Unexpected Strong Acidity Enhancing the Effect in Protic Ionic Liquids Quantified by Equilibrium Acidity Studies: A Crucial Role of Cation Structures on Dictating the Solvation Properties

Exploiting Cation Structure and Water Content in Modulating the Acidity of Ammonium Hydrogen Sulfate Protic Ionic Liquids

In this paper, we investigated the effect of cation structure and water content on proton dissociation in alkylammonium [HSO4]- protic ionic liquids (ILs) doped with 20 wt % water and correlated this with experimental Hammett acidities. For pure systems, increased cation substitution resulted in a reduction in the number of direct anion-anion neighbors leading to larger numbers of small aggregates, which is further enhanced with addition of water. We also observed spontaneous proton dissociation from [HSO4]- to water only for primary amine-based protic ILs, preceded by the formation of an anion trimer motif. Investigation using DFT calculations revealed spontaneous proton dissociation from [HSO4]- to water can occur for each of the protic ILs investigated; however, this is dependent on the size of the anion aggregates. These findings are important in the fields of catalysis and lignocellulosic biomass, where solvent acidity is a crucial parameter in biomass fractionation and lignin chemistry.

Acidity Scale in a Choline Chloride- and Ethylene Glycol-Based Deep Eutectic Solvent and Its Implication on Carbon Dioxide Absorption

An equilibrium acidity (pK_a) scale that comprises 16 Brönsted organic acids, including phenols, carboxylic acids, azoles, and phenylmalononitriles, was established in a choline chloride/EG-based deep eutectic solvent (DES) ([Ch][Cl]:2EG) by ultraviolet-visible (UV-Vis) spectroscopic methods. The established acidity scale spans about 6 pK units in the DES, which is similar to that for these acids in water. The acidity comparisons and linear correlations between the DES and other solvents show that the solvent property of [Ch][Cl]:2EG is quite different from those of amphiphilic protic and dipolar aprotic molecular solvents. The carbon dioxide absorption capabilities as well as apparent absorption kinetics for a series of anion-functionalized DESs ([Ch][X]:2EG) were measured, and the results show that the basicity of comprising anion [X] of choline salt is essential for the maximum carbon dioxide absorption capacity, i.e., a stronger basicity leads to a greater absorption capacity. The possible absorption mechanisms for carbon dioxide absorption in these DESs were also discussed based on the spectroscopic evidence.

Thermodynamics regulated organic hydride/acid pairs as novel organic hydrogen reductants

Organic hydride/acid pairs could realize transformation of N-substituted organic hydrides from hydride reductants to thermodynamics regulated hydrogen reductants on conveniently choosing suitable organic hydrides and acids with various acidities.

Assessing the Structure of Protic Ionic Liquids Based on Triethylammonium and Organic Acid Anions

We present a computational analysis of the short-range structure of three protic ionic liquids based on strong organic acids: trifluoracetate, methanesulfonate, and triflate of triethylammonium. Accurate ab initio computations carried out on the gas-phase dimers show that the protonation of triethylamine is spontaneous. We have identified the anion-cation binding motif that is due to the presence of a strong hydrogen bond and to electrostatic interactions. The strength of the hydrogen bond and the magnitude of the binding energy decrease in the order trifluoroacetate ≳ methanesulfonate > triflate. The corresponding simulations of the bulk phases, obtained using a semiempirical evaluation of the interatomic forces, reveal that on short timescales, the state of the three liquids remains highly ionized and that the gas-phase cation-/anion-binding motif is preserved while no other peculiar structural features seem to emerge.

Silica based ionogels: interface effects with aprotic and protic ionic liquids with lithium

In the frame of the development of solid ionogel electrolytes with enhanced ion transport properties, this paper investigates ionogel systems constituted by ∼80 wt% of ionic liquids (ILs) confined in meso-/macroporous silica monolith materials. The anion-cation coordination for two closely related ILs, either aprotic (AIL) butylmethylpyrrolidinium or protic (PIL) butylpyrrolidinium, both with bis(trifluoromethylsulfonyl)imide (TFSI) anions, with and without lithium cations, is studied in depth. The ILs are confined within silica with well-defined mesoporosities (8 to 16 nm). The effects of this confinement, onto melting points, onto conductivity followed by impedance spectroscopy, and onto lithium-TFSI coordination followed by Raman spectroscopy, are presented. Opposite effects have been observed on the melting temperature: it increased for the AIL (+2 °C) upon confinement, while it decreased for the PIL (-2 °C). With lithium, the confinement led to an increase of the melting temperature (+1 °C) for the PIL and AIL. Regarding ionic conductivities, a relative maximum was observed at 40 °C for a mesopore diameter of 10 nm for the AIL with 0.5 M lithium, while it was not clearly visible for the PIL. These differences are discussed in view of the charge balance at the interface between silanols and ILs: the presence of a PIL, contrary to an AIL, is expected to modify the acidity of the silica. Raman data showed that the coordination number of lithium by TFSI is reduced upon AIL confinement, although this was not observed for PILs. At last, this work highlights the impact of the acidity of a PIL on the chemistry occurring at the interface of the host network within ionogels.