Oxidative delignification of wood in the supercritical carbon dioxide medium. 4. Acetylization of cellulose in the supercritical carbon dioxide medium

It is shown that cellulose can be acetylated with acetic anhydride in supercritical carbon dioxide in the absence of traditional catalysts, toxic solvents, and dilutants. As substrates, three cellulose samples were tested: sulfite and sulfate cellulose and cellulose obtained by low-temperature delignification in a supercritical carbon dioxide medium. The chemical composition of the cellulose diacetate product is characterized.     

Allyl substitution in a supercritical carbon dioxide medium catalyzed by complexes of palladium with phosphite-type ligands

Allyl amination and alkylation catalyzed by a palladium complex-phosphite-type-ligand catalyst in a supercritical carbon dioxide (SC-CO₂) medium is studied. It is demonstrated that that the Pd-containing catalysts dissolved in SC-CO₂ provide a high regioselectivity of the reaction and, thus, offer a convenient method for synthesizing allylamines.     

Formation of long-lived fluorescent merocyanine forms of spiro compounds via their immobilization into polymer matrices from supercritical carbon dioxide medium

It is shown that the supercritical fluid (SCF) impregnation of polymers containing phenylphenol and halide polar groups (polycarbonate, fluoroplastic, poly(vinyl chloride)) with spiro compounds leads to conformational rearrangements resulting in stabilization of merocyanine forms of the photochromes. The effect of the formation of long-lived colored form B of the photochromes immobilized in the polymer matrix in SCF medium, revealed earlier for spinroanthroxazine, is of more general nature: it is observed and investigated by us for a number of spiro compounds SCF-impregnated into polar polymer matrices of different chemical structures. It was found that the colored forms of spiro compounds introduced in halogenated matrices feature intense photoluminescence.     

Solubility of ibuprofen in supercritical carbon dioxide

An experimental apparatus that allows measurement of solubility of solid compounds and liquids in supercritical (SC) fluids in a wide range of temperatures and pressures using a dynamic method is constructed. Experimental data on the solubility of anthracene at 318.1 and 328.1 K and results of the solubility of ibuprofen at 313, 323, and 333 K at 10–30 MPa are obtained. The solubility of ibuprofen in SC CO₂ is described using the Peng-Robinson equation of state. Parameters of binary interactions are obtained for the ibuprofen-supercritical CO₂ system.     

Solubility of ammonium palmitate in supercritical carbon dioxide

The solubility of ammonium palmitate in supercritical carbon dioxide is studied in the dynamic regime at 308.15–333.15 K and 10.0–35.0 MPa. Experimental data are described in the framework of the Peng–Robinson equation of state.     

Conformational lability of ibuprofen in supercritical carbon dioxide

A study of conformational equilibria of ibuprofen in vacuum and supercritical carbon dioxide (SC CO₂) was carried out using methods of quantum chemistry and molecular dynamics simulation. It was found that ibuprofen forms 12 different conformations in SC CO₂, two of them correspond to the structure of the molecule in the crystal lattice of the first and second polymorph. It was shown that the GAFF force field allows one to obtain the characteristics of conformers, almost coinciding with those obtained by quantum chemical calculations at the B3LYP/cc-pVTZ level of theory.     

Benign reduction of carbon nanotube agglomerates using a supercritical carbon dioxide process

A method was developed to deagglomerate commercially available multi-walled carbon nanotube (MWCNT) bundles while maintaining the carbon nanotube aspect ratio. The process utilizes the rapid expansion of a supercritical carbon dioxide/MWCNT mixture to separate large primary carbon nanotube agglomerates. High levels of deagglomeration of Baytubes^® C 150 P and Nanocyl? NC-7000 MWCNT bundles were observed on the macroscale and nanoscale, resulting in 30-fold and 50-fold decreases in bulk density, respectively, with median agglomerate sizes <8 μm in diameter. These results were obtained while retaining the aspect ratio of the as-received nanomaterial, irrespective of the MWCNT agglomerate morphology. It was found that a temperature and pressure of 40 °C and 7.86 MP resulted in maximum deagglomeration without damage to the MWCNTs. Thermodynamic principles were applied to describe the effect of processing variables on the efficiency of the deagglomeration. These results suggest that combining this process with a composite processing step, such as melt compounding, will result in nanocomposites with enhanced electrical properties.     

S-CO2布雷顿循环转化效率分析

以超临界二氧化碳简单回热型布雷顿循环为研究对象,以核电站为应用背景,详细论述了系统循环模型与关键器部件的效率模型建立方法,并利用该模型初步分析了各类工程因素对布雷顿循环效率、系统体积的影响,分析结果表明,循环效率、系统体积对温度、压力、涡轮机械效率、回热器等参数的敏感性存在较大差异,其中增加透平入口温度对缩减系统总体积最为有效,需要建立完善的系统分析模型以进行S-CO₂系统的优化设计。     

Kinetics of coal char gasification in a carbon dioxide medium

Solid fuel samples with different carbon contents are gasified by successively subjecting to pyrolysis in argon and oxidation in carbon dioxide at various temperatures to determine the rate of the chemical reactions and the activation energy required for simulating and optimizing the operation of gas generators. The samples were prepared from bituminous coal, lignite, and anthracite of the Kuznetsk and Kansk-Achinsk coal basins. The gasification of coal char samples in a carbon dioxide medium at 900–1200°C is analyzed by thermogravimetry. The temperature dependences of the weight change rate and gasification time of coal char samples are measured and used to calculate the preexponential factor and activation energy of the carbon oxidation reaction. It is found that, with increasing oxidizing medium temperature from 900 to 1200°C, the gasification time of the coal char samples obtained from anthracite and bituminous coal decrease 8- and 22-fold, respectively. A physicomathematical model of coal char gasification in a fixed bed, with the oxidizing gas diffusing through the ash layer formed, is proposed.     

Extraction of hydrocarbons from carbonaceous rock in supercritical carbon dioxide

Experiments on supercritical CO₂ extraction of organic matter from carbonaceous raw material show that, regardless of its geological origin, the process is characterized by selective extraction of hydrocarbons, especially alkanes and naphthenes. The composition of the supercritical extract makes it possible to use it for manufacturing lubricants.     

Dehydration and desalination of oils using supercritical carbon dioxide

A new technology for purification of oil from water, salts, and solid impurities with supercritical (SC) CO₂ before refining at existing refineries is developed. The proposed technology, along with purifying oil from water, salts, and solid particles, accomplishes its deasphalting and demetallization. The process is implemented on a facility at a pilot plant of the Institute of Petrochemical Processes of the Azerbaijan Academy of Sciences. Preliminary technical-economical calculations show that the energy expenditures on oil extraction with SC-CO₂ are significantly lower than for the existing processes.     

Improvement in dispersion and ionic conductivity of polyether/freeze-dried clay composites using supercritical carbon dioxide as treatment medium

Addition of inorganic fillers to polyether-based electrolytes can lead to an increase in ionic conductivity, a reduction in the degree of crystallinity, and an increase in cation transport number. In order to enhance further conductivity of polyether/clay composites, a dispersion of ??nano-level?? clay in polyether is needed. In this study, we prepared polyether composite filled with freeze-dried clay, which may give a more dispersed state. In a previous study, we reported that supercritical carbon dioxide (scCO₂) treatment medium is an effective method for enhancing the conductivity of polyether/saponite (Sa) composites. Here, we prepared polyether/freeze-dried saponite (fSa) composites and treated them with scCO₂ and studied the effects of freeze-drying and scCO₂ treatment using wide-angle X-ray diffraction (WAXD), transmission electron microscopy (TEM), differential scanning calorimetry, and complex impedance measurements. The WAXD and TEM measurements revealed that the dispersion of Sa in polyether is caused by the freeze-drying pretreatment. Moreover, the combination of freeze-drying and scCO₂ treatment gave rise to homogeneous composites, leading to a significant increase in conductivity. The conductivity of the scCO₂-treated fSa composite was 100 times greater than that of the original Sa composite.     

Regeneration of the active aluminum oxide catalyst in supercritical carbon dioxide

The nature of the compounds inactivating the active aluminum oxide catalyst is studied. The results of the catalyst regeneration by pure supercritical carbon dioxide and by the supercritical carbon dioxide modified with a polar additive (2 wt % dimethyl sulfoxide) are presented.     

Dispersion copolymerization of methyl methacrylate and styrene in supercritical carbon dioxide

A method for producing finely dispersed powders of methyl methacrylate (MMA)-styrene copolymer by radical polymerization in a supercritical carbon dioxide medium (SC-CO₂) was proposed, studied, and experimentally implemented. The dispersing agent (surfactant), which made it possible to obtain nearly monodisperse size distribution of polymer particles, was poly(dimethylsiloxane methacrylate), a SC-CO₂-soluble substance. The copolymer, synthesized with a molecular mass of M _w ～ 36000 in the form of spherical particles with a characteristic size of ～1 μm, exhibited a higher thermal stability as compared to poly(methyl methacrylate) with a similar molecular mass. Varying the percentage ratio between MMA and styrene monomers, it was possible effectively control the integral hydrophobicity and physicomechanical characteristics of the methacrylate-styrene copolymer.     

Immobilization of luminescent nanosilicon in a microfine polytetrafluoroethylene matrix by means of supercritical carbon dioxide

With the use of supercritical carbon dioxide (SC-CO₂), the matrix immobilization of photoluminescent silicon nanocrystals (nc-Si) in polytetrafluoroethylene microparticles (mp-PTFE) is performed, which leads to the formation of mp-PTFE/nc-Si photoluminescent nanocomposite containing ∼10³–10⁴ nc-Si particles per mp-PTFE particle (1–2 μm in size). This approach is based on the effect of polymer swelling in SC-CO₂, efficient SC-CO₂-assisted transport of nanoparticles into the internal free volume of the polymer, and contraction of the nanocomposite after the release of CO₂, an effect that prevents the subsequent agglutination of nanoparticles. Particles of nc-Si photoluminescent in the visible spectrum were synthesized from silicon suboxide powder (SiO _x , x ≈ 1) heated at various temperatures within 25–950°C and then etched in concentrated hydrofluoric acid. The hydrosilylation procedure was used to graft 1-octadecene molecules to the surface of nc-Si particles. As a result, the photoluminescence intensity of nc-Si increased substantially. According to TEM images and small angle X-ray scattering data, the maximum size of nc-Si particles did not exceed 5 nm and 7 nm, respectively, and the core of these nanoparticles consisted of crystalline silicon. The structure and spectral properties of the initial nc-Si particles and synthesized mp-PTFE/nc-Si photoluminescent nanocomposite microparticles were studied.     

Xenogenic bone matrix treated with supercritical carbon dioxide as a potential osteoplastic material

A multistage environmentally safe method for processing xenogenic bone matrix using suband supercritical media has been developed. This method provides fast matrix delipidization in a medium of supercritical carbon dioxide and deproteinization by treatment with a 3% hydrogen peroxide solution followed by washing with subcritical water for removal of polar substances. The final treatment with supercritical carbon dioxide serves to remove residual nonpolar components. A method to control the porosity of xenogenic bone matrix at different stages of its purification has been suggested. Preliminary data indicate that a cellular test system based on an allogenic osteoid cell line Th-1 could be used for evaluation of primary adhesion and proliferation of cells on the surface of purified bone matrix.     

An investigation of the dissolution of acetylsalicylic acid in supercritical carbon dioxide

The dissolution of acetylsalicylic acid in supercritical carbon dioxide (SC–CO₂) is investigated using Fourier transform IR spectroscopy. Temporal dependences of integrated intensities of the absorption band of acetylsalicylic acid (ASA) near 1199 cm^–1 are measured for various ASA charges placed into a cuvette. Molar fractions of ASA in the saturated solution in SC–CO₂ are determined at a temperature of 40°C and pressures of 10.0 and 15.0 MPa, and the components are 2.10 ± 0.25 × 10^–4 and 11.0 ± 1.5 × 10^–4, respectively.     

The ultrasonic-enhanced factor of mass-transfer coefficient in the supercritical carbon dioxide extraction

Based on several hypotheses about the process of supercritical carbon dioxide extraction, the onflow around the solute granule is figured out by the Navier-Stocks equation. In combination with the Higbie’s solute infiltration model, the link between the mass-transfer coefficient and the velocity of flow is found. The mass-transfer coefficient with the ultrasonical effect is compared with that without the ultrasonical effect, and then a new parameter named the ultrasonic-enhanced factor of mass-transfer coefficient is brought forward, which describes the mathematical model of the supercritical carbon dioxide extraction process enhanced by ultrasonic. The model gives out the relationships among the ultrasonical power, the ultrasonical frequency, the radius of solute granule and the ultrasonic-enhanced factor of mass-transfer coefficient. The results calculated by this model fit well with the experimental data, including the extraction of Coix Lacryma-jobi Seed Oil (CLSO) and Coix Lacryma-jobi Seed Ester (CLSE) from coix seeds and the extraction of Eicosapentaenoic Acid (EPA) and Docosahexaenoic Acid (DHA) from the alga by means of the ultrasonic-enhanced supercritical carbon dioxide extraction (USFE) and the supercritical carbon dioxide extraction (SFE) respectively. This proves the rationality of the ultrasonic-enhanced factor model. The model provides a theoretical basis for the application of ultrasonic-enhanced supercritical fluid extraction technique. Supported by the Major Program of the National Natural Science Foundation of China (Grant No. 10674048) and the Natural Science Foundation of Guangdong Province (Grant No. 06025714)