A microporous six-fold interpenetrated hydrogen-bonded organic framework for highly selective separation of C2H4/C2H6

Article abstract of DOI:10.1039/c4cc05506c

A unique six-fold interpenetrated hydrogen-bonded organic framework (HOF) has been developed, for the first time, for highly selective separation of C₂H₄/C₂H₆ at room temperature and normal pressure. This journal is

Full text of DOI:10.1039/c4cc05506c

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A microporous six-fold interpenetrated hydrogen-
bonded organic framework for highly selective
Cite this: Chem. Commun., 2014,
0, 13081
separation of C H /C H †
5
2 4 2 6
Received 17th July 2014,
Accepted 3rd September 2014
a
a
a
b
a
Peng Li, Yabing He, Hadi D. Arman, Rajamani Krishna, Hailong Wang,
c
ad
Linhong Weng and Banglin Chen*
DOI: 10.1039/c4cc05506c
www.rsc.org/chemcomm
A unique six-fold interpenetrated hydrogen-bonded organic frame- strategies have been proposed to achieve preferential adsorption
work (HOF) has been developed, for the first time, for highly selective of olefin over paraffin. One is based on reversible formation of
separation of C H /C H
2 4 2 6
at room temperature and normal pressure. p-complexes of olefins with transition metal cations, and the
other is control of the appropriate pore size and volume for size
As one of the most important petrochemicals, ethylene is used exclusion separation. The former strategy has been successfully
1
0a
12a
widely in the chemical industry and its worldwide production applied in MOFs and POPs to realize high selective adsorp-
exceeds that of any other organic compound (140 million tons tion for ethylene over ethane. Although this selectivity is usually
1
per year by 2010). Thermal cracking of ethane as a feedstock in very high at low pressure, it decreased significantly with increasing
the presence of steam remains one of the most important and pressure possibly due to saturation of the preferential binding sites.
10b,12b
widely employed processes for ethylene production. Due to the The latter strategy often used in propene/propane separation,
similar sizes and volatilities of ethylene and ethane, the traditional however, can hardly be achieved especially for smaller ethylene and
cryogenic distillation technology to separate ethylene from ethane ethane molecules with tiny dimensional difference. Moreover, this
10d
requires distillation columns with over 100 trays under the condi- strategy may result in an inevitable low adsorption capacity.
tions of high pressure (23 bar) and low temperature (À25 1C), Hence, finding new materials for the adsorptive separation of
which has been criticized as the most energy extensive process ethylene–ethane mixtures is very challenging and important.
2
in the petrochemical industry. Therefore, tremendous efforts
Recently, we and several other groups have discovered that
3
have been devoted to develop alternative technologies, such as hydrogen-bonded organic framework (HOF) materials can be used
membrane separation, liquid adsorbent separation and solid as a new class of porous materials for a variety of applications.
adsorbent adsorption separation for ethylene/ethane separation Because HOFs have some obvious advantages such as solvent
at ambient temperature and pressure with lower energy cost. processability and straightforward regeneration by recrystalliza-
In the development of adsorption separation technologies, tion, while they might have different pore surfaces from those of
various porous materials, such as SiO
metal–organic frameworks (MOFs), nanoparticle–MOF compo- might lead to some unique new adsorbents for gas separations.
sites, porous organic polymers (POPs), activated carbons,
and carbon nanotubes, have been explored as solid adsorbents in very challenging C H /C H separation.
4
5
15
6
7
8
9
2
, zeolites, molecular sieves,
well-established porous materials, exploration of HOF materials
1
0
11
12
13
Actually, the first HOF-1 exhibits superior performance to MOFs
1
4
15h
Herein, we report
2
2
2 4
to realize olefin/paraffin separation. Two useful but distinct the synthesis of a robust hydrogen-bonded organic framework
(
HOF-4), which shows a high ideal adsorbed solution theory
a
Department 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
(IAST) selectivity of 14 for ethylene/ethane separation at room
temperature and normal pressure.
The new tetrahedral molecular tecton 2 (Scheme 1) has been
synthesized as a building block based on the following con-
siderations: (i) extended tecton 2 has both tetrahedral symme-
try and diaminopyridine in tecton 1, which is the basic building
unit of HOF-1 and is capable of forming multiple hydrogen
bonds and thus extending into the 3D framework; (ii) generally
speaking, longer ligands will lead to larger voids.
b
c
Van’t Hoff Institute for Molecular Sciences, University of Amsterdam,
Science Park 904, 1098 XH Amsterdam, The Netherlands
Department of Chemistry, Fudan University, Shanghai, 200433,
People’s Republic of China
d
Department of Chemistry, Faculty of Science, King Abdulaziz University,
Jeddah 22254, Saudi Arabia
†
Electronic supplementary information (ESI) available: Synthesis and character-
ization of HOF-4, PXRD, TGA, FTIR spectra, sorption isotherms, and break-
through simulations. CCDC 1010353. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c4cc05506c
The tetrahedral organic building block 2 was readily synthe-
sized in 92% yield by the reaction of the corresponding nitrile
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Scheme 1 Tetrahedral building blocks.
with dicyandiamide (see Scheme S1 in the ESI†). The colorless
needle-like crystals of HOF-4 were easily isolated in 79% yield
by evaporating DMF solution of 2 for a week at room temperature.
1
13
The purity of HOF-4 was confirmed by H NMR and C NMR
spectroscopy, thermogravimetric analysis (TGA), and powder X-ray
diffraction (PXRD) (Fig. S1–S3, ESI†). Single crystal X-ray diffraction
reveals that HOF-4 crystallizes in the monoclinic space group P2/n
and shows a 3D architecture consisting of six equivalent inter-
woven nets of PtS topology.‡ For a single net, the asymmetric unit
consists of only half of the building blocks (Fig. 1a left), and each
building block is connected with six neighbouring ones by 12
strong hydrogen bonds involving the 2,4-diaminotriazine (DAT)
groups (Fig. 1a right, the parameters of hydrogen bonding are
listed in Table S1, ESI†). There exist rhombic channels in the
single net along the [101] direction with an approximate dimen-
sion of 40 Å Â 30 Å along the diagonals (Fig. S4, ESI†). If one
considers the tetrahedral building block to be a four-connected
node in the tetrahedral geometry, and the multiple hydrogen
bonding motif of DAT groups to be a four-connected node in
the square planar geometry, the single net of HOF-4 can then be
2
4
rationalized as a 3D PtS {4 8 } network topology (Fig. 1b). Due to
large void spaces, six equivalent nets interpenetrate each other
via intermolecular pÁÁÁp interactions between the benzene rings
(
Fig. 1c). This high fold net interpenetration is expected to enhance
16–18
Fig. 1 X-ray structure of HOF-4 featuring (a) the basic organic building
block in which the central carbon atoms act as tetrahedral nodes (brown
balls) and centres of multiple hydrogen bonding motifs act as square planar
the framework stability.
direction are completely blocked due to the interpenetration,
The rhombic channels along the [101]
leaving a 1D rectangular channel (3.8 Å Â 8.1 Å) along the b axis
2 4
nodes (cyan balls); (b) a simplified binodal four-connected PtS (4 8 )
(Fig. 1d). The pore spaces within the frameworks encapsulate a topology; (c) six-fold interpenetrated frameworks; (d) the rectangular
channels (3.8 Â 8.1 Å) along the b axis (C, gray; H, white; N, pink).
few disordered DMF solvent molecules. The potential solvent
accessible void space accounts for approximately 42.5% of the
whole crystal volume as estimated by PLATON.
With a slit rectangular channel along the b axis (3.8 Å Â 8.1 Å),
Before examining adsorption properties, the guest solvent mole-
it is reasoned that ‘slim’ C
2
H
4
molecules (3.28 Å Â 4.18 Å Â 4.84 Å) cules in HOF-4 were removed by solvent exchange with acetone and
can access the channel in HOF-4a readily, while relatively ‘fat’ C
2
H
6
then vacuumed at 100 1C to obtain desolvated HOF-4a which is
thermally stable up to 400 1C. The porosity of HOF-4a was evaluated
1
9
molecules (3.81 Å Â 4.08 Å Â 4.82 Å) can hardly get through.
Furthermore, the amino groups residing on the surface wall of the by CO
gas sorption at 196 K (Fig. 2a). The type I isotherm shows a
2
framework might provide stronger hydrogen bonding interactions very sharp uptake at P/P0 o 0.1, indicative of a microporous
with more acidic C H molecules (pK = 44) than C H (pK = 50). material. Because of the flexible nature of the HOF, there exists a
2
4
a
2
6
a
We speculated that the size exclusion effect and hydrogen bonding small degree of sorption hysteresis. The isotherm gives an apparent
2
À1
interactions can work collaboratively to make HOF-4 an ideal Brunauer–Emmett–Teller (BET) surface area of 312 m g (Fig. S5,
material to separate C /C . To test our hypothesis, gas ESI†), which is moderate among a few examples of HOFs with
adsorption experiments were conducted.
2
H
4
2 6
H
1
5d
permanent porosity.
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The pure component isotherm data were fitted with the Langmuir
isotherm model (Fig. S7, ESI†). To understand the binding energy
at low coverage, isosteric heats of adsorption of C H and C H in
2
4
2 6
HOF-4a were calculated. Fig. S8 (ESI†) presents data on the
loading dependence of Qst in HOF-4a. The binding energy for
À1
C
2
H
4
in HOF-4a is 44 kJ mol , which is comparable in magnitude
10g
to those of MgMOF-74 and CoMOF-74. In contrast, the binding
À1
energy for C
that the HOF-4a–C
interaction at low coverage. Because HOF-4a is quite flexible,
2 6
H
in HOF-4a is only about 14 kJ mol , indicating
2 4
H interaction is much stronger than HOF-4a–
2 6
C H
so its pores can be slightly enlarged to accommodate a small
amount of C H during the adsorption process.
2
6
We further performed calculations using the ideal adsorbed
20
solution theory (IAST) of Myers and Prausnitz. Fig. 2c provides a
comparison of the adsorption selectivity of C –C in equi-
2 4
H
2 6
H
molar mixtures as a function of total bulk gas phase pressure in
HOF-4a and three well-known porous materials (MOF materials:
1
0a
10g
8c
FeMOF-74
96 K. It is worthy of note that the adsorption selectivity in respect
of C H /C H for HOF-4a is up to 14 at 1 atm and room
and CoMOF-74; zeolite material: NaETS-10 ) at
2
2
4
2 6
temperature, which not only surpasses the selectivity of the best
MOF materials but is also comparable to that of the best zeolite
material NaETS-10 for such an important separation, highlighting
HOF-4a as a promising material for C
industrial usage. The large pore spaces enable both FeMOF-74
and CoMOF-74 to take up much more C with increasing
2 4 2 6
H /C H separation for
2 6
H
pressure; while the narrow pore sizes in HOF-4a limit its adsorp-
tion capacity for C H even under increasing pressure, so HOF-4a
2
6
is unique for C H /C H separation: the separation selectivity
2
4
2 6
increases with increasing pressure.
In order to further validate the feasibility, breakthrough
simulation experiments were carried out using the established
methodology described in early publications of Krishna (see the
2
0
ESI† for details). The simulated breakthrough curves (Fig. 2d)
clearly show that HOF-4a can efficiently separate C H from the
2
4
C H –C H mixture at room temperature. The more poorly adsorbed
2
4
2 6
saturated C H breaks through earlier and can be recovered in a
2
6
nearly pure form (Fig. S9, ESI†). During the adsorption cycle,
at purities 499% can be recovered for a certain duration.
Once the entire bed is in equilibrium with the partial pressures
= p = 50 kPa, the desorption, or the ‘‘blowdown’’ cycle is
initiated, by applying a vacuum or by purging with inert gas.
9.95% of ethylene can be recovered during the time interval,
and NaETS-10 at 296 K; (d) transient breakthrough of an equimolar which can satisfy the purity requirement for production of
–C
mixture in an adsorber bed packed with HOF-4 in the ethylene as a feedstock in the polymer industry.
adsorption phase of a PSA operation. The inlet gas is maintained at partial
pressures p = p = 50 kPa and at a temperature of 296 K.
2 6
C H
Fig. 2 (a) CO
isotherms for C
2
sorption isotherm at 196 K; (b) single-component sorption
/C in HOF-4a at 296 K (solid symbol: adsorption,
open symbol: desorption); (c) comparison of the IAST calculations of
/C adsorption selectivities for HOF-4, FeMOF-74, CoMOF-74,
p
1
2
2
H
4
2 6
H
9
C
2
H
4
2 6
H
C
2
H
4
2 6
H
In summary, we have prepared and characterized a unique
six-fold interpenetrated HOF-4 material with PtS topology by
using an expanded tetrahedral tecton 2. The high degree of
1
2
Establishment of permanent microporosity in HOF-4 allowed interpenetration not only enhanced the structural integrity but
us to examine its utility as an adsorbent for industrially important also appropriately tuned the channel size to make HOF-4 an
C H /C H separations. Interestingly, the C H uptakes of ideal adsorbent for C H /C H separation. This is the first
2
4
2
6
2
4
2
4
2
6
3
À1
3
À1
1
7.3 cm g at 273 K and 11.1 cm g at 296 K were system- example of a porous hydrogen-bonded organic framework for
atically about three times higher than C
at 273 K and 3.6 cm g at 296 K at 1 atm (Fig. S6, ESI† and which the channel confinement effect and hydrogen bonding
3
À1
2
H
6
uptakes of 5.1 cm g
such an important industrial hydrocarbon separation, during
3
À1
Fig. 2b). This discovery motivated us to examine its feasibility for interactions appear to simultaneously control the uptake of
the industrially important C H /C H separation in more detail. different C2 hydrocarbons. It is believed that this work could
2 4 2 6
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render a new strategy for designing robust HOFs with perma-
nent porosity and promote more investigation on separation of
small hydrocarbons using novel porous organic materials.
This work was supported by the awards from the Welch
Foundation AX-1730.
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‡
Crystal data for HOF-4: C61
group P2 /n, a = 20.212(2) Å, b = 7.725(2) Å, c = 26.666(2) Å, b =
0.606(8)1, V = 3920.81(2) Å , Z = 2, D
F(000) = 1108.0, final R = 0.0976 for I 4 2s(I), wR
GOF = 1.133, CCDC 1010353.
48
H N20, M = 1061.17, monoclinic, space
1
3
À3
9
c
= 0.899 g cm , T = 193(2) K,
= 0.2139 for all data,
1
2
5
964–5972.
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