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Abstract

The single-step production of wax-free liquid hydrocarbons from
syngas (H₂+CO) via
integration of Fischer-Trospch (FT) and hydrocracking catalysts represents an attractive
approach towards 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 α-olefin Fischer-Tropsch primary products were systematically studied on a
Pt/nano-H-ZSM-5 hydrocracking catalyst under H₂
(standard hydrocracking) and syngas (in-situ hydroprocessing) atmospheres. Under H₂, both reactants show indistinguishable reactivity
as 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
α-olefins. Under these conditions,
α-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. 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. These observations were the base for the development of a
Co/Al₂O₃
FT catalyst with enhanced pore transport
to maximize olefin production with high
FT mass-activity. A unique hierarchically organized porosity
enables its tandem integration
with a Pt/ZSM-5 zeolitic hydrotreating catalyst in a spatially distant fashion ‒ permitting a
catalyst-specific temperature adjustment ‒ albeit resembling the case of close active site
proximity ‒ by mitigating secondary reactions of primary FT α-olefin products. This approach
enables the sought conciliation of
in situ
de-waxing with a minimum production of gas
hydrocarbons (18 wt%) and a ca. two-fold higher (50 wt%) selectivity to middle-distillates
compared to tandem pairs based on benchmark mesoporous FT catalysts. An overall 80%
selectivity to liquid hydrocarbons from syngas is attained in one step, attesting for the
potential of this strategy to
increase the carbon efficiency in intensified gas-to-liquid
technologies.