Design And Synthesis Of Ultramicroporous MOFs Based On Sieving Effect And Its Adsorption And Separation Performance For Light Hydrocarbons

The separation of hydrocarbons is of primary importance in the petrochemical industry but remains a challenging process.Hydrocarbon separations have traditionally relied predominantly on costly and energy-intensive heat-driven procedures such as low-temperature distillations.Adsorptive separation based on porous solids represents an alternative technology that is potentially more energy efficient for the separation of some hydrocarbons.In this paper,aiming at the important industrial separation processes,such as the purification and capture of CH₄,the separation and purification of C₂H₄,C₃H₆ and C₄ olefins,we studied the adsorption and separation performance together with the separation mechanism of several ultramicroporous metal-organic framework（MOFs）for C₂H₆/CH₄,CH₄/N₂,C₂H₄/C₂H₆,C₃H₆/C₃H₈ and C4 olefins mixtures systems.The research content of this paper mainly belongs to the cross field of chemical engineering,material chemistry and energy,and has practical significance of important scientific research value.In this thesis,we synthesized a robust nickel-based MOF,Ni-BPZ,featuring one-dimensional（1D）rhombic channels decorated with abundant pyrazole rings.Ni-BPZ exhibits excellent separation performance toward both C₂H₆/CH₄ and CH₄/N₂ binary mixtures.The C₂H₆/CH₄ selectivity of Ni-BPZ is high up to 50.2,exceeding most reported MOF adsorbents,and it simultaneously possesses the remarkable C₂H₆ uptake of 2.46 mmol/g at 298 K and 10k Pa.The CH₄/N₂ selectivity of Ni-BPZ reaches 6.6 and its high CH₄ uptake is 1.56 mmol/g,which is also superior to most high‐performance CH₄ adsorbents.Molecular simulation results show that the 1D rhombic channels of Ni-BPZ are well matched with C₂H₆ molecules,forming multiple C-H???πinteractions with stronger adsorption affinity,which is the key factor for the efficient separation of C₂H₆/CH₄.The C-H???πinteraction of Ni-BPZ with CH₄ is also significantly stronger than the weak dispersion interaction with N₂,leading to the efficient separation of CH₄/N₂.This research shows that MOFs with uniform 1D pore channels can be explored as appropriate adsorbent candidates to achieve efficient equilibrium thermodynamic separations,providing important theoretical guidance to design high-performance MOF adsorbents overcoming the challenges of important chemical separations.In this thesis,aiming at the difference in molecular diameter of C₂H₄/C₂H₆,we rationally controlled the pore apertures by changing the length of the pillar ligands and synthesised the pillar-layer metal-organic frameworks,Co（aip）（pyz）_0.5,which successfully achived the molecular sieving separation of C₂H₄/C₂H₆.At 298 K and 100 k Pa,the adsorption capacity of C₂H₄ on Co（aip）（pyz）_0.5 was 1.78 mmol/g,and the C₂H₆ uptake of Co（aip）（pyz）_0.5 was merely0.14 mmol/g.The C₂H₄/C₂H₆（1:1,v/v）separation selectivity exceeded 2000,surpassing most out-performing MOF adsorbents,which is the major innovation of this paper.The breakthrough experiments confirmed its excellent separation performance under dynamic conditions to produce high purity（97%）of C₂H₄ through a single adsorption-desorption process.Molecular simulations showed that the suitable pore size of Co（aip）（pyz）_0.5 and the high adsorption energy barrier to C₂H₆ are the key factors to realize the efficient sieving and separation of C₂H₄/C₂H₆.In this thesis,we reported the topology-guided design and synthesize of a novel ultramicroporous yttrium-based MOF（Y-dbai）with ftw topology for the first time,and its adsorption and separation performance of C₃H₆/C₃H₈ were studied.At 298 K and 100 k Pa,the adsorption capacities of Y-dbai for C₃H₆ and C₃H₈ are 2.57 mmol/g and 0.10 mmol/g respectively,which achieves the molecular sieving separation of C₃H₆/C₃H₈ together with a high C₃H₆ adsorption capacity.The separation performance exceeds most MOFs adsorbents and overcomes the trade-off between the C₃H₆ adsorption capacity and C₃H₆/C₃H₈ selectivity,which is the major innovation of this paper.The breakthrough experiments confirmed its excellent separation performance under dynamic conditions to produce high purity（97.1%）of C₃H₆ through a single adsorption-desorption process.Molecular simulations showed that the appropriate pore window size of Y-dbai and the electrostatic repulsion between the positive potential of the pore window and C₃H₈ are the key factors for the efficient sieving and separation of C₃H₆/C₃H₈.In this thesis,we further studied the adsorption and separation performance of ultramicroporous yttrium based MOF（Y-dbai）for C4 olefin mixtures C₄H₆/1-C₄H₈/i-C₄H₈.At298 K and 100 k Pa,the adsorption capacities of Y-dbai for C₄H₆,1-C₄H₈ and i-C₄H₈ are 2.88,1.07 and 0.14 mmol/g respectively,which realized the molecular sieving separation of C₄H₆/i-C₄H₈ and 1-C₄H₈/i-C₄H₈.Its Henry’s selectivities of C₄H₆/i-C₄H₈ and 1-C₄H₈/i-C₄H₈ are high up to 521 and 125.7 respectively,surpassing most reported MOFs.Meanwhile,the results of static and dynamic adsorption experiments showed that Y-dbai can separate C₄H₆/1-C₄H₈through thermodynamic-kinetic synergistic effect.The breakthrough experiments confirmed its excellent separation performance for C₄H₆/i-C₄H₈ and C₄H₆/1-C₄H₈ binary mixtures.Molecular simulations showed that the pore window size of Y-dbai is smaller than the minimum cross-sectional size of i-C₄H₈,resulting in a high diffusion barrier to i-C₄H₈ and preventing i-C₄H₈from entering the pore channels.Therefore,efficient screening and separation of C₄H₆/i-C₄H₈and 1-C₄H₈/i-C₄H₈ have been achieved.The difference in the adsorption affinities of amide groups on the ligand towards C₄H₆/1-C₄H₈ and the difference in window size on the diffusion barrier of C₄H₆/1-C₄H₈ have led to efficient thermodynamic kinetic synergistic separation of Y-dbai on C₄H₆/1-C₄H₈.In this research,a series of ultra-microporous MOFs were designed by the pore structure design and pore size regulation strategies,which realized the highly efficient thermodynamic adsorption and separation of C₂H₆/CH₄ and CH₄/N₂,and the molecular sieving separation of C₂H₄/C₂H₆,C₃H₆/C₃H₈ and C₄H₆/1-C₄H₈/i-C₄H₈.The adsorption and separation mechanism was clarified through molecular simulation.It solved the trade-off between the adsorption capacity and adsorption selectivity of existing adsorbents and provided important reference and theoretical guidance for the design and development of high-efficiency MOFs adsorbent.