Introduction

Carbon dioxide (CO2) emissions, a major contributor to climate change, have risen significantly, leading to environmental issues like wildfires, floods, temperature increases, sea-level rise, and ocean acidification (Venturi et al. 2016). The primary source of these emissions is energy consumption across various sectors, with CO2 accounting for around 76% of total greenhouse gases. Projections suggest further increases in CO2 emissions over the next 30 years. While CO2 is a major environmental concern, it also has industrial applications, such as in food and polymer production, making carbon capture essential (Li et al. 2020).

Post-combustion CO2 capture, which allows integration of various separation technologies without disrupting existing processes, is widely adopted in Carbon Capture and Storage (CCS) projects. Flue gases from fossil fuel combustion, containing CO2, nitrogen, and other impurities, are targeted for CO2 extraction (Venturi et al. 2016; Zhang et al. 2018; Guan et al. 2020).

Technologies like amine absorption, adsorption, membrane, and cryogenic separation are used for post-combustion CO2 separation. However, traditional methods face challenges such as high costs, harmful solvents, and low energy efficiency (Baker 2012; Venturi et al. 2016).

Membrane separation has gained interest as a cost-effective, low-energy alternative, providing a small footprint and no need for regeneration. Membranes work by permeating CO2 through pressure differences, with two primary models for describing the process: pore-flow and sorption-diffusion (Zhang et al. 2018; Anh et al. 2021). Both organic and inorganic materials can be used for membrane fabrication, with inorganic materials offering better performance but posing production challenges. Polymeric membranes dominate due to ease of manufacture and good performance for separating CO2 from gases like nitrogen and methane. However, polymeric membranes face a trade-off between permeability and selectivity, with improvements in one often reducing the other, as described by Robeson's upper bound (Zhang et al. 2018; Anh et al. 2021).

Biobased methods

Capturing carbon on a large scale becomes feasible when there is a plentiful supply of low-cost materials that specifically attract carbon dioxide. To meet sustainability standards, researchers have been working on transforming inexpensive and environmentally friendly cellulosic materials into adsorbents and membranes that selectively capture CO₂ (Wu et al. 2018; Anh et al. 2021). Cellulose, along with its derivatives such as cellulose nanofibrils (CNFs) can be utilized as materials that selectively capture carbon dioxide (CO₂) in adsorbents and membranes. CNFs are derived from cellulose, offer a large surface area for mechanical reinforcement and numerous hydroxyl groups for CO₂ capture (Anh et al. 2021).

Metal-organic frameworks

Metal-organic frameworks (MOFs) are crystalline materials made by coordinating metal ions or clusters with organic ligands, forming microporous characteristics (Anh et al. 2021). Zeolitic Imidazolate Frameworks (ZIFs), a type of MOF, offer superior features like high specific surface area, adjustable pore channels, diverse structures, unsaturated sites, and easy functionalization, making them ideal for gas-separation applications. ZIF-8, a widely studied ZIF, has a pore size of 0.34 nm and exhibits molecular-sieving behavior for CO₂, making it a focus of research in gas separation (Anh et al. 2021).

In previous studies, ZIF-8 has been mixed directly with a polymer solution to create Mixed Matrix Membranes (MMMs), aiming to enhance CO₂ permeability and selectivity by leveraging the sieving effects of ZIFs (Guan et al. 2020).

However, in this study, we present a novel strategy that integrates ZIF-8 with nanocellulose-based composite materials to fabricate high-performance membranes for CO₂ capture. Unlike conventional mixed matrix membranes (MMMs) where ZIF-8 is simply dispersed within polymer matrices, our approach leverages the unique properties of nanocellulose, its high surface area, and mechanical reinforcement potential to create a robust and sustainable membrane platform. By combining the molecular sieving ability of ZIF-8 with the bio-derived, tunable matrix and hydroxyl groups of nanocellulose, we demonstrate significant improvements in both CO₂ permeability and selectivity. This synergistic design not only addresses the traditional trade-off between permeability and selectivity often observed in polymeric membranes but also pushes performance closer to, or beyond, the Robeson upper bound. Our results reveal that nanocellulose-ZIF-8 membranes can serve as scalable, environmentally friendly alternatives for post-combustion CO₂ capture technologies. In addition, due to the excellent compatibility and strong intrinsic adhesion of the CNF film to standard printing paper, this low-cost and biobased substrate was used to support the selective membrane layer, providing mechanical strength while reducing the thickness of the selective layer to enhance gas transport efficiency. To the best of our knowledge, the use of standard printing paper as a substrate for nanocellulose-ZIF composite membranes in gas separation applications has not been previously reported, making this approach a novel and scalable solution for CO₂ capture.

Methods

ZIF synthesis method

Initially 2.933 gr of Zn(NO₃)₂⋅6H₂O was added to water and mechanically mixed to prepare the precursor. The ligand solution was prepared by adding 6.48 grams of 2-methylimidazole in water followed by mechanical mixing. Both the solutions were then mixed, and the resultant solution was stirred vigorously for 5 minutes. The final milky solution was allowed to rest for 12 h. It was then centrifuged and finally washed with methanol to obtain the ZIF-8 nanoparticles.

ZIF-8/CNF membrane fabrication

For the fabrication of this membrane, CNF suspension with a specific total solid content was prepared in water. Carboxymethyl cellulose (CMC) was first dissolved in water and then was added to the CNF suspension in 1% w.t% ratio (CMC/CNF) for stabilization. The previously synthesized ZIF-8 in water was added into the CNF-CMC suspension, which was subjected to continuous mechanical mixing for 1 hour. Then final suspension was filtered on a polystyrene mesh to obtain a film and then the film transferred to a printing paper using hot press.

Characterization

To confirm the successful synthesis and integration of ZIF-8 within the nanocellulose-based membranes, a series of characterization techniques were employed. The crystalline structure of ZIF-8 and the composite membranes was analyzed using X-ray diffraction (XRD), while Fourier-transform infrared spectroscopy (FTIR) was used to identify functional groups and confirm chemical interactions. Scanning electron microscopy (SEM) provided insights into the membrane morphology and dispersion of ZIF-8 particles. Additionally, dynamic light scattering (DLS) and zeta potential measurements were conducted to evaluate the particle size distribution and surface charge of the ZIF-8 particles, respectively.

Membrane performance

The gas separation performance of the membranes was evaluated by measuring the permeability of CO₂ and N₂ gases using a gas permeation cell under various relative humidity (RH) conditions. By studying the effect of RH, we aimed to understand how water–membrane interactions impact gas transport behavior and selectivity. In addition, mechanical testing was performed using an Instron tensile test to evaluate the structural integrity of the membranes and the role of CNF as a natural binder for ZIF-8 particles.

Summary of Results

XRD, FTIR, and SEM analyses confirmed the successful synthesis of ZIF-8 with an average particle size below 200 nm, as also validated by DLS measurements. ZIF-CNF composite membranes with varying ZIF-8 loadings (0, 10, 30, 50, 70, and 90 wt%) were successfully fabricated on printing paper substrates. Gas permeability tests were conducted across these compositions to evaluate performance.

Membranes containing 50 wt% and 70 wt% ZIF-8 achieved optimal CO₂ permeability values of 490.9 Barrer and 625.6 Barrer, respectively. Correspondingly, the CO₂/N₂ selectivity values were 19.4 and 16.2. These results demonstrate a favorable trade-off between permeability and selectivity for intermediate ZIF loadings, with 50 wt% offering high selectivity and 70 wt% providing enhanced permeability.

Tensile testing using an Instron system showed a clear improvement in mechanical strength with increasing CNF content. Compared to pure ZIF films, the composite membranes exhibited significantly enhanced tensile strength.This improvement confirms the effective role of CNF as a binder and reinforcing agent.

The gas permeability and CO₂/N₂ selectivity of nanocellulose-based membranes decreased with increasing relative humidity (RH). This trend is attributed to competitive sorption between water vapor and CO₂, as well as membrane swelling at high RH, which disrupts the selective transport pathways. Additionally, the presence of carboxymethyl cellulose (CMC) further influenced membrane performance. Due to its abundant carboxyl and hydroxyl functional groups, CMC enhanced CO₂ affinity through specific interactions, while also contributing to membrane film formation and stability.

Conclusion

This study introduces a novel, sustainable approach for fabricating CO₂-selective membranes by incorporating ZIF-8 nanoparticles into nanocellulose (CNF) matrices, using low-cost printing paper as a substrate. The membranes demonstrated enhanced gas permeability and selectivity, with an optimal performance observed at 50–70 wt% ZIF content. Mechanical tests confirmed that CNF acted as an effective binder, significantly improving the membrane's tensile strength compared to pure ZIF films. Additionally, the inclusion of carboxymethyl cellulose (CMC) contributed functional groups that may enhance CO₂ affinity. Relative humidity was found to influence membrane performance; both permeability and selectivity decreased with increasing RH due to matrix swelling. These findings underscore the potential of biobased composite membranes in CO₂ separation applications, offering a scalable and eco-friendly alternative to conventional materials.

References

Anh N, Ho D, Leo CP (2021) A review on the emerging applications of cellulose, cellulose derivatives and nanocellulose in carbon capture. Environ Res 197:111100. https://doi.org/10.1016/j.envres.2021.111100

Baker RW (2012) Membrane technology and applications. John Wiley & Sons

Guan W, Dai Y, Dong C, et al (2020) Zeolite imidazolate framework (ZIF)-based mixed matrix membranes for CO 2 separation: A review. https://doi.org/10.1002/app.48968

Li Y, Jia P, Xu J, et al (2020) The Aminosilane Functionalization of Cellulose Nanofibrils and the Mechanical and CO 2 Adsorption Characteristics of Their Aerogel. https://doi.org/10.1021/acs.iecr.9b04253

Venturi D, Baschetti MG, Chianese A, et al (2016) Nanocellulose based facilitated transport membranes for CO2 separation Nanocellulose Based Facilitated Transport Membranes for CO 2 Separation. Chem Eng Trans 47:. https://doi.org/10.3303/CET1647059

Wu Y, Zhang Y, Chen N, et al (2018) Effects of amine loading on the properties of cellulose nanofibrils aerogel and its CO 2 capturing performance. https://doi.org/10.1016/j.carbpol.2018.04.017

Zhang X, Zhang T, Wang Y, et al (2018) Mixed-matrix membranes based on Zn/Ni-ZIF-8-PEBA for high performance CO 2 separation. https://doi.org/10.1016/j.memsci.2018.05.004