(594e) Elimination of Acetylene from Ethylene Using Pd1@Zeolite

Selective hydrogenation of acetylene is one of the most effective ways to eliminate the small amount of acetylene (about 1%) from the industrial ethylene stream, but it is still challenging to improve the selectivity to ethylene and avoid formation of ethane. One approach for improving selectivity is to use well-defined catalyst active sites with strong and specific affinity for acetylene over ethylene.

Here, we used a one-pot synthesis method to incorporate single-atom palladium sites within the NaA zeolite. EDA(ethylenediamine) was used to chelate with palladium to prevent it aggregates into nanoparticles. CaA zeolite with lager pore size was prepared after NaA was ion exchanged with CaCl₂. X-ray diffraction (XRD) and N₂-physisorption characterizations were performed to characterize the structure of NaA and CaA zeolite. Diffusion of acetylene and ethylene in NaA was investigated using pressure decay measurements during exposure of acetylene and ethylene gases separately. The results indicated that NaA zeolite showed an improved dynamic acetylene/ethylene selectivity. Besides, CO-DRIFTS and C₂H₄-TPD were also conducted to prove whether the palladium is single atom or not.

Compared with Pd/NaA catalysts prepared by incipient wetness method, Pd₁@NaA(palladium within NaA zeolite) exhibited a much higher selectivity when the acetylene conversion is as high as 99%. In addition, Pd₁@4A showed a higher conversion than Pdn@4A without adding chelate. This is probably because the Pd with large particle size block the small diffusion channel. The improved selectivity of Pd₁@NaA is attributed in part to the small pores of the NaA zeolite (0.4 nm pore size) that can allow discrimination between acetylene and ethylene within the catalyst. We also propose that the single atom Pd sites can facilitate the desorption of ethylene generated from acetylene hydrogenation so as to prevent over-hydrogenation to ethane.