One of the first studies related to glycerol hydrogenolysis was developed by Montassier et al. [40] in the late 1980s. They suggested that over Ru/C catalyst glycerol is first dehydrogen­ated to glyceraldehyde on the metal sites. Next, a dehydroxylation reaction takes place by a nucleophilic reaction of glyceraldehyde with water or with adsorbed — OH species. Finally, hydrogenation of the intermediate yields 1,2-PDO (Figure 9). The main controversial point of this mechanism is the initial dehydrogenation step, which is thermodynamically unfa­vored due to the high hydrogen pressures used [41]. Therefore, in order to shift the equili­brium, glyceraldehyde dehydration should be faster than glycerol dehydrogenation. Otherwise glyceraldehyde would be hydrogenated back to glycerol on the metal sites. Sev­eral authors observed that the addition of a base notably increased glycerol conversion, and this was related to the fact that bases enhance glyceraldehyde dehydration [42—44]. It is inter­esting to point out that when glycerol hydrogenolysis is carried out under alkaline condi­tions, marginal 1,3-PDO selectivities are measured.

Apart from 1,2-PDO, other products stemming from C-C bond cleavage were also report­ed when glycerol hydrogenolysis is conducted under alkaline conditions; mainly, ethyl­ene glycol (EG), methanol and methane. It is suggested that glyceraldehyde can either undergo dehydration or retro-aldolization reactions. The so formed intermediates are hy­drogenated in the last step to yield the products of C-C bond cleavage. Because both the glyceraldehyde dehydration and glyceraldehyde retro-aldol reaction are catalyzed by OH-, the addition of a base increases the glycerol reaction rate but does not improve the selec­tivity to 1,2-PDO [45].