(308b) Non-Oxidative Methane Conversion: Coupling Gas-Phase and Surface Models of Fe Single-Site Catalysts

A better utilization of methane gas would lead to increased environmental and economic performance, particularly for resources that are currently flared. Non-oxidative conversion of methane (NOCM) has especially drawn considerable amount of interest for its simplicity and high selectivity. However, coke accumulation has been the biggest problem for NOCM.

A single site catalyst (SSC) may potentially resolve this issue by preventing adsorbate-adsorbate interaction for a coke growth with its sparsely located active sites [1]. In fact, Guo et al. [2] has shown a promising single site Fe catalyst on SiO₂ which showed stable catalytic activity without forming coke. Additionally, contribution from homogenous reactions to NOCM should not be overlooked because of a high NOCM temperature where homogeneous reactions actively occur. Therefore, we have combined SSC chemistry with gas-phase chemistry to model a NOCM process to comprehensively screen conditions for poly aromatic hydrocarbon (PAH) production which is indicative of coke formation.

Materials and Methods

Kinetic parameters of elementary steps for catalytic methane activation on SSC were calculated by ab initio density functional theory (DFT). A single Fe center with two neighboring carbons was modeled in a 2×2 cristobalite supercell. A Grid-Based Projector-Augmented Wave (GPAW) method with revised Perdew-Burke-Ernzerhof (RPBE) exchange-correlation functional was used in Atomic Simulation Environment (ASE). Barrier energies were calculated by finding a transition state with constrained minimization method.

For the gas-phase chemistry, the detailed kinetic mechanism was obtained from a recent literature study on methane pyrolysis to PAH formation [3]. Finally after combining surface chemistry species and gas-phase species into a single input file, Cantera was used to simulate constant pressure, isothermal batch reactor [4].

Results and Discussion

A complete free energy diagram with barrier energies for surface methane activation was obtained. The elementary reactions include methane dissociation, methyl radical desorption, and H₂ dissociation. The methane dissociation steps had the highest barriers.

Coupled heterogeneous and homogeneous reaction simulation was carried out with kinetic information calculated from barrier energies. The simulation was done for various reaction conditions including reaction time, pressure, inlet hydrogen concentration, and temperature to broadly illustrate different combinations of the controllable parameters. This result quantitatively suggests temperature and time windows for maximum ethylene production which is the most valuable product, and for minimum the coke precursor aromatics formation.

This work provides quantitative guidelines to design stable NOCM system which avoids coke formation. The inclusion of accurate gas-phase kinetics allows for systematic investigation of how the ideal catalyst could perform in a NOCM system.

References

1. Zhang, H., Liu, G., Shi, L., and Ye, J. Energy Mater. 8, 1701343 (2018).
2. Guo, X., Fang, G., Li, G., Ma, H., Fan, H., Yu, L., Ma, C., Wu, X., Deng, D., Wei, M., Tan, D., Si, R., Zhang, S., Li, J., Sun, L., Tang, Z., Pan, X., and Bao, X. Science. 344, 616 (2014).
3. Chu, T., Buras, Z.J., Oßwald, P., Liu, M., Goldman, M., and Green, W.H. Chem. Chem. Phys. In press.
4. Goodwin, D.G., Moffat, H.K., and Speth, R.L. Cantera: An object-oriented software toolkit for chemical kinetics, thermodynamics, and transport processes. https://www.cantera.org, 2018. Version 2.4.0. doi:10.5281/zenodo.170284