Preparation Of Hydrotalcite And Research On Adsorption Capacity

Hydrotalcite-like compounds(HTLcs) are shown highly activity and selectivity in catalytic reaction because of their special composition and structure.However,the applications of the HTLcs have been limited because of fewer species,lower crystallization and higher impurity.Therefore,it is widely studied to synthesize HTLcs by some novel methods.The MgAl-LDHs powders were synthesized by homogeneous precipitation method.The influences of the temperature,time and ion concentration were investigated.With the aid of XRD and SEM technique,the crystal structure and morphology of MgAl-LDHs powders were analyzed.The results show that the MgAl-LDHs powders prepared by the method present a homogeneous and regular hexagonal plate.It is found that the diameter of plate crystal,rather than the morphology,is influenced by temperature,time and ion concentration, indicating that increasing temperature and time only results in larger hexagonal crystal.In addition,with decreasing ion concentration,the crystal size can increase.The effects of solvent property on MgAl-LDHs powders were also studied.The powders were characterized by XRD,SEM,TG-DTG and N₂ adsorption technique.The results show that little addition amount of some alcohols has almost no influence on the crystallinity,thermal stability and morphology of MgAl-LDHs.However,the crystal size of MgAl-LDHs changes apparently, which can decrease from 2 to 1μm.With increasing the addition amount of alcohol,the crystal size of MgAl-LDHs decreases,and the specific surface area of MgAl-LDHs increases.Secondly,MgAl-LDHs nanocrystallites were synthesized by a novel CO₂ gas-liquid reaction method for the first time.XRD,FT-IR,TG-DTG,SEM and TEM measurements strongly characterized the structure,thermal stability and morphology of the powders.The results show that hydrotalcite nanocrystallites with a sheet-like shape could be successfully prepared by the novel method.The average crystalline size distribution of powders was in the range of 60 to 100 nm.In addition,the effects of the aging temperature,aging time and the molar ratio of M²⁺/M³⁺ on the crystal structure and crystal size were also investigated.The results indicate that with increasing the aging temperature and aging time,the crystal structure tends to perfect and the crystal size becomes larger.It is found that the molar ratio of M²⁺/M³⁺ can influence the purity of hydrotalcite.Both high and low molar ratios of M²⁺/M³⁺ can result in impurity easily.The novel method was also applied to the preparation of CoAl-LDHs nanocrystallites and the low crystallinity of CoAl-LDHs powder was obtained.The conditions are necessary to optimized to synthesize good crystallinity of CoAl-LDHs powders.Finally,the MgAl-LDHs and calcined MgAl-LDHs prepared by homogeneous precipitation and the novel method were used to adsorption experiment for methyl orange,in which the adsorption capacity was tested.The results show that powders prepared by the novel method present better effects on removal rate of methyl orange than that by homogeneous precipitation method,just because of the smaller crystal size.Due to the structure memory effect of hydrotalcite,the calcined MgAl-LDHs show more effective adsorption capacity for methyl orange than MgAl-LDHs.In order to obtain the optimum conditions for adsorption methyl orange,the influences of addition amount,adsorption time and temperature on the removal rate were investigated.