Synthesis and Characterization of High Surface Area Flower-Like Ca-Containing Layered Double Hydroxides Mg3-xCaxAl1-NO3

In the present work, we report for the first time the synthesis of high surface area and flower-like Ca-containing LDHs, Mg3-xCaxAl1-NO3. Generally, Ca-containing LDHs tend to form a plate-like morphology and have a very low specific surface area of 10-18 m(2)/g. Such low specific surface area hinders its applications either as adsorbents or as catalysts. However, we found that by inserting certain amount of Ca2+ into the Mg2+ sites, the specific surface area can be significantly increased up to 65-90 m(2)/g. We further proved that such significant increase in specific surface area was attributed to the formation of flower-like particles. In order to obtain a high surface area flower-like Mg3-x,CaxAl1-NO3 LDH, the x should be in the range of 0.1-1.5 and the synthesis pH should be in a very narrow range of 10.5-11. The thermal stability and memory effect of Mg3Al-NO3, Ca(2)AI-NO3, and Mg3-xCaxAl-NO3 were then investigated in detail. Ca2Al-NO3 showed very poor memory effect, and the 500 degrees C calcined sample did not reconstruct its original layered structure even after 12 days. However, Mg2.3Ca07Al1-NO3 has a very good memory effect and it reconstructed its original layered structure in 1 day even after being calcined at 600 degrees C. It is our first time to observe that Ca-containing LDHs possess memory effect. Finally by utilizing the memory effect of Mg3-x,CaxAl1-NO3, a relatively purer Mg3-x,CaxAl1-NO3 LDH was synthesized, which is superior to the direct co-precipitation and anion exchange methods. We expect this work will shed light on the synthesis of Ca-containing LDHs with higher specific surface area. Since the substitution of Mg with Ca will change its surface basicity sites, such LDHs might have potential applications in CO2 adsorption and base catalysis.