The mechanism for H2S scavenging by 1,3,5-hexahydrotriazines explored by DFT

Abstract

Hydrogen sulfide, H2S, stand as one of the major issues in petroleum industry. Aqueous solutions of triazines are the most utilized non-regenerative H2S scavenger. Although they present a theoretical capture capacity of 3 mol of H2S per mole of triazine, it effectively scavenges only 2 mol. Herein, we employed Density Functional Theory at CAM-B3LYP/6–311++G(2d,2p) level of calculation to rationalize the full mechanism for capture of H2S by hexahydro-1,3,5-triazines, in particular the reason why the reaction stops at the second equivalent of H2S. For the capture of the first equivalent of H2S, we found an unprecedented SN1 pathway to be less energetic than SN2, the most commonly accepted in literature. For capture of a second H2S molecule, however, both mechanisms are energetically similar. High barriers were found for the capture of the third molecule of H2S, related to the lower electrophilicity of the carbon atom bonded to nitrogen and sulfur atoms.