Selective Hydrogenolysis of Glycerol to Propylene
Glycol on Supported Pd Catalysts: Promoting Effects of ZnO and Mechanistic
Assessment of Active PdZn Alloy Surfaces
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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<sub>2</sub>) catalysts via an incipient wetness impregnation
method. Glycerol hydrogenolysis turnover rates (normalized per surface
Pd atom measured by H<sub>2</sub> chemisorption) and propylene glycol
selectivity on these PdZn/m-ZrO<sub>2</sub> catalysts depended sensitively
on their Zn/Pd molar ratios, and Zn leaching from the PdZn alloy phases
led to deactivation of PdZn/m-ZrO<sub>2</sub>. Such deactivation was
efficiently inhibited when physical mixtures of Pd/m-ZrO<sub>2</sub> 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<sub>2</sub> concentrations, combined with the primary kinetic
isotope effects (<i>k</i><sub>H</sub><i>/k</i><sub>D</sub> = 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