Electroreduction: A Sustainable and Less Energy-Intensive Approach Compared to Chemical Reduction for Phosphine Oxide Recycling to Phosphine

Although phosphate minerals are the only naturally occurring resources of the industrially important element phosphorus,^[1,2] the industrial extraction of white phosphorous (P₄) from metal phosphates relies on demineralization at high temperatures (ca. 1500 °C) in the presence of coke and it releases large amounts of CO and CO₂.^[3] Due to wide industrial applications of white phosphorus (P₄) as a manufacturing precursor for the production of fertilizers, pesticides, fireworks, and detergents,^[1] a rapid shrinking of natural reserves of this mineral is obvious and thereby demands an efficient recycling strategy. Moreover, organophosphines are versatile phosphine donor ligands in the development of transition metal catalysts for the bulk-scale production of alkenes, functional alcohols, esters, and amines. However, the production of such industrially and commercially important organic compounds is associated with the generation of tons of phosphine oxide per year, a product whose further breakdown is highly energy demanding. This essentially amounts to the need for an economic recycling strategy. At present, lab- and/or industrial-scale recycling of phosphine oxide purely relies on chemical routes using metal hydrides and/or harsh reductive conditions which lead to generation of further reagent wastes. Such harsh reaction conditions and further toxic waste generation demand the development of less energy intensive methods. For the past few years, electro-reduction of phosphine oxides to phosphines remains a pioneering approach to bypass the energy intensive chemical routes. This review summarizes the fundamental studies on electrochemistry of phosphines and phosphine oxides in 1970–1980s and implementation of such knowledge in the development of successful electro-deoxygenation of phosphine oxides in recent times. Research outlook and future scope of this newly developing research area have also been discussed.