A review on solid adsorbents for carbon dioxide capture

Modification of Natural and Synthetic Zeolites for CO2 Capture: Unrevealing the Role of the Compensation Cations

The development of highly effective natural-based adsorbents to face the increasing rates of CO2 production and their delivery to the atmosphere are a big concern nowadays. For such purposes, synthetic and natural zeolites were modified via an ion exchange procedure to enhance the CO2 uptake. Samples were characterized by SEM, EDS, TGA and nitrogen adsorption at 77 K, showing the correct incorporation of the new metals; in addition, the CO2 adsorption isotherms were determined using a gas analyser. During the first stage, the role of the compensation cations for CO2 adsorption was assessed by modifying a pure ZSM-5 synthetic zeolite with different metal precursors present in salt solutions via an ion exchange procedure. Then, five samples were studied; the samples modified with bivalent cation precursors (Zn2+ and Cu2+) presented a higher adsorption uptake than those modified with a monovalent cation (Na+ and K+). Specifically, the substitution of the compensation cations for Cu2+ increased the CO2 capture uptake without affecting the surface properties of the zeolite. The results depict the prevalence of π-cation interactions enhanced by the field gradient induced by divalent cations and their lower ionic radii, if compared to monovalent ones. Subsequently, a natural zeolite was modified considering the best results of the previous phase. This Surface Response Methodology was implemented considering 11 samples by varying the concentration of the copper precursor and the time of the ion exchange procedure. A quantitative quadratic model to predict the adsorption uptake with an R2 of 0.92 was obtained. The results depicted the optimal conditions to modify the used natural zeolite for CO2 capture. The modification procedure implemented increased the CO2 adsorption capacity of the natural zeolite more than 20%, reaching an adsorption capacity of 75.8 mg CO2/g zeolite.

Study of the Electronic Structure of Coronene Doped with Nitrogen Atoms and Its Effect on CO2 Capture

Climate change is a serious global problem. CO2 is of paramount importance in mitigating this environmental problem. Understanding the interaction of CO2 with functionalized carbon structures is essential for designing new materials to aid in efficiently capturing CO2. In this work, the interaction between carbon dioxide (CO2) and coronene models, simulating graphene and the asphaltene moiety, was studied through DFT (CAM-B3LYP-D3) and DLPNO-CCSD(T) methods to investigate the effect of nitrogen doping in two arrangements. Aromaticity, electronic, and topological properties were evaluated using HOMA, HOMO-LUMO gap, QTAIM, and NCI methods. The results show that the adsorption of CO2 in the coronene molecule is dependent on the position of the heteroatom and governed by noncovalent interactions, such as van der Waals and hydrogen bonds. The CO2/N-coronene complex with pyridinic-N is stabilized due to two unconventional hydrogen bonds parallel to the aromatic π system. We hope that the present results can help the synthesis of inhibitors of asphaltene precipitation and better systems for CO2 capture.

Advanced Zeolite-Based Catalysts for CO2 Hydrogenation to Targeted High-Value Chemicals and Fuels

The excessive use of fossil fuels has resulted in elevated CO2 emissions in the atmosphere, significantly impacting the climate and global environment. The catalytic conversion of CO2 into high-value chemicals has been recognized as a promising strategy to mitigate CO2 emissions. Light olefins, aromatics, and alcohols, etc. are widely used high-value chemicals as fuels and chemical synthesis intermediates. To enhance the catalytic efficiency and selectivity for producing these chemicals, various catalysts have been developed. Among them, zeolite-based catalysts have garnered significant attention due to their unique microporous structure, shape-selective catalysis capability, high thermal stability, and tunable acidity. This article focuses on the distinctive structural characteristics of zeolites and their notable representative applications, with particular emphasis on the impact of zeolite structural properties on catalytic performance and reaction mechanism. Additionally, we discuss the current challenges of fabricating highly efficient zeolite-based catalysts and future development prospects in improving the catalytic performance and industrial-scale applications. We propose rational and strategic insights to pave the way for the efficient utilization of CO₂ as a valuable resource.

Exceptional CO2 and H2S adsorption by tuning micro/mesopore ratios with embedded graphene oxide/N-doped carbon quantum dots in MIL-101(Cr): Experimental and computational insights

Herein, a novel nanocomposite was developed to adjust the textural properties of metal-organic frameworks (MOFs) for adsorptive applications. To this end, nitrogen-doped carbon quantum dots/reduced graphene oxide nanocomposite (RC) was embedded into MIL-101(Cr) crystals, named RC-ML-x nanocomposites. The prepared nanoadsorbents were thoroughly characterized by different techniques. Results revealed that the main drawback of microporous MOFs, lack of mesopores, could be solved by embedding RC nanoparticles into MOFs, decreasing the micropore/mesopore volume ratio from 7.71 to 1.15. Optimizing the mesopore volume in RC-ML-1 dramatically improved the surface area and total pore volume by 40 % compared to pristine MIL-101(Cr). Adsorption experiments indicated that the sample containing 1 wt% had outstanding CO2 and H2S adsorption capacity of 25.79 and 34.15 mmol g-1 at 35 and 15 bar in 25 °C, respectively, elevated up to 15.80 % and 19.26 % compared to pristine MIL-101(Cr). This may be attributable to the cumulative effect of suitable micropore/mesopore volume ratio and the creation of the unsaturated metal sites and nitrogen functional groups by RC loading. In addition, the adsorption selectivity in different gas mixtures of CO2/CH4, H2S/CH4, CO2/N2, and H2S/N2 was analyzed by IAST. It was found that the samples containing 10 and 5 wt% had the highest selectivity toward CO2 and H2S, respectively, over N2 and CH4. Considering the simple approach adopted to tune the structure of microporous MOFs to achieve impressive gas adsorption and great cyclic capacity, the proposed RC-ML-x nanocomposites can be potential candidates for the adsorption and separation of CO2 and H2S.

Machine Learning Descriptors for CO2 Capture Materials

The influence of machine learning (ML) on scientific domains continues to grow, and the number of publications at the intersection of ML, CO2 capture, and material science is growing rapidly. Approaches for building ML models vary in both objectives and the methods through which materials are represented (i.e., featurised). Featurisation based on descriptors, being a crucial step in building ML models, is the focus of this review. Metal organic frameworks, ionic liquids, and other materials are discussed in this paper with a focus on the descriptors used in the representation of CO2-capturing materials. It is shown that operating conditions must be included in ML models in which multiple temperatures and/or pressures are used. Material descriptors can be used to differentiate the CO2 capture candidates through descriptors falling under the broad categories of charge and orbital, thermodynamic, structural, and chemical composition-based descriptors. Depending on the application, dataset, and ML model used, these descriptors carry varying degrees of importance in the predictions made. Design strategies can then be derived based on a selection of important features. Overall, this review predicts that ML will play an even greater role in future innovations in CO2 capture.

Facile synthesis of polyethyleneimine-modified cellulose nanocrystal/silica hybrid aerogel for CO2 adsorption

Cellulose nanocrystal (CNC)/silica hybrid aerogel (CSA) was synthesized from CNC and sodium silicate hybridization using the one-step sol-gel method under atmospheric drying. At a weight ratio of CNC to silica of 1:1, the obtained CSA-1 had a highly porous network, a high specific area of 479 m2 g-1, and a CO2 adsorption capacity of 0.25 mmol g-1. Then, polyethyleneimine (PEI) was impregnated on CSA-1 to improve CO2 adsorption performance. The parameters governing CO2 adsorption performance on CSA-PEI, such as temperatures (70-120 °C) and PEI concentrations (40-60 wt%), were investigated systematically. The optimum adsorbent (CSA-PEI50) exhibited an excellent CO2 adsorption capacity of 2.35 mmol g-1 at 70 °C and a PEI concentration of 50 wt%. The adsorption mechanism of CSA-PEI50 was elucidated by analyzing many adsorption kinetic models. The CO2 adsorption behaviors of CSA-PEI at various temperatures and PEI concentrations had the goodness of fit with the Avrami kinetic model, which can correspond to the multiple adsorption mechanism. The Avrami model also showed fractional reaction orders in a range of 0.352-0.613, and the root mean square error is negligible. Moreover, the rate-limiting kinetic analysis showed that film diffusion and intraparticle diffusion resistance controlled the adsorption speed and dominated the subsequent adsorption stages, respectively. The CSA-PEI50 also exhibited excellent stability after ten adsorption-desorption cycles. This study illustrated that CSA-PEI was a potential adsorbent for CO2 capture from flue gas.

Influence of the moisture and ash content in flue gases on the performance of adsorption processes using activated carbons to capture the CO2 for reuse in greenhouses

This work studies the influence of flue gas composition, its moisture and ash content, on the efficiency of a CO2 adsorption/desorption process to capture the CO2 from flue gases along with its subsequent reuse in greenhouse CO2 enrichment (Patent ES2514090). The influence of the inlet flow rate, moisture, and ash content were analysed. The experimental conditions were based on those that are achievable under real operating conditions, namely an inlet flow rate from 1.2 to 4.8 L per minute, humidity from 3 % to 65 %, and an ash concentration from 0 % to 1 %. The results show that the inlet flow had no effect on the adsorption capacity but that there was a reduction in the adsorption capacity at the higher humidity and ash content levels studied, of 10.5 % and 21 %, respectively. The data were used to develop models based on the Langmuir and Freundlich isotherm that fitted the experimental data with a reliability of 100 % and 80.1 %, respectively. This model was used to optimize the combustion gas variables and thus their influence on the final CO2 adsorption/desorption capacity. The techno-economic analysis performed confirmed a total cost reduction of 12 % when using the optimal combustion gas conditions (a relative humidity of 3 % and an ash concentration of 0 %) versus the worst gas conditions (a relative humidity of 65 % and an ash concentration of 1 %), which resulted in a saving of 60 % by avoiding the use of liquified CO2. These results confirm the technical and economic viability of the proposed technology and its potential contribution to improving the environmental and economic sustainability of agricultural food production.

Comparative Review for Enhancing CO2 Capture Efficiency with Mixed Amine Systems and Catalysts

This study investigates methods to enhance the efficiency of CO2 capture using organic amine absorption and compares the performance of traditional and novel amine solvents. It reviews various single-component and mixed amine absorbents, as well as catalysts used in these methods, highlighting the superiority of mixed amine absorbents over single-component amine absorbents in CO2 absorption and desorption. Additionally, the study explores the catalytic mechanisms and effects of catalysts in the CO2 absorption/desorption process with amine solvents and provides an outlook on future research directions. The aim is to promote the widespread adoption of organic amine absorption technology in industrial applications and to contribute to the development of more sustainable and efficient CO2 capture technologies.

Leveraging experimental and computational tools for advancing carbon capture adsorbents research

CO2 emissions have been steadily increasing and have been a major contributor for climate change compelling nations to take decisive action fast. The average global temperature could reach 1.5 °C by 2035 which could cause a significant impact on the environment, if the emissions are left unchecked. Several strategies have been explored of which carbon capture is considered the most suitable for faster deployment. Among different carbon capture solutions, adsorption is considered both practical and sustainable for scale-up. But the development of adsorbents that can exhibit satisfactory performance is typically done through the experimental approach. This hit and trial method is costly and time consuming and often success is not guaranteed. Machine learning (ML) and other computational tools offer an alternate to this approach and is accessible to everyone. Often, the research towards materials focuses on maximizing its performance under simulated conditions. The aim of this study is to present a holistic view on progress in material research for carbon capture and the various tools available in this regard. Thus, in this review, we first present a context on the workflow for carbon capture material development before providing various machine learning and computational tools available to support researchers at each stage of the process. The most popular application of ML models is for predicting material performance and recommends that ML approaches can be utilized wherever possible so that experimentations can be focused on the later stages of the research and development.

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.

Extraction of silica from sugarcane bagasse ash and its utilization in zeolite 4A synthesis for CO2 adsorption

Sugarcane bagasse ash (SCBA) is a solid waste containing a high amount of silica (SiO2) and is suitable to utilize as a silica source for synthesizing zeolite NaA. SCBA is typically calcined at high temperatures before silica extraction. The method is not environmentally friendly because it consumes energy and produces CO2. This work demonstrates an alternative extraction method of SiO2 from SCBA by treating it with hydrochloric (HCl) and sodium hydroxide (NaOH) solution. The obtained mixture was separated by paper filter No. 1 (P) and a combination of paper filter and syringe filter (PS). The solution was neutralized by HCl solution, producing silica (SiO2-P and SiO2-PS) with a purity of 98 wt%. Both SiO2 samples and SCBA were utilized to synthesize zeolite NaA for CO2 adsorption. The CO2 adsorption capacities of NaA-P and NaA-PS were 4.30 and 4.10 mmol gadsorbent -1, in the same range as commercial NaA. The capacity is influenced by the total basicity of zeolite. The CO2 adsorption behavior of all samples correlates well with the Toth model. The CO2 adsorption kinetics agrees well with the pseudo-second-order kinetic model. Overall, this work shows the successful extraction of silica via using a direct NaOH solution, yielding high-purity silica sufficient for synthesizing zeolite NaA, a promising adsorbent of CO2.

Activated carbons-preparation, characterization and their application in CO2 capture: A review

In this paper, we provide a comprehensive review of the latest research trends in terms of the preparation, and characteristics of activated carbons regarding CO2 adsorption applications, with a special focus on future investigation paths. The reported current research trends are primarily closely related to the synthesis conditions (carbonization and physical or chemical activation process), to develop the microporosity and surface area, which are the most important factors affecting the effectiveness of adsorption. Furthermore, we emphasized the importance of regeneration techniques as a factor determining the actual technological and economic suitability of a given material for CO2 capture application. Consequently, this work provides a summary and potential directions for the development of activated carbons (AC). We attempt to create a thorough theoretical foundation for activated carbons while also focusing on identifying and specific statements of the most relevant ongoing research scope that might be advantageous to progress and pursue in the coming years.

Facile Fabrication and Characterization of Amine-Functional Silica Coated Magnetic Iron Oxide Nanoparticles for Aqueous Carbon Dioxide Adsorption

Surface active amine-functionalized silica coated magnetic iron oxide nanoparticles were prepared by a simple two-step process for adsorbing CO2 gas from aqueous medium. First, oleic acid (OA) coated iron oxide magnetic particles (denoted as Fe3O4-OA) were prepared by a simple coprecipitation method. Then, the surface of the Fe3O4-OA particles was coated with silica by using tetraethyl orthosilicate. Finally, aminated Fe3O4/SiO2-NH2 nanoparticles were concomitantly formed by the reactions of 3-aminopropyl triethoxysilane with silica-coated particles. The formation of materials was confirmed by Fourier transform infrared spectral analysis. Transmission electron microscopic analysis revealed both spherical and needle-shaped morphologies of magnetic Fe3O4/SiO2-NH2 particles with an average size of 15 and 68.6 nm, respectively. The saturation magnetization of Fe3O4/SiO2-NH2 nanoparticles was found to be 33.6 emu g-1, measured by a vibrating sample magnetometer at ambient conditions. The crystallinity and average crystallite size (7.0 nm) of the Fe3O4/SiO2-NH2 particles were revealed from X-ray diffraction data analyses. Thermogravimetric analysis exhibited good thermal stability of the nanoadsorbent up to an elevated temperature. Zeta potential measurements revealed pH-sensitive surface activity of Fe3O4/SiO2-NH2 nanoparticles in aqueous medium. The produced magnetic Fe3O4/SiO2-NH2 nanoparticles also exhibited efficient proton capturing activity (92%). The particles were used for magnetically recyclable adsorption of aqueous CO2 at different pH values and temperatures. Fe3O4/SiO2-NH2 nanoparticles demonstrated the highest aqueous CO2 adsorption efficiency (90%) at 40 °C, which is clearly two times higher than that of nonfunctionalized Fe3O4-OA particles.

Valorization of Crab Shells as Potential Sorbent Materials for CO2 Capture

This study delves into the potential advantage of utilizing crab shells as sustainable solid adsorbents for CO2 capture, offering an environmentally friendly alternative to conventional porous adsorbents, such as zeolites, silicas, metal-organic frameworks (MOFs), and porous carbons. The investigation focuses on crab shell waste, which exhibits inherent natural porosity and N-bearing groups, making them promising candidates for CO2 physisorption and chemisorption applications. Selective deproteinization and demineralization treatments were used to enhance textural properties while preserving the natural porous structure of the crab shells. The impact of deproteinization and demineralization treatments on CO2 adsorption and speciation at the atomic scale, via solid-state NMR, and correlated findings with textural properties and biomass composition were investigated. The best-performing sample exhibits a surface area of 36 m2/g and a CO2 adsorption capacity of 0.31 mmol/g at 1 bar and 298 K, representing gains of ∼3.5 and 2, respectively, compared to the pristine crab shell. These results underline the potential of fishing industry wastes as a cost-effective, renewable, and eco-friendly source to produce functional porous adsorbents.

Understanding the Positive Role of Ionic Liquids in CO2 Capture by Poly(ethylenimine)

CO2 capture technology is one of the most important technical methods for significantly mitigating CO2 emissions in a low-carbon context. The recent invention of mixed absorbents comprising poly(ethylenimine) (PEI) and ionic liquids (ILs) provides a novel strategy for efficiently capturing CO2, and this has garnered widespread attention. However, the intermolecular interactions between the IL and other constituents during the CO2 absorption process remain unclear. In this present work, a series of density functional theory (DFT) calculations and molecular dynamics simulations were conducted to investigate the positive role of IL in CO2 capture by PEI. The results showed that the formation of hydrogen bonds between the IL anion and the amino groups of PEI primarily drives the addition of IL to PEI. During the CO2 absorption process, the IL anion not only can absorb CO2 but also exerts a dehydrogenation effect on the amino group of PEI, facilitating enhanced interaction between PEI and CO2. Additionally, the IL substantially reduces the viscosity of PEI, promoting the diffusion of CO2 within the system and enhancing the absorption rate. Based on the information on interaction energy and viscosity, we can easily make theoretical predictions for the optimal proportion of IL to be added. The above results provide fundamental insights to promote the industrial application of the PEI/IL system for CO2 capture.

Ca-Based Layered Double Hydroxides for Environmentally Sustainable Carbon Capture

The process of carbon dioxide capture typically requires a large amount of energy for the separation of carbon dioxide from other gases, which has been a major barrier to the widespread deployment of carbon capture technologies. Innovation of carbon dioxide adsorbents is herein vital for the attainment of a sustainable carbon capture process. In this study, we investigated the electrified synthesis and rejuvenation of calcium-based layered double hydroxides (Ca-based LDHs) as solid adsorbents for CO2. We discovered that the particle morphology and phase purity of the LDHs, along with the presence of secondary phases, can be controlled by tuning the current density during electrodeposition on a porous carbon substrate. The change in phase composition during carbonation and calcination was investigated to unveil the effect of different intercalated anions on the surface basicity and thermal stability of Ca-based LDHs. By decoupling the adsorption of water and CO2, we showed that the adsorbed water largely promoted CO2 adsorption, most likely through a sequential dissolution and reaction pathway. A carbon capture capacity of 4.3 ± 0.5 mmol/g was measured at 30 °C and relative humidity of 40% using 10 vol % CO2 in nitrogen as the feed stream. After CO2 capture occurred, the thermal regeneration step was carried out by directly passing an electric current through the conductive carbon substrate, known as the Joule-heating effect. CO2 was found to start desorbing from the Ca-based LDHs at a temperature as low as 220 °C as opposed to the temperature above 700 °C required for calcium carbonate that forms as part of the Ca-looping capture process. Finally, we evaluated the cumulative energy demand and environmental impact of the LDH-based capture process using a life cycle assessment. We identified the most environmentally concerning step in the process and concluded that the postcombustion CO2 capture using LDH could be advantageous compared with existing technologies.

Enhancing MgO efficiency in CO2 capture: engineered MgO/Mg(OH)2 composites with Cl-, SO42-, and PO43- additives

The formation of a MgCO3 shell hampers CO2 capture efficiency in MgO. Our previous studies developed MgO/Mg(OH)2 composites to facilitate CO2 diffusion, improving capture efficiency. However, MgCO3 still formed along the interfaces. To tackle this issue, we engineered the MgO/Mg(OH)2 interfaces by incorporating Cl-, SO42-, and PO43- additives. Novel MgO-H2O-MgX (X = Cl-, SO42-, and PO43-) composites were synthesized to explore the role of additives in preventing MgCO3 formation. MgO-Mg(OH)2-MgCl2 nano-composites displayed enhanced CO2 adsorption and stability. This breakthrough paves the way for effective bio-inspired strategies in overcoming CO2 transport barriers in MgO-based adsorbents.

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.

Synthesis of Mesoporous Carbon Adsorbents Using Biowaste Crude Glycerol as a Carbon Source via a Hard Template Method for Efficient CO2 Capture

Biowaste utilization as a carbon source and its transformation into porous carbons have been of great interest to promote environmental remediation owing to biowaste's cost-effectiveness and useful physicochemical properties. In this work, crude glycerol (CG) residue from waste cooking oil transesterification was employed to fabricate mesoporous crude glycerol-based porous carbons (mCGPCs) using mesoporous silica (KIT-6) as a template. The obtained mCGPCs were characterized and compared to commercial activated carbon (AC) and CMK-8, a carbon material prepared using sucrose. The study aimed to evaluate the potential of mCGPC as a CO₂ adsorbent and demonstrated its superior adsorption capacity compared to AC and comparable to CMK-8. The X-ray diffraction (XRD) and Raman results clearly depicted the structure of carbon nature with (002) and (100) planes and defect (D) and graphitic (G) bands, respectively. The specific surface area, pore volume, and pore diameter values confirmed the mesoporosity of mCGPC materials. The transmission electron microscopy (TEM) images also clearly revealed the porous nature with the ordered mesopore structure. The mCGPCs, CMK-8, and AC materials were used as CO₂ adsorbents under optimized conditions. The mCGPC adsorption capacity (1.045 mmol/g) is superior to that of AC (0.689 mmol/g) and still comparable to that of CMK-8 (1.8 mmol/g). The thermodynamic analyses of the adsorption phenomena are also carried out. This work demonstrates the successful synthesis of a mesoporous carbon material using a biowaste (CG) and its application as a CO₂ adsorbent.

Engineering of Silica Mesoporous Materials for CO2 Adsorption

Adsorption methods for CO₂ capture are characterized by high selectivity and low energy consumption. Therefore, the engineering of solid supports for efficient CO₂ adsorption attracts research attention. Modification of mesoporous silica materials with tailor-made organic molecules can greatly improve silica's performance in CO₂ capture and separation. In that context, a new derivative of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, possessing an electron-rich condensed aromatic structure and also known for its anti-oxidative properties, was synthesized and applied as a modifying agent of 2D SBA-15, 3D SBA-16, and KIT-6 silicates. The physicochemical properties of the initial and modified materials were studied using nitrogen physisorption and temperature-gravimetric analysis. The adsorption capacity of CO₂ was measured in a dynamic CO₂ adsorption regime. The three modified materials displayed a higher capacity for CO₂ adsorption than the initial ones. Among the studied sorbents, the modified mesoporous SBA-15 silica showed the highest adsorption capacity for CO₂ (3.9 mmol/g). In the presence of 1 vol.% water vapor, the adsorption capacities of the modified materials were enhanced. Total CO₂ desorption from the modified materials was achieved at 80 °C. The obtained silica materials displayed stable performance in five CO₂ adsorption/desorption cycles. The experimental data can be appropriately described by the Yoon-Nelson kinetic model.

Research progress and perspectives on carbon capture, utilization, and storage (CCUS) technologies in China and the USA: a bibliometric analysis

Carbon dioxide capture, utilization, and storage (CCUS) technology is an emerging technology with large-scale emission reduction potential and an essential component of the global response to climate change to achieve net-zero goals. As the two most important countries in global climate governance, it is necessary to review and examine the current status and trends of research in the field of CCUS in China and the USA. This paper uses bibliometric tools to review and analyze peer-reviewed articles in the Web of Science from both countries during 2000-2022. The results show a significant increase in research interest among scholars from both countries. The number of publications in the CCUS field in China and the USA was 1196 and 1302, respectively, showing an increasing trend. China and the USA have become the most influential countries in CCUS. And the USA has a more significant academic influence on a global scale. Furthermore, the research hotspots in the field of CCUS are diverse and differentiated. That is, China and the USA pay attention to different research hotspots or have different focuses in different periods. This paper also finds that new capture materials and technology development, geological storage monitoring and early warning, CO₂ utilization and new energy development, sustainable business models, incentive policies and measures, and public awareness are critical directions for future research in the field of CCUS, to provide a comprehensive review and comparison of CCUS technology development in China and the USA. It helps to gain insight into the research differences and linkages between the two countries in the field of CCUS and identify the research gaps between them. And place some consensus that policymakers can use.

Valorization of lignocellulosic biomass into sustainable materials for adsorption and photocatalytic applications in water and air remediation

An exponential rise in global pollution and industrialization has led to significant economic and environmental problems due to the insufficient application of green technology for the chemical industry and energy production. Nowadays, the scientific and environmental/industrial communities push to apply new sustainable ways and/or materials for energy/environmental applications through the so-called circular (bio)economy. One of today's hottest topics is primarily valorizing available lignocellulosic biomass wastes into valuable materials for energy or environmentally related applications. This review aims to discuss, from both the chemistry and mechanistic points of view, the recent finding reported on the valorization of biomass wastes into valuable carbon materials. The sorption mechanisms using carbon materials prepared from biomass wastes by emphasizing the relationship between the synthesis route or/and surface modification and the retention performance were discussed towards the removal of organic and heavy metal pollutants from water or air (NO_x, CO₂, VOCs, SO₂, and Hg⁰). Photocatalytic nanoparticle-coated biomass-based carbon materials have proved to be successful composites for water remediation. The review discusses and simplifies the most raised interfacial, photonic, and physical mechanisms that might take place on the surface of these composites under light irradiation. Finally, the review examines the economic benefits and circular bioeconomy and the challenges of transferring this technology to more comprehensive applications.

One-Pot Synthesis Method of MIL-96 Monolith and Its CO2 Adsorption Performance

A novel preparation method was proposed for a metal-organic framework (MOF) monolith using a simple one-pot synthesis method. A MOF tubular monolith was successfully prepared by the hydrothermal treatment for an α-Al₂O₃ monolith in an aqueous solution of 1,3,5-benzenetricarboxylic acid and nitric acid without the addition of a metal source. The effects of temperature and the HNO₃ concentration in the synthesis solution on the crystallization behavior of MIL-96 were studied. HNO₃ enhanced the dissolution of the α-Al₂O₃ monolith and the growth of MIL-96. The growth rate of MIL-96 was also influenced by the synthesis temperature; a synthesis temperature of over 453 K was required for crystallization. The CO₂ adsorption capacity of the prepared MIL-96 monoliths was evaluated and found to be comparable to that of the well-grown MIL-96 powdery crystal. Furthermore, the MIL-96 monoliths demonstrated good stability as their adsorption properties were retained even after 2 months of storage under atmospheric conditions.

Carbon dioxide separation and capture by adsorption: a review

Rising adverse impact of climate change caused by anthropogenic activities is calling for advanced methods to reduce carbon dioxide emissions. Here, we review adsorption technologies for carbon dioxide capture with focus on materials, techniques, and processes, additive manufacturing, direct air capture, machine learning, life cycle assessment, commercialization and scale-up.

Development of an Integrated System for Highly Selective Photoenzymatic Synthesis of Formic Acid from CO2

Herein, a Zr-based dual-ligand MOFs with pre-installed Rh complex was employed for NADH regeneration in situ and also used for immobilization of formic acid dehydrogenase (FDH) in order to realize a highly efficient CO₂ fixation system. Then, based on the detailed investigations into the photochemical and electrochemical properties, it is demonstrated that the introduction of the photosensitive meso-tetra(4-carboxyphenyl) porphin (TCPP) ligands increased the catalytic active sites and improved photoelectric properties. Furthermore, the electron mediator Rh complex, anchored on the zirconium-based dual-ligand MOFs, enhanced the efficiency of electron transfer efficiency and facilitated the separation of photogenerated electrons and holes. Compared with UiO-66-NH₂ , Rh-H₂ TCPP-UiO-66-NH₂ exhibits an optimized valence band structure and significantly improved photocatalytic activity for NAD⁺ reduction, resulting the synthesis of formic acid from CO₂ increased from 150 μg mL^-1 (UiO-66-NH₂ ) to 254 μg mL^-1 (Rh-H₂ TCPP-UiO-66-NH₂ ). Moreover, the assembled photocatalyst-enzyme coupled system also allows facile recycling of expensive electron mediator, enzyme, and photocatalyst.

Microporous Borocarbonitrides BxCyNz: Synthesis, Characterization, and Promises for CO2 Capture

Porous borocarbonitrides (denoted BCN) were prepared through pyrolysis of the polymer stemmed from dehydrocoupled ethane 1,2-diamineborane (BH₃NH₂CH₂CH₂NH₂BH₃, EDAB) in the presence of F-127. These materials contain interconnected pores in the nanometer range with a high specific surface area up to 511 m² · g^-1. Gas adsorption of CO₂ demonstrated an interesting uptake (3.23 mmol · g^-1 at 0 °C), a high CO₂/N₂ selectivity as well as a significant recyclability after several adsorption-desorption cycles. For comparison's sake, a synthesized non-porous BCN as well as a commercial BN sample were studied to investigate the role of porosity and carbon doping factors in CO₂ capture. The present work thus tends to demonstrate that the two-step synthesis of microporous BCN adsorbent materials from EDAB using a bottom-up approach (dehydrocoupling followed by pyrolysis at 1100 °C) is relatively simple and interesting.

Carbon Capture Materials in Post-Combustion: Adsorption and Absorption-Based Processes

Global warming and climate changes are among the biggest modern-day environmental problems, the main factor causing these problems is the greenhouse gas effect. The increased concentration of carbon dioxide in the atmosphere resulted in capturing increased amounts of reflected sunlight, causing serious acute and chronic environmental problems. The concentration of carbon dioxide in the atmosphere reached 421 ppm in 2022 as compared to 280 in the 1800s, this increase is attributed to the increased carbon dioxide emissions from the industrial revolution. The release of carbon dioxide into the atmosphere can be minimized by practicing carbon capture utilization and storage methods. Carbon capture utilization and storage (CCUS) has four major methods, namely, pre-combustion, post-combustion, oxyfuel combustion, and direct air capture. It has been reported that applying CCUS can capture up to 95% of the produced carbon dioxide in running power plants. However, a reported cost penalty and efficiency decrease hinder the wide applicability of CCUS. Advancements in the CCSU were made in increasing the efficiency and decreasing the cost of the sorbents. In this review, we highlight the recent developments in utilizing both physical and chemical sorbents to capture carbon. This includes amine-based sorbents, blended absorbents, ionic liquids, metal-organic framework (MOF) adsorbents, zeolites, mesoporous silica materials, alkali-metal adsorbents, carbonaceous materials, and metal oxide/metal oxide-based materials. In addition, a comparison between recently proposed kinetic and thermodynamic models was also introduced. It was concluded from the published studies that amine-based sorbents are considered assuperior carbon-capturing materials, which is attributed to their high stability, multifunctionality, rapid capture, and ability to achieve large sorption capacities. However, more work must be done to reduce their cost as it can be regarded as their main drawback.

Nuclear magnetic resonance studies of carbon dioxide capture

Carbon dioxide capture is an important greenhouse gas mitigation technology that can help limit climate change. The design of improved capture materials requires a detailed understanding of the mechanisms by which carbon dioxide is bound. Nuclear magnetic resonance (NMR) spectroscopy methods have emerged as a powerful probe of CO2 sorption and diffusion in carbon capture materials. In this article, we first review the practical considerations for carrying out NMR measurements on capture materials dosed with CO2 and we then present three case studies that review our recent work on NMR studies of CO2 binding in metal-organic framework materials. We show that simple 13C NMR experiments are often inadequate to determine CO2 binding modes, but that more advanced experiments such as multidimensional NMR experiments and 17O NMR experiments can lead to more conclusive structural assignments. We further discuss how pulsed field gradient (PFG) NMR can be used to explore diffusion of adsorbed CO2 through the porous framework. Finally, we provide an outlook on the challenges and opportunities for the further development of NMR methodologies that can improve our understanding of carbon capture.

Zeolites in Adsorption Processes: State of the Art and Future Prospects

Zeolites have been widely used as catalysts, ion exchangers, and adsorbents since their industrial breakthrough in the 1950s and continue to be state-of the-art adsorbents in many separation processes. Furthermore, their properties make them materials of choice for developing and emerging separation applications. The aim of this review is to put into context the relevance of zeolites and their use and prospects in adsorption technology. It has been divided into three different sections, i.e., zeolites, adsorption on nanoporous materials, and chemical separations by zeolites. In the first section, zeolites are explained in terms of their structure, composition, preparation, and properties, and a brief review of their applications is given. In the second section, the fundamentals of adsorption science are presented, with special attention to its industrial application and our case of interest, which is adsorption on zeolites. Finally, the state-of-the-art relevant separations related to chemical and energy production, in which zeolites have a practical or potential applicability, are presented. The replacement of some of the current separation methods by optimized adsorption processes using zeolites could mean an improvement in terms of sustainability and energy savings. Different separation mechanisms and the underlying adsorption properties that make zeolites interesting for these applications are discussed.

Tailoring of Textural Properties of 3D Reduced Graphene Oxide Composite Monoliths by Using Highly Crosslinked Polymer Particles toward Improved CO2 Sorption

The main constraint on developing a full potential for CO₂ adsorption of 3D composite monoliths made of reduced graphene oxide (rGO) and polymer materials is the lack of control of their textural properties, along with the diffusional limitation to the CO₂ adsorption due to the pronounced polymers' microporosity. In this work, the textural properties of the composites were altered by employing highly crosslinked polymer particles, synthesized by emulsion polymerization in aqueous media. For that aim, waterborne methyl methacrylate (MMA) particles were prepared, in which the crosslinking was induced by using different quantities of divinyl benzene (DVB). Afterward, these particles were combined with rGO platelets and subjected to the reduction-induced self-assembly process. The resulting 3D monolithic porous materials certainly presented improved textural properties, in which the porosity and BET surface area were increased up to 100% with respect to noncrosslinked composites. The crosslinked density of MMA polymer particles was a key parameter controlling the porous properties of the composites. Consequently, higher CO₂ uptake than that of neat GO structures and composites made of noncrosslinked MMA polymer particles was attained. This work demonstrates that a proper control of the microstructure of the polymer particles and their facile introduction within rGO self-assembly 3D structures is a powerful tool to tailor the textural properties of the composites toward improved CO₂ capture performance.

Optimization of CO2 Sorption onto Spent Shale with Diethylenetriamine (DETA) and Ethylenediamine (EDA)

A novel technique was employed to optimize the CO₂ sorption performance of spent shale at elevated pressure-temperature (PT) conditions. Four samples of spent shale prepared from the pyrolysis of oil shale under an anoxic condition were further modified with diethylenetriamine (DETA) and ethylenediamine (EDA) through the impregnation technique to investigate the variations in their physicochemical characteristics and sorption performance. The textural and structural properties of the DETA- and EDA- modified samples revealed a decrease in the surface area from tens of m²/g to a unit of m²/g due to the amine group dispersing into the available pores, but the pore sizes drastically increased to macropores and led to the creation of micropores. The N-H and C-N bonds of amine noticed on the modified samples exhibit remarkable affinity for CO₂ sequestration and are confirmed to be thermally stable at higher temperatures by thermogravimetric (TG) analysis. Furthermore, the maximum sorption capacity of the spent shale increased by about 100% with the DETA modification, and the equilibrium isotherm analyses confirmed the sorption performance to support heterogenous sorption in conjunction with both monolayer and multilayer coverage since they agreed with the Sips, Toth, Langmuir, and Freundlich models. The sorption kinetics confirm that the sorption process is not limited to diffusion, and both physisorption and chemisorption have also occurred. Furthermore, the heat of enthalpy reveals an endothermic reaction observed between the CO₂ and amine-modified samples as a result of the chemical bond, which will require more energy to break down. This investigation reveals that optimization of spent shale with amine functional groups can enhance its sorption behavior and the amine-modified spent shale can be a promising sorbent for CO₂ sequestration from impure steams of the natural gas.

Evaluation of the Stability of Diamine-Appended Mg2(dobpdc) Frameworks to Sulfur Dioxide

Diamine-appended Mg₂(dobpdc) (dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks are a promising class of CO₂ adsorbents, although their stability to SO₂─a trace component of industrially relevant exhaust streams─remains largely untested. Here, we investigate the impact of SO₂ on the stability and CO₂ capture performance of dmpn-Mg₂(dobpdc) (dmpn = 2,2-dimethyl-1,3-propanediamine), a candidate material for carbon capture from coal flue gas. Using SO₂ breakthrough experiments and CO₂ isobar measurements, we find that the material retains 91% of its CO₂ capacity after saturation with a wet simulated flue gas containing representative levels of CO₂ and SO₂, highlighting the robustness of this framework to SO₂ under realistic CO₂ capture conditions. Initial SO₂ cycling experiments suggest dmpn-Mg₂(dobpdc) may achieve a stable operating capacity in the presence of SO₂ after initial passivation. Evaluation of several other diamine-Mg₂(dobpdc) variants reveals that those with primary,primary (1°,1°) diamines, including dmpn-Mg₂(dobpdc), are more robust to humid SO₂ than those featuring primary,secondary (1°,2°) or primary,tertiary (1°,3°) diamines. Based on the solid-state ¹⁵N NMR spectra and density functional theory calculations, we find that under humid conditions, SO₂ reacts with the metal-bound primary amine in 1°,2° and 1°,3° diamine-appended Mg₂(dobpdc) to form a metal-bound bisulfite species that is charge balanced by a primary ammonium cation, thereby facilitating material degradation. In contrast, humid SO₂ reacts with the free end of 1°,1° diamines to form ammonium bisulfite, leaving the metal-diamine bond intact. This structure-property relationship can be used to guide further optimization of these materials for CO₂ capture applications.

Effect of calcium on enhanced carbon capture potential of coal fly ash zeolites. Part II: a study on the adsorption mechanisms

Coal fly ash zeolites (CFAZs) of type X with low (Na-X) and medium (Na-Ca-X) content of calcium were synthesized by alkaline conversion of lignite coal fly ash generated by combustion of lignite with lower and higher limestone amounts, extracted from different coal deposits and burned in separate thermal power plants. The concentration and state of Ca in the zeolites were investigated by energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). CFAZs Na-Ca-X with a medium Ca-content were found to outperform their lower calcium counterparts Na-X in terms of carbon capture capacity. This provoked our research interest in elucidating the role of Ca in the CO₂ retaining mechanism. CFAZs Na-X and Na-Ca-X were studied in consecutive dynamic adsorption cycles, after thermal regeneration at a temperature of 80 °C, at which only the physically adsorbed CO₂ molecules can be released. Desorption was investigated by thermogravimetric TG-DTG analyses over a wide temperature range. In situ Fourier-transform infrared (FTIR) spectroscopy was performed to elucidate the structural features of Na-X and Na-Ca-X CFAZs and the types of their accessible adsorption sites. It was found that the role of Ca is to increase the number of accessible Na⁺ cations in S_II and S_III positions in the zeolites where they can simultaneously adsorb two CO or CO₂ molecules, which benefits their adsorption capacity. This study raises an issue on coal ash classification for the synthesis of zeolites with carbon capture applications.

Experimental and theoretical study of the effect of different functionalities of graphene oxide/polymer composites on selective CO2 capture

There is a constant need for versatile technologies to reduce the continuously increasing concentration of CO₂ in the atmosphere, able to provide effective solutions under different conditions (temperature, pressure) and composition of the flue gas. In this work, a combination of graphene oxide (GO) and functionalized waterborne polymer particles was investigated, as versatile and promising candidates for CO₂ capture application, with the aim to develop an easily scalable, inexpensive, and environmentally friendly CO₂ capture technology. There are huge possibilities of different functional monomers that can be selected to functionalize the polymer particles and to provide CO₂-philicity to the composite nanostructures. Density functional theory (DFT) was employed to gain a deeper understanding of the interactions of these complex composite materials with CO₂ and N₂ molecules, and to build a basis for efficient screening for functional monomers. Estimation of the binding energy between CO₂ and a set of GO/polymer composites, comprising copolymers of methyl methacrylate, n-butyl acrylate, and different functional monomers, shows that it depends strongly on the polymer functionalities. In some cases, there is a lack of cooperative effect of GO. It is explained by a remarkably strong GO-polymer binding, which induced less effective CO₂-polymer interactions. When compared with experimental results, in the cases when the nanocomposite structures presented similar textural properties, the same trends for selective CO₂ capture over N₂ were attained. Besides novel functional materials for CO₂ capture and a deeper understanding of the interactions between CO₂ molecules with various materials, this study additionally demonstrates that DFT calculations can be a shorter route toward the efficient selection of the best functionalization of the composite materials for selective CO₂ capture.

Polyanionic cyano-fullerides for CO2 capture: a DFT prediction

The reaction of C₆₀ fullerene with 'n' molecules (n = 1 to 6) of 1,3-dimethyl-2,3-dihydro-2-cyano-imidazole (IMCN) results in the exothermic formation of imidazolium cation-polyanionic fulleride complexes, (IM⁺)_n⋯((C₆₀(CN)_n)^n-). The binding energy of IM⁺ with (C₆₀(CN)_n)^n- in the imidazolium-fulleride ionic complexes increased from -69.6 kcal mol^-1 for n = 1 to -202.9 kcal mol^-1 for n = 6. The energetics of the complex formation and cation-anion interaction energy data suggest the formation of imidazolium-fulleride ionic liquid (IL) systems. Furthermore, the dimer formation of such ionic complexes showed more exergonic nature due to multiple cooperative electrostatic interactions between oppositely charged species and suggested improved energetics for higher order clusters. The molecular electrostatic potential (MESP) analysis has revealed that the extra 'n' electrons in the ionic complex as well as that in the bare (C₆₀(CN)_n)^n- are delocalized mainly on the unsaturated carbon centers of the fullerene unit, while the CN groups remain as a neutral unit. The MESP minimum (V_min) values of (C₆₀(CN)_n)^n- on the carbon cage have shown that the addition of each CN^- unit on the cage enhances the negative character of V_min by ∼54.7 kcal mol^-1. This enhancement in MESP is comparable to the enhancement observed when one electron is added to C₆₀ to produce (^-62.5 kcal mol^-1) and suggests that adding 'n' CN^- groups to the fullerene cage is equivalent to supplying 'n' electrons to the carbon cage. Also the high capacity of the fullerene cage to hold several electrons can be attributed to the spherical delocalization of them onto the electron deficient carbon cage. The interactive behavior of CO₂ molecules with (IM⁺)_n⋯(C₆₀(CN)_n)^n- systems showed that the interaction becomes stronger from -2.3 kcal mol^-1 for n = 1 to -18.6 kcal mol^-1 for n = 6. From the trianionic fulleride onwards, the C⋯CO₂ noncovalent (nc) interaction changes to C-CO₂ covalent (c) interaction with the development of carboxylate character on the adsorbed CO₂. These results prove that cyano-fullerides are promising candidates for CO₂ capture.

Polyoxometalate-based materials in extraction, and electrochemical and optical detection methods: A review

Polyoxometalates (POMs) as metal-oxide anions have exceptional properties like high negative charges, remarkable redox abilities, unique ligand properties and availability of organic grafting. Moreover, the amenability of POMs to modification with different materials makes them suitable as precursors to further obtain new composites. Due to their unique attributes, POMs and their composites have been utilized as adsorbents, electrodes and catalysts in extraction, and electrochemical and optical detection methods, respectively. A survey of the recent progress and developments of POM-based materials in these methods is therefore desirable, and should be of great interest. In this review article, POM-based materials, their properties as well as their identification methods, and analytical applications as adsorbents, electrodes and catalysts, and corresponding mechanisms of action, where relevant, are reviewed. Some current issues of the utilization of these materials and their future prospects in analytical chemistry are discussed.

Mixed Matrix Membranes Loaded with a Porous Organic Polymer Having Bipyridine Moieties

Mixed matrix membranes (MMMs), derived from three aromatic polyimides (PIs), and an affordable porous organic polymer (POP) having basic bipyridine moieties were prepared. Matrimid and two fluorinated polyimides, which were derived from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride and 2,2′-bis(4-aminophenyl)hexafluoropropane (6F6F) or 2,4,6-trimethyl-m-phenylenediamine (6FTMPD), were employed as polymer matrixes. The used POP was a highly microporous material (surface area of 805 m2 g−1) with excellent thermal and chemical stability. The MMMs showed good compatibility between the PIs and POP, high thermal stabilities and glass transition temperatures superior to those of the neat PI membranes, and good mechanical properties. The addition of POP to the matrix led to an increase in the gas diffusivity and, thus, in permeability, which was associated with an increase in the fractional free volume of MMMs. The increase in permeability was higher for the less permeable matrix. For example, at 30 wt.% of POP, the permeability to CO2 and CH4 of the MMMs increased by 4- and 7-fold for Matrimid and 3- and 4-fold for 6FTMPD. The highest CH4 permeability led to a decrease in CO2/CH4 selectivity. The CO2/N2 separation performance was interesting, as the selectivity remained practically constant. Finally, the POP showed no molecular sieving effect towards the C2H4/C2H6 and C3H6/C3H8 gas pairs, but the permeability increased by about 4-fold and the selectivity was close to that of the matrix. In addition, because the POP can form metal ion bipyridine complexes, modified POP-based MMMs could be employed for olefin/paraffin separations.

CO2 Capture and Low-Temperature Release by Poly(aminoethyl methacrylate) and Derivatives

Poly(aminoethyl methacrylate) (PAEMA), poly(ethylene oxide)-block-(aminoethyl methacrylate) (PEO-PAEMA), and their guanidinylated derivates, poly(guanidine ethyl methacrylate) (PGEMA) and poly(ethylene oxide)-block-(guanidine ethyl methacrylate) (PEO-PGEMA), were prepared to study their capabilities for CO₂ adsorption and release. The polymers of different forms or degree of guanidinylation were thoroughly characterized, and their interaction with CO₂ was studied by NMR and calorimetry. The extent and kinetics of adsorption and desorption of N₂ and CO₂ were investigated by thermogravimetry under controlled gas atmospheres. The materials did not adsorb N₂, whereas CO₂ could be reversibly adsorbed at room temperature and released by an elevated temperature. The most promising polymer was PGEMA with a guanidinylation degree of 7% showing a CO₂ adsorption capacity of 2.4 mmol/g at room temperature and a desorption temperature of 72 °C. The study also revealed relations between the polymer chemical composition and CO₂ adsorption and release characteristics that are useful in future formulations for CO₂ adsorbent polymer materials.

A new comprehensive evaluation indicator of adsorbent for gas separation

To evaluate and compare the performances of different adsorbents for gas separation, an adsorbent comprehensive evaluation indicator (ACEI) is proposed. In the ACEI, the working capacity, selectivity, adsorption rate, regeneration and circulation parameters, stability during adsorption, and tolerance to impurities are considered, and some exponents are introduced to allow the ACEI to adapt to different adsorbent working conditions. To illustrate the applicability of the ACEI, the adsorption of CO₂ and N₂ was investigated on two kinds of activated carbons (ACs), which were used for gas purification and bulk separation. The ACEI is a better overall indicator than other currently used indicators to evaluate the performance of adsorbents.

Model Analysis for the Implementation of a Fast Model Predictive Control Scheme on the Absorption/Stripping CO2 Capture Plants

The purpose of this paper is to investigate the possible implementation of the Fast model predictive control (MPC) scheme for chemical systems. Due to the difficulties associated with complicated dynamic behavior and model sensitivity, which results in considerable offsets, the Fast MPC controller has not been implemented on the CO₂ capture plant based on the absorption/stripping system. The main objective of this work is to evaluate the most appropriate model for implementing the Fast MPC control strategy, which results in fast output responses, negligible offsets, and minimum errors. The steady-state and dynamic simulation models of the CO₂ capture plant are designed in Aspen PLUS. In the System Identification Toolbox, multiple state-space models are identified to achieve a highly accurate model for the Fast MPC controller. The Fast MPC controller is then implemented to evaluate the performance under a setpoint tracking mode with ±5 and ±15% step changes. The results showed that the Fast MPC based on the state-space prediction focus model has on average 7.9 times lower offset than the simulation focus model and 10.4 times lower integral absolute error values. The comparison study concluded that the Fast MPC control strategy performs efficiently using prediction-based focus state-space models for CO₂ capture plants using the absorption/stripping system with minimum offsets and errors.