Thermo-catalytic hydrogenation of CO and/or CO2 to hydrocarbons has been considered as a promising route to produce carbon neutral fuels and has drawn extensive interest in research community recently. Specifically, the production of hydrocarbons from syngas in the presence of CO2 (CO2-rich syngas) through the direct conversion via methanol and dimethyl ether (DME) intermediates is intended to create a cost-competitive method to produce high value fuel products from biomass-based sources. In this contribution, we demonstrate the synthesis of highly branched hydrocarbons from syngas and CO2 hydrogenation in a single reactor system under mild conditions (≤ 220 °C and ≤ 1 MPa) over composite catalysts comprised of commercial Cu-Zn-Al oxide catalyst (CZA) for methanol synthesis, g-Al2O3 catalyst (AlO) for methanol dehydration to DME and supported Cu on BEA zeolite catalyst (CuBEA) for hydrocarbon synthesis. On both reactions of syngas and CO2 hydrogenation, the stacked-bed configuration with a CZA and AlO mixture upstream of the CuBEA catalyst resulted in higher activity than the mixed-bed configuration with a physical mixture of CZA, AlO and CuBEA catalysts. High yields of C5+ hydrocarbon products (up to 40%) was observed from the syngas conversion with a total hydrocarbon gravimetric reactivity (per g CuBEA) comparable or higher than previously reported for DME homologation to hydrocarbons. Near complete conversion of oxygenates (DME and methanol) was achieved in CO2 hydrogenation by optimizing the relative catalyst loadings and reaction conditions with the stacked catalyst bed, yielding unprecedentedly high selectivity to hydrocarbon products with over 94% CO-free C-selectivity of C4+ hydrocarbons. Incorporation of C in CO2 to hydrocarbon products during CO2-rich syngas conversion in a single reactor was demonstrated by cofeeding isotopically labeled 13CO2 and confirming 13C propagation to hydrocarbon products by mass spectrometry. These findings highlight a direct pathway to form highly branched hydrocarbons, a valuable gasoline blendstock or precursor for jet fuel synthesis, from CO2-rich syngas with the ability to tune selectivities and yields by process intensification methods, which will allow for a “market-responsive” biorefinery design to meet demands for a more sustainable route to liquid fuels.

Direct conversion of CO2-rich syngas in a single reactor for the production of highly branched hydrocarbons

ACS Spring 2022

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