Microemulsion-mediated Synthesis of Nanosize Molybdenum Sulfide Coal Liquefaction Catalysts

INTRODUCTION In heterogeneous catalysis, the size of the catalyst is of considerable importance in that the extent of reaction is often inversely proportional to the particle size of the catalyst. As a consequence, various methods have been used to synthesize particles of large specific surface area for catalysis applications (1-3). Most of these methods are based on hydrocolloids (1) and aerosols (2). Recently microemulsion-based synthesis is also attracting attention (3). Haruta et al. (1) prepared molybdenum sulfide particles in the micrometer size range by reacting ammonium molybdate and thioacetamide. Different particle sizes were obtained by varying the pH. In the preparation of iron sulfide catalysts for coal liquefaction, Andres et al. (2) used an aerosol-based method to synthesize iron oxide particles which were used in situ with carbon disulfide to liquefy coal. The liquefaction yield was found to be inversely proportional to the particle size. The synthesis of nanosize particles using inverse microemulsions has been reported for a variety of materials (4,5), among which are the hydrogenation catalysts nickel and cobalt borides (3). and platinum-group metals (6.7). Others worth noting are cadmium sulfide (4.8-lo), copper, lead, and indium sulfides (9.11). cadmium selenide (12). silica (5). and silver halide (13.14). In this communication a microemulsion-based method for the synthesis of molybdenum sulfide nanosize particles is reported for the Fist time. This material is currently under active investigation as a potential coal liquefaction catalyst (15). An inverse microemulsion is a thermodynamically stable, optically isotropic dispersion of microdrops of water in an external oil phase stabilized by a surfactant (1618). The microemulsion systems used in this study are: polyoxyethylene(5)nonylphenylether (NP-S)/cyclohexane/water and NP-S/tetralin/benzyl alcohoYwater. The latter system was used previously in this laboratory as a medium for the synthesis of silica nanoparticles (5). In these systems the water molecules interact with the hydrophilic portion of the surfactant molecules via hydrogen bonding to form inverse micelles. The addition of more water molecules results in the formation of swollen inverse micelles, often referred to as inverse microemulsions. The water pools inside the inverse micelles vary in size depending on the water-to-surfactant molar ratio (R) and are in the size range of 3-30 nm (16-18). Due to the cage-like nature of the water pools, particle growth is limited when particle precipitation is effected in them. Advantage has been taken of this unique property of inverse microemulsions to synthesize nanosize molybdenum sulfide particles in the size range 10-150 nm. The large surface area available on these nanoparticles results in a high yield of hexane-soluble oils when coal is liquefied (15). The chemistry pertaining to the formation of molybdenum sulfide is summarized below: MOO$+ 4S2+ 4H20 = MoS.42+ 8OH(1) (2) MoS42+ 2H+ = MoS3 + H2S