Preliminary assessment of membrane reactors as a means to improve the selectivity of methylamine synthesis

Methylamine synthesis from ammonia and methanol was studied using an amorphous silica-alumina catalyst. A combined thermodynamic and kinetic analysis shows that the selectivity ratio initially observed at high methanol conversion is a kinetic ratio, not a thermodynamic ratio. In some cases in the past this kinetic ratio has been taken to represent thermodynamic equilibrium. However, if the reaction is allowed to continue beyond the point where conversion of methanol and dimethyl ether is essentially complete, the product composition continues to change, albeit at a much lower rate. Eventually the thermodynamic selectivity ratio is obtained. A simple kinetic model was developed that captures this behavior, and this model was used to assess whether a membrane reactor might be used to alter the overall selectivity of methylamine synthesis. Four different membrane reactor configurations were considered. There were operational regimes where each configuration showed advantages, but these either occurred at low conversions or required extremely large reactors. These configurations are limited by currently available catalysts and membrane materials. The impact of membrane reactors could be increased with catalysts that retain high activity during methylamine disproportionation, i.e., after all methanol has been consumed. The development of membrane materials with better permselectivities would also increase the attractiveness of membrane reactor processes.