Natural gas, the cleanest fossil fuel, is an abundant
source of
methane and expected to play an increasingly important role in powering
the world’s economic growth over the energy transition of the
coming decades. Methane has the potential to be a CO2-free
feedstock to cogenerate hydrogen (H2) and added value “building-blocks”
chemicals (e.g., olefins and aromatics) for petrochemistry. In this
review, the two processes (i) the oxidative coupling of methane (OCM)
for production of ethylene and (ii) the nonoxidative methane dehydroaromatization
(MDA) producing hydrogen and benzene are discussed. Both routes convert
methane directly into valuable products, an advantage over the several-steps
syngas route. The performances of various a variety of catalysts reported
during the last 25 years for OCM (MnNaW, La2O3, Li-MgO, etc.) and MDA (M/HZSM-5, M/TNU-9, M/IM-5, M/ITQ-2, M@SiO2, M@CeO2, TaH/SiO2, GaN/SBA15, single-site
M@HZSM-5, bimetallic M-M′/HZSM-5, core–shell structures,
M/Zr(SO4)2 with M = Mo, Fe, Pt) under similar
reaction conditions are compared. The major drawbacks and the strategies
used to mitigate the main challenges related with the performance
of the catalysts in both OCM and MDA reactions are critically revealed.
For instance, the overoxidation in the OCM is mitigated by optimizing
of the operating conditions, using alternative oxidants, and the
application of membrane reactor technology are discussed. In the MDA
reaction, the major issue is the catalyst deactivation by coke formation
and migration and sintering of metallic active phases. Strategies
for robust catalysts, methods for mild coke removal, pretreatment
under reductive atmosphere are presented. Approaches to improve aromatics
yields over coke production by addition of promoters or co-feed reactants
to the MDA catalysts are also discussed.