Evaluation of different potassium salts as activators for hierarchically porous carbons and their applications in CO2 adsorption

Machine learning and experimentally exploring the controversial role of nitrogen in CO2 uptake by waste-derived nitrogen-containing porous carbons

Waste-derived nitrogen-containing porous carbons were widely accepted as promising carbon capture materials. However, roles of nitrogen in CO2 uptake were highly controversial, posing a challenge in designing high CO2 uptake porous carbons. Herein, nitrogen-containing species was firstly introduced into machine learning (ML) models to uncover the complex relationship of nitrogen, micropore and CO2 uptake by combining ML models, DFT computations and experiments. The results revealed that micropore volume (Vmicro) was the most important property influencing CO2 uptake, but was not the only determinant factor. Nitrogen-containing species (pyrrolic/pyridonic-N (N5) and pyridinic-N (N6)) rather than total nitrogen content, also played an essential role. On the one hand, they can enhanced CO2 adsorption by Lewis acid-base and hydrogen bonding. On the other hand, they promoted development of micropores by participating in activation reactions. The model further indicated that excessive N5 (>1.5 wt%) or N6 (>1.7 wt%) led to restriction on developments of micropores, which was attributed to enlargement of pore size, collapses or blockage of micropores. The double edged-sword effect of N5 and N6 on changes of microporous structures was responsible for the long-standing controversy over nitrogen. The result was further verified by synthesizing eight porous carbons with different textural and chemical properties. This study provided not only a new perspective for resolving the controversy of nitrogen in CO2 uptake, but also a graphical user interface prediction software meaningful for designing porous carbons.

Crab shell-based hierarchical micro/meso-porous carbon as an efficient nano-adsorbent for CO2/CH4 separation: experiments and DFT modeling

In this study, we prepared a range of nanoporous carbon nano-adsorbents from crab shells (CSs) using KOH activation and evaluated their suitability for selective adsorption of CO2/CH4 gas mixtures. We employed various characterization techniques, including XRD, FT-IR, SEM, Raman, TGA, and BET analysis, to assess the properties of these nano-adsorbents. Our investigation includes the systematic study of various parameters, such as activation time, activation temperature, and the KOH to CS activating agent ratio. The nanoporous carbons were evaluated for their CO2 adsorption capabilities at 1-10 bar and 25 ℃ condition. The results demonstrated that the CS-2-2-900 sample, activated for 1 h at 900 ℃ with a 2:1 ratio of KOH to CS, exhibited the highest gas adsorption capacity, reaching 7.217 mmol/g at a pressure of 10 bar under room temperature conditions. Additionally, the synthesized CS-2-2-900 sample displayed excellent surface area (914.85 m2/g), a pore volume of 1.1 cm3/g, and an average pore diameter of 4.82 nm. Furthermore, we functionalized the CSs to enhance their selectivity for ammonia adsorption. Using the Myers and Pravnitz theory, we calculated that the FCS-2-2-900 sample exhibited the highest selectivity, reaching 18.99 at 25 ℃ under pressures of up to 10 bar. To gain a more comprehensive understanding of the interactions between the adsorbents and the adsorbed molecules, as well as to identify the active sites involved in the adsorption process, we employed density functional theory (DFT). Our DFT calculations revealed that pyrrolic nitrogen and carboxylic sites played a significant role in enhancing the separation of CO2 in binary mixtures. In summary, nanoporous carbons derived from crab shells outperformed those derived from other waste materials. These functionalized porous nanocarbons represent promising adsorbents for the selective adsorption of CO2 gas in CO2/CH4 mixtures due to their nitrogen content, high porosity, stability, and economic efficiency.

Crosslinked hydrogel-derived carbons activated by trace amounts of aqueous potassium carbonate for carbon dioxide adsorption

A new method for green synthesis of activated carbon using chitosan-based hydrogel precursors is reported. Chitosan-based hydrogel materials are designed to absorb trace amounts of non-toxic and non-corrosive activating agent K2CO3 from dilute aqueous solution. The K2CO3 impregnated hydrogels are further freeze-dried and converted to activated carbons with tuneable pore structure by a single-step pyrolysis. Activated carbon with highest pore volume of 0.76 cm3/g and surface area of 2026 m2/g is obtained by using K2CO3 as low as 0.23 g per gram of chitosan hydrogel. It can adsorb maximum CO2 of 4.2 mmol/g at 25 °C and 1 bar. This study demonstrates that biopolymer hydrogels impregnated with trace amounts of K2CO3 are excellent precursor materials to design high surface area carbons for CO2 capture.

Valorization of orange peel waste to tunable heteroatom-doped hydrochar-derived microporous carbons for selective CO2 adsorption and separation

Constrained by the extortionately expensive carbon sources, low carbon yields, inadequate adsorption capacities, and corrosive chemical activating agents, the commercialization of carbonaceous CO₂ adsorbents remains a challenging task. Herein, potassium oxalate (K₂C₂O₄), an activating agent with less corrosive properties, was used for the synthesis of activated carbons from inexhaustibly available "orange peel biowaste." For the first time, a comprehensive report is presented on the effect of hydrothermal treatment, hydrochar/K₂C₂O₄ ratio, activation temperature, and melamine modification in tailoring the porosity and surface functionalization of activated carbons. The optimized sample, OPMK-900, exhibited large specific surface area ~2130 m²/g; micropore volume ~1.1166 cm³/g, and a high pyrrolic nitrogen content (~ 46.1 %). Notably, melamine played the dual role as a promoter to K₂C₂O₄ porosity generation and a nitrogen dopant, which synergistically led to an efficient CO₂ uptake of ~6.67 mmol/g at 273 K/ 1 bar via micropore-filling mechanism and Lewis acid-base interactions. Moreover, remarkably high IAST CO₂/N₂ selectivity (105 at 273 K and 96 at 298 K) surpasses most of the biomass-derived carbons. Furthermore, the moderately high isosteric heat of adsorption (∆H_ads ~ 38.9 kJ/mol) revealed the physisorption mechanism of adsorption with a limited energy requirement for the regeneration of the spent adsorbents.

Constructing highly porous carbon materials from porous organic polymers for superior CO2 adsorption and separation

The increase in atmospheric carbon dioxide (CO₂) concentration has led to numerous problems related to our living environment, seeking an efficient carbon capture and storage (CCS) strategy associated with low energy consumption and expenditures is highly desirable. Here, we demonstrate a facile approach to synthesize a series of highly porous carbon materials derived from porous organic polymers synthesized from three low-cost isomers of triphenyl using chemical activation with KOH at different temperatures. Compared with the precursor porous organic polymers, the porosity of the prepared porous carbon materials is significantly enhanced with surface areas as high as 3367 m² g^-1 and pore volumes up to 1.224 cm³ g^-1. Notably, such porous carbon materials deliver an exceptionally high CO₂ adsorption capacity of 7.78 mmol g^-1 at 273 K and 1 bar, a value that is superior to most of the previously reported adsorbents. In addition, these porous organic polymers and derived porous carbon materials exhibit high CO₂/N₂ selectivity at ambient conditions. Therefore, the facile construction of highly porous carbon materials from porous organic polymers may offer an efficient strategy for CO₂ adsorption and separation and further mitigates greenhouse effect.

Triple combination of natural microbial action, etching, and gas foaming to synthesize hierarchical porous carbon for efficient adsorption of VOCs

Fungi residue, vinasse, and biogas residue differ from general biomass waste due to natural microbial action. Microbial fermentation helps create natural channels for the permeation of activators and produces proteins for natural nitrogen doping. Inspired by these advantages on porous carbon synthesis, this study adopted dual activators of KOH and KHCO₃ to synthesize porous carbon with different pore ratios for efficient adsorption of volatile organic compounds (VOCs). The fungi residue possessed the least lignin due to the most severe microbial action, contributing to the best pore structures after activation. The etching effect from potassium compounds and gas foaming from the carbonate decomposition contributed to creating hierarchical porous carbon with ultra-high surface area, ca. 1536.8-2326.5 m²/g. However, KHCO₃ addition also caused nitrogen erosion, such that lower adsorption capacity was attained even with a higher surface area when the mass ratio of KOH/KHCO₃ decreased from 2.5:0.5 to 2:1. The maximum adsorption capacities of chlorobenzene (CB) and benzene (PhH) reached 594.0 and 394.3 mg/g, respectively. Pore structure variations after adsorption were evaluated by freeze treatment to discover the adsorption mechanism. The surface area after CB and PhH adsorption decreased 40.3% and 34.5%, respectively. Most of the mesopores might transform into micropores due to the mono/multilayer stacking of adsorbates. The VOC adsorption kinetics were simulated by the Pseudo-first- and -second-order models and Y-N model. This paper provides a new approach for high-value biomass waste utilization after microbial action to synthesize efficient adsorbents for VOCs.

A review on application of activated carbons for carbon dioxide capture: present performance, preparation, and surface modification for further improvement

The atmosphere security and regulation of climate change are being continuously highlighted as a pressing issue. The crisis of climate change owing to the anthropogenic carbon dioxide emission has led many governments at federal and provincial levels to promulgate policies to address this concern. Among them is regulating the carbon dioxide emission from major industrial sources such as power plants, petrochemical industries, cement plants, and other industries that depend on the combustion of fossil fuels for energy to operate. In view of this, various CO2 capture and sequestration technologies have been investigated and presented. From this review, adsorption of CO2 on porous solid materials has been gaining increasing attention due to its cost-effectiveness, ease of application, and comparably low energy demand. Despite the myriad of advanced materials such as zeolites, carbons-based, metal-organic frameworks, mesoporous silicas, and polymers being researched, research on activated carbons (ACs) continue to be in the mainstream. Therefore, this review is endeavored to elucidate the adsorption properties of CO2 on activated carbons derived from different sources. Selective adsorption based on pore size/shape and surface chemistry is investigated. Accordingly, the effect of surface modifications of the ACs with NH3, amines, and metal oxides on adsorption performance toward CO2 is evaluated. The adsorption performance of the activated carbons under humid conditions is also reviewed. Finally, activated carbon-based composite has been surveyed and recommended as a feasible strategy to improve AC adsorption properties toward CO2. The activated carbon surface in the graphical abstract is nitrogen rich modified using ammonia through thermal treatment. The values of CO2 emissions by sources are taken from (Yoro and Daramola 2020).

One-pot hydrothermal preparation of manganese-doped carbon microspheres for effective deep removal of hexavalent chromium from wastewater

Mn-doped activated carbon microspheres (MnO_x/ACS) with super-high adsorption capacities and deep removal capability for hexavalent chromium (Cr(VI)) were successfully prepared via an ammonium persulfate-assisted hydrothermal method followed by potassium oxalate activation using KMnO₄ and sucrose as raw materials. Their -physical and chemical properties, as well as those of Mn-doped non-activated carbon spheres (MnO_x/CS), were characterized by XRD, SEM, TEM, EDS-mapping, XPS, N₂ adsorption-desorption, ICP-AES, and elemental analysis. It was found that the manganese oxide (MnO_x) particles were uniformly embedded within the carbon spheres via layer-by-layer capture, and the MnO_x/ACS exhibited strong redox activity because of the multivalent nature of MnO_x, resulting in excellent adsorption performance via reduction. In particular, MnO_x/ACS-4 with a Mn content of 1.06 wt% and a specific surface area of 1405.7 m² g^-1 achieved a maximum adsorption capacity of 660.7 mg g^-1; this can reduce Cr(VI) content to less than 0.05 mg L^-1, which meets the corresponding Chinese drinking water quality standard when the initial concentration of Cr(VI) is less than 400 mg L^-1. Furthermore, this highly efficient method can be extended to prepare V-, Mo-, or W-doped carbon microspheres with significantly enhanced adsorption performance for Cr(VI) compared to bare activated carbon sphere, indicating their good application prospect for the deep removal for heavy metal ions from wastewater.