Development of zeolite adsorbents for CO₂ separation in achieving carbon neutrality

Abstract

The escalating atmospheric CO₂ concentration has become a global concern due to its substantial influence on climate change, emphasizing the necessity of carbon capture to achieve carbon neutrality. Adsorption-based CO₂ separation is a promising approach for carbon capture, highlighting the importance of developing solid porous materials as effective adsorbents. Among these porous materials, zeolites stand out as promising adsorbents due to their extensively tunable adsorption/separation properties, superior structural stability, non-toxicity, and cost-effectiveness. This review provides a comprehensive overview of the mechanisms, strategies, and prospects for zeolite development in separating CO₂ from critical scenarios, encompassing flue gas (CO₂/N₂), natural/bio/landfill gases (CO₂/CH₄), and air, respectively. This review outlines general mechanisms for CO₂ separation using zeolites, discusses specific strategies for zeolite development, and concludes with a summary of current findings and an outlook for future research.

Introduction

CO₂ is an inevitable product during the combustion of fossil fuels. Over the past few decades, the escalating global energy demand has substantially increased anthropogenic CO₂ emissions. Specifically, global total anthropogenic CO₂ emissions surged from 4.5 GtC yr⁻¹ in the 1960s to 10.9 GtC in 2021, resulting in a cumulative emission of 470 GtC¹. Correspondingly, the surface average atmospheric CO₂ concentration has increased from 317 to 414.7 ppm in this period¹. The continuous rise in atmospheric CO₂ concentration has become a global concern due to its profound influence on climate change, amplifying the frequency and severity of extreme climatic events². Therefore, achieving carbon neutrality is imperative, necessitating efforts to mitigate anthropogenic CO₂ emissions and remove CO₂ from the atmosphere³.

A promising solution for mitigating anthropogenic CO₂ emissions is Carbon Capture, Utilization, and Storage (CCUS)^4,5. It involves capturing CO₂ emitted from large-scale point sources and subsequently utilizing it as an industrial feedstock or storing it underground^4,5. In CCUS, carbon capture is the pivotal stage, constituting over 70% of the total costs^6,7. This stage typically involves capture CO₂ from flue gas (CO₂/N₂ separation) and natural/bio/landfill gases (CO₂/CH₄ separation for methane purification)^8,9,10. In addition, direct air capture (DAC) plays a critical role in removing CO₂ from the atmosphere, serving as an indispensable complement in achieving carbon neutrality^11,12.

Adsorption-based CO₂ separation is a promising approach for carbon capture from these critical scenarios¹³. This approach relies on selective adsorption of CO₂ on the surface of adsorbents (i.e., solid porous materials), driven by their greater adsorption strength, adsorption kinetics, and/or accessibility toward CO₂ compared to other accompanying gas components (e.g., N₂, CH₄, and O₂)^4,14. Notably, the efficacy of this approach primarily hinges on the performance and properties of CO₂ adsorbents, including high CO₂ capacity, high CO₂ selectivity, fast CO₂ adsorption/desorption kinetics, good reusability, non-toxicity, and low-cost^15,16. Hence, developing solid porous materials meeting these criteria is an essential task¹⁷.

One category of promising CO₂ adsorbents is zeolites. Zeolites are microporous (< 2 nm) materials with several advantageous properties, such as non-toxicity, low cost, good structural stability, and uniform aperture size^4,14,16. More importantly, zeolites are crystalline aluminosilicates composed of TO₄ tetrahedra, where T denotes Si (IV) or Al (III) located at the center of the tetrahedra. Each Al(III)O₄ tetrahedron introduces a negative charge to the framework, allowing the incorporation of extra-framework cations for charge compensation^4,14,16. Given that both zeolite framework (oxygen atoms of TO₄) and extra-framework cations can serve as effective adsorption sites, zeolites have superior tunable adsorption properties¹⁸. These inherent merits endow zeolites with a high potential for industrialization, distinguishing them from typical adsorbents such as activated carbons (ACs) and metal-organic frameworks (MOFs)¹⁹.

By employing these advantages, zeolites have been developed as promising adsorbents for CO₂ separation, as summarized in existing review articles^14,20. However, to the best of our knowledge, it lacks of reviews that summarize the specific strategies for developing zeolite adsorbents according to major CO₂ separation applications. To bridge this knowledge gap, this review summarizes specific strategies for zeolite development in separating CO₂ from three major scenarios, including flue gas, natural/bio/landfill gases, and air, respectively. Specifically, the review starts with an introduction of general mechanisms governing CO₂ separation using zeolites, encompassing equilibrium effect, steric effect, kinetic effect, and gating effect. The review proceeds with an in-depth discussion of critical applications of CO₂ separation, unveiling the specific strategies of zeolite developments. Finally, the review concludes with a summary of current findings and an outlook for future research.

Mechanisms for CO₂ separation using zeolites

Equilibrium effect

CO₂ separation by equilibrium effect occurs through its preferential adsorption from gas mixtures (Fig. 1A), driven by the stronger gas-adsorbent interactions of CO₂ than other major components in flue gas (i.e., N₂), natural/bio/landfill gases (i.e., CH₄), and air (i.e., N₂ and O₂)^17,21,22,23. Gas-zeolite interactions are governed by three primary interactions: Van der Waals interactions, electrostatic interactions, and weak chemical reactions^17,21,22,23. Van der Waals interactions are weak interactions occurring “nonspecifically” within all gas-adsorbent systems, typically facilitated by the larger polarizability of gas molecules. Zeolites possess a surface electric field originating from their ionic frameworks, with electrostatic interactions being the primary determinant of gas-zeolite interactions, notably acting on gas molecules with larger polarizability and multipole moments. Weak chemical reactions within zeolites encompass the formation of broad types of bonds, such as π-complexation between gas molecules and specific transition metal cations (e.g., Cu⁺ and Ag⁺), as well as the acid-base interactions between acidic gases (e.g., CO₂) and incorporated basic adsorption sites (e.g., amine groups). Compared to N₂, CH₄, and O₂, CO₂ features considerably greater polarizability, quadrupole moment, and chemical reactivity (Table 1), resulting in stronger interactions with zeolites^17,21,22,23. This characteristic facilitates equilibrium effect-governed CO₂ separation from CO₂/N₂ mixtures, CO₂/CH₄ mixtures, and air. Developing zeolites for equilibrium effect-governed CO₂ separation primarily hinges on manipulating effective adsorption sites that possess stronger interactions with CO₂.

Fig. 1: Schematic representation of the mechanisms for CO₂ separation using zeolites.

The mechanisms include A equilibrium effect, B steric effect, C kinetic effect, and D gating effect.

Table 1 Physical parameters of gas molecules²³

Steric effect (molecular sieving)

Given the smaller molecular size of CO₂ than N₂ and CH₄ (Table 1), CO₂ separation by steric effect can be accomplished using porous materials with an aperture size larger than that of CO₂ but smaller than that of N₂ and CH₄ ²³. Specifically, adsorbents tailored for this purpose enable the exclusive admission of CO₂ into the pores for adsorption, while preventing N₂ and CH₄ from accessing the pores (Fig. 1B)²³. This mechanism is viable for adsorbents like zeolites owing to their uniform aperture size in the crystalline structure (thus also referred to as molecular sieves)²¹. Developing zeolites for steric effect-governed CO₂ separation primarily involves manipulating the aperture size of zeolite frameworks.

Kinetic effect

CO₂ separation by kinetic effect relies on different diffusion rates of gases within porous frameworks (Fig. 1C)^10,20. While kinetic effects have been noted in some instances of CO₂ separation, zeolites designed explicitly for this purpose have not been documented^24,25,26.

Gating effect

CO₂ separation by gating effect depends on specific structural variations in zeolites, which would change the gas accessibility, resulting in exclusive admission and adsorption of CO₂ (Fig. 1D)^{27,28,29,30,31}. These structural variations are collectively triggered by host-guest (i.e., gas-zeolite) interactions and external stimulus (i.e., temperature and pressure), manifested at a specific threshold range^{27,28,29,30,31}. Notably, under a particular working temperature and pressure for CO₂ separation, the threshold for inducing structural variations is exclusively attainable by interacting with CO₂ rather than N₂ and CH₄, owing to the stronger gas-zeolite interactions of CO₂ than that of N₂ and CH₄^{27,28,29,30,31}. For zeolites with a rigid framework, the structural variation typically refers to the facilitated oscillation of the gate-keeping groups (i.e., extra-framework cations) that originally block all the gas diffusion paths within zeolite frameworks, termed molecular trapdoor effect or cation-gating effect²⁸. In addition, zeolites with flexible frameworks undergo structural variations by expanding aperture windows and relocating extra-framework cations, known as gate-breathing effect^29,30. Given the threshold dependency of gating effect, the development of zeolite should focus on investigating the optimal configuration of zeolite frameworks and extra-framework cations that enable effective structural variations under specific working conditions.

General methodologies for developing zeolite adsorbents for CO₂ separation

Synthesis

Artificial zeolites can be synthesized using various methods, typically involving hydrothermal synthesis^32,33, sol-gel synthesis³⁴, dry-gel synthesis³³, vapor phase transport synthesis³⁵, and mechanochemical synthesis³⁶. The synthesis of zeolites is conducted in a mixture containing aluminum sources, silicon sources, zeolite seeds, structure directing agents, mineralizers, and solvents at a specific composition in liquid, sol, solid, or vapor phases. The mixture undergoes nucleation and crystallization at specific temperatures and durations, with or without stirring, microwaves, or ultrasound. The topology, Si/Al ratios, and morphology of produced zeolites can be adjusted by modifying the composition of synthetical mixtures, as well as the conditions of nucleation and crystallization. Subsequently, as-synthesized zeolites are separated and washed with appropriate solvents (primarily H₂O). Finally, as-synthesized zeolites can be calcined in an oxygen-containing atmosphere to remove organic structure directing agents (if used) that occlude the apertures. By employing the aforementioned methodologies, various types of zeolites, especially those with LTA, CHA, MER, MFI, FER, BEA, and FAU structures, have been commercialized as industrial desiccants, catalysts, ion exchangers, and adsorbents³⁷. In the pursuit of carbon neutrality during zeolite production, there is a growing need to develop green synthesis approaches (e.g., approaches at solvent-free, organic-free, and energy-efficient conditions) utilizing sustainable starting materials (e.g., kaolin, diatomite, rectorite, rice husk, corn cob ash, and coal fly ash)^38,39.

Post-synthesis modification

Post-synthesis zeolite modification primarily focuses on modifying extra-framework cations due to their role in tuning the adsorption strength of both cations and zeolite framework (oxygen atoms)¹⁸. Liquid-state ion exchange emerges as the most prevalent method due to its capability to produce well-dispersed isolated metal cations within zeolite frameworks^40,41. This process involves dispersing zeolites in an aqueous solution containing cationic species to be exchanged, followed by a specific duration of ion exchange facilitated by stirring and heating (if needed). Following ion exchange, zeolites are separated, washed with appropriate solvents (primarily H₂O), and may undergo further exchanges or modifications. Solid-state ion exchange is another method for modifying extra-framework cations^40,41. This method involves blending zeolites with ion-containing compounds (e.g., metal salts) to create a homogeneous mixture. The mixture is finally kept in the solid state and then heated to a temperature surpassing the melting point of the ion-containing compounds, allowing the ions to diffuse from the molten salt into the zeolite framework and replace the original cations. After cooling, the ion-exchanged zeolite can be washed and dried. Impregnation is a well-applied methodology for incorporating amine into zeolites, enabling selective CO₂ adsorption through acid (CO₂)–base (amine) interactions. In this process, zeolites are immersed in a solution containing amine compounds. Upon solvent evaporation, the resulting amine-modified zeolite is obtained.

Characterization

The morphology of zeolites can be observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM)⁴². The crystalline and structural information of zeolites, such as cell parameters, phase purity, crystallinity, framework structures, and location of extra-framework cations, are typically characterized by resolving X-ray powder diffraction (XPRD) results. Nuclear powder diffraction (NPD) can also be used to examine cell parameters, phase purity, crystallinity, and framework structures of zeolites. Solid-state nuclear magnetic resonance (SSNMR) and far infrared (FIR) results can provide insights into the location of extra-framework cations^43,44. The resolutions of XPRD and FIR results can be enhanced by employing a synchrotron beamline⁴⁵. The elemental compositions of zeolites, crucial for determining Si/Al ratios and identifying the type and quantity of extra-framework cations, can be analyzed using techniques such as inductively coupled plasma (ICP), energy-dispersive X-ray spectroscopy (EDS), X-ray fluorescence (XRF), or X-ray photoelectron spectroscopy (XPS)⁴⁶. The oxidation states of elements in zeolites, especially metal cations, are primarily characterized by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS)⁴⁷. The presence of amine groups in amine-incorporated zeolites can be characterized using Fourier transform infrared spectroscopy (FTIR), SSNMR, and high-resolution transmission electron microscopy (HRTEM)⁴⁸. Porous properties of zeolites are typically determined using established mathematic models that fit experimentally measured gas adsorption isotherms. Gas adsorption isotherms used to determine porous properties are obtained through single-component static gas adsorption tests conducted at the boiling point of typical probe molecules (e.g., N₂ adsorption at −196 °C)⁴⁹. The mechanisms of CO₂ adsorption in zeolite frameworks can be partially suggested by several methodologies, including FTIR, FIR, SSNMR, NPD, XAS, and synchrotron XPRD^{50,51,52,53,54}. Employing in situ techniques for this characterization would significantly enhance their effectiveness in studying CO₂ adsorption mechanisms. In addition, computational techniques such as density functional theory (DFT) calculations and molecular mechanics can be employed to assist in the characterization of zeolites⁵⁵.

Evaluating the performance of CO₂ adsorption/separation

Capacity and selectivity are critical parameters for evaluating the performance of CO₂ adsorption/separation using zeolites. CO₂ capacity can be determined through single-component and binary adsorption tests conducted at various temperatures and pressures, using pure CO₂ and CO₂-containing gas mixtures, respectively. Adsorption tests are commonly conducted using static (volumetric and gravimetric) or dynamic column breakthrough methods²². In published research, CO₂ capacities are typically reported as numerical values, adsorption isotherms (capacity measured at constant temperature but varying pressures), and adsorption isobars (capacity measured at constant pressure but varying temperatures). The selectivity of CO₂ over the other gas component (e.g., N₂ or CH₄) can be calculated utilizing gas adsorption results obtained experimentally. The selectivity of CO₂ is typically classified as ideal separation factor (ISF)¹⁸, IAST selectivity (IAST)¹⁸, actual separation factor (ASF)¹⁸, and equilibrium selectivity (EQS)²⁰, as detailed in the reference articles.

CO₂ separation from flue gas

Flue gas from power plants typically contains 8−15% of CO₂ and over 75% of N₂, constituting a significant portion of anthropogenic CO₂ emissions⁵⁶. Therefore, developing effective zeolite adsorbents for CO₂/N₂ separation holds great significance toward carbon neutrality. As discussed above, zeolite adsorbents can generally separate CO₂ from N₂ owing to the greater adsorption affinity and/or the smaller molecular size of CO₂ than N₂. The development of zeolites for CO₂/N₂ separation primarily focuses on achieving high CO₂ capacity and selectivity at specific operating conditions (typically within the temperature between 0–40 °C, CO₂ partial pressure between 10–15 kPa, and total pressure of approximately 100 kPa)^57,58,59.

CO₂/N₂ separation by equilibrium effect

Developing zeolites for equilibrium effect-governed CO₂/N₂ separation focuses on manipulating effective adsorption sites that possess stronger interaction with CO₂. Among metal cations-exchanged KFI zeolites (denoted as ZK-5), Liu et al. found that Li-ZK-5 and Na-ZK-5 exhibited higher CO₂ capacity (Table 2, Entry 1) and adsorption strength than K-ZK-5⁶⁰. Their research demonstrates that alkali metal cations (Li⁺, Na⁺, K⁺) are primary CO₂ adsorption sites in ZK-5. Specifically, alkali metal cations with a larger charge-to-size ratio (Li⁺ > Na⁺ > K⁺) can drive stronger gas-cation electrostatic interactions, resulting in higher CO₂ capacity and adsorption strength⁶⁰. Interestingly, an opposite trend was reported by Yang et al. on alkali metal cations (Li⁺, Na⁺, K⁺) -exchanged BEA zeolites⁶¹. K-BEA, containing metal cations with the smallest charge-to-size ratio (i.e., imparting the weakest gas-cation electrostatic interactions), exhibited the highest CO₂ capacity (Table 2, Entry 2) and adsorption strength. This observation suggests that zeolite framework oxygens (O), rather than metal cations, serve as the primary CO₂ adsorption sites in BEA zeolites. Specifically, metal cations with a smaller charge-to-size ratio (i.e., lower electronegativity) would induce more charge on zeolite framework O, resulting in enhanced CO₂-framework interactions^62,63,64. These studies reveal that both metal cations and zeolite framework O can serve as effective CO₂ adsorption sites, while the predominant site varies across different zeolite topologies. Since modifying metal cations would produce contrasting impacts on the adsorption strength of these sites, identifying the predominant site in the specific zeolite adsorbent becomes critical for its further development.

Table 2 CO₂ capacity (at the corresponding partial pressure with a total pressure at approximately 100 kPa) and CO₂/N₂ selectivity of reported prominent zeolite adsorbents for CO₂/N₂ separation

The predominant role of extra-framework metal cations or zeolite framework O as CO₂ adsorption sites can be identified by correlating the trend of CO₂ adsorption strength with the trend of the charge-to-size ratio of metal cations or the mean charge of zeolite framework O. This approach was typically demonstrated by Tao et al. in ion-exchanged LTA zeolites¹⁸. They found that metal cations are the predominant CO₂ adsorption sites in LTA zeolites when the loading is below one CO₂ molecule per unit cell (1 CO₂/U. C.), as evidenced by the increased CO₂ adsorption strength (indicated by DFT binding energy) with the charge-to-size ratio of metal cations (Ce³⁺ > Mn²⁺ > Ca²⁺ > Na⁺, Fig. 2). This finding is also applicable to N₂. Conversely, zeolite framework O were found to be the predominate adsorption site at higher CO₂ loadings (above 1 CO₂/U. C.), as demonstrated by the increased CO₂ adsorption strength (indicated by isosteric heat) with the mean charge of zeolite framework O (Ce-LTA < Mn-LTA < Ca-LTA < Na-LTA, Fig. 2). Notably, in the context of CO₂ and N₂ adsorption at 15 kPa/85 kPa partial pressures, the CO₂ capacity of LTA zeolites generally exceeds one adsorbate molecule per unit cell, nor does N₂ with lower adsorption affinity¹⁸. Therefore, CO₂ and N₂ adsorption, in this case, is governed by gas-framework interactions and gas-cation interactions, respectively. Accordingly, developing LTA zeolites for CO₂/N₂ separation should be directed on ion-exchanging metal cations with a small charge-to-size ratio (e.g., Na⁺, Table 2, Entry 3), which concurrently results in strong CO₂-framework interactions and weak N₂-cation interactions, leading to high CO₂ capacity and high CO₂/N₂ selectivity simultaneously¹⁸.

Fig. 2: Identification of the predominant CO₂ adsorption sites in zeolites.

Relationships between the charge-to-size ratio of metal cations, the mean charge of zeolite framework oxygens, and the CO₂ adsorption strength at different loadings (i.e., 1 and 1.5 CO₂ molecules per unit cell - CO₂/U.C.) in metal cation-exchanged LTA zeolite with Si/Al = 3 (referred to as LTA-3)¹⁸. Copyright 2023 Elsevier B.V.

In summary, developing metal cation ion-exchanged zeolites for equilibrium effect-governed CO₂/N₂ separation primarily involves introducing and tuning the extra-framework metal cations with larger (when cations are predominant adsorption sites for CO₂) or smaller (when zeolite framework O are predominant adsorption sites for CO₂) charge-to-size ratio by ion exchange.

Amine-modified zeolites are also developed for equilibrium effect-governed CO₂/N₂ separation⁶⁵. Typically, CO₂ can be selectively captured by amine groups through acid-base interactions, while N₂ does not interact with amines. Notably, CO₂-amine reactions can occur at relatively high temperatures, allowing amine-modified zeolites to be effective for CO₂ separation at elevated temperatures⁶⁶. This is distinct from metal cation ion-exchanged zeolites, which adsorb CO₂ via physical interactions and are typically more effective at low temperatures⁶⁷. In amine-modified zeolites, the loading of amine significantly affects the CO₂ capacity and CO₂/N₂ selectivity. Figure 3A, B illustrates that the highest CO₂ capacity and CO₂/N₂ selectivity occur at a moderate amine loading^68,69. This observation can be attributed to the trade-off between the quantity of impregnated amine and the (partial) blockage of zeolite micropores it causes¹⁴. Increased amine loadings introduce more amine groups as CO₂ adsorption sites but also lead to more severe micropore blockage, potentially impeding the accessibility of amine groups inside micropores. Notably, mesopores can effectively increase the loading and accessibility of amine groups as CO₂ adsorption sites⁷⁰. Meanwhile, it can also facilitate the diffusion of CO₂, thereby improving adsorption kinetics⁷⁰. Therefore, utilizing mesopore-interconnected zeolites for amine impregnation has been proposed as an effective approach to developing amine-modified zeolites for CO₂/N₂ separation (Fig. 3C, D). This approach can be achieved by preparing lamellar zeolites, hierarchical zeolites, and zeolite nanotubes using large organic structure directing agents during synthesis^70,71,72,73.

Fig. 3: Effect of amine loadings and zeolite mesoporosity on CO₂ adsorption and separation behaviors of amine-modified zeolites.

A CO₂ adsorption capacity (measured in 15% CO₂ balanced by Helium at 101.3 kPa and 35 °C) of aminopropyltriethoxysilane (APTES) and tetraethylenepentamine (TEPA)-loaded BEA zeolites (β-25) with increasing wt% of amine in the impregnation solutions⁶⁸. Copyright 2016 Elsevier B.V. B Ideal separation factors on monoethanol amine (MEA)-loaded BEA zeolites with different MEA loadings (quantified by the wt% of amine in zeolite supports) at 3 kPa and 30 °C⁶⁹. Copyright 2009 Elsevier B.V. C CO₂ adsorption capacity and adsorption kinetics of Polyethylenimine (PEI)-loaded zeolite 13X with and without mesopores in 15% CO₂/85%N₂ at 101.3 kPa and 100 °C⁷². Copyright 2015 Elsevier B.V. D CO₂ adsorption capacity and adsorption kinetics of PEI-loaded mesoporous adsorbents, including zeolite nanotubes (blue), large pore-MCM-41 (green), SBA-15 (yellow), and small pore-MCM-41 (red) in 10% CO₂ balanced by Helium at 101.3 kPa and 30 °C⁷⁰. Copyright 2023 American Chemical Society.

CO₂/N₂ separation by steric effect

By employing zeolites with customized aperture sizes, steric effect emerges as another valuable approach for CO₂/N₂ separation, with partially K⁺-exchanged LTA zeolites as typical examples. Liu et al. and Hao et al. demonstrated that increasing the amount of K⁺ in NaA (Na⁺ form LTA zeolites with Si/Al = 1) by ion exchange (partially K⁺-exchanged NaA designated as NaKA) led to a more pronounced diminishment in N₂ capacity than CO₂, notably increasing CO₂/N₂ selectivity at a critical threshold of cation composition (i.e., 17% K⁺ and 83% Na⁺, designated as NaK(17)A) (Table 2, Entry 4)^74,75. This finding is attributed to the positioning of K⁺ cations near the 8-membered rings (8MRs) of LTA zeolites, which partially block 8MRs and prevent N₂ from entering the pores (Fig. 4A)^74,75,76,77. Further elevating the K⁺ content to 28% led to negligible N₂ capacity but caused a substantial loss in CO₂ capacity due to excessive pore blockage, restricting its effectiveness in CO₂/N₂ separation⁷⁴. In addition, NaKA showcased ineffectiveness in separating CO₂ from binary mixtures under dynamic conditions, with CO₂ breakthrough occurring rapidly⁷⁸. This observation raised concerns about the slow diffusion of CO₂ within NaKA caused by excessive pore blockage, resulting in slow adsorption kinetics. To address these issues, Cheung et al. and Akhtar et al. increased the Si/Al ratio of LTA zeolites from 1 to 1.3 (designated NaK-ZK-4), which reduces the total amount of extra-framework cations, thereby facilitating the admission of CO₂ at higher K⁺ content. This development endows NaK-ZK-4 with high CO₂ capacity, high CO₂/N₂ selectivity (Fig. 4B, and Table 2, Entry 5), and faster adsorption kinetics simultaneously^79,80,81. As a result, developing metal cations-exchanged zeolites for steric effect-governed CO₂/N₂ separation should focus on adjusting the extra-framework cations (via ion-exchange) and framework Si/Al ratios (via synthesis or post-synthetic treatment, see ref. ⁸²) simultaneously, orienting appropriate pore blockage that can achieve exclusive admission of CO₂ with high adsorption capacity and fast adsorption kinetics.

Fig. 4: Schematic representation and adsorption results of CO₂/N₂ separation by steric effect.

A The schematic representation of CO₂/N₂ separation by steric effect on partially K⁺-exchanged NaA (Na⁺ form LTA zeolites with Si/Al = 1)⁷⁴. Copyright 2010 The Royal Society of Chemistry. B CO₂ and N₂ adsorption isotherms of Na-ZK4 (Na⁺ form LTA zeolites with Si/Al = 1.3) and NaK-ZK4 (partially K⁺-exchanged Na-ZK4) at 0 °C⁷⁹. Copyright 2014 American Chemical Society. C The schematic representation of CO₂/N₂ separation by steric effect on iron-containing heteroatomic MOR zeolites. Left: side view, Right: top view⁸³. Copyright 2021 Science.

Besides ion exchange, manipulating the aperture size of zeolites can be accomplished by introducing heteroatoms to zeolite frameworks. Zhou et al. produced iron-contained heteroatomic MOR zeolites via in situ growth as molecular sieves of CO₂. In this adsorbent, the microchannel of the MOR framework was narrowed from around 6.7 to 3.3 Å by incorporating a tiny amount of tetrahedral Fe species, which effectively prevented N₂ from accessing the pores while maintaining rapid admission of CO₂ (Fig. 4C and Table 2, Entry 6)⁸³.

CO₂/N₂ separation by gating effect

By tuning structural variations in zeolites, the exclusive admission of CO₂ can also be accomplished by gating effect at specific working temperatures/pressures. Applying gating effect for CO₂/N₂ separation was first demonstrated by Shang et al. on K⁺-exchanged CHA zeolites (rigid framework with Si/Al = 1, known as r1KCHA), recognized as molecular trapdoor or cation-gating effect. On r1KCHA, the CO₂ and N₂ capacities exhibited a counterintuitive bell-shaped (an initial increase followed by a decrease) isobar as temperatures rose (Fig. 5A)²⁸. The initial increase (of capacity with temperature) is attributed to the facilitated oscillation of K⁺ cations (that originally block all the gas diffusion paths in CHA zeolite as gate-keeping cations) by temperature, promoting the admission of gases. After the temperature reaches a threshold (marked by the peak capacity), the capacity starts decreasing due to the exothermic nature of physisorption^28,84,85. Notably, the oscillation of gate-keeping cations can also be facilitated by gas-cation interactions. Therefore, CO₂, with much stronger gas-cation interactions than N₂, exhibited a substantially lower threshold admission temperature (0 °C) than that of N₂ (60 °C)^28,84,85. This study reveals 0 ^oC as an optimal working temperature for CO₂/N₂ separation using r1KCHA, allowing exclusive admission and maximum uptake of CO₂ (Fig. 5A, B), resulting in exceptionally high CO₂/N₂ selectivity (Table 2, Entry 7)²⁸.

Fig. 5: Schematic representation and adsorption results of CO₂/N₂ separation by gating effect.

A Adsorption isobars of CO₂ and N₂ on r1KCHA (K⁺-exchanged CHA zeolite with Si/Al = 1)²⁸. Copyright 2012 American Chemical Society. B The schematic representation of CO₂/N₂ separation by molecular trapdoor/cation-gating effect on KCHA zeolites¹²⁵. Copyright 2013 Springer Nature Limited. C CO₂ adsorption isotherms at 25 °C on Na⁺ form GIS zeolites at different Si/Al ratios⁸⁶. Copyright 2021 Elsevier B.V. D Mechanisms and influencing factors of gating effect for selective CO₂ adsorption using zeolites³⁰. Copyright 2021 John Wiley & Sons.

Gating effect in flexible zeolites is referred to as gate-breathing effect. Representative cases feature the “two-step” CO₂ adsorption isotherms, exemplified in Fig. 5C^30,86. The first step at low CO₂ (partial) pressures exhibits notably low CO₂ capacity, attributed to hindered CO₂ admission caused by the excessively severe pore blockage^30,86. Upon reaching a threshold pressure, the admission of CO₂ is remarkably facilitated by the narrow-to-wide changes of 8MRs and the relocation of gate-keeping cations that originally block the 8MRs, resulting in considerably higher CO₂ capacity^30,86. Notably, such a threshold pressure was not observed during N₂ adsorption at 0-1 bar, attributed to its weaker gas-cation interactions than CO₂ ^30,86. Accordingly, CO₂/N₂ separation utilizing gate-breathing effect can be achieved when the partial pressure of CO₂ exceeds its threshold value (Table 2, Entry 8–9)²⁷.

Developing zeolites for gating effect-governed CO₂/N₂ separation lies on adjusting their optimal working temperature/pressure that allows exclusive and maximum admission of CO₂, as illustrated in Fig. 5D^84,87. Specifically, increasing the quantity and size of gating cations (e.g., by increasing the Si/Al ratio of zeolite frameworks and/or by ion-exchanging metal cations with smaller charge-to-size ratio) would result in higher critical/threshold admission temperatures/pressures of gating effect for both CO₂ and N₂, vice versa⁸⁴.

CO₂ separation for methane purification

Methane plays a crucial role in the global energy landscape, accounting for over 20% of the global energy demand in the last decade⁸⁸. However, CO₂ always coexists in various methane sources (e.g., typically 0.1–15% in the natural gas⁸⁹, 15–60% in the biogas⁹⁰, and 40–50% in the landfill gas⁹¹), reducing the calorific value of methane and leading to excess CO₂ emissions. Therefore, developing effective zeolite adsorbents for CO₂/CH₄ separation is crucial for achieving carbon neutrality.

Given the greater adsorption affinity and smaller molecular size of CO₂ compared to CH₄ (Table 1), developing zeolites for CO₂/CH₄ separation typically mirrors the approaches used in CO₂/N₂ separation, which have been detailed in the preceding section. Hence, these approaches will not be repeated here unless specific cases warrant further elaboration.

Several studies on equilibrium effect-governed CO₂/CH₄ separation uncover the impact of Si/Al ratios^18,92,93. In LTA zeolites, the adsorption heat of CO₂ increases with the framework Al content (Fig. 6A), while the adsorption heat of CH₄ is less affected (Fig. 6B)⁹². This observation is attributed to the larger polarizability and quadrupole moment of CO₂ compared to CH₄, leading to a more pronounced enhancement in CO₂-zeolite electrostatic interactions with the amplified electrostatic field caused by the increased framework Al content. The higher CH₄ adsorption heat in LTA-1 (Si/Al = 1) compared to counterparts with higher Si/Al ratios may attributed to the interaction between one CH₄ with multiple cations simultaneously, resulting from the large amount of cations present in LTA-1⁹². Overall, higher Al content in LTA zeolites typically yields higher CO₂ capacity and CO₂/CH₄ selectivity (Table 3, Entry 1–2)^18,92,93.

Fig. 6: Effect of zeolite Si/Al ratios on the adsorption strength of CO₂ and CH₄.

Isosteric heat of adsorption in Na⁺ form LTA zeolites with different Si/Al ratios⁹². A CO₂ and B CH₄. Copyright 2009 American Chemical Society.

Table 3 CO₂ capacity (at the corresponding partial pressure with a total pressure at approximately 100 kPa) and CO₂/CH₄ selectivity of reported prominent zeolite adsorbents for CO₂/CH₄ separation

Steric effect-governed CO₂/CH₄ separation has been observed not only in rigid LTA zeolites but also in flexible RHO zeolites^94,95,96. Remarkably, Li⁺ cations, which are generally believed to be too small for pore blockage, can counterintuitively induce steric effect in RHO zeolites. Steric effect in Li-RHO is attributed to the distorted 8MRs caused by Li⁺ located at the 6-membered rings (6MRs) (Fig. 7A, B)⁹⁴. This distortion narrows the gas diffusion pathways, enabling exclusive admission of CO₂ despite a slow diffusion rate (Table 3, Entry 9, and Fig. 7C)⁹⁴. Interestingly, exchanging a minority of Na⁺ or Cs⁺ cations (typically less than 2 cations per unit cell) into Li-RHO leads to expanded 8MRs due to the different location of Na⁺ or Cs⁺ (close to double 8MRs) compared to Li⁺ (Fig. 7A, B)⁹⁴. Although this modification reduces the CO₂/CH₄ selectivity of RHO zeolites (Table 3, Entry 9–10), it effectively facilitates the admission of CO₂ on Na_2.1Li_7.7-RHO and Cs_1.8Li₈-RHO under dynamic conditions (Fig. 7D)⁹⁴. Further increasing the amount of Na⁺, K⁺, and Cs⁺ cations in RHO zeolites would result in pore blockage, accompanied by the cation relocation and framework expansion during CO₂ adsorption, indicating gate-breathing effect-governed CO₂/CH₄ separation in Na-RHO, K-RHO, and Cs-RHO (Table 3, Entry 11–12)^{94,97,98,99,100,101,102}.

Fig. 7: Steric effect-governed CO₂/CH₄ separation in flexible RHO zeolites.

A Top: Cross-sectional views of (left) H-RHO and (right) Li-RHO (Black: Si atoms, Red: O atoms, Orange: Li⁺ cations). Bottom: Views and free diameters of 8MRs in H-RHO, Cs, Li-RHO, and Li-RHO. B Cation locations in RHO zeolites. (Yellow: Double 8MRs for Cs⁺, Red: 8MRs for Na⁺, Blue: 6MRs for Li⁺). Breakthrough curves of CO₂ and CH₄ from C Li-RHO and D Na_2.1Li_7.7-RHO and Cs_1.8Li₈-RHO measured by binary column breakthrough dynamic adsorption tests. Feed conditions: CO₂/CH₄/He = 5/40/55, 100 kPa, 35 °C⁹⁴. Copyright 2016 American Chemical Society.

Direct Air capture (DAC) of CO₂

While the aforementioned CO₂ separation processes are viable for carbon capture from large-scale point sources, CCUS do not work for mobile sources, which account for approximately 50% of total global carbon emissions¹⁰³. Therefore, DAC is essential in achieving carbon neutrality, aiming to directly separate CO₂ from the atmosphere at concentrations around 400 ppm—0.04%^104,105.

Although prominent zeolite adsorbents generally exhibit high CO₂ capacity at moderate concentrations/partial pressures (e.g., 15% and 50%), their capacity diminishes at ultra-low CO₂ concentrations/partial pressures (typically below 1000 ppm—0.1%) owing to relatively weak gas-adsorbent interactions^106,107,108. Therefore, the ongoing development of zeolites for DAC aims to explore effective CO₂ adsorption sites that possess stronger interaction with CO₂^11,12. The adsorption/separation of CO₂ from air is governed by equilibrium effect.

Both extra-framework metal cations and zeolite framework oxygen atoms can serve as CO₂ adsorption sites at low concentrations. Studies by Stuckert et al. reported a higher adsorption heat and capacity of CO₂ (at 395 ppm) on Li-LSX compared to Na⁺ and K⁺-exchanged LSX (Low silica zeolite X, FAU framework, Si/Al = 1, Table 4, Entry 1)¹⁰⁹. Notably, the trend of CO₂ adsorption heat (Li-LSX > Na-LSX > K-LSX) correlates with the charge-to-size ratio of alkali metal cations (Li⁺ > Na⁺ > K⁺), indicating metal cations as the predominant CO₂ adsorption sites¹⁸. Similar findings on metal cation-exchanged LTA zeolites were provided by Tao et al. They elucidated that metal cations (including monovalent alkali metal, divalent alkaline earth metal, and di-, tri-valent transition metal cations) featuring a larger charge-to-size ratio can induce stronger electrostatic and/or back bonding interactions with CO₂ molecules, effectively enhancing the adsorption strength (Fig. 8A). Meanwhile, they also demonstrated a detrimental “shielding effect” among cations with an excessively large charge-to-size ratio. This effect is attributed to excessively strong gas-cation interactions, which hinder the efficient activation and regeneration of adsorbents to remove (pre)adsorbed CO₂ and H₂O, thereby reducing the practical CO₂ capacity. Among their studied LTA zeolites, Ca-LTA featuring Ca²⁺ with an appropriate charge-to-size ratio exhibited the highest CO₂ capacity at 400 ppm (Table 4, Entry 2)¹¹⁰. In addition, Itadani et al. demonstrated that the M²⁺-O^2--M²⁺ species (M²⁺: alkaline earth metal cation) within ion-exchanged MFI zeolites are more effective adsorption sites for low-concentration CO₂ (below 1000 ppm) than single M²⁺ sites (Table 4, Entry 3). This is due to the enhanced electrostatic interactions between M²⁺-O^2--M²⁺ and O^δ-=C^δ+=O^δ- (Fig. 8B)¹¹¹.

Table 4 CO₂ capacity of reported prominent zeolite adsorbents at low concentrations

Fig. 8: Low-concentration CO₂ adsorption behaviors and mechanisms on zeolites with extra-framework metal cations as predominant adsorption sites.

A The relationship between the charge-to-size ratio of metal cations, calculated DFT binding energy with CO₂, and CO₂ capacity (measured at 25 °C and 0.04 kPa) of ion-exchanged LTA zeolites with Si/Al = 3¹¹⁰. Copyright 2023 John Wiley & Sons. B CO₂ adsorption scheme on Ba²⁺-O^2--Ba²⁺ site within MFI zeolite¹¹¹. Copyright © 2016 American Chemical Society.

Zeolite framework O were demonstrated as predominant CO₂ adsorption sites in alkali and alkaline earth metal cation-exchanged zeolite X and Y (FAU framework with Si/Al = 1.2 and 2.6, respectively). This result is evidenced by the consistent trend of CO₂ adsorption heat and the mean charge of zeolite framework O (Fig. 9)¹¹². Notably, this finding contradicts the results observed on zeolite LSX (FAU framework with Si/Al = 1, metal cations as predominant CO₂ adsorption sites), as explained in the previous paragraph¹⁰⁶. This contradiction suggests that the predominant CO₂ adsorption sites can vary in zeolites with different Si/Al ratios^109,112. Nonetheless, directionally adjusting the charge-to-size ratio of metal cations remains the primary strategy in developing metal cation-exchanged zeolites for DAC. For instance, Ba²⁺, with a smaller charge-to-size ratio (i.e., smaller electronegativity) than Mg²⁺ and Ca²⁺, can induce a greater charge on zeolite framework O, resulting in stronger CO₂ adsorption and higher CO₂ capacity in zeolite X (Fig. 9, Table 4, Entry 4)¹¹².

Fig. 9: Low-concentration CO₂ adsorption behaviors on zeolites with framework oxygen atoms as predominant adsorption sites.

The relationship between the mean charge on zeolite framework oxygen atoms and the heat of CO₂ adsorption in alkali and alkaline earth metal cation-exchanged zeolite X (FAU zeolites with Si/Al = 1.2)¹¹². Copyright 2022 John Wiley & Sons.

The effectiveness of metal cations as low-concentration CO₂ adsorption sites is also affected by their specific locations. Oda et al. revealed that the coexistence of two Ca²⁺ at 8MRs and 6MRs locations is pivotal in LTA zeolites (Si/Al = 1) for adsorbing CO₂ at 400 ppm. This configuration enhances the interaction with CO₂ by enabling a single CO₂ molecule to interact with two Ca²⁺ cations simultaneously (Fig. 10A, Table 4, Entry 5)¹¹³. In addition, Fu et al. reported that Na⁺ cations located at the eight-membered ring (8MR) side-pockets of MOR zeolites and Zn²⁺ cations located at double 6MRs in CHA zeolites are primary adsorption sites for CO₂ at 400 ppm (Fig. 10B, C, Table 4, Entry 6–7)^114,115. While the authors did not explicitly provide the underlying reasons, we hypothesized that this finding is attributed to the stronger local electric field generated by cations at these specific locations, thereby augmenting electrostatic interaction with CO₂. Accordingly, zeolites with double 6MRs that arrange one 6-membered ring (6MR) into cages (e.g., CHA, AFX, and AEI frameworks) were reported to effectively accommodate Zn²⁺ cations as CO₂ adsorption sites, while other zeolites without this feature (e.g., LTA and FAU frameworks) do not exhibit such capability (Fig. 10D)¹¹⁴. These observations underscore the importance of further investigations into the impact of cation locations on DAC.

Fig. 10: Effective cation locations for low-concentration CO₂ adsorption in zeolites.

A The adsorption scheme of a singular CO₂ interacts with two Ca²⁺ located at 8MRs and 6MRs of LTA zeolite (Si/Al = 1)¹¹³. Copyright 2021 Royal Society of Chemistry. B Framework and cation locations of MOR zeolites¹¹⁵. Copyright 2022 National Academy of Sciences. C CO₂ capacity at 400 ppm/30 °C on CHA zeolites with and without Zn²⁺ at the double 6MRs site. Copyright 2021 John Wiley & Sons. D CO₂ capacity at 400 ppm/30 °C among different zeolites with and without Zn²⁺ ion exchange. Group I is small-pore zeolites with framework topologies more like the CHA-type. Group II includes the standard low-silica zeolite adsorbents¹¹⁴. Copyright 2021 John Wiley & Sons.

Apart from cation locations, the framework topology of zeolites is another factor that affects low-concentration CO₂ adsorption. Fu et al. demonstrated the pivotal role of confined space in Na⁺ form zeolite for CO₂ adsorption at 400 ppm. Their findings show a diminishing CO₂ capacity (per adsorption site, i.e., Na⁺) with increasingly confined space (Fig. 11). MOR zeolites, with the smallest dimension of confined space of approximately 6.3 Å, exhibited the highest CO₂ capacity per Na⁺¹¹⁵. This phenomenon is attributed to the stronger CO₂-zeolite electrostatic interactions within confined pores, driven by the larger electric field (generated by both extra-framework cations and zeolite frameworks) within smaller confined spaces²¹. Xiang et al. reported a similar phenomenon in zeolite X, where narrowing the pore of zeolite 13X (Na⁺ form FAU zeolite with Si/Al = 1.2) resulted in a higher CO₂ capacity at 400 ppm, which was achieved by introducing Fe atoms to replace T (Si, Al) atoms via in situ synthesis (Table 4, Entry 8)¹¹⁶. These findings suggest that zeolites with smaller cages are more favorable candidates for DAC.

Fig. 11: Effect of the size of zeolite cages on low-concentration CO₂ adsorption.

CO₂ adsorbed per adsorption site (i.e., Na⁺, measured at 400 ppm and 30 °C) among zeolites with different sizes of the confined space. The negligible CO₂ capacity of LTL may be attributed to the insufficient size of the elliptical 8MRs, which restricts the entry of CO₂ molecules¹¹⁵. Copyright 2022 National Academy of Sciences.

Overall, future development of zeolites for DAC should prioritize those with smaller cages and lower Si/Al ratios on the premise of not affecting the accessibility and diffusion of CO₂. Apart from directionally adjusting the charge-to-size ratio of extra-framework cations, developing zeolites for DAC should also focus on zeolite frameworks containing specific cation locations that can drive strong adsorption strength.

Summary and outlook

Zeolites have been developed as effective adsorbents for CO₂ separation from flue gas, natural/bio/landfill gases, and air. The mechanisms for CO₂ separation include equilibrium effect, steric effect, and gating effect. Overall, zeolites designed for equilibrium effect-governed CO₂ separation typically feature high CO₂ capacity and fast adsorption kinetics. In contrast, zeolites designed for steric/gating effect-governed CO₂ separation offer ultra-high selectivity, albeit often compromising CO₂ capacity and adsorption kinetics. Strategies for zeolite development primarily include adjusting the charge-to-size ratio and density of extra-framework metal cations, incorporating functional groups, and modifying the Si/Al ratio, topology, and/or morphology of zeolites. Specific strategies for various applications are detailed in the corresponding sections of this review. By utilizing these strategies, zeolites have been developed to enable high CO₂ capacity, selectivity, and/or fast adsorption kinetics, presenting huge potential for further improvement.

Nevertheless, in practical scenarios, the effectiveness of zeolites for CO₂ separation can vary across different operating conditions (e.g., temperatures, pressures, flow rates, and gas compositions for adsorption/regeneration). For instance, zeolites developed for equilibrium effect-governed CO₂ separation confront the trade-off between CO₂ adsorption strength and regenerability¹¹⁰. In addition, zeolites developed for steric/gating effect-governed CO₂ separation often face the trade-off between CO₂ selectivity and adsorption kinetics⁹⁴. Therefore, developing zeolites for CO₂ separation in practical scenarios necessities detailed adsorption/regeneration conditions of various applications. These conditions should be established based on promising industrial processes (e.g., pressure/vacuum/temperature swing adsorption processes) in future research to provide standardized methodologies for evaluating the CO₂ separation performance of zeolites. Before establishing these standardized methodologies, research should consider a comprehensive evaluation of zeolites to provide more insights for industrial applications.

In addition, the presence of water vapor in the humid feed gas is widely acknowledged as a detrimental factor that can deactivate zeolites as CO₂ adsorbents, attributed to their stronger interaction with H₂O than with CO₂^109,110. Such an issue of CO₂/H₂O selectivity is especially considerable for DAC owing to the high relative humidity and the low CO₂ concentration in its practical operating environments. This issue can be addressed through engineering approaches by utilizing multi-bed systems to incorporate desiccants prior to the zeolite adsorbents¹¹⁷. To develop zeolites capable of CO₂ separation under humid conditions, one approach is to prepare core-shell composites with zeolite cores and hydrophobic outer-shell materials¹¹⁸. Another approach is to develop amine-modified zeolites, whose CO₂ uptake can be enhanced by the presence of water vapor as water participates in the CO₂-amine reactions⁶⁸. But it is worth noting that several amine-based organic compounds have molecular sizes larger than the aperture sizes of most zeolites, rendering the incorporated amine not able to penetrate into the pore and thus less effective to facilitate the selective capture of CO₂ against H₂O occurring inside the porous zeolite. Strategies to create larger pores, such as the development of mesopore-interconnected zeolites, should thus be considered for the incorporation of such amine-based compounds aiming for selective CO₂ uptake.

As a result, zeolites have been developed as one of the most promising categories of adsorbents for adsorption-based CO₂ separation, with great potential for further improvement. By utilizing green synthesis methods to produce zeolite adsorbents developed with high capacity, high selectivity, fast adsorption kinetics, and good reusability, and their subsequent implementation into energy-efficient industrial adsorption/separation processes with renewable energy sources for regeneration, zeolites can be applied for various large-scale gas separation processes (e.g., CO₂ separation for carbon capture, H₂/CO₂ separation for H₂ purification, O₂/N₂/Ar separation for O₂ production, and hydrocarbons separation for fuel production)¹¹⁹, remarkably promoting carbon neutrality.