Reversible, solid state capture of carbon dioxide by hydroxylated amidines

Article abstract of DOI:10.1039/b921688j

Hydroxylated amidine derivatives can capture, store, and release CO ₂ reversibly in the solid state in a quantitative manner under clean and dry conditions at ambient temperature. The Royal Society of Chemistry.

Full text of DOI:10.1039/b921688j

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Reversible, solid state capture of carbon dioxide by hydroxylated
amidinesw
Myungsook Kim and Ji-Woong Park*
Received (in Cambridge, UK) 16th October 2009, Accepted 7th January 2010
First published as an Advance Article on the web 1st February 2010
DOI: 10.1039/b921688j
Hydroxylated amidine derivatives can capture, store, and
release CO2 reversibly in the solid state in a quantitative manner
under clean and dry conditions at ambient temperature.
relatively large molecular mass are expected to capture or
release CO2 in the solid state. The HAM-CO2 salts can be
stored at ambient temperature and pressure and are expected
to generate pure CO2 gas after brief heat treatment.
The HAMs were synthesized via lithiation of the aliphatic
bicyclic amidines 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) followed by
reaction with epoxides (see the ESIw). Combining the amidine
moiety with an alcoholic moiety effectively reduces the
volatility of these compounds. In principle, the reaction of
the lithiated amidines with stoichiometric equivalents of a
molecule with multiple epoxy groups will result in the corres-
ponding polymeric amidines with multiple hydroxyl groups. In
this study, we synthesized mono- and dihydroxyalkylamidines,
which are designated as DBUOH, (DBUOH)2, and
(DBNOH)2, as shown in Fig. 1. Whereas DBUOH was a
liquid, (DBUOH)2 and (DBNOH)2 were obtained as crystalline
solids which melt near 116 and 149 1C, respectively. The HAM
compounds do indeed react with equimolar amounts of CO2 in
the absence of any protic solvent (Fig. 1b). DBUOH-CO2, the
carbonated salt of DBUOH, was obtained as a white solid by
directly bubbling anhydrous CO2 gas into DBUOH. The
carbonate salts of the other two HAMs as well as DBUOH
were conveniently obtained as powders by bubbling CO2 gas
into their THF solutions followed by filtration of the precipitates.
The structures of the three HAMs and the corresponding
alkylcarbonate salts were confirmed by examination of their
1H and 13C NMR spectra (see Fig. 1c and d, respectively for
the DBUOH results; see the ESIw for the data for the other
compounds). The peaks in all NMR spectra were assigned
using the HMQC spectra (see ESIw). The FT-IR spectra of the
alkylcarbonate salts contain peaks due to the out-of-plane
vibration of the carbonate group near 835 cmÀ1 (see Fig. S8w).
The thermogravimetric analysis (TGA) curves for the
carbonate salts of the HAMs indicate that decarbonation of
HAM-CO2 commences in the range 55–65 1C and ends near
90–100 1C (Fig. 2). However, the TGA weight loss occurring
below 120 1C appeared larger than the percentages of CO2 per
amidine moiety in the expected 1 : 1 alkylcarbonate salts of
DBUOH, (DBUOH)2, and (DBNOH)2. It is most likely that
the excess weight loss occurred due to the moisture adsorbed
onto hygroscopic salts while the HAM-CO2 salts were loaded
into TGA sample pans. To exclude moisture from TGA
weight loss data, the carbonated salt of DBUOH was prepared
in situ by flowing anhydrous CO2 gas into the TGA chamber
loaded with a DBUOH sample pan at room temperature. The
TGA curve for the resultant salts showed weight loss
corresponding to the percentage of CO2 in an DBUOH-CO2
adduct (see Fig. S9w). This result indicates that the HAM
Energy-efficient CO2 absorbents are required for capturing
CO2 emitted into indoor or outdoor environments and also for
its storage and reutilization as a carbon resource.1–3 Aqueous
solutions of alkanolamines are the most commonly employed
chemical CO2 absorption systems. However, the use of
aqueous amine solutions has several disadvantages including
high energy consumption due to the high specific heat of water
and the generation of corrosive vapors.1,3 Many studies have
focused on the development of dry amine absorbents to avoid
these disadvantages of aqueous solutions.4–8
Of the various amine types used in CO2 capture, amidine
compounds react with CO2 in the presence of an alcohol to
produce alkylcarbonate salts in the form of a trimolecular
complex consisting of amidine, CO2, and the alcohol in a
1 : 1 : 1 ratio.9–11 The high nucleophilicity and stabilization of
cationic species that result from delocalization in the amidine
moiety enable their complexation with CO2 and the alcohol at
room temperature. The resulting amidinium alkylcarbonate
salts contain less hydrogen bonding than the carbamate or
bicarbonate salts of other amines, so they decompose to
equimolar amounts of amidine, CO2, and the alcohol at
relatively low temperatures.12 It should therefore be possible
to develop new materials for CO2 capture and storage by
exploiting the unique nonaqueous, quantitative, and low
temperature carbonation characteristics of amidines.
Endo et al.13,14 reported that solid polymers containing
amidine moieties in their side chains could capture CO2 under
atmospheric pressure. Although the materials could trap CO2
without supplying water, the presence of a sufficient amount of
water or alcohol is likely to facilitate the amidine–CO2
complexation to enhance the capability of CO2 capture.
Here we report new hydroxylated amidine (HAM) derivatives
that can capture and store CO2 in a quantitative manner under
clean conditions. We synthesized three hydroxyalkylamidines,
each of which contain equal numbers of amidine and hydroxyl
groups that can react with an equimolar amount of CO2 with
no additional protic solvents. In particular, HAMs with a
Department of Materials Science and Engineering, Gwangju Institute
of Science and Technology, 261 Cheomdan-gwagiro, Buk-gu,
Gwangju 500-712, Korea. E-mail: jiwoong@gist.ac.kr;
Fax: (+82) 62-970-2304; Tel: (+82) 62-970-2315
w Electronic supplementary information (ESI) available: Experimental
1
section, H and 13C NMR, HMQC and FT-IR spectra of the HAMs
and the HAM-CO2 salts. TGA of HAM/silica mixtures with various
HAM contents. See DOI: 10.1039/b921688j
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Fig. 3 TGA curves showing the CO2 uptake by neat HAMs
and HAM/silica mixtures under a CO2 flow of 100 mL minÀ1 at 25 1C.
(a) Neat HAM samples (2 mg each), (b) HAM/silica mixtures with a
HAM content of 35%.
Fig. 3 shows the TGA curves that describe the progress of the
complexation reactions between neat HAM samples and CO2;
liquid DBUOH absorbed nearly an equimolar amount of CO2,
whereas, solid (DBUOH)2 and (DBNOH)2 absorbed only
approximately 0.1 equivalent of CO2 during the same duration
of exposure to a CO2 atmosphere. The sluggish solid state
reactions are most likely due to the slow diffusion of CO2 gas
through solid absorbents.
Fig. 1 Reversible CO2 capture by HAMs. (a) The chemical structures
of the HAMs investigated in this study, (b) the carbonation and
decarbonation reactions of the HAMs, (c) and (d) 1H and 13C NMR
spectra of DBUOH and DBUOH-CO2, respectively (see Fig. S5 for
peak assignmentw).
To facilitate the carbonation process without dissolving the
HAMs in a solvent, the solid HAMs must have sufficiently
large surface area. Hence, the HAM compounds were
impregnated into porous supports consisting of silica gel
particles. Solutions of the HAMs in anhydrous THF were
mixed with pre-baked, dry amorphous silica gel particles, and
subsequent solvent evaporation under reduced pressure resulted
in silica-supported HAMs (HAM/silica) as powders. The
liquid DBUOH also becomes a powdery solid on mixing with
silica for contents less than 40%. To test their capture
capacity, the HAM/silica were baked at 110 1C in a TGA
sample pan under a nitrogen stream and then exposed to a
CO2 stream. Fig. 3b shows that the HAMs impregnated into
silica with a content15 of 35% exhibit faster CO2 absorption
with higher efficiencies than the neat samples, the results for
which are shown in Fig. 3a. The silica-impregnated (DBUOH)2
and (DBNOH)2 absorbed CO2 of 0.6–0.8 equivalents to the
amidine moiety, which are much higher than those with the
corresponding neat HAMs. We anticipate that the capture
capacity of the HAM/silica mixtures might be further
improved by impregnating the HAMs into other types of
porous solid supports.
compounds comprised of equal numbers of amidine and
hydroxyl groups form a 1 : 1 complex with CO2 in the absence
of water. Theoretical capture capacities of DBUOH, (DBUOH)2,
and (DBNOH)2 are 224, 225, and 263 mg CO2/g HAM,
respectively.
To confirm that alkylcarbonate salts, not bicarbonate salts,
form upon bubbling CO2 into the HAM solutions, we
obtained two types of salts by bubbling CO2 into two different
solutions of DBUOH in THF without and with one equivalent
of water, respectively, and compared their 1H and 13C NMR
spectra (Fig. S10w). The salts obtained from the anhydrous
DBUOH solution exhibited different peak positions and splitting
patterns in 1H NMR spectra when compared with those
obtained from the solution of a 1 : 1 DBUOH/water mixture.
This result indicates that the salts formed by bubbling CO2 into
anhydrous HAM solutions are exclusively alkylcarbonate forms.
Although the HAMs can form bimolecular complexes with
CO2 in a quantitative manner in their solution or liquid states,
their complexation reactions are sluggish when they are solid.
We studied the reversibility of the CO2 capture and release
behaviors of the HAM/silica mixtures. The changes in sample
weight were recorded in situ on a TGA instrument; the flow gas
and temperature were alternated between two conditions,
100 mL minÀ1 of N2 flow at 65 1C and 100 mL minÀ1 of CO2
flow at 25 1C. The resulting TGA curves are shown in Fig. 4: the
carbonation and decarbonation of the HAMs impregnated into
silica occur reversibly. In all experimental runs, the loss of the
HAMs was negligible. Although the HAM/silica mixtures contain
approximately 65% inactive silica, the CO2 sorption capacities
with respect to the total sorbent weight are still comparable to
those of the other supported amine sorbents reported.16
In conclusion, we have synthesized hydroxyalkylamidine
derivatives that can capture, store, and release CO2 gas in
clean and dry conditions at low temperatures. These new
Fig. 2 TGA curves for the three carbonated salts of the HAMs
generated by bubbling CO2 gas into their liquid or solution states.
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Notes and references
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Fig. 4 Reversible CO2 capture and release by silica-supported HAMs
with a HAM content of 35%. (a) DBUOH/silica, (b) (DBNOH)2/
silica, where capture was performed at 25 1C with a CO2 flow rate of
100 mL minÀ1 and release at 65 1C with a N2 flow rate of 100 mL minÀ1
.
In these graphs, mg CO2/g HS and mg CO2/g H are the weight of CO2
(in mg) per gram of the HAM/silica mixture (HS) and HAM (H),
respectively.
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This work was supported by a Korea Science and Engineering
Foundation (KOSEF) grant [R01-2008-000-12246-0], the BK 21
Program funded by the Korean Government (MEST), and the
Program for Integrated Molecular Systems (PIMS) at GIST in
Korea. We thank Mr Seungkyu Lee and Prof. Jae Il Kim for
helping with high resolution NMR spectra.
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 2507–2509 | 2509