Solid-state exchange of palladium in zeolite NaX

Abstract

The migration of metal ions during solid-state exchange was studied by investigating the effect of grinding force and calcination conditions on the incorporation of Pd in zeolite NaX using various Pd salts as precursors. At temperatures lower than 600°C, it was found that PdCl 2 and Pd(NH 3) 4Cl 2 lead to a significant Pd exchange whereas no Pd incorporation takes place with Pd(NO 3) 2 as precursor. The grinding force influences the Pd 2+ exchange occurring in the subsequent calcination procedure. In the case of Pd(NH 3) 4Cl 2/NaX, the grinding also causes a decomposition of the precursor metal salt to Pd(NH 3) 2Cl 2 and a change in the crystal structure as shown by X-ray diffraction. During the calcination of PdCl 2/NaX samples the precursor salt is transformed according to the following scheme: β − PdC l 2 → 400 ° C α − PdC l 2 → 500 ° C , O 2 PdO → 750 ° C P d 0 The Pd species are incorporated in the zeolite in two stages, one occurring at about 400°C via gas phase transport of Pd 6Cl 12 molecules into the supercages followed by a transfer of fragments to the smaller zeolite cages and a second one occurring between 600 and 700°C via surface migration starting from PdO species.