Study On The Synthesis Of Layered Double Hydroxides By Employing Ammonia And Ammonium Compounds As Precipitants

Coprecipitation is a popular method to synthesize LDHs, which usually uses NaOH or Na2CO3 as precipitant. However, some studies recently reported that LDHs would be also available by using ammonia, ammonium carbonate or ammonium bicarbonate as precipitant. To our regret, these news processes reported, to great extent, are limited to a lab-scale and not suitable for mass production due to the lack of information about the ratio of agents, reaction system and conditions. To establish a promising process for mass production of LDHs using ammonia or ammonium compounds as precipitant, this thesis investigates the probability of synthesis of MgAl-LDHs by employing ammonia, ammonium carbonate, ammonium bicarbonate and their mixture as precipitant. We detected systematically the factors, which affected the structure and property of the products, such as the choice of reaction system, amount of precipitant, the recipe of complicated precipitant, and crystallization conditions, and revealed some rules for the preparation of LDHs.Firstly, we studied the effects of single precipitant and its amount, the recipe of complicated precipitant, plank assembles and ion environment on the formation of MgAl-LDHs. Results showed that the acquired products were MgAl-NO3-LDHs, MgAl-CO3-LDHs and MgAl-Cl-LDHs when ammonia or ammonium bicarbonate, ammonium carbonate, and chloride was used as precipitant, respectively. Then, we studied the influence of the ratio of complicated precipitant on the product. We found it was helpful for the formation of MgAl-NO3-LDHs when increased the amount of ammonia and for the formation of MgAl-CO3-LDHs when increased the amount of ammonium carbonate in complicated precipitant. Also investigated was the influence of crystallization system, temperature, time on the structure and property of the prepared MgAl-LDHs. Results showed that the crystal shape of MgAl-LDHs tended to be uniform with long crystallization time and higher temperature were helpful, and mechanical stir was helpful for the formation of MgAl-LDHs with uniform shape and smaller article size.