Theoretical study of the dynamics and thermal mechanisms of the reaction: Dehydration of glycerol to glycidol

Investigation of the thermal decomposition mechanism of glycerol: the combination of a theoretical study based on the Minnesota functional and experimental support

The multiple thermal decomposition channels of glycerol are calculated at the M06-2X-D3/6-311+G(d,p) level. In addition, the CAM-B3LYP and ωB97X-D functionals are used to show the functional influence on the free energy barrier. For the highly competitive primary channels, the DLPNO-CCSD(T)/CBS method is applied for the energy calculations. The results show that the dominant paths are: (1) breakage of the C-C, C-O, and O-H bonds of glycerol successively to form carbonyl and alkene, and then generation of water, formaldehyde, and acetaldehyde; (2) glycerol undergoing an intramolecular dehydration reaction and producing 3-hydroxypropionaldehyde; it has two subsequent reactions: ① C-C bond fracture occurring to form formaldehyde, acetaldehyde, and water; and ② intramolecular dehydration forming acrolein and water. The ΔG₁ is 65.6 kcal mol^-1 while the ΔG₂ is 65.5 kcal mol^-1 at 101 kPa and 298 K, and fitted rate equations are 1.09 × 10¹³ exp[65.6 × 10³/RT] s^-1 and 8.07 × 10¹² exp[65.4 × 10³/RT] s^-1, respectively. Besides, UPLC and TG-GC/MS are applied complementarily to investigate the anaerobic pyrolysis products of glycerol at different temperatures. The experimental results are consistent with theoretical calculations.

Anharmonic effect of the unimolecular dissociation of Glycerol to Glycidol

The unimolecular dissociation rate constants of the dehydration of Glycerol to Glycidol were calculated at the MP2/6–311G(d,p) level using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The anharmonic effect of the reactions was examined by comparing the rate constants at temperatures (700–3000[Formula: see text]K) of the canonical case and total energies (25654–53089[Formula: see text]cm[Formula: see text]) of the microcanonical system. The calculations showed that high temperatures are required for the reaction to proceed. As the temperatures and total energies increased, the rate of reactions increased. However, the growth rate of the unimolecular dissociation rate constants was high and slower both in the canonical and microcanonical systems. Comparative analysis showed that the anharmonic effect was most significant for the reaction [Formula: see text] and least significant for the reaction [Formula: see text]. The anharmonic effect became more significant as the temperatures and total energies increased. Compared with the microcanonical situation, the anharmonic effect of the canonical system was more pronounced.

Acid catalyzed first dehydration of glycerol at the secondary site: The effect of glycerol conformation

The first dehydration of protonated glycerol taking place at its secondary site was investigated by density functional calculations by considering different conformations of glycerol. Five parallel reaction pathways via different conformers of protonated glycerol were found. One of these pathways leads to a direct formation of protonated 3-hydroxylpropanal (HPA), another one of these pathways produces protonated glycidol, and the other three produce protonated 3-hydroxy-1,1-propanediol (HPD). One of these pathways producing protonated HPD was found to have obviously larger relative reaction rate than other pathways. The dehydration of protonated HPD to afford protonate HPA requires a rather low reaction barrier (12 kcal/mol). These results show that the production of HPA via a stepwise process with protonated HPD as a key intermediate, is energetically favorable than via a one-step concerted process producing HPA.