Abstract: Developing low-cost, energy-efficient CO₂ adsorbents that can be produced at scale is important for advancing China’s carbon peaking and carbon neutrality goals. Ni-MOF-74 exhibits strong CO₂ adsorption capacity owing to its abundant open metal sites. However, its practical application is limited by high preparation costs, the high-temperature and high-pressure conditions required for conventional hydrothermal synthesis, as well as particle agglomeration and mass-transfer limitations. Therefore, developing Ni-MOF-74-based composite adsorbents with high adsorption capacity, rapid mass transfer, and good stability through green and mild synthesis routes remains an urgent scientific challenge. In this study, a hierarchically porous Ni-MOF-74/diatomite composite was synthesized via a reflux condensation method. As a low-cost porous support, diatomite promotes the uniform nucleation and ordered growth of Ni-MOF-74 crystals, effectively suppresses particle agglomeration, and facilitates the construction of a hierarchical micro-meso-macroporous architecture. Zero-length column analysis further demonstrates that diatomite incorporation enhances CO₂ mass-transfer kinetics. Under optimized synthesis conditions, the adsorbent exhibits a CO₂ adsorption of 4.50 mmol·g^-1 at 298 K and 0.1 MPa, a CO₂/N₂ selectivity of 86.76, and an isosteric heat of adsorption of 37.04 kJ·mol^-1, indicating a desirable combination of high adsorption capacity and low regeneration energy requirement. Dynamic breakthrough experiments demonstrate good separation performance and cycling stability of the material in simulated flue gas. After 10 adsorption-desorption cycles, the CO₂ uptake retention remained above 98%. In-situ infrared spectroscopy revealed that CO₂ adsorption mainly arose from the coordination interaction between CO₂ molecules and open Ni²⁺ sites, together with weak physisorption on the pore surfaces. Overall, the Ni-MOF-74/diatomite composite exhibits distinct advantages in low-cost preparation, mass-transfer enhancement, and cycling stability, highlighting its potential for low-energy CO₂ capture applications.

Key words: CO₂ capture, Ni-MOF-74, diatomite, reflux condensation method, hierarchical pore structure