Continuous production of propylene glycol (1,2-propanediol) by the hydrogenolysis of glycerol over a bi-functional Cu–Ru/MgO catalyst†
A series of Cu–Ru/MgO catalysts were developed for the continuous production of propylene glycol from glycerol in a sectionally packed downflow tubular reactor in the vapor phase. Glycerol conversion and the formation of propylene glycol (1,2-propanediol) were continuously monitored at the outlet of the reactor operated under different reaction conditions. At atmospheric pressure, hydroxyacetone (acetol) was obtained as a major reaction product and at higher pressure (≥0.8 MPa), propylene glycol was found to be the main product. Among all the other catalysts, the 6Cu–4Ru/MgO catalyst demonstrated almost complete glycerol conversion (∼98%) with very high propylene glycol selectivity (∼75%) at a much lower H2/glycerol mole ratio (66.8) and reaction pressure (0.8 MPa). The superior activity of the 6Cu–4Ru/MgO catalyst was due to the copper–ruthenium synergy, highly dispersed (3.6%) Ru metal on the catalyst surface, higher BET surface area (258.6 m2 gcat.−1), and small average particle size (∼3 nm) of the catalyst. It was also perceived that ruthenium acted as a hydrogen storage material and this metal propagated hydrogen spillover to the copper metal, which significantly improved propylene glycol selectivity.