.
Angewandte
Communications
DOI: 10.1002/anie.201205362
CO2 Capture
Equimolar CO2 Capture by N-Substituted Amino Acid Salts and
Subsequent Conversion**
An-Hua Liu, Ran Ma, Chan Song, Zhen-Zhen Yang, Ao Yu,* Yu Cai, Liang-Nian He,* Ya-
Nan Zhao, Bing Yu, and Qing-Wen Song
The gas responsible for climate change, CO2, is the subject of
increased attention in both academic and industrial
research.[1] Since controlling anthropogenic CO2 emission
and further reducing the accumulation of CO2 is emerging as
an urgent and challenging research topic, extensive efforts are
being devoted to carbon capture and storage/sequestration
(CCS).[2] In this context, the invention and modification of
new chemicals that can efficiently, selectively, and econom-
ically absorb and separate CO2 from the exhaust formed from
Scheme 1. a) Conventional amine-based scrubbing for CO2 capture
through the ammonium carbamate pathway; b) CO2 capture via the
the burning of fossil fuels appears essential to realize
a practical CCS process.
formation of the carbamic acid rather than the ammonium carbamate
by sodium N-alkylglycinate in PEG.
Conventional technology for the industrial capture of CO2
largely relies on employing aqueous solution of amines.[3] In
academic research, many amine-based scrubbing agents have
been developed for various technologies and processes.[4]
However, there are inherent drawbacks generally associated
with amine absorbents, namely the requirement of two amine
units to capture one CO2 molecule owing to the formation of
ammonium carbamate (Scheme 1a); this increases the energy
required for regeneration. This undesired 2:1 stoichiometry
would be a crucial barrier for improving the capacity of such
amine-based CO2 absorbents.
Several strategies have been proposed for the equimolar
chemisorption of CO2 with an amine absorbent. The groups
led by Jessop,[5] Dai,[6] and others[7] have developed reversible
CO2 capture utilizing a strong nitrogen-containing base in
conjunction with a proton donor. Meanwhile, amidophos-
phoranes were also proved to be capable of capturing one
containing species were essentially required to form the
carbamate/carbonate. Very recently, Brennecke and co-work-
ers[9] designed an ionic liquid (IL) comprising an amino-
functionalized anion and a long-chain alkyl phosphonium
cation to capture CO2 in favor of formation of carbamic acid;
they approached a high capacity of up to almost 1 mole of
CO2 per mole of IL. Despite such great advances, the
development of efficient CCS processes continues to be
appealing. The ultimate goal is a simple, easily prepared,
biocompatible/biodegradable absorbent with high CO2
capacity up to a 1:1 stoichiometry, and thus a lower energy
requirement in the desorption step. In this context, we found
that readily available amino acid salts with a bulky N sub-
stituent have an extremely high capacity approaching almost
equimolar absorption in poly(ethylene glycol) (PEG) solu-
tion (Scheme 1b). Steric-hindrance-controlled CO2 absorp-
tion is assumed to proceed via the carbamic acid rather than
the ammonium carbamate, thus resulting in equimolar
absorption and improved ease of desorption in comparison
with conventional amine absorbents. In particular, the
captured CO2 could be an activated species that could
undergo subsequent conversion to give valuable compounds
smoothly rather than going through a desorption cycle.
Sodium N-alkylglycinates and -alaninates[10] were inves-
tigated to test our proposal about steric-hindrance-controlled
CO2 absorption (Table 1). PEG was selected as a suitable
solvent because the flexible poly(ethylene oxide) chain could
coordinate with alkali-metal cations, thus leading to improved
capacity for counterions.[11] In particular, PEG150 (triethylene
glycol, Mw = 150 Da) showed poor CO2 sorption capacity
alone, implying that only physical interaction between PEG
and CO2 was observed (Table 1, entry 1). As expected,
nonmodified sodium glycinate captured CO2 in a manner
similar to aqueous amines, by forming the ammonium
carbamate in a stoichiometry of one CO2 molecule to two
amino groups (Table 1, entry 2); this salt was detected by
À
equivalent of CO2 through the insertion of CO2 into a P N
bond, resulting in the generation of carbamatophosphor-
anes.[8] Despite the high absorption capacity of amidophos-
phoranes at a 1:1 stoichiometry, additional inter-/intramolec-
ular functional groups such as hydroxy-, amino-, phosphorus-
[*] A.-H. Liu, R. Ma, Z.-Z. Yang, Prof. Dr. L.-N. He, Y.-N. Zhao, B. Yu,
Q.-W. Song
State Key Laboratory and Institute of Elemento-Organic Chemistry
Nankai University, Tianjin, 300071 (P.R. China)
E-mail: heln@nankai.edu.cn
C. Song, Prof. Dr. A. Yu, Y. Cai
College of Chemistry, Nankai University
Tianjin, 300071 (P.R. China)
E-mail: esr@nankai.edu.cn
[**] This work was supported financially by the National Natural
Sciences Foundation of China (no. 21172125), the Ministry of
Science and Technology (2012BAD32B10), the “111” Project of the
Ministry of Education of China (project no. B06005), and the
Committee of Science and Technology of Tianjin.
Supporting information for this article is available on the WWW
11306
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 11306 –11310