Preparation And Properties Of Hydrophobic Silica Aerogels

Silicon dioxide（Si O₂）aerogel is a porous lightweight nanomaterial with a unique three-dimensional mesh structure,which is now widely used in energy conservation and environmental protection,aerospace,biomedicine,petrochemical and other fields because of its excellent properties such as high specific surface area,high porosity,low density and low thermal conductivity.However,due to the high equipment requirements of the current Si O₂aerogel preparation process,which requires high production costs and a long preparation cycle,the prepared samples have a more rigid skeleton structure,poor mechanical properties and strong hydrophilicity,resulting in its application scope being largely limited.Therefore,reducing the cost of Si O₂ aerogels,shortening the preparation time,and introducing chemical groups to make Si O₂ aerogels possess excellent hydrophobic,adsorption,and mechanical properties in addition to their unique properties are the current directions of research and development.The introduction of the hydrophobic group-CH₃can replace the hydrophilic group-OH on the surface of the gel skeleton to improve the hydrophobic properties of the aerogel,and can make the gel skeleton less cross-linked and have compressible space to improve the mechanical properties of the silica aerogel.Therefore,in this thesis,hydrophobic silica aerogels were prepared by sol-gel method using organosiloxanes containing different amounts of-CH₃ as co-precursors and under the condition of atmospheric pressure drying technique,and the aerogel samples were analyzed by FT-IR and XRD for elemental and physical phase analysis,SEM and BET for microscopic morphology and pore structure.The hydrophobic properties of the obtained samples were measured by contact angle measurement,and the adsorption properties were characterized by the adsorption of different organic solvents.To obtain highly hydrophobic silica aerogels using a simple and low-cost preparation method,the effects of the content of silica source containing different amounts of hydrophobic groups,water,ethanol,N,N-dimethylformamide（DMF）,and the p H value of the sol-gel system on the structure and properties of the prepared hydrophobic silica aerogels were analyzed.The research contents and conclusions of the thesis are as follows:（1）Preparation of methyltrimethoxysilane（MTMS）silica aerogels and their properties:tetraethyl orthosilicate（TEOS）and MTMS were used as co-precursors to prepare complete bulk hydrophobic MTMS silica aerogels by atmospheric pressure drying technique and sol-gel method under the action of acid-base catalysts.The effects of molar ratio of co-precursors,water content,ethanol content,DMF content and p H of the sol-gel system on the structure and properties of the samples were investigated.The results showed that the aerogels were prepared with the molar ratio of TEOS:MTMS:Et OH:H₂O:DMF of 1:3:5:60:0.5and the p H of the sol-gel system of 8.5-9.0.The density of the aerogel sample was 0.183g·cm^-3,the volume shrinkage was 2.8%,the porosity reached 90.4%,the specific surface area was 350.874 m²·g^-1,the average pore size was 2.69 nm,and the total adsorption pore volume was 0.221 cm³·g^-1.The adsorption amounts for the organic solvents n-hexane,ethanol and dichloromethane were 5.08,6.42 and 9.79 g·g^-1.and the silica aerogel prepared in this molar ratio possessed a maximum hydrophobic angle of 153.5°.And through the preliminary mechanical property test,it shows that the structure has a certain strength.（2）Preparation of dimethyldimethoxysilane（DMDMS）silica aerogel and its properties:DMDMS containing bis（methylene）was introduced as a co-precursor in the preparation system of MTMS silica aerogel,and DMDMS silica aerogel was obtained by a simple and low-cost sol-gel method and atmospheric pressure drying technique.The skeleton morphology,pore structure,hydrophobicityand adsorption volumes of the obtained DMDMS silica aerogels were investigated by controlling the molar ratio of TEOS and DMDMS.The results showed that the DMDMS silica aerogels were excellent when the molar ratio of TEOS:MTMS:DMDMS:Et OH:H₂O:DMF was 1:3:0.6:5:60:0.5 and the p H of the sol-gel system was 8.5-9.0.The density of the aerogel samples was 0.079 g·cm^-3 and the specific surface area was 261.629 m²·g^-1.With the increase of DMDMS content,the skeleton morphology of the aerogel samples gradually changed from the initial fragmented structure to uniform spherical silica particles containing rough surfaces,and finally to a chain structure composed of spherical silica particles.The aerogel sample contains a large number of hydrophobic group methyl groups and a rough gel skeleton surface,which makes the DMDMS silica aerogel superhydrophobic and adsorption properties.Its hydrophobic angle was 155°,and the adsorption amounts for the organic solvents n-hexane,ethanol,dichloromethane and grease are 6.7,8.6,14.2 and 10.2 g·g^-1,respectively.In this thesis,the silica aerogels were modified by introducing hydrophobic groups to improve their skeleton strength and hydrophobic properties using a simple and low-cost method,which provides a reference for the industrial production of silica aerogels.The modified silica aerogel can be formed into complete blocks and has excellent hydrophobic and adsorption properties,which has a broad application prospect in oil-water separation and other fields.