Introduction of π-complexation into porous aromatic framework for highly selective adsorption of ethylene over ethane

Article abstract of DOI:10.1021/ja502119z

In this work, we demonstrate for the first time the introduction of π-complexation into a porous aromatic framework (PAF), affording significant increase in ethylene uptake capacity, as illustrated in the context of Ag(I) ion functionalized PAF-1, PAF-1-SO₃Ag. IAST calculations using single-component-isotherm data and an equimolar ethylene/ethane ratio at 296 K reveal that PAF-1-SO₃Ag shows exceptionally high ethylene/ethane adsorption selectivity (S_ads: 27 to 125), far surpassing benchmark zeolite and any other MOF reported in literature. The formation of π-complexation between ethylene molecules and Ag(I) ions in PAF-1-SO ₃Ag has been evidenced by the high isosteric heats of adsorption of C₂H₄ and also proved by in situ IR spectroscopy studies. Transient breakthrough experiments, supported by simulations, indicate the feasibility of PAF-1-SO₃Ag for producing 99.95%+ pure C ₂H₄ in a Pressure Swing Adsorption operation. Our work herein thus suggests a new perspective to functionalizing PAFs and other types of advanced porous materials for highly selective adsorption of ethylene over ethane.

Full text of DOI:10.1021/ja502119z

Article
pubs.acs.org/JACS
Introduction of π‑Complexation into Porous Aromatic Framework for
Highly Selective Adsorption of Ethylene over Ethane
Baiyan Li,† Yiming Zhang,† Rajamani Krishna,‡ Kexin Yao,§ Yu Han,§ Zili Wu,∥ Dingxuan Ma,⊥
Zhan Shi,⊥ Tony Pham,† Brian Space,† Jian Liu,# Praveen K. Thallapally,# Jun Liu,#
Matthew Chrzanowski,† and Shengqian Ma*,†
†Department of Chemistry, University of South Florida, 4202 E. Fowler Avenue, Tampa, Florida 33620, United States
‡Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands
§Advanced Membranes and Porous Materials Center, Physical Sciences and Engineering Division, King Abdullah University of
Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia
∥Center for Nanophase Material Sciences and Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee
37831, United States
⊥State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012,
People’s Republic of China
#Pacific Northwest National Laboratory, Richland, Washington 99352, United States
W
S
* Web-Enhanced Feature * Supporting Information
ABSTRACT: In this work, we demonstrate for the first time
the introduction of π-complexation into a porous aromatic
framework (PAF), affording significant increase in ethylene
uptake capacity, as illustrated in the context of Ag(I) ion
functionalized PAF-1, PAF-1-SO3Ag. IAST calculations using
single-component-isotherm data and an equimolar ethylene/
ethane ratio at 296 K reveal that PAF-1-SO3Ag shows
exceptionally high ethylene/ethane adsorption selectivity
(Sads: 27 to 125), far surpassing benchmark zeolite and any
other MOF reported in literature. The formation of π-
complexation between ethylene molecules and Ag(I) ions in PAF-1-SO3Ag has been evidenced by the high isosteric heats of
adsorption of C2H4 and also proved by in situ IR spectroscopy studies. Transient breakthrough experiments, supported by
simulations, indicate the feasibility of PAF-1-SO3Ag for producing 99.95%+ pure C2H4 in a Pressure Swing Adsorption operation.
Our work herein thus suggests a new perspective to functionalizing PAFs and other types of advanced porous materials for highly
selective adsorption of ethylene over ethane.
Over the past decade, advanced porous materials such as
metal−organic frameworks (MOFs)7 and porous organic
polymers (POPs)8 [e.g., porous aromatic frameworks
(PAFs),9 conjugated microporous polymers (CMPs),10 porous
polymer networks (PPNs),11 and porous organic frameworks
(POFs)12] have been explored as new classes of solid
adsorbents for applications in gas storage,13 gas separation,14
carbon capture,15 catalysis,16 and so forth. Compared with
conventional solid adsorbents of zeolites and mesoporous silica
materials,17 MOFs7 and POPs8 feature the amenability of
design and modular nature, adjustable pore sizes, functionaliz-
able pore surfaces, and high surface areas. These features also
make them hold great promise for hydrocarbon separation,18
including the separation of ethylene/ethane mixtures.19
INTRODUCTION
■
Ethylene, one of the most widely used feedstock molecules in
the petrochemical industry, is usually obtained via steam
cracking and thermal decomposition of ethane.1 The similar
molecular sizes and volatilities make the separation of ethylene/
ethane mixtures one of the most challenging chemical
separations at large scale.2 Current technology uses cryogenic
distillation performed under the conditions of high pressure
(23 bar) and low temperature (−25 °C), resulting in an
extremely cost and energy intensive process.3 Extensive efforts
to develop low energy approaches for efficient ethylene/ethane
separation at higher temperature and normal atmospheric
pressure have focused on membrane separation,4 organic
solvent-based sorbents,5 and porous solid adsorbents.6
Among these approaches, porous solid adsorbents attract
particular interest because of their great potential to afford
much lower cost and energy consumption.
Received: March 1, 2014
© XXXX American Chemical Society
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Adsorption-based separation of ethylene/ethane using MOFs
focuses on the preferential interactions between open metal
sites and ethylene molecules, and high ethylene uptake
capacities and ethylene/ethane selectivities have been demon-
strated in MOFs with open metal sites.19a
−f
In comparison with MOFs, POPs, despite the amorphous
nature for most of them, feature robust covalent framework
structures showing high water, moisture, and chemical
stability;8 they could also be readily scaled up using one-pot
reactions. However, the lack of preferential binding sites for
ethylene molecules leads to poor ethylene/ethane adsorption
selectivity.8c,19g A recently reported copper(catecholate)
decorated POP demonstrates enhanced ethylene/ethane
selectivity;20 nevertheless, the absolute selectivity remains low,
presumably due to the moderate interactions between open
Cu(II) sites and ethylene molecules. Therefore, in order to
achieve high ethylene/ethane selectivity in POPs, stronger
binding sites for ethylene molecules are desired.
It has been well-documented that Cu(I) and Ag(I) ions can
form π-complexation with the carbon−carbon double bonds of
olefin molecules in solutions,5,6,21 and these systems have been
employed for absorptive separations of olefins from paraffins,
which are however inefficient because of the poor contact
between the hydrocarbons and the liquid absorbents.5 We
postulate that if such kinds of π-complexation can be
introduced into POP, the π-complexation will afford strong
interactions between the ethylene molecules and the frame-
work, whereas the porous structure of POP can maximize the
contact of between the ethylene molecules and the framework,
thereby resulting in high ethylene/ethane selectivity. In this
contribution, we demonstrate for the first time the introduction
of π-complexation into POPs, as illustrated in the context of
functionalizing the highly porous PAF, PAF-122 with Ag(I)
ions. The resultant PAF-1-SO3Ag not only exhibits significant
enhancement of ethylene uptake capacity compared to the
parent PAF-1, but also demonstrates exceptional ethylene/
ethane adsorption selectivity, far surpassing benchmark zeolite
and any other MOF and POP reported thus far. The formation
of π-complexation between ethylene molecules and Ag(I) ions
in PAF-1-SO3Ag has been proven by heat of adsorption analysis
and in situ IR spectroscopic studies.
Figure 1. N2 sorption isotherms at 77 K for PAF-1 (black), PAF-1-
SO3H (red), and PAF-1-SO3Ag (blue).
The presence of Ag(I) in PAF-1-SO3Ag was confirmed by X-
ray photoelectron spectroscopy (XPS) analysis, which shows a
silver signal at binding energies of 368.8 and 374.8 eV (SI
Figure S2) corresponding to the peaks of Ag 3d5/2 and Ag
3d3/2, respectively. Fourier transform infrared spectroscopy
(FTIR) of PAF-1-SO3Ag shows the obvious characteristic peak
of SO3 group at 1086 and 1186 cm−1, respectively (SI Figure
−
S3). Solid 13C NMR spectra of PAF-1-SO3Ag and PAF-1-SO3H
show similar central carbon atom signals at δ = 65 ppm and the
signals of aromatic carbon (δ = 121 ppm to 147 ppm),
indicating the preservation of framework structure after Ag(I)
ion exchange (SI Figure S4). Inductively coupled plasma mass
spectrometry (ICP-MS) and elemental analysis (EA) indicate
that ∼50% SO3H were exchanged into SO3Ag.
Ethylene and Ethane Adsorption. The low-pressure
ethylene sorption isotherms were collected at 296 K. The
incorporation of Ag(I) ion into PAF-1 results in a significant
enhancement of ethylene adsorption capacity despite the
remarkable decrease in surface area. At 296 K and 1 atm, the
ethylene uptake amounts of PAF-1 and PAF-1-SO3H are 57
and 66 cm3·g−1, respectively (SI Figure S5). In contrast, PAF-1-
SO3Ag exhibits a significantly higher ethylene uptake capacity
of 91 cm3·g−1 (4.1 mmol·g−1) under the same conditions
(Figure 2). PAF-1-SO3Ag surpasses the ethylene uptake
capacity of zeolite 5A25 (∼2.3 mmol·g−1 at 303 K and 1 atm)
and compares to that of zeolite NaX26 (∼4.2 mmol·g−1 at 305
K and 1 atm), two benchmark zeolites widely studied for
ethylene/ethane separation. In addition, PAF-1-SO3Ag outper-
forms the copper(catecholate) decorated POP, CuA10B1,20 in
ethylene uptake, which exhibits an ethylene adsorption amount
RESULTS AND DISCUSSION
■
Materials Preparation and Physicochemical Charac-
terization. PAF-122 [(cross-linked poly tetraphenylmethane)
also known as (a.k.a.) PPN-623] is an amorphous POP
possessing a hypothetical diamondoid-topology structure with
very high surface area and exceptional stability in water/
moisture and acidic/basic media. PAF-1-SO3Ag can be readily
achieved by Ag(I) ion exchange of sulfonate-grafted PAF-1
(hereafter denoted PAF-1-SO3H) following the procedures
reported previously (Supporting Information (SI) Scheme
S1).23a,24a
N2 gas sorption isotherms at 77 K (Figure 1) reveal
Brunauer−Emmett−Teller (BET) surface areas of 4714, 1087,
and 783 m2·g−1 for PAF-1, PAF-1-SO3H, and PAF-1-SO3Ag,
respectively. Pore size distribution analysis (Horvath−Kawazoe
model) indicates that the pore size is reduced from ∼15 Å for
PAF-1 to ∼8 Å for PAF-1-SO3H, whereas the pore size of PAF-
1-SO3Ag is predominantly distributed around ∼8 Å, suggesting
negligible pore size change after the Ag(I) ion exchange process
(SI Figure S1).
Figure 2. C2H4 (black) and C2H6 (red) sorption isotherms of PAF-1-
SO3Ag at 296 K. Filled: adsorption; unfilled: desorption.
B
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of ∼1.8 mmol·g−1 at 0.79 atm and 298 K. The ethylene uptake
capacity of PAF-1-SO3Ag at 296 K and 1 atm is relatively lower
compared to that of some high surface area MOFs possessing
open metal sites (e.g., 7.2 mmol·g−1 of MgMOF-74, 7.2 mmol·
g−1 of Cu-BTC, and 5.8 mmol·g−1 of NOTT-102),19a but the
MOFs containing open metal sites usually experience partial
framework degradation after exposed to moisture, inevitably
leading to drastic decreases in ethylene uptake capacity upon
reuse. In contrast, the fact that PAF-1-SO3Ag was prepared via
ion exchange in aqueous solution suggests its water stability.
This, together with its moisture stability, is further confirmed
by the reproducibility of the ethylene sorption isotherms for
PAF-1-SO3Ag even after exposure to an air environment with
80% humidity for 2 days (SI Figure S6).
and 3.8, respectively. It is worth noting that the ethylene/
ethane adsorption selectivities of PAF-1-SO3Ag are consid-
erably higher than those of zeolite NaX and other MOFs over
the entire pressure range with the adsorption selectivity value at
1 kPa (Sads = 125) even about an order of magnitude higher
(Figure 3).
In practice, the combination of adsorption selectivity and
uptake capacity of gas mixtures contribute to the characteristics
of ethylene/ethane separation.18c,19a Figure 4 shows the IAST
To test the recyclability of PAF-1-SO3Ag, we simulated
temperature and vacuum swings with an ASAP2020 analyzer,
by saturating with ethylene up to 1.1 bar at 296 K followed by a
high vacuum for 3 h at 105 °C. After 5 cycles, there was no
apparent loss in capacity (SI Figure S7), indicating the
complete desorption during each regeneration cycle. Upon
the basis of the differential scanning calorimetry (DSC)
analysis, energies of 2.67 MJ/kg are needed to release ethylene
and regenerate PAF-1-SO3Ag (SI Figure S8).19b
Interestingly, different from the ethylene adsorption, the
trend of ethane uptake by the three samples follows the order
of PAF-1 > PAF-1-SO3H > PAF-1-SO3Ag at 296 K and 1 atm
(SI Figure S9). The smallest ethane uptake amount observed
for PAF-1-SO3Ag is primarily attributed to its lower surface area
when compared with PAF-1 and PAF-1-SO3H. This result also
suggests that the incorporation of Ag(I) ions would not
increase the ethane uptake capacity.
Figure 4. IAST calculations of the C2H4/C2H6 adsorption selectivity
versus the gravimetric uptake capacity of ethylene for adsorption from
an equimolar C2H4/C2H6 mixture at the total bulk gas phase at 296 K
and 100 kPa19a (Note: the uptake capacity of ethylene for FeMOF-74
is at 318 K19b).
calculations of the ethylene/ethane adsorption selectivity versus
the gravimetric uptake capacity of ethylene for adsorption from
an equimolar ethylene/ethane mixture at the total bulk gas
phase at 296 K and 100 kPa for PAF-1-SO3Ag and several
benchmark microporous adsorbent materials.19a Both adsorp-
tion selectivity and gravimetric uptake capacity of PAF-1-SO3Ag
are significantly higher than two important zeolites of NaETS-
1028 and NaX.19a,29 The volumetric ethylene uptake capacity of
PAF-1-SO3Ag (SI Figure S11), which is estimated based on the
density of the compressed PAF-1-SO3Ag pellet, also surpasses
that of NaETS-1028 and NaX.19a,29 Albeit the ethylene uptake
capacity of PAF-1-SO3Ag is lower than that of some MOF
materials, much higher ethylene adsorption selectivity alongside
excellent water stability represent advantages in practice over
most MOFs30 investigated so far.
Ethylene/ethane adsorption selectivities were calculated
using ideal adsorbed solution theory (IAST)27 for PAF-1-
SO3Ag, PAF-1, and PAF-1-SO3H (Figure 3). For an equimolar
Ethylene−Framework Interactions. We reasoned that
the exceptional ethylene adsorption properties of PAF-1-SO3Ag
should stem from the strong interactions between ethylene
molecules and the framework of PAF-1-SO3Ag as a result of the
formation of π-complexation between the d orbitals of Ag(I)
and the π orbitals of carbon−carbon double bonds in
ethylene.5,6 We estimated the isosteric heats of adsorption
(Qst) based upon Clausius−Clapeyron equation by differ-
entiation of the dual-Langmuir−Freundlich fits of the isotherms
at two different temperatures,18c,19a 296 and 318 K(SI Figure
S12) with T-dependent parameters. As shown in Figure 5, at
close to zero loading, the Qst for ethylene in PAF-1-SO3Ag is
106 kJ·mol−1, remarkably higher than that of PAF-1 (14 kJ·
mol−1) and PAF-1-SO3H (23 kJ·mol−1). The Qst for ethylene in
PAF-1-SO3Ag is consistent with that observed in other Ag(I)-
based π-complexation systems,21,31 suggesting the formation of
π-complexation between the ethylene molecules and Ag(I) ions
in PAF-1-SO3Ag. The Qst exceeds that in MOFs with open
Figure 3. Comparison of the IAST calculations for C2H4/C2H6
adsorption selectivities for PAF-1-SO3Ag with PAF-1, PAF-1-SO3H
and other porous materials19a at 296 K.
mixture of ethylene and ethane at 296 K, the adsorption
selectivity (Sads) obtained for PAF-1-SO3Ag is 27 at 100 kPa, far
exceeding those calculated for both PAF-1 (Sads = 0.7) and
PAF-1-SO3H (Sads = 0.88). The ethylene/ethane adsorption
selectivity of PAF-1-SO3Ag at 296 K and 100 kPa is also
significantly higher than those of zeolite NaX,19a the MOFs19a
FeMOF-74 [a.k.a. Fe2(dobdc)], CoMOF-74 [a.k.a.
Co2(dobdc)], MgMOF-74 [a.k.a. Mg2(dobdc)], CuBTC
(a.k.a. HKUST-1) (Figure 3), and the POP CuA10B1,20
exhibiting ethylene/ethane selectivities of 8, 11, 6.4, 5.6, 3.6,
C
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1960 cm−1 (combination mode of CH2 wagging) and 1634
cm−1 (CC stretching), not observed on the PAF-1 and PAF-
1-SO3H, further confirm ethylene adsorption on PAF-1-SO3Ag.
These new IR bands due to adsorbed ethylene persist well after
room temperature desorption, indicating a strong interaction
between ethylene and PAF-1-SO3Ag. The blue-shift of the 
CH2 wagging mode can be attributed to the combinative d−π
and d−π* interaction between Ag and ethylene,33−36 thus
confirming the formation of π-complexation between the
ethylene and Ag(I) ions in PAF-1-SO3Ag.
Ethylene/Ethane Breakthrough Experiments and
Simulations. To evaluate the performance of PAF-1-SO3Ag
in an actual adsorption-based separation process, breakthrough
experiments were performed in which an equimolar ethylene/
ethane mixture was flowed over a packed bed of the solid with a
total flow of 2 mL/min at 296 K. As shown in Figure 7, PAF-1-
SO3Ag can effectively separate an equimolar mixture of
ethylene and ethane into the pure component gases of greater
than 99% purity.
Figure 5. Isosteric heats adsorption, Qst of C2H4 for PAF-1, PAF-1-
SO3H, and PAF-1-SO3Ag.
metal sites, e.g., FeMOF-74 (45 kJ·mol−1)19b or (47 kJ·
mol−1),19a MgMOF-74 (42 kJ·mol−1),19a CoMOF-74 (41 kJ·
mol−1),19a CuBTC (39 kJ·mol−1).19a These results highlight
that, compared with open metal sites, Ag(I) ions can boost the
interactions with ethylene molecules more in a porous
framework via the formation of π-complexation. In contrast
with the high Qst for ethylene, PAF-1-SO3Ag shows a
significantly lower Qst for ethane with a value of 27 kJ·mol−1
(SI Figure S14); thus validating that the Ag(I) ions serve as a
preferential binding sites, selectively adsorbing ethylene over
ethane thereby resulting in high ethylene/ethane adsorption
selectivities.
To further prove the formation of π-complexation between
the ethylene molecules and Ag(I) ions in PAF-1-SO3Ag, in situ
IR measurements of ethylene adsorption at room temperature
were conducted. The CH2 out-of plane wagging mode at 949
cm−1 was found as the most sensitive mode, responding to the
interaction between ethylene and the substrate surface.32 As
shown in Figure 6, ethylene adsorption on PAF-1 and PAF-1-
Figure 7. Experimental data on transient breakthrough of an
equimolar C2H4/C2H6 mixture in an adsorber bed packed with
PAF-1-SO3Ag in the adsorption phase of a PSA operation.
We also carried out breakthrough simulations for C2H4/
C2H6 mixtures in a fixed bed (SI Figure S17) to further
demonstrate the feasibility of producing 99.95%+ pure C2H4 in
a Pressure Swing Adsorption (PSA) operation. The simulated
breakthrough curves are in reasonably good agreement with the
experimental data (SI Figure S18a). During the adsorption
cycle, C2H6 at purities >99% can be recovered for a certain
duration of the adsorption cycle, as indicated by the arrow in SI
Figure S18b. In addition, ethylene of 99.95%+ purity, required
as feedstock to the polymerization reactor, can also be
recovered during the time interval indicated by the arrow in
SI Figure S19 in the desorption cycle. Video animations of the
breakthrough simulations can be viewed in the HTML version
of this work.
Figure 6. IR spectra from ethylene adsorption and desorption on PAF-
1, PAF-1-SO3H, and PAF-1-SO3Ag at room temperature. IR spectrum
from gas phase ethylene is also shown for reference.
CONCLUSIONS
■
In summary, we have demonstrated for the first time the
introduction of π-complexation into POPs for highly selective
adsorption of ethylene over ethane, as illustrated in the context
of Ag(I) ion functionalized porous aromatic framework, PAF-1-
SO3Ag. PAF-1-SO3Ag exhibits significantly higher ethylene/
ethane adsorption selectivity at 296 K than benchmark zeolite
and any other MOF and POP reported in literature. The high
ethylene/ethane adsorption selectivity of PAF-1-SO3Ag is
traceable to the formation of π-complexation between Ag(I)
SO3H exhibits IR features similar to that of gas phase C2H4,
indicating a weak interaction, which is further evidenced by the
complete removal of ethylene IR features after room temper-
ature desorption in helium purge. In contrast, upon initial
adsorption, PAF-1-SO3Ag shows strongly perturbed CH2 mode
at 980 cm−1. The intensity is even comparable with the gas-
phase mode at 949 cm−1 at saturation. Two extra IR features at
D
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grade gases of N2 (99.999%), C2H4 (99.5%), and C2H6 (99.5%) were
used for the collection of respective sorption isotherms.
ions and the double bonds of ethylene molecules, which is
reflected in the high isosteric heats of adsorption of C2H4 and
evidenced by in situ IR spectroscopy studies. The feasibility of
PAF-1-SO3Ag for producing 99.95%+ pure C2H4 in a PSA
operation has been demonstrated by breakthrough experiments
that are supported by simulations. Albeit the light-sensitivity,
utilization of costly Ni-COD catalyst, and high isosteric heats of
adsorption could represent some potential challenges for the
application of PAF-1-SO3Ag in practice, these kinds of issues
could be tackled via some engineering processes. Notwith-
standing, our work presented herein provides a new perspective
to functionalizing POPs for energy-saving ethylene/ethane and
other olefin/paraffin separations. Ongoing work in our
laboratory includes investigating PAF-1-SO3Ag for separations
of other olefin/paraffin mixtures and applying the approach of
π-complexation to functionalizing other types of advanced
porous materials for hydrocarbon separations.
In Situ IR Experiments. IR spectra of ethylene adsorption were
collected using a Thermo Nicolet Nexus 670 spectrometer in diffuse
reflectance mode (DRIFTS). The PAF-1-SO3Ag sample, ca. 5 mg, was
treated in a DRIFTS cell (HC-900, Pike Technologies) at 423 K in
helium (30 mL/min) for 1 h to removal water and other adsorbates.
The sample was then cooled down to room temperature for ethylene
adsorption. The adsorption was conducted by flowing 10% ethylene/
He (30 mL/min) over the sample for 5 min and then desorption was
done in flowing helium. IR spectra were recorded continuously to
follow the surface changes during the adsorption and desorption
process. All reported IR spectra are difference spectra referenced to a
background spectrum collected at room temperature after pretreat-
ment but prior to ethylene adsorption.
Fitting of Pure Component Isotherms. The measured
experimental isotherm data for C2H4, and C2H6 on PAF-1-SO3Ag
were fitted with the dual-Langmuir−Freundlich isotherm model:
bApνA
1 + bApνA
bBpν
B
q = qA,sat
+ q
B,sat 1 + bBpνB
EXPERIMENTAL SECTION
Materials and Syntheses. All starting materials, reagents, and
solvents were purchased from commercial sources (Aldrich, Alfa,
Fisher, and Acros) and used without further purification.
(1)
■
The fit parameters for C2H4 and C2H6 are specified in SI Table S1.
SI Figure S13 presents a comparison of the experimentally determined
component loadings for C2H4 and C2H6 on PAF-1-SO3Ag at 296 K
with the isotherm fits using parameters specified in SI Table S1. The
fits are excellent over the entire range of pressures.
The pure component isotherm data for PAF-1, and PAF-1-SO3H
could be fitted with single site Langmuir model; the fit parameters are
provided in SI Tables S2 and S3, respectively.
Calculations of Adsorption Selectivity. The selectivity of
preferential adsorption of C2H4 (component 1) over C2H6
(component 2) in a mixture containing 1 and 2, can be formally
defined as follows:
Synthesis of Tetrakis(4-bromophenyl)methane. Tetrakis(4-
bromophenyl)methane was synthesized according to the procedures
reported in the literature22 with some minor modification. To a three-
necked round-bottom flask containing bromine (6.4 mL, 19.9 g),
tetraphenylmethane (2.0 g, 6.24 mmol) was added stepwise with small
portions under vigorous stirring at room temperature (25 °C). After
the addition was completed, the resulting solution was stirred for 60
min and then cooled to 0 °C. At 0 °C temperature, ethanol (25 mL)
was added slowly, and the reaction mixture was allowed to warm to
room temperature overnight. Then, the precipitate was filtered off and
washed subsequently with saturated aqueous sodium hydrogensulfite
solution (25 mL) and water (100 mL). After drying at 80 °C for 24 h
under vacuum (80 mbar), tetrakis(4-bromophenyl) methane was
recrystallized in EtOH/CH2Cl2 to afford a yellow solid, yield: 88%.
Synthesis of PAF-1. PAF-1 was synthesized according to the
procedures reported in the literature23a with some minor modification.
Tetrakis(4-bromophenyl)methane (509 mg, 0.8 mmol) was added to a
solution of 2,2′-bipyridyl (565 mg, 3.65 mmol), bis(1,5-
cyclooctadiene)nickel(0) (1.0 g, 3.65 mmol), and 1,5-cyclooctadiene
(0.45 mL, 3.65 mmol) in anhydrous DMF/THF (60 mL/90 mL), and
the mixture was stirred overnight at room temperature under nitrogen
atmosphere. After the reaction, 6 M HCl (60 mL) was added slowly,
and the resulting mixture was stirred for 12 h. The precipitate was
collected by filtration, then washed with methanol and water, and
dried at 150 °C for 24 h under vacuum (80 mbar) to produce PAF-1
as a white powder, yield: 80%.
q1/q2
Sads
=
p /p2
(2)
1
In eq 2, q1 and q2 are the component loadings of the adsorbed phase in
the mixture. The calculations of Sads are based on the use of the Ideal
Adsorbed Solution Theory (IAST) of Myers and Prausnitz.27
Estimation of Isosteric Heats of Adsorption, Qst. The isosteric
heat of adsorption, Qst, were calculated using the Clausius−Clapeyron
equation by differentiation of the dual-Langmuir−Freundlich fits of
the isotherms at two different temperatures, 296 and 318 K with T-
dependent parameters.
⎛
⎜
⎝
⎞
⎟
⎠
∂ ln p
∂T
Q st = RT2
q
(3)
Synthesis of PAF-1-SO3H. PAF-1-SO3H was synthesized
according to the procedures reported in the literature23a,24 with
some minor modification. To an ice-cooled mixture of PAF-1 (100
mg) in dichloromethane (15 mL), chlorosulfonic acid (1.0 mL) was
added dropwise. The resulting mixture was stirred at room
temperature for 3 days. Then, the mixture was poured over ice, and
the solid was collected, washed with water thoroughly, and dried 150
°C for 24 h under vacuum (80 mbar) to produce PAF-1-SO3H as blue
powder, yield: 96%.
Synthesis of PAF-1-SO3Ag. To the 15 mL CH3CN/H2O (1:1)
solution, 100 mg PAF-1-SO3H and 800 mg AgBF4 were added. The
mixture was stirred under room temperature for 48 h, and then the
solid was collected by filtration followed by washing with CH3CN and
water. The whole process was performed carefully under dark
environment. This exchange process was repeated three times, and
then dried at 110 °C under vacuum (80 mbar) for further test, yield:
94%. EA: C: 47.25%; H: 3.19%; N: 0.53%; S: 17.11%; ICP-MS: Ag:
29.20%.
Breakthrough Experiments. In a typical experiment, 400 mg of
PAF-1-SO3Ag was swiftly ground and packed into a quartz column (6
mm I.D. × 220 mm) with silica wool filling the void space. The sample
was in situ activated under vacuum (6.5 × 10−4 Pa) at 110 °C for 2 h.
Then, Helium flow (2 mL/min) was introduced the system to purge
the adsorbent until the temperature of the column was decreased to 23
°C. The breakthrough test was started by introducing a 1:1 C2H4/
C2H6 mixture gas at a total flow rate of 2.0 mL/min and switching off
the He gas. Effluent from the column was monitored using a GC with
a flame ionization detector. The dead volume of this setup was
determined to be 18.6 cm3.
ASSOCIATED CONTENT
■
S
* Supporting Information
Characterization details, additional gas sorption isotherms,
simulated breakthrough curves, TGA plots, XPS and IR spectra
plots, and supporting figures. This material is available free of
charge via the Internet at http://pubs.acs.org.
Gas Adsorption. Gas sorption measurements were performed
using an ASAP 2020 volumetric adsorption analyzer. High-purity
E
dx.doi.org/10.1021/ja502119z | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
W
(14) (a) Li, J.-R.; Sculley, J.; Zhou, H.-C. Chem. Rev. 2012, 112, 869.
(b) Zhu, Y.; Long, H.; Zhang, W. Chem. Mater. 2013, 25, 1630.
(c) Sato, H.; Kosaka, W.; Matsuda, R.; Hori, A.; Hijikata, Y.;
Belosludov, R. V.; Sakaki, S.; Takata, M.; Kitagawa, S. Science 2014,
343, 167.
* Web-Enhanced Feature
Video animations of transient breakthrough for the adsorption/
desorption cycle are available in the HTML version of this
paper.
(15) (a) Liu, J.; Thallapally, P. K.; McGrail, B. P.; Brown, D. R.; Liu,
J. Chem. Soc. Rev. 2012, 41, 2308. (b) Nugent, P.; Belmabkhout, Y.;
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AUTHOR INFORMATION
Corresponding Author
sqma@usf.edu
Notes
■
The authors declare no competing financial interest.
112, 724. (d) Dawson, R.; Stockel, E.; Holst, J. R.; Adams, D. J.;
̈
Cooper, A. I. Energy Environ. Sci. 2011, 4, 4239−4245. (e) An, J.; Rosi,
ACKNOWLEDGMENTS
■
N. L. J. Am. Chem. Soc. 2010, 132, 5578.
The authors acknowledge the University of South Florida for
financial support of this work, and an award from the National
Science Foundation (DMR-1352065) is also acknowledged.
Part of the work including the in situ IR studies was conducted
at the Center for Nanophase Materials Sciences, which is
sponsored at Oak Ridge National Laboratory by the Scientific
User Facility Division, Office of Basic Energy Sciences (BES),
U.S. Department of Energy (DOE). DOE/BES/Division of
Materials Sciences and Engineering (Award No. KC020105-
FWP12152) (P.K.T.) and the National Natural Science
Foundation of China (No. 21371069) (Z.S.) are acknowledged.
We thank Prof. Jeffrey R. Long and Eric Bloch for their kind
help on the calculation of regeneration energies.
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