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Supercritical CO2
Supercritical CO2
We are taught in school that there are three states of matter: solid, liquid and gas. This is sufficient to satisfy most people, even most scientists. But substances cannot always be classified so conveniently as this, for under extreme conditions matter can behave in unfamiliar ways. For instance, neon lighting works by creating plasma, which is neither solid, liquid or gas. Less well known is the supercritical fluid state of matter. Substances have a critical point, which (much like a melting point or boiling point) is defined by a specific temperature and pressure. At the critical point (or any temperature and pressure above it) the distinction between liquid and gas is lost as the substance becomes a supercritical fluid. Viscosity, density, permeability are all in between that of a liquid and a gas.
A schematic of a generic super-critical process. Reprinted from Chemical Engineering and Processing: Process Intensification, 99, Abou-Shehada et al., Tunable solvents: Shades of green, 88-96, Copyright (2016), with permission from Elsevier.
For most substances, the critical point is not realistically achievable in a laboratory setting, only being observed at very high temperature and pressure. However, carbon dioxide becomes a supercritical fluid at about 31 °C and 73 atmospheres of pressure. These conditions can easily be maintained by specialist apparatus, and allow supercritical carbon dioxide (scCO2) to be used as a solvent. Perhaps the most beneficial application of scCO2 is extraction. The isolation of bioactive compounds from harvested plants is possible with scCO2. The conditions can be tuned by varying the temperature or pressure, and sometimes a co-solvent (often methanol) is added to adjust the polarity. Once the extraction is complete, releasing the pressure removes all solvent residue (for carbon dioxide returns to being a gas). This means food-grade and pharmaceutical-grade extracts can be prepared. The carbon dioxide can be recycled within the apparatus, and sourced from the brewing industry or other sources of CO2 waste.
Carbon dioxide may also be used to tune the polarity of another organic solvent, either to improve solubility (e.g. of gaseous reactants), or to lower solubility as an anti-solvent and cause the precipitation of the product. These so-called gas-expanded’ liquids (GXLs) are generally used for materials science and extractions, but could be applied to the crystallisation of pharmaceuticals to dictate their morphology.
Use of gas expanded liquids (GXLs) as solvent. Reprinted from Chemical Engineering and Processing: Process Intensification, 99, Abou-Shehada et al., Tunable solvents: Shades of green, 88-96, Copyright (2016), with permission from Elsevier.
Use of gas expanded liquids (GXLs) as anti-solvent. Reprinted from Chemical Engineering and Processing: Process Intensification, 99, Abou-Shehada et al., Tunable solvents: Shades of green, 88-96, Copyright (2016), with permission from Elsevier.
When synthetic chemistry has been performed in scCO2 in an attempt to minimise the impact of organic solvents, the solubility of catalysts and polar reactants has been a major issue. Attempts to produce hydrophobic perfluorinated ligands for catalysts so that they can be used in scCO2 is arguably worse for the environment than continuing to practice conventional reactions. The energy required to create and sustain a supercritical fluid and the initial outlay on buying the necessary equipment has hampered greater uptake of this technology. A large-scale supercritical fluid facility was constructed, but has since been decommissioned due to the high operational costs.
Additional reading
Additional reading
References
References
Recent advances in supercritical fluid extraction of natural bioactive compounds from natural plant materials: Uwineza, P.A. and Waśkiewicz, A., Molecules 2020, 25, 3847.
Versatile chemoselectivity in Ni-catalyzed multiple bond carbonylations and cyclocarbonylations in CO2-expanded liquids: del Moral, D., Osuna, A.M.B., Córdoba, A., Moretó, J.M., Veciana, J., Ricart, S. and Ventosa, N., Chem. Commun. 2009, 4723-4725.
Gas-expanded liquids: Jessop, P.G. and Subramaniam, B. Chem. Rev. 2007, 107, 2666-2694.
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