How to increase the selectivity of Pd-based catalyst in alkynol hydrogenation: Effect of second metal

The development of selective Pd-based catalyst for semi-hydrogenation of dehydroisophytol (DIP), a C₂₀-alkynol, is reported. A series of unsupported mono- (Pd) and bimetallic (Pd-M) nanoparticles (NPs) with Pd/M molar ratios of 1.5 - 5.0 (M = Ag or Cu) were examined as model systems. The Pd-Ag and Pd-Cu NPs with controlled crystal sizes of ∼3.3 nm were prepared by Ag (Cu) electroless deposition on pre-formed poly(N-vinyl-2-pyrrolidone)-stabilized Pd⁰ colloids. Bimetallic Pd-Ag NPs adopted a core (Pd)-shell (Ag) structure whereas a mixed alloy was attained in the Pd-Cu nanocrystals. A (partial) Pd surface segregation induced by reaction with H₂ was in evidence for the Pd-Ag NPs. A significant increase in selectivity up to 97% (at X_DIP = 99%) to the target alkenol was demonstrated following the incorporation of a second metal. This result is attributed to the dilution of the surface Pd-sites by Ag (Cu) and a modification of the Pd electronic properties. Pd-Ag NPs, having shown the highest selectivity, were further deposited on a structured support based on sintered metal fibers (SMF) coated with ZnO. The improved selectivity achieved over the unsupported Pd-Ag colloidal NPs was retained over the structured catalytic system. The bimetallic Pd _5.0Ag/ZnO/SMF (S_IP = 93%) demonstrated a drastic increase in IP selectivity relative to the monometallic Pd/ZnO/SMF (S_IP = 78%) and state-of-the-art Lindlar catalyst (S_IP = 62%) at DIP conversion ≥99% with the stable activity during 50 h, suggesting catalyst feasibility for selective semi-hydrogenation of long chain alkynols with important applications in the manufacture of vitamins and fine chemicals.