Glycerol Hydrogenolysis to Propylene Glycol and Ethylene Glycol on Zirconia Supported Noble Metal Catalysts

Monoclinic zirconia (m-ZrO₂) supported Ru, Rh, Pt, and Pd nanoparticles with controlled sizes were prepared and examined in glycerol hydrogenolysis to propylene glycol and ethylene glycol at similar conversions in the kinetic regime. Their activity (normalized per exposed surface metal atom, i.e., turnover rate) and selectivity depend sensitively on the nature of the noble metals and their particle size. At a similar size (ca. 2 nm), Ru exhibited a greater turnover rate than Rh, Pt, and Pd, and the rate decreased in the sequence Ru ≫ Rh > Pt > Pd by a factor of about 25 (from 0.035 to 0.0014 mol glycerol (mol surface metal·s)⁻¹) at 473 K and 6.0 MPa H₂. Following such activity sequence, Ru was more prone to catalyze excessive cleavage of C–C bonds, leading to the formation of ethylene glycol and methane, while Pd exhibited the highest selectivity to cleavage of C–O bonds to propylene glycol. Similarly, larger Ru particles possessed higher glycerol hydrogenolysis activity concurrently with higher selectivities to ethylene glycol and especially methane at the expense of propylene glycol in the range of 1.8–4.5 nm. Analysis of kinetics and thermodynamics for the proposed elementary steps involving kinetically relevant glycerol dehydrogenation to glyceraldehyde leads to expressions of glycerol hydrogenolysis rate and selectivity to cleavage of C–O bonds relative to C–C bonds. Together with different effects of reaction temperature and atmosphere of H₂ and N₂ on the activity and selectivity for Ru/m-ZrO₂ and Pt/m-ZrO₂, these results suggest that the observed difference for different noble metals and particle sizes can be attributed to the difference in the strength of adsorption of glycerol and glyceraldehyde, derived from their different availability of unoccupied d orbitals