Molecular Simulation Of CO₂/N₂ Adsorption And Diffusion On Micro-mesoporous Composite Molecular Sieves

Adsorption and separation of carbon dioxide?CO₂?/nitrogen?N₂?has very important practical significance for environmental protection and energy conversion.High efficiency adsorbent is the key factor to determine the separation efficiency.Large specific surface area,sieving effect,strong adsorption capacity,uniform pore size,reproducibility,and good chemical and thermal stability are the advantages of molecular sieves as CO₂ adsorbents.Due to the limitation of pore size,gas diffusion rate of microporous molecular sieves is low,and the adsorption capacity of CO₂ on mesoporous molecular sieves is limited at atmospheric pressure due to the mismatch of pore size and CO₂ molecular size.Introducing mesopores into the structure of microporous molecular sieve to obtain micro-mesoporous composite molecular sieve can improve the gas diffusion performance;grafting amino groups on the structure of micro-mesoporous composite molecular sieve can improve the adsorption capacity of acidic CO₂ gas.A highly efficient CO₂ adsorbent with high adsorption capacity and excellent diffusion performance is obtained.The adsorption and diffusion properties of gas in porous molecular sieve model are studied by molecular simulation.At the same time,the adsorption mechanism,such as adsorption density distribution,adsorption sites and interaction,is deeply understood at the micro level.These simulation studies are expected to provide theoretical basis and guidance for the selection,modification and application of efficient CO₂ adsorbents.?1?Firstly,Monte Carlo simulation and molecular dynamics were used to study the relationship between the structure of FAU,LTA and MFI microporous molecular sieves and the adsorption and diffusion of CO₂/N₂.The models of three typical microporous molecular sieves NaX,NaA and NaZSM-5 were constructed for adsorption simulation.The rationality of the molecular sieve models and the force field parameters were verified by the structural properties and the adsorption isotherms of single component.Under normal temperature and pressure,NaX?Si/Al=1.18?has the largest CO₂ equilibrium adsorption capacity and selectivity?CO₂:N₂=15:85?of 5.61 mmol/g and 295.34,respectively,which is attributed to the larger pore volume of NaX and its stronger interaction with CO₂.Under the same conditions,the diffusion coefficient of CO₂ on the three microporous molecular sieves from high to low is NaX>NaZSM-5>NaA,which is attributed to the larger pore size of NaX?9-10??than NaZSM-5?5.1-5.6??and NaA?4??.Due to the pore size limitation of NaX molecular sieve,the diffusion coefficient of CO₂ is only 2.58?10^-9 m²/s at 298 K and 1000kPa.The single component adsorption density distribution shows that the gas is mainly adsorbed on the octahedral zeolite cage on NaX;eight?cages on the top of NaA crystal cell;as well as the straight channel,Zigzag channel and cross channel of NaZSM-5.?2?Secondly,based on the limitation of pore size on CO₂ diffusion,the whole atom model of NaX/MCM-41 composite molecular sieve was constructed by introducing mesoporous MCM-41 structure into NaX supercell.Under normal temperature and pressure,the CO₂ equilibrium adsorption capacity of NaX/MCM-41 is 2.37 mmol/g,which is 3.76times of MCM-41.This is due to the microporous structure on the mesoporous wall providing more adsorption sites.The order of CO₂/N₂ selectivity from high to low is NaX>NaX/MCM-41>MCM-41.With the increase of pressure,CO₂ changes from surface monolayer adsorption to multi-layer adsorption on MCM-41,accompanied by molecular self-aggregation.At the same time,CO₂ expands from the micropore wall of NaX/MCM-41to the surface and center of mesopores.In addition,the mesoporous channel promotes gas diffusion,and the diffusion coefficient of CO₂ in NaX/MCM-41 is more than 4 times that of NaX.?3?Finally,the amino hydroxyl modification of NaX/MCM-41 was carried out by replacing the silicon hydroxyl group in the structure with 3-aminopropyltriethoxysilane?APTES?,and the APTES-NaX/MCM-41 amino graft modified micro-mesopore composite molecular sieve model was constructed.The weak chemical interaction between amino group and CO₂ improves the adsorption performance.At 298 K and 101.325 kPa,the CO₂equilibrium adsorption capacity of 10-APTES-NM is 6.10 mmol/g,which is 2.57 and 9.53times of NaX/MCM-41 and MCM-41,respectively.The diffusion coefficient of CO₂ on it is still more than 4 times that of NaX.Compared with the mesoporous molecular sieve MCM-41and micro-mesoporous composite molecular sieve NaX/MCM-41,the adsorption performance of CO₂ on 10-APTES-NM is significantly improved,and the diffusion performance is better than that of microporous molecular sieve NaX.