Novel one step preparation of silica supported Pd/Sr and Pd/Ba catalysts via an organometallic precursor: Application in hydrodechlorination and hydrogenation

A gas phase hydrodechlorination (HDC) of chlorobenzene (CB) and 1,2-dichlorobenzene (1,2-DCB) has been examined over Pd/SiO2 (prepared by impregnation with Pd(C2H3O2)(2)) and two alkaline earth metal (AEM=Sr and Ba) promoted Pd/SiO2 catalysts prepared from the organometallic precursor {(DMF)(x)AEMPd(CN)(4)}(infinity) (AEM=Sr, Ba; x=4,3). While Sr/SiO2 or Ba/SiO2 exhibited no measurable HDC activity, the bimetallic catalysts delivered specific HDC rates (per Pd metal surface area) that were up to a factor of 20 times higher than that recorded for Pd/SiO2. The initial fractional dechlorination recorded for Sr-Pd/SiO2 and Ba-Pd/SiO2 was up to two orders of magnitude greater than that for Pd/SiO2. We associate this promotional effect with a surface Pd/AEM synergy that enhances Pd dispersion with a resultant increase in H-2 chemisorption capacity allied to a more effective C-Cl bond activation for hydrogen scission. Bulk and surface catalyst characteristics, pre- and post-reaction, have been probed by IR, BET, TPR, H-2 chemisorption/TPD, XRD, XPS and TEM-EDX analyses. While 1,2-DCB conversion over Pd/SiO2 was lower than that observed for CB due to inhibitory inductive and steric effects, CB and DCB reactivity were comparable over AEM-Pd/SiO2. Each catalyst exhibited a temporal decline in HDC performance that we link to deleterious CI interactions which impact H-2 uptake/release capacity. Although the bimetallic catalysts were less susceptible to deactivation, the samples post-HDC retain an appreciable CI content with a redispersion of both AEM and Pd components and a disruption to the surface electronic characteristics that is apparent from the XPS profiles. The presence of AEM had no effecton benzene hydrogenation performance over freshly activated samples but post-HDC, Pd/SiO2 exhibited depleted hydrogenation activity whereas both bimetallics (notably Sr-Pd/SiO2) generated a significantly enhanced hydrogenation response that we ascribe to a surface restructuring that is beneficial for aromatic reduction. (C) 2008 Elsevier B.V. All rights reserved.