Tetraalkylphosphonium Cations Research Articles

Thermophysical property prediction of anion-functionalized ionic liquids for CO2 capture

We develop a machine learning framework for predicting the density, viscosity, and heat capacity of a family of anion-functionalized ionic liquids for CO2 capture, specifically those with tetraalkylphosphonium cations and aprotic N-heterocyclic anions (AHAs). We screen several feature sets using group contribution-based (GC) descriptors and descriptors extracted from COSMO-RS sigma profiles (SP) to build Support Vector Regression (SVR) and Gradient-Boosted Regression (GBR) machine learning models. Viscosities and densities were modeled based on data sets containing nearly 60 ILs each. The best fit for viscosity used GC-based descriptors and the SVR model, achieving a test set %AARD of 12.5% and R2 of 0.989. Density was modeled using these same descriptors with the SVR model framework and was fitted with a test set %AARD of 1.0%. Heat capacity was fit as a function of molar volume and temperature, a general trend observed for all ILs in a family. Heat capacity predictions could then be made using the density SVR model with a test set accuracy of 3.0 %AARD. With these results, we have developed predictive models which can potentially be used in the design of new advanced ionic liquids for carbon capture.

Unusual polarities of tetraalkylphosphonium and trialkylsulfonium ionic liquids, ylides, and carboxylated zwitterions

Aprotic heterocyclic anions (AHAs) can abstract hydrogen atoms from the methylene groups of the trialkylsulfonium and tetraalkylphosphonium cation. The resulting trialkylsulfonium and tetraalkylphosphonium ylides readily capture CO2 and produce unusual carboxylated zwitterions, in which the distances between the poles, S(P) and O, are 0.274–0.290 nm. The zwitterions are strongly intermolecularly stabilized thanks to the electrostatic attractions between +1.6e(+0.9e) and −0.7e charges. Herein, we report drastic polarity alterations between the states of ionic liquids, ylides, and zwitterions through electronic properties, evolutions of geometries, and interatomic charge transfers. The analysis of the unusual intermolecular attractions was performed in the framework of natural bond orbitals. We propose that a biphasic or triphasic system can be useful to boost the reactions by accounting for the polarities of the intermediates and products. The practical value of the reaction is an enhancement in CO2 chemisorption.

Processing Gray Selenium in Phosphonium-Based Ionic Liquids.

The dissolution of gray selenium in tetraalkylphosphonium acetate ionic liquids was investigated by UV-vis, NMR, and Raman spectroscopy as well as quantum chemical calculations and electrochemical methods. Acetate anions and tetraalkylphosphonium cations facilitate the formation and stabilization of oligoselenides Sen2- and cationic Se species in the ionic liquid phase. Chemical exchange of selenium atoms was demonstrated by variable-temperature 77Se NMR experiments. Additionally, uncharged cycloselenium molecules exist at high selenium concentrations. Upon dilution with ethanol, amorphous red selenium precipitates from the solution. Moreover, crystalline Se1-xTex solid solutions precipitate when elemental tellurium is added to the mixture as confirmed by powder X-ray diffraction and Raman spectroscopy.

Coexistence of Tellurium Cations and Anions in Phosphonium-Based Ionic Liquids.

Elemental tellurium readily dissolves in ionic liquids (ILs) based on tetraalkylphosphonium cations even at temperatures below 100 °C. In the case of ILs with acetate, decanoate, or dicyanamide anions, dark red to purple colored solutions form. A study combining NMR, UV‐Vis and Raman spectroscopy revealed the formation of tellurium anions (Ten)2− with chain lengths up to at least n=5, which are in dynamic equilibrium with each other. Since external influences could be excluded and no evidence of an ionic liquid reaction was found, disproportionation of the tellurium is the only possible dissolution mechanism. Although the spectroscopic detection of tellurium cations in these solutions is difficult, the coexistence of tellurium cations, such as (Te4)2+ and (Te6)4+, and tellurium anions could be proven by cyclic voltammetry and electrodeposition experiments. DFT calculations indicate that electrostatic interactions with the ions of the ILs are sufficient to stabilize both types of tellurium ions in solution.

Preparation, characterization and adsorption properties of tetraalkylphosphonium organobeidellites

New intercalated clay materials based on the combination of four systematically selected alkylphosphonium organocations and beidellite (Bd) were prepared. Particularly, tetraethyl-, tetrabutyl-, tetrahexyl- and tetraoctyl- phosphonium cations were used in the synthesis. The samples were characterized by X-ray diffraction, thermal analysis, infrared spectroscopy and elemental carbon analysis. Further, calculations based on density functional theory method including dispersion corrections (DFT-D3) were carried out in a prediction of stability and detailed characterization of the structure and interactions of the beidellite intercalated models corresponding to four tetraalkylphosphonium cations used in experiment (4PX-Bd; X = 2, 4, 6, 8 / ethyl-, butyl-, hexyl-, octyl-). Calculated interlayer distances (d001) for all four models agreed well with experimental data continuously growing with a length of the alkyl chains of the 4PX cations. The structural stability of 4PX-Bd intercalates also increased with the size of the 4PX cations. Calculated vibrational spectra obtained from ab initio molecular dynamics were in a good agreement with experimental IR spectra. Further, a sorption capacity of synthetized organobeidellites was probed by loading sulfamethoxazole drug and quantified by liquid chromatography method. Adsorption of sulfamethoxazole was the most effective using tetraoctylphosphonium-modified beidellite.

The Effect of Phenyl Substitutions on Microstructures and Dynamics of Tetraalkylphosphonium Bis(trifluoro- methylsulfonyl)imide Ionic Liquids.

Extensive atomistic simulations demonstrated that a gradual substitution of hexyl chains with phenyl groups in tetraalkylphosphonium cations results in remarkable changes in hydrogen bonding interactions, liquid structures and scattering structural functions, and rotational dynamics of hexyl chains and phenyl groups in tetraalkylphosphonium bis(trifluoromethylsulfonyl)imide ionic liquids. Hydrogen donor sites in hexyl chains present competitive characteristics with those in phenyl groups in coordinating anions, as well as their continuous and intermittent hydrogen bonding dynamics. Cation-cation and anion-anion spatial correlations show concomitant shift to short distances with decreased peak intensities with variations of cation structures, whereas cation-anion correlations have a distinct shift to large radial distances due to decreased associations of anions with neighboring cations. These microstructural changes are qualitatively manifested in shifts of prominent peaks for prevalent charge alternations and adjacency correlations between ion species in scattering structural functions. Meanwhile, rotational dynamics of hexyl chains speed up, which, in turn, slow down rotations of phenyl groups, whereas anions exhibit imperceptible changes in their rotational dynamics. These computational results are intrinsically correlated with conformational flexibilities, molecular sizes, and steric hindrance effects of phenyl groups in comparison with hexyl chains, and constrained distributions of anions around cations in heterogeneous ionic environments.

How Molecular Chiralities of Bis(mandelato)borate Anions Affect Their Binding Structures With Alkali Metal Ions and Microstructural Properties in Tetraalkylphosphonium Ionic Liquids.

Spiroborate anion-based inorganic electrolytes and ionic liquids (ILs) have fascinating electrochemical and tribological properties and have received widespread attention in industrial applications. The molecular chiralities of spiroborate anions have a significant effect on the microstructures and macroscopic functionalities of these ionic materials in application and thus deserve fundamental consideration. In the current work, we performed quantum chemistry calculations to address the binding strength and coordination structures of chiral bis(mandelato)borate ([BMB]) anions with representative alkali metal ions, as well as the electronic properties of alkali metal ion-[BMB] ion pair complexes. The optimized [BMB] conformers are categorized into V-shaped, bent, and twisted structures with varied electrostatic potential contours and conformational energies and distinct alkali metal ion-[BMB] binding structures. Alkali metal ions have additional associations with phenyl groups in V-shaped [BMB] conformers owing to preferential cation-π interactions. Furthermore, the effects of the molecular chiralities of [BMB] anions on the thermodynamics and microstructural properties of tetraalkylphosphonium [BMB] ILs were studied by performing extensive atomistic interactions. Oxygen atoms in [BMB] anions have competitive hydrogen bonding interactions with hydrogen atoms in cations depending on the molecular chiralities and steric hindrance effects of [BMB] anions. However, the molecular chiralities of [BMB] anions have a negligible effect on the liquid densities of tetraalkylphosphonium [BMB] ILs and the spatial distributions of boron atoms in anions around phosphorous atoms in cations. Enlarging tetraalkylphosphonium cation sizes leads to enhanced cation-anion intermolecular hydrogen bonding and Coulombic interactions due to enhanced segregation of polar groups in apolar networks in heterogeneous IL matrices, as verified by scattering structural functions.

Phosphorus modified small-pore zeolites and their catalytic performances in ethanol conversion and NH3-SCR reactions

Small-pore zeolites, composed of 8-membered ring windows, exhibit high catalytic performances in the selective catalytic reduction of NOx with ammonia (NH3-SCR) and the light olefin production from methanol or ethanol (MTO or ETO). As these processes are performed at high reaction temperatures in the presence of steam, the enhancement of the thermal and hydrothermal resistances of zeolites is essential for the development of high-performance catalysts that retain their high catalytic activity and selectivity. This paper reports on the syntheses of phosphorus-modified small-pore zeolites with high thermal/hydrothermal stabilities, prepared by the hydrothermal conversion of FAU zeolite in the presence tetraalkylphosphonium cations as both organic structure directing agents and phosphorus modifying agents. Several phosphorus-modified small-pore zeolites with different framework structures and a controlled phosphorus modification degree were synthesized. Moreover, it was found that the phosphorus-modified small-pore zeolites exhibit a good catalytic activity and durability in the NH3-SCR and ETO processes even after severe thermal or hydrothermal treatment.

Hydrogen Bonding and Proton Transfer in Aqueous Toluene Microdroplets Studied by Particle Collision Electrochemistry

AbstractThis report describes hydrogen bonding and protonation in toluene microdroplets (0.4 to 4.0 μm) dispersed in water. Such effects are revealed by studying the two‐electron reduction of tetrachloro‐1,4‐benzoquinone (TCBQ) trapped inside the toluene droplets using particle collision electrochemistry (PCE). This approach allows interrogation of microscopic particles in Brownian motion colliding with an ultramicroelectrode and can provide information when the collision encounter is coupled to an electrochemical event. Upon addition of acids in homogeneous toluene, the half‐wave potential of TCBQ shifts positively (easier reduction), thus implying hydrogen bonding and/or protonation of the reduction product TCBQ2−. The shift is commensurate to the proton donating capacity of each acid, so that, acetic acid (pKa=4.8) displays higher shift than oleic acid (pKa=9.9). However, when the reduction takes place inside toluene droplets, the trend is completely reversed and appears to be driven by preferential partition of acetic acid in water, as opposed to the hydrophobic oleic acid. Furthermore, these effects compete with the underlying binding between TCBQ2− and the tetraalkyl phosphonium cation of the ionic liquid used to enhance the conductivity of toluene. This work aims to illustrate a subtle, yet consequential effect that emerges from micro‐confinement of immiscible phases, as well as the potential of PCE to extract chemical reactivity information at the organic‐water interface.

Microstructures and dynamics of tetraalkylphosphonium chloride ionic liquids.

Atomistic simulations have been performed to investigate the effect of aliphatic chain length in tetraalkylphosphonium cations on liquid morphologies, microscopic ionic structures, and dynamical quantities of tetraalkylphosphonium chloride ionic liquids. The liquid morphologies are characterized by sponge-like interpenetrating polar and apolar networks in ionic liquids consisting of tetraalkylphosphonium cations with short aliphatic chains. The lengthening aliphatic chains in tetraalkylphosphonium cations lead to polar domains consisting of chloride anions and central polar groups in cations being partially or totally segregated in ionic liquid matrices due to a progressive expansion of apolar domains in between. Prominent polarity alternation peaks and adjacency correlation peaks are observed at low and high q range in total X-ray scattering structural functions, respectively, and their peak positions gradually shift to lower q values with lengthening aliphatic chains in tetraalkylphosphonium cations. The charge alternation peaks registered in the intermediate q range exhibit complicated tendencies due to a cancellation of peaks and anti-peaks in partial structural functions for ionic subcomponents. The particular microstructures and liquid morphologies in tetraalkylphosphonium chloride ionic liquids intrinsically contribute to distinct dynamics characterized by mean square displacements, van Hove correlation functions, and non-Gaussian parameters for ionic species in the heterogeneous ionic environment. Most tetraalkylphosphonium cations have higher translational mobilities than their partner anions due to strong coordination of chloride anions with central polar groups in tetraalkylphosphonium cations through strong Coulombic and hydrogen bonding interactions. The increase of aliphatic chain length in tetraalkylphosphonium cations leads to a concomitant shift of van Hove correlation functions and non-Gaussian parameters to larger radial distances and longer time scales, respectively, indicating the enhanced translational dynamical heterogeneities of tetraalkylphosphonium cations and the corresponding chloride anions.

Ionic Liquids as Surfactants for Layered Double Hydroxide Fillers: Effect on the Final Properties of Poly(Butylene Adipate-Co-Terephthalate).

In this work, phosphonium ionic liquids (ILs) based on tetra-alkylphosphonium cations combined with carboxylate, phosphate and phosphinate anions, were used for organic modification of layered double hydroxide (LDH). Two different amounts (2 and 5 wt %) of the organically modified LDHs were mixed with poly(butylene adipate-co-terephthalate) (PBAT) matrix by melt extrusion. All prepared PBAT/IL-modified-LDH composites exhibited increased mechanical properties (20–50% Young’s modulus increase), decreased water vapor permeability (30–50% permeability coefficient reduction), and slight decreased crystallinity (10–30%) compared to the neat PBAT.

The Viscosity and Density of Ionic Liquid + Tetraglyme Mixtures and the Effect of Tetraglyme on CO2 Solubility

We show that the addition of tetraglyme (TG), which is a low viscosity liquid with relatively low vapor pressure, is effective in reducing the viscosity of ionic liquids (ILs). In particular, we measure the viscosities of mixtures of 18 ionic liquids with tetraglyme at temperatures between 278.15 and 323.15 K, with a focus on mixtures that are primarily ionic liquid. Thirteen of the ionic liquids contain aprotic heterocyclic anions (AHA ILs) paired with tetra-alkylphosphonium and imidazolium cations, which we have developed for cofluid vapor compression refrigeration and postcombustion CO2 capture applications. Three bis(trifluoromethylsulfonyl)amide ([Tf2N]−) ionic liquids are included for comparison, as well as trihexyltetradecylphosphonium acetate and trihexyltetradecylphosphonium dicyanamide ([P66614][acetate] and [P66614][DCA]). In addition, we present the densities of trihexyltetradecylphosphonium 1,2,3-triazolide ([P66614][3-Triz]) + tetraglyme mixtures at temperatures between 283.15 and 353.15 K. Finally, we show that the solubility of CO2 in mixtures of [P66614][3-Triz] + 30 mol % tetraglyme and trihexyltetradecylphosphonium 1,2,4-triazolide ([P66614][4-Triz]) + 30 mol % tetraglyme at 313.15, 333.5, and 353.6 K and pressures to 34 bar can be represented reasonably well by a mole fraction weighted sum of the solubilities (on a mole ratio basis) in the two pure components.

(Invited) Electrochemical Properties of Ionic Liquids Containing Aprotic Heterocyclic Anions (AHA ILs) and Their Mixtures with Lithium Salts

Ionic liquids containing aprotic heterocyclic anions, such as pyrrolides, pyrazolides, imidazolides, triazolides and tetrazolides, were originally developed for CO2 capture applications since these anions can chemically react with CO2.1 Subsequently, we have been investigating the potential use of this class of “AHA” ILs for a variety of electrochemical applications. We have synthesized a series of room temperature ionic liquids (RTILs) based on 1-ethyl-3-methylimidazolium ([emim]+) and tetra-alkylphosphonium ([P222n]+) cations with different aprotic heterocyclic anions (AHAs) and characterized them as potential electrolyte candidates.2,3 The density and transport properties of these ILs were measured over the temperature range between 283.15 K and 343.15 K at ambient pressure. The temperature dependence of the transport properties (viscosity, ionic conductivity, self-diffusion coefficient and molar conductivity) was fit well by the Vogel-Fulcher-Tamman (VFT) equation. The ionicity of these ILs was quantified by the ratio of the molar conductivity obtained from the ionic conductivity and molar concentration to that calculated from the self-diffusion coefficients using the Nernst-Einstein equation. Exceptionally high values of this molar conductivity ratio, even greater than 0.9, were reported. The results showed that many of the AHA ILs exhibit very good conductivity for their viscosities, and provided insight into the design of ILs with enhanced dynamics that may be suitable for electrolyte applications. Here we will explore how the addition of lithium AHA salts affects the transport properties of the IL/salt mixtures in order to evaluate the use of AHA ILs for lithium ion battery applications. In particular, we investigate the ionic conductivity and viscosity, as well as anion, cation and lithium diffusivity, as measured by Pulse-Gradient-Spin-Echo NMR spectroscopy. The focus of this investigation is imidazolium cations paired with pyrrolide and pyrazolide based anions. The pure lithium AHA salts, which exhibit some interesting properties themselves, are characterized by a variety of analytical (e.g., TGA, DSC) and spectroscopic (e.g., NMR) techniques. Finally, we explore the influence of additives on viscosity and ionic conductivity, and on anion, cation and lithium diffusivity. If ionic liquids could be used as battery solvents, they could greatly reduce the fire hazard presented by the currently used organic carbonate solvents. B. Gurkan, B. F. Goodrich, E. M. Mindrup, L. E. Ficke, M. Massel, S. Seo, T. P. Senftle, H. Wu, T. W. Rosch, M. F. Glaser, J. K. Shah, E. J. Maginn, J. F. Brennecke, and W. F. Schneider, “Molecular Design of High Capacity, Low Viscosity, Chemically Tunable Ionic Liquids for CO2 Capture,” J. Phys. Chem. Let., 1(24), 2010, 3494-3499.L. Sun, O. Morales-Collazo, H. Xia and J. F. Brennecke, “Effect of Structure on Transport Properties (Viscosity, Ionic Conductivity and Self-diffusion Coefficient) of Aprotic Heterocyclic Anion (AHA) RoomTemperature Ionic Liquids. 1. Variation of Anionic Species,” J. Phys. Chem. B, 2015, 119, 15030-15039.L. Sun, O. Morales-Collazo, H. Xia and J. F. Brennecke, “Effect of Structure on Transport Properties (Viscosity, Ionic Conductivity and Self-diffusion Coefficient) of Aprotic Heterocyclic Anion (AHA) Room Temperature Ionic Liquids. 2. Variation of Alkyl Chain Length in the Phosphonium Cation,” submitted to J. Phys. Chem. B, 2016.

(Invited) Electrochemical Properties of Ionic Liquids Containing Aprotic Heterocyclic Anions (AHA ILs) and Their Mixtures with Lithium Salts

We have previously studied a series of room temperature ionic liquids (RTILs) based on 1-ethyl-3-methylimidazolium ([emim]+) and tetra-alkylphosphonium ([P222n]+) cations with different aprotic heterocyclic anions (AHAs). It has been found that many of the AHA ILs exhibit very good conductivity for their viscosities. The results provide insight into the design of ILs with enhanced dynamics that may be suitable for electrolyte applications (1, 2). Lithium salts based on the bis(trifluoromethylsulfonyl)imide ([TFSI]-), bis(fluorosulfonyl)imide ([FSI]-) and tetrafluoroborate ([BF4]-) anions have been extensively studied in lithium salt/IL mixtures for such applications (3-18). Here we will further explore how the addition of lithium AHA salts, compared to the popular [TFSI]-, [FSI]- and [BF4]--based ones, affects the transport properties of the RTILs in order to evaluate the feasibility of using these AHA ILs for lithium-ion battery applications. In particular, we investigate the viscosity and ionic conductivity, as well as anion, cation and lithium diffusivity, as measured by Pulse-Gradient-Spin-Echo NMR spectroscopy.

Synthesis of phosphorus-modified small-pore zeolites utilizing tetraalkyl phosphonium cations as both structure-directing and phosphorous modification agents

A rational strategy for the synthesis of phosphorus-modified small-pore zeolites by utilizing tetraalkyl phosphonium cations as both a structure-directing agent and a phosphorous modification agent was developed. Initially, we investigated the hydrothermal conversion of FAU zeolites as a starting silica/alumina source in the presence of tetraalkyl phosphonium cations with different structures. Various types of zeolites, including small-pore zeolites with 8-MR windows such as LEV, GIS, AEI, and CHA zeolites, were obtained. Next, we applied the dual-template method with a mixture of the N-containing organic structure-directing agent (OSDA) N,N,N-trimethyl-1-adamantammonium cation and the P-containing OSDA tetraethyl phosphonium cation to the synthesis of phosphorus-modified zeolites with small pores, especially CHA zeolites. Using this method, the proportion of P-containing OSDA/N-containing OSDA in as-synthesized CHA zeolites can be easily controlled. After the calcination of the as-synthesized zeolites, CHA zeolites with different degrees of phosphorus modification were easily obtained without significant pore occlusion or toxification of catalytically active sites. The phosphorus-modified CHA zeolite had high thermal stability and retained its structure even after calcination at 1050 °C for 1 h. In addition, in the selective catalytic reduction of NOx with NH3, the Cu-loaded phosphorus-modified CHA zeolite exhibited NO conversion above 90%, even after hydrothermal treatment at 900 °C for 8 h, indicating extremely high hydrothermal stability.

Tandem mass spectrometry in an electrostatic linear ion trap modified for surface-induced dissociation.

A variety of ion traps are used in mass spectrometry. A key feature shared by most of them is the ability to perform tandem mass spectrometry (MS/MS). The Orbitrap is perhaps the most notable ion trap in which MS/MS has yet to be performed. An electrostatic linear ion trap (ELIT) is analogous to an orbitrap in that ions are trapped using solely electrostatic fields. However, the relatively simple ion motion within an ELIT facilitates analysis of fragment ions produced within the device. In this report, we describe an ELIT to which we have added a target for surface induced dissociation (SID). When combined with our previously described method for isolating a precursor ion trapped in an ELIT,1 this apparatus enables MS/MS to be performed. Measurement of product ion m/z is facilitated by the fact that the ELIT is isochronous over the energy range of 1850-2000 eV so that changes to ion energy during SID do not cause major m/z shifts. We demonstrate MS/MS by isolating and dissociating each compound in a four component mixture of tetraalkylphosphonium cations. We also discuss the optimization of collision energy and the length of time that the SID target is available for collision, two parameters that are important in the performance of these experiments.

Phase-Change Ionic Liquids for Postcombustion CO2 Capture

Phase-change ionic liquids, or PCILs, are salts that are solids at normal flue gas processing temperatures (e.g., 40–80 °C) and that react stoichiometrically and reversibly with CO2 (one mole of CO2 for every mole of salt at typical postcombustion flue gas conditions) to form a liquid. Thus, the melting point of the PCIL–CO2 complex is below that of the pure PCIL. A new concept for CO2 separation technology that uses this key property of PCILs offers the potential to significantly reduce parasitic energy losses incurred from postcombustion CO2 capture by utilizing the heat of fusion (ΔHfus) to provide part of the heat needed to release CO2 from the absorbent. In addition, the phase transition yields almost a step-change absorption isotherm, so only a small pressure or temperature swing is required between the absorber and the stripper. Utilizing aprotic heterocyclic anions (AHAs), the enthalpy of reaction with CO2 can be readily tuned, and the physical properties, such as melting point, can be adjusted by modifying the alkyl chain length of the tetra-alkylphosphonium cation. Here, we present data for four tetrabutylphosphonium salts that exhibit PCIL behavior, as well as detailed measurements of the CO2 solubility, physical properties, phase transition behavior, and water uptake for tetraethylphosphonium benzimidazolide ([P2222][BnIm]). The process based on [P2222][BnIm] has the potential to reduce the amount of energy required for the CO2 capture process substantially compared to the current technology that employs aqueous monoethanolamine (MEA) solvents.

Reversible Phase Transformation Ionic Liquids Based on Ternary Keggin Polyoxometalates

Two new polyoxometalate-based ionic liquids, [TBTP]4PW11VO40 and [TBTP]4PMo11VO40, composed of ternary Keggin polyoxometalates and tetraalkylphosphonium cation have been synthesized and characterized. Their layered structures, thermal stability, conductive performance and electrochemical properties are investigated. The two compounds possess layered structures, and both can exhibit a reversible phase transformation process below 100 °C. Their ionic conductivities are dependent on temperature and follow the Arrhenius behavior. Cyclic voltammetry is carried out to study their electrochemical properties in organic solution. The potential values of redox waves indicate that the oxidability of [TBTP]4PMo11VO40 is greater than that of [TBTP]4PW11VO40.

Atomistic insight into orthoborate-based ionic liquids: force field development and evaluation.

We have developed an all-atomistic force field for a new class of halogen-free chelated orthoborate-phosphonium ionic liquids. The force field is based on an AMBER framework with determination of force field parameters for phosphorus and boron atoms, as well as refinement of several available parameters. The bond and angle force constants were adjusted to fit vibration frequency data derived from both experimental measurements and ab initio calculations. The force field parameters for several dihedral angles were obtained by fitting torsion energy profiles deduced from ab initio calculations. To validate the proposed force field parameters, atomistic simulations were performed for 12 ionic liquids consisting of tetraalkylphosphonium cations and chelated orthoborate anions. The predicted densities for neat ionic liquids and the [P6,6,6,14][BOB] sample, with a water content of approximately 2.3-2.5 wt %, are in excellent agreement with available experimental data. The potential energy components of 12 ionic liquids were discussed in detail. The radial distribution functions and spatial distribution functions were analyzed and visualized to probe the microscopic ionic structures of these ionic liquids. There are mainly four high-probability regions of chelated orthoborate anions distributed around tetraalkylphosphonium cations in the first solvation shell, and such probability distribution functions are strongly influenced by the size of anions.

Amphiphilic and Phase‐Separable Ionic Liquids for Biomass Processing

One main limiting factor for the technoeconomics of future bioprocesses that use ionic liquids (ILs) is the recovery of the expensive and potentially toxic IL. We have demonstrated a new series of phase-separable ionic liquids, based on the hydrophobic tetraalkylphosphonium cation ([PRRRR](+)), that can dissolve lignin in the neat state but also hemicellulose and high-purity cellulose in the form of their electrolyte solutions with dipolar aprotic solvents. For example, the IL trioctylmethylphosphonium acetate ([P8881][OAc]) was demonstrated to dissolve up to 19 wt % of microcrystalline cellulose (MCC) at 60 °C with the addition of 40 wt % of DMSO. It was found that the MCC saturation point is dependent on the molar ratio of DMSO and IL in solution. At the optimum saturation, a ∼1:1 molar ratio of [P8881][OAc] to anhydroglucose units is observed, which demonstrates highly efficient solvation. This is attributed to the positive contribution that these more amphiphilic cation-anion pairs provide, in the context of the Lindman hypothesis. This effective dissolution is further illustrated by solution-state HSQC NMR spectroscopy on MCC. Finally, it is also demonstrated that these electrolytes are phase separable by the addition of aqueous solutions. The addition of 10 % NaOAc solution allows a near quantitative recovery of high-purity [P8881][OAc]. However, increased volumes of aqueous solution reduced the recovery. The regenerated material was found to partially convert into the cellulose II crystalline polymorph.