Characteristics, application and modeling of solid amine adsorbents for CO2 capture: A review

Mechanistic insights into the oxidative degradation of amine-containing CO2 adsorbents

One of the most challenging issues for large-scale implementation of amine-containing adsorbents for CO2 capture, is their propensity to oxidative degradation via radical mechanisms. The nature of the early (primary) oxidation species depends on whether the deactivation took place under humid or dry, aerobic or anaerobic conditions. The current theoretical investigation provides new insights into the reaction mechanisms for such degradation products, specifically imine, aldehyde and CO2, depending on the radical species involved, and the deactivation conditions. A common radical to all reactions referred to as αC•, corresponds to the abstraction of a hydrogen atom from the α-position with respect to an amine group. In dry anaerobic environment, imine formation involving organic radicals R• generated thermally, has an activation barrier of 13.54 kcal mol-1. In humid anaerobic environment, the imine formation in the presence of hydroxyl radicals (HO•) corresponded to much lower activation barriers than organic radicals. However, the generation of HO• radicals would be difficult in the absence of oxygen. Hydroperoxyl radicals (HOO•) occur only in the presence of oxygen, but their formation is facilitated in the presence of humidity. Oxidation of amine to aldehyde occurs in two stages, involving oxygen atom implantation on α-carbon, then the formation of aldehyde and ammonia. In dry aerobic conditions, oxygen implantation involving HOO• has a high activation energy of 19.60 kcal mol-1, while the subsequent reaction into aldehyde has a very low barrier of 2.38 kcal mol-1. In contrast, in humid anaerobic environment, both steps occur in the presence of HO• radicals, with a much lower activation barrier for the first step than the latter (1.52 vs. 22.34 kcal mol-1). More importantly, under humid aerobic condition, amine oxidation is accelerated as HO• and HOO• play complementary roles, with the former facilitating oxygen implantation, while the latter is involved in the carbonyl formation.

Exploration of structured solid amine adsorbents for CO2 capture: PEI-loaded composite foam material

Solid amine adsorbents are widely employed for CO2 capture due to their high selectivity and renewability. However, most existing adsorbents are in powdered form, and current shaping methods often suffer from poor universality, complex synthesis procedures, low mechanical strength, or pore structure degradation-factors that significantly hinder industrial application. To address these challenges, a scalable and cost-effective strategy is proposed for fabricating structured solid amine adsorbents by combining porous materials (e.g. biochar), industrial waste-based sulfur-aluminum cementitious binders, and H2O2-assisted chemical foaming. This process integrates micro-mesopores derived from the porous materials with macropores generated through foaming and hydration, resulting in a 3D interconnected hierarchical pore structure. The structured adsorbent exhibits excellent compressive strength (∼77.33 N), competitive CO2 adsorption capacity (60.04 mg/g at 90°C), high amine utilization (68.52%), and good reusability (only a 7.59% reduction after five cycles). Kinetic analysis reveals that 76.38% of the total CO2 adsorption capacity occurs within the first 2 min, indicating a rapid initial adsorption rate despite structural shaping. Moreover, the proposed method is successfully extended to other carriers such as synthetic resin and nano-fumed silica, demonstrating strong material adaptability. In addition to adsorption capacity, this study emphasizes amine utilization as a key performance metric and introduces a novel evaluation chart for the simultaneous assessment of both parameters. Overall, this work addresses pressing challenges in shaping, mechanical strength, and cost, and offers an effective, scalable pathway for the practical deployment of solid amine adsorbents in CO2 capture technologies.

Adsorption performance of amine-functionalized red mud-based adsorbent for CO2 capture

In this work, an industrial solid waste red mud was resourcefully processed to prepare a red mud-based solid amine adsorbent for efficient CO2 capture. Nitrogen adsorption-desorption isotherms, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy measurement were performed, suggesting that the organic amine was successfully loaded onto the red mud. The results obtained from the CO2 adsorption tests showed that the red mud-based solid amine adsorbent obtained a significantly higher CO2 capacity in comparison to the pristine red mud. Moreover, the type of the organic amine and the loading had a significant impact on the adsorption performance. By comparing the adsorption performance of different organic amine types and loadings, MRM-20-TETA was finally selected as the most effective adsorbent (23.94 mg/g), with only 0.8 mg/g adsorption to the pristine red mud. In parallel, the improved adsorption performance was nearly 29 times higher. In addition, cyclic stability tests were conducted on MRM-20-TETA. It was demonstrated that the adsorption capacity decreased by only 9.9 % after the application of five adsorption-desorption cycles, indicating its wide application potential. The kinetics analysis showed that the Avrami model provided the best fit for the kinetics of the amine-functionalized red mud adsorbent, signifying a combination of physical and chemical adsorption mechanisms. Our work provided valuable insights for efficient CO2 capture and provided a novel technique for the resource utilization of red mud.

Atom-level interaction design between amines and support for achieving efficient and stable CO2 capture

Amine-functionalized adsorbents offer substantial potential for CO2 capture owing to their selectivity and diverse application scenarios. However, their effectiveness is hindered by low efficiency and unstable cyclic performance. Here we introduce an amine-support system designed to achieve efficient and stable CO2 capture. Through atom-level design, each polyethyleneimine (PEI) molecule is precisely impregnated into the cage-like pore of MIL-101(Cr), forming stable composites via strong coordination with unsaturated Cr acid sites within the crystal lattice. The resulting adsorbent demonstrates a low regeneration energy (39.6 kJ/molCO2), excellent cyclic stability (0.18% decay per cycle under dry CO2 regeneration), high CO2 adsorption capacity (4.0 mmol/g), and rapid adsorption kinetics (15 min for saturation at 30 °C). These properties stem from the unique electron-level interaction between the amine and the support, effectively preventing carbamate products' dehydration. This work presents a feasible and promising cost-effective and sustainable CO2 capture strategy.

Unravelling the structure of CO2 in silica adsorbents: an NMR and computational perspective

This comprehensive review describes recent advancements in the use of solid-state NMR-assisted methods and computational modeling strategies to unravel gas adsorption mechanisms and CO2 speciation in porous CO2-adsorbent silica materials at the atomic scale. This work provides new perspectives for the innovative modifications of these materials rendering them more amenable to the use of advanced NMR methods.

Thickness Dependent CO2 Adsorption of Poly(ethyleneimine) Thin Films for Direct Air Capture

Mesoporous silica impregnated with polyethyleneimine (PEI) has been shown to be a suitable material for the direct air capture (DAC) of CO2. Factors such as CO2 concentration, temperature, and amine loading impact overall capture capacity and amine efficiency by altering diffusional resistance and reaction kinetics. When studied in the impregnated 3-dimensional sorbent material, internal diffusion impacts the evaluation of the reaction kinetics at the air/amine interface. In this work, we designed a novel tandem quartz crystal microbalance with dissipation (QCM-D) and polarization modulation infrared reflective absorption spectroscopy (PM-IRRAS) instrument. CO2 adsorption kinetics of the PEI-based amine layer in a 2-dimensional geometry were studied at a variety of film thicknesses (10 nm to 100 nm), temperatures (25 °C to 80 °C), and CO2 concentrations (5 % and 0.04 % by mole fraction). Total CO2 capture capacity increased with film thickness but decreased amine efficiency, as additional diffusional resistance for thicker films limits access to available amine sites. The capture capacity of thick films (>50 nm) is shown to be limited by amine availability, while capture of thin films (<50 nm) is limited by CO2 availability. A 50 nm PEI film was shown to be optimal for capture of 0.04 % (400 ppm) CO2. The adsorption profiles for these conditions were fitted to pseudo-first order and Avrami fractional order models. The reaction process switches between a diffusion limited reaction to a kinetic limited reaction at 80 °C when using 5 % CO2 and 55 °C when using 0.04 % CO2. These results offer accurate analysis of adsorption of CO2 at the air/amine interface of PEI films which can be used for the design of future sorbent materials.

Electron-induced hydroamination of ethane as compared to ethene: implications for the reaction mechanism

The properties of carbonaceous materials with respect to various applications are enhanced by incorporation of nitrogen-containing moieties like, for instance, amino groups. Therefore, processes that allow the introduction of such functional groups into hydrocarbon compounds are of utmost interest. Previous studies have demonstrated that hydroamination reactions which couple amines to unsaturated sites within hydrocarbon molecules do not only proceed in the presence of suitably tailored catalysts but can also be induced and controlled by electron irradiation. However, studies on electron-induced hydroaminations so far were guided by the hypothesis that unsaturated hydrocarbons are required for the reaction while the reaction would be much less efficient in the case of saturated hydrocarbons. The present work evaluates the validity of this hypothesis by post-irradiation thermal desorption experiments that monitor the electron energy-dependent yield of ethylamine after electron irradiation of mixed C2H4:NH3 and C2H6:NH3 ices with the same composition and thickness. The results reveal that, in contrast to the initial assumption, ethylamine is formed with similar efficiency in both mixed ices. From the dependence of the product yields on the electron energy, we conclude that the reaction in both cases is predominantly driven by electron ionization of NH3. Ethylamine is formed via alternative reaction mechanisms by which the resulting NH2˙ radicals add to C2H4 and C2H6, respectively. The similar efficiency of amine formation in unsaturated and saturated hydrocarbons demonstrates that electron irradiation in the presence of NH3 is a more versatile tool for introducing nitrogen into carbonaceous materials than previously anticipated.

Wood-Inspired Ultrafast High-Performance Adsorbents for CO2 Capture

Under favorable regeneration conditions (120 °C, 100% CO₂), ultrafast adsorption kinetics and excellent long-term cycle stability are still the biggest obstacles for amine-based solid CO₂ adsorbents. Inspired by natural wood, a biochar with a highly ordered pore structure and excellent thermal conductivity was prepared and used as a carrier of organic amines to prepare ideal CO₂ adsorbents. The results showed that the prepared adsorbent has a very high adsorption working capacity (4.23 mmol CO₂·g^-1), and its performance remains stable even after 30 adsorption-desorption cycles in the harsh desorption environment (120 °C, 100% CO₂). Due to the existence of the hierarchical structure, the adsorbent exhibited ultra-fast adsorption kinetics, and the reaction rate constant is 37 times higher than that of traditional silica. This adsorbent also showed a very low regeneration heat of 1.64 MJ·kg^-1 (CO₂), which is especially important for the practical application. Therefore, these biochar-based adsorbents derived from natural wood make the CO₂ capture process promising.

Role of fiber density of amine functionalized dendritic fibrous nanosilica on CO2 capture capacity and kinetics

Textural properties of the solid sorbents are critical to tuning their CO2 capture performance. In this work, we studied the effect of fiber density (in turn, pore size, distribution, and accessibility) on CO2 capture capacity and kinetics. CO2 solid sorbents were prepared by physisorption of tetraethylenepentamine (TEPA) molecules on dendritic fibrous nanosilica (DFNS) with varying fiber density. Among the various DFNS, the DFNS with moderate fiber density [DFNS-3] showed the best CO2 capture capacity under the flue gas condition. The maximum CO2 capture capacity achieved was 24.3 wt % (5.53 mmol/g) at 75 °C for DFNS-3 under humid gas conditions. Fiber density also played a role in the kinetics of CO2 capture. DFNS-1 with dense fiber density needed ∼10.4 min to reach 90 % capture capacity, while DFNS-3 (moderate fiber density) needed only 6.4 min, which further decreased to 5.9 min for DFNS-5 with lightly dense fibers. The DFNS-impregnated TEPA also showed good recyclability during 21 adsorption and desorption cycles under humid and dry conditions. The total CO2 capture capacity of DFNS-3 (14.7) in 21 cycles was 108.9 and 105.0 mmol/g under humid and dry conditions, respectively. Adsorption lifetime calculation and recyclability confirmed the fiber density-dependent CO2 capture performance.