Preparation Of Hierarchically Meso/Macroporous Architectures Of Layered Double Hydroxide By Dry Gel Conversion For Water Treatment

Functional materials with hierarchical architecture can make better use of its properties of structural unit, improving their application performance and application scope. Layered double hydroxides have attracted increasing interest due to their potential industrial applications. In order to take full advantage of LDH material properties in the applications, a special attention has been drawn to the preparation of hierarchical architectures of LDHs. For the preparation of LDH hierarchical architectures, templating syntheses are presently the most widely used methods. However, the template method suffers from problems of materials compatibility, process complexity, not environmental friendly. Developing of template-free approach under mild conditions is highly desired and still remains a challenge.(1) A new type of hierarchically porous MgAl-layered double hydroxide (LDH) architecture, with well defined meso-/macroporous structure, was fabricated by dry gel conversion (DGC) method without the need of a surfactant and organic solvent. Predefined amorphous aluminum hydroxide (AAH) gel and anhydrous magnesium sulfate were used as Mg and Al precursor material, respectively, and aqueous ammonia as precipitating agent. This self-assembled hierarchical LDH material exhibits a bi-modal porous structure having a macroporous network with macropore sizes of 10-200 nm and a well defined mesoporous structure of pore size around 3.9 nm in the macroporous framework. LDH crystallites are orientally aligned with a multilayer manner to form the architecture structure.(2) We have studied the formation process and influencing factors of hierarchically porous MgAl-layered double hydroxide (LDH) architecture. The results suggest the preformed AAH gel as raw material is necessary for induce the construction of LDH architecture with the employed synthesis conditions. The provided a template consumpted by self-transformation. Dry gel conversion method limits the hydrotalcite nuclear proliferation that limits the growth direction of hydrotalcite, get layered double hydroxides bent perpendicular to the preformed AAH surface and interconnecte to form large holes. Water and alkali are also important to affect the pH and Nucleation in a weak hydrothermal environment. Through investigation and sample characterization, we deduced the formation process of hierarchically porous MgAl-layered double hydroxide (LDH) architecture:first form a film of water and ammonia in front of the Al surface or internal, and salt is dissolved into the Mg2+, Al3+, ammonia hydrolysis provides OH", then nucleation and crystal growth of LDHs. that resembles a weak thermal environment.(3) As an example of potential applications, the as-synthesized LDH hierarchically meso/macroporous architecture was used as adsorbent for Cr(VI) and MB Removal. LDH architecture exhibits faster elimination and higher adsorption capacity due to the large specific area and abundant pore structure in the meso/macroporous hierarchical architecture, which may benefit the diffusion and provide more adsorption sites for Cr(VI) and MB.