**Exploring the Wet Mechanochemical Synthesis of Mg-Al, Ca-Al, Zn-Al and Cu-Al Layered Double Hydroxides from Oxides, Hydroxides and Basic Carbonates**

#### **Brenda Antoinette Barnard \* and Frederick Johannes Willem Jacobus Labuschagné**

Department of Chemical Engineering, University of Pretoria, Lynnwood Rd, Hatfield, Pretoria 0002, South Africa; johan.labuschagne@up.ac.za

**\*** Correspondence: u14037948@tuks.co.za

Received: 21 September 2020; Accepted: 16 October 2020; Published: 20 October 2020

**Abstract:** The synthesis of Mg-Al, Ca-Al, Zn-Al and Cu-Al layered double hydroxides (LDHs) was investigated with a one-step wet mechanochemical route. The research aims to expand on the mechanochemical synthesis of LDH using a mill designed for wet grinding application. A 10% slurry of solids was added to a Netzsch LME 1 horizontal bead mill and milled for 1 h at 2000 rpm. Milling conditions were selected according to machine limitations and as an initial exploratory starting point. Precursor materials selected consisted of a mixture of oxides, hydroxides and basic carbonates. Samples obtained were divided such that half was filtered and dried at 60 ◦C for 12 h. The remaining half of the samples were further subjected to ageing at 80 ◦C for 24 h as a possible second step to the synthesis procedure. Synthesis conditions, such as selected precursor materials and the MII:MIII ratio, were adapted from existing mechanochemical methods. LDH synthesis prior to ageing was successful with precursor materials observably present within each sample. No Cu-Al LDH was clearly identifiable. Ageing of samples resulted in an increase in the conversion of raw materials to LDH product. The research offers a promising 'green' method for LDH synthesis without the production of environmentally harmful salt effluent. The synthesis technique warrants further exploration with potential for future commercial up-scaling.

**Keywords:** layered double hydroxide; mechanochemistry; bead mill; green chemistry; synthesis; wet grinding
