Selective Hydrogenolysis of Glycerol to Propylene Glycol on Supported Pd Catalysts: Promoting Effects of ZnO and Mechanistic Assessment of Active PdZn Alloy Surfaces

Abstract

Pd catalysts have received increasing attention for selective hydrogenolysis of glycerol to propylene glycol because of their good hydrothermal stability and high selectivity for cleavage of C−O bonds over C−C bonds. Addition of Zn can facilitate glycerol hydrogenolysis to propylene glycol on Pd surface, but the promoting role of Zn, stability of the resulting active PdZn alloys and reaction mechanism remain largely unexplored. Here, we synthesized monoclinic zirconia-supported PdZn (PdZn/m-ZrO 2 ) catalysts via an incipient wetness impregnation method. Glycerol hydrogenolysis turnover rates (normalized per surface Pd atom measured by H 2 chemisorption) and propylene glycol selectivity on these PdZn/m-ZrO 2 catalysts depended sensitively on their Zn/Pd molar ratios, and Zn leaching from the PdZn alloy phases led to deactivation of PdZn/m-ZrO 2 . Such deactivation was efficiently inhibited when physical mixtures of Pd/m-ZrO 2 and ZnO were directly used in glycerol hydrogenolysis, leading to in situ formation of PdZn alloy layers on Pd surfaces with excellent stability and recyclability. Dependence of turnover rates on glycerol and H 2 concentrations, combined with the primary kinetic isotope effects (k H /k D = 2.6 at 493 K), reveals the kinetically relevant step of glycerol hydrogenolysis involving the α-C-H cleavage in 2,3-dihydroxypropanoxide intermediate to glyceraldehyde on PdZn alloys and Pd. Measured rate constants show that the transition state of α-C-H cleavage is more stable because of the stronger oxophilicity of Zn on PdZn alloys than on Pd, which thus facilitates α-C-H cleavage of the Zn-bound intermediate by adjacent Pd on PdZn alloys. Such synergy between Zn and Pd sites accounts for the observed superiority of PdZn alloys to Pd in glycerol hydrogenolysis.

Valorization of Biomass‐derived Small Oxygenates: Kinetics, Mechanisms and Site Requirements of H2‐involved Hydrogenation and Deoxygenation Pathways over Heterogeneous Catalysts

“…The pH of the aqueous environments often dictates the prevalent sequence; a consensus is forming that the acidic environment leads to preferential dehydration of glycerol to acetol while basic or neutral environments favor the dehydrogenation‐initiated pathway to glyceraldehyde (Scheme ). Note that the negative H 2 order measured on PdZn catalysts (almost neutral pH) seems to be consistent with this type of mechanism which perhaps involves quasi‐equilibrated dehydrogenation of adsorbed glycerol to a bound 2,3‐dihydroxypropanoxide species and the kinetically relevant C α ‐H cleavage in this intermediate to form adsorbed glyceraldehyde …”

“…Further, the importance of this direct mechanism seems to vary with the identity of the M 1 phase for a given substrate, in view of the purportedly different kinetic responses to the variation of H 2 pressure in a similar range (1–8 MPa) by different groups. For example, for glycerol hydrogenolysis, Davis and coworkers reported the rate to be proportional to [H 2 ] 0.5 on Pt‐ReO x catalysts, while negative order in H 2 was found on PdZn/m‐ZrO 2 and physical mixtures of Pd/m‐ZrO 2 and ZnO hydrogenolysis; these observations contrast the first order repeatedly found by Tomishige and coworkers and throw the conformation argument by these authors somehow into question. To fully clarify and understand the above discrepancies, a strict synthetic control over the size of the M 2 domain with variation of M 1 identity of a fixed size is required.…”

“…Higher H 2 pressures and glycerol concentrations increased the selectivity to 1,2‐PDO over Cu and Ru catalysts . Higher glycerol concentrations led to improved 1,2‐PDO selectivities and decreased EG selectivities on Cu‐ZnO, pointing to the practical potential of these Cu catalysts for converting realistic concentrated glycerol solutions . Wang et al.…”

“…Sun et al. measured lower ΔG ≠ values for the kinetically relevant step on PdZn alloy surfaces than on Pd and higher adsorption strength (regressed) of 2,3‐dihydroxypropanoxide species on PdZn, leading them to argue that the oxophilic Zn favors the binding of this alkoxide species and exerts a synergy with vicinal Pd sites to stabilize the C α −H cleavage TS from this key intermediate . These authors considered that an electronic effect is unlikely to be responsible for the ten‐fold rate enhancement on PdZn than on Pd, since the electronic state of Pd in the surface alloy actually resembles that of Cu, a much less active metal.…”

“…The measured H/D KIEs with respect to the oxygenate, typically a primary one (ca. 2–3 for glycerol hydrogenolysis,), have not proven very useful in differentiating between the above mechanistic scenarios since a common theme is the activation of C−H (dehydration, dehydrogenation) or C−OH bonds (direct hydrogenolysis, concurrent with C−H formation) which can be affected by the H/D isotopic substitution to commensurate extents. A most thorough review on the selective hydrogenolysis over noble metal+metal oxide modifier catalytic systems was recently composed by Tomishige and coworkers .…”