Disproportionation of hypophosphite and phosphite

Area of Science:

• Inorganic Chemistry
• Solid-State Chemistry
• Materials Science

Background:

• The thermal decomposition and disproportionation of phosphorus oxyanions are crucial for understanding their reactivity and potential applications.
• Sodium hypophosphite (NaH2PO2) and sodium phosphite (NaH2PO3, Na2HPO3) are common inorganic compounds with complex thermal behavior.
• Understanding gas evolution and product formation during heating is essential for process safety and material synthesis.

Purpose of the Study:

• To investigate the disproportionation and oxidation reactions of NaH2PO2, NaH2PO3, Na2HPO3, and their mixtures under thermal stress.
• To elucidate the reaction pathways, identify intermediate and final products, and determine the gaseous byproducts.
• To quantify the efficiency of phosphine (PH3) formation during the disproportionation of NaH2PO2.

Main Methods:

• Thermogravimetric Differential Scanning Calorimetry (TG-DSC) for thermal analysis.
• X-ray Diffraction (XRD) for phase identification of solid products.
• Phosphorus-31 Nuclear Magnetic Resonance (31P NMR) spectroscopy for structural analysis.

Main Results:

• NaH2PO2 disproportionation occurs in three steps, producing PH3, pyrophosphates (e.g., Na4P2O7), and H2, with water acting as an oxidant.
• The maximum PH3 yield from NaH2PO2 disproportionation was found to be 40%, lower than the theoretical 50%, due to oxidation by water.
• Na2HPO3 reacts above 450°C or is oxidized to Na4P2O7 by water below 400°C; NaH2PO3 decomposes to PH3 and pyrophosphates.

Conclusions:

• Water plays a significant role as an oxidant in the thermal decomposition of hypophosphites and phosphites, influencing gas evolution and product distribution.
• The formation of pyrophosphates is a primary outcome, with red phosphorus observed as a decomposition product of PH3 at higher temperatures.
• The study provides detailed insights into the complex thermal chemistry of these sodium phosphorus compounds.