Preparation And CO₂ Adsorption Performance Of Amine-functionalized MCM-41

Global climate changes due to greenhouse effect have raised considerable attention around would. Carbon Capture and Storage (CCS) is one of crucial tasks of academic and industry circles, and carbon capture is a high investment cost and energy-intensive process. Compare to conventional oxyamine process, solid phase adsorption can overcome the problem of adsorbents degradation and equipment corrosion, and amine-functionalized mesoporous adsorbent not only has the potential of reducing costs and energy, but also combines the advantages of chemical and physical adsorbent, it can capture CO2 efficiently.MCM-41 with mature synthesis method and variable pore size was chosen as the substract to exam the effect of pore size and pore volume towards CO2 adsorption performance of amine functionalized adsorbents. An array of MCM-41 with substantially larger average pore diameters was synthesized through adding the swelling agent of 1,3,5-trimethylbenzene (TMB).3-aminopropyltriethoxysilane(APTES) was grafted on pore-expanded MCM-41. The results indicate the swelling agent could increase the pore diameter efficiently, but the order degree and crystallinity decline. Grafted APTES raise the amine content of the adsorbents, but the order degree and crystallinity decline further. When the pore diameter is 7.50 nm, the CO2 adsoption capacity is 0.48 mmol/g. After grafted with APTES, the adsorption capacity reaches 1.16mmol/g, which is 4.8 times as big as standard MCM-41.A series of amine-impregnated MCM-41 adsorbents were synthesized using ethylenediamine (EDA), diethylenetriamine (DETA), tetraethylenepentamine (TEPA) and pentaethylenehexamine (PEHA) and polyethyleneimine (PEI) with two molecular weights as the amine source to study the effect of amine moieties towards CO2 adsorption performance. Typical results include:under 10%CO2/N2, at 35℃, the adsorption capacity of 40wt% TEPA-MCM41 is 1.96 mmol/g. Changing the temperature using temperature-programmed method, the adsorption capcities raise to 4.25 mmol/g; The adsorption capacity of 40wt% PEHA-MCM41 is 2.34 mmol/g, the adsorption capcities could reach 4.5 mmol/g by changing the temperature; The thermal stability of PEI-MCM41s is superior than other adsorbents, and PEI600-MCM-41 exhibit the best regeneration stability. Zero Length Column was used to explore the effect of water vapor towards adsorption performance. The deactive adsorbents can recover with humid mixture gas and stay stable.Thermogravimetric Analysis (TGA) was used to measure the adsorption thermodynamic characteristics of 40wt% TEPA-MCM41 which was chosen as the typical adsorbent under low CO2 partial pressure of the adsorbent. According to the results obtained before,40wt% TEPA-MCM41 shows excellent adsorption capcity, regeneration performance and is easy to synthesize. According to the adsorption capacities of TEPA-MCM41 under different CO2 pressures to calculate the adsorption isotherms. A new method based on Dual-site Langmuir model which can distinguish physical adsorption and chemical adsorption was proposed. Under the same pressure and temperature, the physical adsorption capacity of amine functionalized adsorbents and the adsorption capacity of MCM-41 are propotional with the surface area. The normalized deviations by this model are less than 3%. The physical adsorption enthalpy and chemical adsorption enthalpy (ΔH0) are -25.4 kJ/mol and -47.4 kJ/mol respectively. The saturation adsorption capacity of 40wt% TEPA-MCM41 is 7.79 mmol/g.TGA and Zero Length Column (ZLC) were used to investigate the adsorption kinetics and adsorption/desorption mechnisms. The zwitterionic mechanism of a primary and secondly amine with CO2 for the formation of carbamate. A free base deprotonates the zwitterion and forms the carbamic acid and ammonium carbamate salt. One CO2 molecule reacts with a secondary amine site to form ammonium carbamate directly; The adsorption performance at different temperatures reveals that CO2 adsorption over TEPA-MCM41 and PEHA-MCM41 are under kinetic control while EDA-MCM41 and EDTA-MCM41 are under thermodynamic control below 90 ℃.ZLC was used to obtain the breakthrough curves of MCM-41 and TEPA-MCM41. Avrami’s kinetic model was used to fit the curves. The activation energy of physical and chemical desorption are 15.86 and 57.15 kJ/mol, respectively, the total activation energy (73.02 kJ/mol) is in accordance with TGA result (70.56 kJ/mol); Intraparticle diffusion model and Boyd’s kinetic model were used to study the rate-limiting step. Under higher temperature, intraparticle diffusion controls the desoprtion rate of MCM-41. As for TEPA-MCM41, because there exist chemical reaction, the desorption mechanism is controled by the influence of surface reaction and diffusion at the gas/solid interface in batch systems.