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

The Mn-NaW/TiO 2 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 TiO 2 . Herein, a series of Mn-NaW/TiO 2 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/TiO 2 catalysts with a proper rutile fraction (F R ) of 30–90% could fully initiate the FB-OCM reaction at 760 °C, achieving 26–29% CH 4 conversion and 65–73% C 2 -C 3 selectivity for a feed gas of CH 4 /O 2 (5/1, vol/vol; no dilution gas) at 760–840 °C. The lattice oxygen in the TiO 2 did not directly participate in the FB-OCM reaction, whereas the TiO 2 phase played a crucial role in tuning the lattice oxygen activity of Mn 2 O 3 through the redox cycle of “2Mn 2 O 3  + 4TiO 2  ↔ 4MnTiO 3  + O 2 ”. The higher the rutile phase content in the Mn-NaW/TiO 2 catalysts, the lower the activity of the lattice oxygen in Mn 2 O 3 but conversely the higher the selectivity to C 2 -C 3 selectivity. In addition, the density functional theory (DFT) calculations confirmed that the anatase-rutile junction was conductive to the thermal reduction of Mn 2 O 3 to form MnTiO 3 .