Preparation Of MIL-101 Based Composites With Photothermal Conversion Properties And Study On Their Photocontrolled Desorption Mechanism For Typical VOCs

Adsorption technology on account of low cost,simple operation and VOCs recyclablity,becomes one of the efficient treatment of VOCs technology.As the core of this technology,adsorbent is very important to the treatment efficiency of VOCs.MOFs have large specific surface area,ordered pore structure and surface modifiability,showing a good application prospect in the field of adsorption treatment of VOCs.However,due to the porosity of MOF materials,their thermal conductivity is poor.Therefore,the most commonly used thermal desorption method for adsorbent regeneration often faced desorption difficulties and high energy consumption,which greatly limited the application of MOFs in adsorption and purification of VOCs.On the other hand,GO and Au nanoparticles have good photothermal conversion performance and have been widely used in cancer treatment,seawater desalination,biosensing and other aspects.Considering the high energy consumption of thermal desorption,GO and Au nanosheets with photothermal conversion properties were combined with MIL-101 by one-step hydrothermal method.The desorption of VOCs in the pores of GO@MIL-101 and Au/MIL-101 were triggered by near-infrared light.The mechanism of VOCs desorption by GO@MIL-101 was claimed by in-situ diffuse infrared reflectance spectroscopy,temperature programmed desorption and Lewis acid site on the surface.The main contents of this thesis are as follows:1)Preparation and characterization of GO@MIL-101.GO nanosheets were pre-coordinated with MIL-101 precursor and then GO@MIL-101 was synthesized by one-step hydrothermal method.X-ray diffraction,Transmission electron microscopy,UV-visible-near-infrared spectrophotometer and other characterization methods were used to analyze the crystal composition,morphology and optical absorption properties of the materials.The results showed that MIL-101 was evenly distributed on the GO surface.GO@MIL-101 not only retained the excellent adsorption capacity of MIL-101 to VOCs,but also showed a strong light trapping ability in the whole UV-Vis-NIR region.More importantly,GO can act as a"nanoheater"that converted absorbed sunlight into heat when exposed to UV-Vis or UV-Vis-NIR light,and rapidly transferred localized heat energy to MIL-101.2)Adsorption and photothermal desorption of VOCs by GO@MIL-101.The adsorption and photothermal desorption properties of GO@MIL-101 were tested by using continuous flow fixed light absorption and desorption system with ethyl acetate as a typical pollutant.The results showed that the specific surface area and pore volume of GO@MIL-101 decreased with the increase of GO content,under UV-vis irradiation,the desorption efficiency of ethyl acetate was 99%for 5%GO@MIL-101 which was much higher than MIL-101,and the surface temperature of 5%GO@MIL-101 was 31°C higher than MIL-101.3)Mechanism of GO@MIL-101 photothermal desorption of VOCs:The desorption activation energy of ethyl acetate,the number of Lewis acid sites on the surface of the adsorbent and the adsorption-desorption behavior of ethyl acetate in the adsorbent were calculated by means of temperature-programmed desorption,in-situ diffuse reflectance infrared spectroscopy and surface acidity measurement.The results showed that after GO absorbed light energy,the induced local heat would stimulate the nearly complete release of ethyl acetate adsorbed by MIL-101.This light induced VOCs release could not only mitigate the drawback of MOF on inferior thermal conductivity,but also would provide a potentially low energy strategy for highly efficient regeneration of MOF.4)Property of VOCs adsorption and photothermal desorption of Au/MIL-101:Au³⁺was reduced to ultra-small Au NPs by in-situ impregnation-reduction method,and Au/MIL-101was prepared by deposing Au NPs on MIL-101 surface.With the increase of Au NPs content,the specific surface area and pore volume of Au/MIL-101 decreased,and the adsorption capacity of ethyl acetate decreased slightly.After 1%Au/MIL-101 was exposed to UV-Vis for110 min,the desorption efficiency of ethyl acetate was 94.3%,and the desorption rate also increased.