Partial Oxidation of Methane over Functionalised Zeolites and Zeotypes

Doktorsavhandling, 2018

Partial oxidation of methane to methanol is an industrially important process that allows natural gas and biogas to be utilised for methanol-based production of chemicals or converted to liquid methanol fuel, which make transportation more facile. In this thesis, the activity for partial oxidation of methane to methanol over copper functionalised zeolites and silica supported copper is investigated using a three-step quasi-catalytic approach including catalyst activation, methane reaction and methanol extraction by water. Methanol is produced over isolated copper ions but likely not over copper particles. Under methane oxidation conditions, in situ infrared spectroscopy measurements show the formation of methoxy species adsorbed on the Brønsted acid sites, which is concluded to be an important reaction intermediate in the methanol formation.
 
The dynamic interaction between methanol and copper zeolites was experimentally studied by temperature-programmed desorption of methanol using a chemical flow re-actor and in situ infrared spectroscopy and theoretically by first-principles calculations. It is shown that methoxy species binds strongly to the Brønsted acid sites in the zeolites, which explains the need for the extraction step as to obtain methanol from zeolite-based systems. The results indicate that methanol formation and desorption without using water extraction may require a catalyst with lower acidity.
 
The last part of this thesis investigates the possibility of tuning the catalyst acidity by synthesising so-called zeotypes to replace the zeolites. A boron silicate with chabazite structure and lower acidity compared to zeolitic chabazite was synthesised and functionalised with copper. Upon methane exposure to the copper boron silicate, methoxy species forms and retains on the copper sites, which is promising for methanol production. Further, three iron and/or aluminium containing silicates with MFI structure were synthesised. Through infrared spectroscopic surface analysis, it is shown that their acidity is lower than that of the corresponding zeolite structure, and thus these materials may be potential catalyst candidates that deserves further studies.