Halogenated Cobalt Bis-Dicarbollide Strong Acids as Reusable Homogeneous Catalysts for Fatty Acid Esterification with Methanol or Ethanol

The most commonly used homogeneous catalyst for fatty acid esterification is a corrosive sulphuric acid. However, this requires costly investment in non-corrosive equipment, presents a safety risk, is time consuming, and increases effluent generation. In this study, inorganic 3D heteroborane cluster strong acids are employed for the first time as homogeneous catalysts. Three novel isomeric tetrachlorido and tetrabromido derivatives of 3,3'-<i>commo</i>-bis[undecahydrido-<i>closo</i>-1,2-dicarba-3-cobaltadodecaborate](1-) [<b>1^-</b>] were synthesised and fully characterised using a range of analytical techniques, including NMR, TLC, HPLC, MS, UV-Vis, melting point (MP), CHN analyses, and XRD. Ultimately, H₃O[8,8'-Cl₂-<b>1^-</b>] was identified as the most efficient, reusable, and non-corrosive homogeneous catalyst for the esterification of four fatty acids. The reactions are conducted in an excess of alcohol at reflux. The effective absorption of water vapour provided by the molecular sieves maximises acid conversion. The hydrophobic dye Sudan black B was employed as an acid-base indicator to facilitate a comparison of the <i>H</i>₀ acidity function of sulphuric acid and halogenated heteroboranoic acids when dissolved together in methanol. The ²³Na NMR analysis demonstrated that the application of dry methanol resulted in the displacement of Na⁺ ions from zeolite, which subsequently exchanged the H₃O⁺ ions of the acid. This process led to a gradual reduction in the efficiency of the catalysts, particularly with repeated use. The solution to this issue is to regenerate the catalyst on the ion exchanger following each reaction. In contrast to the published methods, our new approach meets 10 of 12 green chemistry principles.