Triethylamine hydrochloride, also known as triethylamine hcl, has a faint, characteristic amine odor. Its aqueous solution is weakly acidic; when heated, it dissociates into triethylamine and hydrogen chloride. It is commonly used as a protonating agent in organic synthesis and as an intermediate for phase-transfer catalysts, as well as in the synthesis of pharmaceuticals, dyes, and pesticides, and for the preparation of buffer salts in chromatography.

Triethylamine-based catalysts for the melt polymerization of carbonate monomers

Aliphatic polycarbonates are promising biomaterials due to their nonacidic degradation products and customizable properties. By combining specific monomers, the hydrophobicity, degradability, viscoelasticity, and other properties of polycarbonates can be easily tailored. Commercially available monomers include trimethylene carbonate (TMC), neopentylene carbonate (NPC), and 5-benzyloxytrimethylene carbonate (BTMC), and a wide assortment of other carbonate monomers have been synthesized for use in specific applications. Two new catalysts for melt polymerizations of carbonate monomers were identified and compared to the widely used SnOct2 and catalyst-free polymerizations. The purified TMC was used to conduct a series of polymerizations to determine whether trace impurities from the monomer synthesis might be responsible. These polymerizations included catalyst-free conditions as it has been reported that TMC can polymerize without catalyst at temperatures at or above 120 °C. Unexpectedly, the addition of triethylamine hydrochloride (TEA·HCl), which is a by-product of the ethyl chloroformate ring-closing reaction commonly used to prepare cyclic carbonates, proved sufficient to catalyze the complete polymerization of TMC in 20 min at 135 °C. This result was especially interesting as triethylamine hydrochloride is a solid, so it would have a number of advantages as a catalyst including ease of handling and stability.  It is the first reported use of triethylamine hydrochloride as a catalyst for carbonate polymerizations and the first reported use of TEA as a catalyst for melt polymerizations. Both proved capable of catalyzing carbonate polymerization and achieved faster TMC conversion than the widely used SnOct2 catalyst under most conditions tested.[1]

The combined catalytic and initiatory activity of triethylamine hydrochloride is especially noteworthy for two reasons. First, the prevalence of the ethyl chloroformate ring-closing reaction21 in the synthesis of carbonate monomers means that TEA·HCl is a potential impurity in many carbonate monomers, which could lead to undesired auto-polymerization if monomers are not carefully purified. Second, for applications where the decrease in end group fidelity and a corresponding decrease in molecular weight is acceptable, triethylamine hydrochloride has several advantages as a catalyst, including ease of handling and availability. Solid catalysts for carbonate polymerizations are rare, but are easy to mix with the other solid reagents prior to heating and eliminate the safety precautions required to work with volatile toxic or flammable liquids. In addition, TEA proved capable of catalyzing the melt polymerization of TMC at 65 °C, which was too cold for triethylamine hydrochloride catalyzed, SnOct2 catalyzed, or catalyst-free polymerizations to proceed in a reasonable timeframe. These conditions could allow melt polymerization using thermally sensitive end-groups that cannot survive traditional SnOct2 catalyzed ring-opening polymerizations at 110+ °C.

The solubility of triethylamine hydrochloride in ten pure solvents

As a significant additive of lithium-ion batteries, Vinylene carbonate can be synthesized by the reaction of chloroethylene carbonate and triethylamine, along with the generation of a large amount of triethylamine hydrochloride as the byproduct. TEA·HCl was separated from the reaction system by using the standard solid–liquid separation techniques. Undoubtedly, a proportion of solvent and Vinylene carbonate were left on the solid. Currently, triethylamine hydrochloride is neutralized by sodium hydroxide in order to recover triethylamine . However, the neutralization process could have been more environmentally friendly because it produced a lot of waste water whose COD (chemical oxygen demand) and salt concentration were higher and released enormous amounts of carbon dioxide. With the rapid development of the new energy industry, the demand for Vinylene carbonate is increasing yearly, bringing more significant environmental pollution. As a result, the neutralization process runs counter to the concepts of carbon neutral and carbon peaking. Therefore, it is meaningful work to crystallize triethylamine hydrochloride in order to obtain high-purity products. The solubility of TEA·HCl in Methanol and aqueous solutions have been reported in the literature but not in other organic solvents. Moreover, the data have not been correlated, so the corresponding thermodynamic analysis data are unavailable. Therefore, it will be essential for the design and control of the crystallization process to systematically determine the solubility of triethylamine hydrochloride in a wide variety of solvents.[2]

The solubility of Triethylamine hydrochloride in Ethylene carbonate (EC), Propylene carbonate (PC), Dimethyl carbonate (DMC), Vinylene carbonate (VC), Dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), Methyl isobutyl ketone (MIBK), Methanol, 2-Propanol, 1-Octanol were measured from 293.15 K to 348.15 K under ambient pressure via a static method. Subsequently, the solubility data were correlated with the modified Apelblat Equation, the Van't Hoff Equation, the  Equation, and the NRTL model. The average relative deviation (ARD), and root-mean-square deviation (RMSD) were introduced to evaluate the correlated data. The findings indicated that, out of the four models, the modified Apelblat model offered the highest correlation. Moreover, using the Van't Hoff Equation, we determined the Gibbs energy, enthalpy, and entropy of triethylamine hydrochloride dissolution in each solvent examined, all of which point to a dissolving process that is endothermic and entropy-driven. In the temperature range studied, the solubility of TEA·HCl in selected solvents increased with the rising temperature, and the solubility sequence of triethylamine hydrochloride at 298.15 K was Methanol > VC > 2-Propanol > EC > 1-Octanol > DMSO > PC > DMF > MIBK > DMC. In order to understand how TEA·HCl dissolves, density functional theory (DFT) was used to calculate its molecular electrostatic potential (MEP) and solvation free energy. In the meantime, the KAT-LSER model for studying the solvent impact has been presented. This work will provide valuable data references for the purification of TEA·HCl by crystallization.

References

[1]Chesterman, J. P., & Amsden, B. G. (2016). Triethylamine-based catalysts for the melt polymerization of carbonate monomers. Polymer Chemistry, 7(45), 6958–6964. https://doi.org/10.1039/C6PY01248E

[2]Teng, J., Zong, F., Zhang, Z., Wang, L., Sun, X., & Xiang, S. (2023). Measurement, correlation, and analysis of the solubility of triethylamine hydrochloride in ten pure solvents. Journal of Molecular Liquids, 390(Part B), 123040. https://doi.org/10.1016/j.molliq.2023.123040