(679h) Direct Conversion of Renewable CO2-Rich Syngas to High-Octane Hydrocarbons in a Single Reactor

Syngas can be created through renewable carbon sources (biomass, biogas) via known gasification processes. Conversion of syngas to hydrocarbons via intermediates like methanol or dimethyl ether (DME) is known, however further upgrading to high-value hydrocarbon fuel products can be a capitally intensive or costly process due to the higher temperatures and pressures necessary. Here, we used our previously developed approach, termed the "High-Octane Gasoline (HOG) pathway”, to convert DME to linear and branched hydrocarbons over a Cu-modified beta zeolite catalyst (Cu/BEA) with high selectivities to products with high-octane fuel properties under mild conditions (220 °C, 0.75 MPa).¹ Employing the cascade chemistry of syngas to hydrocarbons (STH) through methanol and DME intermediates, we exploited overlapping process conditions in a single reactor with a commercial methanol synthesis catalyst (Cu/ZnO/Al₂O₃, CZA), a methanol dehydration catalyst (g-Al₂O₃), and our DME-to-hydrocarbons catalyst, Cu/BEA. Reactor configurations with CZA and g-Al₂O₃ catalysts mixed with or stacked physically upstream of the Cu/BEA catalyst were investigated and benchmarked against configurations with H/BEA. Although “mixed-bed” configurations had high yields of C₄₊ hydrocarbons, the “stacked-bed” configuration resulted in comparable C₄₊ hydrocarbon yields with higher selectivity towards high-octane C₇ hydrocarbon species, and importantly, lower selectivity towards CO₂. Additionally, comparison to H/BEA demonstrated the importance of the multifunctional Cu species in the zeolite with improvements observed for key metrics such as C₇₊ selectivity and CO₂ selectivity. Further process considerations such as bed composition and reactor pressure and temperature were explored to improve selectivity towards C₄₊ hydrocarbons and away from CO₂ at high conversions and yields. Through control of the catalyst bed compositions and process conditions, we demonstrated that the Cu/BEA activity in the STH reaction can meet or exceed that observed in the DME homologation reaction, while minimized C_1-3 hydrocarbon and CO₂ selectivities and maintaining C_4-7 hydrocarbon selectivity profiles.

The gasification of renewable carbon sources creates a CO₂-rich syngas with CO₂ concentrations ranging from 19-23 %,² therefore co-conversion of CO and CO₂ in our single reactor approach would substantially improve carbon efficiency and overall fuel product yields for a biomass-based syngas conversion process. To model a CO₂-rich syngas feed, we co-fed CO₂ with syngas at H₂:CO:CO₂ ratios ranging from 2:1:0.8 to 2.6:1:0.9. The CO conversion and C₄₊ hydrocarbon yields decreased compared to CO₂-free conditions, but a decrease in CO₂ selectivity was also observed, suggesting CO₂ utilization in the reaction network. Definitive evidence of CO₂ incorporation into the hydrocarbon products was demonstrated with isotopically-labeled ¹³CO₂ experiments, where propagation of ¹³C into the C₄₊ hydrocarbons was confirmed with mass spectrometry. Using a syngas feed composition modeled after the low-cost biomass feedstock of forest residues (H₂:CO:CO₂ ratio of 2.6:1:0.9)², a decrease in CO₂ carbon selectivity below 25 % was observed while C₄₊ hydrocarbon selectivity remaining above 60 %. Here we established the factors to create an intensified process which co-converts CO₂ with syngas to hydrocarbon products and allows for a “market-responsive” biorefinery design,³ producing high octane gasoline or sustainable aviation fuel (SAF) range hydrocarbons to meet demands for a more sustainable route to liquid fuels.

References:

(1) Schaidle, J. A.; Ruddy, D. A.; Habas, S. E.; Pan, M.; Zhang, G.; Miller, J. T.; Hensley, J. E. Conversion of Dimethyl Ether to 2,2,3-Trimethylbutane over a Cu/BEA Catalyst: Role of Cu Sites in Hydrogen Incorporation. ACS Catal. 2015, 5 (3), 1794–1803.

(2) Dupuis, D. P.; Grim, R. G.; Nelson, E.; Tan, E. C. D.; Ruddy, D. A.; Hernandez, S.; Westover, T.; Hensley, J. E.; Carpenter, D. High-Octane Gasoline from Biomass: Experimental, Economic, and Environmental Assessment. Appl. Energy 2019, 241, 25–33.

(3) Ruddy, D. A.; Hensley, J. E.; Nash, C. P.; Tan, E. C. D.; Christensen, E.; Farberow, C. A.; Baddour, F. G.; Van Allsburg, K. M.; Schaidle, J. A. Methanol to High-Octane Gasoline within a Market-Responsive Biorefinery Concept Enabled by Catalysis. Nat. Catal. 2019, 2 (7), 632–640.