Development of a Flat Membrane Microchannel Packed- Bed Reactor for Scalable Aerobic Oxidation of Benzyl Alcohol in Flow

A Teflon AF-2400 flat membrane microchannel reactor was designed and demonstrated for safe and scalable oxidation of solvent-free benzyl alcohol with molecular oxygen on Au- Pd/TiO2 catalyst. The microchannel reactor employed a mesh-supported Teflon AF-2400 flat membrane, with gas and liquid channels on each side. Catalyst particles were packed in the liquid flow channel. Operation with 20 bara pressure difference between the gas and the liquid phases was possible at 120 ºC. Pervaporation of organics through the membrane was experimentally measured to assure that the organic vapour concentration remains below the lower explosive limit during the reaction. The effect of oxygen pressure was studied, and the oxygen was shown to have a positive effect on the oxidation of benzyl alcohol. A conversion of benzyl alcohol of 70% with 71% selectivity to benzaldehyde was obtained at 1150 gcat·s/galcohol, 8.4 bara oxygen pressure and 10 bara liquid pressure. Doubling the membrane thickness led to a 20% drop of oxygen consumption rate, indicating the main oxygen transfer resistance not existing in the membrane. When changing the catalyst particle size and the liquid flow rate, no significant effect was observed on the oxidation reaction rate. An effectiveness factor approach is proposed to predict the effect of oxygen permeation and transverse mass transfer on the catalyst packed in the membrane reactor, which suggests that the oxidation of benzyl alcohol on the highly active Au-Pd/TiO2 catalyst is controlled by the oxygen transverse mass transfer in the bulk liquid within the catalyst bed. Scale-up of the flat membrane microchannel reactor was demonstrated through widening the liquid channel width by ~10 times, which increased the reactor productivity by a factor of 8.