As stated above, the use of acid or base as a co-catalyst gives 1,2-PDO as a main product. To obtain more valuable 1,3-PDO, the most effective approach has shown to be the use of noble metal (Ir, Rh or Pt) combined with oxophilic metals. Shinmi et al. [52] modified Rh/SiO2 cat­alyst with Re, W and Mo. Re addition showed the largest enhancing effect on catalytic activ­ity and also increased the selectivity to 1,3-PDO. The Rh-ReOx/SiO2 (Re/Rh = 0.5) exhibited 22 times higher glycerol conversion (79%) and 37 times higher 1,3-PD yield (11%) than

Rh/SiO2. In a more recent work, an Ir-ReOx/SiO2 (Re/Ir = 1) catalyst prepared by a similar method to that for Rh-ReOx/SiO2 catalyzed the hydrogenolysis of glycerol to 1,3-PDO in a more effectively way (1,3-PDO/1,2-PDO ratio = 11) [72]. Based on characterization results, the authors suggested that oxidized low-valence Re clusters are attached to the Ir or Rh met­al particles. Glycerol is adsorbed on the surface of MO, species (M = Mo, Re and W) at the OH group to form alkoxide. Hydrogen is activated on the noble-metal (Rh or Ir) surface. The alkoxide located on the interface between MO, and the noble-metal surface is attacked by the activated hydrogen species, and the C-O bond neighboring to the C-O-M group is dis­sociated. The hydrolysis of the resulting alkoxide releases the product (see Figure 11). One of the weak point of these catalytic systems is that they are also active in the further hydro — genolysis of both 1,2 and 1,3-PDO to 1-PO.

In summary, Cu based catalysts are active and selective for the production of 1,2-PDO from glycerol. However, if the aim is to produce the more valuable 1,3-PDO, different approaches are required. The used of noble metals combined with low-valence metal oxide seems to be a promising alternative. Nonetheless, there is still room for improvement; both in catalyst design and in process engineering, as PDOs further hydrogenolysis significantly affect the final yields to target products.