Abstract
Adjusting hydrocarbon product distributions in the Fischer–Tropsch (FT) synthesis is of notable significance in the context of so-called X-to-liquids (XTL) technologies. While cobalt catalysts are selective to long-chain paraffin precursors for synthetic jet- and diesel-fuels, lighter (C10–) alkane condensates are less valuable for fuel production. Alternatively, iron carbide-based catalysts are suitable for the coproduction of paraffinic waxes alongside liquid (and gaseous) olefin chemicals; however, their activity for the water–gas-shift reaction (WGSR) is notoriously detrimental when hydrogen-rich syngas feeds, for example, derived from (unconventional) natural gas, are to be converted. Herein the roles of pore architecture and oxide promoters of Lewis basic character on CoRu/Al2O3 FT catalysts are systematically addressed, targeting the development of catalysts with unusually high selectivity to liquid olefins. Both alkali and lanthanide oxides lead to a decrease in turnover frequency. The latter, particularly PrOx, prove effective to boost the selectivity to liquid (C5–10) olefins without undesired WGSR activity. In situ CO-FTIR spectroscopy suggests a dual promotion via both electronic modification of surface Co sites and the inhibition of Lewis acidity on the support, which has direct implications for double-bond isomerization reactivity and thus the regioisomery of liquid olefin products. Density functional theory calculations ascribe oxide promotion to an enhanced competitive adsorption of molecular CO versus hydrogen and olefins on oxide-decorated cobalt surfaces, dampening (secondary) olefin hydrogenation, and suggest an exacerbated metal surface carbophilicity to underlie the undesired induction of WGSR activity by strongly electron-donating alkali oxide promoters. Enhanced pore molecular transport within a multimodal meso-macroporous architecture in combination with PrOx as promoter, at an optimal surface loading of 1 Prat nm–2, results in an unconventional product distribution, reconciling benefits intrinsic to Co- and Fe-based FT catalysts, respectively. A chain-growth probability of 0.75, and thus >70 C% selectivity to C5+ products, is achieved alongside lighter hydrocarbon (C5–10) condensates that are significantly enriched in added-value chemicals (67 C%), predominantly α-olefins but also linear alcohols, remarkably with essentially no CO2 side-production (<1%). Such unusual product distributions, integrating precursors for synthetic fuels and liquid platform chemicals, might be desired to diversify the scope and improve the economics of small-scale gas- and biomass-to-liquid processes.
Highlights
The Fischer−Tropsch (FT) synthesis forms the core of GTL processes, enabling the valorization of natural gas into synthetic hydrocarbon fuels and specialty lubricants via syngas (CO+H2) as a versatile intermediate.[3]
In a future scenario where personal and short-distance transport becomes largely electrified, synthetic GTL fuels are expected to play a central role in the heavy-duty ground transport and aviation sectors, where volumetric energy density considerations make liquid fuels a nearly irreplaceable choice
The combination of pore transport enhancement with surface promotion effects could provide a blueprint toward unconventional FT product distributions uniting high selectivities to C5−10 α-olefin chemicals and heavier paraffinic fuel precursors while retaining the essentially null CO2 side-production inherent to cobalt-based FT catalysts
Summary
The widespread availability of (unconventional) natural gas resources make gas-to-liquid (GTL) technologies an attractive alternative to current refining, which relies on centralized and dwindling crude oil supplies, for the production of fuels and chemicals.[1,2] The Fischer−Tropsch (FT) synthesis forms the core of GTL processes, enabling the valorization of natural gas into synthetic hydrocarbon fuels and specialty lubricants via syngas (CO+H2) as a versatile intermediate.[3]. Designing the pore architecture of Co-based FT catalysts to achieve fast pore transport rates has proven effective to minimize secondary hydrogenation and boost the share of liquid olefins in the products This strategy proved useful to curb undesired overcracking in tandem FT/hydrocracking processes for the direct conversion of syngas into wax-free liquid hydrocarbons.[33,34] The combination of pore transport enhancement with surface promotion effects could provide a blueprint toward unconventional FT product distributions uniting high selectivities to C5−10 α-olefin chemicals and heavier paraffinic fuel precursors while retaining the essentially null CO2 side-production inherent to cobalt-based FT catalysts. Additional experimental and computational details are provided in the Supporting Information
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