The Forgotten Greenhouse Gas

N₂O – A Potent Greenhouse Gas

In a carbocentric political environment, dinitrogen monoxide, is almost the forgotten greenhouse gas. Enormous volumes are released into the atmosphere naturally from soils and the oceans. This accounts for about two-thirds of the atmospheric concentration — agriculture, combustion, and the chemical industry between them account for the other third. Specifically, the large-scale production of adipic acid and several other fine chemicals accounts for a significant volume of the total N₂O flux.

N₂O has an atmospheric lifetime of about 114 years and a global warming potential (GWP) of 298 (given a century-long time horizon). The GWP for CO₂ is just 1, which makes N₂O a potent greenhouse gas. N₂O levels are well over 319 parts per billion by volume compared with pre-industrial levels of 285 ppbv (up 9–10 %). N₂O is not the major contributor to global warming, it is nevertheless one of the six gases, including CO₂, methane sulfur hexafluoride, hydrofluorocarbons and perfluorocarbons, that the United Nations Framework on Climate Change has earmarked for substantial reductions in an attempt to stave off the worst effects on global warming and climate change.

Preventing Release of N₂O

Of the anthropogenic sources, perhaps the two most direct solutions to the problem would be to avoid the release of the gas from livestock urine and to extract it from the gaseous outflows of chemical plants. Preliminary tests have been carried out using unweathered wood, biochar, added to grazing land to adsorb N₂O from livestock urine, but separating N₂O from industrial waste gas, which also contains carbon dioxide, is proving difficult.

DuPont has turned to room temperature ionic liquids (RTILs) as a green solvent for extracting N₂O from waste gases. The process is particularly suited to the waste stream from adipic acid production as the N₂O can be recycled back into the feedstock to boost overall yield and, at the same time, preclude the need to vent this gas into the atmosphere. N₂O is also used as an oxidising agent for converting benzene to phenol, so it could be used in that way too.

Sifting the N₂O from the CO₂

Mark Shiflett and colleagues at the company’s Central Research and Development department in Wilmington, Delaware, USA, explain how they have modeled the mixture N₂O/CO₂/1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) with separation in mind. [bmim][BF₄] is well known as an archetypal RTIL, an ionic substance where the energy of crystallization is too high for it to be a solid at ambient temperatures. As such, it is a liquid and can act as a solvent for a wide range of substances for which potent volatile organic solvents (VOCs) are not available. RTILs also have several major advantages over VOCs in that they have limited volatility, are broadly non-toxic, do not burn and can be relieved of their solutes much more readily than VOCs.

The team’s model of the equilibrium of the separation system shows it to be valid over a temperature range from 296 to 315 K and initial tests show that for both large and small N₂O/CO₂ feed ratios, the two can be separated quite effectively. The actual concentration of RTIL seems to make little difference to final selectivity of the separation but, without it, there is no practical way to separating the two gases. The researchers point out that these preliminary studies could pave the way for an effective industrial separation of these two important gases, although they are yet to identify the specific RTIL that would be most effective in an industrial-scale process.

“Shiflett is one of the world leaders in the study of gas solubilities in ionic liquids,” Ken Seddon, Director of the QUILL Research Centre in Northern Ireland told ChemViews, “This study extends his work to N₂O and although the hydrolytic instability of the tetrafluoroborate anion will preclude its application at the industrial scale, it is clear now what needs to be done to create a practical system.”