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Establishing selectivity-property relationships in the oxidative coupling of methane through combining statistical data analysis with reliable catalyst testing and characterization

Subject Area Technical Chemistry
Term from 2020 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 431705856
 
Final Report Year 2024

Final Report Abstract

The oxidative coupling of methane (OCM) allows the production of C2H4 and C2H6 (C2- hydrocarbons) which have found their application in the modern chemical industry. The main shortcoming of OCM is the low selectivity to C2-hydrocarbons due to their easy oxidation to carbon oxides. Improving the selectivity is a major challenge but is hampered by the uncertainties related to the reaction mechanism due to its complexity. To contribute to this topic, the main objectives of this project were to: (i) understand the role of different oxygen species (lattice oxygen, adsorbed monoatomic or diatomic) in selective and unselective pathways, (ii) provide the basis for the positive effect of steam on catalyst activity and selectivity to C2-hydrocarbons, (iii) establish the catalyst property/composition-relationship for the selective production of C2-hydrocarbons, (iv) identify the upper CH4 conversion limit for achieving C2-selectivity above 90% and (v) improve productivity and selectivity in a chemicallooping mode of OCM. To this end, we applied an approach that included the precise synthesis of bulk and supported materials, their characterization by complementary ex situ/in situ/operando state-of-the-art techniques, partially, in a time-resolved manner as well as mechanistic and kinetic tests under steady-state and transient conditions with isotopic tracers. In addition, own and literature data were analyzed by computer-aided methods. The following main results were obtained. Using the developed protocol for catalyst testing under alternating steam-free and steamcontaining OCM feeds, as well as sophisticated kinetic and mechanistic tests, we identified both reversible and irreversible steam effects on the selectivity to C2-hydrocarbons over the MnOx-Na2WO4/SiO2 system. Since the selectivity-enhancing steam effect was also found for other differently composed catalysts, it may be advantageous to consider it in the design of novel materials for OCM and even for other oxidation reactions. Mechanistically, steam reduces the contribution of direct CH4 oxidation to CO2 due to steam-induced conversion of unselectively adsorbed diatomic oxygen species to selective monoatomic species. Lattice oxygen cannot convert CH4 to C2H6 but oxidizes CH4 exclusively to CO and CO2. The electronegativity of the alkali metal in M(Na, K, Rb or Cs)2WO4 was found to influence the ability of catalysts to form adsorbed oxygen species from O2. This knowledge opens the possibility of influencing product selectivity by controlling the coverage of adsorbed and lattice oxygen via reaction conditions or catalyst composition. Motivated by the elucidated fundamentals of the steam effect, we have shown for the first time that the MnOx-Na2WO4/SiO2 catalyst leads to high C2+-hydrocarbons selectivity of 87.3% at 6.6% CH4 conversion only at 675°C. The steam-mediated improvement was found to increase with decreasing reaction temperature. Our kinetic studies revealed that the apparent activation energy and reaction order of the rate of C2-hydrocarbons formation with respect to O2 are decreased in the presence of steam. The enhancing steam effect was also determined in chemical looping OCM over the same catalyst. Both CH4 conversion and C2- hydrocarbons selectivity were increased. The selectivity to C2H4 of about 53% (the selectivity to C2-hydrocarbons was about 76%) at about 24% CH4 conversion was obtained under optimal reaction conditions. Steam was found to increase the amount of lattice oxygen released by the catalyst due to the reaction of H2O with diatomic oxygen species formed by the recombination of two lattice oxygen species. In addition to the steam effect discussed above, CO2 is another feed component that can indirectly influence product selectivity. We found a volcanic-like correlation between the yield of C2 hydrocarbons and the thermal stability of carbonates. The mechanism developed to explain this dependence considers the involvement of surface carbonates under OCM conditions in the isolation of active oxygen species on the surface of metal oxides. Finally, we developed Gd2O3-based catalysts that can achieve about 90% selectivity to C2+-hydrocarbons at 11% CH4 conversion at 700°C using N2O as oxidant. The nature of the adsorbed oxygen species formed from N2O and, in particular, their binding strength were identified as the key parameters for hindering methane oxidation to carbon oxides. These catalyst properties can be tuned by a metal oxide promoter for Gd2O3. The obtained fundamentals explain the role of oxygen species in controlling product selectivity and can be used to design OCM catalysts that selectively produce C2-hydrocarbons with O2.

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