is 1.27 Å away from the center, which is con-
sistent with previous reports ( 17 , 18 ); in the
presence of H 2 O, H 2 ,andCO 2 , respectively,
stable Na+positions are ~1.33 Å away from the
center (fig. S10). At its stable positions, in con-
trast to the case without Na+in the 8-oxygen
ring, the introduction of Na+decreases the ad-
mission energy barrier for H 2 O but increases
the barrier for CO 2 and H 2 (fig. S11 and table
S5), supporting our hypothesis. Specifically, in
Fig. 2B, the DFT calculation shows a substan-
tial energy decrease (1034 meV) when a polar
H 2 O molecule adsorbs onto the aperture. The
corresponding decreases were 300 meV for the
less polar CO 2 molecule and 98 meV for non-
polar H 2 , indicating a much more favorable
interaction between Na+and H 2 O than with
CO 2 and H 2.
The admission energy barriersDEare
255 meV for CO 2 , 38 meV for H 2 ,and3meV
for H 2 O. Whereas H 2 O encounters a negligible
energy barrier when entering the aperture,
apparently resulting from the favorable inter-
action between polar H 2 OandNa+and its small
size, H 2 and CO 2 must overcome higher energy
barriers, especially for larger CO 2 ,toenterthe
aperture. When further entering thea-cage,
H 2 O shows the largest energy drop (200 meV),
followed by H 2 (68 meV); for CO 2 ,energy
input (229 meV) is needed to enter the cage.
The favorable interaction between H 2 O and
thea-cage also facilitates its fast passage
of the 8-oxygen ring. Overall, both the admis-
sion into the 8-oxygen ring and subsequent
entry into thea-cage energetically favor the
H 2 O molecule.
We applied a WCM to a reaction for the
production of liquid fuels (Fig. 3A). We loaded
copper-zinc-alumina (CZA) catalysts (fig. S12),
which have been commercialized for metha-
nol production, on the outer surface of a WCM
to catalyze CO 2 hydrogenation for methanol
production at 21 to 35 bar and 200° to 250°C
and gas hourly space velocity (GHSV) between
5100 and 10,500 ml gcat–^1 hour–^1. The CZA
catalyst was reduced in pure H 2 in situ at 250°C
and atmospheric pressure for 10 hours with
a ramping rate of 2°C/min in a homemade
apparatus [membrane reactor (MR); fig. S13].
For comparison, the performance of the same
catalysts without incorporation of the WCM
[traditional reactor (TR)] was also evaluated
with the same catalyst packed in a nonperme-
able dense tube with the same dimensions as
the WCM. Among all results obtained (Fig.
3B and figs. S14 and S15), CO 2 conversion (up
to 61.4%) exceeded the equilibrium conver-
sion ( 19 ) after incorporation of the WCM and
was 2.6 to 3.0 times that obtained without
the WCM.
Because NaA zeolite has no catalytic activity
for CO 2 hydrogenation ( 20 ) and only CO 2 and
H 2 were detected with only the WCM in the re-
actor at 35 bar and 250°C, effective in situ water
removalbytheWCMwastheonlyreason
for the drastically increased CO 2 conversion;
moreover, it was the only reason why the ther-
modynamic equilibrium limitation on CO 2 hy-
drogenation was overcome. The adsorption of
water on the catalyst was greatly inhibited by
the low water concentration (less than 2 mol %)
inside the MR, which should have enhanced
the kinetics of the reaction and catalyst activity.
Indeed, the methanol yield (25.1% to 38.9%)
in the MR was 2.8 to 3.5 times that in the TR,
with no obvious difference in product selec-
tivity (figs. S16 and S17). Correspondingly, the
space-time yield (STY) of methanol was highly
promoted, for example, from 339 mg gcat–^1 hour–^1
in the TR to 809 mg gcat–^1 hour–^1 in the MR
with GHSV of 10,500 ml gcat–^1 hour–^1 at 250°C
and 35 bar (fig. S18), the highest value ever re-
ported under similar conditions.
Moreover, because of the in situ water re-
moval (by 95%) from methanol, very-high-purity
methanolwasdirectlyobtainedbysimplycon-
densing liquid products after the reactor—for
example,95.9wt%intheMRversus54.3wt%
in the TR at 230°C and 35 bar (figs. S19 and
S20). The purity can be further increased by
optimizing the MR (see detailed analysis in
supplementary materials). This process could
be expected to save a considerable amount of
energy in purification processes such as dis-
tillation. The selectivities of H 2 Ooverother
components during the reaction (fig. S21) were
consistent with the mixture separation results
(fig. S6).We performed stability tests at the highest
methanol yield (TR, 14.0%; MR, 39.8%) under
conditions of 220°C and 35 bar. We obtained
a minor decrease (4%) of CO 2 conversion (57.2%
to 54.8%) and methanol yield (39.8% to 38.1%)
in the MR, as well as very stable high-purity
methanol (~95 wt %) production for more than
100 hours (Fig. 3C); by contrast, in the TR, in
which catalysts were exposed to large amounts
of product water, we obtained a 10% decrease
of CO 2 conversion (22.7%to 20.4%) and a 20%
decrease of methanol yield (14.0% to 11.2%)
(fig. S22). These results suggest excellent stab-
ility of our WCM under reaction conditions
and effective protection of catalysts from water
poisoning and catalyst sintering. Moreover, no
obvious difference between SEM images of
the WCM (fig. S23) and Brunauer-Emmett-
Teller (BET) surface areas of catalysts (table S6)
before and after reaction supported the above
observation.
We tested our WCM for potential use at
largerscalebyassemblingthree210-mm-long
membranes (Fig. 3D, lower left inset) and load-
ing ~2.8 g of catalysts in one reactor. We ran
bench-scale methanol synthesis by drawing
vacuum on the permeate side (Fig. 3D, upper
inset) to generate driving force for water per-
meation at 250°C and 35 bar (movie S1). CO 2
conversion of 41.0% to 57.6% and methanol
yield of 21.9% to 30.3% were achieved with
the long MR comparable to those for the short
MR; the methanol production rate of the long
MR approached 50 g/day with an averageLiet al.,Science 367 , 667–671 (2020) 7 February 2020 4of5
Fig. 4. Comparisons of catalytic results in the MR using WCMs in this study with TR results from
literature in terms of CO 2 conversion and methanol space-time yield.Data points 1 and 12 are from
( 45 ); 2, ( 46 ); 3, ( 47 ); 4, ( 48 ); 5, ( 49 ); 6, ( 50 ); 7, ( 51 ); 8, ( 52 ); 9, ( 53 ); 10 and 11, ( 54 ); 13, ( 55 ); 14, ( 56 );
15, ( 57 ); 16 to 19, ( 58 ); 20, ( 59 ). Orange stars, TR in this study; purple stars, MR in this study.RESEARCH | REPORT