Carbon-based aerogels and xerogels: Synthesis, properties, oil sorption capacities, and DFT simulations

Abstract

Pristine, hybrid or composite aerogels and xerogels based on carbon allotropes are reviewed. Carbon aerogels applied as sorbents for distinct organic liquids are mainly composed of graphene or graphene oxide (GO) hybrids, carbon nanotubes (CNTs) composites, as well as by hybrids of other carbon nanostructures. Aerogels are generally prepared by the aging of gels (hydrogels or alcogels), drying (mostly by vacuum freeze-drying), and pyrolysis/carbonization. Carbon xerogels are synthesized usually by traditional sol-gel based method from resorcinol and formaldehyde (gels formation), aging, drying (at atmospheric pressure) and pyrolysis/carbonization, being drying method the principal difference between aerogels and xerogels fabrication. A certain number of carbon-containing aerogels can be obtained from natural sources: corn bracts, waste newspaper, bleached softwood kraft pulp, bamboo fiber cotton, or sisal fibers, among others. These precursors are inexpensive, renewable and, on some occasions, wastes are used that would otherwise be discarded. Hybrid and composite carbon aerogels also can be obtained using natural sources as raw material. Outstanding properties of aerogels are high specific surface area, super-compressibility, and high adsorption capacity. Specific properties such as improved surface roughness or magnetic property of aerogels can be tailored by the incorporation of certain nanostructures. Xerogels are less popular as effective adsorption materials, but their fabrication is possible at normal pressure, influencing their cost. Both carbon aerogels and xerogels, due to their exceptional sorbent performance, high selectivity, and reusability, are excellent options for a series of adsorbent purposes. In this paper, an in-depth study of carbon aerogels and xerogels is presented, including their synthesis, use as oil sorbents, properties, and sorption capacities. Additionally, this information is displayed in tabular form (Supplementary Material) for easy comparison.