Microporous Carbon Aerogel Microbeads Produced via Sol-Gel Synthesis and Pyrolysis of Resorcinol-Formaldehyde Aerogel Microbeads by Dropping Method

Abstract

Over the last few decades, different carbon materials for instances activated carbon, carbon nanotube, fullerene, graphene, carbon microbeads and microspheres, have been received considerable attention among researchers. These carbon materials exhibit unique structures, morphologies and properties. Among those materials, carbon microbeads could serve as an excellent candidate for different applications such as water purification, CO2 capture, catalysis, batteries, super capacitors, etc. Several approaches have been employed to synthesize carbon microbeads in spherical form. The most common approach to prepare mesophase carbon beads is the liquid phase carbonization of aromatic hydrocarbon such as coal tar or petroleum pitch and heavy oil residue at elevated temperatures (350-500 °C). However, it has several drawbacks such as low yield (5-10%), tedious extraction process and no control of the bead’s properties. Carbon microspheres have been synthesized by an inverse emulsion polymerization of resorcinol (R) and formaldehyde (F) under basic conditions followed by super-critical or freeze drying and further subjected to pyrolysis. The removal of oil and subsequent drying of the microspheres require large amounts of energy and time. Hence, there is a demand for an easy process to form carbon microbeads.

Here, we adopted sol-gel (aerogel) chemistry to form RF microbeads and convert them into carbon microbeads. The shaping of RF sol into bead form by dropping method is quite challenging and never reported in the literature because of its low viscosity (0.5 Pa.s). Hence, we adjusted the viscosity of the RF sol by adding a polysaccharide-based thickener to achieve RF microbead formation (1-10 Pa.s), followed by sub-critical drying and carbonization. The average sizes of the carbon microbeads are in the range of 1.75-2.02 mm with skeletal densities of 2.0-2.4 g/cm3. Physisorption reveals that carbon beads exhibit type I isotherm, characteristic of microporous materials with little contribution from mesopores. 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 microbeads are spherical and its inner-morphology composed of inter-connected particles grown on the top of the thickening agents’ fibers. Further, continuous production of the RF microbeads in large scale is under investigation.