实验四 沉淀法制备碳酸铝铵

1、实验四 沉淀法制备碳酸铝铵一、实验目的1学习沉淀法制备纳米级粉体的方法2学习如何根据实验原理控制工艺参数以避免副反应3学习常用的材料表征方法二、实验背景碳酸铝铵NH4Al(OH)2CO3是用于制备氧化铝的一种新型前驱体，该前驱体可以制得团聚程度轻、分散性好的Al2O3粉体。制备碳酸铝铵的反应物为硫酸铝铵NH4Al(SO4)2和碳酸氢铵（NH4HCO3），反应式如下：Al3+ + NH4+ + 4HCO3- = NH4Al(OH)2CO3 + 3CO2 + H2O但是，当NH4HCO3浓度低时，会有如下式所示的副反应。Al3+ + 3HCO3- = AlOOH + 3CO2 + H2O因此为保证

2、得到纯度尽量高的碳酸铝铵，必须控制工艺参数抑制副反应。即保证Al3+周围有足量的HCO3-。可从以下几个方面着手：1加料顺序是把NH4Al(SO4)2加到NH4HCO3中还是把NH4HCO3加入到NH4Al(SO4)2去？2反应物的浓度和加料比NH4HCO3和NH4Al(SO4)2溶液的浓度和加料比该如何选择？提示：NH4HCO3在常温下的饱和浓度大约为2 mol·L-1，NH4Al(SO4)2的为0.4 mol·L-1左右。3溶液的pH该调节初始NH4HCO3溶液的pH值到何范围以保证溶液中有足量的HCO3-？溶液的pH值可由氨水调节。提示：2 mol·L-1左

3、右NH4HCO3溶液的pH值约在8左右，0.10.4 mol·L-1之间的NH4Al(SO4)2溶液的pH值在34之间。4加料速度加料速度应根据反应物的浓度的选择而定，为保证混合均匀，加料速度不可太快，但也不宜过慢。为保证混合均匀，在加料和反应过程中应施加搅拌。5产物表征如何表征产物的纯度？提示：目的产物为NH4Al(OH)2CO3，加热到600时基本分解完全，失重率大约为63。副产物为AlOOH，加热到600时基本分解完全，其失重率大约为15。三、仪器和药品仪器：三口烧瓶，搅拌装置，pH计，抽滤装置，研钵，烘箱，热分析仪，X射线衍射仪，红外光谱分析仪，扫描电镜，透射电镜。药品：NH

4、4Al(SO4)2，NH4HCO3，氨水四、实验要求1查阅文献资料，依据实验室提供的条件，设计制备碳酸铝铵的实验方案，并实施实验操作。2以0.01mol NH4Al(SO4)2为基准，确定各反应物的浓度和体积。3提出产物表征方法4写出实验报告五、思考题1为什么加料顺序会对得到的产物有决定性的影响？2如果反应液的pH过低，为何HCO3-的浓度会下降？3为何在文献中NH4HCO3的用量总是过量的？六、参考文献1 林元华，张中太，黄传勇，等. 前驱体热解法制备高纯-Al2O3超细粉体. 硅酸盐学报. 2000，28 (3):268-2712 李继光，孙旭东，张民，等. 碳酸铝铵热分解制备-Al2O3

5、超细粉. 无机材料学报. 1998，13(6):803-8073 付高峰，毕诗文，杨毅宏，等. 碳酸铝铵热分解制取-Al2O3微粉. 中国有色金属学报. 1998，8(增刊2): 64-66Preparation of ammonium aluminum carbonate hydroxide by precipitation methodAims1To learn how to prepared nanometer particles by precipitation route2To learn how to restrain side reaction by optimizing syn

6、thetic parameters according to reaction mechanism.3To learn common characterization method of materialsGuidesAmmonium aluminum carbonate hydroxide (AACH) proves to be an excellent precursor because of low phase transformation temperature of - to -Al2O3, high purity and reduced aggregate of resulting

7、 Al2O3 powders.Generally, AACH is synthesized by solution of ammonium aluminum sulfate (NH4Al(SO4)2 ) and ammonium hydrogen carbonate (NH4HCO3) according to the followed formula。Al3+ + NH4+ + 4HCO3- = NH4Al(OH)2CO3 + 3CO2 + H2OHowever, there is a side reaction when NH4HCO3 is insufficient, as follow

8、s.Al3+ + 3HCO3- = AlOOH + 3CO2 + H2OTherefore, it is necessary to ensure that Al3+ is soaked in enough HCO3- by getting control of synthetic parameters. The following questions are crucial to prepare AACH of high purity.1Dropping orderDropping NH4Al(SO4)2 solution into NH4HCO3 solution or Dropping N

9、H4HCO3 solution into NH4Al(SO4)2solution?2concentration and ratio of reactants How to choose the concentration and ratio of NH4Al(SO4)2 and NH4HCO3?Hints: Saturated solution of NH4HCO3 is about 2M，and that one of NH4Al(SO4)2 is about 0.4M at room temperature.3pH value of NH4HCO3 solutionHow much is

10、the desirable pH value of the starting 2M solution of NH4HCO3? The pH value of NH4HCO3 solution can be adjusted by NH3H2O.Hints: The side reaction arises when the pH value of reaction solution is below 8.5. The pH value of 2M solution of NH4HCO3 is about 8, and that one of 0.4 M solution of NH4Al(SO

11、4)2 is less than 4. 4Dropping rateDropping rate can not be too high in order to homogenize reaction solution. The adequate dropping rate should be coordinated with the reactants concentration. Naturally, Agitation is essential to the precipitation process. 5Characterization of productsHow to charact

12、erization purity of products?Hints: Weight loss of AACH is about 63% at 600, and that one of AlOOH, as side product, is about 15% at 600.Instruments and reagentsInstruments and apparatus: three-necked flask, agitator, pH-meter, extraction filter, mortar and cabinet drier, thermal analyzer, X-ray dif

13、fraction meter, FT-IR spectrometer, scanning electron microscope, transmission electron microscope, .Reagents: NH4Al(SO4)2，NH4HCO3，NH3H2OTasks1Consulting the related papers, you give a scheme of preparing AACH based on the service of lab and put it into practice. 2The reference consumption of NH4Al(

14、SO4)2 is 0.01mol, and you set the concentration and volume of reactants.3You choose characterization method of product.4You write the experiment report.Questions1Why dropping turn is crucial to the synthesis of AACH?2Why concentration of HCO3 decreases with the decreasing of reaction solution?3Why NH4HCO3 is always excessive in