Industrial membranes comprised of a thin selective layer (<100 nm) requires a gutter layer (<100 nm) between the selective layer and the porous support to achieve high permeance for gas separation. The gutter layer materials must be carefully chosen to enhance overall membrane performance, i.e., high permeance and high selectivity. However, the experimental determination of the optimum gutter layer properties is very challenging. Herein we address this need using a three dimensional (3D) computational model to systematically determine the effects of the gutter layer thickness and permeability on membrane performance. A key finding is that the introduction of a gutter layer between the selective layer and porous support can enhance the overall permeance of the penetrant by up to an order of magnitude, but this gain is accompanied by an undesired decrease in selectivity. The analysis also shows for the first time that a maximum increase in permeance with negligible decrease in selectivity is realized when the thickness of the gutter layer is 1-2 times the pore radius. The modeling approach provides clear and practical guidelines for designing ultrathin multilayer composite membranes to achieve high permeance and selectivity for low-cost and energy-efficient molecular separations.
Gas
permeation through ultrathin film composite (uTFC)
membranes can be restricted by the pore size and porosity of
the porous supports, resulting in a reduction in permeance. Although
this geometric restriction has been demonstrated using empirical and
computational models, a systematic experimental validation of the
models is still lacking. This study addresses the gap by preparing
a series of uTFC membranes comprising glassy perfluoropolymers
(such as Teflon AF1600 and Hyflon AD80) as selective layers on top
of a commercial poly(ether sulfone) (PES) microporous support and
investigating the effects of the surface morphology and selective
layer thickness on the gas permeance. The geometric restriction resulting
from the porous support becomes more severe as the selective layer
becomes thinner. For example, the PES support decreased the gas permeance
of a 100-nm-thick Hyflon AD80 film by as much as 42%. The experimental
data agreed well with the modeling results, which convincingly confirms
that porous supports with high porosity and small pores are needed
to prepare high-flux uTFC membranes. This study also
provides a nonintrusive method for determining the pore size and porosity
of support surfaces, despite their great nonuniformity.
Graphene oxide (GO) nanosheets stacked in parallel with subnanometer channels can exhibit an excellent size-sieving ability for membrane-based gas separation. However, gas molecules have to diffuse through the tortuous nanochannels, leading to low permeability. Herein we demonstrate two versatile approaches to modify the GO (before membrane fabrication by vacuum-filtration) to collectively increase gas permeability, etching using hydrogen peroxide to generate in-plane nanopores and acidifying using hydrochloric acid. For example, a membrane prepared at a pH of 5.0 using the 4-h-etched GO (HGO-4h) shows He permeability of 5.3 Barrer and He/CH 4 selectivity of 800, which are 5 times and 1.5 times those of the GO membranes, respectively. Decreasing the pH from 5.0 to 2.0 for HGO-4h enhances He permeability to 57 Barrer and He/CH 4 selectivity to 1,800. The HGO-4h prepared at the pH of 2.0 exhibits separation properties of H 2 / CO 2 , H 2 /N 2 , He/N 2 , and He/CH 4 surpassing their corresponding upper bounds.
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