Metal Phthalocyanines Encapsulated in Faujasite Zeolites for Gas-Phase CO Oxidation

Existing metal-containing porous catalysts have inherent heterogeneity in metal species, rendering it difficult to compare reactivity across varied catalyst formulations without first developing active site quantification protocols. The supercages of faujasite zeolites (FAU) are large enough to confine metal phthalocyanines (MPCs), together serving as a well-defined active center for experimental and computational catalyst characterization. Deviations in zeolite synthesis conditions from prior literature were required to obtain phase-pure FAU. Metal perchloro-, perfluoro-, and perhydrogenated phthalocyanines (MPCCl16, MPCF16, and MPC; M = Cr, Mn, Fe, Co, Ni, Cu, and Zn) were encapsulated into FAU zeolites via hydrothermal synthesis (MPC@FAU) and deposited onto the external surfaces by postsynthetic deposition (MPC/FAU). These MPC@FAU catalysts were tested as catalysts for CO oxidation with dioxygen at 298 K and their reactivity compared to that of silica-supported PdAu nanoparticles and cobalt-nitrogen-doped carbon (Co-N-C). Initial CO2 site time yields were greater than the analogous metal-ion-exchanged zeolites (by similar to 50x). However, this initial activity decreased with time on stream for all MPC samples tested, and the cause of this deactivation is explored herein. Stable CO2 formation rates with time on stream observed over PdAu/SiO2 and Co-N-C suggest that deactivation observed over MPC@FAU samples is distinct and not an artifact of the experimental apparatus. Density functional theory calculations suggest an O-2-activation mechanism, aided by the coadsorption of CO on the pyrrole N of the MPC and an axial ligand that can provide additional electron density to reduce the barrier for O-2 bond breaking; this reaction mechanism is distinct from that over structurally similar metal-nitrogen-doped carbons. Nevertheless, the reactivity of MPC@FAU catalysts for gas-phase CO oxidation with dioxygen at ambient temperature indicates that they may share similar functionality to metal-nitrogen-doped carbons and have the potential to serve as model catalysts for gas-phase chemistries.