The aerobic, water-assisted selective oxidation of methane over platinum-based catalysts
The selective oxidation of methane to C1-oxygenates such as methanol, formaldehyde and dimethylether remains one of the grand challenges of catalysis chemistry, due to the difficulty in attaining high selectivities at high methane conversions. This has been attributed to the higher reactivity of methane oxidation products compared to methane itself, which therefore leads to their facile conversion to CO and CO2 [1]. Strategies that have been adopted to selectively convert methane include product protection (i.e. converting methane to a methylester, which is stable against further oxidation compared to methanol) [2, 3], cyclical methane oxidation using zeolites (i.e. stepwise activation of oxygen, methane and product extraction) [4, 5], the use of mild reaction conditions (i.e. conversion of methane at low temperature using hydrogen peroxide)[6, 7] and the use of copper-exchanged zeolites which enable the catalytic conversion of methane with high methanol selectivities [8-10]. Copper-exchanged zeolites require feeding an excess of methane and still suffer from low methane conversions. The study presented in this thesis is focused on investigating the aerobic, selective oxidation of methane over supported platinum-based catalysts in the presence of water, with the investigation focusing on a) the role of water, b) the effects of the support and c) the effects of alloying platinum on the catalytic activity and oxygenate selectivity. The effects of water were investigated initially in a gas-phase, fixed bed reactor that is equipped with a steam generator. Initial results from the selective oxidation of methane yielded CO2. Accidentally flooding the reactor resulted in the formation of selective oxidation products such as methanol, methoxymethanol and 1,3,5, trioxane, which were observed when the reactor was flooded on purpose. The experiments were then carried out in a specially-constructed trickle-bed reactor that enabled the co-feeding of liquid water. An increase in the H2O/CH4 ratio resulted in an increase in the conversion of methane and in formaldehyde selectivity, which was the favoured product. DFT calculations indicated that formaldehyde was formed from a di s-hydroxy methoxy intermediate, which possibly forms from the hydroxylation of a surface methoxy precursor. Initial catalytic studies involved titania supported catalysts (TiO2, rutile and P25), which deactivated with time, due to deposition of formaldehyde polymeric species. In total, the supports investigated were alumina, carbon, the rutile phase of titania and P25 (a mixture of rutile and anatase). The catalytic activity was in the order of Pt/TiO2-P25 (XCH₄ = 0.6%, Sselective oxidation products = 10%) < Pt/Al2O3 (XCH₄ = 0.5%, SCH₂O = 65%) < Pt/TiO2-Rutile (XCH₄ = 1.0%, SCH₂O = 90%) < Pt/C (XCH₄ = 4.2%, SCH₂O = 99%). Although all catalysts deactivated, alumina underwent phase-transformation to boehmite, which further exacerbated catalyst deactivation. Kinetic experiments indicated that the selective oxidation of methane was limited by the rate of product desorption, which was related to the binding strength of oxygen on the platinum surface. The binding strength of oxygenates on the platinum surface was then tuned by alloying platinum with copper, silver and gold in order to modify the platinum d-band center. The alloys were synthesized targeting a 3:1 Pt:M (M= Cu, Ag or Au) ratio and were supported on the rutile phase of titania. The nanoalloy catalysts were tested in the fixed bed reactor. The selectivity towards formaldehyde was in the order of Pt3Au (64%) = Pt (66%) = Pt3Cu (66%) < Pt3Ag (72%). However, at the highest water partial pressure, the formaldehyde selectivity decreased to 63% for Pt3Ag. The rate of methane conversion was in the order of Pt3Cu (292 µmol/gcat/hr) > Pt3Au (104 µmol/gcat/hr) ³ Pt (95 µmol/gcat/hr) while Pt3Ag was the least active, with a maximum activity of 68 µmol/gcat/hr. Testing Pt3Cu/TiO2 in the trickle bed reactor resulted in a slight increase in the rate of methane conversion (320 µmol/gcat/hr) and an increase in the formaldehyde selectivity (Sformaldehyde = 78%). The performance of the Pt3Ag/TiO2 catalyst was significantly enhanced in the trickle-bed reactor (-rCH₄ = 271 µmol/gcat/hr, Sformaldehyde = 74%). The results presented herein show the possibility of selectively oxidising methane with high selectivity towards formaldehyde. Furthermore, a high conversion of methane (ca. 4-5%) was achieved at fairly high oxygenate selectivities (65-99%), using oxygen as an oxidant.
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