New route to fluorochemicals: Fluorspar activated in water under mild conditions

A team in Oxford has now demonstrated that the fluorine content of acid grade fluorspar (>97% CaF₂) can be harvested in water under mild reaction conditions in the presence of a fluorophilic Lewis acid and oxalic acid serving as Brønsted acid. This follows previous work from the team into solid-state activation of fluoride using mechanical energy.

Their new aqueous reaction pathway is adaptable, depending on the Lewis acid used. With boric acid, the process affords an aqueous solution of tetrafluoroboric acid, that was successfully applied to Balz-Schiemann chemistry. When silica is used instead of boric acid, this scalable process carried out at room temperature provides direct access to aqueous hexafluorosilicilic acid that can be converted into commonly used nucleophilic fluorinating reagents such as potassium fluoride and tetraalkylammonium fluoride salts.

This work represents a new departure for the production of fluorochemicals from fluorspar, as well as lower-purity metspar, because the protocol does not rely on the complex supply chain of hazardous hydrogen fluoride (HF). Considering current efforts to prepare oxalic acid at low cost from CO₂ and biomass, the method may become a viable alternative to the traditional sulfuric acid-dependent HF pathway.

Dr Simon Immo Klose, formerly at the University of Oxford and now at Columbia University (USA), and one of the lead authors of the study, says:

"The most challenging aspect of using fluorspar as a fluoride source is its high stability and low solubility. Unlike table salt, which dissolves readily in water, only a tiny pinch of fluorspar would dissolve in the same amount of water. But, like pulling a thread to unravel an entire sweater, if we can continually remove this tiny amount, we can dissolve kilograms of calcium fluoride under mild conditions, despite its low solubility."

Dr Anirban Mondal, from the Department of Chemistry, University of Oxford, and one of the lead authors of the study, says:

"HF-related accidents serve as a constant reminder of the hazards involved in traditional fluorochemical manufacturing. With our technology, however, we can directly access all the commonly used fluorinating reagents from fluorspar without having to rely on HF and its hazardous supply chain. It is incredibly rewarding to be part of a team working on such a practical, real-world problem with a solution that will immediately have an impact."

Calum Patel, formerly at the University of Oxford and now at FluoRok (UK), and one of the lead authors of the study, says:

"Our first mechanochemical method disclosed in 2023 [Science] led to a new reagent that was studied for its ability to serve as nucleophilic fluoride. We have taken a significantly different, yet complementary approach to processing fluorspar into well-known industrially important fluorinating reagents. Remarkably, fluorspar can be activated in water at low temperature through the cooperative action of oxalic acid and a Lewis acid. This process enables us to access broader classes of fluorochemicals from fluorspar without the need to manufacture HF, such as structurally diverse fluoroarenes used to synthesise agrochemicals."

Lead author Prof Véronique Gouverneur FRS, Department of Chemistry, University of Oxford, who conceived and led this study says:

"A solution to use CaF₂ directly for fluorination chemistry, without the need for HF production, has been sought for decades. This study represents an important step because the protocol developed in Oxford is easy to implement and does not require specialized equipment. It can therefore be used anywhere in academia and industry, minimising carbon emissions by avoiding HF manufacture and enabling supply localisation."