortion, enhanced substrate concentration within the pores, etc. have been proposed to account for the higher intrinsic acidity of bridged Si-(OH)Al hydroxyl groups present in the closed reaction cavity of medium pore zeolites. The high activity of acid zeolites, which in many respects can act as superacidic solids cracking alkanes and producing the skeletal isomerization of other hydrocarbons, contrasts with some ab initio quantum chemistry cluster calculations predicting a high deprotonation energy for these bridging OH groups.16,17 Acid zeolites can also in some cases act as oxidizing materials, whereby in certain reactions radical cations of the guests are generated.10,18-21 This phenomenon appears to be general but is especially notorious when a tight fit of the guest within the internal voids of the zeolite occurs. No convincing explanation for this dual behavior of acid zeolites has yet been given. The reactant molecules must experience the influence of the strong electric fields existing in the cavities which may produce an induced polarization of the guest.22 Detailed maps showing electrostatic potentials have been reported for some zeolites using cluster23 and periodic Hartree-Fock models.24 Besides the influence of the electrostatic potential on the energy levels of the confined molecules, there is another factor of quantum mechanical nature, called electronic confinement,25,26 that may contribute to activate the molecules in the pores and even to change, in some cases, the reaction mechanism one may expect when the reaction occurs in a nonconfined environment. The basic idea regarding electronic confinement is that the orbitals of the molecule inside the zeolite cage are not extended over all the space, as they are in gas phase, but instead within the limits of the zeolite cage. The effect is larger when the cage size becomes similar to the dimensions of the confined molecule. © 1997 American Chemical Society

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Figure 1. Geometry of the different all-silica clusters (terminated by hydrogen atoms) used to simulate zeolites: (a) Beta-zeolite (76 atoms, 20 Si) in the [011] and [100] directions, (b) ZSM-12 zeolite (92 atoms, 24 Si) in the [010] and [100] directions, and (c) ZSM-5 zeolite (84 atoms, 22 Si) in the [010] and [100] directions.

The electronic confinement produces an increase of all the molecular orbital energies, this effect being particularly important for those orbitals which are more diffuse. For the ethylene and aromatic molecules,25 the HOMO seems to be more sensitive to this effect than the antibonding lowest unoccupied molecular orbital (LUMO), and thus a decrease in the HOMOLUMO band gap is expected. In the present work, we have performed ab initio quantum mechanical calculations on toluene confined in the zeolite cavities of Beta, ZSM-12, and ZSM-5. The purpose of this work is to show that inclusion of toluene inside a closed reaction cavity leads to an important increase in the toluene HOMO energy. The consequences of this effect in terms of the reactivity are also discussed. 2. Theoretical Model Most of the theoretical work undertaken to date has focused on the modeling of zeolite active centers and their interaction with substrates. While additional adjustments of this model can be introduced by using larger clusters, not many different conclusions are expected by simply increasing the number of atoms far away from the site. However, in order to study the

effect of confinement, it is necessary to simulate a zeolitic cage surrounding the guest molecule, and therefore, a larger portion of the lattice is required. Depending on each particular zeolite structure, clusters formed by different arrays of atoms are required to define a lattice fragment around the guest molecule. Two contributions are expected to influence the molecular orbitals of the guest molecule upon confinement, the shape of the cavity and its chemical composition. First, calculations on pure silica cavities of different zeolites were carried out to test the effect of the cavity size. Second, calculations on ZSM-5 varying the Al content were performed to test the effect of the chemical composition. Clusters of all-silica zeolites Beta, ZSM-12, and ZSM-5 with 76, 92, and 84 atoms, respectively, were used for the simulations (Figure 1). The silicon atoms were saturated with hydrogen atoms. The Si-H distance of the terminal hydrogens was set to 1.4 Å. The ab initio Hartree-Fock quantum mechanical calculations of the toluene-zeolite assemblies were carried out with the GAUSSIAN94 package,27 using the 3-21G standard basis set.28 The geometry used for toluene was that of the gas phase optimized at the 3-21G level. The geometry of the zeolite cavities has been kept fixed in the ab initio calculations, but in

Zeolite-Guest Supramolecular Systems

J. Phys. Chem. B, Vol. 101, No. 23, 1997 4577

TABLE 1: Force Field and Experimentala Unit Cell Parameters Calculated for Different Zeolites a, Å

b, Å

force field exptl.

12.4644 12.6614

12.4644 12.6614

force field exptl force field exptl a

c, Å

TABLE 2: Force Field Unit Cell Parameters and Volume Calculated for the ZSM-5 Zeolite with Different Al Content

a, deg

b, deg

g, deg

Beta 26.2231 26.4061

90.00 90.00

90.00 90.00

90.00 90.00

24.9374 24.8633

ZSM-12 4.9968 24.1556 5.0124 24.3275

90.00 90.00

107.37 107.72

90.00 90.00

19.9857 20.1072

ZSM-5 19.9034 13.2911 19.8792 13.3691

90.80 90.67

90.00 90.00

90.00 90.00

ZSM-5 (0Al) ZSM-5 (1Al) ZSM-5 (2Al) ZSM-5 (3Al) ZSM-5 (4Al)

a, Å

b, Å

c, Å

19.986 20.069 20.040 20.044 20.013

19.903 19.784 19.797 19.833 19.901

13.291 13.357 13.431 13.424 13.420

a, deg b, deg g, deg 90.80 90.15 90.09 90.18 89.92

90.00 90.04 90.18 90.09 89.71

90.00 89.92 89.88 89.86 89.92

V, Å3 5278.63 5303.36 5328.13 5336.36 5344.56

From ref 33.

Figure 2. ZSM-5 zeolite cluster with different aluminum content: 1 Al (T1 ) Al), 2 Al (T1, T2 ) Al), 3 Al (T1, T2, T3 ) Al), 4 Al (T1, T2, T3, T4 ) Al). The total number of Si + Al atoms in the cluster is 22. These clusters have been used to calculate the effect of the chemical composition of the cluster on the HOMO(tol) energy. The position of the toluene in the cavity is also shown.

order to have a reliable geometry for these clusters (Figure 1), a previous optimization of their geometry has been obtained by using a periodic force field approach. Interatomic force field methods give accurate results in determining lattice parameters and interatomic angles and distances, the GULP29 code being used for this purpose. The interatomic potentials used to model the interactions between the atoms in the structure include a Coulombic interaction, short range pair potentials of the Buckingham type, and a three-body term. The shell model is used to simulate the polarizability of the oxygen atoms.30 In the case of the O-H Brønsted sites an additional Morse potential was included. The short range interactions were considered with a cutoff distance of 10 Å, and the long range Coulombic interactions were calculated by means of the Ewald technique.31 These potentials32 have been successfully used to simulate the structure of zeolites and zeotypes. The optimized cell parameters of the all-silica Beta, ZSM12, and ZSM-5 are given in Table 1 where they are compared to experimental results from which it can be easily seen that these lattice parameters are similar to the XRD results for these zeolites obtained from standard synthesis conditions.33 The clusters modeling the different cavities (Figure 1) were obtained from the force field optimized geometries of each crystal. The same procedure was used to obtain force field optimized geometries for ZSM-5 with different Al content. In this case, the number of Brønsted acid centers in the ZSM-5 unit cells was selected in order to produce the desired number of acid sites in the cavity. The location of the acid sites is indicated in

Figure 3. Distances between the center of the aromatic ring and the different oxygen atoms of the zeolite cluster. These distances give an approximate account of the cavity size of the zeolites used for the calculations. International Zeolite Association code for each zeolite indicated in the figure: *BEA (Beta, Polymorph A), MTW (ZSM12), and MFI (ZSM-5).

Figure 2 and the corresponding lattice parameters obtained after the force field optimization are shown in the Table 2. An increase in the cell volume is observed as the aluminum content increases which is easily understandable in terms of the larger Al-O (average value) distance with respect to that of Si-O (average value). In addition we have also performed some force field optimizations of the zeolite + toluene system. The results indicate that the geometry of the zeolite (without toluene) is practically the same as that of the optimized when the toluene

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TABLE 3: Cartesian Coordinates (x, y, z, in Angstroms) of the Zeolite Clusters (All-Silica) and the Toluene Molecule Located in the Center of the Cavity

Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si O O O O O O O O O O O O O O O O O O O O O O O O O O O O H H H H H H H H H H H H H H H H H H H H H H H

x

BETA y

z

x

ZSM-12 y

z

5.820 9.072 5.815 12.082 5.819 6.690 5.820 9.072 5.815 12.082 5.819 6.690 5.285 15.282 15.274 14.469 15.285 15.282

8.393 10.040 9.073 10.049 12.083 10.851 16.930 15.282 16.250 15.274 13.240 14.472 8.395 9.072 12.082 10.854 16.928 16.251

31.520 27.950 34.560 27.870 34.470 29.720 34.480 38.050 31.450 38.140 31.540 36.290 31.520 34.550 34.470 29.700 34.490 31.450

6.127 6.480 10.578 8.180 6.395 6.172 6.127 6.480 10.578 8.180 6.395 6.494 6.494 14.974 14.629 14.700 14.922 13.021 14.974 14.629 14.700 14.598 13.021 14.598

8.425 9.691 10.623 10.746 10.580 12.661 16.897 15.632 14.700 14.577 14.743 13.025 12.298 8.426 9.694 10.578 12.661 10.725 16.897 15.628 14.745 13.021 14.598 12.301

33.110 30.820 28.010 29.100 34.610 33.000 32.900 35.190 38.000 36.910 31.400 35.600 30.410 33.100 30.820 34.610 33.000 29.000 32.900 35.190 31.400 35.600 37.010 30.410

4.416 6.376 8.503 9.098 6.434 4.411 12.591 12.068 4.410 5.736 4.416 6.376 8.503 9.098 6.434 4.411 12.069 12.592 4.410 5.736 14.730 16.690 16.685

8.403 7.215 10.312 8.637 8.291 9.098 10.358 8.640 12.069 10.693 16.920 18.108 15.011 16.685 17.031 16.224 16.683 14.965 13.254 14.630 7.219 8.404 9.099

31.310 30.960 26.680 28.170 35.570 34.770 26.580 28.050 34.650 28.680 34.690 35.050 39.330 37.840 30.440 31.230 37.960 39.430 31.360 37.330 30.950 31.310 34.770

17.222 7.932 20.085 14.912 15.190 19.205 22.480 11.996 12.674 21.687 16.463 17.173 19.326 4.153 14.430 18.446 21.031 13.196 11.915 20.928 20.272 12.437 18.561 16.503 20.165 14.390 21.235 13.126 12.684 21.799 15.980 18.878 17.801 15.744 16.210 17.866 19.406 13.631 14.160 20.153 20.476 12.367 11.925 21.040 15.221 18.118 20.199 13.596 13.400 19.394 17.280 17.048 18.489 17.829 20.815 20.585 14.453 14.494 14.512 19.816 23.366 23.216 10.818 11.644 11.815 12.212 22.459 22.176 16.514 17.042 19.746 13.779 13.800

13.644 14.298 12.733 14.195 11.461 11.544 10.931 12.600 11.864 10.924 8.706 9.360 7.795 9.257 6.523 6.606 10.114 11.802 6.926 5.986 5.177 6.865 13.187 14.168 11.279 12.769 10.664 12.125 12.395 11.187 12.889 13.128 8.250 9.230 10.245 10.687 6.341 7.831 11.439 11.035 5.726 7.187 7.458 6.250 7.951 8.190 8.792 10.314 6.501 6.097 13.418 15.026 15.509 14.334 13.597 12.818 15.088 14.637 11.239 11.330 9.822 12.068 11.818 13.958 10.737 12.890 11.890 9.623 8.453 9.501 7.772 9.579 6.338

-2.300 6.920 -1.670 6.380 -1.720 6.740 2.150 2.830 -0.170 5.210 -1.890 7.330 -1.260 6.780 -1.310 7.150 -0.360 5.470 0.230 5.620 0.040 5.880 -1.670 6.340 -1.160 6.100 1.230 3.810 1.320 3.660 -1.750 6.520 -1.270 6.750 -1.610 6.810 -0.750 6.510 -0.590 5.660 1.640 4.220 1.720 4.060 -1.340 6.930 -0.380 5.730 -0.180 6.070 -3.700 -2.020 6.430 8.330 -0.810 -3.000 5.370 7.660 -2.940 8.010 2.170 1.720 2.960 3.050 -0.260 -1.030 5.910 5.510 -3.290 8.730 -2.620 8.120 -2.570

x

ZSM-5 y

z

18.464 18.433 16.159 16.124 15.594 15.564 17.595 7.583 18.601 18.582 16.263 16.250 21.412 21.448 23.719 23.750 21.544 21.556 24.490 24.502

8.828 1.127 9.399 0.553 8.730 1.219 6.508 3.448 7.353 2.592 6.500 3.447 .544 9.410 1.175 8.777 3.430 6.515 3.439 6.51

11.180 11.180 9.220 9.220 6.270 6.270 12.980 12.980 2.430 2.430 4.270 4.270 10.890 10.890 8.990 8.990 2.310 2.310 7.100 7.100

17.467 17.436 6.181 16.151 17.668 17.654 18.333 18.313 16.105 16.086 19.913 19.944 16.230 17.830 22.287 22.319 22.513 22.528 23.829 23.844 23.769 23.788 20.089 20.074 21.619 24.241

8.889 1.064 8.782 1.169 6.808 3.139 7.413 2.542 7.380 2.568 1.001 8.953 4.974 4.978 1.078 8.875 3.105 6.842 3.088 6.861 2.584 7.367 6.907 3.039 4.972 4.975

9.960 9.960 7.740 7.740 3.600 3.600 11.900 11.900 5.600 5.600 10.600 10.600 4.730 12.560 9.680 9.680 3.510 3.510 5.710 5.710 8.240 8.240 2.780 2.780 1.950 7.460

18.271 18.240 14.979 16.150 16.111 14.943 14.175 16.081 16.048 14.145 8.151 16.193 16.179 18.138 18.272 18.596 18.579 18.255 15.236 15.230 21.945 21.562 21.601

9.920 .035 8.923 10.812 -0.860 1.035 8.689 9.804 0.146 1.275 6.683 6.736 3.235 3.272 6.770 8.772 1.173 3.176 6.741 3.208 1.151 -0.868 10.822

12.070 12.070 9.850 9.080 9.080 9.840 6.280 5.480 5.470 6.270 14.270 12.980 12.980 14.270 1.180 2.370 2.370 1.180 3.320 3.310 12.060 10.900 10.890

Zeolite-Guest Supramolecular Systems

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TABLE 3 (Continued)

H H H H H H H H H H H H H H H H H C C C C C C C H H H H H H H H

x

BETA y

z

x

ZSM-12 y

z

14.662 16.683 15.488 14.730 16.690 16.685 14.662 16.683 15.488

8.288 12.068 10.713 18.104 16.919 16.224 17.035 13.254 14.610

35.560 34.650 28.720 35.060 34.690 31.230 30.440 31.360 37.280

10.187 10.305 10.720 11.016 10.898 10.483 10.353 10.812 9.867 10.076 11.336 11.126 9.879 9.745 11.343

11.437 10.184 10.092 11.254 12.507 12.599 13.978 9.115 11.508 9.287 11.183 13.404 13.864 14.631 14.415

31.710 32.320 33.650 34.380 33.760 32.430 31.760 34.130 30.680 31.760 35.410 34.320 30.780 32.380 31.630

15.387 19.023 17.220 22.316 11.956 22.607 22.456 10.059 10.884 11.056 11.453 21.699 21.416 19.565 21.571 11.230 12.811 16.803 16.921 17.336 17.632 17.514 17.099 16.969 17.428 16.483 16.693 17.952 17.743 16.495 16.361 17.959

5.488 6.446 5.890 9.906 12.040 4.884 7.131 6.880 9.020 5.799 7.952 6.952 4.685 3.947 5.009 7.218 5.521 9.723 8.469 8.377 9.539 10.793 10.885 12.263 7.401 9.794 7.573 9.468 11.689 12.149 12.917 12.701

-1.130 8.440 7.120 -0.920 6.120 2.580 2.130 3.370 3.460 0.140 -0.630 6.320 5.910 0.120 -0.500 6.540 6.150 1.320 1.930 3.270 3.990 3.380 2.040 1.370 3.740 0.290 1.370 5.020 3.930 0.400 2.000 1.240

x

ZSM-5 y

z

21.983 23.801 24.680 24.704 23.826 21.881 21.899 23.862 24.957 25.882 25.898

8.801 0.086 1.080 8.869 9.871 2.640 7.294 8.776 6.805 3.171 6.763

12.060 8.080 10.030 10.030 8.080 1.180 1.170 4.130 3.490 7.020 7.030

21.146 20.034 18.888 18.855 19.972 21.117 20.054 22.040 18.018 17.962 19.948 21.987 19.297 21.017 19.857

5.302 6.038 5.358 3.977 3.249 3.916 7.555 5.810 5.915 3.468 2.178 3.362 7.943 7.929 7.950

7.220 6.840 6.450 6.430 6.810 7.200 6.850 7.520 6.150 6.130 6.800 7.490 7.530 7.180 5.860

molecule is placed in the middle of the cavities, the differences observed both in the Si-O distances and crystallographic parameters being of the order of 0.01 Å. 3. Results and Discussion 3.1. Effect of the Size of the Zeolite Cage on the Confinement Effect. When studying the toluene-zeolite system, the first aspect to consider is the location of the guest and the part of the lattice that has to be taken as representative of the whole solid. The largest available internal space in each structure was selected as the most convenient cluster. In this way, the fragment of the Beta zeolite presented in Figure 1 corresponds to the intersection of the two perpendicular T-12 channels defining the typical oval shaped cage characteristic of this structure. Likewise, the zeolite ZSM-5 cluster is formed by the intersection of the two T-10 channel systems: the straight and the sinusoidal. The choice for ZSM-12 was more straightforward since, owing to its structure formed by an array of onedimensional T-12 channels, no variations in the geometry of the voids occur throughout the [010] axis of lattice. Previous results regarding confinement effects25,26 predict that the guest stress will be larger as the size of the cage surrounding toluene becomes closer to the dimensions of the organic molecule. The cage size is not an easy parameter to define since their shape is not regular. Nevertheless the zeolites chosen can be listed in decreasing cavity size following the order Beta, ZSM-12, and ZSM-5. Both Beta and ZSM-12 contain T-12 rings, but in the case of Beta, the cavity is formed by the intersection of two T-12 channels which gives the bigger size compared to ZSM-12. Finally, the ZSM-5 cage is formed by the intersection of T-10 channels which are smaller than the ZSM-12 rings. A quantitative measurement which gives an account of the cage size is shown in Figure 3, where histograms showing the number of oxygens at a given distance from the

Figure 4. Toluene HOMO-1 and HOMO energies in the gas phase and upon confinement in different zeolite cages: ab initio HF/3-21G calculations.

cavity center are given. The average values for that distance are 5.348, 5.070, and 5.012 Å in Beta, ZSM-12, and ZSM-5, respectively. The oxygen atoms are more important than the silicon atoms in defining the zeolite cage size because the oxygen atoms project into the void space, effectively hiding the silicon atoms.34 The influence of confinement on toluene has been determined by establishing the distortions on its HOMO by the progressive spatial restrictions caused by this series of structures. In all our calculations, the guest molecule has been placed in the geometrical center of the clusters, keeping the position of the whole array of atoms constant. This is the position where the confinement effect should be smaller.25 The Cartesian coordinates of the atoms of the system are shown in Table 3. Comparison of the binding energy of the HOMO(tol) orbital in the gas phase with the values obtained inside each cluster is provided in Figure 4. From these results it is clear that incorporation of toluene even in a relatively loose reaction cavity like the one defined by the Beta cluster produces a large increase in the HOMO(tol) energy (1.37 eV). The general trend observed in this series is an increase in HOMO(tol) energy with

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Figure 6. Small clusters taken from the different zeolite cavities maintaining the same fixed geometries for the cluster and the toluene as those in the original cavity.

TABLE 4: Toluene HOMO and HOMO-1 Energies upon Interaction with Small Clusters Taken Out of the Corresponding Cavities As Shown in Figure 6a HOMO (eV) HOMO-1 (eV) a

Figure 5. Three-dimensional plots of the toluene HOMO-1 and HOMO in the gas phase and upon confinement in Beta, ZSM-12, and ZSM-5.

decreasing cavity size. Differences in HOMO energies observed between the different cavities are large enough to expect reasonable differences in the chemical behavior of toluene reacting in different zeolites. In other words, if the variation in HOMO(tol) energies influences some parameter related to toluene reactivity, we would expect to see differences in toluene reactivity on zeolites with respect to gas phase. For the same reason different toluene reactivity would be expected on different zeolitic structures. XPS measurements show different protonation energies in different zeolites35 and also that molecular orbital energies change after confinement in different zeolites.36 Quantum chemical calculations of methanol adsorption in zeolites and zeotypes have shown a similar effect.37 The changes in toluene molecular orbital energies have been reported for HOMO and HOMO-1 because they are very close in energy and both of them contribute to explain toluene reactivity. The toluene frontier orbital composition has also been analyzed in order to check whether the energy increase comes from interaction with some of the occupied molecular orbitals of the zeolite, in particular those corresponding to the oxygen lone pair electrons. For this purpose we have depicted three-dimensional plots of the toluene frontier orbitals confined in the different structures. The results are shown in Figure 5, where a comparison with gas phase is also provided. It can be seen that both frontier orbitals have their major components localized in the toluene molecule. In Figure 5 the contribution of the zeolite atoms to the toluene HOMO and HOMO-1 has

gas phase

Beta

ZSM-12

ZSM-5

-8.80 -9.17

-8.69 -8.96

-8.81 -9.08

-8.72 -8.98

The gas phase values are also shown for comparison purposes.

also been calculated but it is too small to be displayed. On the other hand, it is important to note that although some small changes in the composition of these orbitals is observed relative to gas phase, the major HOMO components remain in the para position while those in the HOMO-1 remain in the ortho position (also in the meta, although this position is of less chemical interest here). The relative weight of para and ortho contributions can change from gas phase to confined systems, and this can lead to changes in reactivity, in particular changing the relative amount of toluene alkylation products in the para and ortho positions.38 In the light of these results it could be argued that some confinement effect could also be observed with a smaller cluster without resembling a zeolite cavity. In order to test the validity of this possibility, we have taken a cluster formed by 2 Si atoms out of the cavities simulated, maintaining the geometry of the cluster and keeping the toluene in the same position (Figure 6). We have taken an arbitrary cluster out of each cavity with the only condition of having the toluene molecule as close as possible to the cluster. The results obtained after these calculations are shown in Table 4 where it can be observed that both toluene HOMO and HOMO-1 always have higher energy in the cluster calculation with respect to that in the gas phase case. The differences are less than 0.25 eV which seems to indicate an additive nature of the confinement effect with successive increase in the the number of atoms, the effect being more important when cavities are surrounding the guest molecules. 3.2. Effect of Zeolite Aluminum Content on the Guest Frontier Molecular Orbitals. In order to explore the influence of the Al content in the toluene HOMO energy we have chosen the ZSM-5 cage cluster, and we have consecutively made the replacement Si f Al, H in order to form cages with 1, 2, 3, and 4 Brønsted sites (Figure 2). Aluminum atoms are expected to introduce local effects in the lattice, owing to the presence of charge-balancing countercations and changes in the surface polarity. In particular, the presence of Brønsted sites will produce changes in the electrostatic potential generated inside the ZSM-5 cage which will have an additional effect on the HOMO(tol) energy. Apart from the effect of the cavity size studied in the paragraph above, changes in the electrostatic

Zeolite-Guest Supramolecular Systems

J. Phys. Chem. B, Vol. 101, No. 23, 1997 4581 progress, and they will provide a better understanding of the effect of the zeolite electrostatic potential on the guest molecules. 4. Conclusions

Figure 7. Toluene HOMO-1 and HOMO energies upon confinement inside the ZSM-5 cavity with different aluminum content: ab initio HF/3-21G calculations. The position of the toluene and the chemical substitutions are shown in Figure 2.

potential due to the aluminum atoms will influence the toluene HOMO energy. The direction in which this energy is expected to change can be envisaged as following the lines of argument detailed below. It has been pointed out39 that the electrostatic potential created inside zeolite cages is mainly due to the contribution of the oxygen atoms, the reason for that being that the oxygen atoms project into the void space of the cavity effectively hiding the silicon and aluminum atoms. In a purely siliceous zeolite this negative electrostatic potential will create a given repulsion over the electrons in the toluene molecular orbitals which will tend to increase their energy. If aluminum is introduced into the ZSM-5 cluster, the change O f OH will make the new electrostatic potential less negative due to the proton shielding on the corresponding oxygen atom. The electrostatic repulsion over the electrons in the toluene molecule will be less important and the toluene molecular orbital energies will decrease. In order to address this contribution in the toluene HOMO energy, calculations on the ZSM-5 clusters containing 1, 2, 3, and 4 Brønsted sites were performed. In order to enable direct comparison with the pure silica ZSM-5 cluster, the toluene was kept in the same position as in the all-silica cluster. Once the calculations were done, the toluene Mulliken charge was checked to make sure that no other effects like zeolite-toluene electronic transference is influencing the energy change in the toluene molecular orbitals. The toluene charges in the clusters with 0-4 Al atoms were -0.068, -0.068, -0.100, -0.074, and -0.066 au respectively. These charges are small enough to consider that no neat transference charge is occurring between the two fragments. We have found a remarkable decrease in the HOMO(tol) energy as the number of Al atoms of the cluster increases (Figure 7), and this trend apparently tends to level off for low Si/Al ratios. It follows that there is a decrease in HOMO(tol) energies with increasing Al content. The change in HOMO(tol) energy from the all-silica to the n-Al cluster (n ) 1, 2, 3, 4) is 1.53 eV. This indicates that the chemical effect is as important as the confinement effect. Both effects can nearly cancel each other out in zeolites with low Si/Al ratio, in which case no special features will be expected for the reactivity of toluene or other organic molecules confined in zeolites. On the other hand, the confinement effect will be more important in zeolites with high Si/Al ratio where the increase in the molecular orbital levels is expected to be higher. Again, we have plotted not only the HOMO(tol) energy but also the HOMO-1(tol) energy (Figure 7) due to the importance of both molecular orbitals in reactivity. The effect on both orbitals is the same, decreasing energies as the aluminum content increases. More detailed calculations regarding electrostatic potentials created by zeolites in order to address this contribution in a more sophisticated way are in

Ab initio calculations show that toluene incorporation inside a series of closed reaction cavities emulating zeolite micropores produces a severe modification of its HOMO(tol) energy, depending on the cage size. The increase in HOMO energy is found to be higher as the cavity size decreases, with the overall order being Beta, ZSM-12, ZSM-5. These results show that predictions based on gas phase molecular orbital energy values of the guest may not apply in the interior of microporous solids. The influence of the framework Si/Al ratio of ZSM-5 zeolite on the toluene HOMO has revealed that its energy increases as the Al content in the cluster decreases. Both effects, cage size and Al content, considered together are maximized in zeolites with a high Si/Al ratio in which the rise in the toluene frontier orbital (HOMO and HOMO-1) energy is bigger. The weight of the ortho and para components of the toluene frontier orbitals do not change appreciably with respect to gas phase. Finally it is noted that these results have important consequences in terms of reactivity, particularly for those reactions involving electric charge donation from the guest molecule to the zeolite. These reactions, like proton transfer for example, will be favored in confined systems where the rise in the frontier occupied molecular orbitals will make the electronic transference easier. Acknowledgment. Financial support by the Spanish DGICYT (Project PB93-0370) is gratefully acknowledged. We thank the C4 (Centre de Computacio i Comunicacions de Catalunya) and Centro de Calculo de la Universidad Politecnica de Valencia for the use of their computing facilities. References and Notes (1) Wojciechowski, B. W.; Corma, A. Catalytic Cracking, Catalysts Kinetics and Mechanisms; Marcel Deker: New York, 1984. (2) Thomas, J. M. Angew. Chem., Int. Ed. Engl. 1995, 1995, 913. (3) Thomas, J. M.; Bell, R. G.; Wright, P. A. Bull. Soc. Chim. Fr. 1994, 131, 463. (4) Thomas, J. M. Angew. Chem., Int. Ed. Engl. 1988, 27, 1673. (5) Heterogeneous Catalysis and Fine Chemicals II; Guisnet, M., Barbier, J., Barrault, J., Bouchoule, C., Duprez, D., Pe´rot, G., Montassier, C., Eds.; Elsevier: Amsterdam, 1991; Vol. 59. (6) Heterogeneous Catalysis and Fine Chemicals III; Guisnet, M., Barbier, J., Barrault, J., Bouchoule, C., Duprez, D., Pe´rot, G., Montassier, C., Eds.; Elsevier: Amsterdam, 1993; Vol. 78. (7) Corma, A.; Climent, M. J.; Garcı´a, H.; Primo, J. Appl. Catal. 1990, 59, 333. (8) Climent, M. J.; Corma, A.; Garcı´a, H.; Primo, J. J. Catal. 1991, 130, 138. (9) Armengol, E.; A, C.; Garcia, H.; Primo, J. Appl. Catal. 1995, 126, 391. (10) Cano, M. L.; Corma, A.; Forne´s, V.; Garcı´a, H. J. Phys. Chem. 1995, 99, 4241. (11) Davis, M. E. Acc. Chem. Res. 1993, 26, 111. (12) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Microporous Mater. 1993, 2, 17. (13) Introduction to Zeolite Science and Practice; van Bekkum, H., Flanigen, E. M., Jansen, J. C., Eds.; Elsevier: Amsterdam, 1991. (14) Corma, A. Chem. ReV. 1995, 95, 559. (15) Corma, A.; Forne´s, V.; Navarro, M. T.; Pe´rez-Pariente, J. J. Catal. 1994, 148, 569. (16) Kramer, G. J.; van Santen, R. A. J. Am. Chem. Soc. 1993, 115, 2887. (17) van Santen, R. A.; Kramer, G. J. Chem. ReV. 1995, 95, 637. (18) Ramamurthy, V.; Caspar, J. V.; Corbin, D. R. J. Am. Chem. Soc. 1991, 113, 594. (19) Caspar, J. V.; Ramamurthy, V.; Corbin, D. R. J. Am. Chem. Soc. 1991, 113, 600. (20) Ramamurthy, V.; Eaton, D. F.; Caspar, J. V. Acc. Chem. Res. 1992, 25, 299.

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