Influence of ZIF-95 on structure and gas separation properties of polyimide-based mixed matrix membranes

Highlights

• Matrimid®/ZIF-95 MMMs were synthesized and CO₂, CH₄, and H₂ gas transport studied.

• ZIF-95 crystals were synthesized and homogeneously dispersed in polymer with good interfacial filler-matrix adhesion.

• The gas permeability and selectivity of H₂, CO_2, and CH₄ gases enhanced with increase in the content of ZIF-95.

• The performance of Matrimid®/ZIF-95-30% attained closer to the 2008 upper bound for H₂/CH₄ separation.

• 30 wt % Matrimid®/ZIF-95 mixed matrix membrane shows the best gas permeation properties.

Abstract

Polymeric membranes, especially polyimide membranes, are widely preferred in gas separation processes due to their superior properties such as inherently high permeability, selectivity, and easy processing. Zeolitic imidazolate frames (ZIFs) are crystalline porous materials exhibiting some unique properties, such as tunable pore sizes, thermal and chemical stability.

In this study, a series of polyimide/ZIF-95 mixed matrix membranes (MMMs) were prepared by solution casting method and their structural, thermal, thermomechanical, and permselectivity properties were characterized in detail, by XRD, FTIR, SEM, DLS, DMA, BET surface area and gas permeability measurements. In the first step, ZIF-95 powder was synthesized and characterized then incorporated into polyimide (Matrimid®5218) matrix as the loading amounts of 10, 20, and 30 wt%. SEM analysis showed that ZIF particles were homogeneously distributed into polyimide matrix and there were no agglomeration and cracking and visible holes at interfaces between polymer and filler. Increase in storage modulus of MMMs also implied such strong interfacial interactions between polyimide and ZIF-95 particles. It was found that both permeability and selectivity of CO₂, CH₄, and H₂ gases increased with the increasing of ZIF-95 amount in polymer matrix. The maximum ideal separation factors for the CO₂/CH₄ and H₂/CH₄ were found to be 58.0 and 192.0, respectively. These values are significantly higher than the ideal separation factor of Matrimid®5218. It was also found that the Matrimid®/ZIF-95 (70/30) membrane showed improvement in gas selectivity by 75% and 48% for the CO₂/CH₄ and H₂/CH₄, respectively. Consequently, this study suggests a manufacturing route for MMMs having superior properties and improved permselectivity performances and reports their structure-property relations for potential gas separation, particularly CO₂ and/or natural gas purification, processes.

Introduction

In most processes applied in the fossil fuels combustion, natural gas streams, and chemical industry, the desired purity cannot be achieved in the products during the processes. Separating CO₂, which is an impurity in natural gas, and enriching CH₄ used as fuel, attracts great interest in the natural gas industry. In addition, CO₂ is not a desirable substance as it provides no heating value and CO₂ and other acid gases corrode pipelines and other equipment. On the other hand, carbon dioxide is a greenhouse gas and its presence in atmosphere causes the global warming. Therefore, it is extremely important to obtain technical separation methods with low cost and acceptable yields. Nowadays, membrane technologies are considered as one of the main technics for gas separation applications, since they are cost-effective, environmentally friendly, have good mechanical and chemical resistance, high selectivity, and permeability (Castel et al., 2018; Mixa and Staudt, 2008).

Most non-porous membranes are used in gas separation processes. Generally, these membranes possess an anisotropic construction to achieve better flow properties. Non-porous membranes are composed of a dense film for the diffusion of permeability agents due to the gradient of concentration, electrical potential, or pressure. The diffusivity and solubility of the components within the membrane are the main causes for separating the components in the mixture and are correlated with their relative transport rates within the membrane (Baker, 2012).

Between 1979 and 1980, Monsanto produced the first commercial membrane gas separation systems. Thanks to these systems, nitrogen, argon, and methane in the cleaning gas of ammonia plants are separated from hydrogen. Hydrogen recovery technology was open to development, and a few years later, Monsanto was developed worldwide and the membrane gas separation industry has shown its way (Baker and Low, 2014).

In researches conducted with developing technology, it is seen that polyimides are suitable for gas separation from membranes. However, despite their high selectivity, the same cannot be said for permeability. However, it has been shown that by using suitable diamines and dianhydrides, they can be successfully adjusted by developing both the selectivity and permeability of polymers.

The most significant limitation in the improvement of gas separation membranes is that the balance between selectivity and permeability first defined by Robeson (1991). Pure organic polymeric membranes reach the permeability-selectivity balance defined by Robeson but rarely exceed this limit (Tanh Jeazet et al., 2012). Therefore, the alternative membrane type, accepted as the mixed matrix membrane, has emerged in many scientific studies. There have been substantial developments in the efficiency of polymeric gas separation membranes. Despite all these efforts, as Robeson suggests, the polymeric membranes are not in a position above the compensation curves between gas permeability and selectivity. Therefore, researchers have sought alternative ways to take the gas separation membranes to the upper level. Molecular sieve materials placed in a polymer matrix possess the potential for economic and high-performance gas separation (Shekhawat et al., 2003). The mixed matrix membranes obtained have both easy processability of components and very good gas transport properties of molecular sieve materials. The aim is to prepare membranes with high separation performance with mechanical strength, thermal and chemical stability, and workability (Bernardo et al., 2009).

Matrimid®5218 is one of the commercial grades of polyimides that is extensively used in gas separation membranes due to its high gas permeability value and separation factor, outstanding mechanical properties, and easy solubility in organic solvents. Matrimid®5218 is also a soluble thermoplastic and fully imidized polymer that does not require high temperatures during processing. After solvent evaporation, it generally forms strong, durable, and hard films or coatings (Ayas et al., 2018).

In recent years, research studies have intensified on functional and structurally designed inorganic materials such as carbon molecular sieves (CMS), zeolites, silica, graphene, and metal oxides as membrane fillers. Especially, promising results have been presented for metal-organic frameworks (MOFs) in the literature (Bae et al., 2010). MOFs specifically possess many positive features such as good chemical and thermal stability, high surface area and adsorption capacity, and pore size variety. (Chen et al., 2012; Dai et al., 2012; Deng et al., 2021). Moreover, MOFs show a higher affinity to polymers due to the presence of organic moieties in MOF structures, compared to other alumina-silicate-based inorganic materials such as silica or zeolites. Zeolitic imidazolate frameworks (ZIFs) a class of MOFs produced by reactions of anionic imidazolate binders (ImH) with hydrated transition metal salts in an amide solvent have gained technical and scientific interest in gas separation applications (Askari and Chung, 2013; Phan et al., 2010). Structural variability in MOFs has also led to the synthesis of numerous ZIFs with zeolite-type tetrahedral topologies (Perez et al., 2014). Wang et al. synthesized many zeolitic frameworks in rhombus dodecahedron (RHO) and sodalite (SOD) structures (Wang et al., 2008). ZIF-95, having a tetragonal and neutral frame composed of 128 Zn nodes coordinated tetrahedrally by chlorobenzimidazolate (cbIM) exhibits an exceptional topology (represented by poz) (Assfour et al., 2011; Prakash et al., 2013). Specifically, Aldrich and coworkers showed that ZIF-95 has a specific inlet opening of 3.65 Å, consisting of a large void (24.0 Å). The porous structure and large pore volume of ZIF-95 make it an extremely important material in gas adsorption and purification applications. The crystalline structure of ZIF-95 and its selective adsorption ability for CO₂ could yield high methane purification efficiency. Aldrich et al. reported that ZIF-95 exhibited high performance in CO₂ separation because of the quadrupole contact between ZIF-95 and CO₂. On the other hand, theoretical studies have shown the importance of the presence of ZIF-95 for the storage and separation of H₂. They, therefore, reported that the selective adsorption of H₂ was increased by adding ZIF-95 into a membrane (Aldrich et al., 2019). On the other hand, Dejam et al. investigated flow and diffusion patterns between channel and permeable porous media. They stated that there are differences in flow and diffusion patterns, especially in MMMs (Kou and Dejam, 2019, 2020). Since pressure-driven flows have several possibilities, e.g., flow through the channels of ZIF-95, flow through the torturous paths between particulates matter and pure membrane, and diffusion through membrane material. There are now unifying models in which pressure-driven flow and electroosmotic flow are solved by analytical solutions. Due to the mixing properties of ZIF-95 and membranes, the universal applicability of these unifying models is yet to be proven unequivocally. But according to the studies of Dejam et al., the existence of the permeable porous medium generally enhances the solute transport in the channel.

To the best of our knowledge, an experimental study has not been reported yet on the preparation of MMMs including ZIF-95 in the literature. In this study, a specific ZIF-95 structure was synthesized by taking into account the previously reported results about the atomic level simulations and separation performance of porous membranes. After that, MMMs were prepared based on Matrimid®5218 polymer and nanoporous zeolitic imidazolate frameworks (ZIF-95). Mixed matrix membranes containing these mixtures of materials were used to evaluate separations of important natural gases and hydrogen synthesis (e.g. H₂, CH₄, CO₂). The goal of this study was to exploit the high surface area, regular porosity, adsorption capability, and selectivity of the nanoporous ZIF-95. The experimental section describes the synthesis of the ZIF-95 and mixed matrix membrane materials and pinhole-free membrane preparation. Thermal and morphological properties of membranes were also characterized by Fourier-Transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA) methods. Permselectivity performances of MMMs were then investigated for CO₂/CH₄ and H₂/CH₄ separation. Finally, a discussion section listing the scientific and experimental findings and their conclusions is included.