Selective Palladium-Catalysed Aerobic Oxidation of Alcohols - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 105

4.2.7 Bimetallic Palladium-Catalysed Selective Oxidation
Incorporation of a second metal in palladium catalysts can improve both
alcohol selox stability and selectivity. Typical promoters such as Ag, Bi, Pb
and Sn 60,114-117 enhance oxidation performance towards challenging
substrates such as propylene glycol 118 and allylic and benzylic alcohols.
Wenkin et al. reported that the rate of oxidation of glucose to gluconates was
increased by a factor of 20 over Pd-Bi/C catalysts (Bi:Pd s ¼ 0.1) versus Pd/C
counterparts. 115 In situ XAS and ATR-IR suggested that Bi residing at the
catalyst surface protects palladium from deactivation by either over-
oxidation (a hypothesis since disproved) 70,71,73 or site blocking by aromatic
solvents. 56 Prati and co-workers first reported significant rate enhancements
and resistance to deactivation phenomena in the liquid phase selox of
D -sorbitol to gluconic/gulonic acids upon addition of Au to Pd/C and Pt/C
materials, 119 subsequently extended to polyols and long-chain aliphatic
alcohols. 120 A strong synergy between Pd and Au centres was also
demonstrated by Hutchings and co-workers, who found that Au-Pd alloy
nanoparticles supported on titania exhibited increased reactivity towards a
diverse range of primary, allylic and benzylic alkyl alcohols compared with
monometallic palladium analogues. 121
The versatility of Au-Pd catalysts has also been shown in selox of saturated
hydrocarbons, 122 ethylene glycol, 123 glycerol 124 and methanol, 125 where high
selectivity and resistance to on-stream deactivation were noted.
The effect of Au-Pd composition has been extensively studied for
bimetallic nanoparticles stabilized by PVP surfactants. 126 An optimal Au:Pd
composition of 1:3 was identified for 3 nm particles towards the aqueous-
phase aerobic selox of benzyl alcohol, 1-butanol, 2-butanol, 2-buten-1-ol and
1,4-butanediol; in each case the bimetallic catalysts were superior to
palladium alone. Mertens et al. examined similar systems utilizing 1.9 nm
nanoparticles and an optimal Au content of around 80% was determined for
benzyl alcohol selox. 127 The synergic interaction between Au and Pd there-
fore appears to be dependent on nanoparticle size. It is well-known that the
catalytic activity of Au nanoparticles increases dramatically with a particle
size o 2nm, 128 hence it is interesting to compare phase-separated and
alloyed catalysts systematically. Lee et al. prepared titania-supported Au shell
(five-layer)-Pd core (20 nm) bimetallic nanoparticles for the liquid-phase
selox of crotyl alcohol and systematically studied the evolution of their bulk
and surface properties as a function of thermal processing by in situ XPS,
DRIFTS, EXAFS and XRD and ex situ HRTEM. 129 Limited Au-Pd alloying
occurred below 300 1C in the absence of particle sintering. Higher
temperatures induced bulk and surface alloying, with concomitant sintering
and surface roughening. Migration of Pd atoms from the core to the surface
dramatically enhanced activity and selectivity, with the most active and
selective surface alloy containing 40 at.% Au (Figure 4.11), in excellent
agreement with the surface chemistry of crotyl alcohol, 130 crotonaldehyde
and propene over Au-Pd(111) single-crystal alloys. 64

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