Selectivity Control in Palladium-Catalyzed Alcohol Oxidation through Selective Blocking of Active Sites

Abstract

stabilized metal nanoparticles (NPs) have received wide interest in a number of liquid-phase catalytic transformations but the role of the capping/protective agent is still debated . Operando attenuated total reflection infrared (ATR-IR) spectroscopy enabled us to obtain unprecedented molecular level insights into the selectivity issue induced by the presence of the protective agent by following the liquid-phase benzyl alcohol oxidation on Pd/Al 2 O 3 . Supported Pd NPs protected by polyvinylalcohol (PVA) showed a lower rate of benzaldehyde decarbonylation compared to unprotected Pd nanoparticles and as a result, an improved selectivity toward the aldehyde. In addition, also the further oxidation of benzaldehyde to benzoic acid was reduced by the presence of PVA. In combination with considerations on adsorption site occupancy from CO adsorption, we ascribed this behavior to a selective blocking operated by PVA especially of Pd(111) facets, which are active in the decarbonylation process of benzaldehyde during benzyl alcohol dehydrogenation. metal nanoparticles (e.g. Pt, Pd and Au NPs) have been widely investigated as catalysts.1,4 Extensive efforts have been undertaken to study the correlation between the structure of the active site and the catalytic performance. This feature assumes a primary importance in structure sensitive reactions, where the size and morphology of metal nanoparticles has a strong influence on the catalytic process. In these cases a single atom behaves differently depending on its position within the particle structure. Therefore, from this perspective, the rational design of morphology-and size-controlled nanostructures becomes increasingly important for producing very active and especially, very selective catalysts.

Capping Agent Effect on Pd-Supported Nanoparticles in the Hydrogenation of Furfural

“…Supported Pd nanoparticles were synthesized by the sol-immobilization method [27]. Two aqueous solutions, one containing the metal precursor (Na 2 PdCl 4 , 1 mL of a 1.26 × 10 −4 mol L −1 solution) and one containing the desired capping agent (1 mL of a 1 wt % solution, capping agents/Pd (w/w) = 1), were added to 100 mL of deionized water.…”

“…It was also observed that supported Pd nanoparticles (NPs) protected by PVA led to higher selectivity to benzaldehyde in the liquid phase oxidation of benzyl alcohol, despite the conversion being lower. This was mainly because of the preferential blocking of Pd (111) facets, which have been recognized to facilitate the decarbonylation process [27]. Indeed, Rogers et al proposed that the interaction between the solvent and PVA alters the PVA binding on the metal surface.…”

Untangling the Role of the Capping Agent in Nanocatalysis: Recent Advances and Perspectives

“…More recently, Campisi et al [68] ascribed the improved selectivity in benzyl alcohol oxidation to the presence of PVA on Pd NPs. Investigating the surface site accessibility through DRIFTS-monitored CO adsorption, they attributed the peculiar catalytic behaviour to the preferential blocking of Pd(111) facets, which have been recognized to promote the decarbonylation process [69] by residual PVA (Figure 7).…”

An investigation into bimetallic catalysts for base free oxidation of cellobiose and glucose

“…Generally, it is considered that the activity is a compromise between particle size of the metal and the shielding effect of the PVA ligand . However, in the case of these catalysts the shielding effect did not seem to contribute, either because the maximum amount of PVA used in these catalyst preparations was below the amount required to cause this effect, there is good diffusion of the glucose through the PVA layer, or the PVA is removed under reactions conditions, an effect that we have reported previously .…”