**1. Introduction**

Layered double hydroxides (LDHs) are clay-like minerals commonly referred to as anionic clays with a wide range of physical and chemical properties. They are represented by the general formula [MII1−<sup>x</sup><sup>M</sup>IIIx(OH)2][Xq<sup>−</sup>x/q·H2O] in which MII and MIII represent the selected divalent and trivalent metal elements and [Xq<sup>−</sup>x/q·H2O] denotes the interlayer composition. LDHs often find application in pharmaceuticals, as polymer additives, as additives in cosmetics, and in catalysis. This is due to having variable layer charge density, reactive interlayer space, ion exchange capabilities, a wide range of chemical compositions and rheological properties [1]. LDH materials can be synthesised using various different techniques of which the most common are co-precipitation, reconstruction, hydrothermal methods and urea decomposition-homogenous precipitation. The primary principle associated with these methods include the precipitation of various types of metal ions which makes large scale production difficult. Challenges associated with these methods include differing precipitation rates of metal ions, need for inert environments, production of environmentally harmful waste and high production costs [2]. Novel, 'green' synthesis techniques are therefore often sought after. Recently the use of mechanochemistry as an alternative synthesis procedure has gained wide-spread attention. Mechanochemistry is considered a versatile method of synthesis with the promise of producing LDH materials with unique elemental combinations [3,4]. The most common types of mechanochemical synthesis techniques include single-step or one-pot grinding [5,6], mechano-hydrothermal synthesis [7–10] and two step grinding. Two-step grinding can consist of an initial grinding step followed by an additional treatment step or a second grinding step [11–13]. Grinding of raw materials can be conducted wet, dry or as a paste. Various techniques and combinations involving the wet or dry milling of raw materials have been attempted and found to be successful [2]. Studies have shown that the type of grinding technique can largely a ffect the success of LDH synthesis, with some techniques not producing su fficient mechanical energy for the synthesis to occur readily [11]. Research has indicated that a large amount of mechanochemical methods explored typically involve the use of ball mills, mixer mills or a mortar and pestle as the primary grinding technique [2]. The final properties of LDH are further influenced by the selected method of grinding [14]. It is therefore of interest to expand on the e ffect of milling techniques on the synthesis of LDH materials. The success associated with the formation of an LDH phase for single step grinding procedures are further influenced by the selected starting materials [2]. The use of metallic salts of chlorides or nitrates allows for LDH synthesis but introduces a washing step that could produce an undesirable waste solution [5,6]. The use of hydroxides and oxides eliminates the production of waste solution promoting 'green' synthesis of LDH materials, however, has proven to be challenging [2]. The addition of water to existing grinding techniques, such that wet grinding occurs, is considered unsuitable for solid state chemistry as it may reduce the degree of amorphitization and prevent active site formation [15]. Dry grinding is therefore typically conducted as an initial mechanochemical step when synthesising LDHs. The absence of water allows for su fficient active site formation and amorphitisation. Dry grinding of the precursor materials is regularly used in conjunction with a second synthesis step. A variation of secondary synthesis steps have been explored. LDH materials have successfully been synthesised with the dry grinding of raw materials and agitating the milled material in a solution containing the desired anion for intercalation [16–19]. Similarly, LDH synthesis methods have involved dry grinding followed by washing or thermal treatment of the sample [2,20]. Unique methods have also involved a combination of the initial dry grinding step with that of a wet grinding step [15,21] or methods involving ultrasonic irradiation [22–24]. Limited research has been conducted on single-step or one-pot wet grinding and low conversion rates obtained warrant the need for further research [2,25]. Incomplete conversion or no LDH formation have been attributed to the quantity of water present with insu fficient mechanochemical activation of the precursor materials occurring [15]. The study therefore aims to expand on the one-step wet mechanochemical synthesis of layered double hydroxides, from oxides, hydroxides and basic carbonates, by making use of a Netzsch LME 1 horizontal bead mill. The selected mill is designed specifically for wet grinding application and allows for the continuous, semi batch or batch synthesis of LDH materials. The process could be easily up scaled to produce large volumes of consistent and commercially viable LDH product. Precursor materials and MII:MIII ratios were adapted from mechanochemical techniques in which LDH synthesis was successful [15,17,18,21]. The performance of the selected mill and synthesis conditions could therefore be investigated. Samples obtained were further subjected to ageing at 80 ◦C to determine the e ffects of including a thermal step to the selected mechanochemical method.

## **2. Materials and Methods**
