Electrochemistry Cracks the P–O Bond: Sustainable Reduction of Phosphates to Phosphorus

Elemental white phosphorus (P4), a precursor for many phosphorus-containing commodity chemicals, is conventionally produced by thermal reduction of phosphate ores in an energy-intensive electrical furnace. In this issue of ACS Central Science, Yogesh Surendranath and co-workers establish electrochemical conditions for the activation of strong P−O bonds and report the alternative synthesis of P4 via electroreduction of phosphate salts in molten electrolytes. Phosphorus plays a critical role in biology (e.g., in the backbone of ribonucleic acids DNA and RNA, and the triphosphate of ATP used in cellular energy transfer), agriculture (e.g., as phosphate fertilizers or the herbicide glyphosate), and the chemical industry (e.g., as detergents, pharmaceuticals, and flame retardants). The principal method for extraction of industrially relevant phosphorus is the “wet process”, where phosphate-containing ores are converted to phosphoric acid. Lower-valent phosphorus compounds are instead produced via the intermediacy of P4. 2 This pyrophoric and phosphorescent solid was first isolated by alchemists in the Middle Ages, who obtained it upon distillation of urine from sand and carbon. Interestingly, this early procedure bears striking resemblance to the modern “thermal (Wöhler) process”, wherein P4 is produced by arc furnace treatment of phosphate ores in the presence of carbon coke (C) and silica (SiO2). Here, the carbon acts as a reductant and the silica as an oxide scavenger. The Wöhler process requires temperatures of up to 1500 °C and consumes between 12 and 15 MW h of electricity per ton of P4 produced. 3 The low energetic efficiency and significant carbon footprint of conventional P4 production have motivated numerous efforts to improve sustainability of organophosphorus manufacture. For instance, Cummins and co-workers showed that several phosphorus fine chemicals can be accessed by treatment of phosphoric acid with trichlorosilane, a process that avoids the use of P4 as a precursor. Several reviews and perspectives have since highlighted emerging methods for the synthesis of organophosphorus compounds starting from greener inputs. A complementary effort involves exploring the possibility that phosphate reduction to elemental phosphorus could be driven using sustainable electricity. Building on a recent proof-of-concept report, Surendranath and co-workers have now developed the tools and conceptual framework necessary to enable efficient and selective electrochemical P4 generation from phosphate melts.


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Article Recommendations E lemental white phosphorus (P 4 ), a precursor for many phosphorus-containing commodity chemicals, is conventionally produced by thermal reduction of phosphate ores in an energy-intensive electrical furnace. In this issue of ACS Central Science, Yogesh Surendranath and co-workers establish electrochemical conditions for the activation of strong P−O bonds and report the alternative synthesis of P 4 via electroreduction of phosphate salts in molten electrolytes. 1 Phosphorus plays a critical role in biology (e.g., in the backbone of ribonucleic acids DNA and RNA, and the triphosphate of ATP used in cellular energy transfer), agriculture (e.g., as phosphate fertilizers or the herbicide glyphosate), and the chemical industry (e.g., as detergents, pharmaceuticals, and flame retardants). The principal method for extraction of industrially relevant phosphorus is the "wet process", where phosphate-containing ores are converted to phosphoric acid. 2 Lower-valent phosphorus compounds are instead produced via the intermediacy of P 4 . 2 This pyrophoric and phosphorescent solid was first isolated by alchemists in the Middle Ages, who obtained it upon distillation of urine from sand and carbon. 3 Interestingly, this early procedure bears striking resemblance to the modern "thermal (Woḧler) process", wherein P 4 is produced by arc furnace treatment of phosphate ores in the presence of carbon coke (C) and silica (SiO 2 ). Here, the carbon acts as a reductant and the silica as an oxide scavenger. The Woḧler process requires temperatures of up to 1500°C and consumes between 12 and 15 MW h of electricity per ton of P 4 produced. 3 The low energetic efficiency and significant carbon footprint of conventional P 4 production have motivated numerous efforts to improve sustainability of organophosphorus manufacture. For instance, Cummins and co-workers showed that several phosphorus fine chemicals can be accessed by treatment of phosphoric acid with trichlorosilane, a process that avoids the use of P 4 as a precursor. 4 Several reviews and perspectives have since highlighted emerging methods for the synthesis of organophosphorus compounds starting from greener inputs. 5,6 A complementary effort involves exploring the possibility that phosphate reduction to elemental phosphorus could be driven using sustainable electricity. Building on a recent proof-of-concept report, 7 Surendranath and co-workers have now developed the tools and conceptual framework necessary to enable efficient and selective electrochemical P 4 generation from phosphate melts.
The role of SiO 2 in the traditional Woḧler process can be understood in the context of Lux−Flood acid−base theory, where SiO 2 is a Lux acid (oxide acceptor) and the Surendranath and co-workers have now developed the tools and conceptual framework necessary to enable efficient and selective electrochemical P 4 generation from phosphate melts.
phosphate starting material is a Lux base (oxide donor). The present authors hypothesized that increasing the Lux acidity of the electrolyte could facilitate reductive cleavage of strong P−O bonds. This is analogous to more familiar electrochemical reactions such as O 2 or CO 2 reduction that are promoted in Brønsted−Lowry or Lewis acidic media. They furthermore proposed that addition of oligomeric phosphates containing phosphoryl anhydrides (P−O−P linkages) could increase the Lux acidity of the molten electrolyte in a tunable fashion. Testing of these ideas required construction of a specialized electrochemical cell capable of operation at 800°C, the temperature used to achieve a molten electrolyte ( Figure 1A). Carbon was found to be suitably inert under these forcing conditions; consequently, graphite rods were used as the working, counter, and pseudoreference electrodes (calibrated vs a Na/Na + reference) whereas the bottom of the cell was composed of a glassy carbon crucible. In a pure sodium trimetaphosphate melt (high Lux acidity), cyclic voltammetry reveals the presence of a diffusion-controlled reduction event at 2.4 V vs Na/Na + ( Figure 1C, black trace), proposed to arise from phosphate reduction. Electrolysis at a slightly more negative potential of 2.1 V vs Na/Na + results in collection of pale-yellow pyrophoric crystals in a downstream cold trap, which were spectroscopically confirmed to be pure P 4 ( Figure 1B). The authors estimate an impressive 95% Faradaic efficiency for the 5e − reduction of trimetaphosphate to P 4 .
Building on this success, the authors investigated additional details of the phosphate reduction reaction as a function of the Lux acidity of the electrolyte. The onset of phosphate reduction shifts to progressively more negative potentials as the Lux acidity decreases ( Figure 1C, black to green traces). Furthermore, measurements of open-circuit potential (OCP) under each of these conditions show a linear dependence on the concentration of phosphoryl anhydrides available to act as oxide scavengers (Figure 1C inset). These results confirm that the thermodynamics for P−O cleavage become more favorable as Lux acidity increases, just as the authors originally hypothesized. These efforts parallel another recent advance in sustainable electrosynthesis of silicon nanowires from molten CaSiO 3 . 8 The cell design and electroanalytical methodologies presented herein serve as a blueprint for subsequent studies, and the fundamental insight correlating P−O bond activation

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This work reminds us of the power of electrochemistry to promote activation of very strong bonds and will no doubt inspire future efforts to explore nontraditional substrates under exotic electrolyte conditions.
with Lux acidity is likely applicable to a vast array of other redox transformations. This work reminds us of the power of electrochemistry to promote activation of very strong bonds and will no doubt inspire future efforts to explore nontraditional substrates under exotic electrolyte conditions.