Alternatives to Volatile Organic Solvents
Volatile organic solvents (VOCs) have served chemists well for decades. Without them, progress in the field would have taken a very different route; perhaps a greener, less toxic route, a skeptic might suggest. But, they have so many advantages and have solved so many problems. They have allowed industries to thrive so we perhaps cannot imagine a chemical world without them. And yet, there is always the feeling that there might be better alternatives, non-volatile solvents, ones that do not burn readily, non-toxic materials, ones we can quickly and easily recycle.
Indeed, the pioneering development by Nottingham University’s Martyn Poliakoff and others of supercritical fluids (SCFs) in the 1990s, albeit more than a century after their discovery, as solvents that can be recycled without impurity and room temperature ionic liquids (RTILs) pioneered by Ken Seddon at Queen’s University, Belfast, UK, and other researchers have addressed many of the issues and opened up new synthetic routes and new chemistry. But, there are other types of solvents that might be developed that could address issues in a wholly different manner to conventional VOCs, SCFs, and RTILs. Switchable solvents are a newly emerging class that offers a unique answer.
Jesse Vanderveen, Jeremy Durelle, and Philip Jessop of Queen’s University, Kingston, ON, Canada, are working on switchable-hydrophilicity solvents (SHSs). Such solvents offer the possibility of separation without distillation, the bane of non-volatile organic solvents and a huge plant and energy cost for the industry using VOCs. Jessop and his team offer SHSs as solvents that can be used in combination with water to allow sophisticated chemistry to be undertaken and then, with a figurative flip of the switch, product to be released into a separate phase without the need for VOCs, distillation, or high-pressure equipment.
The team defines an SHS as a material that is miscible with water in one form but immiscible in the other. For instance, they have identified amidine and tertiary amine SHSs that can be switched between those two forms by pumping a small amount of carbon dioxide into the system. “The change in miscibility is due to an acid-base reaction between either hydrated carbon dioxide or carbonic acid in the carbonated water and the SHS, resulting in the hydrophilic bicarbonate salt of the protonated SHS,” the team explains. This kind of change has already been exploited in extracting materials from soybean oil, algae oil, bitumen, and high density polystyrene powder, the team adds.
Environmentally Benign Solvents
The team’s early work led to the discovery of more than half a dozen SHSs, but most of these were too costly to produce to make them commercially viable. These early materials while demonstrating proof of principle neatly, also had toxicity and flammability problems, as well as often being volatile and so not side-stepping the less than green credentials of many of the VOCs they seek to displace. Now, the Ontario researchers have synthesized a range of novel SHSs; secondary and tertiary amines* can be toggled with the carbon dioxide switch described above. They used adapted literature methods for synthesizing similar compounds to produce their new SHSs and have compared each in terms of boiling point, flash point, eutrophication potential, toxicity, and effects on skin and have thus identified the safest and most environmentally benign SHSs.
They have shown that amines that have an added functional group, such as an oxygen-containing group, are less hazardous than the alkylamines. Moreover, in terms of industrial viability, speed of switching is an important factor and they have shown too that secondary amines are much quicker to switch than tertiary amines. “The variety of SHSs identified suggests that amine SHSs can be designed to have ideal properties for a given application,” Jessop says. “Compared to (familiar VOC) toluene, the secondary amine ester SHSs are predicted to be safer for health and the environment,” he adds.
“A company has been created to commercialize these unusual solvents, Switchable Solutions Inc., based in Calgary, Canada,” Jessop told us.
- Design and Evaluation of Switchable-Hydrophilicity Solvents,
J. R. Vanderveen, J. Durelle, P. G. Jessop,
Green Chem. 2013.
*The new SHSs from the Jessop group:
- Ethyl 4-(diethylamino)butanoate
- Ethyl 3-(sec-butylamino)propanoate
- Butyl 3-(isopropylamino)propanoate
- Propyl 3-(sec-butylamino)propanoate