Synthesis And Study Of Hydrophobic Modification Of ZIF-8@mesoSiO₂ Core-shell Materials

Since the industrial revolution of the 1760 s, humans access to energy mainly by burning plenty of fossil fuels. In such process, the accompanying carbon dioxide emission has greatly increased, which causes greenhouse effect result in earth’s surface temperature and sea level rising, land desertification and other extreme climate changes. In May 2013, the vast majority of the world environmental organization announced that the concentration of carbon dioxide has reached a critical level of 400 ppm. This event forced all nations must take measures to reduce carbon dioxide emissions. At the same time, capturing the excess carbon dioxide in air is also an effective method to reduce the greenhouse effect. Among a variety of CO2 capture methods, the adsorption of CO2 by porous adsorbates appear to be more effective due to their low cost, simple operation and the higher energy efficiency, which lead it to be the optimal method in industrial carbon capture in the future. Metal organic frameworks(MOFs) are porous materials consisting of metallic knots and organic linkers, which possess regular frameworks and great specific surface area, have broad application prospects in gas storage, separation, catalysis and other fields. A subclass of MOFs, Zeolitic Imidazolate Frameworks(ZIFs) with regular porous structure like zeolites, has been developed by Yaghi’s group. The apertures and chemical composition of ZIFs can be predicted or controlled through synthetic process, which makes ZIFs materials have good prospects in gas adsorption and separation. However, in the humid environment, water molecule will occupy active sites inside the ZIFs’ materials, which reduce their adsorption performance of CO2 significantly. Currently, two main approaches have been adopted to solve the problem: One is to preparation of hydrophobic ZIFs by in-situ synthesis; and the other one is the functionalization with hydrophobic groups on the surface of ZIFs. The second method has been proved more efficient due to the handleability than that of the first method. This dissertation is devoted to the preparation of core-shell ZIF-8@meso Si O2 materials and the hydrophobic modification in order to improve the materials of CO2 adsorption performance in the moisture environment. The following results has been achieved:1. We synthesized ZIF-8 nanocrystalline with the diameters of 50 ~ 150 nm and shape of granatohedron in aqueous solution at room temperature, by following the method in literature. Similar to Werner St?ber’s method, by controlling hydrolyzation of ethyoxyl silicate in alkaline conditions, we successful synthesized uniformly coated ZIF-8@meso Si O2 core-shell materials with the shell thickness of 25 nm. And the structure characterizations proved that it has a worm-like mesoporous structure in shell and retains the adsorption properties of ZIF-8. The results indicated that this method is applicable to most ZIFs materials. Core-shell materials can be used as a reactant to obtain hollow silicon shell or metal nanoparticles@Si O2 core-shell materials, this provides the possibility for core-shell materials functional synthesis. 2. We successfully obtained a core-shell material with hydrophobic property using dimethyldiethoxysilane as hydrophobic groups to modify the shell of as-prepared ZIF-8@meso Si O2. All the measurement, including the contact angle test, the adsorption of carbon dioxide test, and the water vapor adsorption test, demonstrate that the shell of ZIF-8@meso Si O2 can effectively resist to the invasion of water vapor. Compared with ZIF-8 crystals, CO2 adsorption capacity decreased about 22.6% due to introduction of meso Si O2 shell with low CO2 adsorption capacity, however, the value of water vapor adsorption is only 1/3 of the crystal benefit by the hydrophobic modification process. In vapor treatment, the CO2 adsorption quantity of ZIF-8 crystal fell sharply, but ZIF-8@meso Si O2 with hydrophobic treatment kept 5 times quantity of ZIF-8 in carbon adsorption quantity at this condition as we expected. This method provides a new strategy for MOFs materials to improve the water stability and retain the property in humid condition.