Thermocatalytic hydrogenation of CO2 to hydrocarbons has been considered as a promising route to produce carbon neutral fuels and has drawn extensive interest in research community recently. Even though considerable progress has been achieved for CO2 hydrogenation to C1 products (methane or methanol), synthesis of branched hydrocarbons for gasoline blendstock or jet fuel precursors is still challenging due to side reactions forming C1 products and low selectivity to desired hydrocarbons. In this contribution, we demonstrate the synthesis of highly branched hydrocarbons from CO2 hydrogenation under mild conditions (≤ 220 °C and ≤ 1 MPa) over composite catalysts (Scheme 1) comprised of commercial Cu-Zn-Al oxide catalyst (CZA) for methanol synthesis, g-Al2O3 catalyst (A) for methanol dehydration to dimethyl ether (DME) and supported Cu on BEA zeolite catalyst (CuBEA) for hydrocarbon synthesis. Effect of catalyst bed configuration was investigated with a physical mixture of CZA, A and CuBEA catalyst in one case (mixed bed) and a stacking bed of CZA and A mixture upstream of CuBEA in another case (stacked bed). Even though overall CO2 conversion was similar for the two cases, as expected when the same WHSV relative to CZA was maintained, hydrocarbon formation from the subsequent conversion of methanol and DME over CuBEA was higher in the stacked bed than in the mixed bed configuration. The different performance in hydrocarbon production over the two catalyst bed configurations is attributed to the evolution of methanol/DME concentration along the catalyst bed, and the subsequent effect on the hydrocarbon pool chemistry that occurs in zeolite catalysis, such as CuBEA. Near complete conversion of oxygenates (DME and methanol) was achieved by optimizing the relative catalyst loadings and reaction conditions with the stacked catalyst bed, yielding high selectivity to hydrocarbon products. Over 94% CO-free C-selectivity of C4+ hydrocarbons was achieved, which is unprecedented and advantageous for downstream separation, as CO can be easily separated and recycled to improve hydrocarbon production.

Production of highly branched hydrocarbons from CO2 hydrogenation over composite catalysts

ACS Fall 2021

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