Thermodynamic Stability Determination of MEA-Dithiazine Polymorphs: An Experimental and Computational Exercise

Abstract Substituted 1,3,5-triazines for hydrogen sulfide (H2S) scavenging have established themselves as reliable and economical solutions in the oil and gas industry. Aqueous MEA-triazine is the most used, non-regenerative scavenger in the industry, and its resulting reaction product, MEA-dithiazine, is a ubiquitous and well-studied molecule. However, despite its storied history as a H2S scavenger, only recently were the two reported crystalline polymorphic forms I and II of MEA-dithiazine identified and structurally compared. Polymorphism is defined as different crystal structures/forms of a chemical compound. Physical and chemical properties, e.g., density, melting point, solubility, and stability, arise from the arrangement of the molecules in the solid-state, and thus, understanding the structure/property relationship between polymorphs is crucial. Herein, two polymorphic forms of MEA-dithiazine are experimentally and computationally explored to understand their solid-state properties better. MEA-dithiazine form I and II were analytically characterized by PXRD, DSC, TGA, FTIR, and 1H NMR, and their thermodynamic stability relationship was investigated through a series of analytical and solution-based experiments. Additionally, periodic and molecular Extended Tight-Binding (XTB) along with Density Functional Theory (DFT) calculations were carried out to reveal minimal energy differences between the polymorphs. Determining the thermodynamic stability of polymorphic forms provides direction for expected future dissolution and conversion behavior. Understanding the structural differences in these solid forms should lead to insights into the mechanisms of their formation, possible prevention, and remediation in routine oil and gas operations.