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X Amino Acid Ionic Liquids molecules Article The Proton Dissociation of Bio-Protic Ionic Liquids: [AAE]X Amino Acid Ionic Liquids Ting He 1, Cheng-Bin Hong 2, Peng-Chong Jiao 1, Heng Xiang 1, Yan Zhang 1, Hua-Qiang Cai 1,*, Shuang-Long Wang 2 and Guo-Hong Tao 2,* 1 Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China; [email protected] (T.H.); [email protected] (P.-C.J.); [email protected] (H.X.); [email protected] (Y.Z.) 2 College of Chemistry, Sichuan University, Chengdu 610064, China; [email protected] (C.-B.H.); [email protected] (S.-L.W.) * Correspondence: [email protected] (H.-Q.C.); [email protected] (G.-H.T.); Tel.: +86-28-85470368 (G.-H.T.) Abstract: [AAE]X composed of amino acid ester cations is a sort of typically “bio-based” protic ionic liquids (PILs). They possess potential Brønsted acidity due to the active hydrogens on their cations. The Brønsted acidity of [AAE]X PILs in green solvents (water and ethanol) at room temperature was systematically studied. Various frameworks of amino acid ester cations and four anions were investigated in this work from the viewpoint of structure–property relationship. Four different ways were used to study the acidity. Acid dissociation constants (pKa) of [AAE]X determined by the OIM (overlapping indicator method) were from 7.10 to 7.73 in water and from 8.54 to 9.05 in ethanol. The pKa values determined by the PTM (potential titration method) were from 7.12 to 7.82 in water. Their 1 Hammett acidity function (H0) values (0.05 mol L− ) were about 4.6 in water. In addition, the pKa · values obtained by the DFT (proton-transfer reactions) were from 7.11 to 7.83 in water and from Citation: He, T.; Hong, C.-B.; Jiao, 8.54 to 9.34 in ethanol, respectively. The data revealed that the cationic structures of [AAE]X had P.-C.; Xiang, H.; Zhang, Y.; Cai, H.-Q.; little effect and the anions had no effect on the acidity of [AAE]X. At the same time, the OIM, PTM, Wang, S.-L.; Tao, G.-H. The Proton Hammett method and DFT method were reliable for determining the acidic strength of [AAE]X in Dissociation of Bio-Protic Ionic this study. Liquids: [AAE]X Amino Acid Ionic Liquids. Molecules 2021, 26, 62. Keywords: protic ionic liquids; Brønsted acidity; amino acid ionic liquids; bio-based ionic liquids https://dx.doi.org/10.3390/molecules2 6010062 Academic Editors: Darren A. Walsh, 1. Introduction Issei Nakamura and Derek J. McPhee Received: 20 October 2020 Protic ionic liquids (PILs) are an important subset of ionic liquids (ILs). PILs possess Accepted: 2 December 2020 strong dissolvabilities, high thermal stabilities, designable structures and broad electrochem- Published: 25 December 2020 ical windows [1,2]. They play important roles in fuel cells, electrochemistry, liquid-liquid extraction, gas capture, biological media and so on due to their acidity [3–7]. For the Publisher’s Note: MDPI stays neutral existence of active hydrogen in amino acid cations, proton dissociation occurs in different with regard to jurisdictional claims in solvents to varying degrees [8]. Therefore, PILs have been considered as acidic catalysts for published maps and institutional the replacement of hazardous acids in many catalytic reactions, including the esterification affiliations. reaction, biomass conversion, transformation of CO2 and Diels-Alder reaction. [9–12]. Bio-based ILs have been paid more attention in recent years due to their preferable green characters [13]. Some natural materials, including carboxylate salts, amino acids and sugars or sugar derivatives, have been employed as IL precursors in a green way [14]. Among Copyright: © 2020 by the authors. these natural materials, amino acids and their derivatives are the most abundant natural Licensee MDPI, Basel, Switzerland. sources containing quaternary nitrogens. Amino acid ionic liquids (AAILs) are fascinating This article is an open access article for chemists in view of their close associations with chirality and biomolecules [15,16]. Some distributed under the terms and conditions of the Creative Commons research has found that AAILs may be useful as potential solvents, catalysts, absorbents Attribution (CC BY) license (https:// and selectors, etc. [17,18]. At the same time, AAILs can be used as acidic catalysts in creativecommons.org/licenses/by/4.0/). the esterification of renewable valeric acid, styrene carbonate synthesis under CO2, the alkylation of indoles and so on [19–21]. Moreover, [AAE]X (AAE means the amino acid Molecules 2021, 26, 62. https://dx.doi.org/10.3390/molecules26010062 https://www.mdpi.com/journal/molecules Molecules 2020, 25, x 2 of 14 Molecules 2020, 25, x 2 of 14 nitrogens. Amino acid ionic liquids (AAILs) are fascinating for chemists in view of their close nitrogens.associations Amino with chiralityacid ionic and liquids biomolecules (AAILs) [15,16]. are fascinating Some research for chemists has found in viewthat AAILsof their may close be associationsuseful as potential with chirality solvents, and catalysts, biomolecules absorbents [15,16]. and Someselectors, research etc. [17,18] has found. At the that same AAILs time, may AAILs be usefulcan be as used potential as acidic solvents, catalysts catalysts, in the absorbents esterification and selectors,of renewable etc. [17,18] valeric. At acid, the samestyrene time, carbonate AAILs Molecules 2021, 26, 62 2 of 14 cansynthesis be used under as COacidic2, the catalysts alkylation in ofthe indoles esterification and so onof [19–21].renewable Moreover, valeric [AAE]Xacid, styrene (AAE meanscarbonate the synthesisamino acid under ester CO 2, cations,the alkylation and ofX indolesmeans and the so correspondingon [19–21]. Moreover, anions) [AAE]X AAILs (AAE have means higher the aminothermostabilities acid ester and cations, lower meltingand X points,means as thewell correspondingas lower viscosities anions) than AAILs those ofhave their higher[AA]X thermostabilities(cations are amino and acid) lower analogs melting and,ester points,hence, cations, broaderas and well X meansasprospects lower the viscosities in corresponding acid-involving than anions)those processes of AAILs their [22]. have[AA]X higher thermostabilities and lower melting points, as well as lower viscosities than those of their [AA]X (cations are (cationsIn general, are amino water acid) and analogs ethanol and, are hence, considered broader as grprospectseen media in foracid-involving acidic catalytic processes reactions, [22]. which amino acid) analogs and, hence, broader prospects in acid-involving processes [22]. is oneIn ofgeneral, the “twelve water principles”and ethanol of are green considered chemistr asy gr [23–26].een media Acidic for acidic properties catalytic in solvents reactions, are which very In general, water and ethanol are considered as green media for acidic catalytic isimportant one of the to “twelveindustrially principles” relevant ofreactions green chemistr [27,28]. They [23–26]. common Acidic PILs, properties such as imidazolium in solvents saltsare very and reactions, which is one of the “twelve principles” of green chemistry [23–26]. Acidic importantquaternary to ammonium industrially salts, relevant haveproperties reactions been studied [27,28]. in solvents in manyThe common are acidic very catalytic PILs, important such reactions as to imidazolium industrially in solvents relevantsalts [29,30]. and reactions [27,28]. The quaternaryHowever, the ammonium precursors salts, of [AAE]X havecommon been are studied more PILs, ab in suchundant many as acidic imidazoliumand bio-based catalytic salts reactionscompared and quaternaryin to solvents common [29,30]. ammonium PILs, salts, have been However,which also the possesses precursors Brønsted of [AAE]X acidstudied ity.are Therefore,more in many abundant acidic the proton catalyticand bio-based dissociation reactions compared inof solvents [AAE]X to common[ 29ILs,30 in]. green However,PILs, the precursors of whichsolvents also are possesses interesting Brønsted and important acid[AAE]Xity. forTherefore, are their more appl abundanttheications. proton and However,dissociation bio-based the compared of acidic [AAE]X characteristics to commonILs in green PILs, of which also possesses solvents[AAE]X AAILsare interesting in solvents and are important stillBrønsted lacking. for acidity.their The appl acid Therefore,ications. dissociation However, the proton constant the dissociation (pacidicKa) is characteristics one of of [AAE]X the most ILsof in green solvents are [AAE]Xsignificant AAILs physiochemical in solvents areparameters. stillinteresting lacking. An The and accurate acid important dissociation pKa forvalue their constantis applications.important (pKa) isto However, oneselect of thereaction themost acidic characteristics of significantconditions inphysiochemical catalytic chemistry parameters. [AAE]X[27,31–33]. An AAILs Determining accurate in solvents pK thea arevalue acidity still is lacking.of important ILs in The water acidto andselect dissociation ethanol reaction has constant (pKa) is one of conditionsbecome an inintriguing catalytic topic, chemistry since [27,31–33].thewater most and significant ethaDeterminingnol are physiochemical promising the acidity media of parameters. ILs for in green water chemistry Anand accurate ethanol [30,34– phasKa value is important to become37]. Herein, an intriguing four different topic, methods since waterselect were reactionand employed etha conditionsnol to are study promising in the catalytic Brønsted media chemistry acidityfor green of [27 [AAE]Xchemistry,31–33]. PILs
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