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Catalytic Activity of Carbon Supported Catalysts for Co-Hydrogenation and their Preparation by Oxidative Decomposition of Fe(CO)5
B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Editors), Preparation of Catalysts IV
© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
493
CATALYTIC ACTIVITY OF CARBON SUPPORTED CATALYSTS FOR CO-HYDROGENATION AND THEIR PREPARATION BY OXIDATIVE DECOMPOSITION OF Fe(CO)5 U. Peters, H. Greb, R. Jockers and J. Klein Bergbau-Forschung GmbH, Franz-Fischer-Weg 61, 0-4300 Essen 13, FRG
ABSTRACT Activated carbon supported Fe catalysts were prepared by oxidative decomposition of Fe (CO) from the gas phase. It could be shown that metal distribution and dispersitin are critically determined by the degree of activation of the supports used. Activity and selectivity of these catalysts in respect to CO hydrogenation were determined in a plug flow fixed bed reactor and both show distinct dependencies on dispersion and distribution of the metal phase in the catalyst pellets. INTRODUCTI ON Due to its specific properties activated carbon lends itself as a support for catalysts and has been examined for a great number of different catalysts viz. reactions (ref. 1-8). Of particular interest are the pore structure and the specific surface area which can be varied within a wide range. It should be possible therefore to obtain a high dispersion and, consequently, sufficiently high surface areas of the metal phase. It is generally known that the "history" of a catalyst influences the dispersion and distribution of the metal phase (ref. 9). The influence of the kind of support and of the preparation on the dispersion - and thus on the activity of these catalysts for CO hydrogenation - was observed also in the case of Fe/C catalysts (ref. 2,10). The catalysts presented in this paper were obtained by oxidative decomposition of Fe(CO)5 from the gas phase (ref. 11). Apart from the small number of treatment steps (decomposition of Fe(CO)5 and subsequent reduction) this method, unlike impregnation, offers the chance of attaining any desired Fe contents in the catalysts and of eVidencing the influence of the pore structure of the support on dispersion and distribution of the Fe phase. EXPERIMENTAL The cylindrically shaped activated carbons, serving as support, were made from hardcoal using partial H20 gasification as activation method (ref.12). The percentage of gasified carbon is defined as degree of activation or burn
494
off. Along with the texture and with a burn off diameter and an
increasing burn off there occurs a systematic change both in the specific surface of the support. We used activated carbons of 0 %, 21 %, 35 %, 51 %, and 64 % and grain size of 2 mm average length of 4 mm.
Preparation The catalysts were prepared by decomposition of Fe(CO)5 contained in a N2 gas stream (cFe(CO) = 5 x 10-4 molll) which was mixed with air and then flown through a con~inuOuslY revolved heap of activated carbon. The thus obtained catalysts contained 10 % Fe by wt. Characterization The catalysts were characterized by the following procedures: - The distributions of pore radii were determined by adsorption of methanol and Hg porosimetry. - The metal and metal oxide were characterized by Moessbauer spectroscopy at temperatures of 298 K, 77 K, and 4 K and X-ray diffraction using Cu-Ka radiation (Siemens, Typ K810). - Thermo-gravimetric analyses (TGA) (Mettler type TA 1) were carried out in H2 in the heat range between 298 K and 1170 K, applying a heating rate of 10 K/min. The gaseous products were identified by mass spectroscopy and continuously registered during the experiment. - The dispersion viz. mean particle size of the Fe phase were defined by chemisorption of CO on the reduced catalysts at 298 K (reduction conditions: vH = 100 ml H2/min . g cat; T = 675 K; t = 16 h). The adsorption isotherms we~e determined by the vOlumetric Carlo-Erba apparatus (Sorptomatic 1800). - The Fe distribution within the catalysts pellets was determined by microprobe analysis. CO Hydrogenation CO hydrogenation was carried out in integral conditions in a plug flow fixed bed reactor (10 = 30 rom) charged with 3 g of catalyst. The synthesis gas used was blended, without any further treatment, from the pure gases (CO = 99.997; H2 = 99.999; N2 = 99.996 % by vol) and consisted of 60 % by vol H2, 30% by vol CO and 10 %by vol N2. CO hydrogenation was performed at pressures of 0.1 MPa at a temperature of 550 K and of 1 MPa, 1.5 MPa, and 2 MPa resp. at a temperature of 525 K, with space velocities of 600 h- 1 at normal pressure and 3800 h- 1 at increased synthesis pressure.
495
The products were analyzed on-line by gas chromatography specified in table 1. TABLE 1: Analytical equipment for products of CO hydrogenation analyzed products
Hewlett-Packard Gaschromatograph 5880A, Capillary column (50 m chemical bonded SE 54 fused silica), gas samplig valve, FIO+MSD(He) Temp. program: O°C (20 min)- 4°C/min - 175°C (60 min) packed column (0.6 m Molecular sieve 13 X) gas sampling valve, TCD (Ar), Temp.: O°C 5840A, packed columns (1.2 m Molecular sieve 13 X, 1 m Porapac Q), gas sampling valve, TCD (He) Temp.: 60°C
Hydrocarbons C1 - CIS H CH 4 2, CO, CO 2, H20, CH4, N2
The fact that the integrator of the GC HP 5880 A can be programmed in BASIC permits adaptation of the hydrocarbon product distribution by means of the Schulz-Flory distribution law (e.g. ref. 13) immediately upon analysis. Hydrocarbons were identified by a GC/MS-equipment (HP MSD 5970). RESULTS AND DISCUSSION Characterization Fig. 1 shows the systematic increase of the pore volume along with increasing support burn off. The deposition of about 15 to 16 % by wt Fe203 corresponding to a Fe concentration of about 11 % by wt - has a significant influence on the pore structure (table 2) .
.....
64
Cl
IS) IS)
,
100
51
E
U
u
'"'
35
III
-> E :3
21
0
,., ~
L..O
0
...c
:l
0
III ~
0
a.
1"
Fore Radius Cnm]
Figure 1: pore size distribution of supports
III
496
TABLE 2: Pore characteristics of supports and freshly prepared catalysts Support 0.12 0.12 0.11 0.12 0.13
0 21 35 51 64
No. 4 470 690 910
0.02 0.20 0.26 0.34 0.37
1010
0/10 21/10 35/10 51/10 64/10
Catalyst Fe content Vm~cro3 Vm!cro wt% cm /cm cm /cm 3 11. 5 12 11 11 11 n.d.=
0.1 0.1 0.12 n.d. 0.12 0.2 0.25 0.12 0.27 0.12 not determined
The Moessbauer spectra measured at ambient temperature of the Fe/activated carbon catalysts freshly prepared are completely identical and independent of the support burn off. They are characteristic for superparamagnetic Fe203 (fig. 2) and do not show any signs of splitting even at 4 K. After reduction in H2, 6 hours of atmospheric CO hydrogenation and subsequent reoxidation with oxygen from air there will occur a significant change in the Moessbauer spectrum of the catalyst with a burn off of 0 % (fig. 3a). In this case the characteristic six-peak spectrum of elemental iron will appear. In the case of catalysts with higher support burn off (figures 3b and 3c), the reduced iron is almost completely re-oxidized into a very finely distributed Fe203, which can be concluded from the central doublet. Besides there will occur lines of X-carbide (catalyst 35/10) and of elemental iron (about 2 %, catalyst 64/10).
e
lIS
c:
III L
"
c)
I-
-10 -S 0 5 10 Ve 1oc i ty / mm*s-\
Figure 2: Mossbauer spectra at 298 K of freshly prepared catalysts: a) 0/10; b) 35/10; c) 64/10
-10 -S 0 S 10 Ve 1oc i ty / mm*s-\
Figure 3: Mossbauer spectra at 298 K of reduced and reoxidized catalysts: a) 0/10; b) 35/10; c) 64/10
497
XRD did not give any diffraction peaks for samples of freshly prepared catalysts. After reduction and re-oxidation the intensive diffraction peaks of metallic Fe were only found for a sample of catalyst 0/10. The above results render obvious that certain differences occur between the deposited Fe203 during reduction and that such differences are due to the support burn off. In order to examine this mechanism more in detail we ran thermo-gravimetric tests in a H2 atmosphere. Fig. 4 shows the differential mass losses as a function of the temperature. There are two distinctive types of mass loss diagrams: - The catalysts 0/10 and 21/10 show distinctive maxima below 750 K caused by the release of CO, CO 2, and H20. - The mass loss for catalysts 35/10, 51/10, and 64/10 in the same temperature range is caused by the formation of H20 and CO 2, with no distinctive maximum observed. What strikes with these catalysts is the high mass loss in the temperature range between 720 K and 920 K which is attributed to the gasification of the support under release of CH 4. g ] zsm [m K*g
6T
1.5
1.0 0.5 64/10 .....--=;:=:::....-----7 5 1/10 \---=~-----...::~~=---/
~--'--;''-----r---'--'----.---r---r---(
1100
35/10 21/10 t 0/10 C'
st
,1i
As the above results show, the behaviour during reduction of the Fe203 as well as the interaction between the reduced iron and the support material are influenced by the support burn off. It was demonstrated by CO chemisorption measurements that these differences are mainly due to the Fe phase dispersion. Table 3 contains the amounts of irreversibly adsorbed CO for the different catalysts. The table gives furthermore the relevant values derived for dispersion and mean Fe particle diameters /10/ .
498
TABLE 3: Dispersion and mean Fe particle size determined by CO chemisorption* /10/ Catalyst
CO-Uptake (mmol/gcat)
0/10 21/10 35/10 51/10 64/10
0 0.05 1.27 0.72 0.50
Dispersion
0.025 0.64 0.37 0.25
Fe particle size (nm) Fe/CO= 1/1 Fe/CO= 2/1 30.0 1.2 2.0 3.0
15.0 0.6 1.0 1.5
* Adsorption at 298 K The results of CO chemisorption show that the subdivision of catalysts in two groups, as occurred during thermo-gravimetry, has to be attributed to the discrepant dispersions of the Fe phase. For the first group (b.o. < 35 %) the particles, lead to bulk reduction will, notwithstanding the very small Fe 203 Fe crystals which - as becomes evident by the Moessbauer spectrum of the 0/10 catalyst - do not reoxidize by access of oxygen from air during the lengths of times observed ( 6 month). For the second group of catalysts (b.o. ~ 35 %) there appears a very high Fe dispersion which is stabilized by the supports. From the above observation it can be assumed that Fe203 is deposited mainly in the micro-pores. These micro-pores contribute mainly to the total surface area which is rather small at low support burn offs. In the case of high burn offs (b.o. ~ 35 %) these micro-pores will prevent the metal crystallites from sintering, and stabilize them. In the case of low burn off values the available micro-pore volume will be insufficient for uptaking the quantity of Fe203 applied, thus inducing the formation of great bulk Fe-crystals during reduction. The activity of metallic support catalysts will, however, be critically influenced not only by the dispersion but also by the distribution of the metal within the particles of the support. For this reason we determined the Fe distribution in the catalyst pellets by micro-probe analyses (fig. 5). Up to a burn off of 35 % a homogeneous Fe profile is observed. Higher burn off levels lead to the formation of shell catalysts. As we found out recently (ref. 14) by more detailed examinations, this is due to the higher rate of decomposition of Fe(CO)5 along with increasing burn off.
499
Figure 5: Fe distribution across catalyst pellets a) 0/10; b) 35/10; c) 64/10 CO Hydrogenation Table 4 summarizes the results of CO hydrogenation obtained when using different catalysts at reaction pressures of 0.1 MPa and 2 MPa. TABLE 4: Typical results of CO-hydrogenation for Fe/C-catalysts
Catalyst
0/10 21/10 35/10 51/10 64/10
pressure: O'!lMPa, temperature: 550 K, t = 0.33 h S.v = 630 h chain growth Conversion (%) Alkene/Alkane ratio probability CO
o
37.2 76.4 71.1 42.0
o
17.7 23.6 19.7 19 6
0.022 0.004 0.014 0.045
pressure: 2 MPa, te~~erature: S.v. = 3800 + 200 h 0/10 21/10 35/10 51/10 64/10
o
5.1* 14.1 93.0 91.1
o
5* 13.8 61.6 56.8
* after 4.8 h time on stream
0.0583* 0.083 0.009 0.021
0.05 0.01 0.04 0.13
o 02 0.04 0.22
525 K, t 0.295* 0.472 0.052 0.159
0.45 0.50 0.44 0.46
0.11
= 20
0.503* 0.756 0.132 0.333
h 0.47* 0.57 0.45 0.45
500
As at atmospheric pressure the catalysts are subjected to rapid deactivation, the table gives the initial activity after a reaction time of 20 minutes. The distribution of hydrocarbons can even under these instationary conditions be expressed by the Schulz-Flory law. Independently of the type of catalysts a chain propagation probability of 0.45 has been determined. On the other hand the activity and olefin contents of the hydrocarbons show a strong dependence on the type of catalyst used. Catalyst 35/10, showing the highest Fe dispersion, turned out to be the most active, too, which was up to the expectations. The minimum of olefin contents may also be explained by high dispersion: the a.m. catalyst has at the same time the highest metal surface available for hYdrogenation of the olefins which are primary products of CO-hydrogenation. Besides the formation of linear and branched paraffins and olefins there will also occur formation of BTX-aromates under these reaction conditions.
100 r"1
~
1-1
80
c 0
III
60
L-
III
> 40 c 0
U I
0
u
20 5
Ifj me
[h)5
20
25
Figure 6: CO-conversions of different catalysts with time on stream 2 MPa: v 21/10; 035/10; 051/10; '" 64/10 P = 0.1 MPa: -51/10
p
The deactivation of Fe catalysts during CO hydrogenation is known to be considerably reduced by application of increased synthesis pressures (ref. 15). This is represented on fig. 6 for the catalyst 51/10. Furthermore fig. 6 shows that, on the other hand, higher reaction pressure (2 MPa) will not induce all the catalysts to an increased and permanently high level of activity.
501
Catalysts 0/10 and 21/10 behave, just as during synthesis at atmospheric pressure, inactive or just slightly active which is explained by the low dispersion of the Fe-phase. Catalysts 51/10 and 64/10 show an almost constantly high activity over the entire period under review. Although Fe-dispersion is highest with catalyst 35/10 among all the catalysts concerned. a substantial decline in activity is observed to occur immediately after the reaction started. We believe that this behaviour has to be attributed to some blockage of the iron deposited within the catalyst pellets. by high molecular liquid reaction products. This hypothesis is supported by calculations done by other groups (ref. 16) and is furthermore revealed by the chain propagation probability of 0.57 derived from the Schulz-Flory distribution law. For catalysts 51/10 and 64/10, both of long-lasting activity, chain propagation probabilities amount to 0.45. For these catalysts blockage by long-chain reaction products is excluded due to the altered pore structure and the thin shell wherein the iron is deposited and which implies short diffusion paths. Run under a pressure of p = 2 MPa the reaction does not yield aromates. We suppose that at atmospheric pressure conditions the aromates are coke precursor or are produced by decomposition of graphitic deposites. The existence of graphitic carbon has been evidenced elsewhere (ref. 17) and may be responsible for the rapid deactivation of Fe catalysts at atmospheric pressure conditions. CONCLUSIONS Applying the described method and using appropriate activated carbons as support. it is possible to produce highly active catalysts. During catalysts preparation it turned out that the pore system exerts a strong influence on the dispersion and distribution of the deposited Fe phase. It will have to be clarified by further studies. yielding detailed kinetic data, in how far the activity of catalysts and the behaviour with time on stream are functions of transport mechanisms. ACKNOWLEDGMENTS We would like to thank Mr. B. Gatte and Prof. M. Philips from Pennsylvania State University and Mr. M. Deppe and Prof. M. Rosenberg, Ruhr-Universitat· Bochum. for measuring Mtissbauer spectra and helpful discussions.
502
REFERENCES 1 2 3 4 5 6 7
8 9
10 11 12 13 14 15 16 17
M. Kaminsky, K.Y. Yoon, G.L. Geoffroy, M.A. Vannice J. Catal. 21 (1985), 338 F. Rodriguez-Reinoso, J.D. Lopez-Gonzalez, C. Moreno-Castilla, A. Guerrero-Ruiz, J. Rodriguez-Ramos FUEL 63 (1984), 1089 E. Kikuchi, A. Koizumi, Y Aranishi, Y. Morita J. Japan Petrol. Inst. 25 (1982), 360 A.P.B. Sommen, F. Stoop, K. van der Wiele Appl. Catal. 14 (1985), 277 V.H.J. de Beer, F.J. Derbyshire, C.K. Groot, R. Prins, A.W. Scaroni, J.M. Solar FUEL §l (1984), 1095 A.W. Scaroni, R.G. Jenkins, P.L. Walker, Jr. Appl. Catal. 1i (1985), 173 F.F. Gadallah, R.M. Elofson, P. Mohammed, T. Painter Preparation of Catalysts III (Edited by G. Poncelet, P. Grange, P.A. Jacobs) Elsevier Science Publishers B.V., Amsterdam (1983), 409 J.L. Schmitt, Jr., P.L. Walker, Jr. Carbon 1Q (1972), 87 J.W. Geus Preparation of Catalysts III (Edited by G. Poncelet, P. Grange, P.A. Jacobs) Elsevier Science Publishers B.V., Amsterdam (1983), H.-J. Jung PhD Thesis, Pennsylvania State University (1981) H. Greb, K.-D. Henning, J. Klein, U. Peters DE-PS 33 30 621 H. JUntgen Carbon ~ (1968), 297 G. Henrici-Olive, S. Olive Angew. Chern. 88 (1976), 144 U. Peters, R. Jockers, J. Klein Paper presented at "Carbon '86" F. Fischer, H. Pichler Brennstoff-Chem. 20 (1939), 41 G.A. Huff, Jr., C.N. Satterfield Ind. Eng. Chern. Process Des. Dev. 24 (1985), 986 H.P. Bonzel, H.J. Krebs Surf. Sci. 21 (1980), 499
503
DISCUSSION T. HATTORI: You have mentioned that micro-pore is blocked by Fe203' But the difference in volume of micro-pore is not so large between carbon 21 and 35. What you mentioned is the blockage of pore mouth? R. JOCKERS : Yes, you are rigth. The comparison of available micropore volume of the support and the volume of deposited iron oxide shows that only a small part of the micropore volume can be occupied by Fe-oxide. The drastic decrease of the micropore volumes of the prepared catalysts is a strong indication for pore mouth blockage. J.W. GEUS : 1/ Your results are suggesting that your catalysts prepared from supports of a high burn-off are exhibiting diffusion limitation in the reaction. This can be established by measuring the activity on crushed catalyst tablets. Did you vary the size of your carbon support particles and did that affect the conversion ? 2/ We get a more homogeneous distribution of iron over the support during the decomposition of iron carbonyl by first exposing the support to the carbonyl containing gas at a temperature so low that the carbonyl did not decompose and subsequently rapidly raising the temperature. Since we used a fluidized bed, we could raise the temperature fast. Have you experience with addition of the carbonyl at low temperature and subsequent heating? R. JOCKERS : 1/ Diffusional limitation during reaction may be an explanation for the observed dependence of CO-conversion on burn-off of support. We made activity measurements with a crushed catalyst (burn-off = 35%, particle size = 0.08 - 0.5 mm) and found the same loss of activity with time on stream that we have observed with the uncrushed catalyst. Detailed kinetic measurements are necessary in order to clarify the role of diffusional limitation during CO-hydrogenation. Another explanation for the dependence of activity on burn-off is based on the influence of burn-off on mean iron particle size. Very small Fe-particles result on activated carbons which have 35% burn-off. Possibly these particles are converted to Fe-carbides very fast, and are blocked by inactive carbidic carbon layers in consecutive reactions. 2/ We performed Fe(CO)5-decomposition at various temperatures (273 K - 373 K) and found that the Fe-distribution across the support can be influenced by the decomposition temperature. By example with a burn-off = 35%. there is a change from a homogeneous iron distribution to an egg-shell catalyst, if the decomposition of Fe(CO)5 is carried out at 323 and 373 K, respectively. This fact could be explained by an acceleration of the decomposition rate with increasing temperature. We think that your observation can be explained by a volatilization of adsorbed Fe(CO)5 and a further diffusion of this compound to the centre of support particles during the raise of temperature.
© 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
493
CATALYTIC ACTIVITY OF CARBON SUPPORTED CATALYSTS FOR CO-HYDROGENATION AND THEIR PREPARATION BY OXIDATIVE DECOMPOSITION OF Fe(CO)5 U. Peters, H. Greb, R. Jockers and J. Klein Bergbau-Forschung GmbH, Franz-Fischer-Weg 61, 0-4300 Essen 13, FRG
ABSTRACT Activated carbon supported Fe catalysts were prepared by oxidative decomposition of Fe (CO) from the gas phase. It could be shown that metal distribution and dispersitin are critically determined by the degree of activation of the supports used. Activity and selectivity of these catalysts in respect to CO hydrogenation were determined in a plug flow fixed bed reactor and both show distinct dependencies on dispersion and distribution of the metal phase in the catalyst pellets. INTRODUCTI ON Due to its specific properties activated carbon lends itself as a support for catalysts and has been examined for a great number of different catalysts viz. reactions (ref. 1-8). Of particular interest are the pore structure and the specific surface area which can be varied within a wide range. It should be possible therefore to obtain a high dispersion and, consequently, sufficiently high surface areas of the metal phase. It is generally known that the "history" of a catalyst influences the dispersion and distribution of the metal phase (ref. 9). The influence of the kind of support and of the preparation on the dispersion - and thus on the activity of these catalysts for CO hydrogenation - was observed also in the case of Fe/C catalysts (ref. 2,10). The catalysts presented in this paper were obtained by oxidative decomposition of Fe(CO)5 from the gas phase (ref. 11). Apart from the small number of treatment steps (decomposition of Fe(CO)5 and subsequent reduction) this method, unlike impregnation, offers the chance of attaining any desired Fe contents in the catalysts and of eVidencing the influence of the pore structure of the support on dispersion and distribution of the Fe phase. EXPERIMENTAL The cylindrically shaped activated carbons, serving as support, were made from hardcoal using partial H20 gasification as activation method (ref.12). The percentage of gasified carbon is defined as degree of activation or burn
494
off. Along with the texture and with a burn off diameter and an
increasing burn off there occurs a systematic change both in the specific surface of the support. We used activated carbons of 0 %, 21 %, 35 %, 51 %, and 64 % and grain size of 2 mm average length of 4 mm.
Preparation The catalysts were prepared by decomposition of Fe(CO)5 contained in a N2 gas stream (cFe(CO) = 5 x 10-4 molll) which was mixed with air and then flown through a con~inuOuslY revolved heap of activated carbon. The thus obtained catalysts contained 10 % Fe by wt. Characterization The catalysts were characterized by the following procedures: - The distributions of pore radii were determined by adsorption of methanol and Hg porosimetry. - The metal and metal oxide were characterized by Moessbauer spectroscopy at temperatures of 298 K, 77 K, and 4 K and X-ray diffraction using Cu-Ka radiation (Siemens, Typ K810). - Thermo-gravimetric analyses (TGA) (Mettler type TA 1) were carried out in H2 in the heat range between 298 K and 1170 K, applying a heating rate of 10 K/min. The gaseous products were identified by mass spectroscopy and continuously registered during the experiment. - The dispersion viz. mean particle size of the Fe phase were defined by chemisorption of CO on the reduced catalysts at 298 K (reduction conditions: vH = 100 ml H2/min . g cat; T = 675 K; t = 16 h). The adsorption isotherms we~e determined by the vOlumetric Carlo-Erba apparatus (Sorptomatic 1800). - The Fe distribution within the catalysts pellets was determined by microprobe analysis. CO Hydrogenation CO hydrogenation was carried out in integral conditions in a plug flow fixed bed reactor (10 = 30 rom) charged with 3 g of catalyst. The synthesis gas used was blended, without any further treatment, from the pure gases (CO = 99.997; H2 = 99.999; N2 = 99.996 % by vol) and consisted of 60 % by vol H2, 30% by vol CO and 10 %by vol N2. CO hydrogenation was performed at pressures of 0.1 MPa at a temperature of 550 K and of 1 MPa, 1.5 MPa, and 2 MPa resp. at a temperature of 525 K, with space velocities of 600 h- 1 at normal pressure and 3800 h- 1 at increased synthesis pressure.
495
The products were analyzed on-line by gas chromatography specified in table 1. TABLE 1: Analytical equipment for products of CO hydrogenation analyzed products
Hewlett-Packard Gaschromatograph 5880A, Capillary column (50 m chemical bonded SE 54 fused silica), gas samplig valve, FIO+MSD(He) Temp. program: O°C (20 min)- 4°C/min - 175°C (60 min) packed column (0.6 m Molecular sieve 13 X) gas sampling valve, TCD (Ar), Temp.: O°C 5840A, packed columns (1.2 m Molecular sieve 13 X, 1 m Porapac Q), gas sampling valve, TCD (He) Temp.: 60°C
Hydrocarbons C1 - CIS H CH 4 2, CO, CO 2, H20, CH4, N2
The fact that the integrator of the GC HP 5880 A can be programmed in BASIC permits adaptation of the hydrocarbon product distribution by means of the Schulz-Flory distribution law (e.g. ref. 13) immediately upon analysis. Hydrocarbons were identified by a GC/MS-equipment (HP MSD 5970). RESULTS AND DISCUSSION Characterization Fig. 1 shows the systematic increase of the pore volume along with increasing support burn off. The deposition of about 15 to 16 % by wt Fe203 corresponding to a Fe concentration of about 11 % by wt - has a significant influence on the pore structure (table 2) .
.....
64
Cl
IS) IS)
,
100
51
E
U
u
'"'
35
III
-> E :3
21
0
,., ~
L..O
0
...c
:l
0
III ~
0
a.
1"
Fore Radius Cnm]
Figure 1: pore size distribution of supports
III
496
TABLE 2: Pore characteristics of supports and freshly prepared catalysts Support 0.12 0.12 0.11 0.12 0.13
0 21 35 51 64
No. 4 470 690 910
0.02 0.20 0.26 0.34 0.37
1010
0/10 21/10 35/10 51/10 64/10
Catalyst Fe content Vm~cro3 Vm!cro wt% cm /cm cm /cm 3 11. 5 12 11 11 11 n.d.=
0.1 0.1 0.12 n.d. 0.12 0.2 0.25 0.12 0.27 0.12 not determined
The Moessbauer spectra measured at ambient temperature of the Fe/activated carbon catalysts freshly prepared are completely identical and independent of the support burn off. They are characteristic for superparamagnetic Fe203 (fig. 2) and do not show any signs of splitting even at 4 K. After reduction in H2, 6 hours of atmospheric CO hydrogenation and subsequent reoxidation with oxygen from air there will occur a significant change in the Moessbauer spectrum of the catalyst with a burn off of 0 % (fig. 3a). In this case the characteristic six-peak spectrum of elemental iron will appear. In the case of catalysts with higher support burn off (figures 3b and 3c), the reduced iron is almost completely re-oxidized into a very finely distributed Fe203, which can be concluded from the central doublet. Besides there will occur lines of X-carbide (catalyst 35/10) and of elemental iron (about 2 %, catalyst 64/10).
e
lIS
c:
III L
"
c)
I-
-10 -S 0 5 10 Ve 1oc i ty / mm*s-\
Figure 2: Mossbauer spectra at 298 K of freshly prepared catalysts: a) 0/10; b) 35/10; c) 64/10
-10 -S 0 S 10 Ve 1oc i ty / mm*s-\
Figure 3: Mossbauer spectra at 298 K of reduced and reoxidized catalysts: a) 0/10; b) 35/10; c) 64/10
497
XRD did not give any diffraction peaks for samples of freshly prepared catalysts. After reduction and re-oxidation the intensive diffraction peaks of metallic Fe were only found for a sample of catalyst 0/10. The above results render obvious that certain differences occur between the deposited Fe203 during reduction and that such differences are due to the support burn off. In order to examine this mechanism more in detail we ran thermo-gravimetric tests in a H2 atmosphere. Fig. 4 shows the differential mass losses as a function of the temperature. There are two distinctive types of mass loss diagrams: - The catalysts 0/10 and 21/10 show distinctive maxima below 750 K caused by the release of CO, CO 2, and H20. - The mass loss for catalysts 35/10, 51/10, and 64/10 in the same temperature range is caused by the formation of H20 and CO 2, with no distinctive maximum observed. What strikes with these catalysts is the high mass loss in the temperature range between 720 K and 920 K which is attributed to the gasification of the support under release of CH 4. g ] zsm [m K*g
6T
1.5
1.0 0.5 64/10 .....--=;:=:::....-----7 5 1/10 \---=~-----...::~~=---/
~--'--;''-----r---'--'----.---r---r---(
1100
35/10 21/10 t 0/10 C'
st
,1i
As the above results show, the behaviour during reduction of the Fe203 as well as the interaction between the reduced iron and the support material are influenced by the support burn off. It was demonstrated by CO chemisorption measurements that these differences are mainly due to the Fe phase dispersion. Table 3 contains the amounts of irreversibly adsorbed CO for the different catalysts. The table gives furthermore the relevant values derived for dispersion and mean Fe particle diameters /10/ .
498
TABLE 3: Dispersion and mean Fe particle size determined by CO chemisorption* /10/ Catalyst
CO-Uptake (mmol/gcat)
0/10 21/10 35/10 51/10 64/10
0 0.05 1.27 0.72 0.50
Dispersion
0.025 0.64 0.37 0.25
Fe particle size (nm) Fe/CO= 1/1 Fe/CO= 2/1 30.0 1.2 2.0 3.0
15.0 0.6 1.0 1.5
* Adsorption at 298 K The results of CO chemisorption show that the subdivision of catalysts in two groups, as occurred during thermo-gravimetry, has to be attributed to the discrepant dispersions of the Fe phase. For the first group (b.o. < 35 %) the particles, lead to bulk reduction will, notwithstanding the very small Fe 203 Fe crystals which - as becomes evident by the Moessbauer spectrum of the 0/10 catalyst - do not reoxidize by access of oxygen from air during the lengths of times observed ( 6 month). For the second group of catalysts (b.o. ~ 35 %) there appears a very high Fe dispersion which is stabilized by the supports. From the above observation it can be assumed that Fe203 is deposited mainly in the micro-pores. These micro-pores contribute mainly to the total surface area which is rather small at low support burn offs. In the case of high burn offs (b.o. ~ 35 %) these micro-pores will prevent the metal crystallites from sintering, and stabilize them. In the case of low burn off values the available micro-pore volume will be insufficient for uptaking the quantity of Fe203 applied, thus inducing the formation of great bulk Fe-crystals during reduction. The activity of metallic support catalysts will, however, be critically influenced not only by the dispersion but also by the distribution of the metal within the particles of the support. For this reason we determined the Fe distribution in the catalyst pellets by micro-probe analyses (fig. 5). Up to a burn off of 35 % a homogeneous Fe profile is observed. Higher burn off levels lead to the formation of shell catalysts. As we found out recently (ref. 14) by more detailed examinations, this is due to the higher rate of decomposition of Fe(CO)5 along with increasing burn off.
499
Figure 5: Fe distribution across catalyst pellets a) 0/10; b) 35/10; c) 64/10 CO Hydrogenation Table 4 summarizes the results of CO hydrogenation obtained when using different catalysts at reaction pressures of 0.1 MPa and 2 MPa. TABLE 4: Typical results of CO-hydrogenation for Fe/C-catalysts
Catalyst
0/10 21/10 35/10 51/10 64/10
pressure: O'!lMPa, temperature: 550 K, t = 0.33 h S.v = 630 h chain growth Conversion (%) Alkene/Alkane ratio probability CO
o
37.2 76.4 71.1 42.0
o
17.7 23.6 19.7 19 6
0.022 0.004 0.014 0.045
pressure: 2 MPa, te~~erature: S.v. = 3800 + 200 h 0/10 21/10 35/10 51/10 64/10
o
5.1* 14.1 93.0 91.1
o
5* 13.8 61.6 56.8
* after 4.8 h time on stream
0.0583* 0.083 0.009 0.021
0.05 0.01 0.04 0.13
o 02 0.04 0.22
525 K, t 0.295* 0.472 0.052 0.159
0.45 0.50 0.44 0.46
0.11
= 20
0.503* 0.756 0.132 0.333
h 0.47* 0.57 0.45 0.45
500
As at atmospheric pressure the catalysts are subjected to rapid deactivation, the table gives the initial activity after a reaction time of 20 minutes. The distribution of hydrocarbons can even under these instationary conditions be expressed by the Schulz-Flory law. Independently of the type of catalysts a chain propagation probability of 0.45 has been determined. On the other hand the activity and olefin contents of the hydrocarbons show a strong dependence on the type of catalyst used. Catalyst 35/10, showing the highest Fe dispersion, turned out to be the most active, too, which was up to the expectations. The minimum of olefin contents may also be explained by high dispersion: the a.m. catalyst has at the same time the highest metal surface available for hYdrogenation of the olefins which are primary products of CO-hydrogenation. Besides the formation of linear and branched paraffins and olefins there will also occur formation of BTX-aromates under these reaction conditions.
100 r"1
~
1-1
80
c 0
III
60
L-
III
> 40 c 0
U I
0
u
20 5
Ifj me
[h)5
20
25
Figure 6: CO-conversions of different catalysts with time on stream 2 MPa: v 21/10; 035/10; 051/10; '" 64/10 P = 0.1 MPa: -51/10
p
The deactivation of Fe catalysts during CO hydrogenation is known to be considerably reduced by application of increased synthesis pressures (ref. 15). This is represented on fig. 6 for the catalyst 51/10. Furthermore fig. 6 shows that, on the other hand, higher reaction pressure (2 MPa) will not induce all the catalysts to an increased and permanently high level of activity.
501
Catalysts 0/10 and 21/10 behave, just as during synthesis at atmospheric pressure, inactive or just slightly active which is explained by the low dispersion of the Fe-phase. Catalysts 51/10 and 64/10 show an almost constantly high activity over the entire period under review. Although Fe-dispersion is highest with catalyst 35/10 among all the catalysts concerned. a substantial decline in activity is observed to occur immediately after the reaction started. We believe that this behaviour has to be attributed to some blockage of the iron deposited within the catalyst pellets. by high molecular liquid reaction products. This hypothesis is supported by calculations done by other groups (ref. 16) and is furthermore revealed by the chain propagation probability of 0.57 derived from the Schulz-Flory distribution law. For catalysts 51/10 and 64/10, both of long-lasting activity, chain propagation probabilities amount to 0.45. For these catalysts blockage by long-chain reaction products is excluded due to the altered pore structure and the thin shell wherein the iron is deposited and which implies short diffusion paths. Run under a pressure of p = 2 MPa the reaction does not yield aromates. We suppose that at atmospheric pressure conditions the aromates are coke precursor or are produced by decomposition of graphitic deposites. The existence of graphitic carbon has been evidenced elsewhere (ref. 17) and may be responsible for the rapid deactivation of Fe catalysts at atmospheric pressure conditions. CONCLUSIONS Applying the described method and using appropriate activated carbons as support. it is possible to produce highly active catalysts. During catalysts preparation it turned out that the pore system exerts a strong influence on the dispersion and distribution of the deposited Fe phase. It will have to be clarified by further studies. yielding detailed kinetic data, in how far the activity of catalysts and the behaviour with time on stream are functions of transport mechanisms. ACKNOWLEDGMENTS We would like to thank Mr. B. Gatte and Prof. M. Philips from Pennsylvania State University and Mr. M. Deppe and Prof. M. Rosenberg, Ruhr-Universitat· Bochum. for measuring Mtissbauer spectra and helpful discussions.
502
REFERENCES 1 2 3 4 5 6 7
8 9
10 11 12 13 14 15 16 17
M. Kaminsky, K.Y. Yoon, G.L. Geoffroy, M.A. Vannice J. Catal. 21 (1985), 338 F. Rodriguez-Reinoso, J.D. Lopez-Gonzalez, C. Moreno-Castilla, A. Guerrero-Ruiz, J. Rodriguez-Ramos FUEL 63 (1984), 1089 E. Kikuchi, A. Koizumi, Y Aranishi, Y. Morita J. Japan Petrol. Inst. 25 (1982), 360 A.P.B. Sommen, F. Stoop, K. van der Wiele Appl. Catal. 14 (1985), 277 V.H.J. de Beer, F.J. Derbyshire, C.K. Groot, R. Prins, A.W. Scaroni, J.M. Solar FUEL §l (1984), 1095 A.W. Scaroni, R.G. Jenkins, P.L. Walker, Jr. Appl. Catal. 1i (1985), 173 F.F. Gadallah, R.M. Elofson, P. Mohammed, T. Painter Preparation of Catalysts III (Edited by G. Poncelet, P. Grange, P.A. Jacobs) Elsevier Science Publishers B.V., Amsterdam (1983), 409 J.L. Schmitt, Jr., P.L. Walker, Jr. Carbon 1Q (1972), 87 J.W. Geus Preparation of Catalysts III (Edited by G. Poncelet, P. Grange, P.A. Jacobs) Elsevier Science Publishers B.V., Amsterdam (1983), H.-J. Jung PhD Thesis, Pennsylvania State University (1981) H. Greb, K.-D. Henning, J. Klein, U. Peters DE-PS 33 30 621 H. JUntgen Carbon ~ (1968), 297 G. Henrici-Olive, S. Olive Angew. Chern. 88 (1976), 144 U. Peters, R. Jockers, J. Klein Paper presented at "Carbon '86" F. Fischer, H. Pichler Brennstoff-Chem. 20 (1939), 41 G.A. Huff, Jr., C.N. Satterfield Ind. Eng. Chern. Process Des. Dev. 24 (1985), 986 H.P. Bonzel, H.J. Krebs Surf. Sci. 21 (1980), 499
503
DISCUSSION T. HATTORI: You have mentioned that micro-pore is blocked by Fe203' But the difference in volume of micro-pore is not so large between carbon 21 and 35. What you mentioned is the blockage of pore mouth? R. JOCKERS : Yes, you are rigth. The comparison of available micropore volume of the support and the volume of deposited iron oxide shows that only a small part of the micropore volume can be occupied by Fe-oxide. The drastic decrease of the micropore volumes of the prepared catalysts is a strong indication for pore mouth blockage. J.W. GEUS : 1/ Your results are suggesting that your catalysts prepared from supports of a high burn-off are exhibiting diffusion limitation in the reaction. This can be established by measuring the activity on crushed catalyst tablets. Did you vary the size of your carbon support particles and did that affect the conversion ? 2/ We get a more homogeneous distribution of iron over the support during the decomposition of iron carbonyl by first exposing the support to the carbonyl containing gas at a temperature so low that the carbonyl did not decompose and subsequently rapidly raising the temperature. Since we used a fluidized bed, we could raise the temperature fast. Have you experience with addition of the carbonyl at low temperature and subsequent heating? R. JOCKERS : 1/ Diffusional limitation during reaction may be an explanation for the observed dependence of CO-conversion on burn-off of support. We made activity measurements with a crushed catalyst (burn-off = 35%, particle size = 0.08 - 0.5 mm) and found the same loss of activity with time on stream that we have observed with the uncrushed catalyst. Detailed kinetic measurements are necessary in order to clarify the role of diffusional limitation during CO-hydrogenation. Another explanation for the dependence of activity on burn-off is based on the influence of burn-off on mean iron particle size. Very small Fe-particles result on activated carbons which have 35% burn-off. Possibly these particles are converted to Fe-carbides very fast, and are blocked by inactive carbidic carbon layers in consecutive reactions. 2/ We performed Fe(CO)5-decomposition at various temperatures (273 K - 373 K) and found that the Fe-distribution across the support can be influenced by the decomposition temperature. By example with a burn-off = 35%. there is a change from a homogeneous iron distribution to an egg-shell catalyst, if the decomposition of Fe(CO)5 is carried out at 323 and 373 K, respectively. This fact could be explained by an acceleration of the decomposition rate with increasing temperature. We think that your observation can be explained by a volatilization of adsorbed Fe(CO)5 and a further diffusion of this compound to the centre of support particles during the raise of temperature.