Synthesis And Performance Study Of MOFs Materials For Small Molecule Gases Separation

Light hydrocarbon are essential chemical raw materials and fuels,with diverse sources ranging from petrochemical production to primary energy gases like coal bed methane and shale gas.However,these gaseous energy substances typically come as mixtures,often containing non-combustible gases like carbon dioxide（CO₂）and nitrogen（N₂）.Therefore,separating and purifying these mixtures in industrial production becomes necessary before further applications.Currently,the industry commonly utilizes organic alcoholamine solutions for CO₂ capture,but this method faces challenges such as high energy consumption,environmental pollution,and equipment corrosion.On the other hand,the separation of light hydrocarbon primarily relies on low-temperature rectification technology,which suffers from drawbacks such as high energy consumption,large equipment footprint,and complex process flows.Hence,there is an urgent need to develop pressure swing adsorption（PSA）technology utilizing physical adsorbents to achieve efficient and energy-saving separation of small molecule gases.Metal-organic frameworks（MOFs）are porous crystalline hybrid materials composed of metal ions or clusters and organic ligands.MOFs offer designable structures and facile pore modifications that enable selective adsorption of specific gas molecules,making them excellent physical adsorbents.Among various MOFs,those with stronger interaction forces between microporous structures and gas molecules exhibit exceptional separation capabilities for small molecular gases.Based on the aforementioned considerations,this thesis focuses on the design and synthesis of three types of distinct microporous MOFs with varying topological structures.The modulation of metals or ligands allows precise regulation of the pore environments,thereby achieving efficient separation of different small molecule gases.This thesis is divided into the following three main parts:（1）MOFs materials have broad prospects for application in the field of gas adsorption and separation.However,most MOFs materials exhibit poor water stability,which significantly limits their practical use.In this study,three robust MOFs materials（compounds 1-3）were designed and synthesized based on the hard-soft acid base（HSAB）theory.These compounds utilize H₆BHB（4,4"-benzene-1,3,5-triyl-hexabenzoic acid）as ligands to effectively modulate the charge of the framework by adjusting the metal cluster structure elements,thereby enhancing the specific surface area and CO₂ adsorption capacity of the MOFs materials.Compound 1 features a cationic framework composed of trinuclear In₃O（COO）₆ and H₆BHB ligand,with a BET specific surface area of 301 m²/g.Compound 2 possesses a cationic framework consisting of Fe₃O（COO）₆ clusters and H₆BHB ligands.Due to the lower relative atomic mass of iron atom compared to indium atom,the framework density decreases,resulting in an increased BET surface area of 446 m²/g.Compound 3 is a neutral framework material coordinated by Fe₂Ni O（COO）₆ clusters with H₆BHB,exhibiting the highest BET specific surface area of 808 m²/g.Compounds 1-3 demonstrate excellent chemical and thermal stability,possessing a microporous cage structure and abundant open metal sites.At 298 K and 1 bar,they exhibit significant CO₂ adsorption capacities（28.0,51.5,and 99.6 cm³/g,respectively）.IAST results show that all three MOFs exhibit high separation selectivity toward CO₂ over N₂（35.2,43.2 and 43.2 for CO₂/N₂=0.15/0.85）and CO₂ over CH₄（14.4,11.5 and 10.1 for CO₂/CH₄=0.5/0.5）at298 K,1 bar.Breakthrough experiments further validate the high efficiency of these compounds in separating CO₂/N₂ and CO₂/CH₄.The exceptional stability,low adsorption enthalpy,ease of regeneration,and excellent CO₂/N₂ and CO₂/CH₄separation capabilities of compounds 1-3 make them promising candidates for applications in flue gas separation and natural gas purification.（2）Acetylene（C₂H₂）and carbon dioxide have similar molecular size,shape,and boiling point,making their industrial separation challenging.Therefore,it is crucial to develop effective physical adsorbents for the efficient separation of C₂H₂/CO₂.In this study,two MOF materials（compounds 4-5）with pcu topology were designed and synthesized by pillar-layered strategy.Compound 4 was created by using a 4-pyrazolecarboxylic acid ligand and Ni²⁺to construct a two-dimensional sql layer structure,and then assembled with 4,4’-bipyridine as a pillar ligand,resulting in the formation of framework with pcu topology.Compound 4 possesses abundant adsorption sites in the cage pore,exhibiting a high adsorption capacity for C₂H₂ and enabling the separation of C₂H₂ and CO₂（the selectivity of equimolar C₂H₂/CO₂ is 4.5at 298 K and 1 bar）.To enhance the C₂H₂/CO₂ separation efficiency further,compound5 with same pcu topology was synthesized using a shorter pyrazine as the pillar ligand.By optimizing the pores,compound 5 significantly improved the C₂H₂/CO₂ separation capacity（the selectivity of equimolar C₂H₂/CO₂ increased to 14.0 at 298 K and 1 bar）.Theoretical calculations revealed that subtle differences in host-guest interactions between compound 4 and compound 5 are responsible for their different C₂H₂/CO₂separation selectivity.In single breakthrough test,compound 5 exhibited an adsorption capacity of 2.01 mmol/g for C₂H₂,which was much higher than compound 4（1.16mmol/g）.Additionally,both compounds 4-5 demonstrated excellent cycling performance in continuous cycling breakthrough tests.Due to their low cost,efficient separation performance,excellent stability,and good recycling ability,compounds 4-5hold great potential for industrial applications in the separation of C₂H₂/CO₂.（3）The utilization of MOFs as physical adsorbents in combination with PSA technology is an efficient and energy-saving method for the separation of methanol to olefins（MTO）product and natural gas purification.While most MOF materials come with a high cost,Bio-MOFs made of biomolecules offer the advantage of being low-cost and low-toxicity alternatives,making them highly valuable for industrial applications.In this study,a series of Bio-MOFs（compounds 6-8）based on adenine and Cu²⁺were synthesized by using formic acid,acetic acid,and propionic acid respectively,as terminal ligands through reticular chemistry strategy.The experimental results indicate that changes in the size of the terminal ligands subtly affect the pore environment,leading to significant variations in the adsorption capacity and stability of compounds 6-8.Among them,compound 6 exhibits poor thermal stability,compound8 has low porosity,and the free rotation of ethyl in the pore hinders its separation of light hydrocarbons.On the other hand,compound 7 exhibits the highest specific surface area and excellent gas separation abilities.At 298 K and 1 bar,compound 7 exhibits selectivity of 746 for C₃H₈/CH₄ separation and 10.9 for C₃H₆/C₂H₄ separation.Breakthrough tests conducted at ambient conditions demonstrate that compound 7exhibits excellent separation performance,allowing for the separation of different proportions of C₂H₄/C₃H₆ mixtures（50/50,50/20,and 90/10）in one-step while also exhibiting good recycling capabilities.Compound 7,which employs easily accessible adenine and acetic acid as ligands,offers the advantages of low cost,low toxicity,and facile synthesis,making it highly suitable for industrial utilization.