Efeito de agentes de ativação na geração de Cu2+-oxo-cátions em zeólitas: conversão direta do metano em metanol

Abstract

Methane is the main component of natural gas, a fossil resource with steadily increasing production in Brazil and abroad. On the other hand, the sustainable production of biogas, also rich in methane is strongly encouraged. However, the limited direct use of methane as a fuel or feedstock for hydrogen production has driven the development of efficient technologies to convert it into oxygenated liquid products, such as methanol, a highly valued chemical intermediate. The selective oxidation of hydrocarbons into oxygenated products requires selective catalysts that exhibit high activity under mild temperature and pressure conditions to prevent excessive CO and CO2 formation. In this context, zeolites play a significant role; as microporous materials, they stabilize copper oxo-cation sites, which are responsible for the direct conversion of methane to methanol (MTM). Given this key property of zeolites, this study aimed to investigate the application of copper-ion-exchanged mordenite (MOR), chabazite (SSZ-13), and faujasite (Y) zeolites in this reaction, exploring how the activating agent influences the nature of the resulting Cu2+ oxo-cation. The MTM chemical process comprised three steps: (i) activation of the Cu-zeolites under a flow of air, CO2, O2, or He to form active sites; (ii) reaction with methane; and (iii) off-line extraction of methanol. All the Cu-zeolites studied showed activity in the MTM conversion, even when an inert gas (He or Ar) was used as the activating agent. Based on acidity results, it was determined that efficient methanol production relies on the ratio between Cu2+ sites and Brønsted acid sites, given that methanol yields for the Cu-NaY(2.5) zeolite exceeded those of other Cu-zeolites in the protonic form, particularly when CO2 was used as the activating agent (259 µmol MeOH g⁻¹). Furthermore, the nature of the active species was identified through a theoretical-experimental comparison of in situ UV-Vis spectra with DFT-simulated spectra. Cu-zeolites activated with air, CO2, O2, or Ar exhibit multiple species, with a predominance of the 2[CuOH]⁺ species. Additionally, activation and reoxidation processes indicated that CO2 favors the formation of [Cu3O3]²⁺ trimers in the Cu-NaY(2.5) zeolite.