Fischer-Tropsch Synthesis on Freeze-Cast Hybrid-Backbone Meso-Macroporous Micromonolith Catalysts: Reconciling High Pore Transport Rates with Improved Reaction Heat Management

Fischer-Tropsch synthesis on freeze-cast hybrid-backbone meso-macroporous micromonolith catalysts: reconciling high pore transport rates with improved reaction heat management

Kai Jeske,^a Jonglack Kim,^a Valentina Nese,^a Jochen Joos,^b Nicolas Duyckaerts,^a Norbert Pfänder,^c and Gonzalo Prieto^a,*

^a Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany), ^b Institute for Applied Materials (IAM-WET), Karlsruhe Institute of Technology (KIT), Adenauerring 20b, 76131, Karlsruhe (Germany), ^c Max-Planck-Institut für Chem. Energiekonversion, Stiftstraße 34-36, 45470 Mülheim an der Ruhr (Germany)

* Email: prieto@mpi-muelheim.mpg.de

 

Background

Pore mass transport phenomena in the cobalt-catalyzed Fischer-Tropsch (FT) synthesis are of kinetic significance for the extent of secondary processing of primary 1-olefin products and thus for the overall reaction selectivity under industrially relevant conditions.^[1] This has driven attention to catalysts with large, hierarchically organized porosities, particularly if high selectivity to C₅₊ olefins is targeted.^[2] Highly porous catalysts are, however, intrinsically thermally insulating, which poses a challenge for managing the significant heat of reaction, and the prevention of hot-spots that can have a deleterious impact on selectivity. Common strategies to improve heat dissipation, such as deposition of the catalytically active species within metallic open foams or meshes,^[3] lead to low volumetric catalyst loadings. Albeit barely explored in the field of catalysis, the so-called freeze-casting synthesis route for macroporous materials is compositionally flexible, and thus provides a means for the assembly of hierarchically porous composite catalysts uniting high mass transport rates with thermally conductive skeletons.^[4]

Aim

Here we report the synthesis of micromonolith bodies with a hybrid backbone composed of carbon nanotubes (CNTs) and mesoporous alumina and their application as scaffolds for Co-based FT catalysts.^[5] A honeycomb-shaped and axially oriented macroporous architecture, which closely resembles the structure of conventional macroscopic monolithic catalyst bodies at a micrometer scale, enhances pore mass transport, while the mesoporous ZrO_x/Al₂O₃ oxide backbone component is an excellent support for cobalt nanoparticles. Percolating CNT skeletons contribute low thermal resistance pathways and thus improve the spatial dissipation of heat within the catalyst body, as reflected in the selectivity obtained to liquid olefins at different operation temperatures.

 

Methods

Hierarchically porous monolithic scaffolds with Al₂O₃/CNT (50–50 wt%) hybrid backbones and reference all-alumina backbones were synthesized by unidirectional freeze casting using zirconium acetate as ice growth modulator (Fig. 1a-d). Impregnation yielded supported Ru-promoted Co catalysts which were tested in a fixed-bed reactor setup for Fischer-Tropsch synthesis at 463-503 K, WHSV ~ 1.44 g_CO g_Co^-1 h^-1, P = 20 bar, and H₂/CO = 2.0. Catalysts were characterized by N₂-sorption, H₂-chemisorption, XRD, SEM, HAADF-STEM/EDX and quantitative X-ray tomography.

 

Results and discussion

Figure 1: a) Picture of a CNT-ZrAlO_x monolith body after freeze-casting and annealing. b) High-resolution SEM micrograph showing the structure of the channel walls in a CNT-ZrAlO_x monolith. c,d) Cross-sectional SEM micrographs perpendicular to the monolith axial axis, and e,f) surface-rendered 3D reconstructed X-ray tomograms for CNT-ZrAlO_x micromonoliths freeze cast at cooling rates of -0.5 K min^-1 (c,e) and -10 K min^-1 (d,f). g) Relationship between the volume-averaged macropore diameter and the cooling rate applied during freeze casting for CNT-ZrAlO_x micromonoliths. Error bars correspond to the standard deviation of the average.

 

The reconstructed tomograms for a representative sub-region of monoliths cast at -0.5 and -10 K min^-1 were analysed using a local thickness algorithm to determine the dependency of macropore diameter and macropore wall thickness on the cooling rate (Fig. 1e,f). The macropore diameter along the z-axis was found to be a function of the cooling rate (σ) according to the power law, PD=k· σⁿ, with n being 0.50 (Fig. 1g), suggesting an increasingly higher supercooling at the freezing interface, and higher ice nucleation rates during casting at increasingly higher cooling rates.

Figure 2: a) Anderson-Schulz-Flory hydrocarbon chain-length distribution plots, and b) olefin-to-paraffin molar ratio for hydrocarbon products in the carbon chain length range from C₂ to C₈, for different Co-based Fischer-Tropsch catalysts at 463 K. c) Evolution of the metal-specific Fischer-Tropsch reaction rate (cobalt-time yield) with the reaction temperature for different Co-based Fischer-Tropsch catalysts. d) Olefin-to-paraffin molar ratio for hydrocarbon products in the carbon chain length range from C₂ to C₈, for different Co-based Fischer-Tropsch catalysts at 483 K. WHSV=1.44 g_CO g_Co^-1 h^-1, P=20 bar, H₂/CO=1.0.

The incorporation of CNTs into the skeleton led to a 2.5-fold improvement in the effective thermal conductivity in the composite bodies. The cobalt-loaded structured micromonoliths proved to be effective catalysts for the FT synthesis (Fig. 2). Under industrially relevant reaction conditions, higher rates of evacuation of primary reaction products from the metal active sites through the directional macropore system led to enhanced selectivities to liquid (C₅₊) α-olefin products compared to conventional, microgranulate catalysts based on unimodally mesoporous γ-Al₂O₃ support materials (Fig. 2b). Moreover, the higher effective thermal conductivity associated with the hybrid CNT-ZrAlO_x micromonolith backbone inhibits the development of hotspots within the catalyst body and thus a more gradual increase in the CO conversion rate on increasing the reactor outer wall temperature (Fig. 2c). This prevented undesired secondary hydrogenation of primary olefin products and methanation in a wider range of operation temperatures (Fig. 2d). These findings illustrate the significance of a dual compositional and structural design of multimodally porous bodies by directional freeze casting to produce effective solid catalysts for intensified syngas conversion processes.

 

References

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[3] a) L. Fratalocchi, C. G. Visconti, G. Groppi, L. Lietti and E. Tronconi, Chem. Eng. J., 2018, 349, 829-837. b) M. S. Challiwala, B. A. Wilhite, M. M. Ghouri and N. O. Elbashir, AIChE Journal, 2018, 64, 1723-1731.

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