Selective Coke Combustion by Oxygen Pulsing During Mo/ZSM‐5‐Catalyzed Methane Dehydroaromatization

Abstract

Non‐oxidative methane dehydroaromatization is a promising reaction to directly convert natural gas into aromatic hydrocarbons and hydrogen. Commercialization of this technology is hampered by rapid catalyst deactivation because of coking. A novel approach is presented involving selective oxidation of coke during methane dehydroaromatization at 700 °C. Periodic pulsing of oxygen into the methane feed results in substantially higher cumulative product yield with synthesis gas; a H2/CO ratio close to two is the main side‐product of coke combustion. Using 13C isotope labeling of methane it is demonstrated that oxygen predominantly reacts with molybdenum carbide species. The resulting molybdenum oxides catalyze coke oxidation. Less than one‐fifth of the available oxygen reacts with gaseous methane. Combined with periodic regeneration at 550 °C, this strategy is a significant step forward, towards a process for converting methane into liquid hydrocarbons.

Reversible Nature of Coke Formation on Mo/ZSM‐5 Methane Dehydroaromatization Catalysts

“…[6] There is ag rowing consensus that the active centers are monomeric or dimeric molybdenum (oxy)carbidic species which are stabilized inside the zeolite pores, while larger Mo 2 Cs pecies on the external surface are mere spectators. [10] 4) Use of oxygenpermeable membranes for ac ontrolled supply of as mall amount of oxygen for removal of coke species. [8] 2) Adsorptive or oxidative scavenging of hydrogen for the same purpose.…”

Direct Non‐Oxidative Methane Conversion in a Millisecond Catalytic Wall Reactor

“…Ther eaction, however,i sc hallenged by ah igh-temperature endothermic nature,low C 2+ yields,and coke formation. [5] Then oncatalytic route focuses on CH 4 pyrolysis to achieve high acetylene yield, but temperatures above 1973 Kare required. [5] Then oncatalytic route focuses on CH 4 pyrolysis to achieve high acetylene yield, but temperatures above 1973 Kare required.…”

Comment on “Efficient Conversion of Methane to Aromatics by Coupling Methylation Reaction”

“…2 The main product of coke removal by oxygen was CO, in good agreement with results of our recent study showing that short pulses of oxygen during the MDA reaction decrease the rate of coke deactivation. 3 Ichikawa et al observed that addition of water improves catalyst stability, attributed to steam-reforming of coke species. 4 The reverse Boudouard reaction of carbonaceous deposits with CO 2 can also improve the stability of MDA catalysts.…”