Functionalized Small Molecules Regulating The Effective Aperture Of MOF To Achieve Accurate Screening Of Gas Molecules

At present,with the rapid development of industrialization,CO₂ emissions in the world are increasing year by year,and the greenhouse effect is increasingly intensified.In order to achieve green and sustainable development,emission peak and carbon neutrality have become one of the key work that must be promoted during the“14th Five-Year Plan”period in China.Therefore,the capture and separation of CO₂ is extremely urgent.Compared with the traditional CO₂ separation method,the membrane separation method has the development potential of high separation efficiency and low energy consumption.Mixed matrix membranes（MMMs）that combine the advantages of polymeric and inorganic membranes can theoretically achieve high permeability and high selectivity.Metal-organic framework（MOF）with partially organic structure have better compatibility with polymers than other inorganic materials,and thus can fully play the characteristics of MOF,such as pore size screening.However,the pore size of MOF is generally larger than the average kinetic diameter of gas molecules,which makes it difficult to exert its sieving effect.Therefore,the gas selectivity of MOF-based mixed matrix membranes needs to be further improved.However,the pore size of MOF is generally larger than the average kinetic diameter of gas molecules,which makes it difficult to exert its sieving effect.Therefore,the gas selectivity of MOF-based mixed matrix membranes needs to be further improved.In order to realize the precise sieving of gas molecules by MOF,in situ grafting/polymerization of small molecules rich in CO₂-philic functional groups is proposed in this paper to immobilize the MOF cages.And by adjusting the content of grafted/polymerized molecules,the sieving size of MOF was effectively regulated,and the precise sieving of gas molecules was realized.First,a small molecule polyethylene glycol diglycidyl ester（PEGDGE）with epoxy groups was in situ grafted into NH₂-MIL-101（NMIL-101）with a macroporous cage structure through a ring-opening reaction using a vacuum-assisted method.The pore size of NH₂-MIL-101 was effectively regulated by adjusting the grafting amount of PEGDGE.Pebax-based MMMs with PEGDGE@NMIL-101 composites as fillers were prepared.The effect of PEGDGE on the pore size of MOF and the introduction of CO₂-philic ether oxygen functional group improved the CO₂ diffusion selectivity and solubility selectivity of the membrane respectively,which synergically improved the CO₂ separation performance of the membrane.When the PEGDGE content was 20.8 wt.%and the loading was 5 wt.%,the MMMs had the best CO₂ separation performance,with P_CO2 of 111.6 Barrer andα_CO2/N2 of 72.8,which are improved by 23.8%and30.8%,respectively compared with unmodified MMMs,and the comprehensive performance exceed the Robeson upper bound in 2008.Then,in order to verify the universality of PEGDGE in-situ grafting to control the effective aperture of MOF,the method was applied to NH₂-UIO-66（NUIO-66）with small-cage structure,and the PEGDGE@NUIO-66 composite was prepared.The pore size distribution results show that the PEGDGE grafted into NUIO-66 can successfully adjust the pore size of MOF,and the effective pore size of NUIO-66 decreases with the increase of PEGDGE graft amount.It is proved that PEGDGE modification is universal to adjust the aperture of MOF.When the PEGDGE content is 5.9 wt.%and the loading of PEGDGE@NUIO-66 is 5 wt.%,the selectivity of MMMs is greatly improved,which is 24.8%higher than that of the unmodified membrane.Finally,in order to further optimize the pore size regulation strategy of MOF with small cage structure,dopamine small molecule monomer was introduced into the MOF pore cage by vacuum-assisted method,and then the MOF pore size was regulated by in situ polymerization.The results of pore size distribution showed that the addition of PDA successfully reduced the pore size of UIO-66.With the increase of PDA polymerization,the pore size of UIO-66decreased from 0.68 nm to 0.58 nm,which effectively improves the CO₂ diffusion selectivity of the corresponding membrane.At the same time,the CO₂-philic functional groups（-OH and-NH₂）on the PDA effectively improved the CO₂ dissolution selectivity of the membrane,and the synergistic effect of the two enhanced the CO₂ separation performance of the membrane.When the PDA content is 14.1 wt.%and the loading is 5 wt.%,the MMM has the best gas separation performance,with CO₂ to N₂ selectivity of 70.55,CO₂ permeability of 94.56 Barrer,and the P_CO2 andα_CO2/N2 are improved by 68.9%and 33.1%,respectively,compared with the pure membrane.