Intermediate Product Regulation in Tandem Syngas Conversion to Liquid Fuels

The single-step production of liquid hydrocarbons from syngas (H₂+CO) via the tandem integration of Fischer-Trospch (FT) and hydrocracking (HC) reactions represents an attractive approach towards the intensification of gas-to-liquid (GTL) technologies for the valorization of unconventional and delocalized carbon resources into transportable liquid hydrocarbons, circumventing the wax handling required in conventional GTL processes.[1] However, the integration of two solid catalysts in a single reactor, to steer sequential reactions in tandem poses important challenges. The sharing of a common reaction medium generally means that each catalyst operates under individually suboptimal conditions. Besides, the dichotomy is often faced that close spatial proximity between the tandem catalysts – required for an effective transport of intermediate products between their active sites with minor derivation via undesired reaction pathways – prohibits individual adjustment of other key parameters such as the catalyst operation temperature. The FT/HC process is an excellent showcase for these limitations.

Operating the FT catalyst at the higher temperatures needed for HC, and the strong inhibition of the metal sites on HC catalysts by CO – which enhances secondary cracking and hydrogenolysis pathways – lead to high selectivities to gas (C_4-) hydrocarbons, which severely penalize the efficiency and economy of the tandem process. Our studies with model FT hydrocarbons on a Pt/ZSM-5 HC catalyst showed that, under the syngas atmosphere of the tandem process, α-olefin primary FT products can mitigate this undesired down-shift of the product distribution by curbing secondary cracking.[2] Moreover, we have found that the achievement of sub-micrometer effective transport distances in multimodally porous cobalt-based FT catalysts, enables the "channeling" of primary FT products onto the HC catalyst. In this manner, the two particulate catalysts can operate spatially distant – thus allowing a catalyst-specific temperature adjustment – albeit resembling the case of a nanoscale chemical intimacy between their active sites. The new catalyst and process configurations reconcile an effective wax depletion with high selectivities to middle distillates and minimum production of undesired gas hydrocarbons.[3]

References

[1] a) A. Martinez, G. Prieto, Top. Catal. 52 (2009) 75-90; b) Q. Zhang, K. Cheng, J. Kang, W. Deng, Y. Wang, ChemSusChem 7 (2014) 1251-64 c) S. Sartipi, M. Makkee, F. Kapteijn, J. Gascon, Catal. Sci. Technol. 4 (2014) 893-907; d) J. Li, Y. He, L. Tan, P. Zhang, X. Peng, A. Oruganti, G.Yang, H. Abe, Y. Wang, N. Tsubaki, Nature Catal. 1 (2018) 787–793.

[2] N. Duyckaerts, et al., ACS Catal. 6 (2016) 4229–4238.

[3] N. Duyckaerts, et al., Angew. Chem. Int. Ed. 56 (2017) 11480–11484.