Supercritical Carbon Dioxide as a Green Solvent

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Supercritical Carbon Dioxide as a Green Solvent

Gregory Absillis
June 2009

The majority of chemical processes in chemical synthesis are carried out in solution and organic solvents usually are the first choice. To satisfy this demand, 15 billion kilograms of organic are produced worldwide each year. [1] These solvents often bear considerable risk due to their toxicity, flammability or environmental hazards. Furthermore, they have to be separated from the products and recycled or disposed at the end of the process, increasing the overall production cost. Although they play a valuable role, organic solvents are responsible for significant air, soil and water pollution. One important effort in the development of environmentally friendly chemical processes is the reduction of solvent use through the development of solvent-free processes and more efficient recycling protocols. Since most of these approaches necessitate a pollution prevention program the replacement of organic solvents by environmentally benign reaction media is preferred. Ionic liquids, fluorous biphase systems and water are currently being studied as solvent alternatives. Another actively studied alternative is supercritical carbon dioxide (spCO2). All have potential benefits and disadvantages that need to be considered. [13]

Although supercritical fluids are known since the discovery of the critical temperature 175 years ago, their use as reaction media for organic synthesis emerged only during the last 1015 years. Especially the use of scCO2 as a solvent for selective extraction and synthesis processes attracts considerable interest. Not only is scCO2 used in diverse areas as dry cleaning, dying of fabrics and polymers, metal degreasing and material processing, its application in food chemistry is nowadays even more established. The main commercial example is the production of decaffeinated coffee. [4] This paper aims to analyze the use of scCO2 as a decaffeinating agent for coffee beans and solvent for the synthesis of fluoropolymers. The benefits and disadvantages of scCO2 will be outlined and a comparison to previously used methods will be made.

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1. Supercritical Carbon Dioxide

Figure 1 shows the phase diagram of pure carbon dioxide. At 31.0 °C and 73.75 bar the endpoint on the evaporation line is reached. Beyond this critical point, no distinct liquid or vapor phase is observed and the new supercritical phase exhibits properties that are reminiscent of both states. The favorable diffusivity of a gas is combined with the increased density of a liquid. Due to these unique properties scCO2 is an ideal solvent for many applications. [5]

Figure 1. Phase diagram of pure CO2

General advantages and disadvantages associated with the use of scCO2 [5]: Advantages: • • • • Non-explosive, non-flammable, non-corrosive, relatively non-toxic (significant safety advantage). CO2 is naturally abundant.1 Excellent heat transport capacities: removal of heat from exothermic reactions, avoiding hot spots. Inert towards oxidation, excluding any formation of byproducts from oxidation of the solvent.                                                              If CO2 can be withdrawn from the environment, employed in a process, then returned to the environment ‘clean’, no environmental detriment occurs. However, most of the CO2 employed in processes today is collected from the effluent of ammonia plants or derived from naturally occurring deposits. Ultimately, one should consider the source of CO2 used in a process in order to judge the sustainability of the process. [5] 1

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Simplified work-up procedures. Reduce of organic and hazardous wastes. No residual CO2 after synthesis and processing.

Disadvantages: • • • • High critical pressure. The relatively high energy cost involved in generating and recirculating supercritical CO2. The hazards and costs of high-pressure equipment. The potential damage to the ozone layer....
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