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122 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 generating interest because it produces better quality product than conventional alternatives. 1.1. Physical properties of CO2 The pVT properties of CO2 have been known since the 1930s [2]; extensive data sets are available in the literature and on the web in the form of correlations of density, viscosity, dielectric constant, etc., as functions of temperature and pressure [3]. CO2’s critical pres- sure (and hence its vapor pressure in the ‘near-critical’ or liquid regime) is significantly higher than analogous values for alkane, fluoroalkane or hydrofluoroalkane fluids. CO2’s anomalously high critical pressure is but one result of the effect that CO2’s strong quadrupole moment exerts on its physical properties. While the high critical pressure is problematic, the most unfor- tunate outcome of the effect of quadrupole moment on physical properties was the premise, first advanced during the late 1960s, that CO2 might prove to be a solvent whose strength would rival or surpass that of alkanes and ketones [4]. Because early models em- ployed to calculate CO2’s solvent power relied on a direct relationship between the Hildebrandt solubility parameter (δ) and the square root of the critical pres- sure [(Pc )1/2 ], the solubility parameter of CO2 was over-predicted by 20–100%, leading to early inflated claims as to the potential for using CO2 to replace conventional organic solvents. 1.2. Environmental and safety advantages to use of CO2 in chemical processes Carbon dioxide is non-flammable, a significant safety advantage in using it as a solvent. It is also naturally abundant, with a TLV (threshold limit value for airborne concentration at 298 K to which it is believed that nearly all workers may be repeatedly exposed day after day without adverse effects) of 5000 ppm [5], rendering it less toxic than many other organic solvents (acetone, by comparison, has a TLV of 750 ppm, pentane is 600 ppm, chloroform is 10 ppm [5]). Carbon dioxide is relatively inert towards reactive compounds, another process/environmental advantage (byproducts owing to side reactions with CO2 are relatively rare), but CO2’s relative inert- ness should not be confused with complete inertness. For example, an attempt to conduct a hydrogenation in CO2 over a platinum catalyst at 303 K will un- doubtedly lead to the production of CO, which could poison the catalyst [6]. The same reaction run over a palladium catalyst under the same conditions will by contrast produce lesser amounts of CO as a byproduct [7] and hence knowledge of CO2’s reactivity is vital to its use in green chemistry. Carbon dioxide is clearly a ‘greenhouse gas’, but it is also a naturally abundant material. Like water, if CO2 can be withdrawn from the environment, em- ployed in a process, then returned to the environment ‘clean’, no environmental detriment accrues. How- ever, while CO2 could in theory be extracted from the atmosphere (or the stack gas of a combustion based power plant), most of the CO2 employed in pro- cesses today is collected from the effluent of ammo- nia plants or derived from naturally occurring deposits (e.g. tertiary oil recovery as practiced in the US [8]). Because industrially available CO2 is derived from man-made sources, if CO2 can be isolated within a process one could consider this a form of sequestra- tion, although the sequestered volumes would not be high. Ultimately, one should consider the source of CO2 used in a process in order to adequately judge the sustainability of the process. CO2’s combination of high TLV and high va- por pressure means that residual CO2 left behind in substrates is not a concern with respect to human exposure—the same can certainly not be said to be true for many man-made and naturally-occurring organic compounds. Because there is effectively no liability due to ‘residual’ CO2 in materials following process- ing, CO2 is not considered a solvent requiring process re-evaluation by the US FDA. Only water also enjoys this special situation. Indeed, most of the commercial operations employing CO2 as a solvent were initiated to take advantage of CO2’s particular advantages in products designed for intimate human contact (such as food), or CO2’s non-VOC designation (such as the foaming of thermoplastics). The recent commercial- ization of fabric cleaning using CO2 benefits both from CO2 ’s advantages in human-contact applications and situations where emissions appear unavoidable. The simultaneous use of both hydrogen and oxygen in a reaction is obviously problematic from a safety standpoint, given that H2/O2 mixtures are explosive over a broad concentration range. Addition of CO2 to mixtures of H2 and O2 expands the non-explosivePDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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