Contributed by Philip Jessop, Queen’s University, Kingston, Ontario, Canada
“CO2 is the answer to everything.” That statement started as a joke in my research group but has become more of a philosophy. Society has so much of this compound; it’s one of the most abundant renewable molecules available to us. It’s nonflammable and essentially nontoxic. Why not put it to use? I have no illusions about CO2 utilization being the answer to global warming; it’s not. However, the versatility of CO2 continues to impress me. It can be a solvent, an acid, a catalyst, a trigger for switchable materials, a viscosity modifier, a feedstock, a fire extinguisher, and even part of my beverage at lunchtime! That versatility has made CO2 the star of my research program.
At first glance, my research projects may seem an eclectic group, but they’re all connected to CO2 in some way. We’re working on separation and conversion of biomass-derived chemicals (using supercritical CO2, liquid CO2, CO2-expanded liquids, or CO2-switchable solvents), homogeneously-catalyzed organic reactions (using CO2 as substrate, reagant, solvent, or catalyst), and switchable or stimuli-responsive materials (using CO2 as the trigger or stimulus). Each of these projects feeds the others. The idea of CO2-switchable solvents started from a project on homogeneously-catalyzed CO2 conversion. Now, advances in CO2-switchable solvents help the catalysis projects. Obtaining fuels or chemicals from biomass must be done efficiently if it is to be green and economical. In collaboration with Dr. Pascale Champagne (Civil Engineering, Queen’s University), we’ve developed methods of using CO2 to trigger the aggregation of algae in water, so that the bulk of the water can be easily removed from the algae. Then we can use liquid CO2 or CO2-expanded methanol to extract the lipids from the algae, for use as biofuels. My group has also studied lignin, a renewable source of aromatic molecules. Lignin can be pyrolyzed and single-ring phenols can be separated from the resulting pyrolysis oil using supercritical CO2 rectification. The single-ring phenols can then be methylated to give a mix of anisoles that serve as a sustainable alternative to petroleum-derived anisole, the only aromatic organic solvent to be considered fairly green.
CO2-switchable materials and fluids have been a seemingly limitless supply of avenues for research and interesting applications. One of our most exciting technologies include a method of using CO2-switchable aqueous solutions as draw solutions for forward osmosis; that makes it possible to obtain fresh water from wastewater with most of the energy costs supplied by waste heat rather than electricity. We filed for a patent application in 2010 and now a 20 m3/h demonstration unit is operating in Mississauga, Ontario, and giving us accurate data on performance and energy consumption. We’re also developing greener paints and greener drying agents for organic solvents using CO2-switchable technologies. These projects have been in collaboration with Prof. Michael Cunningham (Chemical Engineering, Queen’s).
However, I’ve found that it isn’t sufficient to come up with a potentially greener process, if the chemicals involved aren’t particularly green. With so many possible molecular structures to choose from, how can we pick the ones that are most likely fit the application AND be fairly green? The classical approach is to design a molecule to meet the needs of the application, make it in the lab, prove it works, and then start figuring out whether it’s green. That’s not good enough because it takes a lot of time, and there’s a pretty good chance that the molecule you’ve made will turn out to be problematic. It’s therefore better to use virtual screening, in which you program a computer to consider tens of thousands of structures that might work, and then use QSARs (quantitative structure activity relationships) to predict all of the physical and chemical properties required to determine whether each compound will meet the performance criteria and how damaging the compound might be to health, safety and the environment (EH&S). Then the computer can reject those compounds that don’t meet the performance criteria and rank the remaining compounds in terms of EH&S impacts. Then all we as experimentalists need to do is check the best 30-50 structures and choose a few that appear simple and inexpensive to make. We then go into the lab and synthesize those few, with the confidence that those molecules have a fair chance of being reasonably green.
Green chemistry research is changing from a niche activity that’s exciting a few researchers to the new standard way of doing things. Any chemistry research developing new technologies needs to demonstrate to potential users that the environmental impact of the process is better than the technologies of the past. I’m delighted that this change of attitude is taking over as the generations change. Students are demanding green chemistry education, and more and more assistant professors are making green chemistry their field of research. My advice to new researchers in green chemistry: find out the real needs of industry. What chemicals or processes are most in need of replacement? Talk to industry representatives you know or you meet at conferences to get their thoughts on industrial needs, or use databases like the Toxic Release Inventory in the US or the National Pollutant Release Inventory in Canada to determine the industries that are causing the greatest releases of problematic chemicals. Choose problems of a larger rather than a smaller scale, so that any improvements you invent will have the greatest benefit to the environment. Use your time, energy, and creativity to solve those problems.
As for me, I’ll continue to follow the path that CO2 has laid out for me. It’s shown itself capable of solving some large-scale problems. Who knows where it will lead me next!
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