Glassy PEEK-WC vs. Rubbery Pebax®1657 Polymers: Effect on the Gas Transport in CuNi-MOF Based Mixed Matrix Membranes

Abstract

Mixed matrix membranes (MMMs) are seen as promising candidates to overcome the fundamental limit of polymeric membranes, known as the so-called Robeson upper bound, which defines the best compromise between permeability and selectivity of neat polymeric membranes. To overcome this limit, the permeability of the filler particles in the MMM must be carefully matched with that of the polymer matrix. The present work shows that it is not sufficient to match only the permeability of the polymer and the dispersed phase, but that one should consider also the individual contributions of the diffusivity and the solubility of the gas in both components. Here we compare the gas transport performance of two different MMMs, containing the metal-organic framework CuNi-MOF in the rubbery Pebax ® 1657 and in the glassy poly(ether-ether-ketone) with cardo moiety, PEEK-WC. The chemical and structural properties of MMMs were investigated by means of FT-IR spectroscopy, scanning electron microscopy and EDX analysis. The influence of MOF on the mechanical and thermal properties of both polymers was investigated by tensile tests and differential scanning calorimetry, respectively. The MOF loading in Pebax ® 1657 increased the ideal H 2 /N 2 selectivity from 6 to 8 thanks to an increased H 2 permeability. In general, the MOF had little effect on the Pebax ® 165 membranes because an increase in gas solubility was neutralized by an equivalent decrease in effective diffusivity. Instead, the addition of MOF to PEEK-WC increases the ideal CO 2 /CH 4 selectivity from 30 to~48 thanks to an increased CO 2 permeability (from 6 to 48 Barrer). The increase in CO 2 permeability and CO 2 /CH 4 selectivity is maintained under mixed gas conditions. natural gas treatment (CO 2 /CH 4 ) [6], and post-combustion carbon capture from flue gas (CO 2 /N 2 ) [7]. This increasing interest is dictated by the advantages of membrane technology compared to traditional gas separation techniques. Gas separation by means of membrane technology is an economical process; it is easily scalable and it can be used in non-drastic temperature and pressure conditions, which are more environmentally friendly. Despite numerous efforts to develop new materials for gas separation, the membrane market still needs to overcome some challenges. In fact, the highly permeable rubbery polymers present low selectivity, and the highly-selective glassy polymers are less permeable. To overcome this trade-off between permeability and selectivity, extensively reported by Robeson in 1991 and in 2008 [8,9], researchers are focusing on the design and development of hybrid membranes based on the combination of two different materials in order to get the advantages of both [10]. According to this concept, a valid strategy is the embedding of porous metal-organic frameworks (MOFs) in the polymer matrix in order to obtain mixed matrix membranes (MMMs) with enhanced gas transport properties [11][12][13][14][15][16]. MOFs are an attractive new class of microporous materials built by the combination o...

Gas Transport in Mixed Matrix Membranes: Two Methods for Time Lag Determination

“…The simplest phenomenological explanation is this that the MOF behaves as a sort of reservoir for the permeating gas species, and the higher the sorption capacity, the longer time lag. A similar observation was recently made by Esposito et al [40] for Ni II 2 (S,S)-hismox, and the diffusion of C 2 H 4 is faster than that of C 2 H 6 , in agreement with its smaller size. On the other hand, the narrower channels are only accessible to C 2 H 4 , and the higher affinity leads to a higher sorption capacity reflected in a higher solubility, but the slower diffusion coefficient in/into these narrower channels and the higher overall sorption capacity result in a much longer time lag and thus a lower effective diffusion coefficient.…”

“…The simplest phenomenological explanation is this that the MOF behaves as a sort of reservoir for the permeating gas species, and the higher the sorption capacity, the longer time lag. A similar observation was recently made by Esposito et al [40] The second explanation is that gases with different sizes and different affinity for the internal voids of the MOF, have access to different sorption sites. Indeed, both C2H4 and C2H6 have access to the relatively large central voids of Cu II 2(S,S)-hismox, and the diffusion of C2H4 is faster than that of C2H6, in agreement with its smaller size.…”

PEEK–WC-Based Mixed Matrix Membranes Containing Polyimine Cages for Gas Separation

“…The latter is a glassy polymer that is soluble in several organic solvents due to the presence of the Cardo-group in the backbone, unlike the classical poly (ether ether ketone) (PEEK) and poly (ether ketone) (PEK) [ 12 ], and its good solubility in low boiling solvents like chloroform is useful for the preparation of dense gas separation membranes by solvent evaporation. Recently, PEEK-WC was loaded with MOF, improving the transport properties in gas separation [ 13 ].…”

The Effect of the Temperature and Moisture to the Permeation Properties of PEO-Based Membranes for Carbon-Dioxide Separation

“…The main disadvantage of pure poly(ethylene oxide) (PEO) is a high affinity towards the crystallization, which negatively affects permeation properties of the membrane. So, a copolymer that contains PEO units as a co-monomer would be a good choice for this application [ 19 ]. This type of the polymer composite may be used either as a self-standing membrane or as an active film coated on the suitable carrier.…”