The pyrolysis of 4,4,8,10-tetramethyl decalin and the influence of molecular structures on oil thermal cracking: A ReaxFF molecular dynamics simulation and DFT study

The molecular structure of different hydrocarbons in crude oil has a significant effect on the thermal cracking mechanism. This study delves into the thermal cracking mechanism, pyrolysis process, and kinetics of 4,4,8,10-tetramethyl decalin (a representative bicyclic sesquiterpane in crude oil) using reactive molecular dynamics simulation (ReaxFF–MD) at the temperature range from 2000 K to 3000 K. Interestingly, the major intermediate products from thermal pyrolysis simulation align closely with the main ion fragments in the mass spectrometry of 4,4,8,10-tetramethyl decalin. The results can be directly used to analyze the pyrolysis mechanism. It reveals five reaction pathways implicated in the initial pyrolysis of 4,4,8,10-tetramethyl decalin: three demethylation reactions (more likely to occur) and two ring-opening reactions. By employing first-order kinetic analysis based on ReaxFF–MD simulations within the temperature range of 2000 K to 3000 K, the apparent activation energies for 4,4,8,10-tetramethyl decalin are calculated and compared with our prior simulation results for n-tetradecane. The initial pyrolysis of 4,4,8,10-tetramethyl decalin predominantly entails the cleavage of C–C bonds in both branched chain and carbon ring systems, resulting in the generation of methyl and high molecular weight ring-structured radicals. Conversely, the initial pyrolysis of n-tetradecane yields low molecular weight straight–chain radicals by cleaving C–C bonds except for the terminal position. This implies that the bicyclic sesquiterpane 4,4,8,10-tetramethyl decalin serves as the favorable precursor for producing methane (CH4) and liquid hydrocarbons (C6–C13) during the primary formation stage of light oil. With an equivalent number of carbon atoms, n-tetradecane serves as the preferred precursor for generating wet gases (C2–C5) at the initial formation stage of wet gases. The insights derived from the ReaxFF–MD simulation offer valuable perspectives for comprehending the process of thermal cracking in crude oil under geological conditions.