A direct glycerol hydrogenolysis mechanism was recently proposed by Yoshinao et al. [50]. The experiments were carried out using Rh-ReOx/SiO2 and Ir-ReOx/SiO2 catalysts at 393 K and 80 bar H2 pressure. The low reaction temperature implies that the dehydration-hydroge­nation route was not further possible, due to the endothermic character of glycerol dehydra­tion and the required activation energy, and suggests the energetically more favored direct hydrogenolysis reaction [51]. They suggested a direct hydride ■ proton mechanism. The se­lected catalysts are able to activate hydrogen easily and to form hydride species. It is pro­posed that glycerol is adsorbed on the surface of ReOx clusters to form alkoxide species. Glycerol can form two adsorbed alkoxides: 2,3-dihydroxypropoxide and 1,3-dihydroxyiso-
propoxide; it is suggested that the formation of 2,3-dihydroxypropoxide is preferred as it re­quires a smaller adsorption cross-section than 1,3-dihydroxyisopropoxide [52]. Next, the hydride attack to the 2-position of 2,3-dihydroxypropoxide gives 1,3-PDO, while the hy­dride attack to the 3-position of 2,3-dihydroxyisopropoxide yields 1,2-PDO. The higher se­lectivity to 1,3-PDO obtained (1,3-PDO/1,2-PDO ratio = 2.7) is explained on the basis of the higher stability of the six membered-ring transition state that leads to the formation of 1,3- PDO as compared to the stability of the seven membered-ring transition state that leads to the formation of 1,2-PDO (Figure 11).

A different direct glycerol hydrogenolysis mechanism was established by Chia et al. [53] try­ing to explain the hydrogenolysis of different polyols and cyclic ethers over a Rh-ReOx/C catalyst. They concluded from DFT calculations that the — OH groups on Re associated with Rh are acidic. The acidic nature of ReOx was also reported before [54]. Such acidic Re sites can donate a proton to the reactant molecule and form carbenium ion transition states. In the case of glycerol hydrogenolysis, the first step involves the formation of a carbocation by pro­tonation-dehydration reaction. This carbocation is stabilized by the formation of a more sta­ble oxocarbenium ion intermediate resulting from the hydride transfer from the primary — CH2OH group. Final hydride transfer step leads to 1,2-PDO or 1,3-PDO [53]. The authors also reported that the secondary carbocation is more stable than the primary carbocation. Nevertheless, higher selectivity to 1,2-PDO was obtained (1,3-PDO/1,2-PDO ratio = 0.65).