Rationally designed micropores within a metal-organic framework for selective sorption of gas molecules.

Abstract

A microporous metal-organic framework, MOF, Cu(FMA)(4,4'-Bpe)0.5 (3a, FMA = fumarate; 4,4'-Bpe = 4,4'-Bpe = trans-bis(4-pyridyl)ethylene) was rationally designed from a primitive cubic net whose pores are tuned by double framework interpenetration. With pore cavities of about 3.6 A, which are interconnected by pore windows of 2.0 x 3.2 A, 3a shows highly selective sorption behaviors of gas molecules.

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Rationally Designed Micropores within a Metal−Organic Framework for
Selective Sorption of Gas Molecules
Banglin Chen,*,†Shengqian Ma,‡Fatima Zapata,†Frank R. Fronczek,|Emil B. Lobkovsky,⊥and
Hong-Cai Zhou‡
Department of Chemistry, UniVersity of Texas-Pan American, Edinburg, Texas 78541-2999,
Department of Chemistry and Biochemistry, Miami UniVersity, Oxford, Ohio 45056, Department
of Chemistry, Louisiana State UniVersity, Baton Rouge, Louisiana 70803-1804, and Department of
Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York
14853-1301
Received August 30, 2006
A microporous metal−organic framework, MOF, Cu(FMA)(4,4’-Bpe)0.5(3a, FMA ) fumarate; 4,4’-Bpe ) 4,4'-Bpe
) trans-bis(4-pyridyl)ethylene) was rationally designed from a primitive cubic net whose pores are tuned by double
framework interpenetration. With pore cavities of about 3.6 Å, which are interconnected by pore windows of 2.0 ×
3.2 Å, 3a shows highly selective sorption behaviors of gas molecules.
Introduction
It has been challenging to rationally design and tune
micropores of porous materials for their selective sorption.1
As their selective sorption behaviors are mainly determined
by size-exclusive effects in which smaller molecules can go
through the microporous channels while larger substrates are
blocked, the exact control of the micropores is of significant
importance for their practical applications in gas separation
and purification. In fact, the breakthrough on the tunable
pores of Molecular Gate adsorbent has initiated industrial
innovation for the economical N2and CO2removal.2
Microporous metal-organic frameworks (MOFs)3have
been rapidly emerging as one unique type of porous material
due to their potential applications in gas storage,4separation,5
and catalysis.6The amenability to design, extraordinary
permanent porosity, and dynamics are characteristic of such
porous materials, leading to some porous materials superior
to traditional inorganic zeolite materials. Although extensive
efforts have been pursued to search for microporous MOFs
for selective sorption of gas molecules, only a few examples
have been reported which were discovered accidentally.7
Recently, we have been focusing on the rational design
strategies to tune micropores for their highly selective
sorption of small molecules. Initiated by the work of Seki,8a
Zaworotko,8dKim,8gand others who have incorporated a
paddle-wheel cluster M2(COO)4with a bicarboxylate and a
bidentate pillar linker to construct 3D porous primitive cubic
* To whom correspondence should be addressed. E-mail: banglin@
utpa.edu
†University of Texas-Pan American.
‡Miami University.
|Louisiana State University.
⊥Cornell University.
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Published on Web 01/20/2007
© 2007 American Chemical Society
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MOFs M(R)(L)0.5‚Gx (M2+) Cu2+and Zn2+, R ) bicar-
boxylate linker, L ) bidentate pillar linker, G ) guest
molecules),8we have successfully tuned the micropores to
about 4.0 × 4.0 Å in Zn(BDC)(4,4’-Bipy)0.5by making use
of framework interpenetration and have applied the first
microporous MOF into the gas chromatographic separation
of alkanes.5eFor separation and purification of smaller gas
molecules such as H2, Ar, N2, and CO2, such micropores
need to be further narrowed down, which we believe can be
simply and readily fulfilled by the different combination of
bicarboxylate R and bidentate pillar linker L and/or multiple
framework interpenetration. As illustrated in Scheme 1, the
pore void space within a doubly interpenetrated primitive
cubic framework (b) can be easily tuned and reduced by
incorporating a shorter pillar linker L and a shorter bicar-
boxylate R to form smaller pore void space in (c) and (d),
respectively. Herein, we report the rational design and
synthesis of three primitive cubic MOFs, Cu(FMA)(Pyz)0.5
(1), Cu(FMA)(4,4’-Bipy)0.5‚0.25H2O (2),8aand Cu(FMA)-
(4,4’-Bpe)0.5‚0.5H2O (3), which are self-assembled by the
incorporation of fumarate (FMA) with pillar linker pyrazine
(Pyz), 4,4-bipyridine(4,4’-Bipy), and 4,4'-Bpe ) trans-bis-
(4-pyridyl)ethylene(4,4’-Bpe), respectively. As revealed in
their single X-ray crystal structures and sorption studies, the
pore void spaces in these MOFs are systematically tuned
simply by changing the pillar linker L in which a mi-
croporous MOF 3 is rationally designed and constructed for
its selective sorption of gas molecules.
Experimental Section
Materials and Methods: All reagents and solvents employed
were commercially available and used as supplied without further
purification. TGA data were obtained on an TGA G500 V5.3 Build
171 instrument with a heating rate of 5 °C/min under a N2
atmosphere. Powder XRD patterns were obtained with a Scintag
X1 powder diffractometer system using KR radiation with a variable
divergent slit and a solid-state detector. The routine power was 1400
W (40 kV, 35 mA). Low-background quartz XRD slides (Gem
Depot, Inc., Pittsburgh, PA) were used. For analyses, powder
samples were dispersed on glass slides.
MOF3:
A solution of
0.285 mmol), H2FMA (0.0331 g, 0.285 mmol), and 4,4’-Bpe
(0.0260 g, 0.143 mmol) in H2O (25 mL) in a 30 mL vial was heated
at 100 °C for 24 h to form blue-green, block-shaped crystals (41
mg, 52%). Anal. Calcd for Cu(C4H2O4)(C12H10N2)0.5‚0.5H2O: C,
Cu(NO3)2‚2.5H2O(0.0663g,
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Scheme 1a
aSchematic illustration of (a), a paddle-wheel M2(COO)4cluster as an
octahedral node, to form (b), a doubly interpenetrated primitive cubic
framework whose pore void spaces can be reduced and tuned by (c), a
shorter pillar organic linker, and (d), a shorter bicarboxylate organic linker.
Chen et al.
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43.24; H, 2.90; N, 5.04. Found: C, 43.33, H, 2.74; N, 5.10%. MOFs
1-2 were synthesized similarly. MOF 3a: Sample of MOF 3 was
heated at 150 °C in vacuum for 24 h to get MOF 3a. TGA indicates
that there is no further weight loss.
Single-Crystal X-ray Crystallography: Intensity data for MOFs
1, 2, and 3a were collected using a Bruker X8 APEX II
diffractometer (Mo radiation) in a cold nitrogen stream. Data
collection and reduction were done using the Bruker Apex2 software
package. Intensity data for MOF 3 were collected using graphite-
monochromated Mo radiation on a Nonius KappaCCD diffracto-
meter fitted with an Oxford Cryostream cooler. Data reduction
included absorption corrections by the multiscan method. Structures
were solved by direct methods and refined by full-matrix least
squares, using SHELXL97. All non-hydrogen atoms were refined
anisotropically. CCDC-616534 (MOF 1), CCDC-616535 (MOF 2),
and CCDC-616536 (MOF 3) contain the supplementary crystal-
lographic data for this paper. The atomic coordinates for this
structure have been deposited with the Cambridge Crystallographic
Data Centre. The coordinates can be obtained, on request, from
the Director, Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge, CB2 1EZ, UK or at www.ccdc.cam.ac.uk/conts/
retrieving.html.
Gas Sorption Measurements: A Beckman Coulter SA3100
surface area analyzer was used to measure gas adsorption. In order
to remove guest solvent molecules in the framework, the fresh
sample soaked in methanol was filtered and vacuumed at 150 °C
overnight. Before the measurement, the sample was vacuumed again
by using the “outgas” function of the surface area analyzer for 2 h
at 150 °C. A sample of 90.0 mg was used for the sorption
measurement and was maintained at 77 K with liquid nitrogen and
at 195 K with an acetone/dry ice slush.
Results and Discussion
All three MOFs were synthesized by hydrothermal reac-
tions of Cu(NO3)2‚2.5H2O with the corresponding organic
linkers at 100 °C for 1 day as blue, block-shaped crystals.
As expected, the frameworks of MOFs 1-3 contain paddle-
wheel binuclear Cu2 units which are bridged by FMA
dianions to form a distorted 2D square grid {Cu2(FMA)2}.
These 2D square grids are further pillared by pyrazine, 4,4’-
bipyridine, and trans-bis(4-pyridyl)ethylene occupying the
axial sites of the Cu2paddle wheels to form 3D MOFs 1-3,
respectively (Figure 1). The topologies of 1-3 can be best
described as an elongated primitive cubic (R-Po) net. The
overall structures are a pair of identical R-Po nets which are
mutually interpenetrated with each other to form 2-fold
interpenetrated 3D frameworks. It is of particular interest to
Figure 1. Crystal structures of frameworks 1 (a, b), 2 (c, d), 3 (e, f), and
3a (g, h) showing doubly interpenetrated primitive cubic nets and corre-
sponding pore void spaces. For (h), 4,4’-Bpe pillar linkers are disordered,
and only one of the possible conformations is shown (Cu, ball; O, red; H,
white).
Figure 2. Gas sorption isotherms of 3a at (a) 77 K (H2, green; N2, blue;
Ar, magenta; CO, yellow) and (b) 195 K (CO2, red; CH4, pink; N2, blue).
Micropores for SelectiWe Sorption of Gas Molecules
Inorganic Chemistry, Vol. 46, No. 4, 2007 1235

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point out that the pore void spaces in these MOFs are
systematically tuned simply by changing the pillar linkers.
Crystal packing indicates that 1 is a nonporous condensed
framework (Figure 1b), while both 2 and 3 contain pore void
space of about 3.6 Å diameter to encapsulate one and two
water molecules, respectively, per doubly interpenetrated
framework unit (Figure 1c and e). The pore void spaces are
disconnected and blocked by the small pore apertures of
about 1.4 × 1.8 Å in 2. However, the pore apertures are
enlarged by incorporating longer pillar linker trans-bis(4-
pyridyl)ethylene in 3 instead of 4,4’-bipyridine as in 2; thus,
a rationally designed microporous MOF 3 with 1D channels
of about 2.0 × 3.2 Å to interconnect the pore cavities (Figure
1f), taken into account the van der Waals radius, is readily
constructed.9
The dehydrated 2 and 3 are thermally stable up to 250 °C,
as shown in their TGA studies, but they still keep crystalline
forms; thus, the X-ray crystal structure of dehydrated 3 (Cu-
(FMA)(4,4’-Bpe)0.5, 3a) was characterized. The X-ray crystal
structure of 3a shows that the framework keeps the same
connectivity, which is slightly shifted. Unlike desolvated Zn-
(BDC)(4,4’-Bipy)0.5, which is significantly compressed and
condensed, 3a is slightly expanded. The volume per formula
unit increases from 1153.4 (3) to 1174.78 Å3(3a), a 1.9%
expansion; thus, the pores become slightly larger (Figure 1h).
Single X-ray and PXRD studies (Supporting Information)
indicate that transformations between 3 and 3a are reversible.
Sorption studies show that dehydrated 2 does not take up
both H2and N2at 77 K, obviously because of the small pore
apertures of 1.4 × 1.8 Å that block the entrance of gas
molecules. However, 3a can take up a significant amount
of 91.8 cm3/g (STP) or 0.8 wt % H2 at 1 atm and 77 K,
exhibiting highly selective sorption to entrap H2(2.8 Å) and
to exclude Ar (3.4 Å), N2(3.64 Å), and CO (3.76 Å) (Figure
2a).10It needs to be noted that, at a higher temperature of
195 K, the pore apertures are enlarged under certain
pressures; thus, 3a absorbs a small amount of N2 (Figure
2b). Such aperture expansion might be attributed to the
framework dynamics in which higher temperature and
pressure will facilitate the entrance of guest molecules
through the pore channels.5eMOF 3a shows selective
sorption of CO2(3.3 Å) over CH4(3.8 Å) at 195 K (Figure
2b).10The different hysteretic sorption behaviors with respect
to H2, N2, CO2, and CH4are due to their different van der
Waals interactions with the host framework.8e
In summary, we have successfully synthesized a rare
example of porous MOFs with rationally designed mi-
cropores for selective gas sorption. It is worth noting that
such microporous materials are of special interest for their
potential applications for the separation and purification of
gas molecules by the pressure swing adsorption (PSA)
technique.11Further research on rational design and construc-
tion of a series of tunable microporous MOFs for PSA
separation of gas molecules is currently underway.
Acknowledgment.
University of Texas-Pan American through a Faculty Re-
search Council award (B.C.) and, in part, by the Welch
Foundation grant (BG-0017) to the Department of Chemistry
at UTPA.
This work was supported by the
Supporting Information Available: X-ray crystallographic data
of 3a, PXRD pattern, and TGA spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC0616434
(9) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(10) Beck, D. W. Zeolite Molecular SieVes; Wiley & Sons: New York,
1974.
(11) Yang, R. T. Gas Separation by Adsorption Processes; Butterworth:
Boston, MA, 1997.
Chen et al.
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