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COMMUNICATION
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Published on 03 February 2012 on http://pubs.rsc.org | doi:10.1039/C2CC17168F
Stepwise adsorption in a mesoporous metal–organic framework:
experimental and computational analysisw
Daqiang Yuan,za Rachel B. Getman,zb Zhangwen Wei,a Randall Q. Snurr*b and
Hong-Cai Zhou*a
Received 18th November 2011, Accepted 3rd February 2012
DOI: 10.1039/c2cc17168f
Stepwise adsorption in a metal–organic framework with both
micro- and meso-pores is caused by adsorbates first filling the
micropores, then adsorbing along the mesopore walls, and finally
filling the mesopores.
Metal–organic frameworks (MOFs) are porous crystalline
materials comprised of metal ions or cluster nodes connected
by organic ‘‘linkers’’.1 The structures can be tuned by combining
different nodes and linkers, leading to a variety of desirable
properties, such as controllable pore size and pore size
distribution,2 large surface areas,3 and tuneable chemical properties
at the pore walls.4 Additionally, MOFs can be designed to contain
special binding sites and functional groups such as coordinatively
unsaturated metal centres, which exhibit high gas affinity5 and are
useful in gas storage and separation.1d,6
Herein we report the structure and adsorption behaviour of
a new MOF, PCN-53 (PCN stands for porous coordination
network), which is composed of benzo-tris-thiophene carboxylate
(BTTC) linkers and Fe3O nodes. The structure of PCN-53 is
unusual among MOFs. Each of the metal cluster nodes has three
coordinatively unsaturated Fe sites, two of which are trivalent
and the other is divalent. In addition, the structure contains both
micro- and meso-pores, with diameters spanning from 9.5 to
22.2 Å; pore sizes are determined by the largest sphere that can
fit inside the pore without overlapping any of the van der
Waals radii of the framework atoms. PCN-53 exhibits rare
stepwise sorption isotherms for N2, CO2, and Ar, which are
ascribed to sequential filling of the pores of different sizes,
shown here by a combination of grand canonical Monte Carlo
(GCMC) simulation and gas adsorption measurements.
PCN-53 was prepared as red octahedral crystals by a solvothermal reaction between benzo-(1,2;3,4;5,6)-tris(thiophene-20 carboxylic acid) (H3BTTC, Fig. 1a) and (NH4)2Fe(SO4)26H2O
at 120 1C for 5 days. Synchrotron radiation X-ray diffraction
studies indicate that PCN-53 crystallizes in the Fd3% m space group
a
Department of Chemistry, Texas A&M University, College Station,
TX 77843, USA. E-mail: zhou@mail.chem.tamu.edu;
Fax: +1-979-8451595
b
Department of Chemical & Biological Engineering,
Northwestern University, Evanston, IL 60208, USA.
E-mail: snurr@northwestern.edu
w Electronic supplementary information (ESI) available: Full experimental and computational details. See DOI: 10.1039/c2cc17168f
z These authors made an equal contribution to this work.
This journal is
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The Royal Society of Chemistry 2012
Fig. 1 (a) The BTTC ligand. (b) The trinuclear iron SBU (red: O;
black: C; yellow: S; green: Fe; turquoise: H). (c) The octahedral cage-I.
(d) The octahedral cage-II. (e) Polyhedra 3D packing in PCN-53, the
yellow ball shows the mesoporous cavity. (f) Accessible surface in PCN-53.
with a unit cell dimension of 50.8 Å. From crystallographic
studies and elemental analysis, the formula is determined as
Fe3O(H2O)3(BTTC)2xS (S represents non-coordinated solvent
molecules). As seen in Fig. 1b, three Fe atoms are bridged by six
carboxylates and one m3-oxygen atom to form the well-known
trigonal prism secondary building unit (SBU) [Fe3O(O2CR)6].7
All three Fe atoms in the SBU are six-coordinate. Bond-valence
and Mössbauer spectroscopic analyses suggest that two of the Fe
ions are FeIII and one is FeII (Fig. S1 in ESIw). This mixed
valence oxo-iron cluster is rare in MOFs.
There are three types of cages in PCN-53. The first type is an
octahedron, with six SBUs at the vertices and six BTTC
ligands across the faces (Fig. 1c). It has triangle-shaped
windows with a m3-O–m3-O distance of 14.75 Å along the
edges and an internal diameter of 9.5 Å. The second type is
octahedral as well. It also has six SBUs at the vertices, but
unlike the first cage, it has twelve BTTC ligands spanning all
twelve edges and connecting to the vertices via two carboxylates
each (Fig. 1d). This cage has an internal diameter of 12.5 Å, and
the triangle windows have 14.75 Å edges. Each triangle face of
the second cage is connected to another triangle face of a
Chem. Commun., 2012, 48, 3297–3299
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neighboring octahedron, resulting in the third, mesoporous
cage, which has an internal diameter of B22.2 Å (yellow sphere
in Fig. 1e). The three types of cages pack in a 6 : 6 : 1 ratio to
form a porous three-dimensional network (see Fig. S22, ESIw).
In the three-dimensional network, the porous structure is also a
continuous percolated pore network (Fig. 1f) with a maximal
opening size of 16 Å in diameter, which is explored by the
Poreblazer V1.2 program.8 If the trinuclear iron SBU serves as
a 6-connected node and BTTC as a 3-connected node, the
structure of PCN-53 can be simplified as a 3,6-c 2-nodal net
with a topology symbol of (426)2(44648610), which represents
a new topology analyzed by the Topos 4.0 program.9 The
solvent accessible void fraction calculated using PLATON
(probe radius = 1.80 Å)10 is 73.1% in the activated structure
(after removal of the coordinated aqua ligands and solvent
molecules). The estimated BET surface area based on adsorption
data is 2817 m2 g1 (Fig. S5, ESIw), which matches very well with
the value of 2780 m2 g1 calculated geometrically from the
crystal structure. On the basis of the N2 sorption isotherm,
PCN-53 has a total pore volume of 1.57 cm3 g1.
Both the geometries of the ligand and trinuclear SBUs of PCN53 are similar to those of MIL-100, which has benzene tricarboxylate (BTC) linkers (BTTC can be viewed as an extended version of
BTC). Additionally, both materials crystallize in the same Fd3% m
space group,11 and it is conceivable that they would be isostructural.
However, PCN-53 possesses a topology quite different from that of
MIL-100. This reminds us of the symmetry-preserving isomerism
observed in our prior work,12 although the two MOFs are not
isomers of each other. Indeed, MIL-100 is constructed based on a
super-tetrahedron formed by four BTC ligands and four SBUs,
while such a super-tetrahedron cannot be found in the structure
of PCN-53, instead, a super-octahedron can be observed.
The presence of micro- and meso-pores, mixed-valence
tri-iron-oxo nodes, open Fe coordination sites at the nodes,
and structural pseudo-isomerism with MIL-100 make PCN-53
an interesting and complex structure. Because of this, we were
prompted to examine the gas uptake behaviour of this MOF.
The activated sample of PCN-53 showed a rare, nearly
reversible, three-step adsorption isotherm for N2 at 77 K
(adsorption isotherms are provided in Fig. 2a, desorption
isotherms are provided in Fig. S4, ESIw). This sorption
behaviour represents a rare ‘‘Type VI’’ isotherm that cannot
be explained by simple multilayer adsorption. In general, steps
in rigid MOFs can be attributed to sorbate–sorbent interactions
as molecules adsorb along the framework surfaces and sorbate–
sorbate interactions as they fill the pores.13 Some flexible MOFs
exhibit stepwise isotherms due to pore ‘‘breathing’’, i.e. opening
and closing of the pores as sorbates adsorb and desorb from
the framework. Experiments and simulations indicate that this
behaviour is due to sorbates sequentially accessing different
pores within the framework as they breathe.14 Reports of rigid
MOFs exhibiting isotherms with more than two steps are rare
and only involve MOFs with a variety of pores and different
pore sizes.3a,15 For example, Jaheon Kim et al. reported a
MOF containing micropores and mesopores that showed a
nearly reversible stepwise isotherm.15b
We find that PCN-53 exhibits triple step isotherms for N2,
CO2, and Ar. Experimental and simulated isotherms for these
gases are shown in Fig. 2a, c, and d. Our simulations capture
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Chem. Commun., 2012, 48, 3297–3299
Fig. 2 Experimental (blue) and simulated (red) adsorption isotherms
for N2 at 77 K (a), N2 at 77 K using a logarithmic pressure axis (b),
CO2 at 195 K (c), and Ar at 87 K (d). Isotherm step labels in (b) are for
the simulated data.
the stepwise behaviour for all three adsorbates, and agreement
with experiment is particularly good for CO2. Differences
between the simulated and experimental isotherms can be
attributed to uncertainty in the sorbate–sorbent interactions
in the simulated model. We find that if we slightly decrease the
magnitude of these interactions, we get better agreement with
experiment (see Fig. S21, ESIw). The first steps in the simulated
isotherms for Ar and N2 occur at very low pressures and are
not evident from the graphs in Fig. 2. We include isotherms
plotted on logarithmic pressure scales in Fig. 2b and Fig. S19
(ESIw) to show all three steps better.
To understand the filling mechanisms in PCN-53, we plotted
snapshots from the N2 simulations at various pressures
(Fig. 3). The filling mechanisms for CO2 and Ar are identical,
and snapshots are provided in Fig. S16–S19 (ESIw). We find
that sorbate molecules adsorb first in the smaller octahedral
cages (snapshot a in Fig. 3) and then in the larger octahedral
Fig. 3 Simulated N2 isotherm at 77 K with snapshots showing the
filling mechanism. N2 fills the octahedral cages I and II first (a and b).
It then adsorbs along the mesopore walls (d and c) before completely
filling the mesopores (e). Snapshots a, b, c, d, and e were taken at 0.1,
0.4, 2, 70, and 105 mmHg.
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principally supported by the NSF/DOE under grant number
NSF/CHE-0822838. Use of the Advanced Photon Source was
supported by the U. S. DOE, Office of Science, Office of Basic
Energy Sciences, under Contract No. DE-AC02-06CH11357. We
thank Prof. Paul A. Lindahl and Dr Ren Miao for Mössbauer
spectra measurement. Some of the calculations were performed
using the National Energy Research Scientific Computing Centre.
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Published on 03 February 2012 on http://pubs.rsc.org | doi:10.1039/C2CC17168F
Notes and references
Fig. 4 Simulated isosteric heat of adsorption (Qst) for N2 in PCN-53,
calculated at 77 K. Letters a–e represent the same pressures as in Fig. 3.
cages (snapshot b). The first steps in the isotherms are due to
sorbate molecules filling these two cages. Sorbates then start
adsorbing in the mesopores, first adsorbing along the octahedron
faces (snapshot c) and then near the nodes (snapshot d). This
causes the second steps in the isotherms. Note that the simulation
results suggest that sorbate molecules sequentially and distinctly
fill these two regions, which is why there is a discernable ‘‘bump’’
in the second step of the simulated isotherm in Fig. 3. The
experimental isotherm does not exhibit this bump, suggesting
that in the experimental system sorbate molecules have less of a
distinction for the octahedron faces and nodes. Finally, sorbates
fill the voids in the mesopores (snapshot e), resulting in the third
steps. The second and third steps are thus caused by sorbate–
sorbent interactions and sorbate–sorbate interactions, respectively,
as is typical in rigid MOFs.13,15c However, the first steps in the
isotherms are caused by adsorption in the micropores. Therefore,
the unique triple-step behaviour is caused by the presence of
micro- and meso-pores in the structure.
Simulated isosteric heats of adsorption (Qst), shown in Fig. 4 for
N2, suggest that each adsorption region within PCN-53 has a
distinct heat of adsorption. The initial heat, corresponding with
the first step in the isotherm, is due to N2 molecules adsorbing in
the small octahedral cages near the sulfur sites. The heat decreases
as N2 molecules fill the octahedral cages, but it increases slightly as
they become saturated due to favourable N2–N2 interactions in the
pores (a). When the octahedral cages saturate (b), nitrogen begins
adsorbing along the faces of the octahedra, and this marks a
dramatic drop in Qst (c), as N2 molecules are less constricted on
the pore surfaces than they are in the pores. Another dramatic
drop is seen when these sites saturate and nitrogen starts adsorbing
near the vertices (d). (However, as stated above, this is washed out
in the experimental isotherms.) After that, the heat increases as N2
molecules fill the mesopores due to favourable N2–N2 interactions
in the void (e). In agreement with our prior work,15c we find
inflections in the simulated isotherms that correspond with
increases in Qst (see Fig. S20, ESIw). However, these are obscured
by the multiple steps. In contrast to our prior work,15c the
dominant features of the isotherms of PCN-53 are the three
steps caused by adsorption in the differently sized pores.
This work was supported by the U. S. Department of Energy
(grants DE-FC36-07GO17033 and DE-FC36-08GO18137)
and the National Science Foundation (CBET-0930079).
The microcrystal diffraction of PCN-53 was carried out with
the assistance of Yu-Sheng Chen at the Advanced Photon Source
on beamline 15ID-B at ChemMatCARS Sector 15, which is
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