Subnanometric palladium zeolite catalysts improve selectivity in alkyne semihydrogenation

Alkyne semihydrogenation to alkenes yields polymer-grade olefins and fine chemicals. However, over state-of-the-art palladium catalysts, such as Lindlar catalysts, this reaction entails complex steps, low atom efficiency and high toxicity due to Pb used for partial poisoning. Because these factors hamper further applications of alkyne semihydrogenation, developing single-metal catalysts with low Pd content without compromising their selectivity may bring both economic and environmental benefits. In this study, we report on the development of such catalysts by downsizing Pd species to nanoclusters (NCs) and, ultimately, to single atoms (SAs) while stabilizing them on layered zeolite carriers. On layered aluminosilicate zeolites MCM-56 and ZSM-5, Pd-SAs, NCs and nanoparticles (NPs, for comparison) were formed by ion-exchange with [PdCl 2 (CH 3 CN) 2 ] to prepare both Pd-NCs and Pd-SA and by impregnation with [Pd(NH 3 ) 4 ](NO 3 ) 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 from 2-10 nm NPs (e.g., 76% 1-octene selectivity at 98% conversion) to NCs smaller than 1 nm (e.g., 1-octene selectivity 92% at 96-98% conversion). In addition, Pd-SAs and Pd-NCs on a layered ZSM-5 support (TOF=3574 h -1 and TOF=4757 h -1 , respectively) outperformed the commercial Lindlar catalyst (TOF=335 h -1 ). These findings demonstrate that supporting Pd-NCs and SAs on 2D zeolites paves the way for greener and more cost-effective alkyne-selective hydrogenation catalysts. • Aluminosilicate layered zeolites stabilize subnanometric Pd species. • Pd subnanometric clusters are more selective than nanoparticles in semihydrogenation. • Pd clusters are one order of magnitude more active than Lindlar catalyst. • Support hydrophilicity likely influences activity and selectivity.