Gas‐phase hydrodechlorination of chlorobenzenes over silica‐supported palladium and palladium–ytterbium

Abstract

AbstractA 5% w/w palladium loading on silica has been achieved via impregnation of the support with Pd(C2H3O2)2 and { (DMF)10Yb2[Pd(CN)4]3} ∞ precursors to deliver monometallic (Pd/SiO2) and bimetallic (Yb–Pd/SiO2) catalyst systems respectively. The catalytic action of each has been assessed in the continuous gas‐phase hydrodechlorination (HDC) of chlorobenzene (CB) and 1,2‐dichlorobenzene (1,2‐DCB) (T = 423 K, inlet chlorine/palladium mol ratio of 5 × 103 h−1) and the hydrogenation of benzene (T = 423 K, inlet C6H6/palladium mol ratio of 35 h−1). Activation of both catalysts delivered similar palladium crystallite size distributions with an average palladium diameter of 5–6 nm where the ytterbium component (in Yb–Pd/SiO2) was present as a thin surface coating. The Pd–Yb bimetallic exhibited significantly higher HDC and hydrogenation activities, the former manifested by significantly greater fractional dechlorinations and benzene selectivities/yields. Yb/SiO2 proved inactive in terms of promoting hydrogen scission or addition and the promotional effect of ytterbium in Yb–Pd/SiO2 is discussed in terms of electron donation and hydrogen transfer via surface YbH2. Under identical reaction conditions, a lower HDC activity was recorded for 1,2‐DCB compared with CB, a response that is attributed to steric constraints allied to the deactivating effect of the second chlorine substituent. Both Pd/SiO2 and Yb–Pd/SiO2 exhibited a decline in HDC activity with time‐on‐stream, but the bimetallic was significantly more resistant to deactivation. Copyright © 2003 John Wiley & Sons, Ltd.