Direct biogas upgrading via CO2 methanation to high-quality biomethane over NiMg/CNT-SiO2 fiber catalysts

Abstract

As an environmentally friendly and renewable alternative fuel, biomethane can be obtained from biogas upgrading; however, the commercial technologies for upgrading are currently an unsustainable global process by releasing CO2 into the atmosphere. Here, this research proposed a novel sustainable method for direct biogas upgrading to high-quality biomethane by catalytic CO2 methanation and focused on the performance improvement of Ni-based catalysts. Carbon nanotubes-silica fiber (CNT-SF) composite as a high-heat transfer fibrous support was successfully synthesized using the Ni/silica fiber (SF) catalyst as a core-fiber seeding structure in ethanol steam reforming. The Ni/CNT-SF catalyst was extensively characterized and investigated in the direct biogas upgrading by CO2 methanation. The results revealed that Ni/CNT-SF catalyst exhibited superior catalytic performance than the Ni/SF and conventional Ni/silica porous (SP) catalysts. This suggested that the Ni/CNT-SF catalyst easily accessed the reactant molecules and exhibited an enhanced metal-support interaction with smaller Ni crystalline size, leading to a higher dispersion of the Ni active site, enhancing the CO2 methanation. Furthermore, the addition of Mg (1–3%wt.) into the Ni/CNT-SF catalyst resulted in a stronger metal-support interaction and increased moderate basic sites, which could suppress the Ni sintering and promote the adsorption and activation of CO2. Under optimum conditions (350 °C, 10 bar, H2/CO2 molar ratio of 4, and GHSV of 24,000 mL⋅g−1⋅h−1), the Ni-2Mg/CNT-SF catalyst achieved the highest CO2 conversion, CH4 selectivity, producing high-quality biomethane with CH4 content at approximately 95%, without CH4 losses.