Pore Space Partitioning MIL-88（Co） For CO₂/CH₄ And CO₂/N₂ Seperation

Nowadays,oil and natural gas resources are supplying the needs of human production and life.Prior to the direct use of these important industrial chemicals,separation and purification processes are required,which rely conventionally on accomplished by repeated distillation–compression cycling of the mixture.These heat-driven separation processes involve huge energy consumption,accounting for more than 40%of the total energy consumption of the chemical industry.Consequently,there is an urgent demand to explore alternative technologies and materials that can efficiently separate and purify these light hydrocarbons under mild conditions.It is well known that the application of advanced adsorption and separation technology based on porous materials can save tremendous energy.Since 1990s,metal organic frameworks（MOFs）,as a new kind of porous materials,have attracted wide attention due to their advantages of designable structure,adjustable pore size and diverse functionality,and have been developing rapidly in gas adsorption and separation.The super large surface area and pore volume of MOFs endow its extremely high adsorption capacity.The adjustability of pore structure,combined with the good chemical and thermal stability enable it to play a role in separation and purification.In the current research,in addition to adding functional groups to improve the separation performance,it is more important to construct MOFs with new topological structure,so it is particularly important to realize the precise control of the material structure.During adsorption,MOFs with a large cavity are expected to promote a greater portion of guest-guest interactions（especially with increasing pressure）among the total combined host-guest and guest-guest interactions.For efficient capture of small gas molecules at ambient conditions,pore space partition is an effective strategy,gas molecules are adsorbed in the appropriate pore space to enhance the interaction between gas molecules and the pore surface to maximize the host-guest interaction.This thesis utilizes pore space partition and optimization strategies to synthesize two isostructural MOF materials,Co-BDC-TPB and Co-DOBDC-TPB,which can effectively separate CO₂/N₂and CO₂/CH₄mixtures with different ratios and flow rates.The crystal structure,stability,gas adsorption and separation properties were characterized by single crystal X-ray diffraction（XRD）,powder X-ray diffraction（PXRD）,penetration experiments,specific surface area and pore size analysis.Firstly,the porous MOF material Co-BDC-TPB was successfully prepared by solvothermal method with 1,4-dicarboxybenzene,1,3,5-tris（4-pyridyl）benzene（TPB）as ligands,N,N-dimethylacetamide（DMA）as solvent,and trinuclear[Co₃（μ₃-OH）N₃（COO）₆]cluster as metal center;Co-DOBDC-TPB,a homogeneous MOF material,was synthesized using the same method with 2,5-dihydroxyterephthalic acid,TPB,and Co Cl₂·6H₂O.Both materials have trigonal bipyramidal cages and a one-dimensional channel along the c axis.They both have good thermal stability and chemical stability,maintains structural stability after activation and adsorption,and can still maintain complete structure and crystal form after exposure to air for three weeks.Secondly,the adsorption properties of the materials for CO₂,CH₄and N₂were measured at different temperatures.The adsorption capacity of Co-BDC-TPB for CO₂was 46.6 cm³g^-1at 298 K 1 bar,and the selectivity for CO₂/CH₄mixtures at 50/50was 5.2,with CO₂/N₂being 17.The adsorption heat of CO₂in Co-BDC-TPB under zero coverage is 19 k J mol^-1,CH₄is 17 k J mol^-1,and N₂is 13.6 k J mol^-1.With the introduction of-OH,the CO₂adsorption capacity increased by 1.1 times to 97.4 cm³g^-1,and the selectivity for CO₂/CH₄at 50/50 was increased to 6.2,while CO₂/N₂was increased to 29.7.Under zero coverage,the adsorption heat of CO₂is 23.6 k J mol^-1,CH₄is 19.3 k J mol^-1,and N₂is 14 k J mol^-1,significantly improving the separation performance.Co-DOBDC-TPB has better adsorption and separation performance than Co-BDC-TPB due to the introduction of hydroxyl groups.On the basis of the original pore separation,hydroxyl functional groups modify the pore surface,and an additional O-H…O（CO₂）hydrogen bond exists,which enhances the interaction force between Co-DOBDC-TPB and CO₂and improves its performance.In order to examine the practical application of materials in separation,breakthrough experiments were conducted on self-made packed columns for CO₂/N₂and CO₂/CH₄mixtures with volume ratios of 15:85 and 50:50.Such breakthrough experiments are expected to simulate industrial separation.The results indicate that Co-BDC-TPB and Co-DOBDC-TPB can effectively separate CO₂/N₂and CO₂/CH₄mixed gases,and the lower adsorption heat allows the material to completely desorb within 10 minutes,indicating that the material has good desorption performance and is conducive to subsequent cyclic regeneration.And the adsorption performance did not significantly decrease after five cycles,indicating that the material has good regeneration performance.