Microwave-Driven Nonoxidative and Selective Conversion of Methane to Ethylene over Mn-Based Catalysts

Abstract

Recent advancements in microwave-driven nonoxidative catalytic synthesis of C₂H₄ from CH₄ coupling offer a promising, energy-efficient, and eco-friendly alternative to conventional methods, where selective heating under microwave irradiation enables comparable conversions at substantially lower bulk temperatures and shorter reaction times. This study explores the performance of an MnO_X-based catalyst supported on CeO₂ and HY zeolite (silica-to-alumina ratio = 5.1) for the nonoxidative coupling of CH₄ (NOCM) under microwave irradiation. Inspired by the well-established efficacy of MnO_X catalyst in oxidative CH₄ coupling (OCM), their application in NOCM has also shown significant performance. The catalytic system achieved 15% CH₄ conversion and 99% selectivity toward C₂ hydrocarbons and maintained 64% selectivity toward C₂H₄ surpassing the yields reported in the literature even at higher temperatures (700-1000 °C). Catalyst performance was correlated with measurements by in situ Raman spectroscopy, and additional characterizations were performed using H₂-temperature programmed reduction , NH₃-temperature programmed desorption, and BET surface area analysis to understand structural changes during the reactions. These findings suggested that Mn functions as active sites for CH₄ activation in nonoxidative environments while also promoting efficient C-C coupling under microwave irradiation.

1

Schematic illustration of the microwave reactor system and the proposed reaction mechanism for MnO_X/CeO₂ catalytic system for nonoxidative methane coupling.

2

XRD pattern of blank supports, fresh and spent catalysts of (a) MnO_X/CeO₂ and (b) MnO_X/HY.

3

H₂ -TPR profiles of blank CeO₂ and HY supports and MnO_X/CeO₂ and MnO_X/HY catalysts.

4

NH₃-TPD profiles of blank CeO₂ and HY supports and MnO_X/CeO₂ and MnO_X/HY catalysts.

5

TGA of (a) blank CeO₂ and MnO_X/CeO₂ fresh and spent catalysts, (b) blank HY support, and MnO_X/HY fresh and spent catalysts.

6

Ex-situ Raman measurement for (a) fresh MnO_X/CeO₂ (b) spent MnO_X/CeO₂ (c) In-situ Raman measurement for fresh MnO_X/CeO₂ as a function of temperature (d) as a function of reaction time at 700 °C.

7

Optimization of reaction temperature to maximize C₂H₄ selectivity.

8

%CH₄ conversion at various reaction temperatures.

9

Selectivity of C₂H₂, C₂H₄, and total C₂s at (a) 650 °C, (b) 700 °C, and (c) 750 °C.

10

%CH₄ conversion of 5 wt % MnO_X/CeO₂ and MnO_X/HY at 700 °C.

11

% C₂H₄ selectivity and total C₂ selectivity using (a) MnO_X/CeO₂ and (b) MnO_X/HY.

12

Influence of gas hourly space velocity (GHSV) on the performance of MnO_X/CeO₂ catalyst at 700 °C.

13

Influence of gas hourly space velocity (GHSV) on the performance of MnO_X/HY catalyst at 700 °C.

14

Product distribution under varying flow rate conditions for 5 wt % MnO_X incorporated (a) CeO₂ and (b) HY-supported catalysts during NOCM at 700 °C. The effect of flow rate on C₂H₄, C₂H₆, and C₂H₂ selectivity is shown for each catalyst system.