Preparation,modification And CO₂ Adsorption Of Beta/KIT-6 Micro/mesoporous Composite Molecular Sieves

The wide using of fossil fuels causes the increase of carbon dioxide level in the atmosphere,leading to serious greenhouse effects.The issue of global warming resulting from increased atmospheric concentration of carbon dioxide（CO₂）is the most important environmental challenge facing the world today.Adsorption approach is one of the most promising methods due to its high selectivity and low energy penalties.Therefore,the preparation of novel efficient adsorbents to capture CO₂ in the environment at lower cost has become a research hotspot.In this paper,micro-/mesoporous composite molecular sieve Beta/KIT-6（BK,where B and K denoteβzeolite and mesoporous molecular sieve KIT-6,respectively）was prepared and then modified by amines.The as-prepared adsorbents were characterized by X-ray powder diffraction（XRD）,scanning electron microscopy（SEM）,Fourier transform infrared spectra（FTIR）,nitrogen adsorption/desorption,thermal gravimetric analysis（TGA）and elemental analysis methods,and then was carried out the investigation of the CO₂ adsorption performance.Beta/KIT-6 was synthesized by post–synthesis method,using nonionic three block copolymer（P123）as directing agents,tetraethyl orthosilicate（TEOS）as the silicon source,and Beta molecular sieve as the partial silica-alumina source.Micro/mesoporous composite sieve BK has a large specific surface area（655.1 m²/g）and total pore volume（1.13 cm³/g）.At60℃,the CO₂ adsorption capacity of BK was 1.268 mmol/g.BK was functionalized with（3-aminopropyl）trimethoxysilane（APTS）by grafting method,followed by the investigation of CO₂ adsorption performance at 40,60,75 and 90℃.With the increase of the loading of organic amine,the adsorption capacity of CO₂ increased first and then decreased,and A-BK-1exhibited the optimum adsorption capacity of 2.456 mmol/g.The optimum adsorption temperature of the modified adsorbent is 60℃ in the range of 40℃ to 90℃.BK was impregnated by tetraethylenepentamine（TEPA）or polyethyleneimine（PEI）with the different percentage.With the increase of the loading of organic amine,the adsorption capacity of CO₂ increased first and then decreased at 60℃.For BK-TEPA-60,the optimum adsorption temperature was 60℃ and the adsorption capacity was 4.21 mmol/g.While for BK-PEI-60,the optimum adsorption temperature was 75℃ and the adsorption capacity was 2.72 mmol/g.APTS and TEPA co-functionalized BK were prepared by a two-step method.The TEPA dispersion was facilitated by the newly formed Si-O-Si networks,and the higher thermal stability and amino group density were obtained.For A-BK-TEPA-50,the optimum adsorption temperature was 60℃,the adsorption capacity was 4.59 mmol/g and the amine efficiency was 0.45.The TEPA impregnated BK had higher adsorption capacity and amine efficiency,while the properties were better after double functionalization.The adsorption kinetic analysis showed that the adsorption of CO₂ by BK was physical adsorption,while the adsorption process of amine modified BK was dominated by physical and chemical adsorption.Four materials were selected for cyclic regeneration experiments and the results showed that all of them have good stability.