Hydrocarbon-Induced Agglomeration of Pd Particles in Pd/HZSM-5

Abstract

The deactivation mechanism of Pd/HZSM-5 catalysts in the conversion of methylcyclopentane (MCP) has been studied. Results obtained with EXAFS and TEM show that agglomeration of Pd particles is the dominant cause of catalyst deactivation: no coke deposition is detected by TPO, 13C-NMR, or TEM. In a freshly reduced 0.88-wt% Pd/HZSM-5 catalyst, triatomic Pd particles are stabilized by protons bridging between Pd3 and the cage wall. During MCP reaction at 250°C the Pd3 clusters coalesce, forming much larger Pd particles of about 20 Å. This agglomeration induces significant changes in the product distribution pattern of the isotope exchange between cyclopentane (CP) and D2. The agglomeration is attributed to "de-anchoring," i.e., the replacement of the protons in Pdn-H+-O−z by carbenium ions. This concept is confirmed by exposing the reduced catalyst to NH3: proton neutralization induces formation of 50-Å large Pd particles. Metal agglomeration is much smaller after exposure to CP instead of MCP; this evidence illustrates the crucial role of the tertiary C atom in MCP. It is concluded that the easy formation of a tertiary carbenium ion is essential for extensive deanchoring. The agglomerated Pd particles are still inside the zeolite for a low-Pd-loading catalyst, but for a catalyst with high Pd loading a significant portion of the secondary Pd particles are on the external surface. In this case the distribution of the isotope-exchange pattern of CP + D2 becomes similar to that of PdSiO2.