Thermal stability of MDM and oxidative decomposition mechanism under the condition of air infiltration: A combined experimental, ReaxFF-MD and DFT study

When siloxanes are used as the working fluids of organic Rankine cycle systems, there is a possibility of air infiltration due to the large vacuum present in the condenser. To address this issue, experimental and simulation studies were conducted on the thermal stability and pyrolysis product characteristics of octamethyltrisiloxane (MDM) in the presence of air. The results indicated that air significantly promotes the decomposition of MDM at the temperatures above 250 °C. Air was helpful in increasing the formation of CH2O, CO, and CO2, and also enhanced polymerization of decomposition product of siloxane, leading to higher yields of MD2M, MD3M, MD4M, D3, D4, and D5. By combining reactive force field molecular dynamics (ReaxFF-MD) simulations with density functional theory (DFT) calculations, the microscopic mechanisms of MDM's oxidative decomposition by O2 in air and H2O under humid conditions were elucidated. O2 can easily combine with unsaturated Si atoms to form Si-O bonds, combine with CH3 radicals to form C-O bonds, and undergoes hydrogen extraction reactions to initiate decomposition. H2O mainly undergoes initial decomposition through CH3 radical hydrogenation and binding with unsaturated Si atoms. The generated O2H, O, and OH radicals tend to bind with Si atoms in molecules and radicals, facilitating polymerization reactions.