CO2 adsorption onto synthetic activated carbon: Kinetic, thermodynamic and regeneration studies

Activated Carbon as an Adsorbent for CO2 Capture: Adsorption, Kinetics, and RSM Modeling

The main objective of this research is to investigate the adsorption isotherms and the optimization of the carbon dioxide (CO2) adsorption process on activated carbon (AC). The AC has been characterized by X-ray diffraction, scanning electron microscopy, and nitrogen (N2) adsorption-desorption. The CO2 isotherms measured under three adsorption temperatures (318, 308, and 298 K) were investigated by the Langmuir and linear driving force model. It is found that the Langmuir model could well predict the adsorption behavior of AC with a correlation factor of about 0.99. The kinetic model shows that the amount of CO2 increases at higher pressures. The response surface methodology (RSM) was applied to investigate the effects of process variables and their interaction on the response to reach the optimal conditions. Based on the analysis of variance, pressure and temperature are the main factors influencing the CO2 adsorption capacity. The effective parameters obtained through this model are found to have a value of p < 0.05. In addition, a semiempirical correlation was developed under the optimal operating conditions which are 25 °C and 9 bar. The results indicated that RSM models provide an effective method for evaluating CO2 adsorption. These results point that AC is a potential adsorbent for CO2 capture.

Unraveling the Potential of Electrochemical pH-Swing Processes for Carbon Dioxide Capture and Utilization

Global warming, driven by the accumulation of anthropogenic greenhouse gases, particularly CO2, in the atmosphere, has garnered significant attention due to its detrimental environmental impacts. To combat this critical issue, the deployment of CO2 capture and utilization (CCU) strategies has been considered as one of the technology-based solutions, leading to extensive scientific and engineering research. Electrochemical pH-swing (EPS) processes offer a promising approach to diverse CCU pathways, such as the delivery of pure CO2 gas, the delivery of bicarbonate (e.g., for microalgae cultivation), and the formation of carbonate minerals. In this study, we discuss several CCU pathways using EPS and provide an in-depth analysis of its mechanisms and potential applications, outlining its limitations from both thermodynamic and kinetic standpoints. The EPS process has demonstrated remarkable capabilities, achieving a CO2 capture efficiency of over 90% and unlocking valuable opportunities for CCU applications. We also develop an initial techno-economic assessment and provide the perspectives and challenges for future development and deployment of EPS. This study sheds light on the integration of EPS with CCU, closing the carbon cycle by effectively utilizing the products generated through the process, such as carbonate minerals and bicarbonate solution. For instance, the bicarbonate product can serve as a viable feedstock for bicarbonate-based microalgae production systems, with the added benefit of reducing costs by 40-80% compared to traditional gaseous CO2 delivery approaches. By integration of electrochemical technologies with CCU methods, this study underscores the immense potential for mitigating CO2 emissions and advancing sustainable practices to combat global warming. This study not only addresses the urgent need for effective solutions but also paves the way for a greener and more sustainable future.

Carbon Capture Using Porous Silica Materials

As the primary greenhouse gas, CO₂ emission has noticeably increased over the past decades resulting in global warming and climate change. Surprisingly, anthropogenic activities have increased atmospheric CO₂ by 50% in less than 200 years, causing more frequent and severe rainfall, snowstorms, flash floods, droughts, heat waves, and rising sea levels in recent times. Hence, reducing the excess CO₂ in the atmosphere is imperative to keep the global average temperature rise below 2 °C. Among many CO₂ mitigation approaches, CO₂ capture using porous materials is considered one of the most promising technologies. Porous solid materials such as carbons, silica, zeolites, hollow fibers, and alumina have been widely investigated in CO₂ capture technologies. Interestingly, porous silica-based materials have recently emerged as excellent candidates for CO₂ capture technologies due to their unique properties, including high surface area, pore volume, easy surface functionalization, excellent thermal, and mechanical stability, and low cost. Therefore, this review comprehensively covers major CO₂ capture processes and their pros and cons, selecting a suitable sorbent, use of liquid amines, and highlights the recent progress of various porous silica materials, including amine-functionalized silica, their reaction mechanisms and synthesis processes. Moreover, CO₂ adsorption capacities, gas selectivity, reusability, current challenges, and future directions of porous silica materials have also been discussed.

On the use of plastic precursors for preparation of activated carbons and their evaluation in CO2 capture for biogas upgrading: a review

In circular economy, useful plastic materials are kept in circulation as opposed to being landfilled, incinerated, or leaked into the natural environment. Pyrolysis is a chemical recycling technique useful for unrecyclable plastic wastes that produce gas, liquid (oil), and solid (char) products. Although the pyrolysis technique has been extensively studied and there are several installations applying it on the industrial scale, no commercial applications for the solid product have been found yet. In this scenario, the use of plastic-based char for the biogas upgrading may be a sustainable way to transform the solid product of pyrolysis into a particularly beneficial material. This paper reviews the preparation and main parameters of the processes affecting the final textural properties of the plastic-based activated carbons. Moreover, the application of those materials for the CO₂ capture in the processes of biogas upgrading is largely discussed.

Valorization of spent disposable wooden chopstick as the CO2 adsorbent for a CO2/H2 mixed gas purification

A series of activated carbons (ACs) derived from spent disposable wooden chopsticks was prepared via steam activation and used to separate carbon dioxide (CO₂) from a CO₂/hydrogen (H₂) mixed gas at atmospheric pressure. A factorial design was employed to investigate the effects of the activation temperature and time as well as their interactions on the production yield of ACs and their CO₂ adsorption capacity. The activation temperature exhibited a much higher impact on both the production yield and the CO₂ adsorption capacity of ACs than the activation time. The interaction of both parameters did not significantly affect the yield of ACs, but did affect the CO₂ adsorption capacity. The optimal preparation condition provided ACs with a desirable yield of around 23.18% and a CO₂ adsorption capacity of 85.19 mg/g at 25 °C and 1 atm and consumed the total energy of 225.28 MJ/kg AC or 116.4 MJ/g-mol CO₂. A H₂ purity of greater than 96.8 mol% was achieved from a mixed gas with low CO₂ concentration (< 20 mol%) during the first 3 min of adsorption and likewise around 90 mol% from a mixed gas with a high CO₂ concentration (> 30 mol%) during the first 2 min. The CO₂ adsorption on the as-prepared ACs proceeded dominantly via multilayer physical adsorption and was affected by both the surface area and micropore volume of the ACs. The adsorption capacity was diminished by around 18% after six adsorption/desorption cycles. The regeneration of the as-prepared chopstick-derived ACs can be easily performed via heating at a low temperature and ambient pressure, suggesting their potential application in the temperature swing adsorption process.

Triggered and controlled release of active gaseous/volatile compounds for active packaging applications of agri-food products: A review

Gaseous and volatile active compounds are versatile to enhance safety and preserve quality of agri-food products during storage and distribution. However, the use of these compounds is limited by their high vapor pressure and/or chemical instability, especially in active packaging (AP) applications. Various approaches for stabilizing and controlling the release of active gaseous/volatile compounds have been developed, including encapsulation (e.g., into supramolecular matrices, polymer-based films, electrospun nonwovens) and triggered release systems involving precursor technology, thereby allowing their safe and effective use in AP applications. In this review, encapsulation technologies of gases (e.g., CO₂ , ClO₂ , SO₂ , ethylene, 1-methylcyclopropene) and volatiles (e.g., ethanol, ethyl formate, essential oils and their constituents) into different solid matrices, polymeric films, and electrospun nonwovens are reviewed, especially with regard to encapsulation mechanisms and controlled release properties. Recent developments on utilizing precursor compounds of bioactive gases/volatiles to enhance their storage stability and better control their release profiles are discussed. The potential applications of these controlled release systems in AP of agri-food products are presented as well.

Synthesis of Silanol-Rich MCM-48 with Mixed Surfactants and Their Application in Acetone Adsorption: Equilibrium, Kinetic, and Thermodynamic Studies

Mesoporous silica MCM-48 with rich silanol was prepared using polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB) as mixed templates, and the dynamic adsorption performance of acetone was evaluated by testing breakthrough curves. The mixed micelle formed by CTAB and PVP, as well as the hydrogen bond between the carbonyl group of PVP and silanol group affected the condensation process of Si-OH group during the formation of mesoporous structure, resulting in the increase of Si-OH group number on the surface of MCM-48. Compared with MCM-48 synthesized by single template (CTAB), the acetone adsorption capacity of MCM-48 (1:3) synthesized by mixed templates (PVP:CTAB = 1:3) improved by 23.86%, which was attributed to the increase of silanol group amount and the decrease of pore size. In addition, Bangham model had the highest goodness of fit to describe the adsorption process among four kinetic models for the adsorbents, conforming to the mechanism of pore diffusion. The Langmuir and Freundlich models were used to fit the adsorption isotherm data, and the Freundlich model could better describe the adsorption of acetone. Freundlich model fitting results showed that MCM-48 with rich silanol had a strong affinity for acetone, and the adsorption of acetone on MCM-48 belonged to multilayer adsorption. The thermodynamic results showed that the adsorption of MCM-48 for acetone was physical adsorption, and the adsorption behavior was exothermic. This work provided insight into how the inherent properties of an adsorbent and environmental factors (including initial concentration and adsorption temperature) affected the adsorption performance of ketones, thus more ideas could be provided for the accurate design of adsorbents. Furthermore, silanol-rich MCM-48 synthesized by mixed templates is expected to be a promising adsorbent for acetone removal.

Novel biosorbents synthesized from fungal and bacterial biomass and their applications in the adsorption of volatile organic compounds

Adsorption is an efficient and low-cost technology used to purify volatile organic compounds (VOCs). In the current study, novel microbial adsorbents were synthesized using cells of lyophilized fungi (Ophiostoma stenoceras LLC) or bacteria (Pseudomonas veronii ZW) that were modified by aminomethylation. Based on the adsorption performance and structural characterization results, the modified fungal biosorbent was the best. Its maximum adsorption capacities for ethyl acetate, α-pinene, and n-hexane were 620, 454, and 374 mg·g^-1, respectively, which were much higher than those of other synthesized biosorbents. The specific surface area of the fungal biosorbent was 20 m²·g^-1, and most of the components were hydrocarbon compounds and polysaccharides. The VOC adsorption process on these synthesized biosorbents was in accordance with the Langmuir isothermal model and the pseudo-first-order kinetic model, thereby suggesting that physical adsorption was the dominant mechanism. The fungal biosorbent could be used for five consecutive VOC sorption-desorption cycles without any obvious decrease in adsorption capacity.

Characterization and Modelling Studies of Activated Carbon Produced from Rubber-Seed Shell Using KOH for CO2 Adsorption

Global warming due to the emission of carbon dioxide (CO2) has become a serious problem in recent times. Although diverse methods have been offered, adsorption using activated carbon (AC) from agriculture waste is regarded to be the most applicable one due to numerous advantages. In this paper, the preparation of AC from rubber-seed shell (RSS), an agriculture residue through chemical activation using potassium hydroxide (KOH), was investigated. The prepared AC was characterized by nitrogen adsorption–desorption isotherms measured in Micrometrices ASAP 2020 and FESEM. The optimal activation conditions were found at an impregnation ratio of 1:2 and carbonized at a temperature of 700 °C for 120 min. Sample A6 is found to yield the largest surface area of 1129.68 m2/g with a mesoporous pore diameter of 3.46 nm, respectively. Using the static volumetric technique evaluated at 25 °C and 1.25 bar, the maximum CO2 adsorption capacity is 43.509 cm3/g. The experimental data were analyzed using several isotherm and kinetic models. Owing to the closeness of regression coefficient (R2) to unity, the Freundlich isotherm and pseudo-second kinetic model provide the best fit to the experimental data suggesting that the RSS AC prepared is an attractive source for CO2 adsorption applications.

Enhanced CO2 capturing over ultra-microporous carbon with nitrogen-active species prepared using one-step carbonization of polybenzoxazine for a sustainable environment

Nitrogen-enriched porous carbon has been a promising material for CO₂ capture in the recent decades. To enhance the performance of CO₂ adsorption, both an N-active site and the textural properties are crucial determinants. Herein, ultra-microporous carbon with N-active species was prepared using two synthesis procedures: 1) one-step carbonization of a polybenzoxazine (PBZ) precursor at 800 °C, and 2) the CO₂ activation process at 900 °C. The activated porous carbon had the higher specific surface area (943 m²/g) and a total pore volume (0.51 cm³/g) compared to un-activated porous carbon (335 m²/g and 0.19 cm³/g, respectively). In addition, the presence of N-active species such as pyridine-N, secondary-N, pyridone-N, and oxide-N in the carbon structures could be clearly observed in the high-resolution XPS spectra. The CO₂ adsorption measurement was performed at 30 and 50 °C under a wide range of pressures (1-7 bar). The maximum amount of CO₂ uptake was ca. 3.59 mmol/g for the activated porous carbon operated at 30 °C and a CO₂ pressure of 7 bar, which was due to the high specific surface area and the large micropore volume. Specifically, carbon with a 3D interconnected pore structure, derived from the sol-gel process of the PBZ precursor, exhibited good structural stability and consequently led to better absorption capability under the high atmospheric pressure of CO₂. The enhanced CO₂ adsorption capability for the as-prepared porous carbon was based on two mechanisms: physisorption as a result of textural properties and chemisorption as a result of the acid-base interaction between the basic N functionality and the acidic CO₂ gas. All results suggested that ultra-microporous carbon with N-active species prepared from polybenzoxazine is a promising adsorbent for CO₂ capture and storage, which can be used at a wide range of pressures and in many applications e.g. flue gas adsorption and natural gas production.

Melamine-formaldehyde derived porous carbons for adsorption of CO2 capture

In this work, we report carbon adsorbents obtained from high nitrogen content melamine-formaldehyde resin as starting material and mesoporous zeolite MCM-41 as template through nanocasting technique. To synthesize different carbon structure adsorbents with improved textural and surface properties, the material undergo carbonization followed by physical activation under CO₂ atmosphere at different temperatures. Characterizations of the adsorbents using SEM, TEM, XPS, nitrogen sorption, CHN, TKN, and TPD have been carried out. Characterization results reveal the development of nanostructured carbon adsorbents with better texture and surface properties as compared to the sample prepared by direct carbonization. Sample prepared at carbonization-activation temperature of 700 °C shows highest basicity, surface area (193.28 m² g^-1) and pore volume (0.32 cm³ g^-1). Performance evaluation of adsorbent was performed thermo gravimetrically at different temperatures and concentrations and was found that the adsorbent synthesized at 700 °C exhibit highest CO₂ uptake of 0.93 mmol g^-1 with nitrogen content of 22.73%. It was found that both surface area and nitrogen functional group have a major impact on adsorption capacity. Physiosorption process was confirmed by a decrease in adsorption capacity with increase in temperature. Three kinetic models and isotherms were used in this study and found that fractional order kinetic model and Freundlich isotherm best fitted with the experimental data. Isotherm study depicts the heterogeneous nature of adsorbent surface. Adsorbent exhibited complete regenerability and was stable over four adsorption-desorption cycles. Low value of isosteric heat of adsorption of 15.75 kJ mol^-1, indicates physiosorption process. Negative value of ΔG⁰ and ΔH⁰ confirms spontaneous, feasible and exothermic nature of adsorption process.

Urea-formaldehyde derived porous carbons for adsorption of CO2

The aim of the research work is to develop high nitrogen content carbon adsorbents with high textural and surface properties using as a precursor urea-formaldehyde resin and as a template mesoporous-zeolite (MCM-41) through a nanocasting technique.

Synthesis of nitrogen doped mesoporous carbons for carbon dioxide capture

Nitrogen doped mesoporous carbons were prepared from melamine-formaldehyde resin and mesoporous silica by nanocasting method followed by their characterization and CO₂ adsorption performance evaluation by fixed-bed experiments.

Resorcinol–formaldehyde based nanostructured carbons for CO2 adsorption: kinetics, isotherm and thermodynamic studies

Silica templated nanostructured carbons were developed from a resorcinol–formaldehyde polymeric precursor by varying the carbonization temperature for capture of CO₂ from simulated flue gas under dynamic flow conditions.

Simulation of hydrogen sulphide absorption in alkaline solution using a packed column

In this work, a simulation tool was developed for hydrogen sulphide (H₂S) removal in an alkaline solution in packed columns working at countercurrent. Modelling takes into account the mass-transfer enhancement due to the reversible reactions between H₂S and the alkaline species (CO(²⁻)(3), HCO⁻(3), and HO⁻) in the liquid film. Many parameters can be controlled by the user such as the gas and liquid inlet H₂S concentrations, the gas and liquid flow rates, the scrubbing liquid pH, the desired H₂S removal efficiency, the temperature, the alkalinity, etc. Since the influence of the hydrodynamic and mass-transfer performances in a packed column is well known, the numerical resolutions performed were dedicated to the study of the influence of the chemical conditions (through the pH and the alkalinity), the temperature and the liquid-to-gas mass flow rate ratio (L/G). A packed column of 3 m equipped with a given random packing material working at countercurrent and steady state has been modelled. The results show that the H₂S removal efficiency increases with the L/G, the pH, the alkalinity and more surprisingly with the temperature. Alkalinity has a very significant effect on the removal efficiency through the mass-transfer enhancement and buffering effect, which limits pH decreasing due to H₂S absorption. This numerical resolution provides a tool for designers and researchers involved in H₂S treatment to understand deeper the process and optimize their processes.

Assessment of CO₂ adsorption capacity on activated carbons by a combination of batch and dynamic tests

In this work, batch and dynamic adsorption tests are coupled for an accurate evaluation of CO2 adsorption performance of three different activated carbons (AC) obtained from olive stones by chemical activation followed by physical activation with CO2 at varying times (i.e., 20, 40, and 60 h). Kinetic and thermodynamic CO2 adsorption tests from simulated flue gas at different temperatures and CO2 pressures are carried out under both batch (a manometric equipment operating with pure CO2) and dynamic (a lab-scale fixed-bed column operating with a CO2/N2 mixture) conditions. The textural characterization of the AC samples shows a direct dependence of both micropore and ultramicropore volume on the activation time; hence, AC60 has the higher contribution. The adsorption tests conducted at 273 and 293 K showed that when CO2 pressure is lower than 0.3 bar, the lower the activation time, the higher CO2 adsorption capacity; a ranking of ω(eq)(AC20) > ω(eq)(AC40) > ω(eq)(AC60) can be exactly defined when T = 293 K. This result is likely ascribed to the narrower pore size distribution of the AC20 sample, whose smaller pores are more effective for CO2 capture at higher temperature and lower CO2 pressure, the latter representing operating conditions of major interest for decarbonation of flue gas effluent. Moreover, the experimental results obtained from dynamic tests confirm the results derived from the batch tests in terms of CO2 adsorption capacity. It is important to highlight the fact that the adsorption of N2 on the synthesized AC samples can be considered to be negligible. Finally, the importance of proper analysis for data characterization and adsorption experimental results is highlighted for the correct assessment of the CO2 removal performance of activated carbons at different CO2 pressures and operating temperatures.