A characterization of Ln-Pd/SiO 2 (Ln=La, Ce, Sm, Eu, Gd and Yb): Correlation of surface chemistry with hydrogenolysis activity

Abstract

The gas phase hydrodechlorination (HDC) of chlorobenzene (CB), 1,2-dichlorobenzene (1,2-DCB) and 1,3-dichlorobenzene (1,3-DCB) has been investigated over Pd/SiO 2 and a series of Ln-Pd/SiO 2 prepared from the organometallic precursor{(DMF) 10Ln 2[Pd(CN) 4] 3} ∞ where Ln = La, Ce, Sm, Eu, Gd and Yb; Pd loading = 5%, w/w. Under identical reaction conditions, the following overall sequence of increasing initial fractional dechlorination has been established: Pd/SiO 2 < Yb-Pd/SiO 2 ≈ Sm-Pd/SiO 2 < Gd-Pd/SiO 2 < La-Pd/SiO 2 ≈ Ce-Pd/SiO 2 < Eu-Pd/SiO 2; reaction over Ln/SiO 2 resulted in a negligible conversion. HDC activity declined with time-on-stream but the Ln-Pd/SiO 2 catalysts maintained a significantly higher fractional HDC than Pd/SiO 2; loss of activity is attributed to deleterious HCl/surface interactions. The pre- and post- reaction catalyst samples have been characterized in terms of BET area, TPR, TEM, H 2 chemisorption/TPD, XRD and XPS analyses. When compared with Pd/SiO 2, Pd is present in the Ln-Pd/SiO 2 samples as much smaller particles, while the lanthanide component is finely dispersed over the surface, i.e. Ln is in intimate contact with Pd. The promotional effect of Ln in Ln-Pd/SiO 2 is attributed to a surface Pd/Ln synergism resulting in an enhancement of surface reactive hydrogen and a more effective C Cl bond activation for hydrogenolytic attack. HDC performance is discussed in terms of surface composition, Pd particle size, Ln electronic structure and H 2 uptake/release dynamics.