INTRODUCTION
Excessive emission of carbon dioxide causes severe environmental issues, e.g. global warming.1,2 Carbon capture, realized by separating CO2 from other gases, e.g. H2, N2, and CH4, has been considered as a feasible strategy to reduce CO2emission. Various technologies, including absorption and membrane separation, are proposed for CO2capture.3-8 Membrane technology has been attracting intensive attention because of its high efficiency, simple operation, environmental friendliness, etc.7-11 Polymeric membranes with high processibility, low energy requirement, and relatively cheap production cost are widely applied in CO2 separation. However, the well-known trade-off between selectivity and permeability hampers the further development of those membranes.12,13 Mixed matrix membranes (MMMs) are formed by continuous phase of polymeric matrixes and dispersed phase of particles.14 The incorporation of porous particles not only offers additional transport channels but also adjusts free volumes by changing polymer arrangements, thereby leading to the improvement of permeability and selectivity.14,15
Many kinds of materials have been applied as fillers to prepare MMMs, e.g. graphene oxide, porous carbons, and metal organic framework (MOFs).16-19 Benefiting from the large surface areas, unique adsorption features, and tailorable apertures, MOF materials are commendable candidates for fabrication of membranes with excellent separation performance.20-28 Gascon’s group substantially improved the capture performance of polyimide membranes by introducing CuBDC nanosheets.29 Jiang et al.prepared the ZIF-8 hollow nanoparticles based MMMs for efficient CO2 capture.30 An ideal MMM with desirable property generally consists of uniformly dispersed fillers and excellent polymer/filler compatibility. The MOF sizes have great influence on the microstructures of MMMs. By reason of the large filler/matrix interfacial areas, nano-sized MOF particles are preferred for preparing MMMs.31-33 Japip et al.demonstrated that the MMMs incorporated with 200-nm ZIF-71 nanoparticles (NPs) exhibited better selectivity than that contained 600-nm fillers.34 However, the excessive surface energy of small NPs may result in filler aggregation. Comparatively, incorporation of large microfillers encounters lower risk of aggregation. Furthermore, the microparticles in MMMs can provide more consecutive transport pathways for gas permeation than nanofillers.35,36
Although the great progresses have been achieved in preparation of MOF-based MMMs, the enhancement of MOF/polymer interactions is still ungently desired for better separation. The weaker binding forces between MOFs and matrixes are more likely to cause the invalid defects, which provide the non-selective pathways and deteriorate the selectivity of MMMs.37 This phenomenon will be further aggravated for the MMMs with large fillers. In order to finely control the interfacial defects, proper modification of MOF fillers is exploited as the key step for ameliorating filler dispersion and remolding interfacial combination.38 The modification is performed through coating or grafting by additional appropriate molecules on MOF surface or at unsaturated metal centers and functional linkers.39-43 Jin et al. incorporated polydopamine-modified ZIF-8 into polyimide (PI) to prepare the MMMs with excellent dispersion and compatibility.39 Shojaeiet al. grafted NH2-UiO-66 NPs by poly methyl methacrylate (PMMA) to reinforce the interfacial affinity of MMMs.41 Li et al. utilized the chelating effect between the metal nodes of MOFs and the ester groups of crosslinked polyethylene oxide (XLPEO) to strengthen the interfacial interactions of MMMs.42 Although chemical modification can ameliorate the dispersion of NPs and improve the interfacial affinity of MMMs, the complicated modification procedures, special requirements to active sites, and risks of blocking MOF pores limit the extensive application.
The large fillers with good compatibility to matrixes, obtained by simple, effective, and versatile synthesis, are highly needed for simultaneously enhancing the permeability and selectivity of MMMs. Herein, we reported employing polymer-embedded MOF (pMOF) microspheres (MSs) as fillers, which were fabricated by one-pot synthesis, to improve the separation performance of MMMs (Figure 1). By adding polymer during crystallization, the polymer chains could be embedded into the MOF materials and could change the MOF NPs to MSs. Because the same polymer embedding as matrix promoted the compatibility of membranes, the prepared MMMs with pMOF fillers displayed improved selectivity for CO2 capture. Meanwhile, since the micrometer-sized MSs offered highly efficient gas transport channels, the permeability of the MMMs greatly increased.