Synthesis of polymer protected Pd–Ag/ZnO catalysts for phenylacetylene hydrogenation

Abstract

A simple and environmentally friendly method, based on sequential adsorption of polyacrylamide (PAM) and transition metal ions (Pd 2+ , Ag + ) on zinc oxide precipitated from water solution, was used to synthesize supported mono-and bimetallic catalysts with various Pd:Ag ratios. The catalyst characterization results indicated that PAM and metal ions are completely adsorbed by zinc oxide, forming polymer-stabilized Pd and Ag nanoparticles of 1-3 nm in size, evenly distributed on the support surface. The catalysts were studied in the hydrogenation of phenylacetylene under mild conditions (0.1 MPa, 40°C). Although Ag-free 1%Pd-PAM/ZnO catalyst presents an interesting catalytic performance (in terms of activity and selectivity), the optimal catalyst was 1%Pd-Ag(3:1)-PAM/ZnO, presenting a selectivity to styrene of 88% at 85.4% conversion of phenylacetylene. For comparison, similar Pd-Ag (3:1) bimetallic catalysts modified with polysaccharides such as pectin (Pec), chitosan (Chit) and 2-hydroxyethylcellulose (HEC) were studied in the hydrogenation process. The catalysts demonstrated nearly the same selectivity to styrene. The activity of the catalysts decreases in the following order: 1%Pd-Ag(3:1)-HEC/ZnO > 1%Pd-Ag(3:1)-PAM/ZnO > 1%Pd-Ag(3:1)-Pec/ZnO > 1%Pd-Ag(3:1)-Chit/ZnO.

Polysaccharide-Stabilized Pd-Ag Nanocatalysts for Hydrogenation of 2-Hexyn-1-Ol

“…In our prior studies [53,56] have been shown that interaction of polymers with metal ions on a support surface led to formation smaller active phase nanoparticles (~2 nm) to compare with those in similar unmodified supported catalysts. The transmission electron microscopy (TEM) image of Pd-HEC/ZnO catalyst showed the formation of finely dispersed palladium nanoparticles of ~2 nm evenly distributed on the surface of zinc oxide modified with HEC (Figure 3).…”

“…The nanocatalyst was prepared by adsorption method, according to procedure described in [53,56]. А 5 mL water solution of 0.9×10 -2 M 2-hydroxyethylcellulose (HEC, 0.0192 g in 5 mL of water) was added dropwise to the aqueous suspension of inorganic sorbent (1 g ZnO in 15 mL of water) and stirred for 2 hours.…”

“…The method of polysaccharide adsorption on an inorganic sorbent followed by metal ions deposition was used to prepare 0.5% bimetallic PdAg polysaccharide-inorganic nanocatalysts [53,56]. 5 mL of 0.9×10 -2 M polysaccharide solution (HEC, Chit or Pec) was added to the aqueous suspension of the support (1 g ZnO in 15 mL of water).…”

Understanding Selective Hydrogenation of Phenylacetylene on PdAg Single Atom Alloy: DFT Insights on Molecule Size and Surface Ensemble Effects

“…Previous experimental results on PA hydrogenation on PdAg alloys show that the catalysts exhibit excellent selectivity towards the production of styrene. At low conversion, the selectivity on Pd and PdAg are similar, but the selectivity on Pd tends drop at high conversion, while PdAg remains highly selective even at high conversion of PA. 8,9,43,44,64 The Pd:Ag ratio in the experimental systems is relatively high and it is reasonable to ask, whether ensembles of Pd-atoms on the surface, such as dimers or trimers, play a role in the hydrogenation process. While the surface of PdAg alloys tends to be terminated by Ag atoms, spectroscopy measurements on adsorbed CO on PdAg surfaces have shown that the formation of isolated Pd-sites on a surface take place only on low Pd surface coverage, typically below 0.002 monolayer.…”

“…10,12,41,42 PA hydrogenation on Pd and PdAg alloy catalysts reveals an interesting trend that, while neat Pd demonstrates reasonably high selectivity in PA hydrogenation, alloying Pd with Ag even enhances the selectivity at high conversion. 43,44 However, the Pd-to-Ag ratio in the PdAg alloys tends to be high, which prompts the question of whether active sites are composed of isolated Pd atoms, as occasionally asserted, or if they rather consist of Pd ensembles.…”