Pore environment engineering in metal-organic frameworks for efficient ethane/ethylene separation

Article abstract of DOI:10.1039/c9ta02822f

Selective adsorption of trace amounts of C₂H₆ from bulk C₂H₄ is a significantly important and extremely challenging task in industry, which requires an adsorbent with specific pore properties. Herein, we describe a strategy for adjusting the pore environment of metal-organic frameworks (MOFs) by introducing different amounts of methyl groups in the channel to enhance the guest-host interaction between C₂H₆ and the framework. To prove this concept, 2,3,5,6-tetramethylterephthalic acid (TMBDC) was deliberately added to a microporous MOF, Ni(BDC)(DABCO)_0.5, affording a series of mixed-ligand materials, Ni(BDC)_1-x(TMBDC)_x(DABCO)_0.5 (x = 0, 0.2, 0.45, 0.71, 1), having different pore environments. Significantly, these mixed-ligand materials demonstrated improved performance in terms of the adsorption capacity of C₂H₆ and C₂H₄ with an unprecedented C₂H₆ uptake of 2.21 mmol g^-1 for Ni(TMBDC)(DABCO)_0.5 at 0.0625 bar and 298 K. With the best theoretical C₂H₆/C₂H₄ selectivity predicted by IAST, Ni(TMBDC)(DABCO)_0.5 exhibited effective separation of C₂H₆/C₂H₄ (1/15, v/v) and great recyclability in five consecutive adsorption/desorption cycles throughout the breakthrough experiment.

Full text of DOI:10.1039/c9ta02822f

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2019, DOI: 10.1039/C9TA02822F.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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JPolueransael odfoMnaotetraiadljsuCshtemmairsgtriynsA
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DOI: 10.1039/C9TA02822F
Journal Name
ARTICLE
Pore Environment Engineering in Metal-Organic Frameworks for
Efficient Ethane/Ethylene Separation
a,b
b
Xun Wang, † Zheng Niu, † Abdullah M. Al-Enizi,c Ayman Nafady, c,d Yufang Wu,a Briana Aguila,b
Received 00th January 20xx,
Accepted 00th January 20xx
Gaurav Verma,b Lukasz Wojtas,b Yu-Sheng Chen,e Zhong Li,*a Shengqian Ma*b
DOI: 10.1039/x0xx00000x
Selective adsorption of trace amount of C2H6 from bulk C2H4 is a significantly important and extemely challenging task in
industry, which requires an adsorbent with specific pore properties. Herein, we describe a strategy for adjusting the pore
environment of metal-organic frameworks (MOFs) by introducing different amounts of methyl groups in the channel to
enhance the guest-host interaction between C2H6 and the framework. To prove this concept, 2,3,5,6-
tetramethylterephthalic acid (TMBDC) was delebertely added into a microporous MOF, Ni(BDC)(DABCO)0.5, affording a series
of mixed-ligand materials, Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45, 0.71, 1) having different pore environments.
Significantly, these mixed-ligand materials demonstrated improved performance in terms of the adsorption capacity of C2H6
and C2H4 with an unprecedented C2H6 uptake of 2.21 mmol/g for Ni(TMBDC)(DABCO)0.5 at 0.0625 bar and 298 K. With the
best theoretical C2H6/C2H4 selectivity predicted by IAST, the Ni(TMBDC)(DABCO)0.5 exhibited effective separation of
C2H6/C2H4 (1/15, v/v) and great recyclability in five consecutive adsorption/desorption cycles throughout the breakthrough
experiment.
www.rsc.org/
adjustable pore environments, and designable framework
structures.14-25 Research on the separation of ethylene/ethane using
MOFs with open metal sites has been widely reported, in which high
Introduction
As the most important raw material for manufacturing plastics,
ethylene (C2H4) is primarily obtained via steam cracking and thermal
decomposition of ethane (C2H6).1 The separation of C2H4/C2H6
mixtures becomes a challenging issue at the large scale because of
their similar molecular sizes and volatilities.2 In industry, the ethylene
is separated from ethane through cryogenic distillation at low
temperature (183 – 258 K) and high pressure (7 – 28 bar), making it
an extremely energy exhaustive process.1, 2 Therefore, separation
approaches that have low cost and energy-saving are highly desirable
in industry. Among several new technologies, adsorptive separation
has attracted extensive interest due to its operational simplicity and
the ability to afford high product purity.3-5 The cornerstone of this
promising technology is a solid adsorbent with specific properties.
Over the past few decades, metal-organic frameworks (MOFs)6-
13 have been explored as one kind of highly efficient solid adsorbent
for gas storage and separation because of their high surface areas,
ethylene uptake capacities and C2H4/C2H6 selectivities have been
achieved.26-32 Furthermore, π-complexation adsorbents,33 such as
Cu(I)-based sorbents34,35 and Ag(I)-based sorbents,36-39 were
developed since these materials can form π-complexation with the
carbon-carbon double bonds of C2H4, resulting in higher ethylene
capacities and C2H4 selectivities over C2H6. Virtually, these materials
preferentially adsorb C2H4 in C2H4/C2H6 mixtures owing to the
interactions between the open metal sites of the MOFs and C2H4 or
formation of π-complexation. Such a behavior is expected to
consume more energy during desorption because these interactions
are stronger than the traditional physisorption.
Practically, the ethylene concentration in the cracked gas feed is
much higher than that of ethane (C2H4:C2H6 = 15:1, v/v) and
consequently large amounts of adsorbents are required, giving rise
to a larger packed-bed column. In this context, it is strategically
essential to develop ethane-selective adsorbents for preferential
adsorption of trace amount of C2H6 from bulk C2H4. Nevertheless,
this class of materials have rarely been reported and most of them
exhibited poor C2H6/C2H4 adsorption capacities and/or separation
performances.41-45 Importantly, simulation and experimental results
revealed that C2H6 interacts more strongly with the material’s
framework than C2H4 owing to the fact that C2H6 has more C-H
bonds.41, 43-46 Inspired by these endeavors, we propose a strategy for
constructing a binding environment in MOFs that favors C2H6 over
a.School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou, 510640, China; E-mail: cezhli@scut.edu.cn
b.Department of Chemistry, University of South Florida, 4202 East Fowler Avenue,
Tampa, Florida, 33620, United States, Fax: +1 813-974-3203; Tel: +1 813-974-
5217; E-mail: sqma@usf.edu
c. Department of Chemistry, College of Science, King Saud University, Riyadh 11451,
Saudi Arabia
d.Chemistry Department, Faculty of Science, Sohag University, Sohag 82524, Egypt
e. ChemMatCARS, Center for Advanced Radiation Sources, The University of
Chicago, 9700 South Cass Avenue, Argonne, Illinois 60439, United States
† These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: Experimental details, TGA
curves, 1H NMR data and single-crystal data. CCDC 1879219. For
ESI and crystallographic data in CIF or other electronic format. See
DOI: 10.1039/x0xx00000x
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(yield: 86% based on DABCO). Elemental Analysis for CV1i1e.8wHA1r3ti.c6lNe OOn4lNinie,
Anal. Calcd. (%) C, 48.47; H, 4.66; N, 4.66. Found: C, 48.57; H, 4.78;
DOI: 10.1039/C9TA02822F
N, 4.56. FTIR (cm-1): 2959 (br), 1613 (m), 1511 (s), 1382 (s), 1229 (w),
1055 (m), 1022 (m).
Synthesis of Ni(BDC)0.55(TMBDC)0.45(DABCO)0.5: a 20 mL vial was
filled with Ni(NO3)2·6H2O (0.093 g, 0.32 mmol), DABCO (0.018 g, 0.16
mmol), TMBDC (0.034 g, 0.15 mmol), BDC (0.025 g, 0.15 mmol) and
8 mL DMF. After heated up to 120 °C for 48 h and purified with DMF
and ethanol, the Ni(BDC)0.55(TMBDC)0.45(DABCO)0.5 was obtained.
(yield: 80% based on DABCO). Elemental Analysis for C12.8H15.6NO4Ni,
Anal. Calcd. (%) C, 50.17; H, 5.10; N, 4.57. Found: C, 49.87; H, 5.46;
N, 4.46. FTIR (cm-1): 2933 (br), 1603 (m), 1548 (w), 1392 (s), 1230 (s),
1053 (m), 1016 (m).
Scheme 1. Schematic illustration of methyl group functionalization in MOF
pore to enhance the interaction toward ethane molecule
C2H4,through adjusting the guest-host interaction between the pores
and C2H6 molecules. as illustrated in Scheme 1.
Synthesis of Ni(BDC)0.29(TMBDC)0.71(DABCO)0.5: a 20 mL vial was filed
with Ni(NO3)2·6H2O (0.093 g, 0.32 mmol), DABCO (0.018 g, 0.16
mmol), TMBDC (0.054 g, 0.24 mmol), BDC (0.01 g, 0.06 mmol) and 8
mL DMF. After heated up to 120 °C for 48 h and purified with DMF
and ethanol, the Ni(BDC)0.29(TMBDC)0.71(DABCO)0.5 was obtained.
(yield: 73% based on DABCO). Elemental Analysis for
C13.84H17.68NO4Ni, Anal. Calcd. (%) C, 51.78; H, 5.51; N, 4.36. Found: C,
51.53; H, 5.90; N, 4.29. FTIR (cm-1): 2929 (br), 1601 (m), 1538 (m),
1441 (s), 1376 (s), 1235 (s), 1061 (m), 1016 (m).
To prove the above concept, we employed
a MOF,
Ni(BDC)(DABCO)0.5
1,4-diazabicyclo[2.2.2]octane) as a prototype host material. The
methyl group functionalized pores were constructed by introducing
,
41, 47-48 (BDC = 1,4-terephthalic acid, DABCO =
different amounts of the TMBDC ligand (TMBDC
tetramethylterephthalic acid) into the framework
=
2,3,5,6-
of
Ni(BDC)(DABCO)0.5 through a mixed-ligand strategy, affording a
series of MOFs, Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45, 0.71,
1). The obtained MOFs retained a similar framework, yet exhibited
varying C2H6/C2H4 adsorption performances with change in the mole
ratio of TMBDC/BDC. The selectivity of C2H6/C2H4 based on ideal
adsorbed solution theory (IAST) using the single component
experimental adsorption isotherms illustrated that the introduction
of methyl group into a MOF can effectively enhance the C2H6/C2H4
separation performance. The designed breakthrough experiments
confirmed the IAST calculated results, whereas the
adsorption/desorption cycling tests implied the robustness and
excellent recyclability of the material.
Synthesis of Ni(TMBDC)(DABCO)0.5: a 20 mL vial was filled with
Ni(NO3)2·6H2O (0.093g, 0.32 mmol), DABCO (0.018g, 0.16 mmol),
TMBDC (0.067 g, 0.3 mmol) and 8 mL DMF. After heated up to 120
°C for 48
h and purified with DMF and ethanol, the
Ni(TMBDC)(DABCO)0.5 was obtained. (yield: 68% based on DABCO).
Elemental Analysis for C15H20NO4Ni, Anal. Calcd. (%) C, 53.41; H, 5.93;
N, 4.15. Found: C, 53.16; H, 6.28; N, 4.19. FTIR (cm-1): 2943 (br), 1603
(m), 1596 (s), 1445 (s), 1376 (s), 1230 (s), 1057 (m), 1010 (m).
Synthesis of Ni(BDC)(DABCO)0.5: For comparison, Ni(BDC)(DABCO)0.5
was prepared according to procedures described elsewhere.41, 47-48
Experimental
Chemicals
Characterization
The N2 adsorption-desorption isotherms were measured at 77 K
using a liquid N2 bath. The surface area was calculated using the
Brunauer-Emmett-Teller equation in the range P/P0 = 0.05 – 0.35,
while the pore size distribution was calculated by the DFT method.
All samples were degassed at 120 °C for 6 h before analysis. Powder
X-ray diffraction (PXRD) were collected on a Bruker AXS D8 Advance
using Cu Kα (λ=1.5406 Å) radiation. The FT-IR data were recorded on
a PerkinElmer Spectrum Two. TGA curves were obtained using
All chemical reagents were purchased from commercial sources
(Sigma-Aldrich, Alfa, Fisher, Acros, TCI et.al.) and used without
further purification. 2,3,5,6-tetramethylterephthalic acid (TMBDC)
was synthesized according to a reported procedure in the literature
with some modification (ESI). 49
Synthesis of Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45, 0.71, 1)
General procedure: a 20 mL vial was charged with Ni(NO3)2·6H2O NETZSCH STA 449F3 Simultaneous Thermal Analyzer (NETZSCH,
(0.093g, 0.32 mmol), DABCO (0.018g, 0.16 mmol), BDC ligands Germany). 1H NMR spectra were performed on a UnityInova 400
(TMBDC and/or BDC) and 8 mL DMF. After vigorous stirring, one drop Spectrometer (400 MHz).
of HNO3 was added. The mixture was sonicated to get a clear solution
and then heated at 120 °C for 48 h to afford green crystals. The Gas adsorption isotherms
crystals were collected by filtration and soaked in DMF to remove The C2H6 and C2H4 adsorption isotherms were collected on a 3 Flex
any residual reactants and then ethanol to exchange the DMF. The Surface Characterization Analyzer (Micromeritics, USA). Before each
obtained Ni(BDC)1-x(TMBDC)x(DABCO)0.5 was kept in ethanol until measurement, the samples were degassed at 120 °C under vacuum
used. The exact ratio of TMBDC/BDC was determined by 1H NMR for 6 h.
after decomposing the MOFs using HCl solution (ESI, Fig. S5 – S9)
Breakthrough experiments
Synthesis of Ni(BDC)0.8(TMBDC)0.2(DABCO)0.5: a 20 mL vial was fikled
with Ni(NO3)2·6H2O (0.093 g, 0.32 mmol), DABCO (0.018 g, 0.16
mmol), TMBDC (0.014 g, 0.06 mmol), BDC (0.04 g, 0.24 mmol) and 8
mL DMF. After heated up to 120 °C for 48 h and purified with DMF
and ethanol, the Ni(BDC)0.8(TMBDC)0.2(DABCO)0.5 was obtained.
2 | J. Name., 2012, 00, 1-3
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Fig. 1 (a) The primary cavity of Ni(TMBDC)(DABCO)0.5; (b)The connection of one
nickel paddlewheel and linkers; (c) pcu topology of Ni(TMBDC)(DABCO)0.5
(green, nickel; grey, carbon; blue, nitrogen; red, oxygen; white, hydrogen)
Fig. 2 (a) PXRD patterns, (b) N2 adsorption-desorption isotherms at 77 K and
(c) Pore size distributions of Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45,
0.71, 1).
The breakthrough curves of the gas mixture C2H6/C2H4 (1:15, v/v)
were collected on a self-assembly experimental apparatus (Fig. S2).
Breakthrough experiments were carried out at 298 K on a gas
chromatography apparatus (GC-9560, Shanghai Huaai), equipped
with a 2 m long Al2O3-packed column with an FID detector. Typically,
~ 200 mg pre-degassed sample was packed into a stainless steel
column with inner dimension of Φ6 × 275 mm. The carrier gas was
N2 having a flow rate of 45 mL/min, and the flow rate of C2H6/C2H4
gas mixture was controlled at 1 mL/min by a mass flow controller
(FMA-A200, America). For the consecutive adsorption/desorption
cycling tests, the sample packed column was regenerated by purging
He at a rate of 30 mL/min at 100 °C for 20 min. After that the gas flow
was switched to the C2H6/C2H4 mixture for the next cycle.
Ni(TMBDC)(DABCO)0.5 was presented and discussed, since all of them
are isostructural. As shown in Figure 1, each nickel paddlewheel is
connected with four linear linkers of TMBDC2- to form two-
dimensional layers, while the DABCO molecules coordinate at the
vertex of the nickel paddlewheels as the pillar to bridge the layers,
thus affording the 3D framework (Fig. 1b). Assuming the nickel
paddlewheel to be the 6-connected nodes and the ligands as linkers,
the Ni(TMBDC)(DABCO)0.5 displays a 6-coodinated network with the
topology of pcu (Fig 1c). The primary cavity of Ni(BDC)1-
x(TMBDC)x(DABCO)0.5 (x = 0.2, 0.45, 0.71, 1) is constructed by eight
nickel paddlewheels, eight BDC2- ligands, and four DABCO pillared
ligands, exhibiting a cubic shaped cavity suitable to fit a sphere with
a diameter of 6.5 – 8.7 Å, which increased along with the decrease of
the TMBDC/BDC ratio.
Results and discussion
Characterization of the materials
Phase purity of the five materials was confirmed by PXRD studies.
As shown in Fig. 2a, the series of materials show high crystallinity and
purity. The PXRD patterns of Ni(BDC)(DABCO)0.5 are in good
agreement with those reported in literatures. 37, 44-46 The Ni(BDC)1-
x(TMBDC)x(DABCO)0.5 (x = 0.2, 0.45, 0.71, 1) retained the same XRD
patterns as that of Ni(BDC)(DABCO)0.5, implying that the introduction
of the TMBDC ligand did not interrupt the crystal phase purity.
Thermal stability of Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45,
0.71, 1) was studied by thermogravimetric analysis. All TGA curves
exhibited the negligible weight loss before 400 °C and great loss
(~65%) after that temperature, which indicated Ni(BDC)1-
x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45, 0.71, 1) were thermally stable
up to 400 °C. (Fig. S2)
Nitrogen adsorption-desorption isotherms at 77 K were used to
investigate the porosity of Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0, 0.2,
0.45, 0.71, 1). As shown in Fig. 2b, all five materials exhibit typical
type-I sorption behaviors, suggesting the microporous structure in
their frameworks. With the increase of the TMBDC/BDC ratio, the
BET surface area and pore volume of Ni(BDC)1-x(TMBDC)x(DABCO)0.5
(x = 0, 0.2, 0.45, 0.71, 1) decreases from 2050 m2/g and 0.80 cm3/g
for Ni(BDC)(DABCO)0.5 to 894 m2/g and 0.39 cm3/g for
Solvothermal reaction of 1,4-terephthalic acid (H2BDC), 2,3,5,6-
tetramethylterephthalic
acid
(TMBDC),
1,4-
Diazabicyclo[2.2.2]octane (DABCO) with Ni(NO3)2·6H2O in DMF at
120 °C yielded green rod-shaped crystals of Ni(BDC)1-
x(TMBDC)x(DABCO)0.5 (x = 0.2, 0.45, 0.71, 1). The single crystal X-ray
diffraction measurement was employed to analyze the structures of
the MOFs, and the results revealed that all mixed-ligand samples
crystallized in the P4/mmm space group (Fig. 1a). The structure of
Table 1 BET surface areas and pore volumes of Ni(BDC)1-
x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45, 0.71, 1)
BET surface area
(m2/g)
2050
Pore volume
(cm3/g)
0.80
Materials
Ni(BDC)(DABCO)0.5
Ni(BDC)0.8(TMBDC)0.2(DABCO)0.5
Ni(BDC)0.55(TMBDC)0.45(DABCO)0.5
Ni(BDC)0.29(TMBDC)0.71(DABCO)0.5
Ni(TMBDC)(DABCO)0.5
1556
0.63
1294
0.53
1084
0.45
894
0.39
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Fig.
4
(a) IAST selectivities of C2H6/C2H4 (1:15) on Ni(BDC)1-
Fig.
3
(a) C2H6 and (b) C2H4 adsorption isotherms of Ni(BDC)1-
x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45, 0.71, 1) and (b) Isosteric heats of C2H6
and C2H4 adsorption on Ni(TMBDC)(DABCO)0.5 at 298 K.
x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45, 0.71, 1) at 298 K.
equilibrium uptake and low-pressure uptake of C2H6 and C2H4 for
Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0.2, 0.45, 0.71, 1) should be
ascribed to the introduction of the TMBDC ligand. In principle, as the
ratio of TMBDC/BDC increases, the amount of methyl groups from
the TMBDC ligand increases, which leads to a stronger interaction
toward the gas molecules. C2H6 molecules, which have more C-H
bonds, are expected to induce stronger interaction with the methyl
groups in the MOF than C2H4, thereby leading to more preferential
adsorption of C2H6 in Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0.2, 0.45,
0.71, 1).
Ni(TMBDC)(DABCO)0.5, respectively (Table 1). The decrease of both
BET surface area and accessible pore volume can be ascribed to the
increased number of methyl groups in the framework (Table 1).
Pore size distribution analysis calculated by the density functional
theory method (DFT) further confirmed the methyl group occupation
in the pores. The results showed that with the increase of the
TMBDC/BDC ratio, the main pore size distribution is shifted from ~8.3
Å for Ni(BDC)(DABCO)0.5 to ~5.9 Å for 100% Ni(TMBDC)(DABCO)0.5
.
Consequently, we have judiciously tuned the pore size and pore
environment of the pristine Ni(BDC)(DABCO)0.5 by introducing the
In order to assess the potential of Ni(BDC)1-x(TMBDC)x(DABCO)0.5
(x = 0.2, 0.45, 0.71, 1) for C2H6/C2H4 separation, ideal adsorbed
solution theory (IAST) was used to predict the C2H6/C2H4 (1:15 v/v)
selectivity for all Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45, 0.71,
1). Before IAST calculation, C2H6 and C2H4 adsorption isotherms at
298 K were fitted by dual-site Langmuir-Freundlich (DSLF) model
(ESI). As shown in Fig. 4a, the IAST selectivity of pristine Ni(BDC)
(DABCO)0.5 is 1.619 at 298 K and 100 kPa. As the ratio of TMBDC/BDC
is increased, the selectivity of Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0.2,
0.45, 0.71, 1) increases and reaches up to 1.985 for
Ni(TMBDC)(DABCO)0.5 at 100 kPa, indicating that it exhibits the best
performance for C2H6/C2H4 separation among these five materials.
The interaction between gas molecules and the host framework was
evaluated by isosteric heat, Qst, which is calculated by the Clausius-
Clapeyron equation (ESI). The obtained Qst of C2H6 is slightly higher
than that of C2H4, further confirming the preferential adsorption of
Ni(TMBDC)(DABCO)0.5 toward C2H6. Additionally, the Qst values of
C2H6 in Ni(TMBDC)(DABCO)0.5 are ~39 kJ/mol, which are higher than
TMBDC ligand into the framework.
TMBDC/BDC ratio, mixed-ligand
x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45, 0.71, 1) were successfully
synthesized, which is anticipated to influence the adsorption and
separation performances of C2H6 and C2H4 to obtain the desirable
C2H6/C2H4 separation selectivity.
The success of constructing methyl group functionalized pores in
the MOFs promoted us to investigate the C2H6 and C2H4 adsorption
performance of Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45, 0.71,
1). Single component adsorption isotherms of both C2H6 and C2H4
(Fig. 3, Fig. S3-S9) were measured at 273 K, 298 K and 308 K. As shown
in Fig. 3, all of the Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0, 0.2, 0.45,
0.71, 1) exhibited preferential adsorption of C2H6 over C2H4. The
equilibrium uptake of C2H6 and C2H4 for Ni(BDC)(DABCO)0.5 can reach
4.36 mmol/g and 3.04 mmol/g at 1 bar, respectively. However, the
C2H6 and C2H4 uptake amounts at the low pressure region (0 –0.3 bar)
are not high, which are insufficient to address the challenging issue
of the industrial separation of C2H6 and C2H4.
Through altering the
materials, Ni(BDC)1-
the reported values including those of Ni(BDC)(DABCO)0.5,
41 IRMOF-
When the concentration of TMBDC ligand increases, the
Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x = 0.2, 0.45, 0.71, 1) takes up an
increasing amount of both ethane and ethylene at 1 bar as well as
the low pressure region (0 – 0.3 bar). The equilibrium uptake of C2H6
and C2H4 for Ni(TMBDC)(DABCO)0.5 reached 5.45 mmol/g and 5.02
mmol/g at 1 bar, respectively, which are comparable with the
previously reported values of other materials.27-28, 45, 54-55 However,
the high C2H6 uptakes in the low pressure region make these
materials promising for the challenging separation of trace amount
of C2H6 from bulk C2H4. Given the low concentration of ethane
(C2H6:C2H4 = 1:15, v/v) in the industrial cracked gas feed, the gas
uptake at 0.0625 bar (P/P0 = 0.0625) should be given more attention.
As shown in Fig. 3(a), Ni(TMBDC)(DABCO)0.5 exhibits an
unprecedented C2H6 uptake of 2.21 mmol/g at 0.0625 bar, which is
higher than 1.47 mmol/g (35 cm3/g) for MAF-49,42 2 mmol/g for
Fe2(dobdc) (318 K),27 and 0.8 mmol/g for Cu-BTC (295 K).54 The higher
8,42 and MAF-3,42 illustrating a strong guest-host interaction between
the framework of Ni(TMBDC)(DABCO)0.5 and C2H6 molecules. Based
on the outstanding IAST calculation results, Ni(TMBDC)(DABCO)0.5
was chosen for further investigations.
To explore the dynamic separation of Ni(TMBDC)(DABCO)0.5 for
the C2H6/C2H4 mixture, a breakthrough experiment was undertaken,
in which the simulated cracked gas feed of C2H6/C2H4 (1/15, v/v) was
used. As shown in Fig. 5a, a clean and sharp separation of C2H6 and
C2H4 was observed. The initial outlet effluent gas contains only C2H4
with high purity (>99.9%), indicating the efficient separation for the
C2H6/C2H4 mixture when Ni(TMBDC)(DABCO)0.5 is used as the
adsorbent in the packed column. The recycle test of
Ni(TMBDC)(DABCO)0.5 was evaluated in the breakthrough apparatus.
Fig. 5b shows the breakthrough curves of five consecutive
adsorption-desorption cycles in
a
column packed with
4 | J. Name., 2012, 00, 1-3
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The authors acknowledge the China ScholarVsihewipArtiCcloe uOnnlciniel
DOI: 10.1039/C9TA02822F
(201606150020), Key Program of National Natural Science
Foundation of China (No. 21436005), the Guangdong Province
Science and Technology Project (No. 2016A020221006), the
United States National Science Foundation (DMR-1352065) and
the University of South Florida for financial support of this work.
Single crystal diffraction data were collected at Argonne
National Laboratory, Advanced Photon Source, Beamline 15-ID-
B of ChemMatCARS, which is principally supported by the
Divisions of Chemistry (CHE) and Materials Research (DMR),
National Science Foundation, under grant number NSF/CHE-
1346572. The use of the PILATUS3 X CdTe 1M detector is
supported by the National Science Foundation under the grant
number NSF/DMR-1531283. Use of the Advanced Photon
Source, an Office of Science User Facility operated for the U.S.
Department of Energy (DOE) Office of Science by Argonne
National Laboratory, was supported by the U.S. DOE under
Contract No. DE-AC02-06CH11357. The authors also extend
their appreciation to the Distinguished Scientist Fellowship
Program (DSFP) at King Saud University for partial funding of
this work.
Fig. 5 Breakthrough curves of C2H6/C2H4 (1:15) in a column packed with
Ni(TMBDC)(DABCO)0.5 at 298 K (a) in a single run; (b) five consecutive
adsorption-desorption cycles.
Ni(TMBDC)(DABCO)0.5. In each cycle, the material was regenerated
completely within 20 min, showing remarkable recyclability and
enhanced stability in the capacity. It should be noted that the
Ni(TMBDC)(DABCO)0.5 packed column was regenerated without
vacuuming, making it feasible for industrial applications.
Conclusions
In summary, a series of Ni(BDC)1-x(TMBDC)x(DABCO)0.5 (x =
0.2, 0.45, 0.71, 1) with controllable methyl group functionalized
pores have been successfully constructed for C2H6/C2H4
separation. The series of MOFs exhibit similar frameworks but
different pore environments. Owing to the adjustment of the
methyl groups in the framework, Ni(BDC)1-x(TMBDC)x(DABCO)0.5
(x =0, 0.2, 0.45, 0.71, 1) shows different guest-host interactions
toward C2H6 and C2H4 than the pristine material, leading to a
different adsorption capacity of both C2H6 and C2H4. Containing
more C-H bonds, the adsorption of C2H6 is highly favoured in
these mixed-ligand materials and an unprecedented C2H6
capacity of 2.21 mmol/g at 0.0625 bar was obtained for
Ni(TMBDC)(DABCO)0.5. With the best IAST selectivity of 1.985
and high Qst of 35 kJ/mol for C2H6, Ni(TMBDC)(DABCO)0.5 was
selected for the breakthrough experiment and recyclability test,
which showed great potential for C2H6/C2H4 (1:15, v/v)
separation. Significantly, this pore environment engineering
approach is most likely to facilitate the design of other novel
MOF materials for other important gas separations.
Notes and references
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the matter
under discussion, limited experimental and spectral data, and
crystallographic data.
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