Selective Aerobic Oxidation of Benzyl Alcohols with Palladium(0) Nanoparticles Suspension in Water

Abstract

This study concerns one of the rare applications of a suspension of palladium nanoparticles in water for oxidation reactions. The aqueous suspension containing well dispersed nanoparticles of 3.85 nm was obtained following a straightforward procedure involving the reduction of Na 2 PdCl 4 with NaBH 4 in the presence of PVP as stabilizing agent. In the way of oxidative catalytic valorisation of lignin, the aqueous suspension was directly applied as catalytic medium for the selective oxidation of vanillic alcohol into vanillin (80 °C, O 2 , 1 h) with more than 90% yield. Reusability of the catalytic medium has been demonstrated, acting as "quasi-homogeneous catalyst". More sophisticated lignin-derived substrates like veratryl alcohol and hydrobenzoin gave yields of 50-80% to the respective aldehyde and ketone. In parallel, this as-synthesized suspension was directly used to prepare a Pd/TiO 2 catalyst, the latter showing less efficiency for the catalytic transformations.

Investigating (Pseudo)-Heterogeneous Pd-Catalysts for Kraft Lignin Depolymerization under Mild Aqueous Basic Conditions

“…Figure 6 presents TEM pictures. As we previously reported, the average particles of 3a were slightly smaller than 1a, with a narrow average size of 1.55 ± 0.39 nm (due to partial dissolution with acidic pH) [18], and with a homogeneous dispersion on the support (Figure 6a). Pd/TiO 2 3b-c prepared showed larger dispersion of particle size, with an average of 2.04 ± 0.97 nm and 2.42 ± 0.57 nm, respectively, as expected for this preparation method (Figure 6b,c).…”

“…Family 1 includes Pd, Au, and PdAu nanoparticles (NPs), stabilized in water by polyvinylpyrrolidone polymer (PVP). These Pd NPs were prepared according to previously reported procedures consisting of reducing Na 2 PdCl 4 by NaBH 4 in the presence of PVP in water [18]. Several Pd to PVP stoichiometries were evaluated from 5 to 25 mol equiv.…”

“…Three different materials were prepared following common procedures. The first one was obtained by the deposition of preformed Pd NPs Pd-PVP 1a on TiO 2 P25 [18]. Among other possible supports such as ZrO 2 or carbons, TiO 2 has been chosen due to its relatively good hydrothermal stability and ability to stabilize Pd nanoparticles with a narrow size distribution [16].…”

“…One possibility (Route a) involves a single-electron lignin dehydrogenative oxidation step at C γ followed by base-catalyzed retro-aldol reaction giving "so-like coniferyl based-structures", delivering the expected compounds, particularly vanillin in our case [24,25]. From our previous studies [18] and literature, we can also propose that Pd favors the oxidation of the benzylic OH group at C α through the formation of Pd-OC α H under basic conditions, followed by hydride abstraction forming the C α = O group (Route b). Then, a second step, well-supported by quantum chemistry, may involve keto-enol tautomerization before C α -C β bond cleavage, delivering acetovanillone [26].…”

“…Our previous work using supported Pt catalysts [16,17], producing limited yields of aromatic compounds, encouraged us to investigate the behavior of homogeneous systems for the oxidative depolymerization of lignin. We selected Pd as metal due to its high oxidative potential as we previously demonstrated on lignin molecular models [18]. Pd-catalysts have been sparsely studied in oxidative depolymerization.…”