Study of catalyst bed composition for the direct synthesis of dimethyl ether from CO2-rich syngas

Abstract

• CO 2 -rich syngas can be directly transformed to DME over CZA:γAl 2 O 3 catalytic beds. • Maximum DME productivity over the beds containing equal amounts of CZA and γAl 2 O 3 . • CO 2 enhances H 2 O production, decreasing carbon conversion and DME productivity. • Cu particle sinterization due to the presence of H 2 O is observed. • In situ H 2 O removal using zeolites enhances catalytic performance for DME production. In this work, we study the direct synthesis of DME using CO 2 -rich syngas, with a CO 2 /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/Al 2 O 3 and γ-Al 2 O 3 , respectively. Our results show that the ratio between each catalytic phase determines the productivity and selectivity to DME, as well CO and CO 2 conversions. Thus, higher total carbon conversions were obtained with the catalytic bed with the highest content of the Cu/Zn/Al 2 O 3 phase. The presence of γ-Al 2 O 3 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 CO 2 in the syngas results in a high production of H 2 O, which after long times on stream result in the deactivation of the Cu/ZnO/Al 2 O 3 catalytic phase due to the sintering of the copper particles. The in situ removal of H 2 O via the addition of an H 2 O sorbent, zeolite 3A, into the catalytic bed, results in a significant enhancement of both carbon conversion and DME productivity.

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