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 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 [PdCl2(CH3CN)2] to prepare both Pd-NCs
and 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 from
2 to 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=3574h−1
and TOF=4757h−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.