CO2 Uptake Performance Research Articles

Hierarchically porous MgO/biochar composites for efficient CO2 capture: Structure, performance and mechanism

For enhancing the CO2 uptake performance of MgO-based materials, hierarchically porous MgO/biochar composites were prepared using a microwave-assisted hydrothermal method. The CO2 adsorption properties was investigated in detail and the adsorption mechanism was revealed by considering structure − function relationships and performing density functional theory calculations. The MgO/biochar-1 had hierarchical pore structure and high specific surface area (1453 m2·g−1) with abundant narrow micropores (<1.0 nm), facilitating the adsorption of CO2. The results demonstrated that MgO/biochar-1 exhibited the highest CO2 capture capacity of 6.65 mol⋅kg−1 (273 K, 1 bar) and excellent selectivity for CO2/N2 (96.70 at CO2/N2 = 15:85, v/v). After 3 cycles, the adsorption capacity of MgO/biochar-1 still remained at 5.70 mol⋅kg−1 (273 K, 1 bar), suggesting the well cycling performance. The results of density functional theory calculation indicated that the oxygen-containing functional groups and MgO particles substantially enhanced the CO2 capture performance due to the enhanced interactions between MgO/biochar and CO2. This study provides novel insights for the construction of efficient and cheap solid adsorbents for CO2 capture.

Entropy‐Driven Carbon Dioxide Capture: The Role of High Salinity and Hydrophobic Monoethanolamine

Addressing atmospheric CO2 levels during the transition to carbon neutrality requires efficient CO2 capture methods. Aqueous amine scrubbing dominates large‐scale flue gas capture but is hampered by the energy‐intensive regeneration step, sorbent loss, and consequent environmental concerns with volatile amines. Herein, hydrophobic non‐volatile alkylated monoethanolamine (MEA) is introduced as a water‐lean CO2 absorbent in brine. The effects of alkylation of MEA, salinity, and aggregation of absorbents on the improved CO2 capture process are systematically investigated. The CO2 absorption facilitates spontaneous self‐aggregation of hydrophobic absorbents, which increases the entropy of water in high‐ion strength solutions. This effect is controlled by the salinity of aqueous solutions, affording comparative gravimetric CO2 uptake performance to benchmark MEA. It is experimentally verified that the hydrophobicity of alkylated MEAs in saline water is responsible for facile absorption, and also for mild regeneration conditions. Therefore, the entropy‐driven approach minimizes absorbent evaporation, corrosion, and decomposition, thus paving the way to realize energy‐efficient carbon capture.

Enhanced CO2 capture and reverse water gas shift reaction using CaO in NaCl-CaCl2 molten salt medium

This study introduces an Integrated Carbon Capture and Utilisation-Reverse Water Gas Shift (ICCU-RWGS) approach, a novel method for in situ CO2 adsorption and conversion, leveraging the synergistic effects of CaO within a NaCl-CaCl2 molten salt blend to enhance CO2 capture and conversion efficiency. Building upon this foundation, we optimize CaO concentration and operating temperature to maximize CO2 uptake and conversion performance. The research focuses on integrating CaO with a NaCl-CaCl2 molten salt blend (mass ratio 4:6) to improve CO2 sorption and conversion performance. Findings show significant enhancements in CO2 uptake and CO yield with the presence of molten salt compared to systems without it. The optimal operating temperature and CaO concentration are identified for maximum CO yield. Characterisation techniques like in-situ infrared spectroscopy, XRD, and SEM provide insights into the behavior of CaO in the molten salt, revealing the solubility of the partial carbonate formed from CO2 and CaO, dispersion of CaO particles, and their morphological characteristics. Overall, the study demonstrates the potential of CaO-molten salt integration in improving ICCU-RWGS process efficiency and contributes to the development of more effective and sustainable ICCU technologies.

Phase transition Ni-Co-Ca-O bifunctional catalysts for high and stable hydrogen production from sorption enhanced steam methane reforming

Sorption enhanced steam methane reforming (SESMR) is an effective method to improve the efficiency of hydrogen production from methane reforming while fixing CO2 to reduce carbon emissions. However, the deactivation of adsorbents during high temperature recycling limits the utilization of SESMR. In this paper, by combining the phase transition process of Ca-Co-O species and Ni-Co alloy, we prepared a series of Ni-Co-Ca-O bifunctional catalysts for SESMR hydrogen production. The 1Ni4.5Co4.5CaO catalyst exhibited high catalytic activity in the SESMR reaction with hydrogen concentration exceeding 99.6 % and complete methane conversion. The catalyst also showed high stability in 50 cycle experiments with no significant reduction in hydrogen production and CO2 uptake performance. The Ni-Co-Ca-O bifunctional catalysts consisted of NiO and Co-Ca-O species, by analyzing the fresh and reacted catalysts, the mechanism of the catalyst with high catalytic activity and stability was revealed. Firstly, the catalyst formed Ni-Co alloy and CaO adsorbent by a reduction reaction, and then the SESMR hydrogen production reaction was carried out. After the complete conversion of CaO to CaCO3, the catalyst was re-formed into NiO and Co-Ca-O species by introducing O2 in the regeneration reaction to carry out a new cycle of reduction → hydrogen production. The synergistic interaction between the Ni-Co alloy enhanced the methane reforming and water gas shift reaction, leading to an increase in hydrogen production and methane conversion. The phase transition of Co from Co-Ca-O species to Ni-Co alloy and back to Co-Ca-O species inhibited the sintering of CaO, which significantly enhanced the cyclic stability of the catalyst. The results of this study are expected to provide a new pathway for the design of SESMR bifunctional catalysts.

Thermodynamic Investigations for Combustion-Assisted Synthesis of Lithium Orthosilicate Powders

The study investigates the combustion-assisted synthesis of lithium orthosilicate (Li4SiO4) powders for potential CO2 capture applications. Technical-grade lithium carbonate and metallic silicon powders were used as starting materials. Synthesis conditions were explored across temperatures ranging from 500 to 900 °C and different holding durations. Thermodynamic modeling using FactSage 8.2 software suggested that Li4SiO4 production is feasible at temperatures of 700 °C and higher with metallic silicon as the silicon source, which was confirmed experimentally. Characterization of the synthesized powders involved X-ray diffraction, specific surface area determination, particle size distribution analysis, scanning electron microscopy, and CO2 uptake tests. Despite having the lowest Li4SiO4 content as 83.7%, the sample synthesized at 700 °C with 45 min of holding time showed the best CO2 uptake performance as 12.80 wt% while having the lowest crystallite size value (126.58 nm), the highest specific surface area value (4.975 m2/g) and the lowest average particle size value (10.85 µm) which are highly effective on the CO2 uptake performance of such solid sorbents. The study concludes that while challenges remain in achieving optimal CO2 capture performance, it lays a foundation for utilizing lithium orthosilicate in carbon capture applications.Graphical

Alumina Incorporation in Self-Supported Poly(ethylenimine) Sorbents for Direct Air Capture.

Self-supported branched poly(ethylenimine) scaffolds with ordered macropores are synthesized with and without Al2O3 powder additive by cross-linking poly(ethylenimine) (PEI) with poly(ethylene glycol) diglycidyl ether (PEGDGE) at -196 °C. The scaffolds' CO2 uptake performance is compared with a conventional sorbent, i.e., PEI impregnated on an Al2O3 support. PEI scaffolds with Al2O3 additive show narrow pore size distribution and thinner pore walls than alumina-free materials, facilitating higher CO2 uptake at conditions relevant to direct air capture. The PEI scaffold containing 6.5 wt % Al2O3 had the highest CO2 uptake of 1.23 mmol/g of sorbent under 50% RH 400 ppm of CO2 conditions. In situ DRIFT spectroscopy and temperature-programmed desorption experiments show a significant CO2 uptake contribution via physisorption as well as carbamic acid formation, with lower CO2 binding energies in PEI scaffolds relative to conventional PEI sorbents, likely a result of a lower population of primary amines due to the amine cross-linking reactions during scaffold synthesis. The PEI scaffold containing 6.5 wt % Al2O3 is estimated to have the lowest desorption energy penalty under humid conditions, 4.6 GJ/tCO2, among the sorbents studied.

Probing Structural Transformations and Degradation Mechanisms by Direct Observation in SIFSIX-3-Ni for Direct Air Capture.

Despite global efforts to reduce carbon dioxide (CO2) emissions, continued industrialization threatens to exacerbate climate change. This work investigates methods to capture CO2, with a focus on the SIFSIX-3-Ni metal-organic framework (MOF) as a direct air capture (DAC) sorbent. SIFSIX-3-Ni exhibits promising CO2 adsorption properties but suffers from degradation processes under accelerated aging, which are akin to column regeneration conditions. Herein, we have grown the largest SIFSIX-3-Ni single crystals to date, facilitating single crystal X-ray diffraction analyses that enabled direct observation of the H2O and CO2 dynamics through adsorption and desorption. In addition, a novel space group (I4/mcm) for the SIFSIX-3-Ni is identified, which provided insights into structural transitions within the framework and elucidated water's role in degrading CO2 uptake performance as the material ages. In situ X-ray scattering methods revealed long-range and local structural transformations associated with CO2 adsorption in the framework pores as well as a temperature-dependent desorption mechanism. Pair distribution function analysis revealed a partial decomposition to form nonporous single-layer nanosheets of edge-sharing nickel oxide octahedra upon aging. The formation of these nanosheets is irreversible and reduces the amount of active material for the CO2 sorption. These findings provide crucial insights for the development of efficient and stable DAC sorbents, effectively reducing greenhouse gases, and suggest avenues for enhancing MOF stability under practical DAC conditions.

Unlocking the potential of N-doped porous Carbon: Facile synthesis and superior CO2 adsorption performance

Porous carbon derived from bio-based sources is highly regarded as a promising adsorbent material, attributed to its exceptional CO2 uptake performance and favorable CO2/N2 selectivity. In this study, we employed a one-step chemical activating technique to fabricate self-heteroatom-doped porous carbon utilizing chitosan as a carbon precursor and KOH as chemical activating reagent. Subsequent activation processes resulted in the porous carbons possessing an excess amount of nitrogen functionality and advanced textural feature. Remarkably, a high CO2 capture amount of 4.40 mmol/g was accomplished at 25 °C and 1 bar, accompanied by a significant CO2/N2 selectivity of 21 for the dual gas blend. The gas adsorption mechanisms were thoroughly discussed, elucidating the factors contributing to the exceptional performance. The superior adsorption and separation capabilities, enhanced recyclability and effective dynamic adsorption capability, establish this N-enriched carbon as a potential adsorbent for CO2 uptake. The present work presents a novel approach for designing high-performance carbon-based CO2 adsorbents.

Comparative investigation on CO2 uptake performance of limestone-derived sorbents with different supports synthesized by impregnated layer solution combustion method

Calcium looping is an effective post-combustion technology for CO2 separation from flue gas due to its low-cost and highly accessible sorbent raw material (i.e., limestone). However, limestone easily occurs serious sintering during multiple high-temperature carbonation/calcination cycles. Here, three types of modification mode, including acetic acid acidification, wet and dry solution-impregnated cigarette butts-assisted combustion methods, were adopted to enhance the cyclic stability of limestone. The impregnated layer solution combustion mode is superior to acid acidification, which is mainly ascribed to the heat released from the combustion of cigarette butt playing the role of thermal pretreatment, thereby stabilizing the pore structure of limestone-derived sorbents. Incorporating different inert supports can further enhance the cyclic stability of limestone-derived sorbents synthesized by wet solution-impregnated cigarette butts-assisted combustion. The effectiveness of several inert supports in improving the cyclic stability of limestone-derived adsorbents is listed as follows: CaZrO3 > MgO > Ca12Al14O33 > CaMn2O4. Moreover, the loading content of CaZrO3 within limestone-derived sorbents was further optimized, and it was found that 10 wt% of CaZrO3 sufficiently provides stable framework to inhibit high-temperature sintering (∼0.525 g-CO2/g-sorbent after 20 cycles). The uniformly dispersed CaZrO3 support at the nanocrystalline scale effectively inhibiting sintering is responsible for the excellent cyclic CO2 capture capability of CaZrO3-supported sorbents.

Construction of three-dimensional porous organic polymers with enhanced CO2 uptake performance via solid-state thermal conversion from tetrahedral benzoxazine-linked precursor

Porous organic polymers (POPs) with three-dimensional (3D) linkages have drawn great interest due to their potential applications in separation, adsorption, and sensing fields. Herein, a new 3D benzoxazine (BZ)-linked porous organic polymer (TPM-BZ-Py POP) was synthesized by linking a tetrahedral benzoxazine (TPM-BZ-Br4) with tetraethynylpyrene (Py-T) via Sonogashira coupling. Specifically, the synthesis of the tetrahedral benzoxazines with four oxazine rings in a single monomer involved the utilization of Schiff base formation, NaBH4 reduction as well as Mannich condensation reactions. To confirm the chemical structure of the benzoxazine monomers and their corresponding polybenzoxazines, Fourier transform infrared (FTIR), proton nuclear magnetic resonance (1H NMR), and carbon nuclear magnetic resonance (13C NMR) spectroscopy techniques were employed. The pore volume and specific surface area of the 3D TPM-BZ-Py POP were determined through N2 adsorption/desorption isotherms, and they were found to be 0.52 cm3/g and 185 m2/g, respectively. In addition, the solid-state chemical transformation occurred during thermal ring-opening polymerization (ROP), and the resulting 3D poly(TPM-BZ-Py) POP displayed higher CO2 capture ability (1.81 mmol/g) compared with TPM-BZ-Py POP (0.60 mmol/g) at room temperature. Such enhancement observed in poly(TPM-BZ-Py) POP was proposed to be caused by the formation of phenolic units and Mannich bridges within the polymer network by taking into consideration the structural transformation during the ring-opening polymerization (ROP). The functionalities in poly(TPM-BZ-Py) have great potential to interact with CO2 molecules through strong intermolecular hydrogen bonding or acid/base interactions, which further supports the benefit of incorporating oxazine rings into POP frameworks.

A g-C3N4 nanotube assisted and (Ce-Mg) co-doped limestone-derived sorbent: Bidirectional diffusion channels and double vacancy mechanism constructed for CO2 capture

A novel (Ce-Mg) co-promoted CaO sorbent with multi-dimensional through-hole structure was fabricated by a hydrothermal template method based on natural limestone, to address the evident decline of CO2 adsorption performance during circulation. A trend of first increasing and then stabilizing was achieved for CO2 long-term durability of the as-prepared sorbent by the hydrothermal recrystallization process. The CO2 absorption capacity of the (Ce-Mg) co-promoted tubular sorbents exceeded that of the reference sample without hydrothermal template treatment by ∼ 400% after 30 cycles. 97% of the initial CO2 capacity can be maintained at the 30th cycle. This significant CO2 uptake capacity can be attributed to the multiple synergistic interactions yielded by incorporating g-C3N4 nanotube template with (Ce-Mg) co-doping. A bidirectional gas diffusion channel was constructed based on the inner-outer multi-dimensional connection structure of g-C3N4, which allowed the sorbent to maintain the prominent specific surface area and pore volume during cycles. After 30 cycles, the specific surface area of the limestone-derived sorbents can still remain at 16.31 m2/g. Furthermore, a double vacancy mechanism that can facilitate the gas transport through both the product layer and the surface layer of CaO grains caused by the co-doping of (Ce-Mg) was proposed, which can also promote the electron transport between CaO and CO2. This synergistic strategy proved useful for improving the CO2 uptake performance and cycle durability of CaO sorbents in practical applications.

Investigation of CO2 sequestration performance and statistical analysis of dye removal efficiency of liquefied hornbeam based carbon foams: Effects of biomass/solvent weight ratio, tar contribution, and chemical activation

The preparation of carbon foams from bio-based polyols was a preferable alternative for the development of new materials from waste resources. The research described the usage of wood processing factory waste for the production of open-cell carbon foams with a high application potential. Bio-based carbon foams were developed by hornbeam tree sawdust, a low-cost and renewable material for dye removal via adsorption and CO2 uptake performance. In this work, the influences of the solvent type, and solvent/biomass weight ratio on the characteristics of carbon foams were investigated in the solvolytic liquefaction process. A chemical activation process with KOH was applied to the foams and this provided high porosity and surface area (up to 1454 m2/g). Carbon foams were found to have comparable compressive strengths; up to 1.080 MPa. As a result of the statistical analysis, according to the variance analysis performed with a confidence level limit of 95%, it was specified that pH had a negative coefficient and was the most significant factor in NH dye removal. According to the experiments carried out within the scope of investigating the usability of carbon foams in the removal of pollutants from gas and liquid systems, HCFTA-3 carbon foam exhibited a maximum removal efficiency of 55.2% for NH dye when pH was 2, and HCFA-3 carbon foam had the highest CO2 holding capacity at 0 °C as measured by 6.53 mmol/g CO2. This study provided an understanding of the sustainable bio-based carbon foams from industrial waste biomass for practical application of green chemistry.

Nickle oxide nanoparticles incorporated flexible and porous carbon nanofiber-based adsorbents for CO2 capture

To capture carbon dioxide (CO2) from the environment, carbonaceous materials are thought to be a convenient source. However, introducing flexibility with high porosity into the carbon nanofibers stays a demanding task. Herein, nickle oxide (NiO) nanoparticles (NPs) incorporated flexible and porous carbon nanofibers were created using an electrospinning process and subsequent carbonization treatment. The optimized (NiC-3) membranes demonstrated superior CO2 adsorption capacity of 2.5 mmol g−1 at 25 °C, strong CO2 gas selectivity (S=20) compared to N2 gas, and incredible cyclic stability. After 10 adsorption-desorption cycles, no major mass change was observed, and the CO2 uptake performance retains >95% of the initial value unveiling the high stability and practical importance of as-synthesized NiC-3 membrane. The incorporation of NiO NPs in the carbon framework not only improved the surface area, porosity, and vacant oxygen sites to capture CO2 molecules but also enhances the flexibility of the CNFs by acting as a plasticiser for single-fibre-crack connection. Profoundly, NiO NPs incorporated porous and flexible CNFs are a good choice for CO2 adsorption applications due to their excellent structural stability, which overcomes the restrictions of annihilation and failure of structure for conventional adsorbents.

Superior Selective CO2 Adsorption and Separation over N2 and CH4 of Porous Carbon Nitride Nanosheets: Insights from GCMC and DFT Simulations.

Development of high-performance materials for the capture and separation of CO2 from the gas mixture is significant to alleviate carbon emission and mitigate the greenhouse effect. In this work, a novel structure of C9N7 slit was developed to explore its CO2 adsorption capacity and selectivity using Grand Canonical Monte Carlo (GCMC) and Density Functional Theory (DFT) calculations. Among varying slit widths, C9N7 with the slit width of 0.7 nm exhibited remarkable CO2 uptake with superior CO2/N2 and CO2/CH4 selectivity. At 1 bar and 298 K, a maximum CO2 adsorption capacity can be obtained as high as 7.06 mmol/g, and the selectivity of CO2/N2 and CO2/CH4 was 41.43 and 18.67, respectively. In the presence of H2O, the CO2 uptake of C9N7 slit decreased slightly as the water content increased, showing better water tolerance. Furthermore, the underlying mechanism of highly selective CO2 adsorption and separation on the C9N7 surface was revealed. The closer the adsorption distance, the stronger the interaction energy between the gas molecule and the C9N7 surface. The strong interaction between the C9N7 nanosheet and the CO2 molecule contributes to its impressive CO2 uptake and selectivity performance, suggesting that the C9N7 slit could be a promising candidate for CO2 capture and separation.

One-pot synthesis of self S-doped porous carbon for efficient CO2 adsorption

Considering the rising environmental and efficiency concerns, the development of porous carbon via a surface decoration approach has received great attention for selectively adsorb CO2 from the flue gas. Herein, self-S-doped porous carbon was prepared from petroleum coke with high sulfur content (PHS) utilizing KOH as an activating agent. The effect of the activating temperatures and the KOH/PHS mass ratio on the physical feature and CO2 uptake performance of the as-synthesis porous carbons was fully examined. The optimal sample (PHS-650-3) presents the specific BET surface area and total pore volume of 1278 m2/g and 0.56 cm3/g, respectively. The PHS-650-3 sample displayed the maximum CO2 adsorption capacity of 4.18 and 5.57 mmol g−1 at 25 and 0 °C and 1 bar thanks to its enhanced textural characteristics and high amount of sulfur functionality on the surface. Last but not least, the as-synthesized S-doped porous carbon shows ideal adsorption solution theory (IAST) CO2/N2 selectivity of 21, fast adsorption kinetics, suitable heat of adsorption value and stable uptake capacity upon consecutive adsorption cycles, claiming its readily potential CO2 adsorption applications.

Rapid room temperature synthesis and CO2 uptake performance of nanocrystalline ZIF-67 and Ni@ZIF-67

Zeolitic Imidazolate Frameworks (ZIFs) are a sub-class of Metal-Organic Frameworks (MOFs) and one of the best porous materials for the selective uptake of CO2. The room temperature synthesis of a series of Ni2+-doped zeolitic imidazolate framework-67 (Ni@ZIF-67) and their application in selective CO2 adsorption. The structure, morphology and surface area of Ni@ZIF-67 nanocrystals have been examined using spectroscopic and surface analytical techniques. Ni2+-doped ZIF-67 (Ni@ZIF-67), have been synthesised by doping 10, 25, and 75 % of Ni2+ ions into ZIF-67. The metal ion ratios of Ni/Co play a prominent role in controlling the particle size and surface area of Ni@ZIF-67. Moreover, the Ni@ZIF-67 shows enhanced uptake of CO2 (17.12 cm3/g (33.65 mg/g)) compared to ZIF-67 (12.60 cm3/g (24.76 mg/g)). In summary, this work shows a novel and eco-friendly technique for the synthesis of Ni@ZIF-67 for improved uptake of CO2.

Design of hierarchically porous carbon frameworks for enhanced CO2 capture performance

To reduce the environmental impact of CO2, CO2 capture has become an area of research interest. Hierarchically porous materials have been certified as efficiency CO2 capture adsorbents. This work is aimed to construct hierarchically porous carbon frameworks (CPI-TPUs) with enhanced CO2 capture performance. CPI-TPUs are derived from polyimide (PI)/thermoplastic polyurethane (TPU) composite aerogels (PI-TPUs) with mesopores and macropores. Accordingly, the obtained CPI-TPU2 exhibits an improved CO2 uptake capacity (2.21 mmol/g, 298 K) compared to the pure CPI and PI-TPUs due to the synergistic effect of hierarchically pores, higher specific area and electron-rich heteroatoms. Specially, CPI-TPUs with pore volume ratio of mesopores/macropores to total pores in a range of 6–21 show high CO2 uptake performance. These CPI-TPUs with high CO2 adsorption capacities and suitable isosteric adsorption heats (Qst) are promising CO2 adsorbents.

In Situ XRD, Raman Characterization, and Kinetic Study of CO2 Capture by Alkali Carbonate-Doped Na4SiO4

Sodium silicate, a new type of CO2 sorbent, has a relatively low cost, but its sorption reactivity is not yet good enough. Alkali carbonate doping is commonly used as an effective means to improve the CO2 uptake reactivity of solid sorbents. In this study, sodium orthosilicate, Na4SiO4, was synthesized and mixed with 5, 10, and 20 mol% of Li2CO3–Na2CO3 or Li2CO3–Na2CO3–K2CO3 as CO2 sorbents. The promotion of alkali carbonates on Na4SiO4 in CO2 capture was characterized using thermal analyses in an 80 vol% CO2–20 vol% N2 atmosphere. The phase evolution and structural transformations during CO2 capture were characterized by in situ XRD and Raman, and the results showed that the intermediate pyrocarbonate, C2O52−, which emerged from alkali carbonates, enhanced the CO2 capture of Na4SiO4 to form Na2CO3 and Na2SiO3 from 100 °C. Isothermal analyses showed that 10 mol% of Li2CO3–Na2CO3 was the optimal additive for Na4SiO4 to attain better CO2 uptake performance. The alkali carbonates were effective in reducing the activation energy for both chemisorption and bulk diffusion, improving the cycle stability of Na4SiO4.

Investigation of K2CO3-modified CaO sorbents for CO2 capture using in-situ X-ray diffraction

Alkali metal salt promoters, such as K2CO3 investigated in this work, are known to affect the cyclic performance of CaO-based sorbents – sometimes positively, sometimes negatively. Thus far, the influence of K2CO3 (and other salt-based promoters) is poorly understood, and detailed investigations on the interaction of K2CO3 with CaO are missing. We find that low amounts of K2CO3 in the sorbents improve the cyclic performance of CaO significantly (up to a factor of three), both under mild and harsh CO2 release conditions, while large amounts of K2CO3 (>1 mol.%) reduce the positive effect on the CO2 uptake performance. Aided by in-situ X-ray diffraction measurements, we observe that the gas environment has an important influence on the stability of K-containing phases in the sorbent. At typical carbonation conditions (650 °C, 15 vol% CO2/N2), CaO and K2CO3 readily form K-Ca double carbonates (K2Ca(CO3)2, which reacts further to form K2Ca2(CO3)3). When the calcination reaction is carried out in pure N2, the K-Ca double carbonates decompose above ∼730 °C, forming K2CO3 that becomes partially volatile near its melting point of ∼900 °C, and leads to a reduction of the K content in the samples with cycling. When the calcination reaction is carried out in a CO2-rich environment, the cyclic loss of K2CO3 is reduced substantially, but the sintering of the sorbent is increased. There are indications that very low quantities of K are incorporated into the CaO structure, and the formation of K-Ca double carbonates appears to influence the structural evolution of the CaCO3 phase formed during carbonation. With very low amounts of K2CO3 present in the sorbent, we observe an increase in pore volume for pore diameters between 20 and 80 nm, which ultimately may explain the improved CO2 uptake performance compared to the CaO sorbent without K2CO3.

Controllable preparation of porous Ca-Mg-Al hydroxides based adsorbents and their CO2 adsorption performances

A series of porous Ca-Mg-Al hydroxides based adsorbents of layered structure were successfully prepared with CaCl2, MgCl2·6H2O and AlCl3·6H2O as the precursors and urea as the precipitant through a hydrothermal method and subsequent KOH alkaline etching treatment, in order to investigate the effects of molar ratio of Mg/Ca, addition amount of graphene oxide and alkaline etching time of KOH solution on the CO2 uptake performance of the resulting adsorbents. The structure and morphology of the samples were characterized by X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FT-IR), and scanning electron microscope (SEM), respectively, and the results showed that the Ca-Mg-Al hydroxides-derived adsorbents presented a hexagonal and layered structure, which conformed to the structure of hydrotalcite-like compounds. CO2 adsorption properties of the adsorbents were measured using a thermogravimetric analyzer, and the adsorption data of the samples were fitted by the first-order kinetic model, pseudo-second-order kinetic model and Elovich model, respectively. Taking advantage of the amphoteric nature of Al species, the adsorbent LTH/GO-12 displayed the favorable uptake performance of 20.04 mg/g and cyclic stability with the retention rate of 93.4% over five cycles. The improving capture performance of LTH/GO-12 was mainly contributed by the development of pore structure and the increase of adsorption active sites because of the removal of Al species by alkaline etching and the adding of graphene oxide.