An overview on trace CO2 removal by advanced physisorbent materials

A Microporous Mn(II) MOF Based on 5-(4H-1,2,4-triazol-4-yl) Isophthalic Acid for CO2/N2 Separation

Removal of carbon dioxide (CO2) from a CO2/N2 mixture by utilizing CO2-selective sorbents is important from the perspective of energy security and environmental sustainability. Herein, a microporous metal-organic framework (MOF) composed of manganese(II) and a bifunctional linker 5-(4H-1,2,4-triazol-4-yl)benzene-1,3-dicarboxylic acid (H2L), [Mn(HL)2] (1) is designed and synthesized using a hydrothermal method. Characterized by single-crystal X-ray diffraction (SCXRD), a microporous channel was found in the structure of compound 1 along the a-axis. Attributed to hydrogen-binding interactions between CO2 molecules and N- and O-donor ligands in its microporous one-dimensional (1D) channel, compound 1 exhibits favorable adsorption of CO2 over N2. Further, verified by experimental breakthrough tests, the CO2/N2 mixture can be separated efficiently. This work provides potential guidance for designing CO2-selective MOFs for CO2/N2 separation.

Improvement in CO2 Capture of Polyamine with Micro-Interfacial System

Polyamines have emerged as a promising class of CO2 absorbents due to their remarkable sequestration capacity. However, their potential industrial application as aqueous absorbents is significantly hindered by a low regeneration efficiency and high energy consumption. To address these issues, this study investigates the use of triethylenetetramine (TETA) and ethylene glycol (EG) to develop a nonaqueous absorbent. The incorporation of EG enhances absorption performance and reduces the regeneration energy needed for TETA, whereas the high viscosity of the absorbent impedes absorption rate, amine efficiency, and regeneration efficiency. In order to enhance CO2 capture, micron-sized reaction units (SiO2@TETA-EG) were developed by encapsulating TETA solution with nanosilica. The SiO2@TETA-EG composite exhibits a large specific surface area (99 m2/g), with a porous shell structure and improved fluidity, which effectively counteracts the negative effects caused by high viscosity. Notably, SiO2@TETA-EG indicates a noticeably higher apparent rate constant of 4.29 min-1 at 323.2 K compared to the TETA-EG solution. Furthermore, SiO2@TETA-EG displays a 28.4% boost in regeneration efficiency while maintaining favorable stability in pore size and shape after regeneration.

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.

Technological Options for Direct Air Capture: A Comparative Process Engineering Review

The direct capture of CO₂ from ambient air presents a means of decelerating the growth of global atmospheric CO₂ concentrations. Considerations relating to process engineering are the focus of this review and have received significantly less attention than those relating to the design of materials for direct air capture (DAC). We summarize minimum thermodynamic energy requirements, second law efficiencies, major unit operations and associated energy requirements, capital and operating expenses, and potential alternative process designs. We also highlight process designs applied toward more concentrated sources of CO₂ that, if extended to lower concentrations, could help move DAC units closer to more economical continuous operation. Addressing shortcomings highlighted here could aid in the design of improved DAC processes that overcome trade-offs between capture performance and DAC cost. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Carbon dioxide capture with zeotype materials

This review describes the application of zeotype materials for the capture of CO2 in different scenarios, the critical parameters defining the adsorption performances, and the challenges of zeolitic adsorbents for CO2 capture.

The inorganic cation-tailored "trapdoor" effect of silicoaluminophosphate zeolite for highly selective CO2 separation

Functional nanoporous materials are widely explored for CO2 separation, in particular, small-pore aluminosilicate zeolites having a "trapdoor" effect. Such an effect allows the specific adsorbate to push away the sited cations inside the window followed by exclusive admission to the zeolite pores, which is more advantageous for highly selective CO2 separation. Herein, we demonstrated that the protonated organic structure-directing agent in the small-pore silicoaluminophosphate (SAPO) RHO zeolite can be directly exchanged with Na+, K+, or Cs+ and that the Na+ form of SAPO-RHO exhibited unprecedented separation for CO2/CH4, superior to all of the nanoporous materials reported to date. Rietveld refinement revealed that Na+ is sited in the center of the single eight-membered ring (s8r), while K+ and Cs+ are sited in the center of the double 8-rings (d8rs). Theoretical calculations showed that the interaction between Na+ and the s8r in SAPO-RHO was stronger than that in aluminosilicate RHO, giving an enhanced "trapdoor" effect and record high selectivity for CO2 with the separation factor of 2196 for CO2/CH4 (0.02/0.98 bar). The separation factor of Na-SAPO-RHO for CO2/N2 was 196, which was the top level among zeolitic materials. This work opens a new avenue for gas separation by using diverse silicoaluminophosphate zeolites in terms of the cation-tailored "trapdoor" effect.

Understanding the Effect of Water on CO2 Adsorption

Carbon capture from large sources and ambient air is one of the most promising strategies to curb the deleterious effect of greenhouse gases. Among different technologies, CO₂ adsorption has drawn widespread attention mostly because of its low energy requirements. Considering that water vapor is a ubiquitous component in air and almost all CO₂-rich industrial gas streams, understanding its impact on CO₂ adsorption is of critical importance. Owing to the large diversity of adsorbents, water plays many different roles from a severe inhibitor of CO₂ adsorption to an excellent promoter. Water may also increase the rate of CO₂ capture or have the opposite effect. In the presence of amine-containing adsorbents, water is even necessary for their long-term stability. The current contribution is a comprehensive review of the effects of water whether in the gas feed or as adsorbent moisture on CO₂ adsorption. For convenience, we discuss the effect of water vapor on CO₂ adsorption over four broadly defined groups of materials separately, namely (i) physical adsorbents, including carbons, zeolites and MOFs, (ii) amine-functionalized adsorbents, and (iii) reactive adsorbents, including metal carbonates and oxides. For each category, the effects of humidity level on CO₂ uptake, selectivity, and adsorption kinetics under different operational conditions are discussed. Whenever possible, findings from different sources are compared, paying particular attention to both similarities and inconsistencies. For completeness, the effect of water on membrane CO₂ separation is also discussed, albeit briefly.

Effective Separation of Prime Olefins from Gas Stream Using Anion Pillared Metal Organic Frameworks: Ideal Adsorbed Solution Theory Studies, Cyclic Application and Stability

The separation of C3H4/C3H6 is one of the most energy intensive and challenging operations, requiring up to 100 theoretical stages, in traditional cryogenic distillation. In this investigation, the potential application of two MOFs (SIFSIX-3-Ni and NbOFFIVE-1-Ni) was tested by studying the adsorption-desorption behaviors at a range of operational temperatures (300–360 K) and pressures (1–100 kPa). Dynamic adsorption breakthrough tests were conducted and the stability and regeneration ability of the MOFs were established after eight consecutive cycles. In order to establish the engineering key parameters, the experimental data were fitted to four isotherm models (Langmuir, Freundlich, Sips and Toth) in addition to the estimation of the thermodynamic properties such as the isosteric heats of adsorption. The selectivity of the separation was tested by applying ideal adsorbed solution theory (IAST). The results revealed that SIFSIX-3-Ni is an effective adsorbent for the separation of 10/90 v/v C3H4/C3H6 under the range of experimental conditions used in this study. The maximum adsorption reported for the same combination was 3.2 mmol g−1. Breakthrough curves confirmed the suitability of this material for the separation with a 10-min gab before the lighter C3H4 is eluted from the column. The separated C3H6 was obtained with a 99.98% purity.

High Purity/Recovery Separation of Propylene from Propyne Using Anion Pillared Metal-Organic Framework: Application of Vacuum Swing Adsorption (VSA)

Propylene is one of the world’s most important basic olefin raw material used in the production of a vast array of polymers and other chemicals. The need for high purity grade of propylene is essential and traditionally achieved by the very energy-intensive cryogenic separation. In this study, a pillared inorganic anion SIF62− was used as a highly selective C3H4 due to the square grid pyrazine-based structure. Single gas adsorption revealed a very high C3H4 uptake value (3.32, 3.12, 2.97 and 2.43 mmol·g−1 at 300, 320, 340 and 360 K, respectively). The values for propylene for the same temperatures were 2.73, 2.64, 2.31 and 1.84 mmol·g−1, respectively. Experimental results were obtained for the two gases fitted using Langmuir and Toth models. The former had a varied degree of representation of the system with a better presentation of the adsorption of the propylene compared to the propyne system. The Toth model regression offered a better fit of the experimental data over the entire range of pressures. The representation and fitting of the models are important to estimate the energy in the form of the isosteric heats of adsorption (Qst), which were found to be 45 and 30 kJ·Kmol−1 for propyne and propylene, respectively. A Higher Qst value reveals strong interactions between the solid and the gas. The dynamic breakthrough for binary mixtures of C3H4/C3H6 (30:70 v/v)) were established. Heavier propylene molecules were eluted first from the column compared to the lighter propyne. Vacuum swing adsorption was best suited for the application of strongly bound materials in adsorbents. A six-step cycle was used for the recovery of high purity C3H4 and C3H6. The VSA system was tested with respect to changing blowdown time and purge time as well as energy requirements. It was found that the increase in purge time had an appositive effect on C3H6 recovery but reduced productivity and recovery. Accordingly, under the experimental conditions used in this study for VSA, the purge time of 600 s was considered a suitable trade-off time for purging. Recovery up to 99%, purity of 98.5% were achieved at a purge time of 600 s. Maximum achieved purity and recovery were 97.4% and 98.5% at 100 s blowdown time. Energy and power consumption varied between 63–70 kWh/ton at the range of purge and blowdown time used. The VSA offers a trade-off and cost-effective technology for the recovery and separation of olefins and paraffin at low pressure and high purity.

CO2 capture on natural zeolite clinoptilolite: Effect of temperature and role of the adsorption sites

In this study, the adsorption capacity of the low-cost zeolite clinoptilolite was investigated for capturing carbon dioxide (CO₂) emitted from industrial processes at moderate temperature. The CO₂ adsorption capacity of clinoptilolite (a commercial natural zeolite) and ion-exchanged (with Na⁺ and Ca²⁺) clinoptilolite were tested under both dynamic (using a fixed-bed reactor operating with 10% vol. CO₂ in N₂) and equilibrium conditions (measuring single component adsorption isotherms). The dynamic CO₂ adsorption capacity of bare clinoptilolite and ion-exchanged clinoptilolite were evaluated in the temperature range from 293 K to 338 K and the obtained breakthrough curves were compared with those of the commercial zeolite 13X (Z13X). Although the adsorption capacity of Z13X exceeded those of bare clinoptilolite and ion-exchanged clinoptilolite at 293 K, the clinoptilolite exhibited the highest CO₂ uptake at a moderate temperature of 338 K (i.e. 25 % higher than Z13X). This feature appears in agreement with the lower isosteric heat of CO₂ adsorption on clinoptilolite compared to the other samples. The surface species affecting the q_iso and adsorption capacity were investigated through the FTIR spectroscopy using CO₂ as probe molecule. As a whole, it has been observed that CO₂ forms linear adducts onto K⁺ and Mg²⁺ cations of the bare clinoptilolite, and carbonate-like species onto its basic sites. With the Na-exchanged clinoptilolite, Na⁺ ions led to a decrease in surface basicity and to the formation of both single (Na⁺···OCO) and dual (Na⁺···OCO⋯Na⁺) cationic sites available for the formation of linear adducts. As a result of the remarkable adsorption capacity of clinoptilolite at 338 K, this material appears to be a promising adsorbent for the direct CO₂ removal from different flue gases sources operating at such temperatures.

Mass Transfer Correlation and Optimization of Carbon Dioxide Capture in a Microchannel Contactor: A Case of CO2-Rich Gas

This work focused on the application of a microchannel contactor for CO2 capture using water as absorbent, especially for the application of CO2-rich gas. The influence of operating conditions (temperature, volumetric flow rate of gas and liquid, and CO2 concentration) on the absorption efficiency and the overall liquid-side volumetric mass transfer coefficient was presented in terms of the main effects and interactions based on the factorial design of experiments. It was found that 70.9% of CO2 capture was achieved under the operating conditions as follows; temperature of 50 °C, CO2 inlet fraction of 53.7%, total gas volumetric flow rate of 150 mL min−1, and adsorbent volumetric flow rate of 1 mL min−1. Outstanding performance of CO2 capture was demonstrated with the overall liquid-side volumetric mass transfer coefficient of 0.26 s−1. Further enhancing the system by using 2.2 M of monoethanolamine in water (1:1 molar ratio of MEA-to-CO2) boosted the absorption efficiency up to 88%.