Metal Phthalocyanines\nEncapsulated in Faujasite Zeolites\nfor Gas-Phase CO Oxidation

Existing metal-containing porous catalysts have inherent\nheterogeneity\nin metal species, rendering it difficult to compare reactivity across\nvaried catalyst formulations without first developing active site\nquantification protocols. The supercages of faujasite zeolites (FAU)\nare large enough to confine metal phthalocyanines (MPCs), together\nserving as a well-defined active center for experimental and computational\ncatalyst characterization. Deviations in zeolite synthesis conditions\nfrom prior literature were required to obtain phase-pure FAU. Metal\nperchloro-, perfluoro-, and perhydrogenated phthalocyanines (MPCCl<sub>16</sub>, MPCF<sub>16</sub>, and MPC; M = Cr, Mn, Fe, Co, Ni, Cu,\nand Zn) were encapsulated into FAU zeolites via hydrothermal synthesis\n(MPC@FAU) and deposited onto the external surfaces by postsynthetic\ndeposition (MPC/FAU). These MPC@FAU catalysts were tested as catalysts\nfor CO oxidation with dioxygen at 298 K and their reactivity compared\nto that of silica-supported PdAu nanoparticles and cobalt–nitrogen-doped\ncarbon (Co–N–C). Initial CO<sub>2</sub> site time yields\nwere greater than the analogous metal-ion-exchanged zeolites (by ∼50×).\nHowever, this initial activity decreased with time on stream for all\nMPC samples tested, and the cause of this deactivation is explored\nherein. Stable CO<sub>2</sub> formation rates with time on stream\nobserved over PdAu/SiO<sub>2</sub> and Co–N–C suggest\nthat deactivation observed over MPC@FAU samples is distinct and not\nan artifact of the experimental apparatus. Density functional theory\ncalculations suggest an O<sub>2</sub>-activation mechanism, aided\nby the coadsorption of CO on the pyrrole N of the MPC and an axial\nligand that can provide additional electron density to reduce the\nbarrier for O<sub>2</sub> bond breaking; this reaction mechanism is\ndistinct from that over structurally similar metal-nitrogen-doped carbons. Nevertheless, the reactivity of MPC@FAU catalysts\nfor gas-phase CO oxidation with dioxygen at ambient temperature indicates\nthat they may share similar functionality to metal–nitrogen-doped\ncarbons and have the potential to serve as model catalysts for gas-phase\nchemistries.