Energy & Fuels 1997, 11, 221-226
221
Poisoning and Inhibition of Dispersed Liquefaction Catalyst Activity by Exposure to Coal Karl Schroeder, Bradley Bockrath,* Ronald Miller, and Henry Davis U.S. Department of Energy Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236-0940 Received June 25, 1996X
The activity of a dispersed MoS2 catalyst prepared in situ from ammonium tetrathiomolybdate was measured in octacosane containing various amounts of Argonne Premium Illinois No. 6 coal using microautoclaves. Conversion of 1-ethylnaphthalene (1-ETN) was followed to measure the extent of hydrogenation and/or rearrangement to 2-ETN. The reactors were charged with 1200 psig of hydrogen at room temperature. After a small amount of coal was added, activity for the acid-catalyzed rearrangement was virtually extinguished. Hydrogenation activity was significantly reduced but not eliminated. The effect on hydrogenation activity was divided into two components. One component was assigned to poisoning and the other to competitive inhibition. The weight of coal required to complete the poisoning process was determined to be 6 times the weight of Mo in the catalyst. The activity that remains for hydrogenating 1-ETN after the poisoning process is complete suffered from competitive inhibition as more coal was added. The amounts of H2S or basic nitrogen compounds that might be released from the coal during liquefaction were determined to be insufficient to account for the extent of deactivation observed.
Introduction Dispersed catalysts, especially those based on the sulfides of molybdenum or iron, are often used in direct coal liquefaction.1 In this application, they serve several functions.2 For MoS2 catalysts in particular, prominent functions include promotion of hydrogenation and the removal of heteroatoms, particularly sulfur, nitrogen, and oxygen. Model compound studies have demonstrated that the sulfides of molybdenum, ruthenium, and iron are very effective in supplying activated molecular hydrogen for the stabilization of free radicals generated by the thermolysis of carbon-carbon and carbon-oxygen bonds.3 This same function has been linked to one of the most critical roles of catalysts during the initial phase of liquefaction: the prevention of retrogressive reactions. These reactions may lead to the formation of high molecular weight residues. Prevention of the recombination of thermally generated radicals by catalytic intervention has been recognized as an important factor in the upgrading of heavy petroleums4 and more recently in coal liquefaction.5,6 In view of the importance of dispersed MoS2 catalysts, investigation of the effect of coal on their activity is much needed. The investigation reported here uses a single model compound to measure both hydrogenation activity and acid-catalyzed isomerization activity as a function of coal addition. * Author to whom correspondence should be addressed (e-mail
[email protected]). X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Derbyshire, F. Energy Fuels 1989, 3, 273-277. (2) Bockrath, B. C.; Finseth, D. H.; Illig, E. G. Fuel Process. Technol. 1986, 12, 175-188. (3) Ikenaga, N.; Kobayashi, Y.; Saeki, S.; Sakota, T.; Watanabe, Y.; Yamada, H.; Suzuki, T. Energy Fuels 1994, 8, 947-952. (4) Bearden, R.; Aldridge, C. L. Energy Prog. 1981, 1, 44-48. (5) Charcosset, H.; Bacaud, R.; Besson, M.; Jeunet, A.; Nickel, B.; Oberson, M. Fuel Process. Technol. 1986, 12, 189-201. (6) Suzuki, T. Energy Fuels 1994, 8, 341-347.
S0887-0624(96)00101-6
The compound selected for use in this work is 1-ethylnaphthalene (1-ETN). Brammer and Weller7 have previously pointed out advantages of using the closely related compound, methylnaphthalene. This single compound was used to follow several catalyst functions: hydrogenation, ring-cracking, and demethylation. In addition to these reactions, we have found that methyl migration may also occur under liquefaction conditions, provided a catalyst of sufficient acidity is present.8 Subsequent trials confirmed that 1-ETN undergoes this positional isomerization more readily than the methyl homologue, thus providing a more sensitive test of catalyst acidity.9 A relatively small amount of the model compound was used in octacosane, which was chosen to serve as a reasonably inert solvent. In all cases, octacosane was the major component in the reaction mixture with a small amount of 1-ETN added to probe the effect of adding small amounts of coal on the activity of MoS2. The methods used here closely follow those developed in an earlier study in which molecular probes were used to investigate the chemical roots of synergistic effects found in coal/oil coprocessing.10 Ammonium tetrathiomolybdate (ATM) was used as a catalyst precursor. It is known to form molybdenum disulfide and become an effective liquefaction catalyst under our reaction conditions.11 The catalyst precursor was added as a powder without effort to impregnate it on the coal. Argonne Premium Illinois No. 6 coal was (7) Brammer, S. T.; Weller, S. W. Fuel Process. Technol. 1979, 2, 155-159. (8) Bockrath, B. C.; Schroeder, K. T.; Smith, M. R. Proc. Int. Conf. Coal Sci. 1989, 2, 667-670. (9) Schroeder, K. T.; Bockrath, B. C.; Smith, M. R.; Davis, H.; Miller, R. D. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1990, 35 (1), 225231. (10) Bockrath, B. C.; Schroeder, K. T.; Smith, M. R. Energy Fuels 1989, 3, 268-272. (11) Wildervanck, J. C.; Jellinek, F. Z. Anorg. Allg. Chem. 1964, 328, 309-318.
This article not subject to U.S. Copyright.
Published 1997 by the American Chemical Society
222 Energy & Fuels, Vol. 11, No. 1, 1997
Schroeder et al.
chosen for use with this catalyst. This seam has been investigated often as a liquefaction feed, and the Argonne Premium sample is well characterized.12 The reactions were carried out in a set of five microautoclaves mounted together and heated simultaneously in a fluidized sand bath. Each reactor in the set of five experienced nearly identical time-temperature profiles. Experiments were then organized so that the concentration of one of the components was varied across a range of five values, taking advantage of the consistency in the heating conditions within the set of five. Experimental Section Batch reactions were performed in a set of five 40-mL 316SS reactors, which were simultaneously immersed in a preheated, fluidized sand bath. About 7 min was required to heat the reactors from room temperature to 425 °C. The reactors were held at reaction temperature for 60 min. Agitation was provided by a pneumatic mechanism which rocked the reactor mount over the top of an arc with a reciprocating motion at about 60 cpm. The reactors were cooled to under 200 °C in
