Dynamic Behavior of Pd/P4VP Catalyst during the Aerobic Oxidation of 2-Propanol: A Simultaneous SAXS/XAS/MS Operando Study

The behavior of a Pd(OAc)2/P4VP catalyst submitted to different pretreatments (prereduced, preoxidized, and untreated) during the aerobic oxidation of 2-propanol to acetone in the gas phase has been investigated. Synchronous, time-resolved, SAXS/XAS/MS techniques coupled with operando DRIFT spectroscopy (which gave information on the destiny of the acetate ligands) and ex situ HR-TEM (to detect the formation of Pd nanoparticles and to obtain their size distribution) were employed to accomplish a dynamical picture of the changes occurring to the Pd phase under transient reaction conditions. In addition, the catalytic performances were qualitatively explored by means of a CATLAB microreactor, with the final aim to establish structure-activity relationships. Our approach clearly demonstrates that highly isolated Pd2+ cationic species, either atomically dispersed or in the form of ultrasmall Pd2+-oxo clusters, are efficient and very stable active sites for the gas-phase aerobic oxidation of 2-propanol to acetone. Noticeably, the behavior of the Pd(OAc)2/P4VP catalyst in reaction conditions is influenced by the nature of the support. On one hand, the presence of the pyridyl functional groups is fundamental to stabilize the cationic Pd2+ species; on the other hand, the porous structure of the P4VP polymer efficiently confines the active Pd2+ species in the presence of the reagents. As such, our catalyst is situated at the confluence between its homogeneous and heterogeneous analogues.