various other sorbents (table S8) ( 10 , 11 , 13 – 16 , 22 ).
Volumetric adsorption capacity is essential
for many practical applications such as sta-
tionary CO 2 capture and pressure swing ad-
sorption (PSA) separation ( 7 , 36 ). Fe-MOR
(0.25) showed the highest volumetric CO 2 uptakes
of 293/219 cm^3 cm−^3 (273/298 K, 1 bar; table
S6) reported, outperforming previous efficient
adsorbents (Fig. 4A and table S8) ( 36 , 37 ). CO 2
adsorption capacity at low pressure, which is
crucial for gas separation, is challenging for
physisorption because of the lower sorption
strength relative to chemisorption ( 10 , 36 , 38 ).
Herein, steep CO 2 sorption isotherms were ob-
served for the Fe-MOR(n) series. Fe-MOR(0.25)
exhibited the steepest ones, affording ultra-
high-CO 2 uptakes at 0.1 bar: 4.55 mmol g−^1
(234 cm^3 cm−^3 ) at 273 K and 3.07 mmol g−^1
(173 cm^3 cm−^3 )at298K(tablesS5andS6).
Large CO 2 uptakes still remained at 0.01 bar—
2.81 mmol g−^1 (145 cm^3 cm−^3 )at273Kand
1.84 mmol g−^1 (103 cm^3 cm−^3 ) at 298 K (tables
S5 and S6)—and were higher than some of
the record adsorbents previously reported
{e.g., 137 cm^3 cm−^3 of [Mg 2 (dobdc)(N 2 H 4 )1.8] at
298 K and 0.15 bar} ( 36 , 38 ).
Fe-MOR(0.1-1.0) monoliths exhibited mark-
edly low Ar, N 2 , and CH 4 uptakes, which grad-
ually decreased with increasing Fe content (fig.
S34). The uptake sequence of CO 2 >>Ar>N 2 >CH 4
is opposite to the kinetic diameter order of CO 2
(3.3 Å)<Ar (3.4 Å)<N 2 (3.64 Å)<CH 4 (3.8 Å),
implying size-exclusive molecular sieving with
high CO 2 /Ar(N 2 or CH 4 ) selectivities (tables S5
and S7). Particularly, ideal adsorbed solution
theory (IAST) calculations (figs. S63 to S65)
predicted the adsorption equilibrium for
the binary mixtures CO 2 /N 2 :15/85 and CO 2 /
CH 4 :50/50, which mimicked the postcombus-
tion CO 2 capture and biogas treatment applica-
tion, respectively ( 8 , 12 ). Fe-MOR(0.25) afforded
very high CO 2 /N 2 and CO 2 /CH 4 IAST selectiv-
ities (10^16 ~10^132 , 0.1~1 bar; fig. S65), greatly
outperforming Fe-MOR(0) (fig. S64), Com-MOR
(fig. S63), and previous efficient physisorbents
(Fig. 4B and table S8). The steep and high CO 2
uptake plus smooth and low N 2 (CH 4 ) sorption
caused a large uncertainty in the IAST cal-
culation, which was attributable to the devia-
tion of the gas mixture from an ideal behavior
[for this reason, only the selectivities of Com-
MOR, Fe-MOR(0), and Fe-MOR(0.25) were cal-
culated] ( 39 ). Despite this, such extraordinary
values highlighted the high selectivities of
Fe-MOR(0.25). Fe-MOR(n) modulated N 2 and
CH 4 adsorption according to their sizes, afford-
ing higher N 2 sorption than CH 4 even at room
temperature (fig. S34). Furthermore, high CO 2 /
Ar selectivities were found over Fe-MOR(n)
(tables S5 and S7), reaching the physical sepa-
ration of CO 2 and Ar with a kinetic diameter
discrepancy of only 0.1 Å.
We also measured the CO 2 ,N 2 , and CH 4
sorption isotherms of Fe-MOR(0.25)-NH 4 Cl
with framework Fe species but non-narrowed
MORmicrochannels (figs. S66 to S74). Fe-
MOR(0.25)-NH 4 Cl demonstrated a slightly
higher CO 2 uptake than Fe-MOR(0.25) (6.10
versus 5.68 mol g−^1 , 273 K, table S5). However,
the CO 2 /N 2 (CH 4 ) selectivities of the former
were much lower than those of the latter (figs.
S65 and S75 and tables S5 and S7). This com-
parison highlighted the crucial role of pre-
cisely narrowed microchannels in the full play
of molecular sieving. Furthermore, the higher
CO 2 (CH 4 )butlowerN 2 uptakes of Fe-MOR
(0.25)-NH 4 Cl relative to Fe-MOR(0) suggested
that the framework Fe species could enhance
the affinity toward the polar C=O/C−H bonds
of CO 2 /CH 4 but not that of the nonpolar N≡N
bond of N 2 (see details in table S9 and supple-
mentary text).
Adsorption behavior at moderate tempera-
tures (313-348 K) is essential for practical ap-
plications in the pre- and postcombustion CO 2
capture ( 3 , 10 , 15 ). Conventionally, adsorp-
tion capacities and selectivities in many CO 2
physisorptive processes rapidly drop with in-
creasing temperature above 303 K ( 10 ). Selec-
tivities of Fe-MOR(n) at 298 K were higher than
or at least close to those at 273 K (tables S5 and
S7), reflecting the preservation of separation
ability at elevated temperatures. Compared
with Fe-MOR(0), Fe-MOR(0.25) still exhibited
high CO 2 uptakes (137/111 cm^3 cm−^3 at 323/373 K,
1 bar) and good CO 2 /N 2 (Ar, CH 4 ) selectivities
at 323 and 373 K (tables S5 to S7).
Isosteric heats of CO 2 adsorption (Qst) as a
function of CO 2 uptakes were analyzed (figs.
S76 and S77). Com-MOR had a low constant
Qstof ~20 kJ mol−^1 for weak physisorption (fig.
S77D). Fe-MOR(0) presented a lower initialQst
than Com-MOR but with an increasing slope
(fig. S76A), giving largerQstat high coverages.
TheQstprofiles of Fe-MOR(0.1 to 1.0) were
similar to that of Fe-MOR(0), and the higher
initialQstcorresponded to the enhanced CO 2
affinity by Fe incorporation (fig. S76). This was
further reflected by the combined Rietveld re-
finements of CO 2 -loaded samples for Fe-MOR(0)
and Fe-MOR(0.25) (figs. S78 to S81). The upward
trend ofQstwith increasing coverage reflected
the enhanced adsorbate-adsorbate interaction
in an energetically“homogeneous”pore struc-
ture and could be ascribed to the formation of
CO 2 clusters inside the channels ( 36 , 40 ). Such
Qstbehavior facilitated both adsorption and
desorption, i.e., the increasingQstalong with
adsorption promoted the adsorption at high
coverages, whereas the decreasingQstalong with
desorption enhanced the exit of gas molecules.
Sorption kinetics of Fe-MOR(0.25) were
analyzed by thermogravimetry in a simulated
flue gas mixture of CO 2 /N 2 :15/85 (fig. S82). Fe-
MOR(0.25) reached 50 and 90% of the satu-
rated CO 2 uptakes within 1.2 and 2.5 min,
respectively. We attributed this rapid adsorp-
tion to the strong CO 2 electrostatic interactionwith tetrahedral Fe species in the narrowed
aperture(Fig.1B).WhenCO 2 molecules entered
the channels, this interaction was weakened
by the larger interpore diameter and avoided
stronger adsorption, consistent with the rela-
tively low initialQst. As expected, desorption
also occurred rapidly and enabled regeneration
at 373 K within 10 min in a dynamic N 2 atmo-
sphere. The sorption on Fe-MOR(0.25)-NH 4 Cl
was considerably slower than Fe-MOR(0.25)
but faster than Fe-MOR(0) (fig. S82). Thus, the
structural character of Fe-MOR(0.25) allowed
adsorption and desorption of CO 2 under dy-
namic conditions, which led to a low energy
penalty. Reproducible CO 2 isotherms of Fe-
MOR(0.25) appeared in a 10-run test, with re-
generation in vacuum at 373 K for 15 min (fig.
S83). Furthermore, only a weak decrease of
the CO 2 uptake with consistently high CO 2 /
N 2 (CH 4 ) IAST selectivities was observed on
Fe-MOR(0.25)-C (fig. S84), hinting at poten-
tially useful regeneration stabilities and re-
cycling sorption.Separation studies
Column breakthrough experiments were per-
formed to evaluate the separation efficiency of
Fe-MOR(0.25) for the dry mixtures of CO 2 /N 2
(15/85) and CO 2 /CH 4 (50/50) at 273 K and at-
mospheric pressure (Fig. 4C). N 2 and CH 4
breakthrough occurred immediately, whereas
CO 2 was retained for a long time. Fe-MOR(0.25)
showed a much longer CO 2 -retaining time rela-
tive to Fe-MOR(0). Fe-MOR(0.25)-NH 4 Cl pro-
vided a longer CO 2 -retaining time but also a
lower separation efficiency (figs. S85 and
S86). The results further highlighted the im-
portance of precisely narrowed microchan-
nels for Fe-MOR(0.25) in practical separation.
Increasing the temperature from 273 to 298 K,
Fe-MOR(0.25) had enhanced separation effi-
ciency, as shown by the longer retention time
of CO 2 (Fig. 4D) attributable to the faster dif-
fusion of CO 2 in the pore of Fe-MOR(0.25) at
the higher temperature (fig. S87). The stable re-
cycling behavior in the CO 2 /CH 4 breakthrough
test confirms the good reversible sorption abil-
ity (fig. S88).
Column breakthrough separation for humid
CO 2 /N 2 (CH 4 )mixturesonFe-MOR(0.25)was
performed at 298 K and compared with two
top-performing adsorbents zeolite 13X and
Mg-MOF-74 (figs. S89 to S96). Breakthrough
curves of Fe-MOR(0.25) remained constant
with increasing relative humidity from 0 to
73% (Fig. 4D). The recyclability test in sepa-
rating highly humid gases demonstrated the
excellent moisture tolerance property of Fe-
MOR(0.25) (Fig. 4E). By contrast, notable in-
fluence of water vapor on the breakthrough
separations for the humid mixtures of CO 2 /
N 2 (CH 4 ) was observed on 13X and Mg-MOF-74
(figs. S93 to S96). This reveals that water vapor
has a negligible effect on the dynamic separationSCIENCEsciencemag.org 16 JULY 2021•VOL 373 ISSUE 6552 319
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