Size-Effect of Pd-(Poly(N-vinyl-2-pyrrolidone)) Nanocatalysts on Selective Hydrogenation of Alkynols with Different Alkyl Chains

We have studied the effect of unsupported Pd nanoparticle (NP) size in the selective C≡C semi -hydrogenation of alkynols with different alkyl chains, i.e., C 16 in dehydroisophytol (DIP) (to isophytol (IP)) vs C 1 in 2-methyl-3-butyn-2-ol (MBY) (to 2-methyl-3-buten-2-ol (MBE)). The Pd NPs were synthesized via colloidal technique with poly( N -vinyl-2-pyrrolidone) (PVP) as stabilizing agent where a range of crystal sizes (2.1–9.8 nm; confirmed by HRTEM) was generated. Both reactions show antipathetic structure sensitivity consistent with higher specific activity ( TOF ) over larger Pd NPs where the structure sensitivity effect is more pronounced for NPs ≤ 3.0 nm. All the Pd NPs exhibit high (≥88%) selectivity to the target alkenol product at almost complete (98%) conversion. Increased IP selectivity ( S IP; X DIP=98% ca. 95%) was observed over smaller (2.1–3.0 nm) Pd NPs while ca. 98% selectivity to MBE ( S MBE; X DIP=98% ) is obtained irrespective of particle size. The kinetic results were consistent with a Langmuir–Hinshelwood model. The observed Pd NPs size effect on catalytic response is ascribed to a contribution of Pd electronic surface modifications, fraction of Pd plane active sites and the steric effects which impact akynol/alkenol adsorption constants. The results obtained in this work provide a powerful tool for catalyst design for industrial applications.