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Modeling of structure and properties of active centers of catalysts on the base of metalorganosiloxanes
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
1175
Modeling of structure and properties of active centers of catalysts on the base of metalorganosiloxanes
A.V.Nemukhin *~ I.M.Kolesnikov b and V.A.Vinokurov b a- Department of Chemistry, Moscow State University, Moscow, 119899, GSP, Russia b - Russian State University of Oil and Gas, Leninsky Prospect, 65, Moscow 117296, Russia Quantum chemical calculations provide support to the reaction mechanisms suggestinq chemisorption of the ethylene molecule on the A104 site of alumophenylsiloxane accompanying a n-bond rupture and a creation of cation-radicals. Studies of elementary chemical processes occurring at the alumosilicate compounds are of importance due to a great significance of these species as catalysts in the conversion of oil fractions~. Along with wellknown solid alumosilicates related soluble substances, in particular alumophenylsiloxane (APS) and metallphenylsiloxanex possess similar structural properties and are viewed as perspective catalysts as well 2. Recent experimental studies 3 show that in the presence of APS in a liquid reaction volume an propilene-benzene interaction occurs yielding propyl-benzene. It is suggested that one of the stages of the process may be related to the chemisorption of unsaturated hydrocarbons on APS ahd MPS accompanying a creation of cation- and anion-radicais. Considerable efforts are being performed to clarify mechanisms of catalytic activity of alumosilicates starting from different viewpoints 4. According to the theory catalysis by polihedra developed by one of the authors 5 active sites of alumosilicate compounds are the tetrahedrons {A104} and {SiO4}. This work presents a quantum-chemical contribution in some support to this idea taking the ethylene-APS system as an example.
1176 All calculations described here have been carried out with the GAMESS package 6. We have considered the interaction of the ethylene molecule with APS by means of a group of atoms representing the local chemisorption site. Namely, the {AIO4} fragment have been selected as an ac,tive species in environments simulating the original APS complex. A special attention has been paid to such parameters as partial charges on atoms, effective electronic configurations and compositions of bonding orbitals. At the preliminary stages the structure of APS, (C6Hs(HO)2SiO)2AI(OSi(OH)2C6Hs), has been studied by using molecular mechanics and semiempirical AM1 quantum chemistry techniques 7 followed by ab initio MO LCAO calculations with the 3-21G basis set. According to these approaches the central part of APS may be considered as a distorted AIO4 tetrahedron with two A1-O bonds directing to the Si(OH)2C6H5 fragments and two AI-O bonds forming a well-known aluminum-oxygen-silicon cycle with a bridged OH group 4. A partial electronic charge on aluminum is estimated as +2.0, on oxygen as-1.2 (on average), and on silicon as +2.4. A composition of the bonding A1-O orbital may be described in terms of natural bond orbitals 8 as 0.35 sp a3 (A1) + 0.95 sp 13 (O) indicating that the orbital is strongly polarized towards the oxygen hybrid. A strongly ionic character of the compound is consistent with the data common to aluminum oxide molecules 9'~~ To simulate an active site of APS, namely, the {A104} fragment, several reduced systems of APS have been considered: A104Li3, AI(OH)4, AI(OSiN)202Li, AI(OH)2(OSiH3). In all cases the tetrahedral arrangement of oxygen atoms placed at a distance 1.72 A from the aluminum center has been assumed. Detailed results will be presented below for the interaction C2H4 + AIO4Li3 although most impirtant qualitative conclusions are similar for all species. The lithium atoms play here the role of "pseudoatoms" often used in attempts to select a finite cluster from an extended system when studying complicated processes like chemisorption 4. In the A104Li3 moiety partial charges on atoms (+ 1.4 on A1 and -0.9 on O) resemble those calculated for APS (+2.0 and-1.2) as well as effective electronic configurations (3s ~ 3p ~~ in A104Li3, versus 3s ~ 3p ~ in APS for aluminum, and 2s19 2p5.0 in A104Li3 versus 2s 1"8 2p 5"4 in APS for oxygen). A composition of the bonding AI-O orbitals in A104Li3 (0.34sp z3 (AI) + 0.94sp 1"6(O)) also reproduces nicely that of APS. We conclude that the local electronic properties of the A104 site are reproduced successfully by the A1 O4Li3 molecule.
1177
Chart I shows geometry arrangements of the entire reactive complex C2H4 + AIO4Li3. The C2v symmetry has been assumed for the system. Other spatial configurations have been rejected since their energies are considerably higher. We have varied only one geometry parameter, a distance R between A1 and a center of the C-C bond in C2H4. Other parameters have been optimized with the RHF/3-21G (a restricted Hartree-Fock model within the 3-21G basis set) wavefunctions for C2H4, and A104Li3 in separate calculations.
H~
J
c
H
CL..
H j
H
t R
O Li /
O ~ ~
A1 ~ ~
/\ \/
O
Li
O
Li Chart 1
Within the natural borM orbital formalism 8 the electronic structure of C2H4 is described as a configuration with the doubly occupied bonding orbitals: [Core]cy2(C-C)[cya(C-H)]4n2(C-C). When interacting with the A104, site of APS or related reduced systems specific changes in the electronic structure of C2H4 are expected. For the goals of the present study the most important is a process of a n-bond rupture in C2H4 which should be reflected by a substantial reduction of population of the n (C-C) orbital. The data presented in Table I show the main result of this work, namely, the changes in total energy and in the electronic structure of the ethylene molecule during its interaction with the A104Li3 species. The numbers have been computed at the RttF/3-21G level of the theory.
1178
Table 1. Interaction energies C2H4 + A104Li3 (AE), total charge on the C2H4 fragment (Q), and population of the n (C-C) orbital (n~) versus intermolecular distance (R) calculated at the RHF/3-21G approximation. R,A
AE, kJ/mol
Q
nn
50.0 5.0 4.0 3.5 3.0 2.75 2.50 2.25
0.0 +3.3 +26.8 - 114. -413. -527. -4601 +54.8
0.0 0.0 +0.194 +0.496 +0.638 +0.656 +0.668 +0.676
2.00 1.98 1.77 < 1.5 --'--
First of all we note that the interaction C2H4 + AIO4Li3 leads to a strongly bound system at the distance R about 2.75 A with a binding energy above 527 kJ/mol. A small barrier in the entrance channel (about 27 kJ/mol) may disappear and the well depth may increase if a geometry relaxation is taken 71-110 account in calculations. A bound C^H + A10 Li complex evidences that a chemisorption may take place when ethylene is contacting APS.
A total natural charge on the ethylene fragment is increasing from zero to a value (+0.68) close to +1 indicating that reaction mechanisms which assume creation of cation-radicals gain some support by these quantum chemical simulations. We also see. that along the reaction pathway a population of the n-orbitaj of C2H4 is reducing from 2.00 to a boundary value of 1.50 generally accepted as a critical one for regularly occupied MO's 8 immediately after passing the potential barrier. We can interpret the result as a rr-bond rupture in this region. An analysis of electron density in terms of natural orbitals shows that .instead of the n(C-C) orbital the ~(C-O) orbitals are getting populated which results in a cycle extending over the entire complex.
1179 In conclusion the present study confirms that the {A104} tetrahedron may serve as an active site of the alumophenylsiloxane complex. According to the quantum chemical model calculations the ethylene molecule reacts efficiently with this site yielding a singly bonded cation-radical as an intermediate species. Acknowledgments The authors thank Prof. J.Almlof for the making these computations possible, Professors F.Weinhold for using their computer programs.
financial support M.Schmidt and
References 1. Kreking neftyanykh frakzyi na tseolitsoderzhschikh katalizatorakh (Cracking of oil fractions on ceolit containing catalysts), ed. S.N.Khadjiev, Khimia, Moscow, 1982, p.280 (in Russian). 2. I.M.Kolesnikov, G.M.Panchenkov and V.A.Tulupov, Zh.Phys.Khim., 1965, 39, 1869 (in Russian) 3. I.M.Kolesnikov and N.N.Belov, Zh.PrikIadnoy Khim., 1990, 63, 162 (in Russian) 4. G.M.Zhidomirov, A.A.Bagaturyanz and I.A.Abronin, Prikiadnaya kvantovaya khimia (Applied Quantum Chemistry), Khimia, Moscow, 1979, p.296 (in Russian) 5. I.M.Kolesnikov, Kinetika i kataliz v gomogennykh i geterogonnykh uglevodorodsoderzhschikh sistemakh (Kinetics and catalysis in homogeneous and heterogeneous hydrocarbon containing systems), Textbook, Moscow Institute of Oil and Gas, Moscow, 1990, p. 198 (in Russian) 6. M.W.Schmidt, K.K.Baldridge, J.A.Boatz, J.H.Jensen, S.Koseki,M.S.Gordon, K.A.Nguyen, T.L.Windus and S.T.Elbert, Quantum Chemistry Program Echange Bulletin, 1990, 52 7. M.J.S.Dewar, E.G.Zoebisch, E.F.Healy and J.J.P.Stewart, J.Amer.Chem.Soc. ,1985, 107, 3902 8. A.E.Reed, R.B.Weinstock and F.Weinhold, J.Chem.Phys., 1985, 83,737 9. A.V.Nemukhin and F.Weinhold, J.Chem.Phys., 1992, 97, 3420 10.A.V.Nemukhin and L.V.Serebrennikov, Uspekhi Khimii, 1993, 62, 566 (Russian Chemical Reviews, 1993, 62, 527)
1175
Modeling of structure and properties of active centers of catalysts on the base of metalorganosiloxanes
A.V.Nemukhin *~ I.M.Kolesnikov b and V.A.Vinokurov b a- Department of Chemistry, Moscow State University, Moscow, 119899, GSP, Russia b - Russian State University of Oil and Gas, Leninsky Prospect, 65, Moscow 117296, Russia Quantum chemical calculations provide support to the reaction mechanisms suggestinq chemisorption of the ethylene molecule on the A104 site of alumophenylsiloxane accompanying a n-bond rupture and a creation of cation-radicals. Studies of elementary chemical processes occurring at the alumosilicate compounds are of importance due to a great significance of these species as catalysts in the conversion of oil fractions~. Along with wellknown solid alumosilicates related soluble substances, in particular alumophenylsiloxane (APS) and metallphenylsiloxanex possess similar structural properties and are viewed as perspective catalysts as well 2. Recent experimental studies 3 show that in the presence of APS in a liquid reaction volume an propilene-benzene interaction occurs yielding propyl-benzene. It is suggested that one of the stages of the process may be related to the chemisorption of unsaturated hydrocarbons on APS ahd MPS accompanying a creation of cation- and anion-radicais. Considerable efforts are being performed to clarify mechanisms of catalytic activity of alumosilicates starting from different viewpoints 4. According to the theory catalysis by polihedra developed by one of the authors 5 active sites of alumosilicate compounds are the tetrahedrons {A104} and {SiO4}. This work presents a quantum-chemical contribution in some support to this idea taking the ethylene-APS system as an example.
1176 All calculations described here have been carried out with the GAMESS package 6. We have considered the interaction of the ethylene molecule with APS by means of a group of atoms representing the local chemisorption site. Namely, the {AIO4} fragment have been selected as an ac,tive species in environments simulating the original APS complex. A special attention has been paid to such parameters as partial charges on atoms, effective electronic configurations and compositions of bonding orbitals. At the preliminary stages the structure of APS, (C6Hs(HO)2SiO)2AI(OSi(OH)2C6Hs), has been studied by using molecular mechanics and semiempirical AM1 quantum chemistry techniques 7 followed by ab initio MO LCAO calculations with the 3-21G basis set. According to these approaches the central part of APS may be considered as a distorted AIO4 tetrahedron with two A1-O bonds directing to the Si(OH)2C6H5 fragments and two AI-O bonds forming a well-known aluminum-oxygen-silicon cycle with a bridged OH group 4. A partial electronic charge on aluminum is estimated as +2.0, on oxygen as-1.2 (on average), and on silicon as +2.4. A composition of the bonding A1-O orbital may be described in terms of natural bond orbitals 8 as 0.35 sp a3 (A1) + 0.95 sp 13 (O) indicating that the orbital is strongly polarized towards the oxygen hybrid. A strongly ionic character of the compound is consistent with the data common to aluminum oxide molecules 9'~~ To simulate an active site of APS, namely, the {A104} fragment, several reduced systems of APS have been considered: A104Li3, AI(OH)4, AI(OSiN)202Li, AI(OH)2(OSiH3). In all cases the tetrahedral arrangement of oxygen atoms placed at a distance 1.72 A from the aluminum center has been assumed. Detailed results will be presented below for the interaction C2H4 + AIO4Li3 although most impirtant qualitative conclusions are similar for all species. The lithium atoms play here the role of "pseudoatoms" often used in attempts to select a finite cluster from an extended system when studying complicated processes like chemisorption 4. In the A104Li3 moiety partial charges on atoms (+ 1.4 on A1 and -0.9 on O) resemble those calculated for APS (+2.0 and-1.2) as well as effective electronic configurations (3s ~ 3p ~~ in A104Li3, versus 3s ~ 3p ~ in APS for aluminum, and 2s19 2p5.0 in A104Li3 versus 2s 1"8 2p 5"4 in APS for oxygen). A composition of the bonding AI-O orbitals in A104Li3 (0.34sp z3 (AI) + 0.94sp 1"6(O)) also reproduces nicely that of APS. We conclude that the local electronic properties of the A104 site are reproduced successfully by the A1 O4Li3 molecule.
1177
Chart I shows geometry arrangements of the entire reactive complex C2H4 + AIO4Li3. The C2v symmetry has been assumed for the system. Other spatial configurations have been rejected since their energies are considerably higher. We have varied only one geometry parameter, a distance R between A1 and a center of the C-C bond in C2H4. Other parameters have been optimized with the RHF/3-21G (a restricted Hartree-Fock model within the 3-21G basis set) wavefunctions for C2H4, and A104Li3 in separate calculations.
H~
J
c
H
CL..
H j
H
t R
O Li /
O ~ ~
A1 ~ ~
/\ \/
O
Li
O
Li Chart 1
Within the natural borM orbital formalism 8 the electronic structure of C2H4 is described as a configuration with the doubly occupied bonding orbitals: [Core]cy2(C-C)[cya(C-H)]4n2(C-C). When interacting with the A104, site of APS or related reduced systems specific changes in the electronic structure of C2H4 are expected. For the goals of the present study the most important is a process of a n-bond rupture in C2H4 which should be reflected by a substantial reduction of population of the n (C-C) orbital. The data presented in Table I show the main result of this work, namely, the changes in total energy and in the electronic structure of the ethylene molecule during its interaction with the A104Li3 species. The numbers have been computed at the RttF/3-21G level of the theory.
1178
Table 1. Interaction energies C2H4 + A104Li3 (AE), total charge on the C2H4 fragment (Q), and population of the n (C-C) orbital (n~) versus intermolecular distance (R) calculated at the RHF/3-21G approximation. R,A
AE, kJ/mol
Q
nn
50.0 5.0 4.0 3.5 3.0 2.75 2.50 2.25
0.0 +3.3 +26.8 - 114. -413. -527. -4601 +54.8
0.0 0.0 +0.194 +0.496 +0.638 +0.656 +0.668 +0.676
2.00 1.98 1.77 < 1.5 --'--
First of all we note that the interaction C2H4 + AIO4Li3 leads to a strongly bound system at the distance R about 2.75 A with a binding energy above 527 kJ/mol. A small barrier in the entrance channel (about 27 kJ/mol) may disappear and the well depth may increase if a geometry relaxation is taken 71-110 account in calculations. A bound C^H + A10 Li complex evidences that a chemisorption may take place when ethylene is contacting APS.
A total natural charge on the ethylene fragment is increasing from zero to a value (+0.68) close to +1 indicating that reaction mechanisms which assume creation of cation-radicals gain some support by these quantum chemical simulations. We also see. that along the reaction pathway a population of the n-orbitaj of C2H4 is reducing from 2.00 to a boundary value of 1.50 generally accepted as a critical one for regularly occupied MO's 8 immediately after passing the potential barrier. We can interpret the result as a rr-bond rupture in this region. An analysis of electron density in terms of natural orbitals shows that .instead of the n(C-C) orbital the ~(C-O) orbitals are getting populated which results in a cycle extending over the entire complex.
1179 In conclusion the present study confirms that the {A104} tetrahedron may serve as an active site of the alumophenylsiloxane complex. According to the quantum chemical model calculations the ethylene molecule reacts efficiently with this site yielding a singly bonded cation-radical as an intermediate species. Acknowledgments The authors thank Prof. J.Almlof for the making these computations possible, Professors F.Weinhold for using their computer programs.
financial support M.Schmidt and
References 1. Kreking neftyanykh frakzyi na tseolitsoderzhschikh katalizatorakh (Cracking of oil fractions on ceolit containing catalysts), ed. S.N.Khadjiev, Khimia, Moscow, 1982, p.280 (in Russian). 2. I.M.Kolesnikov, G.M.Panchenkov and V.A.Tulupov, Zh.Phys.Khim., 1965, 39, 1869 (in Russian) 3. I.M.Kolesnikov and N.N.Belov, Zh.PrikIadnoy Khim., 1990, 63, 162 (in Russian) 4. G.M.Zhidomirov, A.A.Bagaturyanz and I.A.Abronin, Prikiadnaya kvantovaya khimia (Applied Quantum Chemistry), Khimia, Moscow, 1979, p.296 (in Russian) 5. I.M.Kolesnikov, Kinetika i kataliz v gomogennykh i geterogonnykh uglevodorodsoderzhschikh sistemakh (Kinetics and catalysis in homogeneous and heterogeneous hydrocarbon containing systems), Textbook, Moscow Institute of Oil and Gas, Moscow, 1990, p. 198 (in Russian) 6. M.W.Schmidt, K.K.Baldridge, J.A.Boatz, J.H.Jensen, S.Koseki,M.S.Gordon, K.A.Nguyen, T.L.Windus and S.T.Elbert, Quantum Chemistry Program Echange Bulletin, 1990, 52 7. M.J.S.Dewar, E.G.Zoebisch, E.F.Healy and J.J.P.Stewart, J.Amer.Chem.Soc. ,1985, 107, 3902 8. A.E.Reed, R.B.Weinstock and F.Weinhold, J.Chem.Phys., 1985, 83,737 9. A.V.Nemukhin and F.Weinhold, J.Chem.Phys., 1992, 97, 3420 10.A.V.Nemukhin and L.V.Serebrennikov, Uspekhi Khimii, 1993, 62, 566 (Russian Chemical Reviews, 1993, 62, 527)