Influence of Nonionic Surfactant Concentration on Physical Characteristics of Resorcinol-Formaldehyde Carbon Cryogel Microspheres

Influence of Nonionic Surfactant Concentration on Physical Characteristics of Resorcinol-Formaldehyde Carbon Cryogel Microspheres Seong-Ick Kim, Takuji Yamamoto, Akira Endo, Takao Ohmori, and Masaru Nakaiwa National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received January 31, 2006; Accepted April 11, 2006 Abstract: Carbon cryogel microspheres (CC microspheres) are unique porous carbon particles that are synthesized by the inverse emulsion polymerization of a resorcinol-formaldehyde (RF) aqueous solution, followed by freeze-drying and pyrolysis in an inert atmosphere. The effects that the concentration of the nonionic surfactant (SPAN80) used in the inverse emulsion polymerization have on the porous properties of both CC microspheres and their carbon precursors, RF cryogel microspheres (RC microspheres), were studied. The effect of the concentration of SPAN80 on the particle size distributions of the the RC and CC microspheres was also investigated. The mesopore sizes and the mesopore volumes of the RC and CC microspheres increased upon decreasing the concentration of SPAN80. By varying the SPAN80 concentration, the peak radius of the mesopore size distribution of both the RC and CC microspheres can be controlled. The particle sizes of the RC and CC microspheres increased upon increasing the SPAN80 concentration. Keywords: carbon cryogel microsphere, mesoporosity, particle size, SPAN80, inverse emulsion polymerization Introduction 1)RF gels were first synthesized by Pekala and coworkers [1-3] through the sol-gel polycondensation of resorcinol [C 6 H 4 (OH) 2 ] (R) and formaldehyde (HCHO) (F) in an aqueous solution with sodium carbonate (Na 2 CO 3 )(C)as a basic catalyst. Carbon cryogels can be prepared through freeze-drying and pyrolysis of RF gels [4]. Because carbon cryogels possess high BET surface areas and large mesopore volumes, they can be used as, for example, column packing materials for HPLC, adsorbents for gas separation, materials for catalyst supports, and electrode materials for electric double layer capacitors and lithium ion batteries. Carbon cryogels that are prepared in the form of microspheres, with porous properties and particle sizes that can be controlled, are suitable for use as catalyst supporting materials in packed bed reactors and fluidized bed reactors because they possess high mechanical strength and high packing density. Emulsion methods have been To whom all correspondence should be addressed. (e-mail: yamamoto-t@aist.go.jp) used widely for the preparation of spherical inorganic and organic particles [5-8]. In our previous study, carbon cryogel microspheres (CC microspheres) with a high BET surface area and large mesopore volume were successfully synthesized by the inverse emulsion polymerization of a RF aqueous solution using a nonionic surfactant, SPAN80 (sorbitan monooleate, C 24 H 44 O 6 ) [9,10]. In this synthesis, the RF aqueous solution was the dispersed phase; cyclohexane containing SPAN80 was used as the continuous phase. SPAN80 was chosen as an emulsifier in that synthesis because the surfactant has a favorable HLB (hydrophilic-lipophilic balance) value for W/O emulsion. The mesoporosity of the CC microspheres was controlled by varying the concentration of the basic catalyst used in the sol-gel polycondensation [11]. However, precise control over the porous structure of the CC microspheres, by varying the concentration of the catalyst, was difficult, especially at low catalyst concentration, because a subtle change in the amount of the catalyst used for the preparation of the RF solution led to a large change in the mesoporosity of the obtained CC microspheres. It was also reported that the viscosity of the RF solution prior to inverse emulsion polymeri-

Influence of Nonionic Surfactant Concentration on Physical Characteristics of Resorcinol-Formaldehyde Carbon Cryogel Microspheres 485 zation had a strong effect on the particle size distributions of the microspheres obtained [12]. Because the viscosity of an RF solution rapidly increases, due to formation of a network structure in the last stage of a sol-gel polycondensation, depending on the concentration of the basic catalyst, it is often difficult to control the particle size distribution of the resulting microspheres. Based on these findings, a method that allows control over both the mesoporosity and the particle size distribution of the CC microspheres, but does not involve changing the concentration of the basic catalyst in initial RF solution, would be highly applicable for precisely controlling the porous characteristics and morphologies of CC microspheres. From this point of view, changing the composition of the continuous phase is more suitable for such precise control than is modifying the dispersed phase. The concentration of the nonionic surfactant (SPAN80) is considered to be the most significant component in determining the morphologies and porous characteristics of CC microspheres. The objective of the present study was to prepare CC microspheres with controlled porous structures and particle size distributions by varying the concentration of the nonionic surfactant, SPAN80, in inverse emulsion polymerizations. Specifically, the effect that changing the concentration of SPAN80 had on the porous properties of RC and CC microspheres was investigated. These studies aimed to elucidate the relationship between the surfactant concentration and the mesoporosities of both the RC and CC microspheres. The effect of the SPAN80 concentration on the particle size distributions of the microspheres was also examined. Experimental Preparation of RC Microspheres and CC Microspheres Using SPAN80 at Different Concentrations Resorcinol-formaldehyde (RF) solutions were prepared in the same manner as reported previously [9,11]. Sodium carbonate and pure water were used as a basic catalyst and a diluent, respectively, for the sol-gel polycondensation of resorcinol with formaldehyde. The ratios of resorcinol to formaldehyde and resorcinol to diluent were 0.5 mol/mol and 0.25 g/cm 3, respectively. The molar ratio of resorcinol to catalyst (R/C) was fixed at 400 mol/mol. The prepared RF solutions were kept at 298 K for 48 h. Just before the RF solutions lost their fluidity, they were dispersed into cyclohexane solutions containing SPAN80 at different concentrations (1.3, 2.5, 5, and 7.5 vol%), resulting in the formation of inverse emulsions. The concentrations of the RF solutions in cyclohexane were fixed at 10 vol%. The emulsion was agitated at a rate of 300 rpm for 24 h at 333 K to obtain Figure 1. Chemical structures of resorcinol and SPAN80. RF hydrogel microspheres. After exchanging the solvent in the porous RF hydrogel microspheres with t-butanol, the samples were freeze-dried at 263 K for 2 days to obtain RC microspheres. CC microspheres were obtained by pyrolysis of the RC microspheres at 1273 K for 4 h in an inert atmosphere. The chemical structures of resorcinol and SPAN80 are shown in Figure 1. Characterization of RC Microspheres and CC Microspheres The porous properties of the RC microspheres and CC microspheres were examined through nitrogen adsorption experiments using an automatic gas adsorption and desorption apparatus (BEL Japan Inc.; BELSORP mini). The adsorption and desorption isotherms of nitrogen were measured at 77 K. The BET surface areas and mesopore volumes of the samples were evaluated based on the nitrogen adsorption isotherms; the mesopore size distributions were determined by applying the Dollimore- Heal method [13] to the nitrogen desorption isotherms. The samples were observed with an optical microscope (Carton Ltd.; M9251CZNB). The particle size distributions of the microspheres were determined using a laser scattering particle size analyzer (Horiba Ltd.; LA-950). Results and Discussion Optical Micrographs of RC Microspheres and CC Microspheres Figure 2 shows optical micrographs of the RC and CC microspheres prepared using 1.3 vol% of SPAN80. It indicates that spherical RF cryogels were successfully prepared by the inverse emulsion polymerization of the RF aqueous solution in cyclohexane containing SPAN80

486 Seong-Ick Kim, Takuji Yamamoto, Akira Endo, Takao Ohmori, and Masaru Nakaiwa Figure 2. Optical micrographs of the (a) RC and (b) CC microspheres prepared using 1.3 vol% of SPAN 80. as an emulsifier and that the spherical shape of the RC microspheres was maintained after pyrolysis at 1273 K. The particle sizes of the CC microspheres are smaller than those of the RC microspheres; this size change is due to shrinkage of the networks during pyrolysis. Influence of SPAN80 Concentration on Porous Properties of RC and CC Microspheres Figure 3 shows isotherms for the N 2 sorption of the RC microspheres obtained using various SPAN80 concentrations during inverse emulsion polymerization. The mesopore size distributions of the RC microspheres are also shown. The sorption isotherms were classified into type-iv; hysteresis loops associated with capillary condensation in the mesopores were observed. The porous properties of the RC microspheres are shown in Figure 4. The mesopore sizes in Figure 3 and the mesopore volumes in Figure 4 decreased upon increasing the SPAN80 concentration. In contrast, the BET surface areas of the RC microspheres increased upon increasing the SPAN80 concentration. Figure 3 also shows that the peak radius of the mesopore size distributions changed in the range 4.6 6.9 nm upon varying the SPAN80 concentrations. These results indicate that the mesopore size of the RC microspheres can be controlled by varying the concentration of the nonionic surfactant being added to the disperse phase of the inverse emulsion polymerization. Figure 5 shows the nitrogen sorption isotherms of the CC microspheres. These isotherms were also type-iv, indicating that the mesoporosity of RC microspheres was maintained after pyrolysis. Based on Figure 5, the peak radii of the mesopores of the CC microspheres changed within the range 3.1 4.6 nm upon varying the surfactant concentration. The mesopore volumes and the BET surface areas of the CC microspheres (Figure 6) both decreased upon increasing the concentration of SPAN80. The results from Figures 5 and 6 indicate that the concentration of the surfactant used in the inverse emulsion polymerization is an important factor in determining the porous properties of RC and CC micro- Figure 3. Sorption isotherms of nitrogen on RC microspheres at 77 K and pore size distributions of RC microspheres; open symbols, adsorption; closed symbols, desorption. Figure 4. BET surface area and mesopore volume of RC microspheres obtained by inverse emulsion polymerization using SPAN80 as surfactant at different concentrations. spheres. The mesopore volumes and the mesopore sizes of the CC microspheres are smaller than those of the RC microspheres, due to shrinkage of the porous networks during pyrolysis [11]. Yamamoto and coworkers [11] reported that dehydration of the RF solution occurs during inverse emulsion polymerization, and that the amount of water in the cyclohexane phase clearly increases upon increasing the concentration of SPAN80. Therefore, it is expected that the concentration of Na 2 CO 3 used as the catalyst in the RF solution would increase upon increasing the SPAN80 concentration as a direct result of this dehydration during the emulsification. It was previously reported that the porous properties of RF gels and carbon gels depend on the concentration of the basic catalyst used in the sol-gel polycondensation [9,11,14,15]. From the results of SAXS measurements of RF solutions during the gelation process, Tamon and Ishizaka [16] confirmed that the catalyst

Influence of Nonionic Surfactant Concentration on Physical Characteristics of Resorcinol-Formaldehyde Carbon Cryogel Microspheres 487 Figure 5. Nitrogen sorption isotherm of CC microspheres at 77 K and pore size distribution of CC microspheres; open symbols, adsorption; closed symbols, desorption. Figure 6. BET surface area and mesopore volume of CC microspheres obtained by inverse emulsion polymerization using SPAN80 as surfactant at different concentrations. concentration is related to the size of the primary particles forming the cross-linked structure of RF hydrogels, and that the final sizes of the primary particles, which determine the mesopore size of the gels, are in inverse proportion to the catalyst concentration. Accordingly, the relationship between the concentration of SPAN80 and the mesoporosities of the RC and CC microspheres can be explained by considering the increase in the catalyst concentration in the water phase due to dehydration. Influence of SPAN80 Concentration on Particle Size of RC Microspheres and CC Microspheres Figure 7 shows the particle size distributions of the RC microspheres as measured by a laser diffraction particle size analyzer. The particle sizes of the RC microspheres increased upon increasing the surfactant concentration; the median diameters varied from 247 to 424 µm. The Figure 7. Particle size distribution of RC microspheres obtained by inverse emulsion polymerization using SPAN80 as surfactant at different concentrations: the numbers in parentheses refer to median diameters. particle size distributions of the CC microspheres are shown in Figure 8. The particle sizes of the CC microspheres also increased upon increasing the surfactant concentration, the median diameters varied in the range 185 414 µm. The relationship between the concentration of SPAN80 and the particle size of the RC and CC microspheres can be explained as follows. As mentioned above, the catalyst concentration during the sol-gel polycondensation of the RF solution increased upon increasing the SPAN80 concentration due to dehydration during the emulsification. This increase in the catalyst concentration also leads to a drastic increase in the viscosity of the RF solution, because the high concentration of catalyst accelerates the rate of gelation. According to a series of studies performed by Calabrese and coworkers [17,18], the size distributions of droplets and particles of the dispersed phase in an agitated liquid-liquid dispersion system change significantly upon changing the viscosity of the dispersed phase. Horikawa and coworkers [12] also reported that the size distribution of RF carbon aerogel particles increased upon increasing the apparent viscosity of the RF solution used for the emulsion polymerization. Accordingly, a high concentration of SPAN80 resulted in larger particle sizes, while a low concentration of the surfactant produced smaller particle sizes. Conclusions In this study, the influence that the concentration of SPAN80 had on the characteristics of both RC and CC microspheres was studied. The peak radius of the mesopore size distribution varied in the range 4.6 6.9 nm

488 Seong-Ick Kim, Takuji Yamamoto, Akira Endo, Takao Ohmori, and Masaru Nakaiwa Nomenclature C SPAN80 : concentration of SPAN80 in cyclohexane phase [vol%] d : particle diameter [µm] q : amount of adsorbed N 2 [cm 3 (STP) g -1 ] r p : pore radius [nm] S BET : BET specific surface area [m 2 g -1 ] V mes : mesopore volume [cm 3 g -1 ] V p : pore volume [cm 3 g -1 ] References Figure 8. Particle size distribution of CC microspheres obtained by inverse emulsion polymerization using SPAN80 as surfactant at different concentrations; the numbers in parentheses refer to median diameters. upon varying the SPAN80 concentration within the range 1.3 7.5 vol% in the cyclohexane phase. The mesopore size and the mesopore volume of the RC microspheres increased upon decreasing the concentration of SPAN80. The radii of the mesopores of CC microspheres changed within the range 3.1 4.6 nm upon varying the surfactant concentration. The mesopore volume and the BET surface area of the CC microspheres both increased upon decreasing the surfactant concentration. The particle sizes of the RC and CC microspheres increased upon increasing the surfactant concentration; the median diameters varied in the ranges 247 424 µm and 185 414 µm, respectively. From this study, it is obvious that the mesoporosity of CC microspheres can be controlled precisely by varying the concentration of the nonionic surfactant in the inverse emulsion polymerization, without changing the synthetic conditions of the initial RF solution. From the viewpoint of controlling the particle size distribution of CC microspheres, with precise control of the porous properties, our results offer a more effective method for preparing CC microspheres that may be suitable for some special or specific applications, when compared with conventional methods. Acknowledgments We are grateful for the financial support provided by the Japan Society for the Promotion of Science, Grantin-Aid for Young Scientists (B), No. 16710060 (2005). 1. R. W. Pekala, J. Mater. Sci., 24, 3221 (1989). 2. R. W. Pekala, C. T. Alviso, F. M. Kong, and S. S. Hulsey, J. Non-Cryst. Solids, 145, 90 (1992). 3. R. W. Pekala and D. W. Schaefer, Macromolecules, 26, 5487 (1993). 4. H. Tamon, H. Ishizaka, T. Yamamoto, and T. Suzuki, Carbon, 37, 2049 (1999). 5. Y. G. Lee, C. Oh, S. K. Yoo, S. M. Koo, and S. G. Oh, Micropor. Mesopor. Mater., 86, 134 (2005). 6. J. W. Kim, F. Liu, and H. J. Choi, J. Ind. Eng. Chem., 8, 399 (2002). 7. Y. Lee and Y. M. Hahm, J. Ind. Eng. Chem., 9, 713 (2003). 8. D. Tefanec and P. Krajnc, React. Funct. Polym., 65, 37 (2005). 9. T. Yamamoto, T. Sugimoto, T. Suzuki, S. R. Mukai, and H. Tamon, Carbon, 40, 345 (2002). 10. T. Yamamoto, A. Endo, T. Ohmori, and M. Nakaiwa, Carbon, 42, 1671 (2004). 11. T. Yamamoto, A. Endo, T. Ohmori, and M. Nakaiwa, Carbon, 43, 1231 (2005). 12. T. Horikawa, J. Hayashi, and K. Muroyama, Carbon, 42, 169 (2004). 13. D. Dollimore and G. R. Heal, J. Appl. Chem., 14, 109 (1964). 14. H. Tamon, H. Ishizaka, T. Araki, and M. Okazaki, Carbon, 36, 1257 (1998). 15. T. Yamamoto, T. Nishimura, T. Susuki, and H. Tamon, J. Non-Cryst. Solids, 288, 46 (2001). 16. H. Tamon and H. Ishizaka, J. Colloid Interface Sci., 206, 577 (1998). 17.R.V.Calabrese,T.P.K.Chang,andP.T.Dang, AIChE J., 32, 657 (1986). 18.R.V.Calabrese,C.Y.Wang,andN.P.Bryner, AIChE J., 32, 677 (1986).