Mechanistic Study of Selective Oxidation of Dimethyl Ether to Formaldehyde over Alumina-supported Molybdenum Oxide Catalyst

XPS and IR spectroscopies were used to investigate the surface intermediates of dimethyl ether (DME) oxidation to formaldehyde over MoOx/Al2O3 catalyst. The reaction performances were tested by employing three typical reaction conditions, depending on the O2/DME ratio and the reaction temperature. When there was sufficient oxygen present in the reaction media, a terminal or bridged CH3O* species formed by DME dissociation was highly active and rapidly reacted with lattice oxygen to produce formaldehyde, leading to higher selectivity of HCHO. When oxygen was consumed completely or only DME was present in the reaction media, CH3O species bonded to more than two Mo atoms (μ-OCH3) and CH x (x=1–3) species attached to the Mo atoms were observed and the relative ratio of (μ-OCH3) /Mo–CH x was significantly dependent on the reduction degree of MoO x domains. The (μCH3O) species was related to the formation of CH3OH or CO x , and the Mo–CH x species led to the formation of CH4.