Scheme 1. Dehydroaromatization of limonene to p-cymene under a hydrogen atmosphere.

Fig. 1. (a) TEM micrograph of Pd nanoparticles protected by PVP-K90 in water before catalysis (scale bar = 20 nm). Nanoparticles were synthesized according to the method described in Section 2.2.1. (b) Histogram showing the particle size distribution from (a). (c) TEM micrograph of Pd nanoparticles after the first run of recycling experiment, as described in Section 2.4. Scale bar = 20 nm. (d) Histogram showing the particle size distribution from (c). (e) TEM micrograph of Pd nanoparticles stabilized by PVP-K15 after reaction under 2 bar H₂ at 180 °C for 3 h, stirred at 500 rpm, [Pd]=5×10⁻³ mol/L, limonene/catalyst = 50/1 mol/mol. Scale bar = 20 nm.

Scheme 2. Proposed mechanism of limonene conversion over (a) heterogeneous Pd/SiO₂ and (b) soluble Pd catalysts.

Fig. 2. Product distributions for limonene conversion as a function of temperature. Reaction conditions: Reactions were carried out in an autoclave in water containing Pd nanoparticles under 2 bar H₂ for 5 h, stirred at 500 rpm. Further reaction parameters and experimental data are given in Table S1, entries 1–5.

Conversion of limonene using soluble Pd nanoparticles under different conditions^a

Conversion of limonene using hydrogen acceptors and/or using different Pd nanoparticle preparation methods^a