A method
to predict the gas permeability of supported ionic liquid
membranes (SILMs) was established, using as input the pore structure
characteristics of asymmetric ceramic membrane supports and the physicochemical
properties of the bulk ionic liquid (IL) phase. The method was applied
to investigate the effect of IL nanoconfinement on the CO2 and N2 permeability/selectivity properties of novel SILMs
developed on nanofiltration (NF) membranes employing for the first
time the 1-ethyl-3-methylimidazolium and the 1-butyl-3-methylimidazolium
tricyanomethanide ILs as pore modifiers. The selected ILs exhibit
low viscosity, which allows for faster gas solvation rates and ease
of synthesis/purification that makes them attractive for large-scale
production. In parallel, the use of ceramic supports instead of polymeric
ones presents the advantage of operation at elevated temperatures
and pressures and offers the possibility to study the “real”
permeability of the confined IL phase, avoiding additional contributions
from the gas diffusion through the surrounding solid matrix. The developed
SILMs exhibited enhanced CO2 permeability together with
high CO2/N2 separation capacity, though with
distinct variations depending on the alkyl chain length of the 1-alkyl-3-methylimidazolium
cation. Application of the developed methodology allowed discriminating
the contribution of the NF pore structural characteristics on the
SILM performance and unveiled the subtle interplay of diverse IL confinement
effects on the gas permeability stemming from the specific layering
of ion pairs on the nanoporous surface and the phase transition of
the IL at room temperature, dictated by small variations of the IL
cation size.
Zeolitic imidazolate framework ZIF-69 membranes were grown on porous α-alumina substrates via seeded secondary growth and further functionalized by a CO 2 -selective tricyanomethanide anion/alkylmethylimidazolium cation-based ionic liquid (IL) to plug the gaps between the ZIF crystals yet leave the framework pores open for gas diffusion. In this configuration, ZIF intergrain boundaries and defects were repaired by a medium that exhibits high selectivity for CO 2 . As a result, the selectivity of the hybrid membrane was significantly higher than that of as-grown ZIF membranes and, because of the existence of the ZIF channels, the permeability was higher than that corresponding to bulk IL. Specifically, CO 2 permeated 20 times faster than N 2 through the intact ZIF pores and 65 times faster than through the bulk IL phase. The developed membranes at room temperature and under a 2 bar transmembrane pressure exhibited CO 2 permeance of 5.6 × 10 −11 and 3.7 × 10 −11 mol m −2 s −1 Pa −1 and real CO 2 /N 2 selectivities of 44 and 64 for CO 2 /N 2 mixtures consisting of 44% and 75% (v/v) CO 2 , respectively. In addition, on the basis of the experimental evidence from the hybrid membranes, predictions were made on the expected performance of an ideal, crack-free, and homogeneous ZIF-69 membrane. This work provides a promising solution to the challenges associated with defect formation experienced during growth not only of ZIFs but also of other zeolite and inorganic membranes used for CO 2 separation.
Membranes
consisting of ultrathin, oriented, single-wall carbon
nanotube (SWCNT) micropores with a diameter of ∼4 Å were
developed. c-Oriented AFI-type aluminophosphate (AlPO)
films (AlPO4-5 and CoAPO-5), consisting of parallel channels
7.3 Å in diameter, were first fabricated by seeded growth on
macroporous alumina supports, and used as templates for synthesis
of CNTs inside the zeolitic channels by thermal treatment, utilizing
the structure directing agent (amine) occluded in the channels as
carbon source. Incorporation of CNTs inside the AFI channels altered
the transport mechanism of all permeating gases tested, and imposed
a substantial increase in their permeation rates, in comparison to
the AlPO4-5 membrane, despite the pore size reduction due
to nanotube growth. The enhancement of the permeation rates is attributed
to repulsive potentials between gas molecules and occluded nanotubes,
which limit adsorption strength and enhance diffusivity, coupled to
the smooth SWCNT surface that enables fast diffusion through the nanotube
interior. Separation ability, evaluated with respect to H2 and CO2 gases, was enhanced by using polysterene as defect-blocking
medium on both AlPO and CNT/AlPO membranes and was preserved after
CNT growth.
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