Supercritical Medium Research Articles

Electrochemical reduction of CO2 using Pb catalysts synthesized in supercritical medium

Supercritical fluids have been used to obtain Pb/CNT catalysts consisting of Pb nanoparticles (5–10 nm of predominant size) deposited on CNT. The electrocatalytic activity of Pb/CNT catalysts has been studied by electroreduction of CO2 in gas phase using a PEM type cell in continuous operation mode. The influence of current density (8–24 mA cm−2), temperature (40–80 °C), CO2 flowrate (0.02–0.08 L min−1) and anolyte concentration (0.1–0.5 M KHCO3) have been studied in terms of products formation rate. Formic acid has been the main CO2 reduction product, followed by CO and methane, as well as methanol as minority product. The production of formic acid rises by increasing current density and CO2 flowrate within the experimental ranges studied. High CO formation rates have been observed at 80 °C, but also at low CO2 flowrate (0.02 L min−1) and at high anolyte concentration (0.5 M KHCO3). Formation rate of methane improves with increasing current density in the range studied. Regarding methanol, increasing temperature promotes its production, whereas it diminishes at higher anolyte concentration. A remarkable result that has not been reported yet for Pb electrocatalysts is the change in selectivity observed at 80 °C. At this temperature CO is the main CO2 reduction product (instead of formic acid) and the selectivity to methanol formation increases. In addition, it has been observed that Pb/CNT catalysts yield CO2 conversion rates (normalized by metal surface) 10% higher on average than Pt/CNT catalysts, and that the Pb electrocatalysts lead to larger selectivity to methanol formation. Specifically, using Pb catalysts the selectivity to methanol formation was up to 6.7%, which is almost 4 times higher than the maximum one observed with Pt catalysts.

Production of a Novel Material Based on a Collagen–Chitosan Composite and Ibuprofen in a Supercritical Medium

A novel material for biomedical applications was obtained from a collagen–chitosan composite and ibuprofen by a fluid process using supercritical carbon dioxide. The material was characterized by IR and X-ray photoelectron spectroscopy. The applicability of XPS analysis of biomaterials for quantification of loaded therapeutic agents is demonstrated.

Preparation of a Novel Based on a Collagen-Chitosan Composite and Ibuprofen in Supercritical Medium

Preparation of a Novel Based on a Collagen-Chitosan Composite and Ibuprofen in Supercritical Medium

Solubility and data correlation of a reactive disperse dye in a quaternary system of supercritical carbon dioxide with mixed cosolvents

Abstract Mixed cosolvents involving acetone and water were applied in hydrophobically supercritical carbon dioxide medium for modifying the solubility behavior of a new reactive disperse orange SCF-AOL2 dye. Then the equilibrium solubility of the dye solute in the quaternary system of supercritical carbon dioxide with the mixed cosolvents was measured at temperature from 333.2 K to 393.2 K and pressure from 8.0 MPa to 22.0 MPa with an equilibrium time of 60 min in a home-built batch circulation system. The experimental results show that a significant improvement of the dye solubility and its behavior was achieved in the presence of the mixed cosolvents in supercritical carbon dioxide, especially with the tested system pressure in different solubility isotherm curves. Moreover, the achieved experimental data were further correlated, validated and predicted successfully by employing several typically semi-empirical models for the quaternary system with mixed cosolvents. It is most important that a new and specially derived model based on the conventional and modified Mendez-Santiago–Teja ones was also proposed and successfully applied to the quaternary system with a significant improvement and highest accuracy than the employed other ones. Furthermore, better correlations were also achieved from the conventional Chrastil and the Del Valle-Aguilera models than other ones for the employed system.

Obtaining of highly-active catalysts of unsaturated compounds hydrogenation by using supercritical carbon dioxide

Polymer frameworks have been for the first time impregnated with rhodium and palladium compounds in a supercritical carbon dioxide medium. The polymeric carriers were the mesoporous phenol-formaldehyde resin and cross-linked dendrimer networks, based on poly(propylene imine) and poly(amido amine) dendrimers. The metal compounds were rhodium acetylacetonate and palladium carboxylates including their fluoro-substituted derivatives and palladium hexafluoroacetylacetonate. Using the IR-spectroscopy method, we have shown that the metal content in the polymers was in the range of 0.3–2.0 wt.% depending on the polymer nature, cross-linking agent type and cross-linking degree. We have obtained the samples of polymer supports with nanosized metal particles through hydrogen reduction of immobilized rhodium and palladium compounds at P(H2) = 6–10 MPa, T = 60 °C, 4 h time. The average rhodium particle size was 1–2 nm. It has been shown that the obtained nanocomposites were extremely active catalysts for hydrogenation of unsaturated hydrocarbons.

High-pressure phase equilibrium data for carbon dioxide + dichloromethane + acetone and carbon dioxide + dichloromethane + acetone + N-acetylcysteine (NAC)

The N-Acetylcysteine (NAC) is a thiol compound with a strong antioxidant effect and demonstrated preventive action as protector in chronic kidney disease, cancer, pulmonary insufficiency, in the treatment of AIDS and other systemic diseases. Experimental phase equilibrium data provide fundamental information for the micronization supercritical fluid SEDS technique, which is characterized by the reduction of the average particle size leads to changes in physical structure of the compounds processed. There are numerous advantages in particle size reduction and make narrower particle size distribution, the main one being the resulting increased bioavailability. Therefore, the aim of this work was to study the high pressure phase behavior of the ternary system {carbon dioxide (1) + dichloromethane (2) + acetone (3)} and for the quaternary system {carbon dioxide (1) + dichloromethane (2) + acetone (3) + NAC (4)} in order to assist the micronization process in supercritical media. The experiments were performed using a variable volume cell over the temperature range from 308 to 328 K, pressure from 4.7 to 9.87 MPa, and mass ratio dichlorometane/acetone of 1.5:1 for the ternary system. For the quaternary system, the same mass ratio of dichlorometane/acetone was adopted and an amount of 0.001 and 0.004 g cm−3 of NAC was added to solution. Phase transitions of vapor-liquid type (bubble or dew points), in the presence or absence of solid phase were observed. The experimental results of the ternary system were modeled using the Peng-Robinson (PR) equation of state with the Wong-Sandler (PR-WS) mixing rule, providing a good representation of the experimental phase equilibrium data.

Phase behaviour of pseudoternary system (carbon dioxide + ω-pentadecalactone + dichloromethane) at different dichloromethane to ω-pentadecalactone mass ratios

Abstract Experimental phase equilibrium data for the pseudoternary system involving (carbon dioxide + ω-pentadecalactone + dichloromethane) have been measured in order to provide fundamental information to conduct the polymerization reaction in supercritical carbon dioxide medium. The experiments were performed using a variable-volume view cell over the temperature range from 313 K to 343 K, system pressure between 3.6 MPa and 19.4 MPa and different mass ratios of dichloromethane to ω-pentadecalactone (0.5:1, 1:1 and 2:1). Phase transitions of vapour-liquid (bubble and dew point), liquid-liquid and vapour-liquid-liquid types were observed. The experimental results were modelled using the Peng-Robinson (PR) equation of state with the Wong-Sandler (PR-WS) mixing rule, providing a good representation of the experimental phase equilibrium data.

Study of carbonation in a class G Portland cement matrix at supercritical and saturated environments

The carbon dioxide (CO2) is a chemical compound that can be present at subsurface, inside the porous of reservoir rocks containing oil or natural gas, dissolved or free. Being its occurrence natural or artificial, when injected into the reservoir at high pressure in order to improve the oil recovery factor. Regardless the source, it has potential to cause serious problems to the cement used during certain operations in oil wells. When in contact with hydration products of the Portland cement, the CO2 reacts due to a specific phenomenon denominated carbonation. This transformation significantly affects the cementitious composite array, causing changes in the microstructure, chemical compounds and harmful consequences for physical properties, such as porosity/permeability and mechanical resistance. The present study investigated the effects of CO2 under two conditions, supercritical state and dissolved in water, in a class G Portland cement matrix that is used in the oil industry. Samples containing the same formulation were placed at the same time in an autoclave, exposed to mediums with supercritical CO2 or saturated in water and removed after 30 days. In order to corroborate in the discussions and conclusions, the following analyses were conducted: pH indicator for measuring carbonation depth, image analysis to quantify the percentage of the affected area, X-ray diffraction using the Rietveld method for phases identification and quantification, and thermogravimetric analysis in order to confirm the presence of certain compounds. The analysis showed very distinct results, being the samples that were exposed to saturated medium suffering a greater attack. Based on the images analysis, formation of carbonates identified by XRD and quantified by the Rietveld method, the attack on samples subject to the medium saturated with CO2 was 35 percentage points higher when compared with the supercritical medium.

Synthesis of hybrid magnetic carbon nanotubes – C18-modified nano SiO2 under supercritical carbon dioxide media and their analytical potential for solid-phase extraction of pesticides

The synthesis of novel magnetic nanoparticles (MNPs), coated with octadecyl group-modified silica (nano SiO2C18) containing carbon nanotubes (CNTs) under supercritical CO2 medium, is reported. Transmission electron micrographs revealed MNPs decorated the surface of the CNTs and nano SiO2C18. The hybrid nanoparticles were used for the determination of five neonicotinoids and four sulfonylureas pesticides at trace levels by HPLC-UV. A comparative study of analyte adsorption and desorption was conducted with similar magnetic composites prepared by high thermal decomposition and sc-CO2 methods. Both sorbents adsorbed the analytes to some extent, but higher recoveries were obtained by using a mixture of both nanomaterials. This mixture was used for the extraction of neonicotinoids and sulfonylureas pesticides from water and fruit juices samples. MNPs can be re-used ten times at least. Detection limits range from 0.07 to 0.60 μg mL−1, recoveries from 78 to 95%, and relative standard deviations are <5%.

High-pressure phase equilibrium data for systems containing carbon dioxide, ω-pentadecalactone, chloroform and water

Abstract Experimental phase equilibrium data for the pseudobinary system involving (carbon dioxide + ω-pentadecalactone) and for the pseudoternary system involving (carbon dioxide + ω-pentadecalactone + chloroform) have been measured in order to provide fundamental information to conduct the polymerization reaction in supercritical carbon dioxide medium. The experiments were performed using a variable-volume view cell over the temperature range from 313 K to 343 K, system pressure between 5.4 MPa and 26.1 MPa and different mass ratios of chloroform to (ω-pentadecalactone) (0.5:1, 1:1 and 2:1). Phase transitions of vapour-liquid, liquid-liquid and vapour-liquid-liquid types were observed. The experimental results were modelled using the Peng-Robinson (PR) equation of state with the Wong-Sandler (PR-WS) mixing rule, providing a good representation of the experimental phase equilibrium data.

Upgrading of petroleum vacuum residue using a hydrogen-donor solvent with acid-treated carbon

Hydrocarbon/carbon systems can be used to perform the hydrogen transfer reaction in heavy oil upgrading, as alternatives to metal catalysts and molecular hydrogen. In this study, a system comprising a hydrogen-donor solvent (tetralin) and acid-treated activated carbon was established to evaluate its ability as a hydrogen transfer agent to upgrade a vacuum residue (VR). The use of tetralin substantially decreased coke formation from 32.7 wt% to a negligible amount in VR upgrading at 450 °C because the solvent could act as a hydrogen donor and diluent for the coke precursors. The acid-treated activated carbon accelerated dehydrogenation of tetralin through hydrogen transfer in the upgrading, resulting in a higher residue conversion and moderate coke formation. In view of the phase behavior, the hydrogen transfer reaction could also be promoted by increasing the contact between the hydrogen acceptor and donor in the supercritical medium. In the VR upgrading using the activated carbon in supercritical tetralin, complete residue conversion was achieved with 47 wt% light fractions (gas and light oil) and 6 wt% coke at 450 °C and 5.33 MPa. The results indicate that the streams available in oil refinery processes, which are rich in hydrocarbons with hydrogen-donor abilities, have great potential for use in heavy oil upgrading.

Ultrathin Hydrophobic Coatings Obtained on Polyethylene Terephthalate Materials in Supercritical Carbon Dioxide with Co-Solvents

The surface properties of ultradisperse polytetrafluoroethylene coatings on polyethylene terephthalate materials modified in a supercritical carbon dioxide medium with co-solvent additions (aliphatic alcohols) were analyzed. An atomic force microscopy study revealed the peculiarities of the morphology of the hydrophobic coatings formed in the presence of co-solvents. The contribution of the co-solvents to the formation of the surface layer with a low surface energy was evaluated from the surface energy components of the modified polyester material. The stability of the coatings against dry friction was analyzed.

Experimental determination and correlation of the solubility of a new reactive disperse orange SCF-AOL2 dye in pure supercritical carbon dioxide fluid

The solubility as well as its behaviors of a new reactive disperse orange SCF-AOL2 dye in supercritical carbon dioxide fluid, which is a potentially useful special dye for the coloration of natural fibers, was experimentally determined and investigated successfully by employing an improved batch circulation system and a convenient sampling method. The experimental measurements were performed with system temperature ranging from 333.2 K to 393.2 K and pressure from 8.0 MPa to 22.0 MPa at an equilibrium contact time of 60 min. Moreover, the solubility data were correlated and validated by Chrastil, Del Valle-Aguilera and Mendez-Santiago–Teja models in terms of density of pure supercritical carbon dioxide medium. The obtained results show that, the solubility as well as its behaviors of the new reactive disperse SCF-AOL2 dye was significantly influenced by system parameters, in which it increased notably with pressure in different isotherms, as well as with the tested system temperature in different isobars. Furthermore, the obtained experimental data were well correlated and validated by the employed three semi-empirical models at various temperatures and pressures with low values of average absolute relative deviation (AARD %) at 4.36%, 4.35% and 7.30%, respectively.

The Synthesis of Ce1 – xZr x O2 Oxides in Supercritical Alcohols and Catalysts for Carbon Dioxide Reforming of Methane on Their Basis

The effect of the features of starting zirconium compounds on the solvothermal synthesis of mixed Ce–Zr oxides in a flow reactor using ethanol and isopropanol as a supercritical medium was studied. The phase composition and structural properties of the synthesized samples were investigated using X-ray diffraction analysis (XRD) and Raman spectroscopy. The morphology and textural properties were studied by transmission electron microscopy (TEM) and thermal desorption of nitrogen. It was found that the use of zirconium oxychloride or acetate as the starting substances results in the formation of a mixture of phases enriched with cerium or zirconium, whereas the use of zirconium butoxide with acetylacetone as a complexing agent allows obtaining a homogeneous solid solution. The catalytic properties of the synthesized oxides with supported Ni were tested in methane dry reforming (MDR). The catalyst containing a single-phase oxide synthesized in the presence of acetylacetone exhibited much higher activity, selectivity and coking stability.

Production of Sodium Alginate-Based Aerogel Particles Using Supercritical Drying in Units with Different Volumes

A complex study of the production of sodium alginate aerogel particles by emulsion gelation and dripping methods followed by drying in supercritical carbon dioxide is conducted. The factors that affect the characteristics of the obtained materials are determined. The above-mentioned methods are tested on a semiindustrial level using high-pressure homogenization and spraying through pneumatic nozzles. The resulting gel particles are dried in a supercritical carbon dioxide medium using equipment with the volume of drying vessels of 0.25 and 2 L. The necessary characteristics and quality of the obtained aerogels do not deteriorate in the case of production scale-up.

Treatment of Cotton Fabrics by Ammonium Palmitate in a Supercritical CO2 Medium

Experimental results on the solubility of ammonium palmitate as a potential fabric water-repellent agent in supercritical carbon dioxide (SC-CO2)—pure and modified with acetone and dimethyl sulfoxide—are presented. The measurements are performed at temperatures 318.2 and 328.2 K in the pressure range from 10.0 to 32.5 MPa in a dynamic regime. The experimental solubility data are described using the Peng–Robinson equation of state. The results of treatment of various types of cotton fabrics by ammonium palmitate in SC-CO2 are presented. The contact (wetting) angle of the treated samples is determined and the increase in their hydrophobicity is demonstrated.

Improving the photo‐reduction of CO2 to fuels with catalysts synthesized under high pressure: Cu/TiO2

AbstractBACKGROUNDIn previous studies the enhanced activity of TiO2‐based catalysts synthesized in supercritical medium for photocatalytic reduction of CO2 was proved.RESULTSIn this study, Cu/TiO2 photocatalysts were synthesized by hydrothermal methods in supercritical CO2. Two titanium precursors [titanium tetraisopropoxide and diisopropoxititanium bis(acetylacetonate)], two alcohols (ethanol and isopropyl alcohol), and one metal precursor (Cu (II) acetylacetonate) were used in the synthesis. Catalysts produced showed improved properties in comparison with the commercial reference catalyst (Degussa P‐25, Evonik).CONCLUSIONSSpecifically, it has been found that Cu/TiO2 catalysts may yield methane production rates 20 times larger than that of commercial TiO2 catalyst without diminishing CO production rate (about 5 times higher than that of commercial catalyst). This result has been mainly imputed to both the formation of oxygen vacancies during the synthesis in supercritical medium and the high capacity of copper to adsorb and activate CO2 molecules, while preventing CO molecules from reoxidation, and avoiding the competitive reaction of hydrogen formation. © 2017 Society of Chemical Industry

Investigation of carbon phase evolutions in titanium nitride-carbon nanocomposites prepared in supercritical benzene with respect to their lithium storage capacity

Titanium nitride-carbon nanocomposites have been synthesized by the reaction of TiCl4 and NaN3 in supercritical benzene medium that also serves as a carbon source. The as-prepared precursor has been subjected to different heat treatments under ammonia and nitrogen atmospheres. The structure and chemical composition of the synthesized TiN-C nanocomposites are studied by X-ray diffraction (XRD) and CHN elemental analysis. Meanwhile, the nature of carbonaceous species and the respective carbon phase transitions during supercritical process and following heat treatments are further investigated by Raman spectroscopy, time of flight secondary ion mass spectrometry (ToF-SIMS), and their charge-discharge characteristics with respect to lithium storage. After 10 h NH3-treatment at 1000 ˚C carbonaceous phase transforms to graphene layered structure. The highly efficient mixed TiN conducting network and the internal defects between G layers induced by nitrogen doping improve rate capability and cycling performance of G sheets and provides a specific capacity of 381 mAh g-1 at charge/discharge (C/D) rate of 0.2 C. The enhanced electrochemical performance of the SIV nanocomposite is mainly due to improving the electronic/ionic conductivity and reducing charge transfer coefficient and increasing electrochemical surface area that are resulted from anchoring of TiN nanoparticles on graphene sheets.

Modeling Solvation in Supercritical CO2.

In recent decades, a microscopic understanding of solute-solvent intermolecular interactions has been key to advances in technologies based on supercritical carbon dioxide. In many cases, computational work has provided the impetus for new discoveries, shedding new light on important concepts such as the local structure around the solute in the supercritical medium, the influence of the peculiar properties of the latter on the molecular behavior of dissolved substances and, importantly, CO2 -philicity. In this Review, the theoretical work that has been relevant to these developments is surveyed and, by presenting some crucial open questions, the possible routes to achieving further progress based on the interplay between theory and experiments is discussed.

Quantum Chemical Study of CH3 + O2 Combustion Reaction System: Catalytic Effects of Additional CO2 Molecule.

The supercritical carbon dioxide diluent is used to control the temperature and to increase the efficiency in oxycombustion fossil fuel energy technology. It may affect the rates of combustion by altering mechanisms of chemical reactions, compared to the ones at low CO2 concentrations. Here, we investigate potential energy surfaces of the four elementary reactions in the CH3 + O2 reactive system in the presence of one CO2 molecule. In the case of reaction CH3 + O2 → CH2O + OH (R1 channel), van der Waals (vdW) complex formation stabilizes the transition state and reduces the activation barrier by ∼2.2 kcal/mol. Alternatively, covalently bonded CO2 may form a six-membered ring transition state and reduce the activation barrier by ∼0.6 kcal/mol. In case of reaction CH3 + O2 → CH3O + O (R2 channel), covalent participation of CO2 lowers the barrier for the rate limiting step by 3.9 kcal/mol. This is expected to accelerate the R2 process, important for the branching step of the radical chain reaction mechanism. For the reaction CH3 + O2 → CHO + H2O (R3 channel) with covalent participation of CO2, the activation barrier is lowered by 0.5 kcal/mol. The reaction CH2O + OH → CHO + H2O (R4 channel) involves hydrogen abstraction from formaldehyde by OH radical. Its barrier is reduced from 7.1 to 0.8 kcal/mol by formation of vdW complex with spectator CO2. These new findings are expected to improve the kinetic reaction mechanism describing combustion processes in supercritical CO2 medium.