In Situ Hydrocracking of Fischer-Tropsch Hydrocarbons: CO-Prompted Diverging Reaction Pathways for Paraffin and α-Olefin Primary Products

The single-step production of wax-free liquid hydrocarbons from syngas (H-2 + CO) via integration of Fischer-Trospch (FT) and hydrocracking catalysts represents an attractive approach toward process intensification in compact gas-to-liquid technologies. Despite current, intensive efforts on the development of hybrid (multifunctional) catalysts to this end, not much is known about the reactivity of different FT primary products on hydrocracking catalysts under syngas. Using model compounds, the individual and collective reactivities of n-paraffin and alpha-olefin FT primary products were systematically studied on a Pt/nano-H-ZSM-5 hydrocracking catalyst under H-2 (standard hydrocracking) and syngas (in situ hydroprocessing) atmospheres. Under H-2, both reactants show indistinguishable reactivity rapid olefin hydrogenation precedes hydrocracking. Under syngas, however, inhibition of (de)hydrogenation functionalities by CO poisoning of metal sites leads to a notable divergence of the reaction pathways for n-paraffins and alpha-olefins. Under these conditions, alpha-olefins showed enhanced reactivity, as an initial dehydrogenative activation step is not required, and contributed to moderate secondary cracking, likely via enhanced competitive adsorption on the acid sites. Besides, CO poisoning restored the intrinsic activity of the zeolite for the oligomerization of short-chain (alpha-)olefins, providing an additional net chain-growth pathway, which contributes to reducing the overall yield to undesired gas (C4-) hydrocarbons. These findings emphasize the key role of not only the chain-length distribution, but also the olefinic content of the FT primary hydrocarbons for the ultimate product distribution, and suggest guidelines for the design of multifunctional catalysts for the single-step synthesis of liquid hydrocarbons from syngas.