Synthesis Of Pillar-layered Microporous Materials For The Adsorptive Separation Of Light Hydrocarbons

Metal-organic frameworks（MOFs）are a kind of crystal materials constructed by self-assembly of metal ions or metal clusters with organic ligands.They have important applications in chemical,biomedical,energy,catalytic,gas adsorption,energy storage and other fields.Light hydrocarbons are important basic chemical raw materials,and their purity directly affects the downstream process and product quality.According to the challenge of separation light hydrocarbons that have the similar structures and properties,a series of anion-pillared metal-organic framework materials were constructed by pore control strategy to achieve precise regulation of pore size and pore shape,and the performance and mechanism of light hydrocarbon adsorption and separation were investigated,the main research contents are as follows.Firstly,1,2,4,5-tetrakis（4-pyridyl）benzene was used as the organic ligand,and different anion-pillared were used to construct the metal-organic framework materials with precisely adjustable pore size and pore shape,the effect of pore structure on gas adsorption capacity and selectivity were systematically investigated.The results show that the Mo OFOUR-Co-tpb,prepared by using tetrahedral anion-pillared(Mo O₄^2-),has three discrete 1D gourd-like channels with different sizes（channel A:5.5×7.4?²;channel B:4.9×6.9?²;channel C:3.8×4.5?²）.The single-component adsorption isotherm showed that it was able to completely sieve 2,2-dimethylbutane,selectively adsorb linear hexane(n HEX,adsorption capacity of 1.58 mmol g^-1)and mono-branched hexane(3MP,adsorption capacity of 0.96 mmol g^-1)due to the unique pore size and pore shape of Mo OFOUR-Co-tpb,while exclusion of di-branched hexane(22DMB,with an adsorption capacity of 0.13 mmol g^-1).The single-component kinetic adsorption isotherm showed that in Mo OFOUR-Co-tpb,n-hexane occupied the adsorption site preferentially and reached the adsorption equilibrium at 3.5 mins before3-methylpentane,and 3-methylpentane diffused more slowly because the kinetic size was very close to the pore size,and reached the adsorption equilibrium at 11 min.The separation of n-hexane/3-methylpentane was achieved by the synergistic thermodynamic-kinetic effect.The actual separation ability of the material for hexane isomers was further verified by fixed-bed breakthrough experiments,and the linear mono/di-branched hexane isomers could be separated in both 3-and 5-component mixed gas breakthrough experiments,and the real-time RON showed that the material was excellent for obtaining high-RON components.The interaction mechanism between Mo OFOUR-Co-tpb and hexane isomers was investigated by simulations,and the results showed that the unique pore structure and pore size of Mo OFOUR-Co-tpb are the key factors to achieve the separation.three different channels of Mo OFOUR-Co-tpb can selectively accommodate gas molecules:all channels exclude di-branched;channel A can accommodate mono-branched hexane but mono-branched hexanes are slowly diffusing due to the similarity diameter between the pore size and the guest molecule;the channels are completely open to linear hexanes,which allows complete separation of the C6 alkane mixture in Mo OFOUR-Co-tpb.3-methylpentane selectively enters channel A,forming tight binding sites by H bonds which formed between the methyl group of guest molecule and pore wall.In contrast,n-hexane can not only form a tight binding site with the pore wall,but also can further diffuse to the interlayer and form a second site between parallel interlayer benzenes.Molecular dynamics simulations show that the self-diffusion constants of n-hexane is two orders higher than that of 3-methylpentane,further demonstrating that the unique pore structure and size of Mo OFOUR-Co-tpb facilitate the separation of n-hexane/3-methylpentane with the exclusion of 2,2-dimethylbutane.Further exploration of the material’s potential for light hydrocarbons separation revealed that it works well for acetylene/carbon dioxide separation,particularly at low pressures where it achieves a separation selectivity of 96 and a higher selectivity than many benchmark materials at 1 bar,while maintaining excellent acetylene adsorption,achieving compatible adsorption volumes and selectivity.It also has potential for propylene propane separation and the one-step purification of butadiene.