Influência de agentes de ativação na geração de espécies ativas em zeólitas Cu-ZSM-5 – aplicação na conversão de metano a metanol

Abstract

Climate change is a current topic of much debate around the world and is a consequence of the impact of the intensification of the greenhouse effect caused by the accumulation in the atmosphere of gases such as carbon dioxide, nitrous oxide, carbon monoxide, methane (CH4), among others. Therefore, it is necessary to develop ways to add value to these gases and minimize their emissions. In the case of methane, the challenge is its direct conversion to produce methanol via heterogeneous catalysis and in compliance with the principles of green chemistry. The most widely studied catalysts for this reaction are oxocation-exchanged zeolites (CuxOy). The formation of these species is influenced by factors intrinsic to the catalyst preparation process, such as conditions of incorporation of copper cations, activation agent and operating conditions, among others. Therefore, this work aimed to evaluate the generation and nature of copper oxo-cations in ZSM-5 zeolites by varying the activation agent (O2, CO2, Ar or synthetic air) and verify the activity of these species in the conversion direct methane to methanol (MTM). As precursors, NH4-ZSM-5 zeolites (Si/Al = 11,5 and 15) were used, which were subjected to ion exchange with copper acetate. The zeolites were characterized by X-ray diffractometry (XRD), reduction with hydrogen at programmed temperature (H2-RTP), nitrogen physisorption, X-ray fluorescence (FRX), ammonia desorption at programmed temperature (NH3-DTP) and analyzes FTIR in transmission mode with pyridine adsorbed in situ. The reaction process occurred in three stages: (1) activation of Cu-ZSM-5 using CO2, O2, synthetic air and He as activating agents; (2) MTM reaction; (3) ex situ extraction of methanol using ethanol. To identify the active species and their consumption in the reaction, in situ DRS-UV-vis analyzes were carried out. XRD and H2-RTP results showed that the Cu-Z(11,5)_2x24 sample was the most suitable for catalytic tests and more complex characterizations as it has more Cu atoms in exchange positions in the zeolite. The catalytic tests showed that all the activation agents used were capable of generating active species and forming MeOH, however, the species generated by the O2 and CO2 agents produced less methanol than the species generated using synthetic air and an inert gas, showing that with The species generated by O2 and CO2 may have seen overoxidation of methane to CO and CO2 or clusters of Cu oxocations inactive for the DMTM reaction have formed. The DRS-UV-Vis spectra showed the formation of oxocation species with all activating agents under study and also the species that were consumed during the reaction, being intermediates such as µ-(ƞ2:ƞ2)peroxo dicobre and trans-1,2-peroxo dicobre species, [CuxOy], [Cux(OH)x], Z2-Cu(II) and Z[Cu(II)O]. Synthetic air proved to be the most suitable activation agent, as it is the most sustainable and capable of forming species sufficiently active in the DMTM reaction.