均匀沉淀(尿素水解)法合成CO_3^(2-)型MgAl-LDH

SYNTHESIS OF CO_3^(2-) TYPE MgAl-LDH USING HOMOGENEOUS PRECIPITATION(UREA HYDROLYSIS) METHOD

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.

CO2-3 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）e solubility was larger than AI（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（Al） 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（Al） 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.