Directional freeze-cast hybrid-backbone meso-macroporous bodies as micromonolith catalysts for gas-to-liquid processes

Abstract

Materials with spatially organized and multimodal porosities are very attractive in catalysis, as they can reconcile nano-confinement effects in micro- and mesopores with fast molecular transport in wide macropores. However, the associated large pore volumes often result in low overall thermal conductivities, and thus suboptimal heat management in reactions with a high thermal signature, usually with a deleterious impact on the catalytic performance. Here we report the directional freeze-casting assembly of bimodally meso-macroporous micromonolithic bodies with a hybrid backbone composed of intimately bound carbon nanotubes (CNTs) and ZrO_x–Al₂O₃ nanocrystals. A honeycomb-shaped and axially oriented macroporous architecture is achieved through the use of zirconium acetate as an ice growth modulator. (S)TEM and EDX nanospectroscopy show that the nanoscale intimacy between the CNT and oxide backbone components depends on the synthesis route of the mother slurry. As revealed by X-ray tomography, coupled to quantitative image analysis, not only the macrochannel size and wall thickness, but also the extent of axial heterogeneities in macropore diameter and spatial orientation depend on the axial temperature gradient rate during casting. The structured bodies are explored as carriers for cobalt-based catalysts for the Fischer–Tropsch production of synthetic hydrocarbons from syngas, of central significance in intensified X-to-liquid processes. Hybrid CNT-Al₂O₃ backbone micromonolith catalysts show a high selectivity to C_3–8 olefins, owing to the fast evacuation of these primary reaction products from the metal active sites through the directional macropore system. Remarkably, the high olefin selectivity is maintained up to higher operating temperatures compared to reference catalysts based on all-oxide supports, due to a higher effective thermal conductivity which inhibits the development of hotspots under industrially relevant operating conditions.