Modification Of Glucose And G-C₃N₄ Based Carbon Materials And Mixed Matrix Membranes Gas Separation Performance

The continuous increase in the concentration of CO₂ in the atmosphere has severely disrupted the ecological balance of the earth,causing problems such as global warming and reduction of human habitat.At present,the main way of CO₂ emission is the burning of fossil fuels,so the efficient recovery of CO₂ from flue gas has become an important way to achieve carbon neutrality.Membranes separation is a gas separation method with low energy consumption,low cost and no secondary pollution.It has received extensive attention in carbon emission reduction.In this paper,Pebax2533,which has both soft and hard blocks,is selected as the polymer matrix,nitrogen-doped porous carbon materials,nitrogen and sulfur co-doped porous carbon materials,and ZIF-8-grafted few-layer porous g-C₃N₄ are used as the inorganic filler.The purpose of the modification is to enhance the affinity of the mixed matrix membranes to CO₂.Then we prepared a series of mixed matrix membranes with high CO₂ permeability,high CO₂/N₂ selectivity and high operational stability.The details are as follows:（1）Glucose carbon spheres with rich oxygen-containing groups on the surface were selected as precursors,and melamine was used as nitrogen source.Glucose and melamine are hydrothermally prepared to prepare nitrogen-doped carbon spheres.N-doped porous carbon spheres（NPC）with porous structure were prepared by KOH activation method.Then Pebax/NPC mixed matrix membranes were prepared.In the FTIR test,the absorption peaks of glucose aromatization and triazine ring were found,indicating the successful synthesis of nitrogen-doped carbon materials.The BET test showed that NPC has high specific surface area and high microporosity.The CO₂ adsorption test shows that NPC has high adsorption capacity of CO₂.At the same time,SEM and mechanical performance test showed that NPC and polymer have good compatibility and Pebax/NPC mixed matrix membranes has good mechanical properties.The effects of NPC content,feed pressure,test temperature,and CO₂/N₂ mixture（15/85 vol%）on the CO₂ permeability,CO₂/N₂ selectivity and operational stability of Pebax/NPC mixed matrix membranes have been systematically studied.When the NPC content was 5%,the performance was the best.At 25°C and 2 bar,the CO₂ permeability is 552 Barrer,and the CO₂/N₂ selectivity is 56.Compared with the pristine Pebax membranes,the CO₂permeability and CO₂/N₂ selectivity increased by 221%and 107%,respectively.The reason was that the acid-base interaction between the nitrogen-containing groups in NPC and the CO₂,abundant micropores and high specific surface area.The test results of the mixed gas using simulated flue gas CO₂/N₂（15/85 vol%）show that the performance of the mixed matrix membranes in the mixed gas is slightly lower than that in the pure gas,the CO₂ permeability is501 Barrer,CO₂/N₂ selectivity is 53.In addition,under mixed gas conditions,the performance of Pebax/NPC MMMs remained stable within 360 h,exceeding the Robeson upper bound.（2）In order to further improve the affinity of carbon materials for CO₂,N,S-doped porous carbons（NSPC）were prepare by using thiourea as sulfur and nitrogen,and add NSPC to Pebax2533 to prepare Pebax/NSPC MMMs.The BET test shows that NSPC has a high microporosity structure and a high specific surface area.At the same time,the mechanical performance test shows that Pebax/NSPC MMMs have better mechanical properties.During the preparation of NSPC,the influence of the mass ratio of glucose to thiourea and the mass ratio of precursor to activator on the gas separation performance of mixed matrix membranes were studied.The effect of NSPC content in the membranes,feed pressure,test temperature,and CO₂/N₂ mixture（15/85 vol%）on the CO₂ permeability,CO₂/N₂ selectivity and operational stability of the Pebax/NPC mixed matrix membranes were systematically studied.The results show that when the NSPC content is 3%,the performance of the mixed matrix membranes is the best.The CO₂ permeability of Pebax/NSPC-3 MMMs is 589 Barrer and the CO₂/N₂selectivity is 64 at 25°C and 2 bar,which were 244%and 139%higher than pristine Pebax membranes,respectively.This is due to the combination of the acid-base affinity of nitrogen-containing sites with CO₂,the change of the polarity of the carbon structure by the introduction of sulfur,the abundant micropores and the high specific surface area increase the affinity for CO₂.The mixed gas test results show that the performance of the Pebax/NSPC mixed matrix membranes in the mixed gas is slightly lower than that in the pure gas,the permeability of CO₂is 560 Barrer,and the selectivity of CO₂/N₂ is 60.And the performance remained stable in the continuous 360 h test,breaking the Robeson upper bound.（3）Based on the g-C₃N₄ carbon material,firstly,the structure of few-layer porous was generated,then growing ZIF-8 in situ to make an embedded few-layer porous g-C₃N₄/ZIF-8composite material（CNZ）,further improving the selectivity to CO₂/N₂,and then added to Pebax2533 to prepare mixed matrix membranes.The shift of the peak position of nitrogen-containing groups in FTIR indicates that ZIF-8 particles grow in situ on g-C₃N₄.The XRD test showed that the interlayer spacing of the modified carbon material became larger.The effect of the grafting amount of ZIF-8 on the performence of mixed matrix membranes was studied,and the effects of different feed pressure,test temperature and CNZ content on the performance of the mixed matrix membranes were studied.The results show that when the CNZ content is 2%,the performance of the mixed matrix membranes is the best.At 25°C and 2 bar,the CO₂permeability is 470 Barrer,and the CO₂/N₂ selectivity is 96.This is because the edge of g-C₃N₄contains some amino groups which have affinity for CO₂,and the few layers of porous structure produced by peeling reduce the gas transmission resistance,and the introduction of ZIF-8 can also significantly improve the selectivity to CO₂/N₂.Under the simulated flue gas CO₂/N₂（15/85 vol%）,the CO₂ permeability and the CO₂/N₂ selectivity of the Pebax/CNZ-2 mixed matrix membranes is slightly lower than that in the pure gas conditions.The permeability of CO₂ in the gas mixture is 435 Barrer,and the selectivity of CO₂/N₂ is 91.The performance remained stable in the continuous 360 h test under mixed gas conditions,which exceeded the Robeson upper bound.