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Supported Metallic Catalysts Achieved through Graphite Intercalation Compounds
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
479
SUPPORTED METALLIC CATALYSTS ACHIEVED THROUGH GRAPHITE INTERCALATION COMPOUNDS
F. BEGUINl, A. MESSAOUDIl, A. CHAFIK2, J. BARRAULT2 and R. ERRE' 'C.R.S.O.C.I. - C.N.R.S., 1B rue de la FCrollerie, 45071 OrlCans CCdex 02, France. 2Catalyse en Chimie Organique, UnkersitC de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers CCdex, France. SUMMARY New preparations of supported metallic catalysts (Fe, Co, Ni) using a graphite intercalation compound as precursor were described. The graphite-MClx derivatives were reduced by otassium and metallic particles were obtained within a graphite matrix. The reduction a!ot MCl, salt or the decomposition of a metallocene lead to the formation of clusters at the edge of the graphite flakes. Only graphite samples with low specific area were used to obtain the intercalated precursors . Consequently, these materials do not present a very high activity in the hydrogenation of carbon monoxide. However, a marked selectivity to alkenes formation was observed with the graphite supports of the largest specific area. INTRODUCTION The research and the preparation of heterogeneous catalysts were largely developed during the last years. We present here the results of studies for the valorization of C1 molecules (carbon monoxide, methane...) and the development of new processes in fine chemistry. Initially, catalysts were not specifically designed with respect to the complex reactions to which they were applied or as regards high selectivity. Recently, significant advances were obtained in preparing largely multifunctional catalysts, e.g. useful in the selective conversion of syngas to light olefins or alcohols or in hydrofunctionalization reactions, for which several kinds of reactional centers are necessary in the catalyst. For applications in fine chemistry processes, one must develop new selective catalysts taking into account the particular conditions of their use : presence of solvent, of heteroatoms (0,N, S, halogen ...) in the reagents and consequently a possible formation of products (H20, NH3, H2S, HX...) which are well known to modify the classical catalysts. For these reasons,we were interested in a new synthesis of metals dispersed on carbon. Indeed, this support allows us to obtain high-performance catalysts selective either in gas or liquid phases. For the preparation of such catalysts one must take into account the classical concepts of heterogeneous catalysis : geometric and electronic effects, SMSI, effect of a promoting agent ... Inhomogeneous M catalysts are generally obtained by impregnation of a support with a metallic salt dissolved in aqueous solution, then reduced by hydrogen. With
480
graphite as support, a homogeneous dispersion of the metal could be obtained from its intercalation compound, as stated by Volpin (ref.1) to occur with the reduction of the binary graphite MC?, compounds (where M is a transition metal). Braga (ref.2) later claimed that he obtained intercalated metals by reduction of a metallic halide MCl, with the binary Kc8. More recently, Inagaki (ref.3) repeated Braga's work with CoC12 and found that the reaction gives a ferromagnetic ternary graphite-Co-THF compound. However, some authors claimed that whatever the process, a part of the metal is included and not intercalated (ref.4,S). In this paper, we corroborate that, starting either from donor of from acceptor GIC, reduction only yields metallic particles, even under mild conditions. Accordingly, processes based on the chemical reduction of acceptor graphite-MCl, compounds or on the reaction of KC8 with metallic halides or metallocenes constitue a good method for the synthesis of metallic catalysts supported on graphite. These new compounds were tested in the hydrogenation of carbon monoxide. RESULTS - DISCUSSION Reaction of graphite MC1,- (M = Ni,Co.Fe) with metallic potassium Second stage G-MCl, binaries were prepared from the direct reaction of a metallic chloride (FeC13, CoC12, NiC12) with natural graphite under a pressure of chlorine (ref. 6). The G-MCl, compound was then allowed to react with potassium under vacuum at 3 0 0 ° C for about two days. According to some authors, the alkali metal can intercalate the second stage acceptor compound to give a heterostructure (ref.7, 8). We have however found that MC!, and K, located in adjacent domains, are very reactive even at this relatively low temperature, and that the following reaction occurs spontaneously :
The X-ray diffraction diagrams mainly show two sets of lines due to KCl and metallic clusters. After washing with a water/ethanol mixture (l/l), the lines of KC1 are still present, indicating that the products of the reaction are included in the graphite matrix. X.P.S. analysis shows that the main contribution of the 2p core level of the M element is due to a metallic state M" (80%). Compared to the chemical analysis (Table l),the C/M ratio obtained by X.P.S. shows a very weak concentration of the element M at the surface of graphite except with the G - FeC13 sample in which a part of the iron has migrated to the surface during the reduction process. In all cases, chemical analysis and X.P.S. reveal a K/C1 ratio greater that one due to the intercalation of excess potassium in the graphite freed by the reaction. Transmission electron microscopy shows a rather narrow and homogeneous distribution of particles sizes (about 3 0 nm). After exposure to air, the metal in only slightly oxidized, indicating that the particles are protected by the lattice or/and covered by the KC1 produced during the reaction.
481
TABLE 1 Atomic ratios given by chemical analysis (C.A.) and X.P.S. on the products found after reduction of G-CoC12, G-NiC12, G-FeC13 by potassium at 300" C. Sample
C.A.
G-CoC12 G-NiC12 G-FeC13
3.99 5.09 5.33
c/c1 X.P.S. 3.3 1.6 1.1
K/C1 C.A. X.P.S. 1.3 1.4 1.2
1.9 1.4 1.1
C/M C.A. X.P.S.
8.1 10.6 12.2
44.8 120.0 10.5
For catalytic applications, an exfoliation of the host lattice would be essential. To explain this reaction, we propose the following mechanism : the first step is the formation of a biintercalation compound with distinct islands of K and MCl,, since, in the pleated layered structure, the reagents occupy all the interlayered spaces of the host lattice ; then, even at low temperature, the reagents can react together to give metal clusters. Reduction of metallic halides bv K Q The KC8 binary is prepared by the direct reaction of potassium on graphite under vacuum (ref. 9). Due to the electronic transfer to the graphene plane, this compound is a strong reducing agent. If it is allowed to react with a metallic halide MClX (M = Fe,Co,Ni) dissolved in tetrahydrofuran (THF), the reaction occurs spontaneously at room temperature and leads to the formation of metal M" dispersed on graphite :
Three phases were identified by X-ray diffraction : graphite, KCI and the M" metallic species. The X.P.S. spectra of the reaction product show that the 2p core level of M may be attributed essentially to a metallic state (70 to 80%). However X.P.S. analysis reveals a strong concentration of the elements which constitue the KX salt on the surface. In Table 2, the atomic ratios given by chemical analysis are compared to the quantitative results deduced from X.P.S. spectra. The C/K ratio obtained by X.P.S. is always lower than the one given by chemical analysis. The K/X ratio is close to 1, confirming the existence of KCl. The KCl can be completely eliminated by washing with a carefully degased water/ethanol (1/1) mixture, but this treatment which can be used with Ni or Co, is not applicable to the iron derivatives as the metal reacts to give an oxihydroxide FeOOH which transforms to Fe2O3 under vacuum. All three M species are oxidized after exposure to air, proving that the metallic clusters are easily attacked by any reagent. Moreover transmission electronic micrographs show that the metallic particles are localized at the edge of the graphite flakes.
482
TABLE 2 Atomic ratios given by chemical analysis (C.A.) and X.P.S. for the products formed by reduction of some salts in THF solution by Kc8.
Salt
C/K C.A. X.P.S.
coc12 NiBr2 FeC12 FeC13
7.9 8.5 9.6 18.6
1.9 1.5 1.0
1.0
K/X C.A. X.P.S.
C/M C.A. X.P.S.
1.0 0.8 1.1 0.8
14.0 12.1 15.8 36.4
1.1 1.3 1.1 1.0
15.9 8.0 16.0 12.0
DecomDosition of a metallocene bv KC8 The main disadvantage of the previous reaction (2) is the simultaneous formation of KCl which partly inhibits the properties of the metallic element. We have found that under appropriate conditions, the reaction of KC8 with metallocenes (Fe(C5Hg)z and Ni (CgH5)2) dissolved in dimethoxyethane (DME) gives soluble by-products, permitting the preparation of "fine M / C by the following reaction :
The results of chemical analyses are given in Table 3. For the reaction with Fe(CgHg)2, chemical analysis shows an important concentration of residual potassium. In the case of nickelocene, only traces of potassium were detected and two phases were observed by X-ray diffraction : pure metallic nickel Ni" and graphite. TABLE 3 Chemical analysis of the products obtained by the decomposition of metallocenes in DME by KC8. Metallocene
TC
C
K
M
H
Ni(C5H5)2
20 60
69.9 63.2
1.1 1.5
15.2 19.4
1.2 1.7
87.4 C22.5Ko.iNilH4.5 85.8 C25.9K0.1NiiHg.i
Fe(C5H5)2
60
71.0
9.1
10.5
0.5
91.1 C31Ki.2FeiH2.7
X
Global Formula
483
The intensity and the binding energy of the 2p core level of Ni confirm the metallic state. With ferrocene, reaction (3) is not complete because this molecule is stabler than nikelocene : X-ray diffraction clearly proves this results, with small peaks due to metallic iron and others attributed to a high stage intercalation compound. Due to its aromatic character, ferrocene probably first intercalates and reacts in the interlayer space. We tested this hypothesis by studying the direct reaction of KC8 with Fe(CgHg)2, with no solvent. Chemical analysis gives a global formula C28.3K2.3FeH10, equivalent to two KCs molecules react with one C~~.~K~J(F~C~O Thus, H ~ Oapproximately ). Fe(CgHg)2. X.P.S. analysis shows that the iron particles are essentially in the metallic state Fe" : the total decomposition of ferrocene by KCg would occur along the following reaction path :
and the 001X-ray lines with an identity period I, = 12.3 A could correspond to a second stage K2(CgHg)2C16 ternary phase with iron supported by the graphite matrix. At present, we are however unable to explain the exact nature of the intercalated compound found which may contain a polymerization product of the cyclopentadienyl groups.
Hvdrogenation of carbon monoxide For better appreciation of the performances, the new catalysts were tested in the hydrogenation of carbon monoxide. In view of the results presented above, and taking into account the size and distribution of the metallic particles as determined by X-ray diffraction and Transmission Electron Microscopy (ref. lo), only the Co products prepared according to reaction (2) were studied. In fact, the best results were obtained with cobalt using the impregnation method (ref. 11). The distribution of the metal and the occlusion and migration of superficial species, as well as their reduction will greatly depend on the nature of the carbon used for the preparation of KCg. To prepare the supported cobalt, three different carbon materials were chosen : - Ceylon graphite (O.3m2g-') - Graphitized carbon black (LeCarbone Lorraine, Srn2.g-l) - Lonza graphite (reference HSAG, 300m2.gb1) The results are given in table 4. The nature of the carbonaceous support and probably its specific area have a determining effect on the catalytic properties of these solids. In the same experimental conditions, the catalyst from Lonza graphite is 60 times more active than the catalyst from Ceylon graphite. Moreover, the selectivity of Co/Lonza graphite is very different from the other solids as it gives a large amount of hydrocarbons and particularly olefins.
484
TABLE 4 Influence of the carbonaceous support on the properties of the Co/C catalysts used in the hydrogenation of carbon monoxide. P = 1bar, H2/Co = 1, Total flow 3.6 1.h-I. The solids are reduced by hydrogen at 400°C before the reaction. Support Reaction temperature
(“C) Activity ( x l d ) mole h-l g-lCat Selectivity (%)
co2
Ceylon graphite
Graphitized carbon black
Loma Graphite
300
300
280
300
0.08
1
1.8
5
100
92 5.1 1.6
28.7 21.3 4.3 1.2 8.1
Selectivity comparable to 280°C
CHq c2Hq c2Hg c3Hg c3H8 C4H8 C4H10 C5H10 c 6 - c10
1.2
5.6 0.7 6.0 24.0
This behaviour is in good agreement with the structural observations on Co/Lonza graphite indicating well distributed metallic particles localized at the edge of the graphite flakes (ref. 10). With the other supports and owing to the lower specific area, the number of reactional centers at the edge of the flakes is smaller and the particle size larger than in the case of Lorna graphite. A comparison with the selectivity of a catalyst prepared by impregnation of a Lonza graphite with cobalt nitrate and reduced by hydrogen is given in table 5, its total activity being 8.6 x l o 3 mole.h-l.G-l cat.
TABLE 5 Properties of a Co catalyst prepared by impregnation of Lorna graphite with cobalt nitrate and reduced by hydrogen at 410°C. Co 5% / Lonza graphite HSAG 300 (3O0m2.g-l) Temperature of the reaction 240°C. Selectivity (%>
co2 8.5
CHq 53
c2 5
c3 5.6
c4
5.5
c5-c9 21.5
485
Taking into account the differences in the temperatures of reaction, it appears that the impregnated catalyst is 20 to 50 times more active than the solid prepared from KC8. Moreover, our catalyst gives a larger amount of superior hydrocarbons. This behaviour could be due to the presence of potassium as many studies show that an alkali element favors chain growth (ref. 12). Nevertheless this catalyst is particularly selective for alkenes formation (= 30%).
CONCLUSION The three reactions studied in this paper always lead to metallic clusters supported by the graphite matrix in the conditions of our experiments. Depending on the initial binary, the particles are either at the surface or included.Best activity was observed with the catalysts prepared from KC8. However, for the materials prepared from Graphite-MCI,, improved activity could be obtained after previous exfoliation of the graphitic matrix. The relatively low activity of supported cobalt obtained from an intercalation compound could be due to the low specific area of the support or the presence of KCl. Some trials on washed samples or on specimens prepared as in reaction (3) could give more information. The most striking fact with our products a particular selectivity to alkene formation with a support of relatively high specific area. REFERENCES 1
2
3 4 5 6 7 8 9 10 11 12
M.E. Volpin, Yu.N. Novikov, N.D. Lapkina, V.I.Kasatochkin, Yu.T. Struchkov, M.E. Kazakov, R.A. Stukan, V.A.Povitskij, Yu.S. Karimov and A.V. Zvarikina, J. Amer. Chem. SOC.,97 : 12 (1975) 3366. P. Braga, A. Ri amonti, D. Savoia, C. Trombini and A. Umani-Ronchi, J.C.S. Chem. Comm., 8978) 927. M. Inagaki, Y . Shiwashi and Y. Maeda, J. Chim. Phys., 81 (1984) 847. G. Bewer, N. Wichmann and H.P. Boehm, Mat. Sci. and Eng., 31 (1977) 73-76. H. Schafer-Stahl, J.C.S. Dalton (1981) 328. S. Flandrois, J.M. Masson, J.C. Rouillon, J. Gaultier and C. Hauw, Synth. Met., 3 (1981) 1. G. Furdin, L. Hachim, D. GuCrard, A. HCrold, C.R. Acad. Sci., 3 0 1 (1985) 579. R. Erre, F. Bkguin, D. GuCrard, S. Flandrois, Proc. 4th International Carbon Conference, Baden-Baden (1986 516. A. HCrold, Bull. SOC.Chim. Fr., t1955) 999. F. BCguin, A. Messaoudi and R. Erre, submitted to Carbon and A. Messaoudi, Ph. D. Thesis, OrlCans, France (1989). A. Chafik, Ph. D. Thesis, Poitiers, France (1988). J. Abbot, N.J. Clark and B.G. Baker, Appl. Cat., 26 (1986) 141.
479
SUPPORTED METALLIC CATALYSTS ACHIEVED THROUGH GRAPHITE INTERCALATION COMPOUNDS
F. BEGUINl, A. MESSAOUDIl, A. CHAFIK2, J. BARRAULT2 and R. ERRE' 'C.R.S.O.C.I. - C.N.R.S., 1B rue de la FCrollerie, 45071 OrlCans CCdex 02, France. 2Catalyse en Chimie Organique, UnkersitC de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers CCdex, France. SUMMARY New preparations of supported metallic catalysts (Fe, Co, Ni) using a graphite intercalation compound as precursor were described. The graphite-MClx derivatives were reduced by otassium and metallic particles were obtained within a graphite matrix. The reduction a!ot MCl, salt or the decomposition of a metallocene lead to the formation of clusters at the edge of the graphite flakes. Only graphite samples with low specific area were used to obtain the intercalated precursors . Consequently, these materials do not present a very high activity in the hydrogenation of carbon monoxide. However, a marked selectivity to alkenes formation was observed with the graphite supports of the largest specific area. INTRODUCTION The research and the preparation of heterogeneous catalysts were largely developed during the last years. We present here the results of studies for the valorization of C1 molecules (carbon monoxide, methane...) and the development of new processes in fine chemistry. Initially, catalysts were not specifically designed with respect to the complex reactions to which they were applied or as regards high selectivity. Recently, significant advances were obtained in preparing largely multifunctional catalysts, e.g. useful in the selective conversion of syngas to light olefins or alcohols or in hydrofunctionalization reactions, for which several kinds of reactional centers are necessary in the catalyst. For applications in fine chemistry processes, one must develop new selective catalysts taking into account the particular conditions of their use : presence of solvent, of heteroatoms (0,N, S, halogen ...) in the reagents and consequently a possible formation of products (H20, NH3, H2S, HX...) which are well known to modify the classical catalysts. For these reasons,we were interested in a new synthesis of metals dispersed on carbon. Indeed, this support allows us to obtain high-performance catalysts selective either in gas or liquid phases. For the preparation of such catalysts one must take into account the classical concepts of heterogeneous catalysis : geometric and electronic effects, SMSI, effect of a promoting agent ... Inhomogeneous M catalysts are generally obtained by impregnation of a support with a metallic salt dissolved in aqueous solution, then reduced by hydrogen. With
480
graphite as support, a homogeneous dispersion of the metal could be obtained from its intercalation compound, as stated by Volpin (ref.1) to occur with the reduction of the binary graphite MC?, compounds (where M is a transition metal). Braga (ref.2) later claimed that he obtained intercalated metals by reduction of a metallic halide MCl, with the binary Kc8. More recently, Inagaki (ref.3) repeated Braga's work with CoC12 and found that the reaction gives a ferromagnetic ternary graphite-Co-THF compound. However, some authors claimed that whatever the process, a part of the metal is included and not intercalated (ref.4,S). In this paper, we corroborate that, starting either from donor of from acceptor GIC, reduction only yields metallic particles, even under mild conditions. Accordingly, processes based on the chemical reduction of acceptor graphite-MCl, compounds or on the reaction of KC8 with metallic halides or metallocenes constitue a good method for the synthesis of metallic catalysts supported on graphite. These new compounds were tested in the hydrogenation of carbon monoxide. RESULTS - DISCUSSION Reaction of graphite MC1,- (M = Ni,Co.Fe) with metallic potassium Second stage G-MCl, binaries were prepared from the direct reaction of a metallic chloride (FeC13, CoC12, NiC12) with natural graphite under a pressure of chlorine (ref. 6). The G-MCl, compound was then allowed to react with potassium under vacuum at 3 0 0 ° C for about two days. According to some authors, the alkali metal can intercalate the second stage acceptor compound to give a heterostructure (ref.7, 8). We have however found that MC!, and K, located in adjacent domains, are very reactive even at this relatively low temperature, and that the following reaction occurs spontaneously :
The X-ray diffraction diagrams mainly show two sets of lines due to KCl and metallic clusters. After washing with a water/ethanol mixture (l/l), the lines of KC1 are still present, indicating that the products of the reaction are included in the graphite matrix. X.P.S. analysis shows that the main contribution of the 2p core level of the M element is due to a metallic state M" (80%). Compared to the chemical analysis (Table l),the C/M ratio obtained by X.P.S. shows a very weak concentration of the element M at the surface of graphite except with the G - FeC13 sample in which a part of the iron has migrated to the surface during the reduction process. In all cases, chemical analysis and X.P.S. reveal a K/C1 ratio greater that one due to the intercalation of excess potassium in the graphite freed by the reaction. Transmission electron microscopy shows a rather narrow and homogeneous distribution of particles sizes (about 3 0 nm). After exposure to air, the metal in only slightly oxidized, indicating that the particles are protected by the lattice or/and covered by the KC1 produced during the reaction.
481
TABLE 1 Atomic ratios given by chemical analysis (C.A.) and X.P.S. on the products found after reduction of G-CoC12, G-NiC12, G-FeC13 by potassium at 300" C. Sample
C.A.
G-CoC12 G-NiC12 G-FeC13
3.99 5.09 5.33
c/c1 X.P.S. 3.3 1.6 1.1
K/C1 C.A. X.P.S. 1.3 1.4 1.2
1.9 1.4 1.1
C/M C.A. X.P.S.
8.1 10.6 12.2
44.8 120.0 10.5
For catalytic applications, an exfoliation of the host lattice would be essential. To explain this reaction, we propose the following mechanism : the first step is the formation of a biintercalation compound with distinct islands of K and MCl,, since, in the pleated layered structure, the reagents occupy all the interlayered spaces of the host lattice ; then, even at low temperature, the reagents can react together to give metal clusters. Reduction of metallic halides bv K Q The KC8 binary is prepared by the direct reaction of potassium on graphite under vacuum (ref. 9). Due to the electronic transfer to the graphene plane, this compound is a strong reducing agent. If it is allowed to react with a metallic halide MClX (M = Fe,Co,Ni) dissolved in tetrahydrofuran (THF), the reaction occurs spontaneously at room temperature and leads to the formation of metal M" dispersed on graphite :
Three phases were identified by X-ray diffraction : graphite, KCI and the M" metallic species. The X.P.S. spectra of the reaction product show that the 2p core level of M may be attributed essentially to a metallic state (70 to 80%). However X.P.S. analysis reveals a strong concentration of the elements which constitue the KX salt on the surface. In Table 2, the atomic ratios given by chemical analysis are compared to the quantitative results deduced from X.P.S. spectra. The C/K ratio obtained by X.P.S. is always lower than the one given by chemical analysis. The K/X ratio is close to 1, confirming the existence of KCl. The KCl can be completely eliminated by washing with a carefully degased water/ethanol (1/1) mixture, but this treatment which can be used with Ni or Co, is not applicable to the iron derivatives as the metal reacts to give an oxihydroxide FeOOH which transforms to Fe2O3 under vacuum. All three M species are oxidized after exposure to air, proving that the metallic clusters are easily attacked by any reagent. Moreover transmission electronic micrographs show that the metallic particles are localized at the edge of the graphite flakes.
482
TABLE 2 Atomic ratios given by chemical analysis (C.A.) and X.P.S. for the products formed by reduction of some salts in THF solution by Kc8.
Salt
C/K C.A. X.P.S.
coc12 NiBr2 FeC12 FeC13
7.9 8.5 9.6 18.6
1.9 1.5 1.0
1.0
K/X C.A. X.P.S.
C/M C.A. X.P.S.
1.0 0.8 1.1 0.8
14.0 12.1 15.8 36.4
1.1 1.3 1.1 1.0
15.9 8.0 16.0 12.0
DecomDosition of a metallocene bv KC8 The main disadvantage of the previous reaction (2) is the simultaneous formation of KCl which partly inhibits the properties of the metallic element. We have found that under appropriate conditions, the reaction of KC8 with metallocenes (Fe(C5Hg)z and Ni (CgH5)2) dissolved in dimethoxyethane (DME) gives soluble by-products, permitting the preparation of "fine M / C by the following reaction :
The results of chemical analyses are given in Table 3. For the reaction with Fe(CgHg)2, chemical analysis shows an important concentration of residual potassium. In the case of nickelocene, only traces of potassium were detected and two phases were observed by X-ray diffraction : pure metallic nickel Ni" and graphite. TABLE 3 Chemical analysis of the products obtained by the decomposition of metallocenes in DME by KC8. Metallocene
TC
C
K
M
H
Ni(C5H5)2
20 60
69.9 63.2
1.1 1.5
15.2 19.4
1.2 1.7
87.4 C22.5Ko.iNilH4.5 85.8 C25.9K0.1NiiHg.i
Fe(C5H5)2
60
71.0
9.1
10.5
0.5
91.1 C31Ki.2FeiH2.7
X
Global Formula
483
The intensity and the binding energy of the 2p core level of Ni confirm the metallic state. With ferrocene, reaction (3) is not complete because this molecule is stabler than nikelocene : X-ray diffraction clearly proves this results, with small peaks due to metallic iron and others attributed to a high stage intercalation compound. Due to its aromatic character, ferrocene probably first intercalates and reacts in the interlayer space. We tested this hypothesis by studying the direct reaction of KC8 with Fe(CgHg)2, with no solvent. Chemical analysis gives a global formula C28.3K2.3FeH10, equivalent to two KCs molecules react with one C~~.~K~J(F~C~O Thus, H ~ Oapproximately ). Fe(CgHg)2. X.P.S. analysis shows that the iron particles are essentially in the metallic state Fe" : the total decomposition of ferrocene by KCg would occur along the following reaction path :
and the 001X-ray lines with an identity period I, = 12.3 A could correspond to a second stage K2(CgHg)2C16 ternary phase with iron supported by the graphite matrix. At present, we are however unable to explain the exact nature of the intercalated compound found which may contain a polymerization product of the cyclopentadienyl groups.
Hvdrogenation of carbon monoxide For better appreciation of the performances, the new catalysts were tested in the hydrogenation of carbon monoxide. In view of the results presented above, and taking into account the size and distribution of the metallic particles as determined by X-ray diffraction and Transmission Electron Microscopy (ref. lo), only the Co products prepared according to reaction (2) were studied. In fact, the best results were obtained with cobalt using the impregnation method (ref. 11). The distribution of the metal and the occlusion and migration of superficial species, as well as their reduction will greatly depend on the nature of the carbon used for the preparation of KCg. To prepare the supported cobalt, three different carbon materials were chosen : - Ceylon graphite (O.3m2g-') - Graphitized carbon black (LeCarbone Lorraine, Srn2.g-l) - Lonza graphite (reference HSAG, 300m2.gb1) The results are given in table 4. The nature of the carbonaceous support and probably its specific area have a determining effect on the catalytic properties of these solids. In the same experimental conditions, the catalyst from Lonza graphite is 60 times more active than the catalyst from Ceylon graphite. Moreover, the selectivity of Co/Lonza graphite is very different from the other solids as it gives a large amount of hydrocarbons and particularly olefins.
484
TABLE 4 Influence of the carbonaceous support on the properties of the Co/C catalysts used in the hydrogenation of carbon monoxide. P = 1bar, H2/Co = 1, Total flow 3.6 1.h-I. The solids are reduced by hydrogen at 400°C before the reaction. Support Reaction temperature
(“C) Activity ( x l d ) mole h-l g-lCat Selectivity (%)
co2
Ceylon graphite
Graphitized carbon black
Loma Graphite
300
300
280
300
0.08
1
1.8
5
100
92 5.1 1.6
28.7 21.3 4.3 1.2 8.1
Selectivity comparable to 280°C
CHq c2Hq c2Hg c3Hg c3H8 C4H8 C4H10 C5H10 c 6 - c10
1.2
5.6 0.7 6.0 24.0
This behaviour is in good agreement with the structural observations on Co/Lonza graphite indicating well distributed metallic particles localized at the edge of the graphite flakes (ref. 10). With the other supports and owing to the lower specific area, the number of reactional centers at the edge of the flakes is smaller and the particle size larger than in the case of Lorna graphite. A comparison with the selectivity of a catalyst prepared by impregnation of a Lonza graphite with cobalt nitrate and reduced by hydrogen is given in table 5, its total activity being 8.6 x l o 3 mole.h-l.G-l cat.
TABLE 5 Properties of a Co catalyst prepared by impregnation of Lorna graphite with cobalt nitrate and reduced by hydrogen at 410°C. Co 5% / Lonza graphite HSAG 300 (3O0m2.g-l) Temperature of the reaction 240°C. Selectivity (%>
co2 8.5
CHq 53
c2 5
c3 5.6
c4
5.5
c5-c9 21.5
485
Taking into account the differences in the temperatures of reaction, it appears that the impregnated catalyst is 20 to 50 times more active than the solid prepared from KC8. Moreover, our catalyst gives a larger amount of superior hydrocarbons. This behaviour could be due to the presence of potassium as many studies show that an alkali element favors chain growth (ref. 12). Nevertheless this catalyst is particularly selective for alkenes formation (= 30%).
CONCLUSION The three reactions studied in this paper always lead to metallic clusters supported by the graphite matrix in the conditions of our experiments. Depending on the initial binary, the particles are either at the surface or included.Best activity was observed with the catalysts prepared from KC8. However, for the materials prepared from Graphite-MCI,, improved activity could be obtained after previous exfoliation of the graphitic matrix. The relatively low activity of supported cobalt obtained from an intercalation compound could be due to the low specific area of the support or the presence of KCl. Some trials on washed samples or on specimens prepared as in reaction (3) could give more information. The most striking fact with our products a particular selectivity to alkene formation with a support of relatively high specific area. REFERENCES 1
2
3 4 5 6 7 8 9 10 11 12
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