N-Heterocyclic Carbene Modified Palladium Catalysts for the Direct Synthesis of Hydrogen Peroxide

Abstract

Heterogeneous palladium catalysts modified by N-heterocyclic carbenes (NHCs) are shown to be highly effective toward the direct synthesis of hydrogen peroxide (H₂O₂), in the absence of the promoters which are typically required to enhance both activity and selectivity. Catalytic evaluation in a batch regime demonstrated that through careful selection of the N-substituent of the NHC it is possible to greatly enhance catalytic performance when compared to the unmodified analogue and reach concentrations of H₂O₂ rivaling that obtained by state-of-the-art catalysts. The enhanced performance of the modified catalyst, which is retained upon reuse, is attributed to the ability of the NHC to electronically modify Pd speciation.

Figure 1

N-Heterocyclic carbenes as ligands in catalysis and their application for the direct synthesis of hydrogen peroxide.

Figure 2

Effect of Pd:IPr ratio on catalytic performance toward the direct synthesis of H₂O₂. (A) Catalytic activity of NHC-modified 1%Pd/TiO₂ catalysts toward the direct synthesis of H₂O₂ and its subsequent degradation as a function of Pd:IPr molar ratio. (B) CO-DRIFTS spectra of the 1%Pd-IPr/TiO₂ catalysts, as a function of Pd:IPr molar ratio. H₂O₂ direct synthesis reaction conditions: catalyst (0.01 g), H₂O (2.9 g), MeOH (5.6 g), 5% H₂/CO₂ (420 psi), 25% O₂/CO₂ (160 psi), 0.5 h, 2 °C, 1200 rpm. H₂O₂ degradation reaction conditions: catalyst (0.01 g), H₂O₂ (50 wt %, 0.68 g) H₂O (2.22 g), MeOH (5.6 g), 5% H₂/CO₂ (420 psi), 0.5 h, 2 °C, 1200 rpm.

Figure 3

Comparison of catalytic performance toward the direct synthesis of H₂O₂ in addition to a structural and morphological analysis of the 1%Pd/TiO₂ and 1%Pd-IPr(1:1)/TiO₂ catalysts. (A) Catalytic activity as a function of reaction time and (B) over sequential H₂O₂ synthesis reactions. Key: 1%Pd/TiO₂ (blue triangles); 1%Pd-IPr(1:1)/TiO₂ (black squares). TEM micrographs of the (C) 1%Pd/TiO₂ and (D) 1%Pd-IPr(1:1)/TiO₂ catalysts. XPS spectra of (E) Pd(3d) and (F) N(1s) regions for (i) 1%Pd/TiO₂ and (ii) 1%Pd-IPr(1:1)/TiO₂ catalysts. Key: for the Pd 3d spectra Pd²⁺ (purple), Pd⁰ (blue); for the N 1s spectra, imidazolium salt (peach), NHC-Pd moiety (blue). H₂O₂ direct synthesis reaction conditions: catalyst (0.01g), H₂O (2.9 g), MeOH (5.6 g), 5%H₂/CO₂ (420 psi), 25%O₂/CO₂ (160 psi), 0.5 h, 2 °C, 1200 rpm.