Presidential Green Chemistry Challenge: 2002 Academic Award
Professor Eric J. Beckman of the University of Pittsburgh
Design of Non-Fluorous, Highly CO2-Soluble Materials
Innovation and Benefits: Carbon dioxide (CO2) is a nontoxic chemical that can be used as a solvent in many industrial processes. Professor Beckman developed new detergents that allow a broad range of substances to dissolve in CO2. Any process that can now use CO2 may reduce or eliminate the use of hazardous solvents.
Summary of Technology: Carbon dioxide (CO2), an environmentally benign and nonflammable solvent, has been investigated extensively in both academic and industrial settings. Solubility studies performed during the 1980s had suggested that CO2's solvent power was similar to that of n-alkanes, leading to hopes that the chemical industry could use CO2 as a "drop-in" replacement for a wide variety of organic solvents. It was learned that these solubility studies inflated the solvent power value by as much as 20 percent due to the strong quadrupole moment of CO2 and that CO2 is actually a rather feeble solvent compared to alkanes. As the 1980s drew to a close, a number of research groups began to explore the design of CO2-philic materials, that is, compounds that dissolve in CO2 at significantly lower pressures than do their alkyl analogs. These new CO2-philes, primarily fluoropolymers, opened up a host of new applications for CO2 including heterogeneous polymerization, protein extraction, and homogeneous catalysis.
Although fluorinated amphiphiles allow new applications for CO2, their cost (approximately $1 per gram) reduces the economic viability of CO2 processes, particularly given that the use of CO2 requires high-pressure equipment. Furthermore, data have recently shown that fluoroalkyl materials persist in the environment, leading to the withdrawal of certain consumer products from the market. The drawbacks inherent to the use of fluorinated precursors, therefore, have inhibited the commercialization of many new applications for CO2, and the full promise of CO2-based technologies has yet to be realized. To address this need, Professor Eric J. Beckman and his group at the University of Pittsburgh have developed materials that work well, exhibiting miscibility pressures in CO2 that are comparable or lower than fluorinated analogs and yet contain no fluorine.
Drawing from recent studies of the thermodynamics of CO2 mixtures, Professor Beckman hypothesized that CO2-philic materials should contain three primary features: (1) a relatively low glass transition temperature; (2) a relatively low cohesive energy density; and (3) a number of Lewis base groups. Low glass transition temperature correlates to high free volume and high molecular flexibility, which imparts a high entropy of mixing with CO2 (or any solvent). A low cohesive energy density is primarily a result of weak solute–solute interactions, a necessary feature given that CO2 is a rather feeble solvent. Finally, because CO2 is a Lewis acid, the presence of Lewis base groups should create sites for specific favorable interactions with CO2.
Professor Beckman's simple heuristic model was demonstrated on three sets of materials: functional silicones; poly(ether-carbonates); and acetate-functional polyethers. Poly(ether-carbonates) were found to exhibit lower miscibility pressures in CO2 than perfluoropolyethers, yet are biodegradable and 100 times less expensive to prepare. Other families of non-fluorous CO2-philes will inevitably be discovered using this model, further broadening the applicability of CO2 as a greener process solvent.
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