Oxidação do metano em metanol: efeito da natureza do oxidante na estrutura da espécie ativa em Cu-FAU e Cu-CHA

Abstract

The direct oxidation of methane to methanol is carried out with metal-zeolite catalysts in three steps: (I) oxidation of metal oxide with O2; (II) partial oxidation of methane to methanol using active site oxygen, and (III) methanol desorption with water vapour. Generally, the conventional process studied in the literature is carried out in cycles with temperature changes. However, the isothermal cycle is preferable from an economic and operational point of view. Understanding the active species in catalysts is important for the development of materials that allow higher methanol yields. Although the literature presents excellent studies on this reaction, the process has not yet left the laboratory scale. It is necessary to investigate changes in the process that allow improvements to achieve a future viability. Therefore, the general objective of this work is to propose the use of CO2 as an oxidant in an isothermal process at high temperature in the partial oxidation reaction of methane to methanol using the zeolites Cu-FAU and Cu-CHA, and through the technique of diffuse reflectance spectroscopy in the ultraviolet and visible region to compare the effect of this oxidant on the structure of the active species in relation to the traditionally used oxidant, oxygen. The catalysts showed excellent performance in the isothermal process (400 °C), using CO2 as an oxidant. The results achieved were 140 and 132 µmol MeOH/gcat, for Cu-FAU and Cu-CHA, respectively. Through DRS UV-vis in situ it was possible to verify the presence of two species that are able to activate methane in Cu-FAU using CO2 -[Cu3(µ-O)3]2+ and [CuOH]+. However, the use of oxygen only species [CuOH]+ is observed. The Cu-CHA showed only species [CuOH]+ in both oxidants. The results suggest that the combination of oxidant and zeolite structure influences the formation of the active species. The treatment in CO2, in addition to providing active oxygen for the formation of the active site, allows the self-reduction of a copper fraction (Cu2+ → Cu+). The Cu+ formed can migrate into the Cu-FAU zeolite structure and recombine with other atoms to form [Cu3(µ-O)3]2+. The proposal to use carbon dioxide is a step forward in contributing to the study of the direct partial oxidation of methane to methanol.