Relay catalysis in the conversion of syngas

Abstract

China’s energy resource structure is rich in coal but poor in crude oil. For a long time, China has to rely on importing crude oil to meet the domestic demand for liquid fuels (including gasoline, aviation fuel and diesel fuel) and basic chemicals (such as C2–C4 lower olefins, aromatics, and C2+ oxygenates). In 2019, China’s dependence on oil imports has exceeded 70%, which has a negative impact on the social-economic development and national security. Thus, it is very urgent and important to produce liquid fuels and bulk chemicals based on coal resource. Syngas (a mixture of CO and H2) can be largely produced from the non-oil resources, such as coal, gas (natural gas, shale gas, and coal-bed methane), and biomass. From syngas, various hydrocarbon products can be produced as supplement of oil-derived products. The conversion of syngas to target products with high selectivity is the core of C1 chemistry. However, by the classical Fischer-Tropsch synthesis (FTS) route, the product selectivity is limited by the Anderson-Schulz-Flory (ASF) distribution, which results in the low selectivity of target products. In practice, the yields of liquid fuels and bulk chemicals are increased by a secondary process such as (hydro)cracking, isomerization and aromatization. The direct conversion of syngas to target products with high selectivity is attractive but challenging. Recently, the concept of relay catalysis has been successfully applied to the one-step transformation of syngas to liquid fuels and bulk chemicals with ultrahigh selectivity over bifunctional or multifunctional catalysts. The selectivity of target hydrocarbon products can break the limitation of the ASF distribution. In the conversion of syngas to liquid fuels, the used bifunctional catalysts are composed of FT catalysts and zeolites. FT catalysts, such as Fe (Fe x C y ), Co and Ru, are responsible for the activation of CO and the growth of carbon chain. Zeolites are in charge of the hydrocracking and isomerization. High selectivity of middle-distillates, such as gasoline (C5–C11 hydrocarbons), jet fuel (C8–C16 hydrocarbons) and diesel fuel (C10–C20 hydrocarbons), have been obtained via relay catalysis. It was found that the catalytic performances were strongly influenced by the topology, acidity and mesoporosity of zeolites as well as the metal sizes and promoters. The importance of hydrocracking on acid sites and hydrogenolysis on the metal nanoparticles for the selective C–C cleavage has also been analyzed. The selective synthesis of basic chemicals can also be achieved by relay catalysis, where the bifunctional catalysts consist of metal oxides (such as the solution solid ZnO-ZrO2, and the spinel structure of ZnCr2O4, ZnAl2O4) and zeolites (such as SAPO-34, ZSM-5, MOR). The activation of CO to intermediates (CH3OH/DME or ketene) proceeds on the metal oxides and the selective C–C bond formation proceeds on the zeolites. The reaction mechanism of such kind of bifunctional catalysts differs from that of the conventional FT catalysts, where the CO activation and C–C coupling are conducted over different sites. The selectivity of lower olefins and aromatics could reach up to 85% without the formation of undesired methane. Besides, multifunctional catalysts can offer an 80% selectivity towards C2+ oxygenates such as methyl acetate, acetic acid, and ethanol by coupling CO activation and carbonylation. The product selectivity of oxygenates was significantly impacted by the zeolite acidity and the proximity between the two functional components. The present review not only summarizes the major advantages and disadvantages of the relay catalysis in syngas conversion but also offers prospects for future development of relay catalysis in syngas conversion.