Preparation Of Metal-Heteroatom Modified 13X Zeolite And Their Adsorption Performance For Direct Air Capture

Global warming has attracted great attention to CO2 emissions.As one of the most developing prospective emission technologies,direct air capture(DAC)can play a key role in achieving carbon neutrality and can effectively address global CO2 emissions from point and nonpoint sources.13X zeolite has been applied to the adsorption and separation of air CO2 because of their low cost,stability and easy preparation.However,13X zeolite cannot efficiently adsorb and separate CO2/N2 at low CO2 partial pressure based on the difference between zeolite and physical adsorption strength,and is sensitive to water.Based on the above background,this paper takes 13X zeolite as the reference material and constructs transition metal heteroatoms within its nano-pore channels to play a precise role in regulating the pore size of zeolite in order to improve the adsorption selectivity of the zeolite.In addition,the specific location of metal ions in zeolites and how these ions participate in enhancing CO2 adsorption were discussed in depth,and the water resistance,adsorption characteristics at different temperatures and regeneration energy consumption were systematically studied.The main findings of this paper are as follows:(1)The effect of framework Cu species on the precise control of 13X zeolite pore size and the CO2 adsorption performance under different operating conditions was investigated.The Cu heteroatom-containing 13X zeolite(Cu@13X)was prepared by constructing ligand unsaturated divalent metal Cu ions within the 13X molecular sieve nanopore channels using a one-step in situ crystallization method.The results showed that Cu@13X exhibited a high CO2 adsorption capacity(6.9 mmol/g)and CO2/N2 separation selectivity(298)at 25℃ and 1 bar,which were related to the stronger affinity of isolated Cu2+ for CO2 in the framework and smaller micropore size.In addition,Cu@13X has excellent cycling stability and exhibits good CO2 adsorption performance under different operating conditions(temperature,gas concentration and flue gas containing H2O(g),etc.).(2)The adsorption characteristics and regeneration energy consumption of Fe@13X zeolite at different temperatures were investigated.The Fe heteroatom-containing 13X zeolite(Fe@13X)was synthesized by a simple one-step in-situ crystallization method.The results demonstrated that Fe@13X exhibited excellent DAC performance with a CO2 adsorption capacity of 0.64 mmol/g,much higher than that of 13X zeolite in simulated air(0.29 mmol/g),which was attributed to the introduction of Fe atoms effectively narrowing the 13X micropore channel.Furthermore,Fe@13X showed a 3 times higher CO2 production(0.003 kgCO2/kgads·h)and 3.6 times lower desorption energy(0.005 kWh/kgCO2)than 13X zeolite in 400 ppm CO2 in N2.Moreover,Fe@13X exhibited excellent cycle stability in simulated air and maintained its initial CO2 uptake in 10 consecutive cycles.(3)The strengthening mechanism of CO2 adsorption on Fe@13X zeolite was studied.Density functional theory(DFT)calculation results show that the C=C bond in CO2 undergoes active electron transfer with the framework four-coordinated Fe3+,resulting in the π-coordination effect dominates the CO2 adsorption of the zeolite,which enhances the stronger adsorption energy between 13X and CO2 molecules.The results of anti-water adsorption experiments showed that H2O(g)has a certain promoting effect on CO2 adsorption.The CO2 adsorption capacity of Fe@13X was 2 mmol/g under simulated flue gas containing H2O(g)(RH=50%)at 60℃,which was higher than that of dry simulated flue gas(1.65 mmol/g).Moreover,it showed excellent cycle stability in simulated flue gas containing H2O(g)and O2.