Design of Aminopolymer Structure to Enhance Performance and Stability of CO2 Sorbents: Poly(propylenimine) vs Poly(ethylenimine)

Article abstract of DOI:10.1021/jacs.7b00235

Studies on aminopolymer/oxide composite materials for direct CO₂ capture from air have often focused on the prototypical poly(ethylenimine) (PEI) as the aminopolymer. However, it is known that PEI will oxidatively degrade at elevated temperatur

Full text of DOI:10.1021/jacs.7b00235

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Communication
Design of Aminopolymer Structure to Enhance Performance and
Stability of CO2 Sorbents: Poly(propylenimine) vs. Poly(ethylenimine)
Simon H. Pang, Li-Chen Lee, Miles A. Sakwa-Novak, Ryan P Lively, and Christopher W Jones
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b00235 • Publication Date (Web): 28 Feb 2017
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Design of Aminopolymer Structure to Enhance Performance and
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Stability of CO Sorbents: Poly(propylenimine) vs.
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Poly(ethylenimine)
†
†
‡
†
†
Simon H. Pang , Li-Chen Lee , Miles A. Sakwa-Novak , Ryan P. Lively , Christopher W. Jones*
†
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia
30332, United States
‡
Global Thermostat LLC, 311 Ferst Drive, Atlanta, Georgia 30332, United States
Supporting Information Placeholder
bility of neighboring primary amines, for grafted amines with an
ethylenediamine structure. Though aminosilanes with only prima-
ry amines were found to be resistant to oxidation, the amine den-
sity achievable in these amine-grafted materials is typically lower
than in the supported aminopolymer-based materials,
them less desirable for industrial application.
ABSTRACT: Studies on aminopolymer/oxide composite mate-
rials for direct CO capture from air have often focused on the
2
prototypical poly(ethylenimine) (PEI) as the aminopolymer.
However, it is known that PEI will oxidatively degrade at elevated
temperatures. This degradation has been ascribed to the presence
2
5,26
making
of secondary amines, which, when oxidized, lose their CO cap-
2
ture capacity. Here, we demonstrate the use of small molecule
poly(propylenimine) (PPI) in linear and dendritic architectures
In this study, we sought to utilize aminopolymer structures that
had improved stability against oxidative degradation, which we
hypothesized would be dendritic structures that consisted of only
primary and tertiary amines. Linear aminopolymers containing
secondary amines were also explored as control materials. Linear
and dendritic aminopolymers with either ethylene or propylene
spacers (Scheme 1) were supported inside mesoporous silica
SBA-15, a model silica support characterized by long, straight
cylindrical 8 nm diameter pores (Figure S1). The dendritic struc-
tures used in this study, termed EI-den and PI-den, were synthe-
sized in house (see Supporting Information). Unexpectedly, PPI-
based aminopolymers were found to behave quite differently, in
an advantageous way, than traditional PEI-based aminopolymers.
supported in silica as adsorbent materials for direct CO capture
2
from air. Regardless of amine loading or aminopolymer architec-
ture, the PPI-based sorbents are found to be more efficient for
CO capture than PEI-based sorbents. Moreover, PPI is found to
2
be more resistant to oxidative degradation than PEI, even while
containing secondary amines, as supported by FTIR, NMR, and
ESI-MS studies. These results suggest that PPI-based CO2
sorbents may allow for longer sorbent working lifetimes due to an
increased tolerance to sorbent regeneration conditions, and sug-
gest that the presence of secondary amines may not mean that all
aminopolymers will oxidatively degrade.
Scheme 1. Linear and Dendritic Poly(ethylenimine) and
Poly(propylenimine) in this Study
Significant recent research on materials for direct capture of
CO from ambient air has focused on amines supported on oxide
2
1
–3
or polymeric materials. Materials involving impregnation of an
aminopolymer into a porous oxide support such as silica or alu-
mina have been well-studied due to the high density of amines
4–11
present in the material,
leading to high heats of adsorption, and
thus, high CO capture efficiencies, even from ultradilute streams
2
1
2–15
such as ambient air (400 ppm).
mer used in these studies is
poly(ethylenimine), PEI. However, PEI is known to oxidatively
The prototypical aminopoly-
a
randomly-branched
16–19
degrade in the presence of oxygen at elevated temperatures,
leading to development of processes that require sorbent regenera-
tion (CO desorption) be performed in environments free of O ,
2
2
such as vacuum or steam. These regeneration conditions impose
additional costs and can still lead to the possibility of leaching of
20–22
PEI from the material and loss of CO capture capacity.
2
Studies performed on aminosilanes grafted onto oxide supports
have been used to determine the nature of oxidative degradation
1
6,23,24
of various amines.
In particular, primary and tertiary amines
Results from nitrogen physisorption experiments confirm that
the impregnated aminopolymers resided in the mesopores of
SBA-15 (Table S1 and Figure S2). CO
appeared to be stable against oxidation but secondary amines
degraded via imine and amide formation. Furthermore, degraded
capture was performed at
/N ) in a TGA and the
2
secondary amines also negatively impacted the CO capture capa-
35 °C with simulated air (400 ppm CO
2
2
2
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quantity of CO adsorbed was monitored by measuring the mass
increase after the degassed sample was exposed to a flow of simu-
lated air for 3 h. As shown in Figure 1, the amine efficiency and
aminopolymers were impregnated into SBA-15. As expected, the
2
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amine efficiency and CO capture capacity of TETA decreased
2
significantly (Figures 1 and S3), approximately 90%, due to the
presence of secondary amines that were oxidized, whereas PI-den
only lost approximately 50% of its amine efficiency. Unexpected-
ly, TPTA did not lose much efficiency (20% loss) after the oxida-
tive treatment, despite containing secondary amines like TETA.
Additionally, EI-den lost over 90% of its amine efficiency, despite
having no secondary amines. Thus, it appears that aminopolymers
with ethylene linkers were more susceptible to oxidative degrada-
tion than those with propylene linkers. We hypothesize that the
conditions used during oxidative treatment caused thermal rear-
rangement reactions between molecules, resulting in production
of secondary amines that could oxidize EI-den. This point is ex-
plained in greater detail below.
CO capture capacity (Figure S3) increased as a function of amine
2
loading for all molecules studied. Additionally, linear and dendrit-
ic aminopolymers appeared to have similar amine efficiencies for
a given alkyl spacer length when normalized to the number of
amines that are hypothetically active for CO capture under dry
2
conditions, suggesting no significant branching-dependence in
capture capability for these small molecules.
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To investigate the structures created during the oxidative treat-
ment, spectroscopic studies were carried out to examine the
changes in functional groups. Fourier transform infrared (FTIR)
spectra (Figure 2) were normalized to the Si-O-Si framework
-1
bending mode at 460 cm . For all sorbent materials, the expected
-
1
stretches above 2500 cm corresponding to NH, OH, and CH2
stretches were observed (Figure S4). After the oxidative treat-
ment, some of these stretches decreased in intensity, but no other
peaks appeared, as expected. The stretches near 1640, 1570 and
-
1
1
475 cm correspond to vibrations of the aminopolymers and
ammonium carbamate ion pairs formed by capture of CO from
2
2
5,30,32,33
ambient air.
Below this region, the vibrational spectra are
dominated by modes from the silica support, and are therefore not
discussed here.
Figure 1. Amine efficiencies of aminopolymers impregnated in
SBA-15 at 400 ppm CO /N and 35 °C over a range of amine
2
2
loading, normalized to the number of amines that are active for
CO capture under dry conditions (1° and 2° only). Open symbols
2
indicate samples exposed to oxidative treatment at the listed tem-
perature.
It was additionally observed that for both the linear and dendrit-
ic aminopolymers, the poly(propylenimines), TPTA and PI-den,
were more efficient at CO2 capture than the corresponding
poly(ethylenimines), TETA and EI-den. This increased amine
efficiency may be related to the basicity (pKa) of the various
amines and nearest neighbor interactions between the amines. As
the length of the alkyl spacer increased, the pKa of the amines
was previously observed to increase and the extent of nearest
27,28
neighbor interactions decreased.
Since CO capture involves
2
an acid-base interaction, it is possible that the increased basicity
and decreased nearest neighbor interactions associated with in-
creasing alkyl spacer length would increase the CO capture ca-
2
pacity and efficiency.
The CO2 capture capacities and amine efficiencies reported
here (for TPTA, TETA, and PI-den) are higher than or (for EI-
den) equivalent to those for randomly-branched PEI/SBA-15 sys-
tems at similar amine loadings, which reach approximately 0.1
Figure 2. FTIR spectra for SBA-15-supported samples with fresh
aminopolymers (black) and aminopolymers oxidatively-treated at
110 °C for 24 h (red). Spectra were acquired under vacuum and
-
1
normalized to the Si-O-Si framework bending mode at 460 cm .
13,29
mmol CO /mmol N at the highest amine loadings tested.
In
2
-
1
After the oxidative treatment, an intense peak at 1665 cm ap-
peared in the spectra for the ethylenimine sorbents TETA and EI-
den; this peak has been attributed to the stretching mode of C=N
or C=O groups, consistent with oxidation of the
general, small molecule aminopolymers have been shown to have
higher amine efficiencies than randomly-branched PEI due to
their lack of tertiary amines, which are inactive for CO capture
2
3
0
under dry conditions. However, amine efficiencies up to 0.25
17,19,23,34,35
aminopolymers.
This oxidation product is correlated
mmol CO /mmol N have been observed for other PEI/silica com-
2
with the sharp loss in CO capacity. In contrast, the peak at 1665
posites that utilize silica supports with higher pore volume, such
2
-
1
1
2,31
cm did not appear with significant intensity for the propyl-
enimine sorbents TPTA and PI-den, consistent with retention of
as commercially-available CARiACT silicas.
The best amine
efficiency achieved here, 0.21 mmol CO /mmol
N for
2
CO capacity as seen in the results above.
TPTA/SBA-15, approaches this value.
2
To confirm that the oxidation trends observed here were not
due the use of liquid-phase aminopolymers, essentially identical
oxidation experiments were performed after impregnating the
To test the stability of these molecules against oxidative degra-
dation, ultra-zero grade air was bubbled through the liquid amino-
polymers at 110 °C for 24 h, and the resulting oxidatively-treated
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13
SBA-15 with aminopolymer molecules (Figure S7). The results of
the liquid-phase and impregnated sorbent oxidation experiments
resulted in similar changes in all cases – a peak at 1665 cm ap-
peared in the spectra for both TETA and EI-den. Experiments
In contrast, there was very little change in the TPTA H and C
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NMR spectra after oxidative treatment and the m/z = 189 peak
was retained in ESI-MS, corroborating the observations from the
IR spectra that TPTA was not oxidized significantly. Interesting-
ly, some higher and lower molecular weight species were ob-
served in the ESI-MS, indicating that TPTA did undergo some
small amount of thermal rearrangement during the treatment (see
Figure S9 for additional discussion). Thus, from the IR, NMR,
-
1
were also performed with
a
longer chain linear
poly(ethylenimine), pentaethylenehexamine (PEHA), and identi-
cal results to TETA were observed, as expected. TPTA and PI-
den still did not appear to oxidize to a significant extent, but the
intensity of the stretches for TPTA decreased significantly, indi-
cating loss of organic content due to evaporation during the 110
°C thermal treatment.
and ESI-MS studies, the 20% decrease in CO capture capacity
2
for TPTA after the oxidative treatment is likely not associated
with oxidation, but rather reaction between TPTA molecules at
the elevated temperatures within an oxidizing environment.
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To investigate the chemistry occurring during oxidative treat-
ment, solution-phase NMR was performed on the neat and oxida-
tively-treated liquid aminopolymers (Figure 3). Prior to the oxida-
tive treatment, TETA showed peaks at 2.81, 2.74, 2.68 ppm corre-
sponding to the three types of methylene protons in the molecules.
After exposure to the oxidative treatment, many peaks appeared in
the 2.0-4.5 ppm region, indicating the formation of many new
As expected from the IR results, similar oxidation trends were
observed for the dendritic species in H NMR (Figure S10). Par-
1
ticularly striking for EI-den was the appearance of additional
peaks in the 2.0-4.5 and 7.5-8.5 ppm regions, suggesting that sig-
nificant oxidation occurred, in agreement with IR results and the
over 90% decrease in the amine efficiency for sorbents based on
this molecule. Some small additional peaks in the 2.0-4.5 ppm
region also appeared for PI-den.
13
types of methylene protons. C spectra (Figure S8) also indicated
the formation of many types of methylene carbons detected in the
3
5-55 ppm region, supporting the formation of oxidized or ther-
We hypothesize that similar types of oxygen-assisted thermal
rearrangement reactions occurred for all molecules, resulting in
mally-rearranged species. Importantly, peaks in the 8.0-8.3 ppm
region of the H spectrum appeared, suggesting formation of
imine (–CH=N–) and/or amide (–C(=O)NH–) species. These as-
signments are supported by the appearance of carbon shifts at
1
1
intramolecular cyclization and/or alkylamine chain transfer reac-
36,37
tions, as has been suggested previously.
These kinds of reac-
tions could result in generation of secondary amines from the
dendritic species, explaining the oxidation of EI-den. We also
hypothesize that these thermal rearrangements are facilitated by
the presence of primary amines, which have a higher density on
the dendritic structures than the linear structures and may more
easily form degradation products. This helps to explain why there
60-165 ppm, which have also been previously assigned to imine
18
and amide species formed after oxidation of aminopolymers.
was a larger decrease in amine efficiency and CO capture capaci-
2
ty observed after the oxidative treatment for PI-den than TPTA.
Though TPTA appeared to be more resistant to oxidation under
relatively mild (110 °C) conditions, it was still possible to oxidize
the propylene-containing molecule at higher temperature (200
°C). The resulting compound was difficult to dissolve in methanol
for impregnation in SBA-15 or chloroform-d for analysis via
NMR, suggesting that the polarity of the resulting thermally-
rearranged compound was significantly different from the parent
molecule. The methanol-soluble fraction was impregnated into
SBA-15 (Fig. 1) and had essentially no CO capture capacity.
2
However, these harsh, high-temperature thermal treatments are
outside the typical operating envelope for post-combustion CO2
capture and direct air capture.
These findings have important implications for design of
sorbents for CO capture. First, the oft studied randomly-branched
2
poly(ethylenimine), PEI, may be replaced by a molecule with
longer spacing in between the amines, poly(propylenimine), PPI,
1
Figure 3. H NMR spectra for linear aminopolymers TETA and
TPTA, acquired in CDCl (solvent peak indicated by asterisks).
Black, lower: fresh aminopolymers; red, upper: aminopolymers
oxidatively-treated at 110 °C for 24 h. Inset for TPTA: higher
resolution methylene region, showing very little change after the
oxidative treatment.
to increase the CO capacity under flue gas or air capture condi-
2
3
tions due to the increased basicity of the amines in PPI and the
decreased nearest neighbor interactions. This may be achieved
with a randomly-branched, dendritic or linear PPI.
Second, the presence of the accepted “bad actors” for deactiva-
tion of aminopolymers, the secondary amines, may not mean that
the aminopolymer will readily oxidize. That is, it appears that
secondary amines linked by propylene spacers still retain much of
Electrospray ionization mass spectrometry (ESI-MS) experi-
ments were performed to examine the formation of oxidized and
thermally-rearranged products (Figure S9). TETA was character-
ized by a single peak at m/z = 147, corresponding to singly proto-
nated TETA, as expected. After oxidation, this peak disappeared
completely and was replaced by peaks at m/z = 169 and 165, sug-
gesting that all of the TETA molecules had been oxidized. These
masses are consistent with thermal rearrangement and/or dehy-
drogenation reactions and addition of up to two oxygen atoms,
consistent with formation of imines and amides, in agreement
with IR and NMR studies.
their CO capture capability even after exposure to oxidation con-
2
ditions relevant to carbon capture, making them less sensitive to
regeneration conditions. This is consistent with oxidative stability
studies of poly(allylamine), which also has three carbons in be-
tween nearest neighbor amines and which also was not observed
19
to undergo oxidation. However, molecules with amines linked
16,23
by ethylene spacers, be they aminosilanes
branched PEI,
or randomly-
17–19
have the potential to undergo oxidation re-
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gardless of the initial structure due to thermal rearrangement reac-
tions, as was the case shown for EI-den.
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(
CO capture capacities and efficiencies compared to PEI. Moreo-
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ASSOCIATED CONTENT
Supporting Information
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ACS Publication website.
Detailed information about all materials and synthetic procedures,
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AUTHOR INFORMATION
Corresponding Author
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*E-mail: cjones@chbe.gatech.edu
Notes
Funding for the initial work was provided by Global Thermostat,
LLC. Partial support was also provided as part of UNCAGE-ME,
an Energy Frontier Research Center funded by the U.S. Depart-
ment of Energy, Office of Science, Basic Energy Sciences under
award no. DE-SC0012577.
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ACKNOWLEDGMENT
Funding for this work was provided by Global Thermostat, LLC.
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