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Hydrodenitrogenation of quinoline over mechanical mixtures of sulphided cobalt-molybdenum and nickel-molybdenum alumina supported catalysts
Catalysis Today, 10 (1991) 681-687 Elsevier Science Publishers B.V., Amsterdam
681
H drodenitrogenation of quinoline over mechanical mixtures of sus;phided cobalt-molybdenum and nickel-molybdenum alumina supported catalysts C. Moreau, L. Bekakra, R. Durand and P. Geneste Laboratoire de Chimie Organique Physique et Cinetique Chimique Appliqudes, URA CNRS 418, 8 rue Rcole Normale, 34053 Montpellier C&lex 1, France.
Abstract Hydrodenitrogenation of quinoline has been performed over mechanical mixtures of sulphided cobalt-molybdenum and nickel-molybdenum catalysts at 340°C and 70 bar hydrogen pressure. It has been shown that cobalt-molybdenumrich mixtures exhibit higher activity for the overall quinoline conversion. The enhancement of the reactivity can reach a factor of about 3, due mainly to the better hydrogenolysis properties of the catalysts mixtures. These results are of particular interest in hydroprocessing HDN of alkylanilines in the presence of heavier nitrogencontaining compounds.
1. INTRODUCTION In a recent paper, it has been shown that the physical combination of a ruthenium sulphide supported on Y-xeolite with a conventional sulphided nickel molybdate on alumina resulted in a catalytic system which enhanced the activity for the hydrodenitrogenation of quinoline [ 11. Our own results concerning the hydrogenation vs hydrogenolysis properties of sulphided cobalt-molybdenum and nickel-molybdenum alumina supported catalysts towards substituted benxenes have led us to conclude that the differences in reactivity between these two catalysts lay rather in the hydrogenolysis than in the hydrogenation properties [2] : the nickel-molybdenum catalyst has about double the hydrogenating capacity than the cobalt-molybdenum catalyst whatever the substituents on the substrate may be, halogens or heteroatoms. By contrast, the hydrogenolysis properties are about ten-fold more pronounced over the cobaltmolybdebum than over the nickel-molybdenum catalysts, particularly in the case of aromatic C-N bond cleavage. A physical combination of these two conventional hydrotreating catalysts could thus constitute a novel approach to improve the 0920~5861/91/$03.50
0 199 1 Elsevier Science Publishers B.V. All rights reserved.
hydrogenation activity of model compounds where the cleavage of C-N bonds is frequently involved. For this purpose, we have investigated the hydrodenitrogenation of quinoline in the presence of 2,6diethylaniline and hydrogen sulphide at 340°C and 70 bar hydrogen pressure over mechanical mixtures of industrial presulphided cobaltmolybdenum and nickel-molybdenum catalysts. This reaction has been shown to be a catalytic test which is representative of the inhibition of hydrodenitrogenation of alkylanilines in the presence of heavier nitrogen-compounds [3]. It has been shown, in particular, that the hydrodenitrogenation of 2,6diethylaniline is closely dependent on the rates of disappearance of decahydroquinoline and 1,2,3,4-tetrahydroquinoline intermediates [4] 2. EXPERIMENTAL Experiments were carried out in a 0.3 litre stirred autoclave (Autoclave Engineers Magne-Drive) operating in a batch mode and equipped with a system for liquid sampling during the course of the reaction without stopping the agitation. The procedure was typically as follows. An equimolar mixture (0.06 M) of quinoline and 2,6diethylaniline in 60ml of n-decane was poured into the autoclave. The mixture of separately sulphided catalysts (0.6 g) was rapidly added to this solution under nitrogen to avoid contact with air. After it had been purged with nitrogen, hydrogen sulphide was introduced at room temperature up to a pressure of 1 bar. The temperature was increased until it reached 340°C. Hydrogen was then introduced at the required pressure (70 bar). Zero time was taken to be when the agitation began. The alumina supported cobalt-molybdenum and nickel-molybdenum catalysts were Procatalyse HR 306 and HR 346, respectively. They were sulphided separately at atmospheric pressure with a gas mixture of 15 96 H2S and 85% H2 by volume. The catalysts (particle size 0.063-0.125 mm) were heated in flowing H2/H2S (gas flow, 120mbmin) from 20 to 400°C (8”CYmin) and held at 400°C for 4 h, then cooled, and finally swept with nitrogen for 30 min. Analyses were performed on a Girdel30 gas chromatograph equipped with a flame ionisation detector using hydrogen as carrier gas and capillary columns (Chrompack CP Sil5 CB and CP Sil 19 CB, 25 m x 0.22 mm i.d). Products were identified by comparison with authentic samples and by GC-MS analysis. The rate constants were deduced from the experimental plots of concentrations vs time by curve fitting and simulation using Anacin software [5]. Assuming all the reactions to be first order in the organic reactant, this software allowed the determination of the forward and reverse rate constants as well as the adsorption of the reactants.
683
3. RESULTS AND DISCUSSION Table 1 summarixies the rate constants calculated for the different steps of the hydrodenitrogenation of quinoline over the mechanical mixtures of catalysts, according to the reaction network presented in Fig. 1. Table 1 Composition of catalyst mixtures (mg), rate constants for the different steps of quinoline and 2,6diethylaniline HDN (x 104 miril g.cat-l) at 340°C and 70 bar H2. 0
NiMo CoMo
600
200 400
300 300
400 200
600 0
kl k2 kl+k2 k3 k4
28 18 46 950 13
50 38 88 1700 20
53 37 90 1100 14
30 34 64 675 5
34 53 87 1100 12
klk2
1.53
1.30
1.43
0.90
0.64
8 16
8 19
3 12
3 12
* estimated rate constants for hydrogenation of 2,6diethylanibne inhibiting period (k$ and after the inhibiting period (k,,)
during the
Although the differences in reactivity do not exceed a factor of 3, it is interesting to notice a general trend in the plots of the rate constants as a function of the composition of the NiMo/CoMo mixtures (Figs. 2-4). A minimum value is oberved for NiMo-rich mixtures (NiMo/CoMo = 2) whereas a maximum value is observed for CoMo-rich catalyst mixtures (NiMo/CoMo = 0.5). Parallel trends have already been reported in the literature for the hydrodesulphurisation of thiophene over sulphided Ni-Co-MO/Al203 catalysts [6]. A more detailed analysis of the curves given in Figs. 2 and 3 shows that the catalytic effect is more important for the C-N bond cleavage (kl and k3) with the CoMo-rich mixtures, thus confIrming the higher hydrogenolysis activity of the CoMo-based catalysts. The effect is less important on the hydrogenation steps (k2 and k4). This is well accounted for by the ratio of the rate constants klk2 (see Table 1). The hydrogenolysis of the C-N bond of 1,2,3,4_tetrahydroquinoline is favoured over the hydrogenation of the benzene ring for mixtures containing at least 50% of CoMo catalyst.
684
Q
1.2.3.4.THQ
i i
12HQH
5.6.7.8-THQ
Figure 1. Kinetic reaction network for the hydrodenitrogenation of quinoline in the presence of H2S and 2,6diethylaniline (batch reactor, 34O”C, 70 bar Hi>.
booiii
6000
E
400
200
0 NiMo
Figure 2. Plot of the experimental rate constants, kl (0), k2 @) and kq (0) (x 104 min-l.gcat-l) vs the composition (mg) of the NiMo HR 346 + CoMo HR 306 catalytic mixture (batch reactor, 34O”C, 70 bar H2).
685
‘O k3
I 6
lo- \ -*
\
\ \
-r/
._&’ 1
* 608
$8
%
boo 200
60; $$
Figure 3. Plot of the experimental rate constant, k3 (xl&! min ml.g.cat-‘) vs the composition (mg) of the NiMo HR 346 + CoMo HR 306 catalytic mixture (batch reactor, 34O”C, 70 bar H2).
60:
2oo 4oo 200 400 ‘3”oi
60; ;y,“,o
Figure 4. Plot of the sum of experimental rate constants, kl + k;? (x lo4 min’-l.gcat-l) vs the composition (mg) of the NiMo HR 346 + CoMo HR 306 catalytic mixture (batch reactor, 34O”C, 70 bar H2).
686
The important consequence of the greater hydrogenolysis activity of the CoMo-rich catalyst mixtures is that the appearance of hydrocarbons, i.e. the total removal of nitrogen atoms (k3 + k4), results mostly from the denitrogenation of decahydroquinoline (k3). It is thus important that the disappearance of 1,2,3,4tetrahydroquinoline (kl + k2) is not a limiting factor. Even if the NiMo catalyst alone and the mixtures NiMo/CoMo = 1 and 0.5 (Fig. 4) exhibit similar activity because of a probable balance between their hydrogenation and hydrogenolysis properties, the higher hydrogenolysis activity of both NiMolCoMo mixtures is mainly responsible for the improvement of the hydrodenitrogenation of quinoline and its intermediates. Another direct consequence of the improvement of activity is that the inhibition of the hydrodenitrogenation of 2,6diethylaniline in the presence of quinoline was shown to result from the presence of bicyclic nitrogen-containing intermediates such as 1,2,3,4_tetrahydroquinoline and decahydroquinoline. Although the rate constants for the disappearance of 2,6_diethylaniline o(i and ku) are only estimated (Table l), it can be seen that the lower inhibiting effect is observed for the two catalytic mixtures which correspond to the higher disappearance rate constants of both 1,2,3 &etrahydroquinoline and decahydroquinoline. Another remarkable feature is that the high activity of catalyst mixtures was also displayed in hydroprocessing tests carried out on shale oils. The NiMoKoMo = 1 and 0.5 mixtures have shown the best activity for the removal of heteroatoms [7]. A similar observation has already been mentioned in the literature with reference to a mixture of ruthenium sulphide supported on Y-zeolite and a conventional presulphided NiMo-alumina catalyst [l]. However, the origin of this promoting effect is not clearly understood. Harvey and Matheson [l] assumed that a ternary active phase could be present as a RuNiMoS phase. Another possibility was the invocation of a conventional bifunctional mechanism, each part of the catalytic mixture having its own activity. From characterisation studies such as oxygen chemisorption and Infrared spectroscopy of adsorbed NO, Caceres and coworkers [6] have assumed that the differences observed in the HDS activity might be related to the impregnation step of the promoters or to the pretreatment conditions. These ideas could receive some support from the results obtained by Laine [8] by doping conventional CoMo and NiMo catalysts with ruthenium or rhodium. Some of the resulting ternary catalysts display (i) higher activity for the conversion of 1,2,3,4tetrahydroquinoline and (ii) larger differences in the partitioning of hydrocarbons compared to the CoMo or NiMo catalysts. According to these different results and, in the absence of more precise characterisation data on the nature of the active phase, it seems that the higher hydrogenolytic properties of the CoMo-rich catalyst mixtures would favour a bifunctional mechanism. Nevertheless, these assumptions have to be confirmed by other characterisation methods.
687
4. CONCLUSION In this work we have shown that a physical combination of conventional presulphided NiMo and CoMo alumina supported catalysts leads to a substantial promotion effect in the hydrodenitrogenation of quinoline. The reasons for such a promotion effect have not yet been well established. However, this effect is thought to result from either a novel active phase or through a bifunctional mechanism. The hydrodenitrogenation of quinoline is improved in the presence of CoMo-rich catalytic mixtures and, as a consequence, the inhibiting effect on the hydrodenitrogenation of 2,6diethylaniline is reduced. Recent work on upgrading of shale oils confirmed the efficiency of such catalytic mixtures.
Acknowledgments This work was carried out in the framework of the contract “New catalysts for hydrodenitrogenation of heavy cuts” of the “Non nuclear energy” R & D programme of the Commission of the European Communities (EN 3C - 0040 F). It received support from ELF, IFP and TOTAL and from the CNRS-PIRSEM.
5. REFERENCES
T.G. Harvey and T.W. Matheson, J. Catal., 101 (1986) 253, and references therein. C. Moreau, J. Joffre, C. Saenx and P. Genes@ J. Catal., 122 (1990) 448. H. Toulhoat and R. Kessas, Rev. Fr. I.F.P., 41 (1986) 511. C. Moreau, L. Bekakra, R. Durand, N. Zmimita and P. Geneste in “Advances in Hydrotreating Catalysts”, M.L. Occelli and R.G. Anthony Editors, Elsevier Science Publisher B.V., Amsterdam, Studies in Surface Science and Catalysis, 51 (1989) 115. J. Joffre, P. Geneste, A. Guida, G. Sxabo and C. Moreau in “Modelling of Molecular Structures and Properties”, J.L. Rivail, Editor, Elsevier Science Publishers B.V., Amsterdam, Studies in Physical and Theoretical Chemistry, 71(1990) 409. C. Caceres, J.L.G. Fierro, A. Lopez Agudo, F. Severino and J. Laine, J. Catal., 97 (1986) 219. A. Benyamna, C. Bennouna, C. Moreau and P. Geneste, unpublished results. R.M. Laine, New. J. Chem., 11 (1987) 543.
681
H drodenitrogenation of quinoline over mechanical mixtures of sus;phided cobalt-molybdenum and nickel-molybdenum alumina supported catalysts C. Moreau, L. Bekakra, R. Durand and P. Geneste Laboratoire de Chimie Organique Physique et Cinetique Chimique Appliqudes, URA CNRS 418, 8 rue Rcole Normale, 34053 Montpellier C&lex 1, France.
Abstract Hydrodenitrogenation of quinoline has been performed over mechanical mixtures of sulphided cobalt-molybdenum and nickel-molybdenum catalysts at 340°C and 70 bar hydrogen pressure. It has been shown that cobalt-molybdenumrich mixtures exhibit higher activity for the overall quinoline conversion. The enhancement of the reactivity can reach a factor of about 3, due mainly to the better hydrogenolysis properties of the catalysts mixtures. These results are of particular interest in hydroprocessing HDN of alkylanilines in the presence of heavier nitrogencontaining compounds.
1. INTRODUCTION In a recent paper, it has been shown that the physical combination of a ruthenium sulphide supported on Y-xeolite with a conventional sulphided nickel molybdate on alumina resulted in a catalytic system which enhanced the activity for the hydrodenitrogenation of quinoline [ 11. Our own results concerning the hydrogenation vs hydrogenolysis properties of sulphided cobalt-molybdenum and nickel-molybdenum alumina supported catalysts towards substituted benxenes have led us to conclude that the differences in reactivity between these two catalysts lay rather in the hydrogenolysis than in the hydrogenation properties [2] : the nickel-molybdenum catalyst has about double the hydrogenating capacity than the cobalt-molybdenum catalyst whatever the substituents on the substrate may be, halogens or heteroatoms. By contrast, the hydrogenolysis properties are about ten-fold more pronounced over the cobaltmolybdebum than over the nickel-molybdenum catalysts, particularly in the case of aromatic C-N bond cleavage. A physical combination of these two conventional hydrotreating catalysts could thus constitute a novel approach to improve the 0920~5861/91/$03.50
0 199 1 Elsevier Science Publishers B.V. All rights reserved.
hydrogenation activity of model compounds where the cleavage of C-N bonds is frequently involved. For this purpose, we have investigated the hydrodenitrogenation of quinoline in the presence of 2,6diethylaniline and hydrogen sulphide at 340°C and 70 bar hydrogen pressure over mechanical mixtures of industrial presulphided cobaltmolybdenum and nickel-molybdenum catalysts. This reaction has been shown to be a catalytic test which is representative of the inhibition of hydrodenitrogenation of alkylanilines in the presence of heavier nitrogen-compounds [3]. It has been shown, in particular, that the hydrodenitrogenation of 2,6diethylaniline is closely dependent on the rates of disappearance of decahydroquinoline and 1,2,3,4-tetrahydroquinoline intermediates [4] 2. EXPERIMENTAL Experiments were carried out in a 0.3 litre stirred autoclave (Autoclave Engineers Magne-Drive) operating in a batch mode and equipped with a system for liquid sampling during the course of the reaction without stopping the agitation. The procedure was typically as follows. An equimolar mixture (0.06 M) of quinoline and 2,6diethylaniline in 60ml of n-decane was poured into the autoclave. The mixture of separately sulphided catalysts (0.6 g) was rapidly added to this solution under nitrogen to avoid contact with air. After it had been purged with nitrogen, hydrogen sulphide was introduced at room temperature up to a pressure of 1 bar. The temperature was increased until it reached 340°C. Hydrogen was then introduced at the required pressure (70 bar). Zero time was taken to be when the agitation began. The alumina supported cobalt-molybdenum and nickel-molybdenum catalysts were Procatalyse HR 306 and HR 346, respectively. They were sulphided separately at atmospheric pressure with a gas mixture of 15 96 H2S and 85% H2 by volume. The catalysts (particle size 0.063-0.125 mm) were heated in flowing H2/H2S (gas flow, 120mbmin) from 20 to 400°C (8”CYmin) and held at 400°C for 4 h, then cooled, and finally swept with nitrogen for 30 min. Analyses were performed on a Girdel30 gas chromatograph equipped with a flame ionisation detector using hydrogen as carrier gas and capillary columns (Chrompack CP Sil5 CB and CP Sil 19 CB, 25 m x 0.22 mm i.d). Products were identified by comparison with authentic samples and by GC-MS analysis. The rate constants were deduced from the experimental plots of concentrations vs time by curve fitting and simulation using Anacin software [5]. Assuming all the reactions to be first order in the organic reactant, this software allowed the determination of the forward and reverse rate constants as well as the adsorption of the reactants.
683
3. RESULTS AND DISCUSSION Table 1 summarixies the rate constants calculated for the different steps of the hydrodenitrogenation of quinoline over the mechanical mixtures of catalysts, according to the reaction network presented in Fig. 1. Table 1 Composition of catalyst mixtures (mg), rate constants for the different steps of quinoline and 2,6diethylaniline HDN (x 104 miril g.cat-l) at 340°C and 70 bar H2. 0
NiMo CoMo
600
200 400
300 300
400 200
600 0
kl k2 kl+k2 k3 k4
28 18 46 950 13
50 38 88 1700 20
53 37 90 1100 14
30 34 64 675 5
34 53 87 1100 12
klk2
1.53
1.30
1.43
0.90
0.64
8 16
8 19
3 12
3 12
* estimated rate constants for hydrogenation of 2,6diethylanibne inhibiting period (k$ and after the inhibiting period (k,,)
during the
Although the differences in reactivity do not exceed a factor of 3, it is interesting to notice a general trend in the plots of the rate constants as a function of the composition of the NiMo/CoMo mixtures (Figs. 2-4). A minimum value is oberved for NiMo-rich mixtures (NiMo/CoMo = 2) whereas a maximum value is observed for CoMo-rich catalyst mixtures (NiMo/CoMo = 0.5). Parallel trends have already been reported in the literature for the hydrodesulphurisation of thiophene over sulphided Ni-Co-MO/Al203 catalysts [6]. A more detailed analysis of the curves given in Figs. 2 and 3 shows that the catalytic effect is more important for the C-N bond cleavage (kl and k3) with the CoMo-rich mixtures, thus confIrming the higher hydrogenolysis activity of the CoMo-based catalysts. The effect is less important on the hydrogenation steps (k2 and k4). This is well accounted for by the ratio of the rate constants klk2 (see Table 1). The hydrogenolysis of the C-N bond of 1,2,3,4_tetrahydroquinoline is favoured over the hydrogenation of the benzene ring for mixtures containing at least 50% of CoMo catalyst.
684
Q
1.2.3.4.THQ
i i
12HQH
5.6.7.8-THQ
Figure 1. Kinetic reaction network for the hydrodenitrogenation of quinoline in the presence of H2S and 2,6diethylaniline (batch reactor, 34O”C, 70 bar Hi>.
booiii
6000
E
400
200
0 NiMo
Figure 2. Plot of the experimental rate constants, kl (0), k2 @) and kq (0) (x 104 min-l.gcat-l) vs the composition (mg) of the NiMo HR 346 + CoMo HR 306 catalytic mixture (batch reactor, 34O”C, 70 bar H2).
685
‘O k3
I 6
lo- \ -*
\
\ \
-r/
._&’ 1
* 608
$8
%
boo 200
60; $$
Figure 3. Plot of the experimental rate constant, k3 (xl&! min ml.g.cat-‘) vs the composition (mg) of the NiMo HR 346 + CoMo HR 306 catalytic mixture (batch reactor, 34O”C, 70 bar H2).
60:
2oo 4oo 200 400 ‘3”oi
60; ;y,“,o
Figure 4. Plot of the sum of experimental rate constants, kl + k;? (x lo4 min’-l.gcat-l) vs the composition (mg) of the NiMo HR 346 + CoMo HR 306 catalytic mixture (batch reactor, 34O”C, 70 bar H2).
686
The important consequence of the greater hydrogenolysis activity of the CoMo-rich catalyst mixtures is that the appearance of hydrocarbons, i.e. the total removal of nitrogen atoms (k3 + k4), results mostly from the denitrogenation of decahydroquinoline (k3). It is thus important that the disappearance of 1,2,3,4tetrahydroquinoline (kl + k2) is not a limiting factor. Even if the NiMo catalyst alone and the mixtures NiMo/CoMo = 1 and 0.5 (Fig. 4) exhibit similar activity because of a probable balance between their hydrogenation and hydrogenolysis properties, the higher hydrogenolysis activity of both NiMolCoMo mixtures is mainly responsible for the improvement of the hydrodenitrogenation of quinoline and its intermediates. Another direct consequence of the improvement of activity is that the inhibition of the hydrodenitrogenation of 2,6diethylaniline in the presence of quinoline was shown to result from the presence of bicyclic nitrogen-containing intermediates such as 1,2,3,4_tetrahydroquinoline and decahydroquinoline. Although the rate constants for the disappearance of 2,6_diethylaniline o(i and ku) are only estimated (Table l), it can be seen that the lower inhibiting effect is observed for the two catalytic mixtures which correspond to the higher disappearance rate constants of both 1,2,3 &etrahydroquinoline and decahydroquinoline. Another remarkable feature is that the high activity of catalyst mixtures was also displayed in hydroprocessing tests carried out on shale oils. The NiMoKoMo = 1 and 0.5 mixtures have shown the best activity for the removal of heteroatoms [7]. A similar observation has already been mentioned in the literature with reference to a mixture of ruthenium sulphide supported on Y-zeolite and a conventional presulphided NiMo-alumina catalyst [l]. However, the origin of this promoting effect is not clearly understood. Harvey and Matheson [l] assumed that a ternary active phase could be present as a RuNiMoS phase. Another possibility was the invocation of a conventional bifunctional mechanism, each part of the catalytic mixture having its own activity. From characterisation studies such as oxygen chemisorption and Infrared spectroscopy of adsorbed NO, Caceres and coworkers [6] have assumed that the differences observed in the HDS activity might be related to the impregnation step of the promoters or to the pretreatment conditions. These ideas could receive some support from the results obtained by Laine [8] by doping conventional CoMo and NiMo catalysts with ruthenium or rhodium. Some of the resulting ternary catalysts display (i) higher activity for the conversion of 1,2,3,4tetrahydroquinoline and (ii) larger differences in the partitioning of hydrocarbons compared to the CoMo or NiMo catalysts. According to these different results and, in the absence of more precise characterisation data on the nature of the active phase, it seems that the higher hydrogenolytic properties of the CoMo-rich catalyst mixtures would favour a bifunctional mechanism. Nevertheless, these assumptions have to be confirmed by other characterisation methods.
687
4. CONCLUSION In this work we have shown that a physical combination of conventional presulphided NiMo and CoMo alumina supported catalysts leads to a substantial promotion effect in the hydrodenitrogenation of quinoline. The reasons for such a promotion effect have not yet been well established. However, this effect is thought to result from either a novel active phase or through a bifunctional mechanism. The hydrodenitrogenation of quinoline is improved in the presence of CoMo-rich catalytic mixtures and, as a consequence, the inhibiting effect on the hydrodenitrogenation of 2,6diethylaniline is reduced. Recent work on upgrading of shale oils confirmed the efficiency of such catalytic mixtures.
Acknowledgments This work was carried out in the framework of the contract “New catalysts for hydrodenitrogenation of heavy cuts” of the “Non nuclear energy” R & D programme of the Commission of the European Communities (EN 3C - 0040 F). It received support from ELF, IFP and TOTAL and from the CNRS-PIRSEM.
5. REFERENCES
T.G. Harvey and T.W. Matheson, J. Catal., 101 (1986) 253, and references therein. C. Moreau, J. Joffre, C. Saenx and P. Genes@ J. Catal., 122 (1990) 448. H. Toulhoat and R. Kessas, Rev. Fr. I.F.P., 41 (1986) 511. C. Moreau, L. Bekakra, R. Durand, N. Zmimita and P. Geneste in “Advances in Hydrotreating Catalysts”, M.L. Occelli and R.G. Anthony Editors, Elsevier Science Publisher B.V., Amsterdam, Studies in Surface Science and Catalysis, 51 (1989) 115. J. Joffre, P. Geneste, A. Guida, G. Sxabo and C. Moreau in “Modelling of Molecular Structures and Properties”, J.L. Rivail, Editor, Elsevier Science Publishers B.V., Amsterdam, Studies in Physical and Theoretical Chemistry, 71(1990) 409. C. Caceres, J.L.G. Fierro, A. Lopez Agudo, F. Severino and J. Laine, J. Catal., 97 (1986) 219. A. Benyamna, C. Bennouna, C. Moreau and P. Geneste, unpublished results. R.M. Laine, New. J. Chem., 11 (1987) 543.