Abstract
Mixed
matrix composite membranes (MMCMs) hold great potential to realize
efficient CO2 removal from natural gas. However, the
reduction of separation performance arising from the interfacial
defects, significant plasticization and aging effect in the thin films
severely limit their application. Herein,
we fabricated a series of
polyimide MMCMs with
MOF-protruding structure wherein amino-functionalized ZIF-8 nanocrystals
nearly penetrate the thin selective layer.
Through engineering the
interfacial interactions, e.g., covalent or hydrogen bondings, we
successfully fabricated defect-free MMCMs with the thickness ranging
from 140 to 280 nm. The stronger interfacial interactions eliminate the
interfacial defects and restrict the mobility of polymer chains under
high pressure. Accordingly, the MMCM displays a high CO2permeance of 778 GPU and a CO2/CH4selectivity of 34 with significantly improved resistance to
plasticization and aging. Considering the superior performance, we
anticipate our work could provide guidelines on designing advanced MMMs
to tackle critical separations.
Keywords: Natural gas
purifications, mixed matrix membranes, thin-film composite membranes,
interfacial interactions.
Introduction
Efficient CO2 removal from natural gas
has been regarded as one of the
most important processes in modern chemical industries because
the presence of
CO2 can reduce the calorific value and make the natural
gas streams acidic and corrosive.1-3 Among
CO2 separation processes, membrane technologies hold
great potential to siginificantly increase the energy efficiencies and
reduce the environmental impacts compared with traditional separation
technologies such as adsorption and absorption.3-8 To
date, polymeric membranes is dominating the membrane separation market
because of the ease of processing polymers into thin selective
membranes.2,9-11 Nevertheless, the separation
performance of polymeric membranes suffers from the trade-off effect, as
depicted by the Robeson plot for typical gas pairs including
CO2/CH4.12-14 In this
context, it is essential to develop membranes that can outperform
conventional polymeric membranes.
Metal-organic framework (MOF) based mixed matrix membrane (MMM), which
combines the advantages of molecular-sieving capability of microporous
fillers and solution processibility of polymers, has been widely
researched as an effective approach to surpass the performance
upper-bound of polymeric membranes.14-21Recently, MMMs with MOFs
penetrating matrix have attracted extensive attention because the
fillers are expected to dominate the membrane separation performance
which approaches to that from pure MOF crystals.22-25For example, Huang and co-workers fabricated highly H2permeable MMMs with ZIF-7 microplates penetrating the polymer matrix and
yielded a 14-fold higher H2 permeance and 9-fold higher
H2/CO2 selectivity,
respectively.22 Our group has proposed a MMM with
ZIF-8 microcrystals nearly penetrate the polyimide matrix and form a
“Direct-through Channel” structure which achieves nearly 70% of
C3H6/C3H8separation performance from pure ZIF-8 crystals according to the
calculation results of the Maxwell model.24 Overall,
MMMs with MOF-penetrated structure are still in its infancy stage and
usually a self-standing membrane
with large thickness is fabricated in most studies.
For practical application, mixed
matrix composite membranes (MMCMs) with thin selective layers are much
more attractive attributing to the fact that the thin selective layer is
expected to minimize the trans-membrane resistance and thus maximize the
gas permeance.26-28 Till now, fabricating ultrathin
selective films with nano-sized MOF penetrated the selective layer has
been rarely reported. Wang’s group developed MMMs with penetrated
oligomer-grafted MIL-101 nanoparticles serving as gas transport channels
in rubbery poly-vinylamine (PVAm) matrix, with a CO2permeance of 823 GPU and a CO2/N2selectivity of 242 under humidified conditions.23
Glassy polymeric membranes have
excellent mechanical/chemical stabilities which benefit the operation
under extreme conditions such as high
pressure.10,11,29-32 However,
the fabrication of
high-performance MMCMs using glassy polymer as matrix remains elusive.
In theory, there are two critical issues remaining unresolved. Firstly,
non-selective defects or pinholes
can be easily generated at the interfaces when the membrane thickness
reduces to sub-micrometer or nanometer scale due to the weak interfacial
interactions between the fillers and rigid chains of glassy polymers
which are detrimental to the separation
performance.26,27 Secondly, the plasticization and
aging effect would become more serious for thin-selective layer, owing
to the enhanced chain mobility of polymers, leading to drastic decrease
of the overall separation performance.29,33,34
To overcome the above-mentioned issues, we develop a series of MMCMs,
containing MOF-protruding structure wherein the MOF crystals nearly
penetrate the thin selective layer. The thickness of the selective layer
can be reduced to 140-280 nm through engineering the interfacial
hydrogen or covalent bonds, as illustrated in Fig. 1. Attributing to the
affinity of amine groups to CO2 and low permeation
resistance of the selective layer with protruding MOFs, the MMCM yields
a CO2 permeance of 778 GPU which is three-fold higher
than that of polyimide membrane with a
CO2/CH4 selectivity of 34, and
demonstrated improved resistance to plasticization (up to 40 bar) and
aging performance (for 28 days). Considering the superior
CO2 separation performance as well as the good
resistance to plasiticization and aging, we anticipate that the MMCMs
hold great potential for real applications in natural gas purification
processes.