DMSO poses decomposition danger

Chemists are once again raising safety concerns about dimethyl sulfoxide (DMSO), following research that has probed how the popular solvent decomposes when heated. DMSO decomposition has caused many industrial and laboratory explosions since the 1960s, according to accident reports. DMSO is widely used because it is so versatile: it can dissolve polar and nonpolar compounds alike and mixes well with water and organic solvents. It also has a low vapor pressure and low toxicity that make for easier use in the lab and in industrial processes.

But when DMSO is heated close to its boiling point of 189 °C, for example during distillation to recover the solvent, it can rapidly decompose. This generates more heat along with gases such as hydrogen, methane, and carbon dioxide. The decomposition process is also autocatalytic—in other words, molecules formed during decomposition act as catalysts for even more decomposition, triggering a runaway reaction that poses an explosion hazard.

A team led by Yoshikuni Deguchi at Kaneka has now identified some of the breakdown products of DMSO that are responsible for this autocatalytic behavior (Org. Process Res. Dev. 2020, DOI: 10.1021/acs.oprd.0c00113). The researchers hope that this finding will help to raise awareness of the hazards of DMSO.

Deguchi’s team gradually heated DMSO to 430 °C in a pressure vessel under a nitrogen atmosphere, and identified decomposition products including sulfuric acid, methanesulfonic acid, acetic acid, and formic acid. When they heated DMSO at a constant 185 °C, the concentration of these acids increased over time. “Nobody thought these acids could be generated in pure DMSO under an inert atmosphere,” Deguchi says.

Then the researchers spiked pure DMSO samples with traces of these acids before heating, and found that they significantly sped up decomposition. Bubbling air or CO₂ through the solvent had a similar effect. Deguchi’s team is now trying to work out the decomposition mechanism involved in forming these catalytic acids.

The discovery builds on decades of research showing that a wide range of substances, including acids, bases, oxidants, and reductants, can all accelerate DMSO decomposition. In 1985, for example, an explosion at a waste treatment plant that was distilling DMSO and 1-chloro-2,3-epoxypropane caused one injury and one death—just one of many such accidents catalogued in a recent review (Org. Process Res. Dev. 2020, DOI: 10.1021/acs.oprd.0c00159).

“Almost anything you put in DMSO will trigger its decomposition,” says Qiang Yang, a process chemist at Corteva Agriscience who co-authored the review and was not involved in the new research. “DMSO has got so many problems that people should evaluate a safer replacement.”

Yang suggests that chemists should consult the ACS solvent selection tool to find suitable alternatives. “The bottom line is: stay away from DMSO,” he says.