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.