Advances in carbon extraction have increased the availability of light alkanes as domestic carbon sources, motivating studies into their selective conversion to useful chemicals. Among such transformations are selective oxidations, including methane partial oxidation to methanol and the oxidative dehydrogenation (ODH) of alkanes to olefins. Metalloenzymes can limit overoxidation of methane, resulting in high selectivity to methanol and prompting research into synthetic analogs. However, these materials can contain heterogeneities in their metal species, which complicates the development of structure-function relationships. Square planar iron-containing molecular complexes, such as iron phthalocyanine (FePC), catalyze alkane oxidation and contain similar Fe motifs to metalloenzymes and metal nitrogen-doped carbons (M-N-C). The primary binding sites in these materials contain an Fe center bound to four nitrogen atoms (“M-N4” sites). Encapsulation of FePC within the supercages of faujasite zeolite (FePC@FAU) results in Fe-N4 sites that mimic the structure and functionality of metalloenzymes. This presentation reports the kinetics of liquid-phase cyclohexane oxidation with tert-butylhydroperoxide and gas-phase propane ODH over MPC and M-N-C catalysts with varied M-N4 sites. In both reactions, MPC@FAU and M-N-C samples had ~100× higher initial rates than their analogous metal-free samples. Initial cyclohexane oxidation site-time-yields (STYs; per mol M) were higher over M-N-C compared to MPC@FAU, while samples with Fe active sites had the highest initial rates within each group of samples. Initial propylene formation STYs in propane ODH were similar over M-N-C and MPC@FAU. In both chemistries, the apparent activation energies were consistent with previous reports, and apparent reaction orders were <1 for both MPCs and M-N-Cs. Insights gained over MPC@FAU catalysts were used to develop kinetic models and promote MPC@FAU catalysts as useful model catalysts. Techniques and workflows presented herein provide guidance into periodic trends in reactivity that motivate studies of these metals in M-N-Cs that inevitably lack uniform, well-defined active sites.
