I read with interest about the BP and DuPont planned joint venture for biologically synthesized 1-butanol as a potential fuel oxygenate replacement for methyl tert-butyl ether (MTBE) or ethanol (C&EN, June 26, page 9). Several advantages over ethanol were described, such as lower volatility, which allows it to be distributed more economically by pipelines.
One aspect that was not addressed was the possible chemical species from environmental oxidation of 1-butanol to butyraldehyde and butyric acid. Both are less volatile and have lower odor thresholds than acetaldehyde and acetic acid, the oxidation products of ethanol, the current MTBE replacement. The question is would there be sufficient transient butyric acid in drinking water to give it the smell of rancid butter? In Southern California, drinking-water wells were closed because of the odor and/or taste related to the presence of MTBE.
Finally, there is the possibility of volatilization of these odiferous species from water; more likely for butyraldehyde, which has a Henry's law constant of 1.2 × 10-4 atm m3/mol, than it is for butyric acid, which has a Henry's law constant of 2.2 × 10-5 atm m3/mol. Since it is possible that biobutanol could comprise 10% of a fuel mix, the transient concentrations of these odiferous species should be well-characterized so that they do not affect consumers.
James R. Sanborn