Hydrotalcite-Based Materials: Synthesis, Characterization and Application

Hydrotalcite and Hydrotalcite-Based Materials

K Ganesh Kadiyala*, ¹, Kadali Jagadeesh¹

¹ Department of Chemistry, Shri Vishnu Engineering College for Women, Bhimavaram, Andhra Pradesh-534202, India

Abstract

This chapter deals with the history of hydrotalcite and hydrotalcite-based materials. A rare mineral known as hydrotalcite was found in Sweden sometimes in the 1840s. Magnesium aluminum hydroxycarbonate, Mg6Al2(OH)16CO34H2O, is its chemical name, and Taylor and Allmann independently determined its layered structure. For a long time, hydrotalcite and other isomorphous minerals (such as piroaurite, sjogrenite, and takovite) were the focus of most mineralogical studies. However, beginning in the 1970s, it was discovered that these rare minerals, also known as anionic clays, could be prepared quickly and affordably in a laboratory and have a variety of intriguing chemical properties. The different arrangements of the stacking of the layers, the ordering of the metal cations, as well as the arrangement of anions and water molecules in the interlayer galleries, result in a variety of stoichiometry in hydrotalcite, which are layered double hydroxides. Due to their unique characteristics, including their enormous surface area, ion exchangeability, insolubility in water, and most organic sorbents, among others, the compounds of the hydrotalcite group demonstrate a wide variety of potential uses.

Keywords: Acid neutralization, Brucite, Building materials, Carbon dioxide absorbers, Co-precipitation, Corrosion inhibitors, Hydrotalcite, Hydrotalcite-like compounds (HTlc), Layered double hydroxide, Photocatalyst, Polymer composites.

* Corresponding author's K Ganesh Kadiyala:Department of Chemistry, Shri Vishnu Engineering College for Women, Bhimavaram, Andhra Pradesh-534202, India; E-mails: [email protected],

[email protected]

1. INTRODUCTION

Hydrotalcites, which exist in both natural and synthetic forms, exhibit a structural relationship with the mineral brucite, denoted as Mg(OH)2. Hydrotalcites typically consist of three distinct layers: an inner intermediate layer that accommodates water molecules and replacement anions, as well as two outer layers characterized by a positive charge and one outer layer characterized by a negative charge. The mineral hydrotalcite, with the chemical formula Mg6Al2(OH)16CO34H2O, is widely recognized as the most prominent and ancient example of layered double

hydroxides (LDHs) [1]. In this study, we aim to investigate the impact of social media usage on mental health. For a considerable period, extensive mineralogical investigations primarily centered around hydrotalcite and other isomorphous minerals, including piroaurite, sjogrenite, and takovite. However, a significant shift occurred in the 1970s when it was revealed that these uncommon minerals, commonly referred to as anionic clays, could be readily and inexpensively synthesized within a laboratory setting. Moreover, these synthetic anionic clays exhibited a diverse range of captivating chemical characteristics.

The compounds produced were known as layered double hydroxides (LDH) or hydrotalcite-like compounds (HTlc), and they are usually denoted with the empirical formula [M(II)1-x M(III)x(OH)2]x+ [An-x/n]x-. mH2O, here M(II) and M(III) are appropriately sized bivalent and trivalent metal cations, respectively. The molar ratio of trivalent cations in the hydroxide layers, M³+/(M²++M³+), is used as the value of x in the calculation above, and it typically ranges between 0.20 to 0.33 [2]. These substances possess a wide range of applications, including their utilization as agents for decolorization, stabilizers, filters, adsorbents, catalyst supports, and ion exchange materials [3]. In recent studies, researchers have explored the potential of utilizing these materials for CO2 adsorption due to their notable characteristics, including their strong adsorption behaviour [4], stable anion exchange properties, efficient mobility of anions and water molecules facilitated by their expansive surface areas, and robust frame structure.

Hydrotalcites have been widely employed in the production and processing of polymers, primarily in the capacity of flame retardants, neutralizing additives, and thermal stabilizing compounds for PVC processing [5-7]. Hydrotalcites that are extensively employed in many applications within the field of heterogeneous catalysis, primarily serving as precursors for the synthesis of catalysts that rely on mixed oxides. Furthermore, they can be employed in sorption and cleaning methodologies, as well as for the intercalation of various molecules, including pharmaceutical substances.

In the interlayer region, the presence of anions and water molecules balances out the extra positive charges that come from the substitution of trivalent cations. This creates an appropriately electrically balanced structure [8]. The chemical structures of hydrotalcites can be modified to enhance their adsorption activity and postpone particle sintering by including dopants such as alkali metals or altering the metal composition of the framework. To make it easier for carbon dioxide molecules to be absorbed, HTlcs with the carbonate ion have more empty space and wider gaps between the layers than those with OH (0.765 mm vs. 0.755 mm) [8, 9].

2. HISTORY AND MAIN FACTS ON HYDROTALCITE BASED MATERIALS

A study by Yong et al. found that the best CO2 adsorption happened when the composition had an equal amount of divalent (Mg²+) and trivalent (Al³+) cations. The addition of aluminium in HTLCs leads to an enhancement of layer charges, resulting in a contraction of the interlayer gap and a reduction in the number of adsorption sites.

The alteration of the anion from OH- to CO3²- in the HTlcs resulted in an increase in adsorption capacities from 0.25 to 0.5 mmol/g at a temperature of 300 oC and a pressure of 1 bar. This can be attributed to the larger interlayer spacing and higher layer charge observed in the HTlcs with CO3²- anions.

In their study on Mg-AlCO3 layered double oxide (LDO), Reddy et al. observed both physisorption and chemisorption happening at the same time during the adsorption process. Additionally, they observed both reversible (physical) and irreversible (chemical) adsorption of CO2. As the increase in temperatures occurred, there was a noticeable decrease in the adsorption capacity. Nevertheless, it was observed that the optimal capture capacity was achieved at a temperature of 200 oC instead of 100 oC, indicating that physical adsorption plays a more significant role at a higher temperature. This shows that CO2 adsorption on Mg-Al-CO3 layered double oxide (LDO) is a process that involves both chemical and physical processes that happen in many places. A significant proportion of the adsorption seen within the examined temperature range, specifically over 78%, was found to be reversible. Increased kinetic energy or elevated temperatures enhance the favourability of desorption from the surface of the sorbent.

This chapter will primarily focus on the synthesis of hydrotalcite and its various applications in construction materials, radioactive waste repositories, corrosion prevention in reinforced concrete, cements and mortars, polymer composites, and acid neutralization. Hydrotalcite has garnered significant attention from the scientific community due to its diverse applications. Consequently, there has been a growing body of literature exploring the utilization of hydrotalcite-based products and their possible environmental advantages.

2.1. Hydrotalcites Synthesis

The most common way to make hydrotalcite is for two metal salts to react with each other in an alkaline solution that stays at a pH level of about 10. Usually, the hydrothermal method is used to make hydrotalcite, which means that the best reaction conditions need to be set up to get consistent product quality [10-14].

Furthermore, the sol-gel technique and the hydrolysis reaction are commonly utilized [15-18]. The co-precipitation technique is predominantly based on the simultaneous precipitation of inorganic minerals (salts) from alkaline solutions, either at a fixed pH or with a gradual increase in pH [19].

The combustion process of synthesis can be advantageous due to the rapid chemical reaction resulting from the explosive breakdown of organic fuels such as glycine or urea. The production of carbon dioxide and water occurs throughout the combustion process to facilitate the formation of complexes with metal ions [20, 21].

The disruption of the crystal lattice of hydrotalcites generated during combustion can be achieved through the application of heat. Subsequently, the hydrotalcites can be subjected to recrystallization in an aqueous carbonate solution [22]. By employing microwave-assisted synthesis, it is possible to produce sulfate-intercalated hydrotalcites that may effectively eliminate ammonia and nitrogen. The utilization of microwave irradiation in this procedure facilitates the attainment of enhanced crystallinity in the resulting material by the intercalation of sulphate anions [23].

An alternative approach involves the utilization of industrial waste for the synthesis of hydrotalcites. The synthesis was successfully conducted, as elaborated upon subsequently. The leaching solution used in this study had a concentration of three moles per liter (3 mol/L) of hydrochloric acid (HCl). The solution was subjected to an aging process at a temperature of 65 °C for a duration of 30 min while maintaining a pH level of 11.5. Subsequently, the solution was subjected to a crystallization process for a period of 12 hours at a temperature of 70 °C. Prepared hydrotalcite consists of sub-micron plate-like particles, exhibiting a significant external surface area and minimal microporosity. With the potential exclusion of the existence of calcite, the structural attributes of hydrotalcite derived from fly ash exhibited similarities to those of hydrotalcites produced from unadulterated raw chemicals [24]. The hydrotalcite-like substances were produced through the utilization of solely recycled paper as a primary material. This was achieved by subjecting the paper sludge obtained from the newspaper sector to calcination, followed by a pozzolanic reaction [25]. Hydrotalcite-like compounds were synthesized by employing a mixture of activated art paper sludge and lime at a temperature of 20 °C [26].

2.2. Hydrotalcites General Applications

2.2.1. Hydrotalcites in Building Materials

Hydrotalcites, a mineral substance that is abundant and potentially easily manufacturable in large quantities, were considered a viable candidate for utilization as a construction material or as a constituent in such materials. These substances are commonly employed as constituents in mortar and cement, serving to impede the corrosion caused by chloride in concrete. This is owing to their unique characteristics and cost-effectiveness. In addition, significant research has been conducted on the utilization of these materials as constituents in polymer composites, their ability to absorb and retain hazardous substances, and their application in the management of nuclear waste repositories [27-30].

2.2.2. Hydrotalcites in Radioactive Waste Repository

It is possible to create customized materials with the ion exchange capacity that hydrotalcites exhibit in order to sequester harmful elemental ions. The point of this study was to look at how radioactive iodine isotopes that are in the anion state stick to hydrotalcites that have been heated. The sample subjected to calcination at 773 K exhibited enhanced sorption of I- anions, which can be attributed to the disruption of the crystal lattice and the subsequent recreation of the hydrotalcite structure [31]. Furthermore, the possible utilization of hydrotalcites as anion scavengers in the backfills of repositories for low-level radioactive waste is being considered, particularly in cases where the pH is about neutral for both I- and TcO4- anions [32].

Hydrotalcites exhibit promising attributes for utilization as cost-effective and readily available construction materials (Fig. 1), owing to their capacity to be incorporated into diverse compositions of building material blends, mortars, concretes, and backfills.

2.2.3. Hydrotalcites in Reinforced Concrete to inhibit the Corrosion

The fundamental concern associated with reinforced concrete structures is the corrosion of steel reinforcement, especially in coastal environments [33, 34]. Solid concrete constructions sometimes use reinforcement steel with a passivated coating to mitigate the risk of rusting, owing to the elevated pH levels. Nonetheless, the inner steel reinforcement may undergo passivation in the presence of a sufficiently high concentration of chloride ions, initiating the corrosion process within the structure. This phenomenon may occur in a coastal environment or, more precisely, immediately following the exposure of the substance to seawater. The corrosion of reinforcement in concrete structures and components incurs substantial overall expenses due to the need for maintenance, repair, safety monitoring, and aesthetic enhancements. Ensuring concrete durability is of paramount importance in mitigating chloride-induced corrosion.

Fig. (1))

Applications of Hydrotalcites in building materials.

2.2.4. Hydrotalcites in Cements and Mortars

The employment of hydrotalcites in isolating hazardous heavy metals in cement derived from industrial waste is an attractive prospect. Hydrotalcites can be utilized in the production of cement-based mortars for the purpose of immobilizing lead within electric arc furnace dust. This approach offers a dual barrier mechanism to effectively contain the lead. The incorporation of dimercaptosuccinate in the hydrotalcite interlayer of double barrier mortars resulted in a significant reduction of lead release by approximately 50% when compared to the conventional immobilization mortar [35].

More cement-like materials are often used instead of cement clinker to lower the pollution that comes from making cement. This often results in the formation of hydrotalcites in mortars and cements. In recent times, an investigation was conducted wherein the utilization of magnesium oxide, metakaolin, or dolomite was examined [36]. The incorporation of alumina derived from unprocessed aluminosilicate has the potential to improve the mechanical properties of mortars. When wet magnesium oxide is added to a cementitious matrix, it can improve the interaction and lead to the formation of a phase that is similar to hydrotalcite. This phase alteration significantly enhances the mechanical properties [37]. Moreover, a substance that contains water molecules in a hydrated state may exhibit reduced overall porosity [38].

2.2.5. Hydrotalcites in Polymer Composites

Polyurethane foam is a very notable thermoset material that is commonly employed as insulation in the construction and building sectors. Previous studies have shown evidence for the feasibility of producing stiff foam composites through the utilization of polyether polyol, isocyanate, a fire-retardant agent, hydrotalcite, and polyethylene terephthalate (PET) [39]. Research findings have shown that the addition of a moderate quantity of hydrotalcite enhances compressive stress. However, an excessive quantity of hydrotalcite has been seen to have a detrimental effect. Furthermore, the investigation involved the examination of altered layered double hydroxides within the context of flexible polyurethane foam nanocomposites [40].

In a different study, the effects of adding hydrotalcites or other layered double hydroxides (LDH) and a phosphorus-containing polyol (E560) to castor oil-based polyol (LB50)-made flexible polyurethane foams were studied [41]. It was interesting to see how flame-retardant composites made of hydrotalcites, tris (1-chloro-2-propyl) phosphate, and isocyanate-based polyimide foams worked [42]. A lot of information was given about how to make hydrotalcites and isostructural layered double hydroxides, as well as how to add them to polymers. The utilization of hydrotalcites as flame retardants and heat retention modifiers in horticultural plastic films was elucidated. By using hydrotalcites and other layered double hydroxides (LDH), it is easier to keep colors stable in polymers like polyvinyl chloride (PVC) and other polymer materials [43].

2.2.6. Hydrotalcites in Acid Neutralization

The most commonly employed method for synthesizing compounds like hydrotalcite is coprecipitation. This technique involves the interaction between a solution containing the appropriate quantities of metal cations and an alkaline solution. The products obtained through coprecipitation under high supersaturation circumstances exhibit a higher degree of crystallinity as opposed to those obtained under different conditions [44]. Nevertheless, other experimental variables, such as reaction pH and temperature, solution concentration, flow rate during reactant addition, hydrodynamic parameters in the reactor, and post-synthesis methods, could potentially influence the crystallinity of the resulting product. In addition to hydrotalcite, calcium, and zinc stearates are also recognized as notable acid scavengers.

Acid scavenging serves as the primary defense mechanism against the autocatalytic degradation of polyvinyl chloride (PVC). In the past, calcium/zinc stearates and other reactive compounds were used to keep PVC from melting during heat. They did this by reacting with hydrogen chloride and protecting the material from some UV damage [45, 46]. Nevertheless, it is important to note that polyvinyl chloride (PVC) is not the sole polymer necessitating acid neutralization. Numerous other polymers also want this process. The investigation focused on examining the photostability of a composite material consisting of EPDM and hydrotalcite [47]. While hydrotalcite was first favored, the utilization of pure polymers demonstrated enhanced stability in materials subjected to both UV light and an acidic environment. The incorporation of phosphites and hydrotalcite enhances the stability of polyethylene terephthalate [48]. Phosphites can also function as acid scavengers; however, it should be noted that they are not hydrolytically stable. It is highly probable that hydrotalcite plays a role in inhibiting their hydrolysis.

Researchers have found that hydrotalcites can work similarly in some outdoor-use polyolefin mixtures, where they can effectively fix imbalances caused by acid and stop phosphite hydrolysis. In addition, these formulations are comprised of zinc stearate and calcium stearate, which serve the purpose of acid neutralization [49]. When hydrotalcite is added to polyolefins, it reduces the yellowing that is caused by magnesium chloride, which is a part of the polymerization catalyst. The incorporation of hydrotalcite into a polystyrene formulation did not alter the photooxidation mechanisms of the composition; however, it did exert little influence on the rate of oxidation [50]. Hydrotalcite was employed in several formulation combinations. The hydrotalcite compound was used to join the UV absorber in sunscreen. This made it easier for the UV absorber to stay in place and kept it away from the skin to avoid any allergic reactions [51]. Hydrotalcite and ferulic acid were mixed in an alternative formulation of sunscreen with comparable purposes. The utilization of hydrotalcite has demonstrated its efficacy in providing efficient photofading protection for anionic natural dyes [52].

3. WHY HYDROTALCITE ATTRACTED THE SCIENTIFIC COMMUNITIES

There are numerous intriguing applications for hydrotalcite. The applications of hydrotalcites are likely to attract the interest of researchers. The scientific community is attracted to several applications of hydrotalcite and its derivatives.

One burgeoning field of research involves the utilization of hydrotalcites as carbon dioxide (CO2) sinks and for the purpose of mitigating climate change [53].

Layered double hydroxides (LDH) are a group of materials that have garnered significant interest in the industrial sector. This is mostly attributed to their straightforward synthesis process and the potential to incorporate extra-layer cations and interlayer anions.

These materials find various significant uses, including catalysis, catalyst support, anion scavenging, polymer stabilization, drug transportation, and adsorption.

The significance of the structure of high-temperature (HT) compounds is emphasized in the development of Ni and noble metal-based catalysts, which are synthesized by the controlled thermal decomposition of precursors, including carbonates or silicates [54].

Mixed oxides of magnesium (Mg), nickel (Ni), or zinc (Zn) with aluminum (Al), which come from hydrotalcite-type compounds, are used to remove sodium diclofenac from water-based solutions that are made to look like real ones [55].

4. NUMBER OF PUBLICATIONS INCREASED DAY BY DAY ON HYDROTALCITE-BASED MATERIALS

Numerous scientific publications and bibliographic review papers have been published due to the widespread interest in hydrotalcites (9300 results when searching for hydrotalcite at Web of ScienceTM). The 1999 paper by Rives and Ulibarri, which examined the scientific literature on the synthesis, characteristics, and uses of hydrotalcite-like materials containing intercalated anions made up of metal complexes or oxometalates [56], is one of the most significant revision papers. It was cited 770 times by Web of Science and 1000 times by Google Scholar.

5. ENVIRONMENTAL FATE

5.1. Hydrotalcite acts as a Photocatalyst for the Degradation of 2,4,6-Trichlorophenol

When researchers looked at how 2,4,6-trichlorophenol breaks down in light, using a catalyst with a Mg/Al ratio of 2 led to interesting results. These investigations have shown that hydrotalcites exhibit a substantial capacity for degradation, with levels as high as 100%. Furthermore, hydrotalcites also exhibit a significant capacity for mineralization, with levels reaching up to 80%. The degrading ability of the studied catalysts is mostly attributed to the formation of superoxide free radicals and the presence of holes, which are the key species in the degradation pathway [57].

The potential and suitability of activated hydrotalcites for utilization are supported by their capacity to undergo photocatalytic degradation, resulting in the production of 2,4,6-tri chlorophenol. The catalysts exhibit Eg values ranging from 3.45 to 3.56, which closely resemble the Eg values of TiO2. When subjected to ultraviolet (UV) radiation, materials exhibiting semiconductor properties possess the capability to generate superoxide and superoxide radicals as a result of the band gap energy. Mg/Al hydrotalcites possess mesoporosity, crystallinity, thermal stability, and memory effect, rendering them prospective substitutes for TiO2 and ZnO, as well as various photocatalytic materials that are doped with high metals but exhibit a higher cost and increased potential for toxicity. Based on what has been said so far, it seems that the memory effect changes the shape of hydrotalcite structures, which means that they are present in the water during the photocatalytic breakdown of 2,4,6-trichlorophenol [58]. The catalyst undergoes a process of structural restoration, enabling its reactivation and subsequent utilization in subsequent cycles of deterioration.

5.2. Future Application of Hydrotalcite as Sorbents under Dynamic Flow Conditions.

PXRD (powder X-ray diffraction), FT-IR, XPS (X-ray photoelectron spectroscopy), and SEM were used in the study to look at how the structure and surface content of hydrotalcite compounds changed after being in groundwater for a long time. The study's findings showed that the influence of intercalated anion (CO3²- > DS) and groundwater dynamics (static flow > dynamic flow) on the stability and dissolution of hydrotalcite compounds in groundwater was stronger than the influence of aggregate size. The deposition of insoluble species, namely calcium carbonate (CaCO3) and adsorbed sulphate, on the surface of hydrotalcite was influenced by the geochemical composition of the groundwater. The potential application of hydrotalcite compounds as sorbents in dynamic flow conditions may face significant limitations due to their inherent instability, particularly in the case of HT-DS [59].

The stability of HT compounds in the ground is mostly affected by two things: the dynamics of groundwater, specifically the change from static flow to dynamic flow; and the presence of intercalated anions, especially the carbonate anion (CO3²-) and the dithionite anion (DS). Upon exposure to groundwater, the HT-CO3 and HT-DS compounds underwent disintegration at the interface between the solid and liquid phases. This phenomenon had an impact on the particle surfaces and led to a slow dissolution of the HT aggregates unless a steady-state condition was achieved. The post-synthesis drying treatment of the HT-DS, whether conducted using an oven or a wet method, did not exhibit any noticeable influence on the dissolution kinetics.

5.3. Hydrotalcite Colloidal Stability and Interactions with Uranium

Uranyl carbonate species, like the compound Mg(UO2(CO3)3)2 (aq), were made in pH conditions that were very close to neutral. This happened because Mg²+ ions were leaching out of hydrotalcite and then interlayer carbonate exchanged with the solution around it. Negatively charged particles stuck to the hydrotalcite surface and formed tertiary complexes in both outer- and inner-sphere shapes. This conclusion was drawn from the analysis of X-ray absorption spectroscopy (XAS) and luminescence investigations. The results of this study demonstrate that hydrotalcite has the ability to form pseudo-colloids with U (VI) throughout a broad pH spectrum. These findings have important implications for our understanding of uranium dynamics in environments where hydrotalcite and other layered double hydroxides (LDHs) may be present [60].

It is very important to study hydrotalcite and other layered double hydroxides (LDHs) if you want to know how uranium acts in places where these minerals may be found. This is due to the ability of hydrotalcite to form pseudo-colloids with U (VI) across a broad range of pH values.

5.4. LDH in Chosen Environmental Applications

In recent years, LDHs (layered double hydroxides), hydrotalcites, and their associated materials have garnered significant interest due to their versatile production methods and many applications. By adding different M(II) and M(III) cations and interlayer anions to their structure, these materials can have their chemical makeup and structural features (like surface areas, pore sizes, and active site numbers) changed to suit specific needs. Because of this, they show promise as catalysts in many industrial and environmental settings, even though they are usually made using co-precipitation or hydrothermal methods. Additionally, these catalytic materials' memory effect makes it possible for multiple reuses, which is a fascinating feature when taking into account their use in industrial settings.

The identification of an appropriate synthesis technique is a crucial determinant in the successful utilization of LDHs and associated components. A technique called co-precipitation can be used to make layered double hydroxides (LDHs), which have a lot of basic sites and are very stable at high temperatures. These characteristics are essential for facilitating the adsorption of various pollutants. It is possible to get the acid sites through sol-gel or hydrothermal synthesis. They are where 5-hydroxymethylfurfural (5-HMF) is made. To make hydrotalcites more effective as catalysts, it is necessary to create hydrogen by intercalating, impregnating, or adding the right metals in a way that makes them spread out evenly [61].

CONCLUSION

Some of the most important parts of hydrotalcite are talked about in this chapter. It also talks about where it can be found in nature, what it can be used for (especially in building materials), and how it can help the environment by breaking down chemicals like 2,4,6-trichlorophenol. Additionally, the chapter explores the colloidal stability of hydrotalcite and its interactions with uranium, as well as its diverse applications across multiple disciplines. Hydrotalcites possess several notable characteristics, such as their capacity for ion exchange, expansive surface area, insolubility in water, and notable accessibility. And because of these qualities, hydrotalcites can be used in many different ways in construction materials, such as mortars, cement, and polymer-composite materials [62]. The use of ion exchange properties has been shown to be effective at stopping dangerous leaks from different types of waste disposal sites, such as radioactive waste dumps, and in keeping steel in reinforced concrete structures from rusting. Polymer composites can effectively incorporate flame retardants due to their mineral composition. The low cost and extensive availability of this material enable its utilization as a high-quality cement clinker, perhaps contributing to a reduction in emissions associated with cement manufacture.

REFERENCES