Encapsulated Amino-Acid-Based Ionic Liquids for CO2 Capture

Article abstract of DOI:10.1002/ejic.202000364

Ionic liquids have gathered special attention due to their potential for carbon dioxide capture, and their potential as solvents for mitigation of climate change. Following the scope of previous works, amino-acid-based ionic liquids encapsulated (ENILs) into carbonaceous submicrocapsules are here proposed as a novel material for CO₂ capture. The ENILs prepared using tetrabutylphosphonium acetate ([P_4,4,4,4][Ac]), used as reference, (2-hydroxyethyl)trimethylammonium l-phenylalaninate ([N_1,1,1,2(OH)][L-Phe]), (2-hydroxyethyl)trimethylammonium l-prolinate ([N_1,1,1,2(OH)][L-Pro]), and tetrabutylammonium l-prolinate ([N_4,4,4,4][L-Pro]) were characterized by SEM, TEM, elemental analysis, TGA, and BET to assess their morphology, chemical composition, porous structure, and thermal stability. The absorption of CO₂ on these materials was studied up to 0.5 MPa and 343 K. The desorption of CO₂ from the saturated ENILs was evaluated, under mild conditions, evidencing these materials as promising agents for CO₂ capture from post-combustion sources, with high sorption capacity and fast and complete regeneration.

Full text of DOI:10.1002/ejic.202000364

European Journal of Inorganic Chemistry
Accepted Article
Title: Encapsulated amino acid-based ionic liquids for CO2 capture
Authors: Liliana P. Silva, Cristian Moya, Marco Sousa, Ruben
Santiago, Tania E. Sintra, Ana R.F. Carreira, José Palomar,
João A. P. Coutinho, and Pedro Carvalho
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000364
Link to VoR: https://doi.org/10.1002/ejic.202000364
01/2020

10.1002/ejic.202000364
European Journal of Inorganic Chemistry
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Encapsulated amino acid-based ionic liquids for CO2 capture
Liliana P. Silva[a], Cristian Moya[a, b], Marco Sousa[a], Ruben Santiago[b], Tania E. Sintra[a], Ana R.F.
Carreira[a], José Palomar[b], João A. P. Coutinho[a] and Pedro J. Carvalho*[a]
[a]
CICECO – Aveiro Institute of Materials, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
Corresponding author E-mail: quijorge@ua.pt (http://www.ciceco.ua.pt/pcarvalho)
[b]
Secciꢀn de Ingeniería Química (Dpto. Química Física Aplicada), Universidad Autꢀnoma de Madrid, 28049 Madrid, Spain
Supporting information for this article is given via a link at the end of the document.
13] Task-specific ionic liquids (TILs), amino acid-functionalized ILs,
IL-mixed solvents and eutectic solvents, are promising materials
for this purpose. Within chemisorption driven solvents, those
based on amino acids stand out due to their low cost, abundant
Abstract: Ionic liquids have gathered special attention due to their
potential for carbon dioxide capture, and their potential as solvents for
climate changes mitigation. Following the scope of previous works,
amino acid-based ionic liquids encapsulated (ENILs) into
carbonaceous submicrocapsules are here proposed as a novel
material for CO2 capture. The ENILs prepared using
tetrabutylphosphonium acetate ([P4,4,4,4][Ac]), used as reference, (2-
hydroxyethyl)trimethylammonium L-phenylalaninate ([N1,1,1,2(OH)][L-
Phe]), (2-hydroxyethyl)trimethylammonium L-prolinate ([N1,1,1,2(OH)][L-
Pro]) and tetrabutylammonium L-prolinate ([N4,4,4,4][L-Pro] were
characterized by SEM, TEM, elemental analysis, TGA and BET to
assess their morphology, chemical composition, porous structure and
thermal stability. The absorption of CO2 on these materials was
studied up to 0.5 MPa and 343 K. The desorption of CO2 from the
saturated ENILs was evaluated, at mild conditions, evidencing these
materials as promising agents for CO2 capture from post-combustion
sources, with high sorption capacity and fast and complete
regeneration.
availability,
easy
synthesis
and
low
toxicity
and
biodegradability.[11,13,22–26,14–21] However, the use of chemisorption
driven solvents present several drawbacks, such as corrosivity,
high viscosity of the products resultant from the chemical
reaction(s), solvent loss and the high energy requirement for the
regeneration processes, leading to high operational costs and
ultimately leading to uneconomical, and sometimes technical
unfeasible, processes. These drawbacks have hampered the
development of separation units and industrial processes capable
of fulfilling industry demands. The correct selection of the solvent
stands thus, as one of the main challenges for innovative
technological development of acid gases post-combustion
treatment.[27] Furthermore, envisioning the use of green solvents
is indispensable for a clean energy production, anthropogenic
CO2 capture, transportation, and/or reconstitution into value-
added products. Nonetheless, if one aims at developing a
technical and economically viable process for carbon capture, the
pursuit on improving CO2 solubility, viscosity, heat capacity and
mass transfer of sorbents must be addressed from a process
engineering perspective, as highlighted by Leclaire and
Heldebrant[3] in their recent ‘call to arms’ perspective.[3] As
emphasized by the authors, the community should shift focus
from just enhancing equilibrium capacity to focus on the principles
of green chemistry and green engineering, to make Carbon
Capture, Utilization and Storage (CCUS) a reality.
Introduction
Mitigation of climate changes and the use of renewable
energy sources have been two of the most important societal
challenges, particularly when considering the increasing energy
demand and average global temperature due to greenhouse
gases release. Regarding the increasing average global
temperature, predominantly due to accumulation of CO2 in the
atmosphere, innovative post-combustion technologies for CO2
capture stand as indispensable for a cleaner energy production.
Post-combustion stands as the most appealing emission sources
due to an easy retrofit of existent and new technologies into
existing power plants. Although gas separation has long been
used in natural gas industry, counterbalanced by the increased
value of the post-treated streams, post-combustion is still seen as
a non-efficient, costly and, ultimately, unfeasible process.[1–4]
Over the last decade, the research community has been
proposing several approaches for the improvement of CO2
absorption, aiming at developing novel separation technologies
suitable for industrial implementation.[5–10] Among these, chemical
absorption has attracted special interest due to the solvents’ high
capacity, even at post-combustion low CO2 partial pressures.[10–
Although amino acid-based ILs are a promising class of
solvents, CO2 absorption in these solvents presents important
kinetic limitations, resultant from their inherent high viscosity or
the drastic viscosity, increase upon reaction with CO2.[14]
Nonetheless, many have pursued different approaches aiming at
minimizing these limitations, while maintaining the enhanced gas
sorption capacity, such as using a solid phase to immobilize the
solvents and the use of gas-liquid membrane contactors for gas
1
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10.1002/ejic.202000364
European Journal of Inorganic Chemistry
FULL PAPER
absorption processes.[17,28] Recently, the confinement of solvents
in nano-porous matrices has been proposed as a promising
technique for gas separation, with improved mechanical integrity
capable of overcoming the major drawbacks of the bulk solvents,
such as high viscosity and slow gas diffusivity, and ultimately,
requiring reduced quantities of solvent to achieve the
separation.[20,29–36] The encapsulated ionic liquids (ENILs) have
shown to impose no loss of the solvent absorbent capacity with a
drastic enhancement of the CO2 sorption rate, both for physical or
chemical sorption driven solvents.[20,31,36,37]
Results and Discussion
ENILs Characterization
The carbon capsules (Ccap) with a micro/mesoporous shell
structure and hollow core were studied by SEM and TEM. As
depicted in Figure 1 spherical capsules were obtained containing
high carbon content (~90%w/w) and homogeneous morphology
(∼500 nm of diameter and ∼150 nm of shell thickness). Elemental
analysis allowed quantifying the incorporation of IL in the Ccap, by
measuring the percentage of N in the final material, which can be
related to the uptake of the cations and amino acid anions.[38] The
EA results, reported in Table 1, confirm the adequate
incorporation of the ILs (50−62% in mass).
Here, aiming at further enhancing the CO2 solubility and the
fast sorption and desorption kinetics, amino acid-based ILs
encapsulated in hollow carbonaceous submicrocapsules are
evaluated for gas separation at temperatures ranging from 303 to
343 K, for CO2 partial pressures ranging from 0 to 0.5 MPa.
Figure 1. SEM (a) and TEM (b), (c) images of the Ccap used for the ENIL preparation.
Table 1. Characterization of the Ccap and ENILs prepared with different load of
IL.
EA
Textural
properties
%C
%H
%N
IL Load
(%
ABET
(m2/g)
Material
w/w)
Ccap
87.67
58.67
59.57
1.8
5.1
6.8
0.0
0.0
6.3
1721
[P4,4,4,4][Ac]
60.2
51.2
[N1,1,1,2(OH)][L-
Phe]
[N1,1,1,2(OH)][L-Pro]
[N4,4,4,4][L-Pro]
60.83
62.02
6.4
6.7
5.7
8.4
58.9
60.9
Figure 2. Nitrogen adsorption−desorption BET isotherm of capsules used in this
work.
Adsorption–desorption isotherms of N2 at 77 K were used to
analyse the porous structure of the Ccap. The results of the BET
analysis are reported in Table 1 and Figure 2, showing a type IV
adsorption isotherm with an inflection (or Knee) with the
monolayer formation.
They show a highly developed porous structure (ABET
=
1721 m2/g), with contributions of micro- and meso-porosity, as
indicated by the high amount of nitrogen adsorbed in the whole
relative pressure range, which is completely lost after
incorporation of the IL in the Ccap (Table 1); i.e., the IL besides de
2
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10.1002/ejic.202000364
European Journal of Inorganic Chemistry
FULL PAPER
capsule, also completely fills the pores of the capsule structure.
As reported in previous publications,[31,38] the amount of IL
confined inside the carbon capsules can be determined through a
linear regression between the percentage of elemental nitrogen,
obtained by EA, and the weight percentage of IL incorporated on
the support.
the sorption mechanism well described and accepted,[42,43] Shi et
al.[41] have shown that for the tetrabutylphosphonium acetate IL
the anion controls the large CO2 solubility by chemisorption, with
the cation contributing mostly for the physisorption regime. As
depicted in Figure 3 and 4, the L-Prolinate-based IL, a secondary
amine, presents higher CO2 solubility than the L-Phenylalaninate-
base IL, a primary amine, at pressures below 0.5 MPa – inverting
the behaviour observed for higher CO2 pressures. Although,
secondary amines tend to present lower solubility and higher
effective CO2 mass transfer rates than primary amines, the amine
pKa, degree of sterical hindrance or hybridization are known to
present a significant impact on the gas absorption.[45][46,47]
Furthermore, secondary hindered amines present higher
absorption rates than most primary amines but, primary and
secondary amines with similar pKa and hindrance tend to present
similar CO2 solubility.[45] Here, the anion charge delocalization,
anion hinderance, and aromaticity, seem to impose additional
complexity that translates into higher solubilities, in mole fraction,
of the [N1,1,1,2(OH)][L-Pro] and [N4,4,4,4][L-Pro], compared to the
[N1,1,1,2(OH)][L-Phe] and [C4C1im][L-Pro] reported in a previous
publication[20] (Figure S9). Moreover, the higher degree of sterical
hindrance and higher charge delocalization of the [N1,1,1,2(OH)][L-
Pro] cation translates into a lower CO2 sorption capacity, than that
of the [N4,4,4,4][L-Pro]. However, when the molecular weight effect
is removed from the analysis by comparing the solubility in
molalities, the differences and trends of the solubilities change.
As depicted in Figure 4, the [N1,1,1,2(OH)][L-Pro] presents the
highest solubility, at 343 K, followed by the [P4,4,4,4][Ac], [N4,4,4,4][L-
Pro] and [N1,1,1,2(OH)][L-Phe]. Nonetheless, as one would expect,
within the physisorption regime the molecular weight becomes the
dominant factor with the higher molecular weight ILs presenting
higher solubility.[6] The experimental data are reported in Table
S1 in SI.
CO2 solubilities
The sorption isotherms of CO2 in the neat ILs was not
possible to be determined using the apparatus and methodology
here adopted due to the high viscosity of the reaction product(s)
of the carbon dioxide and the ILs. In fact, the initial contact and
absorption of the CO2 in the IL lead to the formation of a solid-like
interface, at the gas-liquid interface, that did not allow for
additional CO2 to absorb and diffuse to the IL bulk. Nonetheless,
when confined within the Ccap the ILs did not present the
behaviour observed in the neat IL solubility measurements, thus,
ENIL sorption measurements were feasible within all the pressure
and temperature ranges evaluated. The only exception was
[P4,4,4,4][Ac] that is solid at temperatures below 333 K. The
encapsulation of IL not only enhances the mass transport
properties but also lets us use ILs that would not reach the
thermodynamic equilibrium under reasonable conditions due to
the kinetic limitations.[20] This improvement is due to the large
increase of the contact surface area between the gas phase and
the amino acid IL, upon dispersion of the absorbent in
submicrodrops,
in
good
agreement
with
previous
results.[20,34,35,39,40]
The sorption isotherms of CO2 in the ENILs were
determined at five temperatures, namely 303, 313, 323, 333 and
343 K, for the amino acid-based ILs and at three temperatures,
namely 333, 343 and 353 K, for the [P4,4,4,4][Ac], as depicted in
Figure 3 – with the CO2 mole fractions calculated based on the
moles of IL encapsulated. The results reported indicate an initial
region with low equilibrium pressure, typical of chemical sorption,
followed by physical absorption regime with a fast increase of the
equilibrium pressure with the CO2 content. Furthermore, the ILs
not only remain liquid during CO2 sorption but also maintain a high
CO2 absorption capacity and kinetics (Figure 4) when
encapsulated. This approach of creating ‘microsolid’ droplets of IL
allows to overcome the problems associated to the use of neat
ILs.
To ensure a successful industrial application one must
ensure an easy and low energy demanding regeneration. Thus,
the sorption capability of ENILs prepared with [P4,4,4,4][Ac],
[N4,4,4,4][L-Pro], [N1,1,1,2(OH)][L-Pro] and [N1,1,1,2(OH)][L-Phe], was
evaluated during four cycles of sorption-desorption. The
desorption cycles were performed after each sorption cycle at 1
Pa, 343 K during 2 h, while the sorption was performed with an
initial pressure of (0.2 ± 0.05) MPa. As depicted in Figure 5, the
regeneration of the ENIL was successful with the solubility
essentially constant after the various regeneration cycles, the
deviations observed being within the experimental uncertainty.
Acetate-based ILs are known to play an important role in
conditioning chemical reactions with CO2 with reaction products
with low viscosity, compared to most chemisorption solvents.[41–
44] Although most of the work focus on the imidazolium cation, with
1
Furthermore, H and 13C NMR was performed after the sorption
cycles showing no degradation of the ILs either due to
3
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10.1002/ejic.202000364
European Journal of Inorganic Chemistry
FULL PAPER
immobilization, solvent evaporation or gas sorption/desorption
cycles, as depicted in Figure S10 of the Supporting Information.
Figure 3. Pressure-composition diagram of the binary systems CO2 + [N1,1,1,2(OH)][L-Pro], CO2 + [N1,1,1,2(OH)][L-Phe], CO2 + [N4,4,4,4][L-Pro] and CO2 + [P4,4,4,4][Ac]. The
lines are guides for the eyes.
Figure 4. Molality – absorption time diagrams of (left) CO2 + [P4,4,4,4][Ac] and (right) the studied ENILs.
4
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10.1002/ejic.202000364
European Journal of Inorganic Chemistry
FULL PAPER
Figure 5. Pressure-composition diagram of the binary systems CO2 + [P4,4,4,4][Ac], CO2 + [N4,4,4,4][L-Pro], CO2 + [N1,1,1,2(OH)][L-Pro] and CO2 + [N1,1,1,2(OH)][L-Phe] (left)
and sorption (333 K and 0.2 MPa for the phosphonium IL and 323 K and 0.2 MPa for the remaining ILs) and desorption (343 K and 1 Pa) cycles (right). The lines
are guides for the eyes.
following the procedure reported by Allen et al.[48] and Santis et al.[49] The
Conclusion
ionic liquids were synthesized by the neutralization of [N4,4,4,4][OH] or
Encapsulated amino acid-based ionic liquids (ENILs) were
prepared for CO2 capture from post-combustion and their
performance evaluated by means of an isochoric pressure cell.
The confinement of specific ILs into the carbon submicrocapsules
resulted in ENILs with high CO2 sorption capacities and thermal
stabilities. Furthermore, contrary to the neat ILs, whose reaction
with CO2 lead to the formation of a solid-like interface that blocked
further gas sorption and diffusion to the IL bulk, the ENILs remain
liquid and absorbing the CO2 within the pressure range studied
with fast sorption kinetics. The incorporation of the ILs into the
carbon submicrocapsules increases their performance when
compared with the neat IL, due to the increase of the surface
contact area, promoting higher mass transfer rates. In addition,
ENILs were successfully regenerated at mild conditions and used
in successive CO2 capture-desorption cycles. Therefore, these
results highlight ENILs as new separating agents with promising
characteristics for industrial application in CO2 capture
technologies, with good thermal stability, high CO2 sorption
capacities, and low energy demanding in regeneration steps.
[N1,1,1,2(OH)][OH] with the respective amino acid/organic acid, namely the L-
phenylalanine, L-proline and acetic acid.
Succinctly, [P4,4,4,4][OH] or [N4,4,4,4][OH] (1 equiv, 40 wt% in aqueous
solution) were added dropwise to an aqueous solution of acetic acid or
respective amino acid, with a molar excess of 1.1 equivalent, at room
temperature. The reaction mixture was constantly stirred at room
temperature for [P4,4,4,4][Ac] and for [N4,4,4,4][L-Pro], and protected from
light for 2 hours, producing the respective ionic liquid and water as the by
product. The water was then removed under reduced pressure (2 kPa).
For [N4,4,4,4][L-Pro], the resultant residue was dissolved in acetonitrile and
filtered to remove the unreacted amino acid. Finally, the acetonitrile was
removed under reduced pressure and the obtained compound was dried
under high vacuum for at least 48 h. The ILs were further dried under high
vacuum (0.1 Pa) and moderate temperature (303 K) for at least 48 hours
in order to remove the remaining solvents. [N1,1,1,2(OH)][OH] (1 equiv, 45
wt% in methanol solution) was added dropwise to an aqueous solution of
amino acid, with a molar excess of 1.1 equivalents, at room temperature
for [N1,1,1,2(OH)][L-Pro] and at 333 K for [N1,1,1,2(OH)][L-Phe]. The reaction
mixture was stirred for 2 hours and protected from light. The solvents were
then removed under reduced pressure (2 kPa).
A solution of
acetonitrile/methanol (9:1, v/v) was then added under vigorous stirring in
order to precipitate the excess of amino acid. The mixture was left stirring
for 1 hour and the excess of amino acid was then filtrated. In a final step,
the acetonitrile and methanol were evaporated initially under reduced
pressure (2 kPa) and then high vacuum (0.1 Pa) and moderate
temperature (303 K) for at least 48 hours. The water mass fraction of the
ionic liquids was determined by coulometric Karl Fischer titration (Metrohm,
model 831) and shown to be less than 0.05 wt%. The structure of all
compounds was evaluated by 1H and 13C NMR spectroscopy, showing
purity higher to 99% for all the ionic structures, as reported in Figures S1
Experimental Section
Materials
The tetrabutylammonium L-prolinate, [N4,4,4,4][L-Pro], the (2-
hydroxyethyl)trimethylammonium L-phenylalaninate, [N1,1,1,2(OH)][L-Phe],
(2-hydroxyethyl)trimethylammonium L-prolinate, [N1,1,1,2(OH)][L-Pro], and
the tetrabutylphosphonium acetate [P4,4,4,4][Ac,] ILs were synthesized
5
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10.1002/ejic.202000364
European Journal of Inorganic Chemistry
FULL PAPER
to S8 in Supporting Information. The properties of four selected ILs in
this work are present in Table 2.
formaldehyde resin was used as carbon precursor to obtain the desired
carbon microcapsules. A mixture of 0.374 g of phenol/g template was
added to the template and maintained in agitation for 14 hours at 373 K
through a rotary evaporated. Then, 0.238 g of paraformaldehyde/ g of
template was added, and the temperature was increased to 403 K and
maintained under vacuum for 24 hours. The resulting material was heated
under a nitrogen atmosphere for 5 hours at 433 K in a vertical furnace, and
then, pyrolyzed at 1123 K for 7 hours. The silica template was removed by
washing it with hydrofluoric acid solution (48 %v/v). The resulting Ccap
support material was washed with deionized water until reach pH 7 and
dried at 373 K for 24 hours. Phenol (99%), paraformaldehyde (95-100%),
aluminium trichloride (95-100%), ammonia (38%), absolute ethanol (99%)
and hydrofluoric acid (48%) used were provided by Panreac.
Tetraethylorthosilicate (98%, TEOS) and octadecyltrimethoxysilane (90%,
C18TMS) were supplied by Sigma–Aldrich. Then, the ENIL materials were
prepared by incipient wetness impregnation with the corresponding ILs,
dissolved in ethanol to reduce the viscosity and assure good dispersion.
To ensure a homogeneous penetration, into the pores, the IL solution was
added drop wise over the Ccap, followed by vacuum evacuation, to remove
the solvent, at 333 K and 1 Pa over 24 h. The procedure followed allowed
to impregnate ~60% of IL (wt., ENIL basis) in the ENIL. The amount of IL
loaded was determined by weight difference between the final ENIL and
Tetrabutylammonium hydroxide ([N4,4,4,4]OH, in aqueous solution at
40 wt%), the tetrabutylphosphonium hydroxide ([P4,4,4,4]OH, in aqueous
solution at 40 wt%), the (2-hydroxyethyl)trimethylammonium hydroxide
([N1,1,1,2(OH)]OH, in methanol solution at 45 wt%), L-phenylalanine (99 wt%
of purity) and acetic acid (99.99 wt% pure) were acquired from Sigma-
Aldrich. L-proline (99 wt% of purity) was acquired from Acros Organics.
Methanol (HPLC grade) and acetonitrile (99.9 wt% of purity) were acquired
from VWR. The water used was ultrapure water, passed by a reverse
osmosis system and further treated with a Milli-Q plus 185 water
purification apparatus.
The sorption experiments were carried out using carbon dioxide
(CO2) supplied by Air Liquide Portugal with a purity of 99.999%.
Preparation of the ENILs
The carbon submicrocapsules (Ccap) were synthesized following the
template method described in detail in previous works.[32,34,36,39,50,51]
A
model of a silica sphere with a solid core and mesoporous shell were used
as template. A volume of 15 mL of tetraethoxysilane (TEOS) was added
to a reaction medium of an ethanolic solution composed of 185 mL of
ethanol, 20 mL of distilled water and 12 mL of ammonia and maintained
with vigorous stirring at 303 K for at least 1 hour. Then, a mixture
containing 17.5 mL of TEOS and C18TMS (2.5:1 v/v) was added to
previous solution to form the mesoporous shell around them. This step
was repeated twice in order to increase the thickness of the mesoporous
shell. After 1 hour of reaction, the resulting material was filtered and
calcinated in air at 823 K for 6 hours. The silica spheres were then
impregnated by a solution consisting on 0.27 g of AlCl3·6H2O per gram of
template and calcinated again under the same conditions. A phenol-
the weight of Ccap
.
Elemental analysis (EA) was used to validate the IL impregnation.
The IL impregnation percentage was derived from the linear relationship,
proposed in a previous publication,[38] using the percentage of elemental
nitrogen, in the supported-IL materials and the weight percentage of the IL
supported on porous carbons. All the ENILs prepared had the
resemblance and behaviour of a powdered material, allowing one to infer
that all the IL was encapsulated.
Table 2. Name, acronym, chemical structure and molecular weight of the four ILs studied in this work.
Name
Acronym
Chemical Structure
Mw /g.mol-1
Tetrabutylphosphonium acetate
[P4,4,4,4][Ac]
318.48
(2-Hydroxyethyl)trimethylammonium L-phenylalaninate
[N1,1,1,2(OH)][L-Phe]
268.27
(2-Hydroxyethyl)trimethylammonium L-prolinate
Tetrabutylammonium L-prolinate
[N1,1,1,2(OH)][L-Pro]
218.32
356.60
[N4,4,4,4][L-Pro]
6
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10.1002/ejic.202000364
European Journal of Inorganic Chemistry
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is removed. The purged sample is transferred to the furnace and to ensure
complete and rapid combustion (oxidation) of the sample the furnace
environment is composed of pure oxygen with a secondary oxygen flow
directed to the sample via a ceramic lance. The combustion gases are
swept from the furnace through a thermoelectric cooler or an anhydrone
reagent, to remove moisture, and collected in a thermostatically controlled
ballast volume. The gases equilibrate and mix in the ballast before a
representative aliquot of the gas is extracted and introduced into a flowing
stream of inert gas for analysis.
Characterization of the ENILs
The morphology and microstructure of Ccap and ENILs were studied
by scanning and transmission electron microscopy (SEM and TEM). SEM
micrographs were obtained using a HITACHI SU-70 operating at 25 kV.
The samples were previously coated with a carbon film using an Emitech
K950X carbon evaporator. TEM analyses were performed by means of a
Philips 420 JEM-2000 FX microscope. The porous structure was
characterized in a Micromeritics apparatus (Tristar II 3020 model) by
nitrogen adsorption−desorption isotherms at 77
K
using the
Thermogravimetric analyses (TGA) of ILs and ENILs were
conducted in a Setaram SETSYS Evolution 1750 thermobalance under
nitrogen atmosphere. A dynamic method was used with a temperature
range from (298–835) K at a heating rate of 10 °C min-1 under a nitrogen
flow of 200 ml min-1. The accuracy of temperature and mass
measurements was 0.1 K and 10-3 mg, respectively. Aluminium pans were
used to hold an initial mass between 4 and 12 mg. In all TGA runs, the
derivative curves (DTG) were obtained for more in-depth interpretation and
DTG temperatures. TDTG (K) was assigned to IL decomposition in the
solid supports from the DTG curve. As depicted in Figure 6, the studied
ILs are thermally stable up to 380 K and afterwards a sharp decomposition
was observed in the temperature range of 380– 680 K. The respective
ENIL exhibits a weight loss with a similar pattern to the neat IL, however
the peak appears a somewhat lower temperatures, denoting worse
thermal stability due to the encapsulation, similar to that reported in a
previous publication.[31]
Brunauer−Emmett−Teller (BET) equation using the methodology reported
in a previous publication and equation 1 (detailed description is given in
the Supporting Information).[32]
푣
푁 푎
푎 푚
푚
푆푎
=
(1)
푣
푚
푆
푚
where Sa is the specific surface area, Na is the Avogadro’s number, Vm is
the amount of gas adsorbed if a monolayer was formed, am is the effective
cross-section area of one adsorbed molecule, 푣푚 is the molar volume of
one adsorbed molecule (22400 mL of volume occupied by mol of
adsorbate gas at STP conditions) and mS is the mass of adsorbent.
Elemental analyses (EA) of Ccap and ENILs were carried out in a
LECO CHNS-932 analyser to obtain C, H and N content. A macro-sized
sample is weighed (2-5 mg) into a ceramic boat, placed in the loader and
transferred to a sealed purge chamber, where entrained atmospheric gas
Figure 6. Thermogravimetric analysis (TGA) of the studied ILs.
0.2%, and valves that allow controlling the gas addition. The experimental
setup, except for the pressure transducer, is placed inside an oven with a
temperature stability of 0.5 K. The pressure transducer is placed outside
the oven to assure no influence of the temperature on the pressure
uncertainty.
CO2 Solubility
The CO2 solubility was evaluated using a constant temperature-
volume equilibrium cell made of stainless steel, reported in detail in
elsewhere.[31] The isochoric equilibrium cell consists of a fixed volume cell
connected to a gas line, as depicted in Figure 7. The gas line, also with a
known volume, consists of a large volume cylinder, a pressure transducer
(Swagelok S Model), able to operate up to 1 MPa with an uncertainty of
A fixed amount of ENIL, whose exact mass is determined by weight
using a high weight/high precision balance (Sartorius LA200P) with an
7
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10.1002/ejic.202000364
European Journal of Inorganic Chemistry
FULL PAPER
accuracy of 1 mg, is introduced into the cell. The ENIL is then kept under
vacuum overnight, while heating at 353 K, in order to remove atmospheric
gases absorbed during manipulation. The gas is introduced on the gas
section (dashed blue section, Figure 7) up to the desired pressure. The
amount of gas (number of moles) present in the gas line is calculated using
the Peng-Robinson EoS knowing the gas-phase volume and system
pressure once the temperature equilibration is reached. The initial
temperature is set to 353 K and once the equilibrium is reached the valve
that connects the gas section with the solvent containing cell is open,
allowing the gas to get in contact with the ENIL. The pressure decay is
then monitored and logged, until new pressure equilibrium is reached.
Knowing the cell gas phase volume and pressure, at equilibrium, the
number of moles is determined. The number of moles adsorbed into the
ENIL is then calculated by the difference to those initial added. The
temperature is then decreased to a new set point allowing for additional
gas to adsorb in the ENIL. Once the new equilibrium is reached the number
of moles adsorbed into the ENIL is again calculated. After the equilibrium
pressure is determined for all temperatures, the IL is regenerated by
applying reduced pressure (1 Pa) and moderate temperature (353 K) for a
period never shorter than 4 hours. Once regenerated the adsorption
procedure is then repeated with a different initial pressure of the gas,
allowing thus, to determine additional equilibrium points and better
describe the system phase diagram. The determined equilibrium points
are remeasured, normally in triplicate, aiming to minimize, identify and
Acknowledgements
This work was developed by Portugal 2020 through European
Regional Development Fund (ERDF), in the frame of Operational
Competitiveness and Internationalization Programme (POCI), in
the scope of the project Smart Green Homes - BOSCH - POCI-
01-0247-FEDER-007678, project CICECO-Aveiro Institute of
Materials (UIDB/50011/2020 & UIDP/50011/2020), financed by
national funds through the FCT/MEC and when appropriate co-
financed by FEDER under the PT2020 Partnership Agreement,
Ministerio de Economía y Competitividad (MINECO) of Spain
(project CTQ2017-89441-R) and Comunidad de Madrid
(PT2018/EMT4348). L.P.S., A.R.F.C and P.J.C. acknowledge
FCT
for
PhD
grant
(SFRH/135976/2018)
and
(SFRH/BD/143612/2019) and contract under the Investigator
FCT 2015 contract number IF/00758/2015, respectively. The
NMR spectrometers are part of the National NMR Network
(PTNMR) and are partially supported by Infrastructure Project Nº
022161 (co-financed by FEDER through COMPETE 2020, POCI
and PORL and FCT through PIDDAC).
Keywords: Post-combustion • Carbon dioxide capture •
Encapsulated ionic liquids • Amino acid-based ionic liquids
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9
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10.1002/ejic.202000364
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FULL PAPER
Entry for the Table of Contents
The absorption of CO2 on amino acid-based ionic liquids, encapsulated (ENILs) into carbonaceous submicrocapsules are proposed
as a novel promising agents for CO2 capture from post-combustion sources, with high sorption capacity and fast and complete
regeneration.
Key Topic: amino acid-based ionic liquids; IL encapsulation into carbonaceous submicrocapsules
10
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