(344i) HY Zeolite Supported Molecular Iridium Catalysts: A Comparison between Single-Site and Pair-Site Catalysts on Zeolites

Highly dispersed metal catalysts on zeolites are economically preferred and offer appealing catalytic properties towards many industrial applications. Although these catalysts have drawn broad attentions, the structure-performance relationship is difficult to conclude, because many of such catalysts are lack of uniformity and undergo structural changes during catalysis.

In this study, we precisely synthesized iridium single-site and pair-site catalysts supported on dealuminated HY zeolites using organometallic precursors. The initial structures of the catalysts after synthesis is characterized by infrared (IR) and extended X-ray absorption fine structure spectra (EXAFS), showing highly uniform single-site and pair-site structures. In-situ spectroscopies were used for monitoring structural changes of the two supported iridium catalysts under various gas environment, including CO or H₂ at temperatures ranging from 298 K to 353 K. We used ethylene hydrogenation as a model reaction and compared the catalytic performance of supported iridium pair-sites with supported single-site Ir species and supported tetrairidium clusters on HY zeolites. The results showed that the pair-site iridium catalysts have a magnitude higher initial activity in ethylene hydrogenation than that of their single-site analogs. On the other hand, the highly unsaturated pair-site structure shows quicker deactivation than tetranuclear clusters. The study offers fundamental knowledge and new chemistry associated with zeolite supported molecular catalysts with different nuclearities.