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Iron Sulfide Catalysts for Coal Liquefaction Prepared Using a Micellar Technique Ajay Chadha, Ramesh K. Sharma, Charter D. Stinespring, and Dady B. Dadyburjor* Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506-6102
We have recently synthesized nanometer-size iron sulfide catalysts using a reverse micellar system. These particles are 40-70 nm in size and were used in laboratory-scale coal-liquefaction experiments. The catalyst particles were impregnated in situ on coal particles. The catalyst loading was 1.67% with respect to coal. The liquefaction run was carried out at 400 °C for 30 min, at a pressure of 1000 psia H2(g) measured at ambient temperature (corresponding to approximately 2000 psia at reaction conditions), in the absence of any solvent or hydrogen donor. The total conversion, as well as the yields of asphaltene plus preasphaltene and oil plus gas, increased after the run, relative to a thermal (noncatalytic) run. The activity of the micellar catalyst is slightly less than that of a nonmicellar catalyst. However, a slightly higher selectivity to oil plus gas is observed with the micellar catalyst. Introduction Recent emphasis has been placed on the development of low-cost “throw-away” iron-based catalysts that would provide the necessary activity for reactions involving low-value, high-volume, solid feedstocks such as coal, without the need to recover the catalyst. In the first stage of the direct liquefaction of coal, the intent is primarily to increase the total conversion of the coal, with only secondary regard to increasing the yield or selectivity of the oil fractions formed. Numerous measures have been taken to enhance the activity of these first-stage catalysts. Increasing the surface area is one of the possible ways. Therefore, an increasing interest has been shown by researchers to develop newer techniques to synthesize small-size catalyst particles, thereby increasing the catalyst surface area and the catalyst dispersion. This allows effective contact of reactants with the catalyst surface, even at low catalyst loading. Our research efforts have been directed toward the use of ferric sulfide as a precursor of catalysts for the first stage of the direct liquefaction of coal. The ferric sulfide (Fe2S3) disproportionates into FeS2 (pyrite, PY), nonstoichiometric FeSx with x ≈ 1 (pyrrhotite, PH), and elemental sulfur (S). The relative amounts of PH and PY and the kind of PH formed (i.e., the value of x) depend upon the time, temperature, gas phase, and mode of disproportionation (Stansberry et al., 1993; Dadyburjor et al., 1994; Liu et al., 1996). The objective of the present work is to synthesize these particles using a microemulsion-based technique, to use them in laboratory-scale direct-coal-liquefaction (DCL) runs, and to compare the performance of the microemulsion-based catalyst to that of our earlier catalysts. Background A microemulsion is a thermodynamically stable dispersed system comprised of at least three components: two immiscible components (a hydrophilic component, e.g., water, and a lipophilic component, e.g, oil) and the third component, usually a surfactant, which stabilizes the system. The phase behavior of microemulsions * To whom correspondence should be addressed. Telephone: (304) 293-2111 ×411. Fax: (304) 293-4139. Email:
[email protected].
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shows that a variety of phases can exist in equilibrium. Each of these phases has a different structure; see, e.g., Smith et al. (1993). In general, when the concentration of the hydrophilic constituent is relatively small, a water-in-oil (W/O)-type microemulsion results. An oilin-water (O/W)-type microemulsion results when the concentration of the hydrophilic constituent is increased past a certain minimum amount. For the synthesis of nanometer-size particles, W/Otype microemulsions are relatively more important (Matijevic, 1993). The concentration of water plays an important role in determining the structure of these microemulsions. In the absence of water or in the presence of a very small amount of water, surfactants dissolved in organic solvents form spheroidal aggregates called reverse micelles. In the presence of larger amounts of water (but not large enough to form O/Wtype microemulsions), the aggregates formed are relatively large and often nonspherical, and the size increases as the molar ratio of water to surfactant (R) increases. In general, reverse micelles correspond to R < 15 (Pileni, 1993). Reverse micelles are relatively small, of nanometer size. Therefore, they can be used as excellent sites to synthesize small particles. Individual micelles can act as nanoscale reactors, in which a desired reaction for particle synthesis can be carried out. Ruckenstein and Karpe (1991) have used reverse micelles to encapsulate enzyme catalysts. Lopez-Quintela and Rivas (1993) have recently proposed a mechanism for the formation of particles in reversed micellar systems. Two microemulsions, each containing an appropriate stoichiometric concentration of one of the precursors A and B, are mixed together. Interchange of reactants in the microemulsion takes place due to collision of micelles. This interchange is very fast, and a virtually instantaneous reaction takes place inside the droplet. The surfactant molecules attached to the surface of the particles prevent excessive growth of the particles and thereby limit the particle size. Hence, the size of the droplets controls the size of the particles. After the formation of the small-sized particles in the reverse micelles is completed, an additional component can be added to break up the reverse micelles. This makes the particles available for use. The magnitude of the concentrations of reactants A and B is important for the formation of particles or the © 1996 American Chemical Society
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(Lopez-Quintela and Rivas, 1993). Changes in this system with pressure and temperature have been studied by Karayigitoglu et al. (1994). Iron colloid catalysts using similar systems have been synthesized and characterized by Martino et al. (1995). Experimental Section
Figure 1. Shape of an AOT molecule forming reverse micelle (Pileni, 1993).
Figure 2. Phase diagram for AOT-water-isooctane (Rosen, 1989).
micellar system itself. High concentrations of precursors (typically greater than 0.1 M) can lead to instability of the microemulsion system. Then the reaction between the precursors may take place outside the micelles. The size of the particles synthesized is not controlled in this case. At lower concentrations (
