Preparation Of ZIF-L Nanosheets By High-gravity Technology And Their Catalytic Properties

MOFs nanosheets（two-dimensional metal-organic frameworks）have attracted more and more attention in recent years.They combine the excellent properties of MOFs with the unique physical and chemical properties of two-dimensional nanomaterials,such as large specific surface area,ultra-thin thickness and abundant reachable active sites.However,the controllable preparation of high-quality MOFs nanosheets remains a great challenge.ZIF-L has become a widely studied MOFs nanosheets because of its excellent chemical properties and wide application fields.In view of this,ZIF-L nanosheets were prepared by high-gravity technology in a rotating packed bed（RPB）reactor in this thesis.The effects of process parameters such as RPB rotating speed,temperature and time on the morphology,size and thickness of the final product were investigated.Moreover,the application performance of ZIF-L nanosheets catalyzed Knoevenagel condensation and PET（Polyethylene terephthalate）degradation were further explored.The main research contents and results are as follows:1.ZIF-L nanosheets were controllably prepared by high gravity method based on a rotating packed bed reactor.The effects of different process conditions on the morphology,size and thickness of the nanosheets were investigated.The study results showed that the reaction temperature has a great influence on the morphology of the synthesized nanosheets.The morphology of the nanosheets can be controlled by changing the temperature within 25-75°C,but the formation of ZIF-L nanosheets is not favorable at a high temperature（such as 75°C）.Under the same growth time,the particle size decreases with the increase of RPB cycling time.The effects of RPB rotating speed and feed flow rate on the size of nanosheets is not obvious.Based on the above studies,the optimal preparation conditions were obtained as follows:the RPB rotating speed of 2500 rpm,the feed flow rate of 150 m L·min^-1,the cycling time of 20 min,the growth time of 45 min and the temperature of 25°C.Under the optimal conditions,the average particle size of the nanosheets prepared by RPB is about 3.7μm,the average thickness is about 130 nm and the BET specific surface area is 308.63 m²·g^-1.Compared with the traditional stirring tank reactor（STR）method,the ZIF-L nanosheets synthesized by high gravity method have smaller size,narrower particle size and thickness distribution,larger specific surface area and shorter synthesis time.2.The application performance of Knoevenagel condensation catalyzed by ZIF-L nanosheets was studied.The effects of different reaction conditions on the catalytic efficiency were explored.The results showed that the conversion percentage of benzaldehyde increased with the increase of the amount of catalyst and malononitrile.The nanosheets with leaf and flower morphologies exhibited higher catalytic efficiency.Aldehydes with different substituents as substrates have great influence on the reaction rate.The optimal catalytic conditions were as follows.When benzaldehyde is selected as the reaction substrate,the molar ratio of ZIF-L to benzaldehyde is 3.5mol%,the molar ratio of benzaldehyde to malononitrile is 1:1.9,and the reaction time was 2 h.In this case,the conversion of benzaldehyde can reach99.25%.Compared with the traditional STR product（87.40%）,the conversion percentage of benzaldehyde of the RPB product（99.25%）is increased by about 12%.In addition,ZIF-L nanosheets have good recycling performance.3.The application performance of catalytic degradation of PET by ZIF-L nanosheets was studied.The effects of different reaction conditions on the degradation of PET were explored.The results showed that the conversion percentage of PET increased with the increase of the amount of catalyst and degradation solvent ethylene glycol（EG）,with the extension of reaction time,and significantly increased with the increase of reaction temperature.The optimal catalytic degradation conditions were the ZIF-L/PET mass ratio of 0.2%and the EG/PET mass ratio of 10.The conversion of PET can reach 95.16%at 195°C for 15 min and can reach to99.86%at 195°C for 30 min.The conversion percentage of PET of RPB product（72.30%）is about 12%higher than that of traditional STR product（59.48%）.