Enhanced protection of PDMS-embedded palladium catalysts by co-embedding of sulphide-scavengers

For Pd-containing hydrodechlorination catalysts, coating with poly(dimethyl siloxane) (PDMS) was proposed earlier as promising protection scheme against poisoning. The PDMS coating can effectively repel non-permeating poisons (such as SO₃^2-) retaining the hydrodechlorination Pd activity. In the present study, the previously achieved protection efficiency was enhanced by incorporation of sulphide scavengers into the polymer. The embedded scavengers were able to bind permeating non-ionic poisons (such as H₂S) during their passage through PDMS prior to Pd contact which ensured an extended catalyst lifetime. Three scavenger types forming non-permeable sulphur species from H₂S - alkaline, oxidative or iron-based compounds - were either incorporated into single-layer coats around individual Pd/Al₂O₃ particles or into a second layer above Pd-containing PDMS films (Pd-PDMS). Hydrodechlorination and hydrogenation were chosen as model reactions, carried out in batch and continuous-flow reactors. Batch tests with all scavenger-containing catalysts showed extended Pd protection compared to scavenger-free catalysts. Solid alkaline compounds (Ca(OH)₂, NaOH, CaO) and MnO₂ showed the highest instantaneous scavenger efficiencies (retained Pd activity=30-60%), while iron-based catalysts, such as nano zero-valent iron (nZVI) or ferrocene (FeCp₂), proved less efficient (1-10%). When stepwise poisoning was applied, the protection efficiency of iron-based and oxidizing compounds was higher in the long term than that of alkaline solids. Long-term experiments in mixed-flow reactors were performed with selected scavengers, revealing the following trend of protection efficiency: CaO₂>Ca(OH)₂>FeCp₂. Under field-simulating conditions using a fixed-bed reactor, the combination of sulphide pre-oxidation in the water phase by H₂O₂ and local scavenger-enhanced Pd protection was successful. The oxidizing agent H₂O₂ does not disturb the Pd-catalysed reduction, while the PDMS-incorporated scavenger considerably extends the catalyst life in the presence of H₂S. This work demonstrates that the scavenger-based protection strategy is an effective means to increase the resistance of PDMS-embedded Pd against permeating poisons.