Methane to Value Added Chemicals Using a Solid Superacid Catalyst

The shale gas revolution has had a significant impact on the production of natural gas and its conversion into higher-value products. The most commonly practiced methane conversion process involves the use of a co-reactant as an oxidant (typically H₂O). Non-oxidative routes involve the use of what is known as a superacid, a compound that has acidity higher than 100% sulfuric acid. A number of superacids are liquids; e.g., HF-SbF₅¹, FSO₃H-SbF₅¹. Recently, gas-phase HBr-AlBr₃² has shown superacidity for the conversion of methane into value added chemicals, but neither liquid- nor gas-phase superacids can serve as the basis of a practical process because separation of the superacid from the products is unrealistic. The literature has reported attempts to prepare solid superacids using sulfated zirconia³, but both poor yields and rapid deactivation have limited their further use. The challenge here is to show that the superacidity that has been demonstrated in gas-phase catalysts can be shown in a solid.

Here, we propose a new solid catalyst based on AlBr₃ supported on zeolites. DRIFTS using pyridine showed presence of an additional Brønsted acid site as compared to the H-ZSM5 base zeolite. NH₃-TPD quantitatively confirmed the presence of more acid sites than base zeolite. This increase in acid site density and creation of a superacid site was able to catalyze methane into higher-value added chemicals such as C₂-C₈hydrocarbons. It appears that the reaction is produced in a step-chain oligo-condensation mechanism. Additional tests on ethane oligomerization showed a similar range of hydrocarbon products, and with a higher TOF, as expected.

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