Fluidized-bed oxidative coupling of methane over the Mn-NaW/TiO₂ catalyst: Effects of TiO₂ phase and interphase

The Mn-NaW/TiO₂ catalyst shows a broad promise for practical application in the fluidized-bed oxidative coupling of methane (FB-OCM) but there is still a lack of understanding of the role of TiO₂. Herein, a series of Mn-NaW/TiO₂ catalysts with tuned anatase and rutile compositions were obtained by varying the calcination temperature and duration and were examined in the FB-OCM reaction. It was found that the Mn-NaW/TiO₂ catalysts with a proper rutile fraction (F_R) of 30–90% could fully initiate the FB-OCM reaction at 760 °C, achieving 26–29% CH₄ conversion and 65–73% C₂-C₃ selectivity for a feed gas of CH₄/O₂ (5/1, vol/vol; no dilution gas) at 760–840 °C. The lattice oxygen in the TiO₂ did not directly participate in the FB-OCM reaction, whereas the TiO₂ phase played a crucial role in tuning the lattice oxygen activity of Mn₂O₃ through the redox cycle of “2Mn₂O₃ + 4TiO₂ ↔ 4MnTiO₃ + O₂”. The higher the rutile phase content in the Mn-NaW/TiO₂ catalysts, the lower the activity of the lattice oxygen in Mn₂O₃ but conversely the higher the selectivity to C₂-C₃ selectivity. In addition, the density functional theory (DFT) calculations confirmed that the anatase-rutile junction was conductive to the thermal reduction of Mn₂O₃ to form MnTiO₃.