Molecular sieving realized with ZIF-8/Matrimid^® mixed-matrix membranes

Abstract

Zeolitic imidazolate frameworks (ZIFs), that have the potential for gas separation, were used as additives in mixed-matrix membranes (MMMs). ZIF-8, which exhibits the sodalite topology, is composed of zinc (II) ion clusters linked by imidazolate ligands. The ZIF-8 pore aperture is 3.4 Å allowing it to readily absorb small molecules such as H₂ and CO₂. ZIF-8/Matrimid^® MMMs were fabricated with loadings up to 80% (w/w), which are much higher than the typical loadings achieved with select zeolite materials. Only at the highest loading did the ZIF-8/Matrimid^® MMM show a loss of mechanical strength, leading to a decrease in flexibility. The ZIF-8/Matrimid^® MMMs permeability properties were tested for H₂, CO₂, O₂, N₂, CH₄, C₃H₈, and gas mixtures of H₂/CO₂ and CO₂/CH₄. The permeability values increased as the ZIF-8 loading increased to 40% (w/w). However, at higher loadings of 50% and 60% (w/w), the permeability decreased for all gases, and the selectivities increased consistent with the influence of the ZIF-8 additive. The ideal selectivities of gas pairs containing small gases, such as H₂/O₂, H₂/CO₂, H₂/CH₄, CO₂/CH₄, CO₂/C₃H₈, and H₂/C₃H₈, showed improvement with the 50% (w/w) ZIF-8 loading, demonstrating a transition from a polymer-driven to a ZIF-8-controlled gas transport process. In control experiments using as-synthesized ZIF-8 with filled pores, there was no transition at 50% (w/w) loading. This may be the first example of an MMM wherein molecular sieving is evident and suggests that additive loadings >50% (w/w) may be required to observe this effect in MMMs.

Introduction

Membrane gas separation technology continues to grow in importance due to advantages such as low capital and operating cost, minimum energy requirements, ease of operation, and environmental friendliness [1]. Membranes are now replacing more traditional gas separation processes, such as cryogenic distillation and absorption [2], [3], and their current applications include hydrogen separation, nitrogen recovery, oxygen and nitrogen enrichment, and natural gas purification [4]. Some significant membrane requirements include durability, mechanical stability at the operating conditions, and excellent permeability and selectivity [5], [6]. However, simultaneously obtaining high permeability and high selectivity remains a challenge. Polymeric membranes have been extensively studied for gas separation applications [7], [8], [9] but, despite efforts to improve polymer separation properties, current polymeric membrane materials have reached a limit in the tradeoff between permeability and selectivity [10]. Inorganic membranes, on the other hand, offer good thermal and chemical stability and high gas flux and selectivity, but are limited by fabrication costs [11]. Thus, finding new membrane materials suitable for industrial separations has become an essential research objective in recent years. The advent of inorganic-organic hybrid membranes combines the processability of polymers and the superior gas separation properties of inorganic materials. Such composites are referred to as mixed-matrix membranes (MMMs) [12], [13]. A desirable MMM consists of well-dispersed particles with as high a loading as possible. Polymers frequently used for commercial gas separations that may be adapted for mixed-matrix membranes include polysulfones, polyarylates, polycarbonates, poly(arylethers), poly(arylketones) and polyimides [10]. Polyimides are especially attractive due to their high gas selectivity and high chemical, thermal, and mechanical resistance [14], [15]. One such polyimide that has been studied extensively for gas separations and MMMs is Matrimid^® (Fig. 1), which has permeability and selectivity properties falling close to the upper bound region of various Robeson plots [16], [17].

Many materials have been used as the inorganic phase in MMMs, including carbon molecular sieves [18], [19], zeolites [20], [21], [22], mesoporous materials [23], activated carbons [24], carbon nanotubes [25], and metal-organic frameworks [26], [27]. While these materials have shown promise in MMM applications, there are still many challenges to overcome. A significant problem is the compatibility of the polymeric and inorganic phases for optimum dispersion and interfacial contact [28], [29] that only allows for moderate loadings of inorganic materials. For example, loadings only of up to about 40% (w/w) are typically achieved for zeolite additives. The low loading is primarily due to poor wetting or interfacial contact between the polymer and zeolite particles. This has been termed “sieve in a cage” [28], [30], and has been observed for several MMMs including those with zeolite 4A [21], [22], [28], [31], [32] and zeolite SSZ-13 [33]. Strategies to create better adhesion at the polymer-zeolite interface include silination using different silane coupling agents [13], [34], [35]. In addition, whisker-like structures formed on the zeolite surface have provided additional roughness for the interlocking of polymer chains with the zeolite [36]. However, these methods did not result in significant improvement in gas permeability and selectivity.

To provide for better wetting and dispersion, mesoporous materials have also been combined with polymers to form MMMs. The premise was that the chains of the polymer could penetrate into the large mesopores (2–30 nm) resulting in better interfacial contact. This concept was demonstrated by MCM-41/polysulfone MMMs, wherein gas permeabilities increased but selectivities remain unchanged [37]. A disadvantage of such membranes can be a loss of selectivity due to the large size of the mesopore. The loss in selectivity might be offset by incorporating micropores into mesoporous materials [38]. In these dual pore systems, the mesopores improve interfacial contact and the micropores provide size and shape selectivity. For example, MMMs were fabricated with a carbon aerogel having both micropores and mesopores in a Matrimid^® matrix. Higher ideal selectivities for H₂/N₂, O₂/N₂, and CO₂/CH₄ gas pairs relative to pure Matrimid^® were observed. The incorporation of nano-sized ZSM-5 embedded in a carbon aerogel to form carbon aerogel-ZSM-5/Matrimid^® MMMs also resulted in higher selectivity for CO₂/CH₄ separation [38].

Another class of porous materials that has been employed in MMMs is the metal-organic frameworks (MOFs). The functional groups of the organic ligands and the metal ions associated with the secondary building units (SBU) may facilitate interactions with the polymer. This new type of zeolitic material is expected to be more flexible in chemical design and diverse in pore size and shape [39], [40], [41]. MOFs are being studied extensively owing to their exceptionally high surface area, controlled porosity, functionalizable pore walls, affinity for specific gases, and flexible chemical composition due to the presence of strong chemical bonds and modifiable organic linking units [42]. These properties make them promising materials for gas storage, gas separations, and catalysis [43]. For gas storage and separation, MOFs can act as molecular sieves due to their rigid frameworks and finite pore sizes allowing for size exclusion of gas molecules [44], [45]. Our research group has explored the use of MOFs as additives in mixed-matrix membranes for gas separations [27], [46]. We fabricated MOF-5/Matrimid^® MMMs that showed increased permeability at high loadings while maintaining constant selectivity compared to Matrimid^® [46]. Cu-BPY-HFS in Matrimid^® showed increased solubility and selectivity for CH₄ gas [27].

Zeolitic imidazolate frameworks (ZIFs) are a sub-family of MOFs that also have tunable pore sizes and chemical functionality [47]. Moreover, ZIFs possess the exceptional chemical stability and rich structural diversity found in zeolites. ZIFs are based on metal imidazolates, and their structures are related to many zeolite frameworks [48]. For example, the zeolitic imidazolate framework number eight (ZIF-8) has sodalite (SOD) topology [39]. The structure of this material is shown in Fig. 2. The five-membered imidazolate ring creates a framework by bridging the Zn(II) centers to the N-atoms in the 1,3-positions of the ring. The 145° angle made by the metal-imidazolate-metal bond is similar to the Si–O–Si bond angle found in many zeolites [49]. In contrast to sodalite, which has no accessible pores, ZIF-8 has a pore aperature of 3.4 Å in diameter allowing it to readily absorb small gas molecules, such as hydrogen and carbon dioxide [41], while the pore cavity has a diameter of 11.1 Å [49]. An attractive feature of ZIF-8 is its apparent thermal stability up to nearly 400 °C. ZIF-8 also exhibits a high surface area of 1300–1600 m²/g [49]. Due to these properties, the ZIF-8 material was chosen as the inorganic additive in Matrimid^® polymer membranes. In this work, ZIF-8 was synthesized and ZIF-8/Matrimid^® MMMs were prepared and characterized as well as tested for their permeability and selectivity properties. The permeability data showed a shift from a polymer-driven to a ZIF-8-controlled gas transport process at high loadings [>50% (w/w)]. Selectivity data showed that the MMMs have an affinity for the transport of small gas molecules due to the sieving effect of ZIF-8.