Carbon Aerogel Beads: Preparation, Characterization and Upscaling

Abstract

In the last several years, there has been an increased interest among researchers in the development of carbon based materials such as activated carbon, carbon beads (micro- and milli-meter sized) and microspheres, carbon nanotubes, graphene, fullerene, etc. Carbon based materials exhibit unique structures, morphologies, properties and so on. Among these materials, carbon aerogels (CAs) are extraordinary materials with high surface area, high porosity with unique micro-structures consisting of micro-pores, etc. CAs find applications in catalysis, chromatography, energy, adsorbents, desalination, etc. Currently, researchers are focused on the development of CAs in the form of spheres and beads instead of monoliths due to the reduction in processing time and equipment costs. Carbon beads (CBs) are produced conventionally from coal or petroleum pitch by carbonizing them at 350-500 °C to achieve mesophase spheres in pitch. The carbonization process is quenched before extensive growth and aggregation of the spheres occurs, and the CBs are separated as solids from soluble pitch by extraction with a solvent. Carbon microspheres of 1 mm to 3 mm are also fabricated by using an inverseemulsion gelation of resorcinol-formaldehyde (RF) solution at high temperatures followed by super-critical drying and subsequent carbonization. The removal of oil and subsequent super-critical drying consume large amount of solvents, energy and time. Hence, there is an increasing demand for an easy process to form CBs. In the current work, we utilized sol-gel chemistry to form RF aerogel beads after ambient pressure drying and converted them using a thermal treatment process into carbon beads. The existing challenge is the shaping of RF sol into beads by a dropping method, which has not been previously reported in the literature because of the low sol viscosity (~ 0.5 Pa.s). Hence, we adjusted the viscosity of the RF sol by adding a polysaccharide-based thickener to achieve RF bead formation (1-10 Pa.s), followed by sub-critical drying and carbonization. The average sizes of the carbon beads are in the range of 1.75-2.02 mm with skeletal densities of 2.0-2.4 g/cm3. The micropore surface area and pore volume are ranging from 735-901 m2/g and 0.28-0.35 cm3/g, respectively. SEM images reveal that the beads are spherical with an inner-morphology composed of inter-connected particles grown on the top of the thickening agents’ fibers. The large scale production of RF aerogel beads by a Jet-Cutting method has several challenges viz., handling of enormous amounts of RF sol and acid bath, process parameter optimization, waste disposal, etc. Hence, we adopted low to high viscous alginate solution (0.5 - 3wt.%) with a viscosity ranging from 0.08 to 10 Pa.s, as a model system to optimize the Jet-Cutting parameters. The diameter of the beads (dbead) is described by: where D is the diameter of the nozzle; ufluid the fluid velocity; n the number of cutting wires; Z the number of cutting tool rotations; and dwire the diameter of the wire. The impact of key process parameters, including the fluid velocity, diameter of the nozzle and number of cutting tool rotations were explored and optimized, leading to the large scale production of RF beads. The properties of the beads will be described.