Study On The Synthesis Of Co-MOF-74 And The Performance Of Co/N₂ Adsorption And Separation

Carbon monoxide（CO）is a key raw material for the production of many high-value chemicals.However,the production of CO is mainly due to the incomplete combustion of carbonaceous substances,which is widely present in various industrial tail gases.Blast furnace gas（BFG）,which is the exhaust gas of steel mills,has the characteristics of large output and low utilization rate.After pretreatment processes such as desulfurization,dust removal and deoxidation,BFG can be converted into a mixture of CO and N₂.Compared with cryogenic separation and solution absorption,the adsorption method has the advantages of low energy consumption and high selectivity,and has become the most promising method for CO/N₂separation.However,the economy of the adsorption method depends on the choice of high-efficiency adsorbents.Traditional adsorbents such as zeolite molecular sieves and activated carbon have low adsorption selectivity for CO/N₂.π-complex adsorbent（Cu⁺）is not suitable for large-scale industrial applications because of its air stability.MOFs containing unsaturated metal sites have been identified as one of the most promising adsorbents for CO adsorption and separation.CO and N₂ have very similar physical properties.However,compared to N₂,which is an inert gas,CO and open metal sites have a quasi-chemical interaction that is better than physical adsorption.However,the open metal sites,as the key sites in the adsorption process,are generally occupied by guest molecules.Therefore,the activation for MOFS containing open metal sites is very important.In this work,we first systematically investigated the activation behavior of Co-MOF-74,which is a typical MOFs containing open metal sites;then we investigated the adsorption perfoemance of Co-MOF-74 on CO and N₂ at different temperatures and pressures,and finally investigated the competitive adsorption of CO/N₂.The specific research conclusions are as follows:（1）The Co-MOF-74 samples activated at different temperatures were prepared by heating and vacuuming at different temperatures to systematically investigate the activation performance of Co-MOF-74.Thermogravimetric analysis（TG）with multi-step temperature programming and isothermal mode was uued to investigate the weight loss in various temperature ranges.In addition,thermogravimetric-mass spectrometry（TG-MS）technology,combined with ¹³C{¹H}and ¹H MAS solid-state NMR spectroscopy were performed to investigate the evolution of the gaseous products.The results show that the removal of guest molecules in the activation process can be divided into two steps:a large amount of water and methanol molecules in the pores are firstly removed in the temperature range of 50-150°C;and the decomposition of bind DMF and ligand molecules coordinated on the open metal site are occurred at 200-350°C.The BET specific surface area for every sample was investigated by the N₂ adsorption and desorption isotherm under 77 K conditions.We found that the BET of Co-MOF-74 increased from 31（Co-MOF-74-25°C）to 1135 m²/g（Co-MOF-74-150°C）,due to the removal of water molecules in the pores,and when the activation temperature continued to rise to 300°C,the BET continued to rise to 1254 m²/g,which was attributed to the decomposition of the bind DMF and terminal ligand coordinated at the open metal site.（2）The DMF molecules,the synthetic solvent of Co-MOF-74,can be removed during methanol activation process,while the DMF molecules and terminal ligands coordinated at the open metal sites can only be decomposed by higher temperature（300°C）.H₂O molecules can occupy the open metal sites of Co-MOF-74 during the filtration and Me OH wash steps under air.As a strong water-absorbing adsorbent,activated Co-MOF-74 can quickly adsorb water molecules in the air.Therefore,in order to maintain the adsorption performance of Co-MOF-74,the sample should be filtered and stored in an inert atmosphere to prevent it from adsorbing water molecules in the air.（3）The effect of activation temperature on CO/N₂ adsorption performance was studied by measuring the CO/N₂ adsorption isotherm at 298 K.The results show that the adsorption amount of CO/N₂ is highly dependent on the activation temperature.With the increase of activation temperature,the adsorption amount of CO and N₂ gradually increased.The adsorption capacity of CO increased from 1.17 mmol/g（Co-MOF-74-25°C）to 5.44 mmol/g（Co-MOF-74-150°C）because of the removal of large amount of water molecules from the pores.The further increase in CO adsorption to 6.07 mmol/g is attributed to the decomposition of the bind DMF and terminal ligand on the open metal site.And the IAST selectivity of CO/N₂was calculated by the ideal adsorption solution theory.The selectivity of Co-MOF-74-300°C in the whole pressure range is higher than other samples.It can be seen that the removal of bind DMF and terminal ligands on open metal sites can greatly improve the CO/N₂ selectivity.Subsequently,the CO/N₂ adsorption isotherms were measured at different temperatures and pressures in order to study the effects of temperature and pressure on the CO/N₂ adsorption performance.Through research,it is found that with the increase of temperature,the adsorption capacity of CO and N₂ gradually decreases.In particular,for the CO isotherm,as the temperature increases,the CO isotherm gradually changes from the I-type isotherm to linear,indicating that the increase in temperature destroys the interaction between Co-MOF-74 and CO molecules.From the CO/N₂ high-pressure adsorption isotherm,it can be seen that the pressure has little effect on the CO adsorption capacity.As the pressure increases from 1 bar to18 bar,the CO adsorption capacity only increases from 6.0 mmol/g to 7.12 mmol.As for the amount of N₂ adsorption,pressure has a greater impact on the amount of N₂ adsorption,which can be attributed to the multilayer adsorption of N₂.The results of CO/N₂ competitive adsorption isotherm show that the affinity of Co-MOF-74 for CO is far better than that of N₂,and the adsorption of CO can replace N₂ molecules in the pores.