Preparation Of Graphene-based Porous Materials And Their CO₂ Adsorption And Separation Properties

To deal with the global warming,sea level rise and other serious climate problems caused by CO2 emissions,the conversion,capture and separation of CO2 have been widely concerned.On the one hand,the micropore filling mechanism of CO2 requires that porous materials with high micropore specific surface area or close to the size of adsorbate molecules will have molecular sieve effect,so as to improve the adsorption capacity and selectivity of CO2.In addition,various heteroatoms(such as nitrogen,oxygen,sulfur,phosphorus and boron)are introduced into the carbon skeleton to enhance the quadrupole moment and polarity of the materials,which can also enhance the CO2 adsorption and selectivity.However,it is a challenging task to directly change the structure of porous materials and dope different heteroatoms.Therefore,we select graphene oxide(GO)rich in a large number of functional groups as the operating platform to carry out functional or non functional modification of GO,so as to introduce other heteroatoms or improve the structure of porous materials.Firstly,the porous carbon materials with iodine adsorption value of 1223.99 mg·g-1 and specific surface area of 1072.08 m2·g-1 were prepared by tubular furnace chemical activation method using semi-coke(SC)as carbon precursor and GO with different mass ratios as raw materials.Fourier transform infrared spectroscopy(FTIR)was used to analyze the change of functional groups in the structure.Raman spectroscopy revealed the change between graphite carbon and amorphous carbon.X-ray diffraction analysis showed that the increase of low angle scattering of graphene based porous carbon indicated the change of pore structure.Static adsorption test showed that the adsorption capacity of CO2 was 5.39 mmol·g-1(298 K,30 bar).The IAST model predicts that the CO2/N2 selectivity of graphene-based porous carbon with 300:1 SC to GO ratio at 1 bar and 298 K is 28.64 at the flue gas composition(CO2/N2=15/85 vol%).To further improve the adsorption capacity and selectivity of CO2,we proposed a new strategy to design and produce graphene-based semi-coke porous carbons with N-rich layered sandwich structure.Covalent functionalization is achieved by condensation and nucleophilic substitution of SC and GO with ethylenediamine(EDA).An appropriate adsorption isotherm model was used to measure and analyze the equilibrium adsorption capacity of CO2.The effect of GO content on CO2 absorption was studied by changing the mass ratio of GO to SC.Graphene-based semi-coke porous carbon with N-rich layered sandwich structure has high specific surface area(701.53 m2·g-1)and a large number of micropores centered at 0.8-2.0 nm.The most obvious is that the synthesized porous carbon materials not only improve the adsorption capacity(7.11 mmol·g-1),298 K),but also have excellent selectivity(34.25).Finally,a novel porous hyper-cross-linked polyimide-UiO-graphene composite adsorbent was prepared by chemical in-situ braiding of 4,4’-oxodiphthalic anhydride(ODPA),2,4,6-trimethyl-1,3-phenylenediamine(DAM),UiO-66-NH2 and GO.A polyimide(PIs)rich in nitrogen and oxides was added to UiO-66-NH2/GO with-NH2 group at the end,and the supermicropores could provide enhanced CO2 affinity.PI-UiO/GO has excellent acid-base resistance,suitable adsorption heat and excellent cycling stability,which indicates that the composite porous material has excellent recycling ability,which provides a variety of possibilities for the application of adsorbent in complex industrial environment.Under 298 K and 1 bar,the breakthrough time of PI-UiO/GO-1 in flue gas reaches 369 s.As expected,the resulting composite(PI-UiO/GO-1)exhibited a three-fold CO2 capacity(8.24 vs 2.8 mmol·g-1 at 298 K and 30 bar),4.2 times higher CO2/N2 selectivity(64.71 vs 15.43),and significantly improved acid-base resistance stability compared with those values of pristine UiO-66-NH2.The improvement of breakthrough time and adsorption capacity proved the applicability of the strategy.This synthesis method has a certain guiding significance for the design of new CO2 capture and flue gas separation materials.