Grafting of polypropylene with N-cyclohexylmaleimide and styrene simultaneously using supercritical CO2

Monomer mixture of styrene (St) and N-cyclohexylmaleimide (ChMI) and initiator benzoyl peroxide (BPO) were first impregnated into isotactic polypropylene (iPP) films simultaneously using supercritical carbon dioxide (SC CO₂) as a solvent and swelling agent at 35.0 °C. The composites were obtained after the monomers were grafted onto the iPP matrix at 70 °C. The effects of various conditions, such as pressure, monomer concentration, and the molar ratio of the two monomers in the soaking process, on the composition of the composites were determined. The molar ratios of St to ChMI in the composites were estimated by Fourier transform infrared spectroscopy. The thermal properties, the morphology, and the mechanical properties of the composites were characterized by different techniques. The results demonstrated that the phase size of the grafted St-ChMI was very small and the phase boundary was very ambiguous. The composites had better thermal stability than the original iPP film. The Young's modulus and tensile strength of the film increased continuously with increasing grafting percentage. The two grafted monomers in the composites had good synergetic effect.     

Cetirizine solubility in supercritical CO2 at different pressures and temperatures

A simple static technique was used to obtain the solubility of cetirizine in supercritical carbon dioxide. The solubility measurements were performed at temperatures and pressures ranging from 308.15 to 338.15 K and 160 to 400 bar, respectively; resulting in mole fractions in the 1.05 × 10⁻⁵ to 4.92 × 10⁻³ range. The Chrastil, Bartle, Kumar & Johnston and the Mendez-Santiago and Teja (MST) models were used to correlate the experimental data. The calculated solubilities showed good agreement with the experimental data in the temperature and pressure ranges studied.     

Gasification of charcoal using supercritical CO2 at high pressures

A novel one-shell high temperature and high pressure semi-continuous reactor has been developed for the study of the Boudouard reaction at temperatures up to 820 °C and pressures up to 32.5 MPa. Semicontinuous gasification of charcoal using supercritical CO₂ has been achieved at conversions up to 90.8% (w/w) at LSHV between 20 and 30 h⁻¹ after 5–9 h. A gasification model is proposed and validated. Effective rates of gasification (1.32 ± 0.12) × 10⁻⁶ to (6.10 ± 2.03) × 10⁻⁵ s⁻¹ were obtained. The results indicated that this method is technically feasible for the on-line production of high pressures streams of CO/CO₂ in the lab for carrying out further chemistries, avoiding the use of CO high pressure bottles.     

Impact of SO2 concentration on the corrosion rate of X70 steel and iron in water-saturated supercritical CO2 mixed with SO2

The corrosion behavior of X70 steel and iron in water-saturated supercritical CO₂ mixed with SO₂ was investigated using weight-loss measurements. As a comparison, the instantaneous corrosion rate in the early stages for iron in the same corrosion environment was measured by resistance relaxation method. Surface analyzes using SEM/EDS, XRD and XPS were applied to study the morphology and chemical composition of the corroded sample surface. Weight-loss method results showed that the corrosion rate of X70 steel samples increased with SO₂ concentration, while the corrosion rate increased before decreasing with SO₂ concentration for iron sample. Comparing resistance relaxation method results with weight-loss method results, it is found that the instantaneous corrosion rate of iron is much higher than the uniform corrosion rate of the iron tablet specimens which are covered with thick corrosion product films after a long period of corrosion. The corrosion product films were mainly composed of FeSO₄ and FeSO₃ hydrates. The possible reaction mechanism under such environment was also analyzed, and the electrochemical reaction between the dissolved SO₂ in the condensed water film with iron is the critical reaction step.     

Impregnation of a triphenylpyrylium cation into zeolite cavities using supercritical CO2

scCO₂ technology was used for the impregnation of microporous zeolites with organic molecules. Specifically, faujasite Y was impregnated with 1,3,5-triphenyl-2-pentene-1,5-dione that after reacting with the acid sites of the zeolite large cavities, formed the 2,4,6-triphenylpyrylium cation. Since the triphenylpyrylium cation has a size higher than the diameter of the zeolite channels, the fabrication method was considered as a ship-in-a-bottle procedure. To optimize the loading, different pressures and temperatures were tested in both the diffusion and cyclisation steps used in the impregnation supercritical process. The obtained impregnated samples were purified either by conventional Soxhlet extraction with dichloromethane or by cleaning with a scCO₂ continuous flow. Loaded amounts of ca. 4–7 wt% were obtained following the supercritical procedure, which resulted more time-effective than the conventional procedure based in the use of organic solvents. Samples obtained were analyzed by thermal analysis, UV–vis and IR spectroscopy and characterization of surface area and micropore volume.     

Extraction of nimbin from neem seeds using supercritical CO2 and a supercritical CO2–methanol mixture

Nimbin, a component found in neem seeds, which is reported to have several valuable medicinal properties including: anti-inflammatory, anti-pyretic, anti-fugal, antihistamine and antiseptic was extracted from neem seeds using supercritical CO₂ and CO₂ with a methanol modifier.Nimbin extraction yields using supercritical carbon dioxide were found to be approximately 85% at 308 K, 23 MPa and a CO₂ flow rate of 0.62 cm³/min for a 2-g sample of neem. An optimum extraction pressure appears to exist at ≈23 MPa and 328 K. Although extraction using a methanol modifier did improve the extraction somewhat, methanol was not found to be an effective modifier for extracting nimbin.Dynamic extraction curves were predicted using three empirical models and a theoretical model. The three empirical models were: a Langmuir gas adsorption model, a first order plus dead time (FOPDT) model and a so-called tⁿ cyclone model used to incorporate sigmoidal curves. The parameters in the empirical models were fitted to the experimental data. The Goto et al. [J. Chem. Eng. Jpn. 31 (1998) 171] theoretical model was compared to the experimental results and was found to fit the data well. The theoretical model shows that the extraction yield depends strongly on the solvent flow rate, that is, external mass transfer or equilibrium is the controlling step of this process.     

Theoretical study on interaction between CO2 and carbonyl compounds: Influence of CO2 on infrared spectroscopy and activity of CO

The interactions between CO₂ and carbonyl compounds at different CO₂ pressures have been studied both experimentally and theoretically. In situ high-pressure FTIR on carbonyl compounds, i.e., acetaldehyde, acetone, and crotonaldehyde, in supercritical CO₂ have been measured at various CO₂ pressures varying from 6 to 22 MPa. In order to get insights into the mechanism, theoretical study has been conducted concerning the effect of CO₂ on frequency shift of CO in acetaldehyde, acetone, benzaldehyde, crotonaldehyde and cinnamaldehyde at different CO₂ pressures. It has been shown that the experimental frequency shifts can be well simulated by the theoretical model calculations using particular structures, in which a carbonyl compound interacts with a few CO₂ molecules, depending on the carbonyl compounds examined, except for acetaldehyde.The interaction energies between CO₂ and those carbonyl compounds are also given. In addition, the effect of CO₂ on hydrogenation of crotonaldehyde and benzaldehyde has been discussed by means of the local softness (s⁺) calculated at CO₂ pressures of 0-22 MPa, which can explain the reactivity difference in the crotonaldehyde and benzaldehyde hydrogenations in supercritical CO₂.     

In situ study of ionic conductivity for polyether-LiCF3SO3 electrolytes with subcritical and supercritical CO2

In situ measurements of the ionic conductivity were performed on polyethers, poly(ethylene oxide) (PEO) and poly(oligo oxyethylene methacrylate) (PMEO), with lithium triflate (LiCF₃SO₃) as crystalline and amorphous electrolytes, and at CO₂ pressures up to 20 MPa. Both PEO and PMEO systems in subcritical and supercritical CO₂ increased more than five fold in ionic conductivity at 40 °C composed to atmospheric pressure. The pressure dependence of the ionic conductivity for PEO electrolytes was positive under CO₂, and increased by two orders of magnitude under pressurization from 0 to 20 MPa, whereas it decreases with increasing pressure of N₂. The enhancement is caused by the plasticizing effect of CO₂ molecules that penetrate into the electrolytes.     

Foaming of polymers with supercritical CO2: An experimental and theoretical study

Microcellular polystyrene (PS) foams and porous structures of the biodegradable poly(d,l-lactic acid) (P_d,lLA) were prepared with the batch foaming technique (pressure quench) using supercritical CO₂ as blowing agent. The effect of pressure, temperature and depressurization rate on the final porous structure was investigated. The results revealed that the size of the pores decreases and their population density increases with pressure increase, or decrease of temperature, and/or increase of the depressurization rate. The results were correlated by combining nucleation theory with NRHB model in order to account for and emphasize the physical mechanism related to nucleation of bubbles inside the supersaturated polymer matrix. A satisfactory agreement between correlations and experimental data was obtained indicating that the nucleation theory yields quantitative correlations when variables such as sorption, degree of plasticization, and surface tension of the system polymer-supercritical fluid are accurately described.     

Regeneration of Pd/TiO2 catalyst deactivated in reductive CCl4 transformations by the treatment with supercritical CO2, ozone in supercritical CO2 or oxygen plasma

Carbonaceous deposits formation was established as the primary reason of Pd/TiO₂ catalyst deactivation during reductive processing of CCl₄ to form hydrodechlorination and oligomerization products. Three methods of carbonaceous deposits elimination were tested: (1) extraction by supercritical CO₂, (2) oxidation by ozone in supercritical CO₂, and (3) low-temperature glow-discharge oxygen plasma treatment. Synchronic thermal analysis confirms effective carbonaceous deposits removal during regeneration by ozone or low temperature glow-discharge oxygen plasma; by XPS deep oxidation of surface Pd after oxidative treatment (by ozone or oxygen plasma) was found. Thus H₂ reduction was proposed as the second step making possible full regeneration of initial catalytic activity of Pd/TiO₂.     

Thermodynamic modeling of dealcoholization of beverages using supercritical CO2: Application to wine samples

The supercritical removal of ethanol from alcoholic beverages (brandy, wine, and cider) was studied using the GC-EoS model to represent the phase equilibria behavior of the CO₂ + beverage mixture. Each alcoholic drink was represented as the ethanol + water mixture with the corresponding ethanol concentration (35 wt% for brandy, 9-12 wt% for different wines and 6 wt% for cider). The thermodynamic modeling was based on an accurate representation of the CO₂ + ethanol and CO₂ + water binary mixtures, and the CO₂ + ethanol + water ternary mixture.The GC-EoS model was employed to simulate the countercurrent supercritical CO₂ dealcoholization of the referred beverages; the results obtained compared good with experimental data from the literature. Thus, the model was used to estimate process conditions to achieve an ethanol content reduction from ca. 10 wt% to values lower than 1 wt%. The model results were tested by carrying out several extraction assays using wine, in a 3 m height packed column at 308 K, pressures in the range of 9-18 MPa and solvent to wine ratio between 9 and 30 kg/kg.     

Nanostructured PVDF-HFP membranes loaded with catalyst obtained by supercritical CO2 assisted techniques

Polymeric catalytic membrane reactors offer a larger flexibility over conventional reactors. The most-used method to generate polymer-based catalytic membranes is the phase inversion that, however, presents some limitations; in particular, the difficulty in generating a uniform distribution of the loaded materials.In this work, we use two new processes for the formation of membranes loaded with catalyst for potential applications in catalysis: supercritical assisted phase inversion and supercritical assisted gel drying, applied to formation of poly(vinylidene fluoride-co-hexafluoropropylene) membranes loaded with palladium nanoparticles. We analyzed the effect of process parameters (polymer concentration, catalyst concentration, pressure, temperature) on the membranes morphology. The supercritical phase inversion process produced cellular asymmetric structures with cell size ranging between 3 and 6 μm and nanoporous homogeneous networks, depending on the process conditions. Palladium nanoparticles homogeneous distributions were obtained only operating at selected process conditions, i.e., pressures larger than 150 bar and temperatures lower than 45 °C.Supercritical gel drying allowed homogeneous nanoporous membranes formation at all the tested process conditions: they were characterized by very high porosity (higher than 90%) and a very uniform catalyst distribution.     

Preparation of nanometer dispersed polypropylene/polystyrene interpenetrating network using supercritical CO2 as a swelling agent

Nanometer dispersed polypropylene/polystyrene (PP/PS) interpenetrating networks (IPNs) have been prepared by the radical polymerization and crosslinking of styrene (St) within supercritical (SC) CO₂-swollen PP substrates. In this method, monomer St, crosslinking agent divinyl benzene (DVB), and the initiator benzoyl peroxide were first impregnated into PP matrix using SC CO₂ as a solvent and swelling agent at 35.0 °C, and then the polymerization and crosslinking were carried out at 120 °C. The composition of the IPNs can be controlled by SC CO₂ pressure, concentrations of St and DVB in the fluid phase. Transmission electron microscopy shows that the PS is homogeneously dispersed in the IPNs and its phase size is in the range of 20-30 nm. The impact strength, tensile strength, and elongation-at-break of the PP/PS IPNs increase with increasing PS percentage in the IPNs.     

Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle

This paper presents experimental results of a 20 kW vertical combustor equipped with a single pf-burner on pulverised coal combustion in air and O₂/CO₂ mixtures with NO_x recycle. Experimental results on combustion performance and NO_x emissions of seven international bituminous coals in air and in O₂/CO₂ mixtures confirm the previous findings of the authors that the O₂ concentration in the O₂/CO₂ mixture has to be 30% or higher to produce matching temperature profiles to those of coal-air combustion while coal combustion in 30% O₂/70% CO₂ leads to better coal burnout and less NO_x emissions than coal combustion in air. Experimental results with NO_x recycle reveal that the reduction of the recycled NO depends on the combustion media, combustion mode (staging or non-staging) and recycling location. Generally, more NO is reduced with coal combustion in 30% O₂/70% CO₂ than with coal combustion in air. Up to 88 and 92% reductions of the recycled NO can be achieved with coal combustion in air and in 30% O₂/70% CO₂ respectively. More NO is reduced with oxidant staging than without oxidant staging when NO is recycled through the burner. Much more NO is reduced when NO recycled through the burner (from 65 to 92%) than when NO is recycled through the staging tertiary oxidant ports (from 33 to 54%). The concentration of the recycled NO has little influence on the reduction efficiency of the recycled NO with both combustion media—air and 30% O₂/70% CO₂.     

Structure,permeability, and rheology of supercritical CO2 dispersed polystyrene-clay nanocomposites

Improvement in clay dispersion and clay-polymer interfacial interactions are keys to producing superior nanocomposites. A supercritical CO₂ (scCO₂) processing method was utilized to pre-disperse commercial organic clays, for further solvent mixing with polystyrene (PS) to form nanocomposites with significant dispersion and interfacial enhancement. The effect of scCO₂ processing on clay pre-dispersion, and clay dispersion and interfacial interaction in nanocomposites were investigated. SEM and WAXD of the clays indicated that after scCO₂ processing the clays lose their long region ordered layer structure appreciably, associated with reduction in particle size. WAXD and TEM of the PS/clay nanocomposites showed that the polymer penetrated into the pre-dispersed clay, leading to a disordered intercalated/exfoliated structure with improved interfacial interaction rather than a disordered intercalated structure as seen with as-received clays. Relationships between those structures, rheological and barrier properties were investigated. The scCO₂-processed nanocomposites showed a plateau in the low-frequency storage modules and increased complex viscosity, each associated with significant clay dispersion in the nanocomposite. With only 1.09% volume fraction of clay, significant reduction (∼49%) of oxygen permeation was achieved.     

Polymer melt rheology with high-pressure CO2 using a novel magnetically levitated sphere rheometer

A magnetically levitated sphere rheometer (MLSR) designed to measure viscosity of fluids exposed to high-pressure carbon dioxide has been developed. This device consists of a magnetic sphere submerged inside a test fluid within a high-pressure housing and levitated at a fixed point. The housing is constructed from an optically transparent sapphire tube. The cylindrical tube can be moved vertically to generate a shear flow around the levitated sphere. The difference in magnetic force required to levitate the sphere at rest and under fluid motion can be directly related to fluid viscosity. Rheological properties, specifically zero shear viscosities, of transparent high-pressure materials can be measured to a precision of about 5% and over a wide range of viscosities. In addition, operation at constant pressure, in concentration regimes from a pure polymer to an equilibrated polymer/supercritical fluid solution, and at shear rates over several orders of magnitude is possible, eliminating many of the disadvantages associated with other high-pressure rheometers. Experiments performed at different temperatures with a poly(dimethylsiloxane) melt at atmospheric pressure are compared with data from a commercial Couette rheometer to demonstrate device sensitivity and viability. Measurements of a PDMS melt plasticized by high-pressure CO₂ are performed to illustrate the utility of the new rheometer under high-pressure conditions. Experimental data are obtained at 30 °C, for pressures up to 20.7 MPa and CO₂ concentrations reaching 30 wt%. Viscosity reductions of nearly two orders of magnitude compared with the pure polymer viscosity at atmospheric pressure are observed. Additionally, the effects of pressure on a polymer/CO₂ system are directly investigated taking advantage of the constant pressure operation mode of the MLSR. This allows us, for the first time in experiments of polymers with supercritical fluids, to decouple the effects of CO₂ concentration and pressure in a single device.     

Vesicle formation of polystyrene-block-poly (ethylene oxide) block copolymers induced by supercritical CO2 treatment

A novel route for a preparation of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) block copolymer vesicles induced by supercritical carbon dioxide (scCO₂) is demonstrated. When PS-b-PEO block copolymer solutions in tetrahydrofuran (THF) are treated with scCO₂ at 70 °C for different times, PS-b-PEO copolymers first assemble into aggregated spheres; then aggregated spheres change into large compound micelles and finally evolve into vesicles. The possible formation mechanism of the vesicles is discussed.     

Porous poly(l-lactic acid) nanocomposite scaffolds prepared by phase inversion using supercritical CO2 as antisolvent

Nanohybrids were fabricated from a poly(l-lactic acid) solution loaded with various concentrations of organically modified montmorillonite. Investigation of the produced composites' morphology by X-ray diffraction analysis, transmission electron microscopy and atomic force microscopy revealed a coexistence of intercalated and exfoliated clay particles, with the latter ones being predominant for low filler loadings. Porous scaffolds of pure and nanocomposite poly(l-lactic acid) were prepared using supercritical CO₂ as antisolvent and the influence of montmorillonite content was examined. It was observed that the final cellular structure was strongly related to the filler content.     

Kinetics and specificity of Lipozyme-catalysed oil hydrolysis in supercritical CO2

Blackcurrant seed oil is rich in linoleic and linolenic acids. Selective enzyme-catalysed oil hydrolysis was studied with aim to obtain different levels of α- and/or γ-linolenic acid in the mixture of liberated fatty acids and in the fraction of di- and monoacylglycerols, making them suitable for special dietary needs. The oil was dissolved in supercritical carbon dioxide flowing through a packed bed reactor (temperature 40 °C, pressure 15–28 MPa, and superficial velocity 0.1–0.7 mm s⁻¹) with Lipozyme^®, a 1,3-specific lipase from Mucor miehei immobilised on a macroporous ion-exchange resin. The enzyme activity was stable as long as water precipitation in the reactor was prevented. The reaction was found to be controlled by both Michaelis–Menten kinetics and mass transfer. The maximum rate of fatty acids liberation per unit amount of enzyme, 2.6 × 10⁻³ mol s⁻¹ kg⁻¹, was achieved at the maximum flow velocity and pressure. Compared to oil, the liberated fatty acids contained more α-linolenic, palmitic and stearic acids, while di- and monoacylglycerols contained increased levels of γ-linolenic and stearidonic acids.     

Photocatalytic activity of porous titania nanocrystals prepared by nanoscale permeation process in supercritical CO2: Effects of supercritical conditions

Porous TiO₂ nanocrystals (PTN) were synthesized using activated carbon templates with supercritical CO₂ by using the nanoscale permeation (NP) process. The photoactivity of PTN was tested by methylene blue (MB) degradation. Compared with the commercially available P-25, all PTN exhibited significant photocatalytic degradation of MB mainly due to their porous structure with high surface area, high hydroxy concentration and small crystalline size. The optimum temperature and pressure are found to be 60 °C and 26 MPa, under which obtained PTN-1 shows the highest photoactivity and slow deactivation for MB degradation after 15 trials.