Ligand Handedness Strategy To Construct Metal-Organic Frameworks For Adsorption Separation

Light hydrocarbons and hexane isomers are vital basic raw materials in petrochemical industry.While bringing huge economic benefits,the similar physical and chemical properties also pose problems of precise separation and purification between components.Currently,cryogenic distillation is the main method for separating light hydrocarbons and hexane isomers,undoubtedly,this heat-driven process is highly energy-intensive.In this context,adsorptive separation by porous solid adsorbents attracts particular recognition because of its advantages of simple operation and low energy consumption,however,the core challenge lies in the advancement of new adsorbents.Emerging metal-organic frameworks （MOFs） hold increasing promise for the separation of light hydrocarbons and alkane isomers owing to their highly tunable pore structures.Most importantly,handedness effect was innovatively utilized to synthesize ultra-microporous MOFs with different topology and pore structures.The discrepancies in topology and pore structure between adsorbents constructed with different chiral ligands are compared in detail.The adsorption performance and mechanism were systematically explored,in hoping of providing a theoretical basis for the design and development of novel adsorbents.In this paper,ligand handedness effect was proposed to construct ultra-microporous MOFs,in which enantiopure malic acid and racemic malic acid were used as organic ligands to prepare homochiral L-MOF （or D-MOF） and racemic chiral DL-MOV with nickel ions and 4,4’-bipyridine,respectively.Interestingly,L-MOF and DL-MOF exhibit remarkable differences in terms of topology as well as pore structure.In L-MOF,continuous onedimensional narrowest channels with cross section of about 4.0 × 4.0 ?² and widest channels with pore size of 5.3×5.5?² run exclusively along the b-direction,while DL-MOF features typical two-dimensional quasi-discrete cavities consisting of expanded molecular cages （5.5× 6.0 × 9.5 ?³） connected by contracted pore windows （3.0 × 4.0 ?²）.The adsorbents’morphology,crystal structure,pore information,and hydrothermal stability were also characterized by SEM,PXRD,CO₂ adsorption-desorption experiments and TGA analysis.Both homochiral L-MOF （D-MOF） and racemic chiral DL-MOF feature abundant carboxy-oxygen and hydroxyl groups,which may be favorable sites for acetylene binding,thus promising the purification and separation of acetylene from multicomponent mixtures.All three chiral adsorbents registered higher C₂H₂ uptakes versus the competing C2-C1 gases（C₂H₄,CO₂ and CH₄）.Dynamic column breakthrough experiments showed that both homochiral and racemic chiral adsorbents are capable of guiding the separation of C₂H₂ from C₂H₂/CO₂,C₂H₂/C₂H₄ and C₂H₂/CH₄ mixtures as well as multicomponent mixtures.Density functional theory calculations revealed the separation mechanism of acetylene preferential adsorption at the molecular level.Compared with the molecular structure difference between acetylene （C₂H₂）,ethylene（C₂H₄） and methane （CH₄）,the difference between propylene （C₃H₆） and propane （C₃H₈）molecules is smaller （<0.4 ?） because both molecules have the same alkyl part.This negligible difference in molecular structure and size renders C₃H₆/C₃H₈ separation a more challenging task.Static adsorption,kinetic experiments and simulation calculations illustrate the inability of L-MOF to distinguish between propylene and propane.In contrast,the racemic chiral DL-MOF has witnessed a remarkable improvement in the separation performance of C₃H₆/C₃H₈.The uptake ratio and Henry selectivity for C₃H₆/C₃H₈ were calculated to be 1.59 and 5.86.Furthermore,DL-MOF exhibited a remarkable kinetic effect,as evident from its kinetic selectivity of 114.2 at 298 K and 1 bar.The calculated equilibrium-kinetic combined selectivity of DL-MOF for C₃H₆/C₃H₈ is 62.6,almost 50 times greater than that observed for homochiral adsorbent,which underscores the effectiveness of ligand handedness strategy in C₃H₆/C₃H₈ separation.The thermodynamic-kinetic synergistic separation mechanism of C₃H₆ and C₃H₈ in the quasi-discrete cavities was carefully studied at the molecular level by density functional theory,Monte Carlo,and molecular dynamics simulation.Hexane isomers are essential components of gasoline,and the extraction of di-branched alkanes from these isomers is necessary for gasoline upgrading to attain high octane ratings.Due to the pore size of L-MOF and DL-MOF being comparable to the dynamic diameter of nHEX （4.3 ?）,3MP （5.0-5.5 ?）,and 22DMB （6.2 ?）,the adsorption separation performance of these materials on hexane isomers was systematically investigated.The difference in molecular size of the hexane isomers endows both L-MOF and DL-MOF with a complete exclude of 22DMB and high capacities for linear nHEX.While the adsorption capacity of 3MP on L-MOF decreases with increasing temperature,in accordance with the laws of thermodynamics,the uptake of 3MP on DL-MOF exhibits an anti-thermodynamic phenomenon by increasing with rising temperature.The unusual phenomenon can be well explained by both the characterization methods of variable-temperature X-ray diffraction and the theoretical model of the instantaneous structural deformation of the framework induced by temperature and guest molecules.The breakthrough experiments suggested that the optimal temperatures for the separation of hexane isomers on L-MOF and DL-MOF were 303 K and 363 K,respectively.