Download 13C-NMR study of acid dissociation constant (pKa) effects on the

13
C-NMR study of acid dissociation constant (pKa) effects on the CO2 absorption and
regeneration of aqueous alkanolpiperidines
Yukio Furukawa1, Hirotaka Koriki1, Daiki Shuto1, Hiroshi Sato2, Yasuro Yamanaka3
1
Department of Chemistry and Biochemistry, Graduate School of Advanced Science and
Engineering, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan
2
Research Laboratory, IHI Corporation, 1, Shin-nakahara-cho, Isogo-ku, Yokohama 2358501,
Japan
3
Energy & Plant Operations, IHI Corporation, 3-1-1, Toyosu, Koto-ku, Tokyo 135-8710, Japan
Abstract
Carbon dioxide (CO2) is a greenhouse gas contributes to global warming and climate change
problem. Post combustion CO2 capture is gaining interest as a practical technology for controlling
greenhouse gas emissions. Aqueous solutions of alkanolamines are used as absorbents for
removing CO2 from flue gas of fossil fueled power plants. An aqueous solution of an alkanolamine
absorbs CO2 at room temperature and releases CO2 at high temperature; CO2 is captured. One of
key parameters for this technology is to use an effective absorbent. The CO2 absorption rate of an
aqueous solution of a tertiary alkanolamine N-methyldiethanolamine (MDEA) is slow, whereas the
CO2 release amount is large. On the other hand, the CO2 absorption rate of an aqueous solution of a
cyclic amine piperazine (PZ) is fast, whereas the CO2 release amount is small. A blend of MDEA
and PZ showed high performance. It has been demonstrated that 13C-NMR spectroscopy is a
powerful tool for qualitative and quantitative analyses of chemical species formed in aqueous
solutions of amines and CO2. It has been elucidated that amines react with CO2, forming ionic
species such as bicarbonate ion (HCO3–), carbonate ion (CO32–), protonated amines, amine
carbamates, and amine carbonates, etc. A tertiary amine does not make its carbamate. Few studies
on the CO2 absorption/regeneration of aqueous cyclic amines among alkanolamines have been
reported. In this paper, we will present 13C-NMR study on concentration changes of chemical
species generated in the course of CO2 absorption (HCO3–/CO32–, amine/protonated amine,
carbonate/protonated carbonate, etc.) and those after release of CO2 upon heating for aqueous
piperidine
derivatives
such
as
2-(hydroxymethyl)piperidine
(2HM-PD),
3(hydroxymethyl)piperidine (3HM-PD), 4-(hydroxymethyl)piperidine (4HM-PD), 2-hydroxymethyl1-methylpiperidine (2HM-1M-PD), 3-hydroxymethyl-1-methylpiperidine (3HM-1M-PD), 1-(2hydroxyethyl)piperidine
(1HE-PD),
2-(2-hydroxyethyl)piperidine
(2HE-PD),
4-(2hydroxyethyl)piperidine (4HE-PD), etc. On the basis of the 13C-NMR results, we will investigate
the pKa effects on the absorption rate of CO2 and the release of CO2 upon heating.
In the experiments of CO2 absorption kinetics, CO2 gas was bubbled through a D2O solution of
an amine at a rate of 150 mL/min at room temperature. The concentration of the aqueous amine
was 20 wt%. In the absorption−regeneration experiments, a D2O solution of the amine bubbled
with CO2 gas at a rate of 150 mL/min for 60 min at room temperature was heated at ca. 93 C for 30
min. The 13C-NMR spectra were measured at room temperature on a JEOL JNM-500ECX 500
MHz NMR spectrometer by using the inverse gated proton decoupling method. Because of a long
spin−lattice relaxation time, the holding time between scans was set at 1 min. The data of 64 scans
1
were accumulated for each spectrum. As the standard of chemical shifts, 1,4-dioxane-d6 (D, 99%)
was added into the sample. The pKa value of each amine was measured at room temperature using
the potentiometric titration method.
The observed 13C-NMR spectra of aqueous amines were assigned on the basis of the data in the
literature. It is not possible to distinguish the signals originating from an amine and its protonated
species, HCO3− and CO32−, etc. because of rapid proton exchanges. No signal due to CO2 was
observed. A small amount of carbonates were observed for all aqueous amines except 2HM-PD. A
small amount of carbamates were observed for all aqueous amines except 2HM-PD and 2HE-PD.
The concentrations of the amine/protonated amine, the carbonate/protonated carbonate, the
carbamate, and the CO32−/HCO3− species were calculated using the band (37−60 ppm) due to the
piperidine ring or the band (59−61 ppm) due to the HO-bonded alkyl chain, the band (160−165
ppm) due to CO2− of the carbonate, the band (164−169 ppm) due to CO2− of the carbamate, and the
band (158−170 ppm) due to the CO32−/HCO3− group. The total concentration of the CO32−/HCO3−,
carbamate, and carbonate/protonated carbonate species was used for the calculation of the apparent
rate of CO2 absorption from 0 to 5 min.
The initial CO2 absorption rates were not dependent on pKa, and in the range between 0.099 and
0.138 mol/L•min. On the other hand, the initial CO2 absorption rates of tertiary amines increased
with increasing pKa because of a base-catalyzed mechanism. The absorption rate of aqueous
piperidine derivatives is governed by other factors such as molecular structure. The loading of CO2
decreased upon heating. The absolute values of decreases in CO2 loading upon heating increased
with decreasing pKa. A similar dependency was reported for the tertiary alkanolamines. The
highest value was 0.651 for 3HM-1M-PD. 3HM-1M-PD showed good performance in the initial
CO2 absorption rate and the CO2 release.
2