“…As mentioned, both Na and W are necessary for this phase transition to occur at the calcination temperature (around 800 °C) of the catalysts [6], and it has been found that the WO 4 2-tetrahedron is very important in the OCM reaction. W-O-Si interactions, or more specifically the interactions between the WO 4 2-tetrahedron and -cristobalite SiO 2 , are crucial in the formation of stable and active surface species [9,19], and the WO 4 2-is further stabilized by Na (or K) [9], which forms Na 2 WO 4 with a distorted WO 4 2-tetrahedron on the support, M a n u s c r i p t potentially part of the active site [20]. Addition of manganese increases the oxygen mobility of the catalysts and enhances exchange between gaseous and surface oxygen [21][22].…”