Abstract
Catalyst systems for the conversion of synthesis gas, which are tolerant to fluctuating CO/CO(2) gas compositions, have great potential for process-technical applications, related to the expected changes in the supply of synthesis gas. Copper-based catalysts usually used in the synthesis of methanol play an important role in this context. We investigated the productivity characteristics for their application in direct dimethyl ether (DME) synthesis as a function of the CO(2)/CO (x) ratio over the complete range from 0 to 1. For this purpose, we compared an industrial Cu/ZnO/Al(2)O(3) methanol catalyst with a self-developed Cu/ZnO/ZrO(2) catalyst prepared by a continuous coprecipitation approach. For DME synthesis, catalysts were combined with two commercial dehydration catalysts, H-FER 20 and γ-Al(2)O(3), respectively. Using a standard testing procedure, we determined the productivity characteristics in a temperature range between 483 K and 523 K in a fixed bed reactor. The combination of Cu/ZnO/ZrO(2) and H-FER 20 provided the highest DME productivity with up to 1017 g(DME) (kg(Cu) h)(-1) at 523 K, 50 bar and 36 000 ml(N) (g h)(-1) and achieved DME productivities higher than 689 g(DME) (kg(Cu) h)(-1) at all investigated CO(2)/CO (x) ratios under the mentioned conditions. With the use of Cu/ZnO/ZrO(2)//H-FER 20 a promising operating range between CO(2)/CO (x) 0.47 and 0.8 was found where CO as well as CO(2) can be converted with high DME selectivity. First results on the long-term stability of the system Cu/ZnO/ZrO(2)//H-FER 20 showed an overall reduction of 27.0% over 545 h time on stream and 14.6% between 200 h and 545 h under variable feed conditions with a consistently high DME selectivity.