Subnanometric palladium zeolite catalysts improve selectivity in alkyne semihydrogenation

ABSTRACT Alkyne semihydrogenation to alkenes yields polymer-grade olefins and fine chemicals. However, over state-ofthe-art palladium catalysts, such as Lindlar catalysts, this reaction entails complex steps, low atom efficiency andhigh toxicity due to Pb used for partial poisoning. Because these factors hamper further applications of alkynesemihydrogenation, developing single-metal catalysts with low Pd content without compromising their selectivitymay bring both economic and environmental benefits. In this study, we report on the development of suchcatalysts by downsizing Pd species to nanoclusters (NCs) and, ultimately, to single atoms (SAs) while stabilizingthem on layered zeolite carriers. On layered aluminosilicate zeolites MCM-56 and ZSM-5, Pd-SAs, NCs andnanoparticles (NPs, for comparison) were formed by ion-exchange with [PdCl2(CH3CN)2] to prepare both Pd-NCsand Pd-SA and by impregnation with [Pd(NH3)4](NO3)2 to prepare Pd-NPs. Over these catalysts, 1-octyne and 2-methyl-3-butyn-2-ol semihydrogenation showed increased alkene selectivity when downsizing Pd particles from2 to 10 nm NPs (e.g., 76 % 1-octene selectivity at 98 % conversion) to NCs smaller than 1 nm (e.g., 1-octene selectivity92 % at 96–98 % conversion). In addition, Pd-SAs and Pd-NCs on a layered ZSM-5 support (TOF=3574h−1and TOF=4757h−1, respectively) outperformed the commercial Lindlar catalyst (TOF=335 h−1). These findingsdemonstrate that supporting Pd-NCs and SAs on 2D zeolites paves the way for greener and more cost-effectivealkyne-selective hydrogenation catalysts.