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)xAEMPd(CN)4}∞ (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 H2 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, H2 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 Cl interactions which impact H2 uptake/release capacity. Although the bimetallic catalysts were less susceptible to deactivation, the samples post-HDC retain an appreciable Cl 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 effect on 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.

Graphical abstractSilica supported Pd/Ba (and Pd/Sr, see Fig.) have been prepared using {(DMF)xBa(or Sr)(CN)4}∞ as precursor. The bimetallics delivered activities that were up to two orders of magnitude greater than that of Pd/SiO2, a response that we attribute to a surface Ba/Pd (and Sr/Pd) synergy that enhances Pd dispersion (and H2 chemisorption capacity) allied to a more effective C–Cl activation for hydrogen attack. Figure optionsDownload full-size imageDownload as PowerPoint slide