采用均匀沉淀法(尿素水解法)合成了高结晶度的CO32-型MgAl-LDH,探索了温度和时间对产物的影响.研究发现,n(Mg)/n(Al)进料比为2时,较低反应温度(如100℃)下很难得到n(Mg)/n(Al)=2的LDH.这可能与Mg((OH)2溶解度较高,Mg2+难以与Al3+一同沉淀有关.为提高产物n(Mg)/n(Al),我们采用提高n(Mg)/n(Al)投料比(如3∶1,4∶1,5∶1)的方法.研究发现,反应时间较短时,可得较纯的LDH相,但产物的n(Mg)/n(Al)远低于进料比;若反应时间延长,会出现水菱镁矿(hydromagnesite)和菱镁矿(magnesite,MgCO3)的杂相.由此推断,为得到高n(Mg)/n(Al)的LDH,不能单纯增加n(Mg)/n(Al)进料比,其他条件如尿素浓度,尤其是反应温度等也需改变.
CO23- type LDH was synthesized by the homogeneous precipitation method (urea hydrolysis) with varied temperature and reaction time. Compositions and structure of LDH obtained revealed that at the temperature of 100 ℃, n(Mg)/n(Al) molar ratio was less than the starting ratio of 2. Under acidic conditions,Mg(OH)2 solubility was larger than Al(OH)3, LDH with lower n(Mg)/n(Al) ratios was obtained. To obtain LDH with higher n(Mg)/n(Al) ratios, the starting n(Mg)/n(A1) ratios were increased to 3: 1, 4∶ 1 and 5:1,but the final LDH product always showed a n(Mg)/n(Al) ratio lower than the initial solution ratio. At an initial ratio of 5: 1, pure LDH phases were formed at short reaction time, but some mixed phases such as hydromagnesite and magnesite appeared with increasing time. Thus increasing the n(Mg)/n(A1) starting ratio only is not a good way, and other conditions such as urea concentration and especially reaction temperature may be important factors influencing the final n(Mg)/n(Al) ratios of the resulting LDH products.
评论