Journal of Membrane Science 361 (2010) 28–37 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Molecular sieving realized with ZIF-8/Matrimid ® mixed-matrix membranes Ma. Josephine C. Ordo ˜ nez, Kenneth J. Balkus Jr., John P. Ferraris, Inga H. Musselman ∗ Department of Chemistry, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080, USA article info Article history: Received 13 October 2009 Received in revised form 7 June 2010 Accepted 10 June 2010 Available online 18 June 2010 Keywords: Mixed-matrix membranes Gas separation ZIF-8 Metal-organic frameworks Zeolitic imidazolate frameworks 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 H 2 and CO 2 . 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 H 2 , CO 2 ,O 2 ,N 2 , CH 4 , C 3 H 8 , and gas mixtures of H 2 /CO 2 and CO 2 /CH 4 . 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 H 2 /O 2 ,H 2 /CO 2 ,H 2 /CH 4 , CO 2 /CH 4 , CO 2 /C 3 H 8 , and H 2 /C 3 H 8 , 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. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Membrane gas separation technology continues to grow in importance due to advantages such as low capital and oper- ating cost, minimum energy requirements, ease of operation, and environmental friendliness [1]. Membranes are now replac- ing more traditional gas separation processes, such as cryogenic distillation and absorption [2,3], and their current applications include hydrogen separation, nitrogen recovery, oxygen and nitro- gen enrichment, and natural gas purification [4]. Some significant membrane requirements include durability, mechanical stabil- ity at the operating conditions, and excellent permeability and selectivity [5,6]. However, simultaneously obtaining high per- meability and high selectivity remains a challenge. Polymeric membranes have been extensively studied for gas separation applications [7–9] but, despite efforts to improve polymer sep- aration properties, current polymeric membrane materials have reached a limit in the tradeoff between permeability and selec- tivity [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 mem- ∗ Corresponding author. Tel.: +1 972 883 2706; fax: +1 972 883 2925. E-mail address: imusselm@utdallas.edu (I.H. Musselman). brane materials suitable for industrial separations has become an essential research objective in recent years. The advent of inorganic-organic hybrid membranes combines the processabil- ity 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. Poly- mers frequently used for commercial gas separations that may be adapted for mixed-matrix membranes include polysulfones, pol- yarylates, polycarbonates, poly(arylethers), poly(arylketones) and polyimides [10]. Polyimides are especially attractive due to their high gas selectivity and high chemical, thermal, and mechani- cal 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 [20–22], meso- porous materials [23], activated carbons [24], carbon nanotubes [25], and metal-organic frameworks [26,27]. While these materi- als have shown promise in MMM applications, there are still many challenges to overcome. A significant problem is the compatibil- ity of the polymeric and inorganic phases for optimum dispersion and interfacial contact [28,29] that only allows for moderate load- ings of inorganic materials. For example, loadings only of up to 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.06.017