Study of catalyst bed composition for the direct synthesis of dimethyl ether from CO 2 -rich syngas

In this work, we study the direct synthesis of DME using CO₂ -rich syngas, with a CO₂/CO ratio similar to that obtained from the gasification of biomass, i.e. , 1.9. We used catalytic beds consisting of physical mixtures of the benchmark catalysts used for the synthesis of methanol from syngas and for methanol dehydration to DME, namely Cu/ZnO/Al₂O₃ and 𝛾-Al₂O₃ , respectively. Our results show that the ratio between each catalytic phase determines the productivity and selectivity to DME, as well CO and CO₂ conversions. Thus, higher total carbon conversions were obtained with the catalytic bed with the highest content of the Cu/Zn/Al₂O₃ phase. The presence of 𝛾-Al₂O₃ allows to exceed the equilibrium conversion of CO for the syngas to methanol synthesis. The highest DME productivity is obtained with the catalytic bed containing equal amounts of both catalytic phases. In addition, we also show that other reaction variables such as temperature, pressure, and contact time also play an important role in terms of DME productivity. The presence of a high fraction of CO₂ in the syngas results in a high production of H₂O, which after long times on stream result in the deactivation of the Cu/ZnO/Al₂O₃ catalytic phase due to the sintering of the copper particles. The in situ removal of H₂O via the addition of an H₂O sorbent, zeolite 3A, into the catalytic bed, results in a significant enhancement of both carbon conversion and DME productivity.