Alumina-supported palladium modified by zeolite Socony Mobil-5 for highly efficient selective hydrogenation of acetylene in excess ethylene

Abstract

Catalytic selective hydrogenation of acetylene plays an indispensable role during polymer-grade ethylene manufacturing. However, highly-efficient removal of acetylene while avoiding overhydrogenation poses a huge challenge, especially in the front-end configurations containing excess H₂ and C₂H₄. To address the issue, our work aims to construct isolated active sites by modifying alumina-supported palladium (Pd/Al₂O₃) with Zeolite Socony Mobil-5 (ZSM-5) characterized by uniform micropores, which would spatially separate active metals without shielding them. A series of Pd/Al₂O₃@mZSM-5 composites were synthesized by adjusting the mass ratio of Pd/Al₂O₃ to ZSM-5 (corresponding to m in Pd/Al₂O₃@mZSM-5, where m = 1:1, 1:2, 1:3). The modification effect is confirmed by various characterizations. In situ diffusion reflectance infrared Fourier transform spectra (DRIFTS) of carbon monoxide indicates that surface Pd species in Pd/Al₂O₃@1:3ZSM-5 are efficiently segregated by the ZSM-5 framework, forming isolated Pd active sites with a partial negative charge. As expected, Pd/Al₂O₃@mZSM-5 composites exhibit much-improved selectivity toward ethylene compared with that of Pd/Al₂O₃. Moreover, the higher the amount of ZSM-5, the greater the selectivity. Notably, Pd/Al₂O₃@1:3ZSM-5 with an ultralow Pd loading of 211 ppm demonstrates exceptional selectivity and extraordinary hydrogenation activity within a broad operating temperature window of 125–250 °C. Specifically, it achieves near-complete conversion at 125 °C and delivers an outstanding specific activity of 8552 mol_C2H2 mol_Pd⁻¹ h⁻¹ at 100 °C, the highest value ever reported under comparable selectivity. Additionally, control experiments using amorphous silica (aSiO₂) and mesoporous Santa Barbara Amorphous-15 (SBA-15) as structural analogs of ZSM-5 highlight the irreplaceable role of microporous architecture in modifying Pd/Al₂O₃ to optimize the performance of acetylene semihydrogenation. According to in situ C₂H₄-DRIFTS, relative to Pd/Al₂O₃, the intensity of 2σ-bonded C₂H₄ is significantly suppressed on Pd/Al₂O₃@1:3ZSM-5 due to the isolated Pd active sites, which accounts for its excellent selectivity.