Preparation of ultra-microporous waste paper carbon aerogel and its CO₂ adsorption performance

The intensification of climate change requires the development of greener and more efficient carbon abatement technologies and products. Conventional CO₂ carbonaceous adsorbent materials derived from solid waste and biomass feedstocks are poorly adsorbed and often require additional activation pore making and functional group introduction to enhance the adsorption performance of the porous carbon, which inevitably results in further growth of the process and further increase in energy consumption. In the work carried out in this paper, different kinds of waste paper were used as raw materials, and after a simple pretreatment and sol-gel carbonization process, highly developed microporous hierarchical porous carbon aerogels were prepared; moreover, KOH could be introduced in situ and activation pore-making could be accomplished synchronously during pyrolysis, which avoided the additional energy consumption of the two-step method. Thermogravimetric (TG), scanning electron microscopy (SEM), specific surface area and pore size analyzer (BET), Fourier transform infrared spectrometer (FT-IR) and fixed bed adsorption rig were used to characterize and test the thermal weight loss properties of the waste paper aerogels, the physicochemical properties of the waste paper carbon aerogels and the CO₂ adsorption properties, respectively, and the results show that the main thermal weight loss process of the waste paper aerogels occurs at around 300 ℃, and is accompanied by the appearance of miscellaneous peaks of heat loss in the low pyrolysis temperature region, and the final mass residual rate is slightly higher than that of cellulose. Scanning electron microscopy showed that the pore structure was well developed and relatively homogeneous, and the surface openings showed a honeycomb-like structure. The printing paper carbon aerogel DYZ-800 prepared at a pyrolysis temperature of 800 ℃ has an ultra-high specific surface area of 1369.94 m²/g (94.28% of microporous specific surface area), a pore volume of 0.59 cm³/g (85.34% of microporous pore volume), and a pore-size distribution that is close to the kinetic diameter of CO₂ molecules (0.4−0.8 nm and containing a large number of super-micropores with a size of 0.7 nm). The results of FT-IR tests revealed the effects of different waste paper types and pyrolysis temperatures on the carbon aerogel skeleton and chemical groups of waste paper, with sample DYZ-800 having a more stable carbon skeleton and a relatively high content of carbon-oxygen (C−O) groups. The maximum CO₂ adsorption capacity of DYZ-800 without modification was 247 mg/g at 0 ℃ (1 bar) and 151 mg/g at 25 ℃ (1 bar), and the CO₂/N₂ adsorption selectivity was 11. The average fluctuation after 7 adsorption and desorption cycles was less than 5%, which showed good regeneration stability. The capture of CO₂ at 10% flue gas concentration on a fixed-bed adsorption bench could also reach 42 mg/g (25 ℃, 1 bar). Among the three different adsorption kinetic models selected, the Bangham pore diffusion model had an excellent fit, demonstrating the great contribution of the well-developed pore structure of waste paper carbon aerogels in the CO₂ kinetic adsorption process. Taken together, these results show that the waste paper carbon aerogel possesses excellent physicochemical properties, and the presence of a large number of micropores (especially ultra-micropores) enables it to exhibit excellent CO₂ adsorption performance, which is superior to that of conventional solid waste and biomass-based carbon materials. All these indicate that the carbon aerogel prepared in this work has great advantages in carbon capture and potentials for further improvement, and this work also provides new ideas for solid waste disposal and resource utilization.