One-step synthesis of dimethyl ether from the gas mixture containing CO₂ with high space velocity

Dimethyl ether (DME) has been considered as a potential hydrogen carrier used in fuel cells; it can also be consumed as a diesel substitute or chemicals. To develop the technique of DME synthesis, a bifunctional Cu-ZnO-Al₂O₃/ZSM5 catalyst is prepared using a coprecipitation method. The reaction characteristics of DME synthesis from syngas at a high space velocity of 15,000mL (g_cath)^-1 are investigated and the effects of reaction temperature, pressure, CO₂ concentration and ZSM5 amount on the synthesis are taken into account. The results suggest that an increase in CO₂ concentration in the feed gas substantially decreases the DME formation. The optimum reaction temperature always occurs at 225°C, regardless of what the pressure is. It is thus recognized that the DME synthesis is governed by two different mechanisms when the reaction temperature varies. At lower reaction temperatures (<225°C) the reaction is dominated by chemical kinetics, whereas thermodynamic equilibrium is the dominant mechanism as the reaction temperature is higher (>225°C). For the CO₂ content of 5vol.% and the pressure of 40atm, the maximum DME yield is 1.89g (g_cath)^-1. It is also found that 0.2g of ZSM5 is sufficient to be blended with 1g of the catalyst for DME synthesis.