Adsorption and kinetics of CO2 gas by molecular sieves prepared from rare earth tailings
Objective In pursuit of carbon neutrality,China is actively advancing the development of a comprehensive carbon capture indus-try cluster.There exists a pressing demand for economically viable and low-cost technologies for the capture,sequestration,and utilization of CO2 gas.Zeolite molecular sieves have garnered significant attention due to their tunable pore structure and selec-tive adsorption properties.However,challenges arise in their widespread adoption and mass production due to issues with selec-tivity,thermal stability during CO2 capture,and the high production costs involved.China possesses vast and diverse quantities of rare earth tailings,yet secondary recovery costs are prohibitive due to immature technology.Mineralogical analysis of these tailings reveals significant concentrations of Si and Al elements,both crucial constituents of synthetic molecular sieves.More-over,active metal elements such as Fe,Ca,and Ce are present in the tailings,which can enhance CO2 gas adsorption.To develop low-cost and highly efficient CO2 adsorbents,we target rare earth tailings,which pose challenges due to their large accu-mulation and difficult management.We analyze the physical and chemical properties of rare earth tailings,synthesize molecular sieves,and determine optimal synthesis ratios and temperatures conducive to CO2 adsorption.The aim is to produce molecular sieves with morphological structures,specific surface areas,and pore structures optimized for CO2 adsorption efficiency.Addi-tionally,we investigate the kinetic equation governing the synthesis of rare earth tailings for CO2 adsorption,aligning with the kinetic equations applicable to molecular sieves synthesized from rare earth tailings.This research contributes to exploring the high-value utilization of rare earth tailings within the context of carbon capture,emphasizing stability,high efficiency,and low cost in CO2 adsorbent development.Methods Thermogravimetric mass spectrometry(TGMS)and thermoanalytical kinetics(TAK)were employed to investigate the mass ratios of silicon and aluminium under optimal synthesis conditions for molecular sieves derived from rare earth tailings.Sur-face micromorphology,specific surface area,and pore structure characterization of these molecular sieves were conducted using scanning electron microscopy(SEM),energy-dispersive X-ray spectroscopy(EDS),and Brunauer-Emmett-Teller(BET)analysis,respectively.Furthermore,the cyclic stability of green synthetic molecular sieves derived from activated rare earth tail-ings solids was assessed through multiple thermal stability experiments conducted under identical operating conditions.Addition-ally,four kinetic models,including the quasi-primary adsorption model,quasi-secondary adsorption model,fractional kinetic model,and Elovich kinetic model,were employed to fit adsorption curves at temperatures of 30℃,50℃,and 70℃.This analy-sis aimed to identify the most effective curve representing the adsorption behavior of rare earth tailings synthetic molecular sieves for CO2 capture.Results and Discussion The findings indicate that the molecular sieve synthesized from rare earth tailings exhibits its highest CO2 adsorption capacity when the silicon-to-aluminium mass ratio is 1∶1.5.Specifically,the adsorption efficiency for CO2 peaked at 0.15 mmol/g when tested at 50℃ and with a 1% CO2 volume fraction.Moreover,the thermal stability of the molecu-lar sieves derived from rare earth tailings was evaluated,revealing consistent and stable adsorption performance over four cycles.To elucidate the CO2 gas adsorption mechanism at various temperatures,quasi-primary kinetics,quasi-secondary kinet-ics,Fractional,and Elovich models were employed to fit the experimental data.Among these models,the Fractional model dem-onstrated the highest degree of fitting,suggesting its suitability for describing CO2 gas adsorption on the molecular sieve surface.Furthermore,the optimal adsorption performance observed at 50 ℃ was attributed to the significant driving force generated by the appropriate operating temperature,facilitating CO2 gas diffusion into the adsorbent with minimal resistance.Conclusion The green synthetic molecular sieve derived from activated rare earth tailings solids exhibits a notable adsorption capacity for CO2 gas.Specifically,under a 1% concentration environment at 50 ℃,the maximum adsorption capacity of the molecular sieve for CO2 gas reached 1.5×10-4mol/g.This adsorption process demonstrates strong selectivity,attributed to the larger molecular polarity of CO2 gas compared to N2.Moreover,the adsorption process of CO2 gas by the concatenated molecular sieves is reversible and exhibits excellent thermal cycle stability.Furthermore,through a kinetic study of adsorption at various temperatures,it was observed that the Fractional equation,with a fit superiority index R2 higher than 0.983 32 across different temperatures,provides a more accurate reflection of the adsorption process of molecular sieves on CO2 gas.This finding offers a valuable kinetic model for the utilization of rare earth tailings in the field of CO2 gas adsorption,paving the way for future research and applications in this area.
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