Equilibrium and Diffusion of CO2 Adsorption on Micro-Mesoporous NaX/MCM-41 via Molecular Simulation

The micro-mesoporous molecular sieve, a promising CO2 adsorbent, possesses a suitable adsorption amount, higher diffusion rate than microporous zeolite, and better thermal stability than the mesoporous one. Here, we proposed and constructed a full atomic mimetic structure of NaX/MCM-41. The structural rationality of models was appropriately verified via the textural properties, X-ray diffraction patterns, and radial distribution functions. Force field parameters and atomic charge were verified to be reasonable by comparing with the reported adsorption isotherms. At 298.15 K and 0.001-101.325 kPa, the maximum adsorption capacity of CO2 on NaX/MCM-41 (Si/Al = 1.4) is 2.53 mmol/g, which is 4.02 times as large as that of MCM-41. The main reason accounting for the excellent adsorption capacity is that NaX/MCM-41 possesses more abundant adsorption sites than MCM-41. The adsorption capacity of the composite molecular sieve is lower than that of NaX due to the partial replacement of the NaX matrix by MCM-41. To reveal the adsorption mechanism in depth, the adsorption density distributions of CO2 in the adsorbents were also investigated under different pressures. The adsorption site of CO2 on NaX predominantly congregates in the octahedral zeolite cage, and CO2 transforms from surface adsorption to multilayer adsorption on MCM-41 with increasing pressure, accompanied by an increase in molecular self-aggregation. Simultaneously, the CO2 layer expands from a microporous wall to the surface and center of mesopores on NaX/MCM-41. Furthermore, NaX/MCM-41 has a higher diffusion coefficient of CO2 than NaX due to the mesoporous channels. This study is expected to provide a favorable theoretical guidance and design basis for the development of highly efficient adsorbents via studying the adsorption and diffusion behavior of CO2 over molecular sieves on the molecular level.