Ionic Liquids for the Separation of Fluorocarbon Refrigerant Mixtures.

This review discusses the research being performed on ionic liquids for the separation of fluorocarbon refrigerant mixtures. Fluorocarbon refrigerants, invented in 1928 by Thomas Midgley Jr., are a unique class of working fluids that are used in a variety of applications including refrigeration. Fluorocarbon refrigerants can be categorized into four generations: chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, and hydrofluoroolefins. Each generation of refrigerants solved a key problem from the previous generation; however, each new generation has relied on more complex mixtures that are often zeotropic, near azeotropic, or azeotropic. The complexity of the refrigerants used and the fact that many refrigerants form azeotropes when mixed makes handling the refrigerants at end of life extremely difficult. Today, less than 3% of refrigerants that enter the market are recycled. This is due to a lack of technology in the refrigerant reclaim market that would allow for these complex, azeotropic refrigerant mixtures to be separated into their components in order to be effectively reused, recycled, and if needed repurposed. As the market for recovering and reclaiming refrigerants continues to grow, there is a strong need for separation technology. Ionic liquids show promise for separating azeotropic refrigerant mixtures as an entrainer in extractive distillation process. Ionic liquids have been investigated with refrigerants for this application since the early 2000s. This review will provide a comprehensive summary of the physical property measurements, equations of state modeling, molecular simulations, separation techniques, and unique materials unitizing ionic liquids for the development of an ionic-liquid-based separation process for azeotropic refrigerant mixtures.

Structure and Dynamics of Hydrofluorocarbon/Ionic Liquid Mixtures: An Experimental and Molecular Dynamics Study.

The physical properties of four ionic liquids (ILs), including 1-n-butyl-3-methylimidazolium tetrafluoroborate ([C4C1im][BF4]), 1-n-butyl-3-methylimidazolium hexafluorophosphate ([C4C1im][PF6]), 1-n-butyl-3-methylimidazolium thiocyanate ([C4C1im][SCN]), and 1-n-hexyl-3-methylimidazolium chloride ([C6C1im][Cl]), and their mixtures with hydrofluorocarbon (HFC) gases HFC-32 (CH2F2), HFC-125 (CHF2CF3), and HFC-410A, a 50/50 wt % mixture of HFC-32 and HFC-125, were studied using molecular dynamics (MD) simulation. Experiments were conducted to measure the density, self-diffusivity, and shear viscosity of HFC/[C4C1im][BF4] system. Extensive analyses were carried out to understand the effect of IL structure on various properties of the HFC/IL mixtures. Density, diffusivity, and viscosity of the pure ILs were calculated and compared with experimental values. The good agreement between computed and experimental results suggests that the applied force fields are reliable. The calculated center of mass (COM) radial distribution functions (RDFs), partial RDFs, spatial distribution functions (SDFs), and coordination numbers (CNs) provide a sense of how the distribution of HFC changes in the liquid mixtures with IL structure. Detailed analysis reveals that selectivity toward HFC-32 and HFC-125 depends on both cation and anion. The molecular insight provided in the current work will help the design of optimal ILs for the separation of azeotropic HFC mixtures.

Cellulose Solubility in Ionic Liquid Mixtures: Temperature, Cosolvent, and Antisolvent Effects.

Select ionic liquids (ILs) dissolve significant quantities of cellulose through disruption and solvation of inter- and intramolecular hydrogen bonds. In this study, thermodynamic solid-liquid equilibrium was measured with microcrystalline cellulose in a model IL, 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIm][DEP]) and mixtures with protic antisolvents and aprotic cosolvents between 40 and 120 °C. The solubility of cellulose in pure [EMIm][DEP] exhibits an asymptotic maximum of approximately 20 mass % above 100 °C. Solubility studies conducted on antisolvent mixtures with [EMIm][DEP] and [BMIm][Cl] indicate that protic solvents, ethanol, methanol, and water, significantly reduce the cellulose capacity of IL mixtures by 38-100% even at small antisolvent loadings (<5 mass %). Alternatively, IL-aprotic cosolvent (dimethyl sulfoxide, dimethylformamide, and 1,3-dimethyl-2-imidazolidinone) mixtures at mass ratios up to 1:1 enhance cellulose dissolution by 20-60% compared to pure [EMIm][DEP] at select temperatures. Interactions between the IL and molecular solvents were investigated by Kamlet-Taft solvatochromic analysis, FTIR, and NMR spectroscopy. The results indicate that preferential solvation of the IL cation and anion by co- and antisolvents impact the ability of IL ions to interact with cellulose thus affecting the cellulose dissolution capacity of IL-solvent mixtures.

Reversible and non-reactive cellulose separations from ionic liquid mixtures with compressed carbon dioxide.

A novel physical (non-reactive) separation of cellulose from an ionic liquid (IL)/cosolvent mixture by compressed carbon dioxide is presented. The precipitation is completely reversible and rapid within small changes of pressure i.e. liquid phase CO2 composition. High pressure phase equilibrium, high pressure NMR, and solid state NMR have been utilized to understand the separation phenomena.

Subcritical CO2 sintering of microspheres of different polymeric materials to fabricate scaffolds for tissue engineering.

The aim of this study was to use CO2 at sub-critical pressures as a tool to sinter 3D, macroporous, microsphere-based scaffolds for bone and cartilage tissue engineering. Porous scaffolds composed of ~200 µm microspheres of either poly(lactic-co-glycolic acid) (PLGA) or polycaprolactone (PCL) were prepared using dense phase CO2 sintering, which were seeded with rat bone marrow mesenchymal stromal cells (rBMSCs), and exposed to either osteogenic (PLGA, PCL) or chondrogenic (PLGA) conditions for 6 weeks. Under osteogenic conditions, the PLGA constructs produced over an order of magnitude more calcium than the PCL constructs, whereas the PCL constructs had far superior mechanical and structural integrity (125 times stiffer than PLGA constructs) at week 6, along with twice the cell content of the PLGA constructs. Chondrogenic cell performance was limited in PLGA constructs, perhaps as a result of the polymer degradation rate being too high. The current study represents the first long-term culture of CO2-sintered microsphere-based scaffolds, and has established important thermodynamic differences in sintering between the selected formulations of PLGA and PCL, with the former requiring adjustment of pressure only, and the latter requiring the adjustment of both pressure and temperature. Based on more straightforward sintering conditions and more favorable cell performance, PLGA may be the material of choice for microspheres in a CO2 sintering application, although a different PLGA formulation with the encapsulation of growth factors, extracellular matrix-derived nanoparticles, and/or buffers in the microspheres may be advantageous for achieving a more superior cell performance than observed here.

Effect of different sintering methods on bioactivity and release of proteins from PLGA microspheres.

Macromolecule release from poly(d,l-lactide-co-glycolide) (PLGA) microspheres has been well-characterized, and is a popular approach for delivering bioactive signals from tissue-engineered scaffolds. However, the effect of some processing solvents, sterilization, and mineral incorporation (when used in concert) on long-term release and bioactivity has seldom been addressed. Understanding these effects is of significant importance for microsphere-based scaffolds, given that these scaffolds are becoming increasingly more popular, yet growth factor activity following sintering and/or sterilization is heretofore unknown. The current study evaluated the 6-week release of transforming growth factor (TGF)-ß3 and bone morphogenetic protein (BMP)-2 from PLGA and PLGA/hydroxyapatite (HAp) microspheres following exposure to ethanol (EtOH), dense phase carbon dioxide (CO2), or ethylene oxide (EtO). EtO was chosen based on its common use in scaffold sterilization, whereas EtOH and CO2 were chosen given their importance in sintering microspheres together to create scaffolds. Release supernatants were then used in an accelerated cell stimulation study with human bone marrow stromal cells (hBMSCs) with monitoring of gene expression for major chondrogenic and osteogenic markers. Results indicated that in microspheres without HAp, EtOH exposure led to the greatest amount of delivery, while those treated with CO2 delivered the least growth factor. In contrast, formulations with HAp released almost half as much protein, regardless of EtOH or CO2 exposure. Notably, EtO exposure was not found to significantly affect the amount of protein released. Cell stimulation studies demonstrated that eluted protein samples performed similarly to positive controls in PLGA-only formulations, and ambiguously in PLGA/HAp composites. In conclusion, the use of EtOH, subcritical CO2, and EtO in microsphere-based scaffolds may have only slight adverse effects, and possibly even desirable effects in some cases, on protein availability and bioactivity.

The future of carbon dioxide for polymer processing in tissue engineering.

The use of CO2 for scaffold fabrication in tissue engineering was popularized in the mid-1990 s as a tool for producing polymeric foam scaffolds, but had fallen out of favor to some extent, in part due to challenges with pore interconnectivity. Pore interconnectivity issues have since been resolved by numerous dedicated studies that have collectively outlined how to control the appropriate parameters to achieve a pore structure desirable for tissue regeneration. In addition to CO2 foaming, several groups have leveraged CO2 as a swelling agent to impregnate scaffolds with drugs and other bioactive additives, and for encapsulation of plasmids within scaffolds for gene delivery. Moreover, in contrast to CO2 foaming, which typically relies on supercritical CO2 at very high pressures, CO2 at much lower pressures has also been used to sinter polymeric microspheres together in the presence of cells to create cell-seeded scaffolds in a single step. CO2 has a number of advantages for polymer processing in tissue engineering, including its ease of use, low cost, and the opportunity to circumvent the use of organic solvents. Building on these advantages, and especially now with the tremendous precedent that has paved the way in defining operating parameters, and making the technology accessible for new groups to adapt, we invite and encourage our colleagues in the field to leverage CO2 as a new tool to enhance their own respective unique capabilities.

Tailoring of processing parameters for sintering microsphere-based scaffolds with dense-phase carbon dioxide.

Microsphere-based polymeric tissue-engineered scaffolds offer the advantage of shape-specific constructs with excellent spatiotemporal control and interconnected porous structures. The use of these highly versatile scaffolds requires a method to sinter the discrete microspheres together into a cohesive network, typically with the use of heat or organic solvents. We previously introduced subcritical CO(2) as a sintering method for microsphere-based scaffolds; here we further explored the effect of processing parameters. Gaseous or subcritical CO(2) was used for making the scaffolds, and various pressures, ratios of lactic acid to glycolic acid in poly(lactic acid-co-glycolic acid), and amounts of NaCl particles were explored. By changing these parameters, scaffolds with different mechanical properties and morphologies were prepared. The preferred range of applied subcritical CO(2) was 15-25 bar. Scaffolds prepared at 25 bar with lower lactic acid ratios and without NaCl particles had a higher stiffness, while the constructs made at 15 bar, lower glycolic acid content, and with salt granules had lower elastic moduli. Human umbilical cord mesenchymal stromal cells (hUCMSCs) seeded on the scaffolds demonstrated that cells penetrate the scaffolds and remain viable. Overall, the study demonstrated the dependence of the optimal CO(2) sintering parameters on the polymer and conditions, and identified desirable CO(2) processing parameters to employ in the sintering of microsphere-based scaffolds as a more benign alternative to heat-sintering or solvent-based sintering methods.

High-pressure phase equilibria with compressed gases.

An apparatus is described that is capable of determining high-pressure vapor-liquid equilibrium, liquid-liquid equilibrium, solid-liquid-vapor equilibrium, vapor-liquid-liquid equilibrium, and mixture critical points and transitions. The device is capable of temperatures to 150 degrees C and pressures to 300 bars (higher with slight modifications). The construction and operation are described in detail and do not require the use of mercury. This method requires very low sample volumes and no analytical equipment nor system-specific calibration. The apparatus was verified by comparison with literature data for the decane-CO(2) mixture and CO(2)-ionic liquid [1-hexyl-3-methyl-imidazolium bis(trifyl)imide)] systems. The experimental data have excellent agreement with the literature data that used different experimental methods. A rigorous error analysis of the system is also presented.

Expanding the useful range of ionic liquids: melting point depression of organic salts with carbon dioxide for biphasic catalytic reactions.

Large and previously unreported melting point depressions (even exceeding DeltaTm of 100 degrees C) were observed for simple ammonium and phosphonium salts in the presence of compressed CO2, bringing them well within the range of typical ionic liquids; possible applications include biphasic catalysis in IL/scCO2 systems as demonstrated for rhodium complex catalyzed hydrogenation, hydroformylation, and hydroboration of 2-vinyl-naphthalene using a CO2-induced molten sample of [NBu4][BF4] as a catalyst phase at temperatures in the range of 55-75 degrees C, i.e. 100 degrees C below the normal melting point of the organic salt.

Carbon dioxide induced separation of ionic liquids and water.

Both hydrophobic and hydrophilic room-temperature ionic liquids can be separated from aqueous solutions with relatively low-pressure gaseous carbon dioxide.

CO(2) as a separation switch for ionic liquid/organic mixtures.

A novel technique to separate ionic liquids from organic compounds is introduced which uses carbon dioxide to induce the formation of an ionic liquid-rich phase and an organic-rich liquid phase in mixtures of methanol and 3-butyl-1-methyl-imidazolium hexafluorophosphate ([C4mim][PF6]). If the temperature is above the critical temperature of CO2 then the methanol-rich phase can become completely miscible with the CO2-rich phase, and this new phase is completely ionic liquid-free. Since CO2 is nonpolar, it is not equipped to solvate ions. As the CO2 dissolves in the methanol/[C4mim][PF6] mixture, the solvent power of the CO2-expanded liquid is significantly reduced, inducing the formation of the second liquid phase that is rich in ionic liquid. This presents a new way to recover products from ionic liquid mixtures and purify organic phases that have been contaminated with ionic liquid. Moreover, these results have important implications for reactions done in CO2/ionic liquid biphasic mixtures.