One-step synthesis of mesoporous ZnO@ZIF-8 composites for CO2, CH4, N2 adsorption and separation with potential application for industrial biogas
Manel Essid, Nadhem Missaoui, Zainab Hassan Alnakhli, Hamza Khari, Houcine Barhoumi, Khadra B. Alomari, Bartosz Dziejarski, Kanagat Kishibayev, Jarosław Serafin
Abstract
The separation of CO₂ and N₂ from CH₄ is essential for improving natural gas quality, while capturing and eliminating CO₂ and CH₄ from the air is crucial for reducing greenhouse gas emissions. This study investigates the performance of a mesoporous ZnO@ZIF-8 composite for these purposes by examining its adsorption equilibrium for CO₂, CH₄, and N₂. The composite was prepared through a mechanical grinding technique utilizing a mortar and pestle and characterized by various analytical methods, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and nitrogen adsorption/desorption measurements. Adsorption isotherms for CO₂, CH₄, and N₂ were recorded at temperatures of 283, 298, 308, and 318 K and pressures up to 100 kPa, with the data being modeled using the Langmuir model. Additionally, separation selectivities for binary mixtures of CO₂/N₂, CO₂/CH₄, and CH₄/N₂, as well as biogas-like mixtures, were evaluated using the Ideal Adsorbed Solution Theory (IAST) model. At 298 K and 100 kPa, the selectivities for equimolar mixtures were found to be 9.13 for CO₂/N₂, 2.51 for CO₂/CH₄, and 3.63 for CH₄/N₂. Notably, the composite exhibited a CO₂ selectivity of 0.1086 at 283 K, which decreased to 0.0953 at 318 K, indicating the influence of temperature on CO₂ adsorption efficiency. Under these conditions, the adsorption capacities were measured at 3.0 mmol/g for CO₂ and 1.3 mmol/g for CH₄. These findings demonstrate the potential of the ZnO@ZIF-8 composite for efficient gas separation, particularly for enhancing biogas quality and contributing to greenhouse gas reduction through selective CO₂ capture.