Development and application of nanoporous carbon materials by catalytic activation of residual solids obtained from biorefinery processes

Abstract

Solid co-products from biofuel technologies have great potential as a promising precursor to replace bituminous coal, conventional raw material for preparing nanoporous activated carbon. There are positive double effects of securing alternative resources that are environmental friendly and economically sustainable as well as valorization of co-products with enhancing the efficiency of the biorefinery process. However, the conversion mechanism has not fully understood due to the complexity of various influence factors, and very little has known about utilization of carbon material from co-products. This research focuses on effects of various reaction parameters on properties of activated carbon product, followed by evaluating performance as a versatile carbon material in further industrial application. First, fast pyrolysis char, obtained from two biomasses at 500°C with a residence time of 1.3 s, was used as a precursor of nanoporous carbon material under different KOH loadings. In order to investigate a potential application as a biosorbent, phenol removal test according to contact time and initial adsorbate concentration was also conducted. After the catalytic activation, a super-active carbon with a large pore volume (1.58 cm3/g) and high specific surface area (2711 m2/g) was successfully prepared. In addition, Langmuir adsorption capacity of the produced activated carbon for phenol was 625 mg/g, which superior to that of AC (500 mg/g), and correlation study revealed that mesopore volume has a positive correlation with adsorption capacity. Second, preparation and modification of activated carbon derived from different precursor chars (fast pyrolysis char and hydrochar) under different atmospheric conditions (N2 and zero air) and by post-acidification (HNO3) were performed. Additionally, an evaluation of their potentials for application as an adsorbent for heavy metal and a carbon electrode was carried out. The specific surface area of the precursor chars was less than 23 m2/g, but it increased to 1515-1879 m2/g for N2-activated carbon and 842-1022 m2/g for air-activated carbon. Changes in aromatic macromolecular structure for each carbon product were observed and conditions of zero air rendered the char more reactive than under conditions of pure N2. A plausible reaction mechanism for this observation was suggested to be the formation of a key intermediate in the presence of excess air. Furthermore, the performance when used in applications for various fields depended on the carbon properties such as specific surface area, functional group, hydrophilicity, and mesopore ratio. Compared to post-modified carbon, the air-activated carbon exhibited high versatility to function as both a Pb2+ adsorbent (~41.1 mg/g) and energy storage material (~185.9 F/g). Lastly, the effects of various parameters, such as feedstock, temperature, reaction time, and catalyst loadings on chemical and structural properties of activated carbon derived from lignin were investigated. The assessment of adsorption capacity for removal of heavy metals (Pb2+ and Cd2+) and organic compounds (phenol, bisphenol-A, and 2,4-dichlorophenoxyacetic acid), and electrochemical properties was also performed. Catalytic reaction occurred from the external surface under mild conditions of less than 750°C and within 1 h, then it transferred to the internal region under more severe reaction conditions with volatile release of de-alkylated aromatics, leading to structure and surface collapse. The maximum BET surface area of 2782 m2/g was obtained under 750°C, 2 h and catalyst ratio of 4. Lignin-derived activated carbon was more efficient for removal of organic pollutants rather than heavy metals due to interaction of π-π bonding. Furthermore, the activated carbon has a potential to be used as a supercapacitor electrode with a specific capacitance up to 214.0 F/g.