Model calculations of electrostatic fields and potentials in faujasite

Silke Megelski, Andreas Lieb, Marc Pauchard, Andreas Drechsler, Stephan Glaus, Christina Debus, Alfred J. Meixner, and Gion Calzaferri. The Journal of...
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J. Phys. Chem. 1985, 89, 2955-2961

2955

Model Calculations of Electrostatic Fields and Potentials in Faujasite Type Zeolites E. Preuss, G. Linden, and M. Peuckert* Institute fur Grenzflachenforschung und Vakuumphysik Kernforschungsanlage Julich, D-5170 Julich, Federal Republic of Germany (Received: July 26, 1984)

Electrostatic fields and potentials inside the pore system of faujasite type zeolites have been calculated. CaZ+-as well as La’+-exchanged zeolites with varying Si:A1 ratios between 1 and 5 were considered. The distributions of Si and AI in the lattice were determined by Monte Carlo statistics considering Loewenstein’s rule as the only restraint. A comparison of fields and potentials on the outer surface of the same zeolites showed that the long-range field effect of the cage structure is minimal relative to the large variations which are introduced by the different possible arrangements of Si, Al, and cations in the zeolite lattice in the proximity of a certain site. Distinct Si-A1 orderings can cause strong acid and base sites.

1. Introduction Zeolites are widely used as catalysts and adsorbents. Their properties with regard to catalytic activity and selectivity and their sorption capacity are mainly determined by the dimensions of the internal pore system (molecular sieve effect).’ Another characteristic which is frequently discussed in connection with the acidity of zeolites and with seemingly unusual reaction paths, e.g. the reaction of NO and NO2 to NO+-N02-, is the electrostatic field e f f e ~ t . ~ Due . ~ to the open zeolite structure having welland cation sites defined and evenly distributed anion sites (A102)(e.g. Ca”, La3+)which are spacially separated, strong electrostatic fields are thought to exist inside the zeolite cages which can polarize adsorbate molecules or create very strong acid sites. In this context, Dempsey et al.“ and Huang et al.’ have performed model calculations on the electrostatic field in the proximity of the cation sites in dehydrated Na+- and Ca2+-exchanged faujasite. In Dempsey’s approach, the distribution of A1 and Si in the zeolite framework was determined by Loewenstein’s rule (each (A102)- unit is tetrahedrally coordinated by four S i 0 2 units, but never by another (A102)- unit) and, as a second restraint, the absence of a dipole field along the main axis of the used building block of two connected sodalite cages (&cages) was required. There is increasing experimental evidence from 29Si nuclear magnetic and neutron diffraction studies” that in faujasite type zeolites Loewenstein’s rule is strictly obeyed but that beyond this restriction long-range ordering of Si and A1 may not exist. In the present study, we further extended the earlier calculations>’ to a three-dimensional description of the electrostatic field and potential inside the supercage (a-cage) of faujasite type zeolites for a wide range of Si:Al ratios. The Si and A1 centers were statistically distributed in accordance with Loewenstein’s rule, but different from Dempsey’s “dipole restriction”, by using a Monte Carlo method. This results in a broad scatter of site distributions for zeolites with a high SkAl ratio, which should more closely model the variety of sites that are found in real Y zeolites. (1) D. W. Breck, “Zeolite Molecular Sieves”, Wiley, New York, 1974, and literature cited therein. (2) J. A. Rabo, Caral. Rev.-Sci. Eng., 23, 293 (1981). (3) J. A. Rabo and M. L. Poutsma, Adu. Chem. Ser., No. 102,284 (1981). (4) E. Dempsey, J . Phys. Chem., 73, 3660 (1969). (5) E. Dempsey, “Molecular Sieves”, Society of Chemical Industry, London, 1968, p 293. (6) P. E. Pickert, J. A. Rabo, E. Dempsey, and V. Schomaker, ‘Proceedings of the 3rd International Congress on Catalysis, Amsterdam, 1964”, North-Holland Publishing Co., Amsterdam, 1965 p 714. (7) Y.-Y. Huang, J. E. Benson, and M. Boudart, I&EC Fundam., 8,346 (1969).

( 8 ) J. Klinowski, S.Ramdas, J. M. Thomas, C. A. Fyfe, and J. S. Hartman, J. Chem. Soc., Faraday Trans. 2,78, 1025 (1982). (9) C. A. Fyfe, J. M. Thomas, J. Klinowski, and G. C. Gobbi, Angew. Chem., 95, 257 (1983). (10) G. D.Stucky and F. G. Dwyer, Eds., ‘Intrazeolite Chemistry”, American Chemical Society, Washington, D.C., 1983, ACS Symp. Ser. No. 218. (11) M. T. Melchior, in ref 10, p 243. (12) A. J. Vega in ref 10, p 217. (13) A. K. Cheetham and M. M. Eddy in ref 10, p 131.

Furthermore, we compared field strength and potential inside the supercage with those on the outer zeolite surface with similar A1 and Si topography, in order to elucidate whether these effects are peculiar to zeolite molecular sieves or whether they fit into a more general discription of oxide surfaces. 2. Model Calculations 2.1. The Faujasite Crystal Lattice. In our calculations dehydrated faujasite crystals of different sizes are used, and we test how the size of the model structure influences the results of the calculations. Figure 1 schematically shows the basic building unit of a faujasite type zeolite structure, which is composed of 10 sodalite cages (&cages) joined by hexagonal prisms, thus forming a large central cavity (a-cage).’ The data basis for the field calculations can be expanded by symmetrically adding more truncated octahedra to the unit depicted in Figure 1. Thus, crystallites of three sizes are created: “small”, “medium”, and “large” with diameters of about 4, 5 , and 7 nm consisting of 10, 30, and 82 sodalite units, respectively. It turns out that the “medium” crystal is already big enough to exclude effects of the size of the model structure on the electrostatic field calculations in the central a-cage, while the “small” model leads to divergent results. Therefore, all computer runs reported in this publication were performed using the ”medium” crystallite. The lines in the model (Figure 1) represent S i U S i or A l U S i bridges with the Si and A1 atoms at the crossing points and in tetrahedral coordination with oxygen. For the calculations a purely ionic structure with 02-,Si4+, and A13+ ions at the respective crystallographic positions is a s s ~ m e d . ~ .The ~ ~ ’distribution ~ of silicon and aluminum within a crystallite leads to a definite result only for a Si:A1 ratio of 1:1, as in zeolite X, when Loewenstein’s rule is accepted as the only restriction. In the case of higher Si:Al ratios (up to 4.7 in this study) several distributions are conceivable and additional restrictions may be introduced into the model in order to define a certain Si-A1 ordering in the crystal lattice. This has been the approach in the work of D e m p ~ e y who , ~ studied faujasites with %:A1 ratios of 1:l and 2:l. In our calculations the reduction of the aluminum ions is performed by Monte Carlo statistics. Starting with a 1:l zeolite X,equal probability for every AI3+ to be substituted by Si4+ is assumed. Thus, for each %:A1 ratio larger than 1 a variety of model crystals with different individual Si-A1 ordering is created. If one substitutes Si4+by A13+ions, excess negative charge from ions is established in the lattice, which is compensated the 02by uni- or multivalent cations like Na+, Ca2+,or La3+. These cations can occupy different sites. Only two kinds of sites will be considered for an ideal dehydrated structure.’J5 Sites I are in the centers of the hexagonal prisms and are always occupied. Sites I1 are about 0.0473 nm above the center of the hexagonal faces of the sodalite units pointing to the center of the a-cage, (14) D. Bonnin and A. P. Legrand, Chem. Phys. Lerr., 30,296 (1975). (15) B. C. Gates, J. R. Katzer, and G. C. A. Schuit, “Chemistry of Catalytic Processes”, McGraw-Hill, New York, 1979, Chapter 1, and literature

cited therein.

0022-365418512089-2955$01.50/0 0 1985 American Chemical Society

2956

The Journal of Physical Chemistry, Vol. 89, No. 13, 1985

,.

Preuss et al. that sites I1 are exposed at the surface and neutrality of the remaining lattice is preserved. Depending on the position of this cut, surfaces of varying roughness can be created. Though these models may be only very primitive representations of a true outer zeolite surface, they should be suitable to show differences between electrostatic fields inside a cage vs. fields extending from a surface. 2.2. Calculation of Electrostatic Field and Potential. The calculations were performed on an IBM 3033 computer. The electrostatic potential at a point j is5J4 uj

Y

Site I

Figure 1. "Small" faujasite crystallite built up by sodalite units and hexagonal prisms. The lines represent Si-0-Si and Si-0-AI bridges; positions of cation sites I and 11, a 3-fold symmetry axis, and the cubic volume (broken lines) used in the model calculations are shown. TABLE I: Number of Cation Sites I1 of Weight 0-3 with 0-3 AI Atoms per Hexagonal Face and Number of Ca*+ and La3+Available To Occupy Sites of Type I1 as a Function of SkAI Ratio for Medium Size Model Crystal weight cation %:AI 0 1 2 3 Ca2+ La3+ 1 .o 120 120 60 1.5 2 13 40 65 84 36 1.71 3 23 39 55 73 29 2.0 8 28 40 44 60 20 2.96 15 45 43 17 31 1 4.0 26 55 28 11 12 33 55 25 7 3 4.7

(Figure 1). These sites I1 lie on 3-fold symmetry axes. On the outer surface of the model crystal semiprisms protrude; the electronic charge of the cations in these site I type prism halves, and also the charges of the six 02-ions at the middle of the prims edges are divided by two. The whole crystal is always neutral. In this investigation only faujasites with Ca2+and La3+ cations are considered. These cations have to be distributed in sites I and 11. With zeolite X (Si:Al = 1:l) all sites are occupied by Ca2+or all sites I and half of the sites I1 are occupied by La3+.l9l5For larger Si:Al ratios it is assumed, in accordance with D e m p ~ e ythat , ~ initially all sites I are occupied; this leads to limiting Si:Al ratios of 5 in the case of Ca2+ and 3 for La3+. The remaining cations are statistically distributed in sites 11. These hexagonal faces of the sodalite units looking into the a-cages may contain 0-3 aluminum atoms and 6-3 silicon atoms, respectively. The number of A1 atoms is used as a statistical weight for the cation occupancy of the different site I1 positions. Thus, firstly the sites with three Al atoms are filled with cations, secondly those with two Al atoms, and so on. If a certain type of hexagonal face cannot be filled completely, Monte Carlo statistics are employed for the filling. Table I shows the number of Ca2+and La3+ cations for a medium model crystal to be distributed into sites of type I1 and the number of hexagonal faces with weight 0-3 (0-3 Al atoms) for Si:Al ratios between 1 and 4.7. From the figures in Table I it follows that all sites I1 with three AI atoms are occupied with Ca2+and the remaining Ca2+ions are distributed into sites with weight 2, while La3+ions are only distributed in sites with weight 3. The computer program was also adapted for another series of runs where cations are nonstatistically positioned in distinct sites I1 inside the central a-cage of the model crystal. For the case of Si:Al = 1 and all sites I and I1 filled with Ca2+ ions, dipole moments in the central a-cage along the four 3-fold symmetry axes are absent; of course, a net dipole moment generally exists for %AI > 1 and asymmetric atom ordering. A further model is constructed in order to simulate the situation on the outer surface of a zeolite crystal. The crystal of medium size described above is cut parallel to the hexagonal faces (see Figure 1) and slightly above the middle of the central a-cage so

=

(1)

xqi,k/ri+kj 1.k

where qi,k is the charge of the ion type i at the lattice point k and r i , k j is the d i s t p c e between the point j and the ion. The electrostatic field E parallel to the vector r' at point j , due to an ion at site k , is s i , k j = qi,k'Fi9kJ/ri,kj 3 (2) The x , y , and z components of this field can be easily calculated sw,i,kj

= zi,kJ/

(r'i,kj)(wj

-

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(3)

, ~ where w = x,y,z and wjare the coordinates of point j and w ~are the coordinates of the ion. The field at point j resulting from all ions is given by sj

=

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+ zy,i,kj

&,i,kJ)

(4)

In the plots presented in the following sections the magnitude of this field is shown. 3. Results Electrostatic field strength and potential are given either with respect to a certain axis through the zeolite crystal or as a three-dimensional plot representing a plane inside the lattice (see Figures 1 and 2). Since site I1 is the main functional internal surface site, the calculations were restricted to a cube just above. the hexagonal face of a sodalite unit and inside the central a-cage of the model structure (see Figure 1). The x coordinate of this cube was normal to one side of the hexagon, the y coordinate pointed to a corner of the hexagon, and the z axis crossed the big cavity. The length of one edge of the cube was 1.446 nm. This edge was divided into 25 points at equal distance of 0.06 nm. Thus, the field and potential calculations were carried out for 15 625 points j in this volume. For example, Figure 3 shows three-dimensional plots of the potential in the hexagonal plane at z = 1 (a) and at the 12membered oxygen ring opening of the large a-cavity at z = 25 (b) in an x-y section of the cube for a Ca2+-exchangedSi:A1 = 1 zeolite. In this case, all four sites I1 facing into the a-cage are occupied by Ca2+ ions. In Figure 3a three peaks resulting from Si4+ ions and three peaks from A13+ ions appear. In the middle of these peaks there is a peak of the Ca2+ ion on site 11. The deepenings arise from 02-ions. Since the potential is truncated at -10 V and the mesh is rather wide, the peaks are somewhat less distinct. However, the main features are still visible. At the window of the large cavity (Figure 3b) there is a flat area of the potential in the center, which is surrounded by peaks of Si4+and AI3' ions and deepenings of 02-ions. A similar plot for an x-y plane was obtained in the center of the a-cage, where the potential is found to be even somewhat lower and the flat area is more extended. Figure 4a shows the potential in a y z section at x = 13 through the center of the cube for the same crystal as in Figure 3. In the hexagonal plane at z = 1 near site I1 the peaks of the A13+and Si4+ions flank the peak of the Ca2+ion. This CaZ+ion is slightly in front of the hexagonal plane. The smooth region in the center of the a-cage extends to the large pore opening in front of the plane at z = 25. The electrostatic field of this section is shown in Figure 4b. The log scale is used since the field strengths at the ion positions are huge. On the other hand, even small variations of the field in the center of the oc-cage appear very pronounced.

The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2957

Faujasite Type Zeolites @ t b x o g o ~ l ~Sodalite Unit

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Figure 3. Three-dimensionalplots of the electrostatic potential for a Ca2+-exchangedSi:Al = 1:l faujasite in an x-y section through the large a-cage close to the hexagonal site I1 ( z = 1) (a) and near the large 12-ring port ( z = 25) (b). All sites I and I1 are occupied by Ca2+ions.

While in Figures 3 and 4 all sites I1 were occupied by CaZ+, Figure 5 shows similar graphs as in Figure 4, but for a model structure with a high Si:AI ratio of 5:l and a specific cation distribution such that there are no Ca2+ions on the sites I1 in the central a-cage of the model. This shows up in the missing Ca2+ peak at z = 1 compared with Figure 4. There is a slight asymmetry in the graphs of Figure 5, since for a Si:A1 ratio of 5:l many of the AI3+ ions are irregularly replaced by Si4+, in contrast to the highly symmetric Si-AI ordering in a 1:l zeolite. Many more graphs like the ones in Figures 3-5 for various Si:AI ratios and different x-y and y-z sections have been calculated. For the sake of simplicity two-dimensional plots of field strength and potential along the z axis are used in the further discussion. The 3-fold axis in the z direction, which cuts through a site of type I1 a t x = y = 13 and z = 1.8, is the most interesting one. Figure 6a shows the potential curves along this axis for Si:AI N 1:1. There is one CaZ+ion in site I1 on this 3-fold axis. In addition

1-3 CaZ+ions are positioned in the other sites I1 of the central large cavity. These runs give the upper four curves. The lower curve arises when there is no CaZ+ion in the large cavity. The potential in the a-cage gets more and more negative the less CaZ+ ions there are in sites I1 of the large cavity. This results from the surrounding negatively charged 02-ions. The magnitude of the electrostatic field along the 3-fold z axis is shown in Figure 6b. The field is slightly greater when only 1-3 Caz+ ions are in the a-cage than with all four sites I1 filled. The lower curve results when no Ca2+ions are in the cavity. In the center of the a-cage and at the pore opening the electrostatic fields are very small and of almost equal magnitude. When the %:AI ratio is increased from 1:1 to 1.7:1, all of the sites I1 in the surrounding of the central large cavity are no longer filled. The Ca2+ions are distributed on the sites I1 by Monte Carlo statistics. Figure 7a shows the potential functions along the four

Preuss et al.

2958 The Journal of Physical Chemistry, Vol, 89, No. 13, 1985

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Figure 4. Three-dimensionalplots of the electrostatic potential (a) and field (b) in a y-z section through the center of the large cavity (x = 13). Same model crystallite as in Figure 3 (Ca2+ions, Si:A1 = 1:l).

Figure 5. y-z plots (x = 13) of electrostatic potential (a) and field (b) along 3-fold axis as in Figure 4, but with vacant sites I1 and only filled sites I (CaZions, Si:A1 = 91).

3-fold axes that cross the a-cage for two different Si-A1 distributions with two or three Ca2+ ions in the a-cage. With three Ca2+ ions one axis goes through an empty site 11, and with two Ca2+there are two axes through unoccupied sites 11, as seen from the low potentials at low z values. In the center of the cavity near z = 13 the values on all four 3-fold axes are equal. But due to the different statistical distributions of the ions in the surrounding crystal lattice the potentials are not equal for the two Si-A1 orderings. In the region near site I1 (z = 2) and at the large pore opening ( z = 21) the scatter of the potential values can also be seen. The variation of the electrostatic field strength is very small for the different runs (Figure 7b), except for the basic difference between empty and filled sites I1 a t small z values. At Si:Al ratios larger than 1 many different arrangements of the Si and A1 atoms in the lattice are possible, as already discussed in section 2.1. The question arises about the influence of this effect on the electrostatic field and potential in the zeolite pore system. For the special case of Si:Al = 1.71 a series of 500 independent

computer rum were done to create various atom distributions. The potential and field were calculated for each of the 500 model crystals along the four 3-fold z axes through the central a-cage. Thus, 2000 potential and field functions were obtained. In Figure 8 the distribution of potential values is shown at three different points on the z axes: at z = 5 (0.194 nm from site 11) (a), a t z = 13 (0.675 nm from site 11) (b), and at z = 21 (1.157 nm from site 11) (c). I n Figure 9a-c similar plots are shown for the electrostatic field strength. The potential distributions (Figure 8) are rather broad and extend even to negative values. Close to site I1 (z = 5, Figure 8a) the distribution shows two maxima, which are characteristic of the two alternative cases where site I1 is either filled by Ca2+(high potential) or empty (low potential). At a larger distance from site I1 this separation disappears, as already seen in the discrete example in Figure 7. The envelopes of the potential curves in Figure 8 show numerous small peaks of almost equal distance along the potential axis. This is a statistical effect. It shows the quantum size of the charges which

The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2959

Faujasite Type Zeolites

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are distributed on distinct sites of the crystal in the surrounding of the supercage. The electrostatic field distribution curves in Figure 9a show this statistical effect, too. The distributions in the center (Figures 8b and 9b) and at the pore mouth of the cy-cage (Figures 8c and 9c) are very similar. The fields at z = 13 and 21 are very small and the distributions rather narrow (Figure 9b,c). Similar plots as in Figures 8 and 9 have been obtained for Si:A1 ratios of 3,4, and 4.7. The results are not shown graphically; only the general conclusions shall be mentioned: in all cases broad distributions are found which are almost identical close to the large pore opening (at z = 21). At z = 5 the relative number of sites I1 which have a high positive potential decreases with increasing Si:Al ratio as one would expect, since the total number of cations that can occupy these sites also decreases. For faujasites with La3+instead of Ca2+cations in sites I and I1 and for Si:Al ratios from 1 to 3 the same broad scatter of potentials and fields is found as with the Ca2+-exchanged zeolite. The major difference is that the potential values span a much wider range from -10 up to 25 V and that the average potential for a given Si:A1 ratio is about 5 V more positive close to an occupied site I1 with La3+ than with Ca2+. The potential along a central 3-fold z axis, as in Figures 2, 6, and 7,is shown in Figure 10 for Ca2+-and La3+-exchangedzeolites with a constant overall Si:AI ratio of 2. From computer runs which generated statistical S i 4 orderings (like in Figure 8) such discrete examples of crystallites were chosen, where the potential curves represent maximum or minimum values and where there is just a single cation (Ca2+or La3+) in site I1 in the central large cavity. Also the potential and electrostatic field that extend from a surface (as described in section 2.1) into vacum were determined. Little difference is found between the situation on an outer surface and inside the a-cage, even when a very rough surface model is used. The main difference is a less wide potential distribution

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for the various Si-A1 orderings and a steeper potential and field decrease to zero as a function of distance from sites of type I1 on the surface. 4. Discussion The model structure that has been used in this study as a data basis for the calculation of the electric field and potential utilizes a number of simplifications with regard to “real” faujasite catalysts. Firstly, it is totally dehydrated. Therefore, the calculated fields are those originating from unshielded ”naked” cations and are maximum values, which are higher than for any iealistic system where coordinated water or hydroxyl groups lower the effective charge. Secondly, the lattice parameter is taken as a constant for all cations and Si:Al ratios, while differences on the order of 2% are found between zeolite X and Y.’ Thirdly, a strongly simplified cation site population pattern is assumed; initially, all sites I inside the hexagonal prisms are occupied, and then the remaining cations are distributed into sites of type I1 protruding from the hexagonal faces into the large cavity: Sites of type I’ and 11’, which are located inside the sodalite units and which are also occupied in real catalysts, namely in rare-earthexchanged zeolites, are not con side red.'^^.'^ Site I1 is assumed to have the same distance from the hexagonal surface for all types of ions as in Ca2+-exchanged faujasite. These latter three simplifications were mainly made in order to allow a close comparison between the different model zeolites and to restrict this comparison to the effect of the cation charge, the Si:Al ratio, and the Si-AI ordering upon the electrostatic field and potential. Thus, it is obvious that the numerical results for these two quantities, which are obtained from the calculations, represent maximum values and that in real Ca2+-and La3+-ex-

2960

The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 -IO

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Figure 8. Distributions of the electrostatic potential (resolution AU = 0.1 V) in the large central cavity along the 3-fold axis ( x = y = 13) at point z = 5 (near the hexagonal site 11) (a), at point z = 13 (in the center) (b), and at z = 21 in the 12-ring port (c). Si:AI = 1.71, d = distance from site 11.

changed zeolites the fields in the pore structure will be less, due to shielding of the cation charges by adsorbed species or to the location of cations inside the sodalite units (in site I' and II').'J6 Since the electric field in the a-cage originates form the special steric separation of Ca2+ (or La3+) cations and (A102)- sites in the zeolite lattice, the size of the model crystallite was thought to be of importance. But, it turned out that hardly any difference is found between a "medium" and a "large" size model with 30 (16)R. M. Barrer and R. M. Gibbons, Trans. Faraday SOC.,61,948 (1965).

I

~

",

5 0

,c .5

2

4

6

8 IO I FIELD I iwAi

12

14

16

IS

Figure 9. Distributions of the electrostatic field (resolution AE = 0.05 V

.&-I)

z =

at the same locations as in Figure 8 ( x = y = 13) at z = 5 (a), 13 (b), and z = 21 (c), Si:Al = 1.71,d = distance from site 11.

and 82 sodalite units, respectively, while a "small" crystal of only 10 sodalite octahedra gave results at variance with the medium and large crystallite. Therefore, all data reported in this paper are those obtained with the medium size model. In an earlier study Dempsey5 had calculated the magnitude of the electrostatic field inside the faujasite structure for Na+-, Ca2+-, Sr2+-,and Ba2+-exchanged zeolites with Si:A1 ratios of 1:l and 2:l. Our results on Ca2+-exchangedfaujasite with Si:AI = 1:l and 2:1, represented in plots like those in Figures 2,6, and 7 , agree with those of Dempsey for the same systems. We have extended the model to rare-earth La3+-exchanged zeolites and have varied the Si:A1 ratio between 1:l and 5:l. This wide Si:AI range covers

.

The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2961

Faujasite Type Zeolites

- 101 f

3

5

site1

7 9 11 13 3-fold z-axis

15

17

19

21

23

[(point No-11 x 06

25

AI

M2+--OH2

I\

4 \\ -

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l

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"

ferent Si-A1 orderings in the surrounding zeolite lattice. With La3+-exchangedzeolite the potential variations were larger than with Ca2+ and were found a t about 15-20 V. On the other hand, the average electrostatic field was only about 3 V nm-l larger in the center of the 12-ring pore opening than at the same distance from a site I1 on the surface of the zeolite. Therefore, it seems as if long-range electric fields indeed exist inside the a-cavity. This field effect has been cited in the catalysis l i t e r a t ~ r ein ~ .order ~ to account for unusually high activities of zeolite catalysts. However, such an effect was found in this study to be much smaller than the field and potential differences that can be induced close to a certain site by various Si-A1 orderings in the neighborhood of this site and by the difference between an occupied and unoccupied site. As a consequence, quite different catalytic sites with a broad range of acidities may exist in a given zeolite.15-22 Acid sites are generally thought to be formed by field-induced dissociation of coordinatively bound e.g.

l

'

5

l

7 9 11 13 3 - f o l d z-axis

'

l

l

l

"

i

l

"

'

15

'

i

17

'

l

19

21

23

25

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Figure 10. Potential along the 3-fold symmetry axis in the z direction for a Si:AI ratio of 2 and (a) one Ca2+ and (b) one La3* cation in the central a-cage for different Si-AI orderings.

zeolites of type X (Si:Al = 1-1.49), type Y (Si:Al = 1.5-3), and dealuminated faujasites (%A1 > 3). Increasing the Si:AI ratio from 1 to 5 led to a slight increase of potential by about 2-3 V (for both Ca2+-and La3+-exchanged faujasite) only close to sites I1 (at z = 5), while at a farther distance from this site in the a-cage center and at the pore mouth this effect is found negligible. When Ca2+cations were exchanged by La3+ at constant Si:A1 ratios, the calculated electrostatic potential close to occupied sites I1 was about 5 V larger than with Ca2+;this effect disappeared away from site I1 or at unoccupied sites. The main difference between Dempsey's and the present work is that for all %:A1 ratios > 1 not a single Si-A1 ordering was used, but rather the Si and A1 and also the cation positions were statistically distributed in the lattice framework observing Loewenstein's rule (no two nearest-neighbor AlO, units) as the only restraint. For a fixed Si:A1 ratio this led to a variety of model systems with varying resulting electric fields and potentials (see Figures 8-10). For constant Si:AI ratios and identical number and location of cations in sites I1 in the central large cavity, potential differences of more than 10 V could be caused by dif-

-

M2+-OH-

+ H+

H+ is then bound to lattice 02-and forms a Brernsted acid site. Therefore, values of field E may be regarded as an indirect measure of the potential acid strength of a cation ~ i t e . ~ ' . ~ ~ As seen from Figures 8 and 10, the electrostatic potential can even attain negative values. These regions with negtive potential values may be regarded as the locations of basic catalytic centers2, They originate from unoccupied sites I1 with two or three (A10,)units in the hexagonal oxygen ring. As seen from the three-dimensional plots, Figures 3-5, basic sites are generally associated with 0" units. While no strong bases can be formed in a structure with a locally balanced distribution of Si, Al, and cations, the heterogeneity inside a distinct zeolite crystal may cause strong basic as well as acidic sites. The dual presence of base and acid sites plays an important role in the proposed mechanisms for the on ZSM-5 conversion of methanol to 0 1 e f i n s ~ ~or- ~aromatics*' ~ type catalysts. In conclusion, a long-range electrostatic field effect seems to be of little significance for the explanation of the catalytic activity of zeolites. Other properties like the molecular sieve e f f e ~ t , l - ' ~ high virtual pressures inside the cavities,2s28or, as shown in this study, the nonuniformity of acid and base sites may be much more important. (17) (1972). 118) (19j (20) 1980. (21) (22)

J. W. Ward, J. Cutul., 13, 321 (1969); 17, 355 (1970); 26, 470 R. Beaumont and D. Barthomeuf. J . Cutal.. 26. 218 (1972). E. Dempsey, J . Cafui.,39, 155 (lb75). B. Imelik, et al., Eds., "Catalysis by. Zeolites", Elsevier, Amsterdam, . .

I

D. Barthomeuf in ref 20, p 55. D. Barthomeuf in "Zeolites: Science and Technology", F. R. Ribeiro, A. E. Rodrigues, C. D. Rollman, and C. Naccache, Eds., Martinus Nijhoff Publishers, The Hague, 1984, p 317. (23) P. A. Jacobs, and W. J. Mortier, Zeolites, 2, 226 (1982). (24) Y . Ono in ref 20, p 19. (25) W. W. Kaeding and S.A. Butter, J . Cutul., 61, 155 (1980). (26) T. Mole, J. Carol., 94, 423 (1983). (27) J. C. Vedrine, P. Dejaifve, E. D. Garbowski, and E. G. Derouane in ref 20, p 29. (28) J. Fraissard in ref 20, p 343.