Deactivation of a Biomass-Derived Zirconium-Doped Phosphorus-Containing Carbon Catalyst in the Production of Dimethyl Ether from Methanol Dehydration

Dehydration of methanol to produce dimethyl ether (DME) was studied at relatively high temperatures (400-600 degrees C) on biomass-derived phosphorus-containing carbon impregnated with a zirconium salt. Highly thermally stable zirconium phosphate surface groups could be obtained on the final catalyst, which were responsible for the high stability and selectivity to DME of the catalyst at temperatures lower than 400 degrees C. However, harder operation conditions, closer to those of the industrial process, were evaluated to analyze the changes of the catalyst surface properties with the reaction temperature and the possible causes of deactivation. Thus, high methanol conversion and selectivity to DME were also observed in the temperature range of 400-600 degrees C, although deactivation was detected. Coke deposition was responsible for a decrease in microporosity and surface concentration of zirconium and phosphorus of the catalyst. Temperature-programmed desorption, P-31 magic angle spinning nuclear magnetic resonance, and X-ray photoelectron spectroscopy results suggest that the Zr-O-P groups from zirconium phosphate species were responsible for the long-term stability of the catalyst and that the C-O-P-type active sites were deactivated very fast. However, coke deposition on Zr-O-P-type active sites caused a slow and irreversible deactivation, while deposited coke on the C-O-P-type active sites was easily eliminated by the oxidative treatment in air. A reaction scheme that accounted for the gas product distribution and the production of coke was proposed. A kinetic model for coke formation as a function of time on stream that successfully represents the experimental results was also propounded, which yielded a value for the activation energy for the production of coke of 124 kJ/mol.