Nitrogen-doped char as a catalyst for wet oxidation of phenol-contaminated water

Abstract

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.

Perspectives of Engineered Biochar for Environmental Applications: A Review

“…275 Nitrogen-doped char has catalytic properties for the wet oxidation of phenol in aqueous solutions. 505 N-doped biochar was used for catalytic oxidation of organic pollutants, including orange G, phenol, sulfamethoxazole, and bisphenol A. Graphitic N in biochar generated at higher temperatures (e.g., 900 °C) showed more reactivity for persulfate activation than PFRs generated at low temperature (e.g., 400 °C). 506 N doping and metal loading methods have been used simultaneously to synthesize a catalyst for the degradation of organic pollutants.…”

Advanced Oxidative Techniques for the Treatment of Aqueous Liquid Effluents from Biomass Thermochemical Conversion Processes: A Review

“…A clear relationship between the oxygen level and the formation of ringed intermediates (excess phenol) vs carboxylic acids (excess oxygen) made it clear that the major mechanism is driven by oxidation and that thermal impact is only second in significance. The complete oxidation pathway is seen in Figure . ,, …”

“…Nitrogen-doped char exhibiting a pyridinic functional group and a site of catalytic activity. Reproduced with permission from ref . Copyright 2021 Springer Nature.…”

“…Nitrogen-doped carbons with identified functional groups. Reproduced with permission from ref . Copyright 2021 Springer Nature.…”

“…Positioning of the nitrogen within the carbon matrix determines the functional group formed. Recent work has indicated that some groups such as the pyridinic group, as seen in Figure , have more catalytic activity than others …”