Postcombustion Capture of CO2 by Diamines Containing One Primary

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Environmental and Carbon Dioxide Issues

Post-combustion capture of CO2 by diamines containing one primary and one tertiary amino group: reaction rate and mechanism Bing Yu, Hai Yu, Qi Yang, Kangkang Li, Long Ji, Rui Zhang, Mallavarapu Megharaj, and Zuliang Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00961 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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Post-combustion capture of CO2 by diamines containing one primary and one tertiary amino group: reaction rate and mechanism Bing Yu1, 2, Hai Yu2*, Qi Yang3, Kangkang Li2, Long Ji2, Rui Zhang4, Mallavarapu, Megharaj1, and Zuliang Chen1*

1. Global Centre for Environmental Remediation, The University of Newcastle, Callaghan, NSW 2308, Australia. 2. CSIRO Energy, 10 Murray Dwyer Circuit, Mayfield West, NSW 2304, Australia. 3. CSIRO Manufacturing Flagship, Clayton, Victoria 3168, Australia 4. College of Chemical Engineering, Xiangtan University, Xiangtan, Hunan, 411105, P.R. China *Corresponding author. Ph: +61-2-49606201.E-mail: [email protected]. *Corresponding author. Ph: +61-2-49139748; Email: [email protected].

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Abstract: Developing ideal amine solvents with a high CO2 absorption capacity and a fast absorption rate is an attractive avenue to advance the amine scrubbing technology. In this regard, diamines bearing one primary and one tertiary amino group (1°/3° diamines) are proposed to be promising solvents for CO2 absorption which can exhibit the fast CO2 absorption rate of primary amines while maintaining the diamines’ intrinsic high absorption capacity. Here, we present a detailed kinetic study of four 1°/3° diamines for CO2 absorption and investigate the relationship between the structure of various 1°/3° diamines and their CO2 absorption rate. Results showed that increasing alkyl spacer between two amino groups within 1°/3° diamines promoted the CO2 absorption rate, while a large decrease in their reactivity with CO2 was observed when the tertiary amino group exists in the cyclic structure. Among these studied 1°/3° diamines, 3-(Dimethylamino)-1-propylamine (DMAPA) displayed the highest absorption rate under relevant conditions, also exhibited higher overall mass transfer coefficients than that of MEA over the entire range of CO2 loadings, and the main reaction routes for CO2 absorption into DMAPA solution via formation of the protonated DMAPA-carbamate were proposed.

Keywords: wetted wall column; absorption; Fourier transform infrared; nuclear magnetic resonance; protonation.

1. Introduction 2 ACS Paragon Plus Environment

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Post-combustion capture of CO2 based on chemical absorption has been demonstrated as a particularly promising emission mitigation strategy for reducing CO2 emissions from coal-fired power station significantly. Chemical absorbents used include amines,1-4 ionic liquids,5 ammonia,6 amino acids,7 amino acid salts,8 carbonates,9 and their mixtures.10-13 Amine scrubbing is widely regarded as the most mature technology for industrial carbon capture in the short term,14 but the technology still requires lowering energy consumption and capital costs.15-17 Currently, developing a promise amine absorbent that possesses a high CO2 absorption capacity and a fast absorption rate but not at the cost of increasing its regeneration heat, is attractive for advancing the amine scrubbing technology, enabling the potential for reducing the parasitic power demand and capital investment for CO2 capture. Amine solvents with optimal CO2 absorption capacity/rate as well as low regeneration heat have been developed mainly via two routes: blended amines18 and novel amines.19 In the first approach, single phase blended primary or secondary amines with tertiary or hindered amine solvents for CO2 absorption has been confirmed by many researchers20, 21 to be able to combine the advantages of each individual amine while limiting their individual problems. Besides, it is believed that this success has triggered further development of tri/qua-amine solvent blends.22, 23

Alternatively, phase-split blended amine solvents have also been demonstrated the

possibility of reducing the regeneration heat, since only the CO2 rich phase is sent to the striper.24 However, the optimal blend ratio and the concentration of mixed amine absorbents is not easy to control.18 In addition, the intermolecular proton transfer process in the blended amine system give rise to a dense hydrogen-bonded network, and thus increases the viscosity of solvent upon the CO2 uptake, which is disadvantageous for the diffusion of CO2 to amine solvents.25 Screening and designing novel amines is the other major route towards promising amine solvents for CO2 absorption. In the last decade, a variety of amine screening tests associated 3 ACS Paragon Plus Environment

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with MEA-based solvents,26 tertiary amines,27 piperidine analogues,28 and piperazine derivatives29 have been conducted. Despite the fact that some amines with certain desirable properties for CO2 absorption have been found, it remains a hurdle for developing an amine solvent with low regeneration energy requirement, fast reaction rate and high absorption capacity via the amine screening test. Additionally, amine screening work is considered to be time-consuming and costly.27 More recently, increasing attention has been concentrated on designing amines with attractive molecular structures which can improve their CO2 absorption properties.30, 31 For example, introducing a hydroxyl group to an amine molecule helps increase amine solubility and boiling point; adjusting the chain length between two different functional groups within an amine molecule gives rise to the variation of electronic effects;32 and varying the amine type can change the CO2 absorption rate and capacity of an amine solvent.11 In particular, introducing an additional amino group to one monoamine molecule and thus forms a diamine, which can achieve a higher CO2 absorption capacity but not at the expense of lowering its CO2 absorption rate compared to those monoamines.11, 33 One example for this scenario is piperazine, which has been widely recognized for its fast reaction kinetics as well as high capacity for CO2 absorption.34, 35 A diamine contains two amino groups in one molecule, and these amino groups can vary among primary (1°), secondary (2°) and tertiary (3°) amino groups. As a result, there are many different combinations (including 1°/1°, 1°/2°, 1°/3°, 2°/2°, 2°/3° and 3°/3°) of diamines in theory. The primary diamines (1°/1°) belong to the category of primary amines, and the primary amines typically feature a fast absorption rate but suffer from a high energy requirement for amine regeneration.32, 36 Therefore, as for those primary diamines (1°/1°) in the absence of the steric hindrance, they are expected to absorb CO2 fast but require a high regeneration energy consumption. The tertiary diamines (3°/3°) typically feature a slow CO2 absorption rate due to their acid-base catalyst reaction mechanism hurdled by the hydration of CO2, which makes 4 ACS Paragon Plus Environment

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them unfavourable for industrial application.37 The presence of secondary amino group promotes the production of undesirable nitrosamines, resulting in the amine absorbent loss and environmental issues.38-40 Moreover, secondary amines appear to be less stable against oxidation than primary and tertiary amines owing to the imine and amide formation.41 In general, to achieve a fast CO2 absorption rate, the existence of a primary or secondary amino group in a diamine molecule is required. To render a low absorption heat and a superior amine stability, incorporating a tertiary amino group while excluding the secondary amino group in a diamine molecule is advantageous. For this reason, we envisioned that the 1°/3° diamines are promising amine candidates for CO2 absorption. Additionally, compared with those monoamines in absence of steric hindrance, a higher CO2 absorption capacity by diamines (including 1°/3° diamines) has been confirmed in several reported work.31, 42 Given that the absorption rate directly relates to the packing requirement in the absorber and it is often considered as an important performance index of an amine solvent for CO2 absorption, the evaluation of the absorption rate is of great significance for the development of the 1°/3° diamines. Recently, the CO2 absorption rates of monoamines and bi-phase solvents,43, 44 haven been intensively studied, but little work has been conducted on the evaluation of the absorption rates of the 1°/3° diamines. Moreover, a thorough investigation of relationships between molecular structure of 1°/3° diamines and their CO2 absorption rate is still limited, and an insight into these relationships can provide the guidance for the screening and design of more suitable 1°/3° diamines for CO2 absorption. In this work, we investigated four 1°/3° diamines with their molecule structures shown in Table S1 in the Supporting Information, and MEA was also studied for validation and comparison. Firstly, we presented the evaluation of the CO2 absorption rate of five amines using a bubble column and a wetted-wall column. Then, the best performing 1°/3° diamine was then selected to investigate its overall mass transfer coefficients at various CO2 loadings by a wetted-wall 5 ACS Paragon Plus Environment

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column. Thirdly, 1H/13C- nuclear magnetic resonance (NMR) analysis was employed for the investigation of protonation events by two different amino groups within this 1°/3° diamine. Finally, the identification of the reaction intermediates under different CO2 loadings by Fourier transform infrared (FT-IR) and

13C-

nuclear magnetic resonance (13C-NMR) was also

conducted, which provided a further evidence to illustrate the underlying mechanism for the reactions between CO2 and this 1°/3° diamine.

2. Experimental section 2.1. Chemicals Monoethanolamine (MEA, 99.0%), N,N-Dimethylethylenediamine (DMEDA, 99.0%), Diethylethylenediamine (DEEDA, ≥99%), and 3-(dimethylamino)-1-propylamine (DMAPA, 99.0%) were all purchased from Sigma Aldrich. 1-(2-hydroxyethyl)-4-aminopiperidine (C4, 99%) was synthesized in-house,31 and all solutions were prepared with deionized water. CO2 gas (99.99%) and N2 gas (99.99%) were purchased from Coregas Australia. 2.2. Apparent CO2 absorption rates and overall mass transfer coefficients A bubble column was employed to measure the apparent CO2 absorption rate in different selected amine absorbents. The schematic of this bubble column was displayed in Figure S1a in the Supporting Information. The absorbent concentration was kept at 1 M for minimizing the impacts on mass transfer by amine’s physical properties, such as viscosity. A mixed gas containing CO2 and N2 was used to simulate a typical flue gas. The total gas flow rate was kept at 5 L min-1 and the CO2 concentration was maintained at 7% (volume/volume) using mass flow controllers (F22824-001, Brooks, USA). The bubble column with the amine solution was immersed in a water bath at 298 K. The outlet CO2 concentration was determined by a gas analyser (VA-3000, HORIBA, Japan). When the CO2 concentrations in outlet and inlet were


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the same, the measurement of CO2 absorption rate was stopped. The apparent CO2 absorption rate was calculated using following equations (1-3):11 𝑄𝑖𝑛 𝐶𝑂2 = 𝑋𝑖𝑛𝑄 𝑄𝑜𝑢𝑡 𝐶𝑂2 =

𝑟=

∆𝑁 ∆𝑡

(1)

(1 ― 𝑋𝑖𝑛)𝑄 1 ― 𝑋𝑜𝑢𝑡

=

― (1 ― 𝑋𝑖𝑛)𝑄

𝑜𝑢𝑡 𝑃(𝑄𝑖𝑛 𝐶𝑂2 ― 𝑄𝐶𝑂2)

𝑅𝑇

(2)

(3)

𝑜𝑢𝑡 where Q is the flow rate of the mixed gas (L min−1); 𝑄𝑖𝑛 𝐶𝑂2 and 𝑄𝐶𝑂2 are the flow rate of inlet

and outlet CO2; Xin and Xout are the CO2 mole fractions of the inlet and outlet gases; ∆t is the CO2 absorption duration (min); r is the CO2 absorption rate (mol min−1); ∆N is the amount of CO2 absorbed during a time interval of ∆t (moles); P is the total pressure in the gas phase (Pa); R is the universal gas constant (8.314 Pa m3mol−1K−1); T is the absolute temperature (K). The overall mass transfer coefficient of CO2 in the diamine solutions was measured using a wetted-wall column contactor, shown in Figure S1b in the Supporting Information. A detailed description of the wetted-wall column can be found in our previous work,36, 45 and the theory of the calculation of the mass transfer coefficients and reaction rate constants can be found in in the Supporting Information. 2.3. Characterization and analysis A solution of 0.7 M HCl was prepared from concentrated HCl (Merck, 32%) in Milli Q water and the concentration confirmed by a titration using NaOH (0.5M). Three stock solutions (50.00 mL each) are made by dissolving appropriate amounts of DMAPA, or DMAPA with a fixed quantity of HCl (0.70 M) in Milli Q water. The concentrations of three stock solutions were: (A) 0.05 M DMAPA and 0 M HCl; (B) 0.05 M DMAPA and 0.112 M HCl; (C) 0.05 M DMAPA and 0.36 M HCl. Then 10 of 2 ml liquid samples which contained 0.05 M DMAPA 7 ACS Paragon Plus Environment

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and various concentrations of HCl were prepared by mixing stock solution A and B or A and C in proportions to achieve the molar ratio of HCl to amine in the solution ranging from 0 to 1.0. The samples were placed into 5.0 mm diameter NMR tubes with a capillary insert containing 13% (w/w) of 1,3,5-trioxane (ACROS, 99%+) in D2O (D2O was used for locking and shimming, and trioxane for chemical shift references). The NMR tubes were capped to prevent any material changes between inside and outside of the NMR tubes. The samples were kept at room temperature for more than two days. Each sample was remained at 25.0°C for 2.6 hours prior to the NMR analysis to ensure the equilibrium of the sample solution was obtained. The chemical shifts of trioxane/D2O references were determined by a calibration with external tetramethylsilane (TMS) as 5.09 ppm (1H NMR) and 93.52 ppm (13C NMR). 1H and 13C NMR spectra were collected using a Bruker Ascend 400MHz NMR instrument operated at a frequency of 400.13 MHz (1H) and 100.62 MHz (13C). The internal temperature of the sample during the spectral acquisitions was kept at 25.0 ºC (± 1.0oC). Water suppression proton 1H NMR spectra were obtained as the sum of 320 scans with an interscan delay of 3.73 seconds and carbon 13C NMR spectra as the sum of 320 scans with an interscan delay of 3.26 seconds. 2-Dimensional HSQC and HMBC spectra were recorded to improve assignment of the relative peaks in the 1H and 13C spectra. The NMR analytical data were processed using TopSin 3.5 software. Relative changes in the chemical shift (ppm) of the peaks were used to determine the protonation events and molecular location of the protonation(s) throughout the titrations. A series of 1 M DMAPA solutions with various CO2 loadings were prepared by bubbling a certain amount of CO2 into 1 M DMAPA solutions for FT-IR (VERTEX 70, Bruker Co. Ltd.) and 13C-NMR analysis, and the detailed experimental procedures for 13C-NMR analysis can be found in our previous work.36

3. Results and discussion


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3.1. The apparent CO2 absorption rate As shown in Figure 1, the apparent CO2 absorption rate decreased as a function of absorption time for all studied amines, and this decreasing trend is related to the depletion of fresh amines by their reactions with CO2 which was continuously bubbled into the amine solutions. Additionally, a diamine - C4 that possesses a cyclic structure appeared to have the lowest CO2 absorption rate among all tested diamines in this study, which performed even worse than the monoamine - MEA. This was probably due to the enhanced steric hindrance for C4 molecule resulted from the existence of cyclic structure. To further examine if the low CO2 absorption rate of C4 was caused by the effect of steric hindrance, the major speciation of C4 solution with various CO2 loadings was identified using FT-IR. As shown in Figure S2 in the Supporting Information, a spectral change at ~1360 cm-1 (νs C=O) corresponding to the characteristic peak of the bicarbonate species (HCO3−) for C4 solution, was appeared after CO2 absorption and gradually increased as the rise of the CO2 loading. Meanwhile, there is an increasing intensity for the peak at 1570 cm-1 (designated to protonated primary amine (-NH3+)) was observed with the increase of CO2 loading. However, no peak related to the carbamate (C−N) stretch appeared in Figure S2 in the Supporting Information, and this indicated that the formation of carbamate species was not involved in the CO2 absorption into C4 solutions. All these results suggested that C4 solution exhibited the typical steric hindered amines’ features for CO2 absorption. Among all other three 1°/3° diamines without cyclic structure, DMEDA exhibited the slowest CO2 absorption rate, but it was still faster than MEA, suggesting that the replacement of a hydroxyl group by a tertiary amino group will be beneficial for improving their reactivity with CO2. This can be explained by the fact that the hydroxyl group exhibited a direct axial electron withdrawing effect and thus lowering the electronic density around the –NH2.46 Another possibility was that the hydroxyl group exerted a strong negative impact via the hydrogen bond formation with –NH2.47 Although the tertiary amino group was also regarded as an electron9 ACS Paragon Plus Environment

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withdrawing group, it is much weaker than the hydroxyl group.26 Considering that CO2 absorption into amine solutions involves an acid−base interaction, the existence of an additional tertiary amino group within 1°/3° diamine molecules provided more reactive sites for protonation that was also possible for increasing CO2 absorption rate.

Figure 1. Comparison of the CO2 absorption rate for different amines at the conditions: amine concentration (1 M), temperature (298 K), CO2 partial pressure (7 kPa). In comparison of DMEDA and DEEDA, the two methyl groups in DMEDA were replaced by the two ethyl groups, which resulted in a slight augment in CO2 absorption rate. This was because that the ethyl groups attached to the nitrogen presented a more profound electron donating effect than that caused by the methyl groups.26 A stronger electron-donating effect by the ethyl groups in DEEDA increased the electron density of the tertiary amino group and thus reducing the axial electron-withdrawing effect on the –NH2 more than that of methyl groups in DMEDA. As the length of the alkyl spacer increased from two carbons in DMEDA to three carbons in DMAPA, a great improvement in the apparent CO2 absorption rate was achieved. This observation was caused by the decreased negative electronic influence between two amino 10 ACS Paragon Plus Environment

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groups. El Hadri et al. studied the influence of the carbon chain length between two amino groups within a diamine molecule, and it was also found that the electronic influence between them is reduced when the distance between two amino groups increased.26 Based on the above discussions, as for linear 1°/3° diamines, the effect of alkyl spacer between two amino groups on the CO2 absorption rate seems to be much larger than that of alkyl’s substitution. It indicated that the negative influence of the tertiary amino group on the –NH2 is dominant, and the adjustment of alkyl types attached to tertiary amino group had no significant impact on the – NH2. Besides, it is important to note that the strong steric hindrance effect may be given rise to as the alkyl with a large molecular weight or cyclic structure attached to the tertiary amino group within 1°/3° diamine molecules. 3.2. The mass transfer and the second-order reaction constants The overall mass transfer coefficients for CO2 absorption into amine solutions have a direct impact on the absorber’s packing requirements in a traditional amine scrubbing process. In addition, it is commonly used to evaluate the CO2 absorption rate in amine solutions. The overall mass transfer coefficients for aqueous amine solutions studied at the temperature of 298 K and amine concentration of 1.0 M. The mass transfer coefficients of the gas phase were calculated based on Eq. (12) - Eq. (16) in the Supporting Information. The mass transfer coefficients of the liquid side can be estimated by Eq. (9) in the Supporting Information and all the values are summarized in Table 1. The overall mass transfer coefficients for all five amines follow an order of DMAPA > DEEDA ≈ DMEDA > MEA > C4. This observation agreed very well with the results obtained from the bubble column. This indicates the general trend of absorption rate for these amines is independent of the gas-liquid contactors, and the reaction of amines and CO2 played a decisive role in determining the apparent CO2 absorption rate. Since the mass transfer coefficient of gas


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phase kg mainly depends on gas ﬂow velocity, their values were maintained at a constant with the value of 24.63 mmol m-2 s-1 due to the fixed gas flow rate. The KG/kl represented the proportion of the mass transfer resistance of the liquid phase in the overall mass transfer resistance and varied from 94.98% to 97.13% in this study. It is evident that the overall mass transfer for CO2 absorption into amine solutions in the wetted wall column was dominated by the mass transfer resistance in the liquid phase. Table 1. Summary of the overall gas-based mass transfer coefficient (KG), individual mass transfer coefficients of gas (kg) and liquid (kl) phases for CO2 absorption into fresh amine solutions (1 M) at 298 K. Amine solutions

KG (mmol m-2 s-1)

kg (mmol m-2 s-1)

kl (mmol m-2 s-1)

KG/kl (%)

MEA

1.108

24.63

1.160

95.50

DMEDA

1.136

24.63

1.191

95.39

DEEDA

1.142

24.63

1.197

95.36

DMAPA

1.235

24.63

1.301

94.98

C4

0.707

24.63

0.728

97.13

To demonstrate that the intrinsic reaction kinetics of amines with CO2 was responsible for the apparent CO2 absorption rates and mass transfer coefficients, we calculated the second order rate constants of CO2 in amine solutions based on Eq. (6) - Eq. (16) in the Supporting Information, with the values given in Table 2. The rate constant of CO2 in MEA solution was used for the validation of the reliability of this method as well as for comparison. The k2 value for MEA is 5626 L mol-1 s-1 at 298 k in this study and agrees wellwith the values obtained by Liu et al.,48,El Hadri et al.,26 and Ali.49 A small difference among these values may be caused by the difference in experimental apparatuses and operating conditions such as the liquid/gas flow rate or the amine concentration. Based on the calculated results of second order rate constants for all 1°/3° diamines shown in Table 2, DMAPA had the highest k2 value of 6997 L


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mol-1 s-1, followed by DEEDA and DMEDA, with their k2 values of 5914 and 5918 L mol-1 s-1, respectively, which were very close. As for C4, its k2 value was only 2292 L mol-1 s-1. This observation was consistent with the results obtained from the bubble column and wetted-wall column, suggesting that the intrinsic reaction kinetics between diamines and CO2 is the decisive factor to determine their absorption rates. In addition, this further confirms that it’s valid to use the above theory to elaborate the relationship between the molecular structure features of these studied amines and their CO2 absorption rates. Summing up the kinetic results for the CO2 absorption into various 1°/3° diamine solutions, DMAPA preformed the best and it was worthy of further investigation. Table 2. The second-order reaction constant k2 from the literature sources and this study

Temperature (K)

k2 (L mol-1 s-1)

Reference

298

5939

48

298

4796

26

298

5520

49

298

5626

This study

DMEDA

298

5914

DEEDA

298

5978

DMAPA

298

6997

C4

298

2292

Amine solutions

MEA

This study

3.3. The mass transfer over the entire range of CO2 loadings Considering the amine scrubbing process for CO2 absorption is continuous and cyclic, and incomplete amine regeneration could occur during CO2 desorption in the stripper. Consequently, it’s important to investigate the influence of the CO2 loading of the absorption liquid on the CO2 mass transfer rate into amine solutions.50 Figure 2 shows the overall mass transfer coefficients for CO2 absorption as a function of CO2 loadings for 2.5 M DMAPA and 13 ACS Paragon Plus Environment

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2.5 M MEA solutions at 298 K. As expected, KG decreases with increasing CO2 loading for both DMAPA and MEA solutions. In comparison with the experimental results of wetted-wall column for MEA, DMAPA exhibited a better performance in terms of KG values over the entire range of CO2 loadings. The comparison between the mass transfer of CO2 into DMAPA and MEA solutions can provide an indication of the packing requirements for the absorber. Due to higher CO2 mass transfer coefficients by DMAPA solutions were obtained than MEA, less gas–liquid contact area would be required to achieve the same amount of CO2 removal from a gas stream at identical driving force.51 This leads to the small process equipment required and thus results in saving of the capital investment in real application. 1.8 1.5

2

KG mol/(m *s*KPa)

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MEA DMAPA

1.2 0.9 0.6 0.3 0.0 0.0

0.2

0.4

0.6

CO2 loading(mol/mol)

0.8

1.0

Figure 2. Overall CO2 mass transfer co-efficients, KG, as a function of CO2 loadings in DMAPA and MEA solutions at 298 K. 3.4. Protonation events of DMAPA Understanding the fundamental CO2 absorption mechanism is of prime importance to unveil the full potential of DMAPA as a competitive amine solvent. DMAPA in this work contains one primary and one tertiary amino groups within one molecule. As a result, the basicity of 14 ACS Paragon Plus Environment

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each amino group will be independently subjected to electron donating and inducing effects from neighbouring atoms. In order to determine the reaction mechanism between CO2 and DMAPA, a thorough investigation of the protonation events for CO2 absorption into DMAPA solutions is necessary. To explore the protonation events of DMAPA, we performed a series of 1H/13C NMR acidbase titrations, and the relative changes in the 1H and 13C chemical shifts were used to interpret protonation

events.

Figure 3a

displays

that

the

chemical

shifts

of

the

three

DMAPA/DMAPAH+ peaks were shifted significantly to lower field as the ratio of acid/amine increased. As shown in the insert picture of Figure 3a, the A1H, B1H, C1H and D1H represent the 1H of four different positions within the DMAPA molecule structure. The shift of the DMAPA/DMAPAH+ peaks downfield in the 1H NMR spectra was caused by the increasing fraction of the DMAPAH+, and the formed ammonium group could act as the electron-acceptor, withdrawing the electron density from the A1H, B1H, C1H and D1H 52. The change in the chemical shift of the A1H (Δδ = 0.04 ppm) is quite similar to that of the C1H (Δδ = 0.05 ppm) shown in Figure 3b, indicating that the both of two amino groups may compete with each other to attract the proton released by the attachment of CO2 to the primary amino group within the DMAPA molecular. Another explanation is that the distance between two amino groups is only three carbons, which is short enough for the proton to exchange between them easily. In addition, a bit lower change in chemical shift with increasing acid for A1H than that of C1H was observed, indicating that the primary amino group within the DMAPA molecular may possess a stronger base than that of the tertiary amino group. Similarly, Figure 3c shows the 13C chemical shifts of the three peaks were shifted to upfield as the ratio of acid/amine increased. In addition, the change in the chemical shift of the carbon (position C) adjacent to the primary amino group (Δδ = 0.13 ppm) was more obvious than that of carbon (position A) adjacent to the tertiary amino group (Δδ = 0.08 ppm) shown in Figure 3d. It can be reasonably postulated that the 15 ACS Paragon Plus Environment

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primary amino group in DMAPA possess a greater protonation constant than that of the tertiary amino group, which was in agreement with the above 1H NMR data.

a

b

c

d

Figure 3. (a) NMR chemical shifts for 0.05 M DMAPA solution with various rations of [HCl]/ [DMAPA] up to 1.0 mol/mol at 25.0 °C: (b) 1H; (c) total chemical shifts of A1H and C1H; 13C; (d) total chemical shifts of A13C and C13C.

3.5. FT-IR and 13C-NMR analysis of DMAPA solutions with various CO2 loadings The partial FT-IR spectroscopy (1800–1000 cm−1) of the 1.0 M DMAPA solution with various CO2 loadings ranging from 0 to 1.0 mol/mol is shown in Figure 4, and five major peaks evolve during CO2 absorption were highlighted by arrows. In comparison with fresh DMAPA solution, the DMAPA-carbamate derivatives (DMAPACOO−) were identified due to several strong peaks in the absorbance bands of 1500– 1300 cm−1 region, which includes the N-COO− stretching vibration (νN - COO 1335 cm−1) of the -


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NCOO− derivative, the symmetric (νs COO , 1440 cm−1) and the asymmetric (νas COO , 1495 cm−1) -

-

vibrations of the COO− moiety. And the intensities of these peaks experienced a continuous increasing trend with the CO2 loading increased, suggesting that the formation of DMAPAcarbamate derivatives was dominant. Furthermore, an absorbance band in the 1571 cm−1 region was gradually increased with the increment of CO2 loadings from 0 to 0.9 mol/mol, suggesting that the protonated primary amine (NH3+) produced increased during the reaction of DMAPA and CO2. The protonated DMAPA at the position of tertiary amino group (NH+) generated on absorption of CO2 may produce a peak in the 1479–1474 cm−1 region. But there was a –CH3/– CH2 asymmetric rocking model within this absorbance band region as well, therefore, it’s difficult to identify the individual vibrational modes due to the possible overlapping of these bands. At CO2 loadings greater than 0.9 mol/mol, the peak located at 1360 cm−1 (-CO2symmetric stretching) appeared, indicating that bicarbonate species were formed.53 This was likely due to an enhanced CO2 hydration and the conversion of carbamate to bicarbonate at high CO2 loadings. Additionally, the pH value of the solution decreased significantly at high loadings, which also benefits the formation of bicarbonate species.54

Figure 4. The FT-IR analysis of DMAPA with various CO2 loadings (ranging from 0.0 to 1.0 moles CO2 /mole amine). 17 ACS Paragon Plus Environment

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The mechanism of CO2 absorption into DMAPA solution was further explored by the

13C-

NMR spectroscopy as shown in Figure 5. As for fresh DMAPA, three peaks marked as a, b and c representing signals of free DMAPA existed in the 13C-NMR spectroscopy. After CO2 absorption, one carbonyl signals were observed at about 164.5 ppm (marked as d), which was assigned to the carbamate, meanwhile, the peaks marked as a', b' and c' representing three carbons in DMAPA-carbamate species appeared and their densities were gradually increased as the increase of the CO2 loading. As the CO2 loading in DMAPA solution reached 0.9 mol/mol, a weak peak at about 161.5 ppm (marked as e) appeared due to formation of the bicarbonate. This provides a further evidence that the carbamate species were the dominant components for the reactions involved in CO2 absorption into DMAPA solution. a′

a d

c

b

DMAPA (DMAPAH+)

d

b′

c′

e CO32- (HCO3-)

DMAPACOO- (H+DMAPACOO-)

c

e

1.0 mol/mol

c′

b′

a′

b

a

0.9 mol/mol 0.8 mol/mol 0.7 mol/mol

CO2 loading increase

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 mol/mol 0.5 mol/mol 0.4 mol/mol 0.3 mol/mol

c′

0.2 mol/mol 0.1 mol/mol

d

b′

b a

c

a′

0.0 mol/mol

Figure 5. The 13C-NMR analysis of DMAPA with various CO2 loadings (ranging from 0.0 to 1.0 moles CO2 /mole amine).

3.6. Mechanism of CO2 absorption into DMAPA solution 18 ACS Paragon Plus Environment

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Based on the above results, the reaction mechanism could be conjectured. The primary amino group in DMAPA first acted as the absorption site for CO2, accompanied with the release of protons during the formation of DMAPA-carbamate. Based on the 1H/13C NMR titrations analysis, the base for the primary amino group within the DMAPA molecule was somewhat stronger than that of the tertiary amino group. However, this did not indicate that the released protons can only be accepted by the primary amino group within the DMAPA molecule. If so, the stoichiometric CO2 absorption capacity for DMAPA was limited to 0.5 mole CO2/mole amine, and it was unlikely to reach about 1.0 mole CO2/mole amine in this study. As a result, both of two amino groups within a DMAPA molecule should act as the proton acceptor to attract the proton released by the deprotonation of the formed zwitterions via the nucleophilic reaction between CO2 and DMAPA shown in Figure 6. In fact, Zhang et al.55 has suggested the existence of an intramolecular proton exchange between the primary and tertiary amino groups within the DMAPA molecule, and Paoletti et al.56 also confirmed that tautomeric forms of the singly protonated diamine exist via an intramolecular proton exchange equilibrium. Based on these findings, we believe that the mono-protonated DMAPA with the protonation occurring at the position of tertiary amino group would continue to react with CO2 to produce the pronated DMAPA-carbamate. Similarly, the mono-DMAPA-carbamate could accept one proton via its tertiary amino group with the formation of the pronated DMAPA-carbamate as well.

O=C=O

O=C=O

Figure 6. The proposed main reaction routes for CO2 absorption into the DMAPA solution. 19 ACS Paragon Plus Environment

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Conclusions In this study, we proposed the diamines bearing one primary and one tertiary amino group (1°/3° diamines) as a potential amine based solvent for CO2 absorption, which did not sacrifice the intrinsic high absorption capacity of diamines to achieve the fast CO2 absorption rate. The determination of the apparent CO2 absorption rate and mass trass coefficients, coupled with the calculations of reaction rate constant of various 1°/3° diamines, enabled the identification of the principal structural features that influence their CO2 absorption rate. First, the existence of cyclic structure for incorporating the tertiary amine may enhance the steric hindrance effect on the primary amino group, thus lowering the CO2 absorption rate. Second, increasing the length of the alkyl spacer between two amino groups within 1°/3° diamine molecules was helpful to reduce the negative electronic influence, resulting in the improvement of the CO2 absorption rate. Third, the effect of alkyl spacer between two amino groups within 1°/3° diamines on the CO2 absorption rate was dominant, when compared with that of alkyl’s substitution on the position of the tertiary amino group. The relationships between the structure of 1°/3° diamines and their CO2 absorption rate identified here could provide the guidance for the 1°/3° diamines screening and design. Based on foregoing kinetic results, DMAPA was identified as a competitive 1°/3° diamine. Then, we performed a series of 1H/13C NMR acid-base titrations to investigate the apparent protonation order of DMAPA and found that the protonation constant for the primary amino group within DMAPA was somewhat stronger than that the tertiary amino group. The FT-IR and

13C-NMR

analysis of DMAPA solution with various CO2

loadings further revealed the protonation of DMAPA and the formation of DMAPA-carbamate derivatives. Finally, the main reaction routes for CO2 absorption into DMAPA solution via formation of the protonated DMAPA-carbamate was proposed.

Conflicts of interest – none 20 ACS Paragon Plus Environment

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Acknowledgements The authors wish to acknowledge financial assistance provided through CSIRO Energy. In addition, the authors are very grateful to Dr. William Conway for his kind suggestions to this work, and we also thank the technical assistance for NMR tests in CSIRO Clayton. Bing Yu would like to thank the University of Newcastle for his International Postgraduate Research Scholarship (UNIPRS), as well as the University of Newcastle Research Scholarship Central 25:75 (UNRSC 25:75) program for supporting his research.

Supporting Information The detailed information on the theory of the calculation of the reaction rate constants and mass transfer coefficients; the molecular structures of amines tested in this work, the schematic diagrams of the bubble column and the wetted-wall column used in this study, and the figure for the FT-IR spectra of C4 solutions with various CO2 loadings. References 1. Yang, X.; Rees, R. J.; Conway, W.; Puxty, G.; Yang, Q.; Winkler, D. A., Computational Modeling and Simulation of CO2 Capture by Aqueous Amines. Chem. Rev. 2017, 117, (14), 9524-9593. 2. Zhang, S.; Du, M.; Shao, P.; Wang, L.; Ye, J.; Chen, J.; Chen, J., Carbonic anhydrase enzymeMOFs composite with a superior catalytic performance to promote CO2 absorption into tertiary amine solution. Environ. Sci. Technol. 2018, 52, (21), 12708-12716. 3. Wang, R.; Liu, S.; Wang, L.; Li, Q.; Zhang, S.; Chen, B.; Jiang, L.; Zhang, Y., Superior energysaving splitter in monoethanolamine-based biphasic solvents for CO2 capture from coal-fired flue gas. Appl. Energy 2019, 242, 302-310. 4. Zhang, X.; Liu, H.; Liang, Z.; Idem, R.; Tontiwachwuthikul, P.; Al-Marri, M. J.; Benamor, A., Reducing energy consumption of CO2 desorption in CO2-loaded aqueous amine solution using Al2O3/HZSM-5 bifunctional catalysts. Appl. energy 2018, 229, 562-576. 5. Dupont, J.; Simon, N. M.; Zanatta, M.; dos Santos, F. P.; Corvo, M. C.; Cabrita, E. J., Carbon dioxide capture by aqueous ionic liquid solutions. ChemSusChem 2017, 10, (24), 4927-4933. 6. Li, K.; Yu, H.; Yan, S.; Feron, P.; Wardhaugh, L.; Tade, M., Technoeconomic assessment of an advanced aqueous ammonia-based postcombustion capture process integrated with a 650-MW coal-fired power station. Environ. Sci. Technol. 2016, 50, (19), 10746-10755. 7. Hussain, M. A.; Soujanya, Y.; Sastry, G. N., Evaluating the efficacy of amino acids as CO2 capturing agents: a first principles investigation. Environ. Sci. Technol. 2011, 45, (19), 8582-8588. 8. Shen, S.; Yang, Y. N.; Bian, Y.; Zhao, Y., Kinetics of CO2 Absorption into Aqueous Basic Amino Acid Salt: Potassium Salt of Lysine Solution. Environ. Sci. Technol. 2016, 50, (4), 2054-63. 21 ACS Paragon Plus Environment

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Postcombustion Capture of CO2 by Diamines Containing One Primary

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