Metal-organic frameworks (MOFs) are crystalline porous materials with potential applications in catalysis, gas storage, and gas separation membranes. For gas separation, MOF membranes require high selectivity and permeability. While mixed matrix membranes (MMMs) combining MOFs with polymers have shown promise, pure MOF membranes are less common due to challenges in achieving high performance and stability. Pure MOF membranes, however, offer the potential for superior performance if their inherent structural flexibility can be addressed. Previous research on CAU-10-based MOFs, specifically CAU-10-PDC, demonstrated good CO₂/CH₄ selectivity but suffered from severe structural deformation under CH₄ exposure, limiting their practical applications. This instability arises from the structural flexibility inherent to many MOFs; linker rotation in ZIF-8, for example, significantly alters the effective pore size, impacting separation performance. This paper investigates a mixed-linker strategy to enhance the stability and performance of CAU-10-based membranes by partially substituting the flexible PDC linker with the more rigid BDC linker, aiming to mitigate the structural deformation and improve long-term operational stability during gas separation. The goal is to create a membrane with both high selectivity and high permeability for CO2/CH4 separation, surpassing the limitations of previous pure MOF membranes.

The literature extensively covers MOFs for various applications, including gas separation. While many MOF-based mixed matrix membranes (MMMs) show good results, pure MOF membranes are less common due to stability issues. Studies on IRMOF-1, MIL-160/CAU-10-F, and ZIF-94 membranes have demonstrated promising CO₂ separation capabilities, but often with limitations in long-term stability or permeability. The issue of structural flexibility in MOFs, highlighted by studies on ZIF-8, is widely recognized. Linker rotation in ZIF-8 leads to an effective aperture size larger than the theoretical pore size, reducing selectivity. Strategies such as rapid heat treatment (RHT) and mixed-linker approaches have shown some success in mitigating this flexibility in ZIF-8. The previous work by the authors on CAU-10-PDC demonstrated high CO₂/CH₄ selectivity but severe instability under CH₄ exposure, motivating the current work on a mixed-linker approach.

The researchers employed a mixed-linker strategy to synthesize CAU-10-PDC-H membranes with varying ratios of PDC and BDC linkers. The seeded growth method was used for membrane fabrication on porous α-alumina substrates. The synthesized materials were characterized using various techniques. FTIR spectroscopy confirmed the presence of both PDC and BDC linkers in the mixed-linker samples. ¹H NMR spectroscopy quantitatively determined the molar percentages of PDC and BDC in the samples. SEM imaging analyzed the morphology and membrane thickness. XRD was used to determine pore limiting diameters (PLDs) and investigate structural changes during gas exposure. In situ XRD and DRIFT spectroscopy monitored structural changes under CO₂ and CH₄ exposure. DFT calculations were performed to model the structures and understand the impact of the mixed linkers. Single-gas and mixed-gas permeation tests were conducted at 2 bar and 35°C using H₂, CO₂, N₂, and CH₄ to evaluate gas separation performance. The performance was evaluated using permeability, selectivity, and separation factor. Langmuir model was used to analyze gas adsorption isotherms. Energy decomposition analysis (EDA) was used to investigate methane adsorption behavior and understand the effect of different linkers on structural stability.

FTIR, NMR, and EA consistently confirmed the successful incorporation of BDC and PDC linkers in the desired ratios. SEM images showed high-quality membranes with thicknesses around 10 µm and intergrown MOF crystals. The PLD decreased with decreasing PDC-to-BDC ratio as confirmed by XRD and DFT calculations. Single-gas permeation tests showed a permeability cutoff between CO₂ and N₂, with CAU-10-PDC-H (7:3) exhibiting the highest permeability for H₂ and CO₂. Mixed-gas permeation tests demonstrated that CAU-10-PDC-H (7:3) displayed the highest separation factor for both CO₂/CH₄ and CO₂/N₂ mixtures. The CAU-10-PDC-H (7:3) membrane achieved a CO₂/CH₄ separation factor of 74.2 and a CO₂ permeability of 1111.1 Barrer at 2 bar and 35°C. In situ XRD experiments revealed that the mixed-linker membranes (especially CAU-10-PDC-H (7:3)) showed significantly improved structural stability under CH₄ exposure compared to the pure CAU-10-PDC membrane. In situ DRIFT spectroscopy indicated reduced and less temporal structural changes in the mixed-linker membranes under CH₄ exposure. DFT calculations supported the experimental findings on structural stability. EDA calculations revealed that the PDC linker exhibits a stronger interaction with methane than the BDC linker, thus explaining the observed difference in the structural change. The CAU-10-PDC-H (7:3) membrane showed exceptional long-term stability during mixed-gas permeation tests, with minimal changes in permeability and separation factors over time.

The results demonstrate the effectiveness of the mixed-linker approach in suppressing the structural flexibility of CAU-10-based membranes. The partial substitution of PDC with BDC significantly enhances the membrane's stability under CH₄ exposure, which was a major limitation of the pure CAU-10-PDC membrane. The optimized CAU-10-PDC-H (7:3) membrane outperforms the pure CAU-10-PDC and CAU-10-H membranes and many other reported MOF membranes in terms of CO₂/CH₄ separation, reaching the state-of-the-art performance. This improved performance is attributed to the optimized pore size and reduced structural flexibility which enhance both selectivity and permeability. The combination of experimental techniques (XRD, DRIFT, SEM, gas permeation measurements) and DFT calculations has provided a comprehensive understanding of the structural changes and the underlying mechanisms that contribute to the improved performance of the mixed-linker membranes. The observed competitive adsorption-diffusion in mixed-gas permeation indicates a strong interplay between different gas molecules during the separation process.

This study successfully demonstrates a mixed-linker strategy for enhancing the structural stability and gas separation performance of CAU-10-based MOF membranes. The incorporation of BDC into the CAU-10-PDC framework significantly improved stability under CH₄ exposure without compromising permeability. The CAU-10-PDC-H (7:3) membrane exhibits superior CO₂/CH₄ separation performance compared to previously reported MOF membranes. This work provides valuable insights into designing high-performance and stable MOF membranes for gas separation applications. Future research could explore other mixed-linker combinations and investigate the scalability and long-term durability of these membranes for industrial applications.

The study focused on a specific set of linkers (PDC and BDC) and a limited range of MOF compositions. The generalizability of the mixed-linker strategy to other MOF systems needs further investigation. The long-term stability tests were conducted for a relatively short period. Longer-term tests are needed to fully assess the long-term durability and aging effects of these membranes. The DFT calculations are computationally intensive and may not fully capture the complexities of the interactions in the real MOF system. The performance of the membranes might be affected by factors such as defects in the membrane structure, which were not specifically considered in this study.