Catalytic wet oxidation (CWO) of aqueous effluents rich in organic compounds is a very promising technology for the treatment of liquid wastes from biomass conversion processes. CWO reactions occur through the formation of free radical species, produced in the presence of an oxidant, which act on organic contaminates in the effluent. Although the reaction is well known, there exists a lack of affordable catalysts to conduct this process at the lower temperatures and pressures in novel bioenergy processes. This study assessed the catalytic effect of nitrogen-doped chars as such an option. Phenol in aqueous solution was used as a model waste effluent. Treatment was conducted at moderate temperatures (190 to 260 °C), oxygen partial pressure of 1 MPa, and reaction times of 15, 30, and 45 min in stainless steel and glass-lined tube reactors. High pressure liquid chromatography (HPLC) analyses of the products quantified phenol and by-product concentrations used in the calculation of reaction activation energy. The char catalyst was studied by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) in order to gain insight into its structure and surface composition. The results indicate that nitrogen-doped char catalysts accelerate the oxidation of phenol by decreasing its reaction activation energy from 82.2 kJ/mol (non-catalyzed) to 40.4 kJ/mol (catalyzed). An analysis from first principles using density functional theory (DFT) was conducted to ascertain which N functional group has the most significant impact on free radical formation in the presence of oxygen. Among all the N functional groups studied, the dipyridinic functional groups showed the most promising characteristics to facilitate the formation of hydroxyl free radicals.
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Special thanks to Jonathan Lomber of WSU Analytical Chemistry Service Center for his assistance in support of this work, Yaime Jefferson for her analytical support, Katie Johnson of the NARA Summer Intern in the Garcia-Perez Laboratory, and Roberto Esquivel for the graphic support. This research used resources from the Center for Institutional Research Computing at Washington State University. PNNL is a multiprogram national laboratory operated for the US DOE by Battelle.
The study was supported by the Sun Grant subproject, Grant No. 128467-G004003. J.-S.M and A.G. were supported by the National Science Foundation under Contract No. CBET-1703052. This work was partially funded by the Joint Center for Deployment and Research in Earth Abundant Materials (JCDREAM) in Washington State.
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• Available treatment of phenol-contaminated water is reviewed, and active carbons are proposed as an inexpensive and robust solution to the shortcomings of existing catalysts.
• Nitrogen-doped char, produced from cellulose, is a sustainable and affordable catalyst for adsorption and removal of phenol from water.
• Pyridinic groups comprise the highest percentage of functional groups in N-doped char.
• Density functional theory analysis confirms that in the presence of oxygen, pyridinic groups favorably produce activated oxygen species which are key to catalytic oxidation of phenol in water.
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Tews, I., Garcia, A., Ayiania, M. et al. Nitrogen-doped char as a catalyst for wet oxidation of phenol-contaminated water. Biomass Conv. Bioref. (2021). https://doi.org/10.1007/s13399-020-01184-0
- Nitrogen-doped chars
- Phenol oxidation
- Oxidation kinetics