Thermal Decomposition Mechanism of 1-Ethyl-3-methylimidazolium Bromide Ionic Liquid

New Insights into the Thermal Stability of 1-Butyl-3-methylimidazolium-Based Ionic Liquids

One of the most promising applications of ionic liquids (ILs) with 1-butyl-3-methylimidazolium (bmim) cation is based on their unique ability to dissolve and fractionate lignocellulosic biomass, allowing for the development of green biorefining technologies. A complete dissolution of lignocellulose requires prolonged treatment at elevated temperatures, which can cause the partial degradation of ILs. In the present study, a combination of various analytical techniques (GC-MS, HPLC-HRMS, 2D-NMR, synchronous thermal analysis) was used for the comprehensive characterization of bmim acetate, chloride, and methyl sulfate degradation products formed at 150 °C during 6- and 24-h thermal treatment. A number of volatile and non-volatile products, including monomeric and dimeric alkyl substituted imidazoles, alcohols, alkyl amines, methyl and butyl acetates, and N-alkylamides, was identified. By thermal lability, ILs can be arranged in the following sequence, coinciding with the decrease in basicity of the anion: [bmim]OAc > [bmim]Cl > [bmim]MeSO4. The accumulation of thermal degradation products in ILs, in turn, affects their physico-chemical properties and thermal stability, and leads to a decrease in the decomposition temperature, a change in the shape of the thermogravimetric curves, and the formation of carbon residue during pyrolysis.

Thermal and Catalytic Decomposition of 2-Hydroxyethylhydrazine and 2-Hydroxyethylhydrazinium Nitrate Ionic Liquid

To develop chemical kinetics models for the combustion of ionic liquid-based monopropellants, identification of the elementary steps in the thermal and catalytic decomposition of components such as 2-hydroxyethylhydrazinium nitrate (HEHN) is needed but is currently not well understood. The first decomposition step in protic ionic liquids such as HEHN is typically the proton transfer from the cation to the anion, resulting in the formation of 2-hydroxyethylhydrazine (HEH) and HNO₃. In the first part of this investigation, the high-temperature thermal decomposition of HEH is probed with flash pyrolysis (<1400 K) and vacuum ultraviolet (10.45 eV) photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS). Next, the investigation into the thermal and catalytic decomposition of HEHN includes two mass spectrometric techniques: (1) tunable VUV-PI-TOFMS (7.4-15 eV) and (2) ambient ionization mass spectrometry utilizing both plasma and laser ionization techniques whereby HEHN is introduced onto a heated inert or iridium catalytic surface and the products are probed. The products can be identified by their masses, their ionization energies, and their collision-induced fragmentation patterns. Formation of product species indicates that catalytic surface recombination is an important reaction process in the decomposition mechanism of HEHN. The products and their possible elementary reaction mechanisms are discussed.

Experimental measurement and prediction of ionic liquid ionisation energies

Ionic liquid (IL) valence electronic structure provides key descriptors for understanding and predicting IL properties. The ionisation energies of 60 ILs are measured and the most readily ionised valence state of each IL (the highest occupied molecular orbital, HOMO) is identified using a combination of X-ray photoelectron spectroscopy (XPS) and synchrotron resonant XPS. A structurally diverse range of cations and anions were studied. The cation gave rise to the HOMO for nine of the 60 ILs presented here, meaning it is energetically more favourable to remove an electron from the cation than the anion. The influence of the cation on the anion electronic structure (and vice versa) were established; the electrostatic effects are well understood and demonstrated to be consistently predictable. We used this knowledge to make predictions of both ionisation energy and HOMO identity for a further 516 ILs, providing a very valuable dataset for benchmarking electronic structure calculations and enabling the development of models linking experimental valence electronic structure descriptors to other IL properties, e.g. electrochemical stability. Furthermore, we provide design rules for the prediction of the electronic structure of ILs.

On-line separation/analysis of Rhodamine B dye based on a solid-phase extraction high performance liquid chromatography self-designed device

A special self-designed device based on poly-1-vinyl-3-pentylimidazole hexafluorophosphate (PILs-C5) solid-phase extraction and high performance liquid chromatography (HPLC) is proposed as a novel method for the on-line separation and analysis of Rhodamine B (RhB) dye. Single factor experiment design and orthogonal experiment design were used to optimize the experimental parameters, such as pH, the amount of PILs-C5, sample volume, flow rate, eluent type, eluent concentration, eluent volume, and the flow rate of eluent. Under the optimized conditions, the linear range was 0.02-2.4 μg mL-1, with the correlation coefficients (R 2) of 0.997. The limit of detection (LOD) and limit of quantification (LOQ) were 0.004 μg mL-1 and 0.02 μg mL-1, respectively. The extraction capacity was 6.22 mg g-1, and enrichment ratio was 15. The extraction mechanism and the post-treatment method of PILs-C5 were also studied. This method was applied to analyze RhB in a wide variety of real samples with satisfactory results.

Molecular Dynamics Simulations and Product Vibrational Spectral Analysis for the Reactions of NO2 with 1-Ethyl-3-methylimidazolium Dicyanamide (EMIM+DCA-), 1-Butyl-3-methylimidazolium Dicyanamide (BMIM+DCA-), and 1-Allyl-3-methylimidazolium Dicyanamide (AMIM+DCA-)

Direct dynamics trajectory simulations were carried out for the NO₂ oxidation of 1-ethyl-3-methylimidazolium dicyanamide (EMIM⁺DCA^-), which were aimed at probing the nature of the primary and secondary reactions in the system. Guided by trajectory results, reaction coordinates and potential energy diagrams were mapped out for NO₂ with EMIM⁺DCA^-, as well as with its analogues 1-butyl-3-methylimidazolium dicyanamide (BMIM⁺DCA^-) and 1-allyl-3-methylimidazolium dicyanamide (AMIM⁺DCA^-). Reactions of the dialkylimidazolium-dicyanamide (DCA) ionic liquids (ILs) are all initiated by proton transfer and/or alkyl abstraction between 1,3-dialkylimidazolium cations and DCA^- anion, of which two exoergic pathways are particularly relevant to their oxidation activities. One pathway is the transfer of a H^β-proton from the ethyl, butyl, or allyl group of the dialkylimidazolium cation to DCA^- that results in the concomitant elimination of the corresponding alkyl as a neutral alkene, and the other pathway is the alkyl abstraction by DCA^- via a second order nucleophilic substitution (SN₂) mechanism. The intra-ion-pair reaction products, including [dialkylimidazolium⁺ - H_C2⁺], alkylimidazole, alkene, alkyl-DCA, HDCA, and DCA^-, react with NO₂ and favor the formation of nitrite (-ONO) complexes over nitro (-NO₂) complexes, albeit the two complex structures have similar formation energies. The exoergic intra-ion-pair reactions in the dialkylimidazolium-DCA ILs account for their significantly higher oxidation activities over the previously reported 1-methyl-4-amino-1,2,4-triazolium dicyanamide [Liu, J.; J. Phys. Chem. B 2019, 123, 2956-2970] and for the relatively higher reactivity of BMIM⁺DCA^- vs AMIM⁺DCA^- as BMIM⁺ has a higher reaction path degeneracy for intra-ion-pair H^β-proton transfer and its H^β-transfer is more energetically favorable. To validate and directly compare our computational results with spectral measurements in the ILs, infrared and Raman spectra of BMIM⁺DCA^- and AMIM⁺DCA^- and their products with NO₂ were calculated using an ionic liquid solvation model. The simulated spectra reproduced all of the vibrational frequencies detected in the reactions of BMIM⁺DCA^- and AMIM⁺DCA^- IL droplets with NO₂ (as reported by Brotton et al. [ J. Phys. Chem. A 2018, 122, 7351-7377] and Lucas et al. [ J. Phys. Chem. A 2019, 123, 400-416]).

Increased flammability hazard when ionic liquid [C6mim][Cl] is exposed to high temperatures

Industrial use of ionic liquids may require exposure to high temperatures. We demonstrate that such applications may result in an increase in flammability hazard due to chemical decomposition. The ionic liquid, 1-hexyl-3-methylimidazolium chloride ([C₆mim][Cl]), was selected as the study sample. The flash point and other properties were measured using a commercially available flash point analyzer, a thermogravimetric analyzer (TGA), Fourier transform infrared spectroscopy (FTIR), an integrated TGA-FTIR system, and pyrolysis-gas chromatography-mass spectrometer. We found that thermal decomposition occurred with the release of chloromethane, 1-chlorohexane, 1-hexene, 1-methylimidazole, and 1-hexylimidazole as [C₆mim][Cl] was heated. Such decomposition changed the components of the residual liquid phase. Vaporization of the [C₆mim][Cl] decomposition products increased the mass loss rate on TGA as [C₆mim][Cl] was heated to high temperatures, resulting in a high concentration of flammable gases and a decrease in the flash point, which increased the flammability hazard.

Thermal stability of dialkylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids: ex situ bulk heating to complement in situ mass spectrometry

Thermal decomposition (TD) products of the ionic liquids (ILs) [CnC1Im][BF4] and [CnC1Im][PF6] ([CnC1Im]+ = 1-alkyl-3-methylimidazolium, [BF4]- = tetrafluoroborate, and [PF6]- = hexafluorophosphate) were prepared, ex situ, by bulk heating experiments in a bespoke setup. The respective products, CnC1(C3N2H2)BF3 and CnC1(C3N2H2)PF5 (1-alkyl-3-methylimidazolium-2-trifluoroborate and 1-alkyl-3-methylimidazolium-2-pentafluorophosphate), were then vaporized and analyzed by direct insertion mass spectrometry (DIMS) in order to identify their characteristic MS signals. During IL DIMS experiments we were subsequently able, in situ, to identify and monitor signals due to both IL vaporization and IL thermal decomposition. These decomposition products have not been observed in situ during previous analytical vaporization studies of similar ILs. The ex situ preparation of TD products is therefore perfectly complimentary to in situ thermal stability measurements. Experimental parameters such as sample surface area to volume ratios are consequently very important for ILs that show competitive vaporization and thermal decomposition. We have explained these experimental factors in terms of Langmuir evaporation and Knudsen effusion-like conditions, allowing us to draw together observations from previous studies to make sense of the literature on IL thermal stability. Hence, the design of experimental setups are crucial and previously overlooked experimental factors.

Quantifying intermolecular interactions of ionic liquids using cohesive energy densities

For ionic liquids (ILs), both the large number of possible cation + anion combinations and their ionic nature provide a unique challenge for understanding intermolecular interactions. Cohesive energy density, ced, is used to quantify the strength of intermolecular interactions for molecular liquids, and is determined using the enthalpy of vaporization. A critical analysis of the experimental challenges and data to obtain ced for ILs is provided. For ILs there are two methods to judge the strength of intermolecular interactions, due to the presence of multiple constituents in the vapour phase of ILs. Firstly, ced_IP, where the ionic vapour constituent is neutral ion pairs, the major constituent of the IL vapour. Secondly, ced_C+A, where the ionic vapour constituents are isolated ions. A ced_IP dataset is presented for 64 ILs. For the first time an experimental ced_C+A, a measure of the strength of the total intermolecular interaction for an IL, is presented. ced_C+A is significantly larger for ILs than ced for most molecular liquids, reflecting the need to break all of the relatively strong electrostatic interactions present in ILs. However, the van der Waals interactions contribute significantly to IL volatility due to the very strong electrostatic interaction in the neutral ion pair ionic vapour. An excellent linear correlation is found between ced_IP and the inverse of the molecular volume. A good linear correlation is found between IL ced_IP and IL Gordon parameter (which are dependent primarily on surface tension). ced values obtained through indirect methods gave similar magnitude values to ced_IP. These findings show that ced_IP is very important for understanding IL intermolecular interactions, in spite of ced_IP not being a measure of the total intermolecular interactions of an IL. In the outlook section, remaining challenges for understanding IL intermolecular interactions are outlined.

Effect of Boron Clusters on the Ignition Reaction of HNO3 and Dicynanamide-Based Ionic Liquids

Many ionic liquids containing the dicynamide anion (DCA^-, formula N(CN)₂^-) exhibit hypergolic ignition when exposed to the common oxidizer nitric acid. However, the ignition delay is often about 10 times longer than the desired 5 ms for rocket applications, so that improvements are desired. Experiments in the past decade have suggested both a mechanism for the early reaction steps and also that additives such as decaborane can reduce the ignition delay. The mechanisms for reactions of nitric acid with both DCA^- and protonated DCAH are considered here, using accurate wave function methods. Complexation of DCA^- or DCAH with borane clusters B₁₀H₁₄ or B₉H₁₄^- is found to modify these mechanisms slightly by changing the nature of some of the intermediate saddle points and by small reductions in the reaction barriers.

Catalytic Decomposition of Hydroxylammonium Nitrate Ionic Liquid: Enhancement of NO Formation

Hydroxylammonium nitrate (HAN) is a promising candidate to replace highly toxic hydrazine in monopropellant thruster space applications. The reactivity of HAN aerosols on heated copper and iridium targets was investigated using tunable vacuum ultraviolet photoionization time-of-flight aerosol mass spectrometry. The reaction products were identified by their mass-to-charge ratios and their ionization energies. Products include NH₃, H₂O, NO, hydroxylamine (HA), HNO₃, and a small amount of NO₂ at high temperature. No N₂O was detected under these experimental conditions, despite the fact that N₂O is one of the expected products according to the generally accepted thermal decomposition mechanism of HAN. Upon introduction of iridium catalyst, a significant enhancement of the NO/HA ratio was observed. This observation indicates that the formation of NO via decomposition of HA is an important pathway in the catalytic decomposition of HAN.

Understanding the thermal decomposition mechanism of a halogen-free chelated orthoborate-based ionic liquid: a combined computational and experimental study

In the last few decades, ionic liquids (ILs) have gained significant attention as lubricants and lubricant additives due to their polar nature, low vapour pressure and tunable physicochemical properties.

Thermal stability of imidazolium-based ionic liquids

This work highlights the factors tuning the thermal stability of imidazolium-based ionic liquids (IL) associated to bis(trifluoromethanesulfonyl)imide anion [NTf2]. The decomposition temperatures (Td) were evaluated by thermogravimetric analyses (TGA) with optimized parameters to obtain reproducible Td. The impact of the alkyl chain length and of the presence of functional groups and unsaturations on Td were evaluated. The thermal behaviour was governed by Van der Waals interactions between alkyl chains, and by inter and intra coulombic interactions such as hydrogen bonds.

Azoniaspiro salts: towards bridging the gap between room-temperature ionic liquids and molten salts

Organic spirocyclic tetraalkylammonium chloride salts exhibit enhanced thermal stabilities relative to traditional dialkylimidazolium ionic liquid analogues.

Near threshold photodissociation study of EMIMBF4 vapor

Photodissociation of the [EMIM][BF₄] ionic liquid vapors following excitation with light in the vacuum ultraviolet region was studied at different liquid temperatures.

Ionothermal synthesis and crystal structure of a new organic-inorganic hybrid compound: catena-poly[bis(1-ethyl-3-methylimidazolium) [hepta-μ-bromido-pentacuprate(I)]]

A new organic-inorganic hybrid compound, catena-poly[bis(1-ethyl-3-methylimidazolium) [μ5-bromido-tri-μ3-bromido-tri-μ2-bromido-pentacuprate(I)]], {(C6H11N2)2[Cu5Br7]}n, has been obtained under ionothermal conditions from a reaction mixture containing Ba(OH)2·8H2O, Cu(OH)2·2H2O, As2O5, 1-ethyl-3-methylimidazolium bromide and distilled water. The crystal structure consists of complex [Cu5Br7](2-) anions arranged in sinusoidal {[Cu5Br7](2-)}n chains running along the a axis, which are surrounded by 1-ethyl-3-methylimidazolium cations. Three of the five unique Br atoms and one of the three Cu(I) atoms occupy special positions with half-occupancy (a mirror plane perpendicular to the b axis, site symmetry m). The Cu(I) ions are in a distorted tetrahedral coordination environment, with four Br atoms at distances ranging from 2.3667 (10) to 2.6197 (13) Å, and an outlier at 3.0283 (12) Å, exceptionally elongated and with a small contribution to the bond-valence sum of only 6.7%. Short C-H···Br contacts build up a three-dimensional network. The Cu···Cu distances within the chain range from 2.8390 (12) to 3.0805 (17) Å, indicating the existence of weak Cu(I)···Cu(I) cuprophilic interactions.

Understanding the evaporation of ionic liquids using the example of 1-ethyl-3-methylimidazolium ethylsulfate

In this work we present a comprehensive temperature-dependence analysis of both the structural and the dynamic properties of a vaporized ionic liquid (1-ethyl-3-methylimidazolium ethylsulfate). This particular ionic liquid is known to be distillable from experimental studies and thus enables us to deepen the understanding of the evaporation mechanism of ionic liquids. We have used ab initio molecular dynamics of one ion pair at three different temperatures to accurately describe the interactions present in this model ionic liquid. By means of radial and spatial distribution functions a large impact on the coordination pattern at 400 K is shown which could explain the transfer of one ion pair from the bulk to the gas phase. Comparison of the free energy surfaces at 300 K and 600 K supports the idea of bulk phase-like and gas phase-like ion pairs. The different coordination patterns caused by the temperature, describing a loosening of the anion side chains, are also well reflected in the power spectra. The lifetime analysis of typical conformations for ionic liquids shows a characteristic behavior at 400 K (temperature close to the experimental evaporation temperature), indicating that conformational changes occur when the ionic liquid is evaporated.

Vapors from Ionic Liquids: Reconciling Simulations with Mass Spectrometric Data

The species involved in the distillation of aprotic ionic liquids are discussed in light of recent simulations and mass spectrometric data obtained by various techniques. New mass spectrometric data collected via laser-induced acoustic desorption and the thermal desorption of ionic liquids are also presented as well as additional DFT calculations. The available evidence of theoretical simulations and mass spectrometric data suggests that the distillation of ionic liquids occurs mainly via neutral ion pairs of composition CnAn [C(+) = cation and A(-) = anion], followed by gas-phase dissociation to lower order ion pairs and then dissociation of hot CA to C(+) and A(-), followed by ion/molecule association events to give [CnAn-1](+) or [Cn-1An](-) ions to a degree that depends on the amount of internal energy deposited into the neutral CnAn clusters upon evaporation.

Ionic liquids--promising but challenging solvents for homogeneous derivatization of cellulose

In the past decade, ionic liquids (ILs) have received enormous interest as solvents for cellulose. They have been studied intensively for fractionation and biorefining of lignocellulosic biomass, for dissolution of the polysaccharide, for preparation of cellulosic fibers, and in particular as reaction media for the homogeneous preparation of highly engineered polysaccharide derivatives. ILs show great potential for application on a commercial scale regarding recyclability, high dissolution power, and their broad structural diversity. However, a critical analysis reveals that these promising features are combined with serious drawbacks that need to be addressed in order to utilize ILs for the efficient synthesis of cellulose derivatives. This review presents a comprehensive overview about chemical modification of cellulose in ILs. Difficulties encountered thereby are discussed critically and current as well as future developments in this field of polysaccharide research are outlined.