Enhancing the Separation Performance of Glassy PPO with the Addition of a Molecular Sieve (ZIF-8): Gas Transport at Various Temperatures

Abstract

In this study, we prepared and characterized composite films formed by amorphous poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and particles of the size-selective Zeolitic Imidazolate Framework 8 (ZIF-8). The aim was to increase the permselectivity properties of pure PPO using readily available materials to enable the possibility to scale-up the technology developed in this work. The preparation protocol established allowed robust membranes with filler loadings as high as 45 wt% to be obtained. The thermal, morphological, and structural properties of the membranes were analyzed via DSC, SEM, TGA, and densitometry. The gas permeability and diffusivity of He, CO2, CH4, and N2 were measured at 35, 50, and 65 °C. The inclusion of ZIF-8 led to a remarkable increase of the gas permeability for all gases, and to a significant decrease of the activation energy of diffusion and permeation. The permeability increased up to +800% at 45 wt% of filler, reaching values of 621 Barrer for He and 449 for CO2 at 35 °C. The ideal size selectivity of the PPO membrane also increased, albeit to a lower extent, and the maximum was reached at a filler loading of 35 wt% (1.5 for He/CO2, 18 for CO2/N2, 17 for CO2/CH4, 27 for He/N2, and 24 for He/CH4). The density of the composite materials followed an additive behavior based on the pure values of PPO and ZIF-8, which indicates good adhesion between the two phases. The permeability and He/CO2 selectivity increased with temperature, which indicates that applications at higher temperatures than those inspected should be encouraged.

Mixed Matrix Membranes Based on Torlon® and ZIF-8 for High-Temperature, Size-Selective Gas Separations

“…Moreover, as reported in Figure S3, no peaks that can be ascribed to decomposition phenomena are depicted in DSC thermograms for neat Torlon ® and composite membranes, confirming the already well known high thermal stability of the used materials [68,71].…”

“…Variations of the polymer glass transition temperature due to the incorporation of the filler were observed also for other ZIF-8 containing MMMs depending on the filler loading, the chemical structure of the polymer, and on the polymer-filler interactions and hindering ability. In ZIF-8 incorporating PPO-based membranes [71], the increase of T g was linear with filler loading and limited to within 1.5% at the highest loading inspected (45%). This suggests that the filler particles have limited hindering ability on the polymer chains segmental motion even for high loads, leading to a small T g variation.…”

“…In Figure 7a,b, we compared the relative permeability variations observed on He and CO 2 permeability after adding ZIF-8 to three glassy polymers, namely Torlon ® , PPO, and PSf at 35 • C [71,72]. Variations are similar for all polymers, especially in the case of CO 2 , while for Helium, the polymer experiencing the highest increase is Torlon ® : this can be attributed to its initial low value of permeability and free volume compared to the other polymers.…”

“…A slight improvement of He/CO 2 and H 2 /CO 2 selectivity was observed after adding ZIF-8 to matrices with relatively low selectivity such as PPO [71], PSf [72], and PPEES [64], while a decrease of selectivity was observed after addition to the more selective matrices such as Matrimid ® [58,59] and PBI [60][61][62].…”

Recent advances in simulating gas permeation through MOF membranes

“…It is important to note that pore size screening can be done to obtain molecular sieving membranes where one type of gas passes through the pores whereas the others do not. [26][27][28] Although the selectivity of molecular sieving membranes such as zeolite imidazolate framework-7 (ZIF-7) was measured to be very high, gas permeability was generally quite low due to the narrow pores. 29 Therefore, MOFs with pore sizes larger than the size of the largest gas molecule can be more preferable to achieve both high selectivity and high permeability.…”