Abstract:
Abstract: So far, some studies have been conducted on preparation of nitrogen-doped (N-doped) active carbon from N-containing biomasses using alkalis as activators. In these studies, the commonly used preparation method was activation with alkali after biomass carbonization. Compared with this method, the one-step carbonization/activation method was simple and apt to reduce energy consumption, but its application in the preparation of N-doped active carbon was not investigated. In this research, N-doped active carbon with ultramicropores was prepared from waste soybean dreg using K2CO3 as activator via one-step carbonization/activation technology. The effects of activation temperature on chemical composition, pore structure, and low-pressure CO2 adsorption performances of the active carbon were investigated. To prepare active carbon, waste soybean dreg with particle size of 0.15-0.90 mm was impregnated with K2CO3 aqueous solution at K2CO3/dreg dry-basis weight ratio of 2:1, and after mixing uniformly, the mixture was sealed and kept for 4 h. Then, it was dried in an oven at 110℃ till constant weight was achieved. Subsequently, the dried mixture was heated to 500-650 ℃ at an average heating rate of 6℃/min and then kept for 75 min. Afterwards, the heated mixture was washed with distilled water until the pH value reached about neutral, and then dried at 110℃ for 12 h to produce active carbon. The obtained samples were subsequently characterized; pore structure and CO2 adsorption performance were measured with volumetric adsorption analyzers, elemental composition was measured with an elemental analyzer, surface chemistry was measured with an X-ray photoelectron spectroscopy, and surface morphology was measured with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). To gain an insight into the mechanism of pore formation, the soybean dreg and K2CO3-impregnated soybean dreg were pyrolyzed and analyzed using a thermogravimetric analyzer coupled with an infra-red spectrometer. The results showed that the technology could be successfully used to prepare N-doped active carbon with ultramicropores. As the activation temperature increased from 560 to 650℃, the N was distributed homogenously both on surface and in bulk of the active carbon; the N content (4.1%-4.4%) varied slightly, but the N chemical state changed. As the activation temperature rose, the specific surface area, total pore volume, and micropore volume of active carbon increased monotonously, but their ultramicropore volume first increased and then decreased. The activated carbon prepared at 600℃ possessed the maximum ultramicropore volume (0.13 mL/g), and the pore diameter of the ultramicropores was mainly in the range from 0.42 to 0.70 nm. The micropore volume, total pore volume, and specific surface area of this kind of carbon were 0.40 mL/g, 0.43 mL/g and 948 m2/g, respectively. After characterizations of these carbon materials, the carbon materials were used for the adsorption of CO2. The CO2 uptakes of the obtained active carbon at low pressures (10, 15 and 20 kPa) first increased and then decreased with the increment in activation temperature, which coincided with the variation trend of their ultramicropore volume. This result indicated that the low-pressure CO2 uptake of the activated carbon prepared at 600℃ was ascribable to its developed ultramicropores. The carbon prepared by activation at 600℃ showed a CO2 uptake of 1.94 mmol/g at 10 kPa and 0℃, which was superior to the corresponding values ever reported for many biomass-based active carbon, indicating that the obtained sample could display a high CO2 uptake at low pressures. Besides, at 10 kPa, this sample displayed a selectivity for CO2/N2, 41.6 and 30.0 respectively at 0℃ and 25℃, which were high compared with the values of many biomass-derived N-deficient active carbon. The high CO2/N2 selectivity of the carbon was owing to the presence of N-containing groups on its surface. Finally, we found that the CO2 adsorption isotherm of the sample activated at 600℃ hardly changed after 5 successive runs of adsorption-desorption, indicating that the sample showed excellent recyclability. From the results derived from the CO2 adsorption tests, it is concluded that the active carbon prepared from N-containing biomass via one-step carbonization/activation technology can display a high uptake of low-pressure CO2, large selectivity of CO2/N2, and excellent recyclability.