Chapter 21 Dispersed Catalysts for Coal Liquefaction Albert S. Hirschon and Robert B. Wilson, Jr.
Downloaded by UNIV LAVAL on November 9, 2015 | http://pubs.acs.org Publication Date: May 6, 1991 | doi: 10.1021/bk-1991-0461.ch021
Inorganic and Organometallic Chemistry Program, 333 Ravenswood Avenue, SRI International, Menlo Park, C A 94025
Dispersed catalysts formed from aqueous metal salt impregnation and organometallic precursor techniques were investigated for conversion of Illinois #6 and lignite coals into soluble products. The organometallic precursors were found to be exceptionally active and provide the greatest yields of toluene soluble products. We believe the key to effective catalysis is the use of catalyst precursors that give high dispersion and do not require high temperature activation. A positive correlation between loss of oxygen in the final product and extent of conversion was found. Low temperature hydrogenations (300°C) of impregnated coals show that the highly dispersed catalysts seem to aid in forming tetrahydrofuran-soluble materials. Results from hydrogenolysis studies suggest that these catalysts will not break weak links in the coal matrix at these low temperatures, so that a possible alternative is that these dispersed catalysts prevent retrogressive reactions. Higher temperature conversions (400-425°C) were also conducted using either tetralin as a donor solvent, or hexadecane as an non-interacting non-donor solvent. The organometallic impregnation gave conversions to toluene-soluble material in the hexadecane which were almost as high as in tetralin, suggesting that with a well dispersed catalyst, expensive conversion solvents are not necessary for high conversions.
Although great progress has been made in converting coal to distillable liquids in high yields, the products are still not competitive with petroleum. A major problem is that under the severe conditions for bond-breaking during coal liquefaction, retrogressive reactions take place that produce char and coal liquids that are difficult to hydrotreat (1-6). For instance, phenolics are thought to polymerize into polymeric furans during the liquefaction process. For low-rank coals, the carboxylates are thought to be an important factor in these retrogressive reactions. Thus the heterocyclic compounds in the coal liquids produced from coal make coal liquids difficult to upgrade. In order to hydrotreat these materials such high temperatures and hydrogen pressures are required that the valuable aromaticcontaining materials in the coal liquid are concurrendy hydrogenated. 0097-6156/91/0461-0273$06.00/0 © 1991 American Chemical Society
In Coal Science II; Schobert, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
COAL SCIENCE II
Downloaded by UNIV LAVAL on November 9, 2015 | http://pubs.acs.org Publication Date: May 6, 1991 | doi: 10.1021/bk-1991-0461.ch021
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Dispersed catalytic liquefaction has several distinct advantages over thermal or conventional supported catalytic liquefaction. In the presence of hydrogen, a suitably dispersed catalyst can provide a highly reducing environment within die coal matrix, thus eliminating the need for a good hydrogen-donating solvent. A n added advantage to these catalysts is that they can promote certain bond cleavage reactions during the liquefaction step. If they can aid in removing the heteroatoms, namely oxygen and nitrogen, during the early stages in coal liquefaction, then the detrimental regressive reactions would be minimized. Thus a better quality coal liquid product would be produced that would be easier and less expensive to hydrotreat M a n y workers have investigated non-supported high dispersion catalysts (2l 16). Most work on dispersed catalysts has focused on metal salts such as ammonium molybdate, pyrites, or oil-soluble catalysts such as molybdenum naphthenate. Derbyshire et al. have conducted considerable research utilizing the (ΝΗ4>2Μοθ4 and (NH4>2MoS4 aqueous impregnation methods at low temperatures where they have shown that these dispersed catalysts can effectively utilize hydrogen to aid i n subsequent conversions (8-10). The problem with most dispersed catalysts that have been tested is that they are only activated at high temperatures. F o r instance, ammonium tetrathiomolybdate decomposes to M0S3 at low temperatures. However, the more active form is M0S2 is formed at much higher temperatures (>350°C) (17-18). In a similar manner, the molybdenum naphthenate needs to be transformed into M0S2. The importance of the correct stoichiometry has been emphasized by Montano et al. (19 20). They have suggested i n work on iron sulfide catalysts that the pyrite (FeS2) must be transformed to pyrrhotite, F e i - S , (0
