Duality of supercritical water pressure on phenol gasification studied by metadynamics simulation and experimental

The gasification of phenol can be accelerated by the use of supercritical water (SCW) during the reaction, but continuously increasing the SCW pressure past a certain threshold will eventually inhibit this reaction. This paper reports the first use of metadynamics simulations to explain this mechanism by describing the waterline structure and fine structure of the phenol dimer better than reactive force field (ReaxFF). Discontinuous experiments showed that when increasing the SCW pressure from 23.5 MPa to 25.5 MPa, the reaction rate increased by 31 %, and when the pressure continued to increase to 29.5 MPa, the reaction rate decreased by 87 %, and then the reaction rate increased again at very high pressure. The simulation of single phenol molecule showed that the underlying reason that supercritical water promoted the gasification of phenol was the formation of waterline structures, which accelerated H transfer, disrupted of C structures, and polarized water molecules, thereby accelerating the gasification reaction. Multiple phenol molecules simulations and experiments showed that excess pressure drove dimer formation, the dimer also reduced the active sites of the benzene ring. A higher pressure formed a more complex dimer structure, which required more energy to destroy. Same law as the experiment, when the density of the supercritical water state in which the multiple phenol molecules is located rises from 0.706 g/cm 3 to 0.868 g/cm 3 , the energy barrier to reach the transition state rises 94 kacl/mol. Metadynamics simulations allowed for a more detailed description of the state of the SCW and its oxidation mechanism, thus describing how the environment influenced the reaction mechanism.