Nanoporous polyporphyrin as adsorbent for hydrogen storage

Article abstract of DOI:10.1021/ma100026f

Novel polyporphyrins with high surface area over 1500 m²/g have been synthesized, and their hydrogen absorption capacities were measured. Porphyrin functionalized with thiophenyl groups was designed as starting monomer; the porphyrin cores offer coordination sites for metal ions which could potentially enhance the interaction with hydrogen for favorable hydrogen storage while the thiophenyl groups are used for cross-linking the monomers under oxidative coupling conditions to yield highly porous networks. These polyporphyrins adsorb up to 5.0 mass % H₂ at 77 K and 65 bar. Compared with metal free polymer, iron(II)-containing polymer shows finite increase in heat of adsorption for hydrogen, suggesting a promising approach for designing high heat of adsorption porous materials.

Full text of DOI:10.1021/ma100026f

Macromolecules 2010, 43, 3325–3330 3325
DOI: 10.1021/ma100026f
Nanoporous Polyporphyrin as Adsorbent for Hydrogen Storage
Jiangbin Xia,† Shengwen Yuan,‡ Zhuo Wang,† Scott Kirklin,‡ Brian Dorney,‡ Di-Jia Liu,*,‡ and
Luping Yu*,†
†
Department of Chemistry and The James Franck Institute, The University of Chicago, 929 E. 57th Street,
Chicago, Illinois 60637, and ‡Chemical Sciences & Engineering Division, Argonne National Laboratory,
9700 S. Cass Ave., Argonne, Illinois 60439
Received January 5, 2010
ABSTRACT: Novel polyporphyrins with high surface area over 1500 m2/g have been synthesized, and their
hydrogen absorption capacities were measured. Porphyrin functionalized with thiophenyl groups was
designed as starting monomer; the porphyrin cores offer coordination sites for metal ions which could
potentially enhance the interaction with hydrogen for favorable hydrogen storage while the thiophenyl
groups are used for cross-linking the monomers under oxidative coupling conditions to yield highly porous
networks. These polyporphyrins adsorb up to 5.0 mass % H2 at 77 K and 65 bar. Compared with metal free
polymer, iron(II)-containing polymer shows finite increase in heat of adsorption for hydrogen, suggesting a
promising approach for designing high heat of adsorption porous materials.
Introduction
With a large number of metal ions could be readily incorporated
into the porphyrin center, metalloporphyrin can serve as an
With high conversion efficiency and zero CO2 emission, hydro-
gen is considered as an ideal energy carrier in future transportation
applications. However, limited on-board capacity of storage media
represents one of the major bottlenecks for implementing H2 as
vehicular fuel. Currently, numerous hydrogen storage materials are
being developed, including metal/chemical hydrides and sorption-
based materials.1 Distinct from hydride-based adsorbents, sorp-
tion-based materials adsorb hydrogen through physisorption with-
out breaking the H-H bond and therefore consume minimum
parasitic energy during H2 extraction. Examples of sorption-based
material include metal-organic frameworks (MOFs),2 enginee-
red carbons,3 porous polymers,4 etc. Among them, porous poly-
mers are attractive in their flexibility in structural modification,
light weight, and high thermal stability. Although porous polymer
does not hold the highest surface area among sorption materials at
present, the combination of low skeleton density and high micro-
pore volume makes it more promising to be engineered to meet
both gravimetric (1.8 kWh/kg) and volumetric (1.3 kWh/L) capa-
city targets simultaneously set by U.S. Department of Energy than
other candidates, even those with higher surface areas. Different
cross-linked porous polymers termed as hyper-cross-linked poly-
mers (HCPs),4a polymers of intrinsic microporosity (PIMs),4b
and covalent organic frameworks (COFs)4c have been developed
for hydrogen storage. At present, the storage capacity at ambient
temperature remains a challenge due to weak interaction between
hydrogen and substrate.1,5
exceptional model system for systematic investigation of the impact
of metal ion on heat of adsorption if a porphyrin group can be
integrated into a high surface area, porous framework. Preparing
high surface area porphyrinic metal organic frameworks (MOF),
however, turned out to be surprisingly difficult.7 Porous network
polymers based on porphyrin8a and phthalocyanine8b via dioxane
formation reactions yielded Brunauer-Emmett-Teller (BET) sur-
face areas below 1000 m2/g. Herein, we report our recent effort in
preparing two highly porous porphyrinic polymers by covalently
cross-linking porphyrin monomers functionalized with thiophenyl
groups (Scheme 1). The thiophenyl groups at the branches of the
monomer allow oxidative polymerization, and the porphyrin cores
provide coordination sites for various metal ions. In addition, the
polyporphyrins we prepared yielded narrow pore size distribution
and high surface area with BET surface area >1500 m2/g.
Furthermore, they demonstrated an interesting yet desirable posi-
tive pressure dependence on hydrogen uptake with capacity larger
than 5.0 mass % at 77 K.
Results and Discussion
Synthesis of Monomers and Porous Polymers. The de-
signed porphyrin monomer was synthesized as shown in
Scheme 1. The 3,5-dibromobenzaldehyde 1 was synthesized
according to the literature procedure.9 The palladium-media-
ted Stille coupling reaction was used to introduce thiophe-
nyl group to yield compound 2. The resulting porphyrin
monomer was polymerized through oxidative coupling of
the terminal thiophenyl groups in anhydrous chloroform
using FeCl3 as the oxidant. The polymer containing Fe(II)
was obtained by polymerization of corresponding Fe(II)-
porphyrin complex.
Structural Characterizations. The structural information
was obtained using Fourier transform infrared (FTIR)
spectrometer. A comparison of the FTIR spectra of these
two polymers and their respective monomers is shown in
Figure 1. Two peaks at 3314 and 965 cm-1 in the spectrum of
To improve storage capacity, chemically modified polymeric
adsorbents with higher H2 adsorption energy than the existing
polymers are needed. Theoretical calculations as well as experimen-
tal results show that metal ions in a porous network could increase
the adsorption energy through d-s electronic orbital interaction.6
A porous polymer framework containing properly designed liga-
tion sites for coordinating and exchanging of transition metal ions
could serve as an ideal platform to study metal-hydrogen interac-
tion in a confined space. Metalloporphyrin is an excellent example.
*Corresponding authors. E-mail: lupingyu@midway.uchicago.edu
(L.Y.), djliu@anl.gov (D.-J.L.).
r 2010 American Chemical Society
Published on Web 03/05/2010
pubs.acs.org/Macromolecules

3326 Macromolecules, Vol. 43, No. 7, 2010
Xia et al.
Figure 1. (a) FTIR spectra for the TTPP, Fe-TTPP, and their corresponding polymers. (b) Expansion of (a) for the region from 500 to 1800 cm-1
.
Scheme 1. Synthesis of Polymer PTTPP and P(Fe-TTPP)
metal free porphyrin (TTPP) are assigned to N-H stretching
and in-plane bending vibration of the pyrrole group, res-
pectively.10 These peaks disappear after forming a Fe-
porphyrin complex (Fe-TTPP). It is known that the band
at 1000-920 cm-1 is metal-sensitive in the porphyrin-metal
complex.11 When Fe(II) ion was inserted into porphyrin ring,
a strong sharp peak at 1003 cm-1 appeared. In TTPP, peaks
at 1442, 1365, and 1236 cm-1 originate from the stretching of
CdC and C-C in the thiophene ring.12 After polymeriza-
tion, these peaks shift to 1452, 1372, and 1241 cm-1, respec-
tively. In addition, the peak intensity at 697 cm-1 related to
in-plane deformation of C-S-C of thiophene ring decreases

Article
Macromolecules, Vol. 43, No. 7, 2010 3327
Figure 2. (a) Nitrogen adsorption (filled symbol)/desorption (open symbol) isotherms for polymer PTTPP (square symbol) and P(Fe-TTPP)
(diamond symbol). (b) Pore size distribution of polymer PTTPP (square symbol) and P(Fe-TTPP) (diamond symbol), calculated using the NLDFT
method.
Figure 3. Excess hydrogen adsorption (solid symbols)/desorption (open symbols) isotherms under 77 K (squares), 87 K (circles), 195 K (triangles), and
298 K (diamonds) for polymer PTTPP (a) and polymer P(Fe-TTPP) (b).
Table 1. Surface Properties of PTTPP and P(Fe-TTPP)
BET surface
area (m2/g)
Langmuir surface
area (m2/g)
total pore
volume (cm3/g)a
micropore
volume (cm3/g)a
dominant pore
diameter (nm)a
PTTPP
P(Fe-TTPP)
1522
1248
2030
1665
0.85
0.68
0.67
0.54
0.85
0.82
a Data calculated from nitrogen adsorption isotherms with the nonlocal density function theory (NLDFT) method.
while the peak intensity at 727 cm-1 (deformation C-S)
increases after polymerization.12
pressures at four different temperatures. The uptake capacity
for polymer PTTPP reaches as high as 5.0 wt % at 77 K and
65 bar, while polymer P(Fe-TTPP) adsorbs up to 4.6 wt %
under the same conditions. Interestingly, we observed that
the adsorption uptake for both polymers at 77 K showed
significant positive pressure dependence up to 60-70 bar,
taking up substantially higher amount of hydrogen than
what would be predicated by the Chahine’s rule (1 wt %/500
m2/g).14 This behavior is strikingly different from that of
more rigid and open structured adsorbents such as activated
carbon or most MOFs which saturates around 10-20 bar at
77 K (see Supporting Information). This suggests that, as
pressure increases, H2 may continually permeate into the
“hidden” micropores and surface area in the polymers as the
result of conformation change under elevated pressure,
possibly due to rotation and stretching of the linking units
in the randomly packed network. Such micropores may not
be accessible to nitrogen at low pressure during surface area
measurement.
Porous Polymer Surface Properties. Surface area and pore
size distribution were characterized with nitrogen as probing
gas at 77 K using a Micromeritics ASAP 2010 system. Both
polymers exhibit type I isotherms, as shown in Figure 2a.
Figure 2b shows the incremental pore volumes as the func-
tion of pore size derived from N2 adsorption isotherms using
a nonlocalized density function theory (NLDFT) method.
The majority of the pore volume of these two polymers is
contributed from the micropores with narrow pore sizes
distribution centered at about 0.8 nm, a desirable dimension
for gas adsorption. The introduction of Fe at the center of the
porphyrin reduced slightly the dominant pore size and the
surface area. Table 1 summarizes the surface properties of
the polymers.
Hydrogen Sorption Capacities and Isosteric Heat of Ad-
sorption. Hydrogen adsorption isotherms of the polymers as
well as reference adsorbents were measured with a modified
Sievert type apparatus.13 Figure 3 shows the excess H2
adsorption capacities measured under various equilibrium
The isosteric heat of adsorption at low coverage region
was derived from the isotherms measured at 195 and 298 K.

3328 Macromolecules, Vol. 43, No. 7, 2010
Xia et al.
Figure 4. Comparison of heat of adsorption of polymer PTTPP (open circles) and P(Fe-TTPP) (solid circles) at constant hydrogen loading
(normalized by surface area).
These two temperatures were chosen because their isotherm
plots are well-separated from which the heat of adsorption at
initial hydrogen adsorption can be more accurately de-
rived, in contrast to the approach from 77 and 87 K
measurements. The heat of adsorption as the function of
hydrogen loading is shown in Figure 4. For better compari-
son between these two polymers with different surface
areas, hydrogen loading (x-axis) was normalized as the
number of H2 adsorbed per unit surface (m2). Introducing
Fe ions at the porphyrin cores shows a finite effect on
increasing the heat of adsorption as indicated by a slightly
larger value of P(Fe-TTPP) than that of PTTPP. The
limited but definitive increase clearly indicates the modi-
fication of interaction of TTPP with H2 by Fe ions albeit a
small magnitude. After the completion of this experimental
study, we were brought to attention of a very recent
theoretical investigation in which nondissociative interac-
tion of dihydrogen with Fe and other transition metal
porphyrins was reported.15 This study suggested a rela-
tively low H2 heat of adsorption improvement over Fe-
porphyrin (1.9 kJ/mol) unless the symmetry is broken by a
substituting ligand. This result appears to agree with our
experimental observation of low impact by Fe. The study
further indicates that other metals, such as Ti and V, may
lead to higher adsorption energy when incorporated into
porphyrin. Since our polyporphyrin polymer provides a
versatile venue for exchanging metals through coordina-
tion chemistry, it can be used for systematic investigation
of different transition metal-hydrogen interaction in a
confined space which is currently underway in our labora-
tories.
Equipment. 1H and 13C NMR spectra were collected on a
Bruker 400 or 500 MHz FT NMR spectrometer. FTIR spectra
were recorded on a Thermo Nicolet Nexus 670 FTIR spectro-
meter. Thermogravimetric analysis (TGA) data were ob-
tained from a Shimadzu TGA-50 with a heating rate of 10
°C/min in a nitrogen atmosphere. Surface area and porosity
were measured using a Micromeritics ASAP 2010 system with
nitrogen as probing gas at 77 K. The isotherms of N2
adsorption/desorption were obtained with high resolution
at low pressure region so that detailed information on pore
size of the polymers could be extracted. Surface area, pore
volumes, and dominant pore size were derived from the N2
adsorption isotherms using BET and Langmuir models and
NLDFT calculation. The hydrogen adsorption isotherms at
77, 87, 195, and 298 K were measured using liquid nitrogen,
liquid Ar, dry ice/acetone, and temperature-controlled water
as coolant with a modified Sievert isotherm apparatus similar
to that reported in the literature.16 Ultrahigh-purity hydro-
gen of 99.9995% was used for H2 update measurement,
and free volume of the sample cell at each temperature was
calibrated with helium (99.9995%). Hydrogen equilibrium
pressure up to 80 bar was applied for the measurement.
A typical measurement error was <(5% of the uptake
value obtained at the high-pressure region. The error reduced
to <(2.5% at the low-pressure region. Commercial soft-
ware, GASPAK, was used to accurately calculate the state
equations of hydrogen and helium under different pressures
and temperatures. For free volume calibration, helium was
dosed to the sample cell in appropriate constant temperature
bath, a series values of free volume of the sample cell were
calculated at different equilibrium pressure (in the range of
2-50 bar), a linear fit was then applied to these values, and the
extrapolated value at zero pressure was used as the free
volume for subsequent H2 uptake calculation.15 A dual
channel high-precision pressure gauge, model CPG 2500 from
Mensor with accuracy of 0.01% FS, was installed in the
system. The reference section of the isotherm apparatus was
encased inside of thermally insulated box with the tempera-
ture controlled by circulating water at constant temperature.
The sample section was immersed into the liquid bath filled
with coolant of different temperature stabilized through
phase transition. The liquid level of the coolant was main-
tained at nearly constant height relative to the sample cell
through insulated dewar and constant top-off. In addition, a
copper sheath with thermal conduction cross section of 5 cm2
was attached to the sample cell through liquid/air inter-
phase to compensate any minor variation of liquid level
change during the measurement. In our study, typical poly-
mer sample sizes ranged from 200 to 500 mg. Before each
isotherm measurement, the sample was weighed after being
heat-treated at 125 °C under vacuum to remove residual
moisture or other trapped gases. The heat of adsorption was
Conclusion
In summary, two polyporphyrins were synthesized with sur-
face area as high as 1522 m2/g. Hydrogen uptake of 5 wt % was
obtained at 77 K and 65 bar. Furthermore, slight increase in heat
of adsorption was observed after insertion of Fe(II) ions into the
porphyrin rings. Such system can be used as a platform for
doping a wide range of transition metals inside of porous frame-
work in a process of continually improving dihydrogen adsorp-
tion energy and capacity.
Experimental Section
Materials. Chemicals were purchased from Aldrich, Alfa
Asia, and Strem and were used without further purification
unless otherwise noted. N,N-Dimethylformamide (DMF) anhy-
drous solvent was distilled from commercial DMF with CaH2.
Pyrrole was distilled before use.

Article
Macromolecules, Vol. 43, No. 7, 2010 3329
derived from the slope of ln P vs 1/T at constant hydrogen
loading using the Clausius-Clapeyron equation:
room temperature and diluted with CH2Cl2, and the filtered
mixture was washed with brine. The organic phase was collected
and dried over MgSO4. After filtration, solvent was removed to
yield the crude product which was then chromatographed on
silica using a hexane/CH2Cl2 (1:1) solvent mixture as the eluent
and gave 0.25 g (60%) yield. CI-MS: Calcd, 1325.5; found (M þ
1)þ, 1326.1. UV/vis (λmax, nm CH2Cl2 ꢀ 105 cm-1 M-1) 285.5
(1.16), 416.5 (1.20), 573.0 (0.121), 612.0 (0.07).
Synthesis of PTTPP (5) and P(Fe-TTPP) (6). Anhydrous
FeCl3 (0.8 g, 5 mmol) was charged into a round-bottom flask.
10 mL of anhydrous CHCl3 was added and stirred to make a
suspension solution. Then a solution of TTPP (0.25 g, 2 ꢀ 10-4
mol) or Fe-TTPP (0.26 g, 2 ꢀ 10-4 mol) in 20 mL of CHCl3 was
added dropwise. The resulting mixture was stirred at room tem-
perature overnight. After that, 200 mL of MeOH was added to the
above mixture and kept stirring for another hour. The precipita-
tion was collected by filtration and washed with MeOH. After
extraction with CH2Cl2, CHCl3, and MeOH/H2O (volume ratio
1:1) in a Soxhlet extractor for 24 h, the product was dried in
vacuum oven at 100 °C overnight. Yield was about 95%.
Hydrogen Isotherm Measurement for Reference Compounds.
Two adsorbent materials, AX-21 and Cu-BTC, which are well
studied in hydrogen storage application, were used as the
references for adsorption isotherm measurement. AX-21 is an
engineered active carbon, and Cu-BTC is a metal-organic
framework material. Their hydrogen adsorption isotherms are
documented in the literature.21-24 We repeated the hydrogen
adsorption of these materials using the identical procedure for
the polyporphyrin samples. The excess hydrogen adsorption
capacities as the function of pressure are plotted in Figure S7a,b
(Supporting Information). These results essentially duplicate
that of previously published. Most importantly, they demon-
strate saturation or nearly saturation at equilibrium pressure of
20-30 bar, which is strong contrast to that shown in Figure 3 for
polymer PTTPP and P(Fe-TTPP).
ꢀ
ꢁ
D ln P
Dð1=TÞ
ΔH ¼ -R
¼ -Rk
ð1Þ
nm
where ΔH is the heat of adsorption, R is ideal gas constant,
P and T are equilibrium pressure and temperature, respectively,
and k is the slop of ln P vs 1/T at constant hydrogen load-
ing nm.17
Synthesis of 3,5-Dibromobenzaldehyde (1). Briefly, a suspen-
sion of 1,3,5-tribromobenzene (4.0 g, 12.7 mmol) in anhydrous
diethyl ether (100 mL) at -78 °C was treated dropwise with
butyllithium (5.3 mL, 13.5 mmol). After 45 min dimethylform-
amide (2.8 g, 38.3 mmol) was added to the mixture, which was
then stirred for a further 1 h. Diluted HCl (40 mL, 2 mol/L) was
added, then organic phase was removed, and a brown solid was
obtained. The crude product was recrystallized from diethyl
ether/hexanes to give needles compound 1 (2.4 g, 70%). 1H
NMR: δ (CDCl3, ppm): 9.91, s, 1H, CHO, 7.94, (d, J = 1.6 Hz,
2H, Ar-H), 7.92, (t, J = 1.6 Hz, 1H, Ar-H), which is consisted
with reported result.9
Synthesis of 3,5-Dithiophen-2-yl-benzaldehyde (2). A 100 mL
round-bottom flask was charged with 3,5-dibromobenzalde-
hyde (1) (1.81 g, 6.85 mmol), tetrakis(triphenylphosphine)palla-
dium(0) (800 mg, 0.692 mmol), 2-(tributylstannyl)thiophene
(6.5 mL, 20.6 mmol), and anhydrous DMF (60 mL) and heated
to 80 °C with stirring for 72 h. The reaction mixture was then
cooled down to room temperature, diluted with Et2O, and fil-
tered through Celite. The organic phase was washed with brine
and water and dried over MgSO4. After filtration, solvent was
removed to yield the crude product which was then chromato-
graphed on silica using a hexane/CH2Cl2(2:1) solvent mixture as
the eluent. The desired product was a crystalline yellow solid
(1.10 g, yield 60%). 1H NMR: δ (CDCl3) 10.09 (1H, s, CHO),
8.06 (1H, t, J = 2.0 Hz, Ar-H), 8.00 (2H, d, J = 2.0 Hz, Ar-H),
7.37-7.15 (6H, m, thiophene-H). 13C NMR: δ (CDCl3) 191.9,
142.5, 137.7, 136.2, 128.8, 128.5, 126.2, 125.8, 124.6. CI-MS:
Calcd, 270.4; found (M þ 1)þ, 271.0. Anal. Calcd for C15H10S2:
C, 66.7%; H, 3.70%. Found: C, 67.6%; H, 3.41%.
Acknowledgment. This work was supported by the U.S.
Department of Energy’s Fuel Cell Technologies program under
the Office of Energy Efficiency and Renewable Energy. The authors
wish to thank GM and Air Products for providing the AX-21
sample, and thank Ms. Desiree White for experimental support.
Synthesis of 5,10,15,20-Tetrakis(3,5-dithiophen-2-ylphenyl)-
porphyrin TTPP (3). An oven-dried, three-necked, 1 L, round-
bottomed flask equipped with a magnetic stirring bar and a
gas-dispersion tube was charged with 3,5-dithiophen-2-yl-
benzaldehyde 2 (2.7 g, 0.01 mol), distilled pyrrole (0.7 mL, 0.01
mol), and dichloromethane (600 mL). The solution was purged
with nitrogen for 15 min. Boron trifluoride diethyl etherate (0.40
mL, 3.1 mmol) was added via syringe, and the flask was wrapped
with aluminum foil to shield it from light. The solution was
stirred under a nitrogen atmosphere at room temperature for 1.5
h, and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (1.7 g,
7.5 mmol) was added directly at one time and keep stirring for
another 1.5 h. Then 6 mL of triethylamine was added to neutralize
the excessive acid. Then concentrated the solvent and purified with
a column. The column was eluted with a mixture of hexane/
CH2Cl2 (1:1) and offered purple crystal-like solid (0.52 g, 20%).
1H NMR: δ (CDCl3, ppm): 9.04 (s, 8H, pyrrole-H), 8.43 (d, J =
1.6 Hz, 8H, Ar-H), 8.27 (t, J = 1.6 Hz, 4H, Ar-H), 7.57-7.3
(24H, m, thiophene-H), -2.67 (s, 2H). 13C NMR: δ (CDCl3)
143.8, 143.5, 133.7, 131.3, 128.4, 125.7, 124.4, 123.0, 119.6. It is not
easy to observe R and β carbon shift in pyrrole ring.18 UV/vis
(λmax, nm CH2Cl2 ꢀ 105 cm-1 M-1): 286.5 (1.37), 423.5 (3.54),
516.5 (0.256), 551.5 (0.106), 590.0 (0.091), 646.0 (0.0581). Anal.
Calcd for C76H46N4S8: C, 71.70%; H, 3.61%; N, 4.40%. Found:
C, 70.67%; H, 3.51%; N, 4.18%.
Supporting Information Available: Text giving 1H and 13
C
NMR, UV-vis spectra, and TGA graph of the related com-
pounds; hydrogen adsorption isotherms for an activated carbon
AX-21 and a MOF Cu-BTC measured with our modified Sievert
apparatus. This material is available free of charge via the
Internet at http://pubs.acs.org.
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