Direct production of olefins from syngas with ultrahigh carbon efficiency

Olefins (C2–4 and C5+ α-olefins) are essential feedstocks for plastics and chemicals. Conventional production relies on naphtha cracking and light-alkane pyrolysis; higher-value long-chain olefins are typically obtained by oligomerization. To diversify away from limited petroleum resources, direct conversion of syngas (H2/CO) via STO/FTO routes is attractive. Two main strategies have emerged: (i) bifunctional oxide–zeolite (OX–ZEO) catalysts, enabling up to ~80% lower-olefins selectivity at <20% CO conversion at ~673 K, and (ii) FTO over promoted Fe or Co carbides, reaching up to ~60% lower-olefins selectivity. A pervasive challenge for both is high CO2 selectivity (30–50%), which lowers carbon efficiency and complicates process operation (e.g., recycling loops and CO2 removal). Recent advances (e.g., hydrophobic FeMn@Si-c; Na/S/Mn-modified hcp Co) mitigate CH4/CO2 to some extent but still suffer from limited carbon efficiency and difficulty achieving high olefins, especially C5+, with minimal C1 by-products at useful conversion. This work targets a catalyst and mechanism that maximize olefins selectivity and yield while minimizing CH4 and CO2 under practical conditions.

Prior STO progress includes OX–ZEO composites where CO activation and C–C coupling occur on separate sites, giving high lower-olefins selectivity but at low conversion and with substantial CO2 formation. Classic FTS over Fe, Co, and Ru tends to give paraffins; only promoted Fe or Co carbides have achieved up to ~60% lower-olefins selectivity. However, both OX–ZEO and metal-carbide FTO typically produce 30–50% CO2, reducing true olefins selectivity when CO2 is counted. Strategies to lower CO2 have focused on tailoring catalyst surface properties and suppressing intrinsic WGS activity. Examples include FeMn@Si-c (total CO2+CH4 ~23% at ~65% olefins, 56.1% CO conversion) and Na/S/Mn-modified hcp Co (54% lower-olefins with 17% CH4 and <3% CO2 at ~1% CO conversion). Despite improvements, achieving high olefins (including C5+) with very low C1 by-products at significant conversion remains challenging. Ruthenium catalysts are classically paraffin-selective; turning Ru to an olefin-selective, low-WGS FTO catalyst would be impactful.

Catalyst synthesis: Ru/SiO2 and Na-Ru/SiO2 catalysts were prepared by incipient wetness impregnation using ruthenium nitrosyl nitrate on aerosol SiO2 (AEROSIL 380). NaNO3 was co-impregnated to achieve Na/Ru molar ratios (0–2); typical Ru loading was 5 wt%. Samples were dried (333 K, 4 h) and calcined in air (673 K, 4 h). Other alkali promoters (Li, K, Rb, Cs) were similarly prepared at promoter/Ru = 0.5. A PVP-assisted route [Na–yRu(P)/SiO2, PVP/Ru mass ratio 10, calcined to remove PVP] increased Ru dispersion; y denotes theoretical Ru wt%. Catalytic evaluation: Fixed-bed reactor (10 mm i.d.), 1 g catalyst (40–60 mesh) diluted with quartz sand (6 g). Pre-reduction: H2, 723 K, 4 h. Reaction: syngas H2/CO = 2/1 (H2/CO/N2 = 64.7/32.3/3), 50 mL·min−1; WHSV 3000 mL·gcat−1·h−1; 533 K; 1.0 MPa, unless otherwise stated. Analysis: online GC (TCD for H2, N2, CO, CH4, CO2; FID for C1–C7 hydrocarbons), with hot (393 K) and cold (273 K) traps; off-line GC for aqueous, oil, wax fractions. Performance reported after ~12 h on stream; balances maintained within 100 ± 5%. CO conversion and selectivity on a carbon basis; oxygenates <1% excluded unless noted. Definitions provided for XCO, Si, Yolefins, rate RCO, and TOFCO. Characterization: Ex situ and in situ XRD; (HR)TEM/HAADF-STEM-EDX for particle size and Na distribution; ICP-OES for Ru content; CO chemisorption for dispersion; H2-TPR; XPS (Ru 3d, C 1s); in situ DRIFTS of CO and ethene adsorption/reactivity (including stepwise hydrogenation at 533 K); XAFS (XANES/EXAFS) at Ru K-edge under reduction (H2) and reaction (H2/CO) conditions; TG; CO-TPSR (CH4 evolution); ethene/propene pulse transient hydrogenation (mass spectrometry); co-feeding ethene with syngas; WGS probe tests by introducing water and by H2-off (CO+H2O) conditions. Computations: DFT calculations (including Bader charge and ethylene adsorption energy/geometry) to assess charge transfer from Na2O to Ru and adsorption strength changes.

- Performance: Na-Ru/SiO2 (Ru ~5 wt%, Na/Ru = 0.5) at 533 K, 1.0 MPa, H2/CO=2 achieved 80.1% olefins selectivity at 45.8% CO conversion with CO2 2.7% and CH4 2.2% (total C1 <5%). ASF chain-growth probability α ≈ 0.75; CH4 far below ASF prediction. In olefins, C5+ fraction 74.5%, including 57.8% C5–11 α-olefins and 16.4% C12–18 α-olefins. - Operating variables: Increasing H2/CO to 5 raised CO conversion to 72.4% while maintaining >70% olefins selectivity and low C1 fraction. Lower WHSV (1500 mL·gcat−1·h−1) gave single-pass olefins yield up to 51.9% at 67.9% CO conversion with 6.7% C1 by-products. Higher pressure reduced olefins selectivity (81.0% at 0.5 MPa to 53.5% at 3 MPa). - Stability: Na-2%Ru(P)/SiO2 maintained ~0.700 molCO·gRu−1·h−1 (TOF ~0.210 s−1), 75–80% olefins selectivity, and <5% C1 by-products over 500–550 h without significant deactivation. - Comparison to unpromoted Ru: Ru/SiO2 (no Na) gave 73.3% CO conversion with 76.5% paraffins and only 16.9% olefins. Introducing Na decreased activity and CH4, increased olefins >70% for Na/Ru ≥0.2, peaking at 80.1% at Na/Ru=0.5. Excessive Na (Na/Ru=2) lowered olefins selectivity and raised C1 to ~30%. - Other alkali promoters: Li→Cs decreased CO conversion but increased long-chain olefins share in the olefins pool from 72.7% (Li) to 83.0% (Cs). - Structure and electronic effects: Under H2 and FTO conditions, RuO2 fully reduced to metallic Ru and remained metallic (in situ XRD/XAFS). Na increased Ru dispersion (TEM, chemisorption) and created electron-rich Ru sites (XPS Ru 3d5/2 shift by −0.5 eV; CO-DRIFTS linear CO band shift 2053→2039 cm−1), enhancing CO adsorption (additional bridge/interface CO bands; higher CO uptake). DFT Bader analysis: each Na2O moiety donates ~0.64|e| to adjacent Ru; ethylene adsorption energy weakened from −1.09 eV to −0.75 eV with Na2O, shortening C=C bond to 1.43 Å, favoring desorption. - Hydrogen reactivity/mobility: In situ DRIFTS during H2 exposure at 533 K showed more persistent COad and lower CH4 formation on Na-Ru/SiO2, indicating suppressed reactivity of chemisorbed H. CO-TPSR showed higher CH4 formation temperature and intensity shift consistent with hindered hydrogenation. Ethene pulse hydrogenation: Ru/SiO2 showed substantial ethene→ethane (R=C2H4/C2H6=5.5), while Na-Ru/SiO2 nearly suppressed hydrogenation (R=22.5). Co-feeding ethene: Ru/SiO2 showed ethane formation rate ↑ by factor 592.8 and C2H4/C2H6 ~0.8; Na-Ru/SiO2 showed C2H4/C2H6 ~113.9 with minimal hydrogenation and limited participation of ethene in chain growth. - WGS/CO2: CO2 selectivity typically <3%. WGS probe: adding H2O under FTO conditions did not change TOF (≈0.102 s−1) or CO2 selectivity (~2.1%); switching to CO+H2O (no H2) dropped TOF to 0.005 s−1, showing negligible intrinsic WGS over metallic Ru phase. - Pilot-scale: Na-2%Ru(P)/SiO2 pellets (pilot fixed-bed, 538 K, 1.0 MPa, WHSV 3000 mL·gcat−1·h−1, H2/CO=2) delivered TOF 0.312 s−1 with 72.5% olefins selectivity, 1.8% CH4, and 2.5% CO2, comparable to microreactor results and superior to most prior Ru catalysts; versus Fe-based FTO, Ru-based system offered higher olefins selectivity (~80% vs ~60%), higher olefins yield (~52% vs ~36%), and lower temperature/pressure.

The study demonstrates that Na promotion converts Ru/SiO2 from a paraffin-selective FTS catalyst to a non-classical FTO catalyst with ultrahigh olefins selectivity and minimal C1 by-products. Mechanistically, Na increases electron density on Ru, strengthens CO adsorption, and reduces the number and mobility of H adsorption sites (likely blocking low-coordination edge/corner Ru sites). The resulting CO-rich, H-lean surface suppresses hydrogenation, favoring β-H elimination for chain termination and inhibiting secondary olefin hydrogenation—thereby producing predominantly primary linear olefins and lowering CH4 formation. The metallic Ru phase remains intact under reaction conditions, and intrinsic WGS activity is negligible, explaining the ultralow CO2 selectivity and high carbon efficiency. Operationally, high olefins selectivity is retained over a broad range of H2/CO ratios and at high conversion/space-time, with long-term stability and scalability validated in a pilot reactor. These findings directly address the field’s challenge of maximizing olefins (including C5+) while minimizing CH4/CO2 at practical conversions, offering a promising route for efficient syngas-to-olefins processes.

Na-promoted metallic Ru nanoparticles on SiO2 deliver a non-classical FTO process with ultrahigh carbon efficiency: olefins selectivity up to 80.1% at ~46% CO conversion with CH4+CO2 <5%, dominated by long-chain α-olefins, and exhibiting excellent stability over 500–550 h. Structural/operando studies and probe reactions reveal that Na induces electron-rich Ru, enhances CO adsorption, and suppresses chemisorbed H reactivity, shifting chain termination toward β-H elimination and minimizing secondary hydrogenation. The catalyst scales to a pilot fixed-bed reactor with high performance under mild conditions. Future work should further reduce Ru loading and overall catalyst cost (e.g., via dispersion-enhancing strategies like PVP), optimize promoter identity/loading, and refine reactor conditions to maintain high olefins selectivity at elevated pressures relevant to industry.

- Use of noble Ru increases catalyst cost; the study notes the need to further decrease Ru loading to improve economic feasibility. - Excessive Na loading (Na/Ru≈2) degrades performance, reducing olefins selectivity and increasing C1 by-products (~30%), indicating a narrow optimal promoter window. - Na promotion reduces overall hydrogenation activity, which can decrease CO conversion at fixed conditions relative to unpromoted Ru, necessitating optimization of space velocity and H2/CO. - Higher pressure (up to 3 MPa) diminishes olefins selectivity (to ~53.5%), suggesting sensitivity to pressure that may constrain some industrial operating windows. - Slight increases in CO2 were observed upon Na addition relative to bare Ru under some conditions, though absolute CO2 remained low (<3%).