New insight into enhancement effect of supercritical water on scrap tire depolymerization: A study based on ReaxFF-MD simulation and DFT method

The enhancement effect of supercritical water (SCW) on scrap tire (ST) depolymerization was investigated based on structural feature of SCW using the reactive force field (ReaxFF) molecular dynamics (MD) simulation and density functional theory (DFT) method. The results show that SCW acts not only in clusters but as non-bonded H2O monomers during ST depolymerization. Moreover, the fraction of clusters and non-bonded monomers determines the proportion of depolymerization products. The strong mutual attraction between oxygen atoms in SCW and carbon atoms in rubber leads to formation of C-O bonds. This causes a sharp decrease in the cracking energy of contiguous C-C bonds, thereby resulting in complete depolymerization of rubber. The impact of SCW on H-2/CO formation was also analyzed. It is indicated that alkoxy groups (R-n-C=O) in rubber are the major precursors of CO. Microscopic pathways of H-2 formation based on ST depolymerization with SCW were described. The optimal range of water/rubber mass ratio ranged from 4 to 6 for more oil and H-2. The theoretical results presented herein are critical for understanding the practicality of using SCW to convert ST to valuable oil and fuel-gas, as well as provide new insight into chemical processes based on SCW.