Synthesis of Nitrogen and Sulfur Codoped Nanoporous Carbons from Algae: Role in CO2 Separation

One-Step Synthesis of Sulfur-Doped Nanoporous Carbons from Lignin with Ultra-High Surface Area, Sulfur Content and CO2 Adsorption Capacity

Lignin is the second-most available biopolymer in nature. In this work, lignin was employed as the carbon precursor for the one-step synthesis of sulfur-doped nanoporous carbons. Sulfur-doped nanoporous carbons have several applications in scientific and technological sectors. In order to synthesize sulfur-doped nanoporous carbons from lignin, sodium thiosulfate was employed as a sulfurizing agent and potassium hydroxide as the activating agent to create porosity. The resultant carbons were characterized by pore textural properties, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The nanoporous carbons possess BET surface areas of 741-3626 m²/g and a total pore volume of 0.5-1.74 cm³/g. The BET surface area of the carbon was one of the highest that was reported for any carbon-based materials. The sulfur contents of the carbons are 1-12.6 at.%, and the key functionalities include S=C, S-C=O, and SO_x. The adsorption isotherms of three gases, CO₂, CH₄, and N₂, were measured at 298 K, with pressure up to 1 bar. In all the carbons, the adsorbed amount was highest for CO₂, followed by CH₄ and N₂. The equilibrium uptake capacity for CO₂ was as high as ~11 mmol/g at 298 K and 760 torr, which is likely the highest among all the porous carbon-based materials reported so far. Ideally adsorbed solution theory (IAST) was employed to calculate the selectivity for CO₂/N₂, CO₂/CH₄, and CH₄/N₂, and some of the carbons reported a very high selectivity value. The overall results suggest that these carbons can potentially be used for gas separation purposes.

CO2 adsorption on pyrolysis char from protein-containing livestock waste: How do proteins affect?

Biogas generation through anaerobic digestion provides an interesting opportunity to valorize some types of animal waste materials whose management is increasingly complicated by legal and environmental restrictions. To successfully expand anaerobic digestion in livestock areas, operational issues such as digestate management must be addressed in an economical and environmentally sustainable way. Biogas upgrading is another necessary stage before intending it to add-value applications. The high concentration of CO₂ in biogas results in a reduced caloric value, so the removal of CO₂ would be beneficial for most end-users. The current work evaluates the CO₂ uptake properties (thermogravimetry study) of low-cost adsorbent materials produced from the animal wastes generated in the livestock area itself, specifically via pyrolysis of poorly biodegradable materials, such as meat and bone meal, and the digestate from manure anaerobic digestion. Therefore, the new element in this study with respect to other studies found in the literature related to biochar-based CO₂ adsorption performance is the presence of high content of pyrolyzed proteins in the adsorbent material. In this work, pyrolyzed chars from both meat and bone meal and co-digested manure have been proven to adsorb CO₂ reversibly, and also the chars produced from their representative pure proteins (collagen and soybean protein), which were evaluated as model compounds for a better understanding of the individual performance of proteins. The ultra-microporosity developed in the protein chars during pyrolysis seems to be the main explanation for such CO₂ uptake capacities, while neither the BET surface area nor N-functionalities on the char surface can properly explain the observed results. Although the CO₂ adsorption capacities of these pristine chars (6-41.0 mg CO₂/g char) are far away from data of commercially activated carbons (~80 mg CO₂/g char), this application opens a new via to integrate and valorize these wastes in the circular economy of the primary sector.

Azo-dye derived oxidized-nitrogen rich carbon sheets with high adsorption capability for dye effluent under both batch and continuous conditions

The removal of methyl blue (MB) from wastewater using graphene and its derivative is very successful due to their high aromaticity which drives adsorption via π-π and electron-donor-acceptor (EDA) interactions; however, graphene is expensive and difficult to synthesize, which limit its practical application. Meanwhile, low aromatic carbon materials (LACM) derived from farm-water and other materials are cheaper and easier to synthesize but have limited π-π and EDA interactions and low adsorption capacity. Herein, we demonstrate that LACM with oxidized-nitrogen (N-O^-) functionality overcomes this limitation via chemisorption of MB through a combination of hydrophobic-hydrophobic interactions and EDA interactions. This is confirmed using XPS analysis of LACM/N-O^- post MB adsorption. Consequently, a remarkable adsorption capacity of 3904 mg g^-1 is achieved under batch condition which is the highest ever reported for any MB adsorbent. Furthermore, LACM/N-O^- works equally well under continuous-flow adsorption conditions which shows its practicability. Amongst several LACM precursors tested, only Azo-dyes are able to generate LACM/N-O^- implying that the NN moiety is key to N-O^- formation. A carbonization temperature of 700 °C generates the highest N-O^- sites hence the highest adsorption capacity. Characterization of LACM/N-O^- is done mainly using BET, XPS, Raman, TGA, and FTIR analysis.