Synthesis and Characterization of Zeolite-Encapsulated Organometallic Complexes That Catalyze Selective Alkane Oxidation

Methane is an unreactive greenhouse gas that is abundant in shale gas. We hypothesize that metal phthalocyanines (MPCs) encapsulated in faujasite (FAU) zeolites (MPC@FAU) can selectively activate methane and other light alkanes, due to their similarities to metal-containing zeolites and MOFs that perform alkane oxidation with nitrous oxide (~393 K). Prior studies have observed heterogeneity in metal species present on existing catalysts, which renders it difficult to draw conclusions about desirable catalyst traits across varied metal identities without first developing careful active site quantification protocols. MPCs contain divalent central metal atoms bound to nitrogen atoms in square planar geometry. FAU zeolites are crystalline microporous aluminosilicates whose pore intersections form supercages (~1.2 nm diameter); these supercages host MPCs (~1.2 nm diameter), and this predictable structure establishes a controlled environment for experimental and computational catalyst characterization. However, undesired MPCs located on the exterior of FAU (MPC/FAU) are unavoidable during synthesis. We have developed a reproducible washing procedure to remove MPC/FAU species, enabling quantification of encapsulated active sites simply via bulk elemental analyses. Copper perfluorophthalocyanine (CuF₁₆PC), was added to FAU zeolites via both hydrothermal synthesis (CuF₁₆PC@FAU) and post-synthetic deposition (CuF₁₆PC/FAU). X-ray diffraction patterns and micropore volumes measured by nitrogen adsorption demonstrate that the obtained materials are crystalline FAU. Samples were washed by repeated vortex mixing and centrifugation using six different solvents. Using ultraviolet-visible spectroscopy (UV-VIS), the amount of CuF₁₆PC in each supernatant was quantified, along with collection of electron paramagnetic resonance (EPR) and diffuse reflectance UV-VIS spectra of the recovered solids, to identify which solvent(s) removed the CuF₁₆PC/FAU species most effectively and how many washes were required. Ongoing studies of this tunable single atom catalyst platform will allow for determination of desired characteristics of alkane oxidation catalysts without the distractions of an ill-defined distribution of metal species.