Preparation of porous carbon nanotube/carbon composite spheres and their adsorption properties

Carbon nanotubes (CNTs) are considered precursors to efficient adsorbents in many fields, including medical hemoperfusion; however, such adsorbents have not yet been realized, mainly because of the lack of scalable processing technologies to transform as-produced CNTs into applicable bulk materials. In this study, an extrusion–spheronization process followed by carbonization was developed to synthesize CNT/carbon composite spheres. A freeze-casting post-treatment in liquid nitrogen was explored to tailor the pore structures. In the freeze-casted tailored composite spheres, hierarchical pore structures with more developed meso- and macro-pores were formed compared to the untreated samples. Scanning electron microscopy observations demonstrated the generation of abundant laminar slit-like macropores after freeze-casting. Nitrogen adsorption-desorption tests and mercury porosimetry analyses indicated that the spheres had pore width distributions ranging from 100 nm to 1000 nm, in addition to highly developed mesopores. The adsorption capacities of the freeze-casted spheres for Vitamin B12 and lysozyme were 52.8 mg/g and 47.1 mg/g, respectively, which were remarkably higher than those of traditional adsorbents such as activated carbon or macroporous resin; the adsorption rates were also significantly higher with the composites. Energy-dispersive X-ray spectral mapping analyses confirmed that the macropores developed by freeze-casting acted as diffusion throughways during the adsorption process.