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Green chemistry: the emergence of a transformative framework 15 Figure 9. Direct synthesis of H2O2 in CO2 over a titanium silicalite catalyst, an alternative to industrial processes using anthraquinone and organic solvents (70). tion (67), and Diels-Alder reactions (68,69). Hydro- gen peroxide can be synthesized directly from H2 and O2 using CO2 as a solvent, eliminating waste from the conventional autoxidation process, and allowing a higher non-explosive limit for the H2O2 mixture (70) (Figure 9). H2O2 produced in CO2 can be used to oxidize propylene to propylene oxide, providing an alternative to the conventional synthesis from chlorohydrin that generates 2 pounds of salt waste per pound of product in addition to large volumes of wastewater (71). In addition to the environmental and safety benefits associated with solvent replace- ment, the tunability of CO2 density often offers additional advantages such as increased reaction rates and control over selectivity (72). The solvent power of CO2 is crucial in the success of a CO2-based application. CO2 is a good solvent for non-polar molecules and some low-molecular weight polar molecules. With few exceptions, highly polar molecules and high-molecular weight compounds are poorly soluble (73). A major area of research has been elucidation of CO2-philicity. Identification of CO2-philic chemical structural motifs has expanded the range of chemistry possible in the medium. CO2- solubility can be imparted by covalently linking CO2- philic groups to a target molecule, adding stabilizers, or using surfactants. The invention of stabilizers for heterogeneous polymerization in CO2 greatly ex- panded the usefulness of CO2 as a medium for polymerization (74,75). Surfactants for water-in- CO2 microemulsions have made it possible to dissolve ionic compounds, a wider range of catalysts, and even biomolecules in CO2 (76). Early research was based on perfluorinated and silicone compounds (77,78), but toxicity concerns led to development of hydro- carbon-based surfactants (79,80), some based on renewable resources (81). Recently, it was shown that ionic liquids can form reverse micelles in super- critical CO2, providing another strategy for dissolving highly polar chemicals in CO2 (82). In the last decade, CO2 has shown success in commercialization and industrial usage. It has been used in dry cleaning, replacing the bioaccumulating chemical perchloroethylene, and in paint spraying applications (83,84). Continuous hydrogenation (85), Friedel-Crafts acylation (86), and hydroformylation (66) processes based on CO2 were developed at the University of Nottingham and, in collaboration with Thomas Swan & Co., have led to the construction of a demonstration plant (87). Water has been increasingly used to replace conventional solvents; the major green chemistry benefits are water’s non-flammability and elimination of volatile emissions. The scope of chemistry that can be carried out in water is vast, as highlighted in extensive reviews of CC bond forming reactions (88,89). Grignard-like reactions are known (90,91). Even dehydration reactions forming esters, ethers, and acetals can be carried out in water using surfactant-like acid catalysts (92). Selective oxidation of various alcohols has been carried out in water, not only obviating the need for organic solvent and facilitating catalyst recycling, but using environmen- tally-friendly O2 as a replacement for stoichiometric toxic metals (93). In some cases increased selectivity can be seen compared to conventional solvents, for example as the result of hydrophobic effects and hydrogen bonding (94,95). Heterogeneous reactions between water-insoluble reactants are sometimes accelerated compared to solvent-free conditions (so- called ‘‘on-water’’ catalysis) (96,97). Polyethylene glycol (PEG) and aqueous solutions of PEG have low toxicity and volatility. PEG is biodegradable but stable to acids, bases, high tem- perature, and some oxidizing and reducing condi- tions. Its solvent properties vary depending on molecular weight but various PEGs can dissolve both organic compounds and inorganic salts. PEG can complex metal ions and be used as a phase- transfer catalyst in biphasic systems (98). A ‘‘smart’’ solvent has been developed, able to alternate between an ionic liquid and a binary mixture of non-ionic constituents depending on exposure to CO2 or N2. The polarity can switch from chloroform-like to dimethylformamide-like. Providing the solvent is suitably inert, it should be possible to induce phase separation of products after reaction, eliminating problems associated with sol- vent waste and recycling (99). An especially fast growing field in the last two decades was research on ionic liquids (ILs), particu- larly those that remain liquid at and below room temperature. The primary green chemistry benefit of ILs is non-volatility, resulting in reduced exposure to industrial workers and easier containment of the solvents. The wide range of room-temperature ILs now known allows these ‘‘neoteric solvents’’ to replace conventional solvents for many synthetic applications (100,101). However, there is growing concern that claims of non-toxicity have not been adequately supported, and that risk analysis and lifecycle considerations are rarely taken into consideration in the development of novel ILsPDF Image | Green chemistry: the emergence of a transformative framework
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