Effects of supports and promoter Ag on Pd catalysts for selective hydrogenation of acetylene

SiO ₂, α-Al ₂O ₃, γ-Al ₂O ₃, ZrO ₂ and CeO ₂ were used as supports and Ag as promoter to study their effects on Pd catalysts for selective hydrogenation of acetylene. The catalysts were prepared by impregnated synthesis and characterized by XRD, BET and TEM. The catalytic reaction was carried out in a fixed-bed reactor. Overall, the low specific surface area supports were better to increase the ethylene selectivity at high conversion rate of acetylene. Among the four Pd catalysts on low specific surface area supports, the catalyst on low specific surface area SiO ₂(LSA-SiO ₂) retained a high ethylene selectivity even at complete conversion, while the other catalysts showed significant decrease in the selectivity at complete conversion. The performance of Pd/LSA-SiO ₂ was important to decrease the loss of ethylene in selective hydrogenation of trace acetylene in ethylene. Addition of Ag to Pd/LSA-SiO ₂ significantly decreased the formation of ethane, C ₄ alkenes and green oil, and improved the ethylene selectivity to 90% when Pd:Ag=1:1 and 1:3(ω). When the ratio of Pd to Ag was above 1, the activity of Pd-Ag bimetallic catalyst was similar to that of Pd monometallic catalyst, and the selectivity of ethylene increased with increasing of amount of Ag. When the ratio of Pd to Ag was below 1, the activity of bimetallic catalyst decreased with increasing of amount of Ag, while the selectivity of ethylene was kept unchanged. The optimum temperature was 200~230°C for 0.02%(ω)Pd-0.02%(ω)Ag/LSA-SiO ₂ to give a high ethylene selectivity and low formation of green oil.