【作者】 张杰；

【导师】 刘壮；

【作者基本信息】 四川大学 ， 化学工艺， 2021， 硕士

【摘要】 层状双金属氢氧化物（LDHs）是一种由正电性的板层及层间阴离子构成的二维材料,其正电性的板层由M²⁺/M³⁺金属的氢氧化物构成,M³⁺的存在使板层具有永久正电性。通过调变金属离子种类、比例,能够构建不同性质、功能的LDH板层。同时,LDHs层间阴离子具有可交换性,通过插层不同的阴离子,可以实现LDHs板层间距的调控以及功能型LDHs颗粒的设计。总之,LDHs作为一种独特的具有正电性板层的二维材料,已被广泛运用于催化、电化学、光学、生物载药、防火材料、膜等领域。值得注意的是,以LDHs颗粒为单元构建的LDHs膜在气体分离、脱盐等方面展现出优异的分离性能,使其在膜领域有潜在应用。此类以LDHs颗粒构建的膜以原位生长法合成,但存在合成的LDHs颗粒有缺陷,颗粒之间结合不紧密等问题,因此膜的分离性能有待提高。LDH单片具有独特的正电性特征、原子级的厚度以及结构组成可调控等优势,是一种优良的构建二维膜的材料。其他的由二维单片构建的二维膜（石墨烯膜、MXene膜、Mo S₂膜和蛭石膜等）已在离子筛分、脱盐和染料截留等方面取得了重大研究突破,但是以LDH单片构建分离膜及膜的传质与分离机理的详细探究与分析尚需进一步研究与完善,LDH膜的研究方兴未艾。因此,利用LDH单片的诸多优势,通过多种手段构建具有稳定且精确结构的高性能LDH膜,探究其传质和分离性能、机理是LDHs研究的新方向,对于LDH膜的研究开发具有推动作用,同时对于扩展二维膜家族具有重要意义。本论文利用LDH正电性的特征以及LDH板层表面丰富的羟基,借助静电相互作用和氢键作用分别构建了氯离子交联的LDH膜和PVA（聚乙烯醇）/LDH有机无机复合膜,并研究了构建的LDH膜的传质与分离性能。本研究的具体工作如下:在第二章中,通过水热法合成了平均粒径在2μm的具有规整正六边形结构的Mg-Al-CO₃^2--LDHs颗粒,进一步通过阴离子交换处理将层间阴离子转换为硝酸根,以硝酸根插层的LDHs颗粒为前体,在甲酰胺中振荡剥离,得到厚度约0.9 nm的LDH板层单片。利用真空抽滤和KCl溶液浸泡处理,成功制备了新型的氯离子交联的LDH膜。本章主要研究了氯离子交联的LDH膜的物质扩散性能。制备的氯离子交联的LDH膜表面光滑,没有缺陷。该膜具有典型的层层堆叠的二维层状结构。由于LDH单片表面富含羟基,制备的氯离子交联的LDH膜呈现出亲水性的特性。由于氯离子与正电性的LDH板层之间的强静电相互作用,在水、甲醇、乙醇、异丙醇、苯、甲苯、邻二甲苯、间二甲苯和对二甲苯中LDH膜的层间距不发生改变,表明氯离子交联的LDH膜良好的稳定性。氯离子交联的LDH膜的理论计算层间距约为1.42 nm,传质通道由LDH膜内部存在LDH单片不规则堆积形成。进一步分析研究了LDH膜通道内的水分子和有机分子的传质特性。结合不同的分子扩散模型分析,发现了LDH膜通道内有机分子符合克努森扩散模型而水分子按冷凝扩散模型的不同传质机制。本章研究工作为LDH膜的有机物与水分子的分离提供了理论基础。在第三章中,利用LDH板层与PVA高分子链之间的氢键相互作用,构建了PVA/LDH复合膜,探究了PVA混入量对于PVA/LDH复合膜结构、亲疏水性及水通量的影响,分析了复合膜的离子扩散性能及染料截留机理。实验结果表明PVA的混入量对复合膜的层层堆叠的层状结构没有影响,但对复合膜的层间距产生影响。由于氢键的作用,复合膜的层间距较之于纯LDH膜有所缩小,复合膜层间距随着PVA含量增加而减小,在PVA含量为15%时达到最小值。PVA含量超过15%后,复合膜层间高分子含量过高,随着PVA含量增加,复合膜层间距增加。由于LDH单片与PVA高分子都属于亲水物质,复合膜表现出亲水性的特征,随着PVA含量变化,复合膜的亲水性基本不变。由于PVA高分子与LDH单片间的强氢键相互作用,抽滤过程中损失量少,不同比例的复合膜的实际混合比与理论混合比相差不大。PVA质量分数为25%的复合膜的纯水通量最大。复合膜对氯化钠、氯化镁、氯化铝、硫酸钠、硫酸镁都具有一定的扩散通量,离子扩散通量受离子价态、离子水合半径和对应盐的阴离子的影响。基于分子筛分,PVA质量分数为25%的复合膜对分子量在300-800 g·mol^-1的染料分子具有优异的截留性能,截留率超过97%,展现出了高效的广谱的染料截留性能。结果表明该复合膜在印染废水处理领域具有广阔的应用前景。综上所述,本文借助真空抽滤法,通过静电作用、氢键作用分别构建了氯离子交联的LDH膜以及PVA/LDH复合膜。研究了膜的物质扩散、截留性能,并详细分析了相关机理。本研究为设计构建高性能LDH分离膜提供了参考,拓宽了LDH膜在分离领域的应用范围。更多还原

【Abstract】 Layered Double Hydroxides（LDHs）,a type of 2D materials composed of positively charged brucite-like layers with charge compensating anions inserted in the adjacent layer.The positively charged layer is composed of hydroxide of M²⁺/M³⁺metal ions.The presence of M³⁺makes the layer permanently positively charged.LDH layers with different properties and functions can be constructed by adjusting the type and proportion of M²⁺/M³⁺metal ions.At the same time,the anions between LDHs layers are exchangeable,Intercalation of different anions can realize the regulation of LDHs lamellar spacing and the design of functional LDHs particles.In a word,LDHs,as a unique two-dimensional material with positive electric layer,are widely used in the fields of catalysis,electrochemistry,optics,biological drug loading,fire retardant materials,membranes,and so on.It is notably that the LDHs membrane constructed from LDHS particles as a unit shows excellent separation performance in gas separation,desalination and other aspects,which makes it have potential applications in the membrane field.This kind of membrane constructed with LDHs particles was synthesized by in-situ growth method,but there were defects in the synthesized LDHs particles,and the particles are not tightly composed,so the separation performance of the membrane needs to be improved.LDH monolayer is an ideal material to fabricate 2D membranes due to its unique positive electrical properties,atomic thickness and adjustable structure composition.Other 2D materials nanosheets construction of 2D membrane such as graphene,MXene,Mo S₂,vermiculite already applicated in ion sieve,desalination,dye separation and made significant breakthroughs.However,the construction of separation membrane with LDH monolayer and the detailed investigation and analysis of the membrane mass transfer and separation mechanism still need further research and improvement.The research on LDH membranes is on the rise.Therefore,the construction of LDH membranes with stable and accurate structures and the exploration of their mass transfer and separation performance and mechanism are the new directions of LDHs research,which can promote the research and development of LDH membranes and is of great significance for the expansion of the 2D membrane family.In this thesis,using the characteristics of LDH positive electricity and the wealthy hydroxyl groups on the surface of LDH monolayer,by means of electrostatic interaction and hydrogen bonding interaction,the chloride ion crosslinking LDH membrane and PVA（polyvinyl alcohol）/LDH organic-inorganic composite membrane were constructed respectively,and the mass transfer and separation performance of the constructed LDH membrane were studied.The specific work of this study is as follows:In Chapter 2,Mg-Al-CO₃^2--LDHs was synthesized by the hydrothermal synthesis method,the LDHs particles were regular hexagonal structure with a mean particle size of 2μm.Further by anion exchange processing converts interlayer anion into nitrate,chose the nitrate intercalated LDHs particles as the precursor,stripping,oscillating in the formamide to get LDH monolayer.A new type of chloride crosslinking LDH membrane was successfully prepared by vacuum filtration and KCl solution immersion.In this chapter,the diffusion properties of chloride ion crosslinking LDH membrane are studied.The chloride ion crosslinking LDH membrane has a smooth surface,no defects and pores.The membrane has a typical 2D lamellar structure,which is stacked by LDH monolayers layer by layer.Due to the LDH monolayer polyhydroxy group on the surface,the prepared chloride crosslinking LDH membrane shows hydrophilic properties.Because of the strong electrostatic interaction between chloride ions and the positively charged LDH plates,the layer spacing of LDH membrane does not change in water,methanol,ethanol,isopropanol,benzene,toluene,o-xylene,m-xylene,and p-xylene,indicating the excellent stability of the chloride ions crosslinking LDH membrane.The theoretical calculation of the layer spacing of chloride crosslinking LDH membrane is about 1.42 nm,the mass transfer channel is formed by irregular accumulation of LDH monolayer in LDH membrane.The mass transfer characteristics of water and organic molecules in LDH membrane channels were further analyzed.Combined with the analysis of different molecular diffusion models,it is found that the organic molecules in the LDH membrane channel conform to the Knudsen diffusion model and the water molecules according to the capillary condensation diffusion model.The research work in this chapter provides a theoretical basis for the separation of organic matter and water molecules in LDH membrane.In Chapter 3,based on the hydrogen bond interaction between LDH plate and PVA polymer,PVA/LDH composite membranes were constructed.The effects of PVA content on the structure,hydrophilicity,and water flux of PVA/LDH composite membrane were investigated,and the mechanism of ions diffusion and dyes rejection of the composite membrane was analyzed.The experimental results show that the amount of PVA has no effect on the layer-by-layer two-dimensional structure of the composite films but dramatically affects the lamellar spacing of the composite films.Compared with pure LDH films,the interlayer spacing of composite films was reduced due to the hydrogen bond formed between PVA and LDH monolayer.With the increase of PVA content,the interlayer spacing of composite films decreased and reached the minimum value when the content of PVA was 15%.When the PVA content exceeds 15%,with the increase of PVA content,the polymer density between the composite films is too large,so the lamellar spacing of the composite films increases.Since both LDH monolayer and PVA polymers belong to hydrophilic substances,the composite membrane shows the characteristics of hydrophilicity,and the change of PVA content has little effect on the hydrophilicity of the composite membrane.Due to the strong hydrogen bond interaction between PVA polymer and LDH monolayer,the mass of loss in the process of filtration is small,and the actual mixing ratio of different proportions of composite membrane is near to the theoretical mixing ratio.The water flux of the composite membrane with an PVA mass fraction of 25%is the largest.The composite membrane has a specific diffusion flux for sodium chloride,magnesium chloride,aluminum chloride,sodium sulfate and magnesium sulfate,and the ion diffusion flux is affected by the valence of the ions,the hydration radius of the ions and the anions of the corresponding salts.Based on molecular sieving,the composite membrane with a PVA mass fraction of 25%has excellent retention performance for dye molecules with a molecular weight range from 300 to 800 g·mol^-1,and the rejection rate is more than 97%.It shows high efficiency and a broad range of dyes retention performance.The results show that the composite membrane has promising application prospects in dyeing wastewater treatment.In conclusion,based on electrostatic interaction and hydrogen bonding,chloride ion crosslinking LDH membrane and PVA/LDH composite membrane was constructed with the vacuum filtration.The membranes’diffusion and interception properties were studied,and the permeation and rejection mechanism were analyzed in detail.This study provides a reference for the design and construction of high-performance LDH separation membranes and broadens the application range of LDH membranes in the separation field.更多还原