Selective oxidation of methane to methanol under mild conditions on Cu-, Fe- and Cu/Fe-based ZSM-5 catalysts

Details

Supervisors

Debecker, Damien P.

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en chimie et bioindustries, à finalité spécialisée

Abstract

enMethane is both a major greenhouse gas and a valuable carbon feedstock, making its conversion into chemicals an attractive prospect. Among the various possible conversion routes, the direct selective oxidation of methane to methanol is particularly promising. However, this reaction poses a challenge due to the stability of the C-H bond and the tendency of methanol to undergo overoxidation once formed. As part of a broader “artificial methanotroph” project aimed at combining a heterogeneous methane oxidation catalyst with an enzyme for the oxidation of methanol to formaldehyde, this master’s thesis focuses on the optimisation of the heterogeneous catalyst. The objective is to develop a ZSM-5-based metal-functionalised catalyst capable of maximising methanol production from methane and H2O2 in the liquid phase under mild conditions compatible with future chemoenzymatic implementation. To achieve this objective, several Fe-, Cu- and Cu/Fe-based ZSM-5 catalysts were prepared and tested, with particular attention paid to the effect of the nature of the metal, the preparation method and the properties of the support. After optimisation of the reaction conditions, reactor configuration, and analytical methods, catalytic tests were carried out in batch mode in a Parr reactor at 30 bar methane pressure and 25-50 °C. Among these catalysts, the bimetallic catalyst (copper and iron, with mass loadings of 2 % and 0.1 % respectively) showed very encouraging performance. It achieved a turnover frequency normalised to the total iron content of 71 molMeOH molFe-1 h-1, more than 12 times higher than the reference catalyst consisting solely of iron (mass loading of 0.25 %). Comparison with catalysts consisting solely of iron and solely of copper at the same loadings confirms that the simultaneous presence of both metals leads to greater methanol productivity. The preparation method also has a significant influence on catalytic performance. Catalysts prepared by ion exchange show higher methanol productivity compared to those prepared by wet impregnation at similar loadings, suggesting that the ion exchange method favours a higher proportion of active species for the oxidation of methane to methanol. Furthermore, the results show that catalysts prepared by ion exchange undergo less metal leaching than those prepared by wet impregnation. However, we highlight potential limitations for the use of these catalysts in the context of chemo-enzymatic catalysis, such as a drop in pH and a deleterious effect of buffers. Overall, this study identifies bimetallic catalysts and the ion-exchange preparation method as promising approaches for the selective oxidation of methane to methanol under mild conditions. These findings provide a relevant basis for the further development of a hybrid chemoenzymatic system for methane valorisation.