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., C16 in dehydroisophytol (DIP) (to isophytol (IP)) vs C1 in 2-methyl3-butyn-2-ol (MBY) (to 2-methyl-3-buten-2-ol (MBE)). The Pd NPs were synthesized via colloidal technique with poly(N-vinyl2-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 (SIP; XDIP=98% ca. 95%) was observed over smaller (2.1−3.0 nm) Pd NPs while ca. 98% selectivity to MBE (SMBE; XDIP=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 Pdplane 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.