A CNDO study of the electronic structure of faujasite type six-rings as

A CNDO study of the electronic structure of faujasite type six-rings as ...https://pubs.acs.org/doi/pdfplus/10.1021/j100470a019(1) NRCC No. 17185. (2)...
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CNDO Study of a Faujasite Six-Ring

The large, isotropic central-atom hyperfine interactions are reminiscent of those of the well-known phosphoranyl radicals,13with which M(0Ac)c would be isoelectronic (at least as far as the valence electrons are concerned). The 31Pisotropic hyperfine interactions in PHI and PF4 are 1456 MHz14 and 3717 MH2,15 respectively, whereas the '19Sn hyperfine interaction in SnH, is -6245 MHz.12 With these data in mind, the I19Sn hyperfine interaction of -23.6 GHz for Sn(0Ac); is not unreasonable. The phosphoranyl radicals also possess CaUrather than Oh symmetry (i.e., the four ligands are equivalent in pairs, axial and equatorial). This low symmetry permits inclusion in the semioccupied orbital of M(np) atomic orbitals, generating considerable hyperfine anisotropy (Table IV). Describing these species as M3+ ions cannot adequately explain this anisotropy. In the case of lead tetraacetate, however, there is very little hyperfine anisotropy, and the spin density in Pb(6p) is only 0.05. Since only 50% of the spin is accounted for, we feel that the free radical is best described as P ~ ( O A C ) ~ rather than Pb3+. The 6s/6p spin-density ratio implies an interbond angle of 920,16and missing spin resides in various ligand orbitals. A possible alternative structure for the lead center is a lead tetraacetate anion having a tetrahedral geometry. Symmetry requirements would prevent a direct contribution of Pb(6p) orbitals to the al semioccupied molecular orbital of such a species, and thus account for the small 207Pb hyperfine anisotropy. The crystal

The Journal of Physical Chemistry, Vol. 83, No. 7, 1979 855

structures of the tetraacetates are, unfortunately, unknown, so that there is no immediately apparent reason for the adoption of a different geometry by the lead radical.

References and Notes (1) NRCC No. 17185. (2) NRCC Research Associate 1977-1979. (3) J. R. Morton and K. F. Preston, "Magnetic Properties of Free Radicals", Landolt-Bornstein, Vol. 9a, H. Fischer and K.-H. Heliwege, Ed., Springer-Verlag, Berlin, 1977, p 263. (4) A. R. Boate, J. R. Morton, and K. F. Preston, J. Magn. Reson., 29, 243 (1978). (5) J. I. Isoya, H. Ishizuka, A. Yamasaki, and S. Fujimara, Chem. Left., 397 (1972). (6) P. W. Schenk, "Handbook of Preparative Inorganic Chemistry", Vol. 1, G. Brauer, Ed., Academic Press, New York, 1963, p 726. (7) A. R. Boate, J. R. Morton, and K. F. Preston, J . Phys. Chem., 80, 2954 (1976). (8) A. R. Boate, J. R. Morton, and K. F. Preston, J . Magn. Reson., 24, 259 (1976). (9) J. R. Morton and K. F. Preston, J . Magn. Reson., 30, 577 (1978). (10) G. S. Jackel and W. Gordy, Phys. Rev., 176, 443 (1968). (11) J. E. Bennett and J. A. Howard, Chem. Phys. Lett., 15, 322 (1972). Our more recent compilation' of $*(O) and ( r 3 )however, , improves the spin count in Sn(CH,),. (12) J. R. Morton and K. F. Preston, Mol. Phys., 30, 1213 (1975). (13) P. J. Krusic, W. Mahler, and J. K. Kochi, J . Am. Chem. SOC.,94, 6033 (1972). (14) A. J. Colussi, J. R. Morton, and K. F. Preston, J . Chem. Phys., 62, 2004 (1975). (15) R. W. Fessenden and R. H. Schuler, J. Chem. phys., 45, 1845 (1966). (16) P. W. Atkins and M. C. R. Symons, "The Structure of Inorganic Radicals", Elsevier, Amsterdam, 1967, p 257.

A CNDO Study of the Electronic Structure of Faujasite Type Six-Rings as Influenced by the Placement of Magnesium and by the Isomorphous Substitution of Aluminum for Silicon W. J . Mortier," Katholieke Universitelt Leuven, Centrum voor Oppervlaktescheikunde en Colloble Scheikunde, De Croylaan 42, 8-3030 Heverlee, Belgium

P. Gieerllngs, C. Van Alsenoy, and H. P. Figeys Free University of Brussels (U.L.B. and V.U.B.), Department of Organic Chemisiy, F.D. Rooseveltlaan 50, B- 1050 Brussel, Belgium (Received June 28, 1978; Revised Manuscript Received December 1 I, 1978)

A CNDO study has been made of the electronic structure of the main cation-exchange site in faujasite-type zeolitic structures, the framework six-ring. The following isolated molecules and ions were considered as models: Si6018H12,(Sis018H12Mg)2t,(Si3A13018H12)3-, and (Si3Al3Ol8HI2Mg)-.Some general statements can be made concerning the properties of the aluminosilicate framework and the six-ring site. First, isomorphous substitution of A1 for Si, or the presence of an exchangeable cation, results in only small variations in framework oxygen charges and a considerable delocalization of these charges is observed. The charge on Si is more highly variable than that on AI. This explains the difference in sensitivities of the SiK, and AlK, emission energies upon isomlorphous substitution. Second, protons attached to aluminate tetrahedra are more acidic than those attached to silicate tetrahedra. Finally, the molecular electrostatic potential (MEP) seems to be very sensitive to residual charges. This influence, as indicated by the features of the MEP pattern, accounts for the fundamentally different adsorption properties of X and Y zeolites toward CO and COz. Further, the increase in the CO stretching frequency observed when passing from X to Y zeolites, both containing divalent cations, is also explained by the MEP.

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The Journal of Physical Chemistry, Vol. 83,

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Mortier et al.

A rings contain no A1 while B rings are mixed A1 and Si. A Si6018H12 CBUsymmetry CSusymmetry

A-Mg

(Si6018H12Mg)2+

B

(Si3A130i&d3-

C3 symmetry

B-Mg

(Si3A130i&izMg)-

C, symmetry

The Si-O(2)-Si and Si-O(3)-Si bond angles were 126.7 and 122.9’, respectively, while those for Si-O(2)-Al and Si-O(3)-Al were 132.8 and 129.5’.

Flgure 1. Drawing of the site I’ six-ring in the faujasite-type structure for a 1:l Si:AI ordering seen from the hexagonal prism side. The 0(1) and O(4) oxygens have a proton attached in the direction of the next neighboring T atom.

= 2.50) zeolites can show markedly different properties. Although it is obvious that the nature of the active sites is closely related to the nature of the cation-exchange sites, the exact form of the relationship is often obscure. Polarization is the first step in the deformation of a molecule during the creation of an active surface intermediate. A detailed study of the electronic structure and electrostatic potential a t the ion-exchange sites may therefore be essential for a better understanding of catalytic reactions a t zeolitic surfaces. Because of the importance of six-ring sites in many zeolite frameworks, this first CNDO study has been made of site I’ in a faujasite-type framework. This is a typical six-ring ion-exchange site. The influence of isomorphous substitution of Si for A1 and the presence of an exchangeable cation (Mg2+)on the electron distribution as well as on the molecular electrostatic potential are considered.

The Six-Ring Model A site I’ six-ring of the faujasite-type framework (shown schematically in Figure 1) was chosen for the calculations. This site is located in the interior of the zeolitic sodalite cage and the six-ring forms one half of the hexagonal prism. The oxygen atom designation is the same as conventionally used for the faujasite-type framework. A single six-ring possesses C3 or CBUsymmetry, depending on the equivalence of the T atoms (T represents tetrahedral Si or Al). Only single crystallographic types of O(1) and O(4) oxygens are present in the structure. The TI0(2), and O(3) atoms are nearly planar. Silicon and aluminum tetrahedra were idealized with Si-0 = 1.61 A and A1-0 = 1.75 A, 0-T-0 = 109.5’, considering the effective ionic radii as given by Shannon and Prewitt.l In the absence of any isomorphous substitution, the framework topology of faujasite obeys the Fd3m space group while for a 1:l Si/A1 ordering, Fd3 is obeyed. The distance least-squares procedure (DLS) as described by Meier and Villiger* was used to obtain the idealized coordinates of both configurations. The starting coordinates were those given by Olson3 for the NaX structure, where partial ordering was observed. For the pure Si form, a unit cell parameter of 24.13 A was used as obtained by extrapolation of the unit cell parameter of a series of hydrated KX and KY zeolites to zero A1 c ~ n t e n t . ~ A Mg-0 distance of 1.96 A was taken, Le., the center of the six-ring for the Al-containing ring, or slightly displaced for the other. The six-ring was terminated by protons a t an 0-H distance of 0.96 A, the bond pointing toward the next neighboring T atom in the framework. Calculations were performed on the six-rings listed below. Note that

T h e Calculation Method CNDO calculations were performed with Pople’s original CNDO/2 parametrization for H and 0.5For the second row elements (Mg, Al, Si) Santry’s parameters were used.6 The 3d orbitals were included in the atomic valence basis set. Their orbital exponent was taken equal to that of the 3s and 3p orbitals (spd basis). This procedure led to convergence problems in several cases. However, when the 3d orbitals were left out of the atomic basis set, good convergence was observed in all cases studied. All the results given in this paper thus refer to calculations without inclusion of 3d orbitals (sp basis). The convergence limit on the electronic energy was set equal to 1 x IO-4 au. As proposed by Giessner-Prettre and Pullman7 the CNDO/2 wave functions were deorthogonalized by the Lowdin procedure.8 Thus we transformed matrix C of the CNDO/2 MO-LCAO coefficients (CAJinto matrix C’ (the deorthogonalized coefficients) via the relation

C’ = S-IPC (1) where S denotes the atomic orbital overlap matrix S,” = J X P X ” d7

(2)

where xPand xu are atomic orbitals. Immediately after the deorthogonalization, a Mulliken population analysisg is performed leading to gross atomic charges and overlap populations. The use of deorthogonalized functions was recommended because dipole moment calculations and electron density plots have proved that these functions yield molecular charge distributions which are in closer agreement with ab initio results than are the CNDO/2 functions.10-12 The molecular electrostatic potential (V)at a given point r in space represents a first-order approximation to the interaction energy of the molecular charge distribution and a unit “test” charge situated a t that point. V(r) is given by13

where the summation over A runs over all the nuclei of the molecule (2, denotes the charge of nucleus A and R A its position) and p ( r )’ represents the molecular electron density a t the point r’. The quantum chemical use of this quantity was introduced by Bonaccorsii3 and since then the MEP has proved to be a useful tool in studies of electrophilic reactivity of a wide variety of molecules (for a review see ref 14). To our knowledge a detailed study of catalysis problems has not been undertaken via the MEP technique hitherto. In order to reduce the computational costs we preferred to calculate the MEP for the structure under study starting from the CNDO/2 wave functions before deorthogonali-

The Journal of Physical Chemistry, Vol. 83,

CNDO Study of a Faujasite Six-Ring

No. 7, 1979 857

TABLE I: CNDO Atomic Charges fin Electrons) A A-Mg B B-Mg Si tetrahedron Si 1.5931 1.5998 1.4035 1.5034

0- Mg

rh

O(1)

O(2) O( 3) O(4) H(O(1)) H(O(4)) Al tetrahedron A1

-0.5600 -0.7268 -0.7595 -0.5591 0.1299 0.1393

-0.5176 -0.7119 -0.7958 -0.5122 0.1674 0.1920

O(1) O(4) H(O(1)) H(O( 4)) cation Mg

A-Mg

n Figure 2. Representation of the band structure of the framework six-rings by a histogram of the number of molecular orbitals vs. the energy level at 0 . l - a ~intervals. Shaded areas represent occupied MO's.

zation. Indeed, if one applies the CNDO/2 approximations to (3) one finally arrives (cf. ref 15 and 16) at the very simple expression (4) where 2, now represents the core charge of nucleus A (use of an atomic valence basis set), P A A the total electronic population on atom A, and YAP an average Coulomb integral (usually taken over valence s functions) between a valence electron on atom A and the Is electron of a hydrogen atom at position r. This simple expression, which is very easy to program, leads to MEP maps which show the same features as the costly ab initio ones.15J6 (Note that for the molecules studied in this article the computing costs for ab initio MlEP maps would be prohibitively large.) The calculations were done with a program originally written for the CDC 6600 computer of the Free University of Brussels. The computation time was further reduced by using the SYMAP program17 which allows an expansion of the grid starting from a limited number of points. This method is justifiable because of the smooth variation of the MEP in the interesting regions. The maps shown in Figure 4 were actually calculated starting from 814 points.

Results and Discussion 1. Stability. Although no d orbitals are included in the present calculations, they are in agreement with calculations already made for XOdn-oxy anions by Hojer et a1.18 The valence levels can be divided in two groups separated by a gap of about 14 eV for rings containing only Si (A rings) and 13 eV for rings containing both Si and A1 (B rings) as shown in Figure 2. Hojer et al. observed a decrease in the gap from the perchlorate to the silicate anion, but did not include A104" in their calculations. The trend seems to continue. A crystal field effect may certainly be expected since our systems carry a charge ranging from -3 to +2. For the oxy anions, the effect of the crystal

0.9480

-0.5938 -0.7268 -0.7219 -0.5842 0.0740 0.0597

-0.5670 -0.7079 -0.7635 -0.5495 0.1185 0.1178

1.2665 -0.6065 -0.5966 0.0189 0.0082

1.2595 -0.5785 -0.5617 0.0534 0.0553 0.8603

field is an upward shift of the orbital levels by about 8 eV per negative charge. Two distinct effects must be dealt with in considering the stability of the ring. These are isomorphous subtitution of A1 for Si (which gives a net destabilization) and the introduction of a cation in the center of the ring (giving a net stabilization). The HOMO energy level is related to increasing charge across the series of six-rings studied (B, -0.96 eV; B-Mg, -9.01 eV; A, -12.81 eV; and A-Mg, -21.03 eV) in that differences of 4.03, 3.80, and 4.11 eV per unit of charge are observed. The effect on the lowest energy level is more irregular. For the same series of six-rings we have -29.89, -40.27, -41.93, and -52.85 eV or differences of 5.19, 1.66, and 5.46 eV per negative charge. The reason for this irregular variation lies in the different effects of isomorphous substitution and the introduction of a cation on this energy level. Specifically, for the replacement of Si by A1 the difference between A and B and the difference between A-Mg and B-Mg were net destabilizations of 4.01 and 4.20 eV per negative charge, respectively. On the other hand, the introduction of a Mg ion has a stabilizing effect. A compared to A-Mg and B compared to B-Mg result in stabilization of 5.46 and 5.19 eV per charge for this lowest orbital level. The stabilizing effect of the introduction of an exchangeable cation exceeds the destabilizing effect of the replacement of Si atom by Al. This effect is an important observation. Zeolites are crystallized from a gel obtained by mixing silicate and aluminate solutions in a highly alkaline medium. It is certainly not obvious how the crystallization proceeds, e.g., the nature of the nucleation centers, the existence of ordered moieties, whether the crystallization involves the reorganization of the gel19 or not, and the nature of the transport between the solution and the "solid" gel phase.20 It is clear from the present calculations that six-rings are stable molecules and that they can be further stabilized by the presence of a cation in the center. It was observed by Gupta Rishi et aL21that if the zeolite A and X synthesis mix contained sufficient NaOH, further addition of NaCl was sufficient to promote crystallization. Ordered moieties may therefore play an important role in the crystallization process. The alkalinity is then only needed to maintain a certain level of aluminate and silicate in solution and the cations to stabilize ordered framework fragments. 2. Charges. The atomic charges before and after deorthogonalization are given in Tables I and 11, respectively. The delocalization of the charges is of great importance, especially since not all oxygens of the A10, tetrahedra (origin of the negative charge excess) need be

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The Journal of Physical Chemistry, Vol. 83, No. 7, 1979

TABLE 11: CNDOIPD Mulliken Population Analysis A. Gross Atomic Charges (in Electrons) A

A-Mg

B

B-Mg

2.0605 -0.7339 -0.9283 -0.9608 -0.7361 0.1709 0.1832

2.0060 -0.6876 -0.9225 -1.0678 -0.6861 0.1202 0.2469

1.7835 -0.7683 -0.9015 -0.8911 -0.7580 0.1039 0.0858

1.8572 -0.7409 - 0.891 3 - 1.0068 -0.7237 0.1572 0.1555

1.8033 1.7466 -0.7602 -0.7324 - 0.7488 -0.7169 0.0327 0.0746 0.0187 0.0757 1.3983

1.2348

B. Overlap Populations

A Si tetrahedron Si-O( 1) Si-O( 2) Si--0(3) Si-O( 4) H-O( 1) H-O( 4)

0.4038 0.4339 0.4043 0.4220 0.5555 0.5614

A-Mg

0.4485 0.3593 0.3070 0.4799 0.5632 0.5721

Al tetrahedron A1-O(1) A1-0( 2) A1-0( 3) A1-0( 4) H-O( 1) H-O( 4)

B

B-Mg

0.2715 0.4458 0.3862 0.2793 0.5286 0.5301

0.3623 0.4009 0.3875 0.3803 0.5531 0.5519

0.3395 0.3478 0.3088 0.3264 0.5129 0.5035

0.3840 0.2510 0.2409 0.3912 0.5295 0.5278

cation Mg-0(3)

0.2020

0.2416

coordinated to an exchangeable cation in the framework. The catalytic activity is traditionally assigned to an increased separation of the surface charges. Zeolites exchanged with multivalent cations indeed show a higher catalytic activity than the monovalent forms, which in some instances show no activity at all. Although the charges obtained by CNDO are exaggerated, their shifts are an indication of the delocalization possibilities. Influence of Mg. A comparison has to be made between A and A-Mg and the B and B-Mg rings. It is observed from Tables I and IIA that the charge differences are fairly small (a few hundredths of a unit charge). The positive charges are delocalized among all atoms, with the exception of the O(3) atoms which are directly coordinated to the Mg ions. It is further observed that in the B rings, the charge shift for Si is larger than for Al. Some comments can also be made concerning the change in overlap populations (Table 1I.B). In all instances, the presence of Mg2+ results in an increase of the overlap populations with the exception of the T-O(3) and T-0(2) overlap. An increase in the overlap indicates a strengthening of the bond. Baur2’ extensively studied the influence of the extraframework cations on the T-0 bond length, based on electrostatic considerations. The T-0 bond length is strongly correlated with the Pauling bond strength. The decrease of the overlap population is an indication of the same effect. However, it is observed here that not only will the directly coordinating oxygens O(3) be subject to bond lengthening, but also the mere distant O(2) oxygens. Influence of Isomorphous Substitution. The influence

Mortier et al.

of isomorphous substitution on the various charges can be followed by comparison of the A and B, as well as the A-Mg and B-Mg six-rings. We again observe a delocalization of the charges (Tables I and 11). An opposite effect is shown by the O(2) and O(3) oxygens. The influence of creating three negative charges is again larger on the Si atoms than on the other atoms. The overlap population also decreases for all bonds with the exception of the Si-0(2) bond, and the Si-0(3) bond in presence of Mg2+. A comment should be made here concerning the intrinsically different behavior of the Si04 and A104 tetrahedra with respect to the properties in the framework. SiK, and AlK, X-ray emission band energies have been measured for a number of zeolites with varying Si/A1 ratio by Patton et al.23 A strong correlation with isomorphous substitution was found for SiK, and a much weaker effect for AlK,. This is in agreement with the present observations since the Si charge, unlike Al, is very sensitive to changes in the framework charge. There is also a difference in acidity between protons attached to A104 or to Si04 tetrahedra (see H charges). The hydrogen atoms attached to Si04tetrahedra show a systematically higher charge than protons attached to A104tetrahedra. It is well known that acidity is related to the A1 content; the higher the Si content, the more acidic the protons.24 An equilibrium geometry cannot be obtained here by adjusting interatomic distances and bonding angles to the lowest total energy because of the extent of the molecule. However, the charge shifts upon variation of one parameter, the others being held constant, do give us an idea of the influence of several factors on the framework charges. As mentioned above the T-0 bond length increases with increasing “bond strength” received by the oxygens.z2 There is also an effect of 0-T-0 and T-0-T bond angles on the T-0 bond length,25so that calculation of an equilibrium geometry becomes even more complex. The effect of the T-0-T bond angle is reflected here by the T-0(2) and T-0(3) overlap populations (Table 1I.B.). The T-0(2) overlap population is indeed larger than the T-0(3) bond overlap, being in agreement with bond shortening upon T-0-T angle widening. It can certainly be concluded that all observed charge shifts are in qualitative agreement with the observed framework properties. 3. The Electrostatic Potential. The molecular electrostatic potential (MEP) will give a qualitative idea of the interaction possibilities of the zeolite surface with adsorbed molecules if the “reactive center” in these adsorbing molecules shows a highly polar character. Indeed, only in that case can the reactive center be approximately described by a point charge model which is finally, in its simplest form (a single unit charge), the basis for the discussion of interaction energies in terms of the MEP. We shall now show how we can apply this model for the study of the interaction possibilities of zeolites with CO and COz, for which a lot of experimental data exist. The reactive centers to be consided in CO and COz are the lone pairs on 0 (in CO and COP)and on C (in CO). These electrons will strongly be attracted by uniform positive regions in the MEP. We will further have to consider the C’O- local dipole in C 0 2 for which an attractive interaction with the zeolite will be favored only if a sign inversion in the potential occurs. The CO dipole of the CO molecule is SO small that it will not be considered as being a reactive site (cf. the very small experimental value for the CO equilibrium dipole moment, 0.122 DZ6). In Figures 3 and 4 the results of several MEP calculations are depicted and the general features of the cal-

CNDO Study

The Journal of Physical Chemistry, Vol. 83,No. 7, 1979 859

of a Faujasite Six-Ring

phenomenon was explained by an end-on adsorption on NaCaY and NaMgY zeolites. Under the same conditions COzis chemisorbed on alkali and alkaline earth exchanged X zeolites and carbonate species were observed which could only be explained by a bidentate adsorption of the form 0( 4 ) *

-

0

C:

4

li

M2+,. ,.,0

1-2

1 distance from center

8

Flgure 3. Molecular electrostatic potential (MEP) along the threefold axis for all six-rings, Negative distances from the center are in the direction of the hexagonal prism, positive in the direction of the cubooctahedron.

culated potential maps are worth comment. The first point of interest is that the MEP is extremely sensitive to the charge on the molecule. This can be seen from Figure 3 where the MEP was calculated perpendicular to the plane of the six-ring along the threefold axis. At 5 8, from the center of the ring the MEP difference is 52 kcal/mol per unit of charge in the absence of Mg2+and 54 kcal/mol in its presence. It is therefore obvious that even small residual charges may have drastic effects on the MEP around the active site and consequently also on its activity. A MEP map was generated through the center of the ring (determined b:y the O(3) oxygens) for the A ring (Figure 4.1) and the B ring (Figure 4.3), respectively. In the case of a negative charge excess, EI positive charge will be highly attracted to the center of the ring, while in the case of a zero charge, the MEP in the center is even slightly positive. A t 2 8, from the A ring (Figure 4.2), the MEP is still positive. This is much more accentuated in the presence of MgZt (figure not shown). At 2 8, from the center of the B-Mg ring having one negative charge (Figure 4.4), positive and negative areas emerge. A positive field is associated with thLe cation, the negative with the O(2) and O(3) oxygens, the MEP above the O(2) oxygens being slightly more negative. This occurrence of regions with positive and negative potential values is clearly related to the presence of a local negative charge excess. A MEP map containing the threefold axis, together with the cation and the O(2) and O(3) oxygens (Figure 4.5), indicates that the MEP above the oxygens is rather symmetric. However, the left and right sides (Figure 4.5) are not symmetric. This is due to the configuration of the ring. The left side (hexagonal prism side) experiences the effect of O(1) oxygens which are closer to the center than the O(4) oxygens a t the other side, the side of the cubooctahedron. The latter side is also very similar to the configuration at the site I1 six-ring (facing the large cavity of the zeolite framework.) Qualitatively, an application of the site I' results to the site I1 and even to other six-ring containing zeolites is reasonable. Let us now correlate the above-mentioned general features of the MEP with the experimental data for adsorption of CO and C 0 2 on zeolites. (1) Adsorption of COz. The experimental data we are concerned with are especially related to the difference in adsorption characteristics between X and Y zeolites. Dehydrated X and Y zeolites both show an interaction with C 0 2as can be seen in the IR spectrum but the nature of the interaction is different.27*28In Y type zeolites a physical adsorption is observed at room temperature. This

The fundamental difference between X and Y zeolites lies in the greater negative charge excess in the framework structures of the X zeolites. Indeed, assuming a statistical distribution of A1 atoms, 70% of the six-rings in Y zeolites contain two A1 atoms (none with three Al) while in X zeolites 70% of the six-rings contain three Al, the remaining fraction only two Ale4The presence of a bivalent cation in Y type zeolites might therefore never create a negative charge excess since the isomorphous substitution of a Si for A1 generates only one negative charge. On the other hand a negative charge excess might emerge in X type zeolites depending on the occupancy of neighboring cation-exchange sites. As seen on the MEP's in Figures 4.2 and 4.4, which were taken in a plane situated at 2 8, above the ring plane, the simultaneous occurrence of regions with positive as well as negative MEP values is only observed in the presence of an excess of negative charge (Figure 4.4). This clearly favors a bidentate adsorption via a CtO- local dipole and disfavors an adsorption of the monodentate type via an oxygen lone pair. This conclusion parallels the experimental observation that a bidentate adsorption might easily occur in X type zeolites. A similar specific interaction with H20 in NaX zeolites as observed by Bertsch and HabgoodZ9can also be explained by the formation of an analogous adsorption complex a t site 11. In H 2 0 the reactive site would then be the 0-H+ local dipole. The small number of sites involved might be explained by the requirement of a local negative charge excess. It was observed that only about 2% of the site I1 sites would be involved. On the other hand, the observed monodentate adsorption of COz on Y type zeolites is in agreement with the uniformly positive MEP values in the absence of negative charge excess (Figure 4.2) favoring a complexation via an oxygen lone pair and disfavoring the formation of a bidentate complex via a local C+O- dipole. Since on Y type zeolites exchanged with divalent cations a negative charge excess is certainly not likely to occur, only one adsorption form (the end-on monodentate adsorption) might be possible. ( 2 ) Adsorption of CO. For CO the experimental data show a specific IR CO stretching absorption band (2200 cm-l) upon interaction of this molecule with multivalent cation-exchanged zeolites,30the frequency being related to the nature of the cation and the type of zeolite. The observed CO frequency was higher for Y than for X type zeolites containing bivalent cations (cf. MgY, 2213 cm-l; MgX, 2205 cm-l). As far as structural data are concerned, a few CO adsorption complexes on six-ring sites were observed by X-ray diffraction methods: on dehydrated Co-exchanged zeolite A,31 dehydrated Ca-exchanged c h a b a ~ i t e and , ~ ~ dehydrated natural offretite (an adsorption site on Mg).33 A linear adsorption complex was observed for CoA and natural offretite. For Ca chabazite a Ca-C-0 angle of 134" was found. For the offretite structure a Mg-C distance of 2.16 A and C-0 distance of 1.13 b, was observed.

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The Journal of Physical Chemistry, Vol. 83, No. 7, 1979

1

0

a

b

3 1

2

c

d

a

f

Flgure 4. MEP parallel to the plane of the six-rings: (1) A, throu h the plane of the q3)oxygens: (2) A, 2 A in Ihs direction of the cubooctahedrm: (3)B. through the plane of the O(3) oxygens: (4) B-Mg, at 2 in the direction of the cubooctahedron; (5) 8-Mg. a plane containing Mg, 0(3),

1

and the threefold a m (horizontal). The right side is the cubooctahedron side, The following classes were considered (values in kcallmol): (a) below -200; (b) -200 to -100; (c) -100 to -75; (d) -75 to - 5 0 (e) -50 to -25 (f) -25 to 0 (g) 0 to 25; (h) 25 to 5 0 (i) 50 to 75; (i) 75 to 100 (k) 100 to 1000: (I)above 1000.

These data can again be rationalized by inspection of the MEP plots, now considering the carbon lone pair of CO as being the "reactive site" of the adsorbing molecule. Indeed, the polarity of the CO molecule being very low, a bidentate adsorption is very unprobable so that monodentate complexes will be formed, preferentially via the carbon lone pair electrons. The preference for complexation via the carbon lone pair could be rationalized hy consideringthat the direction of the experimental dipole moment for CO is C-Of.26 This result was confirmed by

extensive multiconfiguration SCF cal~ulations.3~ In order to explain the enhancement in the CO stretching frequency when passing from complexation with Y type zeolites to X type zeolites we consider again the dependence of the MEP on the residual charge (see for example the intersection of the different curves in Figure 3 a t 2 A from the center of the ring). We observe that the more important the positive charge excess is, the stronger will he the interaction of a CO molecule with the zeolite. As positive charge excess is more likely to occur in Y type zeolites as

CNDO Study of a Faujasite Six-Ring

The Journal of Physical Chemistry, Vol. 83, No. 7, 1979

861

compared to X type ones, we conclude that the stronger References and Notes (1) R. D. Shannon and C. T. Prewitt, Acta Ctystalbgr., Sect. 8 , 25,925 interaction will take place in Y type zeolites. The en(1969). hancement of the 60 stretching frequency for stronger (2) W. M. Meier and H. Villiger, 2 Kristallogr., 129, 411 (1969). interactions (vco in MY > YCO in MX, M being a bivalent (3) D. H. Olson, J . Phys. Chem., 74, 2758 (1970). cation) then parallels the experimental%and t h e ~ r e t i c a l ~ p ~ ~ (4) W. J. Mortier, H. J. Bosmans, J. Phys. Chem., 75, 3327 (1971). (5) J. A. Pople and G. A. Segal, J. Chem. fhys., 44, 3289 (1966). findings on the enhancement of the CN frequency in (6) D. P. Santry and G. A. Segal, J . Chem. fhys., 47, 158 (1967). nitriles on hydrogen bonding. These situations are per(7) C. Giessner-Prettre and A. Pullman, Theor. Chim. Acta, 11, 159 fectly comparable to those in zeolitic CO complexations (1968). (8) P. 0. Lowdin, J. Chem. fhys., 16, 365 (1950). as the hydrogen bonding in nitriles also occurs via a lone (9) R. S. Mulliken, J . Chem. Phys., 23, 1833 (1955). pair in the direction of the internuclear axis. For a (10) D. D. Shillady, F. P. Billingsley, and J. E. Bloor, Theor. Chim. Acta, comparable but more qualitative description based on 21, l(1971). (11) F. A. Van Catledge, J. fhys. Chem., 78, 763 (1974). “classical” electrostatic arguments, see ref 30.

Conclusion In conclusion we can state that the influence of isomorphous substitution and the exchangeable cations on the aluminosilicate framework is such that a considerable delocalization of the charges is achieved. The Si atom charges, unlike Al, are most subject to changes, explaining the sensitivity of the SiK, emission energy lines upon changes in the framework charge. A detailed study of the pattern of the molecular electrostatic potential, especially of the relationship between an excess of negative charges and the simultaneous occurrence of regions with positive and negative potential values, enabled us to explain the fundamental differences between adsorption properties of X and Y zeolites toward CO and COz: a bidentate adsorption between X zeolites and COz, and a monodentate adsorption between Y type zeolites and COz via an oxygen lone pair and for CO via the carbon lone pair. All IR and X-ray data could be rationalized in this way. These findings, together with the different acidities of A1-0-H and Si-0-H protons, explain the basic differences between X and Y zeolites.

Acknowledgment. W.J.M. thanks Professor G. V. Gibbs for interesting discussions. Three of us thank the Belgisch Nationaal Fonds voor Wetenschappelijk Onderzoek for research grants as “aangesteld navorser” (W.J.M.) and “aspirant” (P.G. and C.V.A.). Financial support from the Belgian government (Dienst voor Programmatie van het Wetenschapsbeleid)to the Katholieke Universiteit Leuven is gratefully acknowledged.

(12) H. P. Figeys, P. Geerlings, and C. Van Alsenoy, J . Chem. Soc., Faraday Trans. 2 , 71, 375 (1975). (13) R. Bonaccorsi, E. Scrocco, and J. Tomasi, J. Chem. Phys., 52, 5270 (1970). (14) E. Scrocco and J. Tomasi, Fortschr. Chem. Forschung, 42,97 (1973). (15) C. Giessner-Prettre and A. Pullman, C.R. Acad. Sci., Ser. C , 272, ’ 750 (1972). C. Giessnei-Prettre and A. Pullman, Thew. Chim. Acta, 25,83 (1972); 33. 91 - (1974). ,(17) J. A. Dougehik and D. E. Sheenan, SYMAP, Harvard University, Cambridge, Mass. 02138. (18) G. Hojer, S.Meza-Hojer, and G. Hernandez de Pedro, Chem. fhys. Lett.. 37. 301 11976). (19) 8. P.’McNicol, G. T. Pott, K. R. Loos, and N. Mulder, Adv. Chem. Sei-., No. 121, 152 (1973). (20) C. L. Angell and W. H. Flank, ACS Symp. Ser., 40, 194 (1977). (21) . . B. K. Guota Rishi. S. Viswanathan. G. R. Amladi. A. K. Jain. and M. K. Chapurey, personal communication. (22) W. H. Baur, Amer. Mineral., 56, 1573 (1971). (23) R. L. Patton, E. M. Flanigen, L. G. Dowell, and D. E. Passoja, ACS Symp. Ser., 40, 64 (1977). (24) P. A. Jacobs, ”Carboniogenic Activity of Zeolites”, Elsevier, Amsterdam, 1977. (25) J. A. Tossel and G. V. Gibbs, J. fhys. Chem. Minerals, 2, 21 (1977). (26) J. S. Muenter, J . Mol. Spectrosc., 55, 490 (1975). (27) P. A. Jacobs, F. H. Van Cauwelaert, E. F. Vansant, and J. B. Uytterhoeven, J. Chem. Soc., Faraday Trans. 7 , 69, 1056 (1973). (28) P. A. Jacobs, F. H. Van Cauwelaert, and E. F. Vansant, J . Chem. Soc., Faraday Trans. 7 , 69, 2130 (1973). (29) L. Bertsch and H. W. Habgood J. fhys. Chem., 67, 1621 (1963). (30) C. L. Angell and P. C. Schaffer, J . fhys. Chem., 70, 1413 (1966). (31) P. E. Riley and K. Seff, Inorg. Chem., 13, 1355 (1974). (32) W. J. Mortier, J. J. Pluth, and J. V. Smith, Mat. Res. Bull., 12, 103 (1977). (33) W. J. Mortier, J. J. Pluth, and J. V. Smith, Z. Kristallogr., 144, 32 (1976). (34) F. P. Billingsley and M. Krauss, J. Chem. fhys., 60, 4130 (1974). (35) S. C. White and H. W. Thompson, f r o c . R. Soc. London, Ser. A , 291, 460 (1966). (36) L. A. Curtiss, and J. A. Pople, J . Mol. Spectrosc., 46, 413 (1973). (37) H. P. Figeys, P. Geerlings, and C. Van Alsenoy, in preparation.

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