Synthesis of crystalline layered double hydroxides: Precipitation by using urea hydrolysis and subsequent hydrothermal reactions in aqueous solutions

摘要

The urea method is frequently used for synthesis of highly crystalline layered double hydroxides (LDHs). However, in the Mg-Al system, Mg/Al ratios in products have been often found to be smaller than those of starting solutions. In this study, we emphasized monitoring of Mg/Al ratios to investigate the process of LDH formation, while setting the Mg/Al ratio of the starting solutions at 2.rnLDHs were synthesized by using the urea hydrolysis reaction under hydrothermal conditions. From a starting solution with high metal ion concentration (0.5 M), by-products formed when Mg/Al ratios of products reached about 2; these by-products were most likely hydrated magnesium carbonate hydroxide phases (HM). In contrast, from a low metal ion concentration starting solution (0.01 M), a boehmite-like by-product was detected at an early stage of product formation, but prolonged reaction time or increasing temperature led to product conversion to the LDH phase. Particle size was mainly influenced by metal ion concentration. However, in this study, formation of HM by-products from relatively high metal ion concentration starting solution could not be avoided in one step. Therefore, in order to convert the products to pure LDH phase, product containing HM was reacted hydrothermally in various solutions, yielding almost pure LDH phase. The size of relatively large platelets remained almost the same as that before the hydrothermal reactions, but some or many small broken pieces were present in products after the hydrothermal reactions. Moreover, Mg/Al ratio slightly increased when Na_2CO_3 or NaOH solution was used; with NaOH solution, (001) reflections split.