and the signal atg= 2.0 to Fe species inside
the microporous channels (Fig. 3D) ( 33 – 35 ).
Onthebasisoftheseobservationsandtheunit
cell composition (table S3), combined Rietveld
refinements of Fe-MOR(n) were performed
by collecting the high-resolution powder XRD
and powder neutron diffraction patterns (Fig.
3, E to G; figs. S18 to S24; and table S3). At low
Fe content, the Fe species occupied preferen-
tially theMORframework T sites. Along with
theincreaseofFecontent,moreFespecieswere
located in the 12-MR microchannels ofMOR
zeolite. Partial plugging of Fe species inside
the microchannels caused intermittent nar-
rowing of the 1D 12-MR microchannel of the
MORframework to ~3.3 Å (Fig. 3D). Assum-
ing a random uniform distribution, the low
occupancy density of internal Fe species im-
plies that every several to several hundreds of
unit cells for Fe-MOR(n) series contains one
Fe species inside the microchannels (table S3).
Thus, the tens of micrometers long 1DMOR
pore system (corresponding to tens of thou-
sands of continuous unit cells) varied into
intermittently plugged microchannels, which
could be schematically described as a sequence
of interconnected tubes with narrowed orifices
(Fig.1,BandC).
Quantitative porosity was analyzed by N 2 ,
Ar, and CO 2 sorption experiments (figs. S25
to S27). Fe-MOR(0) demonstrated the typi-
cal type I sorption isotherms for micropores
( 26 , 27 , 34 ). Fe-MOR(0.1 to 1.0) exhibited high
CO 2 uptake but extremely low N 2 (Ar) uptake.
The surface areas of Fe-MOR (0.1 to 1.0) de-
tected by CO 2 sorption at 195 K were 218 to
302 m^2 g−^1 (fig. S27). Such phenomena sug-
gest that Fe-MOR(0.1 to 1.0) zeolites have well-
developed porosity possibly with narrowed
orifices that allow the entry of CO 2 with a
small kinetic diameter (3.3 Å) but retard larger
N 2 (3.64 Å) and even Ar (3.4 Å). Indeed, the
average pore size of Fe-MOR(0) from the
positron annihilation lifetime spectrum (PALS)
was 6.25 Å (fig. S28), near that of typicalMOR
zeolites ( 26 ). By contrast, Fe-MOR(0.25) showed
an apparently reduced average pore size of
5.31 Å. PALS-detected average pore size is re-
garded to be proportional to the accessible
volume in a 3D network ( 16 ), whereas the N 2 /
Ar sorption reflects the orifice variation of
microporous channels. These phenomena sug-
gest an ultrasmall orifice (kinetic diameter:
3.3 to 3.4 Å), matching the precisely narrowed
microchannel structure obtained from the
combined Rietveld refinement results (Fig. 3D).
Such a pore system not only allows the entry of
relatively small CO 2 molecules but also leaves
the unaltered large void volume inside the
channel usable for CO 2 storage (Fig. 1, B and C).
For comparison, another Fe-containingMOR
zeolite, Fe(Alkali)-MOR, was synthesized under
the conventional base co-hydrolysis and crys-
tallization conditions. In addition, with
F e -MOR(0) as the starting material, two counter-
parts, Fe(Ex)-MOR and Fe(Im)-MOR, wereprepared from the traditional ion-exchange
and wet-impregnation methods, respectively.
The three controls are typicalMORzeolites
having large surface areas with open micro-
pores, similar to the commercialMORzeolite
(Com-MOR) (figs. S29 to S33 and table S4),
although external framework–aggregated Fe
species dominated the controls (figs. S32 and
S33) ( 33 – 35 ). This comparison excludes the
possibility that the pore contraction of Fe-
MOR(0.1-1.0) is associated with the external
framework aggregated Fe species, and suggests
that the acid co-hydrolysis route for synthesiz-
ing Fe-MOR(n) described herein plays a vital
role in the formation of zeolite monoliths with
precisely narrowed microchannels.Sorption performance
Single-component gas (CO 2 ,Ar,N 2 , and CH 4 )
sorption isotherms of the aboveMORzeolites
were collected from 273 to 373 K and described
by either the double-site Langmuir isotherm
model or the Langmuir-Freundlich isotherm
model, with the calculation of CO 2 /Ar(N 2
or CH 4 ) selectivities (figs. S34 to S65 and
tables S5 to S7). CO 2 adsorption capacities of
Fe-MOR(0.1 to 1.0) monoliths greatly exceeded
those of Fe-MOR(0), Fe(Ex)-MOR, Fe(Im)-MOR,
Fe(Alkali)-MOR, and Com-MOR (figs. S34 to
S40). Fe-MOR(0.25) exhibited the highest grav-
imetric CO 2 uptakes of 5.68/3.89 mmol g−^1
(273/298 K, 1 bar; table S5), superior or
comparable to those of typical zeolites and318 16 JULY 2021•VOL 373 ISSUE 6552 sciencemag.org SCIENCE
Fig. 4. Gas sorption behavior.(A) Comparison of volumetric CO 2 uptakes at 298 K. (B)ComparisonofCO 2 /N 2 and CO 2 /CH 4 IAST selectivities at 1 bar and 298 K for
the binary mixture of CO 2 /N 2 :15/85 and CO 2 /CH 4 :50/50, respectively. (CandD) Experimental column breakthrough curves for dry (C) and humid (D) CO 2 /N 2 (CH 4 ) separations.
(E) Recyclability under humid CO 2 /CH 4 column breakthrough tests. (FandG) Schematic model (F) and simulation result (G) for the two-bed VSA process.
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