碱金属熔盐掺杂有机镁前驱体合成改性MgO的CO2吸附动力学研究

MgO shows potential for intermediate-temperature CO2 capture from flue gas stream, considering the merits of high theoretical CO2 adsorption capacity, moderate regeneration energy, good working stability and low cost for raw material. However, practical adsorption performances of bulk MgO are limited by its structural characteristics, e.g., low density of basic sites. Besides, the carbonation process of MgO is believed to be limited kinetically, and the intrinsically high lattice enthalpy has restrained its carbonation kinetics. To this end, organometallic magnesium precursors are doped with alkaline metal eutectic mixtures, and calcined to fabricate modified MgO sorbent with improved structural characteristics, better surface property and more basic sites, aiming to enhance the CO2 adsorption capacity and kinetics performances. Firstly, the effect of organometallic magnesium precursors and doped alkaline metal eutectic mixtures on “structure-performance” relationship of the modified MgO sorbent are expounded. Secondly, single particle kinetics model is established to recognize the dominant role between the diffusion effect and chemical reaction of the sorbent in a TGA reactor. Bed kinetics model is also built to elaborate the mass transfer kinetics of the sorbent in a fixed-bed reactor. Lastly, surface reconstruction mechanism is revealed for CO2 adsorption on the modified MgO sorbent by the quantum-chemistry calculation method. Detailed reaction pathways and molecular dynamics are explored on a molecular level, to further deepen the micro-dynamics mechanism. The project will yield fruitful achievements, which will provide significant guidance for the efficient fabrication of MgO sorbent as well as lay a theoretical foundation for application of the sorbent in flue gas treatment.

中温MgO吸附剂具有理论吸附容量高、再生能耗适中、稳定性强和原材料成本低等优势，其在烟气CO2捕集中具有潜在应用前景。但其实际吸附性能受限于MgO结构特性，如：碱性位点密度低。此外，MgO捕获CO2的碳酸化过程受动力学控制，其内在的高晶格能限制了反应速率。为此，本项目采用碱金属熔盐掺杂有机镁前驱体并煅烧合成改性MgO吸附剂，改善微观结构、表面性质和活性位点密度，提高其CO2吸附容量和反应速率。阐明有机镁前驱体类型和熔盐掺杂对吸附剂“结构-性能”关系的影响机制。构建吸附剂与CO2反应的颗粒和床层吸附动力学模型，探明CO2与吸附剂颗粒的扩散-化学反应主导作用机制和传质动力学机理。采用量子化学计算，剖析CO2分子在改性吸附剂表面的重构规律，预测化学反应路径，揭示反应体系的分子动态行为机制并深化微观动力学机理。研究工作为MgO吸附剂高效合成提供指导，同时为其应用于烟气CO2捕集奠定理论基础。

针对MgO吸附剂的CO2吸附容量和速率受限于其结构特性和内在的高晶格能等关键问题，通过前驱体筛选、合成方法优化及碱金属熔盐掺杂，改善其微观结构并调控其活性位点密度。阐明了前驱体、合成方法和碱金属熔盐掺杂对MgO吸附剂的CO2吸附“构效关系”的影响机制，明晰了其CO2吸附动力学机理。.(1) 揭示了前驱体对MgO吸附剂的微观结构和CO2吸附性能的影响机制。低分子量的有机镁前驱体热解扩孔效应明显，易形成孔隙结构好、晶粒尺寸小且表面碱性位点密度高的MgO纳米颗粒，其CO2吸附容量可观。天然镁矿衍生MgO吸附剂的微观结构、表面形貌、碱性位点密度及其CO2吸附性能依赖于矿物前驱体。水菱镁矿和水镁石煅烧MgO的比表面积和孔体积较高，晶粒尺寸较小，表面碱性位点丰富，MgO以片状形态分布，降低了气体扩散阻力并提高了MgO利用效率。.(2) 阐明了合成方法对MgO吸附剂的CO2吸附“构效关系”的影响机制。以固态化学反应法制备的样品表面碱性活性位点密度高，且分布均匀，其表面的弱、中和强碱性活性位点与CO2分子作用形成碳酸氢盐、双齿碳酸盐和单齿碳酸盐。研究发现，MgO吸附剂表面碱性位点密度及其分布特性依赖于煅烧条件。在CO2气氛下中温(500oC)煅烧的MgO吸附剂平均晶粒尺寸小，碱性活性位点密度高，表面活性位点多为强碱性O2-位点，易与CO2结合形成单齿碳酸盐构型。.(3) 考察了MgO吸附剂的CO2吸附动力学特性，发现多孔MgO吸附CO2的过程包括内扩散和CO2在碱性位点表面的物理吸附和化学吸附，其中化学吸附占主导作用。揭示了多孔MgO吸附CO2的传质动力学机理，发现MgO在固定床中吸附CO2的过程由外扩散和内扩散构成，其中内扩散是MgO吸附CO2的速率决定步骤。明晰了碱金属硝酸盐掺杂对MgO吸附CO2动力学的促进作用机理。发现熔盐掺杂MgO的碳酸化过程可分为三个阶段：初始阶段的CO2吸附、快速碳酸化和MgCO3成核及CO2在碳酸化产物层的体相扩散。碱金属硝酸盐掺杂显著提高了MgO吸附剂的CO2吸附容量，NaNO3和KNO3在促进CO2吸附过程中表现出协同效应。.本项目的研究结果丰富了固体吸附剂CO2捕集的理论体系，为高效廉价MgO吸附剂的开发及其在烟气CO2分离中的规模化应用奠定了理论基础。