water permeance did not exhibit large variation
(less than a factor of 3). Relatively high gas per-
meances can be attributed to the presence of
nonselective defects that allow water and gases
to pass at comparable rates (Fig. 1C). High-
density, stabilized seeds on the surface resulted
in the growth of a continuous membrane layer
on the support surface, whereas the penetrated
small seeds in the support pores facilitated
the growth of the crystals in the pores and
subsequent seamless merging. These two inter-
grown layers ensure the high quality of the
WCM with negligible defects and thus high
water/gas selectivities.
To further understand the mechanism of gas
blockage by the WCM, we performed single-
gas permeation measurements for gas molecules
with different sizes after membrane degass-
ing at 200°C for at least 3 days to exclude the
influence of adsorbed water in the zeolitic
nanochannels. Even without adsorbed water in
NaA nanochannels, gas permeances in single-
gas permeation (fig. S8 and table S3) were in
thesamerange(10–^10 to 10–^9 mol m–^2 s–^1 Pa–^1 )
as those in the mixture separation (Fig. 1D and
table S1); this finding suggested that adsorbed
water in mixture separation had negligible
influence on gas permeation and that NaA
zeolite nanochannels effectively blocked gas
permeation.
We then conducted a thorough comparison
of pure gas and water permeabilities with results
on NaA zeolite membranes reported within
thepast20years(Fig.1E).Tohaveafaircom-
parison, we used gas permeability, calculated
as the product of gas permeance and mem-
brane thickness. The pure gas permeabilities
of the WCM were two to three orders of mag-
nitude lower than the reported results and
completely dropped out of their respective per-
meability zones. However, the water perme-
ability of the WCM was two to three orders of
magnitude higher than that of gases and was
well within the water permeability zone, com-
parable to the reported results. These findings,
combined with mixture separation results, in-dicate that the WCM has very high quality and
negligible defects. Moreover, the WCM also ex-
hibited good thermal stability in dry gas up to
400°C (table S4) and good water stability up
to 300°C (fig. S9).
The accepted channel diameter of the NaA
structure, dominated by the 8-oxygen ring (Fig.
2A), is 4.2 Å, as calculated from the interatomic
distance of two opposing oxygen atoms across
the ring ( 15 ). However, Na+, by neutralizing
the negatively charged NaA framework (Fm 3 c
space group witha= 24.555 Å, unit cell com-
position of Na 96 Si 96 Al 96 O 384 ) and positioning
inside zeolite nanocavities with three locations
(Fig. 2A), partially blocks the nanochannels and
decreases the effective aperture size ( 16 , 17 ). We
further speculate, on the basis of our permeation
results, that molecules entering these nano-
channels can be influenced strongly by Na+.The
passage of small, polar water molecules to the
zeolitic nanochannel could be facilitated by Na+,
whereas the passage of larger and less polar mo-
lecules, such as H 2 and CO 2 , would be hindered,Liet al.,Science 367 , 667–671 (2020) 7 February 2020 2of5
Fig. 1. Rationally designed preparation strategy and separation/
permeation properties of water-conduction membrane (WCM).
(A) Schematics of different preparation routes. In route a, 50- to 200-nm
seeds are fixed at high loading density onto and into the support through
dehydration of surface hydroxyl groups as illustrated, for growth of WCM-a
with defects largely suppressed, whereas in route b, these seeds are used
directly for growth of membrane (M-b) with defects. In route c, the seeds
are diluted by a factor of 2 or 10 relative to route a for growth of M-c-02
and M-c-10, respectively. In route d, larger seeds (300 to 400 nm and 400
to 700 nm) are fixed onto and into the support through dehydration for
growth of M-d-300 and M-d-400, respectively. (BandC) Molecular transport
pathway through WCM-a (B) and through membranes prepared by routes
b to d (C). (D) Separation performance of membranes prepared by different
routes for H 2 O/CO 2 /CO/H 2 /MeOH mixture with composition of 1.77 ± 0.14%
/ 23.52% / 0.98% / 73.50% / 0.23 ± 0.02% at 250°C and 21 bar. The
minimum H 2 O/H 2 ,H 2 O/CO, and H 2 O/MeOH selectivities of the WCM are
estimated from the detection limits of GC. The fraction ratio at the top of
each column represents the selectivity ratio from top to bottom. Error bars
denote SD. (E) Permeability of pure gases and water through the WCM at
25° to 200°C and 2 to 8 bar (solid red symbols) and comparison with previous
results (open symbols) ( 23 – 44 ). The shaded area is the permeability zone
of gases and water, based on reported results. Error bars denote SD.RESEARCH | REPORT