8
Pentaerythritol-Based Molecular Sorbent for CO 2 Capturing: A Highly Ecient Wet Scrubbing Agent Showing Proton Shuttling Phenomenon Abdussalam K. Qaroush,* ,Khaleel I. Assaf,* ,Alaa Al-Khateeb, § Fatima Alsoubani, § Enas Nabih, § Carsten Troll, Bernhard Rieger, and Alaa F. Eftaiha* ,§,Department of Chemistry, Faculty of Science, The University of Jordan, Amman 11942, Jordan Department of Life Sciences and Chemistry, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany § Department of Chemistry, The Hashemite University, P.O. Box 150459, Zarqa 13115, Jordan WACKER-Lehrstuhl fü r Makromolekulare Chemie, Technische Universitä t Mü nchen, Lichtenbergstraße 4, 85747 Garching bei Mü nchen, Germany Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106-9510, United States * S Supporting Information ABSTRACT: Pentaerythritol (PE) is considered a biodegradable material that combines the ease of synthesis, nonvolatility, and extra stability under basic conditions (acidic gas sequestration, e.g., CO 2 ), which makes it a useful candidate for postcombustion capture (PCC) application. To overcome corrosion problems associated with CO 2 binding organic liquids, 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 CO 2 capturing. The formation of ionic alkyl organic carbonate (RCO 3 DBUH + ) was conrmed using 13 C NMR (157.4 ppm) and ex situ attenuated total reectanceFourier transform infrared spectroscopy (ATR-FTIR) (two peaks were identied, viz., 1670 and 1630 cm 1 , which were ascribed to the symmetric and asymmetric stretching of both CO and O ··· C ··· O within RCO 3 H and RCO 3 , respectively). The charged adduct was measured using a thermostated beaker coupled with conductivity and pH meter probes. The sorption capacity of a 5.0% PE (w/v) solution was measured volumetrically with high eciencies 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 H 2 O, and simple alcohols, e.g., methanol and ethanol. Moreover, we reported on the newly discovered medium-dependent proton shuttling phenomenon that was veried experimentally and theoretically. C arbon dioxide (CO 2 ), 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 CO 2 concentration to an unprecedented level (up to 430 ppm in 2060) 3 makes boosting CO 2 capturing strategies, viz., precombustion, postcombustion, oxy-fuel com- bustion, and electrochemical separation, necessary to guarantee a sustainable and resilient future. For governmental and industrial sectors, postcombustion capture (PCC) is of particular interest because it does not require additional capital investments in comparison with other sequestration strategies (see the review by Kenarsari et al. 1 and others 4,5 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 CO 2 capturing due to its eectiveness toward dilute CO 2 streams and commercial availability. 911 The chemisorption of CO 2 by aqueous amine-based solvents takes place through the formation of the carbamate anion following a 1:2 mechanism, viz., 1 mol of CO 2 reacts with 2 mol of amine functional groups, or the formation of inorganic bicarbonate ion via a 1:1 mechanism (chemical structures are shown in Scheme 1A,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 absorptiondesorption cycles. 12,13 Ammonia (NH 3 ) scrubbing has several merits over amine-based sorbents in terms of cost, loading capacity and less-corrosive character; however, the high volatility of NH 3 addresses several technical problems. 14,15 This highlights the potential importance of implementing other alternative technologies such as solid sorbents 16 and membranes 17 and paving 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 CO 2 selectively and reversibly at low pressures by the free hydroxyl group(s) of the glucopyranose ring. 2023 Very recently, our group has reported on the supramolecular chemisorption of CO 2 through organic alkyl carbonate formation, adopting a benign-by-design approach using green Received: April 20, 2017 Revised: June 30, 2017 Published: July 3, 2017 Article pubs.acs.org/EF © XXXX American Chemical Society A DOI: 10.1021/acs.energyfuels.7b01125 Energy Fuels XXXX, XXX, XXXXXX

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  • Pentaerythritol-Based Molecular Sorbent for CO2 Capturing: A HighlyEfficient Wet Scrubbing Agent Showing Proton ShuttlingPhenomenonAbdussalam K. Qaroush,*,† Khaleel I. Assaf,*,‡ Ala’a Al-Khateeb,§ Fatima Alsoubani,§ Enas Nabih,§

    Carsten Troll,∥ Bernhard Rieger,∥ and Ala’a F. Eftaiha*,§,⊥

    †Department of Chemistry, Faculty of Science, The University of Jordan, Amman 11942, Jordan‡Department of Life Sciences and Chemistry, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany§Department of Chemistry, The Hashemite University, P.O. Box 150459, Zarqa 13115, Jordan∥WACKER-Lehrstuhl für Makromolekulare Chemie, Technische Universitaẗ München, Lichtenbergstraße 4, 85747 Garching beiMünchen, Germany⊥Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106-9510, United States

    *S Supporting Information

    ABSTRACT: Pentaerythritol (PE) is considered a biodegradable material that combines the ease of synthesis, nonvolatility, andextra stability under basic conditions (acidic gas sequestration, e.g., CO2), which makes it a useful candidate for postcombustioncapture (PCC) application. To overcome corrosion problems associated with CO2 binding organic liquids, a binary mixturecomprised of PE/1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU) (1:4 molar ratio) dissolved in dimethyl sulfoxide (DMSO) wasexploited for CO2 capturing. The formation of ionic alkyl organic carbonate (RCO3

    − DBUH+) was confirmed using 13C NMR(157.4 ppm) and ex situ attenuated total reflectance−Fourier transform infrared spectroscopy (ATR-FTIR) (two peaks wereidentified, viz., 1670 and 1630 cm−1, which were ascribed to the symmetric and asymmetric stretching of both CO andO···C···O− within RCO3H and RCO3

    −, respectively). The charged adduct was measured using a thermostated beaker coupledwith conductivity and pH meter probes. The sorption capacity of a 5.0% PE (w/v) solution was measured volumetrically withhigh 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 newly discovered medium-dependent proton shuttling phenomenon that was verifiedexperimentally and theoretically.

    Carbon dioxide (CO2), is considered to be the mostprominent greenhouse gas, which is attributed as the maincause of anthropogenic climate change.1,2 The expected increasein the atmospheric CO2 concentration to an unprecedented level(up to 430 ppm in 2060)3 makes boosting CO2 capturingstrategies, viz., precombustion, postcombustion, oxy-fuel com-bustion, and electrochemical separation, necessary to guarantee asustainable and resilient future. For governmental and industrialsectors, postcombustion capture (PCC) is of particular interestbecause it does not require additional capital investments incomparison with other sequestration strategies (see the reviewby Kenarsari et al.1 and others4,5 for more details). In this regard,carbon capture and storage or sequestration (CCS)6 togetherwith carbon capture and utilization (CCU)7 are potentialapproaches to mitigate global warming.8

    Amine scrubbing via monoethanolamine (MEA) is the mostmature technology for CO2 capturing due to its effectivenesstoward dilute CO2 streams and commercial availability.

    9−11 Thechemisorption of CO2 by aqueous amine-based solvents takesplace through the formation of the carbamate anion following a1:2 mechanism, viz., 1 mol of CO2 reacts with 2 mol of aminefunctional groups, or the formation of inorganic bicarbonate ionvia a 1:1 mechanism (chemical structures are shown in Scheme

    1A,B). There are several drawbacks associated with amine-basedsorbents such as corrosiveness, toxicity, intensive energyrequired to regenerate aqueous amine solutions, and chemicaldegradation upon successive absorption−desorption cycles.12,13Ammonia (NH3) scrubbing has several merits over amine-basedsorbents in terms of cost, loading capacity and less-corrosivecharacter; however, the high volatility of NH3 addresses severaltechnical problems.14,15 This highlights the potential importanceof implementing other alternative technologies such as solidsorbents16 and membranes17 and paving the way to less energydemanding scrubbing agents upon regeneration through theformation of ionic organic carbonates.18,19 In this context, Sir J.Fraser Stoddart (Chemistry Nobel Laureate, 2016) reported thatcyclodextrin based metal organic frameworks (CD-MOFs)chemisorbed CO2 selectively and reversibly at low pressures bythe free hydroxyl group(s) of the glucopyranose ring.20−23 Veryrecently, our group has reported on the supramolecularchemisorption of CO2 through organic alkyl carbonateformation, adopting a benign-by-design approach using green

    Received: April 20, 2017Revised: June 30, 2017Published: July 3, 2017

    Article

    pubs.acs.org/EF

    © XXXX American Chemical Society A DOI: 10.1021/acs.energyfuels.7b01125Energy Fuels XXXX, XXX, XXX−XXX

    pubs.acs.org/EFhttp://dx.doi.org/10.1021/acs.energyfuels.7b01125

  • chemistry guidelines by applying safer chemicals and the use ofbiorenewables, namely, chitin acetate oligomer dissolved indimethyl sulfoxide (DMSO). The formation of chitin-CO2adduct (Scheme 1C) was confirmed experimentally and verifiedby density functional theory (DFT) calculations. The use ofDMSO (aprotic and hydrogen bond acceptor solvent) wasnecessary to activate the primary hydroxyl group (C-6) of theammonium/amide pyranose repeating units to become moresusceptible toward nucleophilic attack.24,25

    Unlike the previously discussed structural motifs, small organicmolecules combine facile synthesis, well-defined chemicalstructure and the ease of chemical analysis of intermediatesand reaction products. Phillip Jessop’s research group reportedthat exposing a mixture of 1-hexanol and 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU, Scheme 1D) to the atmosphericCO2 resulted in the formation of amidinium hexylcarbonate([DBUH+][−O2COHex]) adduct. The reaction mixture couldbe regenerated by applying external stimuli such as bubblingnitrogen or heating.26 This reversible reaction with CO2 wasexploited to engineer a wide spectrum of species that canreversibly change their ionic character for different applicationssuch as solvents,27 solutes,28 surfactants,29 coagulatable/redispersible polymers,30 catalysts,31 sensors,32 etc. FollowingJessop’s concept, Anugwom et al. reported a switchable ionicliquid comprised of glycerol and DBU to capture acidic gasessuch as CO2 and sulfur dioxide.

    33 However, glycerol can undergodehydration reaction in basic media34 together with theformation of cyclic carbonates,35 which might limit its practicalusage for CO2 capturing in the presence of DBU. In order toeliminate the possibility of side reactions of multifunctionalizedalcohol together with increasing the potential sorption capacityof CO2 compared with other alcohol/DBU mixtures reportedelsewhere in the literature,18,19,33 we have chosen to examine amultiarmed hydroxyl-terminated organic moiety, viz., pentaery-thritol (PE, Scheme 1E) due to the lack of β-H adjacent to thehydroxyl groups. PE is a commercially available small moleculethat was synthesized by Tollens and Wigand in 1891.36 Theability of PE to participate in condensation reactions was utilizedto build up several structural motifs for CO2 fixation.

    37,38

    To the best of our knowledge, this is the first report onexploiting PE as a small molecular sorbent for CO2 capturing inthe presence of DBU through alkyl carbonate formation. Thechemisorption of CO2 by the tetra-functionalized substrate wasinvestigated by using nuclear magnetic resonance (NMR) and exsitu attenuated total reflectance-Fourier transform infrared(ATR-FTIR). The sorption capacity was measured volumetri-cally using in situ ATR-FTIR. Furthermore, DFT was used tounderstand the mechanism and energetics of CO2 sorption atdifferent molar ratios between PE and DBU.

    ■ MATERIALS AND METHODSChemicals. 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+% atomD) was purchased fromACROS Organics. DMSO used for ex situ ATR-IR measurements waspurchased from TEDIA, while the wet and dry (anhydrous) grades usedfor volumetric uptake measurements were obtained from GrüssingGmbH and Sigma-Aldrich, respectively. CO2 (99.95%, Food grade) waspurchased 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 atroom temperature using (AVANCE-III 400 MHz (1H: 400 MHz, 13C:100 MHz)) FT-NMR NanoBay spectrometer (Bruker, Switzerland) inDMSO-d6. In situ attenuated total reflectance-Fourier transforminfrared (ATR-FTIR) measurements were carried out using aMMIR45m RB04-50 (Mettler-Toledo, Switzerland) with an MCTdetector, and a silicon window probe connected via pressure vessel;sampling 3500 to 650 cm−1 at 8 wavenumber resolution; scan option:64; gain: 1×. Ex situ ATR-FTIR spectra were recorded on a BrukerVertex 70-FT-IR spectrometer at room temperature coupled with aVertex Pt-ATR accessory. pH measurements were obtained via an RL150-Russel pH meter. Conductivity measurements were carried outusing a 712 conductometer (Metrohm, Switzerland). Water content wasdetermined using a Karl Fischer titrator (TZ 1753 with Diaphragma,KF1100, TitroLine KF).

    Computational Method. Calculations were performed withinGaussian 09.39 The full optimization was performed using the DFTmethod (B3LYP/6-31+G*). Different starting geometries wereconsidered for each system. Minima were characterized by the absenceof imaginary frequencies. A polarizable continuum model was used forimplicit solvent calculations. pKa calculations were performed in DMSOaccording to the literature.40,41

    ■ EXPERIMENTAL PROCEDURESNMR. In an NMR tube, 30 mg of the PE was dissolved in 0.5 mL of

    DMSO-d6. Upon dissolution, CO2 was bubbled into the NMR tube via along cannula for 20 min. 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 therun of interest. A 1:4 molar solution of PE/DBU was prepared bydissolving 0.5 g of PE in 5 mL of DMSO. Similarly, an appropriateamount of DBU (2.2 mL) was introduced in 5 mL of DMSO. Bothsolutions were mixed and sonicated until complete dissolution occurredand then transferred into the ATR-FTIR autoclave. CO2 was introducedat 25 °C, and the drop-in pressure wasmeasured while scanning every 15s until a constant value (bars) was reached. Initial and final pressures foreach run are shown in Table 1. For correction purposes, CO2 was purgedinto 10 mL of DMSO in the ATR-FTIR autoclave, and the solventcontribution through physisorption of CO2 was measured at thecentered peak of 2337 cm−1. For wet samples, the chemisorbed CO2upon bicarbonate formation was measured within the blank run andcorrected accordingly, and all values are reported in Table 1. For bothMeOH and EtOH, 4 mol of the alcohol was mixed with 2.2 mL of DBUusing the previously mentioned procedures.

    Scheme 1. Chemical Structures of (A) Carbamate-Amine Adduct; forMEA: R1 =H, R2 = CH2OH. (B) Bicarbonate-Amine 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)

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  • ■ RESULTS AND DISCUSSIONIn this work, DMSOwas chosen as a polar aprotic, high dielectricconstant, nonvolatile solvent to solubilize PE (a white crystallinesolid that melts at∼253−258 °C) and to stabilize the anticipatedorganic carbonate adduct (if any) upon bubbling CO2.

    42 As anaside, the high boiling point of DMSO (bp 189 °C) andDBU (bp261 °C) is beneficial upon sorbent regeneration to avoidevaporation losses, which decreases the operational costs andenables greener and sustainable media for such process.Moreover, the smaller specific heat of DMSO compared toH2O (1.96 and 4.18 J·g

    −1·K−1, respectively) makes the use of theformer less energy consuming upon regeneration. It isanticipated that the use of DMSO which has a smaller specificheat (1.96 J·g−1·K−1) will be less energy consuming.

    13C NMR spectra of the neat PE dissolved in DMSO-d6 weresimilar before and after bubbling CO2 with a difference ofemergence of a peak at ca. 124.7 ppm, that corresponds tophysisorbed CO2 (Figure 1A), which highlights the importanceof activating the substrate toward nucleophilic attack prior theexposure to CO2 using an auxiliary base, viz., DBU. Notably,bubbling the base dissolved in DMSO-d6 (used as received withno drying attempted) resulted in a white precipitate, due to the

    reaction between DBU and CO2 in the presence of H2O to formamidinium bicarbonate salt ([DBUH+][−O2COH]), an in-soluble white precipitate.43 The 13C NMR spectrum of theprecipitate dissolved in D2O is shown in the SupportingInformation (Figure S1). This explained the absence of the

    Table 1. Sorption Capacity of Alcohol (PE,MeOH or EtOH)/DBU Dissolved in 10 mL of DMSO Solution Measured by inSitu ATR-FTIR Autoclavea

    run sorbent Pi Pfsorbed

    CO2 (bar)sorption

    capacity (wt %)

    1 DMSO (as received) 4.6 3.2 1.414.662 PE/DBU/DMSO (as

    received)4.6 0 4.6

    3 DMSO (as received) 10.8 8.0 2.815.934 PE/DBU/DMSO (as

    received)10.6 5.6 5.0

    5 DMSO (dry)b 10.8 7.8 3.018.486 PE/DBU/DMSO

    (dry)b10.6 4.8 5.8

    7 MeOH/DBU/DMSO(dry)c

    10.6 5.4 5.2 15.77

    8 EtOH/DBU/DMSOd 10.4 5.0 5.4 21.88aApplied conditions: T = 298 K, Vautoclave = 50.0 mL. The amount ofCO2 absorbed by the binary system was calculated using the equationof state of the ideal gas (PV = nRT). The contribution of DMSO wastaken in consideration while calculating the sorption capacity. Samplecalculation is shown in Supporting Information. bWater contentmeasured by Karl Fischer titrator was 2.6 ppm. cWater contentmeasured by Karl Fischer titrator was 10.2 ppm. dWater contentmeasured by Karl Fischer titrator was 22.6 ppm.

    Figure 1. 13C 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 20min at25 °C.

    Scheme 2. Proposed Chemical Reaction between PE and CO2in the Presence of DBUAssuming 1:4 Reaction Stoichiometrybetween PE and CO2

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

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

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  • CO2 peak (∼124.7 ppm) in the 13C NMR spectrum of the DBUsolution (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 atroom temperature as shown in Scheme 2.The 13C NMR spectrum of the mixture (Figure 1C) showed

    the emergence of a new peak at 157.4 ppm together with a peakshifted from 160.5 to 165.0 ppm upon bubbling CO2. While theformer can be explained by the formation of organic carbonatospecies,20,24,26 the latter shifted peak can be attributed to thebridgehead amidinium carbon of DBUH+ upon complexation tothe PE-carbonato anion. A similar, chemical shift was reported byPhan et al. after bubbling CO2 through a 1-hexanol-DBU binarymixture.44

    1H NMR spectra were measured for PE/DBU binary mixturebefore and after bubbling CO2 (Figure 2). Upon bubbling, thespectrum of the binary mixture showed a broadened peakcentered at 10.8 ppm due to NH proton transfer “shuttling”between of the amidinium cation and the organic carbonate uponhydrogen bonding, which verified the chemisorption of CO2. Tothe best of our knowledge, this is the first experimental proof forproton shuttling phenomenon for CO2-capturing in nonaqueoussolvent as reported very recently by Cantu et al.45

    The formation of alkyl-carbonate was further explored using exsitu ATR-FTIR spectroscopy. In agreement with NMRmeasure-ments, the spectra of the neat materials indicated that neither PEnor DBU reacted with CO2 (Figure S2A,B), although a whiteprecipitate was formed when DBU was exposed to CO2 (vide

    supra). The ATR-FTIR spectra of the reaction mixture beforeand after bubbling CO2 (Figure 3) fortified the chemisorption ofCO2 through the appearance of two peaks at 1670 and 1630 cm

    −1

    ascribed to the symmetric stretching of CO (in the acidic formas a result of proton shuttling, vide infra) and the asymmetricstretching of O···C···O− within the organic carbonate anion,respectively.45

    The interaction between PE/DBU binary system with CO2was further investigated by monitoring the variation intemperature, pH, and conductance as a function of bubblingtime 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 mSthroughout the experiment progress. On one side, the increase inconductance might be attributed to the weaker ion pairingtendency due to increasing the bulkiness of the counteranionafter the chemisorption of CO2 rather than increasing the ionicmobility.46 On the other side, the decrease in pH over timepointed to a proton transfer from the amidinium cation to theorganic carbonato species as verified by DFT calculations (videinfra). Subsequently, as the solution temperature went down dueto reaction completion, the pH and conductance values rose anddropped, respectively.Similarly to a previously reported protocol of volumetric

    uptake experiment,24,25 the sorption capacity was evaluated usingan in situ ATR-FTIR autoclave by following the evolution of theasymmetric stretching frequency of OCO at 1630 cm−1 as afunction of time as shown in Figure 5A. The amount of thechemisorbed CO2 was calculated by plugging the drop-down inpressure (recorded by a digital manometer) in the equation ofstate of the ideal gas (PV = nRT). Correction with respect tosolvent was carried out using neat DMSO (physisorbed). Table 1shows 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 Supporting Information).Doubling the CO2 pressure increased the amount of thephysisorbed CO2 by 2-fold (Run 1 and 3, respectively), as wellas the sorption capacity by ca. 9%., which can be attributed to theenhanced physisorption at higher pressure. Unexpectedly, theremoval of water from the system should decrease the sorptioncapacity (due to the extra contribution of [DBUH+][−O2COH],on the contrary, the increased sorption could be explained by themore favorable physisorption over chemisorption at dryconditions. From a comparison point of view, a dry MeOHbased system (Run 7, Table 1) gave almost the same sorptioncapacity as the wet PE based system (Run 2, Table 1), whichemphasizes the idea of using our system for wet scrubbing inPCC without drying or volatility losses, which is a major concern

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

    Figure 5. (A) Partial in situ IR spectrum for the 5% (w/v) PE/DBU inDMSO as a function of time carried out at 298 K, 4.6 bar. The red asterisk denotesthe followed-up peak at 1630 cm−1 absorption profiles as a function of time. (B) Physisorbed CO2monitored at 2327 cm

    −1 by neat DMSO. (C) Organiccarbonate monitored at 1630 cm−1 using a 2.5% (w/w) (navy) and 5.0% (blue) of PE/DBU dissolved in DMSO.

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  • as in the case of MeOH. Although the EtOH/DBU neat systemhas the previously mentioned limitation (volatility issues), it gavea sorption capacity of 21.88% (Run 8, Table 1), which might beexplained by the higher nucleophilicity of the correspondingalkoxide together with the less repulsion among arms oncecompared to PE/DBU. Assuming a 1:4 reaction stoichiometrybetween PE and CO2 (Scheme 2), the theoretical sorptioncapacity should be 23.62 wt %. However, the volumetric uptakemeasurements provided smaller numbers for both wet and dryconditions at different CO2 pressure as well as sorbentconcentrations. DFT calculations (Table 4, vide infra) indicatedthat the relative equilibrium constants of the first mole of CO2 tobe absorbed are much larger than the second one and so on,which justified the volumetric uptake results.

    The regeneration of PE/DBU binary mixture upon bindingCO2 was achieved either by bubbling N2 gas through the reactionmixture at room temperature, or heating at 100 °C. Thereversibility character was confirmed using 13C NMR, whichindicated a massive reduction in the intensity of the chemicalshift of the carbonate carbon (ca. 157 ppm) in comparison withthe corresponding peak of the central carbon of [DBUH+] (ca.165 ppm).In comparison with CO2 binding organic liquids (CO2BOLs),

    the PE/DBU binary system required only 5.0% PE, while neatliquids are needed in the case of CO2BOLs (Table 2), which willbe reflected on the total cost of the sorbent used versus itsabundance. It is noteworthy that we used diluted PE/DBUsolutions, which will lower the corrosive character oncecompared to neat alcohols/DBU mixtures. For example, theconcentration of DBU required to prepare a 1:1 hexanol/DBUusing a 10 mL of hexanol (as the active material within thesorbent 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 thesame volume. Furthermore, CO2BOLs are more volatile, andthus higher losses upon regeneration are expected. In our system,PE possesses low vapor pressure (bp = 270 °C, 30 mmHg),which makes it useful for the regeneration process, e.g., uponheating. In addition, the use of DMSO as green solvent with ahigh boiling point is another advantage compared to othercommercial solvents.47 Extra merit can be drawn for the studiedmaterial which is its utilization without further purification. Extraprecautions should be taken into consideration in the case ofCO2BOLs. In this context, Heldebrant et al.

    18 reported that theequilibrium constant for the reaction between H2O, DBU, andCO2 with respect to that of MeOH (Keq(H2O)/Keq(MeOH))was 1.43, which emphasizes that the formation of [DBUH+]-[−O2COH] is more preferred over the alkyl carbonate. Once

    Table 2. Comparison between the CO2 Binding Organic Liquids (CO2BOLs) and PE/DBU Binary System

    Jessop’s system PE/DBU in DMSO (this work)

    concentration neat liquids 5.0 wt %

    superbase concentrationa376% (w/v) (relative to MeOH)

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

    volatilitymore volatileb less volatile due to the use of DMSO (bp = 189 °C)(alcohols’ bp

  • compared to EtOH (taken as a model due to its possession of anequivalent number of carbons as in one arm of PE), theKeq(ROH)/Keq(H2O) value is larger for the PE based system,and this makes our system much better at coping in the presenceof water compared to CO2BOLs. One aspect that is superior toCO2BOLs is the sorption capacity of different alkylcarbonates asmeasured by 1H NMR from the reaction of a set of primaryalcohols, DBU, and CO2, which increases from 17.3 to 20.7% as afunction of the molar mass of the homologous series.18,19 In ourcase it was ca. 18.5 wt %.DFT Calculations. The first mechanistic step for the CO2

    capture using alcohols, to form the organic carbonato-species,requires the formation of an alkoxide using a strong base, viz.,DBU.41 For PE, all hydroxyl groups are equivalent, and uponreacting with DBU, they give equivalent alkoxide anions (blacktrace, Figure 2). We have accessed the basicity of each hydroxylgroup through calculating the gas-phase proton affinity (PA) andthe pKa value in DMSO (Table 3). For comparison, PA and pKavalues of DBUH+ (conjugated acid of DBU), H2O, MeOH, andEtOH were also calculated. The calculated pKa values were in agood agreement with experimental data. The PE-(OH)4 showsthe highest PA value of 631.9 kcal mol−1, which is almost twicethat for the formation of PE-(OH)1. Furthermore, the pKa valueswere calculated as 19.85, 32.37, 37.07, and 49.11 for PE-(OH)1,PE-(OH)2, PE-(OH)3, and PE-(OH)4, respectively. Therefore,the order of reactivity toward CO2 capturing follows this order:PE-(OH)1 > PE-(OH)2 > PE-(OH)3 > PE-(OH)4. Interestingly,PE-(OH)1 shows lower basicity (lower pKa value in DMSO)once compared to ethanolate and hydroxide, which clearlyindicates that PE (at least the first arm) has a stronger ability to

    lose the proton and capture CO2 in DMSO once compared toEtOH and H2O. For CO2 capturing purposes, the consecutiveformation of the first carbonate adduct (PE-(CO2)1, vide infra)makes the next nucleophilic attack less susceptible towards CO2.Thermodynamics for the CO2 capturing by PE, MeOH, and

    EtOH, as well as H2O, were calculated in both gas phase andDMSO to mimic our experimental results. Table 4 shows thethermodynamic parameters relative to tert-butanol. In general,the reaction shows favorable enthalpic contributions (ΔH)associated with large entropic penalties (TΔS) (see Table 4 andSupporting Information for the absolute values). The change inthe Gibbs free energy (ΔG) indicated that the consecutive CO2capturing becomes less favorable, viz., PE-(CO2)1 is the mostfavored adduct (Table 4). In the gas phase, PE-(CO2)1 showed amore favorable ΔG value compared to EtOH, but less favorablecompared to H2O and MeOH by only 0.66 and 0.75 kcal mol

    −1,respectively. In DMSO, the trend was different due to its higherdielectric constant compared to gas phase, in which H2O showsthe lowest free energy, followed by MeOH, EtOH, and PE-(OH)1. The same observation was obtained experimentally forH2O and methanol by Jessop and co-workers.

    18 tert-Butanolshowed the least favorable ΔG value, which is known to be soexperimentally, due to both steric and electronic effects.18

    Optimization of the possible adduct(s) formation of the ionicorganic alkylcarbonate anions on each arm of PE’s hydroxylgroups and their complexes with BDUH+ were calculated in bothgas phase and DMSO. The optimized structures in the gas phaseare shown in Figure 6A−D. No significant structural changeswere observed when going from the gas phase to DMSO. In thecase of PE-(CO2)3 and PE-(CO2)4, the depictions of the tri- and

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

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  • tetra-carbonato species showed the occurrence of a protonshuttling between the formed carbonic acid and the DBU whenchanging the medium from gas phase (E) to DMSO (F). Inwhich, the carbonic acid is formed in gas phase, while thecarbonate adduct is formed in DMSO. A similar result wasobserved 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), togetherwith a supporting evidence using an ATR-FTIR spectrum(Figure 3) with a centered peak at 1670 cm−1 as a result ofcarbonic acid contribution. The same phenomenon was reportedby Cantu et al.45 via a single component CO2BOL using ab initiomolecular dynamics simulation.45

    ■ CONCLUSIONSA new binary system comprised of 5.0% (w/v) PE in thepresence of DBU (1:4 molar ratio) dissolved in DMSO wasreported for CO2 capturing. The formation of ionic alkyl organiccarbonate ([DBUH+][−O2COR]) was confirmed using

    13CNMR, ex situ ATR-FTIR. Further, the charged basic species wasconfirmed using a thermostated conductivity and pH metercouples. The sorption capacity of 5.0% PE (w/v) solution wasmeasured volumetrically with high efficiency using an ATR-FTIRautoclave. In addition, DFT was used to justify the reason behindthe different reactivities of all arms toward CO2 capturing both inthe gas phase and DMSO. For the [DBUH+][−O2COR],different substrates were taken into consideration for comparisonreasons, viz., PE together with H2O, and simple alcohols, as inMeOH and EtOH.Moreover, we report on the newly discoveredproton shuttling, medium-dependent phenomenon, that wasverified both experimentally and theoretically.

    ■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.energy-fuels.7b01125.

    13C NMR spectrum of DBU dissolved in DMSO-d6 and[DBUH+][−O2COH] dissolved in D2O after washing withDMSO. Partial ATR-FTIR spectra of PE and DBUdissolved in DMSO before and after bubbling with CO2.Sample calculation of sorption capacity of 5% (w/v) PEsolution (Run 2). Calculated thermodynamic parametersfor the capture of CO2 by PE and the selected alcohols inthe gas phase. (PDF)

    ■ AUTHOR INFORMATIONCorresponding Authors*(A.K.Q.) E-mail: [email protected].*(K.I.A.) E-mail: [email protected].*(A.F.E.) E-mail: [email protected]; [email protected] (Fulbright Visiting Scholar).ORCIDKhaleel I. Assaf: 0000-0003-4331-8492Bernhard Rieger: 0000-0002-0023-884XAla’a F. Eftaiha: 0000-0003-4285-2546FundingFinancial support has been provided by the Deanship ofScientific Research at the Hashemite University.NotesThe authors declare no competing financial interest.

    ■ ACKNOWLEDGMENTSA.F.E. acknowledges the Deanship of Scientific Research at theHashemite University and the Binational Fulbright Commission(BFC) in Jordan. Marina Reiter (TUM, Germany) is acknowl-edged for performing the volumetric uptake measurements usingthe in situ ATR-FTIR autoclaves.

    ■ ABBREVIATIONSPE, pentaerythritol; PCC, postcombustion capture; DBU, 1,8-diazabicyclo-[5,4,0]-undec-7-ene; DMSO, dimethyl sulfoxide;[DBUH+][−O2COR], ionic alkyl organic carbonate; NMR,nuclear magnetic resonance; ATR-FTIR, attenuated totalreflectance-Fourier transform infrared; DFT, density functionaltheory; 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 bindingorganic liquids; Keq, equilibrium constant; PA, proton affinity;ΔH, enthalpy; ΔS, entropy; ΔG, Gibbs free energy

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