A rod-packing microporous hydrogen-bonded organic framework for highly selective separation of C2H2/CO2at room temperature

Article abstract of DOI:10.1002/anie.201410077

Self-assembly of a trigonal building subunit with diaminotriazines (DAT) functional groups leads to a unique rod-packing 3D microporous hydrogen-bonded organic framework (HOF-3). This material shows permanent porosity and demonstrates highly selective sep

Full text of DOI:10.1002/anie.201410077

Angewandte
Chemie
DOI: 10.1002/anie.201410077
Porous Materials
A Rod-Packing Microporous Hydrogen-Bonded Organic Framework
for Highly Selective Separation of C2H2/CO2 at Room Temperature**
Peng Li, Yabing He, Yunfeng Zhao, Linhong Weng, Hailong Wang, Rajamani Krishna, Hui Wu,
Wei Zhou, Michael OꢀKeeffe, Yu Han, and Banglin Chen*
Abstract: Self-assembly of a trigonal building subunit with
diaminotriazines (DAT) functional groups leads to a unique
rod-packing 3D microporous hydrogen-bonded organic
framework (HOF-3). This material shows permanent porosity
and demonstrates highly selective separation of C2H2/CO2 at
ambient temperature and pressure.
cation, and straightforward regeneration and reusage by
simple recrystallization. Because the pore surfaces within
HOFs are apparently different from those within MOFs and
zeolites, exploration of HOF materials might lead to some
unique new adsorbents for gas separations.
Herein we report a new system, HOF-3, constructed from
a new triangular organic linker containing three hydrogen-
bonding motifs of 2,4-diaminotriazinyl (DAT) (Figure 1a).
Unlike those reported HOFs whose porous structures were
built from discrete building units; HOF-3, to the best of our
knowledge, is the first example of HOFs constructed from
one-dimensional rod-packing units exhibiting the srs top-
ology. More importantly, the activated HOF-3a shows
superior selectivity for C2H2/CO2 separation to the estab-
lished MOF materials.
The triangular organic building block 3-DAT, shown in
Figure 1a, can be readily synthesized in 84% yield by the
reaction of the corresponding nitrile with dicyandiamide
(Supporting Information, Scheme S1). The colorless block
crystals of HOF-3 suitable for X-ray diffraction analysis were
grown by slow vapor diffusion of THF into a large vial
containing saturated DMSO solution of this building block
for a week under room temperature. The purity of HOF-3 was
confirmed by 1H NMR and 13C NMR spectroscopy, thermog-
ravimetric analysis (TGA), and powder X-ray diffraction
(PXRD; Supporting Information, Figures S1–S3). The inclu-
sion of the solvent molecules into the framework leads to the
HOF-3 whose phase is different from that of the as-
synthesized 3-DAT. TGA curve indicates that the framework
of HOF-3 can be stable up to 3508C.
D
evelopment of new porous adsorbents is a very important
topic for adsorption-based gas separations. For example,
extensive research on porous metal–organic frameworks
(MOFs) has not only led to a number of new MOFadsorbents
whose separation selectivities and capacities have surpassed
those traditional zeolite materials for some important CO2/
N2, CO2/CH4, CO2/H2, and xylene isomer separations,[1,2] but
also generated a few MOFs to target the very challenging
C2H2/C2H4,[3] C2H4/C2H6, C3H6 and C3H8,[4,5] and CO/N2
separations.[6]
Among diverse gas separations, C2H2/CO2 is another very
challenging mixture. This is because these two gas molecules
have very similar shapes, dimensions (332 ꢀ 334 ꢀ 570 pm
versus 318.9 ꢀ 333.9 ꢀ 536.1 pm), and boiling points (À848C
versus À78.58C).[7] Since the discovery of the first MOF
material for this separation,[8] several MOFs have been
realized for this important gas separation, though the
selectivities are quite low.[9]
Recently we, and several other groups, have realized that
it is feasible to make use of hydrogen bonding interactions to
construct new porous materials which we have termed as
hydrogen bonded organic frameworks (HOFs),[10] providing
the new approach to develop and explore new porous
adsorbents for gas separations. In fact, the first, HOF-1,
exhibits superior C2H2/C2H4 separation to other MOFs.[10h]
Compared with MOFs, HOFs have some advantages includ-
ing solution processability and characterization, easy purifi-
Single-crystal X-ray diffraction reveals that HOF-3 crys-
tallized in the rhombohedral space group R3 and HOF-3 is
a three-dimensional rod-packing porous material with one-
dimensional hexagonal channels of about 7.0 ꢁ in diameter
[*] P. Li,[+] Dr. Y. He,[+] Dr. H. Wang, Prof. Dr. B. Chen
Department of Chemistry, University of Texas at San Antonio
One UTSA Circle, San Antonio, TX 78249-0698 (USA)
E-mail: banglin.chen@utsa.edu
Prof. Dr. H. Wu, Prof. Dr. W. Zhou
NIST Center for Neutron Research
Gaithersburg, MD 20899-6102 (USA)
and
Homepage: http://www.utsa.edu/chem/chen.html
Department of Materials Science and Engineering
University of Maryland, College Park, MD 20742 (USA)
Prof. Dr. L. Weng
Department of Chemistry, Fudan University
220 Handan Rd, Shanghai 200433 (China)
Dr. Y. Zhao, Prof. Dr. Y. Han
Advanced Membranes and Porous Materials Center
Physical Sciences and Engineering Division
King Abdullah University of Science and Technology
Thuwal 23955-6900 (Saudi Arabia)
Prof. Dr. R. Krishna
Van’t Hoff Institute for Molecular Sciences
University of Amsterdam
Science Park 904, 1098 XH Amsterdam (The Netherlands)
[+] These authors contributed equally to this work.
Prof. Dr. M. O’Keeffe
[**] This work was supported by the Welch Foundation (A-1730).
Department of Chemistry and Biochemistry
Arizona State University (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201410077.
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and CO2 at room temperature. The C2H2 uptakes of 58 cm3 gÀ1
at 273 K and 47 cm3 gÀ1 at 296 K are systematically about
twice higher than CO2 uptakes of 31 cm3 gÀ1 at 273 K and
21 cm3 gÀ1 at 296 K at 1 atm (Figure 2a; Supporting Informa-
tion, Figure S6). This is unusual given the fact that these two
gas molecules are comparable in terms of their dimensions
and boiling points.[9]
This discovery motivated us to examine its feasibility for
the industrially important C2H2/CO2 separation in more
details. The pure component isotherm data were fitted with
the dual-Langmuir isotherm model (Supporting Information,
Figure S7). To understand the binding energy at low coverage,
isosteric heats of adsorption of C2H2 and CO2 in HOF-3a are
calculated. Data on the loading dependence of Qst in HOF-3a
are presented in the Supporting Information, Figure S8.
Particularly remarkable is the relatively low value
(< 20 kJmolÀ1) of the isosteric heat of adsorption for C2H2.
To underscore this point, Figure 2b presents a comparison of
the heats of adsorption of C2H2 in various MOFs (UTSA-
30a,[9a] UTSA-50a,[9b] and CuBTC).[5] We note that value of
Qst in HOF-3a is significantly lower than that for MOFs. The
value of Qst in HOF-3a is systematically more than twice
lower than that in CuBTC with coordinately unsaturated
metal sites (Figure 2b). These data highlight that HOF-3a is
a promising material for the separation of C2H2/CO2 with
lower regeneration energy requirement for C2H2.
We further performed calculation using the Ideal Ad-
sorbed Solution Theory (IAST) of Myers and Prausnitz.[12]
Figure S9 presents IAST calculations of the component
loadings for C2H2 and CO2 in binary equimolar mixture as
a function of the total bulk gas-phase pressure at 296 K, and
Figure 2c provides a comparison of the adsorption selectivity
of C2H2/CO2 in equimolar mixtures in HOF-3a and the three
other MOFs at 296 K. We note that adsorption selectivity of
HOF-3a is significantly higher than that of other MOFs.
Though at zero-coverage the selectivity of UTSA-50a (15) is
higher than that of HOF-3a (14), as the pressure increases to
100 kPa, the adsorption selectivity increases to 21 for HOF-
3a, while for MOF materials, the selectivity reduces signifi-
cantly (lower than 6).
Preliminary studies on the breakthrough simulations for
C2H2/CO2 mixtures using the established methodology de-
scribed in early publications of Krishna (Supporting Infor-
mation, Figure S10) indicate that it is feasible for HOF-3a to
separate C2H2 from the C2H2/CO2 mixture at room temper-
ature (Supporting Information, Figure S11).[12b–d] That
encouraged us to evaluate the actual performance of HOF-
3a through the experimental column breakthrough in which
an equimolar C2H2/CO2 mixture was flowed over a packed
column of the HOF-3a solid with a total flow of 5 cm3 minÀ1 at
296 K. As shown in Figure 2d, the separation of C2H2/CO2
mixture gases through a column packed with HOF-3a solid
can be efficiently achieved. To the best of our knowledge, this
is the first example of porous materials whose separation for
C2H2/CO2 mixture has been clearly established by experi-
mental breakthrough, enabling HOF-3a to be a potential
material for industrial acetylene purification application.
To help understand the C2H2/CO2 selectivity in HOF-3a,
we performed detailed computational investigations. We first
Figure 1. X-ray crystal structure of HOF-3 featuring a) the basic
organic building block in which the centers of central benzene ring
(green balls) and centers of three 1,3,5-triazine rings (magenta balls)
act as nodes; b) The H-bonded link between DAT groups showing that
each is joined to two other DAT groups; c) The net of 3-c branch
points as it occurs in the crystal structure; and d) three-dimensional
packing showing the 1D hexagonal channels of about 7.0 ꢀ in diameter
along the c axis (C gray, H white, N pink).
(Figure 1d). The tritopic building unit has a 3-coordinated (3-
c) node (A) at its center shown as a green ball in Figure 1a.
The three DAT groups shown as magenta balls (B) in the
Figure 1a are linked to two other DAT groups by H-bonds as
shown in Figure 1b, so each of these also corresponds to a 3-c
node. The connectivity is A-B3 and B-AB2. The 3-c net is the
ubiquitous srs (Figure 1c), which can be described (see the
Supporting Information) as parallel threefold helical rods (B)
linked by additional 3-c nodes (A) into a 3-D net (Supporting
Information, Figure S4). Initially, we tried to use the discrete
molecule approach to rationalize the topology. However, the
rationalized structure of NaCl topology apparently does not
make much sense and is far away from the pore nature. It was
then termed as a rod-packing topology to suitably describe
the framework channels.[11] The pore spaces within the
frameworks encapsulate certain amount of disordered
DMSO and THF solvent molecules.
The available void space and high thermal stability of
HOF-3 prompts us to examine the gas adsorption properties
of HOF-3. The as-synthesized HOF-3 was exchanged with
acetone several times then evacuated under dynamic vacuum
to obtain desolvated HOF-3a, which does not take up any N2
gas molecules at 77 K. Such phenomena have been commonly
observed in HOF and supramolecular organic framework
(SOF) materials,[10f–h] though the exact reasons are still not
clear. However, the CO2 gas sorption isotherm of HOF-3a at
196 K clearly indicates its microporous nature with Bruna-
uer–Emmett–Teller (BET) surface area of 165.0 m2 gÀ1 (Sup-
porting Information, Figure S5), which is moderate.[10] Inter-
estingly, HOF-3a takes up quite different amounts of C2H2
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much more “localized” than C2H2
in terms of position and orientation,
with the adsorption energy decaying
rapidly if the CO2 molecule is moved
away from the optimized position.
For C2H2, the adsorption potential
surface is much “flatter” in the
region adjacent to the pocket site,
and more than one molecule can be
accommodated on each site simulta-
neously with similar binding
strength. This subtle difference in
the gas/HOF framework interaction
is likely the reason why the uptake
(that is, surface coverage) of C2H2 is
higher than CO2 in HOF-3a at RT.
In summary, we have targeted
a
three-dimensional rod-packing
HOF for highly selective C2H2/CO2
separation at ambient temperature
and pressure. To the best of our
knowledge, this is the first example
of rod-packing porous hydrogen-
bonded organic frameworks. More
importantly, its C2H2/CO2 separation
selectivities are systematically higher
than those found in the well-estab-
lished MOF materials, highlighting
the bright promise of such new
porous HOF materials for gas sepa-
ration. Such preferential adsorption
of HOF-3 for acetylene over carbon
dioxide is attributed to the unique
pockets and pore surfaces within
HOF-3. It is expected that extensive
research endeavors on porous HOFs
Figure 2. a) Sorption isotherms of C2H2 and CO2 of HOF-3a at 296 K; b) comparison of the heats of
adsorption of C2H2 in HOF-3a and various MOFs; c) IAST adsorption selectivities of C2H2/CO2 in
equimolar mixture in HOF-3a and various MOFs at 296 K; and d) experimental column break-
through curve for an equimolar C2H2/CO2 mixture (296 K, 1 bar) in an adsorber bed packed with
HOF-3a.
optimized the HOF-3a structure by first-principles DFT-D
(dispersion-corrected density-functional theory) calcula-
tions,[13] and then carried out grand canonical Monte Carlo
(GCMC) simulations using the classical force field method
(for details, see the Supporting Information). The simulated
PXRD of the optimized HOF-3a structure matches with the
experimental structure of HOF-3a quite well (Supporting
Information, Figure S3), indicating that the results from
simulation studies are quite reasonable. HOF-3a is the
slightly shrunk form of HOF-3, keeping identical framework
connectivity. From the probability distribution of adsorbed
gas molecules generated from the simulations (Supporting
Information, Figure S12), we found that for both C2H2 and
CO2, the adsorption takes place mainly adjacent to the
“pocket” between two H-bonded DAT groups. We then
introduced gas molecules at these sites, and further optimized
the “HOF-3a + gas” structures using DFT-D. We found that
the gas binding is relatively weak in both cases, and of van der
Waals type in nature. The derived static gas binding energies
are 20.9 kJmolÀ1 and 26.3 kJmolÀ1 for C2H2 and CO2,
respectively, in qualitative agreement with the experimental
Qst values. Interestingly, we noticed that the relaxed, adsorbed
CO2 molecules (Supporting Information, Figure S13) are
will facilitate the discoveries of new porous HOFs for gas
separation and other important applications in the near
future.
Experimental Section
Synthesis of 3-DAT: A mixture of 1,3,5-tri(4-cyanophenyl)benzene
(0.90 g, 2.36 mmol), dicyandiamide (0.74 g, 8.80 mmol, Alfa), and
powdered KOH (85%, 0.12 g, 1.82 mmol, Aldrich) in 2-methoxye-
thanol (30 mL, Aldrich) was heated at reflux for 48 h. The resulting
mixture was cooled and filtered. The solid was extracted thoroughly
with hot water. The solid was then rinsed with ethanol and dried in
vacuum to give the target compound as a colorless solid in 84% yield
(1.26 g, 1.99 mmol).
Crystalline material HOF-3 was generated as follows: 3-DAT
(350 mg, 0.55 mmol) was dissolved in DMSO (20 mL) under heating.
The resulting solution was cooled to room temperature and filtered.
The filtrate was divided to 8 small disposable scintillation vials. These
vials were then placed inside a large bottle containing THF as
a diffusion solvent. The bottle was then closed and kept at room
temperature for a week. Colorless block shaped crystals were
1
obtained in 32% yield. H NMR ([D6]DMSO, 300.0 MHz): d = 8.38
(d, J = 8.4 Hz, 6H), 8.06 (s, 3H), 8.01 (d, J = 8.4 Hz, 6H), 6.81 ppm (s,
br, 12H); 13C NMR ([D6]DMSO, 75.4 MHz): d = 169.63, 167.22,
142.18, 140.98, 136.35, 128.17, 126.89, 124.80 ppm; FTIR (neat): n˜ =
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X-ray diffraction data of HOF-3 were collected at 193 K on
a Bruker Apex II CCD diffractometer graphite-monochromatic
enhanced ultra Cu radiation (l = 1.54178 ꢁ). The structure was
solved by direct methods and refined by full-matrix least-squares
methods with the SHELXTL program package. The solvent mole-
cules in as-synthesized HOF-3 crystal are highly disordered. The
SQUEEZE subroutine of the PLATON software suite was used to
remove the scattering from the highly disordered guest molecules.
Crystal data of HOF-3: rhombohedral R3, a = b = 22.5819(7), c =
16.6210(8) ꢁ, V= 7340.2(7) ꢁ3, Z = 3, 1calcd = 0.430 gcmÀ3, crystal size
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Received: October 14, 2014
Published online: && &&, &&&&
Keywords: acetylene separation · adsorption ·
.
hydrogen-bonded organic frameworks · microporous materials ·
selectivity
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Communications
Porous Materials
A 3D microporous rod-packing hydrogen-
bonded organic framework was con-
structed from a trigonal building subunit
with diaminotriazine functional groups.
This framework exhibits highly selective
separation of C2H2/CO2 at ambient tem-
perature and pressure.
P. Li, Y. He, Y. Zhao, L. Weng, H. Wang,
R. Krishna, H. Wu, W. Zhou, M. O’Keeffe,
Y. Han, B. Chen*
&&&& — &&&&
A Rod-Packing Microporous Hydrogen-
Bonded Organic Framework for Highly
Selective Separation of C2H2/CO2 at
Room Temperature
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