Pentaerythritol-Based Molecular Sorbent for CO2 Capturing: A Highly

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Pentaerythritol-Based Molecular Sorbent for CO2 Capturing: A Highly Efficient Wet Scrubbing Agent Showing Proton Shuttling Phenomenon Abdussalam K. Qaroush, Khaleel I. Assaf, Ala’a Al-Khateeb, Fatima Alsoubani, Enas Nabih, Carsten Troll, Bernhard Rieger, and Ala’a Fakhri Eftaiha Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01125 • Publication Date (Web): 03 Jul 2017 Downloaded from http://pubs.acs.org on July 3, 2017

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Pentaerythritol-Based Molecular Sorbent for CO2 Capturing: A Highly Efficient Wet Scrubbing Agent Showing Proton Shuttling Phenomenon Abdussalam K. Qaroush,a,* Khaleel I. Assaf,b,* Ala’a Al-Khateeb,c Fatima Alsoubani,c Enas Nabih,c Carsten Troll,d Bernhard Rieger,d Ala’a F. Eftaihac,e,* a

Department of Chemistry, Faculty of Science, The University of Jordan, Amman 11942, Jordan.

b

Department of Life Sciences and Chemistry, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany. c Department of Chemistry, The Hashemite University, P.O. Box 150459, Zarqa 13115, Jordan. d WACKER-Lehrstuhl für Makromolekulare Chemie, Technische Universität München, Lichtenbergstraße 4, 85747 Garching bei München, Germany. e Department of Chemistry and Biochemistry, University of California Santa Barbara, CA 93106-9510 (Fulbright Visiting Scholar). KEYWORDS. Pentaerythritol, CO2 Capturing, Wet Scrubbing, Proton Shuttling, Quantum Chemical Calculations.

ABSTRACT: Pentaerythritol (PE) is considered as a biodegradable material which combines the ease of synthesis, nonevolatility, and extra stability under basic conditions (acidic gas sequestration, e.g., CO2), that makes it useful candidate for postcombustion capture (PCC) application. To overcome corrosion problems associated with CO2 binding organic, a binary mixture comprised of PE/1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU) (1:4 molar ratio) dissolved in dimethyl sulfoxide (DMSO) was exploited for CO2 capturing. The formation of ionic alkyl organic carbonate (RCO3- DBUH+) was confirmed using 13C NMR (157.4 ppm) and ex situ ATR-FTIR (two peaks were identified, viz., 1670 cm-1 and 1630 cm-1 which were ascribed to the asymmetric stretching of both C=O and O...C...O within RCO3H and RCO3-, respectively). The charged adduct was measured using a thermostatted beaker coupled with conductivity and pH meter probes. The sorption capacity of a 5.0 % PE (w/v) solution was measured volumetrically with high efficiencies as, ca. 16 and 18.5 wt%, for wet and dry conditions, respectively. In addition, density functional theory (DFT) was performed to understand the mechanism of action in the case of H2O, and simple alcohols, e.g., methanol and ethanol. Moreover, we reported on the newlydiscovered medium-dependent proton shuttling phenomenon that was verified experimentally and theoretically via DFT.

Carbon dioxide (CO2), is considered to be the most prominent greenhouse gas, which is attributed as the main cause of anthropogenic climate change.1,2 The expected increase in the atmospheric CO2 concentration to unprecedented level (up to 430 ppm in 2060)3 makes boosting CO2 capturing strategies, viz., pre-combustion, post-combustion, oxy-fuel combustion, and electrochemical separation, necessary to guarantee sustainable and resilient future. For governmental and industrial sectors the postcombustion capture (PCC) is of particular interest for governmental and industrial sectors is the postcombustion capture, because it does not require additional capital investments in comparison with other sequestration strategies (see the review by Kenarsari et al.1 and others4,5 for more details). In this regard, carbon capture and storage or sequestration (CCS)6 together with carbon

capture and utilization (CCU)7 are potential approaches to mitigate global warming.8 Amine scrubbing via monoethanolamine (MEA) is the most mature technology for CO2 capturing due to its effectiveness towards dilute CO2 streams and commercial availability.9–11 The chemisorption of CO2 by aqueous amine-based solvents takes place through the formation of carbamate ion following a 1:2 mechanism, viz., one mole of CO2 reacts with two moles of amine functional groups, or the formation of inorganic bicarbonate ion via 1:1 mechanism (chemical structures are shown in Scheme 1 A and B). There are several drawbacks associated with amine-based sorbents such as corrosiveness, toxicity, intensive energy required to regenerate aqueous amine solutions and chemical degradation upon successive absorption-desorption cycles.12,13 Ammonia (NH3) scrubbing

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has several merits over the amine-based sorbents in terms of cost, loading capacity and less-corrosive character, however, the high volatility of NH3 addresses several technical problems.14,15 This highlights the potential importance of other alternative technologies such as solid sorbents16 and membranes17 or to pave the way to less energy demanding scrubbing agents upon regeneration through the formation of ionic organic carbonates.18,19 In this context, Sir J. Fraser Stoddart (Chemistry Nobel Laureate, 2016) reported that cyclodextrin based metal organic frameworks (CD-MOFs), chemisorbed CO2 selectively and reversibly at low pressures at the primary hydroxyl group of the glucopyranose ring that coordinated to rubidium ions.20–23 Very recently, our group has reported on the supramolecular chemisorption of CO2 through organic alkyl carbonate formation, adopting a benign-by-design approach using green chemistry guidelines by applying safer chemicals and the use of bio-renewables, namely, chitin acetate oligomer dissolved in dimethyl sulfoxide (DMSO). The formation of chitin-CO2 adduct (Scheme 1C) was confirmed experimentally and verified by density functional theory (DFT) calculations. The use of DMSO (aprotic and hydrogen bond acceptor solvent) was necessary to activate the primary hydroxyl group (C-6) of the ammonium/amide pyranose repeating units to become more susceptible towards nucleophilic attack.24,25 Unlike the previously discussed structural motifs, small organic molecules combine facile synthesis, well-defined chemical structure and the ease of chemical analysis of intermediates and reaction products. Phillip Jessop’s research group reported that exposing a mixture of 1hexanol and 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU, Scheme 1D) to the atmospheric CO2 resulted in the formation of amidinium hexylcarbonate ([DBUH+][O2COHex]) adduct. The reaction mixture could be regenerated by applying external stimuli such as bubbling ni-

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trogen or heating.26 This reversible reaction with CO2 was exploited to engineer a wide spectrum of species that can reversibly change their ionic character for different applications such as solvents,27 solutes,28 surfactants,29 coagulatable/redispersible polymers30, catalysts,31 sensors,32 , etc. Following Jessop’s concept, Anugwom et al., reported a switchable ionic liquid comprised of glycerol and DBU to capture acidic gases such as CO2 and sulfur dioxide (SO2).33 However, glycerol can undergo dehydration reaction in basic media34 together with the formation of cyclic carbonates35 which might limit its practical usage for CO2 capturing in the presence of DBU. In order to eliminate the possibility of side reactions of multi-functionalized alcohol together with increasing the potential sorption capacity of CO2 compared with other alcohol/DBU mixtures reported elsewhere in the literature,18,19,33 we have chosen to examine a multi-armed hydroxyl-terminated organic moiety, viz., pentaerythritol (PE, Scheme 1E) due to the lack of β-H adjacent to the hydroxyl groups. PE is a commercially available small molecule that was synthesized by Tollens and Wigand in 1891.36 The ability of PE to participate in condensation reactions was utilized to build up several structural motifs for CO2 fixation.37,38 Herein, to the best of our knowledge, this is the first report on exploiting PE as a small molecular sorbent for CO2 capturing in the presence of DBU through alkyl carbonate formation. The chemisorption of CO2 by the tetrafunctionalized substrate was investigated by using nuclear magnetic resonance (NMR) and ex situ attenuated total reflectance-Fourier transform infra-red (ATR-FTIR). The sorption capacity was measured volumetrically using in situ ATR-FTIR. Furthermore, DFT was used to understand the mechanism and energetics of CO2 sorption at different molar ratios between PE and DBU.

Scheme 1. Chemical structures of: A. Carbamate-amine adduct; for MEA: R1 = H, R2 = CH2OH. B. Bicarbonateamine adduct. C. Chitin-acetate oligomer (x and y are 0.6 and 0.4, respectively). D. 1,8-diazabicyclo-[5,4,0]undec-7-ene (DBU). E. Pentaerythritol (PE). MATERIALS AND METHODS Chemicals Unless otherwise stated, all chemicals were handled under Schlenk line. PE and DBU were purchased from Sigma-Aldrich. Dimethyl sulfoxide-d6 (DMSO-d6, 99.5+% atom D) was purchased from ACROS Organics. DMSO used for ex situ ATR-IR measurements was purchased from M-TEDIA, while the wet and dry (anhydrous) grades used for volumetric uptake measurements were obtained from Grüssing GmbH and Sigma Aldrich, respectively. CO2 (99.95%, Food Grade) was purchased from Advanced

Technical Gases Co. (Amman, Jordan). Methanol (MeOH) and ethanol (EtOH) were purchased from Sigma-Aldrich. Instruments Solution 1H and 13C NMR spectra were collected at room temperature using (AVANCE-III 400 MHz (1H: 400 MHz, 13 C: 100 MHz)) FT-NMR NanoBay spectrometer (Bruker, Switzerland) in DMSO-d6. In situ attenuated total reflectance-Fourier transform infra-red (ATR-FTIR) measurements were carried out using a MMIR45m RB04-50 (Mettler-Toledo, Switzerland) with an MCT Detector, and a silicon window probe connected via pressure vessel; Sampling 3500 to 650 cm−1 at 8 wavenumber resolution; scan


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option: 64; gain: 1x. Ex situ ATR-FTIR spectra were recorded on a Bruker Vertex 70-FT-IR spectrometer at room temperature coupled with a Vertex Pt-ATR accessory. pH measurements were obtained via RL 150-Russel pH meter. Conductivity measurements were carried out using 712 conductometer (Metrohm, Switzerland). Water content was determined using a Karl-Fischer titrator (TZ 1753 with Diaphragma, KF1100, TitroLine KF). Computational Method Calculations were performed within Gaussian 09.39 The full optimization was performed using DFT method (B3LYP/6-31+G*). Different starting geometries were considered for each system. Minima were characterized by the absence of imaginary frequencies. A polarizable continuum model was used for implicit solvent calculations. pKa calculations were performed in DMSO according to literature.40 EXPERIMENTAL PROCEDURE NMR In an NMR tube, 30 mg of the PE was dissolved in 0.5 mL DMSO-d6. Upon dissolution, CO2 was bubbled into the NMR tube via a long cannula for 20 minutes. A white precipitate was formed as a result of [DBUH+][-O2COH, which settles down in the NMR tube. In situ ATR-FTIR Dry or wet solvents were used according to the run of interest. A 1:4 molar solution of PE/DBU was prepared by dissolving 0.5 g of PE in 5 mL DMSO. Similarly, an appropriate amount of DBU (2.2 mL) was introduced in 5 mL DMSO. Both solutions were mixed and sonicated till complete dissolution occurred, then transferred into the ATR-FTIR autoclave. CO2 was introduced at 25 °C, the drop-in pressure was measured while scanning every 15 seconds until a constant value (bars) was reached. Initial and final pressures for each run are shown in Table 1. For correction purposes, CO2 was purged into 10 mL DMSO in the ATR-FTIR autoclave, the solvent contribution through physisorption of CO2 was measured at the centered peak of 2337 cm−1. For wet samples, the chemisorbed CO2 upon bicarbonate formation was measured within the blank run and corrected accordingly, all values are reported in Table 1. For both MeOH and EtOH, 4 moles of the alcohol were mixed with 2.2 mL of DBU using the previously mentioned procedures. RESULTS AND DISCUSSION In this work, DMSO was chosen as a polar aprotic, high dielectric constant, non-volatile solvent to solubilize PE (a white crystalline solid that melts at ~ 253-258 °C) and to stabilize the anticipated organic carbonate adduct (if any)

upon bubbling CO2.41 As an aside, the high boiling point of DMSO (b.p. 189 °C) and of DBU (b.p. 261 °C) is beneficial upon sorbent regeneration to avoid evaporation losses, which decreases the operational costs and enable greener and sustainable media for such process. Moreover, one inherent limitation associated with the aqueous MEA solution upon regeneration is the high specific heat of water (4.18 J.g-1.K-1). It is anticipated that the use of DMSO which has smaller specific heat (1.96 J.g-1.K-1) will be less energy consuming. 13 C-NMR spectra of the neat PE dissolved in DMSO-d6 were similar before and after bubbling CO2 with a difference of emergence of a peak at ca. 124.7 ppm, that corresponds to physisorbed CO2 (Figure 1A), which highlights the importance of activating the substrate toward nucleophilic attack prior the exposure to CO2 using an auxiliary base, viz., DBU. Notably, bubbling the base dissolved in DMSO-d6 (used as received with no drying attempted) resulted in a white precipitate, due to the reaction between DBU and CO2 in the presence of water to form amidinium bicarbonate salt ([DBUH+][-O2COH]), as an insoluble white precipitate.42 The 13C-NMR of the precipitate dissolved in D2O is shown in the Supplementary Information (SI, Figure S1). This explained the absence of the carbon dioxide peak (~ 124.7 ppm) in the 13C-NMR spectrum of the DBU solution (Figure 1B), presumably due to the salting out effect. DBU alkyl-carbonate adduct was synthesized by bubbling CO2 through a 1:4 molar mixture of PE/DBU dissolved in DMSO at room temperature as shown in Scheme 2.

Scheme 2. Proposed chemical reaction between PE and CO2 in the presence of DBU assuming 1:4 reaction stoichiometry between PE and CO2. The 13C-NMR spectrum of the mixture (Figure 1C) showed the emergence of a new peak at 157.4 ppm together with a peak shifted from 160.5 to 165.0 ppm upon bubbling CO2. While the former can be explained by the formation of organic carbonato species,20,24,26 the latter shifted peak can be attributed to the bridgehead amidinium carbon of DBUH+ upon complexation to the PEcarbonato anion. A similar, chemical shift was reported by Phan et al. after bubbling CO2 through 1-hexanol-DBU binary mixture.43


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Figure 1. C-NMR spectra of A. PE. B. DBU and C. 1:4 PE/DBU dissolved in DMSO-d6 before (black) and after (red) bubbling ° CO2 for 20 minutes at 25 C. 1

H-NMR spectra were measured for PE/DBU binary mixture before and after bubbling CO2 (Figure 2). Upon bubbling, the spectrum of the binary mixture showed a broadened peak centered at 10.8 ppm due to NH proton transfer "shuttling" between of the amidinium cation and the organic carbonate upon hydrogen bonding, which verified the chemisorption of CO2. To the best of our knowledge, this is the first experimental proof for proton shuttling phenomenon for CO2-capturing in non-aqueous solvent as reported vey recently by Cantu et al.44

Figure 3. Partial ATR-FTIR spectra of PE/DBU dissolved in DMSO before (black) and after bubbling (red) with CO2.

1

Figure 2. Partial H-NMR spectra in DMSO-d6 of PE/DBU (1:4) mixture before (black) and after (red) bubbling CO2, respectively.

The formation of alkyl-carbonate was further explored using ex situ ATR-FTIR spectroscopy. In agreement with NMR measurements, the spectra of the neat materials indicated that neither PE nor DBU reacted with CO2 (Figure S2A and B), although a white precipitate was formed when DBU was exposed to CO2 (vide supra). The ATRFTIR spectra of the reaction mixture before and after bubbling CO2 (Figure 3) fortified the chemisorption of CO2 through the appearance of two peaks at 1670 and 1630 cm-1 ascribed to the asymmetric stretching of C=O (in the acidic form as a result of proton shuttling, vide infra) and the asymmetric stretching of O...C...O− within the organic carbonate anion, respectively.44

The interaction between PE/DBU binary system with CO2 was further investigated by monitoring the variation in temperature, pH and conductance as a function of bubbling time as shown in Figure 4. Expectedly, the reaction is exothermic. The pH of the solution was decreased to a minimum of 14.88 and the conductance was increased to a maximum of 2.3 mS throughout the experiment progress. On one side, the increase in conductance might be attributed to the weaker ion pairing tendency due to increasing the bulkiness of the counter anion after the chemisorption of CO2 rather than increasing the ionic mobility.45 On the other side, the decrease in pH over time pointing a proton transfer from the amidinium cation to the organic carbonato species as verified by DFT calculations (vide infra). Subsequently, as the solution temperature went down due to reaction completion, the pH and conductance values raised and dropped down, respectively.


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Figure 4. Reaction temperature, conductance and pH of 1:4 PE/DBU mixture dissolved in DMSO as a function of CO2 time.

Similarly to a previously reported protocol of volumetric uptake experiment,24,25 the sorption capacity was evaluated using an in situ ATR-FTIR autoclave by following the evolution of the asymmetric stretching frequency of O=C=O at 1630 cm-1 as function of time as shown in Figure 5A. The amount of chemisorbed CO2 was calculated by plugging the drop-down in pressure (recorded by a digital manometer) in the equation of state of the perfect gas ( ). Correction with respect to solvent was carried out using neat DMSO (physisorbed). Table 1 shows the sorption capacity at different conditions. The uptake of a 5.0 % (w/v) PE solution (Run 2, Table 1) was 4.6 bar (sample calculation is shown in SI). Doubling the CO2 pressure increased the amount of physisorbed CO2 by two-fold (Run 1 and 3, respectively), as well as the sorption capacity by ca. 9 %., which can be attributed to the enhanced physisorption at higher pressure. Unex-

pectedly, the removal of water from the system should decrease the sorption capacity (due to the extra contribution of [DBUH+][-O2COH], on the contrary, the increased sorption could be presumably explained by the more favorable physisorption over chemisorption at dry conditions. From a comparison point of view, dry MeOH based system (Run 7, Table 1) gave almost the same sorption capacity as the wet PE based system (Run 2, Table 1), which emphasizes the idea of using our system for wet scrubbing in PCC without drying or volatility losses which is a major concern as in the case of MeOH. Although EtOH/DBU neat system has the previously mentioned limitation (volatility issues), it gave a sorption capacity of 21.88% (Run 8, Table 1), which might be explained by the higher nucleophilicity of the corresponding alkoxide together with the less repulsion among arms once compared to PE/DBU. Assuming a 1:4 reaction stoichiometry between PE and CO2 (Scheme 2), the theoretical sorption capacity should be 23.62 wt %. However, the volumetric uptake measurements provided smaller numbers for both wet and dry conditions at different CO2 pressure as well as sorbent concentrations. DFT calculations (Table 4, vide infra) indicated the relative equilibrium constants of the first mole of CO2 to be absorbed is much larger than the second one and so on, which justified the volumetric uptake results. The regeneration of PE/DBU binary mixture upon binding CO2 was achieved either by bubbling N2 through the reaction mixture at room temperature, or heating at 100 ̊C. The reversibility character was confirmed using 13C NMR, which indicated a massive reduction in the intensity of the chemical shift of the carbonate carbon (ca. 157 ppm) in comparison with the corresponding peak of the central carbon of [DBUH+] (ca. 165 ppm).

Figure 5. A. Partial in situ IR spectrum for the 5% (w/v) PE/DBU in DMSO as a function of time carried out at 298 K, 4.6 bar. -1 The red asterisk denotes the followed-up peak at 1630 cm Absorption profiles as a function of time: B. Physisorbed CO2 moni-1 -1 tored at 2327 cm by neat DMSO. C. Organic carbonate monitored at 1630 cm using a 2.5% (w/w) (navy) and 5.0% (blue) of PE/DBU dissolved in DMSO.

Table 1. Sorption capacity of alcohol (PE, MeOH or EtOH)/DBU dissolved in 10 mL DMSO solution measured by in-situ ATR-FTIR autoclave. Applied conditions: = 298 K, = 50.0 mL. The amount of CO2 absorbed by the binary system was calculated using the equation of state of the ideal gas . The contribution of DMSO was taken in consideration while calculating the sorption capacity. Sample calculation is shown in SI. Run

Sorbent

Sorbed CO2 (bar)

Sorption capacity (wt%)

1

DMSO (as received)

4.6

3.2

1.4

14.66


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2

PE/DBU/DMSO (as received)

4.6

0

4.6

3

DMSO (as received)

10.8

8.0

2.8

4

PE/DBU/DMSO (as received)

10.6

5.6

5.0

5

DMSO (dry)

10.8

7.8

3.0

6

PE/DBU/DMSO (dry)

10.6

4.8

5.8

7

MeOH/DBU/DMSO (dry)

10.6

5.4

5.2

15.77

8

EtOH/DBU/DMSO

10.4

5.0

5.4

21.88

15.93 a

c

18.48

a

b

a

Water content measured by Karl-Fisher titrator was 2.6 ppm. Water content measured by Karl-Fisher titrator was 10.2 ppm. c Water content measured by Karl-Fisher titrator was 22.6 ppm. b

In comparison with CO2 binding organic liquids (CO2BOLs), the PE/DBU binary system required only 5.0 % PE, while neat liquids are needed in the case of CO2BOLs (Table 2), which will be reflected on the total cost of the sorbent used versus its abundance. It is noteworthy that we used diluted PE/DBU solutions, which will lower the corrosive character once compared to neat alcohols/DBU mixtures. For example, the concentration of DBU required to prepare a 1:1 hexanol/DBU using a 10 mL hexanol (as the active material within the sorbent that bears CO2) is 122% (w/v) compared to 22% (w/v) needed to make a 5.0% (w/v) solution of PE in DMSO of the same volume. Furthermore, CO2BOLs are more volatile, and thus higher losses upon regeneration are expected. In our system, PE possesses low vapor pressure (b.p. = 270 °C, 30 mmHg) which makes it useful for the regeneration process e.g., upon heating. In addition, the used of DMSO as green solvent with a high boiling point is another advantage compared to other commercial solvents.46 An

extra merit can be drawn for the studied material which is it’s utilization without further purification. Extra precautions should be taken into consideration in the case of CO2BOLs. In this context, Heldebrant et al.18 reported that the equilibrium constant for the reaction between H2O, DBU and CO2 with respect to that of MeOH (Keq(H2O)/Keq(MeOH)) was 1.43 which emphasize that the formation of [DBUH+][-O2COH] is more preferred over the alkyl carbonate. Once compared to EtOH (taken as a model due to its possession of an equivalent number of carbons as in one arm of PE), Keq(ROH)/Keq(H2O) value is larger for PE based system, this makes our system much better coping in the presence of water once compared to CO2BOLs. One aspect that was superior to CO2BOLs, is the sorption capacity of different alkylcarbonates as measured by 1H NMR from the reaction of a set of primary alcohols, DBU and CO2, which increases from 17.3 to 20.7% as a function of the molar mass of the homologous series.18,19 In our case it was ca. 18.5 wt%.

Table 2. A comparison between the CO2 binding organic liquids (CO2BOLs) and PE/DBU binary system

Concentration

Superbase concentration

a

Jessop’s system

PE/DBU in DMSO (This work)

Neat liquids

5.0 wt%

376% (w/v) (relative to MeOH) 22% (based on 5 wt% solution) 122% (w/v) (relative to HexOH) More volatile

b

Less volatile due to the use of DMSO (b.p. = 189°C)

Volatility

Drying and ease of handling Keq(ROH)/Keq(H2O) Sorption capacity

c

(alcohols' b.p. < 160 °C)

PE is a solid material with high m.p. (253-258 °C)

Dry conditions, requires extra precautions

DMSO was used as received (wet)

0.44

d

Average of 19%


0.59

e

ca. 18.5%

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(volumetric)

a

A volume of 10 mL n-alcohol (Jessop's) or DMSO (this work). Comparison of boiling points of liquids used is evaluated from C1 to C6 of aliphatic n-alcohols. c Based on DFT Calculations (this work). d Keq(EtOH)/Keq(H2O) value for ethanol. e Keq(ROH)/Keq(H2O) value for the first absorption for PE. twice that for the formation of PE-(OH)1. Furthermore, DFT calculations the pKa values were calculated as 19.85, 32.37, 37.07, and The first mechanistic step for the CO2 capture using al49.11 for PE-(OH)1, PE-(OH)2, PE-(OH)3, and PE-(OH)4, cohols, to form the organic carbonato-species, requires respectively. Therefore, the order of reactivity towards the formation of an alkoxide using a strong base, viz., CO2 capturing follows this order: PE-(OH)1>PEDBU. For PE, all hydroxyl groups are equivalent, and (OH)2>PE-(OH)3>PE-(OH)4. Interestingly, PE-(OH)1 upon reacting with DBU, they give equivalent alkoxide shows lower basicity (lower pKa value in DMSO) once compared to ethanolate and hydroxide, which clearly anions (black trace, Figure 2). We have accessed the baindicates that PE (at least the first arm) has stronger abilsicity of each hydroxyl group through calculating the gasity to lose the proton and capture CO2 in DMSO once phase proton affinity (PA) and the pKa value in DMSO (Table 3). For comparison, PA and pKa values of DBU-H+ compared to EtOH and H2O. For CO2 capturing purposes, (conjugated acid of DBU), H2O, MeOH and EtOH were the consecutive formation of the first carbonate adduct also calculated. The calculated pKa values were in a good (PE-(CO2)1, vide infra) makes the next attack (if any) over agreement with experimental data. The PE-(OH)4 shows CO2 less susceptible towards nucleophilic attack on the the highest PA value of 631.9 kcal mol-1 which is almost electrophilic center of CO2. b

Table 3. Calculated gas-phase proton affinitiesa (PAs) and pKa valuesb in DMSO for different substrates. 1

+

PA/ kcal mol−

pKa

247.66

12.95 (12)

382.96

27.94 (31.4)

373.20

28.00 (29.0)

369.88

28.11 (29.8)

336.65

19.85

449.39

32.37

528.57

37.07

631.90

49.11

+

DBU

DBU + H → DBUH

H2O

OH− + H → H2O

MeOH

CH3O− + H → CH3OH

EtOH

CH3CH2O− + H → CH3CH2OH

PE-(OH)1

C(CH2OH)3(CH2O)− + H → C(CH2OH)4

PE-(OH)2

C(CH2OH)2(CH2O)2 − + H → C(CH2OH)3(CH2O)−

PE-(OH)3

C(CH2OH)(CH2O)3 − + H → C(CH2OH)2(CH2O)2

PE-(OH)4

C(CH2O)4 − + H → C(CH2OH) (CH2O)3

+

+

+

+

2

3

4

+

+

+

2−

3−

c

d

d

d

a

PA was calculated by using the Gaussian 09 software (B3LYP/6-31+G* level of theory) as the negative of the enthalpy change (ΔH) of the gas phase reaction, A−(g) + H+(g) → AH (g). Under standard conditions, the value of the enthalpy of the gas-phase proton was taken as 1.48 kcal mol−1.47 b Calculated pKa values for the conjugated acids in water, see computational method for details. c Taken from an online document provided by D. A. Evans and coworkers, Harvard University.48 d Taken from Ref.49

Thermodynamics for the CO2 capturing by PE, MeOH, and EtOH, as well as H2O, were calculated in both gas phase and DMSO to mimic our experimental results. Table 4 shows the thermodynamic parameters relative to tert-butanol. In general, the reaction shows favorable enthalpic contributions (ΔH) associated with large entropic penalties (TΔS) (see Table 4 and SI for the absolute values). The change in the Gibbs free energy (ΔG) indi-

cated that the consecutive CO2 capturing become less favorable, viz., PE-(CO2)1 is the most favored adduct to form carbonate (Table 4). In gas phase, PE-(CO2)1 showed a more favorable ΔG value compared to EtOH, but less favorable compared to H2O, and MeOH by only 0.66, 0.75 kcal mol−1, respectively. In DMSO, the trend was different due to its higher dielectric constant compared to gas phase, in which H2O shows the lowest free energy, fol-


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lowed by MeOH, EtOH, and PE-(OH)1. The same observation was obtained experimentally for H2O and methanol by Jessop and coworker.18 tert-Butanol showed the least favorable ΔG value, which is known to be so experimentally, due to both steric and electronic effects.18 Table 4. Calculated thermodynamic parametersa (in both gas phase and DMSOb) for the capture of CO2 by PE, H2O, MeOH and EtOH, as well as tert-Butanol (tert-BuOH) according to the following reaction: ROH+CO2+DBU→ → [DBUH+] [−O2COR] a

a

a

ΔH

−TΔS

ΔG

PE-(CO2)1

−4.18 (−5.09)

−0.76 (−0.59)

−4.94 (−5.68)

PE-(CO2)2

−4.05 (−5.28)

−0.34 (-0.20)

−4.39 (−5.48)

PE-(CO2)3

−3.32 (−5.2)

−0.39 (−0.07)

−3.71 (−5.27)

PE-(CO2)4

−1.33 (−3.95)

−0.21 (0.13)

−1.54 (−3.82)

H2O

−3.80 (−4.50)

−1.80 (−2.15)

−5.6 (−6.65)

MeOH

−4.81 (−5.79)

−0.88 (−0.48)

−5.69 (−6.27)

EtOH

−4.28 (−5.30)

−0.50 (−0.94)

−4.78 (−6.24)

tert-BuOH

0.00 (0.00)

0.00 (0.00)

0.00 (0.00)

Page 8 of 12

Optimization of the possible adduct(s) formation of the ionic organic alkylcarbonate anions on each arm of PE’s hydroxyl groups and their complexes with BDUH+ were calculated in both gas phase and DMSO. The optimized structures in gas phase are shown in Figure 6 A-D. No significant structural changes were observed when going from gas phase to DMSO. In the case of PE-(CO2)3 and PE-(CO2)4, the depicts of the tri- and tetra-carbonato species showed the occurrence of proton shuttling phenomenon when DMSO was defined as the implicit solvent in the calculations (see Figure 6F, which did not exist when calculation were performed in the gas phase, 6E). Similar result was observed experimentally by 1H NMR (red trace, Figure 2, broadening of peak corresponding to the amidinium cation-carbonic acid ca. 10 ppm is due to hydrogen bonding), together with a supporting evidence using an ATR-FTIR spectrum (Figure 3) with a centered peak at 1670 cm-1 as a result of carbonic acid contribution. The same phenomenon was reported by Cantu et al.44 via a single component CO2BOL using ab initio molecular dynamics simulation.44

a Values are given relative to tert-Butanol, per CO2 units in kcal mol−1. Values in DMSO are given in parentheses.

+

Figure 6. DFT-optimized structures of PE carbonato: DBUH adducts in the gas phase: A. PE-(CO2)1, B. PE-(CO2)2, C. PE-(CO2)3, D. PE-(CO2)4, respectively. Proton shuttling between the carbonate anion (PE-(CO2)3) and the amidinium cation counterpart


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(circled in the blue ellipse) in gas phase (E) and DMSO (F). Distances between proton-nitrogen and proton-oxygen are shown in blue and red, respectively.

CONCLUSIONS A new binary system comprised of 5.0% (w/v) pentaerythritol (PE) in the presence of 1,8-diazabicyclo-[5,4,0]undec-7-ene (DBU) (1:4 molar ratio) dissolved in dimethyl sulfoxide (DMSO) was reported for CO2 capturing. The formation of ionic alkyl organic carbonate ([DBUH+] [−O2COR]) was confirmed using 13C NMR, ex situ ATRFTIR. Further, the charged basic species was confirmed using a thermostatted conductivity and pH meter couples. The sorption capacity of 5.0 % PE (w/v) solution was measured volumetrically with high efficiency using an ATR-FTIR autoclave. In addition, density functional theory (DFT) was used to justify the reason behind the different reactivity of all arms towards CO2 capturing both in gas phase and DMSO. For the [DBUH+] [−O2COR], different substrates were taken into consideration for comparison reasons, viz., PE together with H2O, and simple alcohols, as in MeOH and EtOH. Moreover, we report on the newly-discovered proton shuttling, medium-dependent phenomenon, that was verified both experimentally and theoretically.

nuclear magnetic resonance; ATR-FTIR, attenuated total reflectance-Fourier transform infra-red; DFT, density functional theory; CCS, carbon capture and storage or sequestration; CCU, carbon capture and utilization; MEA, monoethanolamine; NH3, ammonia; MOFs, metal organic frameworks; SO2, sulfur dioxide; MeOH, methanol; EtOH, ethanol; CO2BOLs, CO2 binding organic liquids; Keq, equilibrium constant; PA, proton affinity; ΔH, enthalpy, ΔS, entropy; ΔG, Gibbs free energy.

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ASSOCIATED CONTENT Supporting Information. 13 C-NMR spectrum of DBU dissolved in DMSO-d6 and + − [DBUH ] [ O2COH] dissolved in D2O after washing with DMSO. Partial ATR-FTIR spectra of PE and DBU dissolved in DMSO before and after bubbling with CO2. Sample Calculation of Sorption Capacity of 5% (w/v) PE solution (Run 2). Calculated thermodynamic parameters for the capture of CO2 by PE and the selected alcohols in the gas phase and DMSO (in parentheses) according to the following reaction: − + ROH+CO2+DBU→ROCO2 ●DBUH . This material is available free of charge via the Internet at http://pubs.acs.org.”

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AUTHOR INFORMATION Corresponding Author

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- AKQ, [email protected] - KIA, [email protected] - AFE, [email protected]; [email protected] (9)

Funding Sources Financial support has been provided by the Deanship of Scientific Research at the Hashemite University.

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ACKNOWLEDGMENT AFE acknowledges the Deanship of Scientific Research at the Hashemite University and the Binational Fulbright Commission (BFC) in Jordan. Marina Reiter (TUM, Germany) is acknowledged for performing the volumetric uptake measurements using the in situ ATR-FTIR autoclaves.

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ABBREVIATIONS PE, pentaerythritol; PCC, postcombustion capture; DBU, 1,8diazabicyclo-[5,4,0]-undec-7-ene; DMSO, dimethyl sulfoxide; + − [DBUH ] [ O2COR], ionic alkyl organic carbonate; NMR,

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Pentaerythritol-Based Molecular Sorbent for CO2 Capturing: A Highly

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