Investigation of the depolymerization process of hydrothermal gasification natural rubber with ReaxFF-MD simulation and DFT computation

This study targets the research gap of degradation mechanism in hydrothermal gasification of recycling waste rubber. The depolymerization process of natural rubber in supercritical water, Gibbs free energy of the reaction pathway and the bond dissociation energy of C–C bonds were determined based on reactive force field molecular dynamics (ReaxFF-MD) simulation and density functional theory (DFT). The bond dissociation energy of the C–C bond between two adjacent repeat units of natural rubber monomer was 213 kJ mol −1 , which was the lowest compared with other C–C bonds. The degradation process of the natural rubber in the hydrothermal gasification was divided into two stages from the simulation. In the first stage, each natural rubber molecule was cleaved into a left end molecular, a right end molecular and a large number of isoprene monomers. In the second stage, the deep cracking of a large number of isoprene monomers occurred. Water molecules could stimulate coke hydrocracking in the hydrothermal gasification of the natural rubber. At 2000 K (simulated temperature), a large number of isoprene monomers produced, which could not be further cracked. At 3000 K, the degradation rate increased by about 29% compared with 2500 K. The degradation efficiency of the natural rubber at the feedstock concentration of 11.20 mass% increased by 10.4% compared with that of 27.45 mass%. This work could provide theoretical support and basic data for recycling of waste rubber products by the hydrothermal gasification.