## 3 Results and Discussion  
### Chemical and Morphological Characterization  
The MOF loading in polymer matrix was evidenced by the distinctive color of obtained membranes, and it was accurately confirmed by the EDX analysis, which revealed the attendance of Cu and Ni metals in both Pebax(r)1657 and PEEK-WC polymers (See SI, Figure S1a,b, respectively). The chemical characterization of neat membranes and MMMs, as well as the interaction between polymer chains and nanoparticles were investigated by FTIR-ATR. Figure 2b,d shows the spectrum for neat Pebax(r)1657 and for Pebax(r)1657/CuNi-MOF membranes. The functional groups of Pebax(r)1657 give characteristic peaks at 3297 cm\({}^{-1}\) for N-H group, at 2943 and 2859 cm\({}^{-1}\) attributed to the asymmetric and symmetric stretching of the C-H bound, 1731 cm\({}^{-1}\) for the symmetric stretches of carboxylate R-O-C=O, while asymmetric R-O-C=O at ca. 1430, 1636 cm\({}^{-1}\) for H-N-C=O and 1099 cm\({}^{-1}\) for -C-O-C.  
Figure 2: SEM image of the samples showing the interface between MOF and polymer phase (**a**) for Pebax®1657 and (**c**) PEEK-WC. FT-IR of neat polymer and MMMs with 23 wt % of CuNi, for Pebax®1657 (**b**) and PEEK-WC (**d**).  
In the spectrum of Pebax(r)1657/CuNi-MOF, characteristic peaks appear at 1578 cm\({}^{-1}\) due to C-C aromatic band of MOF's phenylene, at 1608 cm\({}^{-1}\) of stretching of oxamate ligand of MOF and a peak at 1330 cm\({}^{-1}\), which could be considered diagnostic of the presence of bridging oxamate between Cu and Ni [51]. Finally, the characteristic peaks between 2900 cm\({}^{-1}\) and 2800 cm\({}^{-1}\) due to the symmetric and asymmetric C-H stretching are slightly shifted to lower wavelengths, confirming the interaction of hydrogen bonds at the interface between MOF and Pebax(r)1657, as it was also seen by [PERSON] et al. for Pebax1657/CuBTC mixed matrix membranes [52]. Figure 2d offers a comparison of PEEK-WC membrane and PEEK-WC/CuNi spectrum. The functional groups of PEEK-WC give characteristic peaks at 3052 cm\({}^{-1}\) and 3075 cm\({}^{-1}\) for aromatic C-H stretch, at 1767 cm\({}^{-1}\) for ketonic and esteric C=O stretching, and at 1589 cm\({}^{-1}\) for the C-C aromatic band [53]. In PEEK-WC/CuNi, the peak for the C-C aromatic band is shifted on the lower wavelengths, probably due to the \(\pi\cdot\cdot\pi\) stacking interaction between the benzene rings of polymers and MOF. In fact, when the ring is very conjugated, a weak band can be observed at around 1580 cm\({}^{-1}\), such as the one is visible from the PEEK-WC/CuNi spectrum. The peaks due to the stretch of the aromatic C-H appear to be totally fused with the water peak (3370 cm\({}^{-1}\)) that reveals the relatively hydroscopic nature of the MOFs. The interaction between the Pebax(r)1657 and CuNi-MOF was also confirmed by SEM analysis, and in the enlarged particular of Figure 2a, it is possible to see that polymer phase of Pebax(r)1657 creates a sort of circular compressed polymer region around the MOF, which [PERSON] defined as \"Case I matrix rigidification\" [37]. On the other hand, the distribution of MOF in the PEEK-WC polymer produces a defined and regular network structure (Figure 2c) with a homogenous dispersion of nanoparticles without significant sedimentation across the membranes (See SI Figure S-2). Instead, the addition of MOFs in Pebax(r)1657 shows no significant agglomeration at the lowest 9 wt % that becomes more significant at highest 29 wt % concentration.