Supercritical Fluid Solvents Research Articles

Comparison of Procedures for Isolation of Monoterpene Hydrocarbons from Fresh Needles of Picea abies and Picea omorica

Extraction procedures (steam distillation, supercritical fluid extraction and solvent extraction) for isolation of monoterpene hydrocarbons from fresh needles of Picea abies and Picea omorica were optimised. The procedures were compared with the aim of minimizing consumption of needles and improving the extraction efficiency and repeatability. An influence of homogenisation procedures and storage conditions (liquid nitrogen, -18 and 4 °C) on the total content and composition of essential oils was studied. Cryogenic grinding (liquid nitrogen) combined with the extraction with cold hexane (extraction time 2 h) and subsequent GC-MS determination in freshly homogenised needles gives the best results (1.5-4 times better extraction efficiency, RSD < 10% for P. abies and < 25% for P. omorica). Limits of detections (3 S/N) for individual monoterpene hydrocarbons from units to tens of ng/g and recoveries 97.2-101.4% were found in fresh needles (calculated to fresh weight). While cooling to 4 °C is unacceptable, freezing at -18 °C for the period of 18 days in the dark gives also good results.

The Continuous Acid-Catalyzed Dehydration of Alcohols in Supercritical Fluids: A New Approach to the Cleaner Synthesis of Acetals, Ketals, and Ethers with High Selectivity

We report a new continuous method for forming ethers, acetals and ketals using solid acid catalysts, DELOXAN ASP or AMBERLYST 15, and supercritical fluid solvents. In the case of ether formation, we observe a high selectivity for linear alkyl ethers with little rearrangement to give branched ethers. Such rearrangement is common in conventional syntheses. Our approach is effective for a range of n-alcohols up to n-octanol and also for the secondary alcohol 2-propanol. In the reaction of phenol with an alkylating agent, the continuous reaction can be tuned to give preferential O- or C-alkylation with up to 49% O-alkylation with supercritical propene. We also investigate the synthesis of a range of cyclic ethers and show an improved method for the synthesis of THF from 1,4-butandiol under very mild conditions.

Extraction of lipid‐grown bacterial cells by supercritical fluid and organic solvent to obtain pure medium chain‐length polyhydroxyalkanoates

AbstractA simple two‐step process was developed to extract and purify medium chain‐length polyhydroxyalkanoates (MCL‐PHA) from bacterial cells (Pseudomonas resinovorans) grown on lard and tallow. The process consists of supercritical fluid extraction (SFE) of the lyophilized cells with carbon dioxide to remove lipid impurities, followed by chloroform extraction of the cells to recover the MCL‐PHA. SFE conditions were varied as to temperature (40–100°C), pressure (2000–9000 psi), and carbon dioxide flow rate (0.5–1.5 L/min, expanded gas). Lipid material, usually 2–4%, but in some cases as high as 11%, was extracted from the dried cells by SFE. A pressure range (5000–9000 psi, increased stepwise), a temperature of 60°C, and a carbon dioxide flow of 1.5 L/min were routinely used to extract the bacterial cells (4–5 g) after 3 h. Higher flow rates could shorten the extraction time even more. SFE did not extract MCL‐PHA from the cells. Yield of MCL‐PHA after chloroform extraction at room temperature was a maximum of 42.4% based on dry cell weight. The results show that the two‐step process saves time, uses much less organic solvent, and produces a purer MCL‐PHA biopolymer than previous extraction and purification methods. A more environmentally friendly clean‐up procedure based on SFE and organic solvent recovery was developed to remove contaminating lipid materials from the fermentation biomass, allowing for the recovery of higher purity MCL‐PHA that are suitable for more demanding applications.

Extraction of Polycyclic Aromatic Hydrocarbons from Soils: A Comparison Between Focused Microwave Assisted Extraction, Supercritical Fluid Extraction, Subcritical Solvent Extraction, Sonication and Soxhlet Techniques

Focused microwave, supercritical fluid and subcritical solvent extraction methods were compared with the classical Soxhlet and sonication procedures for routine determination of polycyclic aromatic hydrocarbons (PAHs) in contaminated soil. In each case the extraction efficiency was examined for 5 series (n = 5) of 15 PAHs. Quantitative and pragmatic arguments were considered toward the selection of an appropriated method for routine analysis. The new extraction techniques yielded interesting alternatives to conventional ones, not least since their quantitative results proved more easily reproducible than those afforded either by Soxhlet and sonication. Besides, the automation of extraction systems is a crucial criterion for routine analysis.

Phase Behavior of Polymers in Supercritical Fluid Solvents.

Phase Behavior of Polymers in Supercritical Fluid Solvents.

Homogeneous Organic Reactions as Mechanistic Probes in Supercritical Fluids.

An interesting and intriguing aspect of supercritical fluid solutions is their microscopic inhomogeneities. The most well-known of these are the long-range densities fluctuations that occur in a pure fluid at conditions close to the critical point. However, there are also short-range inhomogeneities around dissolved solutes in supercritical fluids, which are frequently referred to as local density augmentation and local composition enhancements. Many questions have arisen about the potential influences that short-range solvation and long-range density fluctuations might have on complex chemical reaction schemes. In attempts to answer these questions, researchers have sought to study the effect of supercritical fluid solvents on simple unimolecular and bimolecular reactions. In doing so, the reactions themselves have become probes of the molecular structure and the influence of solvation on reactivity in supercritical fluids (SCFs). In this paper, the authors attempt to review those studies that involve simple reactions as probes of molecular structure and its effect on reactivity in SCFs. The paper discusses (1) the potential ways for solvation in supercritical fluids to affect reactivity and (2) the reactions that probe solvation in supercritical fluids, including diffusion-controlled reactions, kinetically-controlled reactions, equilibrium reactions, SCF solvent cage effects, and other important reactions and directions.

Chemical Modification of Metal Oxide Surfaces in Supercritical CO2: The Interaction of Supercritical CO2 with the Adsorbed Water Layer and the Surface Hydroxyl Groups of a Silica Surface

Infrared spectroscopy was used to probe the interaction of CO2 under supercritical fluid (SCF) conditions with a fumed silica. The experimental difficulties associated with CO2 absorption of infrared light in the hydroxyl spectral region are overcome by using deuterated silica or by venting of the CO2 prior to obtaining a spectrum. It is shown that CO2 behaves quite differently from traditional nonaqueous solvents (i.e., carbon tetrachloride, toluene, and cyclohexane) with respect to interactions with the adsorbed layer of water on the surface. A dry silica easily extracts and adsorbs the residual water present in these nonaqueous solvents whereas, in contrast, a dry silica remains dry when placed in contact with the SCF CO2. Moreover, the CO2 extracts the adsorbed water from wet silica. The SCF solvent extracts more surface water into the fluid phase with increasing density, and repeated extraction cycles with SCF CO2 result in the removal of all water from the surface. The interaction of SCF CO2 with th...

Free Radical Chlorination of Alkanes in Supercritical Carbon Dioxide: The Chlorine Atom Cage Effect as a Probe for Enhanced Cage Effects in Supercritical Fluid Solvents

The chlorine atom cage effect was used as a highly sensitive probe for studying the effect of viscosity and the possible role of solvent clusters on cage lifetimes and reactivity for reactions carr...

Supercritical Fluid Tuning of Reactions Rates: the Cis−Trans Isomerization of 4-4‘-Disubstituted Azobenzenes

Remarkable tuning of reaction rates has been achieved for the isomerization of several dye molecules in supercritical fluid solvents using both small pressure changes and small additions of cosolvents. Rates of the thermal cis−trans relaxation were measured spectroscopically following irradiation for three dyes in supercritical carbon dioxide and ethane, pure and with several polar and protic cosolvents. The results are explained in terms of local density and composition enhancements, as well as by a change of mechanism due to strong hydrogen bonding. This study demonstrates the versatility of supercritical fluid solvents both to examine reaction mechanisms and as a means to tune rates.

Supercritical Fluid Extraction Compared with Solvent Method for Incurred Sulfamethazine in Chicken Eggs

ABSTRACTTo obtain eggs containing “normally incurred” sulfamethazine (SMZ), 10 hens were fed a single dose of 75 mg/kg SMZ by capsule. The amount of SMZ in some of the eggs (n = 21) was determined by two extraction methods, supercritical fluid extraction (SFE) and solvent extraction. The mean SMZ values ranged from 0.10 to 0.78 ppm, with an overall mean of 0.32 ppm and no difference (p>0.05) was found between the methods. However, SFE provided a distinct advantage over other methods since the amount of sample manipulation and solvent use and disposal was minimal. Determination of SMZ in the normally incurred eggs over an 8‐day period by SFE showed that levels reached a maximum after the first day, then declined slowly. However, SMZ exceeding 0.10 ppm still occurred 5 days after dosing.

Impact of Backbone Architecture on the Solubility of Fluorocopolymers in Supercritical CO2and Halogenated Supercritical Solvents: Comparison of Poly(vinylidene fluoride-co-22 mol hexafluoropropylene) and Poly(tetrafluoroethylene-co-19 mol hexafluoropropylene)

Cloud-point data to 270 °C and 2800 bar are presented for poly(vinylidene fluoride-co-22.0 mol % hexafluoropropylene) (Fluorel) in supercritical fluid (SCF) C3F6, CClF3, CHF3, and CO2. The impact of backbone architecture on intermolecular interactions and fluorocopolymer solubility is determined by comparing the phase behavior of Fluorel to that of poly(tetrafluoroethylene-co-19.3 mol % hexafluoropropylene) (FEP19) in the same SCF solvents. Good solvents for nonpolar FEP19 (SF6 and C3F8) did not dissolve polar Fluorel even at temperatures in excess of 260 °C. Although the small dipole moments of C3F6 and CClF3 interact favorably with the polar, vinylidene fluoride segments of Fluorel, the cloud-point curves in these two solvents exhibit a sharp rise in cloud-point pressures as the temperature is decreased. Apparently, dipolar interactions between vinylidene fluoride segments are much stronger than Fluorel−C3F6 or Fluorel−CClF3 cross-interactions as the temperature is lowered. In contrast, quadrupolar CO2 and dipolar CHF3 are good solvents for Fluorel due to favorable polar, vinylidene fluoride segment−SCF solvent cross-interactions. However, at or near room temperature, CO2 is a much better solvent for Fluorel than is CHF3, which is speculated to be a result of specific interactions between CO2 and the vinylidene fluoride segments.

Supercritical fluid extraction of acetic acid, alcohols and other amphiphiles from acid-water mixtures

Common supercritical fluid solvents cannot effectively extract highly water-soluble contaminants from either soils or waters. Two questions then arise: (1) how effective is SCF extraction from aqueous solution of contaminants only slightly soluble in water, when highly soluble organics are present; and (2) how will the extractability of the highly soluble contaminant be affected by the slightly soluble one? We have explored SCF extractions of this type, the solvent being CO 2 at 7–17 MPa and temperatures to 323 K, for a range of amphiphilic or surfactant compounds including simple alcohols (1-hexanol and 1-octanol), longer chain oxyalcohols (polyethylene glycol-200 and the methyl ether of polyoxyethylene-3), and a fluorinated polyether carboxylate. Acetic acid was used as the model contaminant of high aqueous solubility. We found that the presence of the amphiphiles decreases the already limited extractability of acetic acid from aqueous solution by SCF-CO 2; this was especially true of materials regarded as nonionic or anionic surfactants. Meanwhile, the presence of acetic acid apparently decreased the extractabilities of alcohols by CO 2, especially for alcohols of higher molecular weight. Possible reasons for this behavior and its implications for SCF extraction of natural waters are discussed.

Supercritical fluid chromatograph for studies of retention and efficiency with a brief study of n-alkanes showing evidence of significant temperature drop with pressure drop

An instrument designed specifically to examine retention and efficiency in packed-column supercritical fluid chromatography is described. Built on a gas chromatograph, it incorporates a syringe pump, helium-actuated injector, flame ionization detector, two outlet restrictors, and direct control of column inlet and outlet pressures. A supercritical fluid solvent injection system provides chromatograms with no solvent peak or solvent-induced effects on retention and efficiency. Mass flow-rates obtained by reading the volumetric displacement at the syringe pump are accurate to within 1.4%, and the repeatability of retention times and plate numbers with the pump operated in constant pressure and constant flow modes is examined. Isopycnic plate height curves for elution of n-dodecane, n-tetradecane, n-hexadecane, and n-octadecane with carbon dioxide at 50°C and a reduced density of 1.0 on a 150×2.0-mm I.D. column packed with 10-μm Spherisorb C 8 show a broad minimum with reduced plate heights of about 2 at reduced velocities from 3 to 7, with sharp increases at higher velocities for the heavier solutes. Retention volumes decrease with pressure drop, which is attributed to a significant temperature drop from the inlet to the outlet of the column.

Simple method for measuring refractive index of supercritical fluids

A high pressure, optical system is presented for measuring the refractive index of a supercritical fluid solvent to within ±3×10−3 at pressures as high as 2500 bar and temperatures of 200 °C. The experimental technique used here is an adaptation of the laser beam displacement method for measuring the refractive index of liquids at ambient pressure. This optical system is used to measure the refractive index of supercritical ethane to pressures of 2000 bar and temperatures of 200 °C.

Dynamics of charge‐transfer state formation in supercritical fluid solvent

AbstractThe rate of the intramolecular charge‐transfer state formation has been determined for 4‐(N, N‐dimethylamino)‐benzonitrile (DMABN) in supercritical CF3H by means of a time‐correlated single‐photon counting technique. The rate constant increases rapidly with increasing CF3H density, indicating the importance of the polar environment offered by the CF3H molecules which are clustering around a DMABN molecule. With increasing clustering number, the activation barrier is lowered and hence the rate increases exponentially. The observation is semiquantitatively interpreted on the basis of a molecular dynamics simulation of clustering in supercritical solutions.

Molecular theory of electronic spectroscopy in nonpolar fluids: Ultrafast solvation dynamics and absorption and emission line shapes

We present a theory of time- and frequency-domain spectroscopy of a dilute nonpolar solute in a nonpolar liquid or supercritical fluid solvent. The solute and solvent molecules are assumed to interact with isotropic pair potentials. These potentials, together with the solute and solvent masses, are the only input in the theory. We arrive at expressions for the absorption and emission line shapes, which include the possibility of motional narrowing, and for the time-resolved fluorescence and transient hole-burning observables, by assuming that the solute’s fluctuating transition frequency describes a Gaussian process. These expressions depend only on the average and variance of the transition frequency distributions in absorption and emission and on the normalized frequency fluctuation time-correlation functions. Within our formalism the former are obtained from the solute-solvent and solvent-solvent radial distribution functions, which are calculated using integral equations. The time-correlation functions involve the time-dependent solute-solvent Green’s function. Its solution depends upon the solute and solvent diffusion constants, which in turn are determined from the radial distribution functions. The theory compares favorably with computer simulation results of the same model. We then investigate the dependence of the various spectroscopic observables on the solvent density, the temperature, and the difference between the ground- and excited-state solute’s pair interaction with the solvent molecules. For example, since our theory for the time-correlation functions captures both their short- and long-time behavior, we can see how the crossover from inertial to diffusive dynamics depends on these variables. Our results are similar to a variety of experiments on solutes in both nonpolar and polar solvents.

Cosolvent tuning of tautomeric equilibrium in supercritical fluids

AbstractCosolvents offer a sensitive tool to tailor equilibria, rates or yields of chemical reactions in supercritical fluid solvents. The tautomeric equilibrium of a Schiff base was chosen as a model system, and small amounts of protic cosolvents shift the position of tautomeric equilibria. The equilibrium was tuned from essentially one tautomer to another by modifying the solvent of pure SCF ethane with less than 2 mol % hexafluoroisopropanol cosolvent. The equilibrium constant was a function of cosolvent concentration and mixture density. A chemical‐physical model of equilibrium constants deviates from the measured values in the near‐critical region, which may have been caused by local composition enhancement of cosolvent around the Schiff base. Decrease in the degree of hydrogen bonding with pressure or density affects the keto‐enol equilibria by decreasing the amount of the keto formation. Thus, solution density, as well as the degree of hydrogen bonding, is manipulated to tune the position of tautomeric equilibria.

Comparison of supercritical fluid extraction and solvent extraction of twenty-two organochlorine pesticides from eggs

The recoveries of twenty-two organochlorine pesticides spiked in egg samples were extracted by supercritical fluid extraction (SFE) using carbon dioxide and solvent extraction using acetonitrile were compared. The extracts were cleaned up with C18 and Florisil SPE columns. At 0.1 ppm spike level, the recoveries were 52 – 100% by SFE and 53 – 93 % by solvent extraction. The limits of detection calculated at 0.01 ppm spike level were 0.072 – 0.006 ppm for SFE and 0.06 - 0.006 ppm for solvent extraction. Both methods show improvement in sample preparation time and solvent consumption from the existing Soxhlet extraction method.

Infrared and Molecular Dynamics Study of D2O Rotational Relaxation in Supercritical CO2and Xe

Supercritical fluid solvents allow continuous tuning of solvent density and therefore continuous control of the number and distance of solvent−solute interactions. Such interactions exert torque on molecules in solution and thereby influence the way in which the rotational orientation of a solute molecule varies with time. In this work, we examine the rotational relaxation of D2O in two supercritical fluid solvents: Xe and CO2. In the former case, there are no electrostatic interactions between the solute and solvent. Since the nonelectrostatic interactions of D2O are very nearly centrosymmetric about the center of mass, Xe is expected to exert little torque on the rotating solute until liquid densities are reached. For D2O dissolved in CO2, the dipole−quadrupole interaction can exert torque on the rotating D2O, thereby hindering its rotation. Molecular dynamics simulations of these systems were used to calculate dipole autocorrelation functions containing only contributions from the solute rotational motion. These were then used to predict the rotation wings of the asymmetric stretching band, ν3, of D2O. Infrared spectra for this band were obtained in both solvents at 110.0 °C as a function of density. In Xe, the experimental and simulated correlation functions indicated that the solute was free to rotate, even at the highest densities examined. In CO2, however, the rotation of D2O was increasingly hindered as the solvent density was increased from gaslike to liquidlike densities. At low densities the correlation functions had negative minima, indicating a partial reversal in the direction of the average transition dipole moment vector as the molecules freely rotated. In CO2, at high densities the functions exhibited simple, monotonic decay, indicating a persistence in the direction of the vector with time and substantially hindered rotation.

Solute−Fluid Coupling and Energy Dissipation in Supercritical Fluids: 9-Cyanoanthracene in C2H6, CO2, and CF3H

We report on the coupling and dissipation of energy between a model fluorescent solute, 9-cyanoanthracene (9CA), and several supercritical fluid solvents. To this end, we have determined experimentally the fluorescence quantum yields and excited-state fluorescence lifetimes for dilute solutions of 9CA in supercritical C2H6, CO2, and CF3H. The 9CA quantum yield is substantially less than unity at lower fluid densities; it approaches unity only at the high-density, liquid-like region. The 9CA excited-state lifetime is also shortened significantly in the low-density region. The radiative (kr) and nonradiative (knr) decay rates for 9CA are found to be strongly density dependent. In the low-density region, the nonradiative rate dominates; however, in the high-density region the 9CA deexcitation follows the radiative pathway. The Strickler−Berg relationship (kr ∝ n2; n = solvent refractive index) holds for 9CA in many normal liquid solvents. However, in supercritical fluids in the low-density regime, the simple...