Breaking the Selectivity-Conversion Limit of Partial Methane Oxidation with Tandem Heterogeneous Catalysts

The inherently unfavorable thermodynamics for the direct partial oxidation of CH4 with O2 limits the system to high selectivities only at low conversions. We demonstrate a tandem strategy capable of circumventing this selectivity-conversion limit by performing sequential oxidation of CH4 to CH3OH over a selective Cu-exchanged zeolite followed by C-alkylation of CH3OH with benzene over an acidic zeolite. Using a small-pore zeolite (SSZ-13, CHA topology) to host the Cu species is essential to achieve increased yields by maximizing CH4-to-CH3OH selectivities while also protecting the final alkylate product from overoxidation via size-exclusion. Cofeeding CH4, oxygen, water, and benzene over a mixture of Cu-SSZ-13 and H-ZSM-5 resulted in 77% toluene selectivity at 663 K and 1 bar compared to only 2% CH3OH selectivity in the absence of benzene under identical conditions at isoconversion. A record productivity of 1.7 μmol min–1 gcat–1 was achieved at 11 bar and 603 K (80% toluene selectivity at 0.37% CH4 conversion), which represents a 30-fold improvement over current continuous processes over Cu-based zeolites. Our findings demonstrate the importance of protecting the methanol product to achieve high selectivities and help close the gap to realize more efficient small-scale CH4 conversion processes.