Mixed Matrix Membranes Produced from a Fluorinated MOF and Pebax for CO₂/H₂ Separation

3.1. XRD Analysis The XRD (X-ray diffraction) analysis depicted in Figure 2 reveals crucial insights into the structural characteristics of the mixed matrix membranes (MMM) containing KAUST-7 MOF. In particular, the diffraction peaks observed in the XRD patterns of the MMM align well with the diffraction peaks expected for the crystalline structure of KAUST-7 MOF, as evidenced by the prominent peak at 2Ɵ (angle of diffraction) around 12.5° [36]. Furthermore, an intriguing observation from the XRD analysis is the gradual broadening and weakening of the characteristic peaks as the KAUST-7 MOF content increases. This phenomenon is noteworthy when compared to the peak intensity and width of the neat KAUST-7 crystals. Peak broadening in XRD patterns suggests a reduction in the crystallite size or an increase in structural disorder within the MMM due to KAUST-7 MOF incorporation. This could be attributed to the interaction between the polymeric matrix and MOF, generating a more disordered arrangement of MOF crystallites inside the composite material. Moreover, the weakening of the characteristic peak (12.6°) indicates a decrease in the crystallinity of the MMM relative to the neat KAUST-7 crystals. This can be related to the disruption of the MOF crystallinity caused by its dispersion inside the polymer and potential interfacial interactions between the polymer and MOF [41]. 3.2. SEM Analysis Metal–organic frameworks (MOF) are modular and tunable highly crystalline porous materials [38]. Figure 3 presents typical SEM micrographs of the KAUST-7 powder. The images confirm that the particle sizes are less than 100 nm, while their agglomerations can be up to several microns in size [38,39]. Figure 4 presents typical images of the MMM for various KAUST-7 concentrations. The membrane thickness is 90–120 microns. For the neat Pebax membrane (Figure 4A), the image shows that the matrix is without voids or defects and exhibits a smooth surface texture. This membrane serves as the base material for the MMM. The structure gets more textured when adding low KAUST-7 nanocrystals (Figure 4B,C). At low KAUST-7 content (5 and 10 wt.%), the SEM images show a homogeneous MOF distribution throughout the polymer. This uniform dispersion suggests efficient mixing and interaction between the MOF nanoparticles and Pebax. Upon closer examination at higher magnification, the MOF nanocrystals are well-embedded within the Pebax matrix without visible defects or voids observed. This “sieve-in-a-cage” structure indicates that the MOFs are encapsulated or surrounded by the polymer, enhancing their dispersion and minimizing the possibility of particle detachment or aggregation. Additionally, a polymer network of a circular pattern (morphology) is another confirmation of good interfacial interaction [40]. However, at higher KAUST-7 content (20 wt.% in Figure 4D), the image reveals notable differences compared to lower concentrations. In this case, clusters or agglomerates of KAUST-7 nanocrystals become apparent within the matrix. This aggregation may lead to non-uniform properties within the MMM, affecting its overall performance. The presence of agglomerated nanofillers alters the MMM morphology, creating irregularities in the structure. This non-uniform morphology can lead to variations in mechanical strength and gas transport properties, affecting the overall MMM performance. In regions where nanofillers agglomerate, voids or gaps are seen in the matrix, compromising the MMM’s structural integrity and barrier properties (preferential channel) potentially leading to decreased performance or failure under lower stress conditions. 3.3. FTIR Analysis To confirm the MOF inclusion in the matrix, FTIR was done and the spectra are reported in Figure 5. The typical Pebax bands are located at 845 cm⁻¹ due to –OH stretching, 1099 cm⁻¹ and 1734 cm⁻¹ related to –C=O (saturated ester groups) and –O–C– (EO segments) [42]. The peaks at 1099 cm⁻¹ and 1734 cm⁻¹ (PEO segments of Pebax and saturated ester groups) are shifted to lower frequencies with lower intensity and higher MOF concentration. For the MMM, the appearance of Nb–O stretching (480 cm⁻¹), Ni–N stretching (450 cm⁻¹), and C=N from pyrazine (1473 cm⁻¹) indicate the presence of KAUST-7. Bands shifting indicates good affinity and strong chemical interactions [43]. This confirms that hydrogen bonding occurs between the H of the pyrazine in KAUST-7 and the EO segments of Pebax. This is why good interaction between both phases was obtained without interfacial defects (Figure 4). 3.4. Thermal Analysis TGA-DTG results were used to determine the thermal stability of the materials (Figure 6). Table 1 presents the temperatures derived from the TGA curves from the first peak in the DTG curves. Typically, temperatures at which 5% (T_d5%) and 10% (T_d10%) weight loss occur are reported (Table 1). These curves also provide insights into the degradation rates. The results of Figure 6A show that KAUST-7 has a much lower thermal stability than Pebax as its T_5% is 265 °C lower (63 vs. 328 °C). This is why the MMM is less stable with increasing MOF content. Figure 6A also shows that KAUST-7 exhibits a distinct weight loss pattern, elucidated by TGA and DTG plots. Initially, the sample undergoes water loss between 30 °C and 150 °C, with a notable decline at 100 °C (Figure 6B-DTG), followed by a plateau extending from 150 °C to 250 °C. The first 8% weight loss (20 °C to 250 °C) corresponds to the loss of two water (7.8 wt.%). Notably, material decomposition initiates above 250 °C, with a prominent peak at 328 °C followed by a smaller one at 720 °C [38]. Despite its decomposition, the metal element content persists, resulting in residues comprising 45% wt. The neat Pebax presents excellent thermal properties since its melting point is around 159 °C. It is also very stable for high-temperature use, as evidenced by its high T_d5% and T_d10%, reaching 328 °C and 360 °C, respectively. For the MMM, negligible weight loss is observed up to 250 °C, suggesting an effective removal of most casting solvents (water and ethanol) through the vacuum drying protocol. Consequently, the onset degradation temperatures marginally decrease with increasing MOF content (T_d5%,T_d10% in Table 2). This trend is expected as higher MOF content leads to increased material degradation, consistent with observations for other MOFs (Cu₃(BTC)₂, UiO-66-NH₂ and MIL-53-NH₂) [43]. Nevertheless, all the MMM have good thermal resistance as their T_5% are all above 181 °C, which is much higher than the normal operating conditions for membrane gas separation. 3.5. Gas Permeation Analysis The permeability for H₂ and CO₂ was determined at 2 bar for two temperatures (35 and 55 °C). The effect of temperature and MOF content are presented in Table 2 and Figure 7. The performances of the neat matrix and MMM were obtained by permeation tests. Based on Table 2 and Figure 7, all the MMM have higher CO₂, H₂ and CO₂/H₂ ideal selectivity compared to the neat matrix. This is expected because KAUST-7 features an ultra-micro aperture size (3.5 Å) and is CO₂-philic. Although H₂ and CO₂ molecules can pass through the MOF, CO₂ permeation is favorized. All the MMM permeabilities are above the neat matrix and increase to a critical MOF content (10 wt.%). This is in agreement with several previous works. Meshkat et al. [43] observed that 10 wt.% was the optimum content for NH₂-MIL-53 in Pebax, leading to a 174% increase in CO₂ permeability over the neat matrix, while Tien-Binh et al. [44] observed that 10–15 wt.% was the optimum MIL-53 concentration in 6FDA-DAM for CO₂/CH₄ separation with 46–47 Barrer of CO₂ permeability and 58–78 of selectivity. These results show that the CO₂ permeability of 10% KAUST-7/Pebax 1657 MMM is 79% and 68% higher than the neat matrix at 35 and 55 °C, respectively, while the H₂ permeabilities are also above the neat matrix values because of MMM porosity. However, in this case, the increases are lower (Table 2). Therefore, the ideal selectivity for CO₂/H₂ was improved by 14% and 29% at 35 and 55 °C, respectively. Nevertheless, the permeability drops at 15% KAUST-7 because of a rigid polymer layer around the MOF and the tortuosity associated with the rigid particles generating lower gas molecules mobility to a high volume of the porosity. When the MOF content reaches 20%, MOF agglomeration generates membranes becoming too rigid and easy to break so that no data can be obtained. 3.6. Comparison of CO₂-Selective MMM Usually, gas separation performances for polymer membranes are compared based on Robeson’s upper bounds [45], but the results are only presented for H₂/CO₂ and not for CO₂/H₂ separation. This is why Figure 8 reports the properties of CO₂-selective membranes compared with the upper bound proposed by Freeman [46]. It can be seen that few CO₂-selective mixed matrix membranes have been studied to compare the CO₂/H₂ selectivity. Nevertheless, some CO₂-selective MMM have been investigated, including PI-Zeolite-5 MMM comprising polyimide (PI-6FDA-TeMPD) with 15 wt.% zeolite (ZSM-5) in liquid sulfolane (SF) [47], Pebax-SAPO-34 MMM with 23% nanofiller [48] and 8 wt.% SiO₂ [49], Pebax MH 1657 with amino group-functionalized MWCNT [33], poly(ethylene oxide) (PEO)-silica of 10 wt.% [50], polyetheramine (PEA)-polyhedral oligomeric silsesquioxane (POSS) MMM [51], Pebax^® 1657 with ZIF-8 particles [52], and azide crosslinked poly(4-methyl-1-pentyne) (PMP) MMM with nano-CuBTC MOF [53]. While some membranes showed enhanced gas permeability with increased filler content, but the CO₂/H₂ selectivity remains unchanged. The conditions also vary across the studies, with temperature and pressure playing significant roles in membrane performance. Finally, these findings underscore the need for tailored MMM design to optimize the CO₂ separation efficiency for diverse industrial applications.