Oxidação direta de metano a metanol sobre espécies [CuxOy] incorporadas em zeólitas ferrierita

Abstract

The development of technologies to minimize the CO2 emissions, which are generated by the indiscriminate burning of fossil fuels, remains a relevant research topic in the context of the planet's sustainability. Thus, it is strategic to avoid the methane’s burning and even its proper emission, therefore being interesting valorize it as raw material to produce chemical and intermediates. In this scenario, the conversion of methane to methanol has gained strong relevance due to its wide application possibilities. However, the strong carbon and hydrogen bond in the methane molecule becomes a challenge in the search for routes to promote its reactivity. Another drawback to overcome is the methanol overoxidation because it is more reactive than methane itself. Considering the promising results of conversion and selectivity of methane to methanol over cationic clusters [CuxOy] incorporated in zeolites, this work evaluated ferrierite zeolites containing those species. The Cu-ferrierites were characterized by X-ray diffraction, nitrogen physisorption, scanning electron microscopy with energy dispersive X-ray spectroscopy, and temperature-programmed reduction with hydrogen. The catalytic evaluation involved three steps: (i) activation of the Cu-ferrierites under air flow at 550 ºC; (ii) reaction of methane to methanol at 210 ºC, and (iii) extraction of the adsorbed methanol. The results confirmed that cationic [CuxOy] complexes were the active species in the conversion of methane to methanol, since physical mixtures of copper oxide with the precursor ferrierites did not show methanol yield. With the aim to identify the active species, the Cu-ferrierites were analyzed in situ by FTIR and DRS UV-Vis in the same conditions used during the activation and reaction steps. The results showed bands attributed to dimeric copper species in the FTIR spectra and, in the DRS-UV-Vis spectra, bands corresponding to cationic complexes bis(μ-oxo) dicopper and μ-(η2:η2)peroxo dicopper, with the last being the precursor of the active species mono(μ-oxo) dicopper.