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Quantum chemical study on the mechanism of CO hydrogenation to oxygenates over vanadium-promoted rhodium catalyst
418
Journal of Molecular Catalysi, 54 (1989)
478 - 483
QUANTUM CHEMICAL STUDY ON THE MECHANISM OF CO HYDROGENATION TO OXYGENATES OVER VANADIUM-PROMOTED RHODIUM CATALYST WANG REN-HU and XU YIN-SHENG* Dalian Institute of Chemical Physics, Academia Sinica, Dalian (China)
Extended Hiickel calculations of CO hydrogenation to oxygenates have been carried out for vanadium-promoted rhodium catalyst. The results of these calculations show that the barrier for CHs migration to Rh-CO to form Rh-COCH3 species is decreased in the case of vanadium-promoted catalyst. A discussion attempts to explain the role of the promoters.
Introduction It is well known that a broad mixture of alkanes and alkenes can be synthesized from CO and Hz over Ni, Ru and Fe metal catalysts. It has been known for some time that the alkylated Co or Fe catalysts can be used to produce appreciable amounts of oxygenates [l]. In 1973, scientists at Union Carbide Company discovered that Fe [2], Mn [3] or alkyl-promoted Rh [4] catalysts can improve the selectivity of CZ oxygenates. It is suggested that the carbonyl insertion reaction (1) : WR)(COW)n
-
WCOR)(Lhn
(1)
is not only an important elementary organometallic reaction, but also one of the key steps in catalytic CO hydrogenation for synthesis of oxygenates. Recently, in our laboratory, Luo [5] has found that high selectivity and reactivity of CZ oxygenates can be obtained over vanadium-promoted rhodium catalysts. An understanding of the role of the promoter in the fundamental steps involved in the catalytic reaction of CO and HZ is of paramount importance. In the present paper, a quantum chemical study of the elementary step of carbonyl insertion in the process of CO hydrogenation over vanadium-promoted rhodium catalyst in the synthesis of oxygenates has been carried out.
*Author to whom correspondence
should be addressed. 0 Elsevier Sequoia/Printed
in The Netherlands
479
Calculation method and model An extended Hiickel semiempirical method has been applied for the calculation 161, and double <-Slater functions were used for the d-orbit& of metallic Rh and V. The parameters used in the calculation are listed in Table 1. Repulsion terms for the inner p electrons of the transition metal have been introduced according to Lhe method of Anders [ 71. In Luo’s paper [5], the mean diameter of a rhodium crystallite was found to be larger than 40 a, so that there are hundreds of rhodium atoms accumulated together. Hence we can imagine that there are metallic crystallites of Rh distributed on the support. According to Sachtler’s model [8], the promotion may take place or locate on the surface of a rhodium crystal with a counterion 02- adsorbed in the vicinity of the promoters. Therefore, the (111) surface of a Rh crystal with promoter V and counterion 02- on it is chosen as the model for the catalyst (Fig. 1). TABLE 1 EHMO parameters for different elements Element
Orbital Exponent ( )
Orbital
Hii (eV)
4.29 (0.5807)
2.135 2.10 4.12 1.97 (0.5685)
5s 5P 4P 4d
8.09 4.57 50.0 12.5
4.70 (0.4755)
1.30 0.875 1.70 (0.7052)
4s 4P 3d
8.81 5.52 11.0
0
2.275 2.276
2s 2P
32.3 14.8
C
1.626 1.625
2s 2P
21.4 11.4
H
1.3
IS
13.6
Rh
V
Fig. 1. Model of catalyst where (0) represents Rh, (@) represents O*-, and (0) represents V. Fig. 2. Reaction path of methyl group migration where (FBO) represents 02-, (@) represents 0, (0) represents Rh, (0 ) represents C, (0) represents V, and ( ) represents H. l
480 TABLE 2 Geometric parameters Bond
Distance between atoms (A)
Angle (“)
Rh-Rh Rh-C C-O C-C
2.69 2.08 1.14 1.54
8 (initial) 60 cp 40 [lo]
It has been confirmed
[9] that the following organometallic
reaction
(2): RhCl(CO)~(PPh~)(C~~)I -
RhClI(CO)(COC~~)(PPh~)
(2)
proceeds via methyl migration as follows: (TF3
Cl,JIIICO
_
1 ‘PPhJ I
CO’
Cl\RbNI ‘“CO 1 ‘PPh, CO’ I
Cl \Rh,COCH3 CO’
1 ‘PPhs
I
In order to design the reaction path of CO hydrogenation to oxygenates, we refer to the organometallic reaction and employ the angle 6 as the reaction coordinate (Fig. 2). The geometric parameters of the catalytic system are listed in Table 2. Results and discussion Path of carbonyl insertion
A plot of potentiaI energy changes along the path of methyl group migration is shown in Fig. 3. According to Uguagliati’s experiments [9], the activation energy of reaction (2) is approximately equal to 6 kcal mol-‘, in agreement with the calculated result.
I..... 60
(&
9
%greel
Fig. 3. Potential energy of methyl migration reaction. The reference energy is the energy for 8 = 60”, Rh catalyst.
481
Comparison of Rh catalyst and V-promoted Rh catalyst in the carbonyl insertion reaction A plot of energy changes of the carbonyl insertion reaction over these two catalysts is presented in Fig. 4. The solid line represents V-promoted Rh catalyst and the broken line corresponds to Rh catalyst. As is clearly shown by the results of calculation, the energy barrier of methyl group migration to Rh-CO to form the Rh-COCH3 species is lowered in Vpromoted Rh catalyst as compared to the case of Rh catalyst.
Fig. 4. Energy changes of carbonyl insertion reaction over two catalysts; (o---O) represents V-promoted Rh catalyst, and (*- - -0) represents Rh catalyst. The reference energy is the same as in Fig. 3. Fig. 5. Molecular conformation
in methyl migratory reaction. Symbols as in Fig. 2.
The role of promoters Why does the reaction of methyl migration to Rh-CO proceed more easily in the V-promoted Rh catalyst? This remains a very interesting question. We know that there are always variations in the components of molecular orbit& in a chemical reaction. In other words, in chemical reactions, some bonds are weakened and broken while the others are strengthened and formed. For example, in Fig. 5, when the angle 9 increases, the methyl group departs from the Rh(3) atom and approaches the C(1) atom. Thus, we can expect that the bond between the methyl group and the Rh(3) atom will be weakened, while the C-C bond will be formed and strengthened. The results of calculation show that in the Rh catalyst, as 6 increases, the components of the Rh(3) atomic orbital in HOMO do decrease markedly. To estimate the difference in the bond strengths between Rh the catalyst and V-promoted Rh catalyst, we calculated the bond energies of CHs-(Rh catalyst) and CHs(V-promoted Rh catalyst) (Table 3). Obviously, the binding energy of CHs-catalyst can be formally constructed from the following equations (3,4): Rh catalyst + CH3 -
CHs-(Rh
V-promoted Rh catalyst + CHs -
catalyst) + El CHs-(V-promoted
(3) Rh catalyst) + E2 (4)
Therefore, we can calculate the difference in binding energy between these two catalysts from (5):
482 TABLE 3 Total energies of methyl-catalyst energies El and E2
for different systems together with estimation of bond
Species
Total energy (eV)
CH3
-123.290 -567.956 -1097.525 -691.998 -1221.275 0.752a 0.460b
Rh catalyst V-promoted Rh catalyst CHa-(Rh catalyst) CHS-(V-promoted Rh catalyst) El E2
aE r = energy of (Rh catalyst + CHs) - energy of (CHa-Rh catalyst). bE2 = energy of (V-promoted Rh catalyst + CHa) - energy of (CHJ-(V-promoted catalyst)).
Rh
De(CHs-V-promoted
(5)
Rh catalyst) - De(CH3-Rh catalyst) = E2 -El
The result shows that the binding energy of CHs-(V-promoted Rh catalyst) is smaller by 0.29 eV than that of the CH,-(Rh catalyst). So less energy is needed to break the bond between the methyl group and V-promoted Rh catalyst as compared with Rh catalyst. To present a detailed discussion, let us analyze the orbital components of the HOMOs of the catalytic system. As the angle 8 varies from 60 to 70” in the Rh catalytic system, the orbital coefficients of d,., and dXz+ of Rh(3) in HOMO are reduced by 0.298 and 0.372 respectively. On the other hand, in the case of V-promoted Rh catalyst, the decrease in orbital coefficients of d,. and d,z _ ,,z is negligible. It is obvious that in the V-promoted catalyst, the bond between methyl and Rh(3) is weaker than in the Rh catalyst owing to the presence of V. Moreover, as 9 varies from 60 to 70”, the increment of d,, of vanadium is about doubled. That is to say, there are more atomic orbit& of vanadium participating in the molecular orbitals of a catalytic system as the methyl group migrates to Rh-CO. A new bond between V and methyl is formed; however it is very weak. Therefore, the barrier of the migration reaction in the V-promoted Rh catalyst decreases. We think that the appearance of vanadium alters the molecular orbitals of the CO/Hz-catalyst system, changes the bonding properties between the adsorbates and catalyst, and improves the methyl migratory reaction. This idea has been confirmed by experiment [5], in which the adsorption capacity of CO on a Rh catalyst is markedly changed on the V-promoted Rh catalyst. We also calculated the processes of CH2 insertion into Rh-CH3 and H addition to Rh---CH, over Rh catalyst and V-promoted Rh catalyst. The calculation results show that there are no marked differences between Rh and V-promoted Rh catalysts in these processes.
483
References 1 D. Gall, E. J. Gibson and C. C. Hall, J. Abpl. Chem., (1952) 371. 2 M. M. Bhasin, W. J. Bartley, P. C. Ellgen and T. P. Wilson, J. Catal., 54 (1978) 120. 3 P. C. Ellgen, W. J. Bartley, M. M. Bhasin and T. P. Wilson, Adv. Chem. Ser., 178 (1979) 147, 4 T. P. Wilson, W. J. Bartley and P. C. Ellgen, in Proc. Symp. Heterogeneous Catalysis Related to Energy Problems, Dalian, China, 1982, paper A27 U. 5 (a) H. Luo, L. Lin and Cui Hua Xue Bao, in press; (b) G. van der Lee, A. G. T. M. Bastein, J. van den Boogert, B. Schuller, H. Luo and V. Ponec, J. Chem. Sot., Faraday Trans., 83 (1987) 2103. 6 Xu Yin-Sheng, He An-Bang and ChewRong, J. Catal. (China), 1 (1980) 49. 7 L. W. Anders and R. S. Hansen, J. Chem. Phys., 59 (1973) 5277. 8 W. M. H. Sachtler,I+oc. 8th Znt. Congr. Catal., Berlin, 1984, Vol. 1,151. 9 P. Uguagliati, A. Palazzi, G. Deganello and U. Belluco, Znorg. Chem., 9 (1970) 724. 10 R. Colton, C. J. Commons and B. F. Hoskins, J. Chem. Sot., Chem. Commun., (1975) 363.
Journal of Molecular Catalysi, 54 (1989)
478 - 483
QUANTUM CHEMICAL STUDY ON THE MECHANISM OF CO HYDROGENATION TO OXYGENATES OVER VANADIUM-PROMOTED RHODIUM CATALYST WANG REN-HU and XU YIN-SHENG* Dalian Institute of Chemical Physics, Academia Sinica, Dalian (China)
Extended Hiickel calculations of CO hydrogenation to oxygenates have been carried out for vanadium-promoted rhodium catalyst. The results of these calculations show that the barrier for CHs migration to Rh-CO to form Rh-COCH3 species is decreased in the case of vanadium-promoted catalyst. A discussion attempts to explain the role of the promoters.
Introduction It is well known that a broad mixture of alkanes and alkenes can be synthesized from CO and Hz over Ni, Ru and Fe metal catalysts. It has been known for some time that the alkylated Co or Fe catalysts can be used to produce appreciable amounts of oxygenates [l]. In 1973, scientists at Union Carbide Company discovered that Fe [2], Mn [3] or alkyl-promoted Rh [4] catalysts can improve the selectivity of CZ oxygenates. It is suggested that the carbonyl insertion reaction (1) : WR)(COW)n
-
WCOR)(Lhn
(1)
is not only an important elementary organometallic reaction, but also one of the key steps in catalytic CO hydrogenation for synthesis of oxygenates. Recently, in our laboratory, Luo [5] has found that high selectivity and reactivity of CZ oxygenates can be obtained over vanadium-promoted rhodium catalysts. An understanding of the role of the promoter in the fundamental steps involved in the catalytic reaction of CO and HZ is of paramount importance. In the present paper, a quantum chemical study of the elementary step of carbonyl insertion in the process of CO hydrogenation over vanadium-promoted rhodium catalyst in the synthesis of oxygenates has been carried out.
*Author to whom correspondence
should be addressed. 0 Elsevier Sequoia/Printed
in The Netherlands
479
Calculation method and model An extended Hiickel semiempirical method has been applied for the calculation 161, and double <-Slater functions were used for the d-orbit& of metallic Rh and V. The parameters used in the calculation are listed in Table 1. Repulsion terms for the inner p electrons of the transition metal have been introduced according to Lhe method of Anders [ 71. In Luo’s paper [5], the mean diameter of a rhodium crystallite was found to be larger than 40 a, so that there are hundreds of rhodium atoms accumulated together. Hence we can imagine that there are metallic crystallites of Rh distributed on the support. According to Sachtler’s model [8], the promotion may take place or locate on the surface of a rhodium crystal with a counterion 02- adsorbed in the vicinity of the promoters. Therefore, the (111) surface of a Rh crystal with promoter V and counterion 02- on it is chosen as the model for the catalyst (Fig. 1). TABLE 1 EHMO parameters for different elements Element
Orbital Exponent ( )
Orbital
Hii (eV)
4.29 (0.5807)
2.135 2.10 4.12 1.97 (0.5685)
5s 5P 4P 4d
8.09 4.57 50.0 12.5
4.70 (0.4755)
1.30 0.875 1.70 (0.7052)
4s 4P 3d
8.81 5.52 11.0
0
2.275 2.276
2s 2P
32.3 14.8
C
1.626 1.625
2s 2P
21.4 11.4
H
1.3
IS
13.6
Rh
V
Fig. 1. Model of catalyst where (0) represents Rh, (@) represents O*-, and (0) represents V. Fig. 2. Reaction path of methyl group migration where (FBO) represents 02-, (@) represents 0, (0) represents Rh, (0 ) represents C, (0) represents V, and ( ) represents H. l
480 TABLE 2 Geometric parameters Bond
Distance between atoms (A)
Angle (“)
Rh-Rh Rh-C C-O C-C
2.69 2.08 1.14 1.54
8 (initial) 60 cp 40 [lo]
It has been confirmed
[9] that the following organometallic
reaction
(2): RhCl(CO)~(PPh~)(C~~)I -
RhClI(CO)(COC~~)(PPh~)
(2)
proceeds via methyl migration as follows: (TF3
Cl,JIIICO
_
1 ‘PPhJ I
CO’
Cl\RbNI ‘“CO 1 ‘PPh, CO’ I
Cl \Rh,COCH3 CO’
1 ‘PPhs
I
In order to design the reaction path of CO hydrogenation to oxygenates, we refer to the organometallic reaction and employ the angle 6 as the reaction coordinate (Fig. 2). The geometric parameters of the catalytic system are listed in Table 2. Results and discussion Path of carbonyl insertion
A plot of potentiaI energy changes along the path of methyl group migration is shown in Fig. 3. According to Uguagliati’s experiments [9], the activation energy of reaction (2) is approximately equal to 6 kcal mol-‘, in agreement with the calculated result.
I..... 60
(&
9
%greel
Fig. 3. Potential energy of methyl migration reaction. The reference energy is the energy for 8 = 60”, Rh catalyst.
481
Comparison of Rh catalyst and V-promoted Rh catalyst in the carbonyl insertion reaction A plot of energy changes of the carbonyl insertion reaction over these two catalysts is presented in Fig. 4. The solid line represents V-promoted Rh catalyst and the broken line corresponds to Rh catalyst. As is clearly shown by the results of calculation, the energy barrier of methyl group migration to Rh-CO to form the Rh-COCH3 species is lowered in Vpromoted Rh catalyst as compared to the case of Rh catalyst.
Fig. 4. Energy changes of carbonyl insertion reaction over two catalysts; (o---O) represents V-promoted Rh catalyst, and (*- - -0) represents Rh catalyst. The reference energy is the same as in Fig. 3. Fig. 5. Molecular conformation
in methyl migratory reaction. Symbols as in Fig. 2.
The role of promoters Why does the reaction of methyl migration to Rh-CO proceed more easily in the V-promoted Rh catalyst? This remains a very interesting question. We know that there are always variations in the components of molecular orbit& in a chemical reaction. In other words, in chemical reactions, some bonds are weakened and broken while the others are strengthened and formed. For example, in Fig. 5, when the angle 9 increases, the methyl group departs from the Rh(3) atom and approaches the C(1) atom. Thus, we can expect that the bond between the methyl group and the Rh(3) atom will be weakened, while the C-C bond will be formed and strengthened. The results of calculation show that in the Rh catalyst, as 6 increases, the components of the Rh(3) atomic orbital in HOMO do decrease markedly. To estimate the difference in the bond strengths between Rh the catalyst and V-promoted Rh catalyst, we calculated the bond energies of CHs-(Rh catalyst) and CHs(V-promoted Rh catalyst) (Table 3). Obviously, the binding energy of CHs-catalyst can be formally constructed from the following equations (3,4): Rh catalyst + CH3 -
CHs-(Rh
V-promoted Rh catalyst + CHs -
catalyst) + El CHs-(V-promoted
(3) Rh catalyst) + E2 (4)
Therefore, we can calculate the difference in binding energy between these two catalysts from (5):
482 TABLE 3 Total energies of methyl-catalyst energies El and E2
for different systems together with estimation of bond
Species
Total energy (eV)
CH3
-123.290 -567.956 -1097.525 -691.998 -1221.275 0.752a 0.460b
Rh catalyst V-promoted Rh catalyst CHa-(Rh catalyst) CHS-(V-promoted Rh catalyst) El E2
aE r = energy of (Rh catalyst + CHs) - energy of (CHa-Rh catalyst). bE2 = energy of (V-promoted Rh catalyst + CHa) - energy of (CHJ-(V-promoted catalyst)).
Rh
De(CHs-V-promoted
(5)
Rh catalyst) - De(CH3-Rh catalyst) = E2 -El
The result shows that the binding energy of CHs-(V-promoted Rh catalyst) is smaller by 0.29 eV than that of the CH,-(Rh catalyst). So less energy is needed to break the bond between the methyl group and V-promoted Rh catalyst as compared with Rh catalyst. To present a detailed discussion, let us analyze the orbital components of the HOMOs of the catalytic system. As the angle 8 varies from 60 to 70” in the Rh catalytic system, the orbital coefficients of d,., and dXz+ of Rh(3) in HOMO are reduced by 0.298 and 0.372 respectively. On the other hand, in the case of V-promoted Rh catalyst, the decrease in orbital coefficients of d,. and d,z _ ,,z is negligible. It is obvious that in the V-promoted catalyst, the bond between methyl and Rh(3) is weaker than in the Rh catalyst owing to the presence of V. Moreover, as 9 varies from 60 to 70”, the increment of d,, of vanadium is about doubled. That is to say, there are more atomic orbit& of vanadium participating in the molecular orbitals of a catalytic system as the methyl group migrates to Rh-CO. A new bond between V and methyl is formed; however it is very weak. Therefore, the barrier of the migration reaction in the V-promoted Rh catalyst decreases. We think that the appearance of vanadium alters the molecular orbitals of the CO/Hz-catalyst system, changes the bonding properties between the adsorbates and catalyst, and improves the methyl migratory reaction. This idea has been confirmed by experiment [5], in which the adsorption capacity of CO on a Rh catalyst is markedly changed on the V-promoted Rh catalyst. We also calculated the processes of CH2 insertion into Rh-CH3 and H addition to Rh---CH, over Rh catalyst and V-promoted Rh catalyst. The calculation results show that there are no marked differences between Rh and V-promoted Rh catalysts in these processes.
483
References 1 D. Gall, E. J. Gibson and C. C. Hall, J. Abpl. Chem., (1952) 371. 2 M. M. Bhasin, W. J. Bartley, P. C. Ellgen and T. P. Wilson, J. Catal., 54 (1978) 120. 3 P. C. Ellgen, W. J. Bartley, M. M. Bhasin and T. P. Wilson, Adv. Chem. Ser., 178 (1979) 147, 4 T. P. Wilson, W. J. Bartley and P. C. Ellgen, in Proc. Symp. Heterogeneous Catalysis Related to Energy Problems, Dalian, China, 1982, paper A27 U. 5 (a) H. Luo, L. Lin and Cui Hua Xue Bao, in press; (b) G. van der Lee, A. G. T. M. Bastein, J. van den Boogert, B. Schuller, H. Luo and V. Ponec, J. Chem. Sot., Faraday Trans., 83 (1987) 2103. 6 Xu Yin-Sheng, He An-Bang and ChewRong, J. Catal. (China), 1 (1980) 49. 7 L. W. Anders and R. S. Hansen, J. Chem. Phys., 59 (1973) 5277. 8 W. M. H. Sachtler,I+oc. 8th Znt. Congr. Catal., Berlin, 1984, Vol. 1,151. 9 P. Uguagliati, A. Palazzi, G. Deganello and U. Belluco, Znorg. Chem., 9 (1970) 724. 10 R. Colton, C. J. Commons and B. F. Hoskins, J. Chem. Sot., Chem. Commun., (1975) 363.