A new method to making fluorine chemical could provide a safer and extra sustainable alternative than present methods by circumventing the need to manufacture and handle with unsafe hydrogen fluoride.
The process, advanced by means of UK researchers, makes use of oxalic acid to reap various fluorochemicals from the mineral fluorspar, which the team says is scalable and works under moderate conditions in water at room temperature.
Fluorochemicals are traditionally derived from the mineral fluorspar, or calcium fluoride, and have wide-ranging applications, together with refrigeration, electric powered automobiles, agrochemicals and pharmaceuticals. Currently, production of fluorine chemicals requires reacting fluorspar with focused sulfuric acid at high temperatures – above 200°C. This produces hydrogen fluoride, that’s used as a feedstock to obtain a number of fluorochemicals. However, hydrogen fluoride is fairly poisonous and corrosive, posing potentially deadly synthesis and dealing with dangers.
Now, Véronique Gouverneur’s lab at the University of Oxford, UK, has advanced a safer process to make numerous commercially important fluorochemicals from fluorspar without having to make hydrogen fluoride or use large amounts of fossil fuel-derived sulfuric acid.
‘The synthesis of fluorochemicals from fluorspar without a reliance on the complex supply chain of hydrogen fluoride is one in all the largest challenges in fluorine chemistry,’ says co-author Anirban Mondal. ‘We have overcome this challenge with methods generating new and regarded fluorinating reagents.’
Setting out to find a moderate alternative to activate fluorspar – to essentially cast off the calcium content and access the fluorine – the team examined numerous acids, landing on a procedure that requires blending three easy-to-take care of solids: fluorspar, oxalic acid and either boric acid or silicon dioxide.
When the three solids were mixed in water at room temperature the oxalic acid reacted with fluorspar to manufacture calcium oxalate, an insoluble salt. Meanwhile, the boric acid or silicon dioxide formed strong boron–fluorine or silicon–fluorine bonds. The calcium oxalate changed into then removed without difficulty through filtration, leaving behind an aqueous solution of either fluoroboric acid or hexafluorosilicic acid.
The team then tested that those solutions could be converted using recognised fluorine chemistries to reap widely used fluorochemicals, such as tetrafluoroboric acid, alkali metal fluorides, tetraalkylammonium fluorides and fluoro(hetero)arenes. ‘This works demonstrates unequivocally that the supply chain of hydrogen fluoride is not vital for the production of fluorochemicals handy upon nucleophilic fluorination,’ says Mondal.
While also the usage of less energy, the technique could also be greater sustainable because oxalic acid can be made from carbon dioxide or biomass, while sulfuric acid required for the traditional approach is mostly derived from fossil fuel manufacturing. Mondal highlights that a undertaking for commercialisation is the modern-day price of oxalic acid, compared with sulfuric acid, however he remains optimistic. ‘Our protocol based on oxalic acid aligns well with the current challenge facing chemical industry along with pathways to decarbonisation and defossilisation,’ he says.
‘We continually think of hydrogen fluoride as the access point to fluorine chemistry, whereas this work demonstrates that there may be extra that may be accomplished with calcium fluoride than previously idea,’ says Alan Brisdon, a fluorine chemist on the University of Manchester, UK. ‘Interestingly, about 25 years ago I wrote an editorial for a fluorine technology in-residence magazine that included speculation that “subjects inclusive of ‘activating’ calcium fluoride … turns into more than just academic interest”.’
However, Brisdon points out that the dimensions of existing hydrogen fluoride manufacturing is large and it’s too early to mention how this new approach may compete. ‘Its overall utility will depend upon what other reagents or chemistry may be developed, but it is an vital step,’ he adds.
