Microporous polyphenylenes with tunable pore size for hydrogen storage

Article abstract of DOI:10.1039/c0cc00235f

A series of highly porous polymers with similar BET surface areas of higher than 1000 m² g^-1 but tunable pore ranging from 0.7 nm to 0.9 nm were synthesized through facile ethynyl trimerization reaction to demonstrate the surface pro

Full text of DOI:10.1039/c0cc00235f

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Microporous polyphenylenes with tunable pore size for hydrogen storagew
Shengwen Yuan,a Brian Dorney,a Desiree White,a Scott Kirklin,a Peter Zapol,ab Luping Yu*c
and Di-Jia Liu*a
Received 5th March 2010, Accepted 21st April 2010
First published as an Advance Article on the web 26th May 2010
DOI: 10.1039/c0cc00235f
A series of highly porous polymers with similar BET surface
areas of higher than 1000 m2 gÀ1 but tunable pore ranging from
0.7 nm to 0.9 nm were synthesized through facile ethynyl
trimerization reaction to demonstrate the surface property-
hydrogen adsorption relationship.
Scheme 1 Structure of the monomers M1–M4.
Microporous organic polymers exhibit unique properties of
report here a new approach of synthesizing porous organic
large surface areas, low skeleton density and high chemical
polymer (POP) with narrowly distributed PD between 0.7 to
stabilities that have applications in gas storage and separation.1–9
0.9 nm and SSA > 1000 m2 gÀ1 using an ethynyl trimerization
Recently, several types of chemical reactions have been used to
reaction. Starting from four simple monomers M1–M4
develop various cross-linked porous organic polymers, such as
listed in Scheme 1, we successfully prepared a series of poly-
dioxane formation for polymers of intrinsic microporosity
phenylenes with tunable diameters ranging from 0.7 nm to
(PIMs),1 palladium catalyzed Sonogashira-Hagihara cross-
0.9 nm. Simple aromatic framework and incremental variation
coupling for poly(aryleneethynylene) microporous polymers,2
of pore size with similar SSA make this group of polymers
Friedel–Crafts reaction for hypercrosslinked polymers,3,9
unique model materials for studying gas-adsorbent interaction
oxidative coupling of thiophenes,4,6b trimerization of ethynyl
with various gases, such as hydrogen, methane, etc.
groups,4 amide or imide formation,6a,d reversible imine forma-
Using dicobalt octacarbonyl as a catalyst, the terminal
tion,7 and homocoupling of aromatic bromides.8 The flexibility
ethynyl groups of the respective monomers were trimerized,
of rational design in molecular structures is a major advantage
and four polymers were readily obtained in quantitative yield.
in producing porous polymers with high surface area and
A representative polymerization to prepare polymer POP-1
narrow pore size distribution. This can be achieved by designing
from M1 is outlined in Scheme 2 (details about synthesis of
monomers with targeted structures and functional groups for
other three polymers POP-2, POP-3, and POP-4 from M2,
polymerization under proper reaction conditions. Since the
M3, and M4, respectively, are shown in Scheme S1 in the
pore size affects the heat of adsorption and diffusion kinetics
Electronic Supplemental Information (ESI)).w While M1 was
purchased commercially, monomers M2, M3 and M4 were
through close interaction between adsorbent and adsorbate,
materials with high specific surface area (SSA) and narrow pore
synthesized by coupling the aromatic bromides and trimethyl-
diameter (PD) are highly desirable for gas adsorption and
silylacetylene under Sonagarshira reaction conditions,
separation. It was suggested that the preferred pore size should
followed by deprotection of the trimethylsilyl (TMS) groups.
be slightly higher than the adsorbate’s van der Waals dimen-
These monomers were designed in such a way that they could
sion9b at 0.6 nm to 0.9 nm in the case of hydrogen.
potentially form porous structures with different pore
Although most porous polymers are amorphous, Cooper
diameters and cross-linking density.
and coworkers have demonstrated the feasibility of system-
The structural properties of these polymers were characterized
atically controlling the pore architecture and a general strategy
by Fourier transform infrared spectroscopy (FTIR) (See ESI).w
of fine-tuning the surface area by copolymerization of mono-
mers with different ‘‘strut lengths’’.2a–c Controlling pore size
The characteristic vibration band of alkynyl carbon-hydrogen at
3300 cmÀ1 is completely diminished for POP-1 and POP-2,
systematically at less than 1 nm dimension is particularly
indicating the completion of the trimerization reaction; whereas
challenging due to limited choice of molecular struts. We
this band is weakly observable for polymers POP-3 and POP-4
a Chemical Sciences & Engineering Division, Argonne National
Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA.
E-mail: djliu@anl.gov; Fax: +1 630 252 4176;
Tel: +1 630 252 4511
b Materials Science Division, Argonne National Laboratory,
9700 S. Cass Avenue, Argonne, IL 60439, USA
c Department of Chemistry & The James Franck Institute,
The University of Chicago, 929 E. 57th Street, Chicago, IL 60637,
USA. E-mail: lupingyu@uchcicago.edu; Fax: +1 773 702 0805;
Tel: +1 773 702 8698
w Electronic supplementary information (ESI) available: Details on
polymer synthesis, characterization, H2 adsorption isotherm measure-
ment and computational modeling. See DOI: 10.1039/c0cc00235f
Scheme 2 Preparation of POP-1 (benzene in red indicates one of the
potential trimerization pathways from ethynyl groups).
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Chem. Commun., 2010, 46, 4547–4549 | 4547

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due to the residual ethynyl groups, suggesting that even higher
surface area maybe achievable should the trimerization be
completed. The thermal gravimetric analysis (TGA) in air showed
high thermal stability for polymers POP-1, POP-2, and POP-3
with no sign of decomposition up to 360 1C. Polymer POP-4 is
also stable up to B330 1C after an initial weight loss of about
10% due to the removal of residual moisture or air upon heating.
Surface properties of these polymers were characterized
with nitrogen adsorption analysis at 77.3 K using a Micro-
meritics ASAP 2010 system. Nitrogen adsorption isotherms
are shown in Fig. 1. The inset of Fig. 1 is the expanded plots at
the low pressure region where nitrogen uptake is most sensitive
to micropores (logarithmic pressure scale was used). The
difference in nitrogen adsorption isotherms among these
polymer samples are distinct. The inflection points of nitrogen
uptake follow the sequence of POP-2oPOP-1oPOP-4o
POP-3, suggesting their micropore diameters follow the same
order. Fig. 2 shows the differential pore volume distributions
as a function of pore width calculated with non-local density
functional theory (NLDFT). Micropores are dominant for all
four polymers. The inset in Fig. 2 represents an expanded view
of pore size distribution from 0.7 to 0.9 nm. A similar
differential pore volume distribution at slightly larger
than POP-3 and POP-4, have more structural flexibility. The
phenylene and diphenylene units of the monomers M1 and M2
could rotate more freely than the phenylene units of M3 and
M4, which linked to three or four other monomers. More
importantly, POP-1 and POP-2 tend to form relatively
flexible structures through extending of M1 or M2, which
can stack and interpenetrate, thus reducing pore sizes. For
POP-3 and even more so for POP-4, a high strain among the
monomers cause the crosslinking to extend three-dimensionally
instead of just planarly, thus leading to a robust three-
dimensional structure with less flexibility than POP-1 and
POP-2, suggesting a higher porosity. The strain arising due to
the high density of ethynyl groups also results in the incompletion
of the trimerization reaction, which was observed for POP3
and POP-4 experimentally. This correlates with the sequence
of porosity POP-2rPOP-1oPOP-4oPOP-3 found through
N2 adsorption analysis. All of the polymers have Brunauer-
Emmet-Teller (BET) specific surface area (SSA) of more than
1000 m2 gÀ1, with the highest value of 1246 m2 gÀ1 for polymer
POP-3. A summary of the surface areas using both BET
model and Langmuir model, together with other structural
properties are listed in Table 1.
Excess hydrogen adsorption capacities of these four
polymers were measured with a modified Sievert isotherm
apparatus.4 Hydrogen adsorption/desorption isotherms were
measured under four different temperatures for each polymer.
Shown in Fig. 3 is a complete set of four isotherms for polymer
POP-3 with hydrogen uptakes up to 3 wt% obtained at 77 K
and 60 bar (a summary of hydrogen adsorption capacities for
POP-3 and other polymers is listed in Table S1 in ESI).w Fig. 4
shows the heats of adsorption derived from the hydrogen
adsorption isotherms at 195 K and 298 K, from which the
initial adsorption energy can be calculated more accurately.
POP-4 and POP-3 exhibited relatively higher initial heat of
adsorption, DHads, around 9 kJ molÀ1, whereas POP-1 and
POP-2 showed values of DHads near 7.5 kJ molÀ1. This
observation offers little correlation between DHads and PDs
obtained from N2 adsorption isotherms. Instead, we found an
interesting correlation between DHads and Np where Np repre-
sents the average number of phenyl groups connected to a
benzene after trimerization. The Np values for POP-2 and
POP-1 of 2.3 and 2.5 are considerably lower than those for
POP-3 and POP-4 of 3 and 3.5, respectively. Quantum
chemical calculations at MP2/6-311++G(2d,2p)//MP2/6-31+G*
level for H2 adsorption on M1, M3 and M4 monomers
simulating different arrangements of neighboring phenyl
groups in the polymers resulted in binding energies of
5.1 kJ molÀ1, 5.5 kJ molÀ1 and 6.0 kJ molÀ1, respectively.
All structures were fully optimized, resulting in a vertical H2 in
the center of aromatic ring (see ESI for details).w Other phenyl
groups adjacent to the H2 could also contribute to the van der
Waals interaction, particularly at low H2 coverage. It is
substantiated by calculations of the binding energies of
´
PDs was also found using Horvath-Kawazoe (H–K) method
(Fig. S3 in ESI).w
A preliminary quantum chemical simulation of the polymer
structures suggests that POP-1 and POP-2, which form fewer
bonds between monomers during the trimerization reaction
Fig. 1 Nitrogen adsorption isotherms measured at 77.3 K for polymers
POP-1 (square), POP-2 (diamond), POP-3 (hexagon), and POP-4
(triangle).
H2 to two monomers, which come out to be 10.6 kJ molÀ1
,
12.6 kJ molÀ1 and 12.9 kJ molÀ1 for M1, M3 and M4,
respectively. Larger Np leads to higher DHads as the order of
POP-4>POP-3>POP-1 Z POP-2 we observed in Fig. 4 at
low hydrogen coverage. Heat of adsorptions at higher coverage
were also extrapolated from isotherms at 77 K and 87 K, the
Fig. 2 Pore size distribution based on NLDFT calculation for POP-1
(square), POP-2 (diamond), POP-3 (hexagon), and POP-4 (triangle).
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Table 1 Surface properties of the polymers
BET Surface
Area/m2 gÀ1
Langmuir Surface
Area/m2 gÀ1
Total Pore
Volume/cm3 gÀ1a
Micropore
Volume/cm3 gÀ1a
DFT Dominant
Pore Diameter/nma
H–K Dominant
Pore Diameter/nmb
POP-1
POP-2
POP-3
POP-4
1031
1013
1246
1033
1387
1378
1515
1402
0.641
0.712
0.729
0.730
0.378
0.341
0.448
0.402
0.77
0.74
0.88
0.81
0.83
0.80
0.90
0.85
a
b
Data calculated from nitrogen adsorption isotherm with NLDFT method. Data calculated from nitrogen adsorption isotherm with H–K method.
PD studied. Further investigation on hydrogen-polymer
interaction in confined space at the molecular level using
advanced characterization methods is underway.
This work was supported by the U.S. Department of
Energy’s Fuel Cell Technologies program under the Office of
Energy Efficiency and Renewal Energy. The authors wish to
thank Dr Shengqian Ma for his helpful discussion and experi-
mental support from Mr Jose R. Regalbuto. Use of Argonne’s
LCRC computational resources is gratefully acknowledged.
Notes and references
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aromatic ethynyl monomers. The polymeric pores are all
narrowly distributed and fine-tuned with
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diameters of various small gas molecules. Isosteric heat of
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to monomer structure than the pore size within the range of
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4547–4549 | 4549