hide
Free keywords:
-
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.
