Synthesis and characterization of novel silica-supported Pd/Yb bimetallic catalysts: Application in gas-phase hydrodechlorination and hydrogenation

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• 作者：Satyakrishna Jujjuri ; Errun Ding ; Elizabeth L. Hommel ; Sheldon G. Shore and Mark A. Keane
• 关键词：Hydrodechlorination ; Pd/SiO₂ ; Silica-supported Yb/Pd ; Chlorobenzene ; Dichlorobenzene ; YbH₂ ; Catalyst deactivation
• 刊名：Journal of Catalysis
• 出版年：2006
• 出版时间：25 April 2006
• 年：2006
• 卷：239
• 期：2
• 页码：486-500
• 全文大小：492 K

文摘

The gas-phase hydrodechlorination (HDC) of chlorobenzene (CB), 1,2-dichlorobenzene (1,2-DCB), and 1,3-dichlorobenzene (1,3-DCB) was investigated over a 5 % w/w Pd/SiO₂ and a series of SiO₂-supported Yb–Pd (5 % w/w Pd and ) catalysts. The Pd/SiO₂ catalyst was prepared by Pd(C₂H₃O₂)₂ impregnation, supported Yb synthesized by contacting SiO₂ with Yb powder in liquid NH₃ and the bimetallic catalysts prepared by two stepwise routes: Pd(C₂H₃O₂)₂ impregnation of Yb/SiO₂ (Pd–Yb/SiO₂-step) or Pd impregnation preceding Yb introduction (Yb–Pd/SiO₂-step) and a single-step simultaneous introduction of Pd and Yb from a {(DMF)₁₀Yb₂[Pd(CN)₄]₃}_∞ precursor (Yb/Pd/SiO₂-sim). Under identical reaction conditions, the following specific initial CB HDC rate sequence was established: Pd–Yb/SiO₂-step (0.21 mol_Cl h⁻¹ m⁻²) ≈ Pd/SiO₂ (0.24 mol_Cl h⁻¹ m⁻²) < Yb/Pd/SiO₂-sim (0.41 mol_Cl h⁻¹ m⁻²) ≈ Yb–Pd/SiO₂-step (0.46 mol_Cl h⁻¹ m⁻²); reaction over Yb/SiO₂ resulted in a negligible conversion. Yb acts as a CB HDC promoter through a surface synergism with Pd; the extent of this promotion depends on the nature of the catalyst precursor and the sequence of metal(s) introduction to the support. The promotional effect of Yb extends to DCB HDC, where Yb–Pd/SiO₂-step outperforms Yb/Pd/SiO₂-sim (with a specific HDC rate 25 times greater than that delivered by Pd/SiO₂) and to catalytic hydrogenation (benzene → cyclohexane). The prereaction and postreaction catalyst samples were characterized in terms of BET area, TPR, TEM-EDX, H₂ chemisorption/TPD, XRD, and XPS measurements. The role of Yb as a promoter is discussed in terms of electron donation and impact on Pd dispersion but is attributed to the action of YbH₂, which serves as an additional source of surface reactive hydrogen; Yb activation of the CCl bond(s) for hydrogenolytic attack is also considered. HDC activity decreased with time-on-stream, an effect that we link to deleterious HCl/catalyst interactions that modify surface composition, leading to a disruption in H₂ uptake/release; XPS and TEM-EDX were used to characterize the residual surface Cl post-HDC.