The Synthesis Of Several Highly Stable MOFs And Their Adsorptive Separation Performance For Small Gaseous Molecules

Global warming and high energy consumption consumed in chemical separation processes are two major environmental and energy problems.Therefore,the development of efficient and energy-saving technology for carbon capture and industrial gaseous small molecule separation is a hot topic at present.In this work,several stable metal-organic framework materials?MOFs?were used to selectively capture CO₂from flue gas and biogas,recover methane from coalbed methane,separate ethylene/ethane,and separate hexane isomers.It mainly involves the preparation and characterization of several stable MOFs,as well as their adsorption performance and separation mechanism for CO₂/N₂,CO₂/CH₄,CH₄/N₂,C₂H₄/C₂H₆and hexane isomers mixtures.This work belongs to the cross research field of chemical engineering,material,energy,and environment,which has important scientific research value and practical significance.A zirconium porphyrin MOF?PCN-222?was synthesized.The water stability and the adsorption phase equilibria of CO₂,CH₄and N₂on PCN-222 at high and low pressures were studied.Results show that the BET surface area of PCN-222 increases by 24.5%instead of decreasing after PCN-222 being soaked in water for 24 h.At 298 K and 100 k Pa,the CO₂/CH₄and CO₂/N₂selectivities on PCN-222 are 4.3 and 73.7,respectively.The CO₂/N₂selectivity of PCN-222 is 4.3 times higher than that of 13X molecular sieve.At 298 K and3000 k Pa,PCN-222 exhibits a high CO₂uptake?13.67 mmol/g?,and the CO₂uptake of PCN-222 is 2.3 times that of 13X molecular sieve.The breakthrough experiments confirm that PCN-222 can effectively separate CO₂/CH₄and CO₂/N₂mixtures.A zirconium based MOF?MIP-202?was prepared by selecting cheap L-aspartic acid as the organic ligand and its adsorptive separation performance for CO₂/CH₄and CO₂/N₂mixtures was studied.Results show that the CO₂/CH₄and CO₂/N₂selectivities of MIP-202 at normal temperature and pressure are 72.9 and 1950000,respectively.MIP-202 exhibits the highest CO₂/N₂selectivity among all reported adsorbents so far.Metropolis Monte Carlo simulation calculations show that CO₂molecule with greater polarizability and quadruple moment tends to occupy the pore walls of large cages with higher polarity,while the less polar CH₄or N₂molecule majorly being adsorbed in the pore walls of small cages with lower polarity.The number of large cages is more than that of small cages in the framework,and the large cages show the preferential adsorption of CO₂over CH₄or N₂,resulting in the ultrahigh CO₂/CH₄and CO₂/N₂selectivities of MIP-202.Meanwhile,MIP-202 has low isoseric heat for CO₂adsorption,good adsorption and desorption cyclic stability and excellent SO₂resistance.The MIP-202 powder was extruded into pellets by using hydroxymethylcellulose as the binder.Compared with MIP-202 powder,the adsorption performance and stability of the MIP-202pellets almost remained unchanged.A strategy for improving CH₄/N₂adsorption selectivity is proposed by designing four centrosymmetric?₂-O groups on the pore wall of MOFs to interact with the four H atoms in CH₄molecules respectively,so as to enhance the adsorption force between MOFs and CH₄molecules.To verify the effectiveness of this strategy,two isomeric Al-based MOFs?CAU-8-BPDC and CAU-21-BPDC?were synthesized.and their CH₄/N₂adsorption selectivities were compared.Results show that unlike CAU-8-BPDC,there are four centrosymmetric?₂-O groups on the pore wall of CAU-21-BPDC.At normal temperature and pressure,the CH₄/N₂selectivity of CAU-21-BPDC is 2.4 times that of CAU-8-BPDC.In addition,it is found that the moisture stability of MOFs is not only related to the bond strength of metal-ligand,the hydrophobicity of pore wall surface,the coordination numbers of metal ions and the interpenetrated degree of framework,but also related to the water vapor uptake of MOFs.The proposed strategy for improving CH₄/N₂adsorption selectivity and the new discovery about the moisture stability of MOFs are the major innovation of this paper.An iron-based MOF?PCN-245?with the interpenetrated structure was synthesized,and its adsorption performance for ethane/ethylene separation was investigated.Results show that PCN-245 exhibits the characteristics of preferentially adsorbing ethane over ethylene at normal temperature and pressure.The simulation calculations show that the interpenetrated structure of PCN-245 provides more preferential adsorption sites for ethane molecules than ethylene molecules,which leads to the preferential adsorption behavior for ethane over ethylene on PCN-245.Therefore,the design and preparation of MOFs with interpenetrated structures can make them have the characteristics of preferentially adsorbing ethane in the separation process of ethane and ethylene.This discovery has important guiding significance for the design of ethane-selective MOFs.A strategy for improving the moisture stability of ethane-selective MOFs is proposed:to design ethane-selective MOFs with strong metal–ligand bonds,strong hydrophobicity,high coordination number for metal centers,and multiple-interpenetrated structure.To verify this strategy,the moisture stability of four ethane-selective MOFs was compared.Results show that the moisture stability of these four MOFs follows the order of Zr-bptc>Ui O-66>PCN-245>Ni?bdc??ted?_0.5.The difference in the bond strength of metal-ligand,the coordination number of metal center,the hydrophobicity of framework,and the degree of interpenetrated framework leads to their different moisture stability.In addition,comparing the structural parameters of ethylene-selective and ethane-selective MOFs,it is found that ethane-selective MOFs had fewer coordinatively unsaturated metal sites than ethylene-selective MOFs.The proposed strategy for improving the moisture stability of ethane-selective MOFs and the new discovery about the characteristic of ethane-selective MOFs are also the major innovation of this paper.A novel iron-based MOF?Fe-6FDCA?with high chemical stability and hydrophobicity was designed and synthesized for the first time,and its adsorption performance for the hexane isomers was studied.Results show that Fe-6FDCA has good separation performance for n-hexane?n HEX?,3-methylpentane?3MP?and 2,2-dimethylbutane?22DMB?.At 303 K and10 k Pa,the selectivity of Fe-6FDCA for n HEX/22DMB is as high as 27.83,which is 5.4-21.5times that of other materials;the selectivities of Fe-6FDCA for n HEX/3MP and 3MP/22DMB are 8.66 and 3.82,respectively,which are at the international advanced level.