Amidoxime porous polymers for CO2 capture

Article abstract of DOI:10.1039/c3ra42433b

CO₂ capture from fossil fuel based electricity generation remains costly since new power plants with monoethanol amine (MEA) as the scrubbing agent are under construction. Amidoximes are known to mimic MEA, and porous polymers with amidoximes could offer a sustainable solution to carbon capture. Here we report the first amidoxime porous polymers (APPs) where aromatic polyamides (aramids) having amidoxime pendant groups were synthesized through low temperature condensation of 4,4′-oxydianiline (ODA) and p-phenylene diamine (p-PDA) with a new type of nitrile-bearing aromatic diacid chloride. The nitrile pendant groups of the polyamides were converted to an amidoxime functionality by a rapid hydroxylamine addition (APP-1 and APP-2). The CO₂ adsorption capacities of these polyamides were measured at low pressure (1 bar) and two different temperatures (273 and 298 K) and high pressure (up to 225 bar-the highest measuring pressure to date) at 318 K. The low pressure CO₂ uptake of APP-1 was found to be 0.32 mmol g ^-1 compared with APP-2 (0.07 mmol g^-1) at 273 K, whereas at high pressure they showed a substantial increase in CO₂ adsorption capacity exhibiting 24.69 and 11.67 mmol g^-1 for APP-1 and APP-2 respectively. Both aramids were found to be solution processable, enabling membrane applications.

Full text of DOI:10.1039/c3ra42433b

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PAPER
Amidoxime porous polymers for CO2 capture
Sonia Zulfiqar,a Sehrish Awan,b Ferdi Karadas,c Mert Atilhan,*c Cafer T. Yavuz*d
and Muhammad Ilyas Sarwar*b
Cite this: RSC Advances, 2013, 3,
17203
CO2 capture from fossil fuel based electricity generation remains costly since new power plants with
monoethanol amine (MEA) as the scrubbing agent are under construction. Amidoximes are known to
mimic MEA, and porous polymers with amidoximes could offer a sustainable solution to carbon capture.
Here we report the first amidoxime porous polymers (APPs) where aromatic polyamides (aramids) having
amidoxime pendant groups were synthesized through low temperature condensation of 4,49-oxydianiline
(ODA) and p-phenylene diamine (p-PDA) with a new type of nitrile-bearing aromatic diacid chloride. The
nitrile pendant groups of the polyamides were converted to an amidoxime functionality by a rapid
hydroxylamine addition (APP-1 and APP-2). The CO2 adsorption capacities of these polyamides were
measured at low pressure (1 bar) and two different temperatures (273 and 298 K) and high pressure (up
to 225 bar – the highest measuring pressure to date) at 318 K. The low pressure CO2 uptake of APP-1 was
found to be 0.32 mmol g21 compared with APP-2 (0.07 mmol g21) at 273 K, whereas at high pressure they
showed a substantial increase in CO2 adsorption capacity exhibiting 24.69 and 11.67 mmol g21 for APP-1
and APP-2 respectively. Both aramids were found to be solution processable, enabling membrane
applications.
Received 16th May 2013,
Accepted 12th July 2013
DOI: 10.1039/c3ra42433b
www.rsc.org/advances
toxicity, degradability, high regeneration energy requirements
and corrosive behavior.3,4
1. Introduction
Climate change is one of the most momentous issues
confronted by humanity and society as a whole. The rise in
global temperatures is expected to initiate a number of dire
consequences, including rising sea levels, alteration in
ecosystems, loss of biodiversity and diminution in crop
yields.1 Therefore, it is desirable to deploy defensive counter-
steps against the imminent threat created by global warming.
This alarming environmental situation has been attributed to
an increasing dependence on the combustion of fossil fuels
which accounts for anthropogenic greenhouse gas emissions.2
However, at present there are no possible alternative energy
sources to completely substitute mineral fuels. Accordingly,
CO2 capture and storage (CCS) is conceived as an integral and
primary action to mitigate the environmental impacts due to
escalating atmospheric CO2 concentration. The conventional
process employed for carbon dioxide capture is reversible
solvent absorption but it possesses many limitations such as
Adsorption is emerging as a superior technology for CO2
capture with possible energy savings relative to other more
established absorption processes.5,6 Various solid sorbents
have been explored as potential adsorbents for CO2 capture.
These include supported amines,7–10 carbon-based sor-
bents,11–18 supported carbonates,19,20 hydrotalcites21 and
zeolites.22–24 Unlike liquids, solid adsorbents can be used
over a wider temperature range, produce less waste during
cycling, and the exhausted material can be disposed of without
excessive environmental precautions. These are also being
exploited in pressure swing adsorption (PSA) systems for the
purification of hydrogen in petrochemical industries.25–27 The
sequestered CO2 can be used as a chemical raw material in the
preparation of dry ice, carbonated beverage manufactur-
ing,28,29 dry cleaning processes30 and in enhanced oil recovery
(EOR) from oil wells, although the emissions are two orders of
magnitude higher (32 000 Mton yr21) than the CO2 market
(250 Mton yr21).31
The interaction between CO2 and N-containing organic
heterocyclic molecules such as pyridine, imidazole and
tetrazole has been investigated,32 as their basic nitrogen
atoms can bind to acidic CO2 molecules by hydrogen bonding.
It is essential to incorporate the molecular specifics of their
CO2-sorbent interactions in order to fabricate superior carbon-
capture systems. Hence, the inclusion of a basic moiety into
the polymers leads to a greater affinity to CO2. Amidoximes are
aDepartment of Physics, COMSATS Institute of Information Technology, Islamabad
44000, Pakistan
bDepartment of Chemistry, Quaid-i-Azam University, Islamabad-45320, Pakistan.
E-mail: ilyassarwar@hotmail.com; Fax: +92-51-90642241; Tel: +92-51-90642132
cDepartment of Chemical Engineering, Qatar University, PO Box: 2713, Doha, Qatar.
E-mail: mert.atilhan@qu.edu.qa; Fax: +974-4403-2491; Tel: +974-4403-4142
dGraduate School of EEWS, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon, 305-701,
Republic of Korea. E-mail: yavuz@kaist.ac.kr; Fax: +82-42-350-2248;
Tel: +82-42-350-1718
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copper(I) cyanide (99.98%), sodium carbonate (99.99%),
hydrochloric acid (37%), ammonium hydroxide (¢99.99%),
hydroxylamine solution (50 wt.% in H2O) (99.9%), 1-methyl-2-
pyrrolidinone (NMP) (¢99.5%) and sodium nitrite (99.999%)
were supplied by Aldrich and used as received.
an important class of organic compounds featuring a fused,
basic amide and oxime functionality. These compounds are
widely used in the pharmaceutical industry and as antitumor
and antimalarial agents, polymers and chelating resins.33–35
Polyamidoximes derived from acrylonitrile-divinylbenzene–
vinylidene chloride copolymer beads with a porous structure
have been reported.36,37 These chelating resins were used as an
ion-exchanger and as an adsorbent to uptake uranium from
sea water.38 The adsorption behavior of poly(acrylamidoxime)
chelating resin for Cu(II), Cd(II), Hg(II), Zn(II), Pb(II), Cr(III), and
U(VI) was investigated. This study shows that this resin had
higher binding capacity to uranyl ions, fast kinetics, and very
good selectivity from binary metal ion mixtures with Cu(II) and
Pb(II).39 Polyacrylonitrile fiber containing amidoxime groups
with high adsorption capacity to Au3+ ions has been
described.40 Poly(amidoxime)–poly(hydroxamic acid) resins
and fibers have also been investigated for the extraction of
uranium from sea water.41 Amidoximes are very similar in
structure to monoethanolamine (MEA), which is a well known
industrial standard for CO2 capture, thus creating interest to
explore their function as CO2 adsorbents.
The aromatic polyamides, commonly referred as aramids,
have been particularly useful as high-performance materials
because of their very high thermal stability, stiffness, specific
strength, and low density.42–44 The inflexible features of such
polymers made them as high-performance materials which is
the main disadvantage in many applications. However, most
aramids suffer poor processability due to limited solubility in
common organic solvents and high glass transition (Tg) and
softening (Ts) temperatures. Therefore, significant efforts have
been made to increase the solubility and processability of
aramids by structural modification. Soluble aramids may open
applications in films, separation membranes, coatings, poly-
mer blends, and composites.45,46 For gas capture applications,
aramids can prove useful with their remarkable strength in
harsh conditions such as power plant exhaust fumes.
2.2. Characterization
FTIR spectra of monomer and polymers were recorded at room
temperature using an Excalibur Series FTIR spectrometer,
model no. FTSW 300 MX, manufactured by BIO-RAD. NMR
spectra were taken at room temperature using a BRUKER
1
spectrometer operating at 300 MHz for H and at 75 MHz for
13C NMR in deuterated dimethyl sulfoxide (DMSO-d6).
Tetramethylsilane was used as an internal reference.
Thermal stability of the polymers was determined by a
Mettler Toledo TGA/SDTA 851e thermogravimetric analyzer at
a heating rate of 10 uC min21 under N2 atmosphere. Glass
transition temperatures of the polymers were determined
using
a
Mettler Toledo DSC 822e differential scanning
calorimeter in N2 at a heating rate of 10 uC min21. X-ray
diffraction measurements were performed on a Rigaku (D/
Max-2500) HR-X-ray diffractometer using Ni-filtered Cu-Ka
radiation at 40 kV and 300 mA. The surface morphology of
powdered polymer samples was investigated by an FEI Nova
230 field emission scanning electron microscope (FESEM).
Nitrogen adsorption–desorption isotherms were measured at
77 K using a Tristar 3020, Micromeritics (USA) porosimetry
analyzer. For surface area determination, the Brunauer–
Emmett–Teller (BET) method was employed using a nitrogen
molecule surface area of 0.162 nm2. The powdered aramid
samples were degassed for surface area analysis at 100 uC for 6
h under vacuum. A Tristar 3000, Micromeritics (USA) CO2
analyzer was used to determine CO2 adsorption capacity at
pressure of 1 bar.
2.3. High pressure CO2 adsorption studies
Gravimetric carbon dioxide adsorption capacities of amidox-
ime porous polymer APP-1 and APP-2, were monitored at 318 K
up to 225 bar using a high-pressure magnetic suspension
We recently showed47 that amidoximes exhibit improved
CO2 capture capacities when compared with similar porous
solids.48–51 The challenge, however, was that the overall
efficacy lacked promise for large scale applications, where
high throughput processes with robust materials are neces-
sary. In the present attempt, we successfully incorporated
amidoxime pendant groups to aramid backbones and
observed a significant increase in CO2 capture capacity. The
new durable solids are found to be viable candidates for power
plant CO2 scrubbing. To the best of our knowledge, aramids or
porous polymers with amidoxime functional groups have
never been reported.
¨
balance (MSB) made by Rubotherm Prazisionsmesstechnik
GmbH. The balance was maintained at 373 K up to 350 bars
with two different operating positions as illustrated in Fig. 1.
Firstly, the measuring cell was loaded with CO2 at the sorption
position. The MSB measured the change in weight of the
sample upon adsorption of high pressure gas on it. The second
position was used to determine the in situ density of the high-
pressure gas needed to calculate the amount of adsorbed gas
on the solid sample in the high-pressure cell. The change in
weight of a sorbent sample is usually measured in the gravity
field upon adsorption from a surrounding gas phase via a
contactless force transmission method; which allows more
stable operation at high pressures. The increase in weight of
the sample within the cell is conveyed through magnetic
suspension coupling from a closed and pressure-proof metal
container to an external microbalance. The cell is provided
with a platinum resistance thermometer, Jumo DMM 5017
Pt100, that records the cell temperature within ¡ 273 K
accuracy. The pressure is measured with a Paroscientific1
Digiquartz 745-3K with an accuracy of 0.01%.
2. Experimental
2.1. Materials
4,49-Oxydianiline (ODA) (¢98%), p-phenylenediamine (p-PDA)
(¢99%), potassium cyanide (98%) and ethanol (99%) were
procured from Fluka. 5-Aminoisophthalic acid (94%),
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Fig. 1 (a) Schematics of the MSB and its operation positions used for sorption and density measurements. (b) Types of isotherms that are common to gas adsorption
experiments.
Typical measurements were conducted by taking 0.25 g of
sample in the holder and evacuating the system for 24 h at 333
K. CO2 was then pressurized with the help of a Teldyne Isco
260D fully automated gas booster and added into the high-
pressure cell, where CO2 adsorption on the sample com-
menced. For each pressure point it took 45 min to reach
equilibrium and once it was reached, 4 different set of
measurements were recorded for a period of 10 min; each
data point was taken at 30 s interval. The total span for each
temperature and pressure point took around 50 min. At the
end of each pressure point, the system automatically moved to
the next pressure measurement point. The optimum pressure
up to 225 bars was used in the present work and at the end of
each isotherm, a hysteresis check was carried out at each
isotherm by collecting desorption data as the system was
depressurized. The adsorption data was evaluated and the
amount of adsorbed CO2 was calculated using the following
relationship:
the sinker; mads = adsorption amount; msample = mass of the
sample; and msinker = mass of the sinker.
The density of gas was recorded in situ by the second
measurement position (Fig. 1) and verified with REFPROP
9.052 for accurate measurements. The mass of empty sinker
was taken at numerous pressures of helium to find out the
buoyancy due to the sinker. The volume of sinker was
computed from the slope of a weight vs. density plot. A blank
reading under vacuum was recorded to measure the mass of
the sinker. The buoyancy correction due to the sample was
achieved by calculating the volume of the sample obtained by
the slope of a weight vs. density plot similar to the helium
measurement of the sinker with the sample. The mass of the
sample was recorded under vacuum and activated charcoal
norit1 RB3 was employed for calibration. The CO2 adsorp-
tion–desorption behavior of both aramids was monitored by
exposing the sample to increasing and decreasing pressure
increments of CO2 at a particular temperature. The sample
and gas were permitted to reach equilibrium at each pressure
point. The corresponding mass change was then corrected for
buoyancy to achieve the adsorption amount (mmoles of CO2
per g of adsorbent).
W+ Wbuoy,sample + Wbuoy,sinker = mads + msample + msinker
(1)
where: W= signal read by the instrument; Wbuoy,sample = Vsample
6 dgas = buoyancy correction due to sample; Vsample = volume
Regardless of the measurement method, gas adsorption
follows certain pre-defined isotherms based on the pore size
and distribution (Fig. 1b). The types I–IV are most common,
although more exist. In a typical Type I isotherm, a Langmuir
of the sample; dgas = density of the gas; Wbuoy,sinker = Vsinker
6
dgas = buoyancy correction due to sinker; Vsinker = volume of
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coverage is observed where gas molecules can only find
enough space to form a simple monolayer. This is typical for
predominantly microporous solids such as activated carbons
and zeolites, with negligible inter-particle voids. Accessibility
of the micropores governs the uptake. Type II is found mostly
in macroporous or non-porous structures where monolayer
coverage (the first saturation) is followed by a multi-layer
adsorption. The isotherm does not reach equilibrium at the
end, pointing to an unrestricted exchange between mono and
multi layer stages. Type III is not too common but nonetheless
exists. It should be considered as Type II without, or with
negligible, initial saturation. Type IV is characteristic of
mesoporous solids and should be considered as the combina-
tion of the capillary filling effects of mesopores with that of
Type II (and possible Type I). The final saturation represents
the lesser contribution of macro or inter-particle pores.
2500–3500 cm21 (acid O–H stretch), 3060 cm21 (aromatic C–H
stretch), 1715 cm21 (CLO stretch), 2246 cm21 (CMN stretch),
1646, 1438 cm21 (aromatic CLC stretch). 1H NMR: dH (300
MHz, DMSO-d6) 9.27(1H, s), 8.71 (2H, s); 13.65 (2H, s). 13C
NMR: dC (75 MHz, DMSO-d6) 112.8, 115.8, 131.1, 135.2, 139.2,
169.5.
2.5. Synthesis of 5-cyanoisophthaloyl chloride (2)
5-Cyanoisophthalic acid (0.11 mol, 22 g) and thionyl chloride
(13 mL) were refluxed for 1 h at 70 uC in a 250 mL round
bottom flask. Excess of thionyl chloride was stripped off by
distillation to obtain acid chloride with good yield (Scheme 1).
Yield (80%); FTIR (KBr) 3088 cm21 (aromatic C–H stretch),
1752 cm21 (CLO stretch), 2244 cm21 (CMN stretch), 1644, 1435
cm21 (aromatic CLC stretch), 692 cm21 (C–Cl stretch). 1H
NMR: dH (300 MHz, DMSO-d6) 9.12(1H, s), 8.66 (2H, s); 13C
NMR: dC (75 MHz, DMSO-d6) 113.2, 115.9, 134.1, 137.3, 141.9,
167.5.
2.4. Synthesis of 5-cyanoisophthalic acid (1)
5-Aminoisophthalic acid (0.02 mol, 3.62 g), conc. HCl (5 mL)
and distilled H2O (5 mL) were placed in a 250 mL round
bottom flask provided with a magnetic stirrer. The reaction
mixture was agitated for 25 min and cooled to 0 uC in an ice-
bath. After complete mixing, NaNO2 solution (prepared by
dissolving 0.023 mol, 1.58 g of NaNO2 in 4 mL of H2O) was
added drop-wise into the flask resulting in yellow colored
diazonium chloride. The diazonium chloride was poured into
hot cyanide solution (prepared by dissolving 0.069 mol, 4.49 g
of KCN and 0.023 mol, 2.06 g of CuCN in 13 mL of water with
Na2CO3 (2 g) at 60 uC for 10 min). Then the mixture was cooled
to room temperature, followed by the addition of conc. HCl
until no more precipitates were formed. The yellow precipi-
tates were filtered off, washed with water and treated with
NH4OH (14 mL). Further filtration and acidification of the
filtrate with conc. HCl produced the light orange precipitate of
5-cyanoisophthalic acid (Scheme 1), which was purified by
recrystallization in water. Finally, the product was dried under
reduced pressure at 80 uC for 24 h. Yield (85%); FTIR (KBr)
2.6. Synthesis of aramid bearing nitrile groups derived from
ODA (3)
ODA (0.05 mol, 10.0 g) and 100 mL of NMP were charged into a
250 mL round bottom flask, equipped with a dropping funnel,
gas inlet tube and a magnetic stirrer. After complete mixing of
the diamine solution, 5-cyanoisophthaloyl chloride (0.05 mol,
11.4 g) dissolved in NMP (100 mL) was added dropwise to the
above solution at 0 uC with constant stirring for 10 min and
then at 25 uC for 24 h in a N2 atmosphere. The reaction
mixture was poured into water with continuous stirring and
precipitates of the resulting polyamide having nitrile pendant
groups were filtered off and thoroughly washed with hot water
and dried under vacuum at 80 uC for 24 h (Scheme 2). Yield
(74%); FTIR (KBr), 3310 cm21 (N–H stretch), 3069 cm21
(aromatic C–H stretch), 1603, 1494 cm21 (aromatic CLC
stretch), 1650 cm21 (CLO stretch), 1214 cm1 (C–O–C stretch),
2234 cm21 (CMN stretch). 1H NMR: dH (300 MHz, DMSO-d6)
7.86 (1H, s), 6.85 (4H, d), 7.63 (4H, d) 8.42 (2H, s), 8.54 (1H, s);
13C NMR: dC (75 MHz, DMSO-d6) 112.6, 115.9, 119.5, 121.0,
129.6, 134.1, 135.2, 138.4 150.6, 165.1.
2.7. Synthesis of APP-1 (4)
The dried polyamide (3) containing nitrile groups (1 g) was
dispersed in water (98.8 g) followed by the addition of
hydroxylamine solution (50 wt.% in H2O) (0.2044 g) with
uninterrupted agitation and refluxing at 100 uC for 4 h. The
contents of the flask were centrifuged to isolate the pre-
cipitates of amidoximated polyamide which were then washed
with distilled water and dried under vacuum at 50 uC for 24 h
(Scheme 2). Yield (68%); FTIR (KBr), 3309 cm21 (N–H stretch),
3075 cm21 (aromatic C–H stretch), 1606, 1498 cm21 (aromatic
CLC stretch), 1654 cm21 (CLN and CLO stretch), 3198 cm21
(O–H stretch), 1407 cm21 (C–N stretch), 1232 cm21 (C–O–C
stretch), 955 cm21 (N–O stretch). 1H NMR: dH (300 MHz,
DMSO-d6) 7.83 (1H, s), 8.62 (1H, s), 5.97 (2H, s), 6.87 (4H, d),
8.50 (1H, s), 8.41 (2H, s), 7.62 (4H, d); 13C NMR: dC (75 MHz,
DMSO-d6) 117.3, 121.1, 122.6, 134.3, 134.9, 135.7, 140.5, 150.5,
164.9, 165.2.
Scheme 1 Synthesis of 5-cyanoisophthalic acid and 5-cyanoisophthaloyl chlor-
ide.
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Scheme 2 Scheme for the synthesis of aramid bearing nitrile and amidoxime
pendant groups derived from ODA.
2.8. Synthesis of aramid bearing nitrile groups derived from
p-PDA (5)
The polymerization of p-PDA (0.05 mol, 5.4 g) and 5-cyanoi-
sophthaloyl chloride (2) (0.05 mol, 11.4 g) was carried out in
NMP. For this purpose, p-PDA was dissolved in NMP (100 mL),
followed by the drop wise addition of 5-cyanoisophthaloyl
chloride (2) dissolved in 100 mL NMP at 0 uC with continuous
stirring for 10 min and then at 25 uC for 24 h in an inert
atmosphere. The polymer was precipitated in water and
filtered off (Scheme 3), thoroughly washed and dried. Yield
(83%); FTIR (KBr), 3308 cm21 (N–H stretch), 3101 cm21
(aromatic C–H stretch), 1613, 1512 cm21 (aromatic CLC
stretch), 1651 cm21 (CLO stretch), 2237 cm21 (CMN stretch).
1H NMR: dH (300 MHz, DMSO-d6) 8.61 (1H, s), 9.64 (1H, s) 8.80
(2H, s) 7.82 (4H, s); 13C NMR: dC (75 MHz, DMSO-d6) 112.6,
118.2, 121.6, 131.4, 132.8, 135.1, 138.2, 164.5.
Scheme 3 Scheme for the synthesis of aramid bearing nitrile and amidoxime
pendant groups derived from p-PDA.
under vacuum for 24 h (Scheme 3). Yield (75%); FTIR (KBr),
3307 cm21 (N–H stretch), 3105 cm21 (aromatic C–H stretch),
1617, 1514 cm21 (aromatic CLC stretch), 1655 cm21 (CLN and
CLO stretch), 3200 cm21 (O–H stretch), 1402 cm21 (C–N
stretch), 938 cm21 (N–O stretch). 1H NMR: dH (300 MHz,
DMSO-d6) 8.42(1H, s), 9.89 (1H, s), 5.86 (2H, s), 8.62(1H, s),
8.52 (2H, s) 7.81 (4H, s); 13C NMR: dC (75 MHz, DMSO-d6)
124.3, 129.6, 132.8, 134.8, 135.3, 138.6, 164.6, 166.0.
2.9. Synthesis of APP-2 (6)
3. Results and discussion
Similarly, an aramid bearing amidoxime groups (APP-2) was
prepared by dispersing polyamide (5) in water and refluxing
with hydroxylamine to convert the nitrile group into an
amidoxime functionality. The precipitates of amidoximated
polyamide were isolated, washed with distilled water and dried
3.1. Monomer and polymer synthesis
Aramids are polymers with exceptional stability and rigidity,
finding good use in applications that require highly resistant
media (e.g. Kevlar1). In membrane or sorbent developments,
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however, soft texture and processability emerge as more
crucial properties. Numerous attempts have been made to
modify the structure of aramids to make them more soluble
and processable.53 Among them, the introduction of different
flexible linkages and kinks (–O–, –SO2–, –CH2–, –C(CF3)2– etc.)
along the back bone led to significant improvement in the
solubility of aramids in many polar aprotic solvents like DMF,
DMAc, DMSO, NMP etc.54 Another interesting approach to
modify the structure of aramids is the incorporation of bulky
substituents as pendant groups in the main chain. So, by
carefully selecting the pendant group, solubility can easily be
promoted without affecting other properties. In this regard,
amidoxime groups may deliver enhanced processability, along
with their use for CO2-philic behavior.
The synthesis involved the conversion of the amine group of
5-aminoisophthalic acid (1) into a nitrile group, which upon
reaction with thionyl chloride produced 5-cyanoisophthaloyl
chloride (2) (Scheme 1). Aramids were then synthesized by
condensing 5-cyanoisophthaloyl chloride with ODA or p-PDA
at low temperature and amidoxime groups were introduced by
converting the nitrile groups using hydroxylamine solution
(Schemes 2 & 3). The structure elucidation of both monomer
and aramids were carried out by FTIR, 1H and 13C NMR
spectroscopy. The resulting aramids APP-1 and APP-2 were
then further characterized for organosolubility, thermal
stability, glass transition temperatures, X-ray diffraction,
FESEM, BET analysis and CO2 adsorption measurements.
Fig. 2 TGA curves of aramids APP-1 and APP-2.
thermal stability of APP-2 was higher than APP-1, which has
weak ether linkages in the backbone thereby exhibiting lower
thermal stability relative to APP-2. The glass-transition
temperature (Tg) of APP-1 was at 97 uC and APP-2 at 242 uC
(Fig. 3). Generally, the inclusion of rigid units along a polymer
backbone restricts the segmental mobility responsible for
increased Tg values.55 On the contrary, flexible linkages reduce
the rigidity and Tgs of the polymers. Therefore, depending on
the structure of the diamine component, the glass-transition
temperatures of the aramids increased with higher rigidity and
symmetry of the polymer backbone. Polyamide APP-2 contain-
ing para units displayed higher Tg values because these units
suppressed the segmental mobility in the polymer chains.
Predictably, polyamide APP-1 having flexible ether linkages
showed relatively lower Tg values on account of enhanced
segmental/molecular mobility.
3.2. Organosolubility
Both the polyamides bearing amidoxime groups were soluble
in polar aprotic solvents such as DMSO, DMF, and NMP while
being insoluble in others such as THF, methanol and
chloroform. Table 1 shows qualitative solubility data of both
the aramids in some common solvents (DMF, NMP, DMSO,
THF, CH3OH, CHCl3 and m-cresol). Polyamide APP-1 derived
from the condensation of ODA and 5-cyanoisophthaloyl
chloride dissolves rapidly at room temperature because of
the flexible ether linkages in the backbone. Alternatively, the
lower solubility of polyamide APP-2 at room temperature is
attributed to the rigid aromatic unit (p-phenylene) in the
polyamide chains.
3.3. Thermal analyses
The thermal stability of CO2 adsorbents is crucial for practical
applications; therefore, thermal behavior of the aramids was
determined through thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) in a N2 atmosphere at
a heating rate of 10 uC min21. The thermograms revealed that
both polymers were thermally stable (Fig. 2). However, the
Table 1 Solubility behavior of aramids containing amidoxime pendant groupsa
Polymer DMSO DMF NMP THF m-cresol Methanol Chloroform
APP-1
APP-2
++
++
++
++
++
++
2
2
+2
2
2
2
2
2
a
Solubility: ++ Soluble at room temperature. +2 Partially soluble at
room temperature. 2 Insoluble.
Fig. 3 DSC thermograms of aramids APP-1 and APP-2.
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respectively. The N2 sorption isotherms of polyamides APP-1
and APP-2 exhibited ‘‘Type III’’ and ‘‘Type II’’ behaviors.
3.6. Field emission scanning electron microscopy
The surface morphology of the aramid samples (4) and (6) was
investigated by field emission scanning electron microscopy.
The micrographs of the polyamides are shown in Fig. 6. The
FESEM image of polyamide APP-1 revealed significant macro-
porosity in the sample whereas polyamide APP-2 has an almost
non-porous structure which may be explained on the basis of
molecular configuration. The micrograph (Fig. 6a) clearly
indicates that APP-1 has more free volume and shows a less
compact structure, therefore it exhibits a more porous nature.
Meanwhile the FESEM image of APP-2 (Fig. 6b) displays a
highly compact structure and less free volume thus signifying
a nonporous morphology. The gas adsorption displayed by
polyamide APP-1 therefore confirmed the porous nature of the
material. On the other hand, polyamide APP-2 demonstrated a
lower N2 uptake owing to the nonporous nature of the sample.
Fig. 4 PXRD patterns of aramids APP-1 and APP-2.
3.4. X-ray diffraction
XRD patterns of the polyamides are illustrated in Fig. 4.
Polyamide APP-1 with ether groups showed a relative decrease
in crystalline behavior owing to the presence of C–O–C linkages
in the polymer backbone that gives rise to less close chain
packing or aggregation giving a weak diffraction pattern.
Conversely, polyamide APP-2 displayed a crystalline peak at
around 17u due to the existence of rigid units in the
macromolecular backbone leading to an efficient close packing
of the polymer chains. Generally, solubility increases with
decreasing crystallinity as evident in the case of polyamide APP-
1 that shows increased solubility in polar solvents at room
temperature compared with polyamide APP-2. Correspondingly,
XRD results were authenticated by thermal analysis wherein
polyamide APP-2 demonstrated higher Tg values.
3.7. Low pressure CO2 adsorption measurements
CO2 uptake by fully organic polymers is emerging as an
alternative for industrial CO2 scrubbing processes.56 To
authenticate the potential of modified aramids as CO2
sorbents, two different types of aramids bearing amidoxime
pendant groups were prepared having flexible and rigid groups
in the polymer backbone for comparative study using a facile
synthetic approach of treating the nitrile pendant group with
hydroxylamine. CO2 uptake of both aramids, APP-1 and APP-2,
was carried out at two different temperatures, 273 K and 298 K,
at a pressure of 1 bar. The CO2 adsorption capacity of APP-1
was found to be 0.32 mmol g21 at 273 K, which decreased to
0.19 mmol g21 with an increase in temperature to 298 K.
Likewise, the adsorption isotherms of APP-2 at 273 K indicated
0.07 mmol g21 uptake of CO2 which also reduced to 0.05 mmol
g21 at 298 K (Fig. 7 and Table 3). The adsorption capacities
decreased drastically when the temperature was increased
from 273 K to 298 K, inferring that the CO2 adsorption capacity
is very much dependant on temperature. These CO2 adsorp-
tion capacities of both aramids are in good agreement with the
3.5. Structural characterization
The porous nature of the polyamides was evaluated by N2
uptake measured at 77 K (Fig. 5). Structural parameters
calculated from N2 adsorption isotherms are presented in
Table 2. The BET surface area was calculated by applying the
Brunauer–Emmett–Teller (BET) model. Polyamides APP-1 and
APP-2 encompassed apparent surface areas of 49.5 m2 g21 and
0.67 m2 g21, respectively. The total pore volumes calculated for
polyamides APP-1 and APP-2 were found to be 0.24 and 0.001
cm3 21, while the average pore sizes were 16.9 and 9.9 nm
g
Fig. 5 N2 isotherms for (a) APP-1 (b) APP-2 measured at 77 K.
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that CO2 can be almost fully recovered during desorption while
the adsorption–desorption isotherms of APP-2 exhibited a
strong hysteresis mainly in the lower pressure range. The
empirical classification of a hysteresis loop was given by
IUPAC in which the shape of the loop is correlated with the
structure of the adsorbent.58 APP-1 and APP-2 both produced
an H4 type of hysteresis loop. H4 type loops are usually
observed with complex materials containing both micropores
and mesopores.59 An H4 hysteresis contains a characteristic
step-down in the desorption branch associated with the
hysteresis loop closure. The hysteretic adsorption is not
reversible in the sense that the ensuing adsorption isotherm
does not retrace the previous desorption isotherm.
Table 2 Textural parameters calculated from N2 adsorption isotherms at 77 K
Parameter
APP-1
APP-2
BET surface area (m2 g21
)
49.5
68.0
16.9
0.24
0.67
1.120
9.90
Langmuir surface area (m2 g21
Average pore size (nm)
)
Pore volume (cm3 g21
)
0.001
fact that the extent of physical adsorption decreases with an
increase in temperature because of the exothermic nature of
the binding.57 The adsorption behavior for both the aramids
showed a substantial chemical interaction with the gas
molecules, which would be attributed to the affinity of the
acidic CO2 molecule towards the basic NH2 group of the
amidoxime pendant group. The reasonable CO2 adsorption
capacity of aramid APP-1 over APP-2 was also due to the
difference in porosity of the material where the presence of
flexible ether linkages in the polymer backbone hinders the
close packing of polymer chains resulting in the availability of
amidoxime functional groups on the surface of the aramid.
Alternatively, APP-2 displayed less affinity towards CO2
adsorption which was ascribed to the rigid aromatic groups
in the aramid backbone leading to close chain packing
responsible for the decrease in porosity. The adsorption
isotherms of both aramids exhibit ‘‘Type I’’ behavior at 273
K and 298 K, depicting a monolayer adsorption.
An adsorption–desorption hysteresis appeared with both the
polymers studied, particularly at low pressure. Ideally,
desorption isotherms should not deviate from the sorption
isotherms. However, as is evident from Fig. 7, the desorption
isotherms generally lie above the excess sorption isotherms,
i.e. a significant hysteresis effect is associated with the
sorption–desorption process. This hysteresis effect indicated
that the sorbent–sorbate system is in a metastable state and as
the pressure decreases, the gas is not readily released to the
extent corresponding to the thermodynamic equilibrium
value. APP-1 showed the smallest deviations between adsorp-
tion and desorption curves (little hysteresis), which indicated
3.8. High pressure CO2 adsorption measurements
CO2 adsorption isotherms (Fig. 8) showed the augmentation in
the adsorption capacity for both the polyamides studied at a
high pressure of 225 bar at 318 K. To the best of our
knowledge, these pressures are the highest ever studied in the
literature. Of course, under these conditions, CO2 is no longer
a gas and the adsorption dynamics differ substantially;
however, our goal is to cover as many parameters as possible
to alleviate the challenges in developing a proper CO2 sorbent
that is immune to extreme conditions. In particular, the
geological capture and pipeline (or container ships) transport
processes require understanding how CO2 behaves with or
without porous structures at high pressures and temperatures.
The CO2 uptakes of APP-1 and APP-2 were found to be 24.69
mmol g21 and 11.67 mmol g21 respectively at 225 bar and 318
K. The results revealed the escalating adsorption for both the
polyamides at high pressure but the remarkable improvement
in APP-1 adsorption above 70 bar is particularly noticeable.
The adsorption isotherms of APP-1 and APP-2 exhibited ‘‘Type
IV’’ behavior, in which multilayer adsorption was observed
after the completion of the first layer. The adsorption
isotherms of both polyamides maintained a clear distinction
between the first layer (0–70 bar region) and multilayer (70–
225 bar region) adsorptions depicting that the adsorbate–
adsorbent interactions present in these polymers are the
Fig. 6 FESEM micrographs of (a) APP-1 (b) APP-2.
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Fig. 7 CO2 gas sorption isotherms at pressure of 1 bar (a) APP-1 at 273 K (b) APP-1 at 298 K and (c) APP-2 at 273 K (d) APP-2 at 298 K.
strongest. No hysteresis loop was observed for both the
polymers at high pressure in contrast to low pressure because,
at low pressure, chemisorption is the primary reason for the
gas to interact, whereas at high pressure the amount of
chemisorbed species decreased.
as promising candidates for CO2 capture and storage. Building
on our earlier work47 we now show that robust chains of
aramids can host CO2-active amidoxime units for enhanced
CO2 capture. The porous amidoxime polymers reported here
are the first examples of this highly promising family of
porous solids. In particular, we explored two types of aramids
bearing amidoxime groups. CO2 adsorption up to 0.32 mmol
g21 at 273 K and 1 bar was found for polyamide APP-1, which
is considerable since aramids are usually non-porous hard
supports and not expected to be CO2-philic. The solubility of
4. Conclusions
Exploitation of polyamides bearing amidoxime functionality as
CO2 scavengers showed that these macromolecules can serve
Table 3 CO2 Adsorption capacity (mmol g21) of aramids at 1 and 225 bar
CO2 Adsorbed (mmol g21) at 1 bar
CO2 Adsorbed (mmol g21) at 225 bar
Aramid
T = 273 K
T = 298 K
T = 318 K
APP-1
APP-2
0.32
0.07
0.19
0.05
24.69
11.67
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Acknowledgements
This work was made possible by NPRP grants # 08-670-1-124
and #5-499-1-088 from the Qatar National Research Fund (a
member of Qatar Foundation). The statements made herein
are solely the responsibility of the authors. We also acknowl-
edge the financial support by grants from the Korea CCS R&D
Center, IWT (NRF-2012-C1AAA001-M1A2A2026588) funded by
the Ministry of Education, Science and Technology of Korean
government, the Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded by the
Ministry
of
Science,
ICT
&
Future
Planning
(2013R1A1A1012998), and the KAIST EEWS Initiative.
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