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The properties and applications of supercritical carbon dioxide

Category: Biochemistry

Subcategory: Chemical engineering

Level: Masters

Pages: 21

Words: 5775

The Properties and Applications of Supercritical Carbon dioxide

Abstract
In chemical sciences, a supercritical fluid is defined as any substance for which the pressure and temperature conditions that are required to maintain its original molecular integrity are beyond critical levels. Supercritical fluids are popular both as solvents and as anti-solvent in processing polymers. Supercritical fluids play an important role in polymer modification, blending of polymers, particle production, in the formation of polymer composites. Moreover, different studies suggest that the qualities of polymers that are processed with SCF are significantly higher than the quality of their counterparts that are processed with conventional toxic solvents. SCO2 is a clean and versatile solvent that has shown promising results in processing polymers compared to the noxious organic solvents or chlorofluorocarbons. SCO2 is popular both for synthesizing and processing polymers owing to its non-toxic and non-flammable properties. Moreover, it is chemically inert and inexpensive. Both these properties enhance its acceptance in the polymer industry, drying and cleaning industry, and for extracting commercial compounds from plants and soils. Hence, it is considered a strong contender for promoting the concept of Green Chemistry. This article provided a systematic review of the physicochemical properties and applications of supercritical carbon dioxide.
Keywords: Review, supercritical carbon dioxide, physicochemical properties, applications, green chemistry
The Properties and Applications of Supercritical Carbon dioxide
Introduction
Background
Over the past few decades, the synthesis and processing of polymers have witnessed radical transitions owing to the development of novel solvent systems. Processed polymers are used exhaustively across various industries. For example, such polymers are often used as vehicles for drug delivery and in separating impurities during the purification of active pharmaceutical ingredients. However, most of the solvents (such as volatile organic solvents and CFCs) that are used in traditional polymer processing are hazardous to the environment. Such solvents can easily access the aquatic bodies through industrial wastes. Increased accumulation of such organic solvents reduces the concentration of DO (dissolved oxygen) in the water bodies and reduces their yield. Hence, chemical engineers and environmental biochemists are exploring different options in reducing the toxic effects associated with polymer processing. One such initiative that has received major attraction is the use of supercritical carbon dioxide (SCO2). SCO2 is routinely used in polymer processing and for extracting different plant products under specific temperatures and pressure. This is because SCO2 is an effective solvent for various non-polar and polar low MW compounds (including polymers such as silicones and fluoropolymers). However, SCO2 is a poor solvent for High MW polymers under ambient conditions. The present study reflects a systemic review of the properties of SCO2 that makes it an ideal solvent for processing polymers and for extracting different biological compounds from plants. The review would also explore the role of SCO2in the field of green chemistry1.
1.2. Concept of Supercritical Fluid
In chemical sciences, a supercritical fluid is defined as any substance for which the pressure and temperature conditions that are required to maintain its original molecular integrity are beyond critical levels. Hence, under a supercritical condition, a liquid or a gaseous compound can maintain a hybrid state. The chemistry of such hybridized states or phases is completely different from their liquid or gaseous chemistry. Hence, Nalawade1(2008) stated; “the special combination of gas-like viscosity and liquid-like density make supercritical fluids an excellent solvent for different applications” (p.21). Supercritical fluids are popular both as solvents and as anti-solvent in processing polymers. Supercritical fluids play an important role in polymer modification, blending of polymers, particle production, in the formation of polymer composites. Moreover, different studies suggest that the qualities of polymers that are processed with SCF are significantly higher than the quality of their counterparts that are processed with conventional toxic solvents.
Supercritical carbon dioxide
Supercritical carbon dioxide is one of the most vividly used supercritical fluids in the polymer industry (Fig 1.1). Nalawade1 stated that SCO2 is a clean and versatile solvent, which has shown promising results in processing polymers compared to noxious organic solvents. SCO2 is popular both for synthesizing and for processing polymers owing to its non-toxic and non-flammable properties. Moreover, it is chemically inert and inexpensive. Both these properties enhance its acceptance in the polymer industry.

Fig 1.1. Concept of supercritical carbon dioxide (Ref:http://www.tst.tw/en/technology-more.php?Key=1)
Advantages and concerns with SCO2
Safety concerns on the environment and humans have imposed significant challenges in the field of chemical engineering. The major emphasis on different process development initiatives (PDI) in chemical engineering pivots on the implementation of sustainable processes. Such stricture also holds true for the polymer industry. Different studies have portrayed the toxic emission levels of organic solvents that are used in polymer processing. In fact, 20 million tonnes of volatile organic compounds (VOCs) are dumped into the atmosphere per year by different industries. The rubber and polymer industry tops the list in emitting such VOCs, while paints and adhesive industry are also accountable for a major chunk of the total VOCs. Hence, alternative strategies are explored in the field of polymer manufacturing to reduce the use and emission of VOCs1.
Two such solutions are available for overcoming such challenges; the introduction of solvent-free systems and the replacement of environmentally toxic solvents with benign compounds. However, solvent-free systems limit the viability of polymer processing due to higher viscosities and mass-transfer limitations. On the other hand, the latter alternative requires an energy-intensive step to remove the solvent once the polymerization is achieved. Hence, volatile solvents are mandated to reduce such energy-intensive processes. However, the risk of environmental and human toxicity is not ruled out for organic solvents1.

Fig 2: Supercritical Extraction process (Ref:http://file.scirp.org/Html/10-2200467_26165.htm)
Although SCO2 is found abundantly in the atmosphere, however; it is also formed as a by-product during the production of ammonia, hydrogen, and ethanol. The critical temperature and pressure for achieving the supercritical conditions for SCO2are estimated to be 304 degrees Kelvin and 7.38 Mpa respectively. Moreover, SCO2 can be easily removed from any system through simple depressurization. The introduction of SCO2 into polymers makes them highly swollen and easily plasticized. Such properties help in processing polymers at much lower temperatures. Finally, removal of SCO2 does not impose the burden of the greenhouse effect because it can be appropriately recovered during polymer processing. SCO2 is readily soluble in different types of polymers. However, its solubility markedly varies with temperature and pressure. Moreover, the solubility of SCO2 is also influenced by its interactions with the chain groups in the respective polymers. The solubility of SCO2in a molten polymer significantly reduces its viscosity by increasing its free volume. Hence, SCO2 can effectively alter the physical properties (density, volume, and diffusivity) of different polymers. Such properties make SCO2 an ideal plasticizing agent in processing polymers under low temperatures (Fig 1.2).
2. Properties of Supercritical Carbon Dioxide (SCO2)
2.1. Density
Most density measurements of SCO2 have been predicted based on the black oil model or empirical correlations. Such models have been developed for oil and natural gas where the composition of carbon dioxide fluctuates between 3% and 20%7. Hence, such fluctuations impose challenges in predicting the density of pure or supercritical carbon dioxide. On the other hand, some authors also estimated the Equation of States to predict the density of SCO2. However, such models are unsuitable for measuring the density of carbon dioxide under conditions of carbon capture or carbon sequestration. Ouyang 7 explored the density of SCO2 under conditions that are expected carbon capture or carbon sequestration. The authors implemented simple and explicit correlations to report the density of carbon dioxide under such conditions. Ouyang (2011) reported the variations in density of carbon dioxide with pressure (between1100 and 9000 psi) and temperature (between 40 and100 degrees centigrade) under carbon capture and carbon sequestration operations7. The study reflected that at a specific temperature, the density of carbon dioxide is increased by an increase in pressure (p<0.05)7. The densities of SCO2 were significantly correlated with that reported in the NIST database and by Bahadori et al8,10.
To recall, Bahadori et al.8,10 under-predicted the densities of SCO2 below 3000 psi and above 7000 psi pressures. Likewise, the Bahadori et al.10 also over-predicted the density of SCO2 at temperatures between 90-degree centigrade and 100-degree centigrade. Recently, Yoon et al.6 carried out Monte Carlo simulations to explore the density of SCO2 based on the local density distribution of subcritical and supercritical carbon dioxide. The authors applied Voronoi tessellation as the statistical measure for their estimations. The authors reflected that the local density distribution of carbon dioxide along the VLE line could be represented by inverse-gamma distribution and normal distribution. The authors proposed an inverted-gamma normal mixture model to present the local density distributions of SCO2. The authors highlighted the density of carbon dioxide to vary between 5 and 10nm-3 during its gaseous phase. Likewise, the density of carbon dioxide was shown to vary between 10 and 15 nm-3 during its liquid phase. The authors also showed that the density of carbon dioxide decreased on either side of these values. The study further reflected that the mean local density was significantly correlated with the principle of effective local density6.

Fig 2.1.1.Monte Carlo Simulations to Predict Density of SCO2 (Ref: Yoon et al.6)
Viscosity
Studies showed that empirical models for predicting the viscosity of pure or supercritical carbon dioxide are difficult12. Ouyang7 estimated the viscosity of supercritical CO2 at carbon sequestration and carbon capture operations and reflected that the viscosity of SCO2 increases with pressure under a specific temperature. On the other hand, it decreases with a decrease in temperature at a fixed pressure. However, such viscosities are always lower than that of water which makes it an effective solvent13,14. Although the prediction of the viscosity of SCO2 by Heidaryan et al.11were correlated with NIST data, the model framed by Ouyang was also consistent with such findings. However, the Ouyang model reduced the chances of average relative error by ten folds for all temperatures. The lower viscosity of SCO2 compared to water makes it a unique solvent for processing polymers1.
High viscosity imposes a significant challenge in processing HMW-polymers. To overcome such a challenge, traditional polymer processing is carried out at higher temperatures because the viscosity of a solution falls with the rise in temperature. However, higher temperatures could also degrade the polymers which imposes further challenge in processing HMW-polymers1. On the contrary, addition of organic solvents such as VOCs could reduce the viscosity at lower temperatures. However, the use of such solvents is limited by environmental and human safety concerns, difficulty in recovery post polymer processing, and reactive nature. In this regard, SCO2 is an effective alternative to organic solvents in processing highly viscous polymers. The respective polymer is plasticized under much lower temperatures than that could be achieved by organic solvents. The efficacy of plasticization is confirmed by the reductions in glass transition temperatures and decreased melting point of the polymer. Reductions in glass transition temperature (GTT) and the melting point of the polymer accounts for their low viscosities. The reduction in GTT occurs at 5 to 20Mpa15.
Diffusion Coefficient
Chen and Rizvi16 showed that the diffusivity and diffusion coefficient of SCO2 depends on the gelatinization of starch and pressure of carbon dioxide. On the other hand, the pressure of carbon dioxide is a function of temperature and pressure16, 18, 19, 20. The authors16 modeled diffusion coefficient of SCO2 as a function of temperature, pressure, and concentration of dissolved carbon dioxide. Raspo et al.17 highlighted that the diffusion coefficient of supercritical carbon dioxide significantly decreases near the upper critical end point. The authors estimated that such upper critical end point of SCO2 to exist between 74 and 77 bars17. Hence, Raspo et al17 complemented the findings of Chen and Rizvi 16 who stated that diffusion coefficient of SCO2was mediated by pressure.
2.4. Solvent properties
Solubility is one of the key determinants for carrying out chemical reactions in SCO2 or any supercritical fluid. Although pure SCO2 is a relatively non-polar solvent, however; it exhibits limited solubility with polar molecules. This is because pure SCO2 has a large molecular quadrupole1, 4. On the contrary, being a linear molecule, SCO2 does not bear any net dipole moment. Such features also impose challenge in dissolving polar and ionic substances in SCO2. However, low viscosity, low surface tension, and a low dielectric constant of SCO2often limit their ability as an effective solvent compared to traditional solvents4.
On the other hand, different studies have reflected that supercritical carbon dioxide is readily soluble in polymers above their glass transition temperature or melting points. Modifiers or co-solvents (such as MeOH and EtOH) are often added to supercritical fluids to improve their solubility with polar molecules. On the contrary, less polar substances can also act as modifiers when they are involved in such reactions which involve more than one reagent. Under such circumstances, the less polar substances (such as SCO2) enhance the solubility of more polar reagents. This property mitigates the need of adding additional cosolvents to the reaction mixture4. On the other hand, fluorinated substituent could greatly enhance the solubility profile of SCO24.
These substances are introduced into SCO2 through a ligand or counterion during organo-metallic catalysis. However, the cost-effectiveness of the reagents limits such practices in industrial settings. The cost burden associated with such approaches is attributed to recycling of the reagents. The solvent properties of SCO2 have significant practical implications. Since SCO2 can be easily removed by depressurization, simple evaporation may be implemented to it can be used to isolate products in total dryness. Such attributes are extremely important during the final steps of pharmaceutical syntheses. During the final steps of pharmaceutical syntheses, it is extremely essential to dry the respective product from traces of the solvent. Particle formation with SCO2 or other supercritical fluids depends on its solubility properties too. Such properties include rapid expansion of supercritical solutions and anti-solvent precipitation of supercritical fluids4.
Compressibility is another feature of fluids that is influenced by their solubility property. The compressibility of SCO2 markedly differs from that of conventional solvents. Large pressure has to be applied to make them compressible or change their density. However, supercritical fluids require only small changes in pressure (near their critical point) to make them compressible (Fig 2.4.1). Such options provide an infinite range of solvent attributes that can be harnessed in driving various chemical reactions with supercritical fluids. To recall, the density of SCO2 or any other supercritical fluid is less than their conventional counterparts. Although such properties could limit solubility, however; it also signifies that the viscosity of such fluids is also lower than their conventional counterparts. As a result, diffusivity in supercritical fluids is higher than that in conventional solvents. On the other hand, higher diffusivity signifies faster rate of diffusion. Hence, solubility attribute of SCO2 is a driving factor for chemical reactions those are diffusion-limited4.
Lewis acidity of SCO2 is the major determinant that influences its solubility across different polymers1. On the other hand, FTIR studies reflect that the carbonyl stretching vibrations of cellulose acetate and polymethylmethacrylate are shifted towards higher wave numbers when the pressure of carbon dioxide was increased. Presence of additional functional groups in the backbone chain reduces the free volume of silicon polymers. In fact, such properties have resulted in creating polymerized soybean oil with SCO24. Such properties significantly influence their interaction with SCO2. For example, fluorinated block polymers have high solubility in SCO2. Such solubility has significant influence in changing the shape and polarity of those polymers3. Moreover, microstructure preparations such as lamellar and bi-continuous phases have provided promising results in increasing the solubility of SCO2 in polar solvents5. Such logic has been successfully implemented in designing fluorinated surfactants and non-fluorinated polymers. Perhaps the solvent properties of SCO2 are best elucidated in SCO2-based extractions2. Russick, Adkins, and Dyck2 showed that SCO2-based extractions canbe successfully used for solvent removal from micro-machined structures.

Fig 2.4.1: Represents the density of SCO2 as a function of its compressibility and solubility (Ref; Peach and Eastoe4)
Surface Tension
Till date, there is no conclusive evidence regarding the surface tension of SCO221. However, certain studies have suggested that the surface tension of SCO2 is either below water or it is negligible22.Low surface tension makes it a unique compound for drying silicon-based semiconductors22, 23, 24, 25. Such properties are harnessed in analytical methods with SCO2.
Microscopic Structure
Saharay et al.26 performed Car-Parrinello simulations to explore the microscopic structure and molecular dynamics of SCO2. The simulations were carried out at 318.15 Kelvin and a density of 0.703 gm/cubic centimeter. The authors explored atomic pair correlation functions about structural dynamics of SCO2.Saharay et al. highlighted that carbon dioxide exists as a marginally non-linear molecule during the supercritical state. As a result, carbon dioxide in its supercritical state exhibit dipole moment. An analysis of the angle distributions between neighboring molecules reflected a distorted T-shaped geometry26. The authors investigated 1st order and 2nd order time correlation functions to assess the reorientation dynamics of the carbon dioxide molecules in the supercritical state. The 1st and 2nd order time correlation functions were estimated to be 620fs and 268fs. Such findings complemented the findings of NMR analysis. The authors also examined the intramolecular vibrations of carbon dioxide in its supercritical state by analyzing the velocity autocorrelation functions of its atoms. The autocorrelation functions revealed a red shift in the frequency spectrum relative to isolated SCO2 molecule26. Such near linear shapes of SCO2 was also confirmed by earlier studies 27, 28, 29. Since SCO2 molecule contains an inversion center, the molecule is also non-polar20. Sun27 demonstrated quasi-periodical coherent structures of SCO2 through FTIR transformation of time series data. The authors showed that the nozzle structure and the experimental conditions affect such coherent structures.
3.The applications of supercritical carbon dioxide
3.1. Extraction of solids and liquids
SCO2 is widely used to extract or remove organic compounds from different solid and liquid matrices. SCO2-based extraction offers unique advantages over conventional separation techniques. Such advantages are attributed to the unique physical properties of any supercritical fluid. These fluids exhibit liquid-like density, while its viscosity and diffusivity parameters fall between gases and liquids. Supercritical fluids can be easily removed by reducing the pressure and evaporation of the solvent. This is because such fluids are gaseous under normal temperature and pressure (NTP). SCO2 is extensively used to decaffeinate coffee and in the production of polythene. Laitinen32 showed that such properties of SCO2 make it an ideal solvent for extracting 6-caprolactam (CPL) from solid and liquid matrices31. The solubility of CPL in SCO2 was estimated to be 17 wt% under ambient temperature and at pressures lesser than 220 bars. Hence, Laitinen (1999) contended the SCO2 could play a significant role in the remediation of contaminated soil from different pollutants.
The pollutants that were studied include phenanthrene, tetrachlorophenol, and pentachlorophenol. Laitinen32 reflected that almost 80% to 90% of the initial concentration of pollutants couldbe extracted with SCO2 under moderate temperatures. The authors further highlighted that supercritical fluids are best suited for remediation of sandy and silt type of soils. This is because such soils have low adsorption capacity and the respective pollutants could be easily extracted or recovered from such soils. However, the binding attribute of respective pollutants with the soil particles and their respective concentrations in the soil strongly influenced their extraction with SCO2. To overcome such drawbacks of SCO2-based extraction, Laitinen32 explored a mechanically agitated countercurrent extraction procedure. The study showed that such innovations improved the extraction kinetics at a high solvent to feed ratios.
3.1.1 Caffeine extraction from coffee beans
In one study, Azevedo et al. 33 reported the kinetics of extraction of caffeine, chlorogenic acids, and coffee oil from green coffee beans. In the experimental model, either pure supercritical carbon dioxide (pSCO2) or SCO2 modified with ethanol and isopropyl alcohol (both 5% w/w) at a temperature of 50 or 60-degree centigrade was used to extract the referred components from green coffee beans. The extraction kinetics was recorded for all the assays at different time intervals for pure and mixed SCO2. Azevedo et al.33 showed that an increase in Mpa translated into the increased extraction of coffee oil with either pure or mixed SCO2. Moreover, when the extraction was carried out with SCO2 modified with isopropyl alcohol, there was a significant change in the yield of caffeine.
Mehr 34 explored the extraction kinetics of caffeine from wet guarana seeds using SCO2. The authors designed a single-pass system to carry out the extractions under different temperatures and pressures. The solubility of caffeine (in SCO2) was explored between 136.1 atm and 272.2 atm and under temperatures of 35 degrees, 45 degrees, and 55 degrees centigrade. The authors highlighted that the solubility of caffeine in SCO2was correlatedwith the density of carbon dioxide. Different thermodynamic relations and the Peng-Robinson equation were used to model the solubility of caffeine in SCO2. Long-term extractions were conducted to estimate the diffusion coefficients of caffeine in ground guarana seeds.
In another study, Wan-Joo et al.35explored SCO2-based selective extraction of caffeine and epigallocatechin gallate from green tea extracts. However, the authors also used water as a cosolvent for ensuring the selective extraction of caffeine in preference to epigallocatechin gallate. The extractions were performed under varying experimental conditions. The authors showed that the caffeine extraction was highest (54%) under experimental conditions of 40-degree centigrade, 400 bars, and 7wt% of water. However, the yield of epigallocatechin gallate under the same experimental conditions was 21%. Hence, the authors highlighted that the SCO2/water-based extraction ensured a relative yield of 2.57 for caffeine in comparison to epigallocatechin gallate. Wan-Joo et al.35 further compared the SCO2-water based relative extraction of caffeine with conventional liquid-solvent extractions (either with water or ethanol). The relative yield of caffeine with water-based extraction was estimated to be 0.88, while that with ethanol was estimated to be 0.24. The authors concluded that the SCO2-water based selective extraction of caffeine from green tea extracts was significantly superior to water-based or ethanol-based extractions.
Wan-Joo et al.35 highlighted that such differences in solvent-based extractions were dependent on the diffusivity gradient between the solvent systems and solute mixture. The findings of Wan-Joo et al.35 are aligned with previous findings. Such authors explored the decaffeination of coffee beans with SCO2 under continuous flow extraction. The rate of decaffeination was estimated as a function of carbon-dioxide flow rate, and variations of temperature and pressure. The rate of decaffeination in the raw beans was determined from the amount of caffeine in the effluent. The authors highlighted that soaking the coffee beans in water before extraction, improved the rate of decaffeination (Fig 3.1). The saturation of SCO2 with water also enhanced the rate of caffeine extraction.

Fig 3.1: Soaking beans in water35
Studies reflect that temperature and pressure increase the rate of decaffeination34. In another study36, the authors acknowledged linear regression model to analyze the extraction rate of caffeine as a function of intraparticle diffusion and external mass transfer. The authors of this study performed a Life cycle assessment study and showed that SCO2 is effective in selective decaffeination. Moreover, such selective decaffeination is effective in preserving the taste of coffee over prolonged periods. These authors endorsed that the partitioning of caffeine between SCO2 and water strongly influenced its extraction in a time-dependent manner. Apart from caffeine, SCO2 can be successfully used to extract pigments from Bixa orellana seeds51.
3.2. Materials Processing
Woods et al.37 highlighted that the role of SCO2 in materials processing couldbe broadly subdivided into three domains. The first domain involves the design of novel surfactants in SCO2. Such designs play a significant role in the synthesisof metallic nanoparticles, porous polymers, and HMW-polymers with superior morphologic attributes. The second domain is attributed to the development of novel polymer processing approaches and implementation of polymer blend technologies. Such approaches ensure synthesis of complex polymer composites or blends. The final domain is attributed to the applications of SCO2 in synthesizing novel biomaterials such as biodegradable polymers and scaffolds (Fig 3.2). Woods et al.37 further highlighted that such modifications and synthesis brought about by SCO2-based processing could not be achieved under any other physical or chemical conditions.

Fig 3.2: Synthesis of Novel Biomaterials with SCO2 [Source Woods et al.37]
Zhang et al.38 highlighted that SCO2 could play a major role in green material processing, apart from its established role of processing polymers and in extracting active pharmaceutical ingredients or organic solutes such as caffeine. The authors defined SCO2 as that form or phase of CO2 that exist at 31.1 degrees centigrade and 7.4Mpa. Hence, the definition and physical attributes ofSCO2portrayed by Zhang et al.38 complemented the definition of SCO2 as provided by Nalawade (2008). Zhang et al.38 highlighted that SCO2 is both a potential solvent and an anti-solvent. Such solvent and anti-solvent properties make it suitable for materials processing. An application of SCO2 in materials processing and synthesis is summarized in Fig (3.3).

Fig 3.3: Application of SCO2 in materials processing and synthesis [Source; Zhang et al.38]
Nalawade1 reported that SCO2 plays a major role in ensuring step-growth polymerizations for the production polycarbonates and polyesters. The major aim of such approaches is to reduce the viscosity of the synthesized polymer. SCO2-assisted polymerization reduces the mass transfer that is a hallmark of other polymerization processes such as melt synthesis. However, viscosity reduction depends upon the solubility of SCO2 in the synthesized polymer. Moreover, Nalawade1 highlighted that SCO2-based polymerization influences the molecular weight and volume of the synthesized polymers. On the other hand, studies suggest that transition of subcritical to supercritical conditions increases the formation of copolymerized carbon dioxide. The increase in copolymerization is attributed to the stability of carbocations under specific temperature and pressure.
Formation of Microcellular Polymers
Different studies suggest that SCO2 is effective in ensuring microcellular foaming of polymers39, 40. Microcellular polymers are referred as those polymers in which the sizes of the polymerizing cells are either equal to or less than 10mm. SCO2 finds ituse as a blowing agent in the formation of microcellular polymers. Such polymers are featured by low weight, increased tensile strength, and improved fatigue life. Microcellular polymers are routinely used in a separation medium, as absorbents, and catalytic supports. SCO2-based microcellular polymers are also used to manufacture different types of delivery devices where biodegradability is recommended. SCO2 has the potential to replace traditional blowing such agents such as VOCs, CFCs, and hydrochlorofluorocarbons. SCO2 is preferred as a blowing agent because it can produce narrow cell size distribution, ease of solvent recovery, high diffusivity, and superior plasticizing property39,40. Different types of microcellular polymers have been prepared with SCO2 in both batch and continuous mode. In both of these methods, carbon dioxide is exposed to its saturation pressure and temperature. Once carbon dioxide reaches it nearly saturated or supersaturated state, it leads to plasticization of the polymers by reducing their glass transition temperatures or melting points closer to the ambient temperatures. A thermodynamic instability is induced by venting the carbon dioxide through depressurization. Such thermodynamic instability supersaturates carbon dioxide and helps it to dissolve in the polymer matrix. As a result, nucleation of the cells takes place and the cells continue to grow until the polymer vitrifies. However, the number of cells and their size distribution depends upon the saturation temperature, the saturation pressures and the rate of depressurization of SCO2.39,40.
Studies suggest that porosity is a key determinant of quality of microcellular polymers. Good control of porosity is achieved through the manipulation of saturation pressure and temperatures. Studies further suggest that the cell number density was increased by decreasing the saturation temperature and increasing the saturation pressure of SCO2. However, cell size reduction was achieved with the opposite phenomenon. The authors highlighted that a high degree of super saturation of dissolved carbon dioxide at high pressure and low temperature accounted for such results. The cell size and structure is also determined by the viscoelastic properties of the polymer. Such properties resist the elongation of cells during growth of cells and restrict the cells to a smaller size.
Polymer blending
Polymer blending refers to the process of mixing more immiscible polymers with each other. However, such mixing is carried out either in a reactive or non-reactive manner. Polymer blending provides an opportunity to ensure tensile strength and rigidity to the individual polymers. In the non-reactive process, polymer blending is achieved by mixing two polymers belonging to two different phases. Such mixing is carried out in the molten state where one phase is referred as the dispersed phase, and the other phase is referred as the continuous phase. The viscosity ratio between disperse phase and continuous phase determines the size of the droplets. When the viscosity ratio is closer to or less than one, then the size of the droplets is the lowest and vice-versa. SCO2 is used to prepare polymer blends in a non-reactive manner41. The blends are prepared by batch mixing and extrusion of the same with SCO2. SCO2 acts as a plasticizing agent. Dissolution of SCO2 in polymer blends decrease shear thinning and causes a superior dispersion of the minor component. Studies suggest that SCO2 is effective in dispersing a minor phase PMMA in the blend of PMMA/PS. SCO2 causes a greater reduction in the viscosity o the minor component and leads to better momentum transfer from the major component that is more viscous compared to the minor component. This results in breaking of the minor component and phase inversion. Phase inversion refers to the transition of high melting polymers from its disperse phase to continuous phase. In the presence of SCO2, the period for phase inversion is shortened. This is because sco2 causes a significant reduction in glass transition temperature of the high-melting polymer. Thus SCO2 can significantly alter the droplet size in polymer blends and plays a significant role in determining the morphology of polymer blends1, 41.
3.3. SCO2 in Drying and Cleaning
The cleaning of metal parts during remanufacturing processes should be environmentally benign43. Hence, newer technologies are explored that has the potential to reduce environmental that stems from such cleaning. In fact, SCO2 is also effective in terminal sterilization. Such findings reinforce its role in drying and cleaning46. Liu and Zhang43explored the potential of SCO2 in cleaning metal parts under different ambient conditions. The authors implemented a second order polynomial model to explore the feasibility of SCO2 as an effective cleaning agent for metal parts under different experimental conditions (such as cleaning temperature, cleaning pressure, flow-rate of carbon dioxide, and cleaning time. The SCO2-based cleaning was carried out in presence or absence of ethyl alcohol that acted as a cosolvent. The authors highlighted that SCO2 was excellent in decontaminating metal parts during the remanufacturing cleaning process. However, the cleaning efficacy of SCO2 was affected by the variations in operating parameters45. Liu and Zhang43concluded that addition of dehydrated ethyl alcohol improved the cleaning efficacy as estimated by cleaning performance index) of SCO2 to 89.30% at a temperature of 75-degree centigrade and 25 MPa pressure. The study reflected that SCO2 could be successfully deployed in decontaminating the surface of metal parts during the remanufacturing cleaning process.
Korzenski44 showed that SCO2 could be used as an effective solvent for semiconductor wafer cleaning. Semiconductor wafers should be decontaminated from photoresists, post-etch process residues, and particulates. Korzenski44 showed that SCO2 significantly removed photoresist from semiconductor wafers. The wafers were processed for two minutes under SCO2/cosolvent formulation at 55-degree centigrade and 4000 psi. SEM analysis reflected that SCO2/cosolvent preparation significantly removed photoresist without causing etching of the Si substrate in the semiconductor wafers. Complete removal or photoresist through UV irradiation or high-dose ion implantation impose significant challenges. High-dose ion implantation results in a solid and tough crust and the resultant carbonized crust make it difficult to remove the photoresist. On the contrary, cosolvents alone are also incompetent in removing the photoresist from the semiconductor wafers.
3.4. Superficial Carbon Dioxide in Green Chemistry
The term “Green Chemistry” was coined by the United States Environment Protection Agency during the 1990s47. The term was framed to promote innovative chemical technologies that have the potential to reduce or eliminate the generation or use of hazardous substances during designing, manufacturing, and usage of chemical products. Hence, the concept of ‘Green Chemistry’ aims to eliminate the adverse environmental effects of chemical reactions or products. Green chemistry endorses that the hazardous effects of a chemical process or chemical reaction should be preferably controlled at its source rather upon its termination. Such mandate emphasizes the introduction of novel solvents in different chemical systems and subsystems. The concept of Green Chemistry is based on three primary philosophies; prevention of waste formation is better than its treatment to cleaner variants, synthetic methods should incorporate all the reactants in a final product to ensure ‘atom efficiency’, and generation of less hazardous products and by-products as a result of a defined chemical reaction48. SCO2 is considered to play a significant role in promoting the concept of “Green Chemistry.” SCO2 is capable of interacting with other problematic chemicals without hazardous effects on the environment or humans. SCO2 has proven itself as an effective reaction medium in the synthesis of Pd-mediated carbon-carbon bond formation, ring closure metathesis, copolymerization reactions, and biotransformation. Studies suggest that liquid near-critical carbon dioxide (ncSCO2) is effective in loading ibuprofen in the mesosporous silica. The resultant material exhibited a high concentration of ibuprofen47.
SCO2 in Pd-catalyzed Carbonylation
Carbonylation refers to a set of reactions that deals with the introduction of carbonyl group in organic molecules. Various types of carbonylation reactions include hydroxyformylation, hydroxyesterification, and hydroxycarboxylation. Most of the carbonylation reactions involve palladium-medium catalysis. However, conversion and regioselectivity have always remained a key concern in Pd-mediated carbonylation. Alkenes are often carbonylated in the presence of Pd to yield different products. Functionalization of alkenes is one of the important aspects of chemistry. Gang et al.49 stated that functionalization of alkenes could be improved by improving atom efficiency. Such findings prompted the philosophy of Green Chemistry for carbonylation of alkenes. The authors suggested that choice of suitable solvent plays a key role in ensuring conversion and regioselectivity of Pd-catalyzed reactions.
In this regard, the role of SCO2 or SCO2/water in promoting Pd-based catalysis of alkenes becomes evident. Gang et al. (2003) showed that that carbonylation of norbene-1 in the presence of MeOH/SCO2 yield three products; cis-exo-diester, cis-exo-chloride ester, and exo-chloronorbornane. The authors further stated that addition of a right amount of a polar solvent improved the dissolution of Pd-catalysts in SCO2. Moreover, such modifications improved the reaction efficiency and regioselectivity. Such findings are aligned with the philosophy of Green Chemistry. To recall, Green Chemistry contends that use of cosolvents improves the efficacy of a chemical reaction and leads to the theformation of the final product through a one-step process50. As a result, formation of hazardous bye-productsis preventedand atom efficiency is also ensured.
On the other hand, radical addition reactions emphasize the successful use of SCO2 as a reaction medium. Since such reactions neither involve nucleophiles nor electrophiles, they can be easily carried out in non-polar solvents. Pd-based catalysis under SCO2 promoted successful radical addition reaction of CO, and carbon tetrachloride with 1-octene. However, when such catalysis was carried out under a normal organic medium (EtOH) in place of SCO2, the products exhibited different selectivity. Gang et al.49 highlighted that the amount of carbonylation achieved under SCO2 was significantly higher than that achieved under EtOH or other organic solvents. This is because CO exhibited higher solubility in SCO2 compared to EtOH. Hence, carbonylation was more favored when SCO2was used as the reaction medium.
Gang et al.49 further showed that SCO2 could prevent competition between carbonylation and dimerization during Pd-based carbonylation of alkynes. Although the addition of alcohol in the reaction medium favors the balance toward carbonylation, however; such benefits are observed up to a certain limit. Beyond such limits, the rate of both carbonylation and dimerization are lowered. The authors further explored the chemoselectivity of amine carbonylation under SCO2and organic solvents. In one such reaction, the authors added amine while studying carbonylation of alkenes. Addition of amines to reaction yield carbamates. The authors contended that carbamates were primarily formed due to carbonylation of amines. Hence, SCO2-based catalysis of alkenes with amines generated carbamates. Carbamates are widely used as pesticide, germicide, and anesthetic.
Gang et al.49 highlighted that the chemoselectivity of amine carbonylation under SCO2 and organic solvents significantly differ from each other. The authors showed that Pd-based catalysis of amines carried under MeOH/SCO2 and oxygen yielded methyl-N-n-butylcarbamate and oxalbutyline. On the other hand, when such reaction was carried out in the absence of SCO2 it yielded only carbamate. Moreover, the authors further highlighted that the ratio of methyl-N-n-butylcarbamate to oxalbutyline could be stringently controlled by varying the concentrations of MeOH. On the contrary, removal of oxygen from the reaction medium led to the formation of oxalbutyline only. Such findings further endorsed the concept of Green Chemistry. This is because such reactions yielded products that were beneficial and less hazardous to the environment. On the other hand, Mayadevi50 implicated the role of SCO2 in hydrogenation reactions. Mayadevi50 highlighted that SCO2 leads to improved reaction rate, enhanced catalytic efficiency, and greater process safety in such reactions. Such findings once again endorse the role of SCO2 in promoting Green Chemistry.
4.Conclusion
To conclude, SCO2 represents a fluidic state of carbon dioxide when it is heldabout or above a critical temperature and pressure. Carbon dioxide usually exists in a gaseous phase at STP and as a solid (dry ice) in the frozen state. However, increasing its temperature and pressure beyond the STP, results in the formation of SCO2. In such supercritical state, it adopts properties in between gases and liquids. In such situations it can fill its holding container like a gas, with a density that of a liquid. The low viscosity and surface tension makes it a unique solvent for various chemical applications. Future studies should try to explore the role of SCO2 in extracting active pharmaceutical ingredients from natural resources. However, its low solubility in polar solvents might mandate designing of polar/non-polar solvents for enhancing the extracting efficacy of the requisite compounds.
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