CHARACTERIZATION OF CE PROMOTED, ZEOLITE-SUPPORTED PD CATALYSTS

Ce-promoted and unpromoted Pd/ZSM5 catalysts were prepared, characteri zed by TPR, XPS and TEM, and tested for the dehydroisomerization of al pha-limonene to p-cymene. At 300 degrees C the conversion is near 100% over all samples, but the selectivity to p-cymene depends on catalyst preparation. Ion exchange in slurries with adjusted pH resulted in ca talysts with much more uniform metal loading than contacting the extru ded zeolite with an aqueous solution of the metal ions. Ce promoted Pd /ZSM5 samples gave higher selectivities to p-cymene than unpromoted Pd /ZSM5. XPS data suggest an interaction between Pd- and CeO2-particles, but hydrolysis of Ce ions to Ce hydroxide gel can lead to formation o f acid sites which catalyze formation of undesired byproducts. Hydroly sis can be minimized by ion exchange at low pH. (C) 1997 Elsevier Scie nce B.V.