Switchable Aqueous Pentaethylenehexamine System for CO2

Subscriber access provided by Kaohsiung Medical University

Article 2

Switchable Aqueous Pentaethylenehexamine System for CO Capture: An Alternative Technology with Industrial Potential

Thai Quoc Bui, Santosh Govind Khokarale, Shashi Kant Shukla, and Jyri-Pekka Mikkola ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01758 • Publication Date (Web): 10 Jun 2018 Downloaded from http://pubs.acs.org on June 10, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 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

ACS Sustainable Chemistry & Engineering

Switchable Aqueous Pentaethylenehexamine System for CO2 Capture: An Alternative Technology with Industrial Potential Thai Q. Bui1,*, Santosh G. Khokarale1, Shashi K. Shukla1, Jyri-Pekka Mikkola1,2,* 1

Technical Chemistry, Department of Chemistry, Chemical-Biological Centre, Umeå

University, Linnaeus 10, SE-90187 Umeå, Sweden 2

Industrial Chemistry & Reaction Engineering, Department of Chemical Engineering, Johan

Gadolin Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, FI-20500 ÅboTurku, Finland * E-mail: [email protected]; [email protected]. Tel.: +46-765651650 * E-mail: [email protected]; [email protected]. Tel.: +46-706200371 KEYWORDS: Aqueous pentaethylenehexamine, PEHA, reversible CO2 capture, carbamate, bicarbonate, Kamlet-Taft parameters, regeneration ABSTRACT Herein we report the application of polyamine pentaethylenehexamine (PEHA, 3,6,9,12tetraazatetradecane-1,14-diamine) in CO2 absorption with both neat PEHA and aqueous solutions thereof. The absorption of molecular CO2 in pure PEHA and in PEHA-water systems resulted in the formation of two chemical species, namely PEHA carbamate and bicarbonate. It was observed that upon formation of these species, both the CO2 absorption capacity and CO2 absorption rate were controlled by the amount of water in the system. During the CO2 absorption, the neat PEHA and 92 wt. % PEHA were capable of forming carbamate species only while other aqueous analogues with higher dilution allowed for the formation of both carbamate and bicarbonate species upon exceeding 8 wt. % water in the mixture. The CO2 uptake steadily increased with an increase in the water concentration in the solvent mixture and reached the maximum value of 0.25 g CO2/g solvent in case of 56 wt. % PEHA in water. However, in case of more dilute systems (i.e. < 56 wt. % PEHA in water), the trend reversed and the CO2 loading 1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

Page 2 of 33

decreased linearly to 0.05 g CO2/g solvent for 11 wt. % PEHA in water. Meanwhile, it usually took shorter time to achieve the full CO2 absorption capacity (equilibrium) with increasing water content in all cases. The

13

C NMR analysis was used to quantify the relative amount of PEHA

carbamate and bicarbonate, respectively, in reaction mixtures. The Kamlet-Taft parameters (α, β, and π*) of aqueous solutions for different concentrations of PEHA were also studied taking advantage of various solvatochromic dyes and correlated with the CO2 absorption capacity. The thermally induced switchable nature of CO2 saturated neat and aqueous PEHA solutions for transformation of ionic PEHA carbamate and bicarbonate moieties to molecular PEHA is also represented. A comparison between aqueous PEHA and aqueous monoethanolamine (industrial solvent) for CO2 capture is reported. Hence, most importantly, a switchable PEHA system is demonstrated for reversible CO2 absorption processes. INTRODUCTION Reducing anthropogenic carbon dioxide (CO2) emissions into the atmosphere has become a critical environmental issue faced by industrialized countries since CO2 is one of the predominant greenhouse gases and CO2 emissions from energy sector are accounting for about 60% of global emissions.1 In 2017, the mean concentration of CO2 (405 ppm)2 was about 46% higher than that recorded in the pre-industrial era (278 ppm),3 with an average growth of 2 ppm/year in the last twenty years. According to the Fifth Assessment Report (Working Group I, 2013) of the Intergovernmental Panel on Climate Change (IPCC),3 the globally averaged Earth’s surface temperature increased by 0.85 oC over the period 1880 to 2012, and the temperature change for the period 2016 to 2035 has been also predicted to reach 0.3 oC to 0.7 oC. A variety of negative consequences caused by global warming have been correlated to various phenomena such as sea level rise, extreme weather and climate change.4 Therefore, it is crucial to find a costeffective technological solution for capturing more CO2 from flue gases or producing less CO2 in industrial processes to mitigate the greenhouse effects and accelerate the shift towards a lowcarbon world. Among the CO2 capture options applied to large point sources such as coal-fired power plants, post-combustion capture based on amines is the most mature technology.5 In general, aqueous monoethanolamine (MEA) solution (30 wt. %) is currently one of solvents extensively used in the post-combustion CO2 capture process from flue gas streams due to the fast CO2 absorption rate, reasonable capacity and low cost of the solvent.6 However, it also suffers from 2 ACS Paragon Plus Environment

Page 3 of 33 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


several major drawbacks such as high energy consumption for regeneration, solvent degradation and equipment corrosion.7 To overcome those deficiencies, many attempts have been done to discover potential candidates as alternatives to MEA. In the area of chemical absorption, polyamines have often received great attention since they contain multiple reactive sites available for CO2 capture leading to the improvements in CO2 uptake, reaction rate and energy penalty.8-12 Recently, pentaethylenehexamine (PEHA), which has two primary and four secondary amine groups in the structure (Figure 1), has been used as the modifier to synthesize solid amine-based sorbents for CO2 adsorption due to its high amine group content per unit mass, high thermal stability and low toxicity.13-17 Nevertheless, there is little detailed information available on the CO2 capture performance of aqueous PEHA. To the best of our knowledge, only three publications18-20 mentioned the utilization of the water-PEHA system for CO2 absorption. In the CO2 capture and utilization (CCU) study, Prakash et al.18,19 used an aqueous PEHA solution for capturing CO2 and continuous production of methanol from the captured CO2 using a homogeneous ruthenium catalyst. Also, Prakash et al.20 presented the aqueous PEHA (~20 wt. %) as one of the screening candidates for capturing CO2 and stabilizing the formate product. The results showed that the CO2 uptake of the aqueous PEHA solution was the highest by mass compared to other screening solvents, namely 0.537 g CO2/g PEHA (2.83 mol CO2/mol PEHA).

Figure 1. Structure of PEHA. The pKa values given here are in water.20 In this study, the CO2 absorption capacity and amine regeneration after CO2 capture in PEHA and

in

aqueous

solutions

of

PEHA

were

examined.

Furthermore,

the

carbamate/bicarbonate/carbonate species distribution in the PEHA-H2O-CO2 systems and correlation between CO2 capacity with polarity parameters (ET(30), α, β, and π*)21-24 were investigated for improved understanding of the CO2 absorption behavior of aqueous PEHA systems. Comparison between aqueous PEHA and aqueous MEA was also reported. Reaction of amines with CO2 in aqueous media

3 ACS Paragon Plus Environment


Page 4 of 33

The general reaction mechanism of unhindered primary/secondary amine with CO2 in aqueous solution is summarized in Scheme 1. Basically, both sterically unhindered primary and secondary amines can act as nucleophiles (Lewis bases) when reacting with CO2 to form a carbamate via zwitterion/carbamic acid intermediate. In the presence of water, the carbamate could be subsequently hydrolyzed to liberate a free amine and, consequently, produce bicarbonate species, thereafter the free amine can again attack on CO2, thus leading to higher CO2 absorption performance of solvent compared to anhydrous conditions. The carbamatebicarbonate equilibrium depends on many parameters such as the structural features of the amine, amine basicity, amine concentration, solution temperature and CO2 partial pressure.25

Scheme 1. Plausible reaction mechanism of an unhindered primary/secondary amine with CO2 in aqueous media. In the case of a polyamine, the reaction mechanism is more complex compared to monoamines because a mixture of different carbamate species can form simultaneously from reactions between CO2 and amine sites. Results of the study on the distribution of the various species in diethylenetriamine (DETA)-H2O-CO2 system using 13C NMR analysis was reported in the literature.26 However, the observed species of triethylenetetramine (TETA)-H2O-CO2 and tetraethylenepentamine (TEPA)-H2O-CO2 systems could obviously not be determined by means of 13C NMR analysis.10 4 ACS Paragon Plus Environment

Page 5 of 33 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


EXPERIMENTAL SECTION Materials PEHA (technical grade, ≈ 100 %), MEA (> 99%), D2O (99.9 atom % D) and K2CO3 (99%) were purchased from Sigma-Aldrich. KHCO3 (> 99%) was supplied by J.T. Baker. CO2 gas was obtained by AGA AB (Linde Group). All of them were used as received. Deionized water was used throughout the experiments. The chemicals or spectroscopic indicator dyes used for the measurement of Kamlet-Taft parameters such as 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1yl)phenolate (Reichardt’s dye), 4-nitroaniline, and N,N-diethyl-4-nitroaniline were obtained from Sigma-Aldrich and used to prepare stock solutions without further purification. Methods Preparation of aqueous PEHA solutions Aqueous PEHA solutions were prepared by mixing about 2 g PEHA with deionized water thus obtaining the different concentrations (wt. %) of PEHA in the solutions. The mixtures were shaken until they became homogeneous. The amount of PEHA and deionized water were taken by using an analytical balance with an accuracy of ±0.1 mg. Measurement of CO2 absorption capacity A CO2 gas stream (30 mL/min) was bubbled into the solution using an immersed needle while it was stirred mechanically at ambient temperature and pressure. The CO2 uptake was determined by weighing both the vial and the immersed needle at regular intervals using an analytical balance with an accuracy of ±0.1 mg until the full saturation absorption capacity of CO2 was reached. However, the CO2 bubbling was stopped after 7 hours in cases of 92 wt. % PEHA and neat PEHA, although they did not reach their full CO2 absorption capacity. Measurement of viscosity Viscosity of sorbents before and after CO2 capture was measured using a Brookfield RV DV1 viscometer which was maintained at controlled 30 oC temperature using a heating immersion circulator (Julabo). Polarity measurement (Kamlet-Taft parameters) The solvatochromic dyes showed in Figure 2 were used to determine ET(30) and Kamlet-Taft parameters, respectively. The stock solutions (10-2 M) of the required dyes were prepared in methanol prior to use. The stock solution was transferred into a glass vial first and then methanol was removed by blowing nitrogen gas to the vial. 2 ml of aqueous PEHA solution was added in 5 ACS Paragon Plus Environment


Page 6 of 33

the vial and the resultant solution was transferred to a quartz cuvette under nitrogen atmosphere and sealed with a septum. The λmax was measured at room temperature using a UV-visible spectrophotometer. Various polarity parameters were obtained by using empirical equations given in Table 1.

Figure 2. The solvatochromic dyes applied in this study.


Page 7 of 33 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


Table 1. Empirical equations to determine the ET(30) and Kamlet-Taft parameters Polarity parameters

Empirical equations

Electronic transition energy

ET(30) (kcal mol-1) = hcνmax = 28591/λmax

[ET(30)]

νmax (equation 1) where λmax is the maximum wavelength

(nm)

= 2.8591

of lowest energy band of Reichardt’s dye

Hydrogen-bond donor acidity

α = [ET(30) – 14.6 (π* – 0.23) – 30.31]/16.5 (equation 2)

(α) Hydrogen-bond acceptor

ν(2) max = 1.035 ν(3) max – 2.8β + 2.64 (equation 3)

basicity (β)

where ν(2) max and ν(3) max are the maximum wave number of the

4-nitroaniline

and

N,N-diethyl-4-nitroaniline

respectively

Polarity index (π*)

ν(3) max = 27.52 – 3.182π* (equation 4)

Quantification of carbamate-carbonate-bicarbonate species The mixture obtained after CO2 capture was further characterized by means of 13C NMR (in D2O) spectroscopy to identify the resulting carbamate and/or (bi)carbonate species. The samples were analyzed with Bruker Avance 400 MHz NMR instrument. For comparison, a neat PEHA sample was also analyzed in the similar manner. To calibrate the chemical shift of carbonate/bicarbonate species in the

13

C NMR spectra, reference solutions were prepared by

dissolving K2CO3, KHCO3, and mixtures of them in different mole percentages in D2O (supporting information Figure S1 for NMR spectra of aqueous K2CO3 and KHCO3 solutions, respectively). All the NMR spectra were assigned using Bruker’s Topspin (3.5 pl7) processing software. The relative amount of carbamate, bicarbonate and carbonate species in PEHA-H2OCO2 reaction mixtures was determined by using a calculation method introduced by Holmes et al.27 as shown in the following equations: n(carbonate) =

. ( . . ) ∗ ()

n(bicarbonate) =

∗ (equation 5)

. ( . . ) ∗ ()

∗ (equation 6)



Page 8 of 33

n(carbamate group) = ∗ (equation 7) R=

( ) () (!)

=

" # $ %& " # $ (!) %

(equation 8)

where δ (ppm) denotes the chemical shift of HCO3−/CO32− in the investigated system (only one single peak was represented both the bicarbonate and carbonate species in the 13C NMR spectra because of the fast proton exchange between them); nCO2 corresponds to the total moles of CO2 absorbed in the PEHA-H2O-CO2 system; the values 167.85 and 160.47 are the chemical shifts of solely K2CO3 and KHCO3, respectively, in aqueous solutions (1 M) used in this work. R gives the ratio of the peak area for the carbamate species and the peak area for the carbonate/bicarbonate species, respectively, in the 13C NMR spectra. Based on the mass balance of carbon, nCO2 can be expressed as follows (physically absorbed CO2 is supposed to be negligible): n(CO2) = n(CO32-) + n(HCO3-) + n(carbamate group) (equation 9) Regeneration of PEHA The switching of CO2 saturated PEHA and its aqueous solutions to pure PEHA through release of molecular CO2 was confirmed. Sample from the CO2 saturated aqueous PEHA solution with 56 wt. % PEHA was heated at 100 oC and 120 oC for both 1 hour and 3 hours on a hot plate. Then, the samples were further analyzed by means of 13C NMR (in D2O) spectroscopy to ascertain the extent of decomposition of carbamate/carbonate/bicarbonate species, respectively. All aqueous PEHA solutions as well as neat PEHA after CO2 capture experiments were also heated at 120 oC for 4 hours and then characterized the same way to confirm PEHA regeneration. Comparison between aqueous PEHA and aqueous MEA for CO2 capture process a) CO2 capture performance Aqueous amine solutions were prepared by dissolving 2 g amine (PEHA, MEA) in water to achieve the desired concentration (30 wt. %). The CO2 uptake was determined by the same method described above (measurement of CO2 absorption capacity). b) Amine volatility and degradation About 7 g pure amine (PEHA, MEA) was heated in an open system (a 16 mm diameter – 8 mL vial) at high temperature (120 oC, 140 oC) using aluminum block for 20 hours. The amine 8 ACS Paragon Plus Environment

Page 9 of 33 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


weights were recorded before and after heating using an analytical balance to evaluate the amine volatility. The amine samples after heating for 20 hours at 120 oC and 140 oC were mixed with water to make 30 wt. % amine solutions (containing 2 g amine) for CO2 capture experiments to evaluate the amine recyclability as well as degradation. The CO2 uptake was determined by the same method described above (measurement of CO2 absorption capacity). The samples after heating were also analyzed using 1H and 13C NMR (in D2O). c) Amine regeneration from amine(30 wt. %)-H2O-CO2 Both amine(PEHA, MEA)-H2O-CO2 systems were heated at 120 oC for 1-4 hours, and the samples after heating were further characterized using 1H and

13

C NMR (in D2O) to exam the

amine regeneration ability. RESULTS AND DISCUSSION Preparation of aqueous PEHA solutions A series of aqueous PEHA solutions containing different amounts (wt. %) of PEHA in water were prepared in order to further determine the Kamlet-Taft parameters and CO2 uptake capacity. An exothermic reaction was observed during mixing of PEHA with water. Measurement of CO2 absorption capacity



Page 10 of 33

Figure 3. CO2 capacity of PEHA and various aqueous PEHA solutions, measured at CO2 flow rate of 30 mL/min, at ambient temperature and pressure. A series of CO2 absorption experiments were carried out with reaction mixtures having different amounts (wt. %) of PEHA in water. Figure 3 depicts the observed CO2 absorption capacities for all these reaction mixtures. It was observed that the CO2 absorption rate as well as the capacity of solutions increased gradually with higher amounts of water in the reaction mixture. The neat PEHA and 92 wt. % PEHA in water absorbed 0.16 and 0.17 g CO2/g solvent, respectively, in 7 h with very low CO2 absorption rate compared to other aqueous PEHA solutions. With further dilution, the CO2 absorption rate and capacity increased steadily up to 56 wt. % PEHA and reached the maximum capacity, i.e. 0.25 g CO2/g solvent in 2 h. However, in case of more dilute systems (< 56 wt. % PEHA), the trend reversed and the CO2 loading decreased linearly to 0.05 g CO2/g solvent in case of 11 wt. % PEHA in water. As indicated by the results above, the amount of water in the system played a vital role in terms of the CO2 absorption capacity, equilibrium time and initial reaction rate. Both neat PEHA and 92 wt. % PEHA in water systems showed very slow CO2 absorption rate. As reported previously,

the

formed

products,

i.e.

CO2-amine

complexes

in

the

form

of

carbamate/(bi)carbonate, usually cause an increase in the viscosity of reaction mixtures because of newly induced extended hydrogen bonded network.28-30 This viscous nature of the reaction mixture led to limited CO2 diffusion and hence decreased the rate and capacity of CO2 absorption in the solvent mixture. Yu et al.31 provided an alternative explanation for the increase in CO2 absorption capacity for aqueous amine solutions. According to them, water allowed the formation of hydronium carbamate and carbamic acid species in the solution, which enhanced the CO2 absorption capacity of the aqueous amine solution. It was observed that upon a further increase in the relative water concentration, the CO2 absorption rate as well as the capacity gradually increased because of more diffusion of CO2 due to decrease in the viscosity of reaction mixture. Notwithstanding the viscosity of the reaction medium decreased probably because the products dissolution in water thus preventing the formation of extended hydrogen bonding networks. However, as shown in Figure 3, an increase in the water concentration above the optimum level (here > 44 wt. % water) in the solution had a significant effect on the CO2 uptake capacity. The CO2 uptake capacity linearly decreased from 0.25 to 0.05 g CO2/g solvent when the concentration of PEHA decreased from 56 to 11 wt. %, respectively, in aqueous PEHA 10 ACS Paragon Plus Environment

Page 11 of 33 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


solutions. The polyamine PEHA is primarily responsible for chemisorption of CO2 and hence as its concentration decreased in the reaction mixture, the CO2 absorption capacity decreased further. Perinu et al.32 described in their report that in case of 2 M aqueous alkanolamine solution, the nitrogen atom of amine formed strong hydrogen bonding with water molecule and was not available to react with bicarbonate, i.e. sodium bicarbonate, to form carbamate species. The plausible reason is the high hydrogen-bond acceptor basicity of the amine and high hydrogen-bond donor acidity of water molecule. Hence, in case of more dilute PEHA aqueous solutions, because of high intrinsic basicity of PEHA, it could be anchored with water molecules instead of strongly interacting with CO2 molecules. Trivedi et al.33 reported that if the water concentration was increased above the limited value, the water could hydrolyze carbamate species back to the amine and molecular CO2 and, consequently, decrease the overall CO2 absorption capacity of the used ethylenediamine containing deep eutectic solvent system. Similarly, Goodrich et al.34 also showed that overly dilute aqueous solutions of amino acid based ionic liquids (ILs) imparted the negative influence on CO2 uptake capacity of their solvent system. Hence, even though addition of water had a positive impact on the performance of the amine solutions in terms of their CO2 absorption, the optimum concentration of water needs to be maintained. However, this optimum concentration varies for each and every solvent system containing different types of amines, eutectic mixtures or ILs. Measurement of viscosity The viscosity of the solvents before and after CO2 capture was measured at 30 oC and is presented in Table 2, respectively. For the PEHA-water miscible mixture, the viscosity increased from 105.2 cP (PEHA) to 318.7 cP (72 wt. % PEHA) and then decreased to 1.47 cP (11 wt. % PEHA) upon increased addition of water. The forming of inter-molecular hydrogen bonding with stronger strength between PEHA and H2O compared to inter- and intra-molecular hydrogen bonding in PEHA could possibly be responsible for the increasing viscosity of the solvent mix at low water concentrations.35,36 In case of dilute systems (≤ 56 wt. % PEHA), there might be enough water in solvent to hydrate the PEHA - in other words, our hypothesis is that water can disrupt inter- and intra-molecular hydrogen bonding in PEHA, hence the viscosity can be reduced close to the corresponding value of water at high water concentrations (11 wt. % PEHA). Considering the solvent mixes after CO2 absorption, the viscosity was higher than the corresponding solvent mixes before CO2 absorption. As already mentioned, the carbamate and/or 11 ACS Paragon Plus Environment


Page 12 of 33

(bi)carbonate products can render the system more viscous. Therefore, it can be observed, as tabulated in Table 2 that pure PEHA, 92 wt. % and 72 wt. % PEHA turned to highly viscous gels upon the CO2 capture process. On the other hand, in case of dilute systems saturated with CO2, since water can dissolve the products efficiently, less viscous solutions were obtained compared to water-lean solution mixtures.

Table 2. Viscosity at 30 oC of absorbents with different wt. % of PEHA Viscosity (cP) Absorbent

Before absorption

PEHA aq. PEHA (92 wt. %) aq. PEHA (72 wt. %) aq. PEHA (56 wt. %) a

105.2

235.2

318.7

100.1

Viscosity (cP) Absorbent

After absorption

a

Highly viscous

aq. PEHA

gelb

(39 wt. %)

Highly viscous

aq. PEHA

gel

b

(30 wt. %)

Highly viscous

aq. PEHA

gelb

(20 wt. %)

Highly viscous liquid

b

aq. PEHA (11 wt. %)

Before

After

absorption

absorptiona

14.66

45.95

6.14

11.46

2.75

3.65

1.47

1.73

After saturated with CO2. bNot measured

Quantification of carbamate-carbonate-bicarbonate species To identify the types of products, i.e. carbamate/(bi)carbonate species after CO2 absorption in neat PEHA and its aqueous solutions, these samples were further characterized by means of the 13

C NMR (in D2O) spectroscopy. Pure PEHA was also analyzed under similar conditions for

comparison purpose. The obtained NMR spectra are displayed in Figures 4 and 5, respectively. As illustrated by the 13C NMR spectra in Figure 4, only carbamate species formed when CO2 gas was bubbled in the mixtures containing neat PEHA and aqueous PEHA solutions containing 92 wt. % PEHA. With further increase in the water amount, 72 wt. % PEHA clearly showed broadened signals of carbamate species which appeared with chemical shifts in between 164.4165 ppm. Splitting was observed in the carbamate region, probably due to having different types of amine sites (primary and secondary) for reaction with CO2 in the PEHA molecule. The aqueous PEHA solutions with more water in their composition, i.e. with 68, 65 and 61 wt. % 12 ACS Paragon Plus Environment

Page 13 of 33 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


PEHA respectively, gave rise to an extra single peak of bicarbonate/carbonate species in between 162.5-160 ppm along with carbamate species. The single peak appeared for both the bicarbonate and carbonate species in the 13C NMR spectra because of the fast proton exchange between them (Figure S1, supporting information).

Figure 4. The

13

C NMR spectra of neat PEHA and CO2 bubbled neat PEHA and aqueous

solutions of PEHA (wt. % PEHA in aqueous solution = 92, 72, 68, 65 and 61) in the 168-158 ppm region. A slight de-shielding of the carbonate/bicarbonate peak was observed as it has been previously reported that this shift is probably caused by the changes in pH of the reaction mixture and species distribution in the bulk of the reaction mixture.25 The chemical shift values of carbamate and (bi)carbonate species shown in this work were in a good agreement with the literature data.19,20 As shown in Figure 5, the 13C NMR spectra of the reaction mixtures in case of decreasing PEHA concentrations in solution showed the signals for both carbamate and (bi)carbonate species. The observed chemical shift values for the reaction mixture after their exposure to CO2 are given in Table 3.



Page 14 of 33

Figure 5. The 13C NMR spectra of reaction mixtures after bubbling CO2 in the aqueous solutions of PEHA (wt. % PEHA in aqueous solution = 56, 39, 30, 20 and 11) in the 168-158 ppm region.

Table 3. The chemical shift values of carbamate/(bi)carbonate species in the PEHA-CO2 and PEHA-H2O-CO2 systems. PEHA (with/without H2O) + CO2

Chemical shift (ppm) Carbamate

(Bi)carbonate

PEHA + CO2

164.72 – 164.23

-

92 wt. % PEHA + CO2

164.78 – 163.71

-

72 wt. % PEHA + CO2

164.72 – 163.42

162.19

68 wt. % PEHA + CO2

164.79 – 163.43

161.24

65 wt. % PEHA + CO2

164.83 – 163.39

160.74

61 wt. % PEHA + CO2

164.46 – 162.76

160.12

56 wt. % PEHA + CO2

164.72 – 163.27

160.21

39 wt. % PEHA + CO2

164.18 – 162.75

159.92

30 wt. % PEHA + CO2

164.47 – 162.95

160.04

20 wt. % PEHA + CO2

164.66 – 163.15

160.14

11 wt. % PEHA + CO2

164.76 – 163.36

160.23


Page 15 of 33 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


After NMR analysis, the relative amounts of carbamate/bicarbonate/carbonate species in CO2 saturated PEHA and its aqueous solutions were further determined. The distribution of carbamate/bicarbonate/carbonate species in the systems after CO2 capture in mole % unit is given in Figure 6.

Figure 6. The relative amount (mole %) of carbamate/bicarbonate/carbonate species in PEHACO2 and PEHA-H2O-CO2 systems. Overall, almost no carbonate could be found in the product mixtures and the bicarbonate species appeared to be the favored one in the carbonate-bicarbonate equilibrium. The carbamateto-(bi)carbonate ratio declined gradually at first and remained stable when the water content of the system increased. Despite this, carbamate was always the main species compared to (bi)carbonate, in all cases. There were approximately 70% moles of CO2 absorbed in the form of carbamate species when the carbamate-(bi)carbonate equilibrium was achieved in the dilute aqueous systems. Even though the decrease in CO2 absorption capacity was due to the increase in the water amount (or decrease in PEHA amount as shown in Figure 3) in the solvent systems, the concentration of (bi)carbonate species increased on the expense of carbamate species. Hence, it seems that excess of water in the reaction mixture probably hydrolyzed unstable carbamate to (bi)carbonate and free amine through the formation of in-situ carbonic acid intermediate (Scheme 1). However, Jou et al.37 previously reported that the (bi)carbonate formation from free 15 ACS Paragon Plus Environment


Page 16 of 33

amine and carbonic acid was the result of hydration of CO2 by water and not from hydrolysis of carbamate. Therefore, in the present work, the source of formation of (bi)carbonate was the result of deprotonation of carbonic acid by free amine in which carbonic acid could be generated either from hydration of CO2 or by hydrolysis of carbamate or both approaches. From the above calculated results, it can consider that the concentration of carbonate species was negligible in the reaction mixture. Based on the mass balance of carbon, total moles of CO2 captured by the solvent can be supposed to equal the total moles of carbamate and bicarbonate species formed in the system. The moles of maximum absorbed CO2 based on 2 g PEHA and calculated values of carbamate/bicarbonate in all the systems are depicted in Figure 7.

Figure 7. Data of total CO2 and carbamate/bicarbonate species in mole unit. When looking at Figure 7, one may observe that the mole fraction of carbamate increased gradually after bubbling CO2 in the reaction mixtures.

The maximum value for moles of

carbamate reached 0.017 in case of 56 wt. % PEHA solution, after which in more dilute cases the value slowly decreased. If we correlate this observation with the CO2 absorption capacity as shown in Figure 3, it seems that the CO2 uptake capacity of aqueous amines is proportional to the amount of carbamate species generated in the reaction mixture. Even though the concentration of carbamate decreased, the total moles of absorbed CO2 slightly increased because of regularly increasing amount of bicarbonate species. Consequently, in case of more dilute aqueous amine solutions, the CO2 absorption capacity is independent on the formation of carbamate and it is 16 ACS Paragon Plus Environment

Page 17 of 33 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


mainly associated with increasing amounts of bicarbonate formed in the reaction mixture. Now, as mentioned previously, the formation of bicarbonate species was the result of interactions of amine with carbonic acid where carbonic acid could be generated in-situ as the result of either hydrolysis of carbamate or hydration of CO2 by water or both. However, as shown in Figure 7, the number of moles of bicarbonate and CO2 slowly decreased when more dilute aqueous PEHA solutions with 30, 20 and 11 wt. % of PEHA in water were prepared. The reason could be that in more dilute solutions, CO2 might possess lower water solubility and hence give rise to less interaction with PEHA molecules. Kamlet-Taft parameters The polarity of the solvent system usually depends on various parameters but not merely on single one of these parameters.38,39 In this work, the different polarity parameters for aqueous PEHA solutions were determined using the Kamlet-Taft empirical polarity scales (α, β and π*), with the solvatochromic probe molecules Reichardt’s dye, 4-nitroaniline and N,N-diethyl-4nitroaniline. The different Kamlet-Taft parameters for aqueous PEHA systems are enlisted in Table 4. Table 4. ET(30), Kamlet-Taft parameters (α, β, π*) and CO2 capacity values for aqueous PEHA solvents Wt. % of

ET(30)

PEHA in water

(kcal/mol)

92

CO2 capacity

α

β

π*

42.7

0.06

0.89

0.95

0.17

72

47.4

0.23

0.70

1.13

0.18

56

51.7

0.37

0.50

1.32

0.25

39

56.2

0.65

0.45

1.34

0.18

30

58.1

0.73

0.30

1.41

0.14

20

59.4

0.83

0.25

1.39

0.10

11

60.4

0.93

0.24

1.36

0.05

0

63.1

1.12

0.18

1.33

-

(g CO2/g solvent)

Overall, with a decrease in PEHA concentration, the CO2 uptake increased gradually up to 56 wt. % PEHA and reached the maximum capacity, i.e. 0.25 g CO2/g solvent, then decreased linearly to 0.05 g CO2/g solvent in case of 11 wt. % PEHA in water. Meanwhile, hydrogen-bond 17 ACS Paragon Plus Environment


Page 18 of 33

donor acidity (α) increased linearly, hydrogen-bond acceptor basicity (β) showed opposite trend with α, and dipolarity/polarizability index (π*) increased steadily up to 56 wt. % PEHA and remained stable for the remaining water-PEHA compositions (< 56 wt % PEHA). The solvent effects on CO2 capacity were examined using a Kamlet-Taft linear solvation energy relationship (LSER) approach that was reported comprehensively by Ab Rani et al.40 The most common form of the Kamlet-Taft LSER for identifying and quantifying solvent effects on solute is shown in the following equation: XYZ = XYZo + a.α + b.β + s.π* (equation 10) where XYZ denotes the solvent-dependent property under study (in this work, the CO2 capacity at ambient conditions); XYZo, a, b and s are solvent-independent coefficients, which are derived from a multiparameter fit to solvent-dependent parameters (α, β, π*). The LSER analyses were carried out using Microsoft Excel applying multiple linear regression analyses. The error associated with each coefficient (XYZo, a, b, s) was appraised by using the p-value and it was decided to accept the correlation equation if each individual coefficient had statistical significance (all p-values < 0.01). The acceptable result along with the associated statistical data and R square value are shown in Table 4 (the numbers in italics represent the p-values for each coefficient). Full results of Kamlet-Taft LSER approach in this work are shown in Table S1.

Table 5. Acceptable result of Kamlet-Taft LSER method for reactions of aqueous PEHA solvents with CO2 LSER equation

R2

CO2 capacity = -1.51(0.0045) + 0.81(0.0021).β + 1.00(0.0037).π*

0.93

The LSER equation shows that the CO2 uptake is mainly governed by β and π* while α indicates insignificant effect. Overall, CO2 capacity shows a positive dependence on both β and π* values of solvent, but π* is slightly more significant. Based on Table 4 and Table 5, we can assume that for aqueous PEHA systems, the optimum CO2 uptake can be reached if both π* value of solvent is similar to that of water (1.33) and β value of solvent is highest as possible. β values 18 ACS Paragon Plus Environment

Page 19 of 33 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


Considering the pKa values of PEHA (pKa = 9.7 and 11 in water), its aqueous solutions usually show high basicity which is further associated with its own hydrogen-bond acceptor ability with solute molecules. The hydrogen-bond acceptor basicity parameter (β) decreased linearly from 0.89 to 0.24 when the concentration of PEHA decreased from 92 to 11 wt. % in the solution. Therefore, it is clear that in case of aqueous PEHA solutions, the β value was mainly governed by the amount of PEHA in the composition of solution. As shown in Table 4, the CO2 absorption capacity of the aqueous PEHA solution (with 92, 72 and 56 wt. % PEHA, respectively) increased even though the β value gradually decreased. Here, with less dilute aqueous PEHA solutions, the CO2 absorption capacity was mainly dependent on the viscosity of the reaction mixture while the basicity of the PEHA molecule played a secondary role. As mentioned previously, an increase in the water concentration reduced the viscosity of the reaction medium because of dissolution of carbamate and/or bicarbonate species in the reaction mixture, and hence more enhanced CO2 diffusion could occur. However, in case of more dilute solutions (< 56 wt. % PEHA), the basicity of the reaction medium was responsible for the CO2 absorption capacity of the solvent system while it was secondarily dependent on the water concentration or the viscosity of medium. As mentioned in Table 4, the β value of aqueous PEHA solutions decreased because of low concentration of PEHA in the solution and, accordingly, the CO2 absorption capacity also decreased. π* values The dipolarity/polarizability parameter (π*) represents the ability of the solvent to induce a dipole in the probe molecule and it generally increases with an increase in the permanent dipoles of the solvent which, in turn, often is the result of the presence of functional groups and delocalized bonds in the solvent. As shown in Table 4, the π* value gradually increased from 0.95 to 1.32 for aqueous PEHA solutions having lower water fraction (92 to 56 wt. % PEHA). Further dilution did not influence significantly the π* value and it remained stable around 1.33 (the π* value for pure water) for the remaining water-PEHA compositions (< 56 wt % PEHA). Here, since water was more polar, and hence could induce the dipoles in the solute more effectively compared to PEHA. Therefore, the amount of water in the system played an important role for explanation of the π* values of aqueous solvents. In cases of the concentrated PEHA solutions (≥ 56 wt. % PEHA), both PEHA and water together caused the induced dipole in the probe molecule, but the effect of water was more important. Therefore, more water in the 19 ACS Paragon Plus Environment


Page 20 of 33

solvent led to higher π* value. The similar values of π* (ca. 1.33) in dilute aqueous PEHA solutions (< 56 wt. % PEHA) compared to pure water were a result of pronounced interactions of the probe molecule with excess water molecules rather than with PEHA-water adducts or PEHA molecules. In terms of CO2 absorption, higher π* values could facilitate dissolution of CO2 (non-polar compound) in polar reaction medium, and hence promote the CO2 absorption process. Therefore, with a decreasing in PEHA concentration (from 92 to 56 wt. % PEHA), along with the benefit from a decreasing in viscosity, an increase in π* value also contributed to an improve in CO2 capacity. However, with further dilution (< 56 wt. % PEHA), even though the π* values were quite similar, the CO2 uptake decreased linearly. As mentioned above, the basicity of solvent played a crucial role in cases of dilute solutions. Regeneration of PEHA The possibility of regeneration of PEHA from its carbamate and (bi)carbonate analogs was also tested following a thermally induced process. Samples of CO2 saturated aqueous PEHA solution with 56 wt. % PEHA were heated initially at 100 oC for 1 hour and the same solution was further heated for additional 3 hours. The existence of both carbamate and (bi)carbonate species was monitored by the

13

C NMR analysis. After 1 hour of thermal treatment, water got

evaporated and as shown in Figure 8, the amount of (bi)carbonate species decreased.


Page 21 of 33 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


Figure 8. The

13

C NMR spectra obtained in regeneration process of CO2 saturated aqueous

PEHA solution (56 wt. % PEHA) to pure PEHA at 100 oC and 120 oC for both 1 and 3 hours. 13C NMR spectra of pure PEHA included for comparison.

Figure 9. The

13

C NMR spectra obtained in regeneration process of CO2 saturated neat PEHA

and its aqueous PEHA solution to pure PEHA at 120 oC for 4 hours.

13

C NMR spectra of pure

PEHA included for comparison. After 3 hours, it was observed that almost all (bi)carbonate species got decomposed. However, it was observed that carbamate derivatives of PEHA remained unaffected at 100 oC and the 13C NMR spectra were found to be non-identical when compared to pure PEHA. When fresh reaction mixture was heated at 120 oC for 1 hour, it was observed that the carbamate species decreased whereas (bi)carbonate species completely disappeared. Upon further heating for 3 hours, carbamate species also almost disappeared and the 13C NMR spectra of the resultant rejuvenated reaction mixture and pure PEHA were found identical. Hence, it was observed that (bi)carbonate and carbamate derivatives of PEHA have different thermal stability. As a conclusion concerning this experiment, PEHA can be regenerated without its decomposition from its CO2-saturated solutions at 120 oC.41 These results prompted us to examine the reversibility of CO2-saturated neat PEHA as well as its aqueous solutions. As shown in Figure 9, just like in the previous experiment, all CO2 saturated reaction mixtures could be regenerated to pure PEHA under thermal treatment and the process was independent on the amount of water used in the composition of the reaction mixture. 21 ACS Paragon Plus Environment


Page 22 of 33

Comparison between aqueous PEHA and aqueous MEA for CO2 capture process As mentioned previously, aqueous MEA (30 wt. %) system has been used widely in the postcombustion CO2 capture process and, hence, we consider it as a standard to evaluate the overall performance of aqueous PEHA – a new solvent for CO2 capture. In this study, aqueous PEHA (30 wt. %) was selected for comparison with aqueous MEA (30 wt. %). a) CO2 capture performance (capacity and rate) Overall, Figure 10 shows that, at the same concentration (30 wt. %), aqueous PEHA provided a better performance in terms of both capacity and absorption rate compared to MEA. Full CO2 uptake loading was reached at 0.138 g CO2/g solvent, near 20% higher than the corresponding value for aqueous MEA (0.117 g CO2/g solvent). In addition, a similar initial CO2 absorption rate (g CO2/g solvent/min) during the first 20 min was observed, but a higher average rate compared to aqueous MEA could be observed during a 60-minute period. The reasons for this better performance (capacity and rate) could be:

Although the PEHA/MEA molecular weight ratio is about 3.8, PEHA has two primary and four secondary amine groups while MEA has only one primary amine group. In theory, both primary and secondary amine groups can react directly with CO2.

PEHA is a stronger base compared to MEA (PEHA: pKa = 9.7 & 11, MEA: pKa = 9.16).20,25 Therefore, it facilitates the forming of carbamate from carbamic acid/zwitterion and amine as well as the forming of bicarbonate from carbonic acid and amine. Due to that, CO2 can react easily with amines and/or dissolve in water easier.

In the presence of water, carbamate species can be hydrolyzed partly to form bicarbonate and release free amine sites that can re-attack on CO2 resulting in higher CO2 uptake. In addition, carbamates from secondary amines are usually less stable than those from primary, hence, PEHA carbamate species are more prone to hydrolysis than MEA carbamate resulting in the fact that more CO2 can be absorbed in the presence of water in case of PEHA.


Page 23 of 33 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


Figure 10. CO2 capacity of aqueous PEHA (30 wt. %) and aqueous MEA (30 wt. %) from fresh amines (PEHA, MEA), amines after heating at 120 oC for 20 h (PEHA*, MEA*) and amines after heating at 140 oC for 20 h (PEHA**, MEA**). b) Amine volatility and degradation In this study, we heated amine (PEHA, MEA) in an open system at high temperature (120 oC, 140 oC) for 20 hours to test their volatility and degradation (Figure 11). Amine degradation:

Figure 11. Amines (PEHA, MEA) before and after heating in open 8 mL vials at 120 oC (a) and 140 oC (b) for 20 hours. As can be seen from Figure 11, PEHA turned from colorless to amber and dark amber at 120 o

C and 140 oC, respectively, while MEA turned to deep brown in both cases. Based on 23 ACS Paragon Plus Environment


Page 24 of 33

discoloration of amines, those results proved indirectly that MEA has lower thermal stability compared to PEHA. Amines after heating were further used to make aqueous solutions (30 wt. %) for CO2 capture and compared to 30 wt. % solutions from corresponding fresh amines to test the amine recyclability. Figure 10 shows that both amines after heat treatment rise to lower capacities compared to corresponding fresh amines but the decrease in the capacity of PEHA was always less than that of MEA under similar conditions. Moreover, PEHA after heating at 140 oC for 20 hours showed even higher capacity compared to fresh MEA. Noticeably, the CO2 uptake for fresh PEHA was similar to PEHA after heating at 120 oC for 20 hours. Those results showed better performance for PEHA compared to MEA and confirmed further PEHA thermal stability, specially at 120 oC – the temperature commonly applied in the stripper for MEA systems.42 The samples were also characterized using 1H and

13

C-NMR (in D2O) after heating. The

NMR spectra are given in Figures S2 and S3, respectively. Some small signals of degradation products can be observed in both PEHA and MEA after heat treatment at 120 oC and 140 oC for 20 hours. Amine volatility: Amine volatility is one of the most important solvent parameters of a post-combustion CO2 capture process. Evaporative loss of amine in flue gas scrubbing will make the process costly and cause a negative influence on the environment. Although MEA has a low vapor pressure, the losses are still about 225 tons/year due to evaporation.43 Compared to MEA, PEHA has a lower vapor pressure even under harsh conditions. Figure 12 shows that even after heating at 120 oC for 20 hours, with initial amount about 7 g amine in a 16 mm diameter – 8 mL vial, evaporative loss of PEHA was only 0.0502 g while that of MEA was near 25 times higher (1.2336 g). Similarly, evaporative loss of MEA was still higher than PEHA at 140 oC for 20 hours although the loss ratio between MEA and PEHA was only 17 times. The reason could be that there are more volatile degradation products from PEHA along with normal PEHA volatility at 140 oC for 20 hours. However, the weight loss of PEHA at 140 o

C (20 hours) was 7 times lower than that of MEA at 120 oC (20 hours). 24 ACS Paragon Plus Environment

Page 25 of 33 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


Figure 12. Evaporative loss of amine (PEHA, MEA) after heating 7 g amine in a 16 mm diameter – 8 mL vial at 120 oC and 140 oC for 4 hours and 20 hours. c) Amine regeneration from amine(30 wt. %)-H2O-CO2 Both amine(PEHA, MEA)-H2O-CO2 systems were heated at 120 oC for 1-4 hours, and the samples after heating were further characterized using 1H and

13

C NMR to exam the amine

regeneration ability. The NMR results (in D2O) are shown in Figures 13 and 14, respectively.



Page 26 of 33

Figure 13. 1H and 13C NMR (in D2O) spectra: a) MEA(30 wt. %)-H2O-CO2; b) 120 oC, 1 hour; c) 120 oC, 2 hours; d) 120 oC, 3 hours; e) 120 oC, 4 hours; f) Pure MEA.

Figure 14. 1H and 13C NMR (in D2O) spectra: a) PEHA(30 wt. %)-H2O-CO2; b) 120 oC, 1 hour; c) 120 oC, 2 hours; d) 120 oC, 3 hours; e) 120 oC, 4 hours; f) Pure PEHA. After heating at 120 oC for 1 hour, the NMR spectra were similar to pure amine in both cases. In other words, bicarbonate and most of carbamate species in amine-H2O-CO2 systems decomposed to regenerate corresponding amines at those conditions. However, even after heating for 4 hours at 120 oC, there still were some remaining MEA carbamate species that could not be reversed into MEA while almost no PEHA carbamate could be detected by NMR after heating for 2 hours at 120 oC. These results show that at 120 oC and in case of PEHA(30 wt. %)H2O-CO2, it is easier to get the amine back compared to MEA(30 wt. %)-H2O-CO2 or it will cost less energy to regenerate amine in case of PEHA. CONCLUSIONS In this work, we studied the CO2 chemisorption ability of neat pentaethylenehexamine (PEHA) and its aqueous mixtures in terms of the absorption capacity and rate. It was evident that PEHA carbamate and bicarbonate formed after absorption of CO2 in aqueous PEHA solutions. Also, the relative concentration of both PEHA and water governed the CO2 absorption capacity and rate as well as the types of chemical species forming in the solution. In general, PEHA was found to be responsible for chemisorption of CO2 in the solution. Meanwhile,


Page 27 of 33 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


addition of water was beneficial since it enabled the dissolution of products in the solution mixture thus leading to a decrease in the viscosity and enhanced CO2 diffusion as well as absorption capacity. The CO2 absorption capacity and rate steadily increased with an increase in the relative water concentration up to 56 wt. % of PEHA in solution. However, in case of more dilute PEHA solutions (< 56 wt. % PEHA) due to lower concentration of PEHA, the CO2 absorption capacity linearly decreased. When the water concentration in the solution increased, the relative amount of bicarbonate gradually increased probably because of hydrolysis of carbamate. The Kamlet-Taft parameters varied depending on the amount of PEHA and water in the solution. The Kamlet-Taft LSER approach showed that for aqueous PEHA systems, the optimum CO2 uptake can be reached if both π* value of solvent is similar to that of water (1.33) and β value of solvent is highest as possible. Both neat PEHA and its aqueous solutions were capable of releasing molecular CO2 to regenerate PEHA without its decomposition under a mild thermal treatment. Upon the regeneration process, the carbamate species decomposed at 120 oC and was found to be more thermally stable compared to the bicarbonate species. The thermally induced PEHA regeneration was not influenced by the amount of water in the solution. At the same concentration (30 wt. %), aqueous PEHA gave rise to more prominent performance in CO2 capture compared to aqueous MEA. Furthermore, PEHA has a lower vapor pressure than MEA, hence, the environmental impact is relatively benign compared to MEA. In conclusion, herein we have successfully explored reversible aqueous PEHA solutions for CO2 absorption, under industrially feasible experimental conditions. ASSOCIATED CONTENT Figure S1. 13C NMR spectra of aqueous K2CO3 and KHCO3 and their mixtures. Figure S2. 1H and 13C NMR (in D2O) spectra: a) pure MEA; b) MEA, 120 oC, 20 hour; c) MEA, 140 oC, 20 hours. Figure S3. 1H and

13

C NMR (in D2O) spectra: a) pure PEHA; b) PEHA, 120 oC, 20 hour; c)

PEHA, 140 oC, 20 hours. Table S1. The results of Kamlet-Taft LSER method for reactions of aqueous PEHA solvents with CO2. AUTHOR INFORMATION 27 ACS Paragon Plus Environment


Page 28 of 33

Corresponding Authors * E-mail: [email protected]; [email protected]. Tel.: +46-765651650 * E-mail: [email protected]; [email protected]. Tel.: +46-706200371 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is part of activities of the Technical Chemistry, Department of Chemistry, ChemicalBiological Centre, Umeå University, Sweden as well as the Johan Gadolin Process Chemistry Centre at Åbo Akademi University in Finland. The Swedish Research Council (Drn: 201604090), Bio4Energy programme, Kempe Foundations and Wallenberg Wood Science Center under auspices of Alice and Knut Wallenberg Foundation are gratefully acknowledged for funding this project. REFERENCES (1) CO2 emissions from fuel combustion highlights. International Energy Agency, 2017; https://www.iea.org/publications/freepublications/publication/CO2EmissionsfromFuelCombustio nHighlights2017.pdf (2) Dlugokencky, E.; Tans P. NOAA/ESRL; www.esrl.noaa.gov/gmd/ccgg/trends/ (3) Stocker, T.F., Qin, D., Plattner, G.-K., Tignor, M., Allen, S.K., Boschung, J., Nauels, A., Xia, Y., Bex, V., Midgley, P.M., Eds. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2013; http://www.climatechange2013.org/images/report/WG1AR5_ALL_FINAL.pdf (4) Kerr, R. A. Global warming is changing the world. Science 2007, 316 (5822), 188-190.


Page 29 of 33 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


(5) 20 years of carbon capture and storage. International Energy Agency, 2016; https://www.iea.org/publications/freepublications/publication/20YearsofCarbonCaptureandStora ge_WEB.pdf (6) Hadri, N. E.; Quang, D. V.; Goetheer, E. L. V.; Zahra, M. R. M. A. Aqueous amine solution characterization for post-combustion CO2 capture process. Applied Energy 2017, 185, 14331449. (7) Sreedhar, I.; Nahar, T.; Venugopal, A.; Srinivas, B. Carbon capture by absorption – Path covered and ahead. Renewable and Sustainable Energy Reviews 2017, 76, 1080-1107. (8) Schäffer, A.; Brechtel, K.; Scheffknecht, G. Comparative study on differently concentrated aqueous solutions of MEA and TETA for CO2 capture from flue gases. Fuel 2012, 101, 148-153. (9) Choi, S. Y.; Nam, S. C.; Yoon, Y. I.; Park, K. T.; Park, S. J. Carbon dioxide absorption into aqueous blends of methyldiethanolamine (MDEA) and alkyl amines containing multiple amino groups. Ind. Eng. Chem. Res. 2014, 53, 14451-14461. (10) Kim, Y. E.; Moon, S. J.; Yoon, Y. I.; Jeong, S. K.; Park, K. T.; Bae, S. T.; Nam, S. C. Heat of absorption and absorption capacity of CO2 in aqueous solutions of amine containing multiple amino groups. Sep. Purif. Technol. 2014, 122, 112-118. (11) Fan, W.; Liu, Y.; Wang, K. Detailed experimental study on the performance of monoethanolamine,

diethanolamine,

and

diethylenetriamine

at

absorption/regeneration

conditions. Journal of Cleaner Production 2016, 125, 296-308. (12) Muchan, P.; Narku-Tetteh, J.; Saiwan, C.; Idem, R.; Supap, T. Effect of number of amine groups in aqueous polyamine solution on carbon dioxide (CO2) capture activities. Sep. Purif. Technol. 2017, 184, 128-134. (13) Wei, L.; Gao, Z.; Jing, Y.; Y. Wang. Adsorption of CO2 from simulated flue gas on pentaethylenehexamine-loaded mesoporous silica support adsorbent. Ind. Eng. Chem. Res. 2013, 52, 14965-14974. (14) Fabiano, T. A.; Soares, V. P.; Andreoli, E. Pentaethylenehexamine-C60 for temperature consistent carbon capture. C 2015, 1, 16-26.



Page 30 of 33

(15) Ji, C.; Huang, X.; Li, L.; Xiao, F.; Zhao, N.; Wei, W. Pentaethylenehexamine-loaded hierarchically porous silica for CO2 adsorption. Materials 2016, 9, 835. (16) Kishor, R.; Ghoshal, A. K. Polyethylenimine functionalized as-synthesized KIT‑6 adsorbent for highly CO2/N2 selective separation. Energy Fuels 2016, 30, 9635-9644. (17) Liu, Y.; Lin, X.; Wu, X.; Liu, M.; Shi, R.; Yu, X. Pentaethylenehexamine loaded SBA-16 for CO2 capture from simulated flue gas. Powder Technol. 2017, 318, 186-192. (18) Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. S. Conversion of CO2 from air into methanol using a polyamine and a homogeneous ruthenium catalyst. J. Am. Chem. Soc. 2016, 138, 778-781. (19) Kar, S.; Sen, R.; Goeppert, A.; Prakash, G. K. S. Integrative CO2 capture and hydrogenation to methanol with reusable catalyst and amine: Toward a carbon neutral methanol economy. J. Am. Chem. Soc. 2018, 140, 1580-1583. (20) Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. S. CO2 capture by amines in aqueous media and its subsequent conversion to formate with reusable ruthenium and iron catalysts. Green Chem. 2016, 18, 5831-5838. (21) Reichard, C. Solvatochromic dyes as solvent polarity indicators. Chem. Rev. 1994, 94, 2319-2358. (22) Taft, R. W.; Kamlet, M. J. The solvatochromic comparison method. 2. The α-scale of solvent hydrogen-bond donor (HBD) acidities. J. Am. Chem. Soc. 1976, 98 (10), 2886-2894. (23) Kamlet, M. J.; Taft, R. W. The solvatochromic comparison method. I. The β-scale of solvent hydrogen-bond acceptor (HBA) basicities. J. Am. Chem. Soc. 1976, 98 (2), 377-383. (24) Kamlet, M. J.; Abboud, J. L.; Taft, R. W. The solvatochromic comparison method. 6. The π* scale of solvent polarities. J. Am. Chem. Soc. 1977, 99 (18), 6027-6038. (25) Kortunov, P. V.; Siskin, M.; Baugh, L. S.; Calabro, D. C. In situ nuclear magnetic resonance mechanistic studies of carbon dioxide reactions with liquid amines in aqueous systems: New insights on carbon capture reaction pathways. Energy Fuels 2015, 29, 5919-5939.


Page 31 of 33 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


(26) Hartono, A.; da Silva, E. F.; Grasdalen, H.; Svendsen, H. F. Qualitative determination of species in DETA-H2O-CO2 system using 13C NMR spectra. Ind. Eng. Chem. Res. 2007, 46, 249254. (27) Holmes II, P. E.; Naaz, M.; Poling, B. E. Ion concentrations in the CO2-NH3-H2O system from 13C NMR spectroscopy. Ind. Eng. Chem. Res. 1998, 37, 3281-3287. (28) Goodrich, B. F.; de la Fuente, J. C.; Gurkan, B. E.; Zadigian, D. J.; Price, E. A.; Huang, Y.; Brennecke, J. F. Experimental measurements of amine-functionalized anion-tethered ionic liquids with carbon dioxide. Ind. Eng. Chem. Res. 2011, 50, 111-118. (29) Zhang, Y. Q.; Zhang, S. J.; Lu, X. M.; Zhou, Q.; Fan, W.; Zhang, X. P. Dual aminofunctionalised phosphonium ionic liquids for CO2 capture. Chem. Eur. J. 2009, 15, 3003-3011. (30) Gutowski, K. E.; Magninn, E. J. Amine-functionalized task-specific ionic liquids: a mechanistic explanation for the dramatic increase in viscosity upon complexation with CO2 from molecular simulation. J. Am. Chem. Soc. 2008, 130, 14690-14704. (31) Yu, J.; Chuang, S. S. C. The role of water in CO2 capture by amine. Ind. Eng. Chem. Res. 2017, 56, 6337-6347. (32) Perinu, C.; Arstad, B.; Bouzga, A. M.; Jens, K. J.

13

C and

15

N NMR characterization of

amine reactivity and solvent effects in CO2 capture. J. Phys. Chem. B 2014, 118, 10167-10174. (33) Trivedi, T. J.; Lee, J. H.; Lee, H. J.; Jeong, Y. K.; Choi, J. W. Deep eutectic solvents as attractive media for CO2 capture. Green Chem. 2016, 18, 2834-2842. (34) Goodrich, B. F.; de la Fuente, J. C.; Gurkan, B. E.; Lopez, Z. K.; Price, E. A.; Huang, Y.; Brennecke, J. F. Effect of water and temperature on absorption of CO2 by amine-functionalized anion-tethered ionic liquids. J. Phys. Chem. B 2011, 115, 9140-9150. (35) Briscoe, B.; Luckham, P.; Zhu, S. The effects of hydrogen bonding upon the viscosity of aqueous poly(vinyl alcohol) solutions. Polymer 2000, 41, 3851-3860. (36) Ma, Y.; Liu, Y.; Su, H.; Wang, L.; Zhang, J. Relationship between hydrogen bond and viscosity for a series of pyridinium ionic liquids: Molecular dynamics and quantum chemistry. J. Mol. Liq. 2018, 255, 176-184. 31 ACS Paragon Plus Environment


Page 32 of 33

(37) Jou, F. Y.; Mather, A. E.; Otto, F. D. Solubility of H2S and CO2 in aqueous methyldiethanolamine solutions. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 539-544. (38) Madeira, P. P.; Passos, H.; Gomes, J.; Coutinho, J. P.; Freire, M. G. Alternative probe for the determination of the hydrogen-bond acidity of ionic liquids and their aqueous solutions. Phys. Chem. Chem. Phys. 2017, 19, 11011-11016. (39) Jessop, P. G.; Jessop, D. A.; Fu, D.; Phan, L.; Solvatochromic parameters for solvents of interest in green chemistry. Green Chem. 2012, 14, 1245-1259. (40) Ab Rani, M. A.; Brant, A.; Crowhurst, L.; Dolan, A.; Lui, M.; Hassan, N. H.; Hallett, J. P.; Hunt, P. A.; Niedermeyer, H.; Perez-Arlandis, J. M.; Schrems, M.; Welton, T.; Wilding, R. Understanding the polarity of ionic liquids. Phys. Chem. Chem. Phys. 2011, 13, 16831–16840. (41) Blasucci, V.; Dilek, C.; Huttenhower, H.; John, E.; Llopis-Mestre, V.; Pollet, P.; Eckert, C. A.; Liotta, C. L. One-component, switchable ionic liquids derived from siloxylated amines. Chem. Commun. 2009, 116-118. (42) Rochelle, G. T. Thermal degradation of amines for CO2 capture. Curr. Opin. Chem. Eng. 2012, 1, 183-190. (43) Heldebrant, D. J.; Koech, P. K.; Glezakou, V. A.; Rousseau, R.; Malhotra, D.; Cantu, D. C. Water-lean

solvents

for

post-combustion

CO2

capture:

Fundamentals,

uncertainties,

opportunities, and outlook. Chem. Rev. 2017, 117, 9594−9624.


Page 33 of 33 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


TOC/ABSTRACT GRAPHIC (For Table of Contents Use Only)

SYNOPSIS A switchable aqueous pentaethylenehexamine system was used as a solvent medium for reversible CO2 absorption processes.


Switchable Aqueous Pentaethylenehexamine System for CO2

Recommend Documents