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

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.

Exploring the Influence of Milling Parameters on the Wet Mechanochemical Synthesis of Mg-Al Layered Double Hydroxides

“…The synthesis method included a wet grinding step followed by an additional ageing step at 80°C for 24 h. LDH synthesis was deemed to be successful with conversion observed to be incomplete. This study serves as a continuation of previous work [28] by expanding on the one-step wet mechanochemical synthesis of Mg-Al LDH, using a NETZSCH LME 1 horizontal bead mill, through varying milling and synthesis parameters. Raw materials selected were a combination of commercial grade hydroxides and oxides, promoting green synthesis and providing insight on commercial viability.…”

“…Selected raw materials were wet batch milled under air atmosphere. Batch set-up was as described and illustrated by [28]. Starting materials selected were commercial grade MgO (86% Chamotte Holdings) and Al(OH) 3 (99.2% Belguam Indal), at a set M 2+ :M 3 ratio of 2:1, facilitating the synthesis of Mg 4 Al 2 (OH) 14 •nH 2 O. MgO was selected to increase system pH such that Al(OH) 3 dissolution was encouraged, and was calcined at 800°C for 1 h to remove any carbonates or hydroxide phases.…”

“…Temperature effects on the synthesis of LDH materials are considered to be complex. Agitation of milled samples at elevated temperatures have resulted in both an increase in crystallinity and conversion of raw materials to LDH product [28]. It could be possible that samples exposed 50°C temperatures exhibit an increase in conversion of raw materials to LDH product as well as improvements on the crystallinity.…”

“…Additionally, LDH synthesis methods have involved dry grinding followed by washing or thermal treatment [3,22], wet grinding [17,23] or methods making use of ultrasonic irradiation [24][25][26]. Limited research has been conducted on single step wet grinding and low conversion rates obtained warrant the need for further research [3,27,28]. Incomplete conversion or little to no LDH formation has previously been attributed to the quantity of water present, with insufficient mechanical activation of raw materials occurring [17].…”

“…A recent study attempted to expand on the wet mechanochemical synthesis of Ca-Al, Mg-Al, Zn-Al and Cu-Al LDH materials by making use of a NETZSCH LME 1 horizontal bead mill [28]. The synthesis method included a wet grinding step followed by an additional ageing step at 80°C for 24 h. LDH synthesis was deemed to be successful with conversion observed to be incomplete.…”