The Structural And Surface Regulation Of Porous Heterostructured Montmorillonite For The Adsorption Of Gaseous Organic Molecules

With the speeding up of industrialization and urbanization in our country, all kinds of organic solvents are used more and more widely. The volatile organic compounds(VOCs) from anthropogenic emission have become a critical component of hazardous air pollutants. Volatile organic compounds have complex composition, and most of them are noxious for environment and human health.The adsorption method is one of the frequently used way to the VOCs pollution control with the advantages of high purification efficiency, simple adsorption equipment, and convenient operation. The selection of adsorbent is the key to the adsorption method. An ideal adsorbent for VOCs removal is usually expected to have sufficiently large and reversible adsoption capacity, large accessible pore volume, low catalytic activity, hydrophobic surface, high thermal and hydrothermal stability, and an easy regeneration capability.Clay minerals belong to a class of hydrated phyllosilicates. As nonmetallic mineral resources, clay minerals are widely distributed in nature with a great deal of species and reserve abundance. The microporous structures of clay minerals are the origin and material base of a variety of particular characteristics and the key object in the modification of clay minerals. Through the porous modification of interlayer space, clay minerals have large specific surface area and pore volume, strong surface activity, stable chemical properties, and high thermal stability. As a result, the pillared interlayer clay minerals are often used as adsorbates and catalysts.Porous heterostructured clay minerals(PHCs) is a kind of pillared interlayered clay minerals. Because of possessing large surface area, unique pore size distribution(PSD) range, high thermal and hydrothermal stability, low cation exchange capacity, and tunable solid acidity, PHCs is a promising candidate for applications as catalysts, adsorbents, carriers, and templates. Until now, studies of PHCs have usually concentrated on the the synthesis, characterization, and applications in liquid phase adsorption and catalysis. However, the pore-forming mechanism, the roles of the synthesis components in the formation of porous structure, and the applications in gas phase adsorption of PHCs are known little.To solve the above problems, montmorillonite(Mt) was used as base clay to synthesize porous heterostructured montmorillonite(PHM). Based on the effects of layer charge of base clay, the chain length and amount of intercalated surfactant, and the ratio between clay and silane, a new non-micellar template model for the poreforming of PHM was proposed. Through the effective control of the composition and structure of PHM, the effects of the structural and surface properties of PHM on its adsorptive performance were investigated. In addition, the effect of the nature of gaseous organic molecule on the adsorptive performance of PHM was studied as well. The main results and conclusions are listed as below:(1) The roles of inorganic clay mineral, organic template agents, and silane in the pore-forming of PHM. The base clay supplied the pillaring objective and space. The effect of layer charge on structural characteristics of PHM within interlayer can be simplified as the variations in diameter and depth dimensions. The alkyl quaternary ammonium salt played a significant role in the formation of porous structure, whose arrangement and orientation was controlled by the base clay. The surfactant changed the hydrophilic silicate surface of mineral host into hydrophobic and enlarged the basal spacing so that the interlayer was accessible for tetraethylorthosilicate(TEOS). The catalysis of neutral amine for the condensation of TEOS led to a more concentrated PSD, especially in mesopore region. TEOS is not only the silicon source for generating the Si framework within the interlayers, but also the reaction medium for the selfassembly process.(2) The non-micellar template model for PHM. In the synthesis process, the(reversed) micelle template almost cannot be formed in TEOS. Therefore, the nonmicellar template model was proposed. In this model, the inorganic base clay changed the congregation type of organic templates within interlayer. The hydrocarbon tails congregated together due to the extrusion and occupation of polymerized TEOS in interlayer space. TEOS monomers entered into the spacer between the aggregates from organic template agents to conduct pillaring reaction, resulting in the formation of porous structure.(3) The structural evolution and porosity regulation of PHM. The PHM samples were composed of a porous silica-intercalated clay and the amorphous silica outside the interlayer space(i.e., extra-lattice silica). The extra-lattice silica lack porosity and has a little contribution to the total porosity of PHM intergrowth. In the process of synthesizing PHM, intercalation and absorption of the polymerized silica proceeded simultaneously and non-preferentially. Despite the nearly invariant value of the most probable pore size(MPPS) of PHM evaluated by nonlocal density functional theory(NLDFT), it does not mean that the PSD of PHM is uncontrollable. A clay host with a high layer charge, a surfactant with an appropriately short alkyl chain, an appropriately small amount of intercalated surfactant, and a low Mt/TEOS ratio are the necessary conditions for synthesizing PHM with better microporosity and vice versa for better mesoporosity.(4) The effects of structural and surface properties of PHM on its adsorptive performance. Through the permanent pillaring of silica and the removal of templates, the specific surface area and porosity of PHM were improved. The improvement led to the larger adsorption quantity to toluene of PHM than that of the pristine Mt, organomontmorillonite(OMt), and PHM precursor in the whole pressure range. Compared with the other Mt-based pillared clay minerals, the PHM has advantages for adsorbing toluene in the middle and high pressure ranges. The surface chemical property of PHM was regulated by carbonaceous coating method with the organic templates as the native carbon source. The concentration of H2SO4 solution affected the textural properties and carbon contents of surface-functionalized porous heterostructured montmorillonite(SF-PHM). The SF-PHM prepared with moderate carbonization conditions possessed a large specific surface area and pore volume. The SF-PHM exhibited a stronger adsorption affinity to toluene compared with untreated PHM in the low pressure region. Accordingly, both the structural characteristics and surface chemical properties of PHM have effect on its adsorptive performance.(5) The effects of the nature of gaseous organic molecules on the adsorptive performance of PHM. Due to the differently dominant interaction in the adsorption process, the adsorptive performances of PHM for nonpolar and polar molecules were different in the low and high pressure ranges. The difference between the adsorptive performances for nonpolar and polar molecules in the low and high pressure regions depended on the the nature of gaseous organic molecules. When adsorbing nonpolar molecules, the polar surface of PHM was changed into nonpolar because of the coverage of nonpolar molecules. Therefore, the change regularity of adsorption quantity of nonpolar molecules was different between in the low and high pressure regions. However, the polar surface of PHM was maintained after adsorbing polar molecules. The order of the adsorption amount between polar molecules thus did not change in the whole pressure region. Generally, the polar gas had a higher monolayer adsorption capacity than that of nonpolar gas due to the polar surface of PHM. Not did all the porous channels participate in the adsorption of organic gas molecules. The smaller kinetic diameters and closer packing of polar molecules induced their lower saturated adsorption capacity compared with nonpolar molecules.The synthesizd various PHM in this research possessed not only the high adsorption quantity, but also high thermal stability, which is a kind of promising candidates for application in VOCs emission control. The results of this work not only provide a theoretical basis for the development and utilization of clay-based adsorption materials, but also have practical applicable meaning for the high value-added utilization of clay mineral resources, and the protection and governance of atmospheric environment.