Extremely Hydrophobic POPs to Access Highly Porous Storage Media and Capturing Agent for Organic Vapors

Article abstract of DOI:10.1016/j.chempr.2018.10.005

Porous organic polymers (POPs) with high surface areas (especially more than 5,000 m² g^?1) are still quite rare. In this work, we implemented the molecular-building-block strategy to isolate three POPs, namely KPOP-1, KPOP-2, and KPO

Full text of DOI:10.1016/j.chempr.2018.10.005

Article
Extremely Hydrophobic POPs to Access Highly
Porous Storage Media and Capturing Agent
for Organic Vapors
Jiangtao Jia, Zhijie Chen, Hao
Jiang, ..., Justyna
ꢀ
Czaban-Jo´ zwiak, Mohamed
Rachid Tchalala, Mohamed
Eddaoudi
mohamed.eddaoudi@kaust.edu.sa
HIGHLIGHTS
Highly porous organic frameworks
with hypothetically approximated
topologies
KPOP-2 surpasses the gravimetric
2012 DOE target for onboard CH4
storage
KPOP-2 shows exclusive potential
for stationary CH4 storage
KPOP-1 shows exceptional
performances for capturing VOCs
Eddaoudi and co-workers implemented the molecular-building-block strategy to
isolate three porous organic polymers (POPs), namely KPOP-1, KPOP-2, and
KPOP-3 (KPOP = KAUST’s POP), exhibiting high surface areas. Impressively, the
methane uptake of KPOP-2 at 298 K and 80 bar is 0.515 g gÀ1, surpassing the
gravimetric 2012 Department of Energy target for onboard CH4 storage.
Interestingly, KPOP-1 and KPOP-2 showed high hydrophobic behavior and high
organic vapor uptakes, ranking them as promising candidates for the removal of
volatile organic compounds at atmospheric conditions.
Jia et al., Chem 5, 1–12
January 10, 2019 ª 2018 Published by Elsevier
Inc.
https://doi.org/10.1016/j.chempr.2018.10.005

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Article
Extremely Hydrophobic POPs to Access
Highly Porous Storage Media and Capturing
Agent for Organic Vapors
Jiangtao Jia,1 Zhijie Chen,1 Hao Jiang,1 Youssef Belmabkhout,1 Georges Mouchaham,1
1
1
2
1
ꢀ
Himanshu Aggarwal, Karim Adil, Edy Abou-Hamad, Justyna Czaban-Jo´ zwiak,
Mohamed Rachid Tchalala,1 and Mohamed Eddaoudi1,3,
*
SUMMARY
The Bigger Picture
The ability to develop advanced
highly porous storage and
separating agents with the
requisite properties, affording
exceptional gravimetric CH4
uptake and aromatics combined
with non-competitive adsorption
of water vapor, is of prime
importance. Successful future
implementation of these
Porous organic polymers (POPs) with high surface areas (especially more than
5,000 m2 ꢀ1) are still quite rare. In this work, we implemented the molecular-
g
building-block strategy to isolate three POPs, namely KPOP-1, KPOP-2, and
KPOP-3 (KPOP = KAUST’s POP), and hypothetically predicted the structures
of the three KPOPs. KPOP-1 and KPOP-2 exhibit high specific BET surface areas
(ca. 5,120 and 5,730 m2
g
ꢀ1) and display outstanding gravimetric methane
storage properties. Remarkably, the methane uptake of KPOP-2 at 298 K and
80 bar is 0.515 g gꢀ1, surpassing the gravimetric 2012 Department of Energy
target for onboard CH4 storage, and KPOP-2 shows the exclusive potential
for stationary CH4 storage. Interestingly, KPOP-1 and KPOP-2 show extremely
hydrophobic behavior combined with high organic vapor uptakes, a desirable
feature enabling effective capturing of volatile organic compounds at room
temperature.
disruptive adsorptive
technologies offers great
potential to address various
United Nations sustainable
development goals pertaining to
clean, reliable, and sustainable
energy and prosperous health.
Herein, we unveil three highly
porous organic frameworks,
KPOP-1, KPOP-2, and KPOP-3
(KPOP stands for KAUST’s porous
organic polymer), which display
outstanding properties for CH4
storage and the capture of volatile
organic compounds to address
many critical societal and
INTRODUCTION
The enduring fast growth of the global economy steered a greater than previous
demand for the consumption of fossil fuels such as petroleum, in turn provoking
the remarkable and uncontrollable escalation of CO2 emission, ground-level air
pollution, and climate change. Natural gas (NG) with methane as the main
component offers a distinct environmentally sustainable alternative, in the short
to medium term, to substitute the conventional petroleum-based fuels and
significantly reduce the adverse emissions responsible for climate change.1,2
Nevertheless, the transportation and storage operations of NG using compres-
sion under high pressure are costly and require numerous demanding preventive
measures. Adsorbed NG (ANG) storage systems encompassing porous materials
in the storage tanks offer great potential to reduce the storage pressure and in-
crease the energy density.3 For this purpose, the appropriate adsorbent material
should have high porosity and high thermal stability, and offer effective mode
for regeneration and recycling while addressing the US Department of Energy
(DOE) released new gravimetric CH4 storage target of 50 wt % in 20123,4 for
onboard applications. It is noteworthy that the gravimetric CH4 storage is the
only driving parameter for a stationary ANG system because of the less con-
strained requirements for space and volume. To achieve the ultimate goal in
enhancing gravimetric storage, the most important metrics for porous materials
are high specific surface area and large pore volume.5
industrial challenges.
Chem 5, 1–12, January 10, 2019 ª 2018 Published by Elsevier Inc.
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The exploration of highly porous materials, and their associated potential applica-
tions, has been an active area of research in academia and industry alike. During
the last two decades, metal-organic frameworks (MOFs) with ultra-high surface
area such as MOF-210,6 NU-110,7 and Al-soc-MOF-18 have been vigorously
explored. However, because of the trade-off between high porosity and stability,
most of the highly porous MOFs showed a lessened chemical stability as the surface
area increased and most of them required impractical activation methods such as
supercritical CO2.9 These MOFs usually cannot preserve their structures under
highly acidic or basic conditions, thus limiting their wide industrial applications.
Only recently, Eddaoudi et al. unveiled a unique isostructural soc-MOF based on
Cr satisfying the challenging high porosity-stability trade-off.10
More recently, porous organic polymers (POPs),11–14 offering superior stability to
MOFs because of their robust covalent bonds, have attracted considerable attention
for gas storage15 as well as for separation.16 Irreversible and fast reaction ap-
proaches are usually applied to synthesize POPs. These methodologies lead often
to amorphous polymers with highly heterogeneous structures. As POP structural
features remain usually unknown, it is very challenging to design and construct
POPs with high surface areas.17–20 Only very few examples of POPs have been re-
ported with ultra-high surface areas comparable with MOFs. For example, partially
crystalline PAF-121 possesses an estimated BET (BET = Brunauer-Emmett-Teller) sur-
face area22 of ca. 5,600 m2 ꢀ1. Another reported POP (PPN-4)23 exhibits an even
g
higher BET specific surface area of around 6,400 m2 gꢀ1. The two aforementioned
POPs were both synthesized using 4-connected (4-c) building blocks in order to
mimic the hypothetical diamond (dia) structure. Nevertheless, the use of building
blocks with higher connectivity, i.e., a higher degree of predictability, is of significant
importance and rarely explored.
The molecular building block (MBB) approach24,25 decidedly permits access to POPs
by the assembly of judiciously preselected organic building blocks that code
geometrical information of targeted underlying nets. Although POPs assembled
from 3-connected (3-c) and 4-c MBBs20,26 have been widely explored, 6-c
MBBs27,28 have rarely been studied in constructing 3D POPs, owing to their complex
conformation and the difficulty to predict obtained structures compared with 3-c and
4-c MBBs. Herein, three 6-c MBBs (Figures 1 and S26–S40), i.e., 1,4-bis(tris(4-bromo-
phenyl)silyl)-benzene (MBB-1), 4,40-bis(tris(4-bromophenyl)silyl)-1,10-biphenyl (MBB-2),
and
50-(3,5-diethynylphenyl)-3,300,5,500-tetraethynyl-20,40,60-trimethyl-1,10:30,100-ter-
phenyl (MBB-3), were designed and synthesized as precursors to deliberately
assemble the targeted 3D POPs. Using these MBBs, three POPs, namely KPOP-1,
KPOP-2, and KPOP-3 (KPOP = KAUST [King Abdullah University of Science and
Technology] POP), were synthesized and show high specific BET surface areas of
5,120, 5,730, and 2,620 m2 gꢀ1, respectively. Thanks to their ultra-high porosity, the
reported POPs displayed records of gravimetric uptakes for CH4, CO2, and O2.
Interestingly, KPOP-1 and -2 also show an extremely hydrophobic behavior and an
exceptional adsorption of large amounts of organic vapors, positioning them as very
promising adsorbent materials for the capture of volatile organic compounds (VOCs).
1Functional Materials Design, Discovery and
Development Research Group, Advanced
Membranes and Porous Materials Center,
Division of Physical Sciences and Engineering,
King Abdullah University of Science and
Technology, Thuwal 23955-6900, Saudi Arabia
2King Abdullah University of Science and
Technology, Core Labs, Thuwal 23955-6900,
Saudi Arabia
RESULTS AND DISCUSSION
The Reticular Chemistry Structure Resource (RCSR) lists the nets with underlying
topologies acs, bcs, crs, lcy, and pcu (Figure S1) as the five edge-transitive nets
constructed by one type of 6-c building units with trigonal prism, octahedral, or
trigonal anti-prism geometry.29,30 On the basis of the MBB conformation analysis,
3Lead Contact
*Correspondence:
mohamed.eddaoudi@kaust.edu.sa
https://doi.org/10.1016/j.chempr.2018.10.005
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Figure 1. Geometrically Optimized Conformations of MBB-1, MBB-2, and MBB-3 by Materials
Studio 8.0 Software with the Forcite Module
Hydrogen atoms are omitted for clarity. Black, carbon atoms; yellow, silicon atoms.
it can be expected that MBB-1 and MBB-2 with twisted trigonal prism conformations
offer great prospective to construct frameworks with acs, bcs, crs, lcy, or pcu under-
lying topologies. MBB-1 and MBB-2 can afford two possible conformations, namely
the trigonal prism or the trigonal anti-prism, by the phenyl rings rotating. MBB-3 may
form a structure with the acs underlying net because acs is the only edge-transitive
net based on linking trigonal prism building units. Replacing the nodes of each net
with MBB-1 and MBB-2, five POP structures can be simulated. However, after geom-
etry optimization by forcite field in Material Studio 8.0, it was found that only acs,
pcu, and lcy nets are theoretically plausible and/or suitable for MBB-1 and MBB-2,
whereas the chemical bonds in POPs based on crs and bcs nets are unreasonable.
With the same method, a structure showing reasonable chemical bonds based on
the acs net and MBB-3 was also simulated (Figure 2). From the calculated results,
it was found that all KPOPs based on acs, pcu, and lcy nets have high theoretical
specific BET surface areas (Table S1), thus prompting our interest to assemble
them experimentally.
Gratifyingly, we successfully synthesized KPOP-1, KPOP-2, and KPOP-3 by employ-
ing MBB-1, MBB-2, and MBB-3, respectively. Indeed, Yamamoto coupling was
applied to synthesize KPOP-1 and KPOP-2, resulting in white powder-like materials
that are insoluble in conventional organic solvents (i.e., dimethylformamide [DMF],
tetrahydrofuran [THF], and dichloromethane). Infrared spectra of KPOP-1 and
KPOP-2 revealed the absence of the signature C–Br bond stretching peak at
1,065 cmꢀ1, suggesting the successful coupling of the monomers (Figures S6
and S7). Energy-dispersive X-ray spectroscopy (EDX) data (Figure S3) showed there
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Figure 2. Simulated Structures of Idealized Crystalline Forms of KPOP-1, KPOP-2, and KPOP-3
Hydrogen atoms are omitted for clarity. Black, carbon atoms; yellow, silicon atoms.
are almost no Br residue signals, attributable to the high complement of polymeri-
zation and the conversion of the ending -Br to -H during quenching of the nickel co-
ordinated complexes by hydrochloric acid.31 The scanning electron microscopy
(SEM) images (Figure S2) of KPOP-1 and KPOP-2 showed aggregated particles
with the size ranging from 200 to 300 nm. It must be noted that the Yamamoto
coupling reaction can form polymers with partial crystallinity.21,23 Consequently,
powder X-ray diffraction (PXRD) studies were performed to assess the crystallinity
of KPOP-1 and KPOP-2 (Figure S5). The two PXRD patterns showed quite broad
peaks showing that both solids are mainly amorphous. In addition, crystal lattices
could not be found from the high-resolution transmission electron microscopy
(HRTEM) images, probably because of the weak crystallinity (Figure S4).
MBB-3 was designed and synthesized for the first time and used to synthesize
KPOP-3 via Eglinton coupling. Reacting MBB-3 with Cu(OAc)2 in pyridine yielded
KPOP-3 as a brown solid that is insoluble in most of the conventional organic
solvents. Infrared spectra of KPOP-3 (Figure S8) did not reveal any peak at
3,280 cmꢀ1 (the characteristic peak of alkyne’s C–H bond stretching), attesting to
the completion of the reaction. SEM images (Figure S2) showed very small aggre-
gated particles and PXRD patterns (Figure S5) revealed no peaks, indicating no
long-range order.
The thermal stability of the three KPOPs was evaluated by performing thermal-
gravimetric analysis under oxygen atmosphere (Figure S9). KPOP-2 has a higher
decomposition temperature (around 440ꢁC) than KPOP-1 (around 370ꢁC), even though
they both have very similar compositions. This could be because KPOP-2 has a relatively
more stable3Dstructurethan KPOP-1. After 600ꢁC, KPOP-1andKPOP-2 decomposeto
form silica. KPOP-3 has the lowest decomposition temperature among the three KPOPs
owing to the active carbon triple bond. From 150ꢁC, KPOP-3’s weight increases by
about 2% perhaps because of the oxidation of the carbon triple bond, and the main
framework starts to decompose around 200ꢁC. After 500ꢁC, about 4% weight residue
was observed. This may be attributed to the formation of CuO from plausibly entrapped
Cu(OAc)2 catalyst in the pores of KPOP-3. The presence of Cu residues was also
confirmed by the EDX analysis of KPOP-3 (Figure S3).
4
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Figure 3. 13C CP MAS and 2D 1H-13C HETCOR NMR
KPOP-1 (A), KPOP-2 (B), and KPOP-3 (C).
To gain further insights into the chemical structures of the POPs, we performed
multinuclear (1H, 13C, and 29Si) solid-state nuclear magnetic resonance (SS-NMR)
spectroscopy studies on KPOP-1, KPOP-2, and KPOP-3. Figure 3A illustrates the
13C cross-polarization magic-angle spinning (CP MAS) and 2D 1H-13C heteronuclear
correlation spectroscopy (HETCOR) NMR for KPOP-1. The signals at d = 142, 135,
131, and 125 ppm indicate the presence of four different aromatic carbon atoms
in KPOP-1 clearly assigned with different colored labels. The 1H MAS NMR shows
a merged peak at d = 7.0 ppm, which can be attributed to the aromatic hydrogen
peak in KPOP-1 (Figure S10). The 2D 1H-13C HETCOR NMR spectrum of
KPOP-1shows different aromatic protons at 6.3 and 7.4 ppm that reveals correla-
tions with the different aromatic carbons. Strong correlations are shown for
black- and green-labeled carbon and protons, indicating the direct bonding that
is reasonable in the structure of KPOP-1. The weak correlations of blue- and
red-labeled carbon atoms with protons demonstrate that there is a long-distance
contact. KPOP-2 shows similar 13C CP MAS spectra with KPOP-1 with the existence
of signals at d = 142, 135, 131, and 125 ppm associated with aromatic carbon atoms.
The 1H MAS NMR also shows a merged proton peak at d = 7.0 ppm corresponding to
the aromatic protons (Figure S12). The 2D NMR 1H-13C HETCOR NMR spectrum
reveals that black- and green-labeled carbon atoms are directly connected to proton
atoms, and the other two carbon atoms have long-distance relation with the proton.
Both KPOP-1 and KPOP-2 have peak at d = ꢀ14 ppm in 29Si CP MAS NMR, confirm-
ing the presence of silicon atoms (Figures S11 and S13).
KPOP-3 exhibits quite different composition in comparison with KPOP-1 and
KPOP-2, as clearly evidenced from the NMR spectrum. The 1H MAS NMR spectrum
displays two signals at 1.7 and 7.0 ppm attributed to the methyl and aromatic pro-
tons, respectively (Figure S14). The 13C CP MAS NMR spectrum (Figure 3C) displays
different signals at 15, 79, 122, 131, 138, and 142 ppm. Additionally, the 2D 1H-13
C
HETCOR NMR spectrum (Figure 3C) shows a correlation between the methyl pro-
tons at 1.7 ppm and their corresponding carbon atoms at 15 ppm. Furthermore,
the peak at 79 ppm shows no correlations with any proton signal assigned to
acetylene carbon atoms. The distribution of aromatic carbon atom signals between
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Figure 4. N2 Isotherms at 77 K
Black, KPOP-1; red, KPOP-2, blue, KPOP-3.
122 and 142 ppm shows a strong correlation with aromatic proton signal at 7 ppm,
confirming their relationship in the predicted structure.
To attest the expected high porosity of KPOPs, we first explored their permanent
porosity by N2 sorption studies at 77 K (Figure 4). The N2 adsorption isotherms
showed that KPOP-1, KPOP-2, and KPOP-3 have very high specific BET surface areas
of ca. 5,120, 5,730, and 2,620 m2 gꢀ1, respectively (Figures S15, S17, and S19). The
single point pore volumes at P/P0 = 0.95 were found to be 2.60, 3.09, and
1.67 cm3gꢀ1, respectively. The pore-size distributions determined via density
functional theory afford the approximation of the possible pore size of the KPOPs
(Figures S16, S18, and S20). It was found that the lowest and highest average pore
sizes are 1.4 and 2.1 nm, respectively, for KPOP-1 and 1.5 and 2.5 nm, respectively,
for KPOP-2; KPOP-3 displayed an average pore size at ca. 1.7 nm. According to the
simulated possible structures of KPOPs, the experimental specific BET surface area
and pore volumes of KPOP-1 and KPOP-2 corroborate with the simulated ones. In
the simulated structures, the calculated BET surface areas of KPOP-1 and KPOP-2
are around 5,300 and 5,900 m2 ꢀ1, respectively, and the calculated pore dimen-
g
sions are both around 1.4 nm. As a result of the plausible amorphous structures
and vague side reactions, KPOP-1 and KPOP-2 have somewhat lower experimental
BET surface areas and larger pore size than the simulated structures.
Interestingly, KPOP-2 has a slightly lower specific BET surface area than PNN-4
)
(6,461 m2 gꢀ1 23 but a similar one to PAF-1 (5,600 m2 gꢀ1),21 the only two reported
POPs displaying ultra-high specific BET surface areas (>5,000 m2 gꢀ1). However, the
pore volume of KPOP-2 is slightly larger than those of PNN-4 and PAF-1 mainly
because of its associated larger pore size. Accordingly, KPOP-2 is expected to offer
greater adsorption and storage performances than the compounds cited above.
KPOP-3 displays lower specific BET but remains high among the POPs constructed
via Eglinton coupling reaction.32
The exceptional porosity features for the aforementioned KPOP compounds
encouraged us to further investigate their associated high-pressure sorption perfor-
mance for different gases, i.e., methane (CH4), oxygen (O2), and carbon dioxide
6
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Table 1. Summary of the Porous Materials with High Absolute Gravimetric Methane Uptake at
298 K
Materials
Methane Uptake
(g gꢀ1
Working Capacity
(g gꢀ1
Reference
)
)
80 bar
0.515
ND
35 bar
0.277
0.274
0.238
0.257
0.255
0.229
0.241
0.234
0.254
5–80 bar
0.469
ND
5–35 bar
0.232
0.231
0.193
0.207
0.211
0.186
0.194
0.192
0.151
KPOP-2
PPN-4
this work
Li et al.,1 Yuan et al.23
Furukawa et al.6
Alezi et al.8
MOF-210
Al-soc-MOF-1
KPOP-1
PAF-1
0.476
0.475
0.450
0.400
ND
0.431
0.425
0.406
0.357
ND
this work
Bracco et al.33
Peng et al.34
Furukawa et al.35
Mason et al.5
Nu-111
COF-102
0.362
0.328
0.32
AX-21
0.225
(active carbon)
KPOP-3
0.262
0.161
0.217
0.116
this work
ND, not determined.
(CO2). The high-pressure CH4 sorption studies were conducted up to 80 bar and at
various temperatures (298, 273, and 258 K).
In the absence of the crystal structure, it is difficult to obtain the theoretical densities
of POPs that are further required to estimate the volumetric CH4 uptake. Therefore,
we aimed first to calculate the skeletal density from the adsorption measurements of
non-adsorbable gas (helium) at 298 K. The crystal-like particle densities (hypotheti-
cal density) of KPOP-1, KPOP-2, and KPOP-3 were then estimated to be ca. 0.310,
0.267, and 0.435 g cmꢀ3, respectively, by combining the skeletal density and the
pore volume, which was obtained experimentally from N2 adsorption isotherm at
77 K (for details of the calculation, see part 8 of the Supplemental Information).5
Hence, the volumetric CH4 uptakes of KPOP-1, KPOP-2, and KPOP-3 at 298 K and
80 bar were estimated to be 195, 192, and 160 cm3 cmꢀ3, respectively. Because
of the lower density of KPOPs than of most MOFs, KPOP-1, KPOP-2, and KPOP-3
showed relatively lower volumetric uptake than benchmark MOF materials such as
HKUST-14 and Al-soc-MOF-1.8 However, KPOP-2 showed a high CH4 gravimetric
uptake associated with a very high working capacity surpassing all of the related
benchmark materials (Table 1). The measured bulk density was also tested by press-
ing the POP solid in a measuring syringe. By measuring the weight and the volume of
the samples, we estimated bulk densities of KPOP-1, KPOP-2, and KPOP-3 to be
0.204, 0.185, and 0.265 g cmꢀ3, respectively. Accordingly, the volumetric capacities
were recalculated as 128, 133, and 97 cm3 cmꢀ3 at 298 K and 80 bar for KPOP-1,
KPOP-2, and KPOP-3, respectively. As expected, the effect of packing led to a less-
ened volumetric capacity.
The total gravimetric uptakes of KPOP-1, KPOP-2, and KPOP-3 at 298 K and 35 bar
are 358, 388, and 226 cm3 gꢀ1, respectively (Figure 5). Several MOFs, for example,
Al-soc-MOF-1,8 NU-111,34and DUT-49,36 also exhibit outstanding CH4 uptakes of
around 361, 337, and 364 cm3 (STP) gꢀ1, respectively, under the same conditions,
although they show inferior performance to KPOP-2. Moreover, the CH4 gravimetric
uptake of KPOP-2 at 298 K and 35 bar (388 cm3 gꢀ1) is even higher than the current
benchmark POP materials (i.e., PAF-1 [330 cm3 gꢀ1] and PPN-4 [384 cm3 gꢀ1]) under
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Figure 5. High-Pressure CH4 Uptake
KPOP-1 (A), KPOP-2 (B), and KPOP-3 (C) at 298, 273, and 258 K, respectively.
the same conditions.23,33 The higher gravimetric CH4 uptake of KPOP-2 compared
with PPN-4 is attributed to its associated exceptional porosity. At 298 K and
80 bar, KPOP-1, KPOP-2, and KPOP-3 show CH4 uptakes of 630, 721, and
367 cm3
g
ꢀ1, respectively. At lower temperature of 258 K, the CH4 uptakes of
KPOP-1, KPOP-2, and KPOP-3 are much higher and estimated to be 826, 953,
and 458 cm3 gꢀ1, respectively (Figure 5). The CH4 uptake of KPOP-2 at 80 bar and
298 K is higher than that of most of the reported MOFs. Most importantly,
KPOP-2 represents the first porous material capable of addressing the DOE target
of 0.5 g gꢀ1 at 298 K and 75 bar. The heat of CH4 adsorption at the lowest CH4 re-
corded with reasonable accuracy for KPOP-1, KPOP-2, and KPOP-3 was also calcu-
lated to be around 16.6, 16.0, and 19.9 kJ molꢀ1, respectively (Figures S21–S23).
As shown in Figure S24, at 298 K, 5–35 bar, and 5–80 bar, the working capacity of
KPOP-2 is 0.232 and 0.469 g gꢀ1, which is similar to PPN-4 and higher than
PAF-1.23,33 Besides, KPOP-1 showed a high volumetric working capacity of
92 (5–35 bar) and 176 (5–80 bar) cm3 cmꢀ3 at 298 K, comparable with PPN-4
(92 cm3 cmꢀ3, 5–35 bar, 298 K).4,8,23 Interestingly, as the temperature decreases
both gravimetric and volumetric working capacities of the KPOPs gradually increase,
as already reported for several highly porous MOFs.8 Table 1 summarizes CH4 up-
takes and working capacities of KPOPs and selected relevant porous materials.
Considering the outstanding performance of KPOP-2 for CH4 storage, we tested its
uptake for CO2 and O2 (Figures S25 and S26), another two commodities that need to
be stored for specific applications. At 298 K and 25 bar, the absolute gravimetric CO2
uptake of 1.51 g gꢀ1 is higher than PPN-4’s, with an uptake of around 1.40 g gꢀ1 un-
der the same conditions. At 298 K and 120 bar, the absolute gravimetric O2 uptake of
around 1.02 g gꢀ1 is much higher than for the Al-soc-MOF-1 (0.84 g gꢀ1, 298 K, 120
bar) and NU-125 (0.56 g gꢀ1, 140 bar).8,37 These results indicate that KPOP-2 is also
an exceptional solid adsorbent for CO2 and O2 storage.
Exhaust VOCs at the workplace or in confined spaces are strongly hazardous to
humans and the environment, and in the real world these exhaust VOCs are always
mixed with humidity.38,39 The ideal method of overcoming this problem is to deploy
hydrophobic porous materials that can enable the selective removal of VOCs while
repelling water. Considering that KPOP-1 and KPOP-2 are highly porous and
encompass mainly hydrophobic organic moieties, we decided to evaluate their
adsorption properties for some VOCs and water. As illustrated in Figure 6, the
benzene uptake of KPOP-1 and KPOP-2 is as high as 2.2 and 2.6 gꢀ1 gꢀ1 (25ꢁC,
8
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Figure 6. Vapor Sorption Isotherms of KPOP-1 and KPOP-2 (for Benzene and Water) at 25ꢁC (P0 is
the Saturation Vapor Pressure)
P/P0 = 0.9), respectively. Other hydrophobic organic vapor adsorbates such as hex-
ane and toluene were also tested (Figure S28). The hexane and toluene uptakes for
KPOP-1 are 1.7 and 2.1 gꢀ1 gꢀ1 (25ꢁC, P/P0 = 0.9), respectively. The corresponding
values for KPOP-2 are 2.1 and 2.4 gꢀ1 gꢀ1 (25ꢁC, P/P0 = 0.9), respectively. The
organic vapor uptake for KPOP-1 and KPOP-2 are pre-eminently higher than most
of the other porous materials such as PAF-121 and CPOP-940 because of their high
pore volume and associated hydrophobic surface. The hydrophobicity of the KPOPs
was also assessed by carrying out water sorption experiments. It was found that the
KPOP-1 and KPOP-2 do not show any water vapor uptake until 85%–90% relative hu-
midity (Figure S27). This clearly attests to the highly hydrophobic character of
KPOP-1 and KPOP-2. Such a behavior can be mainly attributed to the presence of
hydrophobic phenyl rings and silicone moieties in both KPOP-1 and KPOP-2. These
data clearly testify to the distinctive performance of KPOP-1 and KPOP-2 adsorbents
for capturing VOCs from atmospheric air.
Considering the outstanding properties of KPOP-1 and KPOP-2 for CH4 storage and
the capture of VOCs, the stability and adsorption recyclability of the two KPOPs were
also tested. The study attests to the chemical stability KPOP-2 upon soaking in
water, acid solution, and base solution (Figure S42) and shows good adsorption
recyclability for C6H6 (Figure S43). Notably, KPOP-1 seems to undergo pore size dis-
tribution alteration with a higher degree of heterogeneity but without altering the
total pore volume after soaking in water, acid, or base (Figure S41). In fact, the N2
adsorption isotherm at 77 K changed with lower uptake at low relative pressures
(at P/P0 = 0.2, the uptake decreased from 1,250 cm3 to around 850 cm3; at P/P0 =
0.95, the uptake decreased from 1,850 cm3 to around 1,750 cm3). This is plausibly
due to the occurrence of another amorphous network rearrangement of KPOP-1
with a higher degree of heterogeneity. Both KPOP-1 and KPOP-2 proved highly sta-
ble after water vapor adsorption isotherms (Figures S41 and S42).
In conclusion, three highly porous organic polymers were conceived and successfully
synthesized according to the MBB strategy. A thorough and unique topological
analysis delineated the possible hypothetical structures for KPOP-1, KPOP-2, and
KPOP-3. The three KPOPs display high CH4 uptakes. In particular, KPOP-2 showed
Chem 5, 1–12, January 10, 2019
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the highest CH4 storage capacity reported to date, addressing the DOE target at
75 bar and 298 K. More importantly, the unveiled KPOP-2 could be potentially
used for non-mobile storage applications with less constrained space requirements.
KPOP-2 also shows benchmark O2 uptake capacity. More interestingly, KPOP-1 and
KPOP-2 show highly hydrophobic properties and are can be regarded as promising
materials for capturing organic vapors. The findings presented herein represent one
important step in the search for synthetic pathways to unlock the discovery of highly
porous organic polymers with a high degree of structural uniformity and control.
Further studies aiming at exploiting the high hydrophobicity of KPOP-1 and
KPOP-2 in various practical applications are under way.
EXPERIMENTAL PROCEDURES
Synthesis of KPOP-1
To a 100-mL flask, 200 mg (0.73 mmol) of Ni(COD)2, 0.9 mL (0.071 mmol) of COD,
and 114 mg (0.73 mmol) of 2,2-bipyridyl were dissolved in a mixed dry solvent of
DMF/THF (v/v = 10 mL/10 mL) under Ar atmosphere. To this solution, a solution
of MBB-1 (100 mg, 0.093 mmol) in 10 mL of dry THF was added dropwise to the so-
lution at room temperature under Ar. The reaction was reacted for 24 hr at room
temperature, then 5 mL of 6 M HCl solution was added to the solution and stirred
for 8 hr. The white precipitate obtained was filtrated and washed with large amounts
of water, methanol, and THF. The product was dried in vacuo to give a white powder,
KPOP-1 (52 mg, yield = 94%).
Synthesis of KPOP-2
To a 100-mL flask, 200 mg (0.73 mmol) of Ni(COD)2, 0.9 mL (0.071 mmol) of COD,
and 114 mg (0.73 mmol) of 2,2-bipyridyl were dissolved in a mixed dry solvent of
DMF/THF (v/v = 10 mL/10 mL) under Ar atmosphere. To this solution, a solution
of MBB-2 (107 mg, 0.093 mmol) in 10 mL of dry THF was added dropwise to the so-
lution at room temperature under Ar. The reaction was reacted for 24 hr at room
temperature, then 5 mL of 6 M HCl solution was added to the solution and stirred
for 8 hr. The white precipitate obtained was filtrated and washed with large amounts
of water, methanol, and THF. The product was dried in vacuo to give a white powder,
KPOP-2 (54 mg, yield = 93%).
Synthesis of KPOP-3
To a 100-mL flask, 2.5 g (12.5 mmol) of Cu(OAc)2,2H2O was dissolved in 100 mL of
pyridine at 100ꢁC. To this mixture, a solution of MBB-3 (100 mg, 0.20 mmol) in 5 mL
of pyridine was added dropwise to the solution. The reaction was reacted for 1 hr
with stirring. The brown precipitate was filtrated and washed with 1 M HCl, ammo-
nium solution, water, methanol, and THF. Thereafter, the products were solvent
exchanged by methanol and THF many times and dried under vacuum to yield a
brown powder, KPOP-3 (100 mg, yield = 100%).
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 43 fig-
ures, 5 schemes, and 1 table and can be found with this article online at https://doi.
org/10.1016/j.chempr.2018.10.005.
ACKNOWLEDGMENTS
Research reported in this publication was fully supported by the King Abdullah Uni-
versity of Science and Technology (KAUST). We thank Daliang Zhang at the core lab
of KAUST for the HRTEM characterization.
10
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AUTHOR CONTRIBUTIONS
J.J. synthesized the KPOPs. J.J., K.A., H.J., and M.R.T. characterized the materials.
Z.C., Y.B., G.M., and H.A. carried out the adsorption experiments. E.A.-H. carried
out the solid-state NMR experiments. J.J. and J.C.-J. performed the organic synthe-
sis. J.J., H.J., Y.B., K.A., and M.E. wrote the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: June 12, 2018
Revised: July 5, 2018
Accepted: October 11, 2018
Published: November 1, 2018
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