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Ind. Eng. Chem. Res. 2008, 47, 2096-2099
Conversion of Nanomaterial Waste Soot to Recycled Sc2O3 Feedstock for the Synthesis of Metallic Nitride Fullerenes S. Stevenson,*,† Curtis E. Coumbe,† M. Corey Thompson,† H. Louie Coumbe,† J. Paige Phillips,† James L. Buckley,‡ and James H. Wynne‡ Department of Chemistry and Biochemistry, UniVersity of Southern Mississippi, Hattiesburg, Mississippi 39406, and Chemistry DiVision, U.S. NaVal Research Laboratory, Washington, District of Columbia 20375
Herein, we address a need in the industrial and academic communities to reduce costs and environmental impact associated with the synthesis of select carbonaceous nanomaterials. In this effort, we have developed a method to recover Sc2O3 from carbonaceous “waste soot”, thereby alleviating the problem of waste disposal of fullerene depleted soot and tremendously reducing the costs and environmental impact of our synthetic process. The recovery process is based on the thermal oxidation and removal of carbon from waste soot as gaseous byproducts (e.g., CO2) to yield a recycled, reusable feedstock. The economic impact is measured in the cost savings of solid waste disposal fees and expensive reagents, such as scandium and some rare-earth metal oxides. Our recovery method is scalable and simple in design. Waste soot at different stages of thermal oxidation is characterized by thermogravimetric analysis (TGA) to determine optimal temperature and soak parameters. Corresponding X-ray photoelectron spectroscopy (XPS) analysis of these samples indicates a comparable chemical composition of Sc2O3 for recycled samples to virgin Sc2O3 controls. Recovered Sc2O3 material was used in our electric-arc reactor and resulted in statistically comparable fullerene product distributions. 1. Introduction The recent discovery of a new class of materials1 with entrapped metallic nitride clusters in fullerene cages has spawned significant interest in the scientific community. As shown in Figure 1, the prototypical metallic nitride fullerene (MNF) has the generic chemical formula, M3N@C80, where “M” is a Group IIIB transition metal or rare-earth metal.1-5 Although originally an academic curiosity, fundamental research has given way to application research and development (R&D) in academic and industrial research groups worldwide. Much of this interest in MNFs stems from the ability to match the application area with the judicious selection of entrapped metals. Examples include pharmaceutical development of Gd3N@C80 as magnetic resonance imaging (MRI) contrast agents,6,7 Lu3N@C80 as an X-ray contrast agent,8 and Ho3N@C80 as activated radiopharmaceuticals. Nonmedical research and development includes the use of Tb3N@C80 and Er3N@C80 in optical devices9 and Sc3N@C80 in electronic applications.10 In comparison, application development using empty-cage fullerenes (e.g., C60, C70) has been hampered by the absence of entrapped metals that would impart useful properties. Of the MNF family, the highest yielding metallic nitride fullerene is Sc3N@C80. Because of its ease of synthesis, purification,11-13 and availability, Sc3N@C80 is often used as a “practice” compound for developing MNF functionalization chemistry6,7,14-18 of the carbon cage surface. This is especially true for Gd3N@C80,4,19 whose yield is typically 1-2 orders of magnitude lower than Sc3N@C80. Ironically, the two highest yielding MNFs (Sc3N@C80, Lu3N@C80) have the most expensive metal oxide feedstock at ∼$2000.00-$3000.00/kg. For an electric-arc reactor using 1 in. metal oxide packed graphite rods, 1 kg of feedstock will produce only ∼40 packed rods for subsequent vaporization and transformation to soot extract. Of * Corresponding author e-mail:
[email protected]. † University of Southern Mississippi. ‡ U.S. Naval Research Laboratory.
Figure 1. Structure of a Sc3N@C80 metallic nitride fullerene (MNF) extracted from soot.
the subsequent 20 g of extract from 40 rods, only ∼1 g of Sc3N@C80 MNF is produced from 1000 g of feedstock. Hence, 99.9% of the Sc2O3 feedstock being discarded as solid waste after extraction of fullerenes and MNFs. This “waste soot” contains ∼50% by mass of metal content (e.g., Sc metal, Sc2O3) with the remaining ∼50% carbon (e.g., mostly amorphous soot carbon). In our recent paper,20 we have described the environmental impact of discarding scandiumcontaining waste soot and demonstrate a proof-of-concept to recover, recycle, repack, and reuse this recycled Sc feedstock. In this work, we now report and (1) determine optimal thermal parameters via thermogravimetric analysis, (2) utilize X-ray photoelectron spectroscopy to characterize the chemical composition of waste soot at various stages of thermal oxidation, and (3) analyze fullerene product distributions of first- and second-generation soot extracts. 2. Experimental Details Dimensions of the cylindrical electric-arc reactor were 43 cm long and 25 cm in diameter. Prior to vaporization, graphite rods were core-drilled and packed with either Sc2O3 powder (325 mesh, Stanford Materials, CA) for first-generation experiments or recovered Sc-based material from our recycling of waste soot for second-generation experiments. Prior to use in the reactor,
10.1021/ie071250s CCC: $40.75 © 2008 American Chemical Society Published on Web 02/20/2008
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 2097
Figure 2. New production and waste recovery process for recycling scandium.
these packed rods were preheated in a furnace at 1050 °C under He flow for 8 h. Reactor parameters were 300 Torr He, 6 Torr air/min, 220 A, and 38 V. Resulting soot was extracted with o-xylene, and the solvent was removed under reduced pressure to furnish a dried extract, which was washed with solvent (e.g., diethyl ether, acetone). Soot extracts were characterized by highperformance liquid chromatography (HPLC) to determine the type and amount of fullerene material present. Standard HPLC integration software was used to calculate peak areas. HPLC separations were performed on a PYE column (Phenomenex, 4.6 mm x 250 mm) using 1.0 mL/min toluene mobile phase, 360 nm UV detection, and 50 µL injection. The effect of thermal oxidation on waste soot was monitored using thermogravimetric analysis (TGA) to investigate weight changes as a function of temperature and time under a controlled atmosphere. Waste soot was homogenized with a mortar and pestle, and 5-10 mg samples were analyzed in platinum pans under an air environment and at temperature ranges of 221000 °C. TGA analysis was performed with a TA Q500 series instrument. A heat-and-hold method was employed, and the temperature was ramped at a rate of 28 °C/min to 600, 750, and 1000 °C and held for 60 min. Samples of Sc2O3 and waste soot at various stages of thermal oxidation were characterized by X-ray photoelectron spectroscopy (XPS). Acquisition of XPS data was obtained with a Perkin-Elmer 5400 X-ray photoelectron spectrometer. For bulk recovery of Sc material from waste soot, a standard laboratory furnace was used, and the effluent was vented. For these experiments, no significant particulates were observed in either the furnace’s effluent trap or within its interior upon completion. 3. Results and Discussion An overview of our process for recovering Sc material from waste soot is shown in Figure 2. With this scheme, either Sc2O3 or recycled Sc material is packed in a cored rod (stage 1) and subsequently vaporized in an electric-arc reactor to generate soot (stage 2). The composition of produced soot is a mixture of amorphous carbon, metal, metal oxide, and
