Influence of microwave-assisted synthesis temperature on the formation and thermal stability of magnesium phosphate
摘要
This work aims to determine the influence of temperature on the formation of magnesium phosphates during microwave-assisted synthesis. Magnesium phosphates were synthesized from a mixture of basic magnesium carbonate, phosphoric acid, and water. The molar ratio of Mg to P in the initial mixture was set to 1.5, 1.67, and 2.0. The prepared mixtures were placed in a Monowave 300 microwave reactor and synthesized at temperatures ranging from 25 to 200 °C for 2 h. It was determined that newberyite (MgHPO4·3H2O), magnesium phosphate tetrahydrate (Mg3(PO4)2·4H2O), and magnesium phosphate pentahydrate (Mg3(PO4)2·5H2O) can be successfully synthesized using microwave-assisted synthesis. Newberyite was the dominant phase at low temperatures (25–60 °C), forming as a single-phase product at 25 °C and an Mg/P ratio of 1.67. Magnesium phosphate pentahydrate formed at intermediate temperatures (80–140 °C), acting as a metastable intermediate before the formation of magnesium phosphate tetrahydrate and magnesium pyrophosphates. Thermal analysis revealed that synthetic magnesium phosphates were stable up to 200–360 °C, with the formation of amorphous phases at higher temperatures. The final calcination product of newberyite was magnesium pyrophosphate (Mg2P2O7), while both magnesium phosphate tetrahydrate and pentahydrate recrystallized to farringtonite (Mg3(PO4)2. Additionally, in situ XRD experiments were performed, demonstrating the decomposition of synthetic magnesium phosphates over a temperature range of 25–1000 °C.