Toward Sustainable Production of Reduced Phosphorus Compounds

Abstract

Although phosphate is the thermodynamically stable and overwhelmingly abundant form of phosphorus in Nature, the modern phosphorus chemicals industry still relies heavily on white phosphorus (P4) as its central feedstock. In the thermal process, phosphate rock is reduced with coke and silica at temperatures around 1500 °C to produce P4, which is then converted to intermediates such as PCl3 before further conversion to value-added phosphorus compounds. This reaction sequence is effective, but involves the reduction of phosphate under high temperature, followed by additional redox and functionalization steps before many target phosphorus compounds are obtained. We therefore asked whether phosphate-derived feedstocks could be converted more directly into reduced phosphorus compounds, making the production of P4 as an intermediate unnecessary while retaining the synthetic flexibility needed for producing a large variety of phosphorus compounds. That question led to three areas of investigation. The first exploits the strong thermodynamic driving force of Si–O bond formation to achieve oxygen-abstraction reduction of phosphate-derived substrates. This led to the discovery of the bis(trichlorosilyl)phosphide anion, [P(SiCl3)2]− ⁠, a previously unknown P(−3) synthon that can now be accessed from trimetaphosphate, orthophosphate salts, and phosphoric acid. It can be further converted to value-added phosphorus compounds, such as phosphines, hexafluorophosphate, or phosphonium salts. Protonation of [P(SiCl3)2]− gives bis(trichlorosilyl)phosphine, HP(SiCl3)2 ⁠, which undergoes selective radical hydrophosphination and gives synthetic access to primary phosphines, chlorophosphines, phosphinic acids, phosphonic acids, and phosphonium salts. The second strategy uses mechanochemistry to convert condensed phosphates directly to phosphite through hydride phosphorylation. Ball milling with alkali-metal hydrides provides a solvent-free route to a P(III) product while retaining the PO3 framework of the starting material. This route also works with biologically recovered polyphosphate feedstocks, which is important for the recycling of sewage sludge. The same mechanochemical platform further proved useful for the analogous redox-neutral phosphorylation of acetylides, giving alkynyl phosphonates directly from condensed phosphates. Finally, strongly reducing mechanochemical conditions based on supported alkali-metal dispersions gave access to ortho-phosphite, PO33− ⁠, a previously “missing” phosphorus oxoanion. This new anion can be converted further to phosphite salts, tris(trimethylsilyl) phosphite, and alkyl phosphonates. Taken together, these studies show that condensed phosphates and phosphoric acid can be directly used as feedstocks for synthetic phosphorus chemistry, without relying on P4 as an intermediate. This approach enables the identification of reactions that preserve P–O bonds where desirable and creates new opportunities to link phosphorus recovery from waste streams with the synthesis of value-added phosphorus compounds. Determining whether such routes are also more sustainable relative to established manufacturing processes will require a comparative assessment of energy demand, reagent production, waste generation, and process hazards.