Neat CO2 Research Articles

Supercritical fluid chromatography and two-dimensional supercritical fluid chromatography of polar car lubricant additives with neat CO 2 as mobile phase

Car lubricant additives are added to mineral or synthetic base stocks to improve viscosity and resistance to oxidation of the lubricant and to limit wear of engines. Their total amount in the commercial lubricant varies from a few percents to 20–25%. As they belong to various chemical classes and are added to a very complex medium, the base stock, their detailed chromatographic analysis is very difficult and time consuming as it should involve sample treatment and preparative scale separations in order to simplify the sample. The aim of this work is to determine the feasibility of the separation of low molecular weight lubricant additives using various packed columns with pure CO 2 as a mobile phase to enable implementation of flame ionisation detection as universal detector. This is part of a hypernated system including more sophisticated specific detectors, such as AED, FTIR or MS to obtain detailed structural information of compounds. This paper is devoted to the comparison of some stationary phases supposed to provide hydrocarbon group type separation (silica and normal phase) or separations on alkyl-bonded silica in non-aqueous mode of some selected classes of additives in test mixtures or in base stocks. Adsorption chromatography allows partial separation of additives from the base stocks while the direct elution of test additives can only be obtained on reversed phase supports having a very efficient silanol group protection so the interaction of the more polar compounds is much reduced. A two-dimensional scheme of analysis is also described. It combines adsorption chromatography to separate most of the polar additives from the base stock and alkyl-bonded silica for more detailed separation of the additives. However, overlapping between groups of compounds and the lack of resolution between some additives and the base stock should be addressed by the implementing of selective detectors.

Enhanced Selective Extraction of Hexane from Hexane/Soybean Oil Mixture Using Binary Gas Mixtures of Carbon Dioxide

Carbon dioxide (CO2) can effectively separate hexane from a mixture of soybean oil (SBO) and hexane with a slight coextraction of SBO. Previous research demonstrated that CO2 entrained with helium significantly reduced SBO solubility in CO2. In this study, CO2 was mixed with three gases (He, N2, or Ar) (0.5-30 vol %) to decrease SBO solubility while attempting to maintain hexane solubility. The binary gas mixtures (at 25 degrees C and 9.31 MPa) were passed through a 25 wt % hexane/SBO mixture inside a 2.5 m fractionation column. Coextracted SBO was inversely proportional to binary gas concentration, whereas residual hexane in the raffinate was proportional to binary gas concentration. The 10% binary mixture of N2 or Ar was the best compromise to obtain both low residual hexane levels (i.e., 26 ppm) and low SBO coextraction (i.e., only 40 mg). This carry-over of SBO represents a 95% reduction in SBO carry-over compared to neat CO2.

Enhancement of Structural Fluctuation in the Region Connecting Two Kinds of Critical Points in Temperature−Pressure−Composition Three-Dimensional Phase Diagram: Raman Studies of Benzene/CO2 Binary Systems up to Supercritical Region

Pressure dependence of Raman spectra of benzene/CO2 two-component systems was systematically studied at different temperatures and compositions. We estimated the magnitude of inhomogeneous component in Raman bandwidth to get information on the structural fluctuation in the system. It was found that the inhomogeneous bandwidth attains a maximum on an isothermal plane in the temperature-pressure-composition three-dimensional phase diagram when the state point crosses the line connecting the region where the density fluctuation is large (the vicinity of the critical point of neat CO2) and the region where the concentration fluctuation in a binary system is enhanced (the vicinity of the critical solution point). By accumulating such data, we found that the points of large structural fluctuation comprise a sheet that includes the extension line of the gas-liquid equilibrium line in the phase diagram of neat CO2 and the line connecting critical solution points of the two-component system at different temperatures. Interaction between benzene and CO2 molecules in the supercritical region is briefly discussed.

Molecular Interactions and CO2-Philicity in Supercritical CO2. A High-Pressure NMR and Molecular Modeling Study of a Perfluorinated Polymer in scCO2

The carbon and fluorine chemical shifts of mixtures of carbon dioxide and Krytox, a carboxylic acid end-capped perfluorinated polyether used as stabilizer for the dispersion polymerization of methyl methacrylate, have been studied using high-pressure, high-resolution nuclear magnetic resonance. 13C and 19F spectra were measured in the density region between 0.54 and 0.73 g.cm(-3) at 334 K for different solutions of Krytox in scCO2 (0.22, 1.13 and 1.72 w/w %). An in-house developed high-pressure apparatus with the capability to change in situ the sample composition was used for this purpose using a 10 mm polyether ketone NMR tube. The nature of CO2-Krytox interaction was assessed both by comparing the CO2 deltaC variation of neat CO2 with that of mixtures with increasing surfactant composition and by the analysis of Krytox 19F corrected chemical shifts in terms of medium magnetic susceptibility. Ab initio calculations, at the second-order Møller-Plesset level of theory to include the effects of electron correlation, were performed to access and compare the nature of the interactions between CO2 and perfluorinated and nonfluorinated analogue model molecules. Both experimental 13C and 19F HP-NMR results and molecular modeling studies support a F...CO2 site-specific Lewis acid-Lewis base interaction model. A positive entropic variation for the formation of CO2-fluorinated solute complex is advanced as an explanation for the higher solubility of perfluorinated molecules when compared to the nonfluorinated analogues.

Supercritical Fluid Extraction of Microcystin LR from Cyanobacteria using Aqueous Acetonitrile Modifier

The use of supercritical fluids for the analytical extraction of the nonvolatile, higher molecular weight compound, microcystin LR, from cyanobacteria was investigated. The microcystin LR included in this study was sparingly soluble in neat supercritical fluid CO2. However, the microcystin LR was successfully extracted with a ternary mixture (90% CO2, 9.5% acetonitrile, 0.5% water) at 40°C and 250 atm. The polar carbon dioxide‐aqueous acetonitrile fluid system gave a high extraction efficiency for the extraction of the polar microcystin LR from cyanobacteria. In our laboratory, in the past, aqueous acetic acid‐modified CO2 was used for the extraction of microcystins from cyanobacteria and aqueous methanol was also used as a modifier. In this study, we tried a new organic solvent, aqueous acetonitrile as a co‐solvent, so that we could obtain a higher extraction efficiency of the microcystin LR from cyanobacteria than in earlier experiments.

Raman spectral shifts of CO 2 as measure of CO 2-philicity of solutes in supercritical carbon dioxide

We have examined the solute–solvent interactions by measuring Raman spectra of CO 2 in neat supercritical CO 2 (sc-CO 2), and sc-CO 2 containing β-diketones (acetylacetone, trifluoroacetylacetone, and hexafluoroacetylacetone) and uranyl β-diketonato complex [UO 2(hfacac) 2DMSO] (hfacac: hexafluoroacetylacetone, DMSO: dimethyl sulfoxide). As a result, it was found that the Raman bands of sc-CO 2 containing β-diketones are shifted to lower wavenumber compared with those of neat sc-CO 2 and that such a red-shift is not observed in sc-CO 2 containing UO 2(hfacac) 2DMSO. From these results, it was suggested that the Lewis acid–Lewis base (LA–LB) interactions between carbonyl oxygens of β-diketones and CO 2 carbon, and the hydrogen bondings between OH of β-diketones and CO 2 oxygen are formed in sc-CO 2 containing β-diketones, and that the shifts in Raman bonds of sc-CO 2 can be used as the measure of CO 2-philicity of solutes in sc-CO 2. On the basis of such results, we measured Raman spectra of CO 2 in sc-CO 2 containing compounds with different donicity. As a result, we found out that the wavenumbers ( ν l ) of Raman bands of CO 2 correlate with the donor number (DN) of compounds, i.e., ν l = 1281.24–0.0109 DN at 25.0 MPa and 40 °C.

A Direct CO Polymer Electrolyte Membrane Fuel Cell

A good source of energy: The reformation of hydrocarbons generates CO as well as H2. Despite its high reducing activity, CO is not suitable as a fuel in polymer electrolyte membrane fuel cells (PEMFCs) because it is a strong poison for the Pt catalyst used as the anode. A new fuel cell has now been developed with a rhodium octaethylporphyrin ([Rh(oep)]) anode catalyst that uses neat CO as a fuel, and delivers high performance (see graph).

Solvent-modified supercritical CO 2: A beneficial medium for heterogeneously catalyzed oxidation reactions

The selective oxidation of benzyl alcohol to benzaldehyde with molecular oxygen in supercritical carbon dioxide over a commercial 0.5 wt.% Pd/Al 2O 3 catalyst was studied under continuous-flow conditions using toluene as a co-solvent. The whole range from neat CO 2, toluene-modified CO 2, CO 2-expanded toluene to neat toluene was investigated. Adding a small amount of toluene to the feed (molar ratio toluene/alcohol of 1/1) resulted in a strikingly higher reaction rate than in neat CO 2. The turnover frequency reached 2500 h −1 compared to 1500 h −1 without toluene. A too high toluene concentration led to a biphasic reaction mixture and to a remarkably lower catalytic activity (TOF 130 h −1 at a toluene/benzyl alcohol/O 2/CO 2 molar ratio of 16/0.8/0.4/82.8). In all cases, a strong dependence of the reaction rate on the pressure was found, which was closely related to the phase behaviour. The best catalytic activity could be achieved under monophasic conditions due to the elimination of the gas/liquid interface as uncovered by video monitoring combined with ATR-IR spectroscopy. Additional in situ infrared spectroscopic investigations showed that toluene was fully dissolved already at low pressure (120 bar), whereas higher pressure was required (150 bar) to dissolve benzyl alcohol and thus to reach a single phase. Using a higher amount of toluene (five times more toluene than alcohol) led to complete dissolution of the benzyl alcohol in the CO 2/toluene mixture at 130 bar and thus to a higher catalytic activity in this region of lower pressure.

Extraction of lipids and cholesterol from squid oil with supercritical carbon dioxide

The oil obtained from waste squid viscera consists of multi-compounds such as EPA, DHA and other valuable polyunsaturated fatty acids. The refining of this squid oil, using supercritical carbon dioxide plus ethanol, was performed in a semi-continuous flow extractor at 8 to 17 MPa and 25 to 50 °C. When 1.5% w/w ethanol was added to the solvent, the solubility of lipids was increased by up to 50% over the neat CO2 value. The extraction curves indicated mass transfer to be solubility limited. Cholesterol was co-extracted with the lipids but, with its lower solubility, less than 54% appeared in the refined oil. The results of the carbon dioxide/multi-compound squid oil system at applied to the extraction conditions were correlated with the mole fraction of the cholesterol and the density of the pure solvent.

Cu(ii) extraction by supercritical fluid carbon dioxide from a room temperature ionic liquid using fluorinated β-diketones

Copper(II) can be extracted in supercritical CO2 from a room temperature ionic liquid using CO2-philic fluorinated β-diketonate ligands; thanks to the 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIMTf2N) ionic liquid properties, there is no need to add modifiers to the neat supercritical CO2 to reach high extraction efficiencies.

Separation of rosemary antioxidant compounds by supercritical fluid chromatography on coated packed capillary columns

Antioxidant compounds in rosemary extracts obtained by supercritical fluid extraction (SFE) were separated by supercritical fluid chromatography (SFC) on packed capillary columns. The columns contained silica particles coated with SE-54 (5% phenyl, 95% methyl silicone) and Carbowax 20 M [poly(ethylene glycol)]. The use of coated packed capillary columns allowed the separation of polar compounds by SFC with neat CO 2. The SFC conditions were selected on the basis of previous work. High pressures (up to 370 atm; 1 atm = 10,325 Pa) and moderate temperatures (up to 100 °C) were used to separate the compounds responsible for the antioxidant activity such as carnosic acid and carnosol while lower pressures were sufficient to separate the compounds of the essential oil.

Fluorescence of 4-aminophthalimide in supercritical CO2–ethanol mixtures

Steady-state and time-resolved fluorescence studies of 4-aminophthalimide (AP) in neat supercritical CO2 and supercritical CO2–ethanol mixtures at 35 and 45 °C are presented. In neat CO2, the emission maximum of AP shifts to the red upon density increase because of the increase of average number of solvent molecules interacting with the probe. In CO2–ethanol mixtures of different ethanol densities (0.025 and 0.125 M) the tendency upon CO2 density increase is opposite. In mixtures, the CO2 density increase, also increases the probability of exchange of the ethanol molecules interacting with AP by CO2 molecules. This causes a blue shift that is bigger than the red shift caused by density increase. In all the cases, emission spectra were time independent in the nanosecond time range. This allowed to take solvation effects into account using a Langmuir adsorption model, under equilibrium conditions. This is the simplest association model that can semiquantitative describe the results and can successfully explain the lack of solvation entropic effects in emission of AP in the mixtures near the critical density of CO2.

Macromolecular surfactants for supercritical carbon dioxide applications: Synthesis and characterization of fluorinated block copolymers prepared by nitroxide‐mediated radical polymerization

AbstractPoly(perfluorooctyl‐ethylenoxymethylstyrene) (PFDS) and poly(1,1,2,2‐tetrahydroperfluorodecyl acrylate) (PFDA) homopolymers as well as poly(styrene)‐b‐poly(perfluorooctyl‐ethylenoxymethylstyrene) (PS‐b‐PFDS) and poly(styrene)‐b‐poly(1,1,2,2‐tetrahydroperfluorodecyl acrylate) acrylate) (PS‐b‐PFDA) block copolymers of various chain lengths were synthesized by nitroxide‐mediated radical polymerization in the presence of either 2,2,6,6‐tetramethyl‐1‐piperidinyloxy free radical (TEMPO) in the case of FDS monomer or N‐tert‐butyl‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl)‐N‐oxyl (DEPN) in the case of the FDA monomer. The molar composition of the block copolymers was determined by elemental analysis and proton NMR while the blocky structure was checked by SEC analysis in trifluorotoluene. Block copolymers PS‐b‐PFDS (3.6K/60K) and PS‐b‐PFDA (3.7K/43K) were soluble in neat CO2 at moderate pressure and temperature, indicating the formation of micelles. Similar block copolymers with a longer PS block such as PS‐b‐PFDA (9.5K/49K), corresponding to a lower CO2‐philic/CO2‐phobic balance, were insoluble in neat CO2 but could be solubilized in the presence of styrene as a cosolvent. Additionally, surface and bulk properties of PS‐b‐PFDA were investigated, indicating the same surface tension as for the PFDA homopolymer (γLV = 10.3 mN/m) and a bulk nanostructured morphology. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3537–3552, 2004

Decontamination of uranium oxides from solid wastes by supercritical CO 2 fluid leaching method using HNO 3–TBP complex as a reactant

A new method referred to as supercritical CO 2 fluid leaching (SFL), which is based on selective dissolution of uranium oxides with the supercritical CO 2 fluid containing the HNO 3–tri- n-butyl phosphate (TBP) complex, has been developed for the removal and recovery of uranium from solid wastes contaminated by uranium oxides. A recommended SFL procedure has been established consulting experimental results of the complexation efficiency of the uranium oxides and the dissolution efficiency of U(VI)–TBP complex at different pressures. The recommended method consists of complexation process at 60 °C and 15 MPa for 150 min using the supercritical CO 2 containing the HNO 3–TBP complex and dissolution process at 60 °C and 20 MPa using a flow of the neat supercritical CO 2. The complexation–dissolution cycle is repeated twice. The feasibility of the recommended procedure to the removal of uranium from the solid wastes was demonstrated using synthetic wastes that were a mixture of standard sea sand (50 g) and UO 2 or U 3O 8 powders (20–200 mg of U). The decontamination factor of UO 2 or U 3O 8 was determined to be higher than 5×10 2. The solid waste after the SFL treatment was dry and contained enough low concentrations of TBP and HNO 3.

Application of CO2-expanded solvents in heterogeneous catalysis: a case study

CO2-expanded liquid (CXL) solvents are known to provide optimum reaction and environmental benefits for certain homogeneously catalyzed oxidations. In this work, CXL solvent media are applied to heterogeneously catalyzed oxidations. The oxidation of cyclohexene is investigated as a model reaction. Conversion/selectivity measurements were made in a stirred batch reactor on a MCM-41 encapsulated iron porphyrin chloride complex (p=1–127bar, T=25–50°C, t=4–12h), using either dioxygen or iodosylbenzene as oxidant. Neat acetonitrile and CO2-expanded acetonitrile are used as reaction media. The pressures required for reaction studies in CO2-based media were determined by expansion studies as well as cloud point measurements. For the oxidation with dioxygen, conversion and product yields in CO2-expanded acetonitrile (∼30% CO2) almost doubled compared to the neat organic solvent. In contrast, CO2-replacement of the organic solvent had a negligible effect on conversion and selectivity values when using iodosylbenzene as oxidant. Our results indicate that CXLs significantly reduce the use of conventional organic solvents as reaction media and also enhance catalyst stability against leaching. These advantages suggest that CXLs are promising media for developing green heterogeneous catalytic processes.

Supercritical fluid extraction of mixed wastes

This study assesses the feasibility of extraction and separation of uranium and PCBs from contaminated soil samples using a sequential supercritical CO2 extraction method. The contaminated soil was first extracted with neat supercritical CO2 at 150 °C and 200 atm to remove PCBs. Subsequently, complexing agents, tri-n-butylphosphate (TBP) and thenoyltrifluoroacetylacetone (HTTA), were added to remove uranium from the contaminated soils. Using this method, nearly all PCBs were removed, and up to 75% of the uranium in the soil could be removed after multiple extractions. This sequential supercritical fluid extraction method may have a wide range of applications for treating various mixed wastes.

Influence of Carbon Content and End‐Capping of ODS Stationary Phases on the Separation Behavior of Triacylglycerols in LC and SFC

The separation behavior of triacylglycerols (TAGs) on various octadecylsilylated (ODS) silicas was investigated in supercritical fluid chromatography (SFC) and in liquid chromatography (LC). The retention factor of triolein, the selectivity toward the carbon number of saturated TAGs, and the selectivity toward the number of double bonds of TAGs in the stearic‐oleic acid series varied with the carbon content of the ODS‐silicas. In SFC using neat CO2 mobile phase, the residual silanol groups on the silica surface sometimes caused the peak tailing. End‐capping with hexamethyldisilazane vapor at 300°C was of greater advantage than the end‐capping in liquid phase for the deactivation of the silica surface. The retention of TAGs in SFC was discussed using the concept of equivalent carbon number (ECN). The value of ECN of an unsaturated TAG varied not only with the carbon content and the way of end‐capping of the ODS‐silicas, but also with the separation temperature and pressure.

Limited polysulfone solubility in supercritical dimethyl ether with THF and DMF cosolvents

Reported in this short communication are the conditions needed to dissolve up to 1.5 wt.% polysulfone (PSU) (M w=35 000; M n=16 000) in supercritical fluid (SCF) solvents plus cosolvents. Initial experiments showed that PSU does not dissolve in neat CO 2, propane, butane, dimethyl ether (DME), chlorodifluoromethane, or difluoroethane to temperatures as high as 200 °C and pressures of 2100 bar. However, PSU solubility is observed for DME with the addition of 24–65 wt.% tetrahydrofuran (THF) or N, N-dimethyl formamide (DMF), two polar liquid solvents that readily dissolve PSU at room conditions even though PSU melts at ∼188 °C. DMF is a better cosolvent for PSU since DMF has a much higher dipole moment than THF, it has a higher molar density at room temperature, and it has a higher critical temperature which implies that DMF has a higher cohesive energy density than THF. With ∼24 wt.% cosolvent added to DME, the single-phase region extends to 40 °C lower temperatures with DMF compared with THF. Likewise, the cloud-point pressures for 0.15 wt.% PSU in DME with 56 wt.% DMF are ∼400 bar lower than those for a solution with approximately the same overall concentrations but with THF. The solution densities for PSU in DME+THF range from 0.90 to 0.65 g/cm 3, depending on THF concentration and the solution densities for PSU in DME+DMF range from 0.53 to 0.66 g/cm 3, also depending on DMF concentration. Compared with THF solutions, lower DMF solution densities are observed since it takes lower pressures to obtain a single-phase when DMF is used as a cosolvent. The experimental data demonstrate that DME should be considered an anti-solvent to knock PSU out of solution, since significant amounts of cosolvent are needed to dissolve PSU in DME.

Measurement and prediction of LDPE/CO2 solution viscosity

AbstractWhen CO2 is dissolved into a polymer, the viscosity of the polymer is drastically reduced. In this paper, the melt viscosities of low‐density polyethylene (LDPE)/supercritical CO2 solutions were measured with a capillary rheometer equipped at a foaming extruder, where CO2 was injected into a middle of its barrel and dissolved into the molten LDPE. The viscosity measurements were performed by varying the content of CO2 in the range of 0 to 5.0 wt% and temperature in the range of 150°C to 175°C, while monitoring the dissolved CO2 concentration on‐line by Near Infrared spectroscopy. Pressures in the capillary tube were maintained higher than an equilibrium saturation pressure so as to prevent foaming in the tube and to realize single‐phase polymer/CO2 solutions. By measuring the pressure drop and flow rate of polymer running through the tube, the melt viscosities were calculated. The experimental results indicated that the viscosity of LDPE/CO2 solution was reduced to 30% of the neat polymer by dissolving CO2 up to 5.0 wt% at temperature 150°C. A mathematical model was proposed to predict viscosity reduction owing to CO2 dissolution. The model was developed by combining the Cross‐Carreau model with Doolittle's equation in terms of the free volume concept. With the Sanchez‐Lacombe equation of state and the solubility data measured by a magnetic suspension balance, the free volume fractions of LDPE/CO2 solutions were calculated to accommodate the effects of temperature, pressure and CO2 content. The developed model can successfully predict the viscosity of LDPE/CO2 solutions from PVT data of the neat polymer and CO2 solubility data.

Optimization of supercritical fluid extraction for the separation of hyperforin and adhyperforin in St. John’s wort (Hypericum perforatum L.)

Supercritical fluid extraction (SFE) with neat CO2 was optimized for selective removal of hyperforin and adhyperforin from St. John’s wort. High extraction efficiencies were achieved when the fluid density was 0.60 g ml−1 or above. The maximum extraction efficiency of hyperforin and adhyperforin could be achieved with a very mild condition such as 30 °C and 80 atm (density = 0.64 g ml−1). Under these conditions, the amount of hyperforin extracted was comparable to the results of conventional solvent-based extractions (ultrasonic or boiling). The maximum quantity of hyperforin extracted from the St. John’s wort was around 12 mg/g dry plant and the reproducibility of the SFE method was comparatively good (RSD = 10%). Hyperforin and adhyperforin were isolated from the crude SFE extract (using HPLC) with purities of 98.7 and 93.3%, respectively. The identity of the isolated hyperforins was confirmed by NMR and MS methods.