Olefins, particularly lower olefins (C₂₋₄) and long-chain olefins (C₅₊), are crucial building blocks in the chemical industry, primarily used in plastics and basic chemical production. Current commercial methods involve naphtha cracking or alkane pyrolysis, and oligomerization of lower olefins. However, the dwindling petroleum reserves and rising olefin demand necessitate exploring alternative production routes from non-petroleum sources. Fischer-Tropsch to olefins (FTO) presents a highly efficient technology for direct olefin production from syngas (a mixture of H₂ and CO), which can be derived from various renewable and non-renewable feedstocks via gasification/reforming. A major challenge in FTO is achieving high olefin selectivity, especially for the valuable C₅₊ fraction, while minimizing undesirable C₁ by-products (CH₄ and CO₂). Recent advances in direct syngas to olefins (STO) have shown promise, particularly those using bifunctional oxide-zeolite composite catalysts (OX-ZEO) that achieve high lower olefin selectivity but limited CO conversion. Alternatively, FTO processes, while efficient for long-chain olefins, often suffer from high CO₂ selectivity, reducing carbon efficiency. This study addresses this challenge by developing a novel catalyst to maximize olefin yield while minimizing C₁ by-products.

The literature extensively explores direct syngas conversion to olefins (STO). One prominent approach utilizes bifunctional oxide-zeolite composite catalysts (OX-ZEO), where CO activation and C-C coupling occur on separate sites. These catalysts have demonstrated lower olefin selectivities up to ~80% but at low CO conversions (<20%). Fischer-Tropsch to olefins (FTO) offers another direct route, particularly effective for long-chain olefins. Classic Fischer-Tropsch synthesis (FTS) primarily produces heavy saturated hydrocarbons. While promoted iron or cobalt carbide catalysts can enhance lower olefin selectivity (up to 60%), they also show high CO₂ selectivity (30-50%), reducing carbon efficiency. Existing methods thus fall short in simultaneously achieving high olefin selectivity (especially C₅₊), yield, and minimizing CH₄ and CO₂ production. Recent efforts, such as using hydrophobic core-shell FeMn@Si-c catalysts, have improved CO₂ and CH₄ suppression but not to a degree that would allow for high overall carbon efficiency.

This study employed a silica-supported ruthenium (Ru) nanoparticle catalyst promoted with sodium (Na) (Na-Ru/SiO₂). The catalyst was synthesized via incipient wetness impregnation. Various characterization techniques were used to investigate the catalyst structure and properties, including ex situ and in situ X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), in situ X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS). Catalytic performance was evaluated in a continuous flow fixed-bed microreactor under various reaction conditions (temperature, pressure, H₂/CO ratio, space velocity). The effect of Na loading was investigated, as well as the stability of the catalyst over extended periods. Additional experiments probed the reactivity of chemisorbed hydrogen using H₂-TPR, CO-TPSR, and pulse transient hydrogenation experiments with ethylene and propylene. A water-gas-shift (WGS) reaction probe experiment was conducted to assess the catalyst's intrinsic WGS reactivity. For industrial relevance testing, the Na-Ru/SiO₂ catalyst was tested in a pilot-scale fixed-bed reactor.

The Na-Ru/SiO₂ catalyst exhibited remarkably high olefin selectivity (80.1%) at a CO conversion of 45.8%, with CH₄ and CO₂ selectivities limited to 2.2% and 2.7%, respectively. The chain-growth probability (α) was 0.75, indicating suitability for long-chain olefin production. The catalyst showed a high C₅₊ olefin fraction (74.5%), including significant amounts of value-added C₅₋₁₁ α-olefins (57.8%) and detergent-range C₁₂₋₁₈ α-olefins (16.4%). Increasing the H₂/CO ratio increased CO conversion, and while maintaining high olefin selectivity (>70%), decreasing the space velocity increased olefin yield. The catalyst demonstrated high stability over 550 hours of operation. Characterizations revealed that Na promotion led to highly dispersed Ru nanoparticles and electron-rich Ru centers. The Na promoter suppressed the reactivity of chemisorbed H atoms, favoring β-H elimination and olefin formation over paraffin formation and inhibiting secondary hydrogenation. The catalyst showed negligible water-gas-shift (WGS) activity. The addition of polyvinylpyrrolidone (PVP) during catalyst preparation further enhanced Ru dispersion and catalytic activity, enabling the use of a lower Ru loading (Na-2%Ru(P)/SiO₂) while maintaining high performance. A pilot-scale test validated the catalyst's excellent performance (72.5% olefin selectivity, 1.8% CH₄, 2.5% CO₂) under industrially relevant conditions.

The findings demonstrate that the Na-promoted Ru catalyst successfully addresses the long-standing challenge of achieving high olefin selectivity and yield from syngas while simultaneously minimizing the formation of undesirable C₁ by-products. The observed high C₅₊ olefin selectivity is particularly significant, as these olefins are highly valuable chemicals. The ultralow WGS reactivity is a key advantage of this catalyst system, enhancing overall carbon efficiency. The electron-rich Ru surface created by Na promotion, as confirmed by various characterization techniques, is crucial in modulating the hydrogenation capacity, promoting β-H elimination, and suppressing secondary hydrogenation. The success of this catalyst offers a compelling alternative to existing FTO technologies with improved carbon efficiency and industrial scalability.

This study presents a highly effective Na-promoted Ru catalyst for direct syngas conversion to olefins with ultrahigh carbon efficiency. The catalyst achieves exceptional olefin selectivity, particularly for long-chain olefins, while significantly suppressing the formation of CH₄ and CO₂. Its excellent stability and performance in a pilot-scale reactor highlight its potential for industrial applications. Future research should focus on further reducing the Ru loading to enhance cost-effectiveness and explore the applicability of this strategy to other metal catalysts.

While the catalyst demonstrated excellent performance and stability under the tested conditions, further studies are needed to assess its long-term stability under more rigorous industrial conditions and to optimize the catalyst formulation for different syngas compositions. The cost associated with the use of Ru, a noble metal, needs to be further addressed for broader commercial implementation. The study focused primarily on olefin production; further investigation into the potential formation of other by-products could also be beneficial.