CATALYSIS
Na
+
-gated water-conducting nanochannels forboosting CO 2 conversion to liquid fuels
Huazheng Li^1 , Chenglong Qiu^2 , Shoujie Ren1,3, Qiaobei Dong^1 , Shenxiang Zhang^1 , Fanglei Zhou^1 ,
Xinhua Liang^3 , Jianguo Wang^2 , Shiguang Li^4 , Miao Yu^1 *
Robust, gas-impeding water-conduction nanochannels that can sieve water from small gas molecules
such as hydrogen (H 2 ), particularly at high temperature and pressure, are desirable for boosting many
important reactions severely restricted by water (the major by-product) both thermodynamically and
kinetically. Identifying and constructing such nanochannels into large-area separation membranes
without introducing extra defects is challenging. We found that sodium ion (Na+)–gated water-
conduction nanochannels could be created by assembling NaA zeolite crystals into a continuous,
defect-free separation membrane through a rationally designed method. Highly efficient in situ water
removal through water-conduction nanochannels led to a substantial increase in carbon dioxide
(CO 2 ) conversion and methanol yield in CO 2 hydrogenation for methanol production.
P
rotein-constructed water channels ( 1 )or
ion channels ( 2 ) exist in all living or-
ganisms and allow fast permeation of
waterorspecificionswhilerejecting
other ionic species on the basis of size
exclusion and electrostatic repulsion. These
attributes have led scientists to attempt to
createsimilarchannelsasameansofin-
creasing the efficiency of industrial processes.
Experimental strategies to obtain high water
permeability while rejecting hydrated ions in
artificial systems have included the incorpo-
ration of protein-based ( 3 ) or polymer-based
artificial water channels ( 4 , 5 ) into a polymer
matrix or lipid layers, as well as packing of
graphene-based laminates ( 6 – 9 ). Very prom-
ising results in terms of ion rejection for
desalination have been successfully demon-
strated ( 6 , 7 ).
We explored whether nanochannels could
be fabricated that reject small gas molecules
approximately half the size of the hydrated
ions (for example, Na+,6.6Å)athightemper-
atures and pressures for applications in catal-
ysis. For example, by-product water strongly
inhibits the kinetics and thermodynamics of CO 2
hydrogenation to liquid fuels such as methanol
( 10 – 12 ). Gas-rejecting water-conduction nano-
channels could boost the reaction rate and
shift the equilibrium toward product forma-
tion by removing water while retaining reactant
gases and products ( 13 ). However, angstrom-
scale nanochannels that can distinguish water
molecules (kinetic diameter 2.6 Å) from gas
molecules as small as H 2 (kinetic diameter
2.9 Å) are very challenging to precisely engi-
neer. Further, the nanochannel would need to
be assembled without defects into a practical
separation membrane that could operate at
>200°C and >20 bar.
We report gas-impeding water conduction
of NaA zeolitic nanochannels assembled into
a centimeter-scale membrane with negligible
defects through a rationally designed method
(Fig. 1, A and B). Water conduction, which was
confirmed by experimental gas dehydration
at high temperatures and pressures, was likely
the result of a gating effect of Na+located in
the 8-oxygen ring apertures that regulated their
effective size, as supported by theoretical sim-
ulations (Fig. 2). The exceptional benefits of
such a water-conduction membrane (WCM)
were demonstrated for catalytic methanol syn-
thesis from CO 2 hydrogenation at elevated
temperatures (200° to 250°C) and pressures
(21 to 35 bar).
We designed a rational and effective process
(Fig. 1A, route a) to prepare WCMs. We dip-
coated a ceramic hollow-fiber support (length
300 mm, inner diameter 0.75 mm, outer di-
ameter 1.5 mm, pore size 400 nm; fig. S1) with
50- to 200-nm NaA crystals (fig. S2) and then
dried the support at 80°C for at least 3 hours.
Prior to membrane synthesis, the coated sup-
port was subjected to thermal annealing at
200°C overnight. During annealing, physically
loaded nanocrystals were chemically bonded
to the support surface and inside the pores
through dehydration of surface hydroxyl groups
of the zeolite and the support ( 14 ) to ensure
stable and adequate nuclei for inducing mem-
brane growth. After a single cycle of hydro-
thermal growth, membranes without visible
surface defects grew on the support (fig. S3).
The penetration of small crystals, also called
seeds, inside the 400-nm support pores also
induced membrane growth within the pores
and resulted in an indistinguishable boundarybetween the membrane layer and the support.
Themembranethicknesswas3to4mm, as
indicated by energy-dispersive x-ray spectros-
copy (EDX) (fig. S3).
We investigated the separation performance
of our WCM for a H 2 O/CO 2 /CO/H 2 /MeOH gas
mixture (MeOH, methanol) with composition
of 1.77 ± 0.14% / 23.52% / 0.98% / 73.50% / 0.23 ±
0.02% in our homemade apparatus (fig. S4) at
250°C and 21 bar. The mixture was generated
by bubbling a 75% H 2 / 1% CO / 24% CO 2 gas
mixture through a liquid tank containing
1.5 wt % MeOH in H 2 O at 70°C. The selectivity
(permeance ratio) of H 2 O/CO 2 for this mixture
(Fig.1DandtableS1)was~551±33.Themin-
imum selectivities of H 2 O/H 2 ,H 2 O/CO, and
H 2 O/MeOH were 190, 170, and 80, respectively,
as estimated from the detection limits of gas
chromatography (GC), because no H 2 ,CO,and
MeOHweredetectedonthepermeateside.
Relative to previously reported separation re-
sults (fig. S5 and table S2), our membrane
showed two to three orders of magnitude lower
gas permeances but comparable water perme-
ance, and thus considerably higher H 2 O/gas
selectivity, suggesting effective gas-permeation
blockage by the membrane layer (Fig. 1B).
At higher pressures (up to 38 bar), different
temperatures (200° and 250°C), and various
water concentrations (1 to 4.2 mol %), the WCM
maintained excellent mixture separation per-
formance(fig.S6).WhenabinaryH 2 O/gas mix-
ture was used as feed, the selectivity of H 2 O/CO 2
wasashighas10,722±222at250°Cand21bar
(table S1). The much higher CO 2 partial pres-
sure in the binary mixture did not lead to
a proportional increase of its flux and thus
drastically decreased CO 2 permeance (1.39 ×
10 –^11 mol m–^2 s–^1 Pa–^1 ), whereas water per-
meance was almost the same (1.49 × 10–^7 mol
m–^2 s–^1 Pa–^1 ). No obvious selectivity decline
(fig. S7) was detected throughout 12-hour tests,
suggesting high membrane stability.
High-density small seeds (50 to 200 nm) that
had been bonded to the support by annealing
were critical for high membrane quality, as
evidenced by the results of a series of compar-
ative experiments (Fig. 1D and table S1). Water/
gas selectivities near the as-reported values
(fig. S5 and table S2) were obtained after the
elimination of the annealing step (Fig. 1A,
route b), highlighting the importance of an-
nealing treatment. A marked decrease of
water/gas selectivities occurred when the 50-
to 200-nm seed suspensions were diluted by
factors of 2 and 10 (Fig. 1A, route c), indicating
that the initial high-density loading of seeds
was essential. When 300- to 400-nm or 400- to
700-nm seeds were applied, the accumulated
selectivities were a factor of 5 to 8 lower than
with the 50- to 200-nm seeds (Fig. 1A, route d).
The decrease of water/gas selectivity was
mainly the result of the increase of gas per-
meances (by factors of 60 to 300) because theRESEARCH
Liet al.,Science 367 , 667–671 (2020) 7 February 2020 1of5
(^1) Department of Chemical and Biological Engineering, Rensselaer
Polytechnic Institute, Troy, NY 12180, USA.^2 Institute of
Industrial Catalysis, State Key Laboratory Breeding Base of
Green-Chemical Synthesis Technology, College of Chemical
Engineering, Zhejiang University of Technology, Hangzhou
310032, P.R. China.^3 Department of Chemical and Biochemical
Engineering, Missouri University of Science and Technology,
Rolla, MO 65409, USA.^4 Gas Technology Institute, Des Plaines,
IL 60018, USA.
*Corresponding author. Email: [email protected]