Designing silica-coated CoMn-based catalyst for Fischer-Tropsch synthesis to olefins with low CO₂ emission

Highlights

• WGSR-resistant CoMnAl@SiO₂ catalyst was prepared for FTO with low CO₂ selectivity.

• H₂O can easily transfer from Co₂C surface to adjacent hydrophilic SiO₂ sites.

• SiO₂-coating prevents the reabsorption of water on Co₂C site for further WGSR.

• Alkali metals counteract H₂-enrichment effect of SiO₂ and largely improve O/P ratio.

• Low CO₂ emission improved activity, olefins selectivity and carbon-efficiency of FTO.

Abstract

Co₂C nanoprisms exhibit promising catalytic performance for Fischer-Tropsch synthesis to olefins (FTO) but with high CO₂ selectivity (>40%). Herein, silica-coated CoMn-based catalyst was designed to limit CO₂ production and remained Co₂C nanoprisms as active sites unchanged. With a desired coating amount of silica, CO₂ selectivity was significantly suppressed to 15.1 C% while enhancing olefins selectivity from 39.7 C% to 58.8 C%, which also shows at least 160 h of stability. It is suggested the silica-coating not only reduces the adsorption capacity of H₂O, but also promotes the fast transfer of H₂O away from active sites due to the higher adsorption energy of H₂O on SiO₂ surface, thus suppressing water-gas-shift-reaction (WGSR) activity. Moreover, the sodium promoter can counteract the H₂-enrichment effect caused by SiO₂-coating and largely restrain CH₄ formation and olefins hydrogenation. This work provides an effective strategy to suppress CO₂ formation and enhance the carbon efficiency of FTO process.

Introduction

Syngas conversion is a key platform for sustainable production of clean fuels and value-added chemicals from various non-petroleum carbon resources including coal, biomass, nature gas, solid waste and CO₂ [1], [2], [3]. As one of the most important products from syngas conversion, the olefins can be widely applied as key building blocks for the production of plastics and fine chemicals such as polymers, solvents, drugs, cosmetics, and detergents [4], [5]. In the last several years, syngas conversion to olefins has attracted great attention from academic and industrial community, and many significant breakthroughs have been made in the product selectivity control for syngas conversion. Olefins with very high selectivity are available over the carbide catalysts [6], [7], [8], [9] (Fe_xC, Co_xC) or oxide-zeolite bifunctional catalyst systems [10], [11], [12]. For example, de Jong and co-workers [6] reported a Na/S modified supported Fe-based catalyst for Fischer-Tropsch to olefins (FTO) with 61% of lower olefins selectivity at a limited CO conversion (<1.0%). Zhong et al. [7] developed Co₂C nanoprisms with exposed facet of (101) and (020) that exhibits 60.8% lower olefins selectivity and 5.0% CH₄ fraction at 31.8% CO conversion during syngas conversion. Group of Bao [10] and Wang [11] reported oxide-zeolite bifunctional catalysts for syngas conversion to olefins (STO) with ketene or methanol/dimethyl ether (DME) as intermediate, and ~80% of olefins selectivity in hydrocarbons is achieved. However, a large amount of CO₂ (cal. 30–50%) is generally co-produced and the reported high olefins selectivity does not take into account the presence of CO₂, which would significantly decline when calculated on C-atom basis including CO₂. Actually, the production of considerable amount of CO₂ is a common problem for carbide- or metal oxide-involved syngas conversion process due to the same catalyst also works as site for water-gas-shift reaction (WGSR) [13], [14], [15], [16], [17]. The existence of large amount of CO₂ will cause the decrease of catalytic activity and additional energy consumption in unit of heat, compression and separation for industrial process [18], [19]. It is still a grand challenge to suppress CO₂ formation and improve the carbon efficiency in the field of syngas chemistry.

For syngas conversion to hydrocarbons, the O atom obtained via initial C–O cleavage is removed as H₂O and/or CO₂ by chemisorbed hydrogenation (H*) and CO*, respectively [20]. The produced H₂O may adsorb on the active site and cause WGSR to produce extra CO₂. Many attempts are made to suppress the formation of CO₂ by tuning the local chemical environment and active site structure. One of the effective strategies is to tailor the surface hydrophobic and hydrophilic properties of catalysts [8], [13], [21]. For example, Ding and colleagues[8] reported a hydrophobic core-shell FeMn@SiO₂ catalyst, and the total selectivity of CO₂ and CH₄ can be suppressed to less than 22.5% while remaining ~65% of olefins selectivity during FTO reaction. Similar strategies are also reported for Fe-based FTS [21] and Cu-based DME synthesis from syngas [13]. Another approach is to develop newly active structure. Wang et al. [18] prepared a phase-pure ξ- (′)-Fe₂C for FTS, and the CO₂ selectivity can be suppressed to ~5% at 235 °C. Xie et al. [22] reported a Na/S/Mn modified hcp Co for FTO reaction with 54% of lower olefins selectivity and < 3% of CO₂ fraction at 240 °C and 1 bar. Constructing core-shell Zn–Cr @SAPO catalyst [23] or co-feeding CO₂ for ZnO-ZrO₂/H-ZSM-5 bifunctional catalyst [14] are also applied to limit WGSR activity for syngas conversion. Co₂C nanoprisms exhibit promising FTO performance with high olefins selectivity and low methane formation. However, no efficient approach is available to decrease the CO₂ selectivity [24].

Herein, we reported a successful design of silica-coated CoMn-based catalyst with limited CO₂ emission and Co₂C nanoprisms remained as the active sites with high stability. We demonstrated the hydrophilic coating of SiO₂ in close proximity with active sites are crucial to inhibit WGSR activity and improve olefins selectivity. The structure-performance relationship was detailed explored by combining experiment design, structure characterization and DFT calculation.