| In order to clarify the reaction mechanism of pore structure formation and adsorption properties of activated carbon fibers (ACFs) prepared from liquefied wood by zinc chloride activation, zinc chloride activated ACFs (ZACF) were prepared and characterized from liquefied wood under different activating conditions was investigated in this paper. The influence of activating conditions on the surface morphology, pore structure, crystal structure, and surface chemical structure of ZACF were examined. Then the surface fractal dimension of precursor fibers and ZACF was tested under various parameters. According to variations of functional groups, elements and components, the reaction mechanism during activation process of ZACF was discussed. Besides, the relationship between pore structure and adsorption properties as well as the adsorption mechanism of ZACF was studied. The main conclusions as follows:1、With increasing activation temperature and impregnation ratio, specific surface area (SSA) increased but yield declined. However, as activation time was extended, the yield reduced firstly and then increased, whereas an opposite trend can be found on SSA of ZACF. Compared with two-step activation method, one-step activation method was more suitable for preparing ZACF.2、SSA, micropore volume and mesopore volume were improved with increasing activation temperature and impregnation ratio. Ultramicropores and small number of mesopores can be found on the ZACF when the impregnation ratio was low. With improving impregnation ratio, these pores were enlarged and more mesopores were generated.3、Contrary to oxygen element, more carbon element was found on ZACF with increasing the impregnation ratio, activation time and temperature. The relative content of surface oxygen-containing function groups was proportional to activation time and impregnation ratio, while that was inversed to activation temperature.4、Precursor fiber belonged to the amorphous structure. It was aromatized and gradually formed graphite carbon net with the catalytic dehydrogenation of ZnCl2 in the activation process. Lc and La increased and d002 declined correspondingly. Graphite crystallite G-peak and D-peak charactering the disorder degree of materials had appeared in Raman spectra at different temperatures. As the temperature increased, the ratio between D-peak and G-peak decreased, indicating that increasing temperature could improve the orderliness of graphite-like structure on ZACF.5、Precursor fiber and ZACF showed smooth surface without obvious holes and defects. Surface fractal dimension (Ds) range of them calculated by image method were 2.1708～2.2310 and 2.0552～2.3646, respectively. Both of them were closer to two-dimensional plane. Moreover, ZACF owned different Ds at different scales, indicating ZACF had multi-scale fractal. Ds calculated by CCM method presented a good correlation with pore structure parameters of ZACF:the better the porosity, the higher the value of Ds. Thus, using image method from the geometry to describe rough surface of ZACF was feasible to certain scale. Ds of ZACF measured by nitrogen absorption-desorption method was 2.7293～2.9661, indicating that ZACF was provided with strong space filling ability. Ds and microporous structure, as well as characteristic adsorption energy had the same varying trend with the increase of impregnation ratio, indicating that Ds can be used to characterize the development degree of micropores. Compared with image fractal method, the adsorption method could directly describe characteristics of porosity and complex inner structure of ZACF.6、Reaction mechanism of ZACF during activation could be described as follows:Below 327℃, hydroxyl dehydration, dehydroxymethyl and molecular chains fracture occurred inside the fiber. Pyrolysis products such as CH4, HCHO, CH3OH and H2O were found.327℃～700℃ was the main stage of activation, where molecular structure was rearranged due to intramolecular condensation, crosslinking reaction and molecular strand breaking. Carbon network structure was preliminarily formed at about 402℃. In this stage, more products were found, such as H2, H2O, CO, CO2, benzene, phenols, etc. Due to the removal of these substances, surface defects were filled with molten ZnCl2. After activation, cooling and cleaning, more pore left on the surface of ZACF. Above 700℃, the level of aromatization and degree of aromatic ring gradually improved. Pyrolysis products were mainly H2O and H2. Besides, there was a small amount of CO and CO2.7、With increasing impregnation ratio, variation of methylene blue (MB) adsorption presented a similar trend with the change of larger micropores (1.5nm-2nm) and mesopore volume. There was a proportionality between iodine adsorption capacity and impregnation ratio or activation temperature. Based on the adsorption kinetics, pseudo-second order kinetic equation and Langmuir model were suitable to describe the adsorption process for MB onto ZACF, indicating that adsorption rate was mainly controlled by chemical adsorption and MB adsorption onto ZACF belonged to the monolayer adsorption, which was easily carried out. |
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