- Title
- Catalytic partial oxidation of methane to value added products by N₂0 over Fe-Based catalysts at moderate temperatures
- Creator
- Zhao, Guangyu
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2020
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- A laboratory scale process for catalytic partial methane oxidation to methanol and derivatives by N₂O over Fe-based catalysts in a continuous operation mode at moderate temperatures was constructed. Examination of the partial methane oxidation to methanol over Fe-ZSM-5 catalysts at moderate temperatures showed that methanol and formaldehyde were produced at a temperature range from 275 °C to 400 °C with selectivity decreasing with increasing temperature. Incorporation of 0.1 wt% iron to H-ZSM-5 promoted methane and N₂O conversion, as well as the selectivity to C1-oxygenates. Further loading of iron enhanced methane conversion via complete oxidation. With an increase in the CH₄/N₂O ratio, selectivity of both carbon oxides and C1-oxygenates decreased, while the selectivity of ethylene showed an opposite trend. In addition, a relatively high GHSV elevated selectivity to desired products. In order to explore the correlation of structure properties of zeolites to active sites for methanol formation, activity and selectivity over Fe-ZSM-5, Fe-Beta and Fe-FER catalysts were studied at 350 °C for the catalytic partial oxidation of methane. Ammonia adsorption data suggested that among the studied zeolites, H-FER zeolite contained the highest concentration of framework Al atoms which are essential for the formation of active extra-framework Fe species. Fe-FER catalyst contained more active sites for N₂O conversion in comparison to Fe-Beta and Fe-ZSM-5 catalysts, as demonstrated by H₂-TPR profiles and IR spectra of NO adsorbed on the Fe zeolites. The catalytic activity studies showed that Fe-FER was the most active catalyst based on methane and N₂O conversion, and displayed the highest selectivity to C₁-oxygenates and dimethyl ether (DME), while Fe-ZSM-5 obtained the highest selectivity to ethylene among the three catalysts. Fe-ZSM-5 was found to deactivate significantly due to coke formation. Fe-FER catalysts prepared by incipient wetness impregnation (IMP), liquid ion exchange (IE) and solid state ion exchange (SSIE) methods were studied at 350 °C. UV-vis spectra indicated that the main component present in Fe-FER-IMP, Fe-FER-IE and Fe-FER-SSIE was isolated Fe species, dimeric Fe species and oligomeric iron oxides clusters, respectively. The active oxygen sites for the selective conversion of methane were identified by a TPR feature at 220 °C. This sites were also characterised by an infrared band observed at 1872 cm⁻¹ and 1892 cm⁻¹ upon adsorption of NO. The correlation of the area of the unique reduction peak in the H-TPR profiles and the area of a band at 1892 cm⁻¹ in the IR spectra of NO adsorption on the catalysts suggested that the active sites for N₂O decomposition were binuclear Fe species, and Fe-FER-SSIE contained the largest number of active Fe species among the three catalysts. The activity test showed that Fe-FER-SSIE obtained the highest N₂O decomposition at 250 °C but Fe-FER-IE achieved the highest N₂O conversion at elevated temperatures due to the greater likelihood of transformation from active oxygen species to oxygen molecules over Fe-FER-IE. Selectivity to desired products (methanol, formaldehyde, DME and ethylene) over Fe-FER-IMP, Fe-FER-IE and Fe-FER-SSIE was 12.5%, 16.5% and 19.8% (coke took into account), respectively. Formation of active oxygen species from N2O over Fe-FER catalyst prepared by SSIE method at moderate temperatures was studied using spectroscopic and solid characterisation techniques including H₂-TPR, N₂O-TPD and in situ FTIR. These bands observed at 1872 cm⁻¹ and 1892 cm⁻¹ in IR spectra of NO adsorption on catalyst were NO stretching vibrations of NO adsorbed on iron oxygen clusters, present in the zeolite cages and responsible for selective oxidation. It was shown that these oxidised clusters reacted with methane to form oxygenates but at higher temperatures formed molecular oxygen. IR Bands of surface methoxy groups were observed in significant concentration in the FTIR spectra and were suggested to be intermediate species of the selective oxidation of methane. Studies using continuous reactor demonstrated that co-feeding of methane and N₂O promoted generation of desired products from methane conversion by N₂O over Fe-FER catalyst can be enhanced by optimizing the feed ratio of CH₄/N₂O.
- Subject
- catalysis; methane conversion; active sites; methanol; surface oxygen species; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1412919
- Identifier
- uon:36551
- Rights
- Copyright 2020 Guangyu Zhao
- Language
- eng
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