氢氧化钾碱蚀法制备多孔MgAl氢氧化物及其二氧化碳吸附性能

Adsorption of CO_(2) on Porous MgAl Hydroxides Prepared by Alkaline Etching of KOH Solution

目的 类水滑石化合物用于CO_(2)吸附时存在分散性差和有效比表面积低的问题。方法 以硝酸镁和硝酸铝为金属盐前驱体,以尿素为沉淀剂,采用水热法合成具有层状结构的MgAl水滑石,之后利用铝物种的两性性质,通过氢氧化钾碱蚀处理MgAl水滑石脱除部分铝物种以形成孔道,从而获得多孔MgAl氢氧化物并用于二氧化碳的吸附。借助X射线衍射、扫描电镜、低温氮气吸附/脱附、红外分析等研究碱蚀时间对所得MgAl氢氧化物结构形貌的影响。之后,采用热重分析仪测试样品的二氧化碳吸附性能,并且分别采用一阶动力学模型、伪二阶动力学模型对吸附数据进行拟合。结果 与未碱蚀样品比较,当碱蚀时间为12 h时,样品LDH-12的比表面积和孔体积都有所增加,其中比表面积由8.8 m^(2)/g变为16.6 m^(2)/g,这有利于吸附活性位点的暴露;样品LDH-12对二氧化碳有着较优的吸附性能,其吸附容量为19.6 mg/g。结论 碱蚀处理对样品的形貌结构及吸附性能有着很大的影响。适宜的碱蚀时间有利于样品孔结构的进一步发育,使得更多的吸附活性位暴露,从而较大程度地提高其对二氧化碳的吸附能力。

A large amount of CO_(2) has caused adverse effects on people's life,and it is imperative to reduce the concentration of CO_(2) in the atmosphere.As an effective way to capture CO_(2),hydrotalcite-like compounds have problems of poor dispersion and low effective specific surface area for use in CO_(2) adsorption.To address these problems,MgAl layered double hydroxides(LDHs)were firstly prepared via one-pot hydrothermal method with Mg(NO_(3))2·6H_(2)O and Al(NO_(3))3·9H2O as precursors and urea as precipitator.Then,taking advantage of the amphoteric nature of Al species,MgAl hydroxides were treated by alkaline with the sample LDH-0 without alkaline etching, with the alkaline etching time of 12 h, the specific surface area and pore volume of the sample LDH-12 increased, and the specific surface area of LDH-12 changed from 8.8 m^(2)/g to 16.6 m^(2)/g, which was conducive to the exposure of adsorption active sites. In addition, after the alkaline etching treatment, the modified samples still maintained the regular hexagonal morphology, but the layer structure of the samples became thinner with the rougher surface, indicating that the alkaline etching could remove some Al species from the sample. With the prolongation of the alkaline etching time, the sample LDH-24 presented a regular hexagonal ring, and part of the structure began to be destroyed. CO_(2) adsorption performances of the samples were conducted by a thermogravimetric analyzer, and the adsorption data were fitted by the first-order and pseudo-second-order models. Obviously, the adsorption of the alkaline-etched adsorbents was significantly improved, and the sample LDH-12 possessed the superior CO_(2) adsorption capacity of 19.6 mg/g. As the alkaline etching time further increased, the CO_(2) adsorption capacity of LDH-18 declined. Additionally, the fitting coefficient of the pseudo-second-order model was higher than that of the first-order model, which suggested the existence of chemical adsorption. These above results show that the alkaline etching treatment had an obvious effect on the morphology and structure of the samples, and the adsorption performance of the alkaline-etched samples are greatly improved. Moreover, an appropriate alkaline etching time could advance the development of pore structure of the adsorbent and facilitate the contact between CO_(2) molecules and the active sites over the adsorbent, thus promoting CO_(2) capture to a large extent. Consequently, this work will be beneficial to solve the problems in traditional hydrotalcite-like compounds for CO_(2) adsorption and have great potential and important academic significance for fabricating high-efficient CO_(2) adsorbent materials.