Synthesis, Reaction Byproduct Characterization, and Mechanistic Understanding of Hemiformal Hydrogen Sulfide Scavengers: Part 2-Identification of Byproducts, Reaction Mechanisms, and Suppression Thereof

A detailed account of the molecular species identified in the synthesis of alcohol hemiformal H2S scavengers was recently published by the authors. This study focused on the molecular mass and structural characterization of the ultimate products of the scavenging process but was unable to identity the initially formed molecular species upon reaction with hydrogen sulfide. It was conjectured that the reaction of formaldehyde with H2S yielded hydroxymethyl mercaptan but without direct evidence. In this current work, we study the molecular species first present when hemiformal scavengers react with H2S before the formation of a solid polymeric precipitate. The current study of early and later times in the hemiformal/H2S scavenging process showed surprisingly that the first formed species are multi-sulfur dithiols devoid of oxygen. The critical secondary reaction to form a solid polymer is shown here to be the reaction of these dithiols with further formaldehyde released from unreacted hemiformal. This proves hemiformals to be another example of a scavenger whose ultimate reaction products derive from the initial sulfur-containing species reacting with "unreacted"scavenger in the fluid medium. The multi-sulfur dithiols observed likely derive from thioformaldehyde. This transient species is well-known to be highly reactive and can only have been observed by generation followed by trapping this molecular species as a cyclopentadiene Diels-Alder adduct during the photolysis of thietane. The significance of this work is the contribution to understand the structure and mechanism of formation of the (unwanted) solid byproducts. Using the new results presented here on the mechanism of formation and the specific intermediate species involved, better processes can be designed for removal of H2S in both oilfield and other (biogas) systems. In particular, better ways to mitigate the solids that concentrate in field operations can be developed, and this is demonstrated in this paper.