Size control and characterization of spherical carbon aerogel particles from resorcinol–formaldehyde resin
Introduction
Various types of carbon materials, such as activated carbon, molecular sieving carbon, and catalyst carrier, have been produced [1], [2], [3], [4], [5], [6], [7], [8], [9]. The carbon aerogel is one of the unique carbon materials, which has an unusual pore volume. Organic aerogels and carbon aerogels are novel aerogels prepared in the resorcinol–formaldehyde (RF) system as reported by Pekala [10], [11] and Pekala et al. [12]. Since then much research on organic aerogels and carbon aerogels has been briskly carried out [13], [14], [15], [16], [17], [18], [19]. Due to the unique function of the RF carbon aerogels related to their excellent pore characteristics, their promising applications as an adsorbent, catalyst carrier, porous electrode, etc. are expected. Though various applications of the RF carbon aerogels can be considered, how to control and tailor their form and pore properties for specific applications is a difficult task.
In the present study, we aimed to prepare discrete RF carbon aerogel particles with a spherical shape and control their diameter. We synthesized spherical RF aerogel particles by emulsion polymerization followed by supercritical drying with carbon dioxide [20], [21]. The RF carbon aerogel particles were generated by carbonizing the RF aerogel particles at a high temperature under a nitrogen atmosphere. We investigated the influence of the RF sol apparent viscosity on the particle form and particle size distribution, changing the apparent viscosity of the RF sol to be supplied to the cyclohexane solvent containing a surface-active agent during an emulsion polymerization processing. We also investigated the carbonization temperature effect on their pore characteristics.
Section snippets
Synthesis of spherical RF hydrogels
RF hydrogels were synthesized by the polycondensation of resorcinol (R) and formaldehyde (F), using potassium carbonate (K2CO3) (C) as the basic catalyst, and distilled water (W) as the diluent. Spherical RF hydrogel particles were obtained by emulsion polymerization; a viscous RF sol of 20 cm3 was put in a 1000 cm3 cyclohexane solvent mixed with a 10 cm3 surface-active agent (non-ionic sorbitan alkyl ester series, NOF Corp.) under a hydrophile–lipophile balance (HLB=4.3), and the suspension
SEM image and size distribution of RF carbon aerogel particles
Fig. 2a–d shows the SEM images of the particle forms of each sample. These samples were prepared at a stirring speed of 400 rpm for the RF sol apparent viscosities of 0.2, 10, 100, and 500 mPa s, respectively. All samples of the RF hydrogel particles were dried with supercritical carbon dioxide and carbonized at 1073 K for 30 min. The produced RF carbon aerogel particles were all fine spheres, and increased in size with the increasing apparent viscosity of the RF sol.
Fig. 3 shows the variation
Conclusions
We could successfully prepare RF carbon aerogel particles with a truly spherical shape, uniform mesopores, and high BET surface areas, and we could control the particle size of the RF carbon aerogel particles by changing the RF sol apparent viscosity and the stirring speed during the emulsion polymerization processing. The apparent viscosity of the RF sol to be supplied and the stirring speed in the emulsion polymerization processing have significant effects on the size of the generated RF
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