Facile preparation methods of hydrotalcite layered materials and their structural characterization by combined techniques

Highlights

• Hydrotalcites are anionic clays, prepared and modified by many synthetic approaches.

• Inorganic or organic anions are intercalated to obtain many properties/applications.

• Facile and easily scalable preparations toward industrial applications are described.

• LDHs structures, properties and reactivity were characterized by several techniques.

• The perspective of larger LDHs industrial production and applications are discussed.

Abstract

Hydrotalcites are layered double hydroxides (LDHs) of the anionic clay family. They can be obtained by synthesis, with high purity and controlled stoichiometry but with much larger preparation efforts and costs with respect to natural ores. Their properties can be tailored by changing the metals of the layer, or by inserting proper inorganic or organic anions. The reason of the few industrial applications of LDHs might be the difficulty of scaling up their synthesis and anion exchange methods, requiring often cumbersome procedures for filtering, recovery and purification of the final products. Recently, huge efforts were made to obtain facile and easily scalable preparation methods, especially favoured by the almost complete elimination of solvents and water-based procedures. The structures of the nanocomposites were studied by several techniques, from electron microscopy and diffraction to X-ray powder diffraction (the unique structural techniques able to face the low crystallinity of these materials) coupled with others, to obtain an exhaustive view of LDHs structure and reactivity. These advancements and the perspectives of larger LDHs industrial applications are described.

Introduction

Hydrotalcites (HTLCs) are layered double hydroxides (LDHs) with general formula [M(II)_1−xM(III)_x(OH)₂]^x+(Yⁿ⁻)_x/n·mH₂O. M(II) and M(III) are bivalent and trivalent cations, octahedrally coordinated to six hydroxyl groups to form a positively charged layer. The ratio between the divalent and trivalent cations exploits the so called anionic exchange capacity (a.e.c.), typically expressed as meq/g or meq/100 g. The Yⁿ⁻ anions compensate the positive charges, and are placed in the interlayer space. They were discovered at first as natural ores (typically with Mg and Al as metals, substituted by variable amounts of vicariant metals and CO₃²⁻ as anion), showing low cost but low purity and complex chemistry. Conversely, they can be obtained by synthesis with high purity and controlled stoichiometry, but much larger preparation efforts and costs. Their properties can be tailored by changing the metals of the inorganic layer, or by changing the inorganic or organic anions between the inorganic layers. In fact, many metals were tested to partially or fully substituting Al³⁺and Mg²⁺. At the same time, many organic anions were inserted into the layers. The preparation methods span from solution, precipitation, solid-state and mechanochemical methods. These materials were widely investigated at the academic level for the application in almost all fields of science. Inorganic hydrotalcites are widely used in heterogeneous catalysis, [1], [2], [3], [4], [5] flame-retardant additives in polymeric materials, [6], [7], [8] and for wastewater treatment [5], [9], [10]. Many authors proposed the use of LDHs for intercalating bioactive molecules, largely employed in the pharmaceutical and cosmetic field, with the purpose of protecting them by degradation, enhance their water solubility to increase bioavailability, and/or to obtain modified release properties [11], [12], [13]. In the case of sunscreens, the intercalation into LDHs prevents the contact between the organic moiety and the skin and can improve the sunscreen photostability [14], [15], [16]. Compounds like retinoic acid (vitamin A), ascorbic acid (vitamin C) and tocopherol (vitamin E) are all sensitive to light, heat and oxygen when in solution. It was proposed that incorporating the molecules into a layered inorganic lattice might lead to their stabilization, resulting in a wider range of potential applications [17]. A number of cardiovascular, anti-inflammatory agents are either carboxylic acids or carboxylic derivatives (hence negatively charged as ions), so they could be intercalated by ion exchange in a LDH obtaining a controlled release. Furthermore, LDHs possess anti-acid and anti-pepsin properties and many anti-acids products are LDH based. Apart from the potential of using these materials to deliver drugs in vivo, it is possible to control the point of release and the pharmacokinetic profile by selecting the metal ions of the layered structure [18], [19], [20]. Anti-acid performance and pH stability for the intercalated species are also improved by the restriction of molecular interactions and dynamics, and this should also improve long-term stability. Beside the above-mentioned compounds, also surfactants, antibiotics, biomolecules such as amino acids, DNA and ATP, metal complexes and polymeric anions have been inserted into LDHs.

Despite such immense literature, only few “real world” applications of inorganic hydrotalcites are known: anti-acid drugs (see a list at web site https://www.drugs.com/international/hydrotalcite.html), additive for polymers [21] to reduce acidic fumes (i.e. in PVC thermal degradation) and self cleaning TiO₂-LDH coatings [3] for building materials. Conversely, none of the organic-inorganic nanocomposites proposed in the scientific literature was considered. A drawback, and a possible explanation of this lack of technological applications, is the difficulty of scaling up the preparation methods proposed in the literature for the anion exchange. In recent years, huge efforts have been carried out to propose new facile methods (mechanochemical and one-pot approaches) for hydrotalcite preparation and/or exchange, making easier the scale up of the production. In parallel, the structures of the nanocomposites were studied by different techniques, from electron diffraction to X-ray powder diffraction (the unique techniques able to face the low crystallinity of these materials), coupled with other techniques to obtain an exhaustive view of hydrotalcite structure and reactivity. This review describes these advancements and the perspectives for hydrotalcite industrial applications.