超临界流体在超细粉体制备中的应用

对超临界流体用于制备超细粉体的研究现状和主要研究成果进行了综述。介绍了近年来开发的一些新方法和新工艺 ,其中包括超临界溶液快速膨胀 (RESS)法 ,超临界流体脱溶 (SAS)法 ,超临界逆向结晶 (SRC)法 ,超临界干燥 (SD)法和超临界流体化学反应等。分析了这些新工艺新方法的原理、特点 ,技术上的可行性 ,应用前景 ,目前达到的水平与存在的问题 ,及今后发展需解决的关键问题等。     

Solubility of the silver nitrate in supercritical carbon dioxide with ethanol and ethylene glycol as double cosolvents: Experimental determination and correlation

Solubility of the silver nitrate in the supercritical carbon dioxide containing ethanol and ethylene glycol as double cosolvents was measured under certain pressure and temperature range(10–25 MPa, 323.15–333.15 K). The impact of the pressure and temperature on the solubility was also investigated. Based on the experiment data,a correlation model concerning solid's solubility in supercritical fluids was established by combining the solubility parameter with the thermodynamic equation when a binary interaction parameter and a mixed solvent solubility parameter were defined. Experiments show the solubility of AgNO_3 increases with the pressure at a certain temperature. However, the influence of temperature is related to a pressure defined as the turnover pressure(12.3 MPa). When the pressure is higher(or lower) than this turnover pressure, silver nitrate's solubility shows increasing(or decreasing) trend as the temperature rises. Satisfactory accuracy of our presented model was revealed by comparing experimental data with calculated results.     

Synthesis of cerium oxide-based nanostructures in near- and supercritical fluids

Cerium-oxide based nanostructures attract increasing interest for their use in multiple applications. In particular, the substitution of Ce atoms by other elements with lower oxidation state is used to control oxygen vacancies within the oxide structures, which can greatly enhance the material properties for catalysis applications. Among the synthesis approaches, supercritical water (scH₂O) has been proved to be an extremely efficient media for the fast and facile elaboration of pure CeO₂ nanostructures, although substitution was little investigated with this method (precursors’ reactivity, mechanisms, etc.). Here, the influence of two cerium precursors – Ce(NO₃)₃·6H₂O and Ce(NO₃)₆(NH₄)₂ – in scH₂O synthesis was first investigated over the CeO₂ nanostructures synthesis process. Cerium ammonium nitrate was later used as precursor for the synthesis of Ce_1−x−yA_xB_yO_2−δ (A, B = Zr, Pr and/or La; 0 ≤ x, y ≤ 0.1). Supported by ICP analysis quantification, this study proposes an overview of the influences of residence time, temperature and precursors’ concentration over the proportion of the substitution element in the CeO₂ NCs. This work demonstrates a fast and simple process to Ce_1−x−yA_xB_yO_2−δ nanostructures synthesis using scH₂O synthesis.     

Synthesis, characterization and electrocatalytic activity of silver nanorods towards the reduction of benzyl chloride

This paper describes a simple method of synthesizing silver nanorods using the polyol process, where propylene glycol serves both as a reducing agent and as a solvent in the presence of a capping reagent such as polyvinylpyrrolidone (PVP). The diameter and length of silver nanorods could be controlled by changing the AgNO₃/PVP ratio. The end-to-end assembly of the silver nanorods was found. The silver nanorods were characterized by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction and absorption spectroscopy. The catalytic activity of a glassy carbon electrode with Ag nanorods exhibits extraordinary electrocatalytic activities towards the electro-reduction of benzyl chloride.     

Morphology and composition controlled synthesis of flower-like silver nanostructures

Flower-like silver nanostructures with controlled morphology and composition were prepared through wet-chemical synthesis. The reaction rate is simply manipulated by the amount of catalyzing agent ammonia added which is the key point to determine the ratio of hexagonal close-packed (HCP) to face-centered cubic (FCC) phase in silver nanostructures. The existence of formic acid that is the oxidation product of aldehyde group is demonstrated to play a crucial role in achieving the metastable HCP crystal structures by replacing ionic surfactants with polyvinylpyrrolidone (PVP). Utilizing flower-like silver nanostructures as surface-enhanced Raman scattering (SERS) substrates, Raman signal of Rhodamine 6G, or 4-aminothiophenol with concentration as low as 10⁻⁷ M was detected. Moreover, it is demonstrated that phase composition has no direct relation to the SERS enhancing factor which is mainly determined by the amount of hot spots.     

Solubility of chemical warfare agent simulants in supercritical carbon dioxide: experiments and modeling

Solubility data are reported for ethyl phenyl sulfide (EPS) and 2-chloroethyl ethyl sulfide (CEES) in CO₂ at temperatures from 25 to 100 °C. These two sulfide-based compounds are homomorphs for chemical warfare agents (CWAs). Both sulfide–CO₂ mixtures exhibit type-I phase behavior. The maximum in the 100 °C isotherm is approximately 2600 psia for the CEES–CO₂ system and approximately 3400 psia for the EPS–CO₂ system. The Peng–Robinson equation of state (PREOS) is used to model both sulfide–CO₂ mixtures as well as the phase behavior of the 2-chloroethyl methyl sulfide (CEMS)–CO₂ system previously reported in the literature. The Joback–Lydersen group contribution method is used to estimate the critical temperature, critical pressure, and acentric factor for the sulfides. Semi-quantitative estimates of the phase behavior are obtained for the CEES–CO₂ and EPS–CO₂ systems with a constant value of k_ij, the binary interaction parameter, fit to the 75 °C isotherms. However, very poor fits are obtained for the 2-chloroethyl methyl sulfide–CO₂ system regardless of the value of k_ij. On the basis of the high solubility of EPS and CEES in CO₂, supercritical fluid (SCF)-based technology could be used to recycle or recover chemical warfare materials.     

Fabrication of dendritic silver nanostructure using an integration of holographic lithography and electrochemical deposition

Holographic lithography in combination with electrochemical deposition provides a novel and readily controlled method for preparing dendritic three-dimensional silver nanostructure with a large surface area. The scanning electron microscope and the synchrotron radiation X-ray three-dimensional imaging technique are used to characterize the as-prepared silver nanomaterial and demonstrate that the obtained silver has a three-dimensional dendritic morphology. The observed typical widths between the neighbor branches of the dendritic silver nanostructure are 30–50 nm. The configuration of the silver nanostructure is greatly related to the width of gold wires which are fabricated using holographic lithography and serve as substrate of the silver nanostructure. The three-dimensional silver architecture is formed only when silver is electrochemically deposited on 400-nm-width gold.     

Preparation of liposomes using the supercritical anti-solvent (SAS) process and comparison with a conventional method

Two methods to produce liposomes encapsulating a fluorescent marker were compared: the supercritical anti-solvent (SAS) method and a conventional one (Bangham). Liposome size and encapsulation efficiency were measured to assess the methods. Micronized lecithin produced by the SAS process was characterized in terms of particle size, morphology and residual solvent content in order to investigate the influence of experimental parameters (pressure, CO₂/solvent molar ratio and solute concentration). It appears that when the lecithin concentration increases from 15 to 25 wt.%, at 9 MPa and 308 K, larger (20-60 μm) and less aggregated lecithin particles are formed. As concerns liposomes formed from SAS processed lecithin, size distribution curves are mainly bimodal, spreading in the range of 0.1-100 μm. Liposome encapsulation efficiencies are including between 10 and 20%. As concerns the Bangham method, more dispersed liposomes were formed; encapsulation efficiencies were about 20%, and problems of reproducibility have been raised.     

Fabrication of hollow silica particles using copolymeric spheres prepared in supercritical carbon dioxide

Amine functional polymeric spheres were prepared via the dispersion polymerization of 2-dimethylaminoethyl methacrylate and methylmethacrylate in supercritical carbon dioxide, and they were employed efficiently as templates for the fabrication of hollow silica particles. A small amount of divinyl benzene was used as a crosslinking agent to control the morphology of the copolymeric particles from clumpy solid to spherical shape. The assembly of the polymeric spheres with tetraethylorthosilicate (TEOS) through hydrothermal methods produced core-shell type hybrid particles. The whole process required neither surface treatment for the polymeric particles nor addition of any acidic or basic catalyst for the hydrolysis of silica precursor because dimethylamino groups of the copolymeric spheres were able to absorb water and catalyze the hydrolysis of TEOS for the deposition of the silica gel network. The polymeric cores were completely removed by calcination process and silica hollow particles with well-defined structure were obtained finally. The silica hollow particles were characterized by scanning electron microscopy and transmission electron microscopy, which clearly revealed that the silica spheres had the hollow structure with 151 nm wall thickness.     

Modeling solubility in supercritical fluids via the virial equation of state

We examine the utility of the virial equation of state (VEOS) for the prediction of the solubility of hexane in supercritical carbon dioxide. We computed virial coefficients up to fourth-order for this mixture at 353.15 K, using Mayer-sampling Monte Carlo applied to established molecular models for carbon dioxide and hexane. At this temperature, neither the third- nor fourth-order VEOS indicate the presence of a critical point for the mixture. However, the VEOS coupled with an ideal-solution treatment for the coexisting subcritical fluid phase (with parameters determined from coexistence data at one pressure) reproduces much of the solubility curve as previously established by Gibbs-ensemble molecular simulation. Comparison of the third- and fourth-order VEOS indicate that the virial treatment is converged at the conditions where it is applied.     

Study of kinetics of the d-pinitol extraction from carob pods using supercritical CO2

There is a growing interest in the use of d-pinitol (d-3-O-methyl-chiro-inositol) as a food supplement, thus the use of green and efficient process, such as the supercritical fluid extraction (SFE), to isolate d-pinitol from vegetable raw materials is also an interesting area of research and development. The objective of this work was to study the influence of operating conditions on the SFE of d-pinitol from carob pods. The crucial parameters in SFE, like temperature, pressure, CO₂ flow rate and duration of the process, have been studied. Finally, Naik and Barton models have successfully tested in order to fit the experimental extraction curves.     

The synergy of supercritical CO2 and supercritical N2 in foaming of polystyrene for cell nucleation

This study examines the foaming behaviour of polystyrene (PS) blown with supercritical CO₂–N₂ blends. This is achieved by observing their foaming processes in situ using a visualization system within a high-temperature/high-pressure view-cell. Through analyzing the cell nucleation and growth processes, the foaming mechanisms of PS blown with supercritical CO₂–N₂ blends have been studied. It was observed that the 75% CO₂–25% N₂ blend yielded the highest cell densities over a wide processing temperature window, which indicates the high nucleating power of supercritical N₂ and the high foam expanding ability of supercritical CO₂ would produce synergistic effects with that ratio in batch foaming. Also, the presence of supercritical CO₂ increased the solubility of supercritical N₂ in PS, so the concentration of dissolved supercritical N₂ was higher than the prediction by the simple mixing rule. The additional supercritical N₂ further increased the cell nucleation performance. These results provide valuable directions to identify the optimal supercritical CO₂–N₂ composition for the foaming of PS to replace the hazardous blowing agents which are commonly used despite their high flammability or ozone depleting characteristics.     

Batch production of micron size particles from poly(ethylene glycol) using supercritical CO2 as a processing solvent

The major advantage of using supercritical carbon dioxide (CO₂) as a solvent in polymer processing is an enhancement in the free volume of a polymer due to dissolved CO₂, which causes a considerable reduction in the viscosity. This allows spraying the polymer melt at low temperatures to produce micron size particles. We have used supercritical CO₂ as a solvent for the generation of particles from poly(ethylene glycol) (PEG) of different molecular weights. Since PEG is a hydrophilic compound, it is a most commonly used polymer for encapsulating a drug. PEG particles with different properties may allow keeping a good control over the release of the drug. It has been possible to produce particles with different size, size distribution, porosity and shape by varying various process parameters such as molecular weight, temperature, pressure and nozzle diameter. A flow and a solidification model have been applied in order to have a theoretical insight into the role of different parameters.     

Optimization of supercritical extraction of nimbin from neem seeds in presence of methanol as co-solvent

Mathematical modeling and optimization of the extraction of nimbin from neem seeds using supercritical carbon dioxide with methanol as co-solvent is the subject of this study. At first a correlation for Sherwood number (Sh) as a function of Reynolds number (Re) and Schmidt number (Sc) was proposed using a Genetic Algorithm (GA) technique. This correlation was compared to previous correlations and was found to give the most accurate results. Moreover, optimum conditions (temperature, pressure, solvent flow rate and particle diameter) which maximizes the extraction yield have been determined using GA. At the next step, methanol was used as a co-solvent and the dynamic equilibrium constant of solute between the solid phase and the solvent was estimated. By applying the new determined equilibrium constant, good agreement between the model and experimental data was observed.     

Determination of sudan dyes in food samples using supercritical fluid extraction-capillary liquid chromatography

Determination of sudan dyes (Sudan I, Sudan II, Sudan III and Sudan IV) in food samples has been developed by several and different methods. However, sample treatment continues being the most important problem to determine these compounds in real samples. A rapid, sensitive and selective method for the extraction, separation and quantification of sudan dyes in commercial food samples has been developed. The method is based on the combination of a supercritical fluid extraction (SFE) procedure, followed by the analysis of the extracted plug by capillary liquid chromatography (CLC) with diode array detection (DAD). The entire procedure was optimized for the extraction of the samples, separation and detection of the analytes. For the SFE, the effect of CO₂ flow rate, extraction pressure, extraction temperature, equilibration and extraction time and methanol modifier content were studied. Linear responses in the range from 50 to 1000 ng mL⁻¹ with average relative standard deviation lower than 11.6, and detection limits ranging from 23.2 to 42.0 ng mL⁻¹ were obtained for the different sudan dyes. The recoveries were in the range of 91-109% for dyes powder samples.     

Allelopathic properties of the fractions obtained from sunflower leaves using supercritical carbon dioxide: The effect of co-solvent addition

The work described here is a continuation of a previous study centered on the extraction, using supercritical carbon dioxide, of bioactive substances from sunflower leaves of the Helianthus annuus L. variety Arianna. In this study the addition of 9% of ethanol as co-solvent was analyzed. The extraction was carried out (P = 100/400 bar, T = 35/55 °C, ethanol = 9%) in order to analyze the influence of pressure, temperature and sample pre-treatment on the extraction yield and bioactivity of the extracts. The addition of 9% of ethanol to the supercritical solvent enhanced both the extraction yield and the biological activity of the extracts. The best conditions were a pressure of 400 bar and a temperature of 55 °C. In an effort to improve the bioactivity of the extract, a cascade fractionation of the extracts was carried out and this gave different results in terms of biological activities and extraction yields. The phytochemical compositions of the extracts were analyzed by thin layer chromatography. The fractionation that gave the best results was carried out at 90 bar and 40 °C in the first separator. Finally, the effect of extracts on the growth of seeds from different plants was analyzed.     

超临界流体技术在环境科学中的应用

简要介绍了超临界流体萃取、超临界水氧化和超临界流体色谱的基本原理,综述了超临界流体技术在废水处理、固体废物处理、燃煤脱硫和污染监测等方面的应用。     

Optimization of total alkannin yields of Alkanna tinctoria by using sub- and supercritical carbon dioxide extraction

The effects of supercritical carbondioxide extraction was investigated to compare previously validated extraction methods on total alkannin yield with Alkanna tinctoria collected form Antalya, Turkey. A two-step process was used; extraction of alkannin derivatives with supercritical CO₂ followed by alkaline hydrolysis of alkannin derivatives. A Box-Behnken exprerimental design was used to evaluate the effect of three variables, pressure (50-350 bar), temperature (30-80 °C) and CO₂ flow (5-20 g min⁻¹) at 1:30 ratio of alkanna root:CO₂ amount. Response surface analysis revealed that the data were adequately fitted to a second-order polynomial model with R² 0.9665 and the most effective variable was pressure (P ≤ 0.05). Optimum conditions were determined as 80 °C, 175 bar, 5 g min⁻¹ CO₂ flow yielding the highest total alkannins (1.47%) which was higher than conventional hexane extraction (1.24%) providing a solvent-free alternative for industrial production.     

Measurement and calculation of solubility of quinine in supercritical carbon dioxide

Solubility of quinine in supercritical carbon dioxide(SCCO_2) was experimentally measured in the pressure range of 8 to 24 MPa, at three constant temperatures: 308.15 K, 318.15 K and 328.15 K. Measurement was carried out in a semi-dynamic system. Experimental data were correlated by iso-fugacity model(based on cubic equations of state, CEOS), Modified Mendez–Santiago–Teja(MST) and Modified Bartle semi-empirical models. Two cubic equations of state: Peng–Robinson(PR) and Dashtizadeh–Pazuki–Ghotbi–Taghikhani(DPTG) were adopted for calculation of equilibrium parameters in CEOS modeling. Interaction coefficients(k_(ij) l_(ij)) of van der Waals(vdW) mixing rules were considered as the correlation parameters in CEOS-based modeling and their contribution to the accuracy of model was investigated. Average Absolute Relative Deviation(AARD) between correlated and experimental data was calculated and compared as the index of validity and accuracy for different modeling systems. In this basis it was realized that the semi-empirical equations especially Modified MST can accurately support the theoretical studies on phase equilibrium behavior of quinine–SCCO_2 media. Among the cubic equations of state DPGT within two-parametric vd W mixing rules provided the best data fitting and PR within one-parametric vd W mixing rules demonstrated the highest deviation respecting to the experimental data. Overall, in each individual modeling system the best fitting was observed on the data points attained at 318 K, which could be perhaps due to the moderate thermodynamic state of supercritical phase.     

Study of transpiring fluid dynamics in supercritical water oxidation using a transparent reactor

The transpiring wall reactor (TWR) is considered to be one of the most promising reactors because it minimizes both corrosion and salt precipitation problems that seriously hinder the industrialization of supercritical water oxidation technologies. A transparent reactor is built to study the fluid dynamics of transpiring flow, which are the foundation of reactor design and optimization. The results showed that the transpiring flow is anisotropic with respect to the surface of the transpiring wall due to both the static pressure and viscous resistance. Finally, the novel idea of using air as the transpiring fluid instead of water is presented in an attempt to alleviate current TWR problems such as high energy consumption, high volume of pure water consumption, and temperature fluctuation in the reaction area. A series of experiments and theoretical derivations demonstrate that this novel idea is feasible.