Thermodynamic and kinetic studies on OH-involved photo-decarboxylation mechanism for waste cooking oils to biofuels

Decarboxylation process of waste cooking oils (WCOs) has been convinced to be one of the most effective methods to produce biofuels from biomass, but the high yield of correlative decarboxylation reactions used to occur under high temperature and pressure. Now, the moderate photo-decarboxylation was bursting tremendous potentials for solving the extreme situation problems, and the photoinduced active radicals (OH/h+) played the key roles in photo-decarboxylation process. However, the key step of OH-involved photo-decarboxylation has been still ambiguity. Here, we investigated the OH-involved decarboxylation reaction process using DMol3 module based on density functional theory, and the methyl stearate (MS) was chosen as the model reaction for further understanding the MS-decarboxylation controlling step and reaction mechanism. The calculated structures, thermodynamics and kinetics revealed that the MS-decarboxylation belonged to two-OH-induced reaction. With the first OH reaction, methyl hydrogen carbonate (HO(CO)OCH3, MHC), as the most stable intermediate, would exist when OH attacked the adjacent carbon of carbonyl site (C3) of MS. Then, the MHC reacted slowly with the second OH, as the controlling step of MS-decarboxylation processes. Our results indicated that the improved decarboxylation rate of MHC could accelerate the subsequent reactions in OH-involved decarboxylation processes. Finally, OH− attacking MS process was introduced for comparison. Our findings should provide new fundamental insights into OH action mechanism in photo-decarboxylation process for WCOs to biofuels.