Annual carbon dioxide emissions surged to more than 36 billion metric tons last year, causing climate warming and ocean acidification. Capturing some of that CO2 and then converting it into methanol—an alternative transportation fuel and feedstock for chemical synthesis—could be one way to help keep those levels in check. Now, researchers have combined the capture and conversion steps into one continuous process that uses less energy than current methods (J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.9b12711).
Unlike scrubbing concentrated CO2 from smoke stacks, direct air capture (DAC) collects CO2 from ambient air, present at dilute concentrations. DAC uses sorbents such as liquid amines or alkali metal hydroxides that bind CO2 as it contacts the liquid. High temperatures (700–900 °C) drive the CO2 out of the sorbent for recovery, after which the gas can be pressurized and injected underground or hydrogenated to methanol.
In earlier work, Surya Prakash, an organic chemist at the University of Southern California, and his team combined amine-based capture of CO2 and hydrogenation to methanol into a seamless reaction sequence to avoid the energy penalty of the CO2 separation step. “But amines are not well suited for DAC because they are volatile and get oxidized when exposed to the large volumes of air used for DAC,” Prakash says. So the scientists tried integrating the process using alkali hydroxides, which are cheaper than amines and have a much stronger affinity for CO2. “We captured CO2 with potassium hydroxide to form an alkali metal carbonate and reduced it in the presence of a catalyst and H2 to generate potassium formate,” he says, “but then we couldn’t hydrogenate the formate to methanol.”
For the current study, the scientists surmised that alcohols such as ethylene glycol could mediate the hydrogenation pathway from potassium formate to methanol. The researchers bubbled indoor air through a solution of KOH in ethylene glycol in the presence of H2 and a ruthenium catalyst. After 72 hrs at 140 °C, the scientists obtained methanol at 100% yield.
Although most KOH was recoverable, 30% was consumed in an unwanted side reaction. The researchers will aim to avoid that side reaction in future work. Solving these problems and scaling up the process to a commercial level could take 2 to 5 years, Prakash says.
“Industries that already employ hydroxide-based CO2 capture from dilute streams could easily adapt the system to produce methanol, which has many valuable uses,” says David J. Heldebrant, a green chemist at Pacific Northwest National Laboratory. DAC coupled with methanol production could be set up anywhere. If the process is powered with renewable energy, methanol is an ideal way to store excess wind and solar power for later use on calm or cloudy days, he says.