Thermal isomerism and pyrolysis mechanism of α-pinene and β-pinene in a solvent-free solution and ethanol based on density functional theory

Highlights: • The thermal isomerism and pyrolysis mechanism of pinene were calculated by DFT. • Energy barriers of 12 reaction paths and the energy of various isomers were surveyed. • Myrcene to 1-vinyl-3-isopropenyl cyclopentane with regard to R7 is first proposed. • 2-Methyl-1,3-butadiene were formed most probably by the pyrolysis of myrcene. Density functional theory calculation is used to investigate the thermal isomerism and pyrolysis mechanism of α-pinene(1) as important components of turpentine, β-pinene(5), and myrcene(6) in a solvent-free solution and ethanol with 12 reaction paths with the aim of further understanding the reaction mechanism, accelerating the process development, and optimizing the operation of the unit would be helpful. The reaction rates of most of the 12 reaction paths are significantly increased in ethanol. In the reaction paths of R1, R2, and R3 related to the thermal isomerization of α-pinene(1), R1 for the product limonene(2) is the optimal path from the perspective of thermodynamics. Allo-ocimene(4) is more stable than ocimene(3). With regard to the thermal isomerization of β-pinene(5), the energy barriers of R4, R5, and R6 are similar. The energy of the product myrcene(6) is higher than that of pseudolimonene(7), whereas that of limonene(2) is the lowest. The energy barrier of R9 with the product allo-ocimene(4) is higher than that of R4, R5, and R6, making R9 more difficult to react from the perspective of dynamics. The reaction mechanism of the isomerization of myrcene(6) to 1-vinyl-3-isopropenyl cyclopentane(8) with regard to R7 is first proposed. The two isomers for 2-methyl-1,3-butadiene(12) are formed possibly by the pyrolysis of myrcene(6).