(195d) Biogas Sequestration to Carbon Nanomaterials Using Tandem Catalytic Processes

Biogas, composed of CO₂ and CH₄, presents promising, underexplored pathways for greenhouse gas mitigation by sequestering carbon into valuable carbon nanomaterials. Carbon nanofibers (CNFs) and nanotubes (CNTs) are versatile materials that have lifespans significantly longer than conventional chemical products. Biogas conversion to carbon nanomaterials typically relies on dry reforming of CH₄ (DRM) with CO₂ to produce syngas, which then reacts to form C_(s) via the Boudouard and CO hydrogenation reactions. These processes require temperatures >800ºC to limit coke deposition and overcome the thermodynamic stability of CH₄ and CO₂. Discretizing DRM and sequestration circumvents these limitations and enhances solid carbon yield while reducing energy consumption. Here, we present several tandem catalytic strategies for CO₂-assisted and CH₄ into carbon nanomaterials. A one-pot tandem strategy for biogas conversion to CNFs was studied, wherein sequential catalyst beds of Ni/MCM-41and Co₁₂K₅/CeO₂ catalysts were used for the DRM and sequestration steps, respectively, within a single reactor at 600 ºC. This process achieved a catalyst weight gain (CWG) of 1.3 mg CNF/mg Co₁₂K₅/CeO₂ after 5 h. The same catalysts were employed in two separate, sequentially coupled DRM and sequestration reactors, operating at 600ºC and 450ºC, respectively, yielding a maximum CWG of 7.0 mg CNF/mg Co₁₂K₅/CeO₂. Finally, a tandem plasma catalytic-thermocatalytic (PC-TC) scheme was tested. CO₂ and CH₄ were activated within a non-thermal plasma reactor operating at 7 W, before being fed into the sequestration reactor, and a CWG of 0.8 mg CNF/mg Co₁₂K₅/CeO₂ was observed. This tandem PC-TC approach was extended to CO₂-assisted C₂H₆ sequestration into CNTs with the plasma reactor operating at 5 W and the sequestration reactor operating at 750ºC with a Co/γ-Al₂O₃ catalyst. A CWG of 1.2 mg CNT/ Co/γ-Al₂O₃ was obtained. Finally, analyses of the energy costs and CO₂ footprints associated with these processes confirm the advantages of the tandem reactor framework.