Thermal Behavior of Potassium C₁–C₁₂ <i>n</i>‑Alkanoates and Its Relevance to Fischer–Tropsch

Abstract

The thermal behavior of potassium C₁–C₁₂ <i>n</i>-alkanoates (K-carboxylates) were studied over the temperature range <i>T</i>/K = (243 to 873). A number of problems in industrial Fischer–Tropsch facilities were attributed to these compounds, but this study also revealed some beneficial effect that may directly be related to the thermal behavior of potassium methanoate. The unusually low melting point of potassium methanoate, <i>T</i>/K = (442.2 ± 0.3), combined with its thermal stability to <i>T</i>/K ≈ (693), may explain the ease of distribution of the potassium promoter of iron-based Fischer–Tropsch catalysts during synthesis even when potassium promoter is added separately. The C₂–C₁₂ K-carboxylates were all thermally stable at temperatures <i>T</i>/K ≤ (713), and significant mass loss was not observed at <i>T</i>/K ≤ (748). The thermal stability and high melting point of potassium propanoate, <i>T</i>/K = (636.9 ± 0.3) and potassium butanoate, <i>T</i>/K = (623.1 ± 0.3), in particular caused these compounds to be prone to cause pressure drop problems in refining units. The C₄–C₁₂ K-carboxylates melted to a liquid crystal phase first, before clearing at higher temperature. One or more solid–solid transitions were observed in all of the K-carboxylates, with the exception of potassium hexanoate