CO₂ Selective Separation Via Mixed Matrix Membrane Doped With Strongly CO₂-adsorptive MOFs And Derived Porous Materials

Reducing and capturing CO2 from industrial tail gas such as bio-gas and coal-fired flue gas is of great significance to achieve the national goal of carbon neutrality.Compared with the traditional chemical absorption method,physicochemical adsorption and membrane separation technology have the advantages of compact system equipment,low investment cost,low regeneration energy consumption,wide application range,simple and convenient operation process,etc.However,they have difficult bottlenecks such as low CO2 adsorption capacity and permeability coefficient,poor separation selectivity and cycle stability.The research hotspot in the domestic and abroda is how to strengthen technology research and development to promote its industrial application.In this paper,density functional theory calculations,gas isothermal adsorption tests,membrane separation permeation systems and other theoretical calculation or experimental vefitication methods are used to design and develop metal-organic frameworks and derived composite porous materials with strong CO2 adsorption capacity,which are further incorporated on polyethylene glycol(PEG)mixed matrix membranes(MMMs)with high CO2 affinity and flexibility to increase the CO2 permeability and selectivity of MMMs simultaneously.Pyrolyzing the core-shell zinc-cobalt zeolite imidazolate framework to obtain nitrogendoped porous carbon materials with increased CO2 physical adsorption sites.Microscopic characterization revealed that unsaturated metal sites(Me-N2)and pyridine nitrogen groups are the main physical active sites for CO2 adsorption in nitrogen-doped porous carbon materials,which have strong electrostatic interactions with CO2 molecules.The adsorption capacity of nitrogen-doped porous carbon material obtained by pyrolysis at 725℃ increased to 2.13 times of3.22 mmol/g.The ideal adsorption solution theory showed that the CO2 separation selectivity of the material for simulated biogas(CO2/CH4=40:60)increased from 2.11 to 24.3,and the CO2 separation selectivity for simulated coal-fired flue gas(CO2/N2=15:85)increased from 3.71 to40.2.Using polyethyleneimine to modify the nitrogen-doped porous carbon surface to increase the-NH-and-NH2-chemisorption sites and improve CO2 adsorption selectivity.Density functional theory(DFT)calculations revealed that high temperature carbonization resulted in partial fracture of Zn-N,Co-N,C=N and C-N bonds and decomposition of free methyl groups in the bimetallic zinc-cobalt zeolite imidazolate framework.A large number of Me-N2 unsaturated adsorption sites were obtained from the bimetallic zinc-cobalt zeolite imidazolate framework by pyrolysis,which further significantly increased CO2 adsorption capacity.After incororated 40 wt.% polyethylene imide,the CO2 adsorption capacity of adsorbent was increased to 2.6 times of the adsorbent before carbonization.Core-shell Zn/Co zeolite imidazolate framework was grown on a graphene oxide to obtain a composite materials rich in unsaturated Zn-N2-O active sites with selectively adsorption capacity towards CO2 molecule.The composite materials were incorporated into a polyethylene glycol membrane to enhance the CO2 permeability and mass transfer.Compared with saturated Zn-N4 active site,density functional theory calculation revealed that the interaction distance of O=C...Zn on the unsaturated Zn-N2-O site decreases from 4.65 (?) to 3.46 (?),and the corresponding binding energy increased from 34.02 k J/mol to 38.87 k J/mol.After incorporated 5 wt.%composite material,the CO2 permeability of mixed matrix membrane increased by 99.7 % to173.2 Barrer,and the CO2/N2 separation selectivity increased by 66.4 % to 61.9.Incorporating anionic pillared hybrid ultra-microporous materials into polyethylene glycol mixed matrix membranes and constructing a Rubik-like “expressway” channels to enhance CO2 permeability and mass transfer.Density functional theory calculations revealed that the vander Waals force C…F and the hydrogen bond interaction O…H between CO2 molecule and GEFSIX-2-Cu-i crystals can reduce a synergistic effect due to the excellent surface chemical properties of GEFSIX-2-Cu-i ultraporous material.Thus,compared with SIFSIX-2-Cu-i and TIFSIX-2-Cu-i material,GEFSIX-2-Cu-i ultraporous material has the highest binding energy of 34.5k J/mol towards CO2 molecules as well as the highest CO2 adsorption(5.02 mmol/g).1 wt.% GEFSIX-2-Cu-i nanoparticles significantly increased the CO2 permeability of the mixed matrix membranes to 460 Barrer.Growing Cu BDC nanosheets on the surface of multilayer molybdenum disulfide(Mo S2)based on a bottom-up synthesis strategy in order to develop a two-dimensional composite material with metal coordination active sites.The as-synthsized two-dimensional composites can enhance CO2 permeability and selectivity of Pebax-based mixed matrix membranes simultaneously.Microscopic characterization revealed that sulfur atoms on the Mo S2 and copper ions on the Cu BDC-ns formed a coordination complex Cu-S.The two-dimensional composite material could not only improve the CO2 separation selectivity of membarne through the lamellar morphology and narrow pores,but also enhance the CO2 permeability through a large number of metal coordination active sites.Incorporating 2.5 wt.% of the two-dimensional composite into the Pebax-based mixed matrix membrane enhanced the CO2/N2 separation selectivity by 80.2% to 69.Crosslinking two-dimensional metalloporphyrin(Me-TCPP)nanosheets with polyethylene glycol(PEG)semi-interpenetrating polymers to develop a PEG-based mixed matrix membrane,which produced a “rubber band” correction effect and enhanced CO2/N2 separation selectivity.Density functional theory calculations revealed that the N4-Znδ+...Oδ-distance between the metalloporphyrin center(Zn-N4)of the 2D nanosheet Zn-TCPP and the CO2 molecule is as low as 2.81 (?),enabling the binding energy of 19.9 k J/mol.The formation of abundant interfacial coordination bonds between Zn2+ and ester group(O-C=O)induced the “rubber band” stretching effect,which fixed the polymer chain and rearrange it,significantly enhancing CO2/N2 separation selectivity.Gas permeability experiment demonstrated that the CO2/N2 selectivity of the crosslinked polyethylene glycol(PEG)membrane with 2.5 wt.% two-dimensional Zn-TCPP nanosheet was increased to 81.The strong affinity of Zn2+ towards CO2 molecule and the formation of Zn2+-CO2 complex can significantly enhance the CO2 solubility in Zn-TCPP incorporated mixed matrix membranes.Due to abundant polyethyleneglycol groups of polyethylene glycol methacrylate(PEGDME),the CO2 permeability of Zn-TCPP incoporated crosslinked mixed matrix membrane(XLPEGDAco PEGDME)was remarkably increased to 398 Barrer.