Study Of Synthesis And CO₂/N₂ Adsorption And Separation Performance Of CHA Structured Silica-alumina Zeolite

The composition of various gases emitted from the combustion of carbon-containing fossil fuels is extremely complex,of which about 85%is N₂ and 12%is CO₂.Therefore,the separation of CO₂ and N₂ in flue gas is a hot research topic at this stage in order to reduce greenhouse gas emissions.Among them,CHA molecular sieves（Chabazite and SSZ-13）with an octet ring pore structure have attracted much attention and research because of their many remarkable advantages.In this paper,SSZ-13 molecular sieves were prepared by hydrothermal synthesis using N,N,N-trimethyl-1-adamantylammonium hydroxide as a template agent in turn.And Chabazite molecular sieve was successfully prepared by hydrothermal synthesis method using Y-type molecular sieve.The molecular sieves were characterized by XRD,SEM and FT-IR with the aid of ion exchange of cations from the second main group II_A(Mg²⁺,Ca²⁺,Sr²⁺and Ba²⁺)and the second secondary group II_B(Zn²⁺)with the original powders of both in turn and varying their ion exchange solution concentrations（10%-50%）.The isothermal adsorption curves were obtained at 298 K and 0-1000 k Pa by sequentially measuring the adsorption behavior of the samples with different cations and different ion exchange solution concentrations on CO₂ and N₂,and the adsorption isotherm curves were fitted with Langmuir-Freundlich adsorption isotherm curves.The tests showed that the main reasons for the significant changes in the adsorption of CO₂ and N₂ with the change of cation species and ion exchange solution concentrations were the ionic radius size of the introduced cations,the overall number of introduced cations,the cation occupancy and the difference of Si/Al values of the samples.In addition,the mass transfer coefficients（K）,diffusion coefficients（D）and kinetic selectivity（DCO₂/DN₂）of the samples after ion exchange were calculated from the kinetic point of view using the linear driving force model（LDF）for CO₂ and N₂,respectively.The mass transfer coefficients（K）and diffusion coefficients（D）of CO₂ were larger than those of N₂ in the tested samples,while the mass transfer coefficients（K）and diffusion coefficients（D）of CO₂ and N₂ were larger than those of 30%Sr-SSZ-13 in 30%Sr-Chabazite,indicating that the kinetic separation of zeolite with CHA structure after Sr²⁺exchange was excellent.And combined with Aspen Adsorption dynamic simulation of two-tower PSA flue gas separation,after comparing with the simulation results of K-Chabazite and Na-SSZ-13,it can be concluded that in the adsorption separation process of N₂/CO₂=0.85/0.15 mixture,both K-Chabazite and 30%Sr-Chabazite molecular sieves,their performance advantages are greater than those of Na-SSZ-13 and 30%Sr-SSZ-13 molecular sieves.The comparison of the purity and yield of the four adsorbents for N₂ shows that in the field of gas separation,the cation type and the number of cations play a crucial role in gas separation.Meanwhile,the molecular sieve with relatively low Si/Al（Chabazite）is more advantageous than the molecular sieve with the same structure and relatively high Si/Al（SSZ-13）.The simulation results,together with the equilibrium selectivity（α）,separation performance（S）and kinetic selectivity（DCO₂/DN₂）of the adsorbents mentioned above,we can conclude that 30%Sr-Chabazite is selected as the best adsorbent for the PSA process of CO₂ and N₂ separation in flue gas.