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Quantum chemical calculations on cationic positions and adsorption complexes in A-type zeolite
H.K. Beyer, H.G. Karge, I. Kiricsi and J.B. Nagy (Eds.)
Catalysis by Microporous Materials Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All rights reserved.
771
Quantum chemical calculations on cationic positions and adsorption complexes in A-type zeolite aG. Tasi, aI. Kiricsi, aI. Farkas, bL. Nyerges and ~H. F6rster aApplied Chemistry Department, J6zsef Attila University, H-6720 Szeged, Rerrich B. t6r 1., Hungary bDepartment of Medical Chemistry, Albert Szent-Gy6rgyi Medical University, H-6720 Szeged, D6m t6r 8., Hungary Clnstitute of Physical Chemistry, University of Hamburg, Bundesstrasse 45, D-20146 Hamburg, Germany
Semiempirical quantum chemical calculations (MNDO, AM1 and PM3) have been performed for determining the possible positions of exchangeable cations and geometries of sorption complexes of cyclopropane and propene in zeolite A.
1. INTRODUCTION In hydrocarbon processing the A-type zeolites are frequently used as adsorbents for purifying feeds and separating products but never as catalysts due to some inappropriate characteristics (e.g. stability and pore aperture). However, the simplicity of its structure and its well characterized ion-exchange properties make zeolite A suitable to models for investigating the redistribution of cations after various treatments and for studying adsorbentadsorbate interactions (which finally may lead to chemical transformations). Adsorption and reaction of small hydrocarbons have been studied to test the strength of adsorption interactions and catalytic activity of different cations in zeolite A. As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite; (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C =C stretching frequency; (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed; (iv) the rate of cyclopropane isomerization is affected by the cation type; (v) a reactant shape selectivity is observed for the cyclopropane/NaA system; (vi) a peculiar catalytic behaviour is found for LiA; (vii) only Co § ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. The aim of this contribution is to determine the possible cationic positions in zeolite A by
772 means of quantum chemical calculations and to compare them with X-ray results and to describe the interactions between cations and propene or cyclopropane, respectively. 2. METHODS MNDO [5], AM1 [6] and PM3 [7] semiempirical quantum chemical methods were used for the quantitative description of the zeolite A structure, the calculation of the possible positions of exchangeable cations and the geometries of the adsorption complexes. The calculations were performed with the PcMol program system [8]. For theoretical investigations, provided that the phenomena to be studied are determined by the local environment, clusters terminated by hydrogen atoms were used. This assumption is supported by the results discussed in the previous section. The experimentally determined cationic positions are shown in Figure 1. It can be seen that the cations are located near the O4-, 06and O8-rings in the dehydrated zeolite A. From this follows that these oxygen rings can be regarded as the smallest clusters for modelling the structure. To increase the size of our cluster models, quantum chemical calculations were also performed for the pseudo unit cell (sodalite unit) of zeolite A and for the o~ cage of the unit cell. In each case full geometry optimization was performed without symmetry constraints. In the calculations long-range interactions were neglected, the role of which has not been revealed yet [9]. In recent theoretical studies the geometries of the cluster and the adsorbate molecule were regarded as rigid bodies and only the relative orientations and the distances of these units were varied [10]. Unfortunately, these types of calculations did not supply information on the changes of the geometries and the physico-chemical properties of the cluster ("adsorbent") as well as the adsorbate induced by the adsorption interactions. Figure 1. The experimentally determined positions of cations in zeolite A.
3. RESULTS AND DISCUSSION
The structure of faujasites and zeolite A in various ionic forms has been analyzed by X-ray diffraction in detail. The cationic positions can be determined in the fully dehydrated samples using X-ray diffraction, neutron diffraction, far IR spectroscopic, and chemical methods. Since the exchangeable cations are located in the plane or near the plane of the O4-, 06- and O8-rings, these structural subunits can be regarded the most appropriate small cluster models. The O6-ring filling with cations is very often used as a cluster model in quantum chemical calculations [10]. For cluster termination generally hydrogen atoms are used. However, another suitable method would be to apply appropriate pseudo atoms [11].
773 In the case of full geometry optimization, the latter method cannot be applied. Consequently, in this work hydrogen atoms and OH groups are used to terminate the clusters. Full geometry optimization was performed for the O4-, 06- and the O8-rings. Geometrical parameters determined by different methods are listed in Table 1. The table also contains the optimizedgeometry parameters of the Si2A1204H~- cluster in parentheses. Actually, it cannot be expected that the geometrical data of the clusters without exchangeable cations totally correspond to the X-ray diffraction data. In any case, we can conclude that the calculated bond distances, bond angles, and torsional angles are close to the experimentally determined data. It is worth checking whether the four oxygen atoms of the ring are positioned in the same plane. (X-ray diffraction results indicate this.) It can be seen from the data of Table 1 that the improper torsional angle is very close to zero when the cluster is terminated by hydrogen atoms, which means that the ring oxygen atoms are located in the same plane. When the cluster is terminated by OH groups dissimilar results were obtained. Significant deviation can be seen in the case of the MNDO (7.1 ~ and AM1 (18.1 ~ methods. These results show that terminating the clusters with OH groups is not advantageous for full geometry optimization, since the OH groups moving freely can modify the cluster geometry significantly. Table 1 The optimized geometric parameters of the Si2A1204(OH)8:- and Si2A1204H82- clusters. PM3
AM1
MNDO
1.680 (1.715) 1.767( - ) 1.736 (1.757) 1.762( - ) 0.949 (1.477) 0.953 (1.456)
1.607 (1.603) 1.693( - ) 1.708 (1.711) 1.756( - ) 0.933 (1.427) 0.933 (1.475)
124.1 (120.0) 111.2 (108.4) 147.9 (132.5)
116.6 (113.4) 108.7 (107.5) 156.1 (159.2)
Bond length (A) Si-OA1 Si-OH A1-OSi AI-OH SiO-H (Si-H) A10-H (A1-H)
1.653 (1.677) 1.724( - ) 1.784 (1.786) 1.799( - ) 0.946 (1.547) 0.943 (1.602) Bond angle (~
O-Si-O O-AI-O Si-O-A1
123.4 (118.8) 110.2 (118.7) 152.5 (129.4)
Improper torsional angle (o) O4-ring
0.6 (0.0)
18.1 (1.7)
7.1 (1.9)
4.390 (4.224) 4.881 (4.757)
4.487 (4.490) 4.719 (4.681)
Distance (.~) Si-Si A1-A1
4.499 (4.030) 4.929 (4.689) ....
774 The fully optimized geometrical data of the O6-ring terminated by OH groups (Si3A1306(OH)123-) determined by various methods are listed in Table 2. Values in parentheses are referred to the cluster terminated by hydrogen atoms. Table 2 The optimized geometric parameters of the Si3A1306(OH)12 -3 and Si3A1306Hlz -3 clusters. PM3
AM1
MNDO
1.694 (1.719) 1.778( - ) 1.732 (1.756) 1.762( - ) 0.949 (1.487) 0.953 (1.466)
1.602 (1.601) 1.699( - ) 1.702 (1.707) 1.761 ( - ) 0.932 (1.424) 0.934 (1.491)
121.4 (111.6) 111.4 (109.8) 159.0 (132.5)
117.1 (114.1) 111.2 (111.0) 155.6 (161.8)
Bond length (./~) Si-OA1 Si-OH AI-OSi AI-OH SiO-H A10-H
1.643 (1.678) 1.726( - ) 1.773 (1.785) 1.805( - ) 0.946 (1.553) 0.943 (1.611) Bond angle (o)
O-Si-O O-A1-O Si-O-A1
121.3 (112.4) 116.0 (123.0) 178.1 (131.8)
Diagonal average distance (.~) A1-Si
6.817 (6.044)
6.532 (6.161)
6.376 (6.179)
In the case of the sodalite unit, the influence of the rigidity of the skeleton plays an important role. The trial geometry of the sodalite unit was obtained from X-ray data, then the cluster was terminated by hydrogen atoms. We performed full geometry optimization using the semiempirical methods. The optimized geometrical parameters of the O4-ring of the sodalite unit are listed in Table 3, those of the O6-ring in Table 4. Let us compare the data of Tables 1-4. When we terminate a cluster with OH groups each T atom is surrounded by four oxygen atoms. If hydrogen atoms are used for terminating the isolated ring, each T atom is linked to two oxygen atoms. For the sodalite unit hydrogen atoms were used for terminating the system, therefore three oxygen atoms are joined to each T atom. From this aspect the model of the sodalite unit is in between those mentioned before. From MNDO, AM 1 and PM3 results the four oxygen atoms of the O4-ring in the sodalite unit are located in the same plane. For the O6-ring of the sodalite unit, the average diagonal A1-Si distance increases compared to the isolated O6-ring. The next problem to investigate was whether the cationic positions could also be determined in zeolite A and faujasites by semiempirical methods. From this aspect the MNDO method is the best choice, since most of alkali and alkaline earth elements are parameterized within this approximation.
775 Single SCF (so-called reaction coordinate or single point) calculations are proved to be useful in construction of the trial geometry of the interacting molecules (supermolecule) before PM3 AM1 MNDO performing full geometry optimization. In this case one Bond length geometry parameter is changed stepwise and 1 SCF calculation is Si-OA1 1.685 1.709 1.629 performed in each step. A1-OSi 1.796 1.746 1.719 In order to determine the possible positions of the Na + ions, reaction Bond angle (~ coordinate calculations were performed along the C2 axis of the O-Si-O 118.1 116.6 113.9 isolated O4-ring and along the C3 axis O-A1-O 112.7 111.4 109.8 of the isolated O6-ring. The MNDO Si-O-A1 151.8 153.3 157.7 parameters of sodium were obtained Improper torsional angle (o) from literature [12]. During these calculations the geometric parameters O4-ring 0.1 0.0 0.1 were fixed, i.e. only 1 SCF calculation was performed in each Distance (,~) step. The results are shown in Figures 2 and 3. It can be concluded that the Si-Si 4.697 4.633 4.583 local minima of the interaction energy A1-A1 4.852 4.867 4.709 are located symmetrically in front of and behind the plane of O4-ring. The distance of the minimum from the center of the O4-ring is 1.737 ,~. For the O6-ring the local minimum of the interacion energy is in the plane of three 03 oxygen atoms. Table 3 The optimized geometric parameters of the O4-ring in the sodalite unit.
(,~,)
Interaction energy (kJ/mol)
Interaction energy (kJ/mol)
/
-680 -760 -840
0
~Distanee
'-'I',/,,(pm) '30o~~j! so \j 300 )00 "~/
Figure 2. Single point calculations with Na + ion for the isolated O4-ring.
-920 -200 -1-20-~N~
J
120 200
Figure 3. Single point calculations with Na + ion for the isolated O6-ring.
776 Geometries corresponding to the local minima obtained in reaction coordinate calculations are chosen as initial geometries for the full geometry optimization. Some optimized geometry parameters are listed in Table 5. It can be concluded that the picture obtained with reaction coordinate calculations does not change significantly after having performed full geometry optimization. The Na + ion is located at 1.828 ,/~ distance from the plane of the O4-ring. For the O6-ring the Na + ion is located in good approximation in the plane of three 03 oxygen atoms. The out-of-plane angle is 5.1 ~. Comparing the calculated results to the X-ray diffraction data, good correlation may be established. The calculated Na(A)-O3 and Na(H)-O distances are somewhat longer, however, the Na(A)-O~ distance is shorter than the experimental values [13]. For the O6-ring the calculated out-of-plane angle of the Na + ion is in good agreement with the experimental value. Reaction coordinate calculations were also performed for the sodalite unit. The local minimum of the interaction energy calculated along the C~ axis is deeper inside the sodalite unit than outside the cage. It can also be concluded that position A is much more favoured by the Na § ion than position H. Similar calculations were carried Table 4 out for Li+, Mg 2+, C a 2+ and Zn 2+ The optimized geometric parameters of the ions. Selected results obtained for the O6-ring in the sodalite unit. Li + ion/sodalite unit system are shown in Figures 4 and 5. Although the PM3 AM1 MNDO results for the Li + ion are very similar to those of the Na + ion, a very Bond length (,~) important difference is that the Li + ion is located much more closer to the Si-O3A1 1.685 1.709 1.629 framework. For the isolated O4-ring, Si-O2A1 1.681 1.699 1.625 the distances of the Li + and Na + ions A1-O3Si 1.796 1.746 1.719 from the plane of four oxygen atoms A1-O2Si 1.790 1.741 1.712 are 1.251 A and 1.828 A, Bond angle (o) respectively. The calculated Li(A)-O3 distance is 2.065 A. This value is somewhat O 2 - S i - O 3 117.0 116.6 115.7 longer than 1.94 A experimentally O 2 - A I - O 3 118.7 115.7 115.1 determined [14]. The calculated Si-O2-AI 167.9 174.7 176.6 Li(A)-O2 distance (3.053 ,~) is very Si-O3-A1 151.8 153.3 157.7 close to the experimental value (3.09 Diagonal average distance (A) and 3.02 J~). The out-of-plane angle is also smaller for the Li + (1.4 ~ than for AI-Si 6.827 6.791 6.616 the Na + ion (5.1~ This proves also a good agreement between theory and experiment. The calculated T-O3-T bond angle (131 ~ is close to the experimental one (130 ~ in the case of the Li+- containing cluster. These values for the Na§ cluster are 143 ~ and 146~ respectively. The calculated Li(A)-A1 distance was found to be 3.230 ,/~. For the fully optimized Li+/sodalite unit system the total energies showed the following order: Li(H) > Li(G) > Li(A). The differences at 298 K are as follows: AE[Li(H)-Li(A)] = 48 kcal/mol, AE[Li(G)-Li(A)] = 38 kcal/mol and AE[Li(H)-Li(G)] = 10 kcal/mol.
777
Interaefionenergy(kJ/mol)
Interaction energy O0/mol)
-2420 -2400
-2460 -20.t . . . .
300 . . . . ......
70C '
Distanee(pm)
istanee
-2560 .
-200
Figure 4. Single point calculations with Li + ion for O4-ring in the sodalite unit.
.
.
.
.
.
.
.
t 200 400 600
,~,/-
1. -
_
Figure 5. Single point calculations with Li § ion for O6-ring in the sodalite unit.
Adsorption and IR spectroscopic measurements proved that significant structural changes occured in the adsorbed cyclopropane and propene molecules upon interaction with the cations. Similarly, the results of X-ray diffraction measurements revealed that significant changes took place in the geometry of the adsorption sites, first of all in the positions of the cations. In the NaA zeolite the adsorption sites are Table 5 primarily in front of the Na + ions located in Calculated geometric parameters for position A, the O6-ring filled with Na § ion and the isolated 04- and O6-rings filled terminated by hydrogen atoms was chosen as a with Na § ion. model of the adsorption site. The full geometry optimization revealed that the out-of-plane angle Distance (,~) of the Na § ion from the plane of three 03 atoms was 5.1 ~ (see results discussed above). The 2.433 Na(A)-O3 interaction of cyclopropane and propene with this Na(A)-O2 2.730 cluster was studied within the MNDO Na(A)-A1 3.324 approximation using full geometry optimization. Na(H)-O 2.635 In accordance with the experimental results the cyclopropane molecule adsorbs "face-on" on the Out of plane angle (o) sodium ion. The distances between the carbon atoms and the sodium ion are 2.798, 2.793 and Na(A)-303 5.1 2.795 A. The geometry of the sorption complex is shown in Figure 6. As a result of interaction, the out-of-plane angle of the sodium ion increased from 5.1 ~ to 33.7 ~ This means that the cyclopropane molecule pulls the ion out from the plane. The geometry of the adsorption complex formed by Na § ion and propene is shown in Figure 7. The distances between the carbon atoms involved in the double bond and the sodium ion are 2.623 and 2.531 ,~. The out-of-plane angle of the sodium ion from the plane of three 03 atoms was 30.8 ~. It is worth comparing the total energies of these two adsorption
778 complexes. The propene complex is more stable by 12.1 kcal/mol than the cyclopropane one in accordance with our experimental finding that propene adsorbed more strongly than cyclopropane.
~ 279.5pm~~_ ,,, !~ .,'/ ",,ii N~
.
,,,..,~ 135.5pm 253.1pm,: "N~
~
Na p +;.f.."261.2 """ "'"~m
/
Figure 6. The optimized geometry of the cyclopropane adsorption complex.
Figure 7. The optimized geometry of the propene adsorption complex.
4. REFERENCES
1. 2. 3. 4. 5. 6.
H. F6rster and J. Seebode, Zeolites, 3 (1983) 63. G. Tasi, I. Kiricsi, F. Berger and P. Fejes, Acta Phys. et Chem. Szeged, 33 (1987) 99. I. Kiricsi, G. Tasi, P. Fejes and F. Berger, J. Mol. Catal., 51 (1989) 341. G. Tasi, I. Kiricsi, F. Evanics, E. Nagy and P. Fejes, Acta Chim. Hung., 128 (1991) 119. M.J.S. Dewar and W. Thiel, J. Am. Chem. Soc., 99 (1977) 4899. M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, J. Am. Chem. Soc., 107 (1985) 3902. 7. J.J.P. Stewart, J. Comput. Chem., 10 (1989) 209, 221. 8. G. Tasi, I. Pfilink6, J. Halfisz and G. Nfiray-Szab6, Semiempirical Calculations on Microcomputers, CheMicro, Budapest, 1992. 9. E.H. Teunissen, F.B. van Duijneveldt and R.A. van Santen, J. Phys. Chem., 96 (1992) 366. 10. O. Zakharieva-Pencheva, M. Grodzicki, H. Btise and H. F6rster, J. Mol. Struct., 218 (1990) 405. J. Sauer and R. Zahradnik, Int. J. Quantum. Chem., 26 (1984) 793. 11. I. L/tszl6, Int. J. Quantum. Chem., 21 (1982) 813. 12. A.A. Voityuk, Zh. Strukt. Khim., 29 (1988) 138. 13. J.J. Pluth and J.V. Smith, J. Phys. Chem., 83 (1979) 741. 14. Z. Jirgtk, V. Bosacek, S. Vratislav, H. Herden, R. Sch/511ner, W.J. Mortier, L. Gellens and J.B. Uytterhoeven, Zeolites, 3 (1983) 255.
Catalysis by Microporous Materials Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All rights reserved.
771
Quantum chemical calculations on cationic positions and adsorption complexes in A-type zeolite aG. Tasi, aI. Kiricsi, aI. Farkas, bL. Nyerges and ~H. F6rster aApplied Chemistry Department, J6zsef Attila University, H-6720 Szeged, Rerrich B. t6r 1., Hungary bDepartment of Medical Chemistry, Albert Szent-Gy6rgyi Medical University, H-6720 Szeged, D6m t6r 8., Hungary Clnstitute of Physical Chemistry, University of Hamburg, Bundesstrasse 45, D-20146 Hamburg, Germany
Semiempirical quantum chemical calculations (MNDO, AM1 and PM3) have been performed for determining the possible positions of exchangeable cations and geometries of sorption complexes of cyclopropane and propene in zeolite A.
1. INTRODUCTION In hydrocarbon processing the A-type zeolites are frequently used as adsorbents for purifying feeds and separating products but never as catalysts due to some inappropriate characteristics (e.g. stability and pore aperture). However, the simplicity of its structure and its well characterized ion-exchange properties make zeolite A suitable to models for investigating the redistribution of cations after various treatments and for studying adsorbentadsorbate interactions (which finally may lead to chemical transformations). Adsorption and reaction of small hydrocarbons have been studied to test the strength of adsorption interactions and catalytic activity of different cations in zeolite A. As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite; (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C =C stretching frequency; (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed; (iv) the rate of cyclopropane isomerization is affected by the cation type; (v) a reactant shape selectivity is observed for the cyclopropane/NaA system; (vi) a peculiar catalytic behaviour is found for LiA; (vii) only Co § ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. The aim of this contribution is to determine the possible cationic positions in zeolite A by
772 means of quantum chemical calculations and to compare them with X-ray results and to describe the interactions between cations and propene or cyclopropane, respectively. 2. METHODS MNDO [5], AM1 [6] and PM3 [7] semiempirical quantum chemical methods were used for the quantitative description of the zeolite A structure, the calculation of the possible positions of exchangeable cations and the geometries of the adsorption complexes. The calculations were performed with the PcMol program system [8]. For theoretical investigations, provided that the phenomena to be studied are determined by the local environment, clusters terminated by hydrogen atoms were used. This assumption is supported by the results discussed in the previous section. The experimentally determined cationic positions are shown in Figure 1. It can be seen that the cations are located near the O4-, 06and O8-rings in the dehydrated zeolite A. From this follows that these oxygen rings can be regarded as the smallest clusters for modelling the structure. To increase the size of our cluster models, quantum chemical calculations were also performed for the pseudo unit cell (sodalite unit) of zeolite A and for the o~ cage of the unit cell. In each case full geometry optimization was performed without symmetry constraints. In the calculations long-range interactions were neglected, the role of which has not been revealed yet [9]. In recent theoretical studies the geometries of the cluster and the adsorbate molecule were regarded as rigid bodies and only the relative orientations and the distances of these units were varied [10]. Unfortunately, these types of calculations did not supply information on the changes of the geometries and the physico-chemical properties of the cluster ("adsorbent") as well as the adsorbate induced by the adsorption interactions. Figure 1. The experimentally determined positions of cations in zeolite A.
3. RESULTS AND DISCUSSION
The structure of faujasites and zeolite A in various ionic forms has been analyzed by X-ray diffraction in detail. The cationic positions can be determined in the fully dehydrated samples using X-ray diffraction, neutron diffraction, far IR spectroscopic, and chemical methods. Since the exchangeable cations are located in the plane or near the plane of the O4-, 06- and O8-rings, these structural subunits can be regarded the most appropriate small cluster models. The O6-ring filling with cations is very often used as a cluster model in quantum chemical calculations [10]. For cluster termination generally hydrogen atoms are used. However, another suitable method would be to apply appropriate pseudo atoms [11].
773 In the case of full geometry optimization, the latter method cannot be applied. Consequently, in this work hydrogen atoms and OH groups are used to terminate the clusters. Full geometry optimization was performed for the O4-, 06- and the O8-rings. Geometrical parameters determined by different methods are listed in Table 1. The table also contains the optimizedgeometry parameters of the Si2A1204H~- cluster in parentheses. Actually, it cannot be expected that the geometrical data of the clusters without exchangeable cations totally correspond to the X-ray diffraction data. In any case, we can conclude that the calculated bond distances, bond angles, and torsional angles are close to the experimentally determined data. It is worth checking whether the four oxygen atoms of the ring are positioned in the same plane. (X-ray diffraction results indicate this.) It can be seen from the data of Table 1 that the improper torsional angle is very close to zero when the cluster is terminated by hydrogen atoms, which means that the ring oxygen atoms are located in the same plane. When the cluster is terminated by OH groups dissimilar results were obtained. Significant deviation can be seen in the case of the MNDO (7.1 ~ and AM1 (18.1 ~ methods. These results show that terminating the clusters with OH groups is not advantageous for full geometry optimization, since the OH groups moving freely can modify the cluster geometry significantly. Table 1 The optimized geometric parameters of the Si2A1204(OH)8:- and Si2A1204H82- clusters. PM3
AM1
MNDO
1.680 (1.715) 1.767( - ) 1.736 (1.757) 1.762( - ) 0.949 (1.477) 0.953 (1.456)
1.607 (1.603) 1.693( - ) 1.708 (1.711) 1.756( - ) 0.933 (1.427) 0.933 (1.475)
124.1 (120.0) 111.2 (108.4) 147.9 (132.5)
116.6 (113.4) 108.7 (107.5) 156.1 (159.2)
Bond length (A) Si-OA1 Si-OH A1-OSi AI-OH SiO-H (Si-H) A10-H (A1-H)
1.653 (1.677) 1.724( - ) 1.784 (1.786) 1.799( - ) 0.946 (1.547) 0.943 (1.602) Bond angle (~
O-Si-O O-AI-O Si-O-A1
123.4 (118.8) 110.2 (118.7) 152.5 (129.4)
Improper torsional angle (o) O4-ring
0.6 (0.0)
18.1 (1.7)
7.1 (1.9)
4.390 (4.224) 4.881 (4.757)
4.487 (4.490) 4.719 (4.681)
Distance (.~) Si-Si A1-A1
4.499 (4.030) 4.929 (4.689) ....
774 The fully optimized geometrical data of the O6-ring terminated by OH groups (Si3A1306(OH)123-) determined by various methods are listed in Table 2. Values in parentheses are referred to the cluster terminated by hydrogen atoms. Table 2 The optimized geometric parameters of the Si3A1306(OH)12 -3 and Si3A1306Hlz -3 clusters. PM3
AM1
MNDO
1.694 (1.719) 1.778( - ) 1.732 (1.756) 1.762( - ) 0.949 (1.487) 0.953 (1.466)
1.602 (1.601) 1.699( - ) 1.702 (1.707) 1.761 ( - ) 0.932 (1.424) 0.934 (1.491)
121.4 (111.6) 111.4 (109.8) 159.0 (132.5)
117.1 (114.1) 111.2 (111.0) 155.6 (161.8)
Bond length (./~) Si-OA1 Si-OH AI-OSi AI-OH SiO-H A10-H
1.643 (1.678) 1.726( - ) 1.773 (1.785) 1.805( - ) 0.946 (1.553) 0.943 (1.611) Bond angle (o)
O-Si-O O-A1-O Si-O-A1
121.3 (112.4) 116.0 (123.0) 178.1 (131.8)
Diagonal average distance (.~) A1-Si
6.817 (6.044)
6.532 (6.161)
6.376 (6.179)
In the case of the sodalite unit, the influence of the rigidity of the skeleton plays an important role. The trial geometry of the sodalite unit was obtained from X-ray data, then the cluster was terminated by hydrogen atoms. We performed full geometry optimization using the semiempirical methods. The optimized geometrical parameters of the O4-ring of the sodalite unit are listed in Table 3, those of the O6-ring in Table 4. Let us compare the data of Tables 1-4. When we terminate a cluster with OH groups each T atom is surrounded by four oxygen atoms. If hydrogen atoms are used for terminating the isolated ring, each T atom is linked to two oxygen atoms. For the sodalite unit hydrogen atoms were used for terminating the system, therefore three oxygen atoms are joined to each T atom. From this aspect the model of the sodalite unit is in between those mentioned before. From MNDO, AM 1 and PM3 results the four oxygen atoms of the O4-ring in the sodalite unit are located in the same plane. For the O6-ring of the sodalite unit, the average diagonal A1-Si distance increases compared to the isolated O6-ring. The next problem to investigate was whether the cationic positions could also be determined in zeolite A and faujasites by semiempirical methods. From this aspect the MNDO method is the best choice, since most of alkali and alkaline earth elements are parameterized within this approximation.
775 Single SCF (so-called reaction coordinate or single point) calculations are proved to be useful in construction of the trial geometry of the interacting molecules (supermolecule) before PM3 AM1 MNDO performing full geometry optimization. In this case one Bond length geometry parameter is changed stepwise and 1 SCF calculation is Si-OA1 1.685 1.709 1.629 performed in each step. A1-OSi 1.796 1.746 1.719 In order to determine the possible positions of the Na + ions, reaction Bond angle (~ coordinate calculations were performed along the C2 axis of the O-Si-O 118.1 116.6 113.9 isolated O4-ring and along the C3 axis O-A1-O 112.7 111.4 109.8 of the isolated O6-ring. The MNDO Si-O-A1 151.8 153.3 157.7 parameters of sodium were obtained Improper torsional angle (o) from literature [12]. During these calculations the geometric parameters O4-ring 0.1 0.0 0.1 were fixed, i.e. only 1 SCF calculation was performed in each Distance (,~) step. The results are shown in Figures 2 and 3. It can be concluded that the Si-Si 4.697 4.633 4.583 local minima of the interaction energy A1-A1 4.852 4.867 4.709 are located symmetrically in front of and behind the plane of O4-ring. The distance of the minimum from the center of the O4-ring is 1.737 ,~. For the O6-ring the local minimum of the interacion energy is in the plane of three 03 oxygen atoms. Table 3 The optimized geometric parameters of the O4-ring in the sodalite unit.
(,~,)
Interaction energy (kJ/mol)
Interaction energy (kJ/mol)
/
-680 -760 -840
0
~Distanee
'-'I',/,,(pm) '30o~~j! so \j 300 )00 "~/
Figure 2. Single point calculations with Na + ion for the isolated O4-ring.
-920 -200 -1-20-~N~
J
120 200
Figure 3. Single point calculations with Na + ion for the isolated O6-ring.
776 Geometries corresponding to the local minima obtained in reaction coordinate calculations are chosen as initial geometries for the full geometry optimization. Some optimized geometry parameters are listed in Table 5. It can be concluded that the picture obtained with reaction coordinate calculations does not change significantly after having performed full geometry optimization. The Na + ion is located at 1.828 ,/~ distance from the plane of the O4-ring. For the O6-ring the Na + ion is located in good approximation in the plane of three 03 oxygen atoms. The out-of-plane angle is 5.1 ~. Comparing the calculated results to the X-ray diffraction data, good correlation may be established. The calculated Na(A)-O3 and Na(H)-O distances are somewhat longer, however, the Na(A)-O~ distance is shorter than the experimental values [13]. For the O6-ring the calculated out-of-plane angle of the Na + ion is in good agreement with the experimental value. Reaction coordinate calculations were also performed for the sodalite unit. The local minimum of the interaction energy calculated along the C~ axis is deeper inside the sodalite unit than outside the cage. It can also be concluded that position A is much more favoured by the Na § ion than position H. Similar calculations were carried Table 4 out for Li+, Mg 2+, C a 2+ and Zn 2+ The optimized geometric parameters of the ions. Selected results obtained for the O6-ring in the sodalite unit. Li + ion/sodalite unit system are shown in Figures 4 and 5. Although the PM3 AM1 MNDO results for the Li + ion are very similar to those of the Na + ion, a very Bond length (,~) important difference is that the Li + ion is located much more closer to the Si-O3A1 1.685 1.709 1.629 framework. For the isolated O4-ring, Si-O2A1 1.681 1.699 1.625 the distances of the Li + and Na + ions A1-O3Si 1.796 1.746 1.719 from the plane of four oxygen atoms A1-O2Si 1.790 1.741 1.712 are 1.251 A and 1.828 A, Bond angle (o) respectively. The calculated Li(A)-O3 distance is 2.065 A. This value is somewhat O 2 - S i - O 3 117.0 116.6 115.7 longer than 1.94 A experimentally O 2 - A I - O 3 118.7 115.7 115.1 determined [14]. The calculated Si-O2-AI 167.9 174.7 176.6 Li(A)-O2 distance (3.053 ,~) is very Si-O3-A1 151.8 153.3 157.7 close to the experimental value (3.09 Diagonal average distance (A) and 3.02 J~). The out-of-plane angle is also smaller for the Li + (1.4 ~ than for AI-Si 6.827 6.791 6.616 the Na + ion (5.1~ This proves also a good agreement between theory and experiment. The calculated T-O3-T bond angle (131 ~ is close to the experimental one (130 ~ in the case of the Li+- containing cluster. These values for the Na§ cluster are 143 ~ and 146~ respectively. The calculated Li(A)-A1 distance was found to be 3.230 ,/~. For the fully optimized Li+/sodalite unit system the total energies showed the following order: Li(H) > Li(G) > Li(A). The differences at 298 K are as follows: AE[Li(H)-Li(A)] = 48 kcal/mol, AE[Li(G)-Li(A)] = 38 kcal/mol and AE[Li(H)-Li(G)] = 10 kcal/mol.
777
Interaefionenergy(kJ/mol)
Interaction energy O0/mol)
-2420 -2400
-2460 -20.t . . . .
300 . . . . ......
70C '
Distanee(pm)
istanee
-2560 .
-200
Figure 4. Single point calculations with Li + ion for O4-ring in the sodalite unit.
.
.
.
.
.
.
.
t 200 400 600
,~,/-
1. -
_
Figure 5. Single point calculations with Li § ion for O6-ring in the sodalite unit.
Adsorption and IR spectroscopic measurements proved that significant structural changes occured in the adsorbed cyclopropane and propene molecules upon interaction with the cations. Similarly, the results of X-ray diffraction measurements revealed that significant changes took place in the geometry of the adsorption sites, first of all in the positions of the cations. In the NaA zeolite the adsorption sites are Table 5 primarily in front of the Na + ions located in Calculated geometric parameters for position A, the O6-ring filled with Na § ion and the isolated 04- and O6-rings filled terminated by hydrogen atoms was chosen as a with Na § ion. model of the adsorption site. The full geometry optimization revealed that the out-of-plane angle Distance (,~) of the Na § ion from the plane of three 03 atoms was 5.1 ~ (see results discussed above). The 2.433 Na(A)-O3 interaction of cyclopropane and propene with this Na(A)-O2 2.730 cluster was studied within the MNDO Na(A)-A1 3.324 approximation using full geometry optimization. Na(H)-O 2.635 In accordance with the experimental results the cyclopropane molecule adsorbs "face-on" on the Out of plane angle (o) sodium ion. The distances between the carbon atoms and the sodium ion are 2.798, 2.793 and Na(A)-303 5.1 2.795 A. The geometry of the sorption complex is shown in Figure 6. As a result of interaction, the out-of-plane angle of the sodium ion increased from 5.1 ~ to 33.7 ~ This means that the cyclopropane molecule pulls the ion out from the plane. The geometry of the adsorption complex formed by Na § ion and propene is shown in Figure 7. The distances between the carbon atoms involved in the double bond and the sodium ion are 2.623 and 2.531 ,~. The out-of-plane angle of the sodium ion from the plane of three 03 atoms was 30.8 ~. It is worth comparing the total energies of these two adsorption
778 complexes. The propene complex is more stable by 12.1 kcal/mol than the cyclopropane one in accordance with our experimental finding that propene adsorbed more strongly than cyclopropane.
~ 279.5pm~~_ ,,, !~ .,'/ ",,ii N~
.
,,,..,~ 135.5pm 253.1pm,: "N~
~
Na p +;.f.."261.2 """ "'"~m
/
Figure 6. The optimized geometry of the cyclopropane adsorption complex.
Figure 7. The optimized geometry of the propene adsorption complex.
4. REFERENCES
1. 2. 3. 4. 5. 6.
H. F6rster and J. Seebode, Zeolites, 3 (1983) 63. G. Tasi, I. Kiricsi, F. Berger and P. Fejes, Acta Phys. et Chem. Szeged, 33 (1987) 99. I. Kiricsi, G. Tasi, P. Fejes and F. Berger, J. Mol. Catal., 51 (1989) 341. G. Tasi, I. Kiricsi, F. Evanics, E. Nagy and P. Fejes, Acta Chim. Hung., 128 (1991) 119. M.J.S. Dewar and W. Thiel, J. Am. Chem. Soc., 99 (1977) 4899. M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, J. Am. Chem. Soc., 107 (1985) 3902. 7. J.J.P. Stewart, J. Comput. Chem., 10 (1989) 209, 221. 8. G. Tasi, I. Pfilink6, J. Halfisz and G. Nfiray-Szab6, Semiempirical Calculations on Microcomputers, CheMicro, Budapest, 1992. 9. E.H. Teunissen, F.B. van Duijneveldt and R.A. van Santen, J. Phys. Chem., 96 (1992) 366. 10. O. Zakharieva-Pencheva, M. Grodzicki, H. Btise and H. F6rster, J. Mol. Struct., 218 (1990) 405. J. Sauer and R. Zahradnik, Int. J. Quantum. Chem., 26 (1984) 793. 11. I. L/tszl6, Int. J. Quantum. Chem., 21 (1982) 813. 12. A.A. Voityuk, Zh. Strukt. Khim., 29 (1988) 138. 13. J.J. Pluth and J.V. Smith, J. Phys. Chem., 83 (1979) 741. 14. Z. Jirgtk, V. Bosacek, S. Vratislav, H. Herden, R. Sch/511ner, W.J. Mortier, L. Gellens and J.B. Uytterhoeven, Zeolites, 3 (1983) 255.