Brønsted acidity of bio-protic ionic liquids: the acidic scale of [AA]X amino acid ionic liquids

Abstract

Amino acid ionic liquids (AAILs) [AA]X based on amino acid cations are a kind of typical "bio-base" protic ionic liquids (PILs), which are supposed to be acidic ionic liquids. The Brønsted acidity of [AA]X PILs at room temperature were systematically studied for the first time. Acid dissociation constants (pKa) of [AA]X were determined by potentiometric titration method. The first acid dissociation constants (pKa1) are from 1.98 to 2.42. The actual pH values of [AA]X (0.010 mol·L −1 ) obtained from pH meter are from 2.26 to 2.44 which are slightly higher than the calculated pH values according to the above experimental pKa1. The Hammett method performed on UV/Vis spectra with p-nitroaniline as the indicator was used to determine the acid strength of [AA]X. Their H0 values (0.010 mol·L −1 ) are in the range from 2.10 to 2.44. Various frameworks of amino acid cations and five anions (including nitrate (NO3 − ), chloride (Cl − ), perchlorate (ClO4 − ), trifluoromethanesulfonate (OTf − ) and trifluoroacetate (TfA − ) anions) were used to investigate the cationic and anionic effect on the acidity of AAILs. The Brønsted acidity of AAILs depends on the cationic structure, the type of anion and the concentration of [AA]X. In addition, the theoretical pKa1 were studied by the cluster-continuum model using density functional theory (DFT) method. The experimental and theoretical results showed that [AA]X PILs have stronger Brønsted acidity than that of the common PILs prepared by one-pot syntheses.11b Scheme 1 Structures of PILs.

Catalytic Biodiesel Production Mediated by Amino Acid‐Based Protic Salts

“…Additionally, the starting materials of those ILs are synthetic chemicals or heteroaromatic compounds and, therefore, not as green as desired . Amino acid‐based ILs are biodegradable and easy to prepare, have acidities similar to that of H 3 PO 4 , and are thought to be green and sustainable . Although the amino acid‐derived salts were classified as ILs, some of their melting points are higher than 100 °C.…”

“…Amino acid‐based ILs are biodegradable and easy to prepare, have acidities similar to that of H 3 PO 4 , and are thought to be green and sustainable . Although the amino acid‐derived salts were classified as ILs, some of their melting points are higher than 100 °C. For example, the melting points of [Gly][Cl], [Gly][NO 3 ], [Val][NO 3 ], and [Ile][NO 3 ] are 186, 111, 134, and 105 °C, respectively.…”

Characterizing the Effects of a “Switchable Water” Additive on the Aqueous Solubility of Small Molecules

“…Part of the switchable nature of this system might be ascribed to the formation of K 2 HPO 4 and KHCO 3 . Protonation of K 2 HPO 4 is possible, but only a small amount of KH 2 PO 4 is likely to be present because the p K aH of K 2 HPO 4 is 7.21, which is lower than the observed pH of the carbonated solution (approx. 8.0).…”

“…It should be noted that CO 2 causes the protonation of more than just TMBD in this system; K 3 PO 4 is likely also being protonated. The p K aH of phosphate is 12.67, making it more basic than TMBD (p K aH =10.3) and therefore more likely to be protonated by CO 2 . Part of the switchable nature of this system might be ascribed to the formation of K 2 HPO 4 and KHCO 3 .…”

“…It should be noted that CO 2 causes the protonation of more than just TMBD in this system;K 3 PO 4 is likely also being protonated. The pK aH of phosphate is 12.67, [34] making it more basic than TMBD (pK aH = 10.3) and therefore more likely to be protonated by CO 2 .P art of the switchable nature of this system might be ascribed to the formationo fK 2 HPO 4 and KHCO 3 .P rotonation of K 2 HPO 4 is possible, but only as malla mount of KH 2 PO 4 is likely to be presentb ecause the pK aH of K 2 HPO 4 is 7.21, [34] which is lower than the observed pH of the carbonated solution (approx.8 .0). Regardless, all of theses pecies are ionic compounds, so their effects on the ATPS are expectedt ob esimilar.…”