Layered double hydroxide (LDH)-based monolith with interconnected hierarchical channels: enhanced sorption affinity for anionic species

Abstract

Monolithic layered double hydroxides (LDHs) with interconnected channels have been expected to enhance sorption rate as well as increase accumulation of anions. Although powder molding can form easily-handled LDH compacts, interconnected channel formation therein has not been achieved. Herein, we demonstrate cm-scale monolithic LDH-based composites with interconnected hierarchical channels via a spontaneous sol–gel reaction. The synthesis was performed on Mg–Al hydrotalcite-type LDHs starting from metal chlorides aqueous/ethanolic solution with poly(ethylene oxide) incorporated. Addition of propylene oxide triggers a sol–gel reaction to form monolithic xerogels with a formula of [Mg_0.66Al_0.33(OH)₂Cl_0.33·2.92H₂O]·3.1Al(OH)₃. LDH crystals together with aluminum hydroxide crystals homogeneously build up gel skeletons with well-defined hierarchical channels. The interconnected channel in μm range (macrochannel) are formed as a phase-separated structure, whereas the channel in nm range (nanochannel) are as interstices of primary particles. The channel architectures are preserved in the course of rehydration process, affording enhanced sorption affinity for anion species in the process. Both of macro and mesochannels as well as high charge density of the obtained LDHs (Mg/Al = 2.0) contribute to enhanced anion sorption in the monolithic xerogels. The materials obtained here opens up applications of high performance adsorbents and ion-storage free from diffusion limitation.