Carbon dioxide binding organic liquids (CO2BOLs) in a neutral acid form could be used for carbon capture, avoiding a common thickening problem with the substance, researchers at the Pacific Northwest National Laboratory in Washington state have discovered. “Nobody was considering the neutral acid form for these systems before. The conventional idea was that CO2BOLs would always be an ionic liquid, but clearly it doesn’t have to be,” researcher Vassiliki-Alexandra Glezakou said in a PNNL release.
The researchers had observed an issue with the viscosity of CO2BOLs when used for carbon capture. The substance is thin, almost water-like, on its own. However, as it absorbs CO2, it thickens to the consistency of “cold honey,” according to the release. When the substance is used in carbon capture, it also needs to transport the CO2, simply put, cold honey does not travel well through tubes. The energy needed to later remove the CO2 from the substance is increased when the substance is thicker.
The researchers set out to uncover why this viscosity change occurred. They discovered that as the molecules attract CO2, their charges change. “They saw a relation between how the charges moved around on the molecules and their viscosity,” the release says. The movement resulted in an overall neutral molecule that has separated positive and negative charges, what chemists call a zwitterion. “Their analysis also showed them that the zwitterion is what ramps up the viscosity,” the lab said.
Using knowledge of other zwitterions, the researchers wanted to determine if they could move the protons of the molecule in order to produce a neutral molecule. “If this idea panned out, viscosity would no longer be an issue for a non-ionic carbon capture solvent systems,” the release explains. Testing of two variations of CO2BOLS found that they “formed the neutral acid easily.”
