A microporous hydrogen-bonded organic framework: Exceptional stability and highly selective adsorption of gas and liquid

Article abstract of DOI:10.1021/ja403002m

An extremely stable hydrogen-bonded organic framework, HOF-8, was fabricated. HOF-8 is not only thermally stable but also stable in water and common organic solvents. More interestingly, desolvated HOF-8 exhibits high CO₂ adsorption as well as highly selective CO₂ and C ₆H₆ adsorption at ambient temperature.

Full text of DOI:10.1021/ja403002m

Communication
pubs.acs.org/JACS
A Microporous Hydrogen-Bonded Organic Framework: Exceptional
Stability and Highly Selective Adsorption of Gas and Liquid
Xu-Zhong Luo,* Xin-Jian Jia, Ji-Hua Deng, Jin-Lian Zhong, Hui-Jin Liu, Ke-Jun Wang,
and Di-Chang Zhong*
Key Laboratory of Organo-Pharmaceutical Chemistry of Jiangxi Province, Key Laboratory of Jiangxi University for Functional
Material Chemistry, Gannan Normal University, Ganzhou 341000, China
S
* Supporting Information
Scheme 1. Molecular Structure of TPBTC
ABSTRACT: An extremely stable hydrogen-bonded
organic framework, HOF-8, was fabricated. HOF-8 is
not only thermally stable but also stable in water and
common organic solvents. More interestingly, desolvated
HOF-8 exhibits high CO2 adsorption as well as highly
selective CO2 and C6H6 adsorption at ambient temper-
ature.
ecently, the syntheses and property investigations of porous
materials have become one of the hottest research fields in
R
materials science.1 Among these porous materials, porous
organic crystalline materials (POCMs), which possess lower
densities than other porous materials such as zeolites and metal−
organic frameworks (MOFs), have attracted more research
interest in recent years.2 Ordered POCMs usually include
crystalline covalent organic frameworks (COFs) and hydrogen-
bonded organic frameworks (HOFs). The former are connected
by covalent bonds between atoms and show relatively high
stabilities, while the latter are linked by weak non-covalent
interactions such as hydrogen bonds and π−π stacking
interactions and exhibit relatively low stabilities. For HOFs, the
solvent guests play important roles in the construction of the
supermolecular network system. Once the solvent guests are
removed, the supermolecular system is usually broken, and the
HOFs collapse. Therefore, it is unsurprising that although
thousands of HOFs have been reported in the literature during
the last two decades, examples of HOFs with permanent porosity
are very rare.3 In fact, to the best of our knowledge, only three
HOFs (TTBI, HOF-1, and SOF-1, reported by the groups of
More interestingly, desolvated HOF-8 (HOF-8d) exhibits
highly selective CO2 and C6H6 adsorption at ambient temper-
ature. Such exceptional stability coupled with highly selective
adsorption of gas and liquid has not been reported for other
HOFs to date.
TPBTC was synthesized by a typical condensation amidation
reaction process [Scheme S1 in the Supporting Information
1
(SI)]. The IR, 13C NMR and H NMR spectra of TPBTC
revealed the formation of a pure organic building block (Figures
S1, S2, and S3a in the SI). Light-brown block single crystals of
HOF-8 were obtained by recrystallization of the TPBTC from
3:1 (v/v) chloroform/methanol (Figure S4). HOF-8 is insoluble
in water and common organic solvents such as benzene and
hexane.
Single-crystal X-ray diffraction (scXRD) analysis showed that
HOF-8 crystallizes in the C2/c space group (Table S1 in the SI).
As shown in Figure 1a, each TPBTC molecule connects with
three other TPBTC molecules through three pairs of highly
symmetric hydrogen bonds (N1···N6, N3···N2, and N5···N4),
generating a two-dimensional (2D) supermolecular layer along
the plane perpendicular to the ac plane (Figure 1b). This layer is
further stacked together with adjacent two layers through π···π
stacking interactions along the c axis, forming a three-
dimensional (3D) microporous supermolecular structure
(Figure 1c) with a pore size of 6.8 Å × 4.5 Å. The pores are
filled with disordered guest molecules. A solvent-accessible
volume of ∼24.0% was estimated using PLATON.5 Because of
the great disorder of the guest molecules, it was difficult to
Mastalerz,2a Chen,4a and Schroder,4b respectively) show
̈
unusually high thermal stability after the removal of the guest
molecules and exhibit good gas sorption properties. A common
feature of these stable HOFs is that the host molecules can form
strong hydrogen bonds and π−π stacking interactions with each
other. In this case, removal of the solvent guests has a limited
influence on the stability of the supermolecular network
assembled from the host molecules. Inspired by these excellent
works, we synthesized the organic building block N1,N3,N5-
tris(pyridin-4-yl)benzene-1,3,5-tricarboxamide (TPBTC)
(Scheme 1), which contains pyridine N atoms and amide H
atoms as hydrogen-bond acceptors and donors, respectively, for
use in HOF fabrication. Here we report HOF-8, an exceptionally
stable HOF constructed from TPBTC that is not only thermally
stable but also stable in water and common organic solvents.
Received: March 25, 2013
© XXXX American Chemical Society
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Journal of the American Chemical Society
Communication
Figure 2. Variable-temperature PXRD patterns for HOF-8 measured
under N2.
materials; even in MOF materials formed by covalent
(coordinated) bonds, examples that can endure such a high
temperature are very limited.6 The high stability of HOF-8d may
be related to the strong supermolecular system including three
pairs of highly symmetric hydrogen bonds (Figure 1a).
To confirm the contribution of the hydrogen bonds to the
thermal stability of HOF-8, we tried to replace the amide H
atoms with D atoms via a deuteration reaction (see the SI).
Under a nitrogen atmosphere, desolvated TPBTC was dissolved
in anhydrous dimethyl sulfoxide (DMSO), to which n-
butyllithium and heavy water were successively added; enough
heavy water was added to get deuterated TPBTC powder. The
1H NMR spectrum of the deuterated TPBTC powder showed
that the amide H atoms were completely substituted with D
atoms from heavy water (Figure S7). To obtain deuterated
HOF-8 crystals, the deuterated TPBTC was recrystallized in a
mixed solvent of deuterated chloroform and deuterated
methanol (see the SI). The PXRD pattern of the deuterated
HOF-8 was similar to that of HOF-8 (Figure S8), indicating that
the framework of deuterated HOF-8 is the same as that of HOF-
Figure 1. (a) Hydrogen-bonding interactions observed in HOF-8. (b)
2D supermolecular layer structure of HOF-8. (c) 3D supermolecular
microporous structure of HOF-8.
directly assign the position of the solvent guest molecules by
scXRD. To determine the composition of HOF-8, the 1H NMR
spectrum of HOF-8 dissolved in DMSO-d6 was measured at
1
room temperature (Figure S3b). Compared with the H NMR
1
spectrum of TPBTC (Figure S3a), the H NMR spectrum of
HOF-8 contains a new peak at 8.33 ppm, which corresponds to
the chemical shift of the CHCl3 molecule. This result indicates
that the guest molecules in the pores of HOF-8 are CHCl3 and
1
8. The deuterated HOF-8 was further analyzed by H NMR
1
H2O. On the basis of the H NMR spectrum, it was estimated
spectroscopy, which showed that ∼42% of the amide H atoms
were substituted with D atoms (Figure S9). The degree of
substitution is significantly lower than that of the deuterated
TPBTC powder. This may be ascribed to trace H2O in the
deuterated solvents, which led to replacement of some of the D
atoms with H atoms. In addition, moisture in the air also may
have induced this D → H transformation. TGA showed that from
room temperature to 260 °C, the 42% deuterated HOF-8
underwent a guest loss process similar to that of HOF-8, and the
desolvated framework began to decompose upon further heating.
This observation is different from that for desolvated HOF-8,
which remained stable up to 350 °C (Figure 3). The TGA results
indicate that the framework of deuterated HOF-8d shows a
relatively worse thermal stability than that of HOF-8. The
difference in thermal stabilities may be related to the substitution
of the amide H atoms with D atoms, which may weaken the
hydrogen-bonding system within HOF-8. Such a phenomenon is
commonly observed in supermolecular systems of biological
macromolecules.7 Although the comparison of TGA results is
based on HOF-8 and 42% deuterated HOF-8, the conclusion
that the hydrogen bonds within HOF-8 contribute much to the
thermal stability of HOF-8 is undoubtedly confirmed.
that there are ∼0.15 CHCl3 and 3 H2O per TPBTC molecular
unit.
In the room-temperature powder XRD (PXRD) pattern
(Figure S5), all of the measured peaks closely matched those in
the simulated pattern generated from the scXRD data, indicating
that a single phase of HOF-8 was formed. Thermogravimetric
analysis (TGA) revealed that the guest solvent molecules in
HOF-8 can be released upon heating from room temperature to
260 °C (Figure S6a); the weight loss of 13.9% corresponds to
0.15 CHCl3 molecule and three H2O molecules per TPBTC unit
(calculated: 14.1%). No obvious weight loss was observed from
260 to 350 °C, after which the desolvated framework began to
decompose (Figure S6b). To thoroughly understand the stability
of the framework of HOF-8, variable-temperature PXRD data for
HOF-8 were measured from room temperature to 350 °C
(Figure 2). From room temperature to 240 °C, no obvious
change in the PXRD pattern was observed except that the peak at
2θ = 27° shifted slightly as a result of the flexibility and shrinkage
of the framework. When HOF-8 was further heated to 260 °C,
the peaks between 17° and 25° markedly changed, indicating that
after the removal of the guest molecules, the framework of HOF-
8 remarkably changed. HOF-8d shows unbelievably good
stability, as it remained stable even when heated to over 350
°C. To the best of our knowledge, no framework with as high a
stability as HOF-8d has been observed in organic crystalline
To determine the permanent porosity of HOF-8d, N2, H2, and
CO2 sorption measurements under mild conditions were carried
out. The results obviously showed that HOF-8d selectively
adsorbs CO2 rather than N2 and H2 (Figure 4). This highly
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Figure 5. Partial 13C NMR spectra (400 MHz, 298 K, DMSO-d6) of (a)
desolvated HOF-8 and (b) HOF-8d with absorbed benzene.
Figure 3. TGA curves of (a) HOF-8 and (b) 42% deuterated HOF-8.
corresponding to 0.31 benzene molecule absorbed per TPBTC
unit. A general explanation for selective adsorption of hydro-
carbons is size exclusion (steric effect). Obviously, HOF-8d’s
selective adsorption of benzene over the other four types of
hydrocarbons cannot be attributed to size exclusion because all of
the tested hydrocarbons can freely enter the pores of HOF-8d.
The different adsorption behaviors can be ascribed to the
different interactions between the adsorbent (HOF-8) and the
adsorbates (hydrocarbons). As far as benzene is concerned, the
phenyl rings can strongly bind with the phenyl/pyridyl rings of
HOF-8d through π−π* stacking interactions, leading to the
adsorption of benzene by HOF-8d.14 Because n-hexane and
cyclohexane do not contain delocalized π bonds in their
molecular structures, they cannot stack with the phenyl/pyridyl
rings of HOF-8d by π−π* interactions, and thus, they cannot be
adsorbed by HOF-8d. Although toluene and p-xylene can bind
with HOF-8d through π−π* stacking interactions, the stacking
interactions may be very weak because of the steric hindrance by
the methyl group. When HOF-8d with toluene or p-xylene was
treated at 50 °C under vacuum (see the SI), the toluene/p-xylene
molecules easily escaped from the pores of HOF-8d. Therefore,
no adsorption of toluene/p-xylene was observed either. To the
best of our knowledge, although several types of porous materials
(e.g., MOFs, zeolites, and nanotubes) have been developed for
separation of liquid hydrocarbons,15 the use of POCMs for liquid
separation, especially for removal of benzene, has not been
explored to date.16
In summary, we have successfully assembled an extremely
stable hydrogen-bonded organic framework, HOF-8 that is not
only thermally stable but also stable in water and common
organic solvents. More interestingly, desolvated HOF-8 exhibits
high CO2 adsorption as well as highly selective CO2 and C6H6
adsorption at ambient temperature. This result illustrates that
HOF-8 may have potential applications for separation and
purification of gases and solvents to obtain higher-purity
materials for precise analyses. These unique features of HOF-
8, including the exceptional stability and highly selective
adsorption for CO2 and benzene, make it outstanding among
the thousands of HOFs reported in the literature.
Figure 4. N2, H2, and CO2 sorption isotherms for HOF-8d at 298 K.
selective adsorption may be ascribed to the special quadrupole
moment of CO2 (−1.4 × 10−39 C m2).8 It has been reported that
the quadrupole moment of CO2 can induce the framework to
interact with CO2 molecules and thus increase the CO2 binding.9
However, N2 and H2 have small quadrupole moments (4.7 ×
10−40 and 2.2 × 10−40 C m2, respectively)10 thus cannot interact
with and be adsorbed on the pore surface. The sorption
isotherms of CO2 at 298 K exhibit typical type-I behavior, with
the amount of CO2 uptake gradually increasing to 57.3 cm3
(STP)/g at 1.0 atm. This value is higher than that of any other
POCM reported in the literature11 and those of most MOFs
measured under the same conditions.12 The large amount of
CO2 adsorption in HOF-8d may be ascribed to the fact that the
framework is decorated with pyridyl N atoms and amide N
atoms.13
Considering the good stability of HOF-8 in organic solvents,
experiments involving adsorption of five types of hydrocarbons
by HOF-8d were performed (see the SI). HOF-8d samples
adsorbed with different solvents were dissolved in DMSO-d6 and
detected by 13C NMR spectroscopy. The results showed that
HOF-8d cannot adsorb n-hexane, cyclohexane, toluene, or p-
xylene (Figures S10−S14 and Table S3). However, HOF-8d
shows adsorption of benzene. The 13C resonance at 128.3 ppm,
an indicator of benzene, was strongly detected in the 13C NMR
spectrum of HOF-8d with adsorbed benzene (Figure 5). The
TGA curve of HOF-8d with adsorbed benzene showed a weight
loss of 7.52% from room temperature to 355 °C (Figure S6c),
ASSOCIATED CONTENT
* Supporting Information
Experimental section and auxiliary data. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
C
dx.doi.org/10.1021/ja403002m | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Kim, Y.; Yoon, M.; Lim, S.; Park, S. M.; Seo, G.; Kim, K. J. Am. Chem. Soc.
2010, 132, 12200.
(12) (a) Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.;
AUTHOR INFORMATION
Corresponding Author
zhong_dichang@hotmail.com; luoxuzhong@hotmail.com
■
Bloch, E. D.; Herm, Z. R.; Bae, T. H.; Long, J. R. Chem. Rev. 2012, 112,
Notes
́
724. (b) Ferey, G.; Serre, C.; Devic, T.; Maurin, G.; Jobic, H.; Llewellyn,
P. L.; De Weireld, G.; Vimont, A.; Daturi, M.; Chang, J. S. Chem. Soc. Rev.
2011, 40, 550.
The authors declare no competing financial interest.
(13) Zhang, J. P.; Zhang, Y. B.; Lin, J. B.; Chen, X. M. Chem. Rev. 2012,
112, 1001.
(14) Horcajada, P.; Serre, C.; Vallet-Regí, M.; Sebban, M.; Taulelle, F.;
́
Ferey, G. Angew. Chem., Int. Ed. 2006, 45, 5974.
(15) (a) Bloch, E. D.; Queen, W. L.; Krishna, R.; Zadrozny, J. M.;
Brown, C. M.; Long, J. R. Science 2012, 335, 1606. (b) Wu, H. H.; Gong,
Q. H.; Olson, D. H.; Li, J. Chem. Rev. 2012, 112, 836. (c) Maes, M.;
Alaerts, L.; Vermoortele, F.; Ameloot, R.; Couck, S.; Finsy, V.; Denayer,
J. F. M.; De Vos, D. E. J. Am. Chem. Soc. 2010, 132, 2284.
ACKNOWLEDGMENTS
■
This work was financially supported by the Program for New
Century Excellent Talents in University (06-0577), the National
Natural Science Foundation of China (51268002), the Natural
Science Foundation of Jiangxi Province, and the Science
Foundation of the Jiangxi Provincial Office of Education.
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