Researchers have formulated an innovative technique to extract fluorine from fluorspar by utilizing oxalic acid combined with a fluorophilic Lewis acid in water, all under mild reaction conditions. This advancement allows for direct production of fluorochemicals, including popular fluorinating agents, from both fluorspar and lower-quality metspar, which reduces the dependence on the hazardous supply chain of hydrogen fluoride (HF).
At present, the production of all fluorochemicals—vital for various sectors—relies on the highly toxic mineral acid hydrogen fluoride (HF). This acid is generated through a high-energy procedure where naturally occurring fluorspar (CaF2) is treated with concentrated sulfuric acid in extreme conditions. Despite rigorous safety measures, there have been instances of HF leaks that have led to fatalities and significant environmental harm.
A research team in Oxford has recently shown that the fluorine in high-grade fluorspar (>97% CaF2) can be extracted in water under mild conditions, using a fluorophilic Lewis acid and oxalic acid as a Brønsted acid. This builds on their previous research involving the solid-state activation of fluoride through mechanical energy.
Their new method for liquid-phase reactions can be adjusted based on the Lewis acid employed. By using boric acid, they produce an aqueous solution of tetrafluoroboric acid, which has been successfully utilized in Balz-Schiemann chemistry. When silica is substituted for boric acid, this scalable method that operates at room temperature creates an aqueous solution of hexafluorosilicic acid, which can then be transformed into commonly used nucleophilic fluorinating agents like potassium fluoride and tetraalkylammonium fluoride salts.
This research marks a significant shift in producing fluorochemicals from fluorspar and lower-grade metspar, as the method does not depend on the complicated supply chain associated with the dangerous hydrogen fluoride (HF). Given the ongoing projects aimed at producing oxalic acid inexpensively from CO2 and biomass, this technique could serve as a viable substitute for the conventional HF production method reliant on sulfuric acid.
Dr. Simon Immo Klose, previously at the University of Oxford and now at Columbia University (USA), and a lead author of the study, remarks:
“The main challenge in using fluorspar as a source of fluoride is its high stability and poor solubility. Unlike table salt, which dissolves easily in water, only a small amount of fluorspar will dissolve in the same quantity of water. However, by consistently extracting a tiny amount, we can dissolve large quantities of calcium fluoride even under mild conditions despite its low solubility.”
Dr. Anirban Mondal from the Department of Chemistry at the University of Oxford, also a lead author, adds:
“Accidents related to HF remind us of the dangers inherent in traditional fluorochemical production. With our approach, we can obtain all the standard fluorinating agents directly from fluorspar without depending on HF and its perilous supply chain. It’s incredibly gratifying to contribute to a team addressing such a practical and immediate issue with a solution that can have a positive impact.”
Calum Patel, also previously from the University of Oxford and now at FluoRok (UK), another lead author, comments:
“Our initial mechanochemical method presented in 2023 [Science] introduced a new reagent that was investigated for its potential as nucleophilic fluoride. We have adopted a notably different yet complementary strategy to convert fluorspar into recognized industrial fluorinating agents. Remarkably, fluorspar can be activated in water at low temperatures through the combined efforts of oxalic acid and a Lewis acid, allowing us to access a wider array of fluorochemicals from fluorspar without having to manufacture HF, including structurally varied fluoroarenes used in agrochemical synthesis.”
Prof. Véronique Gouverneur FRS, from the Department of Chemistry at the University of Oxford who spearheaded this research, states:
“For decades, we have sought ways to use CaF2 directly for fluorination chemistry without needing HF production. This study marks an essential advancement because the developed protocol is straightforward and doesn’t require specialized equipment, making it applicable in various academic and industrial settings. This also helps reduce carbon emissions by avoiding HF production and promotes local supply chains.”
