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

• Published in: ACS catalysis 2021-08, Vol.11 (15), p.9262-9270
• Main Authors: Dinh, Kimberly T, Sullivan, Mark M, Serna, Pedro, Meyer, Randall J, Román-Leshkov, Yuriy
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.1c02187

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.