Preparation Of Novel Ultramicroporous Carbon Molecular Sieves And Their Adsorptive Separation Performance For Small Molecule Gases

This paper mainly focuses on the important demand for the adsorptive separation of small molecular gases in the fields of petrochemicals,energy,and environmental protection.With the practical application background of capturing methane in low-grade coalbed methane,separating light hydrocarbons,and water harvesting,a comprehensive study is conducted on the preparation and characterization of novel ultramicroporous carbon molecular sieve and its adsorptive separation performance for small molecular gases.The research content of this paper belongs to the interdisciplinary research fields of chemical engineering,materials engineering,and surface science,and has important scientific research value and practical significance.（1）Starch-based ultramicroporous carbons（SCs）were prepared and their adsorption separation performance for CH₄/N₂was studied.An ion in-situ activation method was developed to prepare ultramicroporous carbons.K⁺was uniformly anchored on the surface of SC（H⁺）-x precursors containing abundant polar oxygen functional groups by ion exchange,which not only greatly improved the utilization efficiency of activating ions but also allowed the pore size to be controlled within a narrow range of ultramicropores.Compared with the traditional chemical activation,the amount of KOH can be redu ced by93%.The obtained SCs exhibited a high CH₄ adsorption capacity of up to 1.86 mmol/g at298 K and 100 k Pa,and a CH₄/N₂selectivity of 5.7.Fixed-bed permeation experiments demonstrated that SCs could completely separate CH₄/N₂at room temperature and pressure,and their comprehensive performance ranked among the international advanced level.（2）The effect of different alkali metal ion activations on the pore size control of ultra-microporous carbon molecular sieves（SC-M）was studied.Three types of ions of Na⁺,K⁺,and Rb⁺were anchored on the surface of starch-based hydrochar（SCH）by ion exchange,achieving the regulation of ultramicropore size in the sub-angstrom range of the carbon molecular sieve.The results showed that the three types of ions can be uniformly anchored on the surface of the carbon precursor,and with the increase in the size of the alkali metal ions during activation,the main ultramicropores of SC-Na,SC-K,and SC-Rb were concentrated at 4.7,4.8,and 4.9（?）,respectively.This has significant theoretical and practical implications for the precise control of sub-Angstrom-level pore size in carbon materials.（3）The adsorption and separation performance of C₃ and C₄ analogues by ultra-microporous carbon molecular sieves（SC-M）was studied.The SC-K with pore size centered at 4.8（?）showed excellent separation performance for C₃ and C₄ analogues.At ambient condition,the C₃H₆/C₃H₈uptake ratio of SC-K reached as high as 5.36.The C₄H₆/n-C₄H₈ and C₄H₆/i-C₄H₈ uptake ratio of SC-K reached 3.75 and 4.72,respectively.In contrast,SC-Rb with larger ultramicropore size of 4.9（?）showed co-adsorption of C₃ and C₄ components.Fixed-bed breakthrough experiments verified that SC-K could achieve complete separation of C₃ and C₄ analogues under dynamic conditions.These results confirm that the ultramicroporous carbon molecular sieve prepared in this work has great potential for the size-sieving separation of light hydrocarbons.（4）We developed novel ultramicroporous carbon molecular sieves（SCMSs）and evaluated their adsorption separation performance for a C₃H₆/C₃H₈ mixture.Starch was chosen as the carbon source,and an activating-free method was developed to prepare these ultramicroporous carbon molecular sieves.We proposed a mechanism for the evolution and regulation of sieving pores in carbon materials.By adjusting the hydrothermal synthesis conditions,it is possible to effectively regulate the relative content of defects in hydrochars.The high-temperature decomposition of heteroatoms generates ultramicropores,while the structural rearrangement of the carbon layer induced by thermal driving increases the degree of graphitization and promotes pore shrinking,resulting in the formation of sieving pores.The obtained SCMS-0.2-800 exhibited a C₃H₆ adsorption capacity of 2.54 mmol/g at 298 K and 1bar,while that for C₃H₈ was only 0.08 mmol/g,representing the first carbon molecular sieve for completely size-sieving separation of C₃H₆/C₃H₈ mixture.This work has important scientific significance and practical guiding value in the field of carbon molecular sieves.（5）Super-hydrophilic C₂N-type porous carbons were synthesized and their water vapor adsorption kinetics were investigated.Nitrogen-rich microporous C₂N-type carbons were prepared by thermal polymerization of hexaazatriphenylene（HAT-CN）at 550 or 700℃,exhibiting a high-water vapor uptake of 7.8 mmol/g at 298 K and P/P₀=0.2.Subsequently,ball milling was used to reduce particle size,effectively shortening the diffusion path of water molecules and alleviating mass transfer limitations.The enhanced water vapor adsorption kinetics by ball milling was confirmed by faster thermal adsorption and release signals detected by thermal response measurement.This study has important implications for the preparation of carbon materials with high nitrogen content and improving their adsorption kinetics for polar gases.