Co and Cu ZSM-5 zeolites for the direct production of oxygenates from methane and oxygen

Abstract

The direct partial oxidation of methane to oxygenates still remains one of the greatest challenges in catalysis. Metal-containing zeolites hold great potential for the direct partial oxidation of methane to oxygenates. The nature of the active sites in these materials is, however, still a matter of debate and has been addressed in this PhD thesis for Cu-ZSM-5 and Co-ZSM-5 zeolites. For activated Cu-ZSM-5 zeolites the CT band at 22700 cm-1 is linearly related to the metal loading and to the activity of the samples for the production of methanol from methane and oxygen. Thus, a quantitative structure-performance relationship between the CT band intensity and the amount of methanol produced has been established. The presence of at least two copper species was observed and confirmed by UV-Vis-NIR DRS, TEM and FT-IR measurements. Part of the copper is present as Cu-O nanoclusters on the outer surface. Based on catalytic experiments and FT-IR measurements of adsorbed pivalonitrile we showed that Cu-O species on the outer surface are inactive for methanol production. A minor part of copper is present inside the microporous channels as visualized by FT-IR of adsorbed NO after pivalonitrile adsorption, which most likely gives rise to the CT band at 22700 cm-1. These copper species inside the zeolite channels are involved in the oxidation of methane to methanol. In addition, for the first time the possibility to partially oxidize methane to methanol and formaldehyde at low temperature over Co-ZSM-5 using air is shown. The influence of the preparation method on the nature of the cobalt species is investigated. The catalytic activity and selectivity for methane oxidation as a function of the cobalt speciation is additionally discussed. Based on UV-Vis-NIR and FT-IR spectroscopy, H2-TPR, TEM and kinetic measurements it is concluded that cobalt in ion-exchange positions results mainly in the formation of formaldehyde, while larger Co-oxide particles prepared by impregnation result in the formation of methanol. Often zeolites for methane oxidation consist of small crystals agglomerated to larger entities of typically > 2 ?m. This might hamper the accessibility of the active sites in the zeolite/zeolite aggregates and merits a more detailed investigation. Therefore, the role of pore accessibility on the catalytic performance of Co-ZSM-5 zeolites for the production of oxygenates from methane has been investigated. Alkaline treatment of templated Na-ZSM-5 zeolites with NaOH solutions resulted in the creation of intercrystalline mesopores within the zeolite agglomerates, while preserving the micropore volume and crystallinity. It was found that the zeolite external surface area increased with increasing NaOH concentration and pretreatment time. These alkaline treated samples were loaded with cobalt and a linear relationship between the number of cobalt oxidic species and the external surface area of the zeolite could be established. This in turn leads to a linear relationship between the ZSM-5 surface area and the amount of methanol produced over Co-ZSM-5 from methane and oxygen. Attempts to remove extra framework alumina species by an acid treatment were successful. However, this acid treatment increased, after Co-deposition, the amount of highly dispersed Co2+ inside the ZSM-5 channels, which resulted in a higher selectivity towards formaldehyde.