The catalytic hydrogenolysis of readily available glycerol to 1,2-propanediol (1,2-PD) and 1,3-propanediol (1,3-PD), which provides a new promising synthesis route to produce propanediols, has been extensively studied in the past decades. This paper summarizes the most significant reports regarding the glycerol hydrogenolysis into propanediols. Three reaction routes including the ones working towards 1,2-PD production and the recently proposed one leading to 1,3-PD production have been summarized. The catalysts used for this reaction have been classified into two categories according to the types of the metal components: the transition metal catalysts taking Cu, Ni, and Co as representative metal components and the noble metal catalysts containing Ru, Pt, Ir and Ag. Some inexpensive transition-metal catalysts exhibit high 1,2-PD selectivity and yield under mild reaction conditions, while several noble-metal catalysts are promising in synthesizing the more 2 valuable 1,3-PD. Efficient preparation methods and precise mediation techniques have been systematically developed to synthesize functionalized catalysts on the basis of the metal species in combination with acidic or basic compounds. Other technological aspects such as hydrogen sources, reaction solvents, reactor types and feeding processes were also summed in this paper. The focus of this review is on summarizing the preparation methods and the performance of various catalysts in glycerol hydrogenolysis.

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Gold nanoparticles on a number of supporting materials, including anatase TiO 2 (TiO 2 -A, in 40 nm and 45 μm), rutile TiO 2 (TiO 2 -R), ZrO 2 , Al 2 O 3 , SiO 2 , and activated carbon, were evaluated for hydrodeoxygenation of guaiacol in 6.5 MPa initial H 2 pressure at 300 °C. The presence of gold nanoparticles on the supports did not show distinguishable performance compared to that of the supports alone in the conversion level and in the product distribution, except for that on a TiO 2 -A-40 nm. The lack of marked catalytic activity on supports other than TiO 2 -A-40 nm suggests that Au nanoparticles are not catalytically active on these supports. Most strikingly, the gold nanoparticles on the least-active TiO 2 -A-40 nm support stood out as the best catalyst exhibiting high activity with excellent stability and remarkable selectivity to phenolics from guaiacol hydrodeoxygenation. The conversion of guaiacol (∼43.1%) over gold on the TiO 2 -A-40 nm was about 33 times that (1.3%) over the TiO 2 -A-40 nm alone. The selectivity of phenolics was 87.1%. The products are mainly phenolic compounds with no aromatics and saturated hydrocarbons such as cyclohexane. The gold particle size ranging from 2.7 to 41 nm and water content were found to significantly affect the Au/TiO 2 -A-40 nm catalyst activity but not the product selectivity. The reaction rates of 0.26 and 0.91 (min −1 g-cat −1 cm 3 ) were determined for guaiacol hydrogenation and catechol hydrogenation, respectively. Bimolecular methylation was established as the dominant mechanism for methyl group transfer among the phenolics. Two major pathways of guaiacol hydrogenation to phenolics over the 0.4Au-19 nm/TiO 2 -A-40 nm are proposed: (1) direct hydrogenation of guaiacol to form phenol and methanol, (2) hydrodehydroxylation of catechol intermediate from the transmethylation between guaiacol and phenol.

Supported Cu-containing bimetallic catalysts were prepared and used to convert glycerol to propanediols. The effects of supports, metals, metal loadings, and impregnation sequences were examined. A synergistic effect was observed between Cu and Ag when they were impregnated on g-Al 2 O 3 . Characterizations revealed that the addition of Ag not only resulted in an in situ reduction of CuO, but also improved the dispersion of the Cu species on the support. A CuAg/Al 2 O 3 catalyst with optimal amounts of Cu and Ag (Cu/Ag molar ratio 7 : 3, 2.7 mmol Cu+Ag per gram of g-Al 2 O 3 ) showed a near 100% selectivity to propanediols with a glycerol conversion of about 27% under mild reaction conditions (200 • C, 1.5 MPa initial H 2 pressure, 10 h, (Cu+Ag)/glycerol molar ratio of 3/100). Compared with a commercial copper chromite catalyst commonly used for this reaction, the CuAg/Al 2 O 3 catalyst had much higher activity and did not need a reduction pretreatment.