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

Abstract

Membrane-based processes are taking a more and more prominent position in the search for sustainable and energy-efficient gas separation applications. It is known that the separation performance of pure polymers may significantly be improved by the dispersion of suitable filler materials in the polymer matrix, to produce so-called mixed matrix membranes. In the present work, four different organic cages were dispersed in the poly(ether ether ketone) with cardo group, PEEK-WC. The m-xylyl imine and furanyl imine-based fillers yielded mechanically robust and selective films after silicone coating. Instead, poor dispersion of p-xylyl imine and diphenyl imine cages did not allow the formation of selective films. The H2, He, O2, N2, CH4, and CO2 pure gas permeability of the neat polymer and the MMMs were measured, and the effect of filler was compared with the maximum limits expected for infinitely permeable and impermeable fillers, according to the Maxwell model. Time lag measurements allowed the calculation of the diffusion coefficient and demonstrated that 20 wt % of furanyl imine cage strongly increased the diffusion coefficient of the bulkier gases and decreased the diffusion selectivity, whereas the m-xylyl imine cage slightly increased the diffusion coefficient and improved the size-selectivity. The performance and properties of the membranes were discussed in relation to their composition and morphology.

CO2 Separation by Imide/Imine Organic Cages

“…On the basis of the Maxwell equation (see the Supporting Information for details), the decrease in permeabil-ity for C1 and the increase for C2 suggest that the fillers have a lower and higher permeability than the polymers, respectively. [61] This is also in line with the lower porosity of C1 compared to C2, resulting from the gas-adsorption studies on the two materials (see, e. g., SA BET, CO 2 values at 298 K in Table 1). As for selectivity, both C1 and C2 significantly increase the CO 2 /N 2 selectivity in PEEK-WC (Table 2 and Figure 7a).…”

“…Dense membranes were obtained by slow evaporation of the solvent at 25 °C for 24 h. The resulting MMMs were removed from the Petri dish and their top surface was coated with PDMS Elastosil M 4601 (prepolymer+crosslinker) to cover possible pinhole defects. Further details on MMMs preparation are reported in the Supporting Information, while details of the coating procedure were reported elsewhere [61] …”

“…On the other hand, C2 increases the CO 2 permeability by few percent in PEEK‐WC (Table 2; red square vs. red circle in Figure 7) and about 50 % in Matrimid® 9725 (green square vs. green circle). On the basis of the Maxwell equation (see the Supporting Information for details), the decrease in permeability for C1 and the increase for C2 suggest that the fillers have a lower and higher permeability than the polymers, respectively [61] . This is also in line with the lower porosity of C1 compared to C2 , resulting from the gas‐adsorption studies on the two materials (see, e. g., SA values at 298 K in Table 1).…”

“…[59,60] For the preparation of MMMs, we followed the experimental procedure recently employed by our group in the development of MMMs containing azacryptands as fillers. [61] In particular, the cages were firstly homogeneously dispersed in chloroform, then the required amount of polymeric solution (2 wt.% Matrimid® 9725 or 3 wt.% PEEK-WC in chloroform) was added. C1 and C2, were loaded in the polymer matrix at 20 wt.…”

“…Further details on MMMs preparation are reported in the Supporting Information, while details of the coating procedure were reported elsewhere. [61] Pure gas permeation tests were performed at 298 K, and at a feed pressure of 1 bar in a fixed volume/pressure increase setup using the time-lag method for the determination of the permeability (P), diffusion (D), and solubility coefficients (S). Details of the instrument and the measurement procedure were reported elsewhere.…”