Presence Of Supercritical Carbon Dioxide Research Articles

Hydrolysis of polyethylene terephthalate by ZSM-5 combined with supercritical carbon dioxide under neutral environment

Polyethylene terephthalate (PET) is a kind of stable polyester, which is difficult to be degraded in the natural environment, and the waste PET causes severe environmental problems. To achieve efficient and environmentally friendly hydrolysis of PET, we employed ZSM-5 as a catalyst for neutral hydrolysis of PET in the presence of supercritical carbon dioxide (SCCO2). This study demonstrates that the existence of SCCO2 in the system enhances PET hydrolysis efficiency. The swelling effect of SCCO2 on PET can relax the PET segment and promote the mass transfer of water molecules in the PET bulk phase. However, excessive pressure of SCCO2 leads to plasticization of PET which increases the crystallinity of PET and hinders hydrolysis. The activation energy was measured under different pressures, revealing a decrease in activation energy with increasing pressure below 7.5 MPa. When the pressure exceeds 7.5 MPa, the activation energy increases due to plasticization. Through the density functional theory (DFT) analysis combined with the experimental results, it has been confirmed that Brønsted acid on ZSM-5 plays a more important role than Lewis acid in the system.

Supercritical Carbon Dioxide in Presence of Water for the Valorization of Spent Coffee Grounds: Optimization by Response Surface Methodology and Investigation of Caffeine Extraction Mechanism.

Spent coffee grounds are a promising bioresource that naturally contain around 50 wt% moisture which requires, for a valorization, a drying step of high energy and economic costs. However, the natural water in spent coffee grounds could bring new benefits as a co-solvent during the supercritical CO2 extraction (SC-CO2). This work reports the influence and optimization of pressure (115.9-284.1 bars), temperature (33.2-66.8 °C), and moisture content (6.4-73.6 wt%) on simultaneous extraction of lipids and polar molecules contained in spent coffee grounds by supercritical CO2 (SC-CO2) using Central Composite Rotatable Design and Response Surface Methodology. The results show that for lipids extraction, pressure is the most influent parameter, although the influence of moisture content is statistically negligible. This suggests that water does not act as barrier to CO2 diffusion in the studied area. However, moisture content is the most influent parameter for polar molecules extraction, composed of 99 wt% of caffeine. Mechanism investigations highlight that H2O mainly act by (i) breaking caffeine interactions with chlorogenic acids present in spent coffee grounds matrix and (ii) transferring selectively caffeine without chlorogenic acid by liquid/liquid extraction with SC-CO2. Thus, the experiment for the optimization of lipids and polar molecules extraction is performed at a pressure of 265 bars, a temperature of 55 °C, and a moisture content of 55 wt%.

The Diffusion of TEMPONE Radical in the Graft Copolymer of N-Isopropylacrylamide with Oligolactide in the Presence of Supercritical Carbon Dioxide by In Situ EPR Method

The Diffusion of TEMPONE Radical in the Graft Copolymer of N-Isopropylacrylamide with Oligolactide in the Presence of Supercritical Carbon Dioxide by In Situ EPR Method

Effects of supercritical carbon dioxide sorption on the microstructure of poly(vinylidene fluoride)

Polyvinylidene Fluoride (PVDF) is a high performance polymer with wide technical applications. Often PVDF is in contact with high-pressure fluids which affect its microstructure and properties. Such effects are particularly remarkable in presence of supercritical carbon dioxide. Several PVDF samples with varying molecular weights ranging from 7850 to 109,000 g mol−1 were synthesized. These were subjected to CO2 sorption at 40 °C and 150 bar. At these conditions, negligible swelling was measured. A two-stage sorption process was observed. The equilibrium concentration and the saturation time increased with increasing molecular weight. Investigating the semi-crystalline microstructure by FT-IR, XRD and DSC showed no phase-transition of the crystallites except for the polymer with the lowest molecular weight. However, the size and arrangements of the crystalline lamellae within the amorphous phase seemed to be altered. These observations are discussed with respect to the effect of supercritical CO2 on the mobility of the amorphous chain segments.

Efficient perovskite solar cells processed in supercritical carbon dioxide

Perovskite-based solar cells with high power conversion efficiency were produced by annealing the perovskite layer at low temperature in the presence of supercritical carbon dioxide (scCO2). Photovoltaic devices were produced with perovskite layers annealed at either 50 ℃ or 100 ℃, with and without the assistance of scCO2. For devices annealed at 50 ℃, the use of scCO2 resulted in an 88% increase in the power conversion efficiency (from 9.17% to 17.22%) in comparison to devices annealed without scCO2. For devices annealed at 100 ℃ the use of scCO2 resulted in a smaller (3%) improvement in power conversion efficiency (from 16.94% to 17.52%). The quality of the perovskite layer at each condition was analyzed using x-ray diffraction, scanning electron microscopy and optical absorption. It was determined that the improvement in the observed device power conversion efficiency corresponds to an improvement in the quality of the photoactive perovskite layer.

Measurement and correlation of adsorption equilibria of propylene glycol monomethyl ether acetate on activated carbon in the presence of supercritical carbon dioxide

The adsorption equilibria of propylene glycol monomethyl ether acetate (PGMEA), a major volatile organic compound (VOC) used in the semiconductor industry, on activated carbon were measured in the presence of supercritical carbon dioxide (scCO2) at temperatures from 313 K to 353 K and pressures from 10.0 MPa to 20.0 MPa. The amount of adsorbed PGMEA considerably depended on the pressure and temperature conditions in scCO2, which could be explained by the effects of CO2 density and VOC fugacity. The Dubinin–Astakhov (DA) equation has two fitting parameters, interaction energy between adsorbate and adsorbent (EVOC) and saturated adsorption capacity of VOC on adsorbent (W0,VOC), and was used to correlate the measured adsorption equilibria. Correlation by the DA equation was useful for analyzing the adsorption equilibria of PGMEA on activated carbon in scCO2 by considering the affinity of the adsorbate for the adsorbent and competitive adsorption of CO2.

Thermal oxidation of polypropylene catalyzed by manganese oxide aerogel in oxygen-enriched supercritical carbon dioxide

A new approach to the thermal oxidation of polypropylene in oxygen-enriched supercritical carbon dioxide is presented. Several parameters are revealed to affect the oxidation process, namely, the presence of supercritical carbon dioxide, the presence of Mn2O3 nanoparticles and oxygen to polypropylene mass ratio. It is demonstrated that the use of carbon dioxide enhances the oxidative process at lower oxygen content. Mn2O3 nanoparticles both increase the rate of the oxidation process and influence the product distribution. Gas chromatography-mass spectrometry reveals that acetone, acetic acid and formic acid are among the main products of the oxidation process. Furthermore, the obtained results show that, at a higher oxygen content, the use of manganese oxide aerogel as a catalyst promotes the selective oxidation of polypropylene, with acetic acid being the main liquid product of the process.

Electrochemically active dispersed tungsten oxides obtained from tungsten hexacarbonyl in supercritical carbon dioxide

Electrochemically active nanocrystalline tungsten oxide was synthesized in supercritical carbon dioxide from tungsten hexacarbonyl at 150 °C and 400 bar in the presence of oxygen (partial pressure of 15 bar). The supercritical fluid is a solvent for the precursor (i.e., this is a sc solvothermal synthesis route), whereas the admixed gaseous oxygen serves as an oxidizer, promoting thermal decomposition of the precursor. During the substrate-free synthesis, 200–500 nm aggregates are formed. They consist of smaller grains having the size of about 100 nm. Therefore, a certain structural hierarchy is detected. The electrochemical activity of the as-synthesized particulate material is pronouncedly increasing during both potential cycling and exposure in an aqueous aerated electrolyte. After such a hydration/oxidation process, the electrochemical response of the material shows rather fast and reversible recharging of the entire tungsten-containing phase. This is an indication of facilitated proton transport in bulk of the tungsten oxide phase synthesized in the supercritical carbon dioxide with subsequent hydration/oxidation. Quite differently, the material synthesized at the same temperature only in compressed oxygen (partial pressure of 15 bar) without any presence of supercritical carbon dioxide is highly crystalline one. It does not demonstrate any significant electrochemical rechargeability; neither is the response improving with hydration/oxidation.

A generalized model for predicting adsorption equilibria of various volatile organic compounds on activated carbon in the presence of supercritical carbon dioxide

A generalized model for predicting the adsorption equilibria of various VOCs (volatile organic compounds: acetone, toluene, n-hexane, n-octane, n-decane, methanol, ethanol, 2-propanol, and propylene glycol monomethyl ether (PGME)) on activated carbon in supercritical carbon dioxide (scCO2) was proposed. The two fitting parameters W0,VOC (saturated adsorption capacity of activated carbon for VOC) and EVOC (interaction energy between VOC and adsorbent) of the Dubinin-Astakhov (DA) equation, determined with the correlations to experimental adsorption equilibrium data, were found to be closely related to the density of CO2 and properties of VOCs, which could be generalized based on the physical meanings of each parameter. The generalized DA parameters were able to reproduce the adsorption equilibria of the various VOCs in scCO2 to within 30% of the average relative deviation over a wide range of temperature and pressure conditions for most of the VOCs.

Measurement and modeling of adsorption equilibria of cobalt (III) acetylacetonate on MCM-41 mesoporous silica in the presence of supercritical carbon dioxide with methanol co-solvent

Adsorption equilibria of cobalt (III) acetylacetonate (Co(acac)3) in the presence of supercritical carbon dioxide (scCO2) with methanol co-solvent on MCM-41 mesoporous silica were studied at 313–353 K and 12.5–20.0 MPa from experimental and model calculation viewpoints. The measured data indicated that the amount of the adsorbed metal precursor decreased with increasing pressure and decreasing temperature, which could be explained by CO2 density effects. Correlation by the Dubinin-Astakhov equation was applicable for examining the adsorption equilibria of the metal precursor on the mesoporous silica in scCO2 while considering the molecular size of the precursor, affinity for the adsorbent, and CO2 density effects. Simulated results with the DA equation implied that the methanol co-solvent can affect the adsorption equilibria of Co(acac)3 on the MCM-41 adsorbent in scCO2.

Adsorption kinetics of rhodium (III) acetylacetonate onto mesoporous silica adsorbents in the presence of supercritical carbon dioxide

The adsorption kinetics of rhodium (III) acetylacetonate (Rh(acac)3) onto three types of mesoporous silica adsorbents, MCM-41, HMS, and MSU-H, in the presence of supercritical carbon dioxide (scCO2) at temperatures from 313 to 353 K and at a pressure of 15.0 MPa were studied using fixed-bed adsorption measurements. The breakthrough curves obtained with the measurements were correlated by a mathematical kinetic model using only one fitting parameter, the effective diffusion coefficient of Rh(acac)3 in pores (De). The determined De values depended strongly on the temperature and the types of mesoporous silica used, which could be described using a generalized model that includes the parameters in consideration of the pore structure of the adsorbents (porosity and tortuosity) and the diffusion coefficient of Rh(acac)3 in the bulk scCO2 phase.

Improved synthesis of medium chain triacylglycerol catalyzed by lipase based on use of supercritical carbon dioxide pretreatment

The esterification reaction of glycerol and caprylic acid catalyzed by immobilized lipase has been investigated in the present work quantifying the intensification benefits based on the use of supercritical carbon dioxide pretreatment. Factors influencing the progress of enzymatic reaction such as the pretreatment conditions, reaction time, reaction temperature, molar ratio of the two reactants, type of enzyme and its loading were varied to establish the effect on the progress of reaction. Optimum conditions for pretreatment using supercritical CO2 were established as time of 1 h, pressure of 100 bars and temperature of 50 °C. Use of pretreatment under optimized conditions resulted in a conversion of free fatty acids to tricaprylin as 97.3% in 6 h of reaction time at 50 °C and molar ratio of 4:1 (caprylic acid: glycerol), which was three times higher as compared to the conventional approach (without any pretreatment) performed under same conditions. Two types of immobilized biocatalysts as Novozyme 435 and Lipozyme RM were used and it was established that Novozyme 435 showed better catalytic activity as compared to Lipozyme RM, though importantly both were active even in the presence of supercritical carbon dioxide used as pretreatment. Reuse of both the enzymes up to 15 cycles with supercritical carbon dioxide pretreatment did not affect the activity significantly. The quality analysis of the synthesized product was performed using various physiochemical tests based on the determination of acid value and peroxide value. Overall, it has been established that use of supercritical carbon dioxide pretreatment was more efficient and enhanced the rate of synthesis of tricaprylin significantly as compared to conventional approach. Understanding into effect of reaction and pretreatment parameters also enabled in maximizing the intensification benefits.

Improving adhesion between aramid fibers and natural rubber through morphological and synthetic modification of the fibers

ABSTRACTIn this article, we present new methods to improve the adhesion between poly(paraphenylene terephthalamide) fibers and a natural rubber for tire reinforcement. Fiber pretreatments are applied to create new surface morphologies on the fiber that enables enhanced adhesion between the fiber and rubber matrix. The pretreated fibers are then subject to treatments with coupling agents in the presence of supercritical carbon dioxide (scCO2) in an attempt to permeate the fiber surface and chemically bind the fiber to the rubber matrix. Shear lag analysis using a Kelly Tyson approach is compared to more refined models to evaluate optimum test parameters for fiber pull‐out adhesion tests. The results show that the adhesion increases by approximately 100% when compared to conventional composites. Failure analysis of fiber surface reveals a suppression of interfacial failure. The effects of pretreatments on fiber properties are also characterized, and the optimization between fiber properties, fiber–matrix interface properties, and overall composite properties are discussed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45520.

Supercritical carbon dioxide and imidazolium based ionic liquids applied during the sol–gel process as suitable candidates for the replacement of classical organic solvents

In the present work, a sol-gel process with the precursor tetramethyl orthosilicate (TMOS) is investigated. The gelation was carried out in three different organic solvents. In addition the organic solvents were substituted by six different ionic liquids (ILs). By adding the ILs during the sol-gel process, so called ionogels are formed. The synthesis of such ionogels in supercritical CO2 atmosphere, and its influence on gelation time and the properties of the final ionogels were determined. For the characterization of the ionogels, BET and TGA analysis were carried out. In presence of supercritical carbon dioxide and ionic liquids, the gelation time of silica gels can be decreased significantly. The results illustrate, that organic solvents, typically used in aerogel synthesis, can be replaced completely and that, in presence of CO2, no further catalyst is necessary to promote the gel formation. The results further show that the porous characteristics of the so formed gels can be improved, compared to the classical sol-gel process in presence of organic solvents.

A new empirical model to correlate solute solubility in supercritical carbon dioxide in presence of co-solvent

In this work, correlation of solute solubility in supercritical fluid in presence of co-solvent has been carried out using a new density-dependent empirical equation. The new model has five parameters for each system. The parameters of the new correlation have been obtained for various ternary systems including 1668 experimental solubility data. The overall percent of absolute average deviations (%AARD) are 1.57 and 2.35 using different density calculation methods The comparisons of the new equation and the published models show its good capability in calculating solute solubility in supercritical fluid in presence of co-solvent.

Enhancing the photocatalytic reduction of CO2 through engineering of catalysts with high pressure technology: Pd/TiO2 photocatalysts

Converting carbon dioxide (CO2) to hydrocarbons that can be used as fuel is beneficial from both environmental and economic points of view. However, more efficient catalysts should be developed to bring the process from laboratory to commercial scale. Thus, in this study, in order to improve the CO2 conversion process, Pd doped photocatalysts based on titanium dioxide (Pd/TiO2) are synthesized. The new catalysts are produced from alcohols and precursors of titania and Pd in the presence of supercritical carbon dioxide so the dispersion of metal species (palladium) into the TiO2 lattice is possible. Following this approach, photocatalysts with better or similar features (higher values of crystallinity, surface area, and improved absorbance in visible range) than those of commercial TiO2 have been produced. Moreover, methane and CO production rates up to 22 and 2 times higher than those corresponding to the commercial catalyst, respectively, have been attained with Pd/TiO2 photocatalysts synthesized in this work.

Effect of water–carbon dioxide ratio on the selectivity of phenolic compounds produced from alkali lignin in sub- and supercritical fluid mixtures

Lignin is considered as the most abundant renewable carbon source after cellulose and non-commercialized waste product with constantly growing annual production exceeding 50milliontonsperyear. This article is focused on a newly developed selective synthesis of high-value phenolic products from lignin in presence of supercritical carbon dioxide (scCO2) known as sustainable, non-flammable, naturally abundant, and catalytically active solvent. Depending on the synthesis conditions, such as temperature (250, 300, and 350°C) and water-to-scCO2 ratio (1:5, 1:2, 1:1, and 2:1), high yield of the specific phenolic compounds such as phenol, guaiacols, and vanillin has been achieved. The GCMS analysis reveals the trend of the increased total phenolic yield with temperature and strong dependence of the selectivity on the water-to-scCO2 ratio. The maximum selectivity toward formation of the specific phenolic products, such as guaiacol and vanillin was observed at the highest H2O:scCO2=1:5 ratio. At 350°C the relative yield of guaiacol was ∼39% whereas at 250°C vanillin was a dominant phenolic monomer with a relative yield of ∼33%. Moreover, at the intermediate temperature of 300°C both guaiacol and vanillin were produced with relative yields of ∼28% and ∼13%, respectively. The highest amounts of scCO2 resulted in the highest total relative phenolic yields of ∼79%, ∼71%, and ∼73% at 250, 300, and 350°C, respectively. Demonstrated for the first time, the effect of water-to-scCO2 ratio in the process of alkali lignin liquefaction will have significant impact on selective synthesis of phenolic compounds, their use in synthesis of “green” polymers with desirable properties, and sustainable utilization of both carbon dioxide and lignin waste.

Preparation of natural rubber-based polyol by oxidative degradation under supercritical carbon dioxide for flexible bio-based polyurethane foams

Nowadays, polyurethane foams play a key role in widespread applications, and their market demand is still growing globally. This work aims to prepare hydroxyl telechelic liquid natural rubber, a bio-based polyol, as a sustainable raw material and to enhance elastic properties of soft segment bio-based polyurethane foams. A hydroxyl telechelic liquid natural rubber with a viscosity-average molecular weight of 4.2 × 103 g/mol was successfully produced by oxidative degradation of natural rubber with hydrogen peroxide in the presence of supercritical carbon dioxide, then modifying the chain-end functionality to obtain hydroxyl number of 59 mg KOH/g. The functionality of hydroxyl telechelic liquid natural rubber was confirmed by FT-IR and 1H-NMR. Bio-based polyurethane foams were then prepared by mixing the synthesized hydroxyl telechelic liquid natural rubber with commercial diisocyanate. Morphological properties and thermal stability of polyurethane foams were investigated.

Synthesis of Ru/PDMS nano-composites via supercritial deposition

Nanomaterials consisting of Ru nanoparticles dispersed in polydimethylsiloxane films were synthesized by supercritical deposition. The films were impregnated with the organometallic precursor bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (1,5-cyclooctadiene) ruthenium (II) under thermodynamic control in the presence of supercritical carbon dioxide (scCO2) at 40 °C and 10.34 MPa. The precursor molecules were then converted to metallic Ru by thermal treatment in flowing N2 at ambient pressure, resulting in well-dispersed nanoparticles with diameters of ≈2 nm.

Supercritical Fluid-Driven Polymer Phase Separation for Microlens with Tunable Dimension and Curvature.

Microlenses are highly sought as reliable means for high-resolution optical imaging at low illumination intensities. Plano-convex configuration with tunable dimension and curvature is an essential feature in the microlens fabrication. In this study, we present a facile and green route for preparing well-defined microlenses based on polymer phase separation in the presence of supercritical carbon dioxide (scCO2). The behaviors of linear polymethylmethacrylate protruded from cross-linked silicone network in scCO2 environment are investigated from the perspectives of thermodynamics and kinetics. Microlenses with dimensions from 2 to 15 μm and contact angles from 55° to 112° are successfully obtained through the adjustment of the kinetic conditions and outgassing rate. With the tunable focal length, they exhibit intrinsic function of discerning submicroscale patterns that are unable to be observed directly under optical microscope. Moreover, size confinement on the substrate results in the generation of well-ordered microlens arrays, affording great promise for applications in bioimaging, photolithography, light harvesting, and optical nanosensing.