Energy & Fuels 1994,8, 10-18
10
Nanophase Iron-Based Liquefaction Catalysts: Synthesis, Characterization, and Model Compound Reactivity D. W. Matson,* J. C. Linehan, J. G. Darab, and M. F. Buehler Pacific Northwest Laboratory,+ P.O. Box 999, Richland, Washington 99352 Received July 12, 1993. Revised Manuscript Received October 8, 1993"
Ultrafine nanometer-scale iron-based catalyst precursor powders were generated using two novel technologies, the rapid thermal decomposition of precursors in solution (RTDS) and the modified reverse micelle (MRM) processes. The powders were characterized according to their phase and crystallite size and were evaluated for activity toward C-C bond scission using the model compound, naphthylbibenzylmethane, in the presence of elemental sulfur and 9,lO-dihydrophenanthrene.The catalytic activities of the powders were found to be strongly dependent on their crystallographic phase. RTDS magnetite, six-line ferrihydrite, and ferric oxyhydroxysulfate were found to have very high activity toward conversion of the model compound whereas two-line ferrihydrite and hematite were determined to be poor or mediocre catalyst precursors. MRM magnetite/maghemite was also found to be a relatively good catalyst precursor but exhibited reduced activity when compared to the RTDS magnetite.
Introduction Development of catalyst materials that promote selective C-C bond scission reactions under relatively mild conditions is an important goal in coal liquefaction research. The two types of liquefaction catalysts that have historically shown the best activity for that purpose are metal sulfides, including those of Mo, W, Ni, and Co and metalcontaining acid catalysts (e.g., ZnClz or SnClZ).' Unfortunately, many of the metals contained in these active catalysts are either excessively costly or highly toxic, requiring an expensive catalyst recovery step in order to extract the metal from the residual solids after liquefaction. Recent emphasis has been placed on the development of low cost uthrow-awayniron-based catalysts that would provide the necessary activity without the need to undertake catalyst reco~ery.'*~*~ In addition to having a high degree of activity and bond scission selectivity, a desirable quality for liquefaction catalysts is a high surface area that would promote an intimate contact with the coal substrate and maximize the degree of catalyst/coal interactions. For this reason, well-dispersed, ultrafine catalysts are considered to be a highly desirable form for first-stage liquefaction pr0cesses.l A number of methods have been investigated to maximize the contact between catalyst species and the coal substrate, including the development of catalyst particles in situ after impregnating of the coals with soluble iron-bearing p r e c ~ r s o r s ,and ~ r ~the use of dispersed, high surface area catalyst precursor powders.'?* Again, in order to provide * Author to whom correspondence should be addressed. t Pacific Northwest Laboratory is operated for the US.Department of Energy by Battelle Memorial Institute under contract DE-ACW76RLO 1830. Abstract published in Aduance ACS Abstracts, November 15,1993. (1)Derbyshire, F. Energy Fuels 1989,3, 273-277. (2) Pradhan, V. R.; Herrick, D. E.; Tierney, J. W.; Wender, I. Energy Fuels 1991,5, 712-720. (3) Garg, D.; Givens, E. N. Ind. Eng. Chem. Process Des. Deu. 1982, Q
for an economically viable coal liquefaction process, the cost of the precursor or powder must be low enough to justify its use for this application. At the Pacific Northwest Laboratory (PNL), we have undertaken the development of two processes which offer the capability of generating large quantities of ultrafine iron-bearing coal liquefaction catalyst precursor powders. One is a flow-through hydrothermal method referred to as the rapid thermal decomposition of solutions (RTDS) process.6~7The RTDS method utilizes precursors in very brief exposure of iron-bearing solutions to high-temperature, high-pressure conditions to initiate nucleation of oxide and oxyhydroxide species. The second method, referred to as the modified reverse micelle (MRM) process, utilizes the precipitation of iron-bearing solid particles from iron salt-containing aqueous solutions confined in the nanometer-scale water cores of water-in-oil microemulsions.8 The MRM process differs from other reverse micelle/microemulsionprocesses that have been developed for ultrafine powder production in that the formulation of the surfactant used in the MRM system has been optimized to allow much higher loadings of iron-bearing salts in the aqueous cores without destabilizing the microemulsion solution. Both the RTDS and MRM methods can produce multigram quantities of iron-bearing precursor powders suitable for use in coal liquefaction reactions. Both processes also offer additional flexibility to control the crystallographic phase, the crystallite size, and the particle size of the precursor powders which are generated. In this paper we describe the results of iron-bearing powder production efforts using the RTDS and MRM methods. Preliminary evaluation of the ultrafine powders as precursors for coal liquefaction catalysts was performed using reactions of the model compound naphthylbibenzylmethane at liquefaction conditions. ~~~
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(6) Mataon, D. W.; Linehan, J. C.; Bean, R. M. Mater. Lett. 1992,14, 222-226. (7) Mataon, D. W.; Linehan, J. C.; Geusic, M. E. Part. Sci. Technol. 1993, 10, 143-154. ( 8 ) Darab, J. G.; Pfund, D. M.; Fulton, J. L.; Linehan, J. C.; Capel, M; Ma, Y. Langmuir, in press.
0 1994 American Chemical Society
Energy & Fuels, Vol. 8, No.1,1994 11
Nanophase Zron-Based Liquefaction Catalysts
HEATER aoooc)
I
.I
FEEDSTOCP SOLUTION
I BACKPRESSURE REGULATOR
THERMOCOUPLEJ
NOZZLE PRESSURE
WATER-COOLED SOLID/LIQUID SEPARATION
CONDENSER
Figure 1. Schematic diagram of the RTDS apparatus.
Experimental Section
RTDSPowderFormation. An overviewof the experimental method for producing powders containing nanometer-scale crystallitesby the RTDS technique has been presented previously for a bench-scale apparatus?t7 Using the same principles,a scaledup version of the RTDS apparatus was subsequently assembled at PNL and was used to produce nanometer-scale iron oxide/ oxyhydroxide powders at rates of up to 100 g/h. The essential components of the RTDS apparatus are summarized in Figure 1. Precipitation of simple oxide or oxyhydroxide powders was accomplished by flowing pressurized (5000-8000 psi) solutions containing dissolved iron salts through a short section of smalli.d. high-pressure tubing that was heated either by using a tube furnace or by passage of a dc current through its length. Using these methods, the solutionscontaining dissolved precursor salts were raised from room temperature to as high as 400 "C in as little as 1a. Total residence times at the elevated temperatures were typically less than 5 a, but could be varied depending on the length and internal diameter of the heated tubing and the solution flow rate. Solution temperatures at the downstream end of the heated tube were monitored using a sheathed thermocouple mounted in a tee and exposed to the flowing fluid. Immediately beyond the thermocouple sensor, the reacted solution was passed through a pressure let-down device (e.g., an orifice), a t which point the hydrothermal reactions were quenched. The suspensionssprayed from the pressure let-down orifice were collected in a cooled collection vessel and retained for solids separation. Iron-bearing RTDS suspensions were separated by allowing the particulate fraction to settle, either gravitationally or by centrifugation. If required, additional salts were added to the suspensions to flocculate the particles and assist the settling process. Liquid above the particulate layer was decanted off, the particles were washed with deionized water, and the process was repeated. The resulting solid product was dried under flowing nitrogen or air before being ground in a mortar and pestle. The solutions used for the RTDS processing of materials described here were prepared using commercially available reagent grade nitrate or sulfate salts and deionized water. Unless otherwise noted, the total metals content of solutions prepared for RTDS processing was 0.1 M. Urea (NH2CONH2, typically 0.5 M) was added to some solutions to promote a rapid pH rise by the production of ammonia at the elevated temperatures premnt in the heated region of the RTDS apparatus. A summary
of the conditions and precursors used to generate various ironbearing phases using the RTDS process is included in the Results and Discussion section of this paper. MRM Powder Formation. Water-in-oil reverse micelles or reverse microemulsions consist of nanometer-sized aqueous droplets suspended in a nonpolar continuous organic phase by surfactant shells. They are thermodynamically stable, macroscopically homogeneous, and optically transparent. The size of the water cores in these systems is determined by the watertu-surfactant ratio, W ,and can range from as small as 1nm to much greater than 50 nm in diameter. Larger W values correspond to larger suspended aqueous droplets. If the water cores are sufficientlylarge to overcome the boundary layer effects resulting from the polar groups on the surfactant molecules, they can exhibit the characteristic properties of bulk water. In reverse micelle/microemulsion systems in which the aqueous droplets are of sufficient size to exhibit bulk water properties, a range of aqueous phase reactions, including the chemical precipitation of solid particles, can be accomplished in the micelle cores. In conventional reverse micelle/microemulsion solutions, however, the amount of reactant (dissolved salt) that can be incorporated into these systems is limited because high ionic strengths in the water pools tend to destabilize the solutions and lead to phase separation.9 The procedures involved in the formation of ultrafine ironbearing powders using the MRM process have been summarized previously.8J0J1 These procedures differ from those used by others with conventional reverse micelle or microemulsion s y s t e m ~ ~ ~inJ that ~ J ~aJcosurfactant ~ (sodium dodecyl sulfate or SDS) is used in addition to the primary surfactant (bis(2ethylhexyl) sulfosuccinate sodium salt or AOT) to stabilize the microemulsions even at very high salt concentrations in the aqueous phase. The addition of the SDS cosurfactant (at a 25 ~~
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(9) Hatton, T. A.; Leodids, E.B. Langmuir 1989,5,741-753. (10) Darab, J. G.; Linehan, J. C.; Ma, Y.;Matson, D. W. In Applications
of Synchrotron Radiation Techniques to Materials Science; Perry, D. L.,Shinn, N., Stockbauer, R.,DAmico, K.,Terminello, L., Eds.; Materials Research Society: Pittsburgh, PA, 1993; pp 9-14. (11) Darab, J. G.; Fulton, J. L.; Linehan, J. C. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1993,38,27-33. (12) Fendler, J. H . Chem Rev. 1987,87,877-899. (13) Wilcoxon, J. P.; Baughmann, R. J.; Williamson, R.L. Mater. Res. SOC.Ext. Abstr. 1990, EA-24, 225. (14) Kandori, K.;Kon-No, K.; Kitahara, A. J. J.Colloid Interface Sci. 1988,122,78-82. (15) Lufimpadio, N.;Nagy, J. B.; Derouane, E. G. In Surfactants in
Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 3, pp 1483-1497.
12 Energy &Fuels, Vol. 8,No. 1, 1994 w t % loading relative to the AOT) allows production of reverse microemulsions which can contain over 50 times the amount of ferric ions which would be stable in the corresponding solutions without the cosurfactant. Using the MRM surfactant formulation, nearly 50 w t % of 1M aqueous FeNH4(S04)2solution can be incorporated into a stable microemulaion system. In addition, MRM systems have been generally found to be stable at temperatures over 90 "C, allowing the transfer of many bulk aqueous iron-oxygen compound production methodP to the confining domains of reverse microemulsions. Ferric oxyhydroxide powders were prepared by base precipitation of dissolved ferric saltsfrom homogeneousMRM systems using reagent grade commercially available salts. Bases were added either by bubbling gaseous ammonia through the MRM solutions or by combining the iron-containing solutions with similar MRM dispersionscontaining dissolvedpotassium, sodium, or ammonium hydroxide. Separation of the resulting solid particles from the MRM suspension generally required a high base concentration and/or prolonged exposure to air to facilitate aggregation of the individual crystallites into particles large enough to settle. In a typical preparation of a ferric oxyhydroxide powder from an MRM solution, 1 2 g of SDS and 80 mL of 1.0 M aqueous solution containing Fe(NH4)(S04)2were mixed in a 2-L Erlenmeyer flask. To this slurry, l-L of 0.12 M AOT in isooctane was added with vigorous stirring and gentle heating. After approximately 30 min, an optically transparent, thermodynamically stablemicroemulsionwas obtained. An MRM solution containing sodium hydroxidein place of the metal salt was prepared similarly. Solid particle nucleation was initiated by combining equal volumes of the two MRM solutions. Approximately 10min after mixing the two solutions, the system became cloudy and changed color as the particles grew and aggregated to sufficient size to scatter visible light. After 30 min, the resulting suspensions were transferred to 500 cm3 Nalgene tubes and centrifuged a t 6000 rpm for 10 min. The remaining liquid was decanted off and the compacted powder was washed with isooctane, methylene chloride, and water to remove residual salts and surfactant. The products were recentrifuged after each washing. The resulting powders were dried under vacuum or flowing nitrogen, then ground in a mortar and pestle. A typical yield for the powder preparation as described was 8 g of dried powder. Ferrimagnetic powders were produced by calcination of the MRM-derivedferric oxyhydroxidepowders in air a t temperatures above 200 OC for short times (go%.
extent as an agent for decomposition of the model compound. However, little improvement in selectivity of the cleaved bond was noted relative to the results of the run in which neither catalyst nor sulfur was added (Table 4). Analysis of the products from test runs in which elemental sulfur and RTDS or MRM precursor powders were included in the reaction tube indicated that all of the iron-bearing precursor powderstested improved selectivity of the bond cleavage position relative to the control runs in which no iron-bearing species was present. It was clear, however, that the iron-bearing phase used and its purity strongly affected the conversion yield of the model compound under these test conditions. Among the RTDS precursor powders tested for activity toward model compound consumption, a broad range of catalytic activity was observed which was apparently related to the iron oxideloxyhydroxide phase used as a precursor. Results varied from essentially no catalytic activity for hematite and one of the two-line ferrihydrite samples (
