Lower olefins (C2-C4) and aromatics are the most important building blocks in the current chemical industry. Syngas is an important platform for the transformation of various non-petroleum carbon resources such as natural/shale gas, coal, biomass and even CO2 into chemicals and fuels. Fischer-Tropsch (FT) synthesis is a key reaction for the conversion of syngas to hydrocarbon products. However, the products of FT synthesis usually follow the Anderson-Schulz-Flory distribution, which is non-selective for middle-distillate target products such as lower olefins. The selectivity of lower olefins is typically lower than 60% via FT synthesis, while the selectivity of aromatics is lower than 15%. Here, we report the direct syntheses of lower olefins and aromatics via coupling of syngas to methanol and methanol to lower olefins (MTO) and aromatics (MTA). We found that a Zn-ZrO2 catalyst worked efficiently for the activation of CO to methanol/DME (dimethyl ether) in wide temperature range, nicely matching the subsequent MTO or MTA reaction catalyzed by SAPO-34 or H-ZSM-5. Although the activity of methanol synthesis over Zn-ZrO2 was low at the temperature needed for MTO/MTA reaction (~673 K) due to thermodynamic limitation, the coupling with MTO or MTA reaction could significantly drive the CO conversion by the thermodynamic driving force. We obtained a selectivity of lower olefins of 78% at 30% CO conversion over the Zn-ZrO2/SAPO-34. The selectivity of aromatics was 80% at a 20% CO conversion over the Zn-ZrO2/H-ZSM-5 catalysts. Moreover, the catalyst was very stable. Our deep studies revealed that the oxygen vacancies on ZrO2 surfaces were responsible for the activation of CO, while the surface -Zn-O- pairs functioned for H2 activation. The unique H species generated on Zn-ZrO2 surfaces are responsible for the outstanding lower-olefin selectivity over our bifunctional catalysts. A moderate density of Brønsted acid sites plays a key role in the selective formation of lower olefins or aromatics and the catalyst stability. Furthermore, the two active components should be integrated in proper proximity.

Direct conversion of syngas into lower olefins and aromatics with bifunctional catalysts

255th National Meeting (2018)

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