Insight into the Mechanism of Glycerol Dehydration and Subsequent Pyridine Synthesis

In the present study, glycerol was exploited as the feedstock to synthesize pyridine with ammonia gas as a carrier and reactant through thermal conversion. A density functional theory (DFT) at the M06-2X method was applied to understand the mechanism of glycerol dehydration, the ammonization of oxygenated compounds, and the condensation of imines. The results confirmed that glycerol could be directly converted into pyridine in ammonia atmosphere at 550 °C. The overall view of the thermal conversion mechanism of glycerol was compared with the previous experimental data and the proposed mechanisms, which indicated that the neutral glycerol dehydration process should mainly produce acrolein, acetaldehyde, formaldehyde, and acetol. The produced oxygenated compounds (acrolein and acetaldehyde) can react with ammonia to form imine, which would further undergo Michael addition, a Diels-Alder reaction, deammonization, and dehydrogenation to form pyridine. In a catalytic condition, ZSM-5 not only plays a shape-selective effect on the conversion of glycerol to pyridine but also changes the path of the reaction. The structure of ZSM-5 limits the formation of multisubstituted pyridine products, which is beneficial to the formation of pyridine and monosubstituted pyridine. Protonated vinylamine intermediates may be a critical step limiting pyridine yield and selectivity. The kinetic analysis that is based on transition state theory was consistent with product contribution in experiments. The present study confirmed the selectivity and the conversion route of glycerol to pyridine.