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Selectivity and mechanism of Fischer-Tropsch synthesis with iron and cobalt catalysts
H.E. Curry-Hyde and R.F.Howe (Editors), Natural Gas Conversion 11 0 1994 Elsevier Science B.V. All rights rcservcd.
455
Selectivity and mechanism of Fischer-Tropsch synthesis with iron and cobalt catalysts Hans Schulz, Eric van Steen and Michael Claeys Engler-Bunte-Institute, University of Karlsruhe, KaiserstraDe 12,76128 Karlsruhe, Germany The Fischer-Tropsch CO-hydrogenation has been comparatively performed on iron and cobalt in a gradientless tank reactor. Kinetic coefficients of elementary reactions and their dependence on pm, pco, mZoand T were determined with the help of a model for chain growth-, branching- and termination reactions. Changes of reaction rates of CO consumption, of methane formation, of chain propagation and chain branching are presented and discussed in terms of elemental surface reactions and characteristics of the catalyst base metals. 1. INTRODUCTION
The kinetic regime of Fischer-Tropsch CO-hydrogenation as a type of non Mvial polymerlzation on the surface of a heterogeneous catalyst [ 11 is only established through specific inhibitions of reactions of desorption. Consequently, sets of consecutive reaction steps among adsorbed species lead to chain growth. In this investigation kinetic coeffldents of elementary surface reactions are determined with cobalt and iron catalysts in dependence of the partial pressures of H2, CO, and H20and the reaction temperature through special experimental techniques with a gradientless reactor, special procedures of product sampling and analysis and using the quantitative kinetic model of interconnected surface reactions. 2. EXPERIMENTAL
CO-hydrogenation has been performed in a gradientless operated CSTR reactor with the catalyst particles being suspended in squalane (Fig.1). The flows of hydrogen and carbonmonoxide to the reactor were precisely controlled and measured. A constant reaction pressure was obtained with a pressure controlled make up stream of argon. A reference stream, containing N2 and cyclopropane or neopentane, was added to the hot gaseous product stream. A continuous stream of products, containing liquids, vapours and gases was withdrawn from the reactor at reaction temperature and the liquid fraction collected in a hot trap. From the hot gaseous product stream samples were collected, sealed in glass ampoules and analysed later by gaschromatography. Degree of conversion and yields of all the compounds in the gaseous product stream were directly obtained from the chromatogrammes by referring to the peaks of the reference compounds, nitrogen and cyclopropane. The capillary chromatogrammescovered the range of compounds C1 to %o+ using a temperature programme from -80 to 275 "C. For determination of chain branching only parafinic compounds were obtained via precolumn hydrogenationof olef'ins (and alcohols). With the help of the quantitative model [2] the kinetic coefficients of elementary reactions were calculated from the product composition. The catalysts were prepared by precipitation from nitrate solutions (one exception: a commercial sample of Fe203). Activiation was traced via temperature programmed reduction.
456
'
(PRESSUR CONTROULED),
VM I-:-
vco :
1
41
t"O+
,
CYCLOPROPANE (REFERENCE)
.MIXER ,
\?
CSTR SLURRY REACTOR
Figure 1. Scheme of the Fischer-Tropsch synthesis in a "gradientless" operated slurry reactor
In Figure 2 the reduction profiles of 3 cobalt and 3 iron catalysts are represented together with their composition (mass ratios) and the final degree of reduction obtained at 550 "C. In the cobalt catalysts the thoria and magnesia inhibit the reduction of the cobalt. In this paper mainly the results obtained with the catalysts 100 Co : 10 MgO : 3 T h o 2 : 100 Aerosil are presented. In the iron catalysts copper is added as a reduction promoter and potassium as a chemical promoter. Here mainly the results obtained with the catalyst 100 Fe : 37 M203 : 3 Cu:2 K20 are used. Final Catalyst Red. % 94 100 Co : 465 Silica (Impregnation) 63 100 Co : 10 MgO : 3 T h o 2 : 100 Aerosil 1) 100 Co :20 Tho2: 100 Aerosil 1) 42 98
undef. 29 0
loo 100 T#npmtur. 'C
Fe2°3
100 Fe :766 Mn : 49Cu :5 K20 1) 1OOFe :37 A1203 : 3 C h :2KzO 1)
-
1)
Precipitation from nitrate solutions
Figure 2. Profiles of water formation;temperature programmed reduction of the catalysts. (rg,=40 ml(NTP)/min, p~*=0.25bar, pk=0.75 bar; 2"Umin up to 550°C kept until rH2O=O)
varied
parameter
H2
co H20
T
catalyst
Co-Mg-Th-Aerosil Pe-AI-Cu-K Co-Mg-Th-Aerosil PO-AI-Cu-K Co-Mg-Th-Aerosil Fe-AI-Cu-K Co-Mg-Th-Aerosil Fe
H2
partial pressure, bar
co
H20
2.7 2.5
1.6 1.5 1.6 1.4
2.7
9.9
2.5 10.1 5.1 9.0
2.8 2.5 2.6 0.1
1.2 0.8
T, 'C 210 250 210 250 210 250
457
3. RESULTS A N D DISCUSSION 3.1 Reaction rate of CO-conversion
In FT-CO conversion alternative reaction pathways are visualized:
'CHz
Figure 3. Kinetic scheme of alternative reactions of CO consumption ve reactlOnS0: Reaction (0): CO-conversion on none- IT-hydrogenation sites, producing "extramethane" Reaction (1): CO-dissociationand hydrogenation to form the methylen species 2to react with H to form a CH3 species (reaction 2). or to react with an akyl species R as the chain growth step (reaction 3). ve cOnverSlQn of C& to react with H (reaction 4) to yield FT-methane or to react with CH2 (reaction 5), which is the chain start. So with this scheme we can distinguish 4 modes of CO-consumption: 1. Formation of extra-methane 2. Formation of FT-methane 3. Chain start and 4. Chain growth The rates of CO-conversion to organic compounds are presented in Figure 4. Except for the influence of water, the iron and the cobalt catalyst show the same direction of response of reaction rate to the change of the parameters. It appears, however, that all the responses are much smaller with iron than with cobalt. Amazingly, on cobalt the reaction rate increases with increasing partial pressure of water. It is concluded, that the removal of OH from the surface is not a slow step with cobalt catalysts. The main CO-consumption has to be related to CH2 formation and its addition to growing chains. This reaction now is accelerated I by H2 andslowed down by CO. It is suggested, that the addition of H to alkyliden species is reversible and favoured by PH2 This increases the surface concentration of methyl and higher alkyl groups c and so increases the rate of CH2. 0.4, consumption: -s
+H
CH2 + * 7H3
/\
mr
rm
l:i 0.1
It is consistent with this explanation that the probability of chain growth in Fig. 6 shows
Figure 4. Reaction rate of CO-conversion to organic compounds on cobalt and iron catalysts
458
the opposite tendencies with changes in H2-and CO partial pressures and on the other hand methane selectivity (Fig. 5 ) follows the same trends as the reaction rate (Fig. 4), a l l indicating increased surface concentrations of alkyl and particulary methyl species. 3.2 Methane selectivity
High values of methane selectivity (percent of the organic carbon which is obtained in methane) are mainly thought to be obtained on none-FT-sites (Fig. 3). Methane selectivity is not principally higher with cobalt than with iron (see Fig. 5), however on cobalt methane selectivity responds faster to changes of reaction parameters. Water inhibits methane formation (however increases the reaction rate of CO consumption on cobalt).
3.3 Chaln growth probability Results of chain growth probability are represented in Fig. 6. The values have been determined from the slope of the ASF curves in the range Cg-CI2. The higher the value of growth probability, the stronger will be the inhibition of product desorption. It is concluded, that increasing hydrogen partial pressure and increasing temperature aids product desorption and increasing CO- and H20partial pressures inhibit desorption.
Figure 5. Methane selectivity (carbon basis) on cobalt and iron catalysts, gradientless slurry reactor
Figure 6. Probability of chain growth on cobalt and iron catalysts, gradientless slurry reactor
3.4 Chain branching Chain branching probabiltity of surface species has been calculated from the composition of the hydrogenated product on the basis of the simplified kinetic scheme [2]:
459
The results are presented in Fig. 7 on a logarithmic scale as branching probability versus carbon number of surface species (Spn). Reactions of chain branching are visualized as:
R,
CH + 7H3
/\
m r m r
-
7H-CH3
m
and
+
CH2-CH-CH3
-
R,
R,
/
yH-CH3 yH2
+CH2
-
mr
CH3 R-bH-CH2
(1)
+CH2
I
m
(2)
7777
Tm
Reaction (1) should be pertinent with iron catalysts and reaction (2) has been observed as an additional branching reaction with cobalt catalysts [3,4,5].
CATAl YSTS; Co-Mg-Th-Aerosil Fe-Cu-AI-K
1
LL
0
15
0.2 bo
Co I
I
a
10
Co 2.7 Fe 9.9
1.0
d
0
5
PARAMETERS KEPT CONSTANT
10
15
O''
1
5
10
15
1
5
10
1s
Fe
1.6 1.4
210 250
PCO 2.8 10.1 2.5
210 250
PHz
2.5
1.0
220 e c 0.1
5
10
15
CARBON NUMBER OF SURFACE SPECIES Figure 7. Branching probability in dependence of carbon number of the surface species
460
The shape of these curves in Figure (7) follows the earlier derived equation [2]: Pgbr i = P g b r 4 * (NC i ' 3>* ;i > 3 Pgbr 3 is smaller than expected from the equation. This is assumed to be due to the consumption of the ethyliden as a precursor for ethanol formation: ct3 cy3 /H+ CH3CH20H
CH + YH
--
t+2H CH3CHO The results presented in Figure 7 prove the validity of the above equation for all of the experiments. Decline of branching probability with carbon number is generally observed and attributed to increasing steric hinderance with larger chemisorbed species. The conclusions from Fig.7 are the following. Cobalt generally shows a much higher sensitivity of branching probability against changes of reaction parameters. The nature of the stationary state of the catalyst surface appears to be much more dynamic than that with iron. The general explanation should be that in case of the iron, selectivity is controlled via rather stable inhibitions due to partial coverage with carbon and alkali. Whereas controlling inhibiting effects are more due to reversible competitive chemisorption -in particular of the strongly chemisorbed CO- with cobalt catalysts. Amazingly, the increasing hydrogen partial pressure effects branching with cobalt and iron oppositely: It reduces branching on cobalt and favours branching on iron. CO partial pressure reduces branching with cobalt catalysts strongly, however only minorily with iron. Water partial pressure reduces branching with cobalt, however favours branching with iron. Reaction temperature again influences branching on cobalt and iron oppositely. /\
m
7
T
m
C,H-OHb
4. CONCLUSION The detailed results on branching selectivity and its response to reaction parameter changes will allow more insight to the dynamic FT system and the chemistry at the active sites together with the other new results reported above on further elementary reactions as characterizing the stationary dynamic regime of aliphatic organic compound formation from CO and hydrogen. However, a more profound discussion can only be given later.
ACKNOWLEDGEMENT Financial support of this work from the Deutsche Forschungs Gemeinschaft within the SFB 250-project is greatly appreciated. LITERATURE [ 11 H. Schulz, K,Beck, E. Erich, Proc. "Methane Conv. Symposium", Auckland 1987, (eds. D. Bibby, C. Chang, R. Howe, S. Yurchak) Stud. Surf. Sci. Catal., Vol. 36,457 (Elsevier,
Amsterdam, 1988) [2] H. Schulz, K. Beck, E. Erich, Proc. "9th Int. Congr. on catalysis", Calgary 1988 (eds. M. Phillips, M. Term), Vol. 2, 829, (The Chemical Institute of Canada, Ottawa, 1988) [3] H.Pichler, H.Schulz, B. Rao, Liebigs Ann. Chem. 61 (1968), 68 [4] H. Schulz. B. Rao, M. Elstner, ErdUl u. Kohle (1970), 651 [5] C.Lee, R. B. Anderson, Proc. "8th Int. Congr.on Catalysis", Berlin 1984, (ed. Dechema),Vol. 2, 15 (Verlag Chemie, Weinheim, 1984)
455
Selectivity and mechanism of Fischer-Tropsch synthesis with iron and cobalt catalysts Hans Schulz, Eric van Steen and Michael Claeys Engler-Bunte-Institute, University of Karlsruhe, KaiserstraDe 12,76128 Karlsruhe, Germany The Fischer-Tropsch CO-hydrogenation has been comparatively performed on iron and cobalt in a gradientless tank reactor. Kinetic coefficients of elementary reactions and their dependence on pm, pco, mZoand T were determined with the help of a model for chain growth-, branching- and termination reactions. Changes of reaction rates of CO consumption, of methane formation, of chain propagation and chain branching are presented and discussed in terms of elemental surface reactions and characteristics of the catalyst base metals. 1. INTRODUCTION
The kinetic regime of Fischer-Tropsch CO-hydrogenation as a type of non Mvial polymerlzation on the surface of a heterogeneous catalyst [ 11 is only established through specific inhibitions of reactions of desorption. Consequently, sets of consecutive reaction steps among adsorbed species lead to chain growth. In this investigation kinetic coeffldents of elementary surface reactions are determined with cobalt and iron catalysts in dependence of the partial pressures of H2, CO, and H20and the reaction temperature through special experimental techniques with a gradientless reactor, special procedures of product sampling and analysis and using the quantitative kinetic model of interconnected surface reactions. 2. EXPERIMENTAL
CO-hydrogenation has been performed in a gradientless operated CSTR reactor with the catalyst particles being suspended in squalane (Fig.1). The flows of hydrogen and carbonmonoxide to the reactor were precisely controlled and measured. A constant reaction pressure was obtained with a pressure controlled make up stream of argon. A reference stream, containing N2 and cyclopropane or neopentane, was added to the hot gaseous product stream. A continuous stream of products, containing liquids, vapours and gases was withdrawn from the reactor at reaction temperature and the liquid fraction collected in a hot trap. From the hot gaseous product stream samples were collected, sealed in glass ampoules and analysed later by gaschromatography. Degree of conversion and yields of all the compounds in the gaseous product stream were directly obtained from the chromatogrammes by referring to the peaks of the reference compounds, nitrogen and cyclopropane. The capillary chromatogrammescovered the range of compounds C1 to %o+ using a temperature programme from -80 to 275 "C. For determination of chain branching only parafinic compounds were obtained via precolumn hydrogenationof olef'ins (and alcohols). With the help of the quantitative model [2] the kinetic coefficients of elementary reactions were calculated from the product composition. The catalysts were prepared by precipitation from nitrate solutions (one exception: a commercial sample of Fe203). Activiation was traced via temperature programmed reduction.
456
'
(PRESSUR CONTROULED),
VM I-:-
vco :
1
41
t"O+
,
CYCLOPROPANE (REFERENCE)
.MIXER ,
\?
CSTR SLURRY REACTOR
Figure 1. Scheme of the Fischer-Tropsch synthesis in a "gradientless" operated slurry reactor
In Figure 2 the reduction profiles of 3 cobalt and 3 iron catalysts are represented together with their composition (mass ratios) and the final degree of reduction obtained at 550 "C. In the cobalt catalysts the thoria and magnesia inhibit the reduction of the cobalt. In this paper mainly the results obtained with the catalysts 100 Co : 10 MgO : 3 T h o 2 : 100 Aerosil are presented. In the iron catalysts copper is added as a reduction promoter and potassium as a chemical promoter. Here mainly the results obtained with the catalyst 100 Fe : 37 M203 : 3 Cu:2 K20 are used. Final Catalyst Red. % 94 100 Co : 465 Silica (Impregnation) 63 100 Co : 10 MgO : 3 T h o 2 : 100 Aerosil 1) 100 Co :20 Tho2: 100 Aerosil 1) 42 98
undef. 29 0
loo 100 T#npmtur. 'C
Fe2°3
100 Fe :766 Mn : 49Cu :5 K20 1) 1OOFe :37 A1203 : 3 C h :2KzO 1)
-
1)
Precipitation from nitrate solutions
Figure 2. Profiles of water formation;temperature programmed reduction of the catalysts. (rg,=40 ml(NTP)/min, p~*=0.25bar, pk=0.75 bar; 2"Umin up to 550°C kept until rH2O=O)
varied
parameter
H2
co H20
T
catalyst
Co-Mg-Th-Aerosil Pe-AI-Cu-K Co-Mg-Th-Aerosil PO-AI-Cu-K Co-Mg-Th-Aerosil Fe-AI-Cu-K Co-Mg-Th-Aerosil Fe
H2
partial pressure, bar
co
H20
2.7 2.5
1.6 1.5 1.6 1.4
2.7
9.9
2.5 10.1 5.1 9.0
2.8 2.5 2.6 0.1
1.2 0.8
T, 'C 210 250 210 250 210 250
457
3. RESULTS A N D DISCUSSION 3.1 Reaction rate of CO-conversion
In FT-CO conversion alternative reaction pathways are visualized:
'CHz
Figure 3. Kinetic scheme of alternative reactions of CO consumption ve reactlOnS0: Reaction (0): CO-conversion on none- IT-hydrogenation sites, producing "extramethane" Reaction (1): CO-dissociationand hydrogenation to form the methylen species 2to react with H to form a CH3 species (reaction 2). or to react with an akyl species R as the chain growth step (reaction 3). ve cOnverSlQn of C& to react with H (reaction 4) to yield FT-methane or to react with CH2 (reaction 5), which is the chain start. So with this scheme we can distinguish 4 modes of CO-consumption: 1. Formation of extra-methane 2. Formation of FT-methane 3. Chain start and 4. Chain growth The rates of CO-conversion to organic compounds are presented in Figure 4. Except for the influence of water, the iron and the cobalt catalyst show the same direction of response of reaction rate to the change of the parameters. It appears, however, that all the responses are much smaller with iron than with cobalt. Amazingly, on cobalt the reaction rate increases with increasing partial pressure of water. It is concluded, that the removal of OH from the surface is not a slow step with cobalt catalysts. The main CO-consumption has to be related to CH2 formation and its addition to growing chains. This reaction now is accelerated I by H2 andslowed down by CO. It is suggested, that the addition of H to alkyliden species is reversible and favoured by PH2 This increases the surface concentration of methyl and higher alkyl groups c and so increases the rate of CH2. 0.4, consumption: -s
+H
CH2 + * 7H3
/\
mr
rm
l:i 0.1
It is consistent with this explanation that the probability of chain growth in Fig. 6 shows
Figure 4. Reaction rate of CO-conversion to organic compounds on cobalt and iron catalysts
458
the opposite tendencies with changes in H2-and CO partial pressures and on the other hand methane selectivity (Fig. 5 ) follows the same trends as the reaction rate (Fig. 4), a l l indicating increased surface concentrations of alkyl and particulary methyl species. 3.2 Methane selectivity
High values of methane selectivity (percent of the organic carbon which is obtained in methane) are mainly thought to be obtained on none-FT-sites (Fig. 3). Methane selectivity is not principally higher with cobalt than with iron (see Fig. 5), however on cobalt methane selectivity responds faster to changes of reaction parameters. Water inhibits methane formation (however increases the reaction rate of CO consumption on cobalt).
3.3 Chaln growth probability Results of chain growth probability are represented in Fig. 6. The values have been determined from the slope of the ASF curves in the range Cg-CI2. The higher the value of growth probability, the stronger will be the inhibition of product desorption. It is concluded, that increasing hydrogen partial pressure and increasing temperature aids product desorption and increasing CO- and H20partial pressures inhibit desorption.
Figure 5. Methane selectivity (carbon basis) on cobalt and iron catalysts, gradientless slurry reactor
Figure 6. Probability of chain growth on cobalt and iron catalysts, gradientless slurry reactor
3.4 Chain branching Chain branching probabiltity of surface species has been calculated from the composition of the hydrogenated product on the basis of the simplified kinetic scheme [2]:
459
The results are presented in Fig. 7 on a logarithmic scale as branching probability versus carbon number of surface species (Spn). Reactions of chain branching are visualized as:
R,
CH + 7H3
/\
m r m r
-
7H-CH3
m
and
+
CH2-CH-CH3
-
R,
R,
/
yH-CH3 yH2
+CH2
-
mr
CH3 R-bH-CH2
(1)
+CH2
I
m
(2)
7777
Tm
Reaction (1) should be pertinent with iron catalysts and reaction (2) has been observed as an additional branching reaction with cobalt catalysts [3,4,5].
CATAl YSTS; Co-Mg-Th-Aerosil Fe-Cu-AI-K
1
LL
0
15
0.2 bo
Co I
I
a
10
Co 2.7 Fe 9.9
1.0
d
0
5
PARAMETERS KEPT CONSTANT
10
15
O''
1
5
10
15
1
5
10
1s
Fe
1.6 1.4
210 250
PCO 2.8 10.1 2.5
210 250
PHz
2.5
1.0
220 e c 0.1
5
10
15
CARBON NUMBER OF SURFACE SPECIES Figure 7. Branching probability in dependence of carbon number of the surface species
460
The shape of these curves in Figure (7) follows the earlier derived equation [2]: Pgbr i = P g b r 4 * (NC i ' 3>* ;i > 3 Pgbr 3 is smaller than expected from the equation. This is assumed to be due to the consumption of the ethyliden as a precursor for ethanol formation: ct3 cy3 /H+ CH3CH20H
CH + YH
--
t+2H CH3CHO The results presented in Figure 7 prove the validity of the above equation for all of the experiments. Decline of branching probability with carbon number is generally observed and attributed to increasing steric hinderance with larger chemisorbed species. The conclusions from Fig.7 are the following. Cobalt generally shows a much higher sensitivity of branching probability against changes of reaction parameters. The nature of the stationary state of the catalyst surface appears to be much more dynamic than that with iron. The general explanation should be that in case of the iron, selectivity is controlled via rather stable inhibitions due to partial coverage with carbon and alkali. Whereas controlling inhibiting effects are more due to reversible competitive chemisorption -in particular of the strongly chemisorbed CO- with cobalt catalysts. Amazingly, the increasing hydrogen partial pressure effects branching with cobalt and iron oppositely: It reduces branching on cobalt and favours branching on iron. CO partial pressure reduces branching with cobalt catalysts strongly, however only minorily with iron. Water partial pressure reduces branching with cobalt, however favours branching with iron. Reaction temperature again influences branching on cobalt and iron oppositely. /\
m
7
T
m
C,H-OHb
4. CONCLUSION The detailed results on branching selectivity and its response to reaction parameter changes will allow more insight to the dynamic FT system and the chemistry at the active sites together with the other new results reported above on further elementary reactions as characterizing the stationary dynamic regime of aliphatic organic compound formation from CO and hydrogen. However, a more profound discussion can only be given later.
ACKNOWLEDGEMENT Financial support of this work from the Deutsche Forschungs Gemeinschaft within the SFB 250-project is greatly appreciated. LITERATURE [ 11 H. Schulz, K,Beck, E. Erich, Proc. "Methane Conv. Symposium", Auckland 1987, (eds. D. Bibby, C. Chang, R. Howe, S. Yurchak) Stud. Surf. Sci. Catal., Vol. 36,457 (Elsevier,
Amsterdam, 1988) [2] H. Schulz, K. Beck, E. Erich, Proc. "9th Int. Congr. on catalysis", Calgary 1988 (eds. M. Phillips, M. Term), Vol. 2, 829, (The Chemical Institute of Canada, Ottawa, 1988) [3] H.Pichler, H.Schulz, B. Rao, Liebigs Ann. Chem. 61 (1968), 68 [4] H. Schulz. B. Rao, M. Elstner, ErdUl u. Kohle (1970), 651 [5] C.Lee, R. B. Anderson, Proc. "8th Int. Congr.on Catalysis", Berlin 1984, (ed. Dechema),Vol. 2, 15 (Verlag Chemie, Weinheim, 1984)