A microporous hydrogen-bonded organic framework for highly selective C 2H2/C2H4 separation at ambient temperature

Article abstract of DOI:10.1021/ja2066016

The first microporous hydrogen-bonded organic framework with permanent porosity and exhibiting extraordinarily highly selective adsorptive separation of C₂H₂ and C₂H₄ at ambient temperature has been established.

Full text of DOI:10.1021/ja2066016

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pubs.acs.org/JACS
A Microporous Hydrogen-Bonded Organic Framework for Highly
Selective C H /C H Separation at Ambient Temperature
2
2
2 4
Yabing He, Shengchang Xiang, and Banglin Chen*
Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, United States
S Supporting Information
b
other advantages, such as solution processability and character-
ization, potential high thermal stability, easy purification, and
ABSTRACT: The first microporous hydrogen-bonded
organic framework with permanent porosity and exhibiting
extraordinarily highly selective adsorptive separation of C H
regeneration by simple recrystallization. Motivated by the
10
2
2
pioneering work of Wuest on the construction of HOFs, we plan
to explore HOFs for their potential applications in gas separation.
Herein we report on HOF-1, the first microporous HOF for highly
selective C H /C H separation at ambient temperature.
and C H at ambient temperature has been established.
2
4
2
2
2 4
The organic building block shown in Scheme 1 was synthe-
sized according to the procedure in refs 10 and 13. As pointed out
by Wuest, highly crystalline, colorless needlelike crystals of HOF-1
can be easily obtained by vapor diffusion of dioxane into a
formic acid solution of the organic building block, whose purity
cientific and technological innovations have been always
followed by new discoveries and the realization of novel
S
functional materials. Because porous materials have very impor-
tant and useful industrial applications in gas storage, separation,
1
was confirmed by H NMR spectroscopy, microanalysis, ther-
mogravimetric analysis (TGA), and powder X-ray diffraction
1ꢀ4
and catalysis, there have been extensive research endeavors to
target low-energy-consuming and cost-effective porous materials
for highly efficient gas storage and separation. Such efforts
eventually led to the discovery of the so-called “molecular gate”
adsorbent Engelhard titanium silicate (ETS-4) for economical
(
PXRD). It needs to be mentioned that the guest solvent
molecules can be readily released from room temperature to
170 °C, and the resulting desolvated HOF-1a is also highly
crystalline; its PXRD pattern is right-shifted relative to the as-
synthesized one (Figure S1 in the Supporting Information),
indicating a certain degree of framework flexibility and shrinkage
of the framework. HOF-1a is thermally stable up to 420 °C
5
N and CO removal and the emergence of porous metalꢀ
2
2
organic frameworks (MOFs) and/or porous coordination poly-
mers (PCPs) as very promising energy storage (hydrogen and
natural gas) materials for future vehicles. In fact, a MOF
6
(
Figure S2).
7
The single-crystal X-ray structure of HOF-1 revealed that
methane/natural gas fuel tank has been already developed,
HOF-1 is a three-dimensional porous HOF in which each
organic building block is connected with four neighboring ones
by eight strong hydrogen bonds involving the 2,4-diaminotria-
and several prototypical porous MOFs have been commercia-
8
lized recently by BASF. Because C H /C H separation is a very
2
2
2 4
important industrial process, porous MOF materials for highly
selective adsorptive C H /C H separation have recently at-
10
zine groups (Figure S3a). There exist one-dimensional pores
2
2
2 4
along c axis with a size of ∼8.2 Å based on the van der Waals radii
15
tracted extensive attention.
(
Figure 1a). Topologically, the organic building blocks can be
Although hydrogen-bonded organic frameworks (HOFs)
have been proposed as potential porous materials and a series
of HOFs were structurally characterized about two decades
considered as 8-connected nodes that form a three-dimensional
body-centered cubic bcu {4 6 } network (Figure 1b). The
24 4
framework is further enforced by multiple aromatic πꢀπ inter-
actions among the organic building blocks (Figure S3b). The
collaborative hydrogen bonding and aromatic πꢀπ interactions
have featured the HOF-1 as the rare example of a highly robust
9
,10
ago, no example of such a framework with permanent porosity
by gas/vapor sorption isotherms has been established. This is
mainly because of the weak hydrogen bonding within HOFs,
which makes the framework very difficult to stabilize, so the
structurally “porous” HOFs typically collapse once the solvent
guest molecules are removed after thermal and/or vacuum
activation. Such a failure situation also happened at the early
stage of porous MOF development before the realization of
a few MOFs and/or PCPs with permanent porosity in
10
HOF, as established by Wuest. One of the amine groups within
the 2,4-diaminotriazine moieties is not involved in the hydrogen
bonding and thus is exposed on the pore surfaces for potential
interactions with gas molecules.
Encouraged by the robustness and flexibility of HOF-1, we
examined the permanent porosity of HOF-1a generated by
activation under high vacuum at room temperature for 24 h
and then at 100 °C for another 24 h. Such activation completely
1
1,12
1
997ꢀ1999.
Unlike the very strong covalent bonds and
moderately strong coordination bonds holding together the
frameworks in zeolites and MOFs, which feature their corre-
sponding robustness and dual robustness/flexibility, respectively,
the weaker hydrogen bonding is reasonably expected to stabilize
HOFs with even more flexibility, whose functions for gas
separation might even surpass those of some well-developed
zeolite and MOF materials. Furthermore, HOFs also have some
1
removed the guest molecules, as confirmed by H NMR spec-
troscopy (Figure S4). The porosity was evaluated by CO gas
2
Received: July 15, 2011
Published: August 24, 2011
r 2011 American Chemical Society
14570
dx.doi.org/10.1021/ja2066016
|J. Am. Chem. Soc. 2011, 133, 14570–14573

Journal of the American Chemical Society
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Scheme 1. Organic Building Block Used To Construct
HOF-1
Figure 1. X-ray crystal structure of HOF-1 featuring (a) one-dimen-
sional channels along the c axis with a size of ∼8.2 Å (yellow spheres)
24 4
and (b) three-dimensional body-centered cubic bcu {4 6 } network
topology.
sorption at 196 K (Figure 2a). The isotherm shows a very sharp
uptake at P/P° < 0.1, indicative of a microporous material.
Moreover, the isotherm exhibits a sudden increase at P/P° =
0
.78 and a large hysteretic desorption behavior, suggesting its
framework flexibility and the existence of molecular gates. Such
framework flexibility is attributed to the reversible transforma-
tion between the guest-free HOF-1a and guest-loaded HOF-1,
which was also observed using PXRD (Figure S2) and has been
Figure 2. (a) CO sorption isotherm at 196 K and (b) C H and C H
sorption isotherms at 273 K.
2
2
2
2
4
C H /C H separation selectivity. The moderately high C H
2
2
2
4
2
2
14
observed in flexible MOFs. Thus, the less porous HOF-1a
can be expanded back to porous HOF-1 once gas molecules
have been gradually loaded into the pores. The isotherm gave
an apparent BrunauerꢀEmmettꢀTeller (BET) surface area of
uptake of HOF-1a will make such C H /C H separation
2
2
2 4
practically feasible.
To understand the high C H /C H separation selectivity,
2
2
2 4
adsorption enthalpies based on the virial equation were calcu-
lated (Figures S7 and S8; the virial parameters are summarized in
Table S1). The adsorption enthalpies at zero coverage
2
ꢀ1
3
59.2 m g between relative pressures of 0.10 and 0.31 (Figure
S5), which is moderate.
The establishment of permanent porosity and framework
flexibility in HOF-1a prompted us to examine its gas separation
capacity, particularly for the very important and challenging
(
calculated from the A virial coefficients) are 58.1 and 31.9 kJ
0
1
ꢀ
mol for C H and C H , respectively. The adsorption en-
thalpy of 31.9 kJ mol for C H in HOF-1a is comparable to
2
2
2 4
ꢀ1
2
4
1
6
C H /C H separation at ambient temperature. HOF-1a exhi-
2
2
2
4
those in microporous MOFs. However, the adsorption en-
1
ꢀ
bits hysteretic sorption behaviors for C H at both 273 and 296 K
2
2
thalpy of 58.1 kJ mol for C H in HOF-1a is much higher than
2 2
17
(
Figure 2b and Figure S6). Accordingly, a higher pressure is
needed to open the gate of HOF-1a at 296 K to maximize the
C H uptake. The most significant feature is that HOF-1a takes
those found in microporous MOFs, indicating that the inter-
actions between the guest C H molecules and host HOF-1a
2
2
2
2
framework are very strong. Such very strong interactions might
be attributed to the narrow pore spaces within HOF-1a (the
pores in this activated HOF-1a are much smaller than those of
8.2 Å in the as-synthesized HOF-1 as shown by PXRD) and
hydrogen-bonding interactions of acidic H atoms of the guest
up a much larger amount of C H than C H , indicating that
2
2
2 4
HOF-1a is a very promising microporous material for the C H /
2
2
C H separation at ambient temperature. At 273 K, HOF-1a can
2
4
3
ꢀ1
take up acetylene up to 63.2 cm g (STP), which corresponds
1
8
to 2.1 mol of C H absorbed per mole of HOF-1a. The storage
acetylene molecules and the basic amine groups of HOF-1a.
The calculated Henry C H /C H separation selectivities in
2
2
ꢀ3
density of C H is 0.17 g cm , which is equivalent to the
2
2
2
2
2 4
acetylene density at 16.4 MPa and 82 times greater than the
compression limit for the safe storage of acetylene (0.2 MPa). On
the other hand, HOF-1a can adsorb only 8.3 cm g (STP) of
HOF-1a are 19.3 and 7.9 at 273 and 296 K, respectively, which
are much higher than the values of 4.1 and 5.2 in the recently
3
ꢀ1
0
15
discovered M MOF-3a. Therefore, the collaborative size-
exclusive effect (3.32 Å ꢁ 3.34 Å ꢁ 5.70 Å for C H vs 3.28 Å ꢁ
C H at 1 atm. The C H /C H molar ratio separation selec-
2
4
2
2
2
4
2 2
4.18 Å ꢁ 4.84 Å for C H ) confined by the small narrow pores
19
tivity of 7.6 in HOF-1a is thus significantly higher than those
2
4
0
15
previously reported (<2.5) in M MOFs at 273 K. Such high
C H /C H separation selectivity can be further increased up to
and the preferential stronger interactions of C H with the basic
2 2
2
2
2
4
amine groups of the host framework enable HOF-1a to act as an
extraordinarily highly selective microporous material for the
separation of C H and C H at ambient temperature.
1
4.6 at 296 K for HOF-1a. To the best of our knowledge, HOF-1a
is the best microporous material with such extraordinarily high
2
2
2 4
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dx.doi.org/10.1021/ja2066016 |J. Am. Chem. Soc. 2011, 133, 14570–14573

Journal of the American Chemical Society
COMMUNICATION
Ethylene obtained from natural gas cracking contains a small
amount of acetylene as an impurity, which can serve as a catalyst
poison during ethylene polymerization and also lower the quality
of the resulting polyethylene. In addition, acetylene can form
explosive metal acetylides. It is thus imperative that acetylene in
the ethylene product be reduced to an acceptable level. Current
main commercial approaches to eliminate acetylene in crude
ethylene include partial hydrogenation and solvent extraction.
The former process suffers from the need for a noble-metal
catalyst and the loss of olefins due to the overhydrogenation to
paraffins, while the latter is also disadvantageous in terms of
technical and economical aspects because of the low selectivities
for acetylene over olefins and the significant loss of solvent after
multiple operations. The realization of microporous HOF-1a
with its extraordinarily high capability for C H /C H separation
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