Oxazolidine-Based H₂S and Mercaptan Scavengers: Uncovering the Myths

Oxazolidine-based products were originally used for their biocide properties, but they have become more widely applied recently for scavenging H₂S. Oxazolidine H₂S scavengers offer an oil-soluble alternative to hexahydrotriazine water-based scavengers and may mitigate some of their drawbacks. While the same reactants are used for oxazolidine (an aldehyde and primary amine) as are used for hexahydrotriazines, the synthesis conditions are strictly anhydrous and involve the elimination of water to yield the 5-membered ring heterocyclic species. However, we find that their structural identity is more complex than might originally be suspected. The synthesis can be extended beyond the 2-carbon N,O spacer yielding a 5-membered heterocycle to include the 3-carbon N,O spacer, which leads to a 6-membered 1,3-oxazinane species. Oxazolidine-based scavengers are readily converted into the corresponding hexahydrotriazine and very likely owe a large degree of their scavenger activity to this hydrolytic conversion. Due to this conversion, oxazolidines may not be true “nontriazine” alternatives, but they are still very useful H₂S scavengers, and both their chemistry and H₂S scavenging efficacy deserve further study. This paper contributes to three important aspects of the study of oxazolidine H₂S scavengers. First, this work presents several new results on the chemistry of the two bisoxazolidine H₂S scavengers, 3,3′-methylenebis­[5-methyloxazolidine] (MBO) and 3,3′-methylene-bisoxazolidine (unsubstituted-MBO or US-MBO). Our study focuses on the structure of the scavengers themselves as well as on their reaction with H₂S mechanistically, including the reaction product characterization. Second, a benchmarking reference for the scavenging performance of MBO and US-MBO is presented using a range of standard industry techniques for H₂S scavenger assessment. Third, we go some way toward explaining why the most commercial of the bisoxazolidines is in fact the monoisopropanolamine (MIPA)-derived MBO. This latter result is surprising given the relative availability and cost of monoethanolamine (MEA).