J. of Supercritical Fluids 28 (2004) 121–191 Supercritical and near-critical CO2 in green chemical synthesis and processing Eric J. Beckman∗ Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA 15260, USA Received 19 September 2002; received in revised form 3 February 2003; accepted 24 February 2003 Abstract Carbon dioxide is often promoted as a sustainable solvent, as CO2 is non-flammable, exhibits a relatively low toxicity and is naturally abundant. However, injudicious use of carbon dioxide in a process or product can reduce rather than enhance overall sustainability. This review specifically examines the use of CO2 to create greener processes and products, with a focus on research and commercialization efforts performed since 1995. The literature reveals that use of CO2 has permeated almost all facets of the chemical industry and that careful application of CO2 technology can result in products (and processes) that are cleaner, less expensive and of higher quality. © 2003 Elsevier B.V. All rights reserved. Keywords: Carbon dioxide; Toxicity; Technology 1. Introduction Whereas the use of carbon dioxide as raw ma- terial or solvent could produce product (property) The use of carbon dioxide as a solvent or raw ma- advantages, process (chemistry) advantages, cost ad- terial has been investigated somewhat continuously in vantages, or safety advantages, in this review we will academia and/or industry since 1950; interest in the focus explicitly on uses of CO2 that provide practical use of CO2 in these roles has intensified during the improvements (as defined in Section 1.7) to the sus- past 20 years as large-scale plants using CO2 have tainability (or ‘green’-ness) of a product or process. been brought on line. While supercritical fluids in gen- Carbon dioxide is often promoted as a green solvent, eral exhibit interesting physical properties [1], spe- and its use in this role has permeated throughout the cific interest in CO2 is magnified by its perceived chemical and materials research communities. Here ‘green’ properties—carbon dioxide is non-flammable, we describe recent advances that are both fundamental relatively non-toxic, and relatively inert. In addition, and significant. unlike water, the supercritical regime of CO2 is read- In summary, rather than present a comprehensive ily accessible, given its critical temperature of only review of CO2-based technology, here we focus on 304 K. uses of CO2 that are relatively new and appear to provide ‘green’ advantages. It should be noted that ∗ Tel.: +1-412-624-9641 fax: +1-412-624-9639. there are examples provided in this paper where a E-mail address: [email protected] (E.J. Beckman). CO2-based process is not particularly ‘green’, yet is 0896-8446/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0896-8446(03)00029-9 122 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 generating interest because it produces better quality in CO2 over a platinum catalyst at 303 K will un- product than conventional alternatives. doubtedly lead to the production of CO, which could poison the catalyst [6]. The same reaction run over a 1.1. Physical properties of CO2 palladium catalyst under the same conditions will by contrast produce lesser amounts of CO as a byproduct The pVT properties of CO2 have been known since [7] and hence knowledge of CO2’s reactivity is vital the 1930s [2]; extensive data sets are available in the to its use in green chemistry. literature and on the web in the form of correlations of Carbon dioxide is clearly a ‘greenhouse gas’, but density, viscosity, dielectric constant, etc., as functions it is also a naturally abundant material. Like water, of temperature and pressure [3].CO2’s critical pres- if CO2 can be withdrawn from the environment, em- sure (and hence its vapor pressure in the ‘near-critical’ ployed in a process, then returned to the environment or liquid regime) is significantly higher than analogous ‘clean’, no environmental detriment accrues. How- values for alkane, fluoroalkane or hydrofluoroalkane ever, while CO2 could in theory be extracted from fluids. CO2’s anomalously high critical pressure is but the atmosphere (or the stack gas of a combustion one result of the effect that CO2’s strong quadrupole based power plant), most of the CO2 employed in pro- moment exerts on its physical properties. While the cesses today is collected from the effluent of ammo- high critical pressure is problematic, the most unfor- nia plants or derived from naturally occurring deposits tunate outcome of the effect of quadrupole moment (e.g. tertiary oil recovery as practiced in the US [8]). on physical properties was the premise, first advanced Because industrially available CO2 is derived from during the late 1960s, that CO2 might prove to be a man-made sources, if CO2 can be isolated within a solvent whose strength would rival or surpass that of process one could consider this a form of sequestra- alkanes and ketones [4]. Because early models em- tion, although the sequestered volumes would not be ployed to calculate CO2’s solvent power relied on a high. Ultimately, one should consider the source of direct relationship between the Hildebrandt solubility CO2 used in a process in order to adequately judge parameter (δ) and the square root of the critical pres- the sustainability of the process. 1/2 sure [(Pc) ], the solubility parameter of CO2 was CO2’s combination of high TLV and high va- over-predicted by 20–100%, leading to early inflated por pressure means that residual CO2 left behind in claims as to the potential for using CO2 to replace substrates is not a concern with respect to human conventional organic solvents. exposure—the same can certainly not be said to be true for many man-made and naturally-occurring organic 1.2. Environmental and safety advantages to use of compounds. Because there is effectively no liability CO2 in chemical processes due to ‘residual’ CO2 in materials following process- ing, CO2 is not considered a solvent requiring process Carbon dioxide is non-flammable, a significant re-evaluation by the US FDA. Only water also enjoys safety advantage in using it as a solvent. It is also this special situation. Indeed, most of the commercial naturally abundant, with a TLV (threshold limit value operations employing CO2 as a solvent were initiated for airborne concentration at 298 K to which it is to take advantage of CO2’s particular advantages in believed that nearly all workers may be repeatedly products designed for intimate human contact (such exposed day after day without adverse effects) of as food), or CO2’s non-VOC designation (such as the 5000 ppm [5], rendering it less toxic than many other foaming of thermoplastics). The recent commercial- organic solvents (acetone, by comparison, has a TLV ization of fabric cleaning using CO2 benefits both of 750 ppm, pentane is 600 ppm, chloroform is 10 from CO2’s advantages in human-contact applications ppm [5]). Carbon dioxide is relatively inert towards and situations where emissions appear unavoidable. reactive compounds, another process/environmental The simultaneous use of both hydrogen and oxygen advantage (byproducts owing to side reactions with in a reaction is obviously problematic from a safety CO2 are relatively rare), but CO2’s relative inert- standpoint, given that H2/O2 mixtures are explosive ness should not be confused with complete inertness. over a broad concentration range. Addition of CO2 For example, an attempt to conduct a hydrogenation to mixtures of H2 and O2 expands the non-explosive E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 123 regime (in the gas phase), more so than if either N2 or Whether to use liquid or supercritical CO2 is a water vapor was added [9]. At this point it is not clear choice that actually involves safety as well as chem- to what extent the non-explosive regime will expand istry considerations. While use of supercritical CO2 further as one raises the density of the mixture (and almost always involves use of higher pressure (to hence the heat capacity). achieve the same solubility of a given substrate as In a final intriguing note regarding safety advan- in the liquid case), other factors should also be con- tages inherent to use of CO2 as a solvent, DuPont sidered. First, supercritical CO2 will exhibit a higher scientists [10] discovered that addition of CO2 to compressibility than liquid CO2, and hence the su- tetrafluoroethylene enhances the stability of that no- percritical fluid will be better able to absorb excess toriously difficult-to-handle monomer, although the heat evolved from an exothermic reaction whose rate exact mechanism for the enhanced stability has not suddenly exceeds typical operating conditions. On been published. What has been revealed is that addi- the other hand, use of saturated liquid CO2 (in the tion of CO2 to TFE vapor inhibits runaway decom- presence of the vapor phase) would allow boiling to position and explosion of the monomer. In addition, be used as a means to absorb excess heat. Use of the CO2/TFE mixture behaves like an azeotrope, in supercritical CO2 (versus liquid) could avoid compli- that boiling of a mixture of the two does not signif- cations owing to a phase separation occurring upon icantly change the concentration of either the liquid a departure from established temperature or pressure or the vapor. According to the DuPont patent [10], conditions within a given reactor. For example, if this ‘azeotrope-like’ behavior persists over a wide one is employing a mixture of oxygen, substrate, and concentration range, behavior that is quite unlike that liquid CO2 in a particular process, a sudden drop in of typical azeotropic mixtures.