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The interaction of amine bases on the Lewis acid sites of aluminum oxide — a theoretical study
Surface Science 277 (1992) 389-394 North-Holland
surface science
The interaction of amine bases on the Lewis acid sites of aluminum oxide - a theoretical study Pipsa Hirva and Tapani
A. Pakkanen
Department of Chemistry, Unitiersit‘yof Joensuy
P.O. Box Ill,
SF-80101 Joensuu, Finland
Received 25 January 1992; accepted for publication 30 June 1992
In the present work amine interactions with aluminum oxide surfaces are modeled with quantum chemical methods. The results from our valence calculations with a minimal basis are compared with standard all-electron calculations with different basis sets. The tetrahedral and octahedral Lewis acid sites are considered as active sites for ammonia and pyridine adsorption. The interaction energies are overestimated, since small cluster models cannot represent the effects of neigh~ring acid-base sites. However, qualitative information on the adso~tion on different sites can be obtained. The results show higher acidity and thus higher amine interaction energies for the tetrahedral Lewis acid sites compared to the octahedral ones. The adsorption energies are similar for both ammonia and pyridine indicating that the amine bases are capable of displacing water from the surface.
1. Introduction Inorganic oxides are widely used as carriers for heterogeneous catalysts, since these materials are thermahy stable and suitable for dispersion of catalyti~lIy active com~nents. However, contrary to silica, aluminas are far from being inert materials because of their strong acid-base properties. Furthermore, aluminas exist in a variety of microcrystalline high surface area forms and thus provide rather complex problems especially about the characterization of the surface sites (l,Z]. Theoretical studies are needed to adopt atomic scale knowledge of the various adsorption sites. One of the main features affecting the catalytical properties of aIumina surfaces is the degree of dehydroxyIation. Depending on the pretreatment procedures different surface sites are formed. Spectroscopic studies have revealed at ieast five kinds of surface hydroxyl groups. Moreover, Lewis acid and base sites can be generated after very mild dehydroxylation. At higher temperatures the surface may contain various “defect” sites with
unknown structures. The relative proportions of these sites will depend on the degree of dehydroxylation and on the fact that for a finely divided alumina different crystal faces are exposed in different proportions [2]. Amine bases form active heterogeneous catalysts with transition metal complexes. These catalysts have been found to promote for example alkene hydroformylation and water gas shift reaction [3-61. In this work we have been using ammonia and pyridine to model amine interactions with the Lewis acid sites of aluminum oxide. These interactions can affect the catalyst preparation reactions and therefore the activity of the catalysts. Ammonia adsorption with alumina surfaces has been studied previously using semiempirical methods 171. Simpson has also studied ammonia on different surface sites of silica and alumina with a theoretical method based on the Sanderson’s electronegativity principles [8]. Ammonia and water adsorption have also been modeled on the trigonal aluminum sites of zeolites [9]. Other theoretica studies on alumina concern the
0039~6028/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
390
P. Hirca, T.A. Pakkanen
/ Interaction
of amine bases on the Lewis acid sites of aluminum oxide
acid-base properties of various surface sites [lo131 and the electronic structure of cu-alumina [14-161.
2. Theoretical methods The study of the active sites is a key factor when heterogeneous catalysts are concerned. Experimental methods like IR-spectroscopy have been used in attempt to characterize the nature of different sites during catalyst preparation. Aluminum oxides are particularly problematic because of the variety of possible active sites. Experimental information on Lewis acid sites is especially uncertain. Theoretical methods can give valuable information on the nature of the chemisorption processes and on the relative energies of different sites. Our method of calculation is a frozen core method with different stepwise density approximations for the interactions between the core and valence orbitals [17-191. With this method the computing times can be considerably reduced thus making it possible to calculate larger cluster models. This is particularly important with large adsorbates like pyridine. All computations were made using a minimal basis set (ASA for all stepwise approximations) [19]. The results were compared with standard all-electron calculations with different basis sets. Polarization functions were introduced for aluminum atoms in most cases because of the importance of the aluminum d-orbitals at the surface [16]. The importance of the d-orbitals has also been shown in other inorganic oxides, as in silica (SiO,) [20,21]. Polarization functions on oxygen have shown to have minor effects in silica [22]. The minimal basis set is not likely to give accurate binding energies for the adsorbed molecules because of the large basis set superposition error connected to small basis sets. However, the frozen core method has been shown to diminish the BSSE, since this error is connected to a poor representation of the core orbitals 1231. Furthermore, qualitative comparison of the different sites is possible even at the minimal basis level.
3. Surface models The variety of crystal and surface structures on aluminum oxide form a complication to the experimental and theoretical studies. The fundamental structure of most crystal structures can be described as being a defect spinel, in which the aluminum cations take positions of octahedral and tetrahedral vacant sites [1,24,25]. Alumina surfaces contain various kinds of surface sites including Lewis acid sites (coordinatively unsaturated aluminum atoms), associated basic Al-O sites and at least five kinds of surface hydroxyl groups. However, concerning the amine base interactions on the surface, the Lewis acid sites are the most important adsorption centers. This has been demonstrated experimentally by Majors and Ellis [26]. According to their NMR-experiments pyridine coordinates onto two types of Lewis acid sites, which contain tetrahedrally and octahedrally unsaturated aluminum cations. These sites are generated upon desorption of non-bridged terminal hydroxyl groups. In this study we have adopted small cluster models to represent the tetrahedral and octahedral Lewis acid sites on a spine1 type aluminum oxide. These models are shown in fig. 1. The
Model Ti
Model 01
Model
T2
Model 02
Fig. 1. Cluster models for the tetrahedral (Tl, T2) and octahedral (01, 02) Lewis acid sites on aluminum oxide surfaces.
P. Hirca, T.A. Pakkanen
/ Interaction
391
of amine bases on the Lewis acid sites of aluminum oxide
models Tl and T2 represent the tetrahedral coordination of the aluminum atoms and models 01 and 02 the octahedral coordination. In these models the atoms were frozen to their bulk positions and only the adsorbates were optimized.
4. Water adsorption The associative adsorption of water onto the tetrahedral models Tl and T2 was studied in order to compare the results from our ASA basis set with the standard all-electron calculations. The optimized parameters and the adsorption energies are shown in table 1. The results show shorter water distances from the surface with the split valence basis 3-21G and 3-21G(*) compared to our minimal basis calculations. The interaction energies are also somewhat larger than with the ASA basis set. However, when a larger split valence basis set 6-31G(*) is adopted, the Al-O distance is lenghtened to a same value as with the ASA basis and the interaction energy is considerably reduced. The largest difference in the optimized parameters comes from the bending of
Fig. 2. The basis set dependence on the geometry of adsorbed water on the tetrahedral Lewis acid site 01. For the geometrical parameters, see table 1.
the water molecule from the z-axis orientation, as can be seen in fig. 2. This bending is notable only with the split valence basis; the minimal basis does not show this phenomenon. We also studied the effect of the cluster size on the adsorption energy of water with a larger model T2. This model was only calculated with the minimal basis set. Due to a large cluster size, the change in the charge distribution in the vicin-
Table 1 The optimized parameters and the interaction energies for the adsorption of water on the tetrahedral Lewis acid sites on alumina Model T2
Mode1 Tl
AS A
qW1) q(O4) q(H4) q(H5) Ettot) [au.] Etads) [kJ/mol] Opt. parameters a) rl a T r2 r3 bl b2
1.786 - 0.754 0.426 0.427 - 0.06779 b, 156
ASA( * )
STO-3G
3-21G
3-21G(*)
6-31G(*)
ASA
1.193 - 0.714 0.437 0.438 - 0.06791 b, - 191
1.191 - 0.377 0.289 0.292 - 537.37217 -216
1.474 - 0.818 0.492 0.499 -541.50059 - 267
1.108 - 0.771 0.487 0.494 -541.57805 -257
1.352 - 0.879 0.525 0.525 - 544.42565 - 167
1.584 - 0.162 0.441 0.441 - 0.17342 b, - 228
1.9869 111.6
1.9379 113.3
1.8235 108.3
1.8519 115.1
1.8653 115.3
1.9379 109
46.9 [0.987] [0.987] 124.2 124.2
[:.987] [0.987] 123.4 123.4
0 0.976 0.976 128 123.6
0 0.964 0.968 131 113.9
4.1 0.965 0.967 128.5 116.2
2.5 0.955 0.955 112.7 110.7
1.9437 111.1
LOI [0.987] [0.987] 124.5 124.5
The model symbols refer to the surface models onto which the adsorbate is binding (see fig. 1). Charges (q) were obtained by Mulliken population analysis. a) rl = r(04-All), r2 = r(H4-04), r3 = r(H5-04), a = a(H4-04-H5), bl = b(H4-044All), b2 = b(H5-04-All), Tl = T(H404-All-01). The values in square brackets were not optimized. b, Valence energy.
P. Hirva, T.A. Pakkanen / Interaction of amine bases on the Lewis acid sites of aluminum oxide
392
ity of the adsorption site does not have major influence on the optimimized parameters. On the other hand, the -adsorption energy is somewhat larger when compared to the value of the smaller model within the same basis set. Otherwise, our method seems to give comparable results to the standard all-electron basis calculations. The main advantage in our method is the ability to calculate larger cluster models with reduced computing time, which is important when dealing with large adsorbates like pyridine. In partially ionic materials the neutrality of the cluster is a major requirement for models in adsorption studies. In an ideal crystal the sum of the atomic charges should be zero, for example in alumina 2qCAl) + 3q(O) = 0. In our models the inclusion of aluminum d-orbitals makes the clusters more neutral. An other way of dealing with the neutrality would be the use of pseudoatoms instead of hydrogen termination. However, pseudoatom termination would need careful testing of the models. For qualitative comparison, hydrogen termination should be effective enough.
5. The acid strength of the surface sites The adsorption energy of OH--ion on the Al atom is expected to be the primary index for Lewis acidity. Kawakami and Yoshida have re-
Table 2 The adsorption
of OH -ion on the Lewis acid sites on alumina Model 01
Model Tl ASA( * ) E(tot)
- 67.21961 b,
[a.u.]
E(ads) [kJ/mol] Opt. parameters rl a r2 b T4 T5
ported energy values on idealized a-alumina surfaces by ab initio calculations with the STO-3G basis set [ll]. The models contain only octahedral sites with different coordination for the aluminum atoms. The adsorption energies for the octahedrally coordinated Al-sites range from 279-425 kJ/mol depending on the surrounding of the adsorption site. The estimated energy value for the tetrahedrally coordinated site is 762 kJ/mol as calculated with a partially optimized model. We have carried out calculations for the OHadsorption on different sites in order to test the Lewis acid strength of our models. The results for the optimized parameters and the adsorption energies are shown in table 2. The results from ASA basis were again compared with the 321G(*) values. The most striking difference in the optimized parameters is the variation of the bonding angles of the OH- group to the central aluminum atom. However, there is no experimental data on the geometrical parameters for this adsorption. The interaction energies are again comparable and can give qualitative information on the relative acid strenghts on the tetrahedral and octahedral sites on aluminum oxide. The tetrahedral Lewis acid sites show considerably larger acid strength than the octahedral ones, when considering the smallest models adopted here. On the other hand, the interaction energy
- 721
Model 02 3-21G(*)
ASA( * )
-541.03134
- 100.73916
- 691.92627
- 169.32639
- 705
- 268
- 334
- 734
1.8484 102.5 0.9719 91.2 - 135.6 44.3
1.8528 128 [0.987] [91.09] [ - 90.031 [44.5]
3-21G(*)
ASA( * )
b,
a) 1.767 135 [0.987] [108.7] [-111.91
11801
1.7601 113 0.966 109.8 - 114.8 - 121.2
[-
1.9369 111.1 [0.987] L91.091 144.01 [44.5]
The model symbols refer to the surface models onto which the adsorbate ‘) rl = r(O4-All), r2 = r(04-H4), a = a(H4-04&All), b = b(O4-Al-01) The values in square brackets were not optimized. ‘) Valence energy.
is binding (see fig. 1). T4 = T(04-All-Ol-Hl),
T5 = T(H4-04-All-01).
P. Hirca, T.A. Pakkanen
/ Interaction
of amine bases on the Lewis acid sites of aluminum oxide
for the larger octahedral model 02 is substantially higher than for model 01. The former model shows the co-operative effect of the neighboring Al-atoms on the surface. Due to the vicinity of other unsaturated Al-atoms the interaction is enhanced and no conclusions on the relative acidity of the sites can be made. Generally this kind of co-operativelly generated strong acidity has been assumed to be a reason for the acidity of activated alumina [ll].
6. Adsorption
of amines on the Lewis acid sites
The interaction of amine bases with the Lewis acid sites of aluminum oxide can be expected to have a substantial role in the preparation reactions of amine containing heterogeneous catalysts. This is contrary to silica carrier, where only weak hydrogen bond interactions are possible [E&27].The main aim of this work is to model these interactions with ammonia and pyridine adsorption on different surface models. Since dissociative adsorption is not possible with the heterocyclic amines, only associative adsorption of ammonia has been considered. Table 2 presents the results for the relative acid strengths of the different models. From these results it can be expected that the tetrahedral Al-sites are the main adsorption centers for amine bases. This is indeed the trend with both ammonia and pyridine adsorption energies, as can be seen from the results in table 3. Again the co-operative enhancement of the interaction energy with model 02 is considerably high. The experimental heats of chemisorption of ammonia on alumina are substantially lower than the calculated ones (the highest values reported are about 190 kJ/mol) [8]. This is probably due to the simplified models for the adsorption sites, since the real surfaces contain complicated geometries with various neighboring acid-base interactions. However, the calculated values can be compared at a qualitative level. Table 3 also shows the relative interaction energies for pyridine compared with those of ammonia. These energy values are similar in both cases which indicates that both ammonia and
4 .e 1E -a
a -u
393
P. Hirua, T.A. Pakkanen / Interaction of amine bases on the Lewis acid sites of aluminum oxide
394
pyridine are capable of replacing water on the Lewis acid sites of alumina. Also the N-AI distances are similar, there is only a sligth shortening of the optimized bond lengths in pyridine case. It should be noted, that the torsional angles of the adsorbates relative to the models were optimized in advance by conformational analysis with the CHEM-X molecular modeling program
D81. 7. Conclusions The main aim of this work was to model amine base interactions on aluminum oxide surfaces. Since these surfaces contain rather complicated structures with different kinds of surface sites, only small models of tetrahedral and octahedral Lewis acid sites were considered. However, these small clusters do not represent the long range interactions of neighboring acid-base sites sufficiently well. As a result there is an overestimation of the interaction energies. On the other hand, qualitative comparison of the different sites is possible even with our minimal basis calculations. The tetrahedral sites have a larger Lewis acidity than the octahedral ones and hence the adsorption of ammonia and pyridine shows higher interaction energies with the tetrahedral sites. The co-operative enhancement of the interaction energies with model 02 is an indication of the larger activity of highly dehydroxylated alumina surfaces. Even if the interaction energies are overestimated, the Lewis acid sites can be expected to play a substantial role on the adsorption of amine bases on alumina and therefore on the reactions between amine bases and transition metal complexes. It can be predicted that the catalyst preparation steps are quite different on aluminum oxide carriers than on silica.
References [I] S. Yoshida, Catalysis 506.
in: Theoretical Aspects of Heterogeneous (Van Nostrand-Reinhold, New York, 1990) p.
PI
B.A. Morrow, in: Spectroscopic Characterization of Heterogeneous Catalysts, part A (Elsevier, Amsterdam, 1990) p. 184. Adv. I31 A.T. Jurewicz, L.D. Rollman and D. Whitehurst, Chem. Ser. 132 (1974) 240. I41 B. Fell and A. Geurts, Chem. Ing. Tech. 44 (1972) 708. 0. Krause and M. Joutsimo, 151 L. Alvila, T.A. Pakkanen, US Patent 4,652,539 (1988). 0. Krause and 161 P. Hirva, T.A. Pakkanen, T. Venallinen, L. Alvila, Finn. Pat. 84142 (1991). and M.V. Shimanskaya, [71 M.B. Fleisher, L.O. Golender React. Kinet. Catal. Lett. 24 (1984) 25. Bl H.D. Simpson, J. Phys. Chem. 89 (1985) 982. and V.B. Kazansky, in: Advances in I91 G.M. Zhidomirov Catalysis, Vol. 34 (Academic Press, New York, 1986) p. 131. [lOI H. Kawakami and S. Yoshida, J. Chem. Sot. Faraday Trans. 81 (1985) 1117. I111 H. Kawakami and S. Yoshida, J. Chem. Sot. Faraday Trans. 81 (1985) 1129. [I21 H. Kawakami and S. Yoshida, J. Chem. Sot. Faraday Trans. 82 (1986) 1385. and M.V. Shimanskaya, 1131 M.B. Fleisher, L.O. Golender React. Kinet Catal. Lett. 34 (1987) 137. I141 M. Causa, R. Dovesi, C. Roetti, E. Kotomin and V.R. Saunders, Chem. Phys. Lett. 140 (1987) 120. I151 A.B. Kunz, Mater. Sci. Forum 29 (1988) 1. [I61 M. Causa, R. Dovesi, C. Pisani and C. Roetti, Surf. Sci. 215 (1989) 259. [I71 J.L. Whitten and T.A. Pakkanen, Phys. Rev. B 21 (1980) 4357. ml T.A. Pakkanen, M. Lindblad and R.S. Laitinen, Int. J. Quantum Chem. 29 (1986) 1789. [191 P. Hirva and T.A. Pakkanen, J. Mol. Struct. 184 (1989) 79. Int. J. Quantum Chem. 26 DO1 J. Sauer and R. Zahradnik, (1984) 793. WI S. Grigoras and T.H. Lane, J. Comput. Chem. 8 (1987) 84. BY J.P. Lopez, C.Y. Yang and C.R. Helms, J. Comput. Chem. 8 (1987) 198. D31 P.G. Jasien and W.J. Stevens, J. Chem. Phys. 84 (1986) 3271. 1241 V.A. Ushakov and E.M. Moroz, React. Kinet. Catal. Lett. 24 (1984) 113. B51 M. Niwa, S. Inagaki and Y. Murakami, J. Phys. Chem. 89 (1985) 2550. D61 P.D. Majors and P.D. Ellis, J. Am. Chem. Sot. 109 (1987) 1648. 1271 P. Hirva and T.A. Pakkanen, Surf. Sci. 271 (1992) 530. I281 For the molecular modeling system CHEM-X program by Chemical Design Ltd. (Oxford, UK) was used.
surface science
The interaction of amine bases on the Lewis acid sites of aluminum oxide - a theoretical study Pipsa Hirva and Tapani
A. Pakkanen
Department of Chemistry, Unitiersit‘yof Joensuy
P.O. Box Ill,
SF-80101 Joensuu, Finland
Received 25 January 1992; accepted for publication 30 June 1992
In the present work amine interactions with aluminum oxide surfaces are modeled with quantum chemical methods. The results from our valence calculations with a minimal basis are compared with standard all-electron calculations with different basis sets. The tetrahedral and octahedral Lewis acid sites are considered as active sites for ammonia and pyridine adsorption. The interaction energies are overestimated, since small cluster models cannot represent the effects of neigh~ring acid-base sites. However, qualitative information on the adso~tion on different sites can be obtained. The results show higher acidity and thus higher amine interaction energies for the tetrahedral Lewis acid sites compared to the octahedral ones. The adsorption energies are similar for both ammonia and pyridine indicating that the amine bases are capable of displacing water from the surface.
1. Introduction Inorganic oxides are widely used as carriers for heterogeneous catalysts, since these materials are thermahy stable and suitable for dispersion of catalyti~lIy active com~nents. However, contrary to silica, aluminas are far from being inert materials because of their strong acid-base properties. Furthermore, aluminas exist in a variety of microcrystalline high surface area forms and thus provide rather complex problems especially about the characterization of the surface sites (l,Z]. Theoretical studies are needed to adopt atomic scale knowledge of the various adsorption sites. One of the main features affecting the catalytical properties of aIumina surfaces is the degree of dehydroxyIation. Depending on the pretreatment procedures different surface sites are formed. Spectroscopic studies have revealed at ieast five kinds of surface hydroxyl groups. Moreover, Lewis acid and base sites can be generated after very mild dehydroxylation. At higher temperatures the surface may contain various “defect” sites with
unknown structures. The relative proportions of these sites will depend on the degree of dehydroxylation and on the fact that for a finely divided alumina different crystal faces are exposed in different proportions [2]. Amine bases form active heterogeneous catalysts with transition metal complexes. These catalysts have been found to promote for example alkene hydroformylation and water gas shift reaction [3-61. In this work we have been using ammonia and pyridine to model amine interactions with the Lewis acid sites of aluminum oxide. These interactions can affect the catalyst preparation reactions and therefore the activity of the catalysts. Ammonia adsorption with alumina surfaces has been studied previously using semiempirical methods 171. Simpson has also studied ammonia on different surface sites of silica and alumina with a theoretical method based on the Sanderson’s electronegativity principles [8]. Ammonia and water adsorption have also been modeled on the trigonal aluminum sites of zeolites [9]. Other theoretica studies on alumina concern the
0039~6028/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved
390
P. Hirca, T.A. Pakkanen
/ Interaction
of amine bases on the Lewis acid sites of aluminum oxide
acid-base properties of various surface sites [lo131 and the electronic structure of cu-alumina [14-161.
2. Theoretical methods The study of the active sites is a key factor when heterogeneous catalysts are concerned. Experimental methods like IR-spectroscopy have been used in attempt to characterize the nature of different sites during catalyst preparation. Aluminum oxides are particularly problematic because of the variety of possible active sites. Experimental information on Lewis acid sites is especially uncertain. Theoretical methods can give valuable information on the nature of the chemisorption processes and on the relative energies of different sites. Our method of calculation is a frozen core method with different stepwise density approximations for the interactions between the core and valence orbitals [17-191. With this method the computing times can be considerably reduced thus making it possible to calculate larger cluster models. This is particularly important with large adsorbates like pyridine. All computations were made using a minimal basis set (ASA for all stepwise approximations) [19]. The results were compared with standard all-electron calculations with different basis sets. Polarization functions were introduced for aluminum atoms in most cases because of the importance of the aluminum d-orbitals at the surface [16]. The importance of the d-orbitals has also been shown in other inorganic oxides, as in silica (SiO,) [20,21]. Polarization functions on oxygen have shown to have minor effects in silica [22]. The minimal basis set is not likely to give accurate binding energies for the adsorbed molecules because of the large basis set superposition error connected to small basis sets. However, the frozen core method has been shown to diminish the BSSE, since this error is connected to a poor representation of the core orbitals 1231. Furthermore, qualitative comparison of the different sites is possible even at the minimal basis level.
3. Surface models The variety of crystal and surface structures on aluminum oxide form a complication to the experimental and theoretical studies. The fundamental structure of most crystal structures can be described as being a defect spinel, in which the aluminum cations take positions of octahedral and tetrahedral vacant sites [1,24,25]. Alumina surfaces contain various kinds of surface sites including Lewis acid sites (coordinatively unsaturated aluminum atoms), associated basic Al-O sites and at least five kinds of surface hydroxyl groups. However, concerning the amine base interactions on the surface, the Lewis acid sites are the most important adsorption centers. This has been demonstrated experimentally by Majors and Ellis [26]. According to their NMR-experiments pyridine coordinates onto two types of Lewis acid sites, which contain tetrahedrally and octahedrally unsaturated aluminum cations. These sites are generated upon desorption of non-bridged terminal hydroxyl groups. In this study we have adopted small cluster models to represent the tetrahedral and octahedral Lewis acid sites on a spine1 type aluminum oxide. These models are shown in fig. 1. The
Model Ti
Model 01
Model
T2
Model 02
Fig. 1. Cluster models for the tetrahedral (Tl, T2) and octahedral (01, 02) Lewis acid sites on aluminum oxide surfaces.
P. Hirca, T.A. Pakkanen
/ Interaction
391
of amine bases on the Lewis acid sites of aluminum oxide
models Tl and T2 represent the tetrahedral coordination of the aluminum atoms and models 01 and 02 the octahedral coordination. In these models the atoms were frozen to their bulk positions and only the adsorbates were optimized.
4. Water adsorption The associative adsorption of water onto the tetrahedral models Tl and T2 was studied in order to compare the results from our ASA basis set with the standard all-electron calculations. The optimized parameters and the adsorption energies are shown in table 1. The results show shorter water distances from the surface with the split valence basis 3-21G and 3-21G(*) compared to our minimal basis calculations. The interaction energies are also somewhat larger than with the ASA basis set. However, when a larger split valence basis set 6-31G(*) is adopted, the Al-O distance is lenghtened to a same value as with the ASA basis and the interaction energy is considerably reduced. The largest difference in the optimized parameters comes from the bending of
Fig. 2. The basis set dependence on the geometry of adsorbed water on the tetrahedral Lewis acid site 01. For the geometrical parameters, see table 1.
the water molecule from the z-axis orientation, as can be seen in fig. 2. This bending is notable only with the split valence basis; the minimal basis does not show this phenomenon. We also studied the effect of the cluster size on the adsorption energy of water with a larger model T2. This model was only calculated with the minimal basis set. Due to a large cluster size, the change in the charge distribution in the vicin-
Table 1 The optimized parameters and the interaction energies for the adsorption of water on the tetrahedral Lewis acid sites on alumina Model T2
Mode1 Tl
AS A
qW1) q(O4) q(H4) q(H5) Ettot) [au.] Etads) [kJ/mol] Opt. parameters a) rl a T r2 r3 bl b2
1.786 - 0.754 0.426 0.427 - 0.06779 b, 156
ASA( * )
STO-3G
3-21G
3-21G(*)
6-31G(*)
ASA
1.193 - 0.714 0.437 0.438 - 0.06791 b, - 191
1.191 - 0.377 0.289 0.292 - 537.37217 -216
1.474 - 0.818 0.492 0.499 -541.50059 - 267
1.108 - 0.771 0.487 0.494 -541.57805 -257
1.352 - 0.879 0.525 0.525 - 544.42565 - 167
1.584 - 0.162 0.441 0.441 - 0.17342 b, - 228
1.9869 111.6
1.9379 113.3
1.8235 108.3
1.8519 115.1
1.8653 115.3
1.9379 109
46.9 [0.987] [0.987] 124.2 124.2
[:.987] [0.987] 123.4 123.4
0 0.976 0.976 128 123.6
0 0.964 0.968 131 113.9
4.1 0.965 0.967 128.5 116.2
2.5 0.955 0.955 112.7 110.7
1.9437 111.1
LOI [0.987] [0.987] 124.5 124.5
The model symbols refer to the surface models onto which the adsorbate is binding (see fig. 1). Charges (q) were obtained by Mulliken population analysis. a) rl = r(04-All), r2 = r(H4-04), r3 = r(H5-04), a = a(H4-04-H5), bl = b(H4-044All), b2 = b(H5-04-All), Tl = T(H404-All-01). The values in square brackets were not optimized. b, Valence energy.
P. Hirva, T.A. Pakkanen / Interaction of amine bases on the Lewis acid sites of aluminum oxide
392
ity of the adsorption site does not have major influence on the optimimized parameters. On the other hand, the -adsorption energy is somewhat larger when compared to the value of the smaller model within the same basis set. Otherwise, our method seems to give comparable results to the standard all-electron basis calculations. The main advantage in our method is the ability to calculate larger cluster models with reduced computing time, which is important when dealing with large adsorbates like pyridine. In partially ionic materials the neutrality of the cluster is a major requirement for models in adsorption studies. In an ideal crystal the sum of the atomic charges should be zero, for example in alumina 2qCAl) + 3q(O) = 0. In our models the inclusion of aluminum d-orbitals makes the clusters more neutral. An other way of dealing with the neutrality would be the use of pseudoatoms instead of hydrogen termination. However, pseudoatom termination would need careful testing of the models. For qualitative comparison, hydrogen termination should be effective enough.
5. The acid strength of the surface sites The adsorption energy of OH--ion on the Al atom is expected to be the primary index for Lewis acidity. Kawakami and Yoshida have re-
Table 2 The adsorption
of OH -ion on the Lewis acid sites on alumina Model 01
Model Tl ASA( * ) E(tot)
- 67.21961 b,
[a.u.]
E(ads) [kJ/mol] Opt. parameters rl a r2 b T4 T5
ported energy values on idealized a-alumina surfaces by ab initio calculations with the STO-3G basis set [ll]. The models contain only octahedral sites with different coordination for the aluminum atoms. The adsorption energies for the octahedrally coordinated Al-sites range from 279-425 kJ/mol depending on the surrounding of the adsorption site. The estimated energy value for the tetrahedrally coordinated site is 762 kJ/mol as calculated with a partially optimized model. We have carried out calculations for the OHadsorption on different sites in order to test the Lewis acid strength of our models. The results for the optimized parameters and the adsorption energies are shown in table 2. The results from ASA basis were again compared with the 321G(*) values. The most striking difference in the optimized parameters is the variation of the bonding angles of the OH- group to the central aluminum atom. However, there is no experimental data on the geometrical parameters for this adsorption. The interaction energies are again comparable and can give qualitative information on the relative acid strenghts on the tetrahedral and octahedral sites on aluminum oxide. The tetrahedral Lewis acid sites show considerably larger acid strength than the octahedral ones, when considering the smallest models adopted here. On the other hand, the interaction energy
- 721
Model 02 3-21G(*)
ASA( * )
-541.03134
- 100.73916
- 691.92627
- 169.32639
- 705
- 268
- 334
- 734
1.8484 102.5 0.9719 91.2 - 135.6 44.3
1.8528 128 [0.987] [91.09] [ - 90.031 [44.5]
3-21G(*)
ASA( * )
b,
a) 1.767 135 [0.987] [108.7] [-111.91
11801
1.7601 113 0.966 109.8 - 114.8 - 121.2
[-
1.9369 111.1 [0.987] L91.091 144.01 [44.5]
The model symbols refer to the surface models onto which the adsorbate ‘) rl = r(O4-All), r2 = r(04-H4), a = a(H4-04&All), b = b(O4-Al-01) The values in square brackets were not optimized. ‘) Valence energy.
is binding (see fig. 1). T4 = T(04-All-Ol-Hl),
T5 = T(H4-04-All-01).
P. Hirca, T.A. Pakkanen
/ Interaction
of amine bases on the Lewis acid sites of aluminum oxide
for the larger octahedral model 02 is substantially higher than for model 01. The former model shows the co-operative effect of the neighboring Al-atoms on the surface. Due to the vicinity of other unsaturated Al-atoms the interaction is enhanced and no conclusions on the relative acidity of the sites can be made. Generally this kind of co-operativelly generated strong acidity has been assumed to be a reason for the acidity of activated alumina [ll].
6. Adsorption
of amines on the Lewis acid sites
The interaction of amine bases with the Lewis acid sites of aluminum oxide can be expected to have a substantial role in the preparation reactions of amine containing heterogeneous catalysts. This is contrary to silica carrier, where only weak hydrogen bond interactions are possible [E&27].The main aim of this work is to model these interactions with ammonia and pyridine adsorption on different surface models. Since dissociative adsorption is not possible with the heterocyclic amines, only associative adsorption of ammonia has been considered. Table 2 presents the results for the relative acid strengths of the different models. From these results it can be expected that the tetrahedral Al-sites are the main adsorption centers for amine bases. This is indeed the trend with both ammonia and pyridine adsorption energies, as can be seen from the results in table 3. Again the co-operative enhancement of the interaction energy with model 02 is considerably high. The experimental heats of chemisorption of ammonia on alumina are substantially lower than the calculated ones (the highest values reported are about 190 kJ/mol) [8]. This is probably due to the simplified models for the adsorption sites, since the real surfaces contain complicated geometries with various neighboring acid-base interactions. However, the calculated values can be compared at a qualitative level. Table 3 also shows the relative interaction energies for pyridine compared with those of ammonia. These energy values are similar in both cases which indicates that both ammonia and
4 .e 1E -a
a -u
393
P. Hirua, T.A. Pakkanen / Interaction of amine bases on the Lewis acid sites of aluminum oxide
394
pyridine are capable of replacing water on the Lewis acid sites of alumina. Also the N-AI distances are similar, there is only a sligth shortening of the optimized bond lengths in pyridine case. It should be noted, that the torsional angles of the adsorbates relative to the models were optimized in advance by conformational analysis with the CHEM-X molecular modeling program
D81. 7. Conclusions The main aim of this work was to model amine base interactions on aluminum oxide surfaces. Since these surfaces contain rather complicated structures with different kinds of surface sites, only small models of tetrahedral and octahedral Lewis acid sites were considered. However, these small clusters do not represent the long range interactions of neighboring acid-base sites sufficiently well. As a result there is an overestimation of the interaction energies. On the other hand, qualitative comparison of the different sites is possible even with our minimal basis calculations. The tetrahedral sites have a larger Lewis acidity than the octahedral ones and hence the adsorption of ammonia and pyridine shows higher interaction energies with the tetrahedral sites. The co-operative enhancement of the interaction energies with model 02 is an indication of the larger activity of highly dehydroxylated alumina surfaces. Even if the interaction energies are overestimated, the Lewis acid sites can be expected to play a substantial role on the adsorption of amine bases on alumina and therefore on the reactions between amine bases and transition metal complexes. It can be predicted that the catalyst preparation steps are quite different on aluminum oxide carriers than on silica.
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