Study On Preparation Of Amine-functionalized Adsorbents And Its CO₂ Adsorption Properties

The serious greenhouse effect has aroused widespread concern around the world and CO2 discharged from coal-fired power plants is an important cause of the growing greenhouse effect.Therefore,how to effectively capture CO2 from flue gas and achieve CO2 removal from flue gas of coal-fired power plants is of great significance for mitigating global climate change.Amine-functionalized CO2 adsorbents combine the advantages of the high selectivity,high adsorption capacity of chemisorption and low energy consumption of physical adsorption.Thus,amine-functionalized CO2 adsorbent has shown the broad application prospect in the field of flue gas CO2 capture.The various types of excellent adsorbents have been developed to achieve efficient capture of CO2.However,in the preparation process of the adsorbent,there are still problems such as easy aggregation of organic amines,low adsorption efficiency,and complex preparation process of adsorbent support.Therefore,to solve the above issues,this paper focuses on reducing the agglomeration of organic amines,increasing the dispersion of active components in the pores of the support and simplifying the preparation of the support.By analyzing and optimizing the CO2 adsorption process conditions and the support structure,the relationship between the support structure and the adsorption performance of the amine-functional adsorbent is explored to promote the dispersion of active components in the pores.The amine functionalized adsorption material for CO2 capture with high adsorption performance was developed.Simultaneously,the adsorption mechanism and kinetics of TEPA functionalized adsorbents were discussed initially.The main conclusions are as follows:1.The effects of amine types,amine loading,adsorption temperature,gas flow rate and other process conditions on adsorption behavior was investigated.It was found that porous silicon adsorbents loaded with small molecule organic amines(MEA,DEA,DETA)have significant decrease in the active components during the adsorption and desorption process due to the poor thermal stability,resulting in the low CO2 adsorption capacity.The porous silicon adsorbents loaded with TEPA and PEI have the higher thermal stability and CO2 adsorption capacity.When the TEPA loading is 60 wt.%,the adsorbent has the maximum adsorption capacity of 5.01 mmol/g at 75℃.It was found that adsorption temperature and gas CO2 concentration were important factors affecting the adsorption performance of amine-functionalized adsorbents,while gas flow had little effect on the amount of adsorption.It is found that the deactivation model can better simulate the CO2 breakthrough process under different adsorption conditions.2.A series of amine-functionalized adsorbents with different pore structures were prepared with SBA-15,MCM-4,HPS and 3dd porous silicon support.By comparing and correlating the relationship between the support structure and adsorption performance of different adsorbents,it is found that the specific surface area is not the main factor controlling the adsorption performance of the solid amine adsorbent,both the pore volume and pore size of the support play a crucial role in improving the CO2 adsorption performance of the adsorbent.The larger pore volume in the support can promote the dispersion of active components;the large mesopore structure can provide channels for the active components to enter the mesopores,and can also provide the diffusion channel for CO2 in the adsorbent.Therefore,the adsorption capacity of 3dd and HPS adsorbents is significantly better than that of SBA-15 and MCM-41.And compared with the single mesopore adsorbent,HPS-TEPA60%and 3dd-TEPA60%adsorbents with larger pore volume and hierarchical pore structure have faster adsorption rate and adsorption kinetics,indicating that the larger pore volume and hierarchical pore structure can take dual advantage of the different size distributions in the support to disperse active components and CO2 diffuse gas.Simultaneously,it was found that the uniformly dispersed active components are conducive to the release of the adsorbed CO2,making it easier for the adsorbent to desorption at elevated temperatures,thereby increasing the CO2 regeneration rate and improving its cycle stability.3.Hierarchical mesoporous composite support with pore sizes concentrated at 2.6,6.1 and 23 nm was obtained by physical milling SBA-15 and porous silicon(HPS).The hierarchical mesoporous structure can make TEPA more uniformly dispersed in the adsorbent.The formed adsorbent can retain more residual pores to provide the channel for CO2 diffusion and exert the dual advantages of different size distributions pores to disperse active components and CO2 gas diffusion.When the mass ratio of SBA-15 to HPS is 1:2 and 50 wt.%TEPA is loaded,the adsorbent has the optimal adsorption capacity of 5.05 mmol/g at 75℃ with 15 vol.%CO2,which is significantly increased compared with TEPA modified SBA-15 and HPS adsorbents.The water vapor in the flue gas can significantly improve the CO2 adsorption performance.When the relative humidity of the flue gas is 60%RH,the CO2 adsorption capacity of S2HPS-TEPA50%is increased by 16%.In situ DRIFT shows that the adsorption process of CO2 at the active site follows the zwitterionic mechanism,that is,CO2 forms the zwitterionic intermediate with the primary or secondary amine in the adsorbent under dry conditions,and then the zwitterionic intermediate is deprotonated to form protonation product and carbamate.4.The foamed silicon materials(MCF)with different pore volumes and window sizes were synthesized by introducing the mesitylene(TMB)as the pore expander.With the increase in the mass of TMB,the foam cell size and cell volume of MCF materials increase.When the TMB/P123 ratio is 0.8,MCF-0.8-TEPA60%has an optimal adsorption capacity of 4.75 mmol/g at 75℃with 15 vol.%CO2.The large pore volume and window size promote the uniform dispersion of TEPA in the foam pores,which is conducive to CO2 adsorption.In suit DRIFT and DSC results show that the adsorption temperature affects the dispersion of the active components in the adsorbent.And the adsorbent will expose more adsorption sites at higher temperatures to promote CO2 adsorption.The acidic impurity gas in the flue gas can cause the irreversible deactivation of the adsorbent,which in turn causes the decrease of adsorption regeneration performance of the adsorbent.5.Based on the consideration of simplified the support synthesis method and reduced the preparation cost,hierarchical mesoporous silica(HMS)with the large pore volume(2.44 cm3/g)and double mesoporous structure(8.84 and 24.93 nm)was prepared by the spacer self-assembly sol-gel method.While simplifying the preparation of the support,the dispersion of TEPA in the adsorbent was further improved,thereby the CO2 adsorption capacity was significant increased.Under the condition of 75℃ and 15 vol.%CO2,when the HMS support was loaded with 70 wt.%TEPA,the CO2 adsorption amount was as high as 5.82 mmol/g.In addition,the breakthrough adsorption amount of 4.89 mmol/g accounts for 84%of its saturated adsorption amount,which makes the adsorbent has the high capture efficiency.Simultaneously,the CO2 adsorption capacity can still reach 4.1 mmol/g at 35℃ with 5 vol.%CO2,the excellent adsorption capacity at low temperature and low partial pressure can greatly expand the application range of this adsorbent.In situ DRIFTS results showed that high CO2 concentration can promote the formation of carbamic acid in the reaction process,thereby increasing the CO2 adsorption capacity.It is found that the adsorption activation energy(Eα)of the adsorbent decreases with the increase of CO2 concentration through Arrhenius formula fitting,and it is lower than that of the alcohol amine absorber,indicating that the amine-based adsorbent adsorbed CO2 more easily.