Catalytic Hydrogenolysis of Glycerol to Propylene Glycol over Mixed Oxides Derived from a Hydrotalcite-Type Precursor

Selective hydrogenolysis of glycerol to propylene glycol was performed using an environmentally friendly hydrotalcite-derived mixed-metal oxide catalyst. The Mg/Al, Zn/Al, Ni/Mg/Al, Ni/Co/Mg/Al, and Cu/Zn/Al mixed-metal oxide catalysts were prepared from their corresponding hydrotalcite precursors having M2+/M3+ compositions over the range of 0.5-3.0. The physicochemical properties of the catalysts were studied by X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), NH3 and CO2 temperature-programmed desorption (TPD), and nitrogen adsorption studies. The XRD patterns of pure hydrotalcites exhibited characteristics of hydrotalcite phases, while those of calcined hydrotalcites showed the formation of corresponding metal oxides. The ICP-MS analysis showed agreement between the calculated and actual metal compositions. The prepared catalysts were evaluated for the hydrogenolysis of glycerol to propylene glycol in a Parr reactor. The activity studies indicated a maximum glycerol conversion and selectivity toward propylene glycol in the case of Cu/Zn/Al mixed-metal oxide catalysts. Further, the reaction parameters were optimized with the most active Cu/Zn/Al catalyst, and it was found that at a catalyst concentration of 5% (w/w) of aqueous glycerol, a hydrogen pressure of 200 psig, and 80% glycerol dilution, a maximum glycerol conversion of 52% with 93-94% selectivity toward propylene glycol were obtained.