Enhancement CO₂ separation performance of three-component pebax-1657-based mixed matrix membranes containing ZIF-67/amine functionalized CNT dual nanofillers

Abstract

In this study, a novel class of ternary mixed matrix membranes (MMMs) was developed by incorporating dual nanofillers-ZIF-67 and amine-functionalized multi-walled carbon nanotubes (CNT-NH₂)-into a Pebax-1657 polymer matrix to enhance CO₂ separation performance. The dual nanofillers synergistically combine the high porosity and molecular-sieving characteristics of ZIF-67 with the high aspect ratio (~ 250–400) and efficient gas diffusion pathways provided by CNTs, while amine functionalization facilitates strong interfacial compatibility with the polymer matrix. Structural and morphological analyses using FESEM, FTIR, and XRD confirmed uniform nanoparticle dispersion, strong polymer-filler interactions, and preservation of membrane crystallinity. Gas permeation tests revealed substantial enhancements in both CO₂ permeability and selectivity compared to neat Pebax and binary MMMs. The optimal membrane, containing 2 wt% ZIF-67 and 7 wt% CNT-NH₂, exhibited a CO₂ permeability of 82.13 Barrer and a CO₂/N₂ selectivity of 119.76, that exceed Robeson’s upper bound. Moreover, the fabricated membranes exhibited excellent stability under elevated feed pressures, underscoring their potential for industrial applications such as flue gas treatment and natural gas purification. These results highlight the effectiveness of dual-nanofiller strategy in addressing traditional limitations of MMMs and offer a promising route toward the development of scalable, high-performance gas separation membranes.

Introduction

The increasing emission of carbon dioxide (CO₂), the primary greenhouse gas, has led to severe environmental consequences and raised significant concerns within the global community. Various methods have been developed for CO₂ capture, among which membrane separation processes based on polymeric materials have appeared promising due to their low energy requirements, process simplicity, compact design, and cost-effectiveness¹. However, the inherently moderate permeability and selectivity of polymeric membranes have prompted a shift toward mixed matrix membranes (MMMs). MMMs synergistically combine the advantageous properties of both polymeric and inorganic materials to enhance gas separation performance, mechanical stability, and durability. These properties make MMMs highly suitable for industrial applications, such as natural gas purification, flue gas treatment, and hydrogen recovery².

The first generation of MMMs consisted of binary systems, typically comprising a polymer matrix—most often glassy polymer—and a single filler, usually in the form of microparticles. Over time, more advanced alternatives emerged with the introduction of nanoparticles and the evolution of polymer synthesis techniques. Despite their performance improvements, binary MMMs still face persistent challenges, including nanoparticle aggregation and interfacial defects, such as voids between the filler and the matrix, polymer rigidification near filler particles, and pore blockage within the filler structure. Recent research has shifted toward the development of ternary MMMs, incorporating two different fillers with distinct physical and chemical properties to improve dispersion and enhance gas separation performance. In most cases, both fillers are porous, thereby creating additional gas permeation channels and enhancing the molecular screening ability of the membrane³.

Pebax-1657, a thermoplastic elastomer, has gained considerable attention due to its amphiphilic nature, excellent mechanical properties, and high CO₂-selective performance^4,5,6. Previous studies have extensively explored binary Pebax-based MMM systems, such as Pebax/ZIF-67^7,8,9 or Pebax/CNT^5,10,11, and demonstrated their advantages over conventional polymeric membranes for CO₂/N₂ and CO₂/CH₄ separation. ZIFs, a three-dimensional (3D) metal-organic frameworks (MOFs) featuring high porosity and tunable pore sizes, significantly contributes gas diffusion and selective adsorption¹². Studies demonstrate that incorporating ZIF-67 into Pebax results in microstructural channels that improve permeability and selectivity by providing additional molecular-sized gas transport pathways^9,13. Carbon nanotubes (CNTs) are known for their one-dimensional (1D) porous structure, length-to-diameter ratio of ~ 250–400, high gas transport properties, and remarkable mechanical strength. When incorporated into Pebax, CNTs drastically mitigate mass transfer resistance, facilitate gas transport, and improve mechanical stability^{11,14,15,16,17}.

Pebax has been widely adopted for CO₂ separation due to its intrinsic polar polyether segments that interact favorably with CO₂ molecules while offering sufficient mechanical stability from polyamide domains. Within the ZIF family, ZIF‑67 was selected because it shares an imidazolate framework with ZIF‑8 yet incorporates Co²⁺ centers, which exhibit stronger quadrupole interactions with CO₂ and slightly larger pore apertures (~ 0.34 nm) facilitating higher sorption capacity and faster transport compared to Zn-based analogues^9,18. Furthermore, its higher rigidity and thermal stability are advantageous for maintaining membrane structure under operational conditions.

CNT‑NH₂ was employed to fulfill a complementary role: pristine CNTs provide 1D conductive channels for molecular transport but tend to aggregate and have poor interfacial adhesion in polymer matrices. Amine functionalization introduces –NH₂ moieties capable of hydrogen bonding with the ether groups in Pebax¹⁹, and also coordinates with imidazolate linkers at the ZIF‑67 surface, effectively serving as a physical–chemical bridge between fillers and polymer. Quantitative reports show that amine modification can increase polymer–CNT interfacial shear strength by over 40% and reduce aggregated domains by more than half, as measured via SEM/TEM dispersion analysis²⁰.

Although the development of high-performance MMMs represents a significant leap forward in membrane technology, several challenges remain: (i) optimizing filler ratios and dispersion to prevent agglomeration, (ii) refining fabrication techniques to minimize interfacial flaws, and (iii) ensuring scalability and long-term stability for industrial applications. Continued research efforts aimed to address these challenges will be critical for enabling the widespread adoption of these innovatives for tackling CO₂ emissions and energy efficiency. To the best of the authors’ knowledge, this is the first study to fabricate and investigate ternary Pebax-based MMMs incorporating dual fillers-ZIF-67 and amine-functionalized CNTs-for CO₂ separation.

MMMs incorporating more than one filler phase have been extensively explored for CO₂ separation; however, previous dual- or ternary‐filler systems, including Pebax‐based MMMs, generally combine fillers of similar dimensionality or passive roles, limiting synergistic effects. In this work, the ternary combination of porous ZIF‑67 cages and CNT‑NH₂ in a Pebax matrix is designed to couple complementary transport mechanisms: ZIF‑67 offers rigid microporous cavities with strong CO₂ affinity, while CNT‑NH₂ provides flexible, one-dimensional channels decorated with reactive amino sites that bridge interfacial voids and enhance polymer–filler adhesion. This hybrid architecture enables simultaneous tuning of sorption and diffusion pathways, addressing defects typically observed with rigid nanoparticles alone. Compared to previously reported ternary MMMs^21,22, the present design achieves a distinct interface synergy between dissimilar filler types, offering mechanistic insights into defect mitigation and structure–property relationships beyond simple filler addition.

The objectives of this work are to: (i) synergistically combine 3D (ZIF-67) and 1D (CNT) nanomaterials to increase gas separation performance of MMMs, (ii) promote interfacial compatibility through amine functionalization of CNTs, and (iii) improve nanomaterial dispersion by leveraging the dimensional differences and positive interactions between the two fillers. Given the proven potential of Pebax-1657, ZIF-67, and amine-functionalized CNTs in binary MMM systems, we anticipate that their simultaneous incorporation will lead to breakthrough advancements in ternary MMMs with improved permeability and selectivity, and reduced filler aggregation and interfacial defects. This approach offers a promising pathway for developing efficient membranes to address critical environmental and industrial challenges related to CO₂ removal from flue gas and natural gas streams.

Experimental

Materials

Pebax-1657 (comprising 60 wt% polyamide-6 (PA6) and 40 wt% poly(ethylene oxide) (PEO)) was obtained from Arkema Inc. (France). Chemicals including 2-methylimidazole (2-mIm, ≥ 99.0%), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, ≥ 99.0%), sulfuric acid (H₂SO₄, 98%), nitric acid (HNO₃, 70%), m-phenylenediamine (MPD, 99%), and tetrahydrofuran (THF, ≥ 99.5%) were supplied by Merck Co. Ltd. (Germany). Ethanol (EtOH, 98%) was procured from Dr. Mojallali Co. (Iran), and methanol (MeOH, 99%) was provided by Exir GmbH Co. (Austria). Multi-walled carbon nanotubes (MWCNTs) with a length of 4–20 μm and an external diameter of 10–30 nm were purchased from the Research Institute of Petroleum Industry (RIPI, Iran). High-purity gases—CO₂ and N₂ (each ≥ 99.99%)—were obtained from local suppliers and used as feed gases in gas permeation experiments.

Characterization analysis

The morphological structure of the nanoparticles and membranes was characterized using a field-emission scanning electron microscope (FESEM; TESCAN, Czech Republic). Prior to imaging, all samples were coated with a gold layer approximately 100 Å thick to enhance surface conductivity. Energy-dispersive X-ray (EDX) mapping analysis was utilized to evaluate the dispersion and distribution of nanomaterials within the polymer matrix. X-ray diffraction (XRD; Equinox 3000, France) was employed to examine the crystalline structures of the nanoparticles and membranes. Measurements were conducted at room temperature using CuKα radiation (λ = 1.5542 Å) under operating conditions of 40 mA and 45 kV. The samples were scanned over a 2θ range of 0–90° at a rate of 0.03° (2θ)/min. Fourier transform infrared (FTIR; Nicolet instrument, USA) spectroscopy was used to identify the functional groups and assess interfacial interactions within the MMMs over a wavenumber range of 400–4000 cm^− 1.

Synthesis and characterization of ZIF-67 nanoparticles

ZIF-67 nanoparticles were synthesized via a solvothermal method based on our previous work¹³. Briefly, 0.359 g of Co(NO₃)₂·6H₂O and 0.811 g of 2-mIm were each dissolved in 25 mL of MeOH, then mixed and stirred for 60 s to produce a dark purple solution. The mixture was left undisturbed for 24 h, followed by centrifugation and washing three times with MeOH. The resulting product was dried at 60 °C for 6 h.

The XRD pattern of ZIF-67 (Fig. 1a) revealed distinct, sharp peaks indicative of a highly crystalline material, with prominent reflections at 2θ values corresponding to the (011), (002), (112), (022), (013), and (222) crystal planes. Specifically, the high intensity of the (011) peak at approximately 7.3° (2θ) confirms the well-defined, cubic structure characteristic of ZIF-67. The absence of extraneous peaks indicates high phase purity, suggesting successful synthesis without detectable impurities. FTIR spectroscopy (Fig. 1b) provided further insight into the chemical composition and bonding configurations of the ZIF-67 framework. The peak at 1580 cm^− 1 corresponds to the stretching vibrations of the imidazole rings, while peaks in the range of 950–1430 cm^− 1 are attributed to in-plane bending of C–N bonds. Additional bands in the 600–800 cm^− 1 region confirm out-of-plane bending modes, affirming the structural integrity of the ZIF-67 framework. The presence of aromatic C–H stretching bands at 3130 cm^− 1 and 2920 cm^− 1 further corroborates the inclusion of organic ligands.

The FESEM image (Fig. 1a) demonstrated a uniform, rhombic dodecahedral morphology—consistent with the anticipated crystal habit of ZIF-67. The particles exhibited smooth, well-defined facets with an average particle size of approximately 300 nm, indicating controlled growth and minimal aggregation. Collectively, the XRD data confirming crystallinity and phase purity, the FTIR spectra verifying key functional groups, and the FESEM images confirming morphology and size all support the successful and reproducible synthesis of ZIF-67 nanoparticles. These characteristics underscore the potential of ZIF-67 for applications requiring high crystalline structure, chemical stability, and uniform particle morphology, such as gas separation and catalytic processes.

Fig. 1

The XRD pattern and FESEM image (a), and FTIR spectra (b) of the synthesized ZIF-67.

Amine functionalization and characterization of MWCNTs

Figure 2a demonstrates a schematic representation of the amine functionalization process for MWCNTs. The procedure, based on our previous work¹¹, involved dispersing 5 g of MWCNTs in 600 mL of a concentrated H₂SO₄/HNO₃ mixture (3:1 v/v). The suspension was subjected to alternating sonication and vigorous stirring at specific time intervals (30–60 min) and temperatures (30–80 °C). After oxidation, the mixture was filtered through a Whatman filter (0.2 μm pore size) and washed thoroughly with deionized water until neutral pH was reached. The oxidized MWCNTs were then dried at 80 °C overnight. To introduce amine functionalities, 1 g of oxidized MWCNTs and 100 g of MPD were dispersed in DMF while stirring at 70 °C. The mixture was subsequently sonicated at 80 °C for 5 min. Excess MPD was removed using THF, after which the functionalized MWCNTs were rinsed with deionized water and dried overnight at 80 °C. The final product was labeled CNT-NH₂.

Figure 2b,c present the FTIR spectrum, XRD pattern, and FESEM image of CNT-NH₂. Each component of Fig. 2 highlights significant structural and functional transformations resulting from the functionalization of MWCNTs with amine groups, as well as the chemical and physical changes introduced during the process. The XRD pattern (Fig. 2b) provides insight into the crystalline nature of the synthesized CNT-NH₂. The observed diffraction peaks align with the characteristic graphitic structure of MWCNTs, highlighting their partially ordered crystalline framework. Specifically, the strong and sharp peak near 2θ = 26.1° corresponds to the (002) plane of graphitic carbon, confirming the retention of high structural order even after surface functionalization²³. The broader nature of the peak compared to pristine MWCNTs suggests a disruption in the graphitic lattice, which is likely caused by the introduction of functional groups during the oxidation and amination processes. This disruption is consistent with partial defects introduced on the graphene layers, as well as the incorporation of oxygen- and nitrogen-containing groups. Another small but discernible peak near 2θ = 43° corresponds to the (100) plane of graphitic carbon, reaffirming the graphitic structure²³.

The FESEM image in Fig. 2b illustrates the morphology of CNTs after amination treatment, confirming the formation of CNT-NH₂ structures. The image shows that the tubular structure of the CNTs remain intact after functionalization, confirming the successful preservation of their original morphology. The presence of surface irregularities, likely representing amine functional groups, further supports the successful grafting of these functionalities onto the CNT surface²⁴. This observation suggests that the functionalization process has been effectively carried out, enhancing the chemical activity of CNTs without compromising their structural integrity.

The FTIR spectrum (Fig. 2c) reveals key functional groups that confirm the successful functionalization of the CNTs with amine moieties. A broad absorption band around 3400 cm^{− 1} corresponds to O-H stretching vibrations²⁵, indicating the presence of hydroxyl groups on the CNT surface. This may result from the initial chemical oxidation treatments (using H₂SO₄/HNO₃ in the described synthesis pathway), which introduce carboxylic acid and hydroxyl functionalities onto the CNT surface. The peak in the range of 2800–3000 cm^{− 1} corresponds to C–H stretching vibrations in CNTs, which could originate from sp³-hybridized carbons introduced during the functionalization process²⁶. Additionally, the peak observed near 1720 cm^{− 1} is assigned to the stretching vibration of C = O bonds, further confirming the introduction of carbonyl or carboxylic acid groups on the CNT surface due to the oxidation step²⁷. Prominent absorption peaks in the region of 1500–1600 cm^{− 1} are characteristic of vibrations associated with primary amines. Specifically, the peak around 1580 cm^{− 1} can be attributed to N-H bending vibrations of amine groups, confirming the presence of functional groups introduced through the coupling of MPD during the final functionalization step^28,29. This peak indicates the successful attachment of amine species to the CNT surface. Peaks in the range of 1000–1300 cm^{− 1} are attributed to C–N stretching vibrations of the amine groups³⁰, further supporting the conclusion that the CNTs were functionalized with nitrogen-containing groups. Finally, peaks below 1000 cm^{− 1} reflect skeletal vibrations of the underlying MWCNT structure, along with minor contributions from residual groups³¹.

The schematic representation of the synthesis process (Fig. 2a) depicts the transformations that occur during each step. In the first step, treatment with a mixture of H₂SO₄ and HNO₃ introduces oxygen-containing functional groups, primarily hydroxyl, carboxyl, and carbonyl groups, onto the MWCNT surface. This oxidation step is critical for enhancing the reactivity of CNTs for subsequent functionalization. In the second step, a reaction with MPD introduces amine groups (–NH₂) onto the CNT walls, resulting in CNT-NH₂. This functionalization is confirmed by the presence of strong N–H bending and C–N stretching peaks in the FTIR spectrum, as well as a detectable structural disruption in the XRD pattern.

Fig. 2

A schematic illustration of the synthesis process for CNT-NH₂ (a), the XRD pattern and FESEM image (b), and the FTIR spectrum (c).

Preparation of mixed matrix membranes

The MWCNT-NH₂/ZIF-67/Pebax-1657 MMMs were prepared through a solution-casting process. Pebax-1657 (3 wt%) was dissolved in a water/ethanol mixture (20/80 wt%) at 80 °C under reflux for 4 h. ZIF-67 nanoparticles (1, 2, 4, 6, 8, 10, and 12 wt%, based on the final dried membrane) were dispersed in ethanol and stirred for 1 h, followed by sonication for 30 min. MWCNT-NH₂ nanotubes (1, 3, 5, 7, and 10 wt%, based on the final dried membrane) were added to the ZIF-67 suspension, stirred for 1 h, and then sonicated for 30 min. Approximately 20 wt% of the prepared Pebax-1657 solution was added to the nanomaterials suspension, then stirred for 1 h, and sonicated for 30 min. The remaining Pebax-1657 solution was gradually added to the mixture, then stirred for 2 h and sonicated for 1 h in two steps. The final dope solution was cast into a Petri dish and dried at room temperature for 24 h, followed by drying in an oven at 60 °C for another 24 h. The resulting MMMs were labeled as MZX and MZXCX, where Z refers to ZIF-67, C to CNT-NH₂, and X represents the weight percentages of ZIF-67 and CNT-NH₂ in the dope solution relative to the Pebax content, respectively. The pure Pebax-1657 membrane was prepared using only the first step, while the ZIF-67/Pebax-1657 MMMs were prepared by omitting the third step.

Gas permeation measurement

The single-gas permeabilities and CO₂/N₂ separation performance of the prepared membranes were assessed using a gas permeation setup based on the constant-volume/variable-pressure technique. The permeability of a pure gas was determined using the following equation:

$$P=\frac{{273.15 \times {{10}^{10}}Vl}}{{760AT\left( {\frac{{76{p_0}}}{{14.7}}} \right)}}\left( {\frac{{dp}}{{dt}}} \right)$$

(1)

Here, P represents the gas permeability (in Barrer), V is the constant volume of the container holding the permeate behind the membrane cell (4 cm³), l is the membrane thickness (cm), dp/dt is the rate of pressure increase on the permeate side (mmHg/s), A is the membrane area (12.5 cm²), T is the operating temperature (25 °C), and p₀ is the feed-stream pressure (psia).

Additionally, the ideal gas selectivity of the membranes for a gas pair was calculated using the following equation:

$${\alpha _{{A \mathord{\left/ {\vphantom {A B}} \right. \kern-0pt} B}}}={{{P_A}} \mathord{\left/ {\vphantom {{{P_A}} {{P_B}}}} \right. \kern-0pt} {{P_B}}}$$

(2)

In this equation, A denotes CO₂ molecules, while B represents N₂ molecule.

Results and discussion

Structural properties, chemical interactions, and gas separation performance

The gas separation performance of Pebax/ZIF-67 MMMs was analyzed in relation to their structural, morphological, and gas transport property changes. The study revealed that increasing the ZIF-67 nanoparticle loading from 2 wt% to 12 wt% dramatically affected the polymer matrix structure and morphology, leading to significant variations in both gas permeability and selectivity.

The FTIR spectra of the MMMs (Fig. 3a, b) provided critical insights into the molecular interactions between the Pebax-1657 matrix and the incorporated ZIF-67 nanoparticles or ZIF-67/CNT hybrids, with distinct changes in the C–H (Fig. 3c), C–O–C (Fig. 3d), and C=O (Fig. 3e) regions. In the spectrum shown in Fig. 3c, corresponding to the C-H vibrations of the polymer backbone, a reduction in intensity was observed as the ZIF-67 content increased, reflecting restricted polymer chain mobility due to the incorporation of rigid nanoparticles^9,32. At MZ12, the intensity of C–H decreased by approximately 15% compared to the neat Pebax-1657 membrane, indicating significant structural stiffening. These changes were amplified in membranes containing ZIF-67/CNT hybrids, where a more pronounced reduction in intensity of C–H (~25%) highlighted the improved dispersion and stronger interaction between the polymer and the hybrid nanofillers. Possible interactions between the nanomaterials and the polymer matrix are shown in Fig. 4.

Fig. 3

The FTIR spectra for neat and MMMs in (a) 630–4000 cm^− 1, (b) 630–1750 cm^− 1, (c) C–H, (d) C–O–C etheric, and (e) C=O vibration bands.

In the spectrum shown in Fig. 3(d), associated with C–O–C ether bonds, increasing the ZIF-67 content resulted in both a shift to lower wavenumbers (from 1120 cm^{− 1} in neat Pebax-1657 to ~1115 cm^{− 1} at MZ12) and an increase in peak intensity, indicative of interactions between the oxygen atoms in the polymer matrix and the metal ions in ZIF-67 (as presented in Fig. 4)³³. The C–O–C peak intensity increased by over 20% at MZ12 compared to the pure Pebax membrane, confirming structural reinforcement. For MZ2Cx series membranes, the addition of CNTs further enhanced these effects, with a shift to ~1110 cm^{− 1} and an intensity increase exceeding 30%, demonstrating that CNTs promoted better compatibility and minimized nanoparticle aggregation within the polymer matrix.

Fig. 4

Possible interactions of nanomaterials with the polymer matrix (a) and membrane materials with CO₂ molecules (b).

The spectrum shown in Fig. 3(e), corresponding to carbonyl vibrations (C=O), showed the strongest evidence of interactions, with the peak shifting from 1640 cm^{− 1} in pure Pebax-1657 to ~1630 cm^{− 1} at MZ12, reflecting notable electron density changes due to coordination bond formation between the carbonyl groups and ZIF-67’s metal nodes (as indicated in Fig. 4)⁷. The intensity of the C=O peak increased by 25% in ZIF-67-based membranes and up to 40% in ZIF-67/CNT-based membranes, signifying stronger interactions between the polymer and the hybrid nanofillers. These molecular-level interactions observed in the FTIR spectra strongly correlate with the gas separation performance of the membranes.

The neat membrane exhibits a dense, smooth structure devoid of pathways for efficient selective molecular transport (Fig. 5), resulting in a CO₂ permeability of only 58 Barrer and CO₂/N₂ selectivity of 30 (Fig. 6). In contrast, the MMMs demonstrate significant enhancements in both morphology and performance metrics. Figure 6 demonstrates a distinct initial drop in the permeability data versus a significant increase in selectivity data, as the ZIF-67 loading increases from neat Pebax (0 wt%) to MZ1 (1 wt%). The FTIR spectra reveal significant differences between the neat membrane and the MZ1 membrane (Fig. 3). The neat membrane exhibits characteristic peaks associated with the Pebax polymer, including the strong stretching vibrations of polymer backbone groups such as C=O bonds and ether functionalities (Fig. 3(c) and 3(d)). Upon the introduction of ZIF-67 nanoparticles (MZ1), additional peaks emerge corresponding to characteristic vibrational modes of ZIF-67, particularly in regions attributed to its imidazolate linker and metal-ligand framework (e.g., stretching vibrations of C=N and aromatic C–H bonds)³⁴. One notable observation is the shift and alteration of the intensity in the C = O and N–H stretching bands, indicative of potential interactions between ZIF-67 nanoparticles and the polymer matrix (Fig. 4). The reduction in permeability and increase in selectivity at low filler loading (MZ1) can be linked to changes in the polymer structure due to physical and chemical interactions at the interface between ZIF-67 nanoparticles and the Pebax matrix^34,35. The FTIR spectrum of MZ1 shows evidence of hydrogen bonding and coordination interactions (Fig. 4), which could lead to the formation of rigid interfacial regions in the polymer matrix near the nanoparticles. These regions typically exhibit reduced chain mobility, thereby limiting diffusivity and permeability in the membrane, especially for N₂ gas. This phenomenon explains the observed initial decline in gas permeability for CO₂ and N₂ upon moving from neat to MZ1. To resolve overlapping vibrational bands, FTIR spectra were deconvoluted using Gaussian functions with fitting constraints defined from literature-reported wavenumber ranges. Complete deconvoluted profiles and band-assignment tables are provided in the Supplementary Information (Fig. S1).

Fig. 5

FESEM images of (a–c) MZ2, (d) MZ4, (e) MZ8, and (f) MZ12.

At 2 wt% ZIF-67 loading, FESEM images (Fig. 5(a)-(c)) reveal a uniform dispersion of nanoparticles within the Pebax-1657 matrix. This homogeneous distribution ensures uninterrupted transport pathways for gas molecules³⁶, resulting in relatively higher permeability values for CO₂ (61.74 Barrer) and N₂ (0.72 Barrer) gas compared to MZ1 (Fig. 6). The CO₂/N₂ separation factor for the MZ2 membrane is 85.82, attributed to the molecular sieving effects of ZIF-67 and its interaction with the polymer matrix³⁷. As ZIF-67 content increased from 4 wt% to 12 wt%, the permeability of N₂ increased from 0.77 to 1.03 Barrer (Fig. 6). CO₂ permeability also increased from 58.84 Barrer (at MZ4) to 66.31 Barrer (at MZ12), emphasizing the selective interaction of CO₂ molecules with ZIF-67 ³⁷. At the same time, CO₂/N₂ selectivity steadily decreased from 76.32 to 64.40 (Fig. 6).

Fig. 6

Gas separation performance of MMMs containing ZIF-67 at 4 bar and 25 °C.

At higher ZIF‑67 loadings (>4 wt%), FESEM analysis revealed the formation of localized interfacial voids and occasional microcracks between ZIF‑67 crystals and the Pebax matrix. These morphological changes increase the effective free‑volume fraction, leading to enhanced CO₂ permeability despite saturation of sorption capacity. Concurrently, FTIR spectra showed smaller peak shifts and reduced relative intensity changes compared with lower loadings, indicating weaker polymer-filler interfacial interactions. This reduction in interfacial bonding promotes the development of non-selective permeation pathways through microvoids, resulting in a decline in CO₂/N₂ selectivity at high filler content¹³.

The increase in permeability and the decrease in selectivity observed in the MMMs upon increasing the ZIF-67 nanoparticle loading from 4 wt% to 12 wt% can be explained by the structural changes and polymer-nanoparticle interactions. The FESEM images demonstrate a uniform dispersion of ZIF-67 nanoparticles within the Pebax-1657 matrix (Fig. 5(e) and 5(f)), even at the higher loading of 12 wt%. Unlike cases where nanoparticles aggregation occurs—typically leading to voids or non-selective defects—the absence of aggregation here suggests that the nanoparticles remain well integrated into the polymer matrix^38,39. This uniform distribution ensures that the nanoparticles contribute effectively to the free-volume changes in the matrix without introducing large-scale defects or voids. Nevertheless, the increased filler loading significantly impacts the polymer morphology and gas transport pathways within the membrane. The FESEM images directly illustrate how morphological changes, such as nanoparticle dispersion patterns, affect gas transport properties.

At 8 wt%, the nanoparticle concentration provides sufficient opportunities for polymer-nanoparticle interactions, including hydrogen bonding and coordination effects, which enhance gas separation performance by creating selective transport pathways for CO₂ molecules. These interactions often restrict the mobility of polymer chains near the nanoparticles, contributing to an improved sieving mechanism. Consequently, selectivity remains relatively high at this filler loading. Therefore, the molecular sieving characteristics of the ZIF-67 nanoparticles are optimized due to a favorable balance between polymer chain rigidity and free-volume distribution.

However, as the ZIF-67 content increases to 12 wt%, the influence of nanoparticles on the polymer matrix changes. Although no agglomeration is observed, the higher nanoparticle concentration increases the formation of rigid interfacial regions around the nanoparticles. These rigid regions reduce the flexibility of the polymer chains, leading to localized free-volume increases around the nanoparticles. The larger free-volume zones can create less restrictive pathways for gas molecules, facilitating faster diffusion for non-preferential gases, such as N₂ ^38,40,41. This phenomenon contributes to the increase in permeability observed at higher filler loadings (Fig. 6). Additionally, the excessive incorporation of ZIF-67 may dilute the molecular sieving effect due to the reduced dominance of polymer-filler interactions that are critical for selective gas transport. At high filler concentrations, the overall matrix may lose its optimal distribution of selective transport regions, where CO₂ passes preferentially through facilitated interactions with the nanoparticles^11,40. This reduction in selective molecular pathways leads to the observed decrease in selectivity (Fig. 6). The results presented in Fig. 6 confirm the trade-offs between permeability and selectivity, as ZIF-67 loading increases.

The gas separation performance of MMMs containing ZIF-67/CNT-NH₂ nanoparticles is governed by the microstructural features and nanoparticle dispersion, as demonstrated through cross-sectional FESEM images (Fig. 7) combined with permeability and selectivity data across various CNT-NH₂ loadings (Fig. 8). When CNTs were incorporated as part of the ZIF-67/CNT hybrids, the gas separation performance of MMMs was further enhanced. While ZIF-67 can provide microporous sorption sites and semi-crystalline transport pathways, CNTs can improve dispersion by reducing aggregation tendencies, strengthening Pebax-1657/ZIF-67 compatibility, and creating effective secondary transport channels³². In addition, the interaction between ZIF-67 and CNTs in the polymer matrix is critical to the membrane performance. At low CNT-NH₂ loadings (e.g., MZ2C1 and MZ2C3), FESEM images show a homogeneous dispersion of nanoparticles throughout the polymer matrix, where ZIF-67 and CNT particles are well-distributed without evidence of aggregation (Fig. 7(a)-(f)). This uniformity creates effective molecular transport channels, enhancing CO₂ permeability (from 67.07 Barrer for the neat membrane to 72.19 Barrer for MZ2C1) and CO₂/N₂ selectivity (53.47 to 144.85). These improvements stem from the dual role of ZIF-67/CNT hybrids: the high-speed channels provided by CNTs and the intrinsic molecular sieving properties of ZIF-67. Their high affinity for CO₂ arises from the amine groups on CNTs and functional imidazole rings in ZIF-67 ^42,43,44.

Fig. 7

The cross sectional FESEM images of (a–c) MZ2C1, (d–f) MZ2C3, (g–i) MZ2C5, (j–l) MZ2C7, and (m–o) MZ2C10.

At moderate CNT-NH₂ loadings (e.g., MZ2C7), the FESEM images show small clusters of ZIF-67 particles alongside individually dispersed CNTs (Fig. 7(g) to (i)). These clusters remain relatively small, and CNTs act as structural stabilizers by increasing the number of interfacial voids while preventing severe agglomeration. Consequently, CO₂ permeability and CO₂/N₂ selectivity increase significantly to 82.13 Barrer and 119.76, respectively, compared to MZ2C5. The CNTs play a dual role here, facilitating dispersion while providing secondary gas transport pathways due to their elongated structure and accessible surface area⁴⁵. The data highlight the synergistic behavior between ZIF-67 (2 wt%) and CNT-NH₂ (7 wt%) at this loading, where membrane performance is enhanced without causing substantial trade-offs in selectivity.

Fig. 8

Gas separation performance of MMMs containing 2 wt% ZIF-67 and 1–10 wt% CNT-NH₂ at 4 bar and 25 °C.

At higher CNT-NH₂ loadings (e.g., MZ2C10), the benefits of ZIF-67/CNT synergy are diminished due to severe agglomeration and defect formation. The FESEM images reveal significant agglomeration of nanoparticles within the polymer matrix (Fig. 7(m)-(o)), indicated by the presence of large clusters highlighted in yellow. These agglomerates disrupt the polymer-filler interface, resulting in non-selective, leaky interfaces and structural defects, which hinder the membrane’s ability to maintain high selectivity despite slight permeability gains. The permeability of N₂ and CO₂ increased slightly to 1.26 Barrer and 85.62 Barrer, respectively. As a result, CO₂/N₂ selectivity decreased sharply to 68.11. This reduction in selectivity reflects the trade-off between permeability and selectivity associated with excessive nanoparticle loading. The structural defects observed via FESEM corroborate these trends, emphasizing the challenge of preserving interfacial compatibility at high nanoparticle concentrations despite the presence of CNTs⁴⁴.

Notably, comparing the cross-sectional morphology of neat membranes with that of ZIF-67/CNT-loaded MMMs highlights the transformative impact of fillers. The structure-performance relationship derived from the FESEM imaging correlates directly with the gas separation data, demonstrating the gradual shift from optimal dispersion at low-to-moderate loadings to excessive agglomeration at higher nanoparticle concentrations^44,45. These results underscore the importance of optimizing ZIF-67/CNT loading levels to achieve maximized separation efficiency, while minimizing structural defects.

Effect of feed pressure

The behavior of MMMs containing Pebax-1657, ZIF-67 nanoparticles, and ZIF-67/CNT hybrids under varying feed pressures demonstrates unique gas separation performance that correlate directly with their structural and chemical properties^46,47,48.

The increase in CO₂ permeability observed at moderate upstream pressures is not attributable solely to filler-polymer affinity or enhanced sorption. Rather, three coupled mechanisms contribute to the trends: (i) affinity-driven sorption enhancement due to the combined effects of Pebax’s ether segments, Co²⁺ sites in ZIF‑67, and amine functionalities on CNT‑NH₂, which increase the sorption coefficient via specific quadrupole and hydrogen bonding interactions^5,49; (ii) dual-mode sorption behavior, in which Langmuir‐type sites associated with ZIF‑67 micropores remain partially unsaturated at intermediate pressures, enhancing the Langmuir capacity term in accordance with the Paul–Koros model^50,51; and (iii) pressure‐dependent diffusion changes, wherein moderate CO₂ plasticization of polymer domains increases chain mobility and the diffusion coefficient, especially under single‐gas conditions⁵². At higher pressures, partial site saturation and competitive chain relaxation attenuate this permeability rise. Similar synergistic sorption–diffusion contributions under varying pressures have been documented for CO₂‐affinitive MMMs in the literature⁵³.

In the case of membranes containing only ZIF-67 (MZ series), the experimental results show that increasing feed pressure either has little effect or causes a slight decrease in the permeability of non-polar gases, such as N₂ (Fig. 9). At low loading (e.g., the MZ2 membrane), the N₂ permeability dropped by approximately 19%, from 0.95 Barrer at 6 bar to 0.77 Barrer at 8 bar. For the MZ4 membrane, the N₂ permeability dropped by approximately 14%, from 3.61 Barrer at 6 bar to 3.39 Barrer at 10 bar. These minor reductions result from the pressure-induced compaction of the polymer matrix, which slightly compresses the transport channels^54,55. At high loading (e.g., the MZ12 membrane), the N₂ permeability was 1.04 Barrer at 6 bar and 1.10 Barrer at 10 bar. These results indicate that increased nanoparticle loading enhances the membrane’s resistance to compression and results in negligible changes in the N₂ permeability⁵⁶.

The CO₂ permeability increases as pressure rises (Fig. 9). As pressure increased from 6 bar to 10 bar, the CO₂ permeability of the MZ2 membrane increased from 59.38 Barrer to 66.10 Barrer, and that of the MZ12 membrane increased from 63.85 Barrer to 73.24 Barrer, demonstrating the strong affinity of the polar Pebax polymer and the ZIF-67 framework for CO₂ adsorption while maintaining selective transport pathways at higher pressure⁵⁷.

The selectivity followed the same trend as the CO₂-to-N₂ permeability ratios. With increasing pressure, CO₂ permeability increased in most membranes, whereas N₂ permeability remained almost constant (and in some cases decreased). Therefore, the CO₂/N₂ selectivity remained largely unchanged, occasionally increasing.

For the MZ2C series (Fig. 9), CNTs play an important role in enhancing mechanical stability and gas transport performance. In these membranes, the gas permeabilities are not reduced, confirming that incorporating CNTs improves the stress tolerance of the polymer matrix under pressure and prevents matrix compaction and permeability decrease observed for the membranes without CNT reinforcement. For the MZ2C1 membrane, the permeability of CO₂ and N₂ increased from 72.14 Barrer and 0.88 Barrer at 6 bar to 78.01 Barrer and 1.31 Barrer at 10 bar, respectively. For the MZ2C10 membrane, the permeability of CO₂ and N₂ increased from 84.63 Barrer and 1.26 Barrer at 6 bar to 99.07 Barrer and 1.40 Barrer at 10 bar, respectively. Therefore, because the percentage increase in N₂ permeability was similar to that of CO₂, the CO₂/N₂ selectivity remained nearly constant.

Fig. 9

The effect of pressure on the separation performance of MMMs containing ZIF-67 and those MMMs containing ZIF-67/CNT.

The XRD patterns (Fig. 10) demonstrate the successful integration of ZIF-67 into the polymer matrix, as indicated by the characteristic crystalline peaks of ZIF-67 superimposed on Pebax’s broad amorphous halo^7,9,58. For the MZ2C series, the absence of sharp CNT-specific peaks suggests homogeneous dispersion within the polymer⁵⁹, which can lead to peak broadening and suppression due to strong interfacial interactions. However, because peak disappearance may also arise from low filler content or reduced signal intensity, this interpretation is supported by SEM images (Fig. 7) showing uniform CNT dispersion at both low and high magnification^60,61,62,63. A critical finding is that as ZIF-67 content increases, diffraction intensities grow stronger, although aggregation may occur at higher loadings (>10 wt%). CNTs alleviate this aggregation, maintaining better dispersion across the matrix⁶⁴. This structural synergy underpins the marked improvement in both permeability and selectivity of hybrid ZIF-67/CNT membranes⁵. In all investigated membranes, CNT loading was kept below the threshold typically associated with aggregation. To promote homogeneous distribution, the CNTs were functionalized via mild acid treatment and ultrasonicated in the casting solvent before mixing with the polymer solution, enhancing interfacial compatibility and steric stabilization⁶⁵. Complementary FTIR studies reveal significant chemical interactions between the ZIF-67 fillers and the polymer matrix, particularly at the C=O and C-O-C functional groups of Pebax. These interactions enhance CO₂ adsorption capacity and molecular sieving (Fig. 4). CNTs further strengthen ZIF-67’s integration into the matrix, as evidenced by shifts in peak positions. These chemical and structural findings directly support the pressure performance trends observed during gas permeation trials. Hybrid membranes containing ZIF-67 and CNTs consistently outperform their pure ZIF-67 counterparts in both performance and stability. The synergistic effect provided by CNTs ensures better filler dispersion, improved mechanical properties, and enhanced selective permeability through the polymer matrix⁵⁷. At higher pressures, the driving force for gas diffusion increases, amplifying membrane permeability and selectivity. However, structural integrity and resistance to compaction become limiting factors unless supported by reinforcements such as CNTs^35,57.

Fig. 10

The XRD pattern of ZIF-67, CNT-NH₂, neat Pebax-1657, ZIF-67/Pebax-1657 MMMs, and ZIF-67/CNT-NH₂/Pebax-1657 MMMs.

Evaluation and comparison of gas separation performance

The MMMs demonstrate varying gas separation performance when assessed against the upper bounds for the CO₂/N₂ gas pair (Fig. 11(a)). Under single-gas permeation conditions at 4–10 bar and 25 °C, the developed membranes achieve CO₂/N₂ separation performance close to the 2008 Robeson upper bound, with some data points reaching it. However, their performance remains below the 2019 updated bound. This difference is consistent with the fact that single-gas tests often yield higher selectivity than mixed-gas or humidified conditions due to the absence of competitive sorption and plasticization effects^66,67. Analysis of these membranes provides a detailed understanding of their structure-performance relationships under varying operational conditions. The neat Pebax membrane exhibits poor gas separation performance, positioned considerably below the 2008 Robeson upper bound for both gas pairs. This limitation is primarily attributed to the semi-crystalline nature of Pebax-1657⁶⁸, which restricts gas transport due to limited free volume and ineffective gas transport pathways⁶⁹. The inherent polar groups in Pebax-1657 facilitate CO₂ adsorption to some extent, resulting in higher CO₂ permeability compared to non-polar gases such as N₂³⁹. However, both permeability and selectivity remain insufficient for high-performance applications, rendering Pebax-1657 unsuitable as a standalone membrane material for efficient gas separation.

The addition of ZIF-67 nanoparticles to the Pebax matrix improves both permeability and selectivity toward CO₂, owing to ZIF-67’s intrinsic microporosity and selective adsorption capabilities. ZIF-67 preferentially adsorbs CO₂ molecules due to favorable interactions with polar CO₂ and its framework⁷⁰. This results in increased CO₂ permeability along with improved separation factors in the CO₂/N₂ system. For instance, membranes containing ZIF-67 at various loading levels (2–12 wt%) shift closer to the Robeson upper bound, indicating improved separation efficiency. However, the performance begins to plateau or decline at higher loadings, such as 12 wt% ZIF-67, likely due to aggregation of ZIF-67 particles. These agglomerates disrupt the polymer matrix uniformity, generate inactive transport zones, and reduce the effectiveness of nanoparticle integration. While ZIF-67 enhances gas transport properties up to a certain threshold, its tendency to form non-uniform distributions at excessive loadings underscore the need for further structural modifications.

Incorporating CNTs into the Pebax/ZIF-67 matrices significantly enhances both the selectivity and permeability of the membranes, pushing their performance closer to, or even beyond, the Robeson upper bounds. Membranes containing ZIF-67/CNT hybrids, such as MZ2C7, demonstrate a synergistic effect attributable to the combined presence of ZIF-67 and CNTs. The addition of CNTs resolves the nanoparticle dispersion issue observed in high ZIF-67 loadings while simultaneously contributing to improved mechanical stability and increased free volume within the polymer matrix. This improvement directly increases CO₂ permeability to approximately 82 Barrer and enhances CO₂/N₂ selectivity to nearly 120. CNTs create fast transport pathways for gas molecules⁷¹ and ensure uniform ZIF-67 distribution throughout the matrix, thereby allowing CO₂ molecules to navigate more efficiently while hindering the diffusion of nonpolar gases such as N₂. Additionally, the mechanical reinforcement provided by CNTs mitigates matrix densification under high-pressure feed conditions, further stabilizing overall membrane performance. The pressure-dependent performance trends of these membranes underscore the essential role of feed pressure in determining gas transport characteristics.

Elevated pressures generally increase the driving force for gas diffusion, enhancing CO₂ permeability due to its higher solubility and stronger interactions with polar functional groups in Pebax-1657 and ZIF-67 compared to non-polar gases⁷². This phenomenon becomes more pronounced in membranes containing ZIF-67/CNT hybrids, where synergistic effects increase permeation rates causing without a significant decline in mechanical stability. Conversely, N₂ permeability typically decreases at higher pressures, particularly in Pebax/ZIF-67 membranes, due to plasticization-induced compaction that narrows transport pathways for nonpolar gases⁹. CNT-modified membranes, however, exhibit greater stability under pressure, maintaining higher permeability and selectivity values due to the reinforcement of polymer-filler interactions by CNTs⁷³.

The advanced capabilities of Pebax/ZIF-67/CNT hybrid membranes highlight their potential for industrial applications, such as natural gas purification and CO₂ capture, where high-pressure feed conditions are common. Their ability to achieve near-optimal trade-offs between CO₂ permeability and selectivity, combined with mechanical reinforcement under elevated pressures, distinguishes them from standard Pebax or Pebax/ZIF-67 membranes. Their proximity to the Robeson upper bound for the CO₂/N₂ gas pair demonstrates the suitability of these hybrid membranes for large-scale gas separation applications.

Fig. 11

The gas separation performance comparison of the fabricated membranes with (a) Robeson upper bounds and (b) M_Index (explained in the Supporting information) criteria.

Figure 11(b) illustrates the analysis of the provided M_Index ranges across the MMMs. It reveals key insights into their gas separation performance, reflecting a delicate balance between permeability and selectivity. MMMs with inferior M_Index values, such as MZ6, generally exhibit limited performance due to the absence of effective gas transport pathways originating from the semi-crystalline nature of the Pebax-1657 matrix and/or inadequate polymer–nanoparticle interactions. In the weak M_Index range, membranes such as MZ4 and MZ12 exhibit constrained performance due to issues such as agglomeration leading to interfacial defects, or insufficient interfacial interactions that hinder the formation of continuous transport channels. In contrast, MMMs within the middle M_Index range, exemplified by MZ1, MZ8, and MZ2C3, demonstrate improved performance through optimized nanoparticle loadings that promote synergistic effects within the polymer matrix and creat preferential pathways for CO₂ transport. For instance, homogeneous dispersion of ZIF particles enhances CO₂ permeability, while CNTs contribute to mechanical stability and mitigate surface defects.

The most favorable M_Index values are observed in membranes such as MZ2C1, MZ2C5, and MZ2C7, which achieve an optimal balance between nanoparticle loading (ZIF-67 and CNTs) and polymer-nanoparticle interaction, resulting in superior structural integrity and the creation of effective CO₂ transport pathways. These MMMs leverage the synergistic effects of well-dispersed ZIF-67 particles and CNTs, which provide extended gas transport routes and lead to significantly enhanced CO₂ permeability without compromising selectivity. However, at excessively high nanoparticle loadings (e.g., MZ2C10), the M_Index values decline, reflecting performance deterioration caused by severe CNT agglomeration, which induces structural defects and non-selective voids within the polymer matrix. This analysis underscores the critical role of nanoparticle loading in MMMs, where a carefully controlled balance is essential for achieving optimal performance by enhancing both permeability and selectivity while mitigating the detrimental effects associated with either under- or over-loading nanoparticles.

The CO₂/N₂ separation performance of the best membranes in this study—namely, MZ2C1 and MZ2C7— was compared with that of other ternary MMMs in the literature using an innovative 3D comparison tool. This tool simultaneously provides the permeability and selectivity enhancement factors, the M_Index criterion, and the Score parameter (a positive value indicates crossing the 2008 Robeson upper bound), as shown in Fig. 12. Ternary MMMs including GO/HNT/Pebax-1567, MXene/SiO₂/Pebax-1567 ⁷⁴, porous reduced GO/HNT/Pebax-1657 ⁷⁵, porous GO/POP/Pebax-1657 ⁷⁶, and MXene/UiO-66/Pebax-1657 ⁷⁷ showed both a positive Score and an M_Index value in the good range (0.5 < M_Index < 1.5), while also showing positive permeability and selectivity enhancement factors. As shown in Fig. 12, the membranes fabricated in this work—including MZ2C1 (4 bar) and MZ2C7 at various pressures (4, 6, 8, and 10 bar)—also demonstrated similar results. The excellent performance of the MZ2C7 MMM in the feed pressure range of 4 to 10 bar proves that this membrane is superior to many membranes in the literature and can be useful for industrial applications.

Fig. 12

Comparison with the literature: A 3D plot of CO₂/N₂ separation performance of three-component MMMs, illustrating permeability enhancement, selectivity enhancement, Score (relative to the 2008 Robeson upper bound), and M_Index.

Conclusions

In this work, three-component Pebax-1657-based MMMs were successfully fabricated by incorporating dual nanofillers, ZIF-67 and CNT-NH₂. The incorporation of 3D (ZIF-67) and 1D (CNT-NH₂) nanostructures led to synergistic enhancements in CO₂ separation performance through complementary effects. The structural integrity and compatibility of the hybrid membranes were confirmed via FTIR, XRD, and FESEM analyses, showing strong polymer-filler interactions and uniform dispersion without significant agglomeration up to optimal loading levels. Notably, the membrane with 2 wt% ZIF-67 and 7 wt% CNT-NH₂ (MZ2C7) achieved a CO₂ permeability of 82.13 Barrer and a CO₂/N₂ selectivity of 119.76 at 4 bar and 25 °C, representing substantial improvements over the neat Pebax membrane (58 Barrer and 30, respectively). Feed pressure variation from 4 to 10 bar demonstrated that the hybrid membranes maintained or improved their separation performance under elevated pressures, with the MZ2C7 membrane reaching 99.07 Barrer CO₂ permeability at 10 bar alongside a CO₂/N₂ selectivity of 70.76. These values confirm the structural resilience and superior gas transport stability of the membranes. The performance of the fabricated membranes approached or surpassed the 2019 upper bound for CO₂/N₂ separation, and comparative analysis using the M_Index and enhancement factors confirmed their superiority over many ternary MMMs reported in the literature. Specifically, the MZ2C7 membrane displayed a favorable M_Index and positive Score value, indicating its outstanding permeability-selectivity balance.

Overall, this study establishes the effectiveness of dual-filler strategies in enhancing both the transport and mechanical properties of MMMs. The optimized ZIF-67/CNT-NH₂-loaded Pebax membranes present a promising, scalable solution for high-efficiency CO₂ capture and natural gas purification, particularly under industrial operating conditions where performance stability at elevated pressures is critical.

Building on the promising performance of ZIF-67/CNT dual-filler nanocomposites in enhancing CO₂/N₂ gas separation through Pebax-1657 membranes, future studies could focus on filler decorations, exploring advanced membrane architectures such as thin-film mixed matrix systems, and assessing long-term stability under practical operating conditions. Furthermore, expanding this approach to other industrially relevant gas pairs and evaluating the scalability and economic feasibility of the membrane fabrication process may pave the way for broader adoption in carbon capture and environmental remediation applications.