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Carbide Catalysts: Laser Pyrolysis Synthesis and Catalytic Activity
Guczi, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
CARBIDE CATALYSTS: LASER PYROLYSIS SYNTHESIS AND CATALYTIC
ACTIVITY
J. M.Steneel, P. C. EkYund X-X Bi, B. H. Davis, G.T.Hager andFJ. Derbyshire
Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA
Abstract Ultrafine particle iron catalysts, having particle diameters in the range of 5-20 nm, have been synthesized in carbide and metallic forms by CO, laser pyrolysis of iron pentacarbonyl and ethylene gaseous mixtures. Characterization by bulk and surface spectroscopies has shown that, depending upon the conditions of synthesis, Fe7C3, Fe C or alpha-Fe can be prepared. Sulfiding with dimethyldisulfide converts the UFP's to pyrrhotite having a Fe/S ratio of approximately 0.85. The hydroliquefaction activity of these catalysts is presented and discussed relative to other types of dispersed catalysts. 1. Introduction
The production of distillate fuels and chemicals from nonconvential resources like coals, heavy crudes, tar sands and oil shales will require the development of efficient processes and highly active catalysts. One such process that has been investigated extensively is direct hydroliquefaction. However, for its application, there are strong incentives for the development of dispersed form or ultrafine particle (UFP) catalysts. Until there has been substantial molecular size reduction, the reactant molecules from coal, tars or heavy crudes are too large to access the interior active surface of supported catalysts. In these instances, it has long been realized that the catalyst must be introduced in a form conducive to its intimate dispersion in the feed material (1-5). Over the last few years there has been an appreciable research focus on this topic (6-12). Because of the difficulty of recovering dispersed catalysts for recycle, they must be considered disposable. Hence, disposable Mo-based catalysts may be expensive whereas Fe-based catalysts, if imparted with high activity, may be less expensive. In studies concentrating on enhancing the intimate contact between coal and catalyst (6-8), an increase in the catalytic activity of pyrite was observed as its particle size was soluble iron decreased from 5 micrometers to 50 nm. Oil organometallic~s have also been used aa catalyst precureors
1798 (4,9-12). These compounds are believed to distribute through the coal-oil mixture and to decompose upon heating to form very small particles, enabling their use at concentrations of less than 0.3 wt%. Our research on UFP iron carbides, produced by laser pyrolysis of Fe(CO),, has shown that they possess interesting activity for coal dissolution (13). The present article focuses on their synthesis, characterization and catalytic activity.
Experimental Iron carbide, UFP catalysts were synthesized by laser pyrolysis of Fe(CO), in C,H,. The reactant gas mixture is heated, reacts and forms UFP's rapidly (
Table 1 Analysis of the two coals used in the limefaction experiments. Percent C(daf) H(dafl N(daf) TS(daf) Otdafl' VM(daf1 W. KY #6 82.87 Wyodak 71.02 by difference
'
5.42 5.40
1.72 1.37
5.15 1.00
4.8 21.2
43.1 59.92
Liquefaction expgriments were conducted in 50 ml batch autoclaves at 385OC. The reaction time was 15 minutes, with tetralin as the solvent and a hydrogen atmosphere (5.5 MPa). Typically, autoclaves were charged with 3 g of coal, 5 g of tetralin, and 1 wt. % of the catalyst. Dimethyldisulfide was used to sulfide the catalyst in-situ and was added in 20% excess, calculated on the quantity of sulfur required to form FeSZ. The coal conversions (daf) to soluble products were determined using solubility in pyridine (preasphaltenes), benzene (asphaltenes), and pentane (oils).
1799
Results and Discussion Figure 1 displays typical results obtained from high magnification (300,OOOX) SEM imaging of passivated UFP iron carbides. The particles are spherical in shape and have diameters of 5-10 nm. In general, the distribution of sizes can be maintained within approximately +/- 2 nm for particles with sizes between 5-20 nm. 3.
Figure 1. SEM images of UFP iron carbide. MES and XRD data have shown that three ferromagnetic phases can be synthesized, including a-Fe, Fe3C and Fe7C3. In the case of Fe7C3, the XRD data were necessary for identification since Mossbauer of Fe7C3have not been published. The BET N, surface areas of the UFP's is near 80 m2/g. If a representative particle of Fe7C3 (density of 7.615 g/cc) is 10 nm in diameter and spherical in shape, its external, geometric surface area would be 80 m2/g, in good agreement with the BET surface areas. Hence, the UFP samples are not microporous. XPS data have shown that the surface of the UFP's can be enriched in oxygen (due to passivation and/or synthesis) and/or carbon (due to synthesis), or can be synthesized to contain a negligible amount of oxygen. These surface sensitive data are in agreement with chemical analyses of the UFP's and indicate that the reactant partial pressures during synthesis determine to a large extent their Fe, C and 0 stoichiometry and structure. Upon exposure to controlled He or H2 environments at temperatures as high as 6OO0C, the genesis of surface speciation and the transformation of bulk crystalline forms have also been found to depend on sample preparation conditions. Phases through which the carbide transforms during such treatments include carbides, FeO , Fe,,,,O , Fe30,, Fe,O, (maghemite) and elemental Fe. XRD analyses of sulfided iron carbide UFP's show they contain Fe,-,S pyrrhotite. Energy dispersive x-ray analysis by SEM supports the XRD analysis, providing an approximate stoichiometry of Fe, S , , or Fe7S,. Table 2 shows the conversion of Wyodak subbituminous Coal under hydroliquefaction conditions.
-
1800 Table 2 Conversion analvsis of 1iuuefaction exDeriments. Preoil & Gas IOM aSDhaltenes AsDhaltenes Thermal Fe(CO), Fe,C
Fe&
58.1 53.2 48.9 43.7
15.0 9.3 18.7 22.3
15.1 23.4 21.1 18.0
11.7 14.1 11.3 16.0
Total conversion 41.9 46.8 51.1 56.3
It has been determined that these catalysts can contain several monolayers of carbon on their surface (14) which may, by analogy with the data discussed by Charcosset , et. al. (15), beneficially influence coal dissolution. The UFP catalysts have conversions that are comparable to that observed for Fe(CO), and molybdenum naphthenate. Currently, the dispersion of the UFP's upon their introduction to the coal-tetralin slurry is an important issue to be addressed. 4. References 1) F. Derbyshire, Energy & Fuels, 1, 273(1989). S. Weller and M.G. Pelipetz, Ind. Eng. Chem. 43, 2) 1243 (1951) 3) A. V . cugini, B.R. Utz, and E.A Fromell, Prepr. Pap.- Am. Chem. SOC. , Div Fuel Chem. (1989). 4) D.E. Herrick, J.W. Tierney, I. Wender, G.P. Huffman and F.E. Huggins, Energy & Fuels, Q, 231(1990). D.G. Marriadassou, H. Charcosset, M. Andres and P. Chiche, 5) Fuel, 62, 69(1983). 6) M. Andres, H. Charcosset, P. Chiche, L. Davignon, G.D.
.
Mariadassou, J.P.
7) 8) 9) 10) 11) 12)
13) 14) 15)
.
69 (1983)
Joly
and
s. Pregermain, Fuel, 82,
M. Andres, H. Charcosset, P. Chiche, G. D. Mariadassou, J.P. Joly and S. Pregermain, in Preparation of Catalysts I11 (Elsevier, The Netherlands, 1983), pp. 675-682. S. Pregennain, Fuel Proc. Tech., 12, 155(1986). R.R. Anderson and B.C. Bockrath, Fuel, 62, 320(1984). J.A. Ruether, J.A. Mima, R.M. Kornosky and B.C. Ha, Energy & Fuels, 1, 198(1987). Y. Takemura and K. Okada, Fuel 67, 1548(1988). 0. Yamada, T. Suzuki, J.H Then, T. Ando and Y. Watanabe, Fuel Process. Technol. 11, 297(1985). J.M. Stencel, P.C. Eklund, X.-X. Bi and F.J. Derbyshire, submitted for publication. P.C. Eklund and X.X. Bi, submitted for publication. H. Charcosset, R. Bacaud, M. Besson, A. Jeunet, B. Nickel and M. Oberson, Fuel Proc. Technol. 12, 189(1986).
CARBIDE CATALYSTS: LASER PYROLYSIS SYNTHESIS AND CATALYTIC
ACTIVITY
J. M.Steneel, P. C. EkYund X-X Bi, B. H. Davis, G.T.Hager andFJ. Derbyshire
Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA
Abstract Ultrafine particle iron catalysts, having particle diameters in the range of 5-20 nm, have been synthesized in carbide and metallic forms by CO, laser pyrolysis of iron pentacarbonyl and ethylene gaseous mixtures. Characterization by bulk and surface spectroscopies has shown that, depending upon the conditions of synthesis, Fe7C3, Fe C or alpha-Fe can be prepared. Sulfiding with dimethyldisulfide converts the UFP's to pyrrhotite having a Fe/S ratio of approximately 0.85. The hydroliquefaction activity of these catalysts is presented and discussed relative to other types of dispersed catalysts. 1. Introduction
The production of distillate fuels and chemicals from nonconvential resources like coals, heavy crudes, tar sands and oil shales will require the development of efficient processes and highly active catalysts. One such process that has been investigated extensively is direct hydroliquefaction. However, for its application, there are strong incentives for the development of dispersed form or ultrafine particle (UFP) catalysts. Until there has been substantial molecular size reduction, the reactant molecules from coal, tars or heavy crudes are too large to access the interior active surface of supported catalysts. In these instances, it has long been realized that the catalyst must be introduced in a form conducive to its intimate dispersion in the feed material (1-5). Over the last few years there has been an appreciable research focus on this topic (6-12). Because of the difficulty of recovering dispersed catalysts for recycle, they must be considered disposable. Hence, disposable Mo-based catalysts may be expensive whereas Fe-based catalysts, if imparted with high activity, may be less expensive. In studies concentrating on enhancing the intimate contact between coal and catalyst (6-8), an increase in the catalytic activity of pyrite was observed as its particle size was soluble iron decreased from 5 micrometers to 50 nm. Oil organometallic~s have also been used aa catalyst precureors
1798 (4,9-12). These compounds are believed to distribute through the coal-oil mixture and to decompose upon heating to form very small particles, enabling their use at concentrations of less than 0.3 wt%. Our research on UFP iron carbides, produced by laser pyrolysis of Fe(CO),, has shown that they possess interesting activity for coal dissolution (13). The present article focuses on their synthesis, characterization and catalytic activity.
Experimental Iron carbide, UFP catalysts were synthesized by laser pyrolysis of Fe(CO), in C,H,. The reactant gas mixture is heated, reacts and forms UFP's rapidly (
Table 1 Analysis of the two coals used in the limefaction experiments. Percent C(daf) H(dafl N(daf) TS(daf) Otdafl' VM(daf1 W. KY #6 82.87 Wyodak 71.02 by difference
'
5.42 5.40
1.72 1.37
5.15 1.00
4.8 21.2
43.1 59.92
Liquefaction expgriments were conducted in 50 ml batch autoclaves at 385OC. The reaction time was 15 minutes, with tetralin as the solvent and a hydrogen atmosphere (5.5 MPa). Typically, autoclaves were charged with 3 g of coal, 5 g of tetralin, and 1 wt. % of the catalyst. Dimethyldisulfide was used to sulfide the catalyst in-situ and was added in 20% excess, calculated on the quantity of sulfur required to form FeSZ. The coal conversions (daf) to soluble products were determined using solubility in pyridine (preasphaltenes), benzene (asphaltenes), and pentane (oils).
1799
Results and Discussion Figure 1 displays typical results obtained from high magnification (300,OOOX) SEM imaging of passivated UFP iron carbides. The particles are spherical in shape and have diameters of 5-10 nm. In general, the distribution of sizes can be maintained within approximately +/- 2 nm for particles with sizes between 5-20 nm. 3.
Figure 1. SEM images of UFP iron carbide. MES and XRD data have shown that three ferromagnetic phases can be synthesized, including a-Fe, Fe3C and Fe7C3. In the case of Fe7C3, the XRD data were necessary for identification since Mossbauer of Fe7C3have not been published. The BET N, surface areas of the UFP's is near 80 m2/g. If a representative particle of Fe7C3 (density of 7.615 g/cc) is 10 nm in diameter and spherical in shape, its external, geometric surface area would be 80 m2/g, in good agreement with the BET surface areas. Hence, the UFP samples are not microporous. XPS data have shown that the surface of the UFP's can be enriched in oxygen (due to passivation and/or synthesis) and/or carbon (due to synthesis), or can be synthesized to contain a negligible amount of oxygen. These surface sensitive data are in agreement with chemical analyses of the UFP's and indicate that the reactant partial pressures during synthesis determine to a large extent their Fe, C and 0 stoichiometry and structure. Upon exposure to controlled He or H2 environments at temperatures as high as 6OO0C, the genesis of surface speciation and the transformation of bulk crystalline forms have also been found to depend on sample preparation conditions. Phases through which the carbide transforms during such treatments include carbides, FeO , Fe,,,,O , Fe30,, Fe,O, (maghemite) and elemental Fe. XRD analyses of sulfided iron carbide UFP's show they contain Fe,-,S pyrrhotite. Energy dispersive x-ray analysis by SEM supports the XRD analysis, providing an approximate stoichiometry of Fe, S , , or Fe7S,. Table 2 shows the conversion of Wyodak subbituminous Coal under hydroliquefaction conditions.
-
1800 Table 2 Conversion analvsis of 1iuuefaction exDeriments. Preoil & Gas IOM aSDhaltenes AsDhaltenes Thermal Fe(CO), Fe,C
Fe&
58.1 53.2 48.9 43.7
15.0 9.3 18.7 22.3
15.1 23.4 21.1 18.0
11.7 14.1 11.3 16.0
Total conversion 41.9 46.8 51.1 56.3
It has been determined that these catalysts can contain several monolayers of carbon on their surface (14) which may, by analogy with the data discussed by Charcosset , et. al. (15), beneficially influence coal dissolution. The UFP catalysts have conversions that are comparable to that observed for Fe(CO), and molybdenum naphthenate. Currently, the dispersion of the UFP's upon their introduction to the coal-tetralin slurry is an important issue to be addressed. 4. References 1) F. Derbyshire, Energy & Fuels, 1, 273(1989). S. Weller and M.G. Pelipetz, Ind. Eng. Chem. 43, 2) 1243 (1951) 3) A. V . cugini, B.R. Utz, and E.A Fromell, Prepr. Pap.- Am. Chem. SOC. , Div Fuel Chem. (1989). 4) D.E. Herrick, J.W. Tierney, I. Wender, G.P. Huffman and F.E. Huggins, Energy & Fuels, Q, 231(1990). D.G. Marriadassou, H. Charcosset, M. Andres and P. Chiche, 5) Fuel, 62, 69(1983). 6) M. Andres, H. Charcosset, P. Chiche, L. Davignon, G.D.
.
Mariadassou, J.P.
7) 8) 9) 10) 11) 12)
13) 14) 15)
.
69 (1983)
Joly
and
s. Pregermain, Fuel, 82,
M. Andres, H. Charcosset, P. Chiche, G. D. Mariadassou, J.P. Joly and S. Pregermain, in Preparation of Catalysts I11 (Elsevier, The Netherlands, 1983), pp. 675-682. S. Pregennain, Fuel Proc. Tech., 12, 155(1986). R.R. Anderson and B.C. Bockrath, Fuel, 62, 320(1984). J.A. Ruether, J.A. Mima, R.M. Kornosky and B.C. Ha, Energy & Fuels, 1, 198(1987). Y. Takemura and K. Okada, Fuel 67, 1548(1988). 0. Yamada, T. Suzuki, J.H Then, T. Ando and Y. Watanabe, Fuel Process. Technol. 11, 297(1985). J.M. Stencel, P.C. Eklund, X.-X. Bi and F.J. Derbyshire, submitted for publication. P.C. Eklund and X.X. Bi, submitted for publication. H. Charcosset, R. Bacaud, M. Besson, A. Jeunet, B. Nickel and M. Oberson, Fuel Proc. Technol. 12, 189(1986).