Ind. Eng. Chem. Res. 2006, 45, 1217-1220
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RESEARCH NOTES Production Scaleup of Reverse Micelle Synthesis Shannon A. Morrison,†,‡ Christopher L. Cahill,† Everett E. Carpenter,*,‡ and Vincent G. Harris§ George Washington UniVersity, Washington, District of Columbia 20052, Virginia Commonwealth UniVersity, Richmond, Virginia 23284, and Northeastern UniVersity, Boston, Massachussetts 02115
A wide range of techniques for the successful synthesis of nanosized materials have been developed recently. These procedures are sufficient for normal scientific investigation; however, for these materials to be incorporated into any practical application, the process for making them must be scalable to a larger volume. In this work, we focus on a published recipe for manganese zinc ferrite (MZFO) nanoparticles, which uses the reverse micelle synthesis technique. The normal bench-top synthesis has been scaled by a factor of 40 and successfully adapted to a 30-L pilot plant. The product of this synthesis is similar to the bench-top sample, which is also comparable to a ceramic MZFO standard. Through this work, we have demonstrated that the reverse micelle process is scalable to larger volumes. 1. Introduction During the past two decades, there has been a steady influx of synthetic strategies for the production of nanoparticles.1-6 Many of these methods, such as sol-gel, polyol, precipitation, and reverse micelle synthesis, have the potential for scaleup of the reaction mixture; yet, the problems associated with scaling up the process often have not been addressed.7 Techniques such as reverse micelle synthesis have been used to produce ferrite nanoparticles with narrow size distributions and superior magnetic properties, as compared to bulk ceramic samples;8 yet, because of the limits of laboratory-scale reactions, they have not been fully tested and/or evaluated in device applications. To fully utilize this versatile synthetic technique, it is important to address some of the concerns and demonstrate the scalability of reverse micelle reactions. Reverse micelles are pools of water stabilized by surfactant along the surface of the spheres in a volume of hydrocarbon. In this technique, the ratio of water and surfactant controls the size of the particles and, because of the uniformity of the stabilized water pools, ultimately provides a fairly narrow size distribution. The focus of this work is the development of a scale-up reaction process for the synthesis of magnesium zinc ferrite (MZFO) that has been presented by Morrison et al.9 In that work, initial reactions are performed to address stoichiometry and crystal structure and obtain a synthetic recipe for the synthesis of MZFO. 2. Experimental Section The scaleup of a previously reported synthesis of MZFO9 was undertaken where bis-(2-ethylhexyl) sodium sulfosuccinate (AOT) was used as the surfactant with 2,2,4-trimethylpentane as the oil phase. Stock solutions of AOT (C8) were prepared ahead of time by mixing 1 kg of AOT with a 4-L bottle of 2,2,4-trimethylpentane. A bench-top sample, which, hereafter, * To whom correspondence should be addressed. Tel.: 804 828 7508. Fax: 804 828 8599. E-mail:
[email protected]. † George Washington University. ‡ Virginia Commonwealth University. § Northeastern University.
will be referenced as type A, was prepared for comparison at a reaction volume of 1 L. The metal salts FeCl2, ZnCl2, and MnCl2 were used to provide the metal cations, and ammonium hydroxide was used to adjust the pH. Based on previous experiments, the ratio of the metal salts was chosen so that the relative amounts of manganese, zinc, and iron in the final product would be 1:1:4. For sample A, the metal salts were measureds0.36 g of FeCl2, 0.11 g of MnCl2, and 0.10 g of ZnCl2sand then dissolved in 40.0 mL of degassed water. The aqueous metal was then mixed with 240.0 mL of the AOT (C8) solution until the mixture was no longer turbid. A similar mixture of 17.0 mL of concentrated ammonium hydroxide and 17.0 mL of water was prepared and added to 250.0 mL of the AOT (C8) solution. The ammonia reverse micelle mixture was then added dropwise to the aqueous metal reverse micelle mixture and allowed to react for a total of 2 h. After the reaction had proceeded for 2 h, a volume of methanol (∼400 mL) was added while still stirring the mixture. Aliquots were placed in centrifuge tubes (50-mL volume, six tubes, at 6500 rpm for 10 min), and then the solutions were decanted and the solid material was redispersed in methanol and centrifuged again. This process was repeated with successive washings, using methanol, methanol/water mixtures (75/25, 50/50, and 25/75), and finally water for the remaining four washes. After the initial material was centrifuged the final time and the supernatant was removed, the open tubes were placed in a vacuum chamber, and the process was repeated for any remaining product. The dried nanoparticles were then annealed at 525 °C for 5 h under flowing nitrogen in a tube furnace. Pilot-plant batches (referenced hereafter as sample P) of MZFO were produced using a 30-L unjacketed reactor system (Figure 1) from Chemglass, Inc. The recipe for the pilot-plant synthesis was a scaleup of the bench-top synthesis to a volume of 20 L.9 For these syntheses, the AOT and 2,2,4-trimethylpentane solution was premixed in the pilot plant, using the aforementioned ratios, to produce a volume of 21 L. This solution was degassed using nitrogen gas for at least 30 min, and then 7 L of the AOT (C8) solution was removed. An
10.1021/ie050886l CCC: $33.50 © 2006 American Chemical Society Published on Web 01/04/2006
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Figure 1. Diagram of the pilot plant used in this study.
aqueous metal solution was prepared using 20.8 g of FeCl2, 6.8 g of MnCl2, and 5.5 g of ZnCl2, mixed with 2.3 L of degassed water. The aqueous metal solution was then added to the 14 L of AOT (C8) in the pilot plant, and this was stirred until the mixture was no longer turbid. This required a period of ∼10 min and, during this time, the solution turned a dull yellow color, suggesting the conversion of some divalent iron (Fe2+) to trivalent iron (Fe3+). The remaining 7 L of AOT/oil was mixed with 0.58 L of concentrated ammonia and 0.58 L of water, using an industrial mixer for 5 min, and then it was allowed to settle until the mixture was no longer turbid (∼15 min). This solution was then added to the pilot plant, using a peristaltic pump at its maximum rate of 1.7 L/min, while the system was maintained under nitrogen. After the reaction had proceeded for 2 h, 5 L of methanol was added and mixed with the reverse micelle solution for 15 min. The entire mixture was then allowed to sit for 1 h while the MZFO precursor particles precipitated out of the solution. The top layers of the supernatant were removed and another 10 L of methanol was added and mixed with the particles. The MZFO particulate was allowed to settle out overnight, while under flowing nitrogen, because of the impracticality of centrifuging 20 L in a laboratory-sized centrifuge. The following day, the supernatant was removed using the peristaltic pump, with the particulate material occupying the remaining 2 L of volume. This layer was removed in 500-mL portions for cleaning. The initial wash/centrifugation cycle was methanol, with subsequent washes being methanol/water (50/50 mixture) and then water for three more washes. After the final centrifuge, the supernatant was removed and the material placed in a vacuum chamber to dry. The dried product was then annealed
Figure 2. (a) Transmission electron microscopy (TEM) micrograph of sample A2, showing the agglomeration of particles after firing. (b) Highresolution TEM image, where the diffraction from the crystal lattice indicates the extent of crystallinity of the nanoparticles.
at 525 °C for 5 h under flowing nitrogen to produce the final MZFO nanoparticles. A reference standard of MZFO, denoted herafter as sample S, was produced using ceramic techniques.9 The standards were synthesized by a traditional ceramic method, as described by Rao.10 The metal oxides MnO, ZnO, and R-Fe2O3 were mixed using ball milling and then fired for 20 h at 1150 °C. This process was repeated for a total of 5 times until X-ray diffraction (XRD) indicated no further change in the material. 3. Results High-resolution transmission electron microscopy (TEM) (TECNAI model F20) was used to characterize the morphologies of the annealed nanoparticles. The MZFO nanoparticles were suspended in methanol and agitated in an ultrasonic bath. The TEM sample was prepared by placing a few drops of this suspension on a holey carbon film. The particle size distribution was estimated based on analyzing the bright-field images of randomly selected nanoparticles. The average particle size, as seen in Figure 2, was estimated to be 20 nm. TEM shows that the nanoparticles agglomerate during the annealing. As seen in Figure 2b, the nanoparticles are single crystallites with very little of the dead volume that is normally observed due to amorphous layers on the surface. The elemental composition of the sample was determined using inductively coupled plasma-optical emission spectrometry (ICP-OES) (PE Optima model 4000). Samples for ICP-OES were prepared by dissolving the sample in concentrated nitric acid in an acid digestion bomb at 170 °C. The stoichiometry of S was determined to be Mn0.55Zn0.52Fe1.93O4. The ratio of metals
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Figure 3. Comparison of X-ray diffraction (XRD) results for the benchtop and pilot-plant samples.
Figure 5. Magnetization hysteresis loops taken at 10 K for samples A2, P2, and S.
Figure 4. Magnetization hysteresis loops taken at 300 K for samples A2, P2, and S.
Figure 6. XRD of samples P4 and P5, compared to samples P1 and S, indicating the stability of the as-synthesized nanoparticles over time.
in the final nanoparticles can be controlled by the starting concentrations and the pH. A normal stoichiometry of pilotplant samples was Mn0.60Zn0.50Fe1.90O4. Powder X-ray diffraction (XRD) was performed on the MZFO particles, using a Phillips X-ray diffractometer that used Cu KR radiation from a sealed tube (50 kV, 30 mA, 0.02 step size) source. Analysis of the material revealed a match with the ceramic MZFO standard (Mn,Zn)Fe2O4, indicating the manganese zinc ferrite structure. Figure 3 shows that the pilotplant sample (P1) has broad XRD peaks, which is indicative of small ferrite crystallites.11 When fired at 525 °C for 5 h, the XRD peak positions of sample P2 show good agreement with those from the bench-top sample A1 and the ceramic standard, S. The narrowing of the peaks correlates to an increase in the size of the crystalline region within the particles; however, there is still some peak broadening, when compared to sample S, because the particles are nanosized. Scherrer calculations based on these measurements indicate crystallite sizes of ∼15 nm for all of the nanoparticle samples. Magnetic properties were measured using a Quantum Design model MPMS-5S SQUID magnetometer. Samples were exposed to an applied field of up to 3 T at temperatures of 300 and 10 K. Evaluation of the hysteresis loops at 3 T provides a saturation magnetization of 65 emu/g for sample P2 and 53 emu/g for sample A2. As can be seen in Figure 4, this is a significant increase in saturation magnetization (32 emu/g) from the ceramic standard (sample S). This trend does not hold at low temperatures, as the graph in Figure 5 illustrates; the
saturation magnetizations at 10 K for sample P2 (91 emu/g) and sample A1 (81 emu/g) are within 6% of the S value of 86 emu/g. Extended X-ray absorption fine structure (EXAFS) spectra were collected at the National Synchrotron Light Source at Brookhaven National Laboratory, using the NRL Consortium Beam Lines ×23B and ×11A. A detailed description of the data collection methods and the analytical methodology can be found in Calvin et al.12 EXAFS indicated that the material is single-phase spinel, in agreement with the XRD results.13 4. Discussion The primary difference in the synthesis technique in going from the bench-top level to the pilot-plant level was in the cleanup time. The quantity of material produced in the pilot plant made it prohibitive to use the laboratory centrifuge for each wash. For this reason, the initial washes were performed in situ and the particles were allowed to settle out. This step was generally performed twice in 1 day, meaning it took at least 2 days from initial mixing to the dried powder, whereas the benchtop synthesis, from start to dried powder, could be performed in ∼4 h. XRD of the pilot-plant material (Figure 3) indicated that it was stable during this period; however, this observation raised the question of whether the material would be fine if annealed at a later date. After sample P3 was prepared, some of the material was set aside and annealed 4 days later (sample
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P4) and 9 days later (sample P5). Both materials still produced a clean spinel crystal structure, as observed in the XRD results in Figure 6. An additional concern was the reusability of the various solvents used in the synthesis. In our preliminary work, the solvents were recycled with 90% efficiency, using simple distillations. We recovered both 2,2,4-trimethylpentane and methanol, which were reused in later pilot-plant syntheses as the oil phase and wash solvent, respectively. With a better recovery process and the appropriate equipment, it should be feasible to recover and purify the solvents and surfactant for use in subsequent reactions. 5. Conclusions We have developed a methodology for the synthesis of magnesium zinc ferrite (MZFO) nanoparticles that applies the reverse micelle technique. In scaling the bench-top recipe to a volume ∼40 times greater, we demonstrated that the synthesis is not limited to small volumes. A low-temperature annealing step is required to crystallize the individual particles completely, and the synthesis and isolation process should be scalable through a batch process to an industrial level. Nanoparticles produced in the pilot plant display the same properties as those made on the benchtop. The only limitation in the larger-scale production is the clean up of the nanoparticles, which could easily be overcome with equipment designed to handle this quantity of material. Even if a protracted clean-up step is necessary, the material remains stable over a period of weeks at least, making it a viable process without the addition of nontraditional laboratory equipment. Currently, the cost of the synthesis, excluding time and labor, is primarily determined by the cost of the surfactant, hydrocarbon, metal salts, and wash solvents. Through the recycling of surfactant and solvents, it is possible to reduce the starting material costs by up to 95%, making the cost of wet-chemical-synthesized MZFO comparable to traditionally synthesized ferrite materials.
Literature Cited (1) Wu, M.; Long, J.; Huang, A.; Luo, Y.; Feng, S.; Xu, R. Microemulsion-mediated hydrothermal synthesis and characterization of nanosized rutile and anatase particles. Langmuir 1999, 15, 8822-8825. (2) Chen, X. Z.; Dye, J. L.; Eick, H. A.; Elder, S. H.; Tsai, K. L. Synthesis of Transition-Metal Nitrides from Nanoscale Metal Particles Prepared by Homogeneous Reduction of Metal Halides with an Alkalide. Chem. Mater. 1997, 9, 1172-1176. (3) Petit, C.; Lixon, P.; Pileni, M. P. In situ synthesis of silver nanoclusters. J. Phys. Chem. 1993, 97 (49), 129. (4) Chen, Q.; Rondinone, A. J.; Chakoumakos, B. C.; Zhang, Z. J. Synthesis of superparamagnetic MgFe2O4 nanoparticles by coprecipitation. J. Magn. Magn. Mater. 1999, 194, 1-7. (5) Fang, J.; Wang, J.; Ng, S. C.; Chew, C. H.; Gan, L. M. Preparation and characterization of ultrafine lead titanate powders. J. Mater. Sci. 1999, 34, 1943-1952. (6) Toprak, M. S.; Zhang, Y.; Jo, Y.; Kim, D. K.; Muhammed, M. Nanocrystalline Thermoelectric Alloys: Processing, Chemistry, and Characterization. Diffus. Defect DatasPt. B 2005, 101-102, 197-204. (7) Masala, O.; Seshadri, R. Synthesis Routes for Large Volumes of Nanoparticles. Annu. ReV. Mater. Res. 2004, 34, 41-81. (8) Morrison, S. A.; Carpenter, E. E.; Harris, V. G.; Cahill, C. A. Atomic Engineering of Magnetic Nanoparticles. J. Nanosci. Nanotech. 2005, 5 (9), 1323-1344. (9) Morrison, S. A.; Cahill, C. L.; Carpenter, E. E.; Calvin, S.; Harris, V. G. Preparation and characterization of MnZn-ferrite nanoparticles using reverse micelles. J. Appl. Phys. 2003, 93 (10), 7489-7491. (10) Rao, C. N. R., Chemical Approaches to the Synthesis of Inorganic Materials; Wiley: New York, 1994. (11) West, A. Solid State Chemistry and Its Applications; Wiley: West Sussex, U.K., 1984; p 734. (12) Calvin, S.; Carpenter, E. E.; Ravel, B.; Harris, V. G.; Morrison, S. A. Multiedge Refinement of Extended X-ray-Absorption Fine Structure of Manganese Zinc Ferrite Nanoparticles. Phys. ReV. B 2002, 66, 224405.1224405.13. (13) Calvin, S.; Carpenter, E. E.; Harris, V. G.; Morrison, S. A. Use of Multiple-Edge Refinement of Extended X-ray Absorption Fine Structure to Determine Site Occupancy in Mixed Ferrite Nanoparticles. Appl. Phys. Lett. 2002, 81 (20), 3828-3830.
ReceiVed for reView July 28, 2005 ReVised manuscript receiVed December 5, 2005 Accepted December 13, 2005 IE050886L
