Non-oxidative methane conversion by Fe single site catalysts: quantifying temperature limitations imposed by gas-phase pyrolysis

Abstract

Methane can be directly converted into higher-value chemical species through the non-oxidative conversion of methane (NOCM). Methane to olefins, aromatics, and hydrogen (MTOAH) by an Fe single-site catalyst confined in silica (Fe-SSC) was reported to achieve sustainable methane conversion without experiencing catalyst deactivation by coke. In order to quantitatively investigate the potential of coke-free NOCM by Fe-SSC, we present a methane activation surface mechanism on Fe-SSC, based upon density functional theory (DFT) calculations. This surface microkinetic mechanism was combined with a detailed gas-phase mechanism. The developed MTOAH model was used to investigate temperature limitations, specifically the temperature region where Fe-SSC is effective. At temperatures above ∼1200 °C, gas-phase pyrolysis dominates, and the role of the catalyst is diminished. At temperatures below ∼800 °C, the single-pass conversion is too low to be practical. At intermediate temperatures, the catalyst succeeds in methane activation. A broad parametric analysis suggests that this narrow operable temperature window is a fundamental limitation of NOCM by Fe-SSC. A temperature and space velocity combination where the catalyst effectiveness is maximized was calculated to be 960 °C and 131 h⁻¹, respectively. However, at these conditions, significant quantities of polycyclic aromatic hydrocarbons (PAH) are produced, which are expected to be precursors to coke formation. Among the desirable coke-free conditions where the PAH level is kept under 500 ppm, a combination of 1010 °C and 1709 h⁻¹ is calculated as an optimal condition for an effective catalyst utilization, but it inevitably lowered the predicted methane conversion. The present study emphasizes the importance of acknowledging the temperature limitations in NOCM by Fe-SSC to achieve effective catalyst utilization and coke-free operations.