KINETIC MODELING OF THE PYROLYSIS AND OXIDATION OF PROPYLENE GLYCOL

Abstract

The widespread use of e-cigarettes has attracted significant attention among scientific research communities due to the health risks and toxicity of the e-liquid constituents. Propylene Glycol (PG), which is a key e-liquid ingredient, can undergo thermal degradation and may lead to the formation of dangerous carcinogens. The aim of this research is to investigate the pyrolysis and oxidation of PG using computational chemistry and molecular dynamics (MD). Validation of the simulation results was carried out against experimental work previously executed at AUB’s CSTP laboratory, where PG pyrolysis and oxidation products were analyzed using analytical techniques. Chemical reaction pathways, along with Arrhenius parameters–activation energy and pre-exponential factor–were determined via ReaxFF-MD simulation, and from thermal and kinetic energy calculations using Amsterdam Modeling Suite (ReaxAMS). These showed that formaldehyde (FA) was the dominant product of PG pyrolysis, with other products being acetaldehyde (AA), propanal (PA), acetone (Ace), glyoxal (GA) and methylglyoxal (MGA). PG decayed completely at a rate with activation energy (Ea) = 1.46E+05 J/mol and pre-exponential factor (A0) = 1.86E+12 s−1. Total aldehydes formed at a rate of Ea = 1.45E+05 J/mol and A0 = 1.03E+12 s−1. Individual kinetic rate constants for each product formation were found and compared against experimental results. Relatively low errors in Ea values were obtained, ranging from 2% to 25%. Similar to the experimental results, the Ea of PA formation was the lowest (preferred reaction) indicating that PA was formed most readily. However, more FA was detected since it formed from other aldehydes decaying (besides PG) in the system. For PG oxidation simulations, PG underwent complete pyrolysis, and its products then decayed via cracking and/or oxidation. PG decayed at Ea = 2.35E+05 J/mol and A0 = 4.25E+12 s−1, aldehydes then formed at Ea = 2.94E+04 J/mol and A0 = 3.17E+10 s−1 , and finally decayed with an Ea value of 2.29E+05 J/mol and A0 value of 2.30E+08 s −1. Furthermore, ReaxAMS was used to study key elementary reactions. The first step of pyrolysis, the dehydration of PG to Propylene Oxide (PO), was endergonic, while further conversion reactions of PO were exergonic. The favored reaction with the lowest activation energy was the formation of prop-1-ol from PO. This is significant since prop-1-ol then decayed to form PA and FA, further justifying the abundance of these two products. The findings in this study constitute a critical step in putting forward a comprehensive mathematical model for predicting toxicants in e-cigarette emissions.