A robust microporous metal-organic framework constructed from a flexible organic linker for highly selective sorption of methanol over ethanol and water

Article abstract of DOI:10.1039/c2jm15604k

A microporous metal-organic framework MOF 1 [Zn(L)₂(methanol) ₂] (H₂L = (1R,3S)-1,2,2-trimethyl-3-(pyridin-4- ylcarbamoyl)cyclopentanecarboxylic acid) constructed from a flexible Zn ²⁺ node and a flexible organic linker L was obtained by solvothermal reaction of diamond topology, and exhibits highly selective sorption of methanol over ethanol and water.

Full text of DOI:10.1039/c2jm15604k

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PAPER
A robust microporous metal–organic framework constructed from a flexible
organic linker for highly selective sorption of methanol over ethanol and
water†
b
ab
a
Qian Huang,a Jianfeng Cai,a Hui Wu,cd Yabing He, Banglin Chen
and Guodong Qian
*
*
Received 2nd November 2011, Accepted 30th March 2012
DOI: 10.1039/c2jm15604k
A microporous metal–organic framework MOF 1 [Zn(L)2(methanol)2] (H2L ¼ (1R,3S)-1,2,2-
trimethyl-3-(pyridin-4-ylcarbamoyl)cyclopentanecarboxylic acid) constructed from a flexible Zn2+
node and a flexible organic linker L was obtained by solvothermal reaction of diamond topology, and
exhibits highly selective sorption of methanol over ethanol and water.
a very powerful methodology to tune the small pores within
porous MOFs to direct their highly selective gas/vapor separa-
tion.4 However, framework interpenetration is typically not the
default choice when we assemble rigid SBUs and organic linkers
into their framework structures, mainly because the spatial
arrangements of these SBUs and organic linkers have enabled
them to construct porous MOFs with moderate pore spaces
which cannot provide enough empty space to hold the second
framework. Although framework interpenetration does occur in
some cases, most of such interpenetrated frameworks tend to
have condensed structures because of multiple framework
interpenetration. The importance of porous MOFs with small
pores has motivated the community to explore more method-
ologies to synthesize such porous MOFs with small pores for
their highly selective gas/vapor separation. One of the questions
posted to the community might be: whether or not the isolated
single metal nodes and flexible less conjugated organic linkers
will also have the capacity to construct porous MOFs with
small pores and thus to direct their highly selective gas/vapor
separation. We define such less conjugated organic linkers as
flexible ones because they can easily have conformational
changes, leading to contracted and expanded pores of the
synthesized porous MOFs under thermal/vacuum activation
(generally termed as framework flexible dynamics).3h During
our research on chiral organic linkers for the construction of
nonlinear optical metal–organic frameworks (chiral space
groups are the pre-requisite for nonlinear optical properties), we
developed a new flexible organic linker (1R,3S)-1,2,2-trimethyl-
3-(pyridin-4-ylcarbamoyl)cyclopentanecarboxylate (L) to have
both pyridyl and carboxylate groups for their coordination with
metal ions/clusters to form MOFs. We accidentally realized
a rare example of porous MOF 1 [Zn(L)2(methanol)2] built
from an isolated single Zn2+ as the node and L as the organic
linker of the heavily distorted diamond dia topology for highly
selective sorption of methanol over ethanol and water at room
temperature.
1. Introduction
It took almost one decade to realize a few porous coordination
polymers (PCPs) and/or metal–organic frameworks (MOFs)
whose gas/vapor sorption isotherms were established during the
1990s.1 This is mainly because ‘‘Nature abhors the vacuum’’,
thus the as-synthesized coordination polymers are easily
collapsed to form condensed ones once the solvent molecules
have been removed during thermal and/or vacuum activation.
The establishment of permanent porosities in many porous
MOFs during the 2000s has indicated that the rigid metal-
containing clusters (typically termed as secondary building units
(SBUs)) and highly conjugated rigid organic linkers have played
crucial roles to stabilize the framework structures and thus to
sustain the permanent porosities. Since then, tremendous
research has been carried out to incorporate suitable rigid SBUs
and organic linkers for the construction of porous MOFs and
thus for their important applications in gas storage,2 separa-
tion,3–23 heterogeneous catalysis,24 sensing3c and drug delivery.25
Among the diverse porous MOFs, those with small pores of less
ꢀ
than 4 A are particularly useful for gas/vapor separation. In this
regard, framework interpenetration has been demonstrated as
aState Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor
Materials and Applications, Department of Materials Science
Engineering, Zhejiang University, Hangzhou 310027, China. E-mail:
gdqian@zju.edu.cn
bDepartment of Chemistry, University of Texas at San Antonio, One
UTSA Circle, San Antonio, Texas 78249-0698, USA. E-mail: banglin.
chen@utsa.edu; Fax: (+1) 210-458-7428
&
cNIST Center for Neutron Research, Gaithersburg, Maryland 20899-6102,
USA
dDepartment of Materials Science and Engineering, University of
Maryland, College Park, Maryland 20742, USA
† Electronic supplementary information (ESI) available. CCDC
reference numbers 826452 and 870505 (guest free MOF). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2jm15604k
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2.4 Single-crystal X-ray crystallography
2. Experimental section
Crystal structure analyses: Diffraction data were collected on an
Oxford Xcalibur Gemini Ultra diffractometer with an Atlas
detector using graphite monochromatic Mo-Ka radiation (l ¼
2.1 Materials and methods
All the chemicals were commercially available and used without
further purification. Infrared spectra (IR) were recorded on
a Thermo Fisher Nicolet iS10 spectrometer using KBr pellets.
Elemental analyses for C, H, and N were performed on an
EA1112 microelemental analyzer. Powder X-ray diffraction
(PXRD) patterns were collected in the 2q ¼ 5–60ꢀ range on an
ꢀ
0.71073 A) at 293 K. The structures of MOF 1 was solved by
direct methods and refined with full-matrix least-squares on F2
with the SHELXL-97 program package. Crystal data for MOF
1: molecular formula: C32H46N4O8Zn, formula weight: 680.10,
ꢀ
ꢀ
tetragonal, P43212, a ¼ 14.3035(3) A, b ¼ 14.3035(3) A, c ¼
ꢀ
X’Pert PRO diffractometer with Cu-Ka (l ¼ 1.5420 A) radiation
3
ꢀ
ꢀ
15.9627(6) A, V ¼ 3265.8(2) A , Z ¼ 4, T ¼ 293(2), crystal size ¼
0.32 ꢂ 0.29 ꢂ 0.21 mm3, Dc ¼ 1.383 g cmꢁ3, F(000) ¼ 438, 14844
reflections collected, 2989 independent reflections (Rint ¼ 0.0543)
which were used in all the calculations. After the refinement
cycles, reliability factors were R1 ¼ 0.0616, wR2 ¼ 0.1256 for
[I > 2s(I)], and R1 ¼ 0.0648, wR2 ¼ 0.1270 for all 2989 data.
CCDC 867452 and 870505.
at room temperature. Thermogravimetric analyses (TGA) were
carried out on a TA Instruments SDT Q600 with a heating rate
of 10 C minꢁ1 in N2 atmosphere. Luminescence spectra for the
ꢀ
solid samples were recorded with a Hitachi F4500 fluorescence
spectrometer. The photomultiplier tube voltage was 700 V and
the scan speed was 240 nm minꢁ1. The slit widths were both
2.5 nm for excitation and emission spectra.
3. Results and discussion
2.2 Synthesis of H2L
3.1 Synthesis
To a suspension of PCl5 (8.32 g, 40 mmol) in petroleum ether at
0 ꢀC was added D-camphoric acid (4.74 g, 20 mmol) in portions at
0 ꢀC for 5 h. The solvent was evaporated under reduced pressure
to give the crude product as a pale yellow oil (2).27 A suspension
of 3-pyridinamine (7.52 g, 80 mmol) in CH2Cl2 at 0 ꢀC was added
in portions. The solution was allowed to stir at 0 ꢀC for 5 h and
then warmed to room temperature and allowed to stir overnight.
Then the precipitate was filtered off, the solvent in filtrate was
removed in vacuo and the residue was purified by silica gel
column chromatography using ethyl acetate as the eluent to
The free organic linker H2L can be straightforwardly synthesized
from the precursor D-camphoric acid in three steps as shown in
Scheme 1. The as-synthesized MOF 1 can be readily obtained by
solvothermal reaction ꢀof Zn(NO3)2$6H2O and L in a methanol/
water mixture at 100 C for 48 h as colourless crystals. It was
formulated as [Zn(L)2(MeOH)2] by elemental analysis and
single-crystal X-ray diffraction studies, and the phase purity of
the bulk material was independently confirmed by powder X-ray
diffraction (PXRD).
1
afford a pure white solid (3). H NMR (CDCl3):d 8.24(s, 1H),
8.21(s, 1H), 7.27(t, 1H), 7.15(t, 1H), 2.82(t, 1H), 2.03(m, 1H),
1.93(m, 1H), 1.90(m, 2H), 1.16(s, 3H), 1.04(s, 3H), 0.944(s, 3H).
3.2 Crystal structure descriptions
The X-ray single crystal structure of MOF 1 features a three-
dimensional framework structure in which each Zn2+ is tetrahe-
drally coordinated by four organic linker L whose pyridyl
nitrogen and carboxylate oxygen are involved in the coordina-
tion (Fig. 1a). The overall structure is a three-dimensional
ꢀ
A mixture of 3 and triethylamine was stirred at 50 C for 24 h.
The precipitate was filtered off and the solvent was removed in
vacuo to give the product (1R,3S)-1,2,2-trimethyl-3-(pyridin-4-
ylcarbamoyl)cyclopentanecarboxylic acid (H2L) as a white solid
(80% yield). 1H NMR (DMSO): d 10.06(s, 1H), 8.41(d, 2H),
7.59(d, 2H), 2.92(t, 1H), 2.46(m, 1H), 2.04(m, 1H), 1.76(m, 1H),
1.42(m, 1H), 1.19(s, 6H), 0.78(s, 3H). 13C NMR (DMSO):
179.748, 173.589, 144.045, 141.305, 136.551, 126.424, 123.842,
56.709, 55.074, 46.361, 34.115, 24.501, 23.606, 22.856, 21.841.
MS (ESI), exact mass calcd for C15H20N2O3 [M + H]+, 277.2.
Found 277.1; [M ꢁ H]ꢁ, 275.2, Found 275.0. HRMS (ESI); exact
mass calcd for C15H20N2O3 [M + H]+, 277.1547. Found:
277.1540.
ꢀ
framework exhibiting irregular cages of 3.82 ꢂ 7.12 A (Fig. 1b)
which can hold two methanol molecules per cage (Fig. 1c). The
framework topology can be considered as a highly distorted
diamond one. The void space accounts approximately 15.3% of
the whole crystal volume calculated by PLATON analysis.
3.3 Thermal properties and powder X-ray diffraction studies
To investigate the thermal stability of the framework of 1,
thermogravimetric analyses (TGA) were carried out. As shown
2.3 Synthesis of MOF 1
A mixture of Zn(NO3)2$6H2O (0.119 g, 0.4 mmol), H2L (0.0552
g, 0.2 mmol), methanol (3 mL) and water(0.5 mL) was sealed into
a 20 mL Teflon cup. The vessel was heated at 100 ꢀC for 2 days.
After the mixture was slowly cooled to room temperature,
colorless block crystals were obtained in 60% yield. Elemental
analysis (%). Calcd for C32H46N4O8Zn: C, 56.51; H, 6.82; N,
8.24. Found: C, 56.60; H, 6.32; N, 8.11. IR (KBr, cmꢁ1): 3413(s),
2964(s), 1671(s), 1610(w), 1584(s), 1540(s), 1479(w), 1429(s),
1396(w), 1358(s), 1328(w), 1305(w), 1275(s), 1197(s), 1175(s),
1130(s), 1040(s), 821(s), 699(s), 647(w).
Scheme 1 Synthesis of the organic linker H2L.
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Fig. 1 Single-crystal X-ray structure of 1, showing (a) the isolated single
Zn2+ is coordinated by two pyridyl nitrogen and two carboxylate oxygen
atoms from four organic linkers L, (b) the resulting three-dimensional
ꢀ
framework exhibiting irregular cages of about 3.82 ꢂ 7.12 A viewed
along the c axis, (c) each cage can hold two methanol molecules and (d)
the highly distorted diamond topology (color scheme: C, gray; H, white;
O, red; Zn, pink).
in Fig. S1,† TGA analysis indicates that methanol molecules can
be readily released in the temperature range of 25–160 ꢀC to form
ꢀ
guest-free phase MOF 1a, which is stable up to 260 C.
Fig. 2 The excitation (dashed line) and PL spectra (solid line) of MOF 1
(red), L in solid state (black) and in ethanol solution (10ꢁ4 mol lꢁ1) (blue)
(upper), and vapor sorption isotherms of the MOF 1a for methanol (red),
ethanol (blue) and water (green) (bottom; solid symbols: sorption; empty
symbols: desorption) at 298 K.
Rietveld structural refinement indicates that the activated
MOF 1a keeps the basic framework structure with slight
3
shrinking from the original unit cell volume of 3265.8 A in MOF
ꢀ
3
ꢀ
1 to 3232.9 A in MOF 1a, and all the methanol molecules have
been completely removed from the pores during the activation.
Once the methanol is re-adsorbed into the activated MOF 1a, the
structure is recovered into the original MOF 1 (see ESI† for the
details). These studies indicate that the framework is robust and
the adsorption of MOF 1a for methanol is reversible. A small
amount of secondary impurity phase(s) was observed after
activation (see ESI†), however, the impurity phase has little effect
on the structure restoration from MOF 1a to MOF 1 after
re-adsorption of methanol and on the solvent selectivity of this
new MOF material.
useful microporous material for the separation of methanol over
ethanol and water. Separation of methanol over water might
be of more practical importance among these two potential
applications, and can be utilized to generate high purity meth-
anol by releasing the adsorbed methanol molecules from MOF
materials. Such highly exclusive sorption of methanol over
ethanol and water might be attributed to matching of methanol
molecules into the small cages within MOF 1a which has
enhanced the much stronger interactions between host porous 1a
and guest methanol molecules.
3.4 Luminescent and adsorption properties
The robustness of this framework constructed from a flexible
single metal ion node and flexible organic linker is very unusual.
It highlights the potential to make use of these flexible building
blocks to construct robust porous MOF materials with small
pores and thus for their highly selective gas/vapor separation.
Robust MOFs are those whose structures remain the same after
thermal/vacuum activation (generally can be established by
single and/or powder X-ray diffraction studies) and the activated
ones can take up gas/vapor molecules.
Upon excitation at 276 nm, MOF 1 exhibits intensive blue
luminescence centered at lmax of 321 nm (Fig. 2(upper), red)
which is significantly stronger than the free organic linker H2L in
the solid state (black) and in dilute solution of ethanol (blue).
Such luminescence enhancement might be due to the coordina-
tion of the organic linker L to the Zn2+, which increases the
rigidity of the ligand and thus reduces the loss of energy by
radiationless decay.
The most important and interesting feature of MOF 1 is the
highly selective sorption of methanol over ethanol and water. As
shown in Fig. 2(bottom), the activated MOF 1a (the as-synthe-
sized MOF 1 was thermally activated at 120 ꢀC under high
vacuum overnight to generate activated MOF 1a) takes up
significantly different saturated amounts of methanol, ethanol
and water of 101, 2 and 7 cm3 gꢁ1, respectively, at 298 K. This is
really remarkable, highlighting this new MOF as a potentially
It needs to be mentioned that most of the MOFs generated
from flexible units are easily collapsed and thus non-porous as
exemplified in one of the first such MOFs, Cu(I)[4,40,400,4000-tet-
racyanotetraphenylmethane]-BF4$xC6H5NO2.1b The very few
examples of robust MOFs from such flexible metal ion nodes and
organic linkers are [CuSiF6(4,40-bipyridine)2]$8H2O (single Cu2+
as the flexible node with rigid organic linker 4,40-Bipy)1f and
CuG(4,40-bipyridine)0.5 (G ¼ glutarate; rigid paddle-wheel
10354 | J. Mater. Chem., 2012, 22, 10352–10355
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Cu2(COO)4 node with rigid organic linker 4,40-Bipy and flexible
organic linker glutarate).26 Close structure examination indicates
that the multiple hydrogen bonding and van der Waals interac-
tions among the organic linkers have played important roles in
stabilizing the framework. This accidental discovery in this
reported MOF might motivate more comprehensive research
endeavors on the construction of robust porous MOFs from
a variety of flexible metal ion nodes and organic linkers, and then
on their applications for highly selective gas/vapor separation in
the near future.
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Acknowledgements
We are grateful for financial support for this work from
PCSIRT, the National Natural Science Foundation of China
(Nos. 50625206, 50928201, 50972127 and 51010002). This work
was also partially supported by an Award AX-1730 from Welch
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