Direct Conversion of Syngas into Methyl Acetate, Ethanol, and Ethylene by Relay Catalysis via the Intermediate Dimethyl Ether 

Article Link: https://doi.org/10.1002/anie.201807113

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Catalytic conversion of syngas into C₂₊ compounds via controlled C-C coupling is the core of syngas chemistry. Significant advances have recently been achieved in selective syntheses of liquid fuels, lower olefins, and aromatics from syngas by implementing a reaction-coupling strategy and designing bifunctional catalysts. However, the selective conversion of syngas into C₂₊ oxygenates still remains challenging. C₂₊ oxygenates such as ethanol, acetic acid and methyl acetate are important bulk chemicals having numerous applications. The synthesis of C₂₊ oxygenates from syngas is an atom-efficient route because of the incorporation of oxygen atoms into target products. Modified Rh and bimetallic (e.g., Cu-Co) catalysts containing both CO dissociation and non-dissociation active sites have typically been investigated for the conversion of syngas to C₂₊ oxygenates, but the selectivity of C₂₊ oxygenates is low (usually < 60%). The product distribution in C₂₊ oxygenates is also broad over most catalysts. The development of new methods for direct conversion of syngas into C₂₊ oxygenates with high selectivity is very attractive but challenging.

    The present article reports a new strategy for selective conversion of syngas into C₂₊ oxygenates by designing combinations of catalysts with different functions for relay catalysis. Our idea is that, methanol or dimethyl ether (DME) can undergo C-C coupling via carbonylation with CO into acetic acid (AA) or methyl acetate (MA) catalyzed by zeolite mordenite (H-MOR), and thus it would be promising to realize the direct transformation of syngas into AA or MA by integrating syngas-to-methanol/DME and methanol/DME-to-AA/MA. To the best of our knowledge, so far there has been no success in the direct conversion of syngas into AA/MA using the reaction-coupling strategy.

    We obtained a selectivity of (MA + AA) of ~95% at CO conversion of 4.5% at 473 K by using a combination of Cu-Zn-Al/H-ZSM-5 (for syngas to DME) and H-MOR (for DME carbonylation) catalysts separated by quartz wool (denoted as Cu-Zn-Al/H-ZSM-5│H-MOR). We clarified that the water formed in the conversion of syngas to DME could be removed by water-gas shift reaction, and thus the negative effect of water on H-MOR-catalyzed DME carbonylation could be suppressed. No significant deactivation is observed in 100 h by removing acid sites in 12-MR channels of H-MOR. It is of interest that MA and AA can be synthesized at 603-643 K without significant deactivation by using a combination of spinel-structured ZnAl₂O₄ and H-MOR (ZnAl₂O₄│H-MOR). The selectivity of (MA + AA) is ~85% at CO conversion of 11% at 643 K. DME is the key intermediate and the precise C-C coupling via DME carbonylation results in highly selective formation of MA and AA at both lower and higher temperatures. We have further succeeded in direct conversion of syngas into ethanol and ethylene by using ZnAl₂O₄ and H-MOR combinations with different arrangements. The ZnAl₂O₄│H-MOR│ZnAl₂O₄ provides ethanol with a selectivity of 52%, whereas the ZnAl₂O₄│H-MOR│ZnAl₂O₄│H-MOR offers ethylene. The increase in the proximity between ZnAl₂O₄ and H-MOR increases ethylene selectivity to 65%. It is of high significance that the use of two components, i.e., ZnAl₂O₄ and H-MOR, with different arrangements can result in different products by relay catalysis. We believe that the successful implementation of the strategy of relay catalysis would inspire the development of new catalytic reactions for selective transformations of syngas and CO₂.

    This article was recently published in Angew. Chem. Int. Ed. and was chosen as a Very Important Paper (VIP) based on the evaluation of referees.