No abstract

Electrospinning techniques have become an efficient way to produce continuous and porous carbon nanofibers. In view of CO2 capture as one of the important works for alleviating global warming, this study intended to synthesize polyacrylonitrile (PAN)-based activated carbon nanofibers (ACNFs) using electrospinning processes for CO2 capture. Different structures of PAN-based ACNFs were prepared, including solid, hollow, and porous nanofibers, where poly(methyl methacrylate) (PMMA) was selected as the sacrificing core or pore generator. The results showed that the PMMA could be removed successfully at a carbonization temperature of 900 °C, forming the hollow or porous ACNFs. The diameters of the ACNFs ranged from 500 to 900 nm, and the shell thickness of the hollow ACNFs was approximately 70–110 nm. The solid ACNFs and hollow ACNFs were microporous materials, while the porous ACNFs were characterized by hierarchical pore structures. The hollow ACNFs and porous ACNFs possessed higher specific surface areas than that of the solid ACNFs, while the solid ACNFs exhibited the highest microporosity (94%). The CO2 adsorption capacity on the ACNFs was highly dependent on the ratio of V<0.7 nm to Vt, the ratio of Vmi to Vt, and the N-containing functional groups. The CO2 adsorption breakthrough curves could be curve-fitted well with the Yoon and Nelson model. Furthermore, the 10 cyclic tests demonstrated that the ACNFs are promising adsorbents.

Coaxial electrospinning has been considered a straightforward and convenient method for producing hollow nanofibers. Therefore, the objective of this study was to develop hollow activated carbon nanofibers (HACNFs) for CO2 capture in order to reduce emissions of CO2 to the atmosphere and mitigate global warming. Results showed that the sacrificing core could be decomposed at carbonization temperatures above 900 °C, allowing the formation of hollow nanofibers. The average outer diameters of HACNFs ranged from 550 to 750 nm, with a shell thickness of 75 nm. During the carbonization stage, the denitrogenation reactions were significant, while in the CO2 activation process, the release of carbon oxides became prominent. Therefore, the CO2 activation could increase the percentages of N=C and quaternary N groups. The major nitrogen functionalities on most samples were O=C–NH and quaternary N. However, =C and quaternary N groups were found to be crucial in determining the CO2 adsorption performance. CO2 adsorption on HACNFs occurred due to physical adsorption and was an exothermic reaction. The optimal CO2 adsorption performance was observed for HACNFs carbonized at 900 °C, where 3.03 mmol/g (1 atm) and 0.99 mmol/g (0.15 atm) were measured at 25 °C. The degradation of CO2 uptakes after 10 adsorption−desorption cyclic runs could be maintained within 8.9%.

In this study, polyacrylonitrile (PAN)-based activated nanofiber composites, which were embedded inside zeolitic imidazolate framework-8 (ZIF-8) crystals or ZIF-8-derived carbons (ZDC-850), were fabricated using an electrospinning process, to serve as CO2 adsorbents. The adsorbents were characterized using various techniques. The degree of crystallinity of ZDC-850 totally changed compared to that of ZIF-8. For nanofiber composites, the timing of the ligand decomposition of ZIF-8 significantly affected the material properties. The Zn metals in the ZIF-8/PAN or ZDC-850/PAN could be embedded and protected by the PAN fibers from excess volatilization in the following treatments: ZIF-8 had significant pore volumes in the range of 0.9–1.3 nm, but ZDC-850 and ZIF-8/PAN exhibited a distinct peak at approximately 0.5 nm. The CO2 adsorption capacities at 25 °C and 1 atm followed the order: ZIF-8/PAN (4.20 mmol/g) > ZDC-850 (3.50 mmol/g) > ZDC-850/PAN (3.38 mmol/g) > PAN (2.91 mmol/g) > ZIF-8 (0.88 mmol/g). The slope in the log–linear plot of isosteric heat of adsorption was highly associated with CO2 adsorption performance. Under 1 atm at 25 °C, for Zn metal active sites inside the pores, the pores at approximately 0.5 nm and in C-N (amines) groups could promote CO2 adsorption. At low CO2 pressures, for a good CO2 adsorbent, the carbon content in the adsorbent should be higher than a threshold value. Under this condition, the percentage of ultra-micropore and micropore volumes, as well as the functional groups, such as the quaternary or protonated N (amines), N=C (imines or pyridine-type N), C-OH, and -COOH groups, should be considered as significant factors for CO2 adsorption.