Catalytic hydrodechlorination of chloroarenes over novel transition metal and carbide catalysts.

摘要

The gas phase hydrodechlorination (HDC) of chlorobenzene (CB){09}and dichlorobenzene (DCB) has been performed over Pd/SiO2 and a Yb-Pd/SiO2 synthesized from a novel organometallic precursor {lcub}(DMF)10Yb 2[Pd(CN)4]3{rcub}infinity, DMF = dimethyl formamide; higher HDC activities/selectivities were obtained for CB and DCB over the bimetallic catalyst. Surface and bulk properties of the bimetallic system were studied by preparing a series of catalysts with the same bimetal composition but by varying the catalyst precursor and preparation procedures i.e. simultaneous or stepwise introduction (Pd↔Yb) of the metals to the support. It was found that Yb acts as a HDC promoter through a surface synergism with Pd where the extent of this promotion is dependent on the nature of the catalyst precursor and sequence of metal(s) introduction to the support. As an extension to this work, a series of silica supported Pd-lanthanide (Ln-Pd/SiO 2, Ln = La, Ce, Sm, Eu, Gd and Yb) and Pd-alkaline earth metal (AEM-Pd/SiO 2, AEM = Sr and Ba) samples were prepared from an analogous organometallic precursor, i.e. {lcub}(DMF)10Ln2[Pd(CN) 4]3{rcub}infinity and {lcub}(DMF)10AEM2[Pd(CN) 4]3{rcub}infinity, respectively. A higher activity/selectivity was obtained in each instance (CB and DCB, HDC) over the bimetallic with greater resistance to deactivation (due to chloride poisoning, sintering or coking). The pre- and post-reaction catalyst samples have been characterized in terms of BET area, temperature programmed reduction (TPR), TEM-EDX, H2 chemisorption/temperature programmed desorption (TPD), XRD and XPS. The promotional effect of Ln in Ln-Pd/SiO2 is attributed to surface Pd/Ln synergism resulting in an enhancement of surface reactive hydrogen and a more effective C-Cl bond activation for hydrogenolytic attack. We associate the promotional effect with AEM-Pd/SiO2 to a surface Pd/AEM synergy that enhances Pd dispersion with a resultant increase in H2 chemisorption capacity while electronic effects play a key role in C-Cl bond activation for hydrogen scission. HDC kinetics have been studied in detail for reaction over Pd/SiO2 where the effects of altering hydrogen partial pressure and temperature on HDC activity/selectivity have been considered with a view to process optimization. The feasibility of applying Group VI transition metal (Mo and W) carbides and nitrides as HDC catalysts has also been addressed where HDC performance is correlated with critical catalyst characterization results.