A new class of solvents whose key properties can be rapidly “switched” by the introduction of a common gas could provide a more environmentally-friendly way of producing specialty chemicals for the pharmaceutical and other industries.
A research team from Queen’s University in Canada and the Georgia Institute of Technology in the United States reported on the development of the “switchable solvents” in the August 24th issue of the journal Nature. The first example of what could become a family of such solvents can be changed from a non-ionic liquid to an ionic liquid –– and back again –– with the alternate addition of nitrogen or carbon dioxide.
The ability to rapidly change the key properties of a solvent could allow multiple steps of a chemical reaction to be carried out without the need for removing and replacing solvents. That could potentially reduce pollution, cut cost and speed chemical processing.
“This process could provide a potential tool for benign and economical processing in the manufacture of high-value specialty chemicals,” said Charles Liotta, Georgia Tech’s vice provost for research and graduate studies, Regents professor of chemistry and a member of the team reporting in the journal. “One possible use for these solvents would be for such applications as the manufacture of pharmaceuticals and pharmaceutical precursors, especially for asymmetric or chiral compounds.”
Chemical processing often requires multiple reaction and separation steps, and the type of solvent required for each step may be different. The solvent is therefore usually removed and replaced after each step, contributing to total processing costs, said Charles Eckert, a professor in Georgia Tech’s School of Chemical and Biomolecular Engineering and director of the Specialty Separations Center.
“When you have to add and remove solvents, it’s both expensive and polluting,” he noted. “With this new class of solvents, we would be able to do what are called ‘one-pot syntheses’ –– that is, to carry out several steps in the same container with the same materials without having to do separations in between.”
The switchable solvent system provides a means of reducing the environmental impact from producing pharmaceuticals and other products that are essential to society today, noted Philip Jessop, the paper’s lead author and Canada Research Chair in Green Chemistry at Queen’s University.
“We all want the products of the plastics and pharmaceutical industries, but we don’t want the pollution,” Jessop noted. “Our research is seeking ways to decrease the amount of solvent waste being generated by these companies.”
The reaction begins with a one-to-one mixture of two non-ionic liquids: DBU (1,8-diazabicyclo-[5.4.0]-undec-7-ene) and 1-hexanol. When carbon dioxide is bubbled through the liquid mixture at one atmosphere of pressure, the liquid becomes ionic. The change can be readily reversed, with the ionic liquid converted back to its previous non-ionic state by bubbling nitrogen or argon through it.
The change of properties takes place at room temperature, and can be accelerated by raising the temperature to about 50 degrees Celsius. The change takes place rapidly, as soon as enough of the gas is bubbled through, Eckert said.
“The carbon dioxide actually reacts with a nitrogen atom of the amidine, so the carbon dioxide here is not a solvent –– it’s a reactant,” he explained. “That provides a redistribution of charge that makes the combination ionic. When the nitrogen gas is bubbled through, the carbon dioxide is swept out because it is only weakly bound, so the solvent goes back to its original state.”
The researchers reported that the non-ionic liquid (hexanol and DBU) formed under nitrogen is as nonpolar as chloroform, while the ionic liquid formed under carbon dioxide is as polar as propanoic acid. The researchers demonstrated the polarity changes by testing the solubility of the nonpolar compound decane in each liquid.
The solvent tested by researchers from Queen’s and Georgia Tech is a “proof of concept,” though practical applications aren’t yet known. The work being done by the research team –– which also includes David Heldebrant and Xiowang Li, both from Queen’s University –– is an example of how chemical design principles are facilitating the application of green chemistry.
“We are designing molecules for a specific function,” Eckert explained. “We decide what functions we want, then put atoms together in such a way that we can achieve that function. The collaboration of chemists and chemical engineers at different institutions is what makes it possible to look at both the molecular aspects and the applications.”
Solvents known as ionic liquids are salts that are liquid at room temperature or near-room temperature.
“They tend to have a lot of organic character, and have been widely hailed as environmentally benign because they have no vapor pressure,” Eckert noted. “They have applications where they are beneficial, and they have some unusual properties that we hope to use.”
Eckert and Liotta are recipients of the 2004 Presidential Green Chemistry Challenge Awards, which recognized their collaboration in developing benign tunable solvents that couple reaction and separation processes.
Green chemistry refers to the development of chemical processes and products that reduce or eliminate the use and generation of hazardous substances. Rather than focusing on the natural environment and pollutant chemicals in nature, this type of chemistry seeks to reduce and prevent pollution at its source.
“We’re concerned with pollution prevention rather than treatment,” said Jessop. “That’s a much more economic way to approach the problem.”