Method for Large-Scale Production of Multimetallic Layered Double Hydroxides: Formation Mechanism Discernment

Abstract

A simple, economical, and environmentally friendly method for the production of layered double hydroxides (LDHs) is presented. The synthesis procedure is based on dispersing insoluble metal oxides, adjusting the pH by adding an optimum amount of metal nitrates, and dispersing and aging the product for a short time (6-8 h). The final product does not require washing, opposite to the traditional coprecipitation synthesis procedure. A dissolution-precipitation-recrystallization mechanism is proposed for the formation of LDHs, based on particle size measurements, XRD analyses, radial distribution functions, and 27 Al MAS NMR studies. Solids were characterized by XRD, N 2 physisorption, TGA-DTA, SEM, and TEM, revealing that both LDHs and their calcination products have very similar properties to those prepared by conventional procedures. Pure LDH phase is obtained after 6-8 h; a large, uniform particle size that would usually require prolonged hydrothermal treatments is attained. Surface areas ranged from 32 to 93 m 2 g -1 and from 140 to 230 m 2 g -1 for fresh and calcined samples, respectively. This new method is intended to satisfy the growing demand of LDHs in large-scale applications as catalysts, SO x adsorbents, PVC additives, etc.

Innovative method for hydrocalumite-like compounds' preparation and their evaluation in the transesterification reaction

“…In this context, we have reported previously for hydrotalcite-like compound synthesis, that the material is formed by a combined dissolution-precipitation-recrystallization mechanisms (Valente et al, 2009). By consequence, the presence of calcium hydroxide and calcite can be attributed to the incomplete dissolution of these compounds in commercial hydrated lime.…”

“…The methodology employed in this study was very similar to that reported for hydrotalcite-like compound preparation reported by Valente et al (2008Valente et al ( , 2009 where the raw materials are insoluble oxides, and the method is based on dissociation, hydrolysis, and peptization reactions. Materials are obtained under mild conditions and short synthesis times, using a minimal amount of water providing a sustainable method for obtaining LDH.…”

“…Nevertheless, it is important to recognize that this method requires thermal treatments and prolonged aging times to crystallize HC and HLC, together with intensive washing to eliminate undesirable ions. In this sense, all these issues generate environmental, process and economic drawbacks restricting their large-scale production (Valente et al, 2008(Valente et al, , 2009). An alternative is given by the use of raw materials which do not introduce undesirable anions into the process, adjusting the process variables such as pH, aging temperature and time.…”

Preparation of biomorphic Ni–Al LDHs using cotton from discarded T–shirt as a template and the adsorption capability for Congo red

“…The possible mechanism of growth of LDH has been discussed elsewhere [25,26]. Decomposition of LDH by calcination at high temperature leads to mixed metal oxides (calcined layered double hydroxides, CLDH), which deserve concern as adsorbents because they have the potential to remove anionic dyes.…”

Mesoporous mixed metal oxides derived from P123-templated Mg–Al layered double hydroxides

“…Layered double hydroxides (LDHs) are well-known anionic or hydrotalcite-like clays with the general chemical formula [M 1 À x II M x III (OH) 2 ] x + [A n À ] x/n Á mH 2 O, where M II and M III are divalent and trivalent cations and A n À can be almost any organic or inorganic anion [12]. Their structure is based on M 2 + (OH) 6 octahedral units sharing edges to build M(OH) 2 brucite-like layers.…”