Lab Electrospinning System| Development of multi-channel nanofibrousmolecular sieves with aerogel structure forefficient carbon dioxide capture

Dr. Zhu Jie's Team at Shanghai University of Engineering Science: Multi-channel Nanofibrous Molecular Sieves with Aerogel Structure for Efficient CO2 Capture

Inherently microporous polymers (PIMs), with their abundant micropores and high specific surface area, show great promise for CO2 adsorption and separation applications. However, conventional PIM solid adsorbents face practical limitations due to their single-pore structure and powdery nature. There is an urgent need to develop flexible PIM-based porous solid adsorbents with superior adsorption capacity.

Recently, Dr. Zhu Jie's team at Shanghai University of Engineering Science published their latest research titled "Development of multi-channel nanofibrous molecular sieves with aerogel structure for efficient carbon dioxide capture" in *Chemical Engineering Journal*. The researchers employed an electrospinning method combined with a non-solvent induced phase separation strategy to prepare amidoxime-modified PIM-1 (AO-PIM-1) nanofibers with aerogel structure (Figures 1a, b), followed by in-situ spray crosslinking with epoxy monomers to enhance structural stability.The study systematically compared the morphology and structure of electrospun fibers made from AO-PIM-1 with different molecular weights, ultimately obtaining AO-PIM-1-A nanofibrous molecular sieves with excellent mechanical properties (2.2 MPa), high specific surface area (445 m²/g), and outstanding CO2 adsorption performance.

Figure 1: Electrospinning process, contact angle, and morphological characterization of AO-PIM-1 nanofibers

To address the competitive adsorption between CO2 and water molecules in the environment, the team developed hydrophobic AO-PIM-1 nanofibers with water contact angles (WCA) exceeding 140° (Figure 1c), enabling more CO2 molecules to access the surface under high humidity conditions and thereby enhancing CO2 adsorption performance.Transmission electron microscopy (TEM) revealed the rough surface of aerogel-structured AO-PIM-1 nanofibers (Figure 1d), with interconnected pore structures clearly visible in the TEM images. Scanning electron microscopy (SEM) images (Figures 1e-g) showed that the AO-PIM-1 nanofibrous molecular sieves contained overlapping macropores (8-10 μm, Figure 1e) and uniformly distributed mesopores (30-60 nm) on both the surface and interior of the nanofibers (Figures 1f, g).

Figure 2: Channel construction and regulation of AO-PIM-1 nanofibrous molecular sieves with different molecular weights

The formation of continuous porous structures in fibers is closely related to polymer chain entanglement (i.e., degree of polymerization, polymer concentration, and phase separation process). During electrospinning phase separation, increasing polymer molecular weight significantly enhances chain entanglement, leading to uneven polymer network permeation in the spinning solution. This highly entangled state creates greater viscous resistance (FV) during fiber formation, making it difficult for Coulomb forces (FE) to drive molecular chain stretching, resulting in unstable jets and challenges in achieving stable pore structure control.

Accordingly, the researchers synthesized three AO-PIM-1 powders with different molecular weights: Powder A (weight-average molecular weight ≈47 kDa), Powder B (≈63 kDa), and Powder C (≈96 kDa). Fibers were then prepared via electrospinning at 25% polymer concentration (relative humidity=50±5%), yielding AO-PIM-1-A, AO-PIM-1-B, and AO-PIM-1-C. Among these, AO-PIM-1-A exhibited the most regular and uniform internal pore structure with the highest porosity, displaying distinct aerogel characteristics (Figure 2).

Figure 3: Crosslinking and pore structure characterization of AO-PIM-1 nanofibrous molecular sieves

To further enhance the structural stability of AO-PIM-1 nanofibrous molecular sieves, the researchers performed in-situ spray crosslinking using 1,4-butanediol diglycidyl ether (BDDE) as the crosslinker. As shown in Figure 3a, various crosslinked AO-PIM-1 materials with different degrees of substitution (DS) could be designed through ring-opening reactions between amino groups in AO-PIM-1 and epoxy groups in BDDE. Molecular dynamics simulations modeled the free volume fraction (FFV) inside crosslinked AO-PIM-1 (Figure 3b). FFV showed a clear decreasing trend with increasing DS, as crosslinking brings polymer chains closer together, reducing porosity.Based on the design concept of high free volume fraction, the researchers prepared AO-PIM-1-A nanofibrous molecular sieves with different molecular weights at DS=25% for subsequent characterization. 

Compared to the original powder samples, all fibrous molecular sieve adsorbents exhibited slight adsorption hysteresis (Figures 3e-f), confirming the hierarchical porous structure containing micropores (<2 nm), mesopores (2-50 nm), and macropores (>50 nm).Further analysis revealed a significant positive correlation between the specific surface area of AO-PIM-1 materials and their molecular weight. Under the same molecular weight conditions, fibrous samples generally exhibited higher specific surface areas than their powder precursors. Density functional theory (DFT) calculations of pore size distribution showed that the relative content of micropores increased significantly after fiber formation, with particularly notable optimization in the 0.6-0.7 nm range for AO-PIM-1-A1 and AO-PIM-1-B1.The low-molecular-weight AO-PIM-1-A1 nanofibrous molecular sieve showed the most significant increases in BET specific surface area (445 m²/g, up 31.96%) and micropore surface area (377 m²/g, up 35.16%) compared to its powder form, indicating great potential for high gas adsorption performance.

Figure 4: Mechanical properties and CO2 adsorption performance of crosslinked AO-PIM-1 nanofibrous molecular sieves

Moreover, crosslinked AO-PIM-1 nanofibrous molecular sieves demonstrated good mechanical properties and excellent CO2 adsorption performance (Figure 4). The AO-PIM-1-A1 sample exhibited outstanding mechanical strength with a tensile strength of 2.2 MPa. In terms of CO2 adsorption, the AO-PIM-1-A fibrous molecular sieves with different molecular weights showed CO2 uptake capacities of 2.45, 2.68, and 2.74 mmol CO2/g, respectively. Benefiting from its uniform pore structure, the AO-PIM-1-A1 nanofibrous molecular sieve achieved a 15.02% increase in CO2 adsorption capacity compared to its powder form.In conclusion, the crosslinked AO-PIM-1 nanofibrous molecular sieves with aerogel structure prepared via non-solvent induced phase separation pore-forming technology show broad application prospects in CO2 adsorption and related fields.

Paper link: https://www.sciencedirect.com/science/article/abs/pii/S1385894725039403