The Oxidative Dehydrogenation of Methanol to Formaldehyde over Silver Catalysts in Relation to the Oxygen-silver Interaction

The properties of silver in the oxidative dehydrogenation of methanol were studied in a flow reactor under near industrial conditions. The influences of temperature, concentration of both reactants, gas velocity, space velocity, the form of the silver catalyst and surface composition of the catalyst were studied. A model for the reaction is proposed which is based on the experimental observations and on the nature'of suggested that different oxygen in the reactions to CO, Cop and conversion to CO. INTRODUCTION Oxidative dehydrogenation of the interaction of silver with oxygen, It is species on the silver surface play different roles HiCO. Gas phase reactions only contribute to the methanol using silver as a catalyst is a wellknown industrial process for the production of formaldehyde. At one atmosphere with temperatures around 6OO"C, it is possible with an excess of methanol relative to oxygen to reach a selectivity of 90% towards formaldehyde with almost complete conversion of the methanol. The main by-products are CO, C02, H2 and H20. The catalytic behaviour of silver has been reviewed by Clayton and Norval [I]. Much research has been done using as catalyst both pure silver and silver supported on low area-alumina or on alumina-silicates. It is still not clear how the process on the silver surface takes place, especially under industrial conditions. Gavrillin and Popov [Zl have reported that an unstable temperature range exists due to the exothermic nature of the reaction. Above the temperatures corresponding to this instability, conversion of the oxygen of the reaction mixture is complete. Several workers have stated that the process is diffusion controlled above 500°C [3-S]. It has been shown that the presence of oxygen is necessary for reaction to occur [7]; i.e. no further reaction occurs on the silver in that part of the bed in which the oxygen has been exhausted. Kaliya and his coworkers [6,8,91 have reported that selective oxidation (Equation 1) and dehydrogenation (Equation 2): CH3OH + 0.502 -f t$CD + H 0 2